It’s either an unaccounted-for background or it’s new physics. In either case, it’s complicated, for sure.
The CDF Collaboration, at Fermilab’s Tevatron accelerator, has submitted for publication a new paper describing a subsample of proton-antiproton collision events in which there is at least one muon produced far from the primary proton-antiproton interaction. This subsample is not yet described by known processes, including the effects of detector/reconstruction failures, and is starting to cause somewhat of a sensation in the high energy physics community. As a member of CDF, I can tell you this analysis has gotten some rather intense scrutiny in the past several months!
The excess subsample is called a “ghost” sample in the paper, and is characterized by the fact that there can be several muons whose direction of travel lies within 37 degrees of the primary (highest energy) one, and the distribution of muon “impact parameters” has a long tail, out to several centimeters. The impact parameter is a measure of how far away from the main event vertex the particle was produced, and so these extra muons appear to come from the decay of some sort of particle with a lifetime much longer than that of the b quark.
Is this new physics? Or did CDF underestimate the rate of background processes leading to this sort of observation?
The subsample in question came to light in the course of the measurement of the b quark pair production cross section, which is proportional to the rate at which events with a b quark and a b antiquark are produced. This measurement is done in at least two ways. Unlike the lighter quarks, the b quark lifetime is long enough that it flies a few millimeters through space before decaying. So in one method, one looks for “secondary vertices”, distinct from the primary location in space where the proton and antiproton collided. These secondary vertices are where the b quarks decay to several hadrons, or possibly leptons such as muons.
The muon is the heavier cousin of the electron. It has a mass of 106 MeV, compared with 0.511 MeV for an electron. Being so much heavier, and having a lifetime of 2.2 microseconds, when it is produced, it tends to travel through very thick layers of material before stopping. Indeed, in modern large collider physics experiments, the muon detection systems lie outside the rest of the detector, effectively using it as shielding from all the other particles produced in the collisions, which are mainly pions. Pions are hadrons, consisting of a quark and an antiquark. Pions tend to “shower” and leave their energy in the heavy layers of material (typically lead and steel) in the calorimeters. But sometimes they can “punch through” and leave hits in the muon detection system, fooling us. That’s one clear component of this ghost sample. If we have badly underestimated this, it could account for all of it, but that is not too likely as far as we can tell.
The b quark decays to muons offer another way to measure not only the pair production cross section but the “mixing” of b quarks. Due to a subtlety in the weak interaction that we need not explore here, a b quark (or more properly speaking a B hadron) flying along can change spontaneously into its own antiparticle. This can lead to events where, if both b quarks decay to muons, they can have the same electric charge if one b quark has flipped to the charge opposite that with which it was produced.
In the analysis presented in the paper, the authors (a group from Frascati/Harvard led by Paolo Giromini) have ostensibly resolved a long standing disagreement between the results from two different methods to measure the b pair cross section. If one selects events with two muons, but tighten the previously used requirements on where the muons come from, demanding that they emanate from near the main event vertex, they find much better agreement between the two different measurements of the b pair cross section. But this implies that the previous measurements suffered from a large, unaccounted-for background. What is it?
The paper is an exploration of this ghost background, and the conclusion is that we can’t explain it at this point. So the collaboration has published what we’ve learned so far, in hopes that other experiments, especially our neighbors around the ring at D0, can look at their data and tell us if they see this sort of thing too. This is the most important first question to be answered, and if they do see it, I can tell you what will happen: all hell will break loose in the field. If they don’t, well, we have more work to do in figuring out just how this ghost background comes about.
If this is the first observation of some sort of new physics, then it is tremendously exciting and very, very weird. Though, oddly, not entirely unanticiapted. Neal Weiner and Nima Arkani-Hamed have a recent paper out where they predicted “lepton jets” not unlike what we are seeing. Kind of makes the hair stand up on the back of your neck…but let’s not get too far ahead of ourselves just yet. I am sure the theorists are already very busy!
If you want more details, there is a very nice and much more physicist-oriented post by Tommaso Dorigo available at his blog.