CMS pursues the Higgs boson

The CMS collaboration has announced its first results on the measurement of the W⁺W^– production cross-section and on the related search for the Higgs boson in proton–proton collisions at the LHC at 7 TeV in the centre-of-mass. This is the first paper by CMS that includes searches for the Higgs boson.

The data used for the analysis, recorded in the 2010 LHC proton runs, corresponds to an integrated luminosity of 35.5 ± 3.9 pb^–1. Each W is observed through its decay into a charged lepton (electron or muon) measured in CMS; the corresponding neutrino escapes undetected (figure 1). Thirteen candidates were reconstructed in the data, while a total background was estimated of 3.29 ± 0.45 (stat.) ± 1.09 (syst.) events. The production cross-section was measured to be 41.1±15.3 (stat.) ± 5.8 (syst.) ± 4.5 (lumi.) pb. This is in good agreement with the prediction from next-to-leading-order QCD calculations.

The result is of particular interest because it is highly sensitive to possible non-Standard Model contributions to the self-interactions of the W bosons, specifically the WWγ and WWZ triple gauge boson couplings. It is also an irreducible background in the decay of the Higgs boson to W⁺W^– .

The data from the first year’s LHC run are insufficient to be sensitive to the Standard Model Higgs; for that, at least ten times the number of collisions will be necessary. However, a possible extension of the Standard Model incorporates a fourth family of fermions with very large masses. The presence of this fourth family substantially increases the production cross-section of Higgs bosons via gluon fusion and also alters the decay branching fractions. Higgs production could be enhanced by a factor of about nine, such that Higgs particles could be observed in the 2010 CMS data sample.

To select H→ W⁺W^– candidates, the CMS analysis uses additional observables such as the transverse momentum of the leptons, the azimuthal angle difference between the dileptons, and the dilepton mass. The analysis found no evidence of the Higgs boson in the 2010 data sample. This enables the Higgs mass range of 144–207 GeV/c² to be excluded, at a 95% confidence level, for physics models that include a fourth generation of leptons and quarks. This constraint is more stringent for high masses than similar results from the Tevatron collider at Fermilab.