MATT STRASSLER
Of Particular Significance

Blog – Of Particular Significance

Remember That “Blow” to Supersymmetry (And Other Theories)?

According to the BBC, it was a heavy blow.  According to a member of the LHCb experiment quoted in the article, it put the theory “in the hospital.”  The reality?  Nobody even suffered a scratch.

On Monday I wrote about a new measurement by the LHCb experiment at the Large Hadron Collider [LHC] of a rare process, reported at the HCP 2012 conference in Kyoto (a link to the talks can be found at this link), in which a B_s meson decays to a muon and an anti-muon (click here for more details of the physics process.)  It’s a very important measurement, definitely!  But whilelistening to theorist Gino Isidori’s talk in which he briefly discussed this measurement, I was a little puzzled about an inconsistency between what LHCb had done and said in the past, and what they had done and were saying now, in particular as was reported/implied by the BBC.

When I chatted with him later, Isidori reminded me exactly what LHCb reported in March, and how it compares to what they report now.

  • March: LHCb reported that at most 4.5 per billion B_s mesons decay this way (at 95% confidence)
  • November: LHCb reports about 3.5 per billion B_s mesons decay this way, and at 95% confidence the rate is at least 1.1 per billion and at most 6.4 per billion.

Notice that the new measurement raises the upper limit on how often this process occurs.  This upward shift is not an indication of a problem; it’s probably just an ordinary statistical effect that arises from having small amounts of data.  But it makes the constraints from this measurement — on the many variants of supersymmetry, and on other theories — a little weaker, if anything.  Most of the supersymmetric (and other) models that would be constrained by this measurement lead to a higher rate than in the Standard Model (where it is predicted to be about 3.2 ± 0.3 per billion).  So a higher upper limit means fewer of these variants are excluded by the data.  All in all,  the constraints on supersymmetric (and other) models are little changed, and perhaps somewhat weaker than they were in March.  Isidori and his colleagues have worked this out, and I’ll try to get details from them next week.

No need to take my word for it, or even Isidori’s.  Professor Michelangelo Mangano of CERN, apparently having spoken independently to other experts, made exactly the same point on page 41 of his summary talk concluding the conference.  (By the way, it was a great talk, and I recommend that experts read it.) 

Well!  So much for the big BBC headline!  [Most likely the public will never learn about this; a news report describing more accurately what this really means for supersymmetry etc. probably will not appear at all on the BBC, and even if it does, it certainly won’t get a big attention-grabbing headline.   It’s sad that this inherent bias in media reporting ensures the public gets an unhealthy dose of incorrect scientific information and rarely gets the antidote.]

None of this at all diminishes LHCb’s accomplishment!  They’ve made a great measurement, for which they deserve big congratulations.  A round of applause, please!!  But let’s not overstate its immediate impact.  As the measurement becomes more precise, its impact will gradually become greater, and even more so when it is combined with similar measurements from ATLAS and CMS.

But meanwhile, there were powerful and truly new constraints on supersymmetry (and other theories) reported at this conference, and they came from ATLAS and CMS, in their searches for effects from superpartner (and other) particles.  I told you about a small number of these searches a couple of days ago (by the way, I learned meanwhile that CMS has a search that is similar to the one I mentioned from ATLAS  involving bottom quarks) and maybe I’ll point out a few others next week, if I have the energy.

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON November 16, 2012

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Why Theories Don’t Go Into Hospitals

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. (more…)

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON November 14, 2012

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Summary of New Higgs Results

As I warned you to expect, there wasn’t anything particularly dramatic in the new results, reported at the HCP conference in Kyoto, on the Higgs-like particle.  There just wasn’t enough new data relative to what was available in July for that to happen — a bit less than double for most measurements.  That said, there were some interesting developments.  Here’s are the summary plots from ATLAS (left) and CMS (right); things are a bit more consistent with the Standard Model Higgs hypothesis (especially for decays to tau’s, b’s and W’s) than they were before, but the precision of the measurements is still pretty low.

The measurements looking for signals of a Higgs particle at ATLAS (left) and CMS (right). For each process, the dot is the measurement of the rate relative to the Standard Model expectation, and the horizontal bar gives the one-standard-deviation uncertainty. Note all measurements are within two standard deviations of expectations. Some measurements (photons for CMS and ATLAS, ZZ* –> 4 leptons for ATLAS) were not updated for this conference. For details, see the previous post.

Also notable: that the Tevatron experiments’ measurement of Higgs decaying to bottom quarks is now smaller than before and more in line with Standard Model expectations; that the ratio of W to Z coupling strengths is consistent with a simple generic Higgs [i.e custodial symmetry works with uncertainties of 30%]; and at 2.5 standard deviations this is not a parity-odd spin-zero particle, while it remains consistent with being the parity-even spin-zero particle that we’d expect a simple Higgs to be.

For many more details, see my previous post from Kyoto.

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON November 14, 2012

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Higgs Results at Kyoto

At the HCP conference in Kyoto, we’re expecting to see some updated (and perhaps new) measurements concerning the recently discovered Higgs-like particle.  Presentations from the ATLAS and CMS experiments at the Large Hadron Collider [LHC] are coming shortly.

First things first — based on the questions I’m getting from reporters and other non-experts, I need to emphasize right away that it is very unlikely that anything dramatic is going to happen here.  ATLAS and CMS only just co-discovered this particle in July; the amount of data that is available now is only about twice as much as was available in July, and that is nowhere near enough for the big questions about the Higgs-like particle to be resolved.  We will not know after today whether it is a Higgs at all, whether it is a Standard Model Higgs or not, or whether any particular speculative idea beyond the Standard Model is now excluded.  What we are likely to see is some incremental changes in what we know.

Knowledge about nature does not come easy.  We discovered the top quark in 1995, and we are still learning about its properties today — there were over a dozen talks about the top quark earlier in this conference.  And we will still be learning important things about the Higgs during the coming few decades.  We’ve no choice but to be patient.

But here are the new results (below, SM is short-hand for “Standard Model”); see here for a discussion of the main July results: (more…)

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON November 14, 2012

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Theory Killers at the HCP conference

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! (more…)

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON November 13, 2012

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“Supersymmetry Dealt a Blow”?

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. (more…)

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON November 13, 2012

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