The meaning of the title of Clara Moskowitz’s new article for the public, “Dark Matter Mystery May Soon Be Solved“, all lies in the word “may”. It may. It may not.
According to the article, “the answer to this cosmic mystery could come within the next three or four years, scientists say.”
I have to admit that this kind of phraseology, which one often sees in the press in reports about science, drives me a bit nuts. Which scientists? How many of them? You can’t tell from this line whether this is something that a group of three or four mavericks are claiming, or whether it is conventional wisdom shared by most of the community. And “the answer… could come…”? Interpreted literally it is content-free: yes, the answer could come in the next few years, or not — but you don’t need any scientists to tell you that. If one interprets it more optimistically — that it is intended to imply that the answer will very likelycome within the next three or four years — then I think it is far from clear what fraction of the experts will agree with that statement.
Rather than debate the claim, let’s start with the physics. What will determine how long it takes to discover what dark matter is made from? (more…)
After a hiatus for a hurricane and a trip to a conference in Asia, I am adding one more article to my series on How the Higgs Field Works, following my series of articles on Fields and Particles. (These sets of articles require a little math and physics background, the sort you’d get in your first few months of a beginning university or pre-university physics class. I’m still thinking about how to structure a similar set of articles that require no math or physics; that’s much harder, of course!)
This article contains the most elaborate equations and concepts that I’ve had to introduce to my readers, so it won’t be suitable for everyone (though it still only requires some first-year physics/math.) But on the other hand, it seems necessary for me to write it, since it’s the only place that I’ve explained not only why the Higgs field can give mass to the known particles, but why it (or something very much like it) must do so.
(Note that in these articles I’m mainly concentrating on the simplest type of Higgs, the Standard Model Higgs field and particle. However, most of the basic concepts in these articles apply even for more complicated cases.)
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 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 Bs→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.)
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, MA, 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.
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.)
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 MA 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 MA 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.
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.
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.
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…)
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.
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.
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…)
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:
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…)
Today the LHCb experiment, for the first time, announced evidence for the existence of this process, using their data collected in both 2011 and 2012. In the Standard Model (the equations that describe the known particles and forces), it is predicted that about one in about 300,000,000 B_s mesons (hadrons containing a bottom quark and a strange anti-quark, or vice versa) should decay in this fashion. The measurement that LHCb has made is completely consistent with this prediction.
[In detail, the Standard Model predicts (3.54 ± 0.30) × 10-9 {including mixing effects} and LHCb measures (3.2 +1.5[−1.2])× 10-9.] The measurement is at the level of 3.5 standard deviations — evidence, but not yet a convincing observation. LHCb excludes a rate of zero at much better than 95% confidence; their 95% confidence lower bound on the process is 1.1 × 10-9.]
The detailed implications of this result will take a while to work through, but the general implication is easy to state: the Standard Model has survived another test. And the constraints from LHC data on speculative ideas that predict particles and forces beyond those of the Standard Model have become tighter. Many variants of these speculative ideas would have affected this process, and the more precisely the data matches the Standard Model prediction, the more of these variants are excluded by the data.
Note Added: CMS says they a very good chance of being able to confirm LHCb’s result using the full 2012 data; not sure what ATLAS says about their chances.
2nd Note Added: Gino Isidori, in his talk on the theoretical perspective on heavy flavor physics, emphasized that a future high-precision measurement of this process will be very important; it can be predicted with high precision, and interesting variants of speculative ideas often have effects on its rate that are between 10% and 100%.
