Highlights at Beach 2002

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Search for invisibly-decaying Higgs Bosons at LEP
André G. Holzner L3 collaboration
HEP 2005 Europhysics Conference July 21-27
2005Lisboa, Portugal
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Overview

• Theoretical motivation
• Signal process
• Experimental signatures
• Cuts against Backgrounds
• Analysis Strategies
• Individual experiment's results
• LEP combination
• Summary and Outlook

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Theoretical motivation

• In some models, the Higgs can decay invisibly,
  e.g. into

• Neutralinos
• Fourth generation neutrinos
• neutrinos in extra dimension theories
• majorons
• general additional scalar gauge singlet

? Search for such decays in a model
independent way

• two signatures - two jets missing mass
  - two charged leptons missing mass

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Signal process

• if such a Higgs has to fulfill its duties
  (generation of mass through coupling), the
  Standard Model production mechanisms should be
  important

• searches assume standard Model production
  mechanism

? experimental signatures determined by Z
decay modes

• interpretations assume standard model cross
  section

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Experimental Signatures
BR
Topology
Channel
Z ? qq H ? invisible
70
10
Z ? ll H ? invisible
20
Z ? ?? H ? invisible
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Cuts against backgrounds
Leptonic channel
Hadronic channel

• two acoplanar jets / leptons
• require missing momentum into central region
• visible mass compatible with Z mass
• recoil mass is Higgs candidate mass

Essential cuts/techniques

Main (irreducible) backgrounds Note there are
more than these diagrams contributing to the same
final states !
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Analysis strategy I

• Preselection using simple variables, removing
  'easy' backgrounds
• Check data quality, further selection, more
  sophisticated variables
• Construction of a discriminating variable,
  hopefully close to the optimum
• Exclusion/Observation confidence levels from MC
  trials (frequentist probability)

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Analysis strategy II
Experiment Final discriminantHadronic Leptonic Final discriminantHadronic Leptonic Discriminant / Feature
ALEPH counting counting Neural network / Sliding mass window
DELPHI recoil mass recoil mass Iterative discriminant /Hadronic analysis split into low and high mass
L3 likelihood recoil mass Likelihood /Hadronic analysis split into low and high mass
OPAL recoil mass not (yet) included Likelihood / Hadronic analysis split into 2 jets and 3 jets
Opal has however published a generic search ee-
? ZS0 ? eeS0 / µµS0
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ALEPH
no evidence for a signal seen
PL B499 (2001) 53 / PL B526 (2002) 191
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DELPHI
hadronic channel
no evidence for a signal seen
EPJ C32 (2004) 475
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DELPHI

• Interpretation in a Majoron modelAdd a complex
  singlet field to the SM? after mixing, one
  has two Higgs bosons H and S

tan ß 10
mixing parameter
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L3
no evidence for a signal seen
PL B609 (2005) 35
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OPAL
2 jets selection
3 jets selection
no evidence for a signal seen
OPAL PN 472
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Indvidual experiments' results

• No Higgs signals foundLimits _at_ 95 CL

Experiment Limit (GeV) obs. exp. Limit (GeV) obs. exp. status reference
ALEPH 114.1 112.6 Published PL B526 (2002) 191
DELPHI 112.1 110.5 Published EPJ C32 (2004) 475
L3 112.3 111.6 Published PL B609 (2005) 35
OPAL 107.0 107.4 Preliminary OPAL PN 472
Delphi gives interpretation in the context of
Majoron models and MSSM modified mH-max scenario
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LEP combination (from 2001)
LEP Limit on relative production rate
Cross section exclusion
LEP preliminary
LEP preliminary
mH gt 114.4 GeV (95 CL)(partially based on
unpublished results)
no evidence of a signal seen in LEP combination
hep-ex/0107032
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Summary Outlook

• No hints of invisibly-decaying Higgs seen by any
  for the four LEP experiments
• ADL have published their results, final Opal
  results seem to come soon
• final LEP combinations (based on published
  results of all four LEP experiments) soon ? Will
  increase limit by few GeV

• Searches for invisible Higgs bosons at hadron
  colliders are difficult
• Continue search at the linear collider ?