CMS and ATLAS present their results

CMS results are being presented by Jim Olsen of Princeton University.

CMS has magnet problems this year due to cooling system problems but was able to record 3/4 of the data with the magnet on.

The diboson excess widely discussed this summer is, perhaps not surprisingly, not confirmed.  Same for the old dilepton excesses.

With certain assumptions, limits on gluinos jump from 1.3 TeV – 1.4 TeV to 1.6-1.7 TeV.

Big improvement in limits on “Black Holes” or anything else dramatic at very high energy (as we saw also in my post yesterday about ATLAS multijet events.)

Top-primes — limits jump to about 950 GeV relative to 800, again with assumptions.

Some new limits on invisible particles.  W’ resonances ruled out up to 4.2 TeV if they decay to leptons, to 2.4 TeV if they decay to top quark + bottom antiquark (with assumptions.)  No dijet bumps or other unusual dijet behavior.  No dilepton bumps up to 2.5 – 3.1 TeV for simple assumptions.

Diphotons (with 2.6 inverse fb of data)! (Olsen shows an event at 745 GeV).  All diphoton events used.  Peak?  Yes!!  BUT: local 2.6 standard deviations, and with the look elsewhere effect, only 1.2 standard deviations. Not impressive.   Such a peak is not inconsistent with previous results, but doesn’t look like a signal.  Still… combining old and new data we see a signal at 3 standard deviations local, 1.7 standard deviations globally after look elsewhere effect.

Also the peak is rather ragged, though this doesn’t imply anything in particular; it is worth noting.  If you assume the peak comes from a wider bump, the significance goes down.

Now on to ATLAS, with results presented by Marumi Kado (from the French Laboratoire de l’Accelerateur Lineaire and Orsay).

ATLAS has 1.2-1.5 times more useable data than CMS.  This could be important.

Look for Higgs in four leptons.  Big statistical fluke!  They see fewer events than expected! This is, of course, no big deal… if you expect 6 events it is no surprise if you happen to see 2.

No peak in two Z’s at higher mass (i.e. no heavy Higgs seen.)  Some improvement in searches for Heavy Higgs particles decaying to taus at higher mass.

Limits on gluinos (with assumptions) go from 1.2-1.4 TeV to 1.4-1.8 TeV. (Got an improvement by looking for boosted top quarks in the case where gluinos decay to top quarks.)  Bottom squarks (with assumptions) — limits go from 650 GeV to 850 GeV.

The excess in Z + jets + invisible particles in high energy events remains in Run 2, a little smaller than in Run 1 but still there.  [Run 1: 10 expected, 29 observed; Run 2: 10 expected, 21 observed.] CMS still doesn’t see it.  What’s the story here?

Dijets (as I wrote about yesterday.)  Kado shows the highest-energy dijet event ever observed by humans.  Nothing unusual in photon + jet. Nothing in dileptons — limits on typical Z’ bosons in the 3-3.4 TeV range, W’ decaying to leptons limited up to 4.1 TeV,

DIPHOTONS. Here we go.

A completely generic search for photon pairs; nothing special or unusual.  Looking for bump with narrow width up to large width.  3.6 standard deviations local, global significance is 1.9 standard deviations.  Looks amusingly similar to the first hint of a Higgs bump from four years ago!  Large width preferred, as much as 45 GeV. Local significance goes up to 3.9 standard deviations, 2.3 after look elsewhere.  Mass about 750 GeV.  Hmm.  No indication as to why they should have been more efficient than in Run 1, or why such an excess wouldn’t have been seen at Run 1.

WW or ZW or ZZ where there was an excess in Run 1.  As with CMS, no excess seen in Run 2.  WH,ZH: Nothing unusual.

Ok, now for the questions. The diphoton bump seen, with moderate significance in ATLAS and low significance at CMS, is very interesting, but without more information and more thought and discussion, it’s premature to say anything definitive.

Kado says: Run 1 two-photon data was reanalyzed by ATLAS and it is compatible with the Run 2 bump for large width at 1.4 standard deviations, less compatible for narrow width at more than 2 standard deviations.  They have not combined Run 1 and Run 2 data yet.

Kado says: the diphoton excess events look like the background, with no sign of extra energetic jets, invisible particles, etc; nothing that indicates a signal with widely different properties sitting over the standard two-photon background.  (Obviously — if it had been otherwise they could have used this to reduce background and claim something more significant.)  There are about 40 events in the peak region (but how wide is he taking it to be?) Olsen: CMS has 10 events in the same region, too little to say much.

Conclusion?  The Standard Model isn’t dead yet… but we need to watch this closely… or think of another question.




14 responses to “CMS and ATLAS present their results

  1. Pingback: LHC Run 2 First Results | Not Even Wrong

  2. Pingback: CMS y ATLAS observan un pequeño exceso en difotones a 750 GeV | Bosón de Higgs | La Ciencia de la Mula Francis

  3. Thanks for keeping us all up to date Matt!

  4. I am surprised to see all these results presented at this time, as I thought Run 2 had not collected enough data to say something meaningful. All appear to be in the straight line of the standard model.

    I think we should wait to have more data, next summer or so, to have enough statistics to see if something new can appear at 13 TeV;

    At least, at this stage, no sign of supersymmetric particle…

  5. Dear Matt, good to see you writing about particles. If you really think that the significance of the bumps is unimpressive, why don’t you accept my 10-to-1 bet that this particle near 750 GeV will be discovered?

  6. Curious George

    The Standard Model isn’t dead yet. It is flexible enough; it incorporated a massive neutrino after starting with a massless one.

    • I don’t know, it doesn’t explain where neutrino mass comes from does it? The standard model tolerates it but it’s quite obvious something’s being missed.

  7. But would he pay out if he lost?

  8. @Albert Z: you mean Lubos?

  9. I really don’t like this idea of physicists placing bets. I know Stephen Hawking has done it a few times. It just seems like a trashy gimmick. I can see no scientific motivation for Lubos’s stance. Let’s just wait and see what comes of this.

  10. Of course.

    I hope some well-heeled science aficionado bets him $100K that the “hint” goes the way of most “hints”.

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