2012 may well turn out to be The Year of The Higgs. Right now we have very little knowledge about this particle, but that may change dramatically over the year. As I described in my previous post, we’re coming toward the end of Phase 1 of the Higgs search (where the ATLAS and CMS experiments at the Large Hadron Collider [LHC] search for the simplest possible form of the Higgs particle, the Standard Model Higgs, or SM Higgs for short.) And we’re also starting up Phase 2 of the Higgs search. As discussed in my Cosmic Variance guest post, and in more detail in my most recent post, if a particle resembling the SM Higgs is found, Phase 2 involves checking its details and determining as well as possible whether it is or isn’t precisely what is predicted by the Standard Model. If no such particle is found, Phase 2 involves searching widely for the many other types of Higgs particles that nature might or might not possess. Fortunately, despite these apparently divergent aims, the two possible branches of Phase 2 involve asking some of the same experimental questions (see Figure 3 of the most recent post), and so we can start on Phase 2 before even finishing Phase 1. And that is happening now.
One of the things that has to be done in Phase 2 is to search for decays of the Higgs particle that are not among the decays predicted to occur in the Standard Model. ["Decay" = "a disintegration of one particle into two or more". Click here for an introduction.] Such “exotic” decays are thought of as particularly plausible, because a lightweight Higgs (below about 150 GeV/c2 or so) is a very sensitive creature. It is very easy for new particles and/or forces to alter the Higgs’ properties, perhaps causing changes in how (or how often) it is produced, and to what (and with what probability) it may decay. As shown in a large number of papers, written by quite a variety of particle physics theorists, there are many, many types of possible exotic decays, and they can arise for many reasons. If you’re curious what kind of exotic decays might occur, I gave a few examples in my now somewhat out-of-date analysis of what the summer’s Higgs searches imply. The basic logic of how unusual Higgs decays might arise is still correct in the cases described, but there are many, many more possibilities too. I’ll have to write a long article about the options in the coming month or so.
Another thing I could recommend, especially to graduate students and to those laypersons who are willing to sit through a certain amount of technical mumbo-jumbo enclosed within a largely non-technical discussion, is the first 8 (or even 20.5) minutes of a lecture I gave in 2010 to graduate students who were not Large Hadron Collider [LHC] experts. (If it loads too slowly you can download it from this page; it is my June 18th lecture.)
You might ask,“but if there were new particles and forces that could affect Higgs production or its decays, wouldn’t we have seen signs of them already at other experiments or at least at the LHC itself?” The answer is “no!” Because a lightweight Higgs is so sensitive, the new particles and forces required to alter it may easily be so weakly interacting with ordinary matter, or so heavy, that they can evade detection at all previous experiments and so far at the LHC. The first place they will be discovered is in the properties of the Higgs particle. And so it is very important to measure how the Higgs particle behaves as carefully and precisely as possible!
There are three good reasons why one of the most important strategies to be used in Phase 2 will be to check whether the Higgs has exotic decays — perhaps common, or perhaps rare.
- If we don’t find an SM-like Higgs in Phase 1, it might well be that although there is a Higgs there with SM or near-SM production rates, its decays are not those expected in the Standard Model, but are somehow “exotic.”
- Conversely, if we do find an SM-like Higgs in Phase 1, we know now that it can only lie above 600 GeV/c2 (but this is disfavored) or in the 115-127 GeV/c2 range, in which case (being lightweight) it may very easily have rare or even common exotic decays.
- And if current hints of an SM-like Higgs at 125 GeV/c2 turn out to be the real thing, we still only know from current data that it roughly looks like an SM-Higgs. If it decays as expected only, say, 85% of the time, and decays exotically, say, 15% of the time, we wouldn’t know that yet. Our data sets are still far too small, and the hints far too uncertain, to be able to rule that out.
At the very least, then, we should be prepared to look for exotic Higgs decays that are perhaps common, or perhaps somewhat rare. These searches might very well be the first to reveal a breakdown of the Standard Model! So we should be preparing for them, and designing them to be as powerful as possible.
“But slow down,” you might say. “We’re not even sure yet that the Higgs particle is there at all; we’re still arguing over whether what’s been seen near 125 GeV/c2 is a mirage. In 2012 that mirage may take solid form, but even if it does, there will be just barely enough data for confidence in its reality. At such an early stage, how could we ever hope to detect something that the Higgs particle does only rarely?”
Ah. We need to remember that the Phase 1 searches for the SM Higgs particle themselves rely on things that a Higgs particle does only very rarely. The most important search strategies for the Higgs in 2011 and 2012 will be to look for its decays to two photons and its decays to “four leptons” (shorthand for two lepton/anti-lepton pairs.) If the Higgs has a mass of 125 GeV/c2, it decays to two photons only 0.2% of the time. It decays to four leptons only 0.01% of the time. Even its decay to a lepton, anti-lepton, neutrino and anti-neutrino, which is also somewhat useful, is only a few percent effect. All of the decays of the SM Higgs that we can hope to actually measure are rare. The common ones, such as the decay to a bottom quark/anti-quark pair or to a tau lepton/anti-lepton pair, are almost completely drowned in huge backgrounds. (More accurately, there are in fact clever ways to measure decays to bottom quarks or to taus, but only by using rare production processes, so these are rare too, for a different reason.)
It’s well worth going through these numbers a bit more carefully. If indeed there is an SM-like Higgs at 125 GeV/c2, produced at the rates expected in the Standard Model, then the number of Higgs bosons produced so far at ATLAS and CMS (separately!) is
the probability of making a Higgs in a 7 TeV proton-proton collision:
times the number of proton-proton collisions in 2011 at ATLAS or CMS:
equals the number of Higgs particles produced at ATLAS and at CMS
Let me say that again. If nature sports a SM-like Higgs particle with a mass of 125 GeV/c2, the number produced so far at ATLAS and CMS is already approaching one hundred thousand in each experiment.
Most of these Higgs particles were not noticed; they disappeared under huge backgrounds, as the planet Venus disappears in bright sunlight. A few thousand or so decayed (via two W particles) to a lepton, an anti-lepton, a neutrino and an antineutrino; those can be detected, but the backgrounds are large and the measurement is hard. The number of Higgs particles that decay to two photons is something like a few hundred in each experiment, though not all are detected. The number that decay to four leptons is a handful, and some are lost. It’s the last two very rare processes that give the most striking information in the search for the SM Higgs.
But maybe this new Higgs particle is not a Standard Model Higgs particle after all, but just looks similar at first glance. Maybe a few dozen, or a few hundred, or a few thousand of the 90,000 Higgs bosons produced in 2011 decayed in an unexpected way, or in one of several unexpected ways. And maybe one of those types of decays gives an experimental signature that we can discover rather easily, giving a signal that stands out above background. All the experimenters might need to do … is look. Maybe they don’t even need more data; just searching through the 2011 data might be enough. All the focus last year was on pushing Phase 1 of the Higgs search as far as possible. But Phase 2 can start with this year’s questions about last year’s data. It could even lead to a striking discovery before we get far into 2012. [I hope some of the experimenters at ATLAS and CMS are reading this.]
Of course we should also get as much information about the Higgs as possible from the new 2012 data, starting in March or April, which with some good fortune might bring 3 to 4 times as many collisions as in 2011. These may occur at a slightly higher energy per collision (8 TeV versus 7 last year) which would give slightly higher Higgs production rates. If there are some exotic decays happening, we obviously want the LHC experiments to collect as many of them as possible in order to maximize the chances that they can be identified when the data is analyzed. But this is where the trouble starts.
The trigger. Remember the trigger?
Read about the trigger here. You can’t deeply understand the Large Hadron Collider and how science is done there if you don’t understand the trigger. It all starts from there.
The trigger involves the real-time trashing of 99.9999% of the data that the LHC is collecting. As quickly as the data comes in, all but 1 in a million collisions is discarded irretrievably. There’s no choice. Let’s go through these numbers too.
Last year, 20,000,000 times a second, two bunches of about 100,000,000,000 protons crossed through each other at the center of CMS and at the center of ATLAS; and in each bunch crossing the number of proton-proton collisions which occurred simultaneously was between an average of 5 or so (at the beginning of the year) and an average of 15 (at the end of the year.)
As an aside, note that this means that every time a potentially interesting proton-proton collision occurred at ATLAS or CMS in 2011, the detector was measuring the debris from not just this possibly interesting collision but from as many as 20 others that happened all at the same time. Almost always those other collisions were very boring — just glancing blows that make small sprays of particles — but they do clutter up the detector with uninteresting particles that have to be removed later, to the extent possible, in the data analysis stage. These multiple proton-proton collisions in the same bunch crossing are called “pile-up”. [In addition, there are also particles, of rather low energy, still running around from collisions that occurred during previous bunch-crossings, and they're actually even worse, I'm told.] Pile-up makes measurements harder — not so much harder that one should avoid pile-up at the expense of fewer collisions, but one has to actively deal with it. The effect of pile-up is a little like electronic static on a video screen (or “snow” on an old-style TV) making it harder to see the image; you can still see it, but some of your ability to see detail is lost, and you may want to use special image processing techniques to clean it up.
Given this amount of pile-up, the total number of collisions toward the end of 2011 was about 300,000,000 per second, of which it is possible to store a few hundred per second, for a total of a few billion (1,000,000,000) collisions stored in 2011 for data analysis. The rest of the half a million billion collisions (570,000,000,000,000) may as well never have happened; they’re gone, forever. All of these numbers apply for ATLAS and CMS separately.
This year, the LHC is going to collect more data — probably not by having more frequent bunch crossings (the only other practical option would be 40,000,000 crossings per second), but probably by arranging for an average of 30 – 40 proton-proton collisions in each bunch crossing. In other words, the pile-up may double. On top of that, the energy per collision may be slightly higher, meaning the probability of any particular interesting-looking proton-proton collision will be higher. That’s good because the things you want to discover happen more often; but it’s still a problem for the trigger, because the rate for interesting-looking-but-actually-boring stuff goes up too… and since that stuff is so vastly more common, it’s what determines how aggressive the trigger’s decisions have to be.
So both the higher energy and the higher pileup will cause new challenges for the trigger. That means that the trigger will have to throw out collisions in 2012 that it would have kept in 2011. (Stated more accurately, the trigger is constantly being adjusted for changing conditions: the more interesting-looking collisions there are, and the more “noise” there is from pile-up, the tighter are the criteria that a collision has to satisfy for the trigger system to decide to keep it.)
Now, for some types of searches for new phenomena, this does not matter much. But the Higgs is a lightweight particle, so its decays typically produce a rather small amount of energy and momentum in particles moving perpendicular to the beam. And that isn’t very distinctive. [Specifically, if you add up the ``scalar transverse momentum'' (take each particle, measure the amount of its momentum perpendicular to the beam --- just the magnitude, not the direction --- and add the numbers up) for all the particles that are produced in a Higgs event, you will get only about 100 -- 200 GeV or so. Unfortunately, a proton-proton collision can in many other ways generate that much transverse momentum --- not easily... the probability is in the 1/20,000-1/200,000 range... but that's far too often. Remember only 1/1,000,000 events can be stored, and not all of them can be aimed at studying the Higgs -- the LHC needs to look for other new phenomena too, and make precise measurements of known particles, like the top quark.)]
What this means is that collisions that make a Higgs particle tend to sit in a dangerous place — right about where the trigger’s long knives do their slicing. So if you don’t think about the trigger carefully… ouch!
Even in 2011, the trigger threw quite a few of those 90,000 or so Higgs particles away. That was ok for Phase 1, the search for the SM Higgs. The SM Higgs can be produced in several ways, and then can decay in several ways. But not all combinations of production and decay have any hope of being discovered; for some combinations, backgrounds are far too large. Over the years there have been many studies that tell us which combinations are promising, and for each of them, there are logical pathways built into the current trigger strategies at ATLAS and CMS that allow for them to be studied.
But what about exotic decays? There are perhaps two dozen possibilities that we have to consider, maybe more. The theoretical studies of how to discover them have in some cases never been performed, and in other cases were performed in a context that is slightly but importantly different from the context of 2012. [One of the most important common differences is that many studies were done to examine what would happen if the Higgs were not found in Phase 1 because the probability of a certain exotic decay is near 100%. In this case the exotic decay would be common, but the Higgs mass would be unknown. But in fact we find ourselves in a different situation, with a Higgs that may be in the process of being found in Phase 1, with a mass that (if it is there) is known to be close to 125 GeV/c2, and for which any exotic decay (given that the decays expected in the SM must be common, to make the discovery of this SM-like Higgs possible) must be rare --- at most, say, 10-15% of all Higgs decays, and perhaps much, much smaller! Knowing the Higgs' mass makes the search easier, but the smaller maximum probability for the decay makes it harder; together they certainly make the search different, different enough that the previous studies may not apply.] So we find ourselves in the unfortunate position of not yet knowing precisely how to look for large classes of exotic decays.
But we certainly want to know this information in advance! Otherwise, how can we be sure how best to adjust the trigger strategies? How can we try to collect the right fraction of the data starting in March 2012, if we don’t know what strategies we’re going to want to use at the time of the data analyses, some of which won’t be done until 2013?!
I personally believe that we need to act quickly — that a number of theory studies of how to detect exotic Higgs decays are needed, now. [I hope some of my theorist colleagues are reading this.]
Certain things have to be done faster than others. The most urgent (which is what I have been working on for the past month, in consultation with some members of CMS and separately with some members of ATLAS — the two detectors have trigger systems which are different in crucial ways, and the strategies they will use may end up being quite different) is to understand which types of exotic decays are most likely to cause a problem for the trigger systems, such that collisions that make a Higgs with this type of decay would often be discarded under present trigger strategies. Conversely, for these problematic cases, it is urgent to understand what types of tricks can be used to make sure that more of these collisions pass the trigger. The good news is that the number of dangerous cases isn’t quite as large as I initially feared, and in the cases that worry me, it appears tentatively that there might be some tricks that can be played. But the jury is still out.
Moreover, adjusting the trigger is very complicated and controversial, because any time you choose to play some tricks that allow the trigger to keep a class of events that you were going to throw away, you have to choose to throw away other events that you were planning to keep. It’s close to a zero-sum game. So this is not something to be done lightly; you have to find the right compromise, and long arguments ensue within the experimental collaborations as to how best to do that.
Fortunately, even if in 2012 ATLAS and CMS don’t trigger on Higgs exotic decays as well as they possibly could, there is a safety net, a fall-back position. As various people often remind me, the production of a Higgs from a collision of a quark and anti-quark that makes a Higgs along with a W or Z particle (see the third row of Figure 2 of my article on Higgs production) offers a guarantee: no matter how the Higgs decays, the ~22% of the time that the W decays to a lepton and a neutrino, and the ~25% of the time that the Z decays to a neutrino/anti-neutrino or to a lepton/anti-lepton pair, offer up something that the trigger system can easily recognize — a lepton and/or anti-lepton, or “missing energy” (actually “missing transverse momentum”, or, more intuitively, a sign that something you can detect must be recoiling against something you have failed to detect — the neutrinos in this case.) But unfortunately the production of a Higgs in association with a W or Z is only about 5% of Higgs production (for an SM Higgs of 125 GeV; it’s a bit less for a heavier one and a bit more for a lighter one.) So if we have to rely on the safety net, only about 1% (and probably somewhat less when all is said and done) of Higgs bosons produced at the LHC will be triggered in this way. That means that although we’d have something like 400,000 Higgs bosons produced at each experiment in 2012, and perhaps as many as 40,000 or so decaying exotically, we’d only have 400 exotic decays stored permanently for later data analysis. Clearly, anything we can do to increase that 400 toward 4000 is worth doing! Especially since the fraction of exotic decays might be much smaller than 10%, making that 400 in the safety net more like 40 or 4. So yes, there’s a fall-back position, but we shouldn’t fall back until we’re beaten. And my own studies suggest we’re nowhere near beaten yet — though the time constraints of getting ready for the start of the 2012 data-taking mean we do have to work very fast.
[It is a pleasure to thank especially Oliver Buchmueller, Alex Tapper, Yuri Gershtein, Eva Halkiadakis, Andy Haas, Kyle Cranmer, Elliot Lipeles, Henry Lubatti, Dan Ventura, Guido Ciapetti, Barbara Mele, Stefano Giagu, Kathryn Zurek, Scott Thomas, David Shih, Jared Evans, Yevgeny Kats, Neal Weiner, Nima Arkani-Hamed, Josh Ruderman, Tomer Volansky, Itay Yavin, and others for many useful discussions on this and closely related topics.]