CERN-LHC. Search for new neutral Higgs bosons through the process where a heavy scalar(pseudoscalar) decays to a Z boson and a pseudoscalar(scalar), itself decaying to a pair of b quarks/anti-quarks. Based on a sample of proton-proton collisions at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 35.9 fb−1. Upper limits at 95% confidence level are set on the production cross section times branching fraction, with masses of the resonances ranging up to 1000 GeV. The results are interpreted in the context of the two-Higgs-doublet model.
The main background processes, in decreasing order of importance, are Z boson production through the Drell–Yan (DY) process and top quark pair production where both top quarks decay leptonically (fully leptonic tt). Single top quark, diboson, triboson, ttV (V = W, Z), W+jets, and SM Higgs boson production are also considered. The contribution from quantum chromodynamics (QCD) multijet events with jets misidentified as leptons constitutes a negligible background after requiring a pair of well-identified leptons. The dominant contribution arises from the production of a Z boson in association with two b quark jets, which yields the same final state as the signal process when the Z boson decays leptonically.
Simulated background samples are generated at next-to-leading order (NLO) precision in QCD. In particular, the DY, ttV, W+jets, triboson, and part of the diboson background samples are produced with MadGraph5 amc@nlo version 188.8.131.52 with the FxFx procedure for NLO jet merging and MAD SPIN to properly propagate spin information in the matrix element of the process. The tt, single top quark, SM Higgs boson production, and the remaining diboson background samples are produced with POWHEG version 2.
Simulated signal samples of 207 different mass hypotheses are produced with mH and mA ranging from 120 to 1000 GeV and from 30 to 1000 GeV, respectively. These samples are produced using MadGraph5 amc@nlo version 2.3.2 interfaced with the parton shower generator PYTHIA 8.212. The parton distribution function (PDF) set used is NNPDF 3.0 at leading order (LO) in the four-flavour scheme, and the factorisation and renormalisation scales are estimated dynamically. The underlying event tune is CUEPT8M1, derived from the MONASH tune.
Jets are reconstructed using a particle flow (PF) technique.
PF candidates are clustered to form jets using the anti-kT clustering algorithm with a distance parameter of 0.4.
Region of interest is defined as:
- two leptons and two b-tagged jets
- (Pt_l1, Pt_l2) > (25GeV,15GeV) for the ee and mue channels
- (Pt_l1, Pt_l2) > (20GeV,10GeV) for the mumu channel
- (Pt_l1, Pt_l2) > (25GeV,10GeV) for the emu channel
- abs(eta_mu) < 2.4
- abs(eta_el) < 2.5
- pT_jet > 20GeV
- abs(eta_jet) < 2.4
- 70 GeV < m_ll < 110 GeV
- Pt_miss < 80 GeV