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Volume 21 Friday, June 5, 1998 Number 11 ~ In Theory For every fundamental particle, the theory of supersymmetry proposes a supersymmetric partner, a “sparticle” in SUSY jargon. The electron, e, for example, has a SUSY spartner, the selectron e.˜ Would every Fermilab theorist have a SUSY “stheorist”? Probably not, but theorists worldwide will be watching Tevatron Run II for the first experimental evidence for SUSY. The Fermilab Stheory Group INSIDE f 2 Supersymmetry 7 Supersymmetry Workshop 8 MiniBooNE 10 Fishing 12 Paperless Papers 14 CDF Party The Fermilab Theory Group supersymmetry …and the physics of Run II by Meher Antia Without resorting to tricky mathematics or exotic physics, anyone can see that much of the stuff the world is made of has mass. Things can STANDARD be touched and felt; they have some bulk, unlike, say, light which is intangible and MODEL immaterial. But why? Why are some things in the universe, like quarks and leptons, chunky and massive and others, like photons and Mass = ~ 3 MeV ~ 1.2 GeV 175 (6) GeV gluons, ethereal and massless? up charm top photon ~ 4 MeV 95 (15) MeV ~ 4.8 GeV Answers to questions like these are usually bottom gluon provided by the Standard Model, to date the down strange 91.186 (2) GeV best framework to < 15 eV < 100 KeV < 35 MeV describe the patchwork of elementary particles electron neutrino muon neutrino tau neutrino Z boson and forces of nature. .511 MeV 106.76 MeV 1.784 GeV 80.36 (13) GeV However, in its simplest form, the Standard Model disappoints. It electron muon tau W boson clearly states that all particles should be I II III massless, just like the Three Generations of Matter photon. How, you might ask, can the Standard Model retain its reputation as the best In its simplest form, the description of nature if Standard Model fails to it makes a prediction account for different that so blatantly flies in particle masses. To solve the face of even the least this problem, theorists perceptive observer? have invoked The Standard supersymmetry, a Model retains its theory that provides a position because there supersymmetric particle is a fix to the mass partner, or “spartner” problem. A particle for each of the known fundamental particles. called the Higgs boson, yet to be observed, is conjured as part of a mechanism to bestow mass upon particles in a way that is consistent with experimental results. Still, the fix of the 2 FermiNews June 5, 1998 Supersymmetry could manifest itself in many different ways. Theorists, including Fermilab’s Joe Lykken, left, are still on the fence about exactly what form it might take. “That’s why we need our experimental colleagues,” says Lykken, shown here with theorist Xerxes Tata, of the University of Hawaii, attending CDF’s 20th birthday party on May 20. Photo by Jenny Mullins Higgs comes burdened with difficulties of its symmetry; these symmetries are restrictions on own. Happily, though, even those difficulties how they behave and interact with each other. may be resolved by a theory known as Unfortunately, the Higgs boson has no such supersymmetry. symmetry to slim it down, and it gets heavier The problem of introducing the Higgs into and heavier as the Standard Model is called the Standard Model is symptomatic of a more upon to explain physics at smaller and smaller general deficiency in an otherwise stunningly length scales. successful model. The Standard Model was The easiest way for theorists to stop the designed to describe particle interactions at out-of-control growth of the Higgs was to length scales of particles like the electron or the introduce a symmetry, and supersymmetry was quarks. These, less than billionths of millimeters the one they chose to cure the Higgs’s weight in size, are undoubtedly tiny, but are still far problem. Developed about 20 years ago, from zero. The problem with the Standard supersymmetry proposed giving the Higgs a Model occurs when physicists try to understand partner—a supersymmetric partner called the particle interactions at length scales close to Higgsino—identical to it in every way except zero. At that point, infinite quantities pop up in for its spin. The Higgsino could then act as a the energies and masses of particles. Infinities sort of personal trainer to the Higgs, not are meaningless not only to physics, but also to allowing it to do things that made it too common sense; it is impossible to have a massive, and keeping it nice and lean to be particle with an infinite mass, for example. of service to the Standard Model. Sometimes, the Standard Model can take But supersymmetry and the Higgsino came care of these infinities by a process called along with much excess baggage. Not only was renormalization: getting the infinite quantity to there a supersymmetric partner for the Higgs, cancel out with some other infinite quantity there had to be supersymmetric partners for all with an opposite sign. Most of the Standard other particles. All these new supersymmetric Model particles are prevented from being partners would have exactly the same properties infinitely massive by having some sort of as their twins, again with the exception of spin. FermiNews June 5, 1998 3 Theorists have invented particle partners before. In the 1930s, to reconcile quantum mechanics with relativity, Paul Dirac proposed siblings for all known particles, identical to the existing particles in every way but with opposite charges. The mysterious siblings— antimatter—are now as commonplace as they were once exotic. …Any particles with the same mass and charge of everyday electrons and Fermilab Archives quarks would Spartners for all The spartners, however, have been much already have Physicists have had their share of fun in more elusive. It was clear from the very naming the supersymmetric particles. Some get beginning that any particles with the same mass been found. an “s” put in front of their ordinary names, the and charge of everyday electrons and quarks sypersymmetric partner (or spartner) of the would already have been found. As no such As no such electron is the selectron, that of the quark is particles have turned up, supersymmetry is the squark. Other particles, like the Higgs, said to be a broken symmetry. Something has particles have get suffixed by an “ino”: the spartner of the caused the masses of the spartners to be very photon is the photino, that of the gluon is the different from the regular particles. In fact, turned up, gluino, and so on. In essence, supersymmetry such a symmetry breaking happens quite claims, we have only seen half the particles that naturally in the theory at extremely high supersymmetry exist. The rest are lurking in the wings, waiting energies, energies beyond the realm of even the to be discovered. most powerful particle accelerator imaginable. is said to be a Stupendous as this may sound, there has But even though the pairs of particles are split been a precedent, points out Fermilab physicist apart and given different masses at high broken symmetry. Joe Lykken. In the 1930s, when trying to energies, they must have a way of reconcile quantum mechanics with relativity, communicating that information down to the Paul Dirac found that, for his theory to make more earthly scales that are accessible to us. sense, all known particles had to have siblings. How exactly this communication might take These siblings were identical to the existing place is not well understood. One possibility is particles in every way, except that they had gravity. Another is that the interaction might opposite charges. The mysterious siblings— occur through some new particle, one similar antimatter—were soon found. They are now to the photon or the gauge bosons that fly back as commonplace as they were once exotic and and forth to carry the weak force. have been snugly accommodated into particle physics. 4 FermiNews June 5, 1998 They can’t hide forever However supersymmetry breaking transmits itself to our world, the result is that if the spartners were to be hunted down in particle accelerators they would be quite heavy. Supersymmetry does not predict exact masses for the various squarks and sleptons. Thus, there is no need to panic and abandon the theory because none of the spartners have yet been found. If they do exist, though, they cannot keep hiding forever. Although there is theoretically no limit on how heavy the spartners can get, after a certain point they no longer perform the function for which they were originally invoked. After all, the supersymmetric particles were supposed to prevent the Higgs from getting too hefty, but if they themselves are not trim enough, their existence serves no purpose. There is a sort of natural mass range within which they are expected to be found. Tantalizingly, the current generation of particle accelerators is just at the edge of that expected range. At Fermilab, the hunt for sparticles is already on. There are dozens of different ways in which these sparticles can make their presence known to experimenters, depending on exactly how supersymmetry theory is formulated. All sparticles, except perhaps the lightest one, are unstable; they decay either into other sparticles or into the regular quarks and leptons that we already know. If they decay into particles of the ordinary kind, it could be extremely difficult to Photo by Jenny Mullins confirm that they were indeed produced from supersymmetry and not from some known the missing energy; only then can it be Harvard graduate Standard Model process. Some signatures — attributed to the LSP. Like all particle searches, student and CDF the presence of lots of electrons and muons this one requires combing through staggering collaborator Maria in the detector, for example — would be clear amounts of data.

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