Searches for new physics in the first year of running of LEP2, at energies of
161 and 172 GeV, are summarized. After a short review of WW results and their
implications on new physics, searches for the Higgs boson and SUSY particles,
analyses of four-jet final states and constraints on possible explanations for
the HERA high Q**2 anomaly are discussed in turn.Comment: LaTeX, 15 pages, 7 eps figures, needs iopconf1.sty, included. To
  appear in the proceedings of "Beyond the Desert", Castle Ringberg, Tegernsee,
  Germany, 8-14 June 199

Miquel, Ramon

English

arXiv

Searches for new physics in the first year of running of LEP2, at energies of 161 and 172 GeV, are summarized. After a short review of WW results and their implications on new physics, searches for the Higgs boson and SUSY particles, analyses of four-jet final states and constraints on possible explanations for the HERA high Q**2 anomaly are discussed in turn

Miquel, R

CERN Document Server

New physics at LEP2 1Ramon Miquel 2Dept. ECM and IFAEUniv. de Barcelona, Facultat de FsicaDiagonal 647, E-08028 Barcelona (Spain)Abstract. Searches for new physics in the rst year of run-ning of LEP2, at energies of 161 and 172 GeV, are summarized.After a short review of WW results and their implications onnew physics, searches for the Higgs boson and SUSY particles,analyses of four-jet nal states and constraints on possible ex-planations for the HERA high Q2 anomaly are discussed in turn.1. IntroductionThe second phase of LEP, LEP2, designed to run at energies above the W-pair production threshold started operation in 1996, with two short runs,one at a center-of-mass energy of 161 GeV and another at about 172 GeV.The four LEP experiments, ALEPH, DELPHI, L3 and OPAL, collectedabout 10 pb−1 of luminosity each in each of the two runs.The main purpose of the 161 GeV run was to measure the W-pairproduction cross section very close to threshold. This cross section is verysensitive to the value of the W mass and it provides a measurement withgood precision. Searches for new particles were also performed at thisenergy, but they have already been superseded by the searches done at172 GeV soon after.In the 172 GeV run, the mass of the W gauge boson was determineddirectly, through the measurement of the invariant mass of its decay prod-ucts. Also a rst look at the structure of the trilinear gauge-boson vertices1 To appear in the proceedings of \Beyond the Desert", Castle Ringberg, Tegernsee,Germany, 8-14 June 1997.2 E-mail address: miquel@ecm.ub.es1was attempted. A short summary of W physics in the rst year of LEP2and its implications on limits for new physics can be found in section 2.Results from the searches in the 172 GeV run for Higgs bosons (bothfrom the Standard Model and from Supersymmetry models) and for super-symmetric particles are summarized in sections 3 and 4, respectively.In the 1995 LEP run at a center-of-mass energy about 130 GeV, theALEPH collaboration reported an excess of four-jet events with sum ofjet-pair invariant masses close to 105 GeV [1]. The analyses of ALEPHand the other LEP collaborations on this topic both in the 161 GeV runand in the 172 GeV run are discussed in section 5.The reports of the H1 and ZEUS collaborations at HERA hinting atpossible new physics in electron-quark interactions [2] has prompted severalanalyses of electron-positron anihilation into quarks that are sensitive tosome types of new physics that could explain the HERA events. Section 6summarizes the status after analysing the 1996 data.Finally, section 7 contains a summary of the talk. It should be notedthat, unless specied otherwise, all results are to be understood as stillpreliminary.2. W physicsThe precise determination of the W mass serves as a stringent test of theStandard Model, since it can be predicted from the known values of theother parameters of the Standard Model. The only large uncertainty comesfrom the lack of knowledge of the Higgs boson mass, and therefore, mea-suring MW precisely enough, one can get information on the value of MH .The measurement of the W cross section close to the W-pair productionthreshold provides a clean and precise way to determine the W mass, sincethe phase-space factors appearing in the cross section expression dependstrongly on the W mass.All four LEP experiments have already published their nal results onthe WW cross section production at 161 GeV [3]. The combined value forthe WW cross section is [4]WW (161:3 GeV) = (3:69 0:45) pb :The error is dominated by statistics. From the total WW cross section atthreshold, the W mass is obtained by using a Standard Model calculationthat relates the WW cross section near threshold to its mass. The resultingW mass is:MW (161 run) =(80:40+0:22−0:21  0:03GeV ;2where the last error reflects the current uncertainty on the LEP beamenergy.Above the WW threshold, the most ecient way of determining the Wmass is by measuring the invariant mass distribution of its decay products,either a charged lepton and a neutrino or a pair of jets. The detectorresolution being far too poor to obtain a reasonably narrow invariant massdistribution, energy and momentum conservation have to be imposed in allchannels to improve the resolution.The preliminary determinations of the four experiments agree very wellwith each other and the combined value reads [4]MW (172 run) = (80:37 0:18exp  0:05theo  0:03beam) GeV :The dominant error includes statistical and purely experimental errors, thesecond one is the estimate of the uncertainty due to soft QCD eects andthe third comes from the beam energy uncertainty.The two numbers can be combined [4] to give the preliminary W massresult from the rst year of running of LEP2:MW = (80:38 0:14) GeV :This value is compared in g. 1 with the other measurements of MW , bothdirect at the Tevatron and indirect at LEP1 and SLD.The agreement of all measurements is perfect. The current LEP2 erroris already quite good but still much larger than the 40 MeV uncertaintyin the indirect measurement. To get a direct determination of MW withthis sort of accuracy and to compare it against the indirect measurement,which assumes the Minimal Standard Model, is the next challenge for bothLEP2 and the Tevatron.The other important topic of WW physics at LEP2 is the search forpossible anomalous couplings between a pair of Ws and a Z or a pho-ton. However, these studies require the highest possible energy as well assubstantial amount of integrated luminosity. The results available so far,which include only the semileptonic WW decays after the 172 GeV run,only improve marginally on the limits obtained previously at the Tevatron.3. Higgs searchThe main production process for the Standard Model Higgs at LEP2 is itsproduction associated with an on-shell Z. The cross section goes down toa fraction of a picobarn once the Higgs mass reaches aboutps− 100 GeV.This is, more or less, the discovery limit for a xed center-of-mass energy.Since the LEP1 limit stands at about 65 GeV, it is clear that the 161 GeV3W-Boson Mass  [GeV]mW  [GeV]χ2/DoF: 0.0 / 180.0 80.2 80.4 80.6 80.8pp-colliders 80.37 ± 0.10LEP2 80.38 ± 0.14Average(world) 80.37 ± 0.08LEP1/SLD 80.323 ± 0.042State: m97Figure 1. Comparison of the direct W mass measurements at the Tevatronand LEP2 and the indirect determination using LEP1 and SLD data.run was not useful for Standard Model Higgs search, while the 172 GeVrun started to extend the search region.The main decay channel for a Higgs boson with mass around 70 GeV isto a pair of b quarks. Therefore, according to the Z branching ratios, thenal state ZH will consist 70% of the times of four jets, two of them bs;20% of the times of two b jets and missing energy; and 10% of the timesof two b jets and two charged leptons. The four-jet channel is both themost aboundant and the most dicult to separate from the background.Good b-tagging capabilities are mandatory and all the LEP experimentshave invested recently in new and more powerful silicon vertex detectors.Typical eciencies in the four-jet channel are around 30% with very lowbackground contamination. No experiment has found any evidence foran excess in any of the channels that have been searched. Typical 95%4Actual CLAverage CLLEP2 onlya) L31-CL = 5 %MH (GeV)1-CLb)95 % C.L. Line69.5 GeVL3MH (GeV)Expected events10-310-210-1160 65 70 75 8005101560 65 70 75 80Figure 2. a) Exclusion condence level as a function of the Higgs massfor L3. b) Expected number of Standard Model Higgs events in L3 as afunction of the Higgs mass. The arrow points at the mass excluded at the95% condence limit.5condence level limits on the mass of the Standard Model Higgs bosonstand at around 70{71 GeV [5]. Figure 2 shows the number of Higgs bosonsexpected by L3 as a function of MH , and the limit taking into account theobserved candidates (compatible with background expectations).In all Supersymmetry models at least two Higgs doublets appear, giv-ing rise to ve physical states: two charged Higgses, two neutral CP-evenHigges (h;H) and a neutral CP-odd Higgs (A). In most SUSY theoriesthere is an upper limit to the mass of the lightest CP-even neutral Higgs,h, which is around 150 GeV [6]. Furthermore, in many models, its mass isbelow 100 GeV, suitable for its search at LEP2.The lightest CP-even Higgs can be produced in association with a Z,in a process very similar to its Standard Model counterpart, or in associa-tion with the CP-odd state, A. Both processes are complementary, in thesense that in the regions of the SUSY parameter space in which one crosssection is small the other is large and viceversa, so that the overall rate ofh production remains sizable in all regions with Mh;MA < 60− 65 GeV.Since all neutral Higgses decay predominantly to b jets, the channel hAinvolves identifying a four-jet event with four bs in the nal state. As inthe case of the Standard Model Higgs, no excess has been found [5] in anychannel. Absolute 95% condence level mass limits on both h and A havebeen set at about 62.5 GeV for all values of tan  1, where tan is theratio of the vacuum expectation value of the Higgs doublet that gives massto up-type particles to that of the doublet that gives mass to down-typeparticles. Figure 3 shows the region in the plane MA − tan excluded bythe DELPHI collaboration.4. Supersymmetry searchesSupersymmetry models not only predict more fundamental scalars. Theypredict for every particle in the Standard Model another particle withspin diering by 1=2, the supersymmetric partner. The partners of thecharged Higgses and Ws are generally called charginos, while the partnersof the neutral Higgses and neutral electroweak vector bosons are knownas neutralinos. Charginos and neutralinos are expected to be the lightestSUSY particles. In particular, the lightest neutralino, , is supposed to bethe lightest supersymmetric particle (LSP) in most models with gravity-mediated SUSY breaking.Searches have been performed for chargino pairs (+−), neutralinopairs (0), where 0 is the second lightest neutralino, scalar-lepton pairs(~l+~l−), scalar-top pairs (~t1~t1), etc. If R-parity is conserved, then the LSP,assumed to be , is stable and does not interact in the detector, so that61Figure 3. Regions of the MA{tan region excluded by the DELPHIcollaboration under several assumptions.7the main experimental signature for the production of SUSY particles isthe presence of large missing energy in the event.Several topologies involving jets and/or leptons and missing energy havebeen searched for by the four collaborations with negative results. There-fore, limits on the masses of the SUSY particles have been put. The crosssections used to derive the limits on the masses and the masses themselvesdepend on many of the parameteres of the SUSY models, making dicultthe task of giving absolute limits.In most models, one can set a limit on the mass of the lightest charginoclose to the kinematical limit, about 85 GeV. If, however, the scalar neu-trino mass is light (below 100 GeV), the limit degrades because the crosssection for chargino production decreases, due to the diagram with a t-channel scalar neutrino exchange. The mass limit for the lightest neutralinois only about 24 GeV, assuming heavy scalar leptons. The result of thecombined searches for charginos and neutralinos is customarily displayedas excluded areas in the M2 −  plot, where M2 and  are gauge- andHiggs-mass parameters appearing in the SUSY lagrangian. One such plot,showing the regions excluded by ALEPH, can be seen in g. 4.Limits for scalar partners of leptons from 53 GeV (for ~) to 75 GeV(for ~e) have been put. Assuming that the lightest scalar top decays 100%of the times to c, the limit on its mass is about 65 GeV for the value ofthe stop mixing angle which results in the smallest cross section. For someother values, the limit goes up to 73 GeV, as can be seen in g. 5 fromOPAL [7]. More details on all these limits can be found, for instance, inref. [9].In models in which supersymmetry is broken via gauge interactions,the gravitino (SUSY partner of the graviton) is the LSP. Then, if  is thenext-to-lightest SUSY particle, the production of a pair  can result in anal state γγ ~G ~G, that is, two acoplanar photons plus missing energy, sincethe gravitinos stay undetected. Searches for these kind of events have beenunsuccesful and limits on m around 72 GeV have been set within someparticular models [10].5. Four-jet anomalyIn the autumn 1995 run at energies close to 130 GeV, the ALEPH col-laboration reported an excess of four-jet events in which the sum of thetwo jet-pair invariant masses that diered the least was peaked at about105 GeV, as shown in g. 6 from ref. [1]. The excess was not conrmedby the other three LEP collaborations. Four-jet events have been studiedby all LEP collaborations at both runs at 161 GeV and 172 GeV. While80100200300-300 -150 0 150 300µ (GeV)M2 (GeV)tanβ = √2M ν∼ = 200 GeVLEP 1.5χ±χ′kinematiclimitLEP 1√s up to 172 GeVpreliminaryALEPHFigure 4. Regions of the M2{ region excluded by the ALEPH collabo-ration for tan =p2 and scalar neutrino mass of 200 GeV.902040608040 50 60 70 80m ( t~ 1 ) [ GeV ]m ( χ~10  ) [ GeV ]OPALt~ → c χ~ 10(b)θ t~  = 0.0θ t~  = 0.98excludedFigure 5. Regions in the m~t1−m plane excluded by OPAL [7] for severalvalues of the stop mixing angle, assuming the decay goes as ~t1 ! c 100%of the times. The cross-hatched area had already been excluded at LEP1,while the single-hatched area has been excluded by D0 [8].10Figure 6. Distribution of the sum of the invariant masses of the twojet pairs with smallest invariant mass dierence in four-jet events, fromALEPH at 130 GeV [1].11√s=133-172 GeV0510152025303520 40 60 80 100 120 140 160 180 200ΣM for minimum δMNexp=90.2Ndata=9302468101220 40 60 80 100 120 140 160 180 200ΣM for minimum δMNexp=30.3Ndata=37Figure 7. Distribution of the sum of the invariant masses of the two jetpairs with smallest invariant mass dierence in four-jet events at 130, 161and 172 GeV put together, from DELPHI, L3 and OPAL (top plot) andfrom ALEPH (bottom plot).12ALEPH seems to conrm their initial nding, although with lower sig-nicance, DELPHI, L3 and OPAL continue to see no deviation from theStandard Model expectations. A LEP-wide working group on the sub-ject has concluded [11] that the four experiments are able to select andreconstruct four-jet events with similar eciencies and mass resolutions.Figure 7 shows the combined results at all energies from 130 to 172 GeVfor DLO (that is, DELPHI, L3 and OPAL combined) and for ALEPH. Thediscrepancy is clear.The study concludes that the probability for the ALEPH observation tobe due to a statistical fluctuation is about 7 10−4, while the probabilityfor both the ALEPH and the DLO observations to be compatible with asignal at 105 GeV is also about 7 10−4! It is clear that, so far, the originof the eect is not understood.6. Constraints on possible explanations of HERA eventsIf the excess of high Q2 events seen in H1 and ZEUS at HERA [2] is dueto new physics aecting the coupling between quarks and electrons, therecould be also eects in the process e+e− ! qq, very well studied at LEP.The OPAL collaboration has investigated two scenarios [12], either with thepresence of four-fermion contact iteractions between e− e− q − q, or withthe exchange of a scalar particle in the t-channel of the reaction e+e− ! qq.By measuring the cross section for hadron production and, separately,the ratio of bb nal states to all hadronic nal states, both at 161 andat 172 GeV, OPAL has put limits on the mass scale appearing in thecontact-term lagrangian between 1.0 and 2.5 TeV, depending on the typeof interaction and on whether the interaction aects one up-type quark orone down-type quark. It should be noted that, in spite of the huge statisticsaccumulated at LEP1, this kind of search is more sensitive at LEP2 becauseat the Z peak the interference between the new amplitude (purely real) andthe dominant Z amplitude (almost purely imaginary) almost vanishes andone is left with the purely new eect squared.If a t-channel exchange of a scalar leptoquark or a scalar quark violatingR-parity is assumed, some constraints on possible models explaining theHERA excess events can be obtained. The constraints will become trulysevere once the 1997 data is analysed.7. SummaryAfter analysing the data from the rst year of LEP2, no deviation from theStandard Model predictions has been found:13 The direct measurement of the W mass agrees with the previous in-direct determinations using LEP1/SLD data and assuming the Stan-dard Model. Limits on Higgs and Supersymmetry particles have been greatly ex-tended. For example, the Standard Model Higgs has to be heavierthan 71 GeV, the SUSY Higgses h and A heavier than 62.5 GeV,and the lightest chargino heavier than 85 GeV in most of the SUSYparameter space. The four jet anomaly reported by ALEPH in 1995 remains a mistery,having been conrmed by ALEPH in 1996 but not seen by any of theother LEP experiments. Analysing the reaction e+e− ! qq the LEP experiments can start toget interesting constraints on models trying to explain the high Q2HERA eventsAcknowledgmentsIt has been a great pleasure to attend this conference in such a nice set-ting and with such an attendance. I am most thankful to Prof. Klapdor-Kleingrothaus for his invitation to give this talk and to him and all histeam for making the whole week so enjoyable. I would also like to thankDr. Michael Schmitt for his help in the preparation of this talk.References[1] Buskulic D et al (ALEPH Collaboration) 1996 Z. Phys. C71 179[2] Adlo C et al (H1 Collaboration) 1997 Z. Phys. C74 191Breitweg J et al (ZEUS Collaboration) 1997 Z. Phys. C74 207[3] Barate R et al (ALEPH Collaboration) 1997 Phys. Lett. B401 347Abreu P et al (DELPHI Collaboration) 1997 Phys. Lett. B397 158Acciarri M et al (L3 Collaboration) 1997 Phys. Lett. B398 223Ackersta K et al (OPAL Collaboration) 1996 Phys. Lett. B389 416[4] Abbaneo D et al (LEP Electroweak Working Group) A Combination ofPreliminary Electroweak Measurements and Constraints on the StandardModel, LEPEWWG/97-01[5] Barate R et al (ALEPH Collaboration) 1997 CERN-PPE/97-070Acciarri M et al (L3 Collaboration) 1997 CERN-PPE/97-081[6] Kane G L, Kolda C and Wells J D 1993 Phys. Rev. Lett. 70 2686Espinosa J R and Quiros M 1993 Phys. Lett. B302 5114[7] Ackersta K et al (OPAL Collaboration) 1997 CERN-PPE/97-046[8] Abachi F et al (D0 Collaboration) 1996 Phys. Rev. Lett. 76 2222[9] Cerutti F (1997) to appear in Proceedings of the SUSY 97 conference,Philadelphia[10] Lopez J and Nanopoulos D (1997) Phys. Rev. D55 4450[11] Bizouard M A et al (LEP Working Group on Four Jets) Report of the LEPWorking Group on four-jet production in e+e− collisions at centre-of-mass energies of 130{172 GeV, ALEPH 97-056 PHYSIC 97-050, DEL-PHI 97-57 PHYS 707, L3 note 2090, OPAL Technical Note TN 486[12] Komamiya S LEPC presentation, CERN, February 25th 1997.15

New Physics at LEP2

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