Searches for neutral Higgs bosons in e+e− collisions at centre-of-mass energies from 192 to 202 GeV
Introduction
Searches for neutral Higgs bosons of the Standard Model, its minimal supersymmetric extension (MSSM), and extensions allowing Higgs boson decays into invisible final states were performed using data collected by the ALEPH detector at LEP during 1999. The data sample was taken at four centre-of-mass energies, 191.6, 195.5, 199.5, and 201.6 GeV at which 28.9, 79.9, 86.3, and 41.9 pb−1 of data were collected, respectively. The total data sample corresponding to an integrated luminosity of was analysed to search for topologies arising from the e+e−→hZ Higgsstrahlung process supplemented by W and Z gauge boson fusion, and from the e+e−→hA associated pair-production process of the MSSM. The production cross section of the Higgsstrahlung process in the MSSM is reduced by a factor sin2(β−α), where tanβ is the ratio of the vacuum expectation values of the two Higgs doublets and α is the mixing angle in the CP-even Higgs sector. The hA production cross section is proportional to cos2(β−α). For an invisibly decaying Higgs boson, the observable rate of the Higgsstrahlung process can be expressed as ξ2σSM(e+e−→hZ), where ξ2 is the product of the branching ratio to invisible decays and a model-dependent factor which reduces the cross section with respect to that in the Standard Model.
Searches for neutral Higgs bosons with the ALEPH detector have already been carried out up to a centre-of-mass energy of 188.6GeV [1], [2]; no evidence of a signal was found. A lower limit at 95% confidence level (CL) was set at 92.9 GeV/c2 on the Standard Model Higgs boson mass. In the MSSM for the benchmark parameter set with maximal stop mixing [3], lower limits of 82.5 GeV/c2 and 82.6 GeV/c2 were derived on the masses of the h and A Higgs bosons, respectively. For an invisibly decaying Higgs boson, a lower limit of 95.4 GeV/c2 was set, for a production cross section equal to that in the Standard Model. Similar searches have been performed by the other LEP experiments [4].
The higher centre-of-mass energies and integrated luminosity in the 1999 data substantially increase the experimental sensitivity for the detection of Higgs bosons with respect to previous results. Nevertheless, the background processes are the same as those described in Refs. [1], [2]. The theoretical framework, the event selections, the study of systematic uncertainties, and the result extraction are therefore very similar to those previously described. The differences mainly consist of reoptimization of event selections, the introduction of new discriminating variables, and improvements related to the simulation of signal processes.
Section snippets
ALEPH detector
The components of the ALEPH detector that are most relevant for the analyses presented here are summarized in this section. A more detailed description of the detector can be found in Ref. [5] and its performance in Ref. [6].
Three coaxial tracking devices are located inside a solenoidal superconducting coil which produces an axial magnetic field of 1.5 T. The vertex detector (VDET) [7] consists of two cylindrical layers of silicon wafers situated at average radii of 6.3 and 11.0 cm. Charged
Signal and background simulation
For each of the four centre-of-mass energies, fully simulated samples of signal and background events were generated. The hzha program [12], used to generate the signal events, was also used to compute the signal cross sections, the Higgs boson decay branching fractions, and the radiative corrections to the Higgs boson masses in the MSSM. The most recent version contains, in particular, the latest refinements on these radiative corrections [3], [13], [14] and a more complete simulation of the he
Selection overlaps
Events selected by more than one analysis were treated with a well-defined prescription to remove overlaps. This procedure prevents any given event from affecting the confidence level calculations more than once. The potential overlaps of the searches for different topologies arising from the Higgsstrahlung process were suppressed by assigning a precedence order to the four selections, and by rejecting any candidate event already selected by a higher precedence search. The order of the
Conclusions
Searches for neutral Higgs bosons in e+e− collisions at centre-of-mass energies from 191.6 to 201.6GeV have been carried out with the ALEPH detector using an integrated luminosity of 237pb−1. The major event topologies expected from the hZ process for visibly as well as invisibly decaying Higgs bosons, the weak boson fusion process, and the hA process have been analysed. The data were combined with the 176pb−1 collected in 1998 at a centre-of-mass energy of 188.6 GeV.
A 95% CL lower limit on the
Acknowledgements
We wish to congratulate our colleagues from the accelerator divisions for the very successful operation of LEP at high energies. We are indebted to the engineers and technicians in all our institutions for their contribution to the excellent performance of ALEPH. Those of us from non-member countries thank CERN for its hospitality.
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