PDF《CERN-PH-EP-2011-228》

arXiv:1203.

1303v3 [hep-ex]

28 Jun

2012 CERN-PH-EP-2011-228 Submitted to Physical Review Letters Search for a light Higgs boson decaying to long-lived weakly-interacting particles in proton-proton collisions at √ s =

7 TeV with the ATLAS detector The ATLAS Collaboration A search for the decay of a light Higgs (120 ?

140 GeV) to a pair of weakly-interacting, long-lived particles in 1.94 fb?1 of proton-proton collisions at √ s =7 TeV recorded in

2011 by the ATLAS detector is presented. The search strategy requires that both long-lived particles decay inside the muon spectrometer. No excess of events is observed above the expected background and limits on the Higgs boson production times branching ratio to weakly-interacting, long-lived particles are derived as a function of the particle proper decay length. PACS numbers: 12.60.-i,14.80.Da A Higgs boson [1C3] below

140 GeV is particularly sen- sitive to new physics. Many extensions of the Standard Model (SM) include neutral, weakly-coupled particles that can be long-lived [4, 5] and to which the Higgs may decay. These long-lived particles occur in many models, including gauge-mediated extensions of the Minimal Su- persymmetric Standard Model (MSSM) [6], MSSM with R-parity violation [7], inelastic dark matter [8] and the Hidden Valley (HV) scenario [9]. This letter presents the ?rst ATLAS search for the Higgs decay, h0 → πvπv, to two identical neutral particles (πv) that have a displaced decay to fermion?anti-fermion pairs. As a benchmark we take an HV model [9] in which the SM is weakly coupled, by a heavy communicator par- ticle, to a hidden sector that includes a pseudoscalar, the πv. Due to the helicity suppression of pseudoscalar de- cays to low-mass ff pairs, the πv decays predominantly to heavy fermions, bb, cc and τ+ τ? in the ratio 85:5:8 %. The weak coupling between the two sectors leads the πv to have a long lifetime. Other, non-HV, models with the identical signature, where the πv is replaced with an- other weakly-interacting scalar or pseudoscalar particle, are discussed in Ref. [4, 10]. Both Tevatron experiments, CDF and D0, performed similar searches for displaced decays in their respective tracking volumes, which lim- ited the proper decay length range they could explore to a few hundred millimeters [11, 12]. In many of these beyond-the-SM scenarios, the life- time of the neutral states is not speci?ed and can have a very large range. The current search covers a range of expected proper decay lengths extending to about

20 m by exploiting the size and layout of the ATLAS muon spectrometer. Consequently the experimental challenge is to develop signature-driven triggers to select displaced decays throughout the ATLAS detector volume [13]. This analysis requires both πv decays to occur near the outer radius of the hadronic calorimeter (r ?

4 m) or in the muon spectrometer (MS). Such decays give a (η, φ) cluster of charged and neutral hadrons in the MS. Requiring both πv'

s to have this decay topology improves background rejection. The analysis uses spe- cialized tracking and vertex reconstruction algorithms, described below, to reconstruct vertices in the MS. The analysis strategy takes advantage of the kinematics of the gluon fusion production mechanism and subsequent two-body decay, h0 → πvπv, which results in events with back-to-back πv'

s, by requiring two well separated ver- tices (?R ≡ (?η)2 + (?φ)2 >

2) [14] in the MS. The data used in this analysis were collected in the ?rst half of

2011 with the LHC operating at