Elsevier

Physics Letters B

Volume 544, Issues 1–2, 19 September 2002, Pages 44-56
Physics Letters B

Search for associated production of massive states decaying into two photons in e+e annihilations at s=88–209 GeV

https://doi.org/10.1016/S0370-2693(02)02472-3Get rights and content

Abstract

A search is performed for production of short-lived particles in e+eXY, with Xγγ and Y→ff̄, for scalar X and scalar or vector Y. Model-independent limits in the range of 25–60 femtobarns are presented on σ(e+e→XY)×B(X→γγ)×B(Y→ff̄) for centre-of-mass energies in the range 205–207 GeV. The data from all LEP centre-of-mass energies 88–209 GeV are also interpreted in the context of fermiophobic Higgs boson models, for which a lower mass limit of 105.5 GeV is obtained for a “benchmark” fermiophobic Higgs boson.

Introduction

This Letter presents the results of two types of search for the production of a di-photon system recoiling from another massive scalar or vector object. The searches are sensitive to the processes e+eXY, with Xγγ and Y→ff̄, where ff̄ is a hadronic system (jets), a pair of charged leptons, or neutrinos resulting in missing energy. In the general search mode X must be a scalar, Y can be any scalar or vector particle of any mass, and both particles must be short-lived so that they decay close to the interaction point. The other search mode is referred to as the h0Z0 search; it requires Y to be a Z0 boson and is applied to data taken at all energies. The data used for these searches were recorded by the OPAL detector at centre-of-mass energies (Ecm) 88–209 GeV, the entire energy range achieved at LEP.

These searches are largely motivated by “fermiophobic” scenarios where one of the Higgs bosons decays primarily into a boson pair. In the fermiophobic interpretation, Y would be a Z0 and X a Higgs boson decaying into two photons. Indeed, the Higgs boson predicted in the Standard Model decays into two photons via a quark- or W-boson loop [1], but with a rate too low for observation of the process at LEP luminosities. Processes e+eh0Z0→γγff̄ at near-Standard Model production rate and having large di-photon branching ratios have been predicted in a number of alternative theories [2], [3], [4], [5], [6]; here h0 refers to the lightest neutral scalar boson where extended Higgs sector models are discussed. A particularly natural situation for fermiophobic Higgs bosons occurs in two Higgs doublet models (2HDM) [7] of “Type-I”, where one Higgs doublet couples only to bosons. Because there are different fermiophobic models, it is not possible to present search results for the entire parameter space of the various theories. In the present Letter a benchmark fermiophobic model is defined as having Standard Model production strength and a Higgs boson di-photon branching fraction calculated by turning off the fermion couplings to the Higgs boson in the Standard Model.

The OPAL Collaboration has presented searches similar to those reported here for LEP energies up to Ecm=189 GeV [8], [9], [10], [11]; this Letter extends those searches with the addition of data taken at Ecm=192–209 GeV. Fermiophobic Higgs boson searches have also been presented by other LEP collaborations [12], [13], [14] and by hadron collider experiments [15], [16]. To date, no evidence of a fermiophobic Higgs boson has been seen.

Section snippets

Data, simulated backgrounds and signals

The data used in this analysis were recorded using the OPAL detector [17] at LEP. The 1999 data consisted of 217.0±0.7 pb−1 collected at Ecm=192–202 GeV. The 2000 LEP data consisted of 211.1±0.8 pb−1 collected at Ecm=200–209 GeV, with the majority of the data taken at 205 and 207 GeV. The data sets are summarized in Table 1.

The backgrounds from Standard Model processes were modelled using Monte Carlo simulations at s=192, 196, 202, and 206 GeV for the 1999 and 2000 data. Simulated events were

Event selection

The analysis described in the following is identical to the one used in the Letter for OPAL data taken at 189 GeV [8]. Slightly different analysis cuts were used on the lower energy data sets, as described in the earlier publications [9], [10], [11].

For events to be selected as candidates they are required to have at least two photons recoiling from some other system decaying into one of the following three topologies:

  • (A)

    a qq̄ pair (“Hadronic Channel”), or

  • (B)

    one or two charged leptons (“Leptonic

Results

For the 1999 and 2000 data, the numbers of events passing the cuts are listed for the three recoil topologies in Table 2. There are no events in which more than one photon pair satisfying the cuts was found. The numbers of candidates passing cuts are generally in good agreement with the expected numbers of Standard Model backgrounds; this was also the case in earlier OPAL publications for the lower Ecm [8], [9], [10], [11]. The one noteworthy discrepancy is for cut (C1) in the missing energy

Conclusions

A search for the production of Higgs bosons and other new particles of narrow width and decaying to photon pairs is performed using e+e annihilation data with Ecm=192–209 GeV combined with 88–189 GeV data from previous OPAL searches. Model independent upper limits are obtained for Ecm∼206 GeV on σ(e+e→XY)×B(X→γγ)×B(Y→ff̄), where limits of 25–60 fb are obtained over 10<MX<180 GeV, for 10<MY<200 GeV and MX+MY>MZ. The limits are valid for Y either a scalar or vector particle, provided that the Y

Acknowledgements

We thank A.G. Akeroyd, L. Brücher, and R. Santos for helpful discussions. We particularly wish to thank the SL Division for the efficient operation of the LEP accelerator at all energies and for their close cooperation with our experimental group. In addition to the support staff at our own institutions we are pleased to acknowledge the Department of Energy, USA, National Science Foundation, USA, Particle Physics and Astronomy Research Council, UK, Natural Sciences and Engineering Research

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    1

    And at TRIUMF, Vancouver, Canada V6T 2A3.

    2

    And Royal Society University Research Fellow.

    7

    And Research Institute for Particle and Nuclear Physics, Budapest, Hungary.

    3

    And Institute of Nuclear Research, Debrecen, Hungary.

    12

    Now at University of Toronto, Department of Physics, Toronto, Canada.

    9

    And CERN, EP Div, 1211 Geneva 23.

    13

    Current address Bergische Universität, Wuppertal, Germany.

    10

    And Universitaire Instelling Antwerpen, Physics Department, B-2610 Antwerpen, Belgium.

    6

    And MPI München.

    8

    Now at University of Liverpool, Department of Physics, Liverpool L69 3BX, UK.

    4

    And Heisenberg Fellow.

    5

    And Department of Experimental Physics, Lajos Kossuth University, Debrecen, Hungary.

    14

    Now at Brookhaven National Laboratory, Upton, NY 11973, USA.

    11

    Now at University of Kansas, Department of Physics and Astronomy, Lawrence, KS 66045, USA.

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