One of the missions of the LEP program was the search for the Standard Model
Higgs Boson. The skillful operation of the machine in the year 2000, the final
year of operation, has allowed the four collaborations ALEPH, DELPHI, L3 and
OPAL to collect 536/pb of data at center-of-mass energies of 206 GeV or higher.
This data is used to probe the existence of the Higgs boson up to a mass of
around 115 GeV. Tantalizing candidates have been observed in excess over the
Standard Model predictions, but without enough statistical power to claim a
discovery. A Higgs boson lighter than 114.4 GeV is hence excluded at 95%
confidence level.Comment: 5 pages, 6 figures, to be published in the proceedings of the "5th
  International Conference on Hyperons, Charm and Beauty Hadrons

Acciarri

AndréG. Holzner

Barate

Englert

Grünewald

Guralnik

Higgs

Janot

English

arXiv

One of the missions of the LEP programme was the search for the Standard Model Higgs Boson. The skilful operation of the machine in the year 2000, the final year of operation, has allowed the four collaborations ALEPH, DELPHI, L3 and OPAL to collect 536/pb of data at center-of-mass energies of 206 GeV or higher. This data is used to probe the existence of the Higgs boson up to a mass of around 115 GeV. Tantalizing candidates have been observed in excess over the Standard Model predictions, but without enough statistical power to claim a discovery. A Higgs boson lighter than 114.4 GeV is hence excluded at 95% confidence level

Holzner, A G

CERN Document Server

1Search for the Standard Model Higgs Boson at LEPAndre G. HolzneraaInstitut fu¨r Teilchenphysik, ETH Ho¨nggerbergCH-8093 Zu¨rich, SwitzerlandEmail: Andre.Holzner@cern.chOne of the missions of the LEP program was the search for the Standard Model Higgs Boson. The skillfuloperation of the machine in the year 2000, the final year of operation, has allowed the four collaborations ALEPH,DELPHI, L3 and OPAL to collect 536 pb−1 of data at center-of-mass energies of 206 GeV or higher. This datais used to probe the existence of the Higgs boson up to a mass of around 115 GeV. Tantalizing candidates havebeen observed in excess over the Standard Model predictions, but without enough statistical power to claim adiscovery. A Higgs boson lighter than 114.4 GeV is hence excluded at 95% confidence level.1. IntroductionThe Standard Model (SM) of electroweak inter-actions is one of the most successful theories: Itdescribes the physics at the highest energy scalesknown today with unprecedented precision. How-ever, the SU(2) local gauge symmetry breaking(or equivalently, the existence of massive fermionsand gauge bosons) is still not understood. TheHiggs mechanism [1] is the most widely acceptedmechanism allowing for massive fundamental par-ticles while preserving the local gauge invarianceof the theory. It predicts a new scalar parti-cle, the Higgs boson. Its mass is essentially un-known, theoretical considerations give an upperlimit of about 800 GeV [2]. Recent ts of elec-troweak parameters to the data favor a light Higgs(mH < 193 GeV at 95% condence level) [3].2. Experimental Signatures and Event Se-lectionThe dominant production mechanisms at LEPare the Higgs strahlung e+e− ! HZ and the weakboson fusion (Fig. 1). The Higgs strahlung dia-gram is the dominant one up to Higgs masses ofaboutps−mZ where the fusion diagram becomesimportant. The cross section as function of theHiggs mass for a center-of-mass energy of 207 GeVis shown in Fig. 2 (left).The Higgs boson decays mainly into a pair ofthe heaviest particle allowed by kinematics. Thisis due to the fact that the couplings of particles tothe Higgs are the stronger the higher their mass.Fig. 2 (right) shows the branching ratios as func-tion of the Higgs mass. In the mass range acces-sible at LEP, the Higgs decays mainly into a pairof b-quarks, e.g. 74% at mH = 115 GeV.The search analyses are inspired by theHZ!bbZ nal state, although the fusion diagramand the interference with the Higgs strahlung di-agram and non-b decay modes are included inthe eciency calculation. The experimental sig-natures are determined by the Higgs and Z decaymodes. In the following paragraphs, the mainsearch channels are summarized.2.1. Four-jet channelThe high branching ratio Z ! qq of about 70%makes this channel the most abundant one. Themost important backgrounds are W pair produc-tion (  17pb) where one di-jet pair happens tohave a mass close to the Z mass and e+e− ! qq(  81pb) with hard gluon radiation.2.2. Jets + missing energy channelEvents in this channel are characterized bya missing mass close to the Z mass and twoacoplanar b-jets. The major backgrounds aree+e− ! qqe (  3:2pb) where one electron es-capes detection along the beam pipe and e+e− !W+W− ! qq or e+e− ! W+W− ! qq2e-e+Z*HZe-e+W,ZW,Zνe,e+νe,e-HFigure 1. The dominant Higgs production mechanisms in e+e− collisions at LEP energies: Higgs strahlung(left) and weak boson fusion (right). (GeV)Hm0 20 40 60 80 100 120cross section (pb)10-310-210-11Total cross sectionHZ onlyWW fusion only=207.0 GeVsZ-ms (GeV)Hm60 70 80 90 100 110 120Higgs branching ratio10-310-210-11 bbcc-τ+τ*W*W*Z*ZggFigure 2. Higgs production cross section at LEP for a center-of-mass energy of 207 GeV as function ofthe Higgs mass (left). The solid line shows the total cross section, the ne dashed line the cross sectionof the W fusion diagram and the coarse dashed line the cross section from the Higgs strahlung process.The arrow indicates the kinematic limitps − mH. The right plot shows the Higgs branching ratios asfunction of the Higgs mass. For masses reachable at LEP, the bb decay mode is dominant, while +−is the second most abundant. The cross sections and branching ratios were calculated using the HZHAgenerator [4].events where the lepton is not identied. Pairproduction of Z bosons (  1:3pb) and the pro-cess e+e− ! qq(γ) with undetected initial stateradiation of photons constitute further importantbackgrounds.2.3. Jets + leptons channelEvents in this channel have two leptons with adilepton invariant mass close to the Z mass andtwo b-jets. The relatively low branching ratio ofZ ! ‘+‘−, 3.4% per lepton flavor, makes this asmall statistics channel. However, the most im-portant background is Z pair production (whereone Z decays into quarks and the other decaysinto leptons) which also has a small cross section,thus making this signature extremely clean. Allfour experiments also include the case where theHiggs decays into a pair of  leptons and the Z3decays hadronically.Fig. 3 shows the reconstructed Higgs mass ofall LEP experiments and all search channels com-bined at a level of the selection where the numberof expected signal events above 109 GeV is abouttwice the number of expected background eventsin the same region. Note that these mass distri-butions are not used directly to draw quantitativeconclusions about the presence or absence of a sig-nal. This is done by combining the reconstructedmass with other event properties.012345670 20 40 60 80 100 120Reconstructed Mass mH [GeV/c2]Events / 3 GeV/c2√s– = 200-210 GeVLEP tightbackgroundhZ Signal(mh=115 GeV)allcnd= 18bgd= 13.97sgl= 2.9> 109 GeV41.212.21Figure 3. Higgs candidate mass (mrecH ) distribu-tion after tight cuts for the combination of allfour LEP experiments and all search channels [5].The light histogram represents the expected back-ground, the dark histogram is the signal (mH=115 GeV) on top of the background while the dotsare the data. The cuts were chosen such thatthe ratio of the number of expected signal andbackground events above mrecH = 109 GeV is ap-proximately 2:1 (2.21 signal and 1.21 backgroundevents where 4 events are observed in data).3. Statistical Method and QuantitativeResultsAfter a more or less tight event selection, usu-ally a final discriminant is constructed from sev-eral variables of which each shows some separa-tion power between signal and background. Thedistribution of this nal discriminant is used tocalculate the following likelihood ratio Q:Q(x) =L(xjs + b)L(xjb) (1)where L(xjs + b) is the likelihood of x underthe signal+background hypothesis (similarly forL(xjb)). If the nal variable distribution is binnedinto a histogram, L is the product of the corre-sponding Poisson probabilities over all histogrambins. The variable x usually represents the ob-served data but Q can also be calculated for theexpected background. Fig. 4 shows the evolutionof −2 lnQ as function of the Higgs mass hypoth-esis for the individual LEP experiments. TheALEPH result shows some excess, mainly fromthe four-jet channel, in the mass region around115 GeV, which is however not observed by theother experiments. Fig. 5 shows the combinationof all LEP experiments and search channels. Thedata are on the signal-like side of the backgroundbut are never more than 2 away from it.Condence levels are calculated by comparingthe data to a large number of Gedanken exper-iments, obtained from the expected signal andbackground nal discriminant distributions: CLbis the fraction of background experiments beingmore background like (Q < Qdata) while CLs+bis the fraction of signal+background experimentsbeing more background like than the data. Valuesof 1−CLb < 5:7  10−7(2:7  10−3) are interpretedas 5 discovery (3 evidence). Higgs masses forwhich CLs = CLs+b=CLb < 0:05 are excluded at95% condence level.The evolution of 1 − CLb as function of theHiggs mass hypothesis is shown in Fig. 6. Thevalues of 1− CLb at mH= 116 GeV for the com-bination and the individual experiments are givenin Table 1.4-10-50510152025100 102 104 106 108 110 112 114 116 118 120mH(GeV/c2)-2 ln(Q)ObservedExpected backgroundExpected signal + backgroundALEPH-10-50510152025100 102 104 106 108 110 112 114 116 118 120mH(GeV/c2)-2 ln(Q)ObservedExpected backgroundExpected signal + backgroundDELPHI-10-50510152025100 102 104 106 108 110 112 114 116 118 120mH(GeV/c2)-2 ln(Q)ObservedExpected backgroundExpected signal + backgroundL3-10-50510152025100 102 104 106 108 110 112 114 116 118 120mH(GeV/c2)-2 ln(Q)ObservedExpected backgroundExpected signal + backgroundOPALFigure 4. Evolution of−2 lnQ as function of the Higgs mass hypothesis of the individual LEP experimentsfor all search channels combined [5]. The solid line corresponds to the observed data, the dashed lineto the expected background and the dash-dotted line to the expected signal+background (for a slidingHiggs mass hypothesis). The dark and light bands indicate the 1 and 2 deviations from the background.4. SummaryA search for the Standard Model Higgs Bosonat LEP using 2461 pb−1 of data (at center-of-mass energies of 189 GeV and higher) was per-formed. In the highest energy data sample, theALEPH collaboration observed some highly sig-nicant candidates around a Higgs mass of 115GeV in the four-jet channel. The probability forthe background to generate such a fluctuation is0.24%. However, the other LEP experiments donot observe any candidates with a local signalover background ratio greater than one. A lowerbound on the Higgs mass is set at 114.4 GeV at95% condence level [5].All the results quoted here are preliminary,however since two of the four experiments pub-lished their nal analyses [6,7], one has submit-ted its nal publication [8] and one submitted itsnal results to conferences [9], the nal combinedLEP result is expected to be similar.REFERENCES1. P.W. Higgs, Phys. Lett. 12 (1964) 132; F.Englert and R. Brout, Phys. Rev. Lett. 13(1964) 321; G.S. Guralnik, C.R. Hagen, andT.W.B. Kibble, Phys. Rev. Lett. 13 (1964)585.2. T. Hambye and K. Riesselmann, SM Higgsmass bounds from theory, hep-ph/97084163. M. Gru¨newald, Electroweak Physics, talk5-10-50510152025100 102 104 106 108 110 112 114 116 118 120mH(GeV/c2)-2 ln(Q)ObservedExpected backgroundExpected signal + backgroundLEPFigure 5. Evolution of −2 lnQ as function ofthe Higgs mass hypothesis of all four LEP ex-periments and all search channels combined [5].The meaning of the lines and bands is the sameas in Fig. 4. The data is never more than twostandard deviations away from the backgroundover the mass range shown. The distance betweenthe background and signal+background lines is ameasure of the sensitivity (or separation powerbetween background and signal+background) ofthe analysis. The observation crosses the expec-tation from signal+background around 116 GeV,however the separation power is very small at thismass.given at ICHEP 2002, Amsterdam, July 24-31, 20024. P. Janot, The HZHA generator, in Physics atLEP2, CERN Report 96-01 (1996)5. The LEP Working Group for Higgs BosonSearches, Search for the Standard ModelHiggs Boson at LEP, LHWG Note/2002-016. ALEPH Collab., R. Barate et al., Phys. Lett.B526 (2002) 191.7. L3 Collab., M. Acciarri et al., Phys. Lett.B517 (2001) 319.8. OPAL Collab., OPAL Physics Report PR367(July 2002), to be submitted to Eur. Phys.Journal C.9. DELPHI Collab., DELPHI 2002-041 CONF575 (July 2002),10-510-410-310-210-11100 102 104 106 108 110 112 114 116 118 120mH(GeV/c2)1-CLb3σ2σ4σLEPObservedExpected for signal+backgroundExpected for backgroundFigure 6. Probability 1 − CLb as function ofthe Higgs mass hypothesis. The solid line cor-responds to the observed data, the dash-dottedline corresponds to the expectation from a signal(at each mass hypothesis) and the dashed hori-zontal line at 1 − CLb = 0:5 is the expectationfrom background. The solid horizontal lines indi-cate the probabilities for a 2,3 and 4 deviationfrom the background.1− CLbALEPH 0.00241DELPHI 0.874L3 0.348OPAL 0.543Four-jet 0.0570All but four-jet 0.368LEP 0.099Table 1Probability 1 − CLb at mH= 116 GeV for theindividual experiments, the LEP combined resultand the four-jet and all other channels [5]. Smallvalues indicate a deviation from the backgroundin the signal-like direction.

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