Search for SUperSYmmetric particles at LEP2

ELSEVIER

Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 75-85

PROCEEDINGS SUPPLEMENTS

Search for SUperSYmmetric Particles at LEP2 F. Cerutti a a C E R N - P P E Division 1211 Geneve 23 A review of results on searches for supersymmetric particles with the data collected at LEP2 for centre-of-mass energies up to 172 GeV is reported in the following. In addition to the "classical" X LSP scenario with R parity conserving some developments have been carried out in searches for topologies with Gravitino LSP scenario, typical of Gauge Mediated SUSY breaking models, and with R-parity violation. No signal has been found in any of the searched topologies and limits on the parameters of the supersymmetric models have been derived.

1. I n t r o d u c t i o n The Supersymmetric Extension of the Standard Model [1] (SUSY) is believed to be the only way out of the hierarchy problem which can naturally bring to the unification of the known interactions at large energy scale and at the same time does not conflict with the precision tests of the Standard model performed at centre-of-mass energies up to the Z mass. Among the SUSY models the one which is most supported by the theoretical community is the Minimal one (MSSM), just because it introduces a reasonable number of new parameters. The basic idea of the MSSM is to introduce for every known SM particle a SUSY one which differs by 1/2 spin units. So for any SM fermion helicity (fL, fR) two scalar fields are introduced (j~, jR). The Higgs sector requires at least 2 Higgs doublets which give the mass to the two weak isospin sectors. The SM bosons have spin 1/2 SUSY patters called gauginos and higgsinos. They mix together giving the mass eigenstates called charginos (;~+ and ) ~ ) and neutralinos (X, X', X" and X'"). The beauty of tile MSSM is that the interaction of all the new SUSY particles are required to be the same as their SM partners. If SUSY was unbroken also the masses would be the same but experimental observation definitively requires SUSY to be broken. This is the more obscure and complex part of the model. In SUSY mass terms are, in principle, free parameters of the theory (as well as the mixing angles between the different fields which give the mass eigenstates) provided 0920-5632/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0920-5632(97)00645-2

they are not too large ( M s u s Y _< 1 TeV) in order to do not spoil the solution of the hierarchy problem. Some additional hypotheses are in general added when the negative result of a search in a given experimental topology is translated into an exclusion in the MSSM parameter space. The first relation that is usually assumed in the MSSM is the unification of the gaugino masses at the GUT scale. This condition implies the relation at the electro-weak scale 3 a M2 = 5 tan 20w M1 - a8 sin 20w M3 where M1, M2 and M3 are the bino, the wino and the gluino masses, respectively. Any result given in the following quoted as an MSSM exclusion will assume this condition. The other condition which is sometimes assumed is the unification of the scalar masses at GUT scale. This common mass is referred to as m0. When this condition is assumed all the sfermion masses are function of only 3 SUSY parameters M2, m0 and tan ~3, where tan/3 is the ratio between the vacuum expectation values of the two Higgs fields. I will refer to CMSSM (Constrained MSSM) when the result assumes this condition too. In the SUSY Lagrangian, terms which allow the decay of a SUSY particle into a pair of SM ones are naturally present. These terms are strongly constrained by the present data. In particular the limits on the proton lifetime requires a fine tuning of these couplings which is considered unnatural. For this reason it is usually assumed that

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a discrete symmetry which forbids these terms is present in nature; this is achieved by imposing the conservation of a multiplicative quantum number defined as R p -- ( - 1 ) 3s+L+2S. The main consequence of this symmetry, called R-parity, is that all the SUSY particles must be pair-produced and that the Lightest Supersymmetric Particles (LSP) is stable. Since cosmological constraints require LSP to be neutral [2] the main experimental characteristic of R-parity conserving SUSY is the presence in the final state of at least two neutral weak-interacting particles which escapes the detector and give at LEP2 the characteristic missing energy and missing momentum signature. In the framework of R-parity conserving SUSY two main exceptions could invalidate this assumption: first if the LSP coupling to the other SUSY particles is very weak (G LSP scenario characteristic of Gauge Mediated SUSY Breaking models [3,4]) the SUSY particles can acquire a long lifetime and decay outside the detectors, second if the LSP is a light gluino a final state without missing energy is possible. The first hypothesis will be considered in Section 3 where the results concerning analysis optimised to deal with the LSP scenario are given. For what concerns the light gluino scenario recent papers [5,6] have pushed the lower limits on the gluino mass up to about 5 GeV. No special searches have been developed at LEP2 to deal with this possibility. If R-parity is broken all the SUSY particles can decay to the SM ones so that missing energy and momentum are not necessarily present in the detected final state. It has been pointed out that some of the R-parity violating terms [7] could be different from zero without causing the undesired short proton lifetime. The presence of R-parity violating terms has a strong impact on the experimental topologies and therefore requires dedicated searches that will be briefly describe in Section 4. In this paper an overview of the results obtained by the 4 experiments ALEPH, DELPHI, L3 and OPAL by analysing the data collected at LEP2 (1996 run) is given. In this data taking period the machine delivered a luminosity of about 11 pb -1 at 161 GeV and of about 11 pb -1 at 172

GeV. Some of the presented results make also use of the 6 pb -1 of data delivered by LEP in 1995 at v/~ ,~ 134 GeV. All the limits given in the following are at 95 % confidence level and most of the presented results have to be considered as preliminary.

2. S U S Y w i t h R-parity conservation: the X L S P scenario As already stated the main experimental signature of the production of SUSY particles at LEP2 is missing energy and momentum. From the experimental point of view the main parameter which characterises the signal topologies is the mass difference between the produced SUSY particle and the LSP, hereafter called AM. Other parameters affect the visible topology like the final state branching ratios (BR's) and, to a lesser extent, the mass of the produced SUSY particles. The main guidance in designing the analysis for a given final state is therefore AM. In the small A M region the signal events are characterised by a small multiplicity, small visible mass and energy and they look very similar to 2photon events (e+e - -+ e + e - v 7 --+ e + e - f ] ) . For the high AM region the backgrounds come from 2-fermion production through Z or 7 exchange and from 4-fermion processes (WW, Wev, ZT*). These processes have no or small missing mass and energy and a large visible mass. They are more easy to fight with respect to the 2-photon background because of the smaller production cross section. The background prediction for the 2- and 4-fermion processes is reliable and they could be subtracted. Since the resulting background is small in most of the analysis it has been decided to do not subtract it. The main exception is the W + W - ~ e+v/-P background in the acoplanar lepton selection which is almost an irreducible one and therefore has been subtracted. For the future high-luminosity runs the reduction of the background to a negligible level will be very hard so that probably the background will be subtracted also in other topologies. A general feature of all the analysis is to define different selections for the low and the high AM regions. In each analysis the cuts are optimised

E Cerutti/Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 75-85

following the N95 prescription [8]. The choice of the combination of analyses to apply depending on AM, BR's and other parameters is done following this prescription too.

2.1. Charginos and Neutralinos Charginos are produced at LEP2 through virtual Z or 3' s-channel exchange and are expected to have large cross section (of the order of few picobarns). The presence of the negative interference with the t-channel sneutrino exchange can decrease the cross section significantly when the chargino coupling to the sneutrino is large, i.e. when the chargino is gaugino like M2 << I#[ (where # is the supersymmetric Higgs mass term). In this region the chargino production cross section rapidly decreases when the sneutrino becomes light. The chargino mass and field composition is determined, in the MSSM, by the 3 parameters M2, # and tan ft. The exclusion limits on chargino are usually given in the M2 - # plane for fixed values of M~ and tan/L All LEP experiments performed searches for the lightest chargino by looking at the three possible final states: acoplanar leptons (not necessary of the same flavour), 4-jets plus missing energy, 2-jets plus lepton plus missing energy. The weight of these final states depends on the chargino BR's. They are usually assumed to be the same of the W but leptonic or hadronic BR's could be enhanced if the sleptons or the squarks would be light. The selection efficiency for the different topologies are quite similar making the results almost insensitive to the chargino BR's. Typical efficiencies are of the order of 10-75 % for 5 < AM < 80 GeV (where the lowest efficiency is reached at the boundary of the AM range): The only case where the efficiency can be very small is when the sneutrino is lighter than the chargino but very close in mass to it (less than 3 GeV), this correspond to the production of two very soft leptons that can be experimentally invisible. The cross-section excluded by the ALEPH experiment at vG = 172 GeV in the charginoneutralino mass plane is shown in Fig. 1. This plot assumes W-like chargino BR's. The neutralino searches are complementary to the chargino ones and can improve the sensitivity in

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Figure 1. Chargino cross section upper limit in pb at v ~ = 172 GeV from ALEPH in the charginoneutralino mass plane. This plot assumes W-like chargino BR's.

the mixed region region (M2 ,~ #) where the chargino cross section can be small for light sneutrinos and in the higgsino region (M2 >> [#[) where the neutralino cross section and efficiency are large. The main interest is for the XX production via Z exchange. The production cross section can be enhanced thanks to the selectron t-channel diagram, if the selectron is light. The X' decay mainly into X~, Xq(lor X'Y depending on the X and X' field composition. Selections were developed in order to cope with these possible topologies. The chargino and neutraiino exclusions can be translated in exclusion in the M2 - # plane once the slepton masses and tan/~ are fixed. An example of such an exclusion plot is given in Fig. 2 where the ALEPH result is shown (in this plot tan/~ = v/2 and M~ = 200 GeV are assumed). A region of particular interest is the so called Supersymmetric region which corresponds to small M2

E Cerutti/Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 75-85

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and # and tan 3 "~ 1. In this region the chargino mass is close to the W one while the two lightest neutralinos are light and the two heaviest ones have masses close to the Z one. A L E P H gets rid of the region us!,ng, data collected at 130-136 GeV and selecting X X ~ x ' Z * x ' --+ x x Z * 7 7 • DELPHI and OPAL have performed new searches in which they look for deviation of the W W cross section from the standard model one in data collected at 172 GeV. This is possible because the chargino cross section in this region of the parameter space is about half of the W W one. The result of the OPAL analysis is shown as the hatched area in Fig. 3; it assumes CMSSM, t a n 3 = 1 and minimum m0, i.e. the minimum scalar mass compatible with the exclusion limits derived by slepton searches.

2.2. Sleptons, Stop and S b o t t o m The sleptons can be produced at LEP2 via the photon and the Z s-channel exchange. The selectron production can receive also contribution

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/l [GeV] Figure 3. Limit in the M2 - # plane for minimum m o and t a n 3 = 1 from OPAL. This plot assumes CMSSM. The light-shaded region represents the limit derived from the LEP1 and LEP1.5 results while the dark-shaded one represents the limit derived from the LEP2 standard chargino and neutralino searches. The hatched region represent the improvement to the limit given by the W-like search at LEP2 described in the text.

from the neutralino t-channel. The experimental signature for these particles is the presence of two acoplanar leptons of the same flavour in the final states (where leptons can be also tau's). The most difficult case is the tau one where the tan decays into hadronic final states. Typical efficiencies are of the order of 40-70 % for A M in the range 5-75 GeV for smuons and selectron. For staus efficiencies of the order of 30-40 % are reached for A M in the range 20-50 GeV. The 70'~e background is particularly dangerous in the low A M region while the W W background is almost an irreducible one for the high A M selection and has been subtracted. No signal has been found by the four LEP experiments and the exclusion limits obtained by OPAL are given in Fig. 4 for stau and smuon and in Fig. 5 for selectrons. In the CMSSM the

E Cerutti/Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 75-85

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squarks are expected to be heavier than sleptons. Stringent limits on the squark masses are derived by Tevatron experiments [9,10] under the hypothesis of degenerated flavours and large A M . The stop and the sbottom are the only two flavours for which a non negligible mixing is possible (due to the sizable mass of the SM partner). For this reason special searches looking for these particles have been performed at LEP2. Stops and sbottoms are pair-produced at LEP2 through Z and 3' s-channel exchange. The coupling of these two squarks to the Z strongly depends on the mixing angle (Omix) which determines the mass eigenstates composition in terms of the left- and rightsquarks fields. The stop decouples from the Z for 0mix ~ 56 ° while the sbottom decouples for 0.~ix ~ 68 °. It is interesting to look for stops and sbottoms when they are lighter than the chargino (at LEP2 the chargino cross section is larger than the stop one for a large region of the MSSM parameter space). Under this hypothesis the dominant de-

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cay channels are { -+ cx or t --+ bg~ and/~ --+ bx. Both stop decays are expected to have quite a small width [11] (_ 1 eV for the first one and _< 100 KeV for the second one) much smaller than the typical hadronization scale. For this reason the stop is expected to hadronise before decaying and the second channel is expected to be dominant as soon as it is accessible (Mr, + MB <_ M~). For the sbottom the decay b -~ b)¢ is expected to have a width comparable with the hadronization scale so no firm statement can be made about its hadronization. These channels are looked for by selecting acoplanar jets plus missing energy or jets plus leptons plus missing energy• Typical selection efficiencies are of the order of 30-70 % for A M in the 5-60 GeV range for all the channels. No signal in excess to the standard model expectation has been found for these topologies and these results has been translated in exclusion limits shown in Figs. 6, 7 and 8. The results are plotted for two

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different values of the mixing angle. 80 ~D

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2.3. Limit on the L S P The limits on charginos and neutralinos can be translated into limits on the LSP assuming that the other SUSY-particles are heavy. This is shown in Fig. 9 where the L3 result is plotted, this result assumes a large scalar mass mo = 500 GeV. The large m0 assumption can be relaxed if CMSSM is assumed and if the information coming for slepton searches are added. As an example the LSP limit for minimum too, i.e. the minimum scalar mass compatible with the exclusion limits derived by slepton searches, obtained by OPAL is about 12 GeV. 2.4. T h e L E P - S U S Y working group Since 1997 it has been decided to combine the results obtained by the four LEP experiments in

Figure 7. Limit for t -~ b£P in the scalar-top scalar-neutrino mass plane from ALEPH. The excluded regions for two values of the mixing angle are shown. The light shaded area, labelled as LEP1 Exclusion, shows scalar-neutrino mass excluded by LEP1 data.

order to increase to sensitivity to SUSY signals. A LEP-SUSY working group has been appointed and the first exercise has been the combination of the "stable" sleptons results described in Section 3. In future the LEP-SUSY working group is planning to combine all the SUSY results coming from the 4 experiments. 3. T h e G LSP scenario The main feature of Gauge Mediated SUSY Breaking models (GMSB) [3,4] is the fact that the LSP is expected to be the gravitino with a very small mass. This implies that all the other SUSY particles can decay, without violate Rparity, into the gravitino plus SM particles. So the neutralino becomes visible through its decay X ~ G')'. The lifetime of the next-to-lightest supersymmetric particle (NLSP) is related to the

E Cerutti/Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 75-85

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Figure 9. Limit on the LSP as a function of tan fl from L3. A 95 % CL lower limit of 24.6 GeV is reached assuming heavy sfermions mo > 500 GeV.

gravitino mass and can become very long if the gravitino is light. The relation between the lifetime of the NLSP and the SUSY-breaking energy scale v ~ is given by the formula [13]:

DELPHI has developed dedicated chargino search where the neutralino decay into photon is explicitly requested. The experimental topologies are the same as the "standard" chargino searches but in addition energetic isolated photons are requested. This allows the improvement of the selection efficiency with respect to the stable X case in the low AM region. No signal has been found in this topology and a chargino mass up to 72 GeV has been excluded provided that M~ > M~+. The main signature for neutralino production at LEP2 is the presence of a single photon (e+e- ~ GX --+ G G T ) or two acoplanar photons ( e + e - -~ XX ~ GG'y3') with missing energy and mass in the detector (if the neutralino lifetime is short enough). The search for acoplanar 7's is also useful in the standard X LSP scenario since pair-produced X' could decay into X'Y. The GX production cross section strongly depends on the G mass. The XX production cross

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This kind of model received a strong boost thanks to the "famous" CDF event [14] where two high energy electrons and two high energy photons with large missing transverse energy have been observed; the interpretation of this event in GMSB models is qq ~ ~ --+ x x e + e - --+ GG-yTe+e - .

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section depends on the X mass and on the 6R mass (since in these models the X is expected to be almost pure bino). The main SM background to these searches is the process e+e - -+ ZT('y) vPT(7); the estimate of these processes is based on two different Monte Carlo codes: KORALZ and NUNUGPV 1. Some criticisms have been raised during this workshop [15] about the "correctness of these estimates. At present the KORALZ results seems to be more reliable than the NUNUGPV ones and an official number on the theoretical systematic uncertainty of KOtLELZ predictions will be released soon by the authors. The estimate of the background proposed by [15] appears to be not completely correct since it neglected the contribution of a 3rd (undetected) ISR photon. Typical selection efficiency for the acoplanar photon topology in the GMSB framework are of the order of 70 %. No deviations from the SM expectation has been found in both single and two photon channels. This negative result can be translated in limits in the M x - M~ R mass plane under the assumption of a pure bino X. The ALEPH experimental result as well a comparison with two GMSB models [3,4] are shown in Fig. 10. In GMSB theories the next to lightest SUSY particle can be charged and can acquire a long lifetime decaying outside the detector. This would give rise at LEP2 to a striking signal: very heavy charged particles pair-produced in e+e - annihilation traversing the full detector and decaying outside it. Such a signature is too beautiful not to be searched for. ALEPH and DELPHI developed searches designed to tag such events based mainly on kinematic and specific ionization for ALEPH and on specific ionization and Cerenkov detector for DELPHI. No events have been found with an expected SM background of less than 1 event per experiment. This has been translated in limits on the mass of stable smuon or staus (since their production cross-section depends only on their mass and the helicity of their SM partner). The DELPHI experiments also looked for IA more accurate study of the S M background and a comparison between the different Monte Carlo codes available on the market ( K O R A L Z , N U N U G P V , C O M P H E P , G R A C E ) is in progress in the L E P - S U S Y working group.

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decay of the sleptons into the apparatus to extend the range of gravitin0 mass to which their results are sensitive from 100 eV (search for "stable" sleptons) to about 3 eV (decay in the detector added). Inside the LEP-SUSY working group the ALEPH and DELPHI results have been combined [16] and the results of the combination is shown in Fig.ll. From the plot is easy to infer the excluded "stable" staus (or smuons) masses which are 73 GeV for rR and 74 GeV for ~L. 4. S U S Y with tt parity violation In SUSY terms which can violate lepton and baryon numbers and allow the decay of a SUSY particle in two SM ones are naturally present. They are of the form: . +AijkLiQjD~ . . . . +AijkU~DjD~ L R p v o( AijhLi L jE~ These terms allow a very fast proton decay if the lambda's are not fine tuned to very small values. The R p conservation imposes all the lambda to be zero, solving the proton lifetime "puzzle" without any fine tuning. However the possibility to have some of the lambda significantly different

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E Cerutti/Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 75-85

Charginos and Neutralinos (Rp via LLE)

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from zero without violating the actual experimental bounds is still open. For example if only one of the three terms is non zero the proton is still stable. For this reason separate analyses have been developed assuming that only the LLE or the LQD or the UDD term is different from zero. Up to know only preliminary results for the LLE term from A L E P H and for the LQD term from DELPHI are available but more complete results on LQD an UDD terms are in preparation and will be published soon. If Rp is violated the LSP in no longer stable and can decay into SM particles. If the dominant term is the LLE one, the typical signature is a large number of leptons in the final state (and no missing energy). The A L E P H search has given a negative result. The worst efficiency is obtained when a maximum number of tau's is present in the final state. This conservative efficiency has been used when deriving the A L E P H result shown in Fig.12. D E L P H I performed a search for scalar-top with R-parity violation as-

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suming that the LQD term A131 is different from zero. In this framework the stop can decay into an electron plus a d-quark. The searched experimental topology consists of two jets, two electron and no missing energy. No signal has been found in the data and a limit on the stop mass of about i 68 GeV has been derived (provided A131 > 10-4 and 0~ = O) 5. S u m m a r y o f t h e r e s u l t s

The four LEP experiments have searched for supersymmetry in a large variety of experimental topologies in order to cover classical (X LSP scenario) and more exotic (G LSP and R p violation) scenarios in data collected up to v/~ = 172 GeV. A good agreement between the expectation evaluated on the basis of the SM and the data selected samples has been found in all these searches. The results of the analyses have therefore been translated into exclusions in the SUSY parameter space. For Rp-conserving SUSY with

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Table 1 Summary of the SUSY results at LEP2 for the X LSP scenario with R p conservation. Assumptions Particle 95 % CL Mass Lower Limit GeV/c ~ [

)~± Heavy 9 scenario ]

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Stop and Sbottom 72 65 72

X LSP a summary of the results in the framework of the MSSM is reported in Table 1. The chargino limits get worse for light sneutrinos and eventually vanish for M~± - M~ < 3 GeV. For the G LSP scenario the kinematic limit for the chargino mass exclusion has almost been reached. The "preferred" region for the CDF event has been half-excluded by the results of the single experiments and a 95 % CL lower limit on the neutralino mass of about 71 GeV has been set for the two models described in [3,4]. The "stable" charged sleptons (smuons or staus) have been excluded for masses up to 73-74 GeV thanks to the combination of the ALEPH and DELPHI results. In R-parity violating searches strong limits have been put for the LLE dominant coupling scenario. DELPHI has excluded a scalar-top mass smaller than 68 GeV assuming a dominant LQD term of the type A'13x and a mixing angle 0r = 0. More complete searches for the LQD and UUD scenarios are on the way and the results will be presented soon.

AM>10GeV0 r=0 ° AM > 10 GeV 0F = 56 ° A M _> 10 GeV 0~ = 0 °

6. Conclusion and Acknowledgements The four experiments ALEPH, DELPHI, L3 and OPAL have searched for Supersymmetry signals at LEP2 analysing data collected in e+e collisions at centre-of-mass energies up to 172 GeV. No signal have been found in a large variety of experimental topologies giving a large exclusion in the SUSY parameters space. The charginos and neutralinos searches allow the exclusion of ~+ for masses up to the kinematic limit if large mo is assumed. For other SUSY particles due to smaller cross sections the obtained limits are still far from the kinematic threshold allowed by the LEP2 energy reached up to now. This year run (summer 1997) will hopefully allow the collection of large luminosities (~ 50 pb -1 per experiment) at V~ = 183 GeV. In the (unfortunate) case that no SUSY signals will be found the kinematic limit (,,~ 91 GeV) will be reached for almost all the SUSY particle masses under the MSSM assumptions, thanks also to the combination of the results between the four LEP experiments which will be performed by the LEP-SUSY working group. For what concerns Gauge Mediated SUSY

F. Cerutti/Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 75-85

Breaking theories, i.e. the light G scenario, a lot of work has been done to improve the search sensitivity to these topologies especially after the presentation of the puzzling CDF event. Search for acoplanar photon allow the exclusion of at least half of the CDF "preferred region". The 1997 run should give a final answer to the possibility that the CDF event is a manifestation of GMSB. A lot of effort has been put on R-parity violating searches, since in this scenario all the "classic" SUSY searches would fail because of the absence of missing energy. Some results for the LLE and LQD couplings have already been presented by ALEPH and DELPHI. A more complete study of R-parity violation topology is under study and the results will be presented soon. I want to thank all the people from the 4 LEP experiments who helped me in preparing this talk namely: V. Buesher, J. Dann, L. Duflot, L. Moneta, J. Nachtman, B. Orejudos, M. Schmitt and D. Zerwas for the ALEPH experiment; A. Lipniacka, W. De Boer and P. Kluit for the DELPHI experiment; S. Rosier for the L3 experiment; and P. Giacomelli, S. Koma.miya, H. Neal and G. Wilson for the OPAL experiment. REFERENCES

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