A 2.1 sigma excess of MSSM Higgs!

CDF recently released the results of a search for Higgs boson decays to tau pairs in one inverse femtobarns of Run II data.

The tau-tau final state of Higgs decay (h->tau tau) presents some interesting features: despite the smallish branching fraction (i.e. the probability of disintegrating into those particles: it is of the order of 10%), the good signature offered by taus allow us to search for direct production of neutral higgs bosons (gg->h), as opposed to the h->bb decay (branching fraction of the order of 80%) which is totally impossible to discriminate from the QCD background gg->bb unless one searches for associated production gg->bbh->bbbb, which displays the sufficiently rare signature of four b-jets.

The payoff is simple: direct production is much more frequent than associated production, and so the expected event rate is way larger. The figure on the left shows the expected cross section for producing a MSSM neutral higgs boson (either the h, or the H, or the A – the three neutral members of the company of five higgs bosons predicted by the so-called Minimal Super Symmetric Model) as a function of the particle mass, for a particular value of the tan(beta) parameter – a detail I won’t delve into discussing here. What one should bring home from this plot is that the cross section is large: for a 120 GeV MSSM Higgs boson the production rate could be as high as 50,000 events per inverse femtobarn of collected data!

I mentioned a “good” signature of tau leptons at hadron colliders, but that has to be taken cum granu salis: indeed, taus are much less clean objects than their lighter brothers electrons and muons, since the taus have a mass (1.77 GeV) which is large enough to allow the creation of light hadrons from their decay. What’s worse, the mass dictates a very short lifetime, with the result that taus typically travel less than a millimeter before disintegrating into electrons and neutrinos (relatively easy to see), muons and neutrinos(ditto), or hadrons and a neutrino -and in the latter case, they produce a narrow jet with few particles.

A jet is the most common thing you can be looking at when you smash protons against antiprotons: but if you collect events with an electron or a muon, and observe that opposite the triggering lepton there is  a narrow jet, containing only one or three charged tracks (the tau is charged, so two tracks is out of the question),  you may indeed be looking at the production of a tau-tau event. And QCD -the strong interaction which usually directs the course of actions during the hadronic collision- does not produce tau pairs at all! So if you really have two tau decays in your event, you can only be looking at an electroweak-produced object, such as a Z boson, or a virtual photon, or -hopefully- a higgs boson.

The analysis does not stop there. You need to reconstruct the mass of the object that produced the two taus. And since each tau decay has produced at least one energetic neutrino (through one of the following chains:  tau-> e nu_e nu_tau, or tau-> mu nu_mu nu_tau, or tau-> nu_tau + hadrons), you are in trouble. However, experimentalists are skilled at these kinds of problems, and indeed we have figured out how to reconstruct acceptably the mass of the originating body. 

In this plot you can see the reconstructed tau-tau mass for the selected events (black points with error bars), compared with the soup of expected background sources. You see in blue a prominent Z decay signal, the largest source of tau pairs, roughly peaking at a mass a bit smaller than the nominal Z mass value (91 GeV). The next largest background source, in light blue, is due to jet fakes, caused by QCD processes where jets mimic the tau signature on the jet as well as the lepton candidate opposite to it. Then there is a small contamination from other electroweak processes – such as W pair production, for instance. And finally, with some fantasy one can see an excess due to MSSM higgs bosons, sitting at about 160 GeV of reconstructed mass.

The significance of the bump at 160 GeV is not overwhelming – it is estimated at 2.1 sigma. However, it is suggestive. If you think that the overhyped LEP II excess of Standard Model Higgs boson candidates at 114 GeV ended up being a mere 1.7 sigma effect, there would be room for getting excited. But let’s keep our feet on the ground…

Indeed, the bump is most likely a fluctuation.  Unconfirmed voices claim that D0 sees a deficit of events where CDF sees an excess. However, let us dream for one minute.

If neutral higgs particles were the source of the intriguing excess in the plot above, they should be detectable in other ways. The most straightforward way to detect them is the already cited associated production gg->bbh->bbbb process. No recent blessed results are available from that search yet. Instead, let me dig out a plot from D0, showing their excess of Z->bb events. Remember, if the higgs is produced directly, it has a branching fraction to b-quark pairs, and should be present as an excess in a plot such as D0’s, albeit in modicum quantities.

Here you can see the recent D0 plot. Interesting! They have an excess at 160 GeV above QCD backgrounds (which are subtracted in this plot). I wonder if D0 is considering the use of that plot – or better, an updated, larger-statistics version of it – for setting limits on MSSM higgs bosons, or claiming a signal!

My question is not irrelevant, although slightly half-joking as is my usual style. Indeed, Julien and I are just about to bless our own Z->bb signal, based on half an inverse femtobarn of data. Of course I cannot show you the plot, but guess what ? We also see a hint at 160 GeV!

I got you curious, didn’t I ? You will have to wait for three more weeks to see the Z->bb plot by CDF, unfortunately!