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EPJ manuscript No. (will be inserted by the editor)

Gaugino searches and constraints on

Paolo Azzurri Scuola Normale Superiore, Piazza dei Cavalieri 7, 56100 Pisa, Italy.

Received: date / Revised version: date

Abstract. The negative outcome of gaugino searches at LEP has been one of the most disappointing results for fans of supersymmetric models. In the framework of minimal supersymmetric models with GUT unification assumptions, the combined absence of supersymmetry and Higgs signals in the LEP data, sets stringent constraints on the models parameter space, and lower limits on the mass of the lightest and of other supersymmetric . All limits are given at 95% Confidence Level.

PACS. 11.30.Pb Supersymmetry – 12.60.Jv Supersymmetric models – 13.66.Hk Production of non- particles in e-e+ interactions – 14.80.Ly Supersymmetric partners of known particles

1 Gaugino searches searches come from four- events as W-pairs,Z-pairs, Weν and Zee. In the case of low ∆M, main backgrounds + The production of gauginos in e e−collision has certainly arise from two collisions as γγ ττ, [2]. In been the most expected signal of supersymmetry (SUSY) scenarios where ∆M is smaller that 3GeV→ /c2 the γγ back- during the last ten years. In Minimal Supersymmetric ex- ground can only be reduced by requiring the presence of tensions of the Standard Model (MSSM models [1]), gaug- an initial state radiation (ISR) photon with large trans- inos are the fermionic of Standard Model verse momentum, that will also ensure the signal trigger- (SM) gauge and Higgs , making up two charged ing, but at the expense of a signal selection efficiency lower gauginos ( χ1±,2) and four neutral gauginos (neu- than 3%. Finally when ∆M is below the produc- 0 tion threshold, charginos can be detected as heavy stable tralinos χ1,2,3,4). In models where SUSY breaking is me- diated by gravity, and with unification assumptions at the charged particles [3]. A very nice -pair candidate 0 is shown in fig. 1. GUT scale, the lightest neutralino (χ1), is the natural lightest SUSY (LSP). When R-parity further con- serves and number, the LSP is stable and 1.2 Neutralino-pair searches weakly interacting, and is of interest as a possible cosmo- logical component of non-baryonic cold dark . The Signal topologies arising from pair productions of heavy SUSY models considered in the following will assume a can be quite different due to a variety of de- neutralino-LSP, GUT unification in all sectors and con- cay channels and to possible cascade processes. The main servation of R-parity, implying that SUSY particles can searched topology arises from only be produced in pairs. + 0 0 0 0 e e− χ χ ffχ χ (2) → i 1→ 1 1 and gives final states with acoplanar jets and acoplanar 1.1 Chargino-pair searches . When two fermion pairs are produced as from + 0 0 0 0 e e− χ χ ffffχ χ (3) In chargino-pair productions each produced chargino will → i j → 1 1 decay to a fermion-pair and a LSP according to the chain the searched final states are multi-leptons or multi-jets with missing energy. Finally to cope also with possible + + 0 0 0 0 e e− χ χ− ff 0χ ff 0χ . (1) radiative χ χ γ decays signals as → → 1 1 i → j + 0 0 0 0 0 e e− χ χ ffγχ χ (4) The χ1 pair will carry away undetected energy while the → i j → 1 1 two fermion-pairs (ff 0ff0) can give rise to three main topolo- are also searched for, with jets, and missing en- gies with (i) two leptons (`ν`ν), (ii) one lepton and two ergy. Again different searches are performed according to jets (qq`ν), or (iii) multi-jets (qqqq). The relative amount possible ∆M mass differences between the heavy neutrali- 0 of visible and missing energy will depend mainly on the nos and the χ1-LSP, with SM backgrounds arising mainly mass difference ∆M between the chargino and the LSP. from four-fermion processes at large ∆M, and from two- For large ∆M values the main SM backgrounds for the photon collisions at small ∆M [2]. 2 Paolo Azzurri: Gaugino searches and constraints on supersymmetry

DALI_F2 ECM=195.6 Pch=35.0 Efl=45.5 Ewi=15.5 Eha=18.8 zd3001_2 Run=50140 Evt=18802 ALEPH Nch=6 EV1=.942 EV2=.271 EV3=.028 ThT=.943 99−07−05 :39 Detb= E1FFFF 4.5Gev EC 5.4 Gev EC 7.7Gev HC 3.8 Gev HC

(φ −0 )*SIN(θ)

o o o−xox−

x o 16. GeV θ=180 θ=0

Fig. 1. A perfect chargino-pair candidate event recorded by ALEPH at √s=196 GeV. The event display shows an isolated with 8.5 GeV/c momentum and an hadronic monojet with a 16 GeV/c2 invariant mass. The total missing invariant mass in the event amounts to 148 GeV/c2, with a transverse missing energy component of 21 GeV. The possible Standard Model source of such an event is a W-pair decay WW∗ τ( µνν)νqq, where the hadronic decay of the second W is extremely off-shell. → →

2 Lower LSP mass limits gauginos. Using dedicated searches of different decay topolo- gies and direct searches, the MSSM parameter In the LEP data up to √s=209 GeV none of the SUSY space can, however, be covered in a way similar to the searches has revealed an excess of signal events over the case of heavy , and the LSP limit still holds at expected SM processes. Therefore upper limits on gaugino m(χ+) > 39 GeV/c2,withtanβ 1[2]. pair production cross sections have been set at the level of ≈ less than a pb for all main topologies, when kinematically accessible. These cross section limits can be translated in 2.3 Exclusions from Higgs searches exclusion domains in the constrained MSSM parameter space (tan β, µ, M2,m0), and from here in lower mass lim- The negative outcome of searches [4] allows its on the LSP and other SUSY particles. to exclude MSSM regions with tan β<2 3, bringing the LSP mass limit at m(χ0) > 50 GeV/c2 with− tan β 3in the case of heavy sfermions, and at m(χ0) > 45 GeV≈ /c2 2.1 Limits with heavy sfermions with tan β 10 in the case of light sfermions, where possi- ble sneutrino-chargino≥ mass degeneracies limit the exclu- When sfermions are heavier than electroweak bosons they sion reach [2]. do not interfere with the gaugino production and decay. In this scenario the expected gaugino production cross sec- tions are in the 1-10 pb range. The excluded parameter 2.4 Effects of sfermion mixing space corresponds roughly to domains where the chargino mass is within the kinematic production limit, i.e. m(χ+) The possible effects of mixing in the third family of sfermions √s/2 100 GeV/c2. The corresponding limit on the LSP≤ (˜τ,b˜,˜t) have also been considered, and open more mass mass can≈ be set at m(χ0) > 39 GeV/c2,attanβ 1[2]. degenerate scenarios where standard searches lose their ≈ sensitivity. The sensitivity is recovered with more dedi- cated searches, in particular for lightτ ˜1 signals in gaugino 2.2 Limits with light sfermions cascade decays, and the final LSP mass limits are only slightly deteriorated to m(χ0) > 37 GeV/c2 at tan β 1, The effects of light sfermions (with a GUT unified mass and are maintained at m(χ0) > 45 GeV/c2 at large tan≈ β, m0) are significant both in the production and decay of including Higgs exclusions [2]. Paolo Azzurri: Gaugino searches and constraints on supersymmetry 3

A summary of LSP mass limits in the constrained the derivation of the mass limits. The proper inclusion of MSSM is shown in table 1, with different hypothesis and these corrections could deteriorate the LSP mass limits at constraints. the level of 1-2 GeV/c2, that sets the level of precision of the quoted results.

Table 1. Summary of lower mass limits on the LSP

0 3 Lower mass limits on other SUSY particles m(χ1) limit any tan β with Higgs constraints heavy sfermions >39 GeV/c2 >50 GeV/c2 Using the same exclusions of the constrained MSSM pa- 2 2 rameter space used to derive the LSP limits, lower mass any m0 >39 GeV/c >45 GeV/c with ˜f mixing >37 GeV/c2 >45 GeV/c2 limits on other SUSY particles can be derived, and are shown in table 2, again under the assumption of uni- fied light sfermions and using the constraints from Higgs searches [2].

2.5 Limits in minimal supergravity Table 2. Other lower mass limits on SUSY particles

In the framework of minimal supergravity (mSUGRA) the SUSY particle lower mass limit with MSSM parameters are further reduced and the LSP mass any m0 and Higgs constraints limits, with light sfermions, mixing and Higgs constraints, + 2 0 2 chargino m(χ ) > 90 100 GeV/c improves to m(χ ) > 50 60 GeV/c , depending on the − 2 − sneutrino m(˜ν) > 85 95 GeV/c top mass [5], as shown in fig. 2. − 2 right selectron m(˜eR) > 75 95 GeV/c − 2 left selectron m(˜eL) > 110 GeV/c

< 2 2 ADLO preliminary Any A0, m0 1 TeV/c , Mtop= 175 GeV/c

) 80 2 75 µ > 0 70 References 65 2 A0=0, Mtop= 175 GeV/c 1. H. P. Nilles, Phys. Rep.110, (1984) 1; H. E. Haber, (GeV/c

60 2 lim χ A =0, M = 180 GeV/c 55 0 top G. L. Kane, Phys. Rep.117, (1985) 76. M 2. ALEPH Coll.,Absolute mass lower limit for the lightest neu- 50 + 45 tralino of the MSSM from e e− data at √s up to 209 GeV 40 CERN-EP/2003-077 (submitted to Phys. Lett. B). 35 Excluded at 95% C.L. DELPHI Coll., Searches for supersymmetric particles in + 30 e e− collisions up to 208 GeV and interpretation of the re- 10 20 30 40 tanβ sults within the MSSM CERN-EP/2003-007 (accepted by Eur. Phys. J. C).

) 80 + 2 75 µ < 0 L3 Coll., Search for Supersymmetry in e e−collisions at √s= 70 202-208 GeV L3 note 2731. OPAL Coll., Search for Chargino and Neutralino Production 65 2 A0=0, Mtop= 175 GeV/c

(GeV/c at √s=192-209 GeV at LEP OPAL Note PN503. 60 2 lim χ A =0, M = 180 GeV/c 55 0 top 3. ALEPH Coll.,Search for charginos nearly mass degenerate

M + 50 with the lightest neutralino in e e−collisions at centre-of- 45 mass energies up to 209 GeV Phys. Lett. B 533 (2002) 40 223-236. 35 Excluded at 95% C.L. DELPHI Coll.,Search for charginos nearly mass-degenerate 30 10 20 30 40 with the lightest neutralino Eur. Phys. J. C 11 (1999) 1; tanβ Phys. Lett. B 485 (2000) 95. L3 Coll.,Search for charginos with a small mass difference to Fig. 2. Combined LSP lower mass limits in mSUGRA, as a the lightest supersymmetric particle at √s=189 GeV Phys. Lett. B 482 (2000) 31-42. function of tan β for µ>0 (top), and µ<0 (bottom). OPAL Coll.,Search for Nearly Mass-Degenerate Charginos and Neutralinos at LEP CERN-EP/2002-063 (submitted to Eur. Phys. J. C).

2.6 Radiative corrections 4. LEP Higgs Working Group, Search for the Standard Model Higgs Boson CERN-EP/2003-11 (submitted to Phys. Lett. Radiative corrections to the gaugino unification relation B). 5. LEP SUSY Working Group, LSP mass limit in Minimal M1 = 5 tan2 θ2 1 and to the tree level gaugino masses M2 3 W ≈ 2 SUGRA (LEPSUSYWG/02-06.2). are at the level of 5% and are not taken into account in