| Themes > Science > Physics > Elementary particle physics > Elementary particle physics Today > Elementary particle physics Today > Searching for New Physics |
Testing the Standard Model and probing for new phenomena at accelerators can be roughly categorized by three approaches: high energy, high precision, and high intensity. The most direct way to find new physics is to go to higher energy and explore completely uncharted territory. The Fermilab Tevatron currently has the highest center-of- mass energy of any accelerator in the world. It is the only existing facility where top quarks can be produced and where there still remains the possibility that other new high-mass phenomena might be discovered. The Main Injector upgrade will increase the Tevatron's intensity and allow a better look at the top quark's properties. Pushing the high- energy frontier ever forward is the lifeblood of elementary particle physics. Beyond the Tevatron, one must take large enough steps to ensure a significant new discovery potential. In that regard, the SSC energy of 40 TeV represented a factor of twenty increase over the Tevatron, and was chosen to allow thorough exploration of electroweak symmetry breaking, including discovery of the Higgs over its entire mass range. The European Laboratory for Particle Physics' (CERN) Large Hadron Collider (LHC), with an energy of 14 TeV, represents a significant step beyond the Tevatron on the energy frontier. Although the LHC is not as energetic as the SSC, it has considerable discovery potential. A TeV- scale electron-positron collider would also extend our discovery potential and would be well-suited for thorough investigations of new phenomena. Complementary to high-energy searches are high precision studies of the Standard Model. In this approach, one tests the consistency of standard-model predictions through precision experiments. Such studies allow us to refine our understanding of the Standard Model. In addition, any deviation from expectations would indirectly signal the presence of new physics. Examples of precision measurements include the W and Z masses, the electroweak mixing angle, as well as the quark mixing angles. Of particular importance are plans to measure the W mass to an accuracy of about 50 MeV (better than 0.1%) both at the Tevatron with the Main Injector upgrade, and at LEP II, along with the ongoing effort at SLAC to measure the electroweak mixing angle with similar accuracy using polarized electrons. The third means of testing the Standard Model and hunting for new physics involves studies of very rare, or even forbidden processes, including CP violation. At accelerators, such experiments require high intensity. Traditionally, the æ and K mesons have been used because of their relatively long lifetimes and copious production rates. Indeed, K decays presently provide our only evidence for CP violation. They also indirectly probe for new physics at the 200 TeV scale, a domain well beyond the reach of our highest energy accelerators. Ongoing experiments at BNL and Fermilab continue to push the search for rare K decays to unprecedented levels and probe for the origin of CP violation. Rare decays of the bottom and charm quarks as well as the tau lepton are starting to reach significant limits. For example, the CLEO collaboration at CESR recently found the first evidence for rare radiative b quark decays. Studies of B mesons (that contain b quarks) are particularly exciting because they open a new window to CP violation. Indeed, the standard model of CP violation predicts relatively large effects in B decays. Studies of these predictions will be possible at high-luminosity electron-positron B factories as well as at high-energy hadron colliders. Other examples of exotic phenomena that require high rates or massive detectors include neutrino oscillations from one type to another, non- standard CP violation searches and proton decay. Proton decay experiments are particularly impressive because they are our most direct window to physics at the grand unification mass scale. Indeed, present bounds on the proton lifetime already test physics at 1015 GeV. A joint Japan-U.S. experiment presently under construction at the Kamioka mine in Japan should push the proton lifetime search more than a factor of ten. Discovery of any reaction forbidden by the Standard Model would revolutionize physics and open up many new avenues of investigation. A well-balanced experimental program must include this three-pronged approach of high energy, high precision, and high intensity experiments, along with a variety of complementary non-accelerator initiatives. Only in that way can we hope to broaden our frontiers and increase our chances for discovery. What then are the most compelling questions and issues which currently drive our experimental program in high-energy physics and how can they be best addressed? As representative of the many exciting questions still to be answered by particle physics we propose the following list and briefly indicate with what facilities these questions may be answered. |
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