I spent three days last week at the Phenomenology 2012 Symposium in Pittsburgh, known as Pheno 2012. Phenomenology specifically refers to the practice of predicting and analyzing the results of particle physics experiments, and the symposium looked at the possibility of a "new physics"—things not predicted by the Standard Model of particles and interactions—that might show up in experiments like the Large Hadron Collider (LHC).
Particle physics is the study of the particles that make up ordinary matter—quarks, electrons, and so forth—along with their more exotic cousins produced in high-energy collisions. Accelerators bring particles up to high speeds and smash them together for the purposes of probing the internal structure of atomic nuclei and the forces holding matter together. The weak and strong nuclear forces play important roles at high energies—along with potentially interesting new forces and new particles yet undiscovered.
The energies involved are sufficient to access the quarks that are normally locked away inside protons and neutrons. But they also make it possible to create new particles—a process described in part by Albert Einstein's famous formula E = mc2. If you put enough energy into the collisions, some of it will get converted to mass in the form of a particle.
(Because of the relationship between mass and energy provided by relativity, particle physicists find it convenient to write masses in units of energy, specifically electron volts [eV]. For example, electrons have masses of about 0.5 MeV [a half-million eV], while protons and neutrons are around 940 MeV in mass.)
So why is new physics required? A number of reasons were discussed, including the mysterious dark matter that makes up about 80 percent of all the matter in the Universe. (Dark matter is demonstrably not made up of ordinary particles—not quarks, electrons, or any of the other constituents of normal matter). Another reason is that, while the Standard Model provides a good description of the electromagnetic and nuclear forces, it does not include gravitation.
I attended 21 plenary talks and 23 smaller presentations at Pheno 2012, so I hope you will forgive me for not summarizing everything I heard. Instead, I'll highlight some of the major themes of the symposium. Since no major new results were announced (partly for reasons I'll explain shortly), the conference felt more like a "State of Particle Physics" event. Here's what that state looks like—and how it may change in the year ahead.
Hunting the Higgs
A number of talks focused on the search for the Higgs boson, the particle that gives mass to the W and Z bosons (the carriers of the weak nuclear force that mediates radioactive decay). The Higgs is predicted by the Standard Model and by other theories, but its existence is still unconfirmed by experiment. However, results from the ATLAS (A Toroidal LHC ApparatuS) and CMS (Compact Muon Solenoid) detectors at the LHC, along with CDF (Collider Detector at Fermilab) show a possible detection at nearly the same energy. The LHC is Higgs-hunting by smashing together streams of protons at high fractions of the speed of light; Fermilab's Tevatron collider used protons and antiprotons.
As emphasized in many of the presentations, particles don't have name tags. Detectors can't automatically distinguish between each thing produced in high-energy collisions. So the job of particle phenomenology is often taking the huge number of detections and attempting to work backward to the particles that triggered them. This is challenging in the case of the Higgs, as it's by theory believed to be both electrically neutral (meaning it carries no electromagnetic signature) and short-lived (decaying rapidly into other particles). The SM doesn't predict exactly what the Higgs mass should be, either, though it is constrained by the masses of other particles, including the W boson. That mass has been measured by the Tevatron to high precision, as Weiming Yao from Lawrence Berkeley National Laboratory explained.
Furthermore, the SM predicts a variety of possible decay processes for the Higgs, including disintegration into two photons, two W bosons, or two Z bosons. Dieter Zeppenfeld of Karlsruhe Institute of Technology described the possible two-Z decay as a "gold-plated" part of the energy spectrum: it's hard to confuse that kind of decay with any other process. In fact, the LHC was able to use it to rule out a wide range of possible Higgs masses—there is no signal at all corresponding to decay to two Z bosons, to a high degree of confidence.
On the other hand, all three detectors—ATLAS, CMS, and CDF—found a small signal corresponding to the two-photon or two-W decay modes. The rest energy of the detection is around 125 GeV (125 billion eV), with a bit of statistical wiggle room on either side, as Mia Tosi of the University of Padova explained. All three detectors finding something at this energy seems suggestive, at least to this non-specialist. But when pressed, Alex Martyniuk of the University of Victoria wisely declined to specify a probability that this finding is real. Speaking to a few experts after the session, the consensus seems to be that the signal is probably real, but there may be some hidden systematic problem shared among the detectors. It's an unlikely problem, but one that can't be completely discounted.
The 2012 run of the LHC will increase the amount of data in the hunt by a factor of 10, which should settle whether the 125 GeV detection is real or not. Alas, the Tevatron shut down after the 2011 run for budgetary reasons, so the United States no longer has a collider capable of participating in the Higgs hunt.
As a final note, not only is it possible that the 125 GeV signal will vanish, but it is still possible the Higgs boson doesn't exist at all. While this would be a major blow to the SM, it's not the end of physics as we know it. We would, however, need an additional bit of new physics to explain why the W and Z bosons behave as they do. I'll return to this issue in the last section.
If you knew SUSY like I know SUSY
While many of the conference participants were very pleased with the progress of the LHC, another group seemed a little less happy. The Higgs boson is also a particle within a framework called supersymmetry (SUSY, usually pronounced "SOOsee"), an elegant extension to the Standard Model that helps resolve some of its difficulties (which are too involved to get into here). Most of SM physics is left as it stands, but SUSY pairs each particle of the SM with a supersymmetric partner. These have unfortunately comic names: quarks are paired with squarks, W bosons are partnered with winos (pronounced "WEEnos"), and so forth.
Despite the goofiness in nomenclature, SUSY is serious theory. It may help resolve the dark matter problem. However, the simplest version of SUSY, called the minimally supersymmetric standard model (MSSM), predicts a mass range for the Higgs boson that is significantly smaller than 125 GeV—a problem if the latest results hold up under the 2012 LHC run.
A major aspect of SUSY is that the symmetry between ordinary particles and their supersymmetric partners must be broken, or we should have observed these additional particles in colliders by now. This is a major concern. Even though SUSY has been around in one form or another since 1966 and became a viable physical model through work in the 1970s and '80s, we've not spotted a single SUSY particle thus far. As pointed out by Rahmat Rahmat (yes, that's his name—in my notes I list him as Rahmat2) and Csaba Csaki, respectively from University of Mississippi and Cornell University, the LHC should have detected some signature of SUSY by now, especially if the MSSM is correct. As Csaki said, "SUSY is a wonderful woman who does not return my letters. It makes you wonder if she even exists!"
SUSY is far from dead, even with a 125 GeV Higgs boson. But solving its problems requires modifications, many of which depart from the elegance of the original theory. One resolution in particular is known as "fine-tuning"—as the name suggests, some parameters of the theory can be tuned more or less by hand until they match what is observed experimentally. Even many SUSY supporters seem uncomfortable with that idea, so they are looking for other, more natural ways to salvage the theory. These include (for example) variations with multiple Higgs particles. As with the Higgs boson, many researchers are waiting for the 2012 LHC results, which may reveal SUSY particles hiding in the data.
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