[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.)
A few days back I told you things were going very well so far in 2012 at the Large Hadron Collider [LHC] — that not only had the increase in energy (from last year’s 7 TeV of energy per proton-proton collision up to 8 TeV) gone well but the collision rate (the number of collisions per second) had already been brought back up to last year’s level. And I pointed out that the current collision rate had been achieved using fewer bunches of protons than last year, which meant it could eventually be increased further, by putting more bunches in. But I didn’t know when they’d take that step — in particular, whether it would be anytime soon.
The proton beams at the LHC aren’t continuous; as of now, they consist of over a thousand bunches, each containing something like 100,000,000,000 protons. Two bunches are arranged to hit head on every 50 billionths of a second, and in each bunch crossing occur 10 to 40 virtually simultaneous proton-proton collisions.
Well, they did it the next day! Since Wednesday the number of bunches per beam has been about 1380, same as late last year, and the collision rate jumped up by over 25%, just like that! In fact they brought it even a bit higher (not sure exactly how) to within 15 – 20% of this year’s final target. And they’ve had some long runs, as long as nearly 10 hours, showing the accelerator remains very stable.
Three cheers for the accelerator physicists! Now the experimentalists just have to assure they can extract quality data from a firehose.
Every time I turn around, the Large Hadron Collider [LHC] is breaking another record. Since 2012 data-taking began April 5th, the news has been improving steadily and rapidly. Over the weekend, the accelerator physicists brought proton-proton collision rates — the total number of collisions per second — back up to the same level as last year’s record, and even a bit beyond. The LHC produced over these two days more than six times as much data as was obtained in all of 2010, and a quarter of what was obtained in the first three months of 2011. Granted, the accelerator physicists won’t keep up this torrid pace every day; there are times (typically weekends and overnights) where the LHC produces lots of data, and other times where the focus is on improving the accelerator and much less data is gathered. But so far the signs are very good indeed. Some experts were unsure whether the move from 7 TeV to 8 TeV energy per collision would be smooth, but fortunately it seems that the accelerator is performing very well at the new energy.
Two differences from last year that are important to know:
The simultaneous collision rate (or “pile-up”) is higher: the current high collision rate per second is being obtained with a smaller number of bunches of protons per beam than at the end of last year, by about 25%, which means the number of simultaneous collisions has gone up by about 25%, to as many as 30 collisions or so happening at virtually the same instant. (Pile-up will be a recurrent theme (and perhaps thorn) in 2012.) But the use of fewer bunches at the present time means that more bunches can be added later, which will allow another increase in the collision rate per second sometime this year (I’m not sure when.)
The challenges ahead for the ATLAS and CMS experiments to handle this firehose of data, especially the pile-up, are not to be minimized. I’ve written about one of the big challenges here; there are others. But better to have to face these difficulties than to be sitting idly, twiddling their thumbs and waiting for data to start showing up! Also, the more data they get in the early part of 2012, the sooner Phase 1 of the search for the Higgs particle will come to a close — and the more likely that July 2012 will see significant news, perhaps fairly convincing news, concerning the hint of a Higgs particle with a mass of about 125 GeV/c2.
Still early days in the 2012 data-taking run, which just started a couple of weeks ago, but already the Large Hadron Collider [LHC] accelerator wizards, operating the machine at 8 TeV of energy per proton-proton collision (compared to last year’s 7 TeV) have brought the collision rates back up nearly to where they were last year. This is very good news, in that it indicates there are no significant unexpected technical problems preventing the accelerator from operating at the high collision rates that are required this year. And the experiments are already starting to collect useful data at 8 TeV.
The challenges for the experiments of operating at 8 TeV and at the 2012 high collision rate are significant. One challenge is modeling. To understand how their experiments are working, well enough that they can tell the difference between a new physical phenomenon and a badly understood part of their detector, the experimenters have to run an enormous amount of computer simulation, modeling the beams, the collisions, and the detector itself. Well, 8 TeV isn’t 7 TeV; all of last year’s modeling was fine for last year’s data, but not for this year’s. So a lot of computers are running at full tilt right now, helping to ensure that all of the needed simulations for 8 TeV are finished before they’re needed for the first round of 2012 data analysis that will be taking place in the late spring and early summer.
Another challenge is “pile-up.” The LHC proton beams are not continuous; they consist of up to about 1300 bunches of protons, each bunch containing something like 100,000,000,000 protons. Collisions in each detector occur whenever two bunches pass through each other, every 50 nanoseconds (billionths of a second). With the beam settings that were seen late in 2011 and that will continue to intensify in 2012, every time two bunches cross at the center of the big experiments ATLAS and CMS, an average of 10 to 20 proton-proton collisions occur essentially simultaneously. That means that every proton-proton collision in which something interesting happens is doused in the debris from a dozen uninteresting ones. Moreover, some of the debris from all these collisions hangs around for a while, creating electronic noise that obscures measurements of future collisions. One of the questions for 2012 is how much of a nagging problem the increasing pile-up will pose for some of the more delicate measurements — especially study of Higgs particle decays, both expected ones and exotic ones, and searches for relatively light-weight new particles with low production rates, such as particles created only via the weak nuclear force (e.g.supersymmetric partners of the W, Z and Higgs particles.)
But I have a lot of confidence in my colleagues; barring a really nasty surprise, they’ll manage pretty well, as they did last year. And so far, so good!
On Monday, during the concluding session of the SEARCH Workshop on Large Hadron Collider [LHC] physics (see also here for a second post), and at the start of the panel discussion involving a group of six theorists, Michael Peskin, professor of theoretical particle physics at the Stanford Linear Accelerator Center[and my Ph.D. advisor] opened the panel with a few powerpoint slides. He entitled them: “My Advice in Four Slogans” — the advice in question being aimed at experimentalists at ATLAS and CMS (the two general-purpose experiments at the LHC) as to how they ought best to search for new phenomena at the LHC in 2012, and beyond. Since I agree strongly with his points (as I believe most LHC theory experts do), I thought I’d tell you those four slogans and explain what they mean, at least to me. [I’m told the panel discussion will be posted online soon.]
1. No Boson Left Behind
There is a tendency in the LHC experimental community to assume that the new particles that we are looking for are heavy — heavier than any we’ve ever produced before. However, it is equally possible that there are unknown particles that are rather lightweight, but have evaded detection because they interact very weakly with the particles that we already know about, and in particular very weakly with the quarks and antiquarks and gluons that make up the proton.
Peskin’s advice is thus a warning: don’t just rush ahead to look for the heavy particles; remember the lightweight but hard-to-find particles you may have missed.
The word “boson” here is a minor point, I think. All particles are either fermions or bosons; I’d personally say that Peskin’s slogan applies to certain fermions too.
2. Exclude Triangles Not Points
The meaning of this slogan is a less obscure than the slogan itself. Its general message is this: if one is looking for signs of a new hypothetical particle which
is produced mostly or always in particle-antiparticle pairs, and
can decay in multiple ways,
one has to remember to search for collisions where the particle decays one way and the antiparticle decays a different way; the probability for this to occur can be high. Most LHC searches have so far been aimed at those cases where both particle and anti-particle decay in the same way. This approach can in some cases be quite inefficient. In fact, to search efficiently, one must combine all the different search strategies.
Now what does this have to do with triangles and points? If you’d like to know, jump to the very end of this post, where I explain the example that motivated this wording of the slogan. For those not interested in those technical details, let’s go to the next slogan.
3. Higgs Implies Higgs in BSM
[The Standard Model is the set of equations used to predict the behavior of all the known particles and forces, along with the simplest possible type of Higgs particle (the Standard Model Higgs.) Any other phenomenon is by definition Beyond the Standard Model: BSM.]
[And yes, one may think of the LHC as a machine for converting theorists’ B(SM) speculations into (BS)M speculations.]
One of the main goals of the LHC is to find evidence of one or more types of Higgs particles that may be found in nature. There are two main phases to this search, Phase 1 being the search for the “Standard Model Higgs”, and Phase 2 depending on the result of Phase 1. You can read more about this here.
Peskin’s point is that the Higgs particle may itself be a beacon, signalling new phenomena not predicted by the Standard Model. It is common in many BSM theories that there are new ways of producing the Higgs particle, typically in decays of as-yet-unknown heavy particles. Some of the resulting phenomena may be quite easy to discover, if one simply remembers to look!
Think what a coup it would be to discover not only the Higgs particle but also an unexpected way of making it! Two Nobel prize-winning discoveries for the price of one!!
Another equally important way to read this slogan (and I’m not sure why Peskin didn’t mention it — maybe it was too obvious, and indeed every panel member said something about this during the following discussion) is that everything about the Higgs particle needs to be studied in very great detail. Most BSM theories predict that the Higgs particle will behave differently from what is predicted in the Standard Model, possibly in subtle ways, possibly in dramatic ways. Either its production mechanisms or its decay rates, or both, may easily be altered. So we should not assume that a Higgs particle that looks at first like a Standard Model Higgs actually is a Standard Model Higgs. (I’ve written about this here, here and here.) Even a particle that looks very much like a Standard Model Higgs may offer, through precise measurements, the first opportunity to dethrone the Standard Model.
4. BSM Hides Beneath Top
At the Tevatron, the LHC’s predecessor, top quark/anti-quark pairs were first discovered, but were rather rare. But the LHC has so much energy per collision that it has no trouble producing these particles. ATLAS and CMS have each witnessed about 800,000 top quark/anti-quark pairs so far.
Of course, this is great news, because the huge amount of LHC data on top quarks from 2011 allowed measurements of the top quark’s properties that are far more precise than we had previously. (I wrote about this here.) But there’s a drawback. Certain types of new phenomena that might be present in nature may be very hard to recognize, because the rare collisions that contain them look too similar to the common collisions that contain a top quark/anti-quark pair.
Peskin’s message is that the LHCexperimenters need to do very precise measurements of all the data from collisions that appear to contain the debris from top quarks, just in case it’s a little bit different from what the Standard Model predicts.
A classic example of this problem involves the search for a supersymmetric partner of a top quark, the “top squark”. Unlike the t’ quark that I described a couple of slogans back, which would be produced with a fairly high rate and would be relatively easy to notice, top squarks would be produced with a rate that is several times smaller. [Technically, this has to do with the fact that the t’ would have spin-1/2 and the top squark would have spin 0.] Unfortunately, if the mass of the top squark is not very different from the mass of the top quark, then collisions that produce top squarks may look very similar indeed to ones that produce top quarks, and it may be a big struggle to separate them in the data. The only way to do it is to work hard — to make very precise measurements and perhaps better calculations that can allow one to tell the subtle differences between a pile of data that contains both top quark/anti-quark pairs and top squark/anti-squark pairs, and a pile of data that contains no squarks at all.
Following up on slogan #2: An example with a triangle.
Ok, now let’s see why the second slogan has something to do with triangles.
One type of particle that has been widely hypothesized over the years is a heavy version of the top quark, often given the unimaginative name of “top-prime.” For short, top is written t, so top-prime is written t’. The t’ may decay in various possible ways. I won’t list all of them, but three important ones that show up in many speculative theories are
t’ → W particle + bottom quark (t’ → Wb)
t’ → Z particle + top quark (t’ → Zt)
t’ → Higgs particle + top quark (t’ → ht)
But we don’t know how often t’ quarks decay to Wb, or to Zt, or to ht; that’s something we’ll have to measure. [Let’s call the probability that a t’ decays to Wb “P1”, and similarly define P2 and P3 for Zt and ht].
Of course we have to look for the darn thing first; maybe there is no t’. Unfortunately, how we should look for it depends on P1,P2, and P3, which we don’t know. For instance, if P1 is much larger than P2 and P3, then we should look for collisions that show signs of producing a t’ quark and a t‘ antiquark decaying as t’ → W+ b and t‘ → W–b. Or if P2 is much larger than P1 and P3, we should look for t’ → Zt and t‘ → Z t.
Peskin's triangle for a t' quark; at each vertex the probabilty for the decay labeling the vertex is 100%, while at dead center all three decays are equally probable. One must search in a way that is sensitive to all the possibilities.
Peskin has drawn this problem of three unknown probabilities, whose sum is 1, as a triangle. The three vertices of the triangle, labeled by Wb, Zt and ht, represent three extreme cases: P1=1 and P2=P3=0; P2=1 and P1=P3=0; and P3=1, P1=P2=0. Each point inside this triangle represents different possible non-zero values for P1, P2 and P3 (with P1+P2+P3 assumed to be 1.) The center of the triangle is P1=P2=P3=1/3.
Peskin’s point is that if the experiments only look for collisions where both quark and antiquark decay in the same way
t’ → W+ b and t‘ → W–b;
t’ → Zt and t‘ → Z t;
t’ → ht and t‘ → h t;
which is what they’ve done so far, then they’ll only be sensitive to the cases for which P1 is by far the largest, P2 is by far the largest, or P3 is by far the largest — the regions near the vertices of the triangle. But we know a number of very reasonable theories with P1=1/2 and P2=P3=1/4 — a point deep inside the triangle. So the experimenters are not yet looking efficiently for this case. Peskin is saying that to cover the whole triangle, one has add three more searches, for
t’ → W+ b and t‘ → Z t, or t’ → W–b and t’ → Zt;
t’ → W+ b and t‘ → h t, or t‘ → W– b and t’ → ht;
t’ → Zt and t‘ → h t, or t’ → ht or t‘ → Z t;
so as to cover that case (and more generally, the whole triangle) efficiently. Moreover, no one search is very effective; one has to combine them all six searches together.
His logic is quite general. If you have a particle that decays in four different ways, the same logic applies but for a tetrahedron, and you need ten searches; if two different ways, it’s a line segment, and you need three searches.
Before I do that, let me make a remark. There is a big difference between healthy skepticism and political denialism. I get the impression that some people who are writing or reading other blogs misinterpret my caution with regard to experimental results as being somehow a political and unreasonable bias against the Higgs particle being present, either at a mass of 125 GeV/c2 or at all. That’s ridiculous. All that is going on is that I simply am not convinced yet by the data. I’m a careful scientist… period. And you’ll see that I’m consistent; later in this post I will advise you not to over-react negatively to what ATLAS didn’t see.
From the LHC, CMS had already presented its main results from 2011 data on the Standard Model Higgs search, so there wasn’t much new from them. But there are three searches which CMS had presented in December that ATLAS had not (at least not with the full 2011 data). Today, ATLAS presented results on them all.
What did we learn?
The Tevatron experiments see a combined 2.2 standard deviation [2.2 “sigma”] excess in their search, consistent with a Standard Model Higgs particle with a mass anywhere in the range of 115 to 135 GeV/c2. This is not inconsistent with the Higgs hints that we saw in December from the LHC experiments. Here I am being perhaps overly careful in not saying, more positively, “it is consistent with the Higgs hints…” only because this measurement is intrinsically too crude to allow us to narrow in on 124-126 GeV, where ATLAS and CMS see their hints. In short, the Tevatron measurement could, in the end, turn out to indicate a Higgs at a different mass than the one indicated by the current ATLAS and CMS hints. Anyway, it’s a minor and mostly a semantic point.
The results from ATLAS were a bit of a shock. In all three processes on which ATLAS reported, CMS has presented results already, and in each case CMS saw a small excess (1 standard deviation [1″sigma”], which is small indeed.) But ATLAS reported today that it sees essentially no excess in any of the three, and even a deficit in one of them for low mass. This has a big effect.
First, it allows ATLAS to exclude a Standard Model Higgs all the way up to 122 GeV/c2 (except for a little window 1 GeV/c2 wide centered at 118) and down to 129 GeV/c2. The only large window left for the Standard Model Higgs particle is 122-129, more or less centered around the hint at 126 GeV/c2 that they saw in December.
But second, the significance of the December hint, when combined with the new data that shows no excesses in these three new processes, drops by about a full standard deviation. That’s a pretty big drop.
What does it all mean?
I think it basically means, roughly, status quo. We got some positive information and some negative information today, and none of it is that easy to interpret. So I think we are roughly where we were before, except that we probably no longer have to worry about any Standard Model Higgs below 122 GeV/c2. Before today we had a decent hint of a Standard Model-like Higgs particle with a mass around 125 GeV/c2; we still have it. Let me explain what I mean.
There are easy (relatively!) searches for the Higgs, and there are hard ones. The easy searches are the ones where the backgrounds are relatively simple and the signal is a narrow peak on a plot. There are two:
Higgs decaying to photons
Higgs decaying to two lepton/anti-lepton pairs (often called “four leptons” for short)
Results on these were presented by both ATLAS and CMS back in December. The hard searches are the ones where the backgrounds are rather complicated and the signal is quite broad, so that a mistake in estimating a background can either create a fake signal or hide a real one. There are three (mainly) for a lightweight Higgs:
Higgs decaying to a lepton, an anti-lepton, a neutrino and an anti-neutrino
Higgs decaying to a tau lepton/anti-lepton pair
Higgs decaying to a bottom quark/anti-quark pair
These are the three that ATLAS reported on today (where they saw no sign of a Higgs signal), and that CMS presented back in December (and saw a small excess in all three.) [ATLAS presented a result on the first one in December, but only using part of their data; it showed a small excess at the time, but not now.] The third process is the main one in which CDF and DZero reported an excess today, though the first one also plays a role in interpreting that excess.
In other words, everything we learned today had to do with the difficult searches — the ones that are hard to perform, hard to interpret, and hard to check. And everything we learned was 1 or 2 sigma information; not very compelling even statistically.
For this reason,
I would not conclude that the new Tevatron results make the 125 GeV Higgs case much stronger
I would not conclude that the new ATLAS results make the 125 GeV Higgs case much weaker
If, like me, you discount the difficult Higgs searches somewhat relative to the easy Higgs ones, then almost nothing has changed, as far as the current Higgs hints, after today’s up and down information. The excess in the two easy searches at ATLAS is still there, and there are excesses at CMS at least in the two-photon search. Even from the beginning, I gave you good reasons to think the ATLAS’s easy-search excesses were a bit larger than they should be, probably due to an upward statistical fluctuation in the background. Conversely I think now that one should not overstate how bad today’s ATLAS news is for the Higgs hints. It’s still quite reasonable to think there may be a Standard Model Higgs there at 125 GeV/c2. There’s some evidence in its favor, and it’s certainly not ruled out at this point. (Whereas now, almost all other masses are.)
So as usual I advise patience and calm and no hyperventilating; the 2012 data will settle the issue. Either there is a Standard Model Higgs with a mass within a few percent of 125 GeV/c2 , or we’ll soon be fanning out in Phase 2 of the Higgs search, looking for all the other types of Higgs particles that might be out there.
[UPDATE: Tevatron results start a few paragraphs down; LHC results will appear soon]
[2nd UPDATE: ATLAS new results added: the big unexpected news. As far as I can tell CMS, which got its results out much earlier in the year, didn’t add anything very new in its talk today.]
[3rd UPDATE: some figures from the talks added]
[4th UPDATE: more understanding of the ATLAS lack of excesses in new channels, and what it does to the overall excess at 125 GeV; reduction in local significance from about 3.5 sigma to about 2.5, and with look-elsewhere effect, now the probability the whole thing is an accident is 10%, not 1%. Thanks to a comment for pointing out how large the effect was.]
This morning there are were several talks about the Higgs at the Moriond Electroweak conference. There will be were talks coming from the Tevatron experiments CDF and DZero; we expected new results on the search for the Higgs particle from each experiment separately, and combined together. There were also talks from the Large Hadron Collider [LHC] experiments CMS and ATLAS. It wasn’t widely known how much new we’d see; they don’t have any more data than they had in December, since the LHC has been on winter shut-down since then, but ATLAS especially still hasn’t presented all of the results based on its 2011 data, so they may present new information. The expectation was that the impact of today’s new results would be incremental; whatever we learned today wouldn’t dramatically change the situation. The Tevatron results will certainly cause a minor ruckus, though, because there will surely be controversy about them, by their very nature. I gave you a sense for that yesterday. They aren’t likely to convince doubters. But they might provide more pieces of evidence in favor of a lightweight Higgs (though not necessarily at the value of around 125 GeV/c2 currently preferred by ATLAS and CMS; see below.)
There are two things I didn’t explain yesterday that are probably worth knowing about.
First, if you look at Figure 2 in my post from yesterday, you’ll notice that the shape of the Higgs signal at the Tevatron experiments is very broad. It doesn’t have a nice sharp peak at the mass of the Higgs (115 GeV in the figure). This is because (as I discussed yesterday) it is hard to measure jets very precisely. For this reason CDF and DZero will be able to address the question: “is there or is there not a lightweight Higgs-like particle”, but they will not easily be able to address the question “is its mass 115 GeV, 120 GeV, 125 GeV or 130 GeV?” very well. So we’re really talking about them addressing something only slightly beyond a Yes-No question — and one which requires them to understand their backgrounds really well. This is to be contrasted with the two-photon and four-lepton results from ATLAS and CMS, which with more data are the only measurements, in my view, that can really hope to establish a signal of a Higgs particle in a completely convincing way. These are the only measurements that will see something that could not be mimicked by a mis-estimated background.
Second, the key to the CDF and DZero measurements is being able to identify jets that come from a bottom quark or anti-quark — a technique which is called “b-tagging the jets” — because, as I described yesterday, they are looking for Higgs decays to a bottom quark and a bottom antiquark, so they want to keep events that have two b-tagged jets and throw away others. I have finished a new short article that explains the basic principles are behind b-tagging, so you can get an idea of what the experimenters are actually doing to enhance the Higgs signal and reduce their backgrounds. Now b-tagging is never perfect; you will miss some jets from bottom quarks, and accidentally pick up some that don’t come from bottom quarks. But one part of making the Tevatron measurement involves making their b-tagging techniques better and better. CDF, at least, has already claimed in public that they’ve done this.
Will update this after information becomes available and when time permits.
UPDATES: New Tevatron Results and New ATLAS Results
New Tevatron Results
Tevatron claims a lightweight Higgs; to be precise, the combination of the two experiments CDF and DZero is incompatible with the absence of a lightweight Higgs at 2.2 standard deviations (or “sigmas”), after the look elsewhere effect. CDF sees a larger effect than DZero; but the CDF data analysis method seems more aggressive. But both methods are far too complicated for me to evaluate.
The combination of DZero and CDF results from the Tevatron shows that their observed limit on the Higgs production rate as a function of its mass (solid line) lies about two sigma above the expected limit in the absence of any Higgs (dashed line) indicating an excess of events that appears consistent with a Higgs signal roughly in the 115-135 GeV mass range. By itself this result is not confidence-inspiring, but it does add weight to what we know from ATLAS and CMS at the LHC.
2.2 sigma is not much, and excesses of this size come and go all the time. We even saw that several times this past year. But you can certainly view today’s result from the Tevatron experiments as another step forward toward a convincing case, when you combine it with what ATLAS and CMS currently see. At minimum, assuming that the Higgs particle is of Standard Model type (the simplest possible type of Higgs particle), what CDF and DZero claim is certainly consistent with the moderate evidence that ATLAS and CMS are observing.
There’s more content in that statement than you might think. For example, if there were two Higgs particles, rather than one, the rate for the process CDF and DZero are measuring could easily be reduced somewhat relative to the Standard Model. In this case they wouldn’t have found even the hint they’ve got. (I explained why yesterday, toward the end of the post.) Meanwhile the process that ATLAS and CMS are measuring might not be reduced in such a scenario, and could even be larger — so it would certainly be possible, if there were a non-Standard-Model-like Higgs at 125 GeV, for ATLAS and CMS to see some evidence, and CDF and DZero to see none. That has not happened. If you take the CDF and DZero hint seriously, it points — vaguely — toward a lightweight Standard-Model-like Higgs. Or more accurately, it does not point away from a lightweight Standard-Model-like Higgs.
However, we do have to keep in mind that, as I noted, CDF and DZero can only say the Higgs mass seems as though it might be in the range 115 to 135 GeV; they cannot nail it down better than that, using their methods, for the reasons I explained earlier. So their result is consistent with a Standard Model Higgs particle at 125 GeV, which would agree with the hints at ATLAS and CMS, but it is also consistent with one at 120 GeV, which would not agree. Thus Tevatron bolsters the case for a lightweight Higgs, but would be consistent both with the current hints at LHC and with other parts of the range that the LHC experiments have not yet excluded. If the current ATLAS and CMS hints went away with more data, the Tevatron results might still be correct, and in that case ATLAS and CMS would start seeing hints at a different mass.
First, as with any measurement, these results cannot automatically be assumed to be correct; indeed most small excesses go away when more data is accumulated, either because they are statistical fluctuations or because of errors that get tracked down — but unfortunately we will not get any more data from the now-closed Tevatron to see if that will happen. The plausibility of Tevatron’s claims needs to be evaluated, and (in contrast to the two photon and four lepton results from ATLAS and CMS, which are relatively straightforward to understand) this won’t be easy or uncontroversial. The CDF and DZero people did a very fancy analysis with all sorts of clever tricks, which has the advantage that it makes the measurement much more powerful, but the disadvantage of making it obscure to those who didn’t perform it.
One other caveat is that we will have to be a little cautious literally combining results from Tevatron with those from the LHC. There’s no sense in which[this statement is factually incorrect as stated, as commenters from CDF are pointing out; there are indeed several senses in which it was done blind. I should have been more precise about what was meant, which was more of a general knowledge of how difficult it is to avoid bias in determining the backgrounds for this measurement. Let me add that this is not meant to suggest anything about CDF, or DZero, in particular; doing any measurement of this type is extraordinarily difficult, and those who did it deserve applause. But they’re still human.] the Tevatron result was done `blind’; it was done with full knowledge that LHC already has a hint at 125, and since the Tevatron is closed and all its data is final, this is Tevatron’s last chance (essentially) to contribute to the Higgs particle search. Combining experiments is fine if they are truly independent; if they are not, you are at risk of bolstering what you believe because you believe it, rather than because nature says it.
New ATLAS results
ATLAS has now almost caught up with CMS, in that its searches for Higgs particles decaying to two photons and to two lepton/anti-lepton pairs (or “four leptons” for short) have now been supplemented by (preliminary! i.e., not yet publication-ready) results in searches for Higgs particles decaying to
a lepton, anti-lepton, neutrino and anti-neutrino
a tau lepton/anti-lepton pair
a bottom quark/anti-quark pair (which is what CDF and DZero looked for too)
(The only analysis ATLAS is missing is the one that CMS added in January, separating out events with two photons along with two jets.) In contrast to the CMS experiment, which found small excesses (just 1 sigma) above expectation in each of these three channels, ATLAS does not. [And I’ve been reminded to point out that the first channel has changed; in December, with 40% of the data analyzed, there was a small excess.] So CDF and DZero’s results from today take us a step forward toward a convincing case, while ATLAS’s result takes us a small step backward. That’s par for the course in science when you’re squinting to see something that’s barely visible.
In the same search as performed by CDF and DZero, and in the same region where they see an excess, ATLAS sees no excess at all; but ATLAS has less data and is currently less sensitive to this channel than CDF and DZero, so there is no clear contradiction.
But one can’t get too excited about this. Statistics are still so low in these measurements that it would be easy for this to happen. And determining the backgrounds in these measurements is tough. If you make a mistake in a background estimation, you could make a small excess appear where there really isn’t one, or you could make a real excess disappear. It cuts both ways.
But actually there is a really important result coming out of ATLAS today; it is the deficit of events in the search for the Higgs decaying to a tau lepton/anti-lepton pairs. For a putative Higgs below 120 GeV, ATLAS sees even fewer tau lepton/anti-lepton events than it expected from pure background — in other words, the background appears to have fluctuated low. But this means there is not likely to be a Standard Model-like Higgs signal there, because the likelihood that the background plus a Higgs signal would have fluctuated very low is small. [UPDATE: actually, looking again, I think I am somewhat overstating the importance of this deficit in taus compared to the lack of excess in the other two channels, which is also important. To be quantitative about this would require more information. In any case, the conclusion is the same.]And this allows ATLAS to exclude new regions in the mass range for the Standard Model Higgs, at 95% confidence!
This is very important! One of the things that I have complained about with regard to those who’ve overplayed the December Higgs hints is that you can’t really say that the evidence for a Higgs around 125 GeV is good if you can’t start excluding both above and below that mass. Well, ATLAS has started to do that. Granted, it isn’t 99% exclusion, and since this is the Higgs we’re talking about, we need high standards. But at 95% confidence, ATLAS now excludes, for a Standard Model Higgs, 110-117.5, 118.5-122.5, and 129-539 GeV. Said better, if there is a Standard Model Higgs in nature, ATLAS alone restricts it (to 95% confidence only, however) to the range 117.5 – 118.5 GeV or 122 – 129 GeV.
ATLAS, just from its own data alone, excludes (pink-shaded regions) the Standard Model Higgs particle at 95% confidence (but not yet at 99%) across the entire allowed range except around 118 GeV and between 122 and 129 GeV, where the two-photon and four-lepton searches provide some positive evidence. What is shown is how large a Higgs signal can be excluded, in units of the Standard Model expectation, as a function of the Higgs mass. Anywhere the solid line dips below the dotted line marked "1" is a place where the Standard Model is 95% excluded. The red dotted line indicates how well this experiment would perform, on average, if there were no Standard Model Higgs signal.
The window is closing. Not only has ATLAS completely excluded the old hints of a Standard Model Higgs at 115 from the LEP collider, it seems it has probably excluded CMS’s hint around 120, which was the next best option for the Higgs after 125. And as far as I can tell, this is coming mainly from the tau lepton/anti-lepton measurementAs I said above in an update, I think it is really a mix of all three channels… hard to be quantitative about that without talking to the experts.
So if the Standard Model Higgs is what nature has to offer us, we’re probably down to a tiny little slice around 118 GeV for which there’s no evidence, and a window that has 125 GeV smack in the middle of it, where the evidence, though not much stronger today, if we include both Tevatron and ATLAS, than it was yesterday, is certainly no weaker.
UPDATE: Well, it’s been pointed out to me by the first commenter that the last statement is misleading, because it doesn’t emphasize how the ATLAS excess at 126 GeV has decreased substantially in significance. Somehow I thought originally that the decrease was marginal. But it isn’t.
The statistics numbers as I think I have them now: What was previously about 3.5 sigma local significance for the Higgs-like peak at 125 GeV is now down to 2.5, and what seemed only 1% likely in December to be a fluctuation is now 10% likely.
There is an issue, however, with combining many measurements. Of course the two-photon and four-lepton results from ATLAS are the same as before, and they are just as significant; nothing changed. But the other three measurements came in low, and that pulls the significance of the combination down. However, I must remind you again how difficult the last three measurements are. I would trust the first two before the last three. So I think we should be careful not to overinterpret this change. When you combine what you trust most with what you trust least, you reduce your confidence in what you have.
That said, it also indicates why one should be very cautious with small amounts of data.
Comparison of the December ATLAS results (left), combining all measurements that were available at the time, with the March 2012 ATLAS results (right). I've lined them up as best I could, given the scales were slightly different. What is shown is how large a Higgs signal can be excluded, in units of the Standard Model expectation, as a function of the Higgs mass. Anywhere the solid line dips below the dotted line marked "1" is a place where the Standard Model is 95% excluded. Compared to December, there is much more excluded and the height of the peak at 126 GeV is noticeably lower.
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
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 amuch 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.
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.
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.
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.
Among the bridges that I hope to build, as I develop this website, is one connecting what we know today about nature with how we know it. After all, you’re reading my depiction of nature, based on how I think nature works. I can try to assure you that my depiction is the mainstream viewpoint at the forefront of the research field — but you may still wonder if this website is legitimate, or if I might just be full of hot air, or if I might simply be mistaken. Well, my confidence in what I’m saying doesn’t come from having trained at some fancy university or my degree or from having been in the business for over 20 years. It comes from the data… in short, from nature itself.
So it’s important, I think, to link the data to the ideas and concepts, when it’s possible to do that.
You’ve heard the famous statement that “a proton is made from two up quarks and a down quark”. But in this basic article, and this somewhat more advanced one, and in Wednesday’s post where I went into some details about what we know about proton structure, I’ve claimed to you that protons are actually chock full of particles, most of which carry a tiny fraction of the proton’s energy, and most of which are gluons, with a lot of quarks and antiquarks. [If this sounds unfamiliar, you should read those articles and posts before reading this one, which is a follow-up.] And I claimed that these complications make a big difference at the Large Hadron Collider [LHC].
So should you take my word for this? You don’t have to. Let me show you evidence. From LHC data. Here’s an article defending the main claim’s of Wednesday’s post. It’s a near-final draft, still needing some proofreading perhaps, and probably some clarification, but I think it is fully readable now. Enjoy it (and please feel free to give me feedback on its clarity, so I can improve it), or wait for the final version next week, as you see fit. And have a great weekend!
Two plots, differing only in the range for the vertical axis, showing the relative likelihood of striking a gluon or an up or down quark or antiquark carrying a fraction x of the proton's energy. At low x gluons dominate (and quarks and antiquarks become equally likely, and numerous, though far less so than gluons), while quarks dominate (but are very rare) at moderate x. Plotted using a Mathematica package (Trout and Olness, 2000) based on CTEQ5L results; somewhat out of date, but accurate enough for today's purposes.
The two plots in the Figure show exactly the same thing, just with a different vertical scale, so that certain things that are hard to see on one plot are clearer on the other. And what they show is this: if a proton is flying toward you in a Large Hadron Collider [LHC] proton beam, and you strike something inside that proton, how likely are you to have hit an up quark, or down quark, or gluon, or up antiquark, or down antiquark, that carries a fraction x of the proton’s energy? From these plots we can learn: (more…)
