The first results on searches performed by ALEPH on the 1999 data sample are
presented here. They are based on an integrated luminosity of about 54 pb$^-1$
collected at the two centre-of-mass energies of 192 and 196 GeV. Preliminary
results on searches for supersymmetric particles and for the neutral Higgs
bosons are shown.Comment: 3 page

Cerutti, F.

English

arXiv

The first results on searches performed by ALEPH on the 1999 data sample are presented here. They are based on an integrated luminosity of about 54 pb$^-1$ collected at the two centre-of-mass energies of 192 and 196 GeV. Preliminary results on searches for supersymmetric particles and for the neutral Higgs bosons are shown.The first results on searches performed by ALEPH on the 1999 data sample are presented here. They are based on an integrated luminosity of about 54 pb$^-1$ collected at the two centre-of-mass energies of 192 and 196 GeV. Preliminary results on searches for supersymmetric particles and for the neutral Higgs bosons are shown

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arXiv:hep-ex/9910031v1  15 Oct 1999Is there new physics in the 1999 ALEPHdata ?F. Cerutti11 CERN - EP Division 1211 Geneva 23 - CH and Laboratori Nazionali di FrascatiDell’INFN - ITE-mail: Fabio.Cerutti@cern.chAbstractThe first results on searches performed by ALEPH on the 1999 data sample arepresented here. They are based on an integrated luminosity of about 54 pb−1collected at the two centre-of-mass energies of 192 and 196 GeV. Preliminaryresults on searches for supersymmetric particles and for the neutral Higgs bosonsare shown.1. IntroductionThe LEP accelerator moved away from the Z peakin 1995 towards unexplored domains in high energyphysics. In this new phase of LEP the experimentalinterest of the four LEP collaborations has beenfocused, in addition to the precise measurements ofthe W properties, on the search for new phenomenawithin (the Higgs boson) and beyond the StandardModel (SM). Since then every year the performanceof LEP has gradually improved both in energy andin luminosity.In 1998 ALEPH has collected 175 pb−1 at acentre-of-mass energy of about 189 GeV. In 1999LEP has performed an excellent start up delivering,up to now, an integrated luminosity of about 54pb−1 almost equally shared between the two centre-of-mass energies of 192 and 196 GeV. This startup is the best one since LEP has been switched onand it seems reasonable to expect that this year themachine will be able to deliver more than 200 pb−1at centre-of-mass energies up to 200 GeV.In this paper the fresh results on searchesobtained by ALEPH by analysing this first bunchof data are given. The two main streams of theALEPH searches are updated starting from searchesfor Supersymmetric (SUSY) particles in the threepossible SUSY breaking scenarios, followed bythe update of the neutral Higgs bosons hunt forboth the SM and the Minimal Supersymmetricextension of the SM (MSSM) scenarios. Needlessto stress the preliminary content of this paper. Asgeneral comment all the limits given here refer to aconfidence level of 95%. A more detailed descriptionof the results reported in this paper is given in [1, 2].2. Search for SUSY.This section describes the update of the SUSY re-sults at 192-196 GeV in the three different sce-narios: gravity mediated MSSM, Gauge MediatedSusy Breaking (GMSB) and Supersymmetry withR-parity violating coupling (RPV). More details onthe ALEPH analyses on which these results arebased can be found in [3, 4, 5, 6]2.1. Search for SUSY in the MSSM scenario.In this update the SUSY searches can be dividedinto two categories: particles which have a smallcross section close to thresholds (this is the case forthe scalars, like sleptons and squarks) and particleswhich have a quite large cross section close tothreshold (this is the case of charginos when thesleptons are quite heavy).For the first category the new data, due tothe small additional luminosity, are not expectedto largely improve the sensitivity of the 189GeV sample reported in [3]. For both sleptonsand squark searches a good agreement betweenexpected background from the SM processes anddata candidates has been observed and new limitshave been obtained by combining all the data upto the maximal energy. As expected the updatedlower limits on the sleptons and squarks massesare not much different from the ones obtained at189 GeV. The improvement on the 189 GeV limitsranges between 0 and 1 GeV.The situation is quite different for the secondcategory where the increase in the centre-off-massenergy is expected to give a sizable improvementon the sensitivity to the gaugino masses. Forlarge slepton masses in absence of a signal, the2chargino exclusion goes up to the kinematic limit(i.e., half of the centre-of-mass energy) with fewinverse pb of collected luminosity, which means thatan improvement of about 3.5 GeV on the charginomass lower limit is expected. Both chargino andneutralino searches did not find any deviation fromthe SM expected background and new limits havebeen derived under the assumption of large sleptonmasses (m0 > 500 GeV): chargino masses lowerthan ∼98 GeV are excluded for a large fraction ofthe MSSM parameter space and the LSP mass lowerlimit is increased to about 34 GeV.2.2. Search for SUSY in the GMSB scenario.In the GMSB scenario the lightest neutralino isexpected to decay, mainly, into a photon plusa gravitino. If the lifetime associated to thisdecay is small the experimental topology consistsof two acoplanar and energetic photon. Thissearch has been updated and one candidate hasbeen observed to be compared with 0.7 backgroundevents expected from SM processes (mainly γνν¯.Another characteristic topology of GMSB are longlived sleptons. This analysis has been updatedon 192-196 GeV sample and no events survivedthe cuts, to be compared with about 0.2 expectedbackground events.2.3. Search for SUSY in the RPV scenario.In this scenario the LSP is expected to decay in SMparticles. In this case the experimental signaturedepends crucially on the type of the dominant RPVcoupling. Three main types of RPV couplings areexpected: LLE, LQD and UDD. In each searchonly one RPV coupling is assumed to be present.Searches for all the possible dominating couplingshave been updated and no deviation from theSM has been observed. Similarly to the R-parityconserving scenario only the chargino mass limitsare improved sensibly with respect to 189 GeVresults reaching the new kinematic limit of about98 GeV.3. Search for the neutral Higgs bosons.The searches for the SM and MSSM neutral Higgsbosons have been updated. The update of thesearch for the SM Higgs is of particular interestsince in 1998 data ALEPH has observed a slightexcess of Higgs candidates [7]. The probability ofsuch a fluctuation happening in the SM backgroundwas at least of the order of 4%, once the systematicuncertainties were taken into account. Moreoverthis small excess was more driven by low massTable 1. Expected SM background, Expected signalevents (Mh=95 GeV) and number of data candidatesfor the SM Higgs search at 192-196 GeV for the differentchannels and for the two main streams of ALEPHanalyses (cut based and neural network based).Channel N-bkg N-sig N-obshqq NN/Cut 8.5 / 6.6 4.6 / 3.7 7 / 5hνν NN/Cut 2.4 / 2.7 1.6 / 1.4 3 / 4hℓℓ Cut 5.5 0.8 7qqττ NN 1.7 0.4 1Total NN/Cut 18.1 / 16.1 7.4 / 6.3 18 / 17candidates than by “golden” high mass ones. Thebest Higgs mass hypothesis compatible with thisexcess was around 95 GeV. The combination withthe other LEP experiments did not clarify thesituation since also OPAL observed a slight excessof events [8].For a 95 GeV Higgs the production cross sectionwould increase from about 180 fb at 189 GeV toabout 320 fb at 196 GeV. The preliminary resultsat 192-196 GeV for the different topologies and forthe to main streams of the ALEPH analyses arelisted in table 3. A good agreement between theSM background and the data candidates is found.These data alone would allow a lower limit onthe Higgs mass of about 96 GeV. The use of a subsetof the full data sample is not correct; the mostsensitive sample must be used to derive the limitand this sample corresponds to the full set of datacollected from 189 to 196 GeV. With this samplea preliminary lower limit of 94.9 GeV on the SMHiggs mass is obtained (to be compared with anexpected one of 97.4 GeV).As an optimistic remark I want to mention thepresence in the 196 GeV data sample of a verynice hZ → bbℓℓ with a reconstructed Higgs mass ofabout 100 GeV. Unfortunately this nice candidateis also compatible with an off-shell ZZ(γ); morecandidates like this are welcome in the ongoingLEP2 run.In the MSSM the Higgs production and decayis very similar to the SM case for sin2(β − α) ∼ 1.For small sin2(β − α), hA associated productionbecomes relevant and final state topologies like 4b’sand bbττ are searched for. No excess of candidatesis found in both topologies. In the 4b’s onecandidate is observed with 1.5 expected backgroundand in the bbττ no candidates are observed with1.0 expected background. The exclusion in the mhtanβ plane has been updated and a new lower limit3of 83.8 GeV on the mass of lightest MSSM Higgshas been obtained (valid for tanβ > 1).4. Conclusions.The data collected by ALEPH in the first periodof the 1999 run has been analysed to search fornew physics. In SUSY no deviation from the SMhas been observed in the three different studiedscenarios (MSSM, GMSB and RPV). Due to thelow luminosity collected up to now (less than 1/3 ofthe 1998 run) only the chargino limits are sensiblyimproved: Mχ± ≥ 98 GeV for large m0.For the SM Higgs the excess observed in lastyear data sample has not been confirmed. A SMHiggs lighter then 94.9 GeV is excluded at 95% CLby the ALEPH data. The typical MSSM Higgstopologies have been updated with no deviationfrom the SM and a lower limit of 83.8 GeV on Mhhas been derived.I want to conclude with an optimistic remark:the LEP experiments are now exploring a veryinteresting region both for the SM Higgs (the actualpreferred value of the EW fit is below 100 GeV) andfor SUSY where the “natural” SUSY mass spectrumis expected to be close to the EW energy scale (i.e.,very close to the LEP2 energy scale). Up to knowwe didn’t succeed to enter into this new domain butthe chance that these last two years of LEP runningcould bring us there is still very high.References[1] The ALEPH collaboration, Search forSupersymmetric particles with the ALEPHdetector at√s ∼ 192− 196GeV.,ALEPH-CONF 99-050, HEP-EPS’99 Abstract7 425.[2] The ALEPH collaboration, Search for theneutral Higgs bosons at√s up to 196 GeV.ALEPH-CONF 99-047, HEP-EPS’99 Abstract7 408.[3] The ALEPH collaboration, Search for sleptonsand squarks at√s = 188.6 GeV.ALEPH-CONF 99-041, HEP-EPS’99 Abstract7 423.[4] The ALEPH collaboration, Search forCharginos and Neutralinos in e+e- Collisionsat√s = 188.6 GeV and Mass Limit for theLightest Neutralino. ALEPH-CONF 99-006,HEP-EPS’99 Abstract 7 415.[5] Search for Gauge Mediated SUSY Breakingtopologies at√s = 189 GeV. ALEPH-CONF99-002, HEP-EPS’99 Abstract 7 420.[6] Search for R-Parity violating decays of SUSYparticles at√s = 189 GeV. ALEPH-CONF99-007, HEP-EPS’99 Abstract 7 426.[7] The ALEPH collaboration, Search for theHiggs bosons of the Standard Model and toMSSM in e+e− collisions at√s = 188.6 GeV.ALEPH-CONF 99-053, HEP-EPS’99 Abstract7 428.[8] The ALEPH, DELPHI, OPAL and L3collaborations, Search for the Higgs bosons:preliminary combined results from the fourLEP experiments at√s ∼ 189 GeV.ALEPH-CONF 99-052, HEP-EPS’99.

Is there new physics in the 1999 ALEPH data ?

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