CMS measures multiboson production processes
The CMS collaboration submitted for publication last week an interesting new result, where proton-proton collision data collected by the experiment during the last execution of the Large Hadron Collider was scanned for very specific events involving featuring a weak boson (W or Z) along with two energetic photons. The rate of these rare processes has been measured and found to agree well with the predictions of the Standard Model of particle physics.
Now that you’ve read the summary above, I’m wondering what else I can say to keep you interested. Indeed, weak bosons were discovered 38 years ago, and we already know their properties very well, thanks to a number of measurements and past studies. As for photons, they are THE most common particle in the universe, so really, can’t these scientists at CERN find a better use of their time?
But wait. We’re not talking about a boson and a photon over there. It is a single proton-proton collision materializing from nothing a heavy W or Z boson, associated with two very energetic gamma rays. It is an extremely rare phenomenon, in fact – a phenomenon which occurs only once in a trillion collisions. Physicists are really interested in very rare events of this type, because when something is extremely rare as produced by standard means, its study may be the best way to reveal whether something other also contributes to the total rate. This is what we call “measuring zero”: if we find zero (or something going back and forth) then everything is as expected, but if we find something other than zero, we have discovered something. If we instead study the common reactions according to the Standard Model, it would be much more difficult to reveal that something else is contributing to it. In other words, it is much easier to find a needle in an empty barn than a needle in a barn full of hay!
Besides the intrinsic value of studying rare reactions, let’s take a look at what produces them, and maybe I can express even more the fear I feel when I consider them. An AW or Z boson can be produced in proton-proton collisions when a quark and an antiquark annihilate each other. It already sounds weird: where was the antiquark to begin with? Well, inside the proton!
In fact, the proton does not only contain the so-called “valence” quarks (two high-type quarks and one low-type quark): its image as a “static quark model” of three quarks linked by gluons is not all the story. When you hit a proton with something else hard and small, you may have the chance to directly hit one of its less obvious constituents – a quark or an antiquark of the “sea”. These particles are also genuine quarks and antiquarks that these particles come from, but they share a smaller fraction of the total proton momentum than the valence ones, on average. So it takes a lot of energy (what the LHC offers) to build a W or Z boson by a hit between a quark and an antiquark.
Once produced, the W boson can easily emit photons on its own, as it is electromagnetically charged. The Z boson is instead neutral, so it cannot emit photons. So how do you get a pair of photons accompanying the Z? Well, these can be produced by the two charged leptons in which the Z sometimes decays (6% of the time a Z boson decays into an electron-positron pair or a muon-antimuon pair). Both the electron and the muon are charged, so they occasionally emit photons.
But there is a third way to get photons in the final state of the event: this occurs through what is called “QED initial state radiation”, another phenomenon that is worth mentioning. . In general, it is not surprising that the quark and the antiquark have a chance to spit out an energetic photon before annihilating into a W or a Z: they too have an electric charge, and therefore they “couple” to the photon, although with lower intensity than that of leptons (quarks have fractional charges, the coupling is therefore weaker). But the funny thing is that it is precisely because of this emission of energy in the form of a gamma ray that the quark pair can annihilate themselves into a W or Z boson! In fact, they are likely to do so if they have too much energy to start with.
It goes like this: a quark and an antiquark are on a collision course as the protons get closer. Their quantum numbers are exactly the right ones to allow annihilation: they have a total color charge of zero, and their total electric charge is 1 (if they are about to produce a W) or 0 (for a Z). . But collectively they have much more energy than the residual mass of the boson they want to produce. Then, in a thoughtful maneuver, one of the quarks spits out the extra energy in the form of a quantum of electromagnetic radiation. A zeptosecond later, the pair merge happily and a W or Z boson is produced.
Below are two Feynman diagrams describing the processes by which a W or Z can be produced with two photons. In addition to knowing that time flows from left to right in these diagrams, as the spatial dimension is illustrated by the movement of the lines on the vertical axis, you are now fully able to understand what is depicted. You can then easily appreciate that each of these diagrams tells a different story!
Come to the measure, which is described in detail in the CMS pre-printingI think you won’t be very interested in knowing the exact cross-sectional value that the two phenomena were found to have. After all, CMS couldn’t find any needles in the (tiny) haystack this time! I will only show one graph from the post, which details the distribution of the data after meaningful signal enrichment selection.
The graphs in the top row show the data selected by Wγγ when W decays into an electron (left) or muon (right), and the data selected in the bottom row with decays into electrons (at left) or muons (right), as black dots in a transverse momentum distribution of the photon pair system. Contributing processes are represented by filled histograms. Orange is the signal component. As you can see in the lower “data on prediction” insets, the data conforms reasonably well to the sum of the Standard Model processes; a new physical signal (blue line) could produce excess events at a high diphotonic pulse.
Tommaso Dorigo (see his personal web page here) is an experimental particle physicist working for the INFN and the University of Padua, and collaborates with the CMS experience at the CERN LHC. He coordinates the FASHION collaboration, a group of physicists and computer scientists from eight institutions in Europe and the United States who aim to enable end-to-end optimization of detector design with differentiable programming. Dorigo is the journal editor Physics Reviews and Open physics. In 2016, Dorigo published the book “Anomaly! The physics of colliders and the search for new phenomena at Fermilab“, an overview of the sociology of major experiments in particle physics. You can get a copy of the book on Amazon, or contact them for a free pdf copy if you have limited financial means.