LHC Run 2 First Results

First results using the full data from Run 2 at 13 TeV will be presented tomorrow at CERN at 15:00 Geneva time, with a live webcast available here. For some relevant commentary, see Tommaso Dorigo and Matt Strassler.

Among relatively reliable rumor sources, Jester is tweeting about a supposed excess at 750 GeV in the diphoton spectrum. We’ll see tomorrow, but the problem with this is that it would be hard to understand why such a thing didn’t show up in Run 1 at 8 TeV. Tommaso explains why it is only at higher masses that one expects the Run 2 data to be competitive with that from Run 1, and suggests that what to look for is a 2 TeV excess in the dijet spectrum, since there were already hints of such a thing in the Run 1 data.

Matt describes 13 TeV results recently published by the ATLAS and CMS groups looking for exotic behavior at very high mass (predicted for instance by various theories of extra dimensions). Nothing there.

One other thing to look for is whether Gordon Kane will get his Nobel Prize. He’s predicting a 1.5 TeV gluino, with current limits around 1.4 TeV, and this year’s Run 2 data perhaps enough to push those up a bit. It may though be that such analyses will be among those that take longer, not appearing until the Moriond conference in March.

Update: CMS went first, results now publicly available here. Tommaso was pulling our leg, the 2 GeV Run 1 excesses have gone away. There is a diphoton excess at 766 GeV, but an unimpressive one (2.6 sigma locally, 1.2 sigma with look elsewhere effect).

Gluino mass limits have moved up, some as high as 1.7 TeV. Presumably Kane is now at work on new string theory predictions.

Bottom line: nothing beyond the SM so far.

Update: ATLAS next. No gluinos up to 1.8 TeV. 2.2 sigma for the 2 TeV excess that CMS doesn’t see.

They also see an excess in diphotons around 750 GeV, 3.6 sigma local significance, 1.9 sigma with look-elsewhere. So, starts to look interesting if combined with CMS, the rumor was right. They also reanalyzed the Run 1 data, nothing there at 750 GeV, no combination of Run 1 and Run 2.

Results available here.

Bottom line: only thing interesting is the possible 750 GeV diphoton excess. One can predict a flood of theory papers with models predicting such a thing. Will have to wait until at least next summer though to see if this gets confirmed or goes away.

Commentary from Matt Strassler here.

Update: As expected, best explanation and discussion of the implications of the diphoton excess is from Jester, see here.

Reasons to be excited: naively combining CMS and ATLAS gives something of 4 sigma significance, people are making the analogy with the early Higgs signal. Reasons to be less excited: in the case of the early Higgs signal, the tentative signal was what was expected from the Higgs, and we had very good reasons to believe there was a Higgs roughly in that mass range. Here I know of no well-motivated models that predict this: extraordinary claims require extraordinary evidence, and this is not that.

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13 Responses to LHC Run 2 First Results

  1. Peter Woit says:

    Tom Weidig,
    Not interested in betting against this (especially not at 10-to-1 odds). I do think the likelihood is now stronger that this will go away than that it will survive, but it would be fantastic if this were true: the non-standard model physics we’ve been waiting to see for 40 years, at an energy where the LHC can start to study it.

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  3. vmarko says:

    If the 750 GeV event eventually turns out to be really a new particle, I am sure that Gordon Kane will easily modify his ST model such that it… hmm… will have had predicted… precisely that particle, several years ago. 🙂

    And if you ask him how is that even possible, he’ll give the (by now famous) answer: “go and read my paper, it’s all written there.”

    Best, 🙂

  4. NeapTide says:

    Nice, hope it is really true.

    LUX (the dark matter experiment) also published a nice paper this week. They seem to have calibrated way better than ever before, and aren’t seeing the low-mass WIMPs that were claimed.

  5. Peter Woit says:

    I don’t think Kane has shown interest in “predicting” unexpected things like this. So far his claims have been about string theory “predicting” the Higgs, and “predicting” that the Higgs will behave like the standard model says, as well as a long history of “predictions” of superpartner masses.

    A couple interesting things to watch for over the next year:
    1. Once Kane moves his “prediction” of the gluino mass up again in coming months, who will invite him to give a talk explaining why string theory “predicts” a 2 TeV gluino, and not mentioning any of the earlier string theory “predictions” of the gluino mass that marched up from a few 100 GeV years ago to 1.5 TeV in Munich recently? Will the organizers of the Munich conference publish in their proceedings the news of the falsification of string theory? Will Physics Today publish an update on the “supertestability” of string theory?

    2. Any reasonable hope for vindication of SUSY at the LHC is rapidly disappearing, at what point will all those besides Kane who advertised SUSY at the LHC as a “test of string theory” admit failure and face up to the implications of this failure?

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  7. Ralph Dratman says:

    “One can predict a flood of theory papers with models predicting such a thing.”

    Does your theory of theory paper flooding give any quantitative predictions? There might be some surprisingly energetic papers in flight right now, from a University far, far away.

  8. Cosmonut says:

    “Gluino mass limits have moved up, some as high as 1.7 TeV. Presumably Kane is now at work on new string theory predictions.”

    In an universe where string theorists are not continually revising their predictions, they do not exist because nobody was willing to sponsor a program of research which promised a theory of everything and delivered nothing after 40 years.

    This is a demonstration of how useful the Anthropic Principle is for explaining properties of the universe that we observe. 🙂 🙂

  9. RandomPaddy says:

    How many times has a maybe-maybe-not bump like this appeared before? Are there examples where it did and didn’t result in a new particle being ultimately confirmed?

  10. Peter Woit says:

    Random Paddy,
    Jester refers in his posting to early hints of a Higgs that disappeared
    “It is a very large effect, but we have already seen this large fluctuations at the LHC that vanished into thin air (remember 145 GeV Higgs?).”
    I think one can probably come up with quite a few such examples, there’s a reason that people in this field have adopted the “5 sigma” standard. In this case it’s tricky how to take into account the two different experiments (they haven’t combined their data), as well as the fact that you’re looking at lots of different channels, so increasing the probability of seeing a fluctuation in one of them. I doubt that at this point the statistical significance of this is near 5 sigma. In addition, the fact that this doesn’t correspond to anything theoretically well motivated argues for added caution (and also, added excitement if it works out).

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