Search for New Physics at LEP 2

Eilam Gross Department of Particle Physics, Weizmann Institute of Science, Rehovot 76100, ISRAEL

Abstract The results of the search for Higgs bosons, Charginos, Neutralinos, Sleptons, Squarks

and light Gravitinos with the LEP accelerator at 130-172 GeV center-of-mass energy are briefly described. Prospects for Standard Model Higgs search at higher center-of-mass energies are also given.

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Introduction Since November 1995 the LEP e+e- collider at CERN collected about 2.7,2.5,10 and 11 pb-1 integrated luminosity per experiment (ALEPH, DELPHI, L3 and OPAL) at centre-of-mass energies ( yls) of 130,136,161 and 172 GeV (LEP 2) respectively.

This provided an opportunity to search for new particles, in particular Higgs bosons and traces of Supersymmetry, at these higher energies. The preliminary results of these searches, based on presentations of the 1 72 GeV run results at the LEP 2 jamboree on February 25th, 1997 [l] are reported below.

In models invoking Supersymmetry (SUSY) each elementary particle is accompanied by a supersymmetric partner whose spin differs by half a unit. SUSY models require a minimum of two Higgs doublets to generate the masses of bosons and fermions. In SUSY models with two Higgs doublets, such as the Minimal Supersymmetric Standard Model (MSSM), the fields of the fermionic partners of the w± and of the charged Higgs bosons, H±, mix to form two mass eigenstates, the charginos xt2. The fields of the partners of the 1, of the zo and of the neutral Higgs bosons mix to form four mass eigenstates, the neutralinos x? (i = 1 , . . . , 4, in increasing mass order). Each massive fermion (lepton or quark) has two scalar partners, the right- and left-handed scalar fermion (sleptons and squarks) denoted lP. and lL, according to the helicity states of their non-SUSY partners.

Most search results are interpreted within the framework of the MSSM where the gaugino­higgsino sector of the theory is completely determined by three parameters: M2, the universal mass of all gauginos at the unification scale; µ, the mass coupling strength between the two Higgs superfields; and tan {3, the ratio (v2) / (v1) of the vacuum expectation values of the two Higgs doublets. For a given set of these parameters there are, at tree level, unique relations [2] that determine masses and coupling constants of all gauginos. The scanned regions of the parameters were (e.g. for DELPHI) 0 ::S M2 ::S 800 GeV, -400 ::S µ ::S 400 GeV, for three values of tan {3: tan f3 = 1 .0, tan f3 = 1.5, and tan f3 = 35. A more general case of all values of tan f3 > 1 was also considered. These scanned ranges of M2 and µ are large enough so that exclusion regions change negligible for larger ranges. A golden rule that usually works is that Charginos and Neutralinos are Higgsino-like in the lµI < M2 region and couple predominantly to the Z. Other assumptions that play a role in the sfermions sector are that all diagonal scalar sfermions squared masses equal at the GUT scale with a common mass parameter m0 and that all :3-linear couplings, A;, which couples the Higgs with two squarks are equal. With all the a.hove assumptions one gets that all squarks but stop are nearly mass degenerate and somewhat heavier than the sleptons.

In the following we will make the assumption that a new multiplicative quantum number, R-parity which discriminates between ordinary and supersymmetric particles, is conserved and that the lightest neutralino, x�, is the lightest supersymmetric particle (LSP). R-parity con­servation implies that supersymmetric particles are always pair-produced and always decay, through cascade decays, to ordinary particles and X�X�· The x� is stable and escapes detection due to its weakly interacting nature. Therefore a characteristic signature of all events containing supersymmetric particles when R-parity is conserved is missing energy and unbalanced momen­tum. Recently models predicting a light Gravitino (Gauge Mediated SUSY Breaking models, GMSB) are very popular. In these models the lightest neutralino will decay to a gravitino and a photon, leading to final states with isolated photons accompanied by missing energy.

In this report the searches for SUSY signature and Standard Model Higgs bosons at yls =

130 - 172 Ge V are summarised.

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Background The main sources of background for the SUSY signature are:

• Two photon processes where lots of energy goes undetected down the beam pipe are eliminated by a cut on the transverse momentum with respect to the beam (PT).

• Four-fermions processes are very problematic backgrounds because they are irreducible. ZZ production has still a low cross section (around 1 pb at 172 GeV), however processes like e+e- -+ Z"/1Z -+ e+e-vii are irreducible source of background. The most prob­lematic four fermion processes are those which involve a lepton in the final state which goes undetected down the beam pipe. Such are e+ e- -+ W ev or e+ e- -+ WW -+ qij'lv where the lepton escapes detection. The WW production cross section is closed to its peak around 172 GeV ( 12 pb) . The only way to reduce WW background is via b-tagging (especially in Higgs searches, taking advantage of the fact that Higgs decays mostly to bb) .

• Radiative multihadronic events and radiative di-lepton events where the photon goes into the beam pipe are eliminated by cuts on energy flow around the beam, or cuts on the direction of the missing momentum.

• The main background to GMSB signature are double bremmstrahulang events ( e+ e- -+ vii·n). This background is reduced by requiring that both photons will be energetic or by a cut on the recoil mass of the visible system.

Charginos and N eutralinos If charginos are light enough, they can be pair-produced in e+e- collisions through / or Z exchange in the s-channel and through sneutrino (ii) exchange in the t-channel. The production cross section is fairly large unless the sneutrino is light (low m0 ) in which case destructive interference may occur between the s-channel and t-channel diagrams [3] .

The details of chargino decay depend on the parameters of the mixing and the masses of the scalar partners of the ordinary fermions [3]. The lightest chargino xt can decay into a neutralino x� and a lepton pair: xt -+ x�f+v, or a neutralino and a quark pair: xt -+ )(�qq' through virtual W, slepton (f), or scalar quark (q) emission. The effects of the latter are ignored with the assumption that scalar quarks are very heavy. xt decay via virtual w emission is dominant in most of the MSSM parameter space; however, in some regions of the parameters space the xt decay via the w boson is suppressed and the dominant decay mode may be 5(�£+11 via a virtual l or ii.

Due to the energy and momentum carried away by the invisible )(�, chargino events are characterised by large missing energy and t ransverse momentum imbalance.

Neutralino pairs (x?x�) can be produced through an s-channel virtual Z boson or by t­channel selectron (e) exchange. At LEP 2 the second lightest neutralino )(� produced in con­junction with a x� could give the first direct signal for neutralinos, since single photon events from e+e- -+ x�xh suffer from background from e+e- -+ Vii/. At LEP 1 limits have been obtained from measurements of the width of the Z boson and direct searches.

If the )(� is the lightest visible SUSY particle, it would decay into )(�£+£- or )(�qq via a z·, a Higgs boson, a scalar lepton, or a scalar quark. This leads to a similar experimental topology to that for chargino events.

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The most important parameters governing the charginos and neutralinos detection efficiency are the masses mx+ , mx' and the mass difference between the chargino or neutralino and the lightest neutralino: flM+ = (mxi - mx� ) and flMo = (m;<g - mx� ) respectively. The other important parameters are the decay branching fractions. For large flM the visible energy is large and the detection efficiency lies between 40-80%. For small flM the detection efficiencies lie between 10-35%. Interpreting the null results of the above searches in the MSSM framework exclusion regions in the µ - M2 plane were derived. The resulting lower limits on the chargino mass for flM+ > 10 GeV are given in table 1 for large and small m0.

I Experiment II Large mo I Small mo II DELPHI 84.5 (tg/3 > 1.0) 71.3 L3 73.6 (tg/3 > 1 .0, M2 < 1 TeV) ALEPH 79.0 (mv = 200) OPAL 85.0 (tg/3 > 1.5) 70.1

Table 1 : Lower limits on the Chargino mass (in GeV) for large and small m0 and flM+ > 10 GeV

Limits on the Lightest Neutralino Neutralinos might be candidates for dark matter in the universe. Therefore it is of utter importance to set a lower limit on their mass. The most problematic experimental scenario occurs when tan/3 = 1 .000 and M2 � µ � 0. In this region the two chargino masses are close to the W boson mass (Mx� "'" Mxt "'" Mw ) , the two lightest neutralinos are almost massless (Mx� "'" Mxg "'" 0) and the other two neutralino masses are close to the Z boson mass (Mx� "'" Mx� "'" Mz). In this region one of the two lightest neutralinos is an almost pure photino and the other one is an almost pure Higgsino, hence x�xg production in e+e- collisions is suppressed. The heavy neutralinos xg and x� are mixtures of a Zino and the other Higgsino. To be sensitive to the region near M2 = µ = 0, the ALEPH and 13 collaborations studied xgxg or x�xg production with the subsequent decays xg.4 -+ Z(•lxg and xg -+ ;\}y. The topology of events due to these processes is similar to that of e+e- -+ Z1 events except that the photon energy is smaller than in the e+e- -+ Z1 case. At ..jS well above the W pair threshold chargino pair production cross section is typically much larger than the neutralino pair (xgxg or x�xg) production cross sections near M2 = µ = 0. At ..jS = 172 GeV, the sum of the cross sections of the four chargino pair production processes (e+e- -+ xtxl ", xtx2, xtx2 and xtxl) near M2 = µ = 0 is as large as 6 pb, whereas the W pair production cross section is about 13 pb. In the Mrµ region considered here the event shape of the chargino pair events (xtx;- , xtx2, xtx2 or xtxl) is similar to that of ordinary w boson pair events, because each chargino decays into an on-mass shell or' almost on-mass shell W plus an almost massless neutralino with small momentum. These events tend to have somewhat larger missing energy than the ordinary W pair events, as the neutralinos tend to have small but significant momentum. Large neutralino momentum in the rest frame of the chargino is favoured in order to gain phase space in the two body decay of xt -+ W(•J+ x�- On the other hand, the W boson tends to stay near its mass-shell. These two effects determine the momentum spectrum of neutralinos. OPAL and DELPHI searched for an excess of w+w- like events with respect to the expected number of Standard Model events (mainly W boson pairs) . The resulting lower limits on the lightest neutralino are given in table 2 for heavy m0• For light m0 only OPAL performed the analysis and the limit weakens in a significant way to mx� > 12.3 Ge V .

Search for Light Gravitinos. In Gauge Mediated Supersymmetry models, the Gravitino,

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I Experiment II mo I Limit on LSP neutralino II DELPHI 1000 17.0 L3 500 24.6 ALEPH 200 22.0 OPAL 200 24.0

Table 2: Lower limits on the lightest Neutralino mass (in Ge V) for large m0 (given in the second column in Ge V)

G, might be the LSP which couples directly to x� and I· The Gravitinos are produced in association with two photons via an exchange of a selectron in the t-channel in the process e+e- -+ X�X� -+ CC11. The topology is therefore of two acoplanar photons accompanied by missing energy, due to the presence of the undetected gravitinos. The background from c+e- -+ vii// is reduced by requiring that the recoil mass of the observed system be not compatible with a Z boson or that both photons will be energetic. Interpreting the search results following ref [4], OPAL and ALEPH find mxi > 7L0(72.3) GeV respectively, with x� -+ G1. These searches are first of their kind and there is no doubt that GMSB models are becoming more and more popular amongst theoreticians and experimentalists these days.

Sleptons. Sleptons could be pair-produced through s-channel zo or I exchange.

Scalar electrons (selectrons, e) could also be produced through t-channel neutralino exchange (if the neutralino is gaugino-like) enhancing the selectron production cross-section compared to the scalar muon (smuon, ji,) and scalar tau (stau, f) production cross-sections. The next­to-lightest neutralino x� and the lightest charginos. xr are usually assumed to be heavier than the sleptons so the dominant slepton decay mode is expected to be l± -+ e± + x� .

The resulting topology is that of an acoplanar, acolinear pair of leptons with large missing energy. The search efficiency is dependent on Llm = (mi± - mxi ) and ranges between 20-70% for selectrons and smuons and 0-30% for staus.

In the MSSM the SUSY partners of the right handed leptons are always lighter than the SUSY partners of the left handed leptons.

Lower limits were derived in the mxi - mi domain. For a massless X� the following lower limits were derived: m.;, > 70.5 GeV (L3) ,me > 76 GeV (ALEPH) and m.;, > 76 GeV (OPAL).

Stop. The scalar top quark (stop, t), the bosonic partner of the top quark, could be the lightest charged supersymmetric (SUSY) particle. The supersymmetric partners of the right­handed and left-handed top quarks (tL and tR) mix, and the resultant two mass eigenstates (t1 and t2) have a large mass splitting due to the large mass of the top. The lighter mass eigenstate (t1 ) could be lighter than any other charged SUSY particle, and lighter than the top quark itself. The stop quark p�r-pr_?duction cro�s-section depends on the stop mass, m;, , and the mixing angle emix, where ti = k cos emix + tR sin emix· The minimum cross section is obtained at a mixing angle of 0.98 radians where the stop decouples from the Z boson. The maximum cross section is obtained

Two decay modes of the stop quark were considered. t1 -+ c + x� and if the chargino is lighter than the stop the following decay is also possible t1 -+ bX"i -+ blii. The experimental signature for t1 events would therefore be an acoplanar two-jet topology (possibly accompanied

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by two leptons) with large missing transverse momentum with respect to the beam a.xis. The efficiency is dependent on the visible mass, i.e. on t;.M = m1 - mxi in the first case and on mi - m;; in the second case.The results are given in table 3:

· 1 Experiment II ti --+ c + x� I t1 --+ bzv L3 58.3 (AM > 10) ALEPH 68.0(AM > 15) 69.0(m;; = 55) OPAL 64.8(AM > 10) 55.8(m1 - m;; > 10)

Table 3: Lower limits on t1 mass (in GeV) valid for any mixing

Search for the Standard Model Higgs Boson. Higher centre-of-mass energies increase the sensitivity of the searches fbr Higgs bosons towards higher masses. The main production process for the SM Higgs boson is e+ e- --+ HZ although at 172 Ge V there are already contribu­tions of a few percents from WW and ZZ fusion processes to e+e- --+ Hvii and e+e- --+ H e+e­respectively. The dominant decay mode of the Higgs boson is H --+ bb, with a branching ratio of approximately 86%. Other relevant decay modes are: H --+ rr (8%), H --+ cc (4%), and H --+ gluons (2%) . In the mass range of interest, these branching ratios exhibit only a mild dependence on the Higgs boson mass. The main topologies are 4-jets (qijbb, about 60%) with a typical efficiency of 25%, the missing energy (Hvii, 20%, 2 acoplanar jets accompanied with large missing transverse momentum) with a typical efficiency of 40% and the leptonic channels (Hee; H µµ, 3.3% for the electronic and 3.3% for the muonic channel) . These channels have a very clean topology of 2 isolated electrons (or muons) recoiling against two acoplanar jets. Due to the clear signature of these channels they exhibit high efficiencies of up to 75%. Note that all the typical efficiencies are given for a 70 Ge V Higgs at 172 Ge V centre-of-mass en­ergy. As already mentioned in the background section, b-tagging is essential in order to reduce four-fermions background taking advantage of the almost dominant decay mode of the Higgs to bb. The rrqq final states are important for Supersymmetric Higgs searches, and play no important role for the SM Higgs search, though they are taken into account. In that sense, it is interesting to notice that LEP 2 allows for the first time the use of all channels in the search for the Higgs boson (due to the relatively low background from hadronic final states) . For 172 GeV CM energy, a few candidate events were observed by the different experiments. DELPHI and OPAL observed a candidate in the four jets channel (with a mass of 68.8 and 54.l GeV respectivel:y) . ALEPH observed no candidate events. L3 have two analyses. The one that was presented in the LEP jamboree on Feb 97, on which this note is based on, and just recently they changed their analysis completely towards an analysis which does not use the Higgs mass in order to discriminate between the signal and the data. In the original analysis they had one electronic candidate and two hadronic candidates. Those candidates remained in the new analysis as their most significant candidates, but now they allow many more candidates which get a low weight.

All candidate with their mass resolution were taken into account when deriving a lower limit on the Higgs Boson mass. The results are shown in table 4. For L3 both the jamboree and an updated result (especially delivered for this talk) are given. As usual in this note, all numbers are preliminary.

In an attempt to combine all the search results of all experiments taking all candidates into account assuming Gaussian mass resolution and using the method described in ref [5] one

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I Experiment II 95% CL lower limit on the SM Higgs Boson Mass II L3 66.4 (Feb 1997) L3 68.2 (Mar 1997) ALEPH 70.7 OPAL 68.8

Table 4: 95% CL lower limit on the Standard Model Higgs Boson mass. All numbers are preliminary.

arrives at a combined limit of 77 GeV. This result is not accurate but serves as an estimate till the official All-LEP result, which is under work, will be published.

Standard Model Higgs Boson Search Prospects. Based on the yellow book LEP 2 report, one can estimate the sensitivity of LEP to detect (or exclude) the Higgs Boson at higher energies. The result is shown in the following figure, from which one can conclude that for about 70 pb-1 1uminosity per experiment one reaches the maximum sensitivity for exclusion which is mH = 92, 98, 105 GeV /c2 for y'S = 185, 192, 200 GeV respectively. However, for a Su discovery one gains only if one increases the center-of-mass energy to 200 GeV and then with a modest luminosity of about 150 pb-1 one can discover a SM Higgs boson with a mass of 100 GeV /c2 which is a very powerful achievement. The loss of sensitivity at y'S = 200 GeV for lower luminosities is due to the fact that the cross section for Higgs production is higher for y'S = 185 GeV up to about mH = 80 GeV /c2 and also because of the severe ZZ background at higher center-of-mass energies, in particular for mH = mz.

-� 200 :e 180 = 160 e 14-0 1 120 :! 100 ... ii. 80 i 60 ...l 40 20 0 so

E. Grou

_ , .. ""' 192C.V .•••••. 200C.V

60 70 80 90 100 110 m8 (GeV)

.. - 200 i,180 i 160 e 14-0 1i 120 "" ll 100 ... !. 80 i 60

...l 4-0 20 0 so 60 70 80 90

Figure 1: SM Higgs sensitivity for higher centre-of-mass energies

E. Grots

100 110 m8 (GeV)

Conclusions. Although no new physics was found at LEP 2.0 (130-172 GeV centre-of­mass energy) some improvements were obtained on mass limits of supersymmetric particles. In particular, for large common mass scale, mo and tanf3 > 1 .0, charginos lighter than 85 GeV /c2 and a lightest SUSY particle (x8) with a mass smaller than 24.0 GeV /c2 were excluded at the 95% C.L. The lower limit on the Higgs Boson mass was raised significantly to 70.7 GeV /c2 .

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The LEP machine performed smoothly and promised bright search opportunities at 185 GeV center-of-mass energies.

Acknowledgements I would like to thank the following physicists who helped me in preparation of this talk: M. Schmitt, P. Janot (ALEPH), D. W. Venus, F. llichard (DELPHI). M. Pieri (L3)

References

[l] CERN Particle Physics Special Seminar 25/2/97, results from the LEP 172 GeV Run, OPAL presented by S.Komamiya, ALEPH presented by Glen Cowan L3 presented by Marco Pieri and DELPHI presented by Francois Richard.

[2] S. Katsanevas and S. Melanchroinos, published in the LEP200 workshop report, CERN 96-01 . The formulae found in the following references are employed: Ref. 6 and A. Bartl, H. Fraas, and W. Majerotto, Nucl. Phys. B278 ( 1986) 1 , and Z. Phys. C34 (1987) 411 .

[3] A. Bartl, H. Fraas and W. Majerotto, Z. Phys. C30 ( 1986) 441; A. Bartl, H. Fraas and W. Majerotto, Z. Phys. C41 (1988) 475; A. Bartl, H. Fraas, W. Majerotto and B. Mosslacher, Z. Phys. C55 ( 1992) 257.

[4] S. Dimopoulos et al. , Phys. Rev. Lett. 76 ( 1996) 3494; D.R. Stump, M. Wiest, C.P. Yuan, Phys. Rev. D54 ( 1996) 1936; S. Ambrosanio et al., Phys. Rev. D54 ( 1996) 5395; J. Ellis, J. L. Lopez, D.V. Nanopoulos Phys.Lett.B397:88-93, 1997, Phys.Lett .B394:354-358,1997; J. L. Lopez, D.V. Nanopoulos,Phys.Rev.D55:5813-5825,1997.

[5] P. Bock, Determination of exclusion limits for particle production using different decay channels with different efficiencies, mass resolutions and backgrounds. Heidelberg University Preprint, HD-PY-96/05 (1996)

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