More details about the CDF bump

These days the next round of LHC results is beginning to emerge but for at least another week all eyes will be on the Tevatron. Recall that the CDF collaboration is observing an unexplained bumpy feature in the invariant mass spectrum of jet pairs in events with a W boson. Last Thursday CDF released an online note describing the latest update based on 7.3 fb-1 of data. The significance of the excess has increased to 4.1 sigma. The note describes a number of additional checks that the authors of the analysis have made to exclude background mismodeling as the origin of the bump. In particular, it seems that neither the standard model top quark, nor a difference of jet energy scales between quark and gluon jets can be responsible for the excess. Moreover, the excess persists (albeit a bit smaller) when a different Monte Carlo program is used to simulate the background. All in all, currently none of the known sources can explain the peak in a way consistent with all data. It must be a more subtle detector effect, or new physics.

Furthermore, the note presents a number of kinematic distributions of the events in the
window 115 < M_JJ < 175 GeV where the excess is the largest (thanks guys!). One plot makes you jump in your chair: It shows the invariant mass of the sum of the 4-vectors of the 2 jets, the lepton, and the neutrino, the latter reconstructed from the missing energy. If they all originate from one mother resonance, as suggested by a class of models, the invariant mass should reproduce that resonance mass. Indeed, the plot above shows a clear excess just below 300 GeV. This hints at another heavy particle being produced at the Tevatron, which then decays to a W boson and a 150 GeV particle who is directly responsible for the dijet bump. However, the plot on the right, which shows the same distribution but without subtracting the background, tells you that the standard model also peaks around 300 GeV (as a results of the imposed cuts) which makes the peak less trustworthy. If not for that, we would already be dancing on the streets and indulging in wild orgies.

It's also worth looking at another plot showing the transverse mass of the lepton + neutrino system. If the two come from a decay of a W boson, as tacitly assumed in the analysis, that distribution should have an endpoint at m_W = 80 GeV. Since most of the excess is below 80 GeV we can conclude that most of the times the 150 GeV resonance is accompanied by a genuine W boson. This excludes a more exotic class of models I mentioned where the lepton and the neutrino originate from the same particle that is responsible for the bump.

What's next? We are of course dying to hear the story from D0 and from the LHC experiments. The update from D0 is imminent. We expect it to be announced on June 10 at the Wine&Cheese seminar in Fermilab (as a general fact, wine facilitates communication between experiment and theory). As for the LHC, a blog post on Quantum Diaries points to an ATLAS note based on 33 pb-1 of data which roughly repeats the CDF analysis and finds no excess. However this means nothing: ATLAS simply had no right to see the hypothetical CDF bump in their 2010 data. First of all, the larger production cross section at the LHC (5 to 40 times, depending on the production mode) combined with the better efficiency does not make up for the 200 times smaller luminosity. Moreover, the W+jets background at the LHC is about 40 times larger. For these reasons you need a larger data set to make any conclusive statement. Suppose the CDF excess indeed originates from a 300 GeV mother resonance. If that resonance is produced by gluon-gluon collisions then the LHC should be able to see the excess already in 200 pb-1, which is the amount of data used in the most recent analyses. I'm sure hundreds of people are looking into this as we speak and as soon as the peak appears it will be pasted into Peter Woit's blog ;-) If, on the other hand, that resonance is produced in quark-antiquark collisions then we need to wait for 1 inverse femtobarn to see a significant excess at the LHC. In any case, the floor should be swept by the end of this summer, one way or another.