© Matt Strassler [Dec. 4, 2014]
In this article, we’ll see how things can go wrong with the trigger system at experiments like ATLAS and CMS at the Large Hadron Collider [LHC], and also how one can work around the associated challenges. Specifically, we’ll look at Higgs particles decaying to unknown long-lived particles, and how that can make a mess of things.
Suppose, as might well be true, that one in a hundred Higgs particles decays to a pair of as-yet unknown particles — let’s call them “X” particles. And further suppose that X particles have a rather “long” lifetime, on particle timescales, that allows them to travel a meter or so on average before they decay, let’s say to a quark and an anti-quark. Any high-energy quark or antiquark then makes a “jet” of “hadrons” (a spray of particles, each of which is made from quarks, anti-quarks and gluons). But since the X has traveled some distance before it decays, the jets that it produces — unlike most jets, which start at the proton-proton collision point — appear in the middle of nowhere.
To be more precise, imagine a proton-proton collision such as shown in Figure 1, in which two gluons, one from each proton, collide head on, and make a Higgs particle, plus an extra gluon, which is kicked off and makes a jet of its own. So far, this is perfectly ordinary Higgs particle production. Now, however, this is followed by something unusual: Higgs decays to two X particles immediately (or rather, after a billionth of a trillionth of a second), and much later, after a few billionths of a second, each X decays to quark + anti-quark.
What will this look like to a detector like ATLAS or CMS? Well, it depends exactly where the two X’s decay — and remember, even though X particles have a common average lifetime, each individual X decays at a random time. But perhaps one such collision might look to a detector roughly like Figure 2. In this figure you are looking at ATLAS or CMS from the perspective of the beampipe, with the colliding protons coming straight into and out of the screen.
At upper left is what the event would look like if you had a perfect detector that could see all particle tracks. You notice three sprays of particles: one jet from the collision point due to the final gluon, and one additional spray from each of the decaying X particles… in fact, if you were to look closely, you’d see each of those sprays from an X particle has two sub-sprays, one from the quark and one from the anti-quark to which the X decays. But this won’t be so obvious to the detector. At lower right is what ATLAS or CMS experimentalists would see in ideal circumstances, with lots of time to analyze the event carefully. But the problem is that the trigger system doesn’t have much time. At upper right is what it would know about the event at “Level 1”, the first stage of triggering. It has to decide, in less than a millisecond, whether or not to keep the event based only on a rough sense of what is seen in the calorimeters and muon system; the tracker is not used, because it takes too long to read out its data. At lower left is what the full trigger would see if Level 1 gives its ok; now the tracker information is available, but there isn’t time to go looking for tracks that don’t start near the collision point, such as the tracks (which I colored blue) that are present at the right side of the tracker.
The good news is that a collision like this cannot be directly mimicked by any real physical process that occurs involving known particles. If you saw a collision event like this (more precisely, if you saw a computer’s picture of the event, as inferred from the data that the detector recorded) in enough detail, you’d be pretty excited [though still cautious, because weird sprays of particles can also happen when a hadron collides with a wire in the detector … so you do have to look closely.]
But here’s the big problem. The standard trigger methods used to select which events to keep and which ones to discard would discard this event. It would be lost forever, using standard methods. That’s because from the Level 1 trigger point of view this is just an event with three ordinary uninteresting low-energy jets. And there are a gigantic number of events with three low-energy jets produced every second at the LHC. An example is the one shown in Figure 3, where a quark and anti-quark collide and scatter, spitting off a gluon, and giving a three jet event which appears to the detector as shown in Figure 4. You see that the Level 1 trigger sees something rather similar in Figure 2 and Figure 4, even though the reality of what is happening is very different. If one looked at these two events in detail, one could easily tell these two collisions are qualitatively different. But if the Level 1 trigger cannot distinguish them, then the baby, looking just like the bath water, will be tossed out!
And so, good-bye, discovery…
…unless we bring in a new trigger method. Fortunately, collisions like the one in Figure 2 can in fact be saved, using techniques (some of which Kathryn Zurek and I suggested in our first work on this subject) that the ATLAS and CMS experiments pioneered. The first key observation is that even the Level 1 view of the event (Figure 2, upper right) shows that the jet at the top is particularly narrow… and there is something else interesting that makes narrow jets — a tau lepton that decays to hadrons. Ordinary jets can be narrow too, but that’s rare. So the Level 1 trigger has a strategy that says: if we see three jets that have energy below, say, 100 GeV, we’ll discard the event, but if one of the jets is narrow, we’ll accept the event even if the jet only has energy of 50 GeV. Again, this is a strategy that was developed to find tau leptons, not long-lived particles — but so what? It can be repurposed in this context!
So now we’re in business; the event has survived Level 1! At the next stage, (Figure 2, lower left), the trigger has time to look at the tracking information, and will notice that two of the three jets have no tracks. That’s a bit unusual. Even more unusual is that one of the jets has energy in the outer “hadronic” calorimeter but little or none in the inner “electromagnetic” calorimeter. So one or both of these facts can now be used by the trigger as an excuse to keep the event, so that humans can look at it later. Saved!
This event, however, will not be alone. All sorts of weird events, having nothing to do with Higgs decays, will be in the same pile as the one in Figure 2. There will be some with detector problems (electronic noise that made a fake jet, tracker failures which removed all the tracks from jets in the upper right quadrant of the detector, etc.) and others with weird effects (e.g. a fake jet produced when a muon, produced when a stray proton from the beam hit some piece of the accelerator, entered the detector and hit something in the calorimeter.) There will also be a few ordinary three-jet events, in which two of three jets had nothing but neutral hadrons (so no tracks) and none of these were hadrons that decay to photons (so no energy in the electromagnetic calorimeter) and one of the jets happened to be particularly narrow, all by chance. So the experimentalists will have to work hard to sift through those events and find the interesting ones, if any, perhaps looking for the hard-to-see vertex in Figure 2, lower right, where the blue tracks emerge just at the edge of the tracker. But at least they have a chance! If the trigger had thrown away the event, all would be lost!!
Starting in 2006 (and with my participation until the LHC started taking data), the ATLAS experiment developed this trigger strategy, along with a few others, and recently used it to look for events in which two X’s decayed in the hadronic calorimeter. While the probability that both X’s will decay in this part of the detector is relatively small, it’s big enough that ATLAS had a decent shot at a discovery. Unfortunately, they didn’t make one, and instead they could only put limits on this process. As shown in Figure 5, they were able to put limits for X particles with masses of 10-40 GeV: for certain lifetimes, at most 1 in a few tens of Higgs particles can have decayed to two X particles. But 1 in a hundred is still allowed, or even 1 in ten if the lifetimes are a little longer or shorter.
Of course it would be very interesting if ATLAS folks could look for one decay in the hadronic calorimeter and a second decay somewhere else, either in the tracker or in the muon system (which at ATLAS is a sort of limited tracker for any particle which gets that far; this is not true of CMS.) I hope we’ll see this search sometime. The backgrounds will be larger but the amount of signal can go way up too… and you might actually see a X decay by eye…
Both ATLAS and CMS have made a few other interesting measurements of long-lived particles (I gave links to some of them early in this post), and in most of them so far, special triggers were used. There are other methods that don’t require such special triggers, but can still benefit from them. One other trick up CMS’s sleeve is in its parked data, which was obtained in 2012 using a trick to allow the trigger to keep more data than would otherwise have been possible. One trigger strategy used for the parked data was to identify two high-energy jets that might have accompanied a Higgs particle when it is produced in the scattering of two quarks. For this strategy, one ignores the Higgs decay, more or less, and focuses on the jets created by the two quarks that scattered. However, the events collected using parked data are lying mostly unexplored, still waiting for someone to analyze them and look for a sign of an X particle decay (or of anything else unusual that a Higgs particle might have done, other than decay to undetectable particles.) I hope this parked data won’t sit gathering dust until after 2018.
Run 2, starting in May 2015 with higher collision energy and higher collision rates, will raise a whole new set of trigger challenges for Higgs studies and for long-lived particles. Time is quickly running out to improve any triggers in place for 2015. So while I was at CERN in November 2014, I gave a talk to members of the ATLAS experiment, and a similar talk to members of the CMS experiment, on why long-lived particle searches are so important, and on what triggers and analysis strategies might be useful. This was followed in both cases by extensive and fruitful discussions about the challenges and opportunities that lie ahead in Run 2. Many details and subtleties about how the experiments and their triggers actually work — the hardware, the software, and the decisions humans make about how to use them — have to be considered with care. I left CERN with renewed optimism that we will see a significantly broader range of searches for long-lived particles in Run 2 than we did in Run 1.