The Hadronic Tau Decay Signature of a Heavy Charged Higgs Boson at Lhc

The hadronic tau decay channel offers by far the best signature for heavy charged Higgs boson search at the LHC in the large tanβ region. By exploiting the distinct polarization of the tau and its large transverse mass, along with the accompanying missing–pT , one can probe for a charged Higgs boson up to a mass of about 600 GeV in an essentially background-free environment. The transverse mass distribution of the tau jet also provides a fairly unambiguous estimate of the charged Higgs boson mass. CERN-TH/99-136 May 1999 The charged Higgs boson carries the hallmark of a two Higgs doublet model and in particular the minimal supersymmetric Standard Model (MSSM). The two complex doublets correspond to eight scalar states, three of which are absorbed as Goldstone bosons. This leaves five physical particles – the two neutral scalars, a pseudoscalar and a pair of charged Higgs bosons H [1]. While one of these neutral Higgs bosons may be hard to distinguish from that of the Standard Model, the H carries a distinctive signature of the supersymmetric Higgs sector. Therefore the charged Higgs boson search is of considerable importance in probing the SUSY Higgs sector. The charged Higgs boson couplings to fermions are given, in the diagonal Cabibbo-KobayashiMaskawa matrix approximation, by L = g √ 2mW H {cot β mui ūidiL + tanβ mdi ūidiR + tanβ mli ν̄iliR}+H.C. , (1) where i denotes the generation index and tanβ represents the ratio of the vacuum expectation values of the two Higgs doublets. The QCD corrections are taken into account, in the leading log approximation, by substituting the quark mass parameters by their running masses evaluated at the H mass scale [2]. It is clear from (1) that, for a charged Higgs boson lighter than the top quark, H → τν (2) is the dominant decay mode over the large tanβ region. This is in striking constrast to the universal decay branching fractions of W into each fermion pair. This difference has been utilized to search for H in the top quark decay data of the CDF experiment in the τ channel at the Tevatron and obtain modest limits on the H mass in the large tanβ region [3]. The search can be extended up to mH ≃ mt and over a wider region of tanβ at the upgraded Tevatron and LHC, particularly by exploiting the opposite states of τ polarization coming from charged Higgs and W boson decays [4],[5]. But the search for a heavy charged Higgs boson, mH > mt, at a hadron collider is generally considered very hard, because in this case the dominant decay mode H → tb̄ , (3) suffers from a large QCD background. The dominant production process for a heavy H at the LHC is its associated production with a top quark via gluon-b quark fusion gb̄ → Ht̄ (4) or the related process of gluon-gluon fusion gg → Ht̄b . (5) Integrating out the kinematic variables of the final state b quark in the second case results in a similar size of cross-section (within a factor of 2) as the first. This is as expected since mb ≪ mt, mH . The prospect of a heavy H search at the LHC was considered in [6],[7] by