It seems that it's the season for Tevatron data analysis, as a set of papers is doing the rounds based on the full set of data from its most recent run (another run, its last, is in progress that will apparently double the total data). In March came word that there was an odd asymmetry in the production of top quarks that could be explained by a number of particles that have been limited to the realm of theory. Now there's been a paper that suggests the data might contain hints of the decay of something that nobody had predicted, a particle with a mass of about 144GeV that doesn't behave the way theorists predict the Higgs should.

The new work is described in a paper that has been placed on the arXiv preprint server; it has been submitted for peer review and publication, but hasn't passed these hurdles yet. The work builds on an earlier paper in which the data from the CDF detector was scanned to look for the production of the particles that carry the weak force, the W and Z bosons.

These are sometimes produced in pairs during the collisions, but neither of them live very long before decaying. As a result, the particles themselves don't reach the detectors; instead, their decay products do. But it is possible to trace the path of these more stable products back to their point of origin, and sum up all the energy they carry in order to figure out when they originated from the decay as a single particle, and how much that particle must weigh (we've done a detailed description of this process).

Since we know how much the W and Z boson weigh, we can figure out when jets of particles originated in a single location, and calculate the weight of their source, matching it to the W and Z bosons. If all goes well, there should be a peak in the data at the right energy to match these known particles. What the new paper does is repeat the whole process and try to determine if there are any bumps at energies that don't match up with the production of W and Z bosons.

This sounds simple, but spotting the events that may represent W/Z production isn't easy. The authors first looked for a lepton—either an electron or its heavier relative, the muon—traveling above a certain energy cutoff, which is the product of a W decay. To find a potential partner (a W, Z, or something more exotic), the authors looked for two jets of particles above an even higher energy threshold, produced when the partner decays. And then they had to get rid of events that produced two leptons, since those can be produced by many other processes.

But selecting the right events is only part of the work involved. Even though a lot of potential confounding events were eliminated during the selection, there are a lot of processes that produce signals that look like this, with a lepton and a pair of particle jets. As seen in the figure below, the authors used a variety of particle models to figure out how much of the signal would be produced by processes like quantum chromodynamics, top quark decay, and the decay of regular W/W and W/Z pairs. For the most part, the signal from the Tevatron matched up with that predicted by the models almost exactly.

But, if you look carefully at the area from about 120-160 GeV, a slight deviation from predictions is apparent. This becomes obvious when you subtract the predicted background from the signal. This clearly shows a bump at around 80GeV that is the product of W/W and W/Z pairs. But it also shows a similar bump centered at 144 GeV—and we don't know anything that should live there. Some models of the Higgs suggest that it might weigh in that range, but none of them produce the sort of decay pattern that the authors were examining.

How significant is this? The authors look to answer that question in a variety of ways, like summing the uncertainties in their models of background events and fitting Gaussian curves to random energies in their data. In the end, they conclude, "the probability to observe an excess larger than in the data is 7.6 × 10-4, corresponding to a significance of 3.2 standard deviations." Put another way, they state, "We find a statistically significant disagreement with current theoretical predictions."

How significant is that statistically significant disagreement? The authors looked at a number of ways to test significance, and the signal passed most of them, so it's clearly real in that sense. Whether it represents something real or a statistical fluke, however, will require more work. The easiest way to judge is by looking at the data from the CDF's partner, as the Dzero detector should have picked up an equivalent signal. Later this year, it should be possible to perform the analysis on far more data, which could make the bump vanish into the background. And, ultimately, the LHC will produce far more data.

But in the meantime, this would seem to be an open invitation for the theorists to get busy. Because until those other analyses weigh in on the probability of this signal being real, the best way of evaluating its significance is to have a plausible explanation for what might be causing it.