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Science Today: A New View of Flavor in Supersymmetry Models

Many exciting possibilities exist for physics at the 100 GeV to 1 TeV scale, and one of the most intriguing is the possibility of supersymmetry. In supersymmetry, there is a new partner state with a different spin for every state in the Standard Model. For each gauge boson, there is a spin-1/2 particle called a gaugino; for each quark, there is a scalar quark (or squark) and for each lepton there is a scalar lepton (or slepton).

Theorists invoke supersymmetry to explain the energy scale of weak interactions. This explanation works only if the new particles predicted by supersymmetry—the gauginos, squarks and sleptons—have masses below about 1,000 GeV. However, that makes these particles light enough to create other problems. We know that the weak interactions mix different quark species. This is a mechanism for the decay of K and B mesons and, in more complicated processes, for the mixing of neutral K mesons and neutral B mesons with their antiparticles. The BaBar experiment at SLAC has measured b quark mixing in many ways and found that the Standard Model description works quite well. However, if the mass terms of squarks and sleptons mix the partners of different quarks or leptons, this can translate into a new source of quark mixing. This source, which arises from the breaking of supersymmetry, need have no relation to the source from the Standard Model weak interactions. Slepton mixing can induce new processes that are forbidden in the Standard Model, such as the decays of a muon to an electron and a photon or a tau to a muon and photon. These are not seen in experiments. BaBar has recently placed new, stringent limits on exotic tau decays.

This is a long-standing problem known as the supersymmetric flavor problem. In building a model with supersymmetry, it is necessary for forbid or suppress mass terms that mix the squarks and sleptons. In some models, this is done by making the squarks and sleptons degenerate. Specific schemes of supersymmetry breaking, such as gauge mediation, gaugino mediation, or anomaly mediation, can explain this degeneracy. In other models, such as "minimal supergravity," one just imposes the constraint by fiat.

During my visit to SLAC and Stanford, I have been collaborating with with Graham Kribs, of the University of Oregon, and Erich Poppitz, of the University of Toronto, to explore a new possibility. We postulate that the minimal supersymmetric extension of the Standard Model is enhanced with a new symmetry, called an R-symmetry, under which Standard Model particles and their superpartners carry different charges. This symmetry remarkably forbids the most dangerous flavor-changing mass terms. However, it also forbids some mass terms that are needed in the usual description of supersymmetric particles. So our idea produces some unconventional predictions.

Gauginos are quite different with an R-symmetry. The usual mass terms, which make the gauginos Majorana fermions, are forbidden. It is possible to give masses to the gauginos by adding additional fermions to the theory. In 2002, Patrick Fox (of Fermilab), Ann Nelson (of the University of Washington), and I studied this possibility and saw that it is easy to make the gauginos a factor 5 to 10 heavier than the squarks and sleptons.

What Kribs, Poppitz and I have found is that these heavier gauginos allow for a dramatic suppression of those flavor changing processes that are not already explicitly forbidden by R-symmetry. As a consequence, the mass eigenstates of the squarks need not be aligned at all with the mass eigenstates of the quarks. Squarks and sleptons could have very different masses and could decay to different quarks or leptons, and yet no signal would have shown up yet in existing experiments. This would lead to dramatic flavor violation when squarks and sleptons are produced directly at LHC and ILC. I am now investigating this effect with Roni Harnik, Mina Arvanitaki and other colleagues at SLAC.

Intriguingly, our theory also suggests new contributions to neutral B meson mixing and to muon decay to electron plus photon that might be observed in more precise experiments. New precision studies of these flavor violating processes may yield exciting signals, giving insight into the nature of physics at the weak scale.

óNeal Weiner, SLAC Today, November 15, 2007

Neal Weiner works at New York University and is currently visiting SLAC and Stanford.