Theoretical and experimental works. Fermilab is a laboratory where advances in particle physics, astrophysics and cosmology converge. At Fermilab, theoretical physicists work hand-in-hand with experimenters. Theorists play a crucial role in making connections among the numerous discoveries made by experiments around the world each year. At the most fundamental level, all forces and particles may be related. So far, no one has realized Einstein's dream of a single grand unified force. Supersymmetry and superstring Theory are two approaches that may point in the right direction. Observations of the fundamental interactions of quarks and gluons in high-energy particle collisions at Fermilab's Tevatron and the Large Hadron Collider at CERN will show the way.

The calculation of phenomena that arise from these high-energy collisions is a difficult undertaking. It commands the use of a wide range of advanced mathematical methods and the development of sophisticated and detailed computer simulations. Members of the Fermilab Theory Department provide input. Based on the knowledge gained by the experiments, they also make predictions of new physics phenomena such as supersymmetric particles and extra dimensions of space.

Scientists know that the Standard Model of quarks and leptons and their interactions is incomplete. Fermilab theorists help create, shape and define theories of new physics that may lie beyond the present Standard Model. They study means to test different aspects of these ideas that are best suited for observation at the Tevatron, the LHC and future colliders.

Top Quark. Physicists observed the first proton-antiproton collisions produced by the Tevatron on Oct. 13, 1985. Researchers at the CDF experiment and at DZero, which began operating later in 1992, have used the Tevatron to study matter at ever smaller scales. On March 2, 1995, physicists at CDF and DZero announced the discovery of the top quark. Researchers in both collaborations had statistically proven observation of the top quark in collisions at their detectors.

The top quark, which is as heavy as a gold atom but much smaller than a proton, was the last undiscovered quark of the six predicted to exist by current scientific theory. Scientists worldwide had sought the top quark since the discovery of the bottom quark at Fermilab through fixed-target experiments in 1977.

Both collaborations were subsequently able to measure the mass of the top quark to high precision. Particle physicists measure particle masses to verify their particle models. Knowing the value of the top quark mass has allowed physicists to zero in on the mass of the undiscovered Higgs boson, a crucial component of the theoretical framework of particle physics.

Bottom Quark. In 1977, an experiment led by physicist and Nobel laureate Leon Lederman at Fermilab provided the first evidence for the existence of the bottom quark, an essential ingredient in the theoretical framework called the Standard Model. Using a fixed-target experiment, the collaboration discovered a particle, which they called an upsilon. It was composed of a previously unobserved kind of quark, the bottom quark, and its antimatter partner, the antibottom quark. The bottom quark was then the heaviest sub-nuclear particle ever observed, weighing in at 10 times the mass of a proton.

Physicists at Brookhaven National Laboratory and SLAC National Accelerator Laboratory had in 1974 discovered particles made up of charm quarks and their antimatter partners, anticharm quarks. The discovery of the bottom quark provided important proof of the speculation that all matter is made up of quarks.

When certain subatomic particles called kaons decay, they break into a charged pion, a neutrino and either an electron or its antimatter counterpart, a positron. In the absence of CP violation, the number of electrons and positrons created in these decays would be equal. However, scientists have observed that the scales tip slightly toward decay into electrons. This provides proof that CP violation can lead to an excess of matter over antimatter. If this process occurred in the early universe, all of the positrons would annihilate upon encountering electrons. But after all of the positrons had disappeared, some matter would remain.

This result gives credence to the theory that CP violation allowed all of us to exist. But the effects of the process that causes an excess of matter from kaon decays are too small to complete the picture. The observed difference is orders of magnitude away from explaining asymmetry in the universe. This is one of the reasons that scientists are so interested in observing CP violation in other places, like neutrinos.

Tau Neutrino. The Nu Tau or DONUT collaboration at Fermilab announced on July 21, 2000, the first direct evidence for the subatomic particle called the tau neutrino, the third kind of neutrino known to particle physicists. Although earlier experiments had produced convincing indirect evidence for the particle’s existence, no one had directly observed a tau neutrino, an almost massless particle carrying no electric charge and barely interacting with surrounding matter.

The collaboration reported 12 instances of a neutrino interacting with an atomic nucleus to produce a charged particle called a tau lepton, the signature of a tau neutrino. To make this find, they aimed Fermilab’s intense beam of neutrinos across a 3-foot-long target of iron plates sandwiched with emulsion, similar to photographic film, which recorded the particle interactions. In the target, one out of 1 trillion tau neutrinos interacted with an iron nucleus and produced a tau lepton, which left its 1-millimeter-long tell-tale track in the emulsion. Physicists needed about three years of painstaking work to identify the tracks revealing a tau lepton and its decay, the key to exposing the tau neutrino’s secret existence.

Experimenters at Fermilab’s NuTeV, Neutrinos at the Tevatron, experiment discovered an imbalance of neutrinos and muons emerging from high-energy collisions of neutrinos with target nuclei in a 700-ton detector.

The results of generations of particle experiments with other particles have yielded precise predictions for the value of this ratio, which characterizes the interactions of particles with the weak force, one of the four fundamental forces of nature. But neutrinos did not fall into line with those expectations.

Experimenters using the Large Electron Positron at CERN, the European particle physics laboratory, measured the same neutrino interaction in a different particle reaction. They saw the same discrepancy, although with less precision. If the discrepancy is real, it could be another indication that neutrinos truly are different.

By Vasil Sidorov on February 2, 2009 from Fermilab  

E-mail: sidorovvasil@gmail.com

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