Pauline Gagnon's blog post: I’m a Higgs gatherer, not a Higgs hunter!
Ever since four theorists published their “Higgs hunter’s guide” in 1990, people have been referring to the Higgs hunt, as if we were a bunch of bloodthirsty physicists out for a kill. I’d say, if the search for the Higgs is a safari, at most it’s going to be a photo safari!
The way it works is purely based on statistics. With the ATLAS and CMS detectors, we collect millions of “events” every day, each event being a snapshot of what happened after two protons collided in the Large Hadron Collider (LHC). All sorts of particles are created out of the highly concentrated energy coming from these head-on collisions, some more often than others. Whatever happens often, we know very well by now, having had plenty of opportunities to study them in the past. What’s left now is everything that is hard to do, like catching rare events such as those containing Higgs bosons. And for this, we have to sort through huge numbers of events to find them.
The heavy particles created out of these collisions are highly unstable and soon decay into smaller, stable particles. Each event looks very much like a mini firework with debris flying out in all directions. If a Higgs boson is created, it will readily decay into smaller particles. To identify it, we need to reconstruct it from its decay products.
The Higgs boson has been rather elusive so far and no one really knows what it will look like. But if the Higgs boson exists, and if it is the one predicted by the Standard Model, then we know how to set traps to catch some. Each trap, or analysis in our jargon, is a set of selection criteria based on what is unique in the “signal” events, namely a specific decay of a Higgs boson.
For example, a Higgs boson could decay into two Z bosons, and those could each produce a pair of electrons or muons. We would then look for events containing two pairs of electrons or muons. But it is also possible to just produce a pair of Z bosons directly, and these events would look just like the decay of a Higgs boson. Or sometimes, decay products are mis-identified as electrons or muons, mimicking an event similar to those we are interested in.
The difficulty is to catch as many events corresponding to the decay of a Higgs boson as possible while rejecting the vast majority of other types of events that are produced more abundantly, what we call the background. It is as if we want to take a photo of a small flash of light in bright daylight! We need to set special filters to minimise the amount of background light coming through, while still allowing the tiny flash to be seen.
Fortunately, we know quite well what the Standard Model predicts. So in the end, all we do is count the number of selected events to see if we find more than what is expected solely from the background.
In the end, we take all selected events and look at the distributions of some of their attributes. The distribution we most often use is the value of the mass reconstructed from all the decay products. Events coming from the background will not have a particular mass but if the decay products all come from the same particle, a Higgs boson for instance, they will start clustering in one spot. And that would be the clear sign that we have found it.
But even if one day we claim to have found the Higgs, we will never be able to tell with complete certainty if one particular event (taken from all those we selected) came from a Higgs boson decay or from some background process. It is only when looking at all the collected events that we can see the excess, and not for any particular event.
So just as with blueberry picking, you need to gather many of them before it starts making a bump at the bottom of your pail. Having grown up in the northern part of Quebec in Canada, I know a great deal about blueberry picking. Hopefully, this will give me an edge in the Higgs gathering race!
An event captured by the ATLAS detector in the search for a Higgs boson decaying into two Z bosons, one of which goes into a pair of electrons (shown in yellow in the small picture at the bottom left corner) and two muons (the two tracks shown in red). There is no way to tell if this particular event comes from a Higgs decay or simply from a background event such as one containing two Z bosons. Only after gathering many such events and studying their properties will we be able to tell if some of them came from a Higgs boson decay.
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