Search for New Particles at LEP

S. Rosier-Lees LAPP (INPPS-CNRS) Annecy-Le-Vieux - France

Abstract

The LEP energy upgrade up to fi =189 GeV has allowed us to extend substantially the potential of searches for new physics. Results on searches for Higgs bosoms and supersymmetric particles obtained by the ALEPH, DELPHI, L3, and OPAL experiments are reported. No evidence of any signal is observed. Therefore, new limits on the Higgs boson masses as well as on the masses of the various supersymmetric particles are derived. They significantly improve those obtained either at LEPl or LEP1.5. The LEPBOO discovery potential for the neutral Higgs bosons is also shown.

@ 1998 by S. Rosier-Lees.

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. . be discovered at LEP200 when running at &=200 GeV and assuming an integrated luminosity of 200 pb-’ collected by each experiment. LEP PRELIMINARY

Individual Limit LEP Combined

Table 1: Individual and LEP combined observed and expected mass limits for the Stan- dard Model Higgs boson [3], up to fi = 183 GeV.

4 _?I: 80 82 84 86 88 90 92 94 10 80 82 84 86 88 90 92 mH(GeV/

z5 k --- HZ-Signal(mn=85GeV)

1.5

0.5

0 0 20 40 60 80 loo 0 mF(GeV)

Figure 1: Mass distribution for the candidate events selected by the OPAL experiment in the searches for e+e- + HZ at center-of-mass energies up to 183 GeV [3].

I,,,I,,,,,,/,I lo 80 82 84 86 88 90 92 94. 80 82 84 86 88 90 92 94 mH(GeV/c’) mH(Ge V/c2) 2.2 The MSSM Higgs Bosons Figure 2: Average expected (dashed lines) and observed (solid lines) confidence levels, CL,, obtained In the MSSM, all SUSY particle masses, their couplings, and their production cross sec- from combining the results of the four LEP Collaborations using the four statistical methods. The tions and decay widths are predicted in terms of only six free independent parameters: intersections of the curves with the horizontal lines at 0.05 define the 95% confidence level lower rnilz (the common gaugino mass parameter at the GUT scale), me (the common mass for bounds, ezpected and observed, for the mass of the 5X4 Higgs boson. scalar fermions at the GUT scale), p (the higgsino mixing parameter), tan/3 (the ratio of the vacuum expectation values of the two Higgs doublets), mA (the mass of the neutral CP-odd Higgs boson A), and A (the trilinear coupling in the Higgs sector at the GUT scale); the renormalization group equations are used to determine the parameters at low

-411- h ] Mh.a Lower Limit (GeV/c2) ]

PRELIMINARY Range of ~ Lower ’ Individual Limit LEP Combined (ADLO) y#p#p h (Obs.) 70.7-74.4 78.8 (Exp.) 67.4-70.3 76.3 A (Obs.) 71.0-76.1 79.1 (ExP.) 68.4-72.0 76.3

Table 4: Individual and LEP combined mass limits for the neutral h and A bosons [3].

background contamination. The 95% C.L. individual limits on the charged Higgs boson

Mu*=60 GeV/c’ r-o,r+u, T+V,SC cssi: Efficiency 24% 42% 40% Expected events 9.2 30.1 99.4 Observed events ] 6 1 28 1 93 0 100 200 300 400 500 mA (GeV/c2) Table 5: Typical efficiencies and number of expected and observed events obtained by the L3 experiment in the charged Higgs boson searches at a c.m. energy of 183 GeV (141.

Individual Limit (ADLO) LEP Combmed 10 11

Table 6: Individual and LEP combined mass limits for the charged Higgs bosons.

mass is shown as a function of the branching ratio Br(H* + r*vT) in Fig. 4. Combining all LEP results, a lower limit on the charged Higgs boson mass of 68 GeV is established at the 95% C.L., assuming that the sum BR(H+ + r’+uT) + Br(H+ + cS) is equal to one. It represents a gain of almost 10 GeV with respect to the individual limits, Table 6.

0 20 40 60 80 loo 120 140 3 Other SUSY Particles mh (GeV/c2) The supersymmetric particles searched for at LEP2, not counting the Higgs bosons, are Figure 3: LEP combined exclusion domains (method D) in the plane (MA, tan,@ (top) or sleptons, stops, sbottoms, charginos, and neutralinos as detailed in Refs. [17] and [18]. (Mh, tan,@ (bottom) by the LEP direct searches for e+e- + hZ and hA at center-of-mass The most popular SUSY breaking mechanisms are the gravity and the gauge mediated energies up to 183 GeV assuming either a maximal mixing or no mixing [3] and [IS]. SUSY breaking models (GMSB). The results of SUSY searches at LEP are interpreted in the framework of these two models. In general, to avoid lepton and baryon number

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Table 8: Individual and LEP combined mass limits for &, @s, and ?s up to fi = 183 GeV for AM greater than 20 GeV, and assuming tar@ = 2 and p = -200 GeV for Es.

masses of right- and left-handed sleptons can be used to combine results of the searches for acoplanar leptons (muons, electrons together) [20] coming from the .!?+?Lprocess when the CR and the XT are mass degenerate (AM below 3 GeV). The result is shown in Fig. 5 (bottom right), for tar@ = 2 and p = -100 GeV; this value of p minimizes the product of cross section times branching fraction for the process e+e- -+ Fnd~ with t?~+ exy for vanishing AM. In this case, a scalar lepton mass limit of 65 GeV/c2 is set independently of AM; this limit holds for higher values of tar@.

Scalar quarks The squarks of the two first generations should be abundantly produced at the Tevatron which already places limits far above the kinematical reach of LEPZ. Due to a large Yukawa coupling, the mass of one of the top squarks can be significantly smaller compared to those of the other scalar quarks; the large mixing between the left and right stops, proportional to mt, leads to a large splitting of the two mass eigenstates. The lighter stop ii could be within the discovery range of LEPB. In addition, for large tan/3 values ( 2 30), the mixing angle in the sbottom sector can also be large and the lighter sbottom bl could also be within the reach of LEP2. Stop and sbottom production at LEP proceed via Z/y exchange in the s-channel; the production cross sections depend on the squark mass and the squark mixing angle cos 0,, and at cos 0, N 0.57(0.39) the stop (sbottom) decouples from the Z-boson and the cross section is minimal; the dominant decay modes of the ii are expected to be ii + cxy or ii -+ bDZ+; both of these decay modes have been searched for at LEP. The dominant decay mode of bi is expected to be bi + b,yy. 40 50 60 70 80 90 Under the assumption of R-parity conservation, the xi and the 5 are invisible in the detector; thus stop or sbottom pair events are characterized by two acoplanar jets or two 9 [GeV/c2] acoplanar jets plus two leptons, with missing energy. When the 21 + cxy decay mode is dominant, the corresponding decay width is small enough for the stop to hadronize into Figure 5: Combined exclusion contours for scalar electrons, muons, and taus [25] as a a colorless “stop hadron” before decaying; this feature has been implemented by the LEP function of the LSP mass and the limit for sleptons obtained by the ALEPH experiment experiments in their Monte Carlo programs. No excess of events was observed by any of [20] when combining all selectron and smuon searches up to 183 GeV (bottom-right). the four LEP experiments (26]- [29] in the searches for stop and sbottom. Typical sensitiv- ities obtained by the OPAL [29] experiment in the various channels are listed in Table 9. All LEP results have been combined [25] and are summarized in Table 10, for AM greater

-415- I , I I Table 11: Typical efficiencies and number of expected and observed events obtained by DELPIU+L3+OPAL , x+x- Exclusion limit the DELPHI experiment in the chargino searches at a c.m. energy of 183 GeV (321.

AM 2 3 GeV/c2. The DELPHI experiment has excluded regions for lower AM ranges as depicted in Fig. 7 (bottom). Masses lower than 80 GeV/c2 are excluded by all LEP experiments for the next-to-lightest neutralino xi assuming Mz less than 1500 GeV/c2. This limit holds only for higgsino-like & since the coupling to the Z-boson vanishes for gaugino-like neutralinos. ~ Gaugino-like chargino (I/J] > Mz): the cross section depends strongly on the scalar neutrino mass. For 50 < rnc 5 80 GeV/c2, the destructive interference term reduces the cross section by one order of magnitude compared to what is expected for mi, > 300 GeV/c2. When the two-body decay x: + 1*P is dominant, the relevant AM becomes equal to mx: -mc. Therefore, the limit on the chargino mass will clearly depend on the rnc value. For large rnc values (2 300 GeV/c’), the situation is the ideal one: we benefit there from the best detection sensitivity and the highest cross section production. This is the reason why the kinematical limit is reached with only a few pb-i. Up to now, including the recent 189 GeV data, LEP experiments exclude a chargino mass below 94.3 GeV/c2, irrespective of tan/3. Extending the study to all rnG values where the chargino leptonic decay is enhanced for low slepton masses, dedicated analyses were then developed by the LEP experiments. However, it appears that the chargino cannot be directly detected when it is degenerate in mass with the sneutrino since the soft leptons produced in the final state escape detection. In this case, the LEPl limit of 45 GeV/c2 remains, that is deduced from the Z-boson total decay width measurement which is insensitive to the chargino decay pattern. Recently, this limit has been improved in two independent ways: (i) the ALEPH experiment excludes mx: below 51 GeV/c2 by measuring the invisible W-boson width, within some model assumptions [31]; and (ii) the L3 experiment [34] 40 45 50 55 60 65 70 75 80 85 exploits the complementarity of the scalar lepton searches, excluding chargino masses lb-$ (GeVk2) below 57.2 GeV/c2 irrespective of tan/3 and mn, as depicted in Fig. 7 (top right). Figure 7: Top left: LEP combined [25] lower limit on the chargino mass for different AM Indirect limits on the 2: mass values at c.m. energies up to 183 GeV. Top right: lower limit on the chargino mass for Finally, let us discuss the neutralino-LSP mass limits. Since the neutralino ~7 is the different field content, as a function of tar@, by the L3 experiment [34]. Bottom: excluded LSP and escapes detection, direct searches for the LSP neutralino cannot be performed domains in the plane (mxb, AM) in the very low AM range obtained by DELPHI [33] at at LEPZ. However, indirect limits have been derived from the constraints on chargino- a c.m. energy up to 183 GeV. neutralino and slepton searches. As detailed previously, the limits depend on the slepton masses, in particular on rnfi. Large sneutrino mass: For high tanp values, the chargino search provides the most

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3.2 Search for Charginos and Neutralinos Assuming No R-Parity Conservation In supersymmetry, R-parity violation can occur through the following Yukawa coupling terms in the superpotential

where L and Q are the SU(2) doublet lepton and quark superfields and E, U, D are the singlet superfields with i, j, k as the generation indices. The simultaneous presence of the last two types of terms would induce fast proton decay, which is excluded experimentally. Therefore, it is generally assumed that among the 45 R-parity violating coupling terms only one (or more generally only one type) dominates. The three types (X, X’, A”) have been studied at LEPS. The main consequence of R-parity violation is that the LSP is no longer stable. The R-parity violating couplings under consideration are strong enough so that the LSP decays within the detector (it requires a strength of the coupling greater than 10e3 which is within the existing upper bound obtained from low energy experiments). The L-violating X type coupling terms will give rise to multilepton final states, for instance, a Xjj, term may induce the decay of a neutralino LSP to final states such as &1~& (via virtual slepton or sneutrino exchange); the L-violating X’ type coupling terms involve multijets with leptons or with some missing energy. Finally, the B-violating X” type coupling terms lead to events with multijet signatures and without missing energy. In what follows the process involving chargino pair production as well as neutralino pair production (gyd but also aygi) have been studied in which R-parity violation man- ifests itself only in the decay of the LSP. No signal above the expected Standard Model backgrounds was detected at LEP [37]- [40] m any of the topologies investigated, result- ing in mass limits or constraints on the parameters of the MSSM. As shown by ALEPH in Fig. 9 [37], the kinematic limit for the chargino is reached everywhere, in particular in the deep-higgsino region in contrast to the case of R-parity conservation. Assuming a dominant Xr3acoupling (channel providing the worst sensitivity for this type of coupling), the LEP experiments [38]- [40] h ave derived a lower limit on the LSP mass equal to 26.8 GeV/c’ irrespective of tanp.

3.3 The Light Gravitino Scenario 3 50 0 50 100 150 200 A class of supersymmetric models in which the gravitino is the lightest supersymmetric PPW particle has recently received renewed attention, since the observation by the CDF experiment [41] of one event containing two electrons, two photons, and a large transverse missing energy. Within this scenario (G is the LSP), the phenomenology will depend on the nature of the next-to-lightest supersymmetric particle. At LEP, one distinguishes two cases: the NLSP is either the lightest neutralino (xy) or a slepton (1). Figure 9: Regions excluded in the (p,Ms) plane for the three X type couplings. The dotted line is the chargino kinematic limit.

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Iapun $day aq p[noQs QNQM ‘(L)&n t -a+a uo!$xal aQ$ol anp puno@Dvq aIqF3npay 08 us JaAarnoQ SF aIaQ$ ‘saInye@s 30 spu~y asay? JOT 3aInyeu8!s L8Iaua Bu~ssy snId Uo$oQd a@!s pus uo$oQd 0~1 aQ% 0% d[aAyadsaI peaI suoyaoal oh? asaQ& .(s/Aa P-o~ N E$q ‘9 v@g haA 103 dpo Iawq aQ3) 3f~L t $X t -a+a pwe fjf$L t AXAX t -a+a :sassazoId OM$ aQ$ “03 (qd 1 -) Z&J-~ l't? pa$D!paId are suo!$Das &O.TD uoynpoId aSIy 'O!I'SUaDS OII!~!AEJs $Q8!1 aQ'j UI '$/A a Ma3 8 UVQ'J IaIIwus S! SSEUI OU!$!AVI8 aQ$ Se uoos se I~UTS L[q!SqSau s! auq$ajy ouqw$nau aQ$ ‘sasodlnd Iw!yvId ION '3iL t $ 'ou!~!ilwS 8 pw uoloqd 1? OJU! h?3ap 0% papadxa s! ouQw$nau $salQSq aQ% '&wa3s B QDns III ADLO limits stable slcptons &=130-183 GeV Felcini, Pascal Gay, Paolo Giacomelli, Fabiola Gianotti, Eilam Gross, Peter Igo Kemenes, , \ Patrick Janot, Luc Pape, and Pierpaolo Rebecchi from the ALEPH, DELPHI, L3, and OPAL experiments as well as all the members of the Higgs and SUSY LEP working

Kink and Large IP groups for their great help in preparing this talk. I am especially grateful to G. Coignet and J. F. Grivaz for the numerous interesting discussions and for a careful reading of the report. It is a pleasure for me to thank the organizers of the SSI for the very stimulating session and for the very pleasant atmosphere at the conference.

References

[I] P. Janot, “Searching for Higgs bosons at LEPl and LEP2,” Perspectives on Higgs Physics ZZ,World Scientific Publishing Company, ed. G. L. Kane.

[2] G. Qua&, Electroweak physics, talk given on behalf of the LEP Electroweak working group, LEPC, 15 Sept. 1998. [3] The LEP HIGGS Working Group, Abstracts 577-582, ICHEP98, Vancouver, July 1998. F. Di Lodovico, Report from the LEP Higgs working group, talk given on Figure 11: Left: LEP combined upper limit on the production cross section for stable behalf of the LEP HIGGS working group, LEPC, 15 Sept. 1998. sleptons as a function of the mass of the sleptons. Right: Exclusion region in the (rr~~,~~) plane by DELPHI. The positive slope hatched area shows the region excluded by the small [4] ALEPH collaboration, Abstract 903, ICHEP98, Vancouver, July 1998; DELPHI col- impact parameter search. The negative slope hatched area shows the region excluded by laboration, Abstract 219, ICHEP98, Vancouver, July 1998; L3 collaboration, Ab- the combination of the large impact parameter and kink searches. stract 485, ICHEP98, Vancouver, July 1998; OPAL collaboration, Abstract 1069, ICHEP98, Vancouver, July 1998.

[5] E. Gross et al., Prospects for the Higgs Boson Search in Electron-Positron Collisions elude Higgs massesbelow 89.8 GeV/c’. New limits in the SUSY sector have been derived, at LEP 200, CERN-EP/98-094. in particular for the neutral Higgs bosons h and A , masses below 77 ,78 GeV/c2(respec- tively) are excluded for tar@ values greater than 0.8. With the coming data foreseen at [6] ALEPH collaboration, Phys. Lett. B 440, 403 (1998); Phys. Lett. B 447, 336 (1999). fi N 200 GeV, the SM Higgs with a mass up to 100-110 GeV/c2 can be discovered at [7] ALEPH collaboration, Abstract 897, ICHEP98, Vancouver, July 1998. LEP2. LEP2 is also a unique place for SUSY searches: Due to the good detector performances [8] DELPHI collaboration, Abstract 200, ICHEP98, Vancouver, July 1998. and the relatively good knowledge of the background reactions, a large domain is covered and all channels are exploited. With the absence of any signal, LEP2 excluded chargino [9] DELPHI collaboration, Abstract 209, ICHEP98, Vancouver, July 1998. masses up to the kinematic limit (94.3 GeV/c2) for most of the SUSY parameter space and derived an absolute limit equal to 57.2 GeV/c’. In addition, an indirect and absolute [lo] L3 collaboration, Phys. Lett. B 436, 389 (1998) lower limit, equal to 28 GeV/c’, on the Lightest SUSY Particle has been derived in the [ll] OPAL collaboration, Abstract 355, ICHEP98, Vancouver, July 1998 framework of the CMSSM. With future LEP runs, chargino massesup to 100 GeV/c’will be explored and a lower mass limit on the LSP of the order of N 35 GeV/c2will be [12] ALEPH collaboration, Abstract 900, ICHEP98, Vancouver, July 1998. reached. [13] DELPHI collaboration, Abstract 214, ICHEP98, Vancouver, July 1998. Acknowledgments [14] L3 collaboration, Phys. Lett. B 444, 516 (1998). Preparing such a review needs a lot of discussion and interaction with many colleagues. [15] OPAL collaboration, Abstract 1069, ICHEP98, Vancouver, July 1998. For this reason I would like to thank Wim deBoer, Laurent Duflot, Alvise Favara, Marta

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