INTRODUCTION

The Standard Model of particle physics regroups our knowledge of the elementary particles and their interactions. Over the years, experimental data collected by various experiments in high energy physics has confirmed the predictions of the Standard Model to an outstanding precision. But, despite this success, the Standard Model fails to give an answer to the question “What is the origin of mass?”

The Standard Model relies on the process of spontaneous symmetry breaking (see right plot) to generate mass to the elementary particle. Without it, the elementary particle would indeed remain massless. The mechanism of spontaneous symmetry breaking is also involved in other area such as superconductivity. When applied to particle physics, it leads to the production of a scalar particle named the Higgs boson.

For many years, the Higgs boson has been searched by experiments based at CERN using the LEP collider as well as at FNAL with the TeVatron. The LEP program was terminated in 2001 without conclusive evidence for the Higgs boson but yield a lower limit on its mass of 114.4 GeV/c².  Since 2001, the TeVatron experiments are searching for a signal but a large amount of data is still required. The beginning of the LHC in 2007 will allow continuing the search.

Looking for the Higgs at LHC

Phenomenology in brief

The Higgs boson can be produced at the LHC through various processes depending on its mass. As It can be seen on Fig .2, the production is dominated by the gluon–gluon fusion over a large range of possible mass. The associated production processes such as qq→HW, qq→HZ, qq/gg→ttH as well as qq/gg→ tbH have much lower cross sections. The production through the vector boson fusion qq→qqH (10% of the gg →H cross section) is also important.

The Higgs boson has many decay channels which branching ratios also depend on its mass. The Fig .3 shows the branching fraction s of the Higgs boson as a function of its mass. For masses below 130 GeV/c², the Higgs decays mainly in a pair of b quarks. The decay into a pair of tau leptons is also important in that region of mass. The search in the 2 photons final state is relevant for a Higgs mass lower than a 150 GeV/c². For larger masses, the decay is almost entirely through the H→WW*/WW and H→ZZ*/ZZ processes.

Search strategy

A light Higgs (mass < 150 GeV/c²) can be searched in the 2 photons, 2 b quarks, 2 vector bosons (WW*/ZZ*) and 2 tau leptons decay channels. The Higgs decaying into 2 photons has the advantage of having a clean signature in the hadronic environment of the LHC where the excellent energy resolution of the CMS electromagnetic calorimeter can be exploited. The channel H→ZZ*→ 4 leptons has an even cleaner signature due to a lower jet background. The H→bb can only be exploited in the case of associated production due to large multi-jet background and the rather low mass resolution of the b-quark system. The vector boson fusion production is showing interesting perspective with the Higgs decaying into 2 tau leptons and WW* final state. Because of the absence of colour exchange in the central hard process, this production mechanism leads to a low jet activity in the central region of the detector which can be exploited. The significance corresponding to these channels can be seen on Fig .4.

In the case of a heavier Higgs, the WW*/WW and ZZ*/ZZ final state yield a very high sensitivity. The vector boson fusion production is large and can be fully exploited.

Figure 4: Expected statistical significance with 30 fb-1 for the Standard Model Higgs as a function of its mass. The Next To Leading (NLO) cross sections for both signal and background were used for the inclusive H to 2 photons, H decaying to ZZ*/ZZ going to 4 electrons as well as the Higgs decaying into WW*/WW going to leptons

INTERESTS of the group

The Imperial College group is actively taking part of the Higgs searches at CMS. The group has been working on key channels to look for the Standard Model Higgs, within the framework of the Minimal Supersymmetric (MSSM) extension as well as in the extra-dimension model. Here follows a list of the channels studied by the group:

Search for the Standard Model Higgs:

Search for the Higgs boson through the vector boson fusion production and in its tau leptons decay (lepton-jet). People: Alexandre Nikitenko and Costas Foudas.

Search for the Higgs within the MSSM:

Search for the A/H bosons in their tau leptons decay channel (the tau leptons decay hadronically). People: Alexandre Nikitenko, Konstantinos A. Petridis and Stuart Wakefield.

Search for the A Higgs boson in its decay into the light Higgs (h) and the Z gauge boson.

Search for the Higgs in Extradimension model:

Search for Radion decay into two Higgs bosons with the production of two photons and two b quarks in the final state within the Randall-Sundrum model for extra-dimension. People: Alexandre Nikitenko