So now the second shoe drops, and the earth shakes a bit (3.9 earthquake just outside Berkeley prior to this talk.) Yesterday we had the first report from the CMS experiment’s search for multi-lepton events at the LHC (Large Hadron Collider). (See yesterday’s post.) The emphasis yesterday was on events that show evidence of substantial momentum and energy carried off by undetectable particles such as neutrinos (such evidence involves an imbalance of momentum among the detectable particles, or `missing energy’ as it is often [misleadingly] called). Today the emphasis is on events with large overall energy (not exactly true but close enough for the moment), including both the energy of detectable particles (the leptons and any jets from quarks or gluons) and the evident energy of anything undetectable. One could already tell from yesterday’s table of event rates that today’s talk at the Berkeley supersymmetry workshop would be worth paying attention to.
But before we begin, maybe I should make my own opinion perfectly clear to the reader. How high does this story rate on the scale?
- Particle physicists perhaps should be interested, maybe even intrigued, but definitely not excited. As do most small excesses, this one will probably disappear as more data becomes available.
- Other scientists should basically ignore this. The excess will probably disappear soon enough.
- I can’t see why the general public should pay any heed to this, as the excess will probably disappear — with the exception of those who are specifically curious as to why particle physicists are paying close attention. Those of you who are in this category have a nice opportunity to learn why multi-lepton searches are a powerful, though tricky, way to look for new physics.
Why do excesses so often vanish? Because sometimes they are just statistical fluctuations; when you do 100 different measurements, as is the case at the LHC, there’s a good chance that one of them will do something that the average measurement won’t do, namely, give you initially an excess of events. [Imagine a hundred people flipping coins; it wouldn't be that unusual if one of those hundred coins landed on the same side the first seven or eight times it was flipped.] The other reason is that experimentalists and theorists are human and make errors, often missing some subtle aspect either of their experiment or of their experimental subject, or introducing a bias of some sort. I wrote about this just a few days ago (in regard to superluminal neutrinos — and the situation here is even more likely to be due to a statistical effect than is OPERA’s neutrino claim.)
So, in that context, here we go.
Today’s talk, by Rutgers University’s postdoctoral researcher Richard Gray [who has his office down the hall from me and has been very hard at work for many months], was entitled “CMS searches for R-parity violating Supersymmetry” Why is the multi-lepton search sitting under this title? What is R-parity-violating supersymmetry, and why does it often give multiple leptons?
You can read about R-parity (one of the three key assumptions of standard variants of supersymmetry) here; the point is that it assures that the lightest superpartner (LSP) is stable. By a circuitous chain of logic, which depends on the other two assumptions, this assures the LSP is undetectable, and that all collisions that produce superpartners have two undetectable LSPs at the end of the day. This in turn implies that most of these events have large amounts of `missing energy’, or more accurately, an imbalance of momentum among the detectable particles that can only be made up by the undetected momentum from the undetectable LSPs.
If R-parity is violated, however, then the LSP decays. And if the decays are fast enough, occurring after the LSP travels only a microscopic distance, its energy is not undetectable after all. In this case, rather than a lot of missing energy, one expects very little missing energy and a lot of detectable energy. [In the language of yesterday's post's table, one expects low to medium MET and high HT, instead of high MET; the variable which is most stable is actually ST, see below.]
Relevant for today’s purposes is that supersymmetry with R-parity violation can produce multiple leptons. One way this can happen (and by no means the only possibility!!) is shown in Figure 3 of my article on supersymmetry and multileptons.
However, we really should not get caught up with R-parity violation or supersymmetry here. This particular speculative idea is only the stated motivation for the search; but it may well not be the source of any observed excess in multi-lepton events. Many other new-physics processes could generate multi-leptons too. What is so good about a multi-lepton search is that it is broadly applicable, as are many of the search strategies (even the standard supersymmetry search) used at the LHC. It casts a wide net, and could catch fish that are not supersymmetric, and perhaps of types never considered by theoretical physicists.
Now, what did CMS observe?
Today’s talk was packed with information, and there’s no way to process it all in a few minutes. So consider this a first pass only.
The key difference between yesterday’s talk and today was the way exactly the same events were divided up. Yesterday they were classified by two variables: HT and MET. Today they are classified differently, by ST. Vaguely, roughly, and incorrectly, one might say that HT measures the energy in jets, MET the energy in undetected particles, and ST the total energy in everything produced in the collision. Correctly stated [you can skip this if you don't want to know...]
- HT is the [ready???] sum of the absolute values of the transverse momenta of all the jets (read quarks and gluons) where “transverse” means the components of the momentum perpendicular to the beam.
- MET is the [careful...] absolute value of the sum of the transverse momenta of all the jets and the leptons and anti-leptons. It should be about equal to the absolute value of the sum of the transverse momenta of all the undetectable objects.
- ST is the sum of the absolute values of the transverse momenta of all the jets and the leptons and anti-leptons plus the MET. This variable (also sometimes called effective-mass) is the most robust variable for detecting new particles, as it does not much depend on how those particles decay.
So take the same events as yesterday, divide them up by their ST (is it bigger or smaller than 300 or 600 GeV), and ask (as yesterday) if there are any electron-positron or muon-antimuon pairs, and if so, ask if there is a pair that might have come from a Z particle decay. And you get the following tables. For four leptons, you get two unusual events. The speaker makes the point better than I could (Figure 1).
Something to watch, but nothing more for now.
So now what about the three-lepton events? There were several bins that were a bit high in yesterday’s talk. Here they are reorganized, and they mostly end up in two bins, as seen in Figure 2.
Now there were a number of very interesting plots (but not enough!) that give additional insights into these events. I just picked a couple that seem most revealing to me. First, take the events that have an electron-positron or muon-antimuon pair ["DY1"] and separate the events into those that have a pair from a Z and those that don’t; and now make plots of the ST distribution for both sets. Here’s what CMS finds:
What does this mean? Well, of course it might mean that CMS is having trouble modeling its backgrounds. But if it is new physics, we learn a lot right away. The amount of energy being released in these events is low. For instance, if new 500 GeV particles were being produced, and most of their mass-energy were released in their decays, we would expect a higher distribution of ST. So any new physics associated with this excess lies with somewhat lighter particles [or with heavy particles that do not release much of their energy, because they decay to stable undetectable particles that have just slightly smaller masses.]
That’s not all; in one of the backup slides, one finds, for the channel in yesterday’s talk that had the largest number of excess events, a distribution of the number of jets (those surrogates for quarks, antiquarks and gluons) with moderate to high energy. The number is typically zero or one.
And this tells us that it is unlikely that new particles that feel the strong nuclear force could be responsible for this signal; such particles, if produced in pairs, would easily produce two jets or more (see Figure 3 of this post), but not easily one or less. [Another possibility (see above) is that the jets produced are very low-energy, because the produced heavy particles decay to other heavy particles, of very similar mass, that do not themselves feel the strong nuclear force.]
What options remain? One would be the production of particles that feel only electromagnetic and weak nuclear forces, and have masses perhaps in the 250–400 GeV range [a very rough guess, I'd need to do simulations to see if that's really right.] (Crude example: something similar to Chargino-Neutralino production [see Figure 2 of this post], but with a higher production rate.) Another would be the production of heavy particles that do feel the strong nuclear force but that don’t release that much energy because their masses lie just above those to which they decay. (Crude example: A form of quasi-universal Extra Dimensions, modified to allow for the trileptons to carry a lot of energy.) Maybe there are singly-produced particles in the 1 TeV mass range that can give this effect too, but that tentatively seems difficult, based on a few of the other plots in the talk. I’m sure theorists will come up with many other possible explanations.
There are more plots in the talk to discuss, and more things to say about plots I’d like to see, but I’ll stop at this point: this seems like more than enough for one post. We can’t do too much more with only a handful of events. Now let’s wait for ATLAS to weigh in, and for CMS and ATLAS both to analyze the 150% more data that they’ve already recorded. Then we’ll see if this excess disappears like a shimmering mirage, or takes on a sharper outline like an much-needed oasis in the desert.