Monday, 30 May 2011

Theorists vs. the CDF bump

Almost 2 months ago the CDF collaboration published their analysis of the events with exactly 2 jets, 1 lepton, and missing energy. These are vastly dominated by boring Standard Model processes where the W boson is produced together with jets and subsequently decays to an electron or a muon and a neutrino. A surprising feature showed up in the distribution of the invariant mass of the jet pairs. After subtracting the Standard Model background, CDF observed a bump near 150 GeV with a significance of 3.2 sigma. Obviously, theorists rushed to interpret the bump in term of physics beyond the standard model. The CDF result hints to a new particle with a mass of around 150 GeV, a significant coupling to the light quarks and a tiny coupling to leptons; the remaining details are left up to our imagination . Here is a selection of the educated guesses that appeared in about 50 papers to date.

The first thing that comes to mind is Z' - a new neutral gauge boson coupled to the left-handed quarks. This is a valid possibility provided Z' is leptophobic, that is to say, its coupling to electrons is less than about 0.05 to avoid constraints from the LEP experiment. There is some tension with the constraints from the UA2 experiment that was operating some 30 years before christ and made a search for a narrow Z' in the dijet channel. The UA2 limits on the Z'-quark coupling translate to a constraint on the W+Z' cross section at the Tevatron that allows one to explain only about 60 percent of the events observed by CDF. However, given the large uncertainties involved in the CDF measurement and in interpreting the UA2 results, the Z' option remains open. One should also note that nothing in the data tells us the new particle is a vector boson, it could just as well be a scalar.

To ease the UA2 constraints one can turn to another class of model. Quite generally, the Tevatron may produce a ≥ 250 GeV mother resonance who decays to a W boson and a 150 GeV daughter resonance. The latter subsequently decays to 2 jets who are observed by CDF. Several proposals for the mother and daughter exist: a technirho meson decaying to a technipion and a W in a version of technicolor, a sbottom decaying to a stop and a W in R-parity violating supersymmetry, a charged Higgs decaying to a neutral Higgs and a W in two-Higgs doublet models, a weak doublet color octet in the Manohar-Wise model, etc. The striking prediction of this class of models is that not only the invariant mass of the jets but also of the entire final state should display a resonance. CDF looked at the invariant mass of the 2 jets + lepton + missing energy vector and found it consistent with background only, but it is not clear if this excludes the presence of a mother resonance (the presence of the missing energy introduces larger systematic uncertainties than for the jet pair mass).

One can also imagine a more intricate class of models where the lepton and the missing energy in the CDF excess events come not from a usual W boson but from some other particle decaying to an electron and a neutrino. For example, this paper explains the excess by a production of a pair of supersymmetric winos of which one decays, via R-parity violation, to a charged lepton and a neutrino, and the other decays to 2 jets. This possibility may be excluded by analysis the distribution of the transverse mass of the lepton+missing energy subsystem.

Finally, one should mention those who are trying to spoil the party. From the very beginning many have cast doubts on the CDF analysis as it requires a perfect control over the overwhelming Standard Model backgrounds. One thing is that even the Standard Model W/Z peak in the observed jet mass spectrum, arising due to the well known contribution of the WW and WZ production processes, does not seem to be very well described by the simulations. Furthermore, by eye it seems that shifting the jet energy scale a few percent upwards, which would correspond to shifting the whole data curve to the right, allows one to get rid of the excess (the authors of the analysis reply that raising the jet energy scale makes additional events pass the analysis cuts, so that naive shifting of the curve is not correct; they say a 3 sigma excess persists even when the JES is scaled up by 7 percent). Another attempted explanation is that the apparent excess is in reality the Standard Model top quark. When a top quark decays hadronically, t → W b → jjb, the invariant mass of the 2 light jets of course peaks at the W boson mass of 80 GeV, however the invariant mass of the b-jet and one of the light jets has the distribution peaking near 150 GeV (the endpoint is Sqrt[mTop^2- mW^2] = 155 GeV), suspiciously close to the CDF bump. Thus, the excess may be due to the semileptonic t-tbar or single top production where one or more additional jets are missed at the detector, assuming the Monte Carlo simulations of that background have been (rather grossly) mismodeled.

So this is where we stand today. The situation may or may not be clarified when more data arrive. The updates from CDF and D0 are imminent. Someone will call a bluff? Or someone is holding an ace up his sleeve? Stay tuned for the next episode.


Anonymous said...

And some papers try to give one explanation to both ie 150GeV excess and top forward backward asymmetry. Could YOu comment on these?

Jester said...

I'm not aware of a model where the connection is natural. It's always two independent couplings, one responsible for the bump, and the other for the forward-backward asymmetry.

Jester said...

Oops, I forgot about Nelson et al. 1104.2030. In that case one could complain that the symmetry is topsy turvy, but this is a good example of what you're asking. Maybe there exist other examples I'm not aware of.