supersymmetry

Higgs Symposium: A More Careful Summary

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My rather hasty, breathless and inconsistent summaries (#1, #2 and #3) of last week’s talks at the excellent Higgs Symposium (held at the University of Edinburgh, as part of the new Higgs Center for Theoretical Physics) clearly had their limitations.  So I thought it might be useful to give a more organized overview, with more … Read more

Conclusion of the Higgs Symposium

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By almost all measures, the Higgs Symposium at the University of Edinburgh, as part of the new Higgs Centre for Theoretical Physics, was a great success.  The only negative was that Professor Peter Higgs himself had a bad cold this week, and had to cancel his talk, as well as missing the majority of the talks by others.  Obviously all of us in attendance were very disappointed not to hear directly from him, and we wish him a speedy recovery.

Other than this big hole in the schedule, the talks given at the symposium seemed to me to form a coherent summary of where we are right now in our understanding of the Higgs field and particle.  They were full of interesting material, and wonderfully complementary to one another.  This motivates me to try to provide, for non-experts, some future articles on what the conference attendees had to say.  But to write such articles well takes time.  So for now, here’s the quick version summarizing the last few talks, along the lines of the summaries I wrote (here and here) of the earlier talks.  The slides from all the talks are posted here.

Here we go:

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It’s (not) The End of the World

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The December solstice has come and gone at 11:11 a.m. London time (6:11 a.m New York time). That’s the moment when the north pole of the Earth points most away from the sun, and the south pole points most toward it. Because it’s followed by a weekend and then Christmas Eve, it marks the end … Read more

Details Behind Last Week’s Supersymmetry Story

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Last week, I promised you I’d fill in the details of my statement that the recent measurement (of the rare process in which a Bs meson decays to a muon and an anti-muon — read here for the physics behind this process) by the LHCb experiment at the Large Hadron Collider [LHC] had virtually no … Read more

Why Theories Don’t Go Into Hospitals

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I’m always amused at how very reasonable remarks so often generate attacks from unreasonable people.  I wrote a perfectly ordinary post about what one does and doesn’t learn from LHCb’s important new measurement at the Large Hadron Collider [LHC] (and in fact I overstated the significance of the result — more on that later), and somehow I touched off a mini-firestorm.  Well, that just indicates how essential it is to have calm people expressing sensible points of view.  When people become so politicized that they can’t distinguish propaganda from science, that’s not good.

Forget supersymmetry — because none of my remarks have anything to do with this theory in particular, and the theory doesn’t deserve the excessive attention it’s getting.  Take any theory: call it Theory X.  Extra dimensions; compositeness of quarks and leptons; non-commutative spacetime; grand unification; your-theory-here.  The idea behind theory X may be very clever, but as always, there are many variants of theory X, because an idea is almost never precise enough to permit a unique realization.  Each variant makes definite predictions, but keep in mind that detailed experimental predictions may very well differ greatly from variant to variant.

Now, here is a logical fact:  one of two options is true.

  • Option A: One variant of theory X is “correct” (its predictions agree with nature) while all other variants are “wrong” (disagree with nature)
  • Option B: All variants of theory X are wrong.

Nature is what it is; there are no other options (and this is not the place for a discussion about this basic scientific assumption, so pace, please, philosophers.). [More precisely about option A: the space of variants is continuous, so the correct statement is that an arbitrary small region in this space is correct; you can put in the correct calculus vocabulary as you like.  I’ll stick with the imprecise language for brevity.]

For either option, as more and more data is collected, more and more variants of theory X will become “dead” — excluded because of a disagreement with data.  Therefore — obviously! — a reduction in the number of live (i.e. unexcluded) models always takes place over time.  And this has absolutely no bearing on whether, at the end, all variants of X will be dead, or one (or perhaps several very similar ones) are still alive.

And thus it makes absolutely no sense to describe, as a “blow to theory X” — in particular, to the idea behind theory X — a measurement that excludes (“kills”) even a big fraction, but not virtually all, of the variants of theory X.  It’s certainly a blow to those variants; in fact, it is a fatal blow for them.  But it does nothing to distinguish between Option A and Option B.  It only tells us that if Option A is true, the variant of X that will be alive at the end is not among the ones that have just been killed.

This isn’t rocket science, folks.  It’s logic.  [Well – As a commenter points out, it’s  not “logic” in the strictest sense; but it is basic scientific reasoning.] And if we take theory X to be the Standard Model itself, I’ve just described its history.

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Theory Killers at the HCP conference

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There were many interesting results presented yesterday at the HCP conference in Kyoto, and they were both too numerous and too detailed for me to completely absorb as yet — a follow-up will clearly be needed.  But a few are obviously so important that I want to point them out now.

First, both ATLAS and CMS, the two general purpose experiments at the Large Hadron Collider [LHC], produced important new results on “multileptons”.  Based on a significant fraction of their 2012 data, they looked for signs of new phenomena that would appear as proton-proton collisions that produce at least three leptons or anti-leptons, or even (in unusual combinations and/or along with other unusual things) two leptons or anti-leptons.  (I’ll just summarize this class of studies as “multileptons” for the purpose of this brief post and be more specific at a later date.) ATLAS used about 50% more data than CMS, but CMS had a more intricate analysis of their data, so I believe the results were similar where they can be compared.  [By the way, the CMS result was approved to be shown at this conference under extreme conditions; at least two of the major players in the analysis had no power or internet for over a week following Hurricane Sandy!]

The bottom line is simple: neither CMS nor ATLAS sees any significant deviation from what is predicted by the Standard Model.  And this now kills off another bunch of variants of many different speculative ideas.  The details are extremely complicated to describe, but essentially, what’s dead is any theory variant that leads to many proton-proton collisions containing

  • two or more top quark/anti-quark pairs
  • multiple W and Z particles
  • two or more as-yet unknown moderately heavy particles that often decay to muons, electrons and/or their anti-particles
  • new moderately heavy particles that decay to many tau leptons

and probably a few others I’m forgetting. While multilepton searches (especially those for 3 or more leptons) are often touted as a great way to look for supersymmetry in particular, that description vastly understates their power — they are a great way to look for many different types of phenomena not predicted in the Standard Model.  (This is something that a number of scientists at Rutgers University have been emphasizing in talks and papers.)  And both experiments have demonstrated this with various interpretations of their results; CMS has over a dozen of them!

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“Supersymmetry Dealt a Blow”?

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One of the challenges of being a science journalist is conveying not only the content of a new scientific result but also the feel of what it means.  The prominent article in the BBC about the new measurement by the LHCb experiment at the Large Hadron Collider [LHC]  (reported yesterday at the HCP conference in Kyoto — I briefly described this result yesterday) could have been worse.  But it has a couple of real problems characterizing the implications of the new measurement, so I’d like to comment on it.

The measurement is of how often B_s mesons (hadrons containing a bottom quark and a strange anti-quark, or vice versa, along with many quark/anti-quark pairs and gluons) decay to a muon and an anti-muon.  This process (which I described last year — only about one in 300,000,000 B_s mesons decays this way) has three nice features:

Yesterday the LHCb experiment reported the evidence for this process, at a rate that is consistent (but see below) with the prediction of the Standard Model.

The worst thing about the BBC article is the headline, “Supersymmetry theory dealt a blow” (though that’s presumably the editor’s fault, as much as or more than the author’s) and the ensuing prose, “The finding deals a significant blow to the theory of physics known as supersymmetry.”  What’s wrong with it?  It’s certainly true that the measurement means that many variants of supersymmetry (of which there are a vast number) are now inconsistent with what we know about nature.  But what does it mean to say a theory has suffered a blow? and why supersymmetry?

First of all, whatever this new measurement means, there’s rather little scientific reason to single out supersymmetry.  The rough consistency of the measurement with the prediction of the Standard Model is a “blow” (see below) against a wide variety of speculative ideas that introduce new particles and forces.  It would be better simply to say that it is a blow for the Standard Model — the model to beat — and not against any speculative idea in particular.  Supersymmetry is by no means the only idea that is now more constrained than before.  The only reason to single it out is sociological — there are an especially large number of zealots who love supersymmetry and an equal number of zealots who hate it.

Now about the word “blow”.  New measurements usually don’t deal blows to ideas, or to a general theory like supersymmetry.  That’s just not what they do.  They might deal blows to individual physicists who might have a very particular idea of exactly which variant of the general idea might be present in nature; certain individuals are surely more disappointed than they were before yesterday.   But typically, great ideas are relatively flexible.  (There are exceptions — the discovery of a Higgs particle was a huge blow to the idea behind “technicolor” — but in my career I’ve seen very few.)  It is better to think of each new measurement as part of a process of cornering a great idea, not striking and injuring it — the way a person looking for treasure might gradually rule out possibilities for where it might be located.

Then there’s the LHCb scientist who is quoted as saying that “Supersymmetry may not be dead but these latest results have certainly put it into hospital”; well…  Aside from the fact that this isn’t accurate scientifically (as John Ellis points out at the end of the article), it’s just not a meaningful or helpful way to think about what’s going on at the LHC.

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A Real Workshop

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In the field of particle physics, the word “workshop” has a rather broad usage; some workshops are just conferences with a little bit of time for discussion or some other additional feature.  But some workshops are about WORK…. typically morning-til-night work.  This includes the one I just attended at the Perimeter Institute (PI) in Waterloo, Canada, which brought particle experimentalists from the CMS experiment (one of the two general-purpose experiments at the Large Hadron Collider [LHC] — the other being ATLAS) together with some particle theorists like myself.  In fact, it was one of the most productive workshops I’ve ever participated in.

The workshop was organized by the PI’s young theoretical particle physics professors, Philip Schuster and Natalia Toro, along with CMS’s current spokesman Joseph Incandela and physics coordinator Greg Landsberg. (Incandela, professor at the University of California at Santa Barbara, is now famous for giving CMS’s talk July 4th announcing the observation of a Higgs-like particle; ATLAS’s talk was given by Fabiola Gianotti. Landsberg is a senior professor at Brown University.) Other participants included many of the current “conveners” from CMS — typically very experienced and skilled people who’ve been selected to help supervise segments of the research program — and a couple of dozen LHC theorists, mostly under the age of 40, who are experienced in communicating with LHC experimenters about their measurements. 

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