Standard Model Higgs Searches at the LHC

The search for the Higgs boson, a key component in the Standard Model description of electroweak symmetry breaking, is a major goal of the physics program at the Large Hadron Collider (LHC) at CERN. Both ATLAS and CMS, experimental collaborations at LHC, have done extensive simulation and performance studies to optimize their strategies for this search. Some of these studies are discussed, as well as the resulting discovery potential (as a function of the Higgs boson mass), for the main experimental signatures that will be addressed by the two collaborations.


Introduction
The Higgs mechanism [1] is central to the Standard Model of particle physics (SM).The existence of the Higgs field maintains the theory weakly interacting up to high energy scales and prevents some processes from violating unitarity.In the process of spontaneous breaking of electroweak symmetry, the weak bosons W ± and Z 0 , as well as leptons and quarks, acquire mass through interactions with the Higgs field.In the SM, the simplest form of the Higgs mechanism is assumed, which predicts the existence of a single scalar particle, the Higgs boson and a single free parameter, its mass (m H ). The discovery of this particle would provide experimental evidence for the Higgs mechanism.The discovery and study of the mechanism of electroweak symmetry breaking is one of the main goals of the ATLAS [2] and CMS [3] experiments, operating at the Large Hadron Collider (LHC).Many models of physics beyond the SM predict a more complex Higgs sector, covered elsewhere [4].
Direct searches for the Higgs boson produced in association with a Z 0 were performed in the Large Electron Positron collider (LEP).These resulted in the exclusion of the Higgs boson in the mass range up to 114. 4 GeV at 95% confidence level [5].On the other hand, precision fits [6] of electroweak observables, including data from the LEP and Tevatron colliders, provide an indirect estimate of the Higgs boson mass, assuming the SM scenario.The latest fit results give m H = 84 +34 −26 GeV/c 2 , or the one-sided 95% confidence-level limit m H < 154 GeV.Including the LEP direct search results, this limit increases to 185 GeV/c 2 .Recent combined results from the Tevatron experiments have, for the first time, excluded the hypothesis of a Higgs boson mass around 170 GeV [7] at 95% confidence level.Although the expected sensitivity of Tevatron experiments is not enough to make a 5σ discovery of the SM Higgs boson [8], it is enough to exclude it out up to m H ∼ 200 GeV/c 2 at 95% confidence level, or to make a 3σ observation.

Higgs boson searches in ATLAS and CMS
The Higgs boson production at the LHC is dominated by the gluon-gluon fusion process, described at leading order through a heavy-quark loop.The next-to-leading order cross section for this process is 37.6 pb, for m H = 120 GeV/c 2 .The Higgs boson can also be produced by Vector Boson Fusion (VBF) with a cross section of 4.25 pb, or by associated production with a W ± , a Z 0 , or a t t quark pair, with 3.19 pb for the three processes and m H = 120 GeV/c 2 (cross sections calculated at next-to-leading order using parton density function sets CTEQ6M and CTEQ6L1 [9]).
The Higgs boson branching ratio is strongly dependent on its mass.At m H < ∼ 135 GeV/c 2 , the main decay mode is to a b b pair (BR = 81%), followed by the decay to a τ + τ − pair (BR ∼ 8%).For a small but important interval of m H , the Higgs boson decays to a pair of photons with a small branching ratio.At higher masses, the decay to a pair of (possibly off-shell) W ± or Z 0 bosons becomes dominant.
The most abundant signal topologies, containing b b pairs are unfortunately hard to separate from the large QCD background.The following gives a summary of the discovery channels being investigated at the LHC.In all cases, full detector simulation was used, which included realistic descriptions of the material budget and detector geometry.The trigger response was also realistically simulated, and systematic uncertainties were estimated.Next-to-leading order cross sections were used whenever available, and used to normalise simulated event samples.

Higgs boson decay to four leptons:
This channel provides excellent sensitivity for a wide range of m H above 130 GeV/c 2 , except for the interval between 2m W and 2m Z , where the Higgs boson branching ratio is dominated by H → W + W − .The main background is pp → ZZ ( * ) → 4l ± .Other backgrounds, such as Zb b, ZW , t t, and Z + X are effectively suppressed by the analysis event selection.Both CMS and ATLAS rely on selecting events which contain pairs of electrons or muons with opposite charge.At least one Z 0 is expected to be on mass shell, and the two-lepton invariant mass is used to reject fake and misidentified leptons.A clear four-lepton mass peak above a flat background is expected in this channel (see figure 1, left), thus allowing a good background estimation from the peak sidebands.Two factors are especially significant for analyses of this channel: the lepton reconstruction efficiency and the invariant-mass resolution.Both experiments show similar experimental sensitivities in this channel, with the total signal significance for each experiment expected to be in excess of 10 for an integrated luminosity of 30 f b −1 (see figure 1, centre).

Higgs boson decay to two photons: H → γγ
In spite of the low Higgs boson branching ratio to two photons (around 0.2% for m H = 120 GeV/c 2 ), this channel shows very good sensitivity in a range of m H between the LEP limit and m H ∼ 140 GeV/c 2 .The main irreducible background is the direct γγ production.Signal appears as a sharp peak over a smoothly falling di-photon invariant mass distribution.Reducible instrumental backgrounds such as γ + jet and multijet production, where one or more particles are misidentified as photons, are important due to the large QCD cross sections.Both experiments need to take photon conversions into account, since ∼ 60% of Higgs events contain at least one converted photon.
Both collaborations classify the selected events into several categories, according to the event kinematics or topology.In this way, the event selection can be optimized, in order to maximize the background discrimination without degrading the efficiency for selecting signal events.In ATLAS, a significance of about 3.5σ is expected for 10 f b −1 , assuming m H = 120 GeV/c 2 .With an optimized analysis employing a neural network, CMS expects up to 10σ after collecting 30 f b −1 and a 5σ after accumulating 8 f b −1 (see figure 1, right).Analyses are also being developed to target the associated production channels ZH, W H and t tH.

Higgs boson decay to a tau pair: H
This channel currently provides one of the best sensitivities at low m H . Analyses rely on the topology of VBF events to provide additional rejection against SM backgrounds.In this process, the Higgs boson is radiated by W ± or Z 0 bosons exchanged between the interacting partons.Due to the lack of colour flow between the two interacting partons, the characteristic event topology consists of two relatively forward jets with a rapidity gap in between, containing little hadronic activity.Both ATLAS and CMS investigate this channel for all final states, in which at least one tau decays to an electron or muon plus neutrinos.The final state where both taus decay hadronically was also investigated by ATLAS and proved to be feasible, but there is as yet no detailed sensitivity estimates.The dominant background in all cases is Z + jets with the Z boson decaying to τ + τ − .Both collaborations have developed data-driven methods to estimate these backgrounds.Other backgrounds are W + N jets, t t, and di-jet events.
The τ + τ − invariant mass reconstruction requires an approximation, in which the τ ± is assumed to be collinear with the visible lepton (in τ → e or τ → µ decays).The resulting Higgs mass resolution can be observed in figure 2 (left).The expected sensitivity for this channel, currently reaches up to 5σ for 30 f b −1 .

Higgs boson decay to a W boson pair: H
The Higgs boson decay to a W + W − pair provides the most sensitive search channel in the mass range 2m W < m H < 2m Z , where this decay mode has a branching ratio above 95%.Analyses have concentrated on final states containing electrons or muons from the W ± decay.Contrary to the remaining channels, though, the presence of high transverse momentum neutrinos makes it unfeasible to obtain a Higgs mass peak.The transverse mass [10] can still be calculated and used in the event selection (see figure 2, right).Analyses in this channel then need to rely on a very good knowledge of the background shape and normalisation.
The dominant backgrounds come from events containing a W-boson pair, most importantly W + W − and t t production.The H → W + W − decay creates a correlation between the W spins in the Higgs reference frame which translates into an angular correlation between the leptons emitted in the W-bosons decay.This is exploited to suppress the W + W − background.The t t background can be effectively suppressed by a veto on central jets (jets with a small pseudorapidity value).Separate analyses are performed for the cases where there are no high transverse momentum jets present in the event and where there are two additional jets, directed at the gluon fusion and the VBF production modes, respectively.For a Higgs boson mass close 160 GeV/c 2 a sensitivity in excess of 10σ is expected for each experiment for 30 f b −1 of integrated luminosity.Additional channels being explored are the VBF H → W + W − → lν + 2jets and the associated production channels W ± H and t tH.

Summary and Outlook
The expected sensitivity of ATLAS and CMS to a SM Higgs boson was evaluated by both collaborations.Analyses in both experiments show similar sensitivity in most channels.The sensitivity for SM Higgs discovery in the various channels is illustrated by figure 3 for the CMS case.An integrated luminosity of less than 10 f b −1 should be enough to make a 5σ discovery in the mass range above the LEP limit.Preliminary ATLAS studies indicate that an integrated  ∼ m H < ∼ 460GeV/c 2 at 95% confidence level.Other analyses, not described here, may contribute to further enhancements of the sensitivity.

Figure 3 :
Figure 3: SM Higgs discovery potential for CMS assuming an integrated luminosty of 30 f b −1 (left) and integrated luminosity needed for a 5σ Higgs discovery, versus Higgs boson mass [10]).

luminosity of 2
f b −1 would be enough to exclude the Higgs boson in the range 121GeV/c 2 <