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Preliminary Results from LEP 1.5 High Energy RunAnna Lipniacka E-mail address: [email protected] of StockholmIn November 1995 for the �rst time LEP provided e+e� collisions well above Z0peak. The luminosity of � 6 pb�1 per experiment at energies 130-140 GeV wasprovided. In this report preliminary results of four LEP experiments on ElectroWeak model tests and searches for New Phenomena are presented.1 IntroductionAt the end of 1995 LEP data taking period the centre of mass energy was raised to130 GeV, 136 GeV and 140 GeV. Luminosities collected per experiment at these energieswere respectively 3pb�1, 2:9pb�1 and 0:03pb�1. Interactions at these energies are verydi�erent from those at Z0 pole. The interactions dominate with a cross-section of �10 nb. Cross-section for quark and lepton pair production are two orders of magnitudesmaller then at the Z0 pole and the e�ect of initial state radiation (ISR) leading to theradiative return to Z0 is very large . Figure 1 show an example of this process observedby the DELPHI detector. Out of many Electroweak and QCD measurements obtainedby four LEP experiments only measurements of cross-sections of two fermion �nal statesand resulting Electro-Weak model tests together with new limits on anomalous W andZ coupling will be presented in this report.Higher energy provided opportunities for searches for new particles with no back-ground from e+e� !W+W� threshold. New data expanded signi�cantly the accessibledomain of SUSY searches. In this report searches guided by the Minimal Supersym-metric Standard Model (MSSM) [1] for super-partners of the top quark (stop) and forcharginos and neutralinos which are super-partners of neutral and charged Higgs andgauge bosons and charged are emphasised.A search of two-jet mass resonances in a four jet �nal states motivated by pair pro-duction of e+e� ! A0+ h0 of Higgs bosons predicted by MSSM lead to an observationof an excess of four jet events reported by ALEPH [2]. Similar searches performed byother LEP collaborations will be presented.2 Electro-Weak ResultsTwo fermion �nal states ( q�q, �+��, e+e�, �+��) are selected after requiring a su�-ciently high cut ( � 60 GeV) on the visible mass of all reconstructed particle above acertain polar angle. This cut rejects the background from interactions. In additionlepton selection [8] is applied to select leptonic �nal states. In �gure 2 the distributionof the number of q�q( ) and �+��( ) as a functions of the energy of the fermion pair isc Institute of Physics, SAS, Bratislava, Slovakia 1
2 Anna Lipniackashown [3]. A peak at 90 GeV corresponding to the radiative return to Z0 is clearly visi-ble together with a peak at the CMS energy corresponding to the non-radiative events.In the High Energy Run the luminosity was measured with the typical accuracy of 1%, the accuracy of cross-sections for the radiative and non radiative processes shownin �gure 3 and in the Table 1 [4] is limited statistically. A good agreement with theElectro-Weak model prediction is observed and results are consistent among four LEPexperiments.Total cross-sections (pb) at p(s) = 130 GeV, except of DELPHI � and � results which areinclude p(s) = 136 GeV data. Nonradiative cross-section is for events with s0=s > 0:9 forALEPH and DELPHI, s0=s > 0:8 for OPAL and p(s0) > 110 GeV for L3. The polar anglecut in ee( ) is jcos(�)j< 0:9 for ALEPH and jcos(�)j< 0:7 for other experiments.Process ALEPH DELPHI L3 OPALq�q( ) 343� 11� 8 330� 12 343� 11 318� 12� 6q�q 75� 6� 3 82� 6 67� 6� 4ee( ) 47� 4 52� 5 50� 4� 1ee 219� 9� 1 38:5� 4:0 43� 4 42� 4� 1��( ) 24� 3� 1 26� 4 26� 4 23� 4� 3�� 10� 2� 0:4 7:5� 1:3 7:7� 2 9:5� 1:8� 0:5��( ) 25� 3� 1 25� 4 26� 5�� 12� 3� 1 9:4� 3 8:0� 2� 1��( ) 9� 2 12� 4 11� 3 3:3� 1� 0:1The shape of the q�q( ) cross-section as function of CMS energy (p(s)) can beused to determine in a model independent way the � Z0 interference term (jh).ALEPH, DELPHI and L3 [4] used the S-matrix [9] ansatz to parametrise the cross-section in terms of �h = �(s;MZ ; jh). In the Standard Model jh = 0:22 with mtop =180GeV=c2;mH = 300GeV=c2. Thanks to the high energy data the correlation betweenthe value of MZ and jh are reduced in the �t. The values of jh = �0:39 � 0:44 andMZ = 91:199� 0:012 are obtained by ALEPH.At p(s)=130-140 GeV a process of Z0 radiative return with Z0 decaying to ���e+e� ! ���( ) can be detected by a single high energetic (� 30 � 40 GeV ) . Thesingle cross-section is a standard tool for the model independent neutrino counting.N� = 3:1� 0:6� 0:1 was obtained by DELPHI [5]. The angular and energy spectrumof single can be used to limit possible anomalous trilinear coupling of gauge bosonsZV where V = ( ; Z0) Such a vertex vanishes in the tree-level of the Standard Modeland can be described in a model independent way with four form-factors hVi=1;4 [10].DELPHI data [5] where used to put a 95 % con�dence level limit: jh 1;3j < 3:8 andjh 2;4j < 9:1. The limit on h 1;3 is close to the one set by Tevatron experiments [11].In the Standard Model the cross-section for e�W �nal state at 130-140 GeVps is ex-pected to be in a range of 0.15-0.2 pb which would lead to 0.9-1.2 event of e+e� ! e�Wto occur at the high energy run. At this energies the dominating single W productionmechanism is �W � collision. With 1 candidate event observed by DELPHI a limit can
Results from LEP High Energy Run 3be put on parameters � and � describing the trilinear gauge boson coupling WW .In the SM � = 0 and � =1. The 95 % con�dence level limit from DELPHI is j� j < 0:6if � =1 and �9 < (� � 1) < 7 if j� j < 0.3 SearchesChargino (�+�) and neutralino (�0i=1;4) can pairs can be produced through an s-channelvirtual boson or via t-channel 11 sneutrino (selectron) exchange, with cross-sections ofthe order of several pb. For low sneutrino masses the sneutrino mediated t-channel in-terferes destructively with the s-channel diminishing chargino production cross-section.Within MSSM the most probable production - decay chain is in case of charginos:e+e� ! �+�� !W �W ��01�01and;e+e� ! �01�02 ! Z��01�01in case of neutralinos.The lightest neutralino �01 is assumed to be stable and weakly interacting and thusescapes detection. Production of neutralino pairs results in events with missing energyand momentum with acoplanar pairs of jets or leptons in the �nal state. In chargino pairproduction a �nal state consisting of one charged lepton and two jets is also possible.The detection e�ciency depends on the mass of �01 and on the mass di�erence between�+� (or �02) and �01. For mass di�erences of � 5 GeV ( degenerate case) the detectione�ciency is near 10 % and is rising to � 70 % for mass di�erence of 30 GeV.DELPHI observes 1 candidate for the leptonic neutralino or chargino decay (�gure5 ). It is an acoplanar electron pair with the invariant mass of 4.6 GeV=c2 and visiblept = 15GeV=c. The candidate is compatible with the background of 0.2 events expectedfrom four-fermion processes. Other LEP experiments do not observe any candidates.Limits on chargino mass obtained by ALEPH [6] are presented at �gure 4. In thedegenerate case, where chargino is a pure Higgsino and does not couple to sneutrinos aninteresting limit on the chargino mass is obtained by DELPHI (�gure 4 ). Neutralino andchargino searches allowed for signi�cant increase of excluded area of MSSM parametersspace beyond LEP 1 limits [12] as it is ilustrated on �gure 5 .The super-partner of the top quark (the stop ) could be the lightest scalar quark inthe MSSM. Because of the large mass of the top quark, the scalar partners of its twochirality states, sR and sL, are expected to mix [13], and the lightest physical state canbe signi�cantly below the typical mass of scalar quarks, and thus accessible at LEP. Ifkinematically allowed, the stop is expected to decay into c�01 with a branching ratio closeto unity which leads to events with two jets, missing energy and momentum, similar tothe �01�02 �nal state. The expected cross section for scalar top quark production is inthe pb range close to the kinematic limit, but may be reduced if left-right mixing causede-coupling from Z0 in the s-channel, leaving only the photon contribution. Figure6 shows the mass combinations excluded, at the 95% con�dence level, together withresults from previous searches at LEP1 and the Tevatron [14]. The results are shownfor the two extreme cases of stop mixing.
4 Anna Lipniacka4 Four Jet EventsIn the MSSM one can expect pair production of e+e� ! A0 + h0 with a subsequentdecays ofA0 and h0 into b-quarks. Search for four b-quarks �nal states which starts withthe life-time tagging of b-quarks gave negative results [7] in all four LEP experiments.ALEPH collaboration reported an excess of four-jet events with high dijet mass pair asillustrated on �gure 7. The probability of this excess to be a statistical uctuation wascalculated to be P = 0:1%. The cross-section of possible dijet resonances production is3.1�1.7 pb. No other experiment reported a similar excess (�gure 8), L3 put a 95 %con�dence limit for cross-section of possible pair production of 55 GeV dijet resonancesto be below 3.1 pb. However results of other experiments are still compatible with theALEPH result with probability of 5 % [2].5 Summary and ConclusionsThe LEP 1.5 high energy run provided plenty of interesting results in Electroweaksector and extended signi�cantly available domain of new particles searches. Althoughno SUSY particles has been found up to now, the new area of MSSM parameter spacewas explored. LEP run at 161 GeV starting in the summer 96 will provide next stepin the MSSM model exploration (and may be discovery of SUSY particles) and hint asolution of ALEPH four-jet puzzle.6 AcknowledgementsI would like to thank the Organisers of the Hadron'96 for inviting me to this inspiringConference. I am grateful to Gigi Rolandi of ALEPH, David Stickland of L3 andDavid Ward of OPAL for providing me with results prior to the publication. MyDELPHI colleagues from the University of Stockholm: Klas Hultqvist, Reino Keranen,Per Andersson and Richard Jacobsson helped me to obtain results and �gures presentedin this report. References[1] H.P. Nilles: Phys. Rep. 110 (1984) 1;H.E. Haber: Phys. Rep. 117 (1985) 75;[2] L.Rolandi: Aleph Collaboration, CERN LEP 1.5 seminar (Dec 1995) ;P.Janot: Aleph Collaboration, CERN PPE seminar (March 1996) ;[3] The DELPHI Collaboration: DELPHI note 96-29 PHYS 603 (1996) ;[4] The ALEPH Collaboration: CERN-PPE/96-45 (1996) ;The DELPHI Collaboration: DELPHI note 96-29 PHYS 603 (1996) ;The L3 Collaboration: CERN-PPE/95-191 (1995) ;The OPAL Collaboration: DELPHI note PN201 (1995) ;[5] The DELPHI Collaboration: DELPHI note 96-20 PHYS 595 (1996) ;
Results from LEP High Energy Run 5[6] The ALEPH Collaboration: CERN PPE 96-10 (1996) ;[7] L.Rolandi: Aleph Collaboration, CERN LEP 1.5 seminar (Dec 1995) ;H. Dijkstra: DELPHI Collaboration, CERN LEP 1.5 seminar (Dec 1995) ;D. Stickland: L3 Collaboration, CERN LEP 1.5 seminar (Dec 1995) ;[8] DELPHI Collaboration, P.Abreau et al.: CERN-PPE-95-104 (December 1995) ;ALEPH Collaboration, D.Buskulic et al.: Z. Phys. C92 (1994) 539;[9] A. Leike, T. Riemann and J. Rose: Phys. Lett. B 273 (1991) 513;[10] U. Baur and E. Berger : Phys. Rev. D47 (1991) 4889;[11] F. Abe et al.: Phys.Rev. Lett. 74 (1995) 1941;S. Abachi et al.: Phys.Rev. Lett. 75 (1995) 1034;[12] ALEPH Coll., D. Decamp et al.: Phys. Rep. 216 (1992) 253;L3 Collaboration: M. Acciarri et al.: Phys.Let.B 350 (95) 109;A. Lopez-Fernandez, DELPHI note 92-95: Dallas ( PHYS 206) ;OPAL Collab.,: OPAL Physics Note PN194 (July 1995) ;[13] M. Drees and K. Hikasa: Phys.Let.B 252 (90) 127;[14] D0 Collaboration, S. Abachi et al: FERMILAB-PUB-96-003-E (January1996) ;
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An event of radiative return to Z0 (e+e� ! Z0 ) at CMS energy 130 GeV. A 40 GeV initialstate radiation photon was registered in the Forward Elecromagnetic Calorimeter.
Results from LEP High Energy Run 7Reduced energy
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Number of e+e� ! q�q events (upper plot) and e+e� ! �+�� (lower plot) as a function ofs' - e�ective energy of the fermion pair.
8 Anna Lipniacka
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60 70 80 90 100 110 120 130 140 150Total cross-sections of various e+e� �nal states as a function of the CMS energy (s0=s > 0:01),measured by OPAL. The curves show predictions of ZFITTER for multihadrons and muon pair�nal states, that of ALIBABA for the e+e�, and that of an O(�3 for the �nal state. Thee+e� �nal state cross-section is measured with a polar angle cut (jcos(��e )j < 0:7).
Results from LEP High Energy Run 9
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Two upper plots show ALEPH limit on the chargino mass as a function of � for four valuesof sneutrino mass and as a function of sneutrino mass (lower plot). The lowest plot shows thecross-section for the chargino pair production as a function of chargino mass in the \degeneratedcase", when chargino is a pure Higgsino an does not couple to sneutrinos. A limit on thechargino mass obtained by DELPHI in this case is indicated.
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The upper plot shows the DELPHI �01�02 or �+�� candidate, an acoplanar electron pair withmomenta 8 GeV/c and 12 GeV/c and invariant mass of 4.6 GeV=c2. The event can come fromthe process e+e�� > e+e��� (0.2 events expected) The lower plot shows regions in the ��M2plane excluded from neutralino search at 95% con�dence for tan(�)=1.5. The solid, dashed,and dotted lines correspond to m0=1 TeV/c2, MZ , and 30 GeV/c2 , respectively. The shadedareas show the regions excluded by LEP 1 for the same three values of m0 . The thin line isthe kinematic limit for �01�02 production at ps = 136 GeV .
Results from LEP High Energy Run 11
Exclusion regions in the mstop � �01 plane at the 95% con�dence level. Two di�erent mixingscenarios are considered. Also shown are results from previous searches at LEP1 and theTevatron.
12 Anna Lipniacka
The distribution of the sum of the dijet masses for selected four-jet events with high dijetmasses obtained by ALEPH. Shaded area is the Monte Carlo prediction
Results from LEP High Energy Run 13
The upper plot shows distribution of the sum of the dijet masses for selected four-jet eventswith high dijet masses obtained by DELPHI. Shaded area is the Monte Carlo prediction. Thelower plot shows the expected h0A0 production signal with the same cuts and cross-section of3 pb.
