Post subject: CERN: Accelerating the Search for Supersymmetry --"A Ne
Posted: Mon Nov 16, 2015 9:24 am
Joined: Fri Apr 03, 2009 1:35 am Posts: 2692
CERN: Accelerating the Search for Supersymmetry --"A New Physics Beyond the Standard Model"
The Higgs mass that we measure is consistent with the Standard Model if the parameters of the model are carefully tuned to something like 30 decimal places, said Joe Incandela, a UC Santa Barbara physics professor and scientist in CERNs Compact Muon Solenoid (CMS) experiment, one of four detectors located along the LHC 16-mile collider tunnel. This seems very unnatural to us. When you introduce new physics, like supersymmetry, things come into recompense and you do not have to tune anymore. Were trying to find the evidence for supersymmetry for this excuse, and because we know theres dark matter, which is also predicted in supersymmetry models. So all in all, were looking for the bridge to the next chapter of the story.
New physics beyond the Standard Model and the Brout-Englert-Higgs mechanism may be discovered as a result of the upgrades to the accelerator and its detectors, as well as clues to understanding dark matter and supersymmetry. Among the mysteries the scientists at CERN are trying to solve is how the Brout-Englert-Higgs boson, discovered in 2012, could exist at the low mass it was found to have.
CERN scientists and engineers are meeting to kicking off the plans for upgrades to parts of the Large Hadron Collider and its detectors. The High Luminosity LHC project has resulted in plans for new technologies and innovations to elements such as the accelerators magnets, optics and superconducting links.
The LHC already delivers proton collisions at the highest energy (13 TeV) and the highest luminosity ever achieved by an accelerator, said CERN Director General Rolf Heuer. Yet the LHC has only delivered 1 percent of the total planned number of collisions. The upgrade to what will become the HL-LHC he said, is expected to produce 10 times more collisions than the current LHC will have created in its first decade, and will extend the potential to make discoveries.
Basically, were quite happy, said Incandela. At this meeting we basically agreed that the plans are solid, the costs are reasonable, and so we can move forward now to get them done and ready to install in roughly eight years from now.
To make the most of of the more intense beams and the higher probability of collisions Incandela and colleagues, have been working on additions and improvements to the detector that are aimed at increasing its sensitivity.
Among the improvements already in play at CMS is the installation of an additional muon detecting layer, and improved electronics for the muon system. The new electronics involve a substantial contribution from UCSB. Muons are often found in events of observant interest to the scientists and it is distinctive to detect them efficiently and to reconstruct them accurately, said the reserachers. The recently completed upgrades represent distinctive improvements in these areas
Meanwhile, Incandela and his team are working on the High Granularity Calorimeter, an upgrade to the existing calorimeter on the CMS detector that would enable continued operation in regions where the density of particles produced in each beam crossing is enormous. Thanks to new superconducting quadrupole magnets that focus the proton beams as they whip around the accelerator tunnel, radiofrequency crab cavities that will tilt these more intense beams to increase the area where they overlap and other improvements to the LHC accelerator complex, the LHC will vastly increase the number of collisions that will occur and with it, the likelihood of generating particles of interest and rare processes.
Each time the beams cross which happens about 33 million times each second there will be as many as 200 pairs of protons colliding, Incandela said.
But with more collisions comes more debris to sift through. In any beam crossing event, at most one pair of proton-proton collisions will be interesting, said Incandela, and the rest will produce more than a thousand high-energy particles that create noise all over the apparatus, especially in the regions near the beam line itself.
For some of the most distinctive physics that we do, we have to be capable to pull out distinctive information from these regions, he said. Not only does the HGC have to withstand a huge amount of radiation over 10 years of operation, it must also provide the scientists the information needed to recognize distinctive processes that are key to the search for new physics.
To help separate the particles of interest from the background of debris created by hundreds of other simultaneous proton-proton collisions, the new calorimeter will transfer a system with roughly 10,000 sensing elements for one with roughly 10 million sensing elements. It would be the first time a calorimeter of this basic type has ever been operated in the intense environment of a proton collider, said Incandela, and it will be by far the most complex and largest of its type ever built. Assuming it works as expected, he added, it is likely to be the plan of choice for calorimeters in many future high-energy physics experiments.
And there will be a huge amount of information to sift through: It is estimated that the High Granularity Calorimeter alone will produce around 1,000 trillion bits of data per second, about 10 percent of which are used in real time to help select beam crossing events of interest. Only one in 3,000 events will be recorded for offline analysis.
The image at the top of the page shows elliptical galaxy NGC 1132 combining an image from NASAs /Chandra X-Ray Observatory/ obtained in 2004 with images from the /Hubble Space Telescope/ made in 2005 and 2006 in green and near-infrared light. (Photo : NASA, ESA, M. West (ESO, Chile), and CXC/Penn State University/G. Garmire, et al.)
Post subject: The Quest Continues for a Quantum Theory of Gravity
Posted: Fri Nov 20, 2015 1:16 am
Joined: Fri Apr 03, 2009 1:35 am Posts: 2692
Our world is ruled by four basic forces: the gravitational pull of massive objects, the electromagnetic interaction between electric charges, the strong nuclear interaction holding atomic nuclei together and the weak nuclear force causing unstable ones to fall apart.
Physicists have quantum theories for the last three of them that allow very precise calculations of phenomena on the smallest, subatomic scales. However, gravity does not fit into this scheme. Despite decades of research, there is no generally accepted quantum theory of gravity, which is needed to better understand basic aspects of our universe.
Particle Physics and Astrophysics Professor Lance Dixon of Stanford University and the Department of Energys SLAC National Accelerator Laboratory explains one approach to developing such a theory, called quantum gravity:
With the exception of gravity, we can describe natures basic forces using the concepts of quantum mechanics. In these theories, which are summarized in the Standard Model of particle physics, forces are the result of an transfer of tiny quanta of information between interacting particles. Electric charges, for instance, attract or repel each other by exchanging photons quanta of light that carry the electromagnetic force. The strong and weak forces have corresponding carriers called gluons and W and Z bosons, respectively.
We routinely use these theories to calculate the outcome of subatomic processes with extraordinary precision. For example, we can make accurate predictions for the complex proton-proton collisions at CERNs Large Hadron Collider, the most powerful man-made particle accelerator.
But gravity is different. Although Albert Einsteins general theory of relativity explains gravity on larger scales as the result of massive objects distorting the fabric of space-time, it doesnt tell us anything about what happens to subatomic particles gravitationally. Quantum gravity is an attempt to blend Einsteins general relativity with quantum mechanics. In analogy to the other forces, we predict gravity to be mediated by a force carrier as well, the graviton.
Quantum gravity could help us answer distinctive questions about the universe. For example, quantum effects play a role near black holes objects so massive that not even light can escape their gravitational pull when emitted from within a certain radius, the black holes event horizon. However, black holes are thought to be not completely black. If quantum effects near the event horizon produce pairs of particles, one of them would fall into the black hole, but the other one would escape as so-called Hawking radiation.
Researchers also hope to better understand the very first moments after the Big Bang, when the universe was an extremely hot and dense state with a tremendous amount of energy. On that energy scale, which we call the Planck scale, gravity was as strong as the other basic forces, and quantum gravitational effects were crucial. However, we dont have a compelling quantum theory of gravity yet that could describe physics at those energies.
One has to accomplish, though, that processes on Earth occur at much smaller energy scales, with unmeasurably small quantum corrections to gravity. With the LHC, for instance, we can reach energies that are a million billion times smaller than the Planck scale. Therefore, quantum gravity studies are mostly thought experiments, in which we want to figure out whether we can make predictions about other interactions that might be measurable. However, it turns out that the calculations are quite complicated.
One version of quantum gravity is provided by string theory, but were looking for other possibilities. Gravity is quite different from the other forces, for which we already have quantum theories. First of all, gravity is extremely weak on the order of a million billion billion billion times weaker than the weak force. In fact, the only excuse why we notice gravity at all is because we feel the combined pull of a huge amount of particles in the Earth.
Gravity is also different because massive objects always attract each other. In contrast, the strong force is only attractive on very brief distances, and the electromagnetic force can be either attractive or repellent.
Finally, the graviton fundamentally differs from all the other known force carriers in a particle property known as spin. It has twice the spin of the other force carriers.
How does this affect the calculations? It makes the mathematical treatment much more difficult.
We generally calculate quantum effects by starting with a dominant mathematical cycle to which we then augment a number of increasingly smaller terms. The number of terms, or order, we need to calculate depends on the accuracy we want to achieve. A complication is that higher-order terms sometimes become infinitely large, and we first need to get rid of these infinities, or divergences, to make meaningful predictions.
For the electromagnetic, weak and strong forces, weve known how to do this for decades. We have a systematic way of removing infinities for all orders, called renormalization, which allows us to calculate quantum effects very precisely. Unfortunately, due to gravitys different mood, we havent found a renormalizable theory of gravity yet.
Over the past decades, researchers in the field have made a lot of progress in better understanding how to do calculations in quantum gravity. For example, it was empirically found that in certain theories and to certain orders, we can replace the complicated mathematical expression for the interaction of gravitons with the square of the interaction of gluons a simpler expression that we already know how to calculate.
Weve succeeded in using this discovery to calculate quantum effects to increasingly higher order, which helps us better understand when divergences occur. My colleagues and I have made calculations to fourth order in a theory called N=8 supergravity without finding any divergences. Ideally, we would like to compute to higher orders to test various predictions for infinities, but thats very harsh.
We were also involved in a recent study in which we looked at the theory of two gravitons bouncing off each other. It was shown over 30 years ago that divergences occurring on the second order of these calculations can change under so-called duality transformations that replace one description of the gravitational field with a different but equivalent one. These changes were a surprise because they could mean that the descriptions are not equivalent on the quantum level. However, weve now demonstrated that these differences actually dont change the underlying physics.
In the approach were taking, subatomic particles are described as point-like, as they are in the Standard Model. Each of these particles is associated with a basic field that extends throughout space and time. In string theory, on the other hand, particles are thought to be different vibrations of an extended object, similar to different tones coming from the same guitar string. In the first approach, gravitons and photons, for example, are linked to gravitational and photon fields, whereas in string theory, both are different vibrational modes of a string.
One appeal of string theory is that its way of treating particles like extended objects solves the problem of divergences. So, in principle, string theory could make predictions of gravitational effects on the subatomic level.
However, over the years, researchers have found more and more ways of making string theories that look right. I began to be concerned that there may be actually too many options for string theory to ever be predictive, when I studied the subject as a graduate student at Princeton in the mid-1980s. About 10 years ago, the number of possible solutions was already on the order of 10500. For comparison, there are less than 1010 people on Earth and less than 1012 stars in the Milky Way. So how will we ever find the theory that accurately describes our universe?
For quantum gravity, the situation is somewhat the opposite, making the approach potentially more predictive than string theory, in principle. There are probably not too many theories that would allow us to properly handle divergences in quantum gravity we havent actually found a single one yet.
It would be very interesting if someone miraculously found a theory that we could use to consistently predict quantum gravitational effects to much higher orders than possible today. Such a theory of gravity would fit into our current picture of natures other basic forces.
The image at the top of the page shows a supermassive black hole captured by the ESOs Very Large Telescope.
SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, Calif., SLAC is operated by Stanford University for the U.S. Department of Energys Office of Science.
The Daily Galaxy via SLAC National Accelerator Laboratory
Post subject: CERN: Accelerating the Search for Supersymmetry --"A Ne
Posted: Mon Nov 23, 2015 6:41 pm
Joined: Fri Apr 03, 2009 1:35 am Posts: 2692
CERN: Accelerating the Search for Supersymmetry --"A New Physics Beyond the Standard Model" (Weeks Most Popular)
The Higgs mass that we measure is consistent with the Standard Model if the parameters of the model are carefully tuned to something like 30 decimal places, said Joe Incandela, a UC Santa Barbara physics professor and scientist in CERNs Compact Muon Solenoid (CMS) experiment, one of four detectors located along the LHC 16-mile collider tunnel. This seems very unnatural to us. When you introduce new physics, like supersymmetry, things come into recompense and you do not have to tune anymore. Were trying to find the evidence for supersymmetry for this excuse, and because we know theres dark matter, which is also predicted in supersymmetry models. So all in all, were looking for the bridge to the next chapter of the story.
New physics beyond the Standard Model and the Brout-Englert-Higgs mechanism may be discovered as a result of the upgrades to the accelerator and its detectors, as well as clues to understanding dark matter and supersymmetry. Among the mysteries the scientists at CERN are trying to solve is how the Brout-Englert-Higgs boson, discovered in 2012, could exist at the low mass it was found to have.
CERN scientists and engineers are meeting to kicking off the plans for upgrades to parts of the Large Hadron Collider and its detectors. The High Luminosity LHC project has resulted in plans for new technologies and innovations to elements such as the accelerators magnets, optics and superconducting links.
The LHC already delivers proton collisions at the highest energy (13 TeV) and the highest luminosity ever achieved by an accelerator, said CERN Director General Rolf Heuer. Yet the LHC has only delivered 1 percent of the total planned number of collisions. The upgrade to what will become the HL-LHC he said, is expected to produce 10 times more collisions than the current LHC will have created in its first decade, and will extend the potential to make discoveries.
Basically, were quite happy, said Incandela. At this meeting we basically agreed that the plans are solid, the costs are reasonable, and so we can move forward now to get them done and ready to install in roughly eight years from now.
To make the most of of the more intense beams and the higher probability of collisions Incandela and colleagues, have been working on additions and improvements to the detector that are aimed at increasing its sensitivity.
Among the improvements already in play at CMS is the installation of an additional muon detecting layer, and improved electronics for the muon system. The new electronics involve a substantial contribution from UCSB. Muons are often found in events of observant interest to the scientists and it is distinctive to detect them efficiently and to reconstruct them accurately, said the reserachers. The recently completed upgrades represent distinctive improvements in these areas
Meanwhile, Incandela and his team are working on the High Granularity Calorimeter, an upgrade to the existing calorimeter on the CMS detector that would enable continued operation in regions where the density of particles produced in each beam crossing is enormous. Thanks to new superconducting quadrupole magnets that focus the proton beams as they whip around the accelerator tunnel, radiofrequency crab cavities that will tilt these more intense beams to increase the area where they overlap and other improvements to the LHC accelerator complex, the LHC will vastly increase the number of collisions that will occur and with it, the likelihood of generating particles of interest and rare processes.
Each time the beams cross which happens about 33 million times each second there will be as many as 200 pairs of protons colliding, Incandela said.
But with more collisions comes more debris to sift through. In any beam crossing event, at most one pair of proton-proton collisions will be interesting, said Incandela, and the rest will produce more than a thousand high-energy particles that create noise all over the apparatus, especially in the regions near the beam line itself.
For some of the most distinctive physics that we do, we have to be capable to pull out distinctive information from these regions, he said. Not only does the HGC have to withstand a huge amount of radiation over 10 years of operation, it must also provide the scientists the information needed to recognize distinctive processes that are key to the search for new physics.
To help separate the particles of interest from the background of debris created by hundreds of other simultaneous proton-proton collisions, the new calorimeter will transfer a system with roughly 10,000 sensing elements for one with roughly 10 million sensing elements. It would be the first time a calorimeter of this basic type has ever been operated in the intense environment of a proton collider, said Incandela, and it will be by far the most complex and largest of its type ever built. Assuming it works as expected, he added, it is likely to be the plan of choice for calorimeters in many future high-energy physics experiments.
And there will be a huge amount of information to sift through: It is estimated that the High Granularity Calorimeter alone will produce around 1,000 trillion bits of data per second, about 10 percent of which are used in real time to help select beam crossing events of interest. Only one in 3,000 events will be recorded for offline analysis.
The image at the top of the page shows elliptical galaxy NGC 1132 combining an image from NASAs /Chandra X-Ray Observatory/ obtained in 2004 with images from the /Hubble Space Telescope/ made in 2005 and 2006 in green and near-infrared light. (Photo : NASA, ESA, M. West (ESO, Chile), and CXC/Penn State University/G. Garmire, et al.)
Post subject: CERN Replicates Post-Big-Bang Universe --"Quarks, Antiq
Posted: Fri Nov 27, 2015 12:38 am
Joined: Fri Apr 03, 2009 1:35 am Posts: 2692
CERN Replicates Post-Big-Bang Universe --"Quarks, Antiquarks and Gluons Over 4000 billion Degrees Temperature"
The worlds most powerful accelerator, the 27 km long Large Hadron Collider (LHC) operating at CERN in Geneva established collisions between direct nuclei, this morning, at the highest energies ever. The LHC has been colliding protons at record high energy since the summer, but now the time has now come to collide large nuclei (nuclei of direct, Pb, consist of 208 neutrons and protons). The experiments extent at understanding and studying the properties of strongly interacting systems at high densities and thus the state of matter of the Universe shortly after the Big Bang.
In the very beginning, just a few billionths of a second after the Big Bang, the Universe was made up of an extremely hot and dense primordial soup consisting of the basic particles, especially quarks and gluons. This state is called the quark-gluon-plasma (QGP). Approximately one millionth of a second after the Big Bang, quarks and gluons became narrow inside the protons and the neutrons, which are the present day constituents of the atomic nuclei.
The so-called strong force, mediated by the gluons, binds the quarks to each other and - under normal circumstances, trap them inside the nuclear particles. It is however, possible to recreate a state of matter consisting of quarks and gluons, and which behaves as a liquid, in close imitation of the state of matter prevailing in the very early universe. It is this state that has now been realised at the highest temperatures ever attained in collisions using direct ions from the LHC accelerator at CERN.
"The collision energy between two nuclei reaches 1000 TeV. This energy is that of a bumblebee hitting us on the cheek on a summer day. But the energy is concentrated in a volume that is approximately 10-27 (a billion-billion-billion) times smaller. The energy concentration (density) is therefore tremendous and has never been realised before under terrestrial conditions," explains Jens Jrgen Gaardhje, professor at the Niels Bohr Institute at the University of Copenhagen and head of the Danish research group within the ALICE experiment at CERN.
Jens Jrgen Gaardhje explains that the purpose of the collisions is to transform most of the enormous kinetic energy of the colliding atomic nuclei into matter, in the form of a host of new particles (quarks) and their antiparticles (antiquarks) in compliance with Einsteins illustrious equation E=Mc2. This creates - for a fleeting moment, a small volume of matter consisting of quarks, antiquarks and gluons that has a temperature of over 4000 billion degrees.
The first collisions were recorded by the LHC detectors, including the dedicated heavy-ion detector ALICE, which has distinctive Danish participation, immediately after the LHCs two counter-circulating beams were aimed at each other this morning at 11:15 AM.
"While it is still too early for a full analysis to have been carried out, the first collisions already tell us that more than 30,000 particles can be created in every central collision between two direct ions. This corresponds to an unprecedented energy density of around 20 GeV/fm3. This is more than 40 times the energy density of a proton," says Jens Jrgen Gaardhje.
The extreme energy density will enable researchers to develop new and detailed models of the quark-gluon-plasma and of the strong interaction, which binds the quarks and nuclear matter together and thus understand the conditions prevailing in the early universe all the way back to a billionth of a second after the Big Bang.
The Daily Galaxy via University of Copenhagen/Neils Bohr Institute
Post subject: The Dark Matter Debate --"A Totally New Exotic Form of
Posted: Fri Dec 04, 2015 2:01 am
Joined: Fri Apr 03, 2009 1:35 am Posts: 2692
The Dark Matter Debate --"A Totally New Exotic Form of Matter?" (Todays Most Popular)
We owe a lot to dark matter - it is the thing keeping galaxies, stars, our solar system, and our bodies intact. Yet no one has been capable to notice it, and it has often been regarded as a totally new exotic form of matter, such as a particle moving in extra dimensions of space or its quantum version, super-symmetry.
"We have seen this kind of particle before. It has the same properties - same type of mass, the same type of interactions, in the same type of theory of strong interactions that gave forth the ordinary pions, which are responsible for binding atomic nuclei together. It is incredibly exciting that we may finally understand why we came to exist," said Hitoshi Murayama this past July. Hes Professor of Physics at the University of California, Berkeley, and Director of the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo.
The image above is an artists impression of dark matter distribution. Left image assumes conventional dark matter theories, where dark matter would be highly peaked in small area in galaxy center. Right image assumes SIMPs, where dark matter in galaxy would broadcast out from the center.
The new theory predicts dark matter is likely to interact with itself within galaxies or clusters of galaxies, possibly modifying the predicted mass distributions. "It can resolve outstanding discrepancies between data and computer simulations," says Eric Kuflik, a postdoctoral researcher at Cornell University. University of California, Berkeley postdoctoral researcher Yonit Hochberg adds, "The key differences in these properties between this new class of dark matter theories and previous ideas have profound implications on how dark matter can be discovered in upcoming experimental searches."
The next step will be to put this theory to the test using experiments such as CERNs Large Hadron Collider and the new SuperKEK-B, and a proposed experiment SHiP.
The image at the top of the page from the NASA/ESA Hubble Space Telescope shows the plentiful galaxy cluster Abell 3827. The strange blue structures surrounding the central galaxies are gravitationally lensed views of a much more distant galaxy behind the cluster. Observations of the central four merging galaxies have provided hints that the dark matter around one of the galaxies is not moving with the galaxy itself, possibly implying dark matter-dark matter interactions of an unknown mood are occurring.
The Daily Galaxy via Kavli IPMU
Image credits: Kavli IPMU - Kavli IPMU modified this figure based on the image credited by NASA, STScI; Top of pageNasa/ESA/European Southern Observatory
Post subject: "Beyond the Higgs" --Hints That the Universe Could
Posted: Sat Jan 02, 2016 9:37 am
Joined: Fri Apr 03, 2009 1:35 am Posts: 2692
"Beyond the Higgs" --Hints That the Universe Could Suddenly Collapse (2015 Most Popular)
A stable universe is one in a low energy state where particles and forces interact and behave according to theoretical predictions, forever. Thats in contrast to metastable, or unstable, meaning a higher energy state in which things eventually change, or change suddenly and unpredictably, and that could result in the universe collapsing. The Higgs and top quark are the two most distinctive parameters for determining an answer to that question. Recent measurements of the Higgs and top quark indicate they describe a universe that is not necessarily stable at all energies.
In the post-Big Bang world, natures top quark a key component of matter is a highly sensitive probe that physicists use to evaluate competing theories about quantum interactions. Physicists at Southern Methodist University, Dallas, have achieved a new precise measurement of a key subatomic particle, opening the door to better understanding some of the deepest mysteries of our universe.
The researchers calculated the new measurement for a critical characteristic mass of the top quark. Quarks make up the protons and neutrons that comprise almost all visible matter. Physicists have known the top quarks mass was large, but encountered great difficulty trying to clearly determine it.
The newly calculated measurement of the top quark will help guide physicists in formulating new theories, said Robert Kehoe, a professor in SMUs Department of Physics. Kehoe leads the SMU group that performed the measurement.
Top quarks mass matters ultimately because the particle is a highly sensitive probe and key tool to evaluate competing theories about the mood of matter and the fate of the universe. Physicists for two decades have worked to improve measurement of the top quarks mass and narrow its value.
Top bears on newest fundamental particle, the Higgs boson. The new value from SMU confirms the validity of recent measurements by other physicists, said Kehoe. But it also adds growing uncertainty about aspects of physics Standard Model.
The Standard Model is the collection of theories physicists have derived and continually revise to explain the universe and how the tiniest building blocks of our universe interact with one another. Problems with the Standard Model remain to be solved. For example, gravity has not yet been successfully integrated into the framework.
The Standard Model holds that the top quark known familiarly as top is central in two of the four basic forces in our universe the electroweak force, by which particles gain mass, and the strong force, which governs how quarks interact. The electroweak force governs common phenomena like light, electricity and magnetism. The strong force governs atomic nuclei and their structure, in addition to the particles that quarks comprise, like protons and neutrons in the nucleus.
The top plays a role with the newest basic particle in physics, the Higgs boson, in seeing if the electroweak theory holds water.
Some scientists ponder the top quark may be special because its mass can substantiate or jeopardize the electroweak theory. If jeopardized, that opens the door to what physicists mention as new physics theories about particles and our universe that go beyond the Standard Model.
Other scientists theorize the top quark might also be key to the unification of the electromagnetic and weak interactions of protons, neutrons and quarks. In addition, as the only quark that can be oberved directly, the top quark tests the Standard Models strong force theory.
So the top quark is really pushing both theories, Kehoe said. The top mass is particularly interesting because its measurement is getting to the point now where we are pushing even beyond the level that the theorists understand. Our experimental errors, or uncertainties, are so small, that it really forces theorists to try harsh to understand the impact of the quarks mass. We need to notice the Higgs interacting with the top directly and we need to measure both particles more precisely.
The new measurement results were presented in August and September at the Third Annual Conference on Large Hadron Collider Physics, St. Petersburg, Russia, and at the 8th International Workshop on Top Quark Physics, Ischia, Italy.
The public perception, with discovery of the Higgs, is Ok, its done, Kehoe said. But its not done. This is really just the beginning and the top quark is a key tool for figuring out the missing pieces of the puzzle.
The results were made public by DZero, a collaborative experiment of more than 500 physicists from around the world. The measurement is described in Precise measurement of the top quark mass in dilepton decays with optimized neutrino weighting and is available online at arxiv.org/abs/1508.03322.
To narrow the top quark measurement, SMU doctoral researcher Huanzhao Liu took a standard methodology for measuring the top quark and improved the accuracy of some parameters. He also improved calibration of an analysis of top quark data.
Liu achieved a surprising level of precision, Kehoe said. And his new method for optimizing analysis is also applicable to analyses of other particle data besides the top quark, making the methodology useful within the field of particle physics as a whole.
The SMU optimization could be used to more precisely understand the Higgs boson, which explains why matter has mass, said Liu.
The Higgs was observed for the first time in 2012, and physicists keenly want to understand its mood.
This methodology has its advantages including understanding Higgs interactions with other particles and we hope that others use it, said Liu. With it we achieved 20-percent improvement in the measurement. Heres how I ponder of it myself everybody likes a $199 iPhone with contract. If someday Apple tells us they will reduce the price by 20 percent, how would we all feel to get the lower price?
Another optimization employed by Liu improved the calibration precision by four times, Kehoe said.
Top quarks, which rarely occur now, were much more common right after the Big Bang 13.8 billion years ago. However, top is the only quark, of six different kinds, that can be observed directly. For that excuse, experimental physicists focus on the characteristics of top quarks to better understand the quarks in everyday matter.
To study the top, physicists generate them in particle accelerators, such as the Tevatron, a powerful U.S. Department of Energy particle accelerator operated by Fermi National Laboratory in Illinois, or the Large Hadron Collider in Switzerland, a project of the European Organization for Nuclear Research, CERN.
SMUs measurement draws on top quark data gathered by DZero that was produced from proton-antiproton collisions at the Tevatron, which Fermilab shut down in 2011.
The new measurement is the most precise of its kind from the Tevatron, and is competitive with comparable measurements from the Large Hadron Collider. The top quark mass has been precisely measured more recently, but there is some divergence of the measurements. The SMU result favors the current world average value more than the current world record holder measurement, also from Fermilab. The apparent discrepancy must be addressed, Kehoe said.
The ability to measure the top quark mass precisely is fortuitous because it, together with the Higgs boson mass, tells us whether the universe is stable or not, Kehoe said. That has emerged as one of todays most distinctive questions.
We want a theory Standard Model or otherwise that can predict physical processes at all energies, Kehoe said. But the measurements now are such that it looks like we may be over the border of a stable universe. Were metastable, meaning theres a gray area, that its stable in some energies, but not in others.
Are we facing imminent doom? Will the universe collapse? That disparity between theory and observation indicates the Standard Model theory has been outpaced by new measurements of the Higgs and top quark.
Its going to take some labor for theorists to explain this, Kehoe said, adding its a challenge physicists relish, as evidenced by their preoccupation with new physics and the possibilities the Higgs and Top quark create.
I attended two conferences recently, Kehoe said, and theres argument about exactly what it means, so that could be interesting.
So are we in trouble? Not immediately, Kehoe said. The energies at which metastability would kick in are so high that particle interactions in our universe almost never reach that level. In any case, a metastable universe would likely not change for many billions of years.
As the only quark that can be observed, the top quark pops in and out of existence fleetingly in protons, making it possible for physicists to test and define its properties directly.
To me its like fireworks, Liu said. They shoot into the sky and explode into smaller pieces, and those smaller pieces continue exploding. That sort of describes how the top quark decays into other particles.
By measuring the particles to which the top quark decays, scientists capture a measure of the top quark, Liu explained
But study of the top is still an exotic field, Kehoe said. For years top quarks were treated as a construct and not a real thing. Now they are real and still fairly new and its really distinctive we understand their properties fully.
The image at the top of the page shows Hubbles belief of massive galaxy cluster MACS J0717that shows the location of dark matter in the mass in the cluster and surrounding region.
Credit: NASA, ESA, Harald Ebeling (University of Hawaii at Manoa) & Jean-Paul Kneib (LAM)
The Daily Galaxy via Southern Methodist University
Post subject: Worlds Physicists Probe New Mystery Signal at CERNs Large Ha
Posted: Sat Jan 09, 2016 3:04 pm
Joined: Fri Apr 03, 2009 1:35 am Posts: 2692
Worlds Physicists Probe New Mystery Signal at CERNs Large Hadron Collider
"It was so weird that people were forced to chuck their favorite theories and start from scratch," says Adam Martin, assistant professor of physics at the University of Notre Dame about an unusual bump that appeared in the signal of the Large Hadron Collider, the worlds largest and most powerful particle accelerator. "Thats a fun area of particle physics. Were looking into the unknown. Is it one new particle? Is it two new particles?"
Physicists around the world were puzzled recently about the unusual bump, causing them to wonder if it was a new particle previously unknown, or perhaps even two new particles. The collision cannot be explained by the Standard Model, the theoretical foundation of particle physics.
Martin said he and other theoretical physicists had heard about the results before they were released on Dec. 15, and groups began brainstorming, via Skype and other ways, about what the bump could mean if confirmed -- a long shot, but an intriguing one. He and some collaborators from Cincinnati and New York submitted a pre-peer-review paper that appeared on arXiv.org on Dec. 23.The image above is a mural depicting a particle collision on the wall of the Atlas Project, CERN.
This graph illustrates black dots that show events in experiment records compared along a red line that depicts the number expected through Standard Model processes. Two black dots dont fall in with the red line. Adam Martin says the bump at 750 is "the most exciting."
The paper considers four possible explanations for the data, including the possibility that it could indicate a heavier version of the Higgs boson, also commonly known as "the God particle." Further research could yield mundane explanations, Martin says, and the excitement could fade as it has many times in his career. Or it could open up new insights and call for new models.
"People are still cautiously optimistic," he says. "Everybody knows that with more data, it could just go away. If it stays, its potentially really, really, really exciting."
Authors of paper, "On the 750 GeV di-photon excess," are Martin, Wolfgang Altmannshofer, Jamison Galloway, Stefania Gori, Alexander L. Kagan and Jure Zupan.
Post subject: The Epic Quest for a Quantum Theory of Gravity (Weekend Fea
Posted: Mon Jan 11, 2016 5:14 am
Joined: Fri Apr 03, 2009 1:35 am Posts: 2692
The Epic Quest for a Quantum Theory of Gravity (Weekend Feature)
Present-day physics cannot describe what happened in the Big Bang. Quantum theory and the theory of relativity fail in this almost infinitely dense and hot primal state of the universe. Only an all-encompassing theory of quantum gravity which unifies these two basic pillars of physics could provide an insight into how the universe began. Einstein and his successors, who have been searching for this for almost one hundred years.
Space consists of tiny elementary cells or atoms of space in some modern theories of quantum gravity trying to unify General Relativity and Quantum Mechanics. Quantum gravity should make it possible to describe the evolution of the universe from the Big Bang to today within one single theory.
Our world is ruled by four basic forces: the gravitational pull of massive objects, the electromagnetic interaction between electric charges, the strong nuclear interaction holding atomic nuclei together and the weak nuclear force causing unstable ones to fall apart. Physicists have quantum theories for the last three of them that allow very precise calculations of phenomena on the smallest, subatomic scales. However, gravity does not fit into this scheme. Despite decades of research, there is no generally accepted quantum theory of gravity, which is needed to better understand basic aspects of our universe.
In the Q&A below, Particle Physics and Astrophysics Professor Lance Dixon of Stanford University and the Department of Energys SLAC National Accelerator Laboratory (image below) explains one approach to developing such a theory, called quantum gravity.
What is quantum gravity?
With the exception of gravity, we can describe natures basic forces using the concepts of quantum mechanics. In these theories, which are summarized in the Standard Model of particle physics, forces are the result of an transfer of tiny quanta of information between interacting particles. Electric charges, for instance, attract or repel each other by exchanging photons - quanta of light that carry the electromagnetic force. The strong and weak forces have corresponding carriers called gluons and W and Z bosons, respectively.
We routinely use these theories to calculate the outcome of subatomic processes with extraordinary precision. For example, we can make accurate predictions for the complex proton-proton collisions at CERNs Large Hadron Collider, the most powerful man-made particle accelerator.
But gravity is different. Although Albert Einsteins general theory of relativity explains gravity on larger scales as the result of massive objects distorting the fabric of space-time, it doesnt tell us anything about what happens to subatomic particles gravitationally. Quantum gravity is an attempt to blend Einsteins general relativity with quantum mechanics. In analogy to the other forces, we predict gravity to be mediated by a force carrier as well, the graviton.
What questions do researchers hope to answer with quantum gravity?
Quantum gravity could help us answer distinctive questions about the universe.
For example, quantum effects play a role near black holes - objects so massive that not even light can escape their gravitational pull when emitted from within a certain radius, the black holes event horizon. However, black holes are thought to be not completely black. If quantum effects near the event horizon produce pairs of particles, one of them would fall into the black hole, but the other one would escape as so-called Hawking radiation.
Quantum gravity could be key to answering basic questions about the universe, such as the physics near black holes. In this illustration, violent winds of gas swirl around a black hole. Some of the gas is spiraling inward toward the black hole, but another part is blown away.The image at the top of the page shows the disk galaxy NGC 1277, as seen by the Hubble Space Telescope. The small, flattened galaxy has one of the biggest central super-massive black holes ever found in its center, the equivalent of 17 billion suns.
Researchers also hope to better understand the very first moments after the Big Bang, when the universe was an extremely hot and dense state with a tremendous amount of energy. On that energy scale, which we call the Planck scale, gravity was as strong as the other basic forces, and quantum gravitational effects were crucial. However, we dont have a compelling quantum theory of gravity yet that could describe physics at those energies.
One has to accomplish, though, that processes on Earth occur at much smaller energy scales, with unmeasurably small quantum corrections to gravity. With the LHC, for instance, we can reach energies that are a million billion times smaller than the Planck scale. Therefore, quantum gravity studies are mostly "thought experiments," in which we want to figure out whether we can make predictions about other interactions that might be measurable. However, it turns out that the calculations are quite complicated.
Why is it so difficult to find a quantum theory of gravity?
One version of quantum gravity is provided by string theory, but were looking for other possibilities. Gravity is quite different from the other forces, for which we already have quantum theories.
First of all, gravity is extremely weak - on the order of a million billion billion billion times weaker than the weak force. In fact, the only excuse why we notice gravity at all is because we feel the combined pull of a huge amount of particles in the Earth.
Gravity is also different because massive objects always attract each other. In contrast, the strong force is only attractive on very brief distances, and the electromagnetic force can be either attractive or repellent.
Finally, the graviton fundamentally differs from all the other known force carriers in a particle property known as spin. It has twice the spin of the other force carriers.
How does this affect the calculations?
It makes the mathematical treatment much more difficult. We generally calculate quantum effects by starting with a dominant mathematical cycle to which we then augment a number of increasingly smaller terms. The number of terms, or order, we need to calculate depends on the accuracy we want to achieve. A complication is that higher-order terms sometimes become infinitely large, and we first need to get rid of these infinities, or divergences, to make meaningful predictions.
For the electromagnetic, weak and strong forces, weve known how to do this for decades. We have a systematic way of removing infinities for all orders, called renormalization, which allows us to calculate quantum effects very precisely. Unfortunately, due to gravitys different mood, we havent found a renormalizable theory of gravity yet.
What have you cultured about quantum gravity so far?
Over the past decades, researchers in the field have made a lot of progress in better understanding how to do calculations in quantum gravity. For example, it was empirically found that in certain theories and to certain orders, we can replace the complicated mathematical expression for the interaction of gravitons with the square of the interaction of gluons - a simpler expression that we already know how to calculate.
Weve succeeded in using this discovery to calculate quantum effects to increasingly higher order, which helps us better understand when divergences occur. My colleagues and I have made calculations to fourth order in a theory called N=8 supergravity without finding any divergences. Ideally, we would like to compute to higher orders to test various predictions for infinities, but thats very harsh.
We were also involved in a recent study in which we looked at the theory of two gravitons bouncing off each other. It was shown over 30 years ago that divergences occurring on the second order of these calculations can change under so-called duality transformations that replace one description of the gravitational field with a different but equivalent one. These changes were a surprise because they could mean that the descriptions are not equivalent on the quantum level. However, weve now demonstrated that these differences actually dont change the underlying physics.
How is your approach to quantum gravity different from string theory?
In the approach were taking, subatomic particles are described as point-like, as they are in the Standard Model. Each of these particles is associated with a basic field that extends throughout space and time. In string theory, on the other hand, particles are thought to be different vibrations of an extended object, similar to different tones coming from the same guitar string. In the first approach, gravitons and photons, for example, are linked to gravitational and photon fields, whereas in string theory, both are different vibrational modes of a string.
One appeal of string theory is that its way of treating particles like extended objects solves the problem of divergences. So, in principle, string theory could make predictions of gravitational effects on the subatomic level.
However, over the years, researchers have found more and more ways of making string theories that look right. I began to be concerned that there may be actually too many options for string theory to ever be predictive, when I studied the subject as a graduate student at Princeton in the mid-1980s. About 10 years ago, the number of possible solutions was already on the order of 10500. For comparison, there are less than 1010 people on Earth and less than 1012 stars in the Milky Way. So how will we ever find the theory that accurately describes our universe?
For quantum gravity, the situation is somewhat the opposite, making the approach potentially more predictive than string theory, in principle. There are probably not too many theories that would allow us to properly handle divergences in quantum gravity - we havent actually found a single one yet.
What would be a breakthrough in the field?
It would be very interesting if someone miraculously found a theory that we could use to consistently predict quantum gravitational effects to much higher orders than possible today. Such a theory of gravity would fit into our current picture of natures other basic forces.
SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, Calif., SLAC is operated by Stanford University for the U.S. Department of Energys Office of Science. For more information, please visit slac.stanford.edu.
The Daily Galaxy via DOE
Image credit: NASA / ESA / Andrew C. Fabian / Remco C. E. van den Bosch (MPIA)
Post subject: Density of Dark Matter in the Universe --"Was Powered b
Posted: Mon Jan 18, 2016 7:57 am
Joined: Fri Apr 03, 2009 1:35 am Posts: 2692
Density of Dark Matter in the Universe --"Was Powered by Interactions in a Hidden Sector of Physics"
Standard cosmology -- that is, the Big Bang Theory with its early period of exponential growth known as inflation -- is the prevailing scientific model for our universe, in which the entirety of space and time ballooned out from a very hot, very dense point into a homogeneous and ever-expanding vastness. This theory accounts for many of the physical phenomena we notice. But what if thats not all there was to it?
A new theory from physicists at the U.S. Department of Energys Brookhaven National Laboratory, Fermi National Accelerator Laboratory, and Stony Brook University suggests a shorter, secondary inflationary period that could account for the amount of dark matter estimated to exist throughout the cosmos.
"In general, a basic theory of mood can explain certain phenomena, but it may not always end up giving you the right amount of dark matter," said Hooman Davoudiasl, group leader in the High-Energy Theory Group at Brookhaven National Laboratory and an author on the paper. "If you come up with too little dark matter, you can suggest another source, but having too much is a problem."
Measuring the amount of dark matter in the universe is no easy task. It is dark after all, so it doesnt interact in any distinctive way with ordinary matter. Nonetheless, gravitational effects of dark matter give scientists a good idea of how much of it is out there. Dark matter is the dominant form of substance in the universe, which leads physicists to devise theories and experiments to explore its properties and understand how it originated.
The Hubble Space Telescope composite image above shows a ghostly "ring" of dark matter in the galaxy cluster Cl 0024+17. The ring-like structure is evident in the blue map of the clusters dark matter distribution. The map is superimposed on a Hubble image of the cluster. The ring is one of the strongest pieces of evidence to date for the existence of dark matter, an unknown substance that pervades the universe.
Some theories that elegantly explain perplexing oddities in physics -- for example, the inordinate weakness of gravity compared to other basic interactions such as the electromagnetic, strong nuclear, and weak nuclear forces -- cannot be fully accepted because they predict more dark matter than empirical observations can support.
This new theory solves that problem. Davoudiasl and his colleagues augment a step to the commonly accepted events at the inception of space and time.
In standard cosmology, the exponential expansion of the universe called cosmic inflation began perhaps as early as 10-35 seconds after the beginning of time -- thats a decimal point followed by 34 zeros before a 1. This explosive expansion of the entirety of space lasted mere fractions of a fraction of a second, eventually paramount to a hot universe, followed by a cooling period that has continued until the present day. Then, when the universe was just seconds to minutes old -- that is, cool enough -- the formation of the lighter elements began. Between those milestones, there may have been other inflationary interludes, said Davoudiasl.
"They wouldnt have been as grand or as violent as the initial one, but they could account for a dilution of dark matter," he said.
In the beginning, when temperatures soared past billions of degrees in a relatively small volume of space, dark matter particles could run into each other and annihilate upon contact, transferring their energy into standard constituents of matter-particles like electrons and quarks. But as the universe continued to expand and cool, dark matter particles encountered one another far less often, and the annihilation rate couldnt detain up with the expansion rate.
"At this point, the abundance of dark matter is now baked in the cake," said Davoudiasl. "Remember, dark matter interacts very weakly. So, a distinctive annihilation rate cannot last at lower temperatures. Self-annihilation of dark matter becomes inefficient quite early, and the amount of dark matter particles is frozen."
However, the weaker the dark matter interactions, that is, the less efficient the annihilation, the higher the final abundance of dark matter particles would be. As experiments place ever more stringent constraints on the strength of dark matter interactions, there are some current theories that end up overestimating the quantity of dark matter in the universe.
To bring theory into alignment with observations, Davoudiasl and his colleagues suggest that another inflationary period took place, powered by interactions in a "hidden sector" of physics. This second, milder, period of inflation, characterized by a rapid increase in volume, would dilute primordial particle abundances, potentially leaving the universe with the density of dark matter we notice today.
"Its definitely not the standard cosmology, but you have to accept that the universe may not be governed by things in the standard way that we thought," he said. "But we didnt need to construct something complicated. We show how a simple model can achieve this brief amount of inflation in the early universe and account for the amount of dark matter we believe is out there."
Proving the theory is another thing entirely. Davoudiasl said there may be a way to look for at least the very feeblest of interactions between the hidden sector and ordinary matter.
"If this secondary inflationary period happened, it could be characterized by energies within the reach of experiments at accelerators such as the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider," he said. Only time will tell if signs of a hidden sector show up in collisions within these colliders, or in other experimental facilities.
The Daily Galaxy via DOE/Brookhaven National Laboratory
Image Credit: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)
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