Not Even Wrong

There’s a new paper out this evening from a large collaboration entitled Implications of Initial LHC Searches for Supersymmetry. Instead of just adding it to the bottom of my recent posting, I thought it would be a good idea to start a new one, and add a bit more explanation of what is going on.

For a good news story from today by Kate McAlpine, see this at Physics World. For excellent more technical explanations, see the latest blog postings at Tommaso Dorigo’s blog (today) and at Resonaances (yesterday). Physics World, Tommaso and the new arXiv preprint discuss published results from CMS and ATLAS, while Resonaances discusses even more stringent preliminary limits on SUSY from ATLAS made public last week at Aspen.

Tommaso also refers to a 2008 guest blog posting by Ben Allanach explaining how statistical predictions for SUSY masses were being made, adopting various simplifying assumptions (CMSSM) and assuming supersymmetry solves the problems it is advertised as solving (muon g-2 anomaly, dark matter, etc.). Allanach discusses the 2008 version of this kind of calculation by the same group that has just put out a new, 2/22/2011 version this evening.

The usual model for how science is done is that theorists make predictions before an experiment is done, then when the experimental results come in, they get compared to the predictions. That’s not quite what is going on here, where as far as I can tell, the new paper doesn’t directly compare the 2008 predictions to the new experimental results. Instead, the new experimental results are used to make new predictions. Since a large part of the parameter space favored in the 2008 predictions has now been ruled out, the new ones move the favored part of parameter space up to higher particle masses. The authors do make clear what is going on, showing on their plots a “snowflake” where the 2008 best-fit value was, and “stars” for where the new best-fit values are based on data from the two experiments. Note that the paper does not include the latest, stronger results from ATLAS announced last week, which presumably would move the “stars” up to even higher mass.

While the question this paper addresses about where supersymmetry might be given that it hasn’t been seen yet is interesting, it leaves unaddressed the more conventional question: do the LHC experimental results show that the theoretical predictions about supersymmetry made in 2008 before the machine was turned on were wrong? This is a statistical question, so should have a statistical answer. Assuming that the LHC continues to not see supersymmetry as it collects more data, I’m interested in the question of how the experimental data will falsify the theory. Will its proponents just keep calculating statistical predictions of higher and higher masses as lower ones get ruled out? Most will undoubtedly at some point throw in the towel, although there will be some who will never, never, never, never give up (see here):

SUSY may still be there even if it remains invisible to the LHC, indeed. And yes, I don’t hide that I will be convinced that SUSY is there even if the LHC doesn’t find it. The LHC will only confirm or exclude effects at particular regimes – usually low energy but it’s not quite accurate a description of the regime that may be excluded.

What I have been scared for several years is the pseudoscientific propaganda of your kind trying to claim – without any justification – that not seeing SUSY at the LHC should imply that physicists shouldn’t be allowed to work on SUSY or believe that it is a key feature of our Universe. There are many reasons to think it’s the case and theorists whom I consider any good will continue to treat SUSY as an essential feature whether or not it shows up at the LHC.

Update: See figure 1 of this evening’s What if the LHC does not find supersymmetry in the sqrt(s)=7 TeV run? to see how how much of the predicted region of superpartner masses was ruled out by initial LHC results, and how much of the rest is likely to be ruled out during by the 2011-2 7 TeV run.

Update: There’s a very new up-to-the-minute survey of LHC results concentrating on supersymmetry by John Ellis here. Unfortunately no figures that superimpose CMS/ATLAS exclusion regions on the statistically favored regions for supersymmetry that are discussed (based on assuming supersymmetry explains dark matter and the muon g-2 anomaly). It does look like this year’s data should be able to convincingly rule out the idea that supersymmetry explains both of these phenomena.

Update: The ATLAS results providing the strongest limits so far on SUSY are now out, see the paper here.

This week the Aspen Center for Physics is hosting one of the first of this year’s “Winter Conferences” where results from last year’s LHC run are being reported. Appropriately, the title of the conference is New Data from the Energy Frontier. The most dramatic result has to do with what is not being seen: any evidence of supersymmetry, with new limits reported today by ATLAS. The new ATLAS results rule out gluino masses up to 7-800 GeV, improving on the first limits of this kind from CMS which were about 600 GeV.

For more detailed discussion courtesy of the blogosphere, see Resonaances and Cosmic Variance. For some indication of what this means for string theory, Michael Dine’s lecture notes for his talks on “What LHC might tell us about String Theory” at last summer’s TASI summer school are now out, with the title Supersymmetry from the top down. These lecture notes start off with a section very unusual for this kind of thing, entitled “Reasons for Skepticism”. and he notes:

Our enthusiasm for supersymmetry, however, should be tempered by the realization that from existing
data – including early LHC data – there are, as we will discuss, reasons for skepticism.

For some historical perspective about what pre-LHC expectations were, I happened to run across today a copy of Witten’s lecture notes from a string theory conference at Hangzhou in 2002, where he gives the muon magnetic moment discrepancy as one piece of evidence for supersymmetry, and says:

Assuming this discrepancy holds up, we would expect to interpret it in terms of new particles, but these are highly constrained; one explanation that does work is supersymmetry, with masses of new particles of order 200 – 300 GeV.

Of course, even the minimal supersymmetric extension of the Standard Model is ferociously complicated, with over a hundred unknown parameters, so all quoted limits make various simplifying assumptions. Relating LHC data to limits on supersymmetry will be a subject keeping many physicists busy for the next few years, for more about this, see this talk at Aspen by Jay Wacker. He doesn’t expect this year’s run to as dramatically increase limits on gluinos as last year’s run did, describing early results as “full coverage up to 300 GeV, reach up to 600 GeV”, increasing to “full coverage up to 375 GeV, reach up to 800 GeV” after an inverse femtobarn of data is analyzed (that’s the official LHC goal for 2011, although it’s hoped they can double or triple that).

The last sentence of his last slide refers to something that I’ve always worried about, but am not expert enough to know whether such a worry is serious. He describes the web-site http://LHCNewPhysics.org where simplified models based on supersymmetry and other BSM ideas are given, and notes:

ATLAS studying 10 Simplified Models from 0 in August. Changing their triggers.

The worry I’m not so sure about is to what extent the LHC detector triggers are being optimized to look for supersymmetry, potentially missing un-expected non-Standard Model physics. Since there were always reasons to be skeptical of LHC-scale supersymmetry, and these have now become so compelling that even Michael Dine is writing about them, one hopes that the trigger designers will keep that in mind.

Meanwhile, back at the LHC, powering tests are finished, the ring is closed and will be put through full tests of its operational cycle the next couple of days. Official start of beams for this year is planned for Monday.

Update: More details about the latest on this at Resonannces.

Update: More from Tommaso Dorigo (LHC Excludes SUSY Theories, Theorists Clinch Hands), and a Physics World article by Kate McAlpine here. Tommaso links to a 2008 posting by Ben Allanach that discusses predictions for SUSY masses made (using various assumptions one can read about there) around that time. One of these, by a large group including John Ellis, predicted that 50 inverse picobarns at 10 TeV would be enough to explore most of the region they expected SUSY masses to be in, at 68% confidence level. The latest data, which is about that luminosity but at 7 TeV, does rule out much of that region, with the most likely SUSY mass right around the boundary of the region ruled out by ATLAS (although the tan(beta) values are different). According to the Physics World article:

John Ellis of CERN and King’s College London disagrees that the LHC results cause any new problems for supersymmetry. Because the LHC collides strongly interacting quarks and gluons inside the protons, it can most easily produce their strongly interacting counterparts, the squarks and gluinos. However, in many models the supersymmetric partners of the electrons, muons and photons are lighter, and their masses could still be near the electroweak scale, he says.