Matt Strassler [Aug. 13, 2012]
I told you a few days ago about the workshop I attended recently at the Perimeter Institute (PI), which brought a number of particle theorists together with members of the CMS experiment, one of the two general purpose experiments operating at the Large Hadron Collider [LHC]. I described the four main areas of discussion, and mentioned a fifth issue that underlies them all: triggering. Today I’m going to explain to you one of two big advances in triggering at CMS that have recently been made public, the one called “Data Parking”. And I’ll also describe my small role in Data Parking over the last ten months, which will explain, in retrospect, some of the other articles that have appeared on this site during that period.
[This article is almost entirely about CMS, but CMS's competitor, ATLAS, has also made public that they have something essentially identical (I think!) to Data Parking, which they call "Delayed Data Streaming".]
This is a rather long article. It has to be. It’s not super-technical, but it does involve some of the more subtle issues for doing physics at the LHC, so I have to explain a lot of different things to make the article self-contained. Sorry about that, but I hope you’ll find it’s worth it in the end. If you’re short on time, read the next section, and perhaps the one following.
The Basic Idea Behind Data Parking or Delayed Data Streaming
First, let me remind you briefly what “Triggering” is all about (you can read more about it here.) Each of the two beams of protons at the LHC is made from fourteen hundred little bunches, each one containing a hundred thousand million protons; two bunches cross inside CMS [and inside ATLAS] 20 million times a second. During each bunch-crossing, an average of twenty or so proton-proton collisions occur essentially simultaneously. (The presence of so many collision per crossing is called “pile-up“.) After each crossing, particles stream out from all those collisions and create electronic signals in CMS [or ATLAS] as they pass through all the electronic sensors inside the experiments. With so many bunch-crossings each second, the result is a huge deluge of data, too much to handle; if all of this data were stored it would vastly exceed the computing capabilities available. Until recently, CMS could only store the data from 350 of these 20 million bunch-crossings occurring each second. All the rest had to be dumped in the trash — more specifically, all the data from most crossings is overwritten, and lost forever.
The reason this is acceptable is that the vast majority of proton-proton collisions generate only phenomena that physicists are already familiar with; what’s thrown away doesn’t contain any new or useful information. And conversely, most new phenomena that might be present at the LHC and that we’re really searching for often lead to proton-proton collisions that are easily identifiable as unusual and potentially interesting. For this reason, an automated system — the trigger — can and must quickly scan the data from the collisions and decide which bunch-crossings contain an interesting collision worth keeping. I have to emphasize that this isn’t optional: it is necessary. If the LHC only produced 350 bunch-crossings per second, it would produce a few Higgs particles per year, instead of the few per minute that we’re getting now. And that wouldn’t be enough to discover the particle, much less study it in detail. So the LHC must make more collisions that can be stored, and CMS and ATLAS and the other experiments have to choose judiciously the ones to keep.
But the trigger is only as smart and unbiased as the people who program it, and there’s always the risk of throwing out the gold with the gravel, or at least being less efficient at keeping the gold than one would like. Everyone in the field knows this, and the experimentalists spend a lot of their time and personnel worrying about and tinkering with and testing the triggering strategies that they use.
Data Parking at CMS (and the Delayed Data Stream at ATLAS) takes advantage of the fact that the computing bottleneck for dealing with all this data is not data storage, but data processing. The experiments only have enough computing power to process about 300 – 400 bunch-crossings per second. But at some point the experimenters concluded that they could afford to store more than this, as long as they had time to process it later. That would never happen if the LHC were running continuously, because all the computers needed to process the stored data from the previous year would instead be needed to process the new data from the current year. But the 2013-2014 shutdown of the LHC, for repairs and for upgrading the energy from 8 TeV toward 14 TeV, allows for the following possibility: record and store extra data in 2012, but don’t process it until 2013, when there won’t be additional data coming in. It’s like catching more fish faster than you can possibly clean and cook them — a complete waste of effort — until you realize that summer’s coming to an end, and there’s a huge freezer next door in which you can store the extra fish until winter, when you won’t be fishing and will have time to process them.
So recently CMS, and apparently ATLAS, started storing the data from hundreds more bunch-crossings per second than they did previously; CMS, in particular, is taking 350 crossings per second for immediate processing, and storing 300 extra (and considering adding more) for processing in 2013/2014. These 300 extra are ones they select using novel methods that would previously have been impractical, aimed at signals of new phenomena that are not that distinctive (as least from the trigger’s point of view) and could otherwise have been missed.
Think about it: It’s the equivalent of running the LHC for an extra year, while using a bizarre triggering method for selecting events for storage, one that would never ordinarily have been acceptable! It represents a new opportunity for making (or at least assisting in) discoveries, obtained at minimal financial cost.
Data Parking and (Exotic) Higgs Decays
Needless to say, Data Parking for CMS was an idea that originated within the experiment; only experts would know the details of computer allocation within CMS. But by lucky chance, the possibility of Data Parking was mentioned during a small one-day workshop that I attended in London at Imperial College last September, involving just a handful of theorists and CMS experimentalists. A faculty member at Imperial, who gave the opening talk laying out the goals for the coming months, mentioned that CMS had the capability to store extra data, and asked if any of the theorists thought this capability would be useful for anything in particular. I practically leapt out of my chair!! I’ve been worrying for years about several types of phenomena that I was pretty sure the experiments would start throwing away, once the collision rate became as high as it is now.
Among the worries I had, the one that seemed most urgent involved the Higgs particle. (Recall that back in September we had no reliable experimental hints for the particle yet, but theorists have been thinking about its possible properties for decades.) The point is this: while the experimentalists have (justifiably) paid a great deal of attention to setting up triggering methods that will catch Higgs particles that decay in a fashion that we expect (i.e., in ways predicted for the simplest possible Higgs particle, the “Standard Model Higgs”, and many of its variants), not much attention has been paid to looking for Higgs particles that decay in an unexpected, “exotic” way, such as to new particles that we haven’t even discovered yet that in turn decay to known particles. And the list of possible exotic decay patterns is very, very long! so there’s a lot to think about.
Of course, it is possible the Higgs does not ever decay in an exotic way — so looking for such a process is arguably a long-shot with low odds. But a light-weight Higgs particle, such as one with a mass of 125 GeV, is a very sensitive creature, easily affected by new phenomena — so it’s not a stretch to imagine exotic decays might be present. And correspondingly, the particles emerging from an unexpected exotic decay of a light-weight Higgs often would have too little energy and would be insufficiently distinctive for the collision to be selected by the trigger system for permanent storage. So if we were to assume such decays probably don’t exist and consequently didn’t actively work to assure the trigger would fire when signs of such a decay appeared in a bunch-crossing, well — we might miss a discovery, and even incorrectly draw the conclusion the Higgs particle is of the simplest type, when in fact it isn’t.
Data Parking is the perfect opportunity, and that’s why I jumped out of my chair when I heard about it. If you have a fixed amount of data that you can afford to store each second, any attempt to trigger in a new way means you have to give up one of your old ways of triggering, at least to a degree. It’s a zero-sum game. And you can’t realistically expect people to give up what they consider to be high-odds triggering strategies for what they consider to be low-odds triggering strategies. The community has its biases about the odds, and though I worry a lot about these biases, I can’t say with any certainty that I’m sure those biases are wrong. But if there’s extra storage available, a low-odds but high-payoff opportunity is absolutely one of the things you should consider using it for. So that’s why I’ve been thinking so hard about exotic Higgs decays this year; there are posts on it here, here, here, here, here, here and here. I wrote extensively in those posts about the potential importance of triggering strategies aimed at exotic Higgs decays — but I couldn’t at that time explain that the idea was that this data would be parked, and that I had been working closely with CMS experts at Imperial and elsewhere on the matter. [I've also been talking with ATLAS experts as well about the triggering risks, but because ATLAS and CMS's trigger systems are so different, the two experiments have to solve this problem in completely different ways.]
Triggering is one of the most important and subtle aspects of doing physics research at the LHC experiments, and the reason I was able to contribute to this project, which had to be done very rapidly in order for it to be useful during the 2012 data-taking period, is that I have some experience. I once did some research (co-authored with Kathryn Zurek, now a professor at Michigan) in which it was emphasized (among many other things) that long-lived particles that would decay in flight while traversing ATLAS, CMS and other similar experiments arise much more commonly in theories than is widely appreciated, and that, if present, they might not be noticed by existing trigger methods. This observation inspired a couple of my then-colleagues at the University of Washington, who were members of ATLAS, to join with their ATLAS collaborators at the University of Rome La Sapienza and elsewhere to develop trigger methods specifically aimed at closing this gap in the ATLAS trigger strategy. I worked very closely with them for a long time — indeed I’m a co-author on the relevant ATLAS note — until ATLAS started collecting data and I was no longer allowed to discuss details with them. It is very satisfying to me that this effort has produced a real scientific result — the first one from the LHC that begins to constrain the possibility that the newly-found Higgs-like particle (or anything like it) sometimes decays to as-yet unknown long-lived particles. (ATLAS has a second search completed now. Two related searches, also partly inspired by our 2006 papers, were done at the Tevatron, one by DZero and one by CDF; and at ATLAS and CMS as well as DZero and CDF there are also searches for many other types of more-easily observed long-lived particles [see for example here] that don’t require a specialized trigger strategy.) I was in Seattle back in February to serve on the thesis committee of the University of Washington graduate student who did the bulk of this research, which required not only the development of the new trigger but also entirely new and remarkable methods of using the ATLAS muon detector, ones for which it was not originally designed. His excellent work represents a real success story of interchange among theorists and experimentalists in particle physics.
Now I’ll go into more detail. But before I do, a few definitions:
“Transverse momentum”. Momentum is something that has a magnitude and a direction; crudely, it tells you where an object is going and how hard it would be to stop it. Transverse means perpendicular, and at the LHC it refers to perpendicular to the direction of the incoming proton beams. Transverse momentum tells you the part of the momentum that is perpendicular to the beams, as opposed to “longitudinal momentum”, which is along the beam. Transverse momentum is useful because although both transverse momentum and longitudinal momentum should add up to zero after a proton-proton collision at the LHC, there are always particles that travel near or in the beam-pipe after the collision that can’t be measured, so longitudinal momentum conservation can’t be checked. Only transverse momentum conservation can be checked.
“Visible transverse `energy'”. This really isn’t energy, but is instead the sum of the magnitudes of the transverse momenta of the observed energetic particles. It gives a measure of the degree to which particles are flying out sideways, in all directions, after a proton-proton collision.
“Missing transverse `energy'”. This really isn’t energy either. First, you add up (as vectors, including magnitudes and directions) the transverse momenta of all of the observed particles in the event to get the total visible transverse momentum (NOT the visible transverse `energy’!) Since transverse momentum is conserved, if the total visible transverse momentum isn’t zero then there must have been “invisible” transverse momentum to cancel it: momentum carried off by one or more unobserved particles (such as neutrinos or hypothetical dark-matter particles that can travel through the detector without leaving a trace.) The magnitude of the total invisible transverse momentum is called the “missing” transverse energy.
A Few Details About How Triggering is Done
Ok, now back to triggering.
The ATLAS and CMS triggering systems are very similar in some ways, very different in others. Both of them (speaking very crudely) divide the triggering process that looks at the data from each individual bunch crossing into two steps, with similar goals — but the differences in execution are significant:
Level 1 (L1): first, hardware, part of it programmable (and therefore called firmware) is used to make an extremely rapid and crude analysis of the data coming out of a bunch-crossing, looking for any sign of something interesting. L1 is the key bottleneck for the trigger; of the 20 million bunch crossings per second, it can afford only to choose 100,000, i.e. half of one percent. Because of the high speed required, neither experiment can access anything about particle tracks during L1 analysis; only the distribution of energy can be studied, along with a few additional pieces of information, such as whether there might be electron, positrons, muons, anti-muons or photons present. Both ATLAS and CMS are able to study jets (the sprays of particles created by high-energy quarks and gluons), but while ATLAS records only the transverse momentum of each jet, CMS also records some information about the jets’ angles relative to the incoming proton beams.
Beyond Level 1: Then the 100,000 bunch-crossings per second that pass L1 are sent to the next level. There, software begins to take over, but what that software can and can’t do is determined in part by the hardware of the experiment, and by how long it takes to read the data out of the hardware. CMS can in principle look at any aspect of the data at this stage, which they call the High Level Trigger (HLT) though they are still limited by the time it takes to read out and analyze the data from various portions of the experiment. ATLAS has more hardware constraints, and divides this process into Level 2 (L2) and the Event Filter (EF). At L2 only the data from certain parts of the detector, ones that contain information that appeared to be interesting at L1, can be obtained; and only if this subset of the data looks interesting at L2 will the study of the data proceed to the EF, where all the data can be accessed. Again, only a fraction of a percent of the bunch-crossings that pass through L1 will also make it through this second stage to be stored permanently.
More on Higgs Decays: Increasing the Baseline
Ok — now back to possibly-exotic Higgs decays. What’s the relevance of Data Parking?
To explain this I need to first tell you how one can trigger on these exotic Higgs decays using standard methods. Imagine a bunch-crossing in which an exotically-decaying Higgs is produced; stuff flies out from the Higgs decay, and contributes to the electronic signals in the experiment. Will the trigger recognize the resulting data as “interesting”? Clearly that will depend on exactly how the Higgs particle has decayed, so one can’t answer this in general; it has to be case by case.
Unfortunately, there are some types of exotic decays that ordinary trigger methods will almost entirely bypass — trust me, I studied this all winter, and I know it’s true. Does that mean that such decays will be 100% lost? No; there is a baseline of about 1%. And to understand what Data Parking does in this context, one has to understand the baseline first.
What forms the baseline? About 5% of Higgs particles are expected to be produced along with a W or Z particle. About 22% of the time, a W particle will decay to a lightweight charged lepton (more precisely, to an electron, positron, muon or anti-muon), which, if it is energetic enough, will usually be recognized by the trigger system as a good reason to store the data from the corresponding bunch-crossing in which it appears. Similarly, 6% of Z particles decay to an electron-positron pair or a muon-antimuon pair, and these will usually be stored. In these cases, the decision to store the bunch-crossing is completely independent of how the Higgs particle itself decays. No matter how crazy and untriggerable is the Higgs decay, the decay of the W or Z in these collisions will assure the data from about 1% of the exotic Higgs decays will be recorded.
But a 1% baseline is rather disappointing. Can it be increased? The answer is yes, not enormously but by enough to be useful, in the presence of data parking. And here’s how.
The basic idea is to use another way that Higgs particles are produced, in the process (called vector-boson-fusion, or VBF) in which two quarks (one from each proton) collide, and in the middle of their collision a Higgs particle is produced. (This is described as the p p –> q q H process here.) The two quarks survive and scatter out of the beam, forming jets that enter into CMS or ATLAS at high energy but at a small angle relative to the beam, and thus at small transverse momentum. The presence of these two jets, like the presence of the charged lepton in the baseline events, is completely independent of how the Higgs particle decays. So all that has to be done is have the trigger system identify the two jets (which are called “tagging jets”, since they tag the collision as possibly due to Higgs particle production), and if they have the right characteristics, store the data from that bunch-crossing.
There’s only one problem with this idea: It doesn’t work. You can’t apply this idea at Level 1, either at ATLAS or CMS. There are multiple problems. First, it’s so common that a proton-proton collision produces two jets of this type — or that two simultaneous collisions produce jets such that the bunch-crossing as a whole has two jets of this type — that if you tried to select events in this way you’d choke the Level 1 system. Second, neither ATLAS nor CMS (for different reasons) can select the jets that you’d ideally want, because of the compromises built into the design of their Level 1 trigger systems to make them sufficiently fast. There’s nothing you can do about those design issues; only new technology and a rebuild of the hardware can change this.
Fortunately, there’s a simple work-around. How CMS uses it is too long and technical a story for this article, but what I found in my studies is that no matter (almost) how the Higgs particle decays, if you combine the transverse energy of the two tagging jets with the transverse energy of the particles that come out from the Higgs decay, you often get a reasonable amount of visible and/or missing transverse energy, enough to be interesting for the trigger. This is shown in the figure, where the probability distribution for the sum of the visible transverse energy and the missing transverse energy is plotted, for twenty different types of exotic Higgs decays produced through vector-boson fusion. You can see that the distribution is almost identical in every case.
The CMS people who were leading this project had to do a tremendous amount of work very quickly, and then convince their CMS colleagues; I don’t know enough of the details of that process to describe it properly, so I leave it to them to describe if they choose. In the end, as revealed at the ICHEP conference in July [see page 13 of Stephanie Beauceron's talk (pdf slides)], CMS’s new Data Parking strategy does in fact include a “VBF” trigger, based in part on the above observation, that identifies a certain fraction of Higgs particles produced with two jets (many of which, though not all, are produced in vector boson fusion) independent of how those Higgs particles decay. This gives another way of agnostically triggering on Higgs particles, and I think I can fairly say, without revealing inside information (since you can basically guess that the whole thing wouldn’t be worth doing if the following weren’t true), that doing so at least doubles the number of baseline events.
Is this useful? Just because you have more data doesn’t mean that you can actually find anything new in it. This was the complaint from some members of CMS and ATLAS during the spring, so several of my young theory colleagues and I set to work to answer that complaint. Though we didn’t have enough people working on it and had very little time to produce results — a month or two, where typically one would work for three to six months on something like this — we did find cases where we could be sure that the new baseline events will be useful, and even cases where they are probably more useful than the original baseline events for making a discovery. In other words, we argued, it’s worth putting this trigger in: the data obtained will definitely be useful in at least some cases. [Big thanks to Tomer Volansky, Jessie Shelton, Andrey Katz, Rouven Essig, David Curtin, Prerit Jaiswal, and Raffaele Tito D’Agnolo.]
How well this effort will really pay off is something we’ll only start to know a year from now, when the data is reconstructed and data analysis takes place. It’s a bit of a gamble, but I think it’s a wise one. Needless to say, I’ll be way more than simply thrilled if the new trigger contributes to a discovery of an exotic Higgs decay. But I’ll still be pretty happy, even if the searches for exotic decays find none, and only put a limit on how often they can happen, if the new trigger helps CMS make the resulting limits considerably stronger than would otherwise have been possible. As far as I’m concerned, helping to optimize the capability of an existing experiment to make measurements of profound scientific importance is honorable work for a theorist.
Other Opportunities with the Trigger at ATLAS and CMS
The CMS talk at ICHEP that I mentioned above also reveals several other types of phenomena being funneled into Data Parking:
- Events with four or more jets above 50 GeV of transverse momentum each
- Events with a possible low-momentum tau lepton/anti-lepton pair
- Events with a very low-momentum muon/anti-muon pair
- Events with a lot of visible transverse energy, or a lot of missing transverse energy (as calculated only from the jets, electrons, photons and hadronically decaying tau leptons in the event, specifically ignoring spread-out distributions of energy)
As far as I know, those are the only public details. I’d encourage my fellow theorists to put some time into thinking about how these unexpected and valuable data sets might be put to best use.
One of the things that was discussed actively at the Perimeter Institute workshop was how to make use of a possible increase in the Data Parking capability at CMS. There were a number of interesting suggestions. But there’s very little time to make the best of this opportunity, because already almost half of the data for the year has already been collected. The reason there’s still any time at all is that data-taking on proton-proton collisions for the year has been extended; it was expected to end in October, but now will continue through December, with the collisions of lead nuclei that were scheduled for November moved into early 2013, and the shutdown of the LHC starting in March. That not only gives a big boost to the amount of data expected but also makes it worth trying to find additional Data Parking possibilities over the next month. Natural opportunities include looking for other ways to trigger on unexpected Higgs decays (now focusing on the specific particles produced in the decays, and less on increasing the baseline) and on ways to find rarely-produced low-mass particles, such as the W/Z/t/H partner particles predicted by naturalness considerations and discussed in Tuesday’s post. It’s probably too late to do anything new about triggering on exotic particles (also discussed in Tuesday’s post); the methods required would take too long to develop.
At ATLAS, it’s not yet clear what’s being put in the Delayed Data Stream, but the fact that it involves over a hundred bunch-crossings per second is clear from this plot, which shows the trigger rates for this year in thousands of bunch-crossings per second (“kHz”), with the amount of delayed data slowly increasing from late May. From the labels on the plot, it would seem they are focusing the delayed data on events with jets (“hadrons”) and on events with low-momentum muons (which often come from bottom quark decays), but that’s guesswork on my part. Equally important, it would also appear they’ve increased the amount of non-delayed data that they are storing to 500 bunch-crossings per second. I look forward to seeing additional public information from ATLAS.
So you see from this that doing science at the LHC doesn’t involve the experimenters sitting back in their chairs, twiddling dials, and waiting for the data to come out of the computer. They’re constantly working, day in and day out, to improve their methods, pushing them to the limit, and beyond. It’s a pleasure and a privilege to work with such creative and intelligent people as they seek to make the best of this magnificent machine.
[I am grateful to a number of members of CMS, especially to Oliver Buchmueller, for many useful conversations about these issues.]