The status of the search at LEP2 for the Higgs in the Standard Model (SM) and
in the minimal supersymmetric extension of the Standard Model MSSM) is
reviewed. A preliminary lower limit of 95.5/c^2 at 95% C.L. on the SM Higgs is
obtained after a preliminary analysis of the data collected at sqrt(s)= 189
GeV. For standard choices of MSSM parameter sets, the search for the neutral
Higgs bosons h and A leads to preliminary 95% C.L. exclusion lower limits of
83.5GeV/c^2 and 84.5 GeV/c^2, respectively.Comment: 6 page

Pepe-Altarelli, M.

English

arXiv

The status of the search at LEP2 for the Higgs in the Standard Model (SM) and in the minimal supersymmetric extension of the Standard Model MSSM) is reviewed. A preliminary lower limit of 95.5/c^2 at 95% C.L. on the SM Higgs is obtained after a preliminary analysis of the data collected at sqrt(s)= 189 GeV. For standard choices of MSSM parameter sets, the search for the neutral Higgs bosons h and A leads to preliminary 95% C.L. exclusion lower limits of 83.5GeV/c^2 and 84.5 GeV/c^2, respectively.The status of the search at LEP2 for the Higgs in the Standard Model (SM) and in the minimal supersymmetric extension of the Standard Model MSSM) is reviewed. A preliminary lower limit of 95.5/c^2 at 95% C.L. on the SM Higgs is obtained after a preliminary analysis of the data collected at sqrt(s)= 189 GeV. For standard choices of MSSM parameter sets, the search for the neutral Higgs bosons h and A leads to preliminary 95% C.L. exclusion lower limits of 83.5GeV/c^2 and 84.5 GeV/c^2, respectively

Pepe Altarelli, Monica

CERN Document Server

arXiv:hep-ex/9904006v1  7 Apr 1999HIGGS SEARCHES AND PROSPECTS FROM LEP2MONICA PEPE ALTARELLILaboratori Nazionali dell”INFN, Via E.Fermi 40, I-00044 FRASCATIand CERN, EP DivisionE-mail: monica.pepe.altarelli@cern.chThe status of the search at LEP2 for the Higgs in the standard model (SM) andin the minimal supersymmetric extension of the standard model (MSSM) is re-viewed. A preliminary lower limit of 95.5 GeV/c2 at 95% C.L. on the SM Higgs isobtained after a preliminary analysis of the data collected at√s = 189 GeV. Forstandard choices of MSSM parameter sets, the search for the neutral Higgs bosonsh and A leads to preliminary 95% C.L. exclusion lower limits of 83.5 GeV/c2 and84.5 GeV/c2 , respectively.1 IntroductionAfter reviewing the indirect information on the Higgs mass based on preciseelectroweak measurements performed at LEP1, SLD and at the TEVATRON,I will discuss the mechanisms of Higgs production and decay and the strategyadopted to search for the neutral Higgs boson (in the SM and in the MSSM) atLEP2 1. I will summarise the results based on the analysis of approximately170 pb−1 collected by each LEP experiment at√s = 189 GeV updated to themore recent Winter Conferences numbers 2. In the end I will briefly discussthe prospects for Higgs discovery at LEP2.2 Higgs mass from precision electroweak measurements andfrom theoretical argumentsThe aim of precision electroweak tests is to prove the SM beyond the treelevel plus pure QED and QCD corrections and to derive constraints on itsfundamental parameters. Through loop corrections, the SM predictions forthe electroweak observables depend on the top mass via terms of order GF/M2tand on the Higgs mass via logarithmic terms. Therefore from a comparison ofthe theoretical predictions 3, computed to a sufficient precision to match theexperimental capabilities and the data for the numerous observables whichhave been measured, the consistency of the theory is checked and constraintson MH are placed, once the measurement of Mt from the TEVATRON isinput. The present 95% C.L. upper limit on the Higgs mass in the SM is 4,2MH < 220GeV/c2 , (1)win99: submitted to World Scientific on February 11, 2014 1if one makes due allowance for unknown higher loop uncertainties in the anal-ysis. The corresponding central value is still rather imprecise:MH = 71+75−42 ± 5GeV/c2 . (2)The range given by Eq.1 may be compared with the one derived from the-oretical arguments 5. It is well known that in the SM with only one Higgsdoublet a lower limit on the Higgs mass MH can be derived from the require-ment of vacuum stability. This limit is a function of the energy scale Λ wherethe model breaks down and new physics appears. Similarly an upper boundon MH is obtained from the requirement that up to the scale Λ no Landaupole appears. If, for example, the SM has to remain valid up to the scaleΛ ≃MGUT, then it is required that 135 < MH < 180 GeV/c2.In the MSSM two Higgs doublets are introduced, in order to give massesto the up-type quarks on the one hand and to the down-type quarks andcharged leptons on the other. The Higgs particle spectrum therefore consistsof five physical states: two CP-even neutral scalars (h,A), one CP-odd neutralpseudo-scalar (A) and a charged Higgs boson pair (H±). Of these, h and Acould be detectable at LEP2 6. In fact, at tree-level h is predicted to be lighterthan the Z. However, radiative corrections to Mh7, which are proportional tothe fourth power of the top mass, shift the upper limit ofMh to approximately135 GeV/c2 , depending on the MSSM parameters.3 Higgs production and decayAt LEP2, the dominant mechanism for producing the standard model Higgsboson is the so-called Higgs-strahlung process e+e− → HZ 8,9, with smallercontributions from the WW and ZZ fusion processes leading to Hνeν¯e andHe+e− final states, respectively. A sizeable cross section (few 0.1 pb) is ob-tained up to MH ∼√s−MZ, so that an energy larger than 190 GeV is neededto extend the search above MH ≃MZ. For example the production cross sec-tion at√s = 189 GeV forMH = 95 GeV/c2 is 0.18 pb, which for an integratedluminosity L=170 pb−1/exp. gives 30 signal events per experiment.For the MSSM Higgs the main production mechanisms are the Higgs-strahlung process e+e− → hZ, as for the SM Higgs, and the associated pairproduction e+e− → hA 10. The corresponding cross sections may be writtenin terms of the SM Higgs-strahlung cross section, σSM, and of the cross sectionσSMννfor the process Z∗ → νν asσ(e+e− → Zh) = sin2(β − α)σSM (3)σ(e+e− → hA) ∝ cos2(β − α)σSMνν.win99: submitted to World Scientific on February 11, 2014 2The parameter tanβ gives the ratio of the vacuum expectation values of thetwo Higgs doublets and α is a mixing angle in the CP-even sector.The Higgs-strahlung hZ process occurs at large sin2(β − α), i.e., at smalltanβ. Conversely, at small sin2(β−α), i.e., at large tanβ, when hZ productiondies out, the associated hA production becomes the dominant mechanism withrates similar to the previous case. In this region the masses of h and A areapproximately equal.For masses below∼ 110 GeV/c2 , the SM Higgs decays into bb in approxi-mately 85% of the cases and into τ+τ− in approximately 8% of the cases. Sim-ilar branching ratios (BR) are expected for the MSSM Higgs bosons. AboveMH ∼ 135 GeV/c2 , the BR into W and Z pairs becomes dominant.4 Searches at LEP2While at LEP1 energies the signal to noise ratio was as small as 10−6 due tothe very high qq cross section, at LEP2 the signal to noise ratio is much morefavourable, increasing to ≃ 1%. In order to reduce this background, mainlydue to W pair production, qq (with two gluons or two additional photonsin the final state) and ZZ events, use is made of b-tagging techniques whichexploit the large BR of the Higgs into bb. For MH ≃ MZ, as is the case forthe expected experimental sensitivity, ZZ production represents an irreduciblesource of background since the Z decays into bb in 15% of the cases.The following event topologies are studied:i) The leptonic channel (Z→ e+e−, µ+µ−, H→ bb) which represents 7% ofthe Higgs-strahlung cross section. These events are characterised by twoenergetic leptons with an invariant mass close to MZ and a recoil massequal to MH. Because of the clear experimental signature, no b-tag isnecessary and therefore the signal efficiency is high, typically ∼ 75%.ii) The missing energy channel (Z→ νν, H→ bb) comprising ≃ 20% ofthe Higgs-strahlung cross section. This channel is characterised by amissing mass consistent with MZ and two b-jets. The selection efficiencyis ≃ 35%.iii) The four jet channel (Z→ qq, H→ bb) which is not as distinctive as thetwo previous topologies but compensates for this drawback with its largeBR of ≃ 64%. The efficiency for this channel is typically ≃ 40%.iv) The τ+τ−qq channel (Z→ τ+τ−, H→ qq and vice-versa) with a ≃ 9%BR. The event topology includes two hadronic jets and two oppositely-win99: submitted to World Scientific on February 11, 2014 3charged, low multiplicity jets due to neutrinos from the τ decays. Thesignal efficiency is of the order of 25%.The b-tagging algorithms are based on the long lifetime of weakly decay-ing b-hadrons, on jet shape variables such as charged multiplicity or boostedsphericity and on high pt leptons from semileptonic b decays. The b-jet iden-tification is improved by combining information from the different b-taggingalgorithms with tools like neural-networks and likelihoods. Typically, for a60% signal efficiency, the WW background, which has no b-content, is sup-pressed by a factor over 100, and the qq and ZZ backgrounds by approximatelya factor 10. With respect to the b-tagging algorithms developed for the mea-surement at LEP1 of Rb, the b fraction of Z hadronic decays, the performancesat LEP2 have improved by almost a factor of 2, due to vertex detectors withan extended solid angle coverage and to more efficient b-tagging techniques.All the analyses developed for the standard model Higgs produced via theHiggs-strahlung mechanism can be used with no modification for the super-symmetric case, provided that the Higgs decays to standard model particles(bb, τ+τ−). The results can then be reinterpreted in the MSSM context, bysimply rescaling the number of expected events by the factor sin2(β − α).For the pair production process, the signal consists of events with fourb-quark jets or a τ+τ− pair recoiling against a pair of b-quark jets.5 Results and prospectsTable 1 shows the number of selected events in the data for the SM Higgssearch, the expected number of background events and the expected numbersof signal events assuming MH = 95 GeV/c2 2,11,12,13,14.Table 1. Standard Model Higgs search. Number of observed events in the data nobs, ex-pected number of background events nback and expected numbers of signal events nsigassuming MH = 95 GeV/c2 for the four LEP experiments and for their combination. Alsoshown are the number of events observed and expected by the four experiments combinedin the mass window ∆MH = 92 − 96 GeV/c2 .nobs nback nsigALEPH 53 44.8 13.8DELPHI 26 31.3 10.1L3 30 30.3 9.9OPAL 50 43.9 12.6Total 159 150 46.4∆MH = 92 − 96 GeV/c2 47 37.5 24.6As can be observed from Table 1, an excess of events is observed bywin99: submitted to World Scientific on February 11, 2014 4Table 2. Observed 95% C.L. lower limits on MH. Also shown are the limits predicted bythe simulation if no signal were present.Observed Expectedlimit (GeV/c2 ) limit(GeV/c2 )ALEPH 90.2 95.7DELPHI 95.2 94.8L3 95.2 94.4OPAL 91.0 94.9ALEPH 11 and OPAL 14 which, in the case of OPAL, is concentrated inthe mass region around MH ≃ MZ, while for ALEPH it is distributed overhigher masses, typically ≥ 95 GeV/c2 . These results translate into the lowerlimits shown in Table 2, together with the sensitivity (expected limit) of eachexperiment.Table 3 shows the preliminary 95% C.L. lower limits on Mh and MA forthe four LEP experiments 2,11,12,13,14, as well as the derived excluded rangesof tanβ for both no mixing and maximal mixing in the scalar-top sector.In the years 1999 to 2000 LEP2 is expected to deliver a luminosity largerthan 200 pb−1 per experiment at a centre-of-mass energy eventually as highas ∼ 200 GeV. These data should allow to discover a SM Higgs of 107 GeV/c2or to exclude a Higgs lighter than ∼108 GeV/c2 15,16. This is a particularlyinteresting region to explore, given the present indication for a light Higgsfrom the standard model fit of the electroweak precision data. The sensitivityto the Higgs in the MSSM will reach ∼ 90 GeV/c2 for the high tanβ regionand ∼ 108 GeV/c2 for tanβ ≃ 1, therefore allowing good coverage of theMSSM plane.Table 3. Observed 95% C.L. lower limits on Mh and MA. Also shown are the derivedexcluded ranges of tan β. The mass limits are given for tan β > 1, except for those ofDELPHI, given for tan β > 0.5.Mh (GeV/c2 ) MA (GeV/c2 ) tan β tan βmax. mixing no mixingALEPH 80.8 81.2 - 1 < tan β < 2.2DELPHI 83.5 84.5 0.9 < tan β < 1.5 0.6 < tan β < 2.6L3 77.0 78.0 1. < tanβ < 1.5 1. < tanβ < 2.6OPAL 74.8 76.5 - 0.81 < tan β < 2.19win99: submitted to World Scientific on February 11, 2014 5AcknowledgmentsI would like to thank Cesareo Dominguez and Raul Viollier for their greathospitality and excellent organization of the Workshop and “Maestro” PatrickJanot for his precious advice and for carefully reading this manuscript.References1. For other more detailed reviews on the subject, see, e.g. P.Janot inPerspectives on Higgs Physics II, Advanced Series on Direction in HighEnergy Physic, Vol.17 (1997), ed. by G.L.Kane, 104;F.Richard, hep-ex/9810045, 28 Oct. 1998.2. M.Felcini, Rencontres de Moriond on ElectroWeak Interactions and Uni-fied Theories, 13-20 March 1999, Les Arcs, France.3. Reports of the working group on precision calculation for the Z resonance,ed. by D.Bardin, W.Hollik and G.Passarino, CERN Yellow Report 95-03,March 1995 and references therein.4. LEP EW Working Group, available athttp://www.cern.ch/LEPEWWG/plots/winter99/.5. See, e.g., T.Hambye, K.Riesselmann, DESY-97-152, Aug. 1997.6. See, e.g. Physics at LEP2, ed. by G.Altarelli, T.Sjostrand and F.Zwirner,CERN 96-01, Vol.1 (1996) 351.7. J.Ellis, G.Ridolfi and F.Zwirner, Phys.Lett. B257 (1991) 83; Phys.Lett.B262 (1991) 477.8. B.L.Ioffe and V.A.Khoze, Sov.J.Part.Nucl.9 (1978) 50.9. J.D.Bjorken, Proc. Summer Institute on particle Physics, SLAC Report198 (1976).10. J.F.Gunion and H.E.Haber, Nucl.Phys. B272 (1986) 1; Nucl.Phys.B278 (1986) 449 and Nucl.Phys. B307 (1988) 445.11. ALEPH Collaboration, Contribution to 1999 Winter Conferences,ALEPH 99-007, CONF 99-003, March 1999.12. DELPHI Collaboration, Contribution to 1999 Winter Conferences, DEL-PHI 99-8 CNF 208, March 1999.13. L3 Collaboration, Contributions to 1999 Winter Conferences, L3 Note2382, 12 March 1999; L3 Note 2383, 15 March 1999.14. OPAL Collaboration, Contribution to 1999 Winter Conferences, OPALPN382, March 12, 1999.15. E.Gross, A.L.Read and D.Lellouch, CERN-EP/98-094.16. P.Janot in Proceedings of the Workshop on LEP-SPS Performance, Cha-monix IX, Jan. 1999, 222.win99: submitted to World Scientific on February 11, 2014 6

Higgs Searches and Prospects from LEP2

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