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.
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