Eur. Phys. J. C (2007) 49: 457-472
https://doi.org/10.1140/epjc/s10052-006-0170-x

Regular Article - Experimental Physics

Search for invisibly decaying Higgs bosons with large decay width using the OPAL detector at LEP

¹ School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, UK
² Dipartimento di Fisica dell’ Università di Bologna and INFN, 40126, Bologna, Italy
³ Physikalisches Institut, Universität Bonn, 53115, Bonn, Germany
⁴ Department of Physics, University of California, Riverside, CA, 92521, USA
⁵ Cavendish Laboratory, Cambridge, CB3 0HE, UK
⁶ Ottawa-Carleton Institute for Physics, Department of Physics, Carleton University, Ottawa, Ontario, K1S 5B6, Canada
⁷ CERN, European Organisation for Nuclear Research, 1211, Geneva 23, Switzerland
⁸ Enrico Fermi Institute and Department of Physics, University of Chicago, Chicago, IL, 60637, USA
⁹ Fakultät für Physik, Albert-Ludwigs-Universität Freiburg, 79104, Freiburg, Germany
¹⁰ Physikalisches Institut, Universität Heidelberg, 69120, Heidelberg, Germany
¹¹ Department of Physics, Indiana University, Bloomington, IN, 47405, USA
¹² Queen Mary and Westfield College, University of London, London, E1 4NS, UK
¹³ III Physikalisches Institut, Technische Hochschule Aachen, Sommerfeldstrasse 26–28, 52056, Aachen, Germany
¹⁴ University College London, London, WC1E 6BT, UK
¹⁵ Department of Physics, Schuster Laboratory, The University, Manchester, M13 9PL, UK
¹⁶ Department of Physics, University of Maryland, College Park, MD, 20742, USA
¹⁷ Laboratoire de Physique Nucléaire, Université de Montréal, Montréal, Québec, H3C 3J7, Canada
¹⁸ Department of Physics, University of Oregon, Eugene, OR, 97403, USA
¹⁹ CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK
²⁰ Department of Physics, Technion – Israel Institute of Technology, Haifa, 32000, Israel
²¹ Department of Physics and Astronomy, Tel Aviv University, Tel Aviv, 69978, Israel
²² International Centre for Elementary Particle Physics and Department of Physics, University of Tokyo, Tokyo, 113-0033, Japan
²³ Kobe University, Kobe, 657-8501, Japan
²⁴ Particle Physics Department, Weizmann Institute of Science, Rehovot, 76100, Israel
²⁵ Institut für Experimentalphysik , Universität Hamburg/DESY, Notkestrasse 85, 22607, Hamburg, Germany
²⁶ Department of Physics, University of Victoria, P O Box 3055, Victoria, BC, V8W 3P6, Canada
²⁷ Department of Physics, University of British Columbia, Vancouver, BC, V6T 1Z1, Canada
²⁸ Department of Physics, University of Alberta, Edmonton, AB, T6G 2J1, Canada
²⁹ Research Institute for Particle and Nuclear Physics, 1525, Budapest, PO Box 49, Hungary
³⁰ Institute of Nuclear Research, 4001, Debrecen, PO Box 51, Hungary
³¹ Sektion Physik, Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748, Garching, Germany
³² Max-Planck-Institute für Physik, Föhringer Ring 6, 80805, München, Germany
³³ Department of Physics, Yale University, New Haven, CT, 06520, USA
³⁴ TRIUMF, Vancouver, Canada V6T 2A3,
³⁵ University of Alberta, Edmonton, Alberta, Canada
³⁶ Institute of Nuclear Research, Debrecen, Hungary
³⁷ Institute of Physics, Academy of Sciences of the Czech Republic, 18221, Prague, Czech Republic
³⁸ Department of Experimental Physics, University of Debrecen, Debrecen, Hungary
³⁹ MPI München, München, Germany
⁴⁰ Research Institute for Particle and Nuclear Physics, Budapest, Hungary
⁴¹ Dept of Physics, University of Liverpool, Liverpool, L69 3BX, UK
⁴² Dept. Physics, University of Illinois at Urbana-Champaign, Urbana-Champaign, USA
⁴³ Manchester University, Manchester, M13 9PL, UK
⁴⁴ Dept of Physics and Astronomy, University of Kansas, Lawrence, KS, 66045, USA
⁴⁵ Dept of Physics, University of Toronto, Toronto, Canada
⁴⁶ Bergische Universität, Wuppertal, Germany
⁴⁷ University of Mining and Metallurgy, Cracow, Poland
⁴⁸ University of California, San Diego, USA
⁴⁹ The University of Melbourne, Victoria, Australia
⁵⁰ IPHE Université de Lausanne, 1015, Lausanne, Switzerland
⁵¹ IEKP Universität Karlsruhe, Karlsruhe, Germany
⁵² Physics Department, University of Antwerpen, 2610, Antwerpen, Belgium
⁵³ Technische Universität, Dresden, Germany
⁵⁴ High Energy Accelerator Research Organisation (KEK), Tsukuba, Ibaraki, Japan
⁵⁵ University of Pennsylvania, Philadelphia, Pennsylvania, USA
⁵⁶ TRIUMF, Vancouver, Canada
⁵⁷ CERN, Geneva 23, Switzerland
⁵⁸ DESY, Hamburg, Germany

^* e-mail: David.Plane@cern.ch

Received: 8 August 2006
Revised: 30 November 2006
Published online: 6 December 2006

Abstract

This paper describes a topological search for an invisibly decaying Higgs boson, H, produced via the Bjorken process (e⁺e^-→HZ). The analysis is based on data recorded using the OPAL detector at LEP at centre-of-mass energies from 183 to 209 GeV corresponding to a total integrated luminosity of 629 pb^-1. In the analysis only hadronic decays of the Z boson are considered. A scan over Higgs boson masses from 1 to 120 GeV and decay widths from 1 to 3000 GeV revealed no indication for a signal in the data. From a likelihood ratio of expected signal and standard model background we determine upper limits on cross-section times branching ratio to an invisible final state. For moderate Higgs boson decay widths, these range from about 0.07 pb (M_H=60 GeV) to 0.57 pb (M_H=114 GeV). For decay widths above 200 GeV the upper limits are of the order of 0.15 pb. The results can be interpreted in general scenarios predicting a large invisible decay width of the Higgs boson. As an example we interpret the results in the so-called stealthy Higgs scenario. The limits from this analysis exclude a large part of the parameter range of this scenario experimentally accessible at LEP 2.