supersymmetry – Page 3 – Of Particular Significance

Details Behind Last Week’s Supersymmetry Story

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 B_s 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 effect on the constraints on any speculative theories, including supersymmetry, contrary to the statements in the press and by a certain LHCb member. Today I’m providing you with some sources for this statement.

A number of my colleagues have tasked themselves with keeping track of how measurements at the Large Hadron Collider and elsewhere are affecting certain subclasses of variants of the supersymmetry. They call themselves the “Mastercode Project”; here’s their website. They’re not the only ones looking at this, but among them is Professor Gino Isidori, whom I was talking to last week, so I’ve gotten this information from him. I quote from the MasterCode website regarding last week’s result from LHCb: “The new measurement provides a valuable new constraint on the supersymmetric parameter space, but the observation of a Standard Model-like branching fraction for the B_s→μ⁺μ^– decay is quite consistent with supersymmetry. In fact, a Standard Model-like branching fraction of this decay was expected in constrained supersymmetric models like the CMSSM or NUHM1 (see, e.g., the recent MasterCode results for further details). As a result, the favoured regions in the parameter space of these models do not change significantly after the inclusion of the new constraint. ”

Now before I explain what this means, it’s important to have some terminology, running from most general to most specific.

Supersymmetry: the general theory that space and time are more subtle than they appear to be, and as a result, for every known particle in nature there is a corresponding superpartner particle with the property that either the particle is a boson and its superpartner a fermion, or vice versa. (Read more about supersymmetry here, and more about fermions and bosons here and here.)
TeV-Scale Supersymmetry: the masses of each particle and its superpartner do not differ by more than 1 TeV/c² (recall that the proton-proton collisions at the LHC have energy of 8 TeV in 2012, and the mass of the new Higgs-like particle is about 125 GeV/c² = 0.125 TeV/c².) In this case, signs of supersymmetry should be discovered by the Large Hadron Collider [LHC]; you can read here about how LHC experimentalists usually try to do this (and about the limitations of their simplest methods, which they are beginning to go beyond).
MSSM — Minimal supersymmetric Standard Model: a subclass of supersymmetric theories in which the only particles in nature are the known particles and their superpartners, along with a total of 5 Higgs particles and their superpatners.
CMSSM (Constrained MSSM): a much smaller subclass of variants of the MSSM in which the masses of the superpartners are assumed to be related to each other in particular ways (the details are technical and not essential, so I’ll skip them.)
NUHM1 (Non-Uniform Higgs Mass variant of the MSSM): another small subclass of the MSSM variants, slightly more general than the CMSSM.

Keep in mind that

ruling out the CMSSM or NUHM1 does not mean that the MSSM is ruled out;
ruling out the MSSM does not mean that supersymmetry at the TeV scale is ruled out;
ruling out supersymmetry at the TeV scale does not mean that supersymmetry is ruled out.

Among the many goals of the LHC is to find or rule out supersymmetry at the TeV scale. (It cannot hope to rule out supersymmetry altogether; that would presumably require a vastly more powerful collider that won’t likely be built for centuries, if ever.) It’s not enough to rule out the CMSSM, or the NUMH1, or even the MSSM. Similar statements apply for other speculative ideas that propose as yet unknown particles and forces; it’s not enough for the LHC to rule out just the simplest variants of these ideas.

Now if it turns out that supersymmetry is part of nature, rather few of my colleagues expect the variant we find to be contained within the CMSSM or NUHM1; and personally (though I’m probably in the minority) I have long doubted that it would be contained within the MSSM. Nevertheless, it is instructive to look at how LHC data is impacting the CMSSM and the NUHM1 subclasses of supersymmetry variants. One just must be careful not to over-interpret; the exclusion of most variants in the CMSSM is not an indication that most variants of TeV-scale supersymmetry as a whole are excluded.

Now in this context, let’s see how the new measurement that was announced last week affects the CMSSM and the NUHM1. In Figure 1 is a plot showing the allowed variants of the CMSSM and the NUMH1, as a function of two quantities: on the horizontal axis, M_A, which if large is (approximately but essentially) the mass of four of the five Higgs particles in the MSSM, and on the vertical axis, tan β, the ratio of the values of the two non-zero Higgs fields that are required in the MSSM. In solid red and solid blue are the one-standard-deviation and two-standard-deviation allowed regions after the new LHCb measurement is accounted for; any variant of the theory not sitting inside the blue region is excluded by the data. The dashed bands show the same thing before the new LHCb measurement. Since the dashed and solid blue bands are right on top of each other, you see there’s almost no effect at all. That’s what was behind my claim last Friday.

HCP_LHCbEffect — Fig. 1: Constraints on the CMSSM (left) and NUHM1 (right) subclasses of supersymmetric theories, before and after the HCP conference of last week. The quantities on the horizontal and vertical axes are explained in the text. In both plots: solid red (blue) give the constraints at one (two) standard deviations; variants outside the blue curve are excluded. Dashed red (blue) are the same limits before the new LHCb measurement. Notice there is almost no change.

But please, don’t misinterpret what I’m saying (or my colleagues) as suggesting that the LHC’s data has had no impact on the list of possible variants of supersymmetry! Far from it! Many variants are excluded, and many popular (but not necessarily more likely) subclasses of variants of supersymmetry have been pushed into regions that many would consider corners. The only statement in Figure 1 is that the new LHCb measurement didn’t make these corners smaller. But to see how things have changed since before the LHC began, look at Figure 2, which shows how the LHC as a whole — all the measurements from LHCb, ATLAS and CMS taken together — have affected the CMSSM and NUMH1 since 2009. (The CMSSM and NUHM1 also make assumptions about where dark matter comes from, so even effects of the dark matter measurements from the XENON100 experiment are included here.)

LHC_1fb_Effect — Fig. 2: as in Figure 1, except that the dashed lines give the constraints on the CMSSM and NUHM1 before the LHC began taking data, and the solid line gives the constraints after the data taken through early summer 2011 was analyzed. Notice the scale on the horizontal axis is different from that of Figure 1.

Figure 2 is a similar plot to Figure 1 — but this time, solid blue and red indicate the impact of LHC data as of summer 2011, and the dashed blue and red indicate the situation before the LHC started. Now compare the dashed blue line in Figure 2 (before the LHC) with the solid blue line in Figure 1 (now); note the scale on the horizontal axis is different!. You’ll see that in the CMSSM it was possible before the LHC to have M_A as low as 350 GeV/c², but now it has to be over 900 GeV/c², which many would consider a rather high value. In the NUHM1 there’s been a similar shift from 150 to about 300 GeV/c², not yet so high but still a significant increase. And meanwhile, while almost any value of tan β from 2 up to 60 was allowed before the LHC, this number is now limited to a smaller range. For example, if M_A were below 900 GeV/c², then the CMSSM would be excluded and the NUHM1 would be allowed only for tan β < 30 or so. This upper limit on tan β is mainly caused by the similar LHCb measurement presented back in March (and mentioned by me on Friday), and by similar ones from the CMS, CDF and ATLAS experiments.

But clearly there are plenty of variants within the NUHM1 that remain viable. And the NUHM1 is not representative of the full range of possibilities within the MSSM, so even if the NUHM1 were excluded, we’d still have a long way to go to exclude the MSSM, much less all of TeV-scale supersymmetry. In short, it’s neither all nor nothing. Yes, a lot of progress has been made; LHC data (and data from other sources) have ruled out a lot of variants of TeV-scale supersymmetry. But no, we’re not yet close to ruling out the full range of variants.

Please note that I’m not telling you this because I’m some devotee of supersymmetry who believes deeply in his heart that we’ll someday find it, and is trying to persuade you not to give up. I’m just laying out for you the facts on the ground. Do you imagine that I’m happy that a long, painful slog lies ahead, during which particle physicists — theorists and experimentalists — will painstakingly cover all the possible variants of supersymmetry, and slowly but surely determine whether or not supersymmetry is absent at the TeV scale? Don’t you think my life and that of my colleagues would be a lot easier if we could snap our fingers and with one or two quick measurements settle the question of whether supersymmetry is a fact of nature or not? Unfortunately, things don’t work that way. You should simply ignore the irresponsible grand statements you will see in the press and on various blogs; indeed, sweeping remarks are a sign of careless thinking, and you should beware. The truth is that only through very hard work — by the experts who make the measurements, by those who advise them on which measurements to make, and by those who do the calculations that are the ingredients for studies like the MasterCode Project — can we hope to settle profound questions about nature.

POSTED BY Matt Strassler

ON November 21, 2012

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…)

POSTED BY Matt Strassler

ON November 14, 2012

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…)

POSTED BY Matt Strassler

ON November 13, 2012

“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:

it can be well-predicted in the Standard Model (the equations we use to describe the known particles and forces, including the simplest type of Higgs particle)
it is relatively straightforward to measure, and
it is very sensitive to effects of as-yet unknown particles and forces.

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…)

POSTED BY Matt Strassler

ON November 13, 2012

A Real Workshop

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. (more…)

POSTED BY Matt Strassler

ON August 7, 2012

A Very Brief Comment About Yesterday’s Post

I got a question after yesterday’s post that motivates me to make a comment.

My post yesterday said that “the LHC is gradually moving from broad searches to precision tests.”

The question I received was “Does this mean that the LHC experiments are giving up on looking for supersymmetry [for now]?”

The answer is: “Of course not.” There’s a complete logical disconnect between those two statements. The first would imply the second ONLY if it were true that the way to find supersymmetry (or anything else new) was in broad searches rather than in precision measurements. But that premise is false.

New particles and forces (such as, but not limited to, those predicted by supersymmetry) are easy to find in broad searches if they generate collisions that look very distinctive and are much more common than similar collisions predicted by known phenomena.

New particles and forces (such as, but not limited to, those predicted by supersymmetry) are impossible to find in broad searches if they generate collisions that are either not so distinctive or are not very common compared to similar events predicted by known phenomena. For these you need to measure and predict known phenomena much more precisely.

Some variants of supersymmetry (including many of the more popular ones) generate large distinctive signals. Some don’t. Broad searches only rule out the first class (and I should mention that not all the broad searches have even been done yet.)

The same goes for many other theories with as-yet unknown particles and forces. There’s nothing special about supersymmetry in this regard.

So no, the new phase of the LHC research program is not about giving up on looking for this or that. It’s about working even harder than before, in order to find what might be hiding a bit below the surface. In fact, that was the major topic of this weekend’s workshop (including my own talk).

POSTED BY Matt Strassler

ON July 17, 2012

SEARCH Workshop Panel Discussion on LHC Posted Online

The final panel discussion at the Maryland SEARCH workshop — six theoretical particle physicists talking about the 2011 experimental results from the Large Hadron Collider [LHC] and looking ahead to the 2012 data — has finally been posted online, along with the rest of the presentations at the workshop. I wrote about the workshop, which took place in mid-March, here and here. In the latter post, I wrote:

The workshop concluded with a panel discussion — the only point during the entire workshop when theorists were formally asked to say something. The panel consisted of Michael Peskin (senior statesman [and my Ph.D. advisor] famous for many reasons, including fundamental work on the implications of highly precise measurements ), Nima Arkani-Hamed (junior statesman, and famous for helping develop several revolutionary new ways of approaching the hierarchy problem), Riccardo Rattazzi (also famous for conceptual advances in dealing with the hierarchy problem), Gavin Salam (famous for his work advancing the applications of the theory of quarks and gluons, including revolutionary methods for dealing with jets), and myself (famous for talking too much… though come to think of it, that was true of the whole panel, except Gavin.) And Raman Sundrum, one of the organizers (and famous for his collaboration with Lisa Randall in introducing “warped” extra dimensions, and also anomaly-mediated supersymmetry breaking [which was competitive with a paper by Rattazzi and his colleagues]) informally participated too. (more…)

POSTED BY Matt Strassler

ON May 4, 2012

Is Supersymmetry Ruled Out Yet?

[A Heads Up: I’m giving a public lecture about the LHC on Saturday, April 28th, 1 p.m. New York time/10 a.m. Pacific, through the MICA Popular Talks series, held online at the Large Auditorium on StellaNova, Second Life; should you miss it, both audio and slides will be posted for you to look at later.]

Is supersymmetry, as a symmetry that might explain some of the puzzling aspects of particle physics at the energy scales accessible to the Large Hadron Collider [LHC], ruled out yet? If the only thing you’re interested in is the answer to precisely that question, let me not waste your time: the answer is “not yet”. But a more interesting answer is that many simple variants of supersymmetry are either ruled out or near death.

Still, the problem with supersymmetry — and indeed with any really good idea, such as extra dimensions, or a composite Higgs particle — is that such a basic idea typically can be realized in many different ways. Pizza is a great idea too, but there are a million ways to make one, so you can’t conclude that nobody makes pizza in town just because you can’t smell tomatoes. Similarly, to rule out supersymmetry as an idea, you can’t be satisfied by ruling out the most popular forms of supersymmetry that theorists have invented; you have to rule out all its possible variants. This will take a while, probably a decade.

That said, many of the simplest and popular variants of supersymmetry no longer work very well or at all. This is because of two things: (click here to read the rest of the article.)

POSTED BY Matt Strassler

ON April 24, 2012

News from La Thuile, with Much More to Come

At various conferences in the late fall, the Large Hadron Collider [LHC] experiments ATLAS and CMS showed us many measurements that they made using data they took in spring and summer of 2011. But during the fall their data sets increased in size by a factor of two and a half! So far this year the only results we’d seen that involved the 2011 full data set had been ones needed in the search for the Higgs particle. Last week, that started to change.

The spring flood is just beginning. Many new experimental results from the LHC were announced at La Thuile this past week, some only using part of the 2011 data but a few using all of it, and more and more will be coming every day for the next couple of weeks. And there are also new results coming from the (now-closed) Tevatron experiments CDF and DZero, which are completing many analyses that use their full data set. In particular, we’re expecting them to report on their best crack at the Higgs particle later this week. They can only hope to create controversy; they certainly won’t be able to settle the issue as to whether there is or isn’t a Higgs particle with a mass of about 125 GeV/c², as hints from ATLAS and CMS seem to indicate. But all indications are that it will be an interesting week on the Higgs front.

The Top Quark Checks In

CMS5_topWpol — Fig. 1: In the Higgs mechanism, the W particle gets its mass from the non-zero average value of the Higgs field. A precise test of this idea arises as follows. When the top quark decays to a bottom quark and a W particle, and the W then decays to an anti-neutrino and an electron or muon, the probability that the electron or muon travels in a particular direction can be predicted assuming the Higgs mechanism. The data above shows excellent agreement between theory and experiment, validating the notion of the Higgs field.

There are now many new measurements of the properties of the top quark, poking and prodding it from all sides (figuratively) to see if it behaves as expected within the “Standard Model of particle physics” [the equations that we use to describe all of the known particles and forces of nature.] And so far, disappointingly for those of us hoping for clues as to why the top quark is so much heavier than the other quarks, there’s no sign of anything amiss with those equations. Top quarks and anti-quarks are produced in pairs more or less as expected, with the expected rate, and moving in the expected directions with the expected amount of energy. Top quark decay to a W particle and a bottom quark also agrees, in detail, with theoretical expectation. Specifically (see Figure 1) the orientation of the W’s intrinsic angular momentum (called its “spin”, technically), a key test of the Standard Model in general and of the Higgs mechanism in particular, agrees very well with theoretical predictions. Meanwhile there’s no sign that there are unexpected ways of producing top quarks, nor any sign of particles that are heavy cousins of the top quark.

One particularly striking result from CMS relates to the unexpectedly large asymmetry in the production of top quarks observed at the Tevatron experiments, which I’ve previously written about in detail. The number of top quarks produced moving roughly in the same direction as the proton beam is expected theoretically to be only very slightly larger than the number moving roughly in the same direction as the anti-proton beam, but instead both CDF and DZero observe a much larger effect. This significant apparent discrepancy between their measurement and the prediction of the Standard Model has generated lots of interest and hope that perhaps we are seeing a crack in the Standard Model’s equations.

Well, it isn’t so easy for CMS and ATLAS to make the same measurement, because the LHC has two proton beams, so it is symmetric front-to-back, unlike the Tevatron with its proton beam and anti-proton beam. But still, there are other related asymmetries that LHC experiments can measure. And CMS has now looked with its full 2011 data set, and observes… nothing: for a particular charge asymmetry that they can measure, they find an asymmetry of 0.4% +- 1.0% +- 1.2% (the first number is the best estimate and the latter two numbers are the statistical and systematic uncertainties on that estimate). The Standard Model predicts something of order a percent or so, while many attempts to explain the Tevatron result might have predicted an effect of several percent. (ATLAS has presented a similar measurement but only using part of the 2011 data set, so it has much larger uncertainties at present.)

Now CMS is not measuring quite the same thing as CDF and DZero, so the CMS result is not in direct conflict with the Tevatron measurements. But if new phenomena were present that were causing the CDF and DZero’s anomalously large asymmetry, we’d expect that by now they’d be starting to show up, at least a little bit, in this CMS measurement. The fact that CMS sees not a hint of anything unexpected considerably weakens the overall case that the Tevatron excess asymmetry might have an exciting explanation. It suggests rather that the whole effect is really a problem with the interpretation of the Tevatron measurements themselves, or with the ways that the equations of the Standard Model are used to predict them. That is of course disappointing, but it is still far too early to declare the case closed.

There’s also a subtle connection here with the recent bolstering by CDF of the LHCb experiment’s claim that CP violation is present in the decays of particles called “D mesons”. (D mesons are hadrons containing a charm quark [or anti-quark], an up or down anti-quark [or quark], and [as for all hadrons] lots of additional gluons and quark/anti-quark pairs.) The problem is that theorists, who used to be quite sure that any such CP violation in D mesons would indicate the presence of new phenomena not predicted by the Standard Model, are no longer so sure. So one needs corroborating information from somewhere, showing some other related phenomenon, before getting too excited.

One place that such information might have come from is the top quark. If there is something surprising in charm quarks (but not in bottom quarks) one might easily imagine that perhaps there is something new affecting all up-type quarks (the up quark, charm quark and top quark) more than the down-type quarks (down, strange and bottom.) [Read here about the known elementary particles and how they are organized.] In other words, if the charm quark is different from expectations and the bottom quark is not, it would seem quite reasonable that the top quark would be even more different from expectations. But unfortunately, the results from this week suggest the top quark, to the level of precision that can currently be mustered, is behaving very much as the Standard Model predicted it would.

Meanwhile Nothing Else Checks In

Meanwhile, in the direct search for new particles not predicted by the Standard Model, there were a number of new results from CMS and ATLAS at La Thuile. The talks on these subjects went flying by; there was far too little information presented to allow understanding of any details, and so without fully studying the corresponding papers I can’t say anything more intelligent yet than that they didn’t see anything amiss. But of course, as I’ve suggested many times, searches of this type wouldn’t be shown so soon after the data was collected if they indicated any discrepancy with theoretical prediction, unless the discrepancy was spectacularly convincing. More likely, they would be delayed a few weeks or even months, while they were double- and triple-checked, and perhaps even held back for more data to be collected to clarify the situation. So we are left with the question as to which of the other measurements that weren’t shown are appearing later because, well, some things take longer than others, and which ones (if any) are being actively held back because they are more … interesting. At this preliminary stage in the conference season it’s too early to start that guessing game.

ATLAS5_Zprime — Fig. 2: The search for a heavy particle that, like a Z particle, can decay to an electron/positron pair or a muon/anti-muon pair now excludes such particles to well over 1.5 TeV/c-squared. The Z particle itself is the bump at 90 GeV; any new particle would appear as a bump elsewhere in the plot. But above the Z mass, the data (black dots) show a smooth curve with no significant bumps.

So here’s a few words about what ATLAS and CMS didn’t see. Several classic searches for supersymmetry and other theories that resemble it (in that they show signs of invisible particles, jets from high-energy quarks and gluons, and something rare like a lepton or two or a photon), were updated by CMS for the full or near-full data set. Searches for heavy versions of the top and bottom quark were shown by ATLAS and CMS. ATLAS sought heavy versions of the Z particle (see Figure 2) that decay to a high energy electron/positron pair or muon/anti-muon pair; with their full 2011 data set, they now exclude particles of this type up to masses (depending on the precise details of the particle) of 1.75-1.95 TeV/c². Meanwhile CMS looked for heavy versions of the W particle that can decay to an electron or muon and something invisible; the exclusions reach out above 2.5 TeV/c². Other CMS searches using the full data set included ones seeking new particles decaying to two Z particles, or to a W and a Z. ATLAS looked for a variety of exotic particles, and CMS looked for events that are very energetic and produce many known particles at once. Most of these searches were actually ones we’d seen before, just updated with more data, but a few of them were entirely new.

Two CMS searches worth noting involved looking for new undetectable particles recoiling against a single jet or a single photon. These put very interesting constraints on dark matter that are complementary to the searches that have been going on elsewhere, deep underground. Using vats of liquid xenon or bubble chambers or solid-state devices, physicists have been looking for the very rare process in which a dark matter particle, one among the vast ocean of dark matter particles in which our galaxy is immersed, bumps into an atomic nucleus inside a detector and makes a tiny little signal for physicists to detect. Remarkable and successful as their search techniques are, there are two obvious contexts in which they work very poorly. If dark matter particles are very lightweight, much lighter than a few GeV/c², the effect of one hitting a nucleus becomes very hard to detect. Or if the nature of the interaction of dark matter with ordinary matter is such that it depends on the spin (the intrinsic angular momentum) of a nucleus rather than on how many protons and neutrons the nucleus contains, then the probability of a collision becomes much, much lower. But in either case, as long as dark matter is affected by the weak nuclear force, the LHC can produce dark matter particles, and though ATLAS and CMS can’t detect them, they can detect particles that might sometimes recoil against them, such as a photon or a jet. So CMS was quite proud to show that their results are complementary to those other classes of experiments.

CMS5_WIMPs — Fig. 3: Limits on dark matter candidates that feel the weak nuclear force and can interact with ordinary matter. The horizontal axis gives the dark matter particle's mass, the vertical mass its probability to hit a proton or neutron. The region above each curve is excluded. All curves shown other than those marked "CMS" are from underground experiments searching for dark matter particles hitting an atomic nucleus. CMS searches for a jet or a photon recoiling against something undetectable provide (left) the best limits on "spin-independent" interactions for masses below 3.5 GeV/c-squared, and (right) the best limits on "spin-dependent" interactions for all masses up to a TeV/c-squared.

Finally, I made a moderately big deal back in October about a small excess in multi-leptons (collisions that produce three or more electrons, muons, positrons [anti-electrons] or antimuons, which are a good place to look for new phenomena), though I warned you in bold red letters that most small excesses go away with more data. A snippet of an update was shown at La Thuile, and from what I said earlier about results that appear early in the conference season, you know that’s bad news. Suffice it to say that although discrepancies with theoretical predictions remain, the ones seen in October apparently haven’t become more striking. The caveat that most small excesses go away applies, so far, to this data set as well. We’ll keep watching.

CMS5_multilep — Fig. 4: The updated multilepton search at CMS shows (black solid curve) a two standard deviation excess compared to expectations (black dotted curve) in at least some regimes in the plane of the gluino mass (vertical axis) versus the chargino mass (horizontal axis) in a particular class of models. But had last fall's excess been a sign of new physics, the current excess would presumably have been larger.

Stay tuned for much more in the coming weeks!

POSTED BY Matt Strassler

ON March 5, 2012

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