Desperately Seeking SUSY!

Posted on March 3, 2013 by Glen Mark Martin

Yesterday, the 2013 installment of Rencontres de Moriond got underway at La Thuile, in the Aosta Valley of the Italian Alps, roughly where Italy, France, and Switzerland come together. (La Thuile is smack-dab between Lyon, France and Milano, Italy, just southeast of Geneva, Switzerland. I would gladly part with a few organs to be there.) This high energy particle physics conference, which runs through March 14, gathers together some of the best and brightest experimentalists and theorists from the world of particle physics.

The sessions from Wednesday, March 6 will be webcast, and are anticipated to include the latest results of analysis of data from CERN‘s Large Hadron Collider, including the latest info regarding the quest to determine whether the “Higgs-like boson” announced last July 4 is indeed the Standard Model Higgs boson. (Keep in mind that, as of around Valentine’s Day, the LHC is offline until 2015 for repairs and upgrades.) While the July 4, 2012 announcement was a major milestone, further data was still needed to verify that the newly-discovered boson is in fact a spin-0 boson (as opposed to a spin-1 boson such as a photon, W or Z boson, or spin-2 such as the hypothetical graviton), and whether the branching ratios for its decay products are as predicted by the Standard Model. (Gamma-gamma counts were a bit high at that point, although still within error bars, and the rare tau-anti-tau signal had yet to be observed by CMS.) I don’t expect breakthrough-level announcements on this front just yet, as that will likely have to wait until more data is collected after the LHC upgrade, but the picture painted by the data collected thus far should start to grow a bit more clear as statistical fluctuations start to get dampened by larger and larger datasets.

But what I am really interested in seeing is news about  supersymmetry (or SUSY for short). To see why, it might be fruitful to step back and take a look at the announcements from the last major conference like this.

Back in November of 2012, theoretical and experimental particle physicists from around the globe, especially those involved with experiments at CERN‘s Large Hadron Collider, gathered in Kyoto, Japan to attend the Hadron Collider Physics Conference (HCP 2012). Events such as this are a big deal for physicists. Not only are they golden opportunities to network with colleagues from around the world, swapping phone numbers, email addresses, and ideas, but they are also prime opportunities to present results.

And boy were there results.

The results are perhaps not as earth-shaking as July’s announcement of the tentative discovery of the Higgs boson, but big nonetheless. There was more analysis related to characterizing the Higgs discovery based upon more data, the observation of a new composite particle, observation of D^0 \mbox{--} \bar{D}^0 mixing, and, perhaps biggest at all, data which casts the body of theories known as supersymmetry into further doubt (or not: more on that in a bit).

So, what the heck is SUSY, anyway?

I’ll start with the big one, SUSY. But perhaps I should first clarify what the heck it is.

Supersymmetry (or SUSY for short) is a concept conceived in the early seventies. SUSY is a proposed approach to extending the Standard Model of Particle Physics in such a way that, for every particle in the Standard Model, there exists a corresponding supersymmetric partner particle of opposite spin type. For each SM fermion (electron, neutrino, quark), there is a supersymmetric boson (selectron, sneutrino, squark). For each SM boson (photon, W or Z boson, Higgs), there is a corresponding supersymmetric fermion (photino, wino, zino, Higgsino). Ordinarily, the superpartners would be expected to have the same masses as their SM partners, so they would have been observed already; but, via a spontaneous symmetry breaking process, the superpartners are imbued with higher mass. Most versions of SUSY models put the masses for the lighter superpartners (such as sneutrinos and gluinos) within a range detectable by the LHC.

The beauty of SUSY models is that they provide mechanisms for explaining some phenomena which the Standard Model fails to answer, such as the Hierarchy Problem and providing candidate particles for dark matter. By the early eighties, SUSY theories had grown quite popular among theoretical physicists, so much so that they came to be incorporated into many variants of string theory (to such a degree that many, but not all, versions of string theory depend upon SUSY being correct). All that has been left has been the detection of these supersymmetric partner particles.

Yes, and?

Well, thus far, results from the LHC have not been very promising for SUSY. With each data release, the parameter space of possible SUSY models gets more and more constrained as various SUSY versions get ruled out. Already, portions of the parameter space which make SUSY useful for addressing the Hierarchy Problem have largely been ruled out. The latest round of results released in Kyoto are no different. However, as the blog postings listed below by Professor Matt Strassler point out below, not all of the results reported at Kyoto were a blow to SUSY.  While more mass ranges for potential supersymmetric partners have been excluded, revised figures regarding the decay of Bs mesons actually relax other constraints on the SUSY parameter space. In short, the results announced at Kyoto put us no closer to knowing whether SUSY is valid or not.

Meanwhile, on the Higgs front….

Nothing earth-shattering was reported on the Higgs front. Results were presented based upon twice as much data as was used for the July 4 announcement. Error bars were tightened at bit, and the data was looking a little better for the tau-anti-tau channel. Still more data is needed.

A new particle!

Well, it isn’t a new elementary particle, but rather a composite particle made up of quarks. It was the discovery of a plethora of such particles in the fifties and the sixties which lead to the formulation of quark theory.

New particle-like structure confirmed at the LHC | symmetry magazine

Mix-masters

The other big new out of Kyoto was the observation of neutral D meson mixing (D^0 \mbox{--} \bar{D}^0). Neutral meson mixing had long been predicted, and had in fact been confirmed with kaons, B-mesons, and strange B-mesons, but this was a first for D-mesons.

Basically, what is happening here is that what starts off as a neutral D-meson can later be detected as a neutral anti-D-meson. This oscillation occurs for exactly the same reason that neutrino flavor oscillation can take place: the mass and flavor eigenstates are different. But that is a discussion for another time.

Summing Up, and Looking Forward

So that was the news out of Kyoto in November. I’m a bit late getting around to commenting on it (life getting in the way and such); but, with Moriond getting underway, it seemed worth a look back.  Now to wait and see what tidbits trickle out from the Alps….

About Glen Mark Martin

MCSE-Messaging. Exchange Administrator at the University of Texas at Austin. Unrepentant armchair physicist.
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