Potential for Higgs Physics at the LHC and Super-LHC

The Large Hadron Collider (LHC) at CERN and the two multipurpose detectors, Atlas and CMS, have been built in order to discover the Higgs boson, if it exists, and explore the theoretical landscape beyond the standard model [1, 2]. The LHC will collide protons with unprecedented center-of-mass energy ( √ s = 14 TeV) and luminosity (10 cms); the Atlas and CMS detectors will record these interactions with ∼10 individual electronic readouts per event. The first collisions are expected in 2007, with a possible luminosity upgrade around 2015. Observation of the Higgs boson is key to confirming the description of electroweak symmetry breaking in the standard model. The standard model Higgs sector has only one free parameter: the mass of the Higgs boson, mH . Masses below 114.4 GeV/c have been directly excluded by LEP Higgs searches at the 95% confidence-level [3]. Indirect evidence of the Higgs mass, through electroweak precision measurements, indicate a light Higgs (mH ∼ 185 GeV/c), though the theory remains valid until about 1 TeV/c [4]. For a variety of reasons, it is reasonable to expect that supersymmetry is manifest in nature. The minimal supersymmetric extension of the standard model (MSSM) requires an extended Higgs sector with two Higgs doublets, corresponding to five physically observable Higgs boson resonances. The MSSM Higgs sector is typically parametrized by the ratio of the vacuum expectation values of the two doublets, tanβ, and the mass of the neutral, CP-odd Higgs boson, mA. Large radiative corrections extend the upper-bound on the mass of the lightest Higgs from its Born-level value mZ to about 133 GeV/c . Explicit CP-violation in the MSSM complicates matters slightly [5]. At the LHC, production cross-section for the standard model Higgs bosons falls from about 30 pb near the LEP limit to less than 0.1 pb near mH ≈ 1 TeV. The production is dominated by the gluon-fusion process, followed by weak boson fusion, and associated production with weak bosons and heavy quarks. The decay is dominated by heavy lepton pairs (bb̄ and ττ), for masses less than 2mW , and pairs of weak bosons once above threshold [6]. Due to the enormous rate of QCD processes at the LHC, at least one high-pT lepton or photon or very large missing pT is needed to trigger the event. Furthermore, due to the high design luminosity of the LHC, an average of 23 soft p-p interactions are expected per bunch crossing – a phenomenon referred to as “pile-up” – which makes the LHC environment particularly challenging. This pile-up effect will be even more severe at the Super-LHC.