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Supersymmetry is a Theory that Predicts a New Heavy and Stable Particle Called Neutralino
:: 10 July, 2008
“We are looking at the heavens, and using the very biggest things to help up predict what will happen with the very smallest things,” David Toback tells. Toback is a professor at Texas A&M University in College Station, and he believes that there is a way to combine cosmology and particle physics in a way that can help us learn more about the universe.
“We’re interested in the dark matter question,” Toback continues. “Our current best guess is that the particles we know and love only make up about four percent of the stuff in the universe. Twenty-three percent of the universe is dark matter. The rest is dark energy. But I’m interested in dark matter, which should be made of particles. We want to know the properties of the bulk of the matter in the universe. This is a question that interests both cosmologists and particle physicists.”
Toback and his colleagues at Texas A&M, Richard Arnowitt, Bhaskar Dutta, Alfredo Gurrola, Teruki Kamon and Abram Krislock, have been working on a model that allows them to use information obtained from the Large Hadron Collider (LHC) to predict the amount of dark matter left over from the beginning of the universe. Their work is published “Determining the Dark Matter Relic Density in the Minimal Supergravity Stau-Neutralino Coannihilation Region at the Large Hadron Collider.”
“Our goal is to see whether our understanding of particles in the universe, the theory of supersymmetry, is correct. If it is, it will explain one of the most important questions in particle physics and cosmology in one fell swoop,” Toback says.
Supersymmetry is a theory that predicts that all elementary particles with spin are paired to other particles whose spin differs by half a unit. “One of the things that makes it special,” Toback says, “is that supersymmetry is a theory that predicts new particles. And one of the particles predicted is called a neutralino.” Neutralinos are thought to be heavy and stable, and they represent the leading candidate to explain the amount of cold dark matter indirectly detected in the universe.
The problem is that no one has been able to measure dark matter directly yet. This is where the LHC comes in. This $6 billion project is scheduled to begin operation later this summer, smashing protons into each other. The LHC is the largest and highest energy particle accelerator in the world, and Toback thinks that there’s a good chance that neutralinos could be produced from the collisions between protons. The data produced by the LHC will be made available to scientists around the world, including the team at Texas A&M.
“If our results are correct we now know much better where to look for this dark matter particle at the LHC,” Toback explains. “We’ve used precision data from astronomy to calculate what it would look like at the LHC, and how quickly we should be able to discover and measure it.” He and his colleagues have even gone so far to be show that with their measurements with LHC data they would be able to predict the amount of dark matter in the universe. This could be compared to what is seen from the WMAP satellite. “If we get the same answer,” he continues, “that would give us enormous confidence that the supersymmetry model is correct. If nature shows this, it would be remarkable.”
Toback says that the work he is doing with his peers at Texas A&M could make a connection between particle physics and cosmology. “If this works out, we could do real, honest to goodness cosmology at the LHC. And we’d be able to use cosmology to make particle physics predictions.”
Note for Dark Matter
In physics and cosmology, dark matter is matter that does not interact with the electromagnetic force, but whose presence can be inferred from gravitational effects on visible matter. According to present observations of structures larger than galaxies, as well as Big Bang cosmology, dark matter accounts for the vast majority of mass in the observable universe. The observed phenomena which imply the presence of dark matter include the rotational speeds of galaxies, orbital velocities of galaxies in clusters, gravitational lensing of background objects by galaxy clusters such as the Bullet cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. Dark matter also plays a central role in structure formation and galaxy evolution, and has measurable effects on the anisotropy of the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation: the remainder is called the "dark matter component."
The dark matter component has vastly more mass than the "visible" component of the universe. At present, the density of ordinary baryons and radiation in the universe is estimated to be equivalent to about one hydrogen atom per cubic meter of space. Only about 4% of the total energy density in the universe (as inferred from gravitational effects) can be seen directly. About 22% is thought to be composed of dark matter. The remaining 74% is thought to consist of dark energy, an even stranger component, distributed diffusely in space. Some hard-to-detect baryonic matter makes a contribution to dark matter but constitutes only a small portion. Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics. It has been noted that the names "dark matter" and "dark energy" serve mainly as expressions of human ignorance, much as the marking of early maps with "terra incognita."
Note for Dark Energy
In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. Assuming the existence of dark energy is the most popular way to explain recent observations that the universe appears to be expanding at an accelerating rate. In the standard model of cosmology, dark energy currently accounts for 73% of the total mass-energy of the universe.
Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space, are usually also included in the cosmological constant. The cosmological constant is physically equivalent to vacuum energy. Scalar fields which do change in space can be difficult to distinguish from a cosmological constant, because the change may be extremely slow.
High-precision measurements of the expansion of the universe are required to understand how the expansion rate changes over time. In general relativity, the evolution of the expansion rate is parameterized by the cosmological equation of state. Measuring the equation of state of dark energy is one of the biggest efforts in observational cosmology today.
Adding the cosmological constant to cosmology's standard FLRW metric leads to the Lambda-CDM model, which has been referred to as the "standard model" of cosmology because of its precise agreement with observations. Dark energy has been used as a crucial ingredient in a recent attempt to formulate a cyclic model for the universe.
Note for Supersymmetry
In particle physics, supersymmetry is a symmetry that relates elementary particles of one spin to another particle that differs by half a unit of spin and are known as superpartners. In other words, in a supersymmetric theory, for every type of boson there exists a corresponding type of fermion, and vice-versa.
As of 2008 there is no direct evidence that supersymmetry is a symmetry of nature. Since superpartners of the particles of the Standard Model have not been observed, supersymmetry, if it exists, must be a broken symmetry allowing the 'sparticles' to be heavy.
If supersymmetry exists close to the TeV energy scale, it allows the solution of two major puzzles in particle physics. One is the hierarchy problem - on theoretical grounds there are huge expected corrections to the particles' masses, which without fine-tuning will make them much larger than they are in nature. Another problem is the unification of the weak interactions, the strong interactions and electromagnetism. Another advantage of supersymmetry is that supersymmetric quantum field theory can sometimes be solved. Supersymmetry is also a consequence of most versions of string theory, though it can exist in nature even if string theory is wrong.
About Large Hadron Collider
The Large Hadron Collider (LHC) is a particle accelerator complex that will collide opposing beams of 7 TeV protons together in order to explore the validity and limitations of the highly successful current theoretical picture for particle physics, the standard model, which is known however to break down at sufficiently high energy. It is being built by the European Organization for Nuclear Research (CERN), and lies under the Franco-Swiss border near Geneva, Switzerland, where it is undergoing commissioning while being cooled down to its final operating temperature of approximately 2K. The first beams are due for injection in August 2008, with the first collisions planned to take place about two months later. The LHC will become the world's largest and highest-energy particle accelerator. The LHC is being funded and built in collaboration with over two thousand physicists from thirty-four countries as well as hundreds of universities and laboratories.
When activated, it is theorized that the collider will produce the elusive Higgs boson, the observation of which could confirm the predictions and "missing links" in the Standard Model of physics and could explain how other elementary particles acquire properties such as mass. The verification of the existence of the Higgs boson would be a significant step in the search for a Grand Unified Theory, which seeks to unify three of the four known fundamental forces: electromagnetism, the strong nuclear force and the weak nuclear force, leaving out only gravity. The Higgs boson may also help to explain why gravitation is so weak compared to the other three forces. In addition to the Higgs boson, other theorized novel particles that might be produced, and for which searches are planned, include strangelets, micro black holes, magnetic monopoles and supersymmetric particles.
Tags: Toback , Texas A&M University in College Station , dark matter , dark energy , Texas A&M , Richard Arnowitt , Bhaskar Dutta , Alfredo Gurrola , Teruki Kamon , Abram Krislock , Large Hadron Collider (LHC) , supersymmetry , Looking for neutralinos at the Large Hadron Collider ,
