Monday, 20 June 2011

Meanwhile at the LHC

The excitement about the CDF bump is subsiding so we can relax and look back at the LHC. A lot is going on there, although the best of the action will occur later this summer.
  • As everyone knows, the luminosity collected by ATLAS and CMS just passed the 1 fb-1 benchmark which had originally been the goal for the entire 2011. The LHC management is running a policy of making careful projections which can later be spectacularly surpassed. We all agree it's better than the previous policy of bold projections and spectacular catastrophes.
  • Unofficial predictions for the luminosity at the end of 2011 are Gaussian distributed with a peak around 5 fb-1. It means that, this year we'll probably learn whether the Standard Model Higgs exists or not! That's massive.
  • Several results using a good chunk of this year's data, around 200 pb-1, have already emerged from ATLAS on the occasion of the PLHC conference in Perugia 2 weeks ago. The most interesting result is the t-tbar invariant mass spectrum. The most disappointing, too. If Tevatron's excess in the top quark forward-backward asymmetry is due to a 1-2 TeV heavy Kaluza-Klein gluon the t-tbar spectrum should display a peak or an excess at the tail. Alas, nothing there, as you can see.
  • About the update on searches for Z' decaying to leptons, see Tommaso's blog. There's also a new search for W' in the muon + missing energy channel. In those cases also nothing, only vague rumors.
  • Meanwhile SUSY searches are continuing at full steam. New analyses in the jets+met and jets+lepton+met channels are out. From the plot you can read that, for equal squark and gluino masses, the limit on the masses is above the magic threshold of 1 TeV. If the squarks are decoupled the limit on the gluino mass is slightly less stringent, about 750 GeV, and similalry the other way around. Children take about 6 years to realize Santa Claus does not exist, the LHC may be quicker than that.
  • Theorists, on the other hand, are trying to understand the deeper meaning of the LHC limits on SUSY. The conclusion is that SUSY must be just behind the corner, just a little bit more, one last effort, and we'll see it. One should note that preference of the global fits for light superparticle masses is driven by one measurement: the long standing 3 sigma excess in the muon anomalous magnetic moment. Interestingly, a recent paper reevaluates the theoretical contributions to the muon g-2 and concludes there is no excess whatsoever. I am not in a position to judge whether the paper is correct, drop your comment if you are.
  • Meanwhile, ATLAS and CMS keep posting papers on arXiv which use only the meager last year's harvest of 35pb-1. At this point it feels like offering ZX Spectrum in an Apple store.
The next big dump of LHC results is bound to happen for the EPS conference in Grenoble end of July. Expect numerous new physics analyses using 1.x inverse femtobarn of data. It'll rock!


Kea said...

Zombies never die, arrrggggghhhh!

Robert L. Oldershaw said...

In science, do negative results obligate one to decrease one's confidence in the tested model, and the assumptions underlying it?

Is adjusting the model, and its underlying assumptions, so as to conform to problematic empirical results considered to be scientifically defensible?

How many adjustments are considered acceptable before serious doubt sets in?

Is the "WIMPs" dark matter hypothesis falsifiable in any real scientific sense?

What about "string/M theory"?

Can SUSY be adjusted arbitrarily?

Anonymous said...

As for the analogy, I'd rather have the Spectrum.

Anonymous said...

Hey Peter, you interested in publishing your theory in book form? We have special deals for crackpots!

Anonymous said...

"My experimental results call into question Einstein's interpretation of E = mc^2. Maybe we need a serious paradigm shift concerning the gravitational force. "

You have got to be kidding me...

Anonymous said...

The SUSY exclusion plot has a massless lightest neutralino, which seems like a rather special corner in the parameter space. I was wondering how the bounds on the squark and gluino masses would change as one dials up the LSP mass?

Herbi said...

You should maybe also mention the extensive analysis by Jegerlehner et al on (g-2) from June 7th. They come to the conclusion that there is a 4.1 discrepancy between the SM and exp.

Herbi said...

Jegerlehner et al put out an extensive paper (over 90 pages) on the arXiv June 7th on an HLS model calculation of (g-2). They find a 4.1sigma discrepancy between TH and EXP. I can also not judge if this is correct.

Anonymous said...

Meanwhile at Dzero,

the likesign dimuon charge asymmetry was upgraded to a 3.9 sigma excess.

Anonymous said...

Robert: It's a matter of nomenclature in my opinion.

First of all, the "negative" SUSY results are not so negative. They are exclusions of particle masses. The MSSM does not predict it's parameters, they must be measured, just like Newton's theory does not predict the mass of the Earth. The results on SUSY tell us that superpartners cannot be lighter than some bound.

The reason they are thought to be light is that if they are then they more neatly solve the problems that the model was meant to solve. But that says nothing of the framework of SUSY.

We normally rename models when they have been sufficiently changed, for example there is the MSSM, and the NMSSM, but when speaking colloquially we will call a huge range of theories "SUSY".

"Serious doubt" that you mention would probably apply to the specific instantiation of "TeV SUSY" if we did not see anything at the LHC. Then the community would likely start to take a different explanation for the problems that TeV SUSY addresses as the front runner. Maybe some version of Technicolor. But whatever is there should be seen at the LHC and give clues, since we know the problems lie at the TeV scale.

WIMPs is too generic a word in my opinion to be easily falsifiable. "WIMPs" appplies to many dark matter candidates from many different SUSY theories. However it could be falsified if we were to find that 100% of the dark matter is made of something with a distinctly different signal, coupling to photons in the case of axions, or late decay in the case of FIMPs for example. Particularly something that has been named already by its authors. Even if were to find that WIMPs were a specific type of particle, my guess is that the community would start to say "MSSM neutralino of such and such a composition" even in its colloquialisms over WIMP as time went by and we became more certain. We stick with generics in early stages as broad names help us to keep a broad view.

SUSY cannot be adjusted arbitrarily. In my opinion it is the name of a symmetry group first and foremost. This group has certain representations and different numbers of supercharges, but these are well understood and in the case of supercharges, finite. This is totally fixed, like the Lorentz group of Special Relativity. How you apply this group to your particular theory is up to you, and is much more adjustable. Like the situation in the Standard Model where we can choose the representations that the fermions live in in the symmetry groups. There are "minimal" or "natural" choices for this, but there is no reason these should be the whole story.

String/M theory is a different matter. Definitive falsification of SUSY would be a huge problem for string theory as a fundamental theory, since we mostly need super strings in what we know. See it as a calculational tool, in the AdS/CFT setting, and the question becomes much more subtle. Either way, it is teaching us a huge amount about the possible forms of theories, just like the formal setting of GR did.

Did that help?