A Novel Higgs Discovery Channel
Jack Gunion and Radovan Dermisek have recently published papers on a novel Higgs scenario:
h → a a → 4 τ
In this case, the Higgs boson, h, has properties similar to the standard model Higgs boson, and the authors suppose a mass of about 100 GeV. The pseudoscalar Higgs boson, a, is much lighter: in order to escape bounds from LEP, it must decay predominantly to tau pairs. For details, see Many Light Higgs Bosons in the NMSSM (arXiv:0811.3537) and A light CP-odd Higgs boson and the muon anomalous magnetic moment (arXiv:0808.2509) and references therein.
Four-tau final states are certainly very rare in a hadron collider, and the kinematics of this channel should lead to a very unusual topology. If we take Mh = 100 GeV and Ma = 4 GeV, then each a boson would be highly boosted in the lab frame, and the tau pair would be highly collimated. In the special case that both taus decay leptonically, one has a pair of oppositely-charged, isolated leptons, possibly with different flavors (i.e., e+μ). Their invariant mass would be low, since Ma is low (by assumption) and there are four neutrinos in a double-leptonic tau pair decay. The transverse momentum of the tau pair, recognized as a lepton pair (ee or μμ or eμ) would be high, however, compared to standard model processes such as Drell-Yan production of tau pairs. Furthermore, there are two a Higgs bosons per event, so in an ideal case there would be four isolated leptons in a highly distinctive topology. The dream topology would be along the lines of two e+μ- pairs, each with high transverse momentum, on opposite sides of the detector, isolated. I can’t imagine there would be any significant background for such topologies, nor do I see any special reconstruction problems. My intuition says that the high pT of the a boson should render the leptons from the tau decays sufficiently energetic that they can be reconstructed and would provide an adequate trigger – but this requires a simulation to be sure.
What is the rate? Let’s consider the production mechanism with the highest cross section – gluon fusion. (For an excellent discussion, see a post by Tommaso Dorigo from May of this year.) At the Tevatron, the cross section is about 1.6 pb. So we might take σ×Br(h→aa) = 1 pb, which is pessimistic according to Gunion and Dermisek. This means that 4000 h→aa events have been produced in 4 fb-1 of CDF or D0 data. The leptonic branching ratio of τ leptons is about 0.18 (per species, ie, each for e and μ final states or 0.36 for both together). The number of produced events in various purely leptonic final states would be:
- 4 events as eeee
- 4 events as μμμμ
- 16 events as eμ on both sides
- 67 events as any allowed combination of e and μ
These numbers are not insubstantial, but is the acceptance and efficiency high enough?
Trigger efficiency: we would want two leptons with pT > 8 GeV or so. The typical lepton pT would be around 8 GeV, I believe: very roughly, pT,lept approx ⅓×pT,τ = ⅓×½×pT,a = ⅓×½×½×Mh = 8 GeV. The transverse momentum of the h is not insubstantial and will help push two of the leptons above threshold, so let’s say the trigger efficiency is 50%. It would be easy for at least one of the leptons to fall outside the acceptance, which I would guess is something like 20%. Finally, one would need to identify all four leptons, with at least some loose criteria including isolation, and maybe that costs another factor of ½. Throwing these factors around in a wild fashion, the acceptance × efficiency seems to be of order 5-10%. This means there is little hope for a large sample of “dream” events (eμ twice), but it is not crazy to imagine gathering a sample of, say, five events in all topologies, given that around 67 have been produced, hypothetically. Since this is a counting experiment, the search would succeed only if the number of expected background events is fewer than one, but I’ll bet that would be the case.
So, is this search viable? CDF and D0 have already published very nice papers searching for h → τ+ τ-, so the expertise is at hand… Any takers?
PS. Recent CDF papers using the tau final state to search for new particles include Search for Doubly Charged Higgs Bosons with Lepton-Flavor-Violating Decays involving Tau Leptons (arXiv:0808.2161) and Search for neutral MSSM Higgs bosons decaying to tau pairs in p-pbar collisions at sqrt(s)=1.96 TeV (arXiv:hep-ex/0508051), with more on the way…
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