Of Particular Significance

Searches for New Phenomena: Results from Mumbai Conference Today

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 08/22/2011

Real Time Updates at end of post:

(all times Mumbai)

  • 12:15: update 1  (top quarks seem to behave mostly as expected)
  • 13:00: update 2 (more ways in which top quarks behave as expected)
  • 13:15 update 3 (general thoughts on the lack of new top-quark surprises)
  • 14:15 one huge summary of everything new at LHC begins — oof!
  • 14:45-15:0o a summary of the huge summary — for what it is worth
  • 15:30 Tevatron: only a few updates from Grenoble, nothing too striking.
  • 20:00 GRRR: still waiting for the organizers to post today’s slides so I can review them and understand the details!!!

No blockbusters today!  But way too much, way too fast.  Will have to look at the slides carefully once they finally post them, and then probably will still have to dig into the details of the new results to learn anything really interesting.


Ok, getting ready for an interesting day.  On the morning agenda: measurements of things we think we know — W and Z particles, photons, and top quarks — looking for surprises.   In the case of the top quark, we have a very good reason to be looking closely — the “Forward-Backward Asymmetry” for top-quark production, which should be small in the Standard Model, is measured at the Tevatron, by both CDF and D0,  to be moderately large.   This suggests a possibility that new physics is affecting top quarks.  So although the LHC experiments can’t make exactly the same measurement as the Tevatron to check for the same discrepancy, it is very natural for them to look for something else odd about the top quark in their data.   So far nothing strange has shown up, but … perhaps today?

After that, in the afternoon, we’ll have results on searches for various speculative ideas, including supersymmetry, extra dimensions, and presumably a few other things.  Won’t know the details until the talks are given… but I expect we’ll see quite a few new results.  With so many results, the chances are that at least one of them will get people talking… although there is a tendency for exciting results to not be shown for quite a while, for reasons detailed in this article.   If there’s anything really exciting in the data, we might not see if for a few more months, until the data set doubles again.


12:15 — Albert De Roeck from CMS reviews lots of top quark measurements from Tevatron and LHC — but focuses on precision measurements of production rates.  Flies through everything involving possible new physics.  Advances on many fronts, but plots zipped by, no time to really look at them and understand whether there’s anything funny in them.  Only a few new results for this conference.   Most interesting technically, at first glance, is search for heavy particle with 1 – 1.5 TeV mass decaying to top quark and top anti-quark, using new “boosted-particle” techniques.  Searches for various types of fourth-generation quarks (usually called top-prime or bottom-prime) putting lower limits on their masses in the 400-450 GeV range.  Nothing new on the top quark forward-backward asymmetry, the one discrepancy from the Standard Model.  Nothing obviously to get excited about since the Grenoble conference… but need to look at the actual slides (not yet posted.)

13:00 — Yuji Takeuchi reviews other types of measurements from Tevatron and LHC involving the top quark (mass, charge, details of decays, spin effects) — nothing violating Standard Model expectations.  ATLAS has a nice new measurement of W helicity in top quark decay, and another on the correlation of the spins of the top quark and top anti-quark when they are produced together.

13:15 — I should say that today’s results on top quarks, while not surprising after the Grenoble conference last month showed almost nothing new with the top quark, are in general somewhat disappointing.  The Tevatron measurement of the top-quark forward-backward asymmetry, which appears even now to be in conflict with the Standard Model, has been our best hope so far for something indicating a new phenomenon involving the top quark.  And we have many reasons to expect the top quark might not behave as predicted — it is by far the heaviest quark, and the reason for its very large mass is not known.  Equivalently, it is the particle which feels the Higgs field more strongly than any other particle known.   So there are many speculative theories of new physics in which the top quark plays a special role, which tends to be reflected in some novel effect on the top quark’s properties.  Yet except for the forward-backward asymmetry, nothing else odd about the top quark has shown up yet to provide more evidence that something is amiss with the Standard Model.  Granted, these are still early days at the LHC, and the various measurements from ATLAS and CMS will improve and proliferate with more data and more time… so there will be plenty more to watch for over the next 18 months…

14:15 Here we go — one talk on everything the LHC has done on new physics.  Henri Bachacou presents.

14:45 Ok, this was only a moderate update of what we saw in more detail in Grenoble.  There were a few new results, but not that many we hadn’t seen before.  And it all went by far too fast, with far too few details, for any thoughtful analysis.  But with only 30 minutes, what could the speaker do?  He should have had double the time.  (Why weren’t the LHC speakers more numerous and given more time at this conference??!!)

The talk itself was very nice, the results presented were spectacular, and the speaker was very careful to make precise statements, which I always appreciate.  Basically: Standard Supersymmetry with a large jets-plus-missing-energy signal is excluded for squarks and gluinos up to the 1 TeV range — putting such models in serious trouble.  The same is true for the version of supersymmetry known as “minimal gauge mediation” with that produce 2 photons in every event (and some new and interesting limits on the case where often there is only one photon per event.)  Limits on heavy Z-like and W-like particles are in the 1.8 and 2.3 TeV range, as are particles decaying to two photons.  Technicolor particles decaying to WZ are excluded up to 380 GeV.  No sign of any excess in events with two charged leptons or two charged anti-leptons.  Nothing in the classic searches for invisible sectors and/or extra dimensions; no deviations at high energy in the distribution with energy of charged lepton-antilepton or two-photon events.   No huge signals of lots of quarks or leptons produced at once, as expected from black holes.  And certain powerful searches for long-lived particles and slowly-moving particles came up empty.  So — the experiments have done a fantastic job!  Unfortunately, nothing obvious has popped out.

BUT — as the speaker emphasized, there are many classes of searches that have not been done.  Many variants of supersymmetry would not have shown up in current searches (my perspective on that is here) and will have to be looked for in future analyses with different strategies.  Also, he emphasized, with the large data sets expected by fall, entirely new methods of analysis will become possible.  So for the next round of conferences, expect results involving ultra-high-energy leptons (with 1 TeV or so of energy), “boosted” objects (a  technique for finding high-energy W and Z particles and top quarks, and perhaps Higgs  and other new particles), and searches for a number of less obvious signatures of new physics.

I’m going to have to look at all of this again once they post the slides, and frankly the slides were almost useless anyway, given how much material had to be squeezed in such a short talk.  Really, if one wants to see anything more subtle than the bottom line, what one has to do is read the corresponding papers and notes — and that will be my goal for the next couple of weeks.

15:30 In contrast to the LHC speaker, the Tevatron speaker Alberto Annovi got to spend most of his time on a relatively small number of results. A significant fraction of the time was spent on the widely reported (one might say widely over-reported) W-plus-two-jets excess that CDF sees and D0 does not.  I remain unconvinced.  Nothing else too dramatic that we didn’t see in Grenoble.


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3 Responses

  1. Dear Leonard Lerner,

    thanks very much for this nice and clear explanations, now I see 🙂

    (I`ve heard from somewhere else that Prof. Strassler is busy moving to another house and writing 4 papers at present; so it is natural that he has not that much time to come here …)


  2. I will reply to the above question, given I am intersted on participating in an informed discussion of particle physics. There are 3 doublets of quarks, of which all hadronic (heavy) matter is made, thats six quarks overall. The first two, called u and d are the only real (as opposed to virtual) contributors to the matter we see around us. Their masses are of the order of 1-3 MeV. Next its the strange quark, about 100 MeV, the charmed quark, about 1GeV, and the bottom quark 4 GeV. The first two doublets are composed of quarks of roughly the same mass. The bottom quark’s partner, the top quark, is 175 GeV however, about the mass of a gold nucleus. Physicists at the moment like to think that these particles condense out of some kind of symmetric theory, which will explain all their masses. If so, where does this incredible range of numbers 1:175000 come from? Most numbers in a symmetric theory come out roughly equal. For example, after the discovery of the bottom quark, the last quark was widely expected to have a mass of order 15GeV. The coupling to the Higgs field is large for the top quark simply because the probability of a Higgs interaction with the quarks, electrons, etc is postulated proportional to the mass squared. So any funny effects in the Higgs field are expected to be reflected there. Mind you this coupling is simply postulated in the Higgs theory to be equal to the mass, for there is no other known way to make sense of the masses of particles in the electroweak theory than assume they come out of an interaction. Its no prediction, more an exasperation, and could very well be wrong

  3. Dear Prof. Strassler,

    I like these real-time reports a lot, thank You very much for doing this :-).
    Could You explain in a bit more detail why the large mass (and corresponding coupling to the higgs field(s)) is considered unexpected and what the candidate models which possibly could explain that are?
    If You are too busy with other things, just ignore the question …


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