Preliminary results of measurements of fermion-pair production at LEP2 are
used to derive limits on new physics phenomena. Combinations of the cross
sections and asymmetrie of the 4 LEP collaborations are interpreted in terms
four-fermion contact interactions, exchange of Z' and leptoquarks. Results on
the search for extra dimensions are also presented.Comment: 4 pages, 3 figures, 2 tables; Proceedings of the Conference "QCD and
  High Energy Hadronic Interactions", Rencontres de Moriond, 18 - 24 March 200

Riemann, Sabine

English

arXiv

Preliminary results of measurements of fermion-pair production at LEP2 are used to derive limits on new physics phenomena. Combinations of the cross sections and asymmetrie of the 4 LEP collaborations are interpreted in terms four-fermion contact interactions, exchange of Z' and leptoquarks. Results on the search for extra dimensions are also presented.Preliminary results of measurements of fermion-pair production at LEP2 are used to derive limits on new physics phenomena. Combinations of the cross sections and asymmetrie of the 4 LEP collaborations are interpreted in terms four-fermion contact interactions, exchange of Z' and leptoquarks. Results on the search for extra dimensions are also presented

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SEARCH FOR EXOTIC PHYSICS WITH FOUR-FERMION COUPLING AT LEP SABINE RIEMANN DESY Zeuthen, Platanenallee 6, D-15738 Zeuthen, Gerrnay Preliminary results of measurements of fermion-pair production at LEP2 a.re used to derive limits on new physics phenomena. Combinations of the cross sections and asymmetries of the 4 LEP collaborations are interpreted in tem1s four-fermion contact interactions, exchange of Z1 and leptoquarks. Results on the search for extra dimensions. a.re also presented. 1 Introduction: Fermion-Pair Production at LEP2 In the years from 1995 to 2000 LEP delivered an integrated luminosity of 700 pb-1 to each experiment at c.m.s. energies between 130 GeV and 209 GeV. This allows a precise measurement of the process e+e- � ff and therefore a test of the Standard Model at high energies. The individual experiments' analyses of cross-sections are combined for the e+ e- � qq, µ+ µ-, r+r- , bb, cc channels, forward-backward asymmetry measurements are combined for the µ+µ-, r+r- , bb, cc final states. The averages are made for the samples of events with JSi > 0.85./8 in order to suppress a 'pollution' of potential signals of new physics by radiative returns to the Z peak. A detailed description of the combination procedure can be found in Reference 1 and references therein. The results are shown in Figure 1. The combination of published and preliminary results of the Rq = (l'q/(l'had and A'}8 measurements 1 are shown in Figure 2. 2 Interpretations Although all measurements confirm the Standard Model predictions so far, new physics phe­nomena could be hidden in small deviations from theoretical expectations. A fit to the combined data including a variety of new physic models may reveal new effects or to place limits on their parameters. All limits presented here are given at the 953 confidence level. 7 1  preliminary � ;1--.---�t =� t .;=,.� I 1 20 1 40 1 60 1 80 200 220 ..JS (GeV) "E 0.6 1 � 0.4 a;i "E � 0.2 0 u. preliminary t Figure 1: Combined LEP results on cross sections and forward-backward a.symmetries for qq, µ+µ- and r+r­fina.I states a.s a function of centre-of-mass energy. r:t' 0.26 LEP preliminary 0.24 Rb 0.22 .-*"· ···-. .. .. 0.2 ." 0.18 0.16 0.14 .Js' Ns >."-) . 0.85 80 100 120 �"0·32 LEP preliminary 0.3 0.28 RC 0.26 0.24 0.22 0.2 0.18 0.16 0.14 , . .  .Js' Ns > <.;. � • 0.85 140 160 180 200 -.Is (GeV) 80 100 120 140 160 180 200 -.Is (GeV) i <( 0.6 .6 0.4 0.2 0 //" -0.2 .Js· 1.Js >. u.;, . 0.85 80 100 120 140 160 180 200 b' 1.2 l "' 0.8 1 u 0.6 0.4 0.2 0 -./s (GeV) LEP preliminary � A� ,,----i--... -.. ····+· ···-... -" . I / .I. .Js'Ns > O i . 0.85 120 140 160 180 200 -./s (GeV) Figure 2: Combined LEP results on Ro, R.,, and on forward-backward a.symmetry for bb and cc fina.I states a.s a function of centre-of-mass energy. Table 1: 95% confidence level lower limits on the Z' mass and x, ,P, 'f/, L-R and SSM models. Model X I 1/; I 7J I L-R I SSM I m�'T' (GeV /c2 ) 715 I 478 I 454 I 862 I 2090 I 2. 1 Contact Interactions Four fermion contact interactions 2 parametrise interactions beyond the Standard Model by an effective scale, A.. Models can be defined by assuming different helicity couplings between initial and final state currents with either constructive ( +) or destructive(-) interfence between Standard Model and contact term. No deviations e oc 1/ A. 2 from the Standard Model predictions have been obtained for the considered final states. Bounds on e were derived and converted into limits on A.+ and A.- . The results are shown in Figures 3 for averaged and combined µ+µ- and r+r- final states assuming lepton universality, for bb and for cc final states. LF.JI Combined Preliminary A. At LL HlO 16.l RR 9.6 15.3 VY 17.2 25.3 AA 13.9 21.8 RL 9.0 10.7 LR 9.0 10.7 VO 13.9 22.3 AO l3.8 15.0 11 20. A' (TeV) 20. At (TeV) bb - LEP Combined preliminary LL RR w AA LR RL VO AO 1 5 TeV K A+ 1 5 TeV cc - LEP Combined Preliminary LL RR w AA LR RL VO AO 15 TeV Figure 3: Limits on the scale of Contact Interactions in e+e- -+ 1+1-, bb and cc using LEP combined results from 133 to 209GeV. 2.2 Extra Z Bosons Additional heavy gauge bosons, Z', are predicted by many theories. The combined hadronic and leptonic cross sections and leptonic forward-backward asymmetries were used to fit the data to models from an E5 GUT or from left-right symmetries 3 . No evidence for the existence of a Z' was found. Table 1 presents limits on the mass mz• for the special models x, 1/;, 7J and L-R and for the Sequential Standard Model (SSM) , that proposes a Z' with exactly the same couplings to fermions like the Z. 2.3 Leptoquarks Leptoquarks mediate lepton-quark transitions. Following the basics in References 4, scalar (S) and vector (V) leptoquarks could be exchanged in the u- or t-channel of the process e+e- -+ qq. Assuming a coupling of electromagnetic strength, g = J41l"aem, and one type of leptoquarks much lighter than the others, limits on the masses of leptoquarks are derived and presented in Table 2: Lower limits on the LQ mass assuming 9£,R = V4ro. For S1;2 (L) no limit can be set. j LQ type I So(L) -+ eu So(R) --t eu So(R) --t ed S1 (L) --t eu, ed S1;2 (L) --t eil S1;2 (R) --t eil, ed S112(L) --t ed I 789 II VidL) --t ed 639 V1;2 (R) --t eu, ed 210 V1;2 (L) --t eu 364 V0(L) --t ed 189 Vo(R) --t ed 240 V0 (R) --t eil - Vi (L) --t eil, ed I 305 I 227 176 1070 167 497 664 Table 2. In most cases the kinematical limits of searches for direct leptoquark production at LEP are exceeded by this indirect search. Further, the bounds presented here, complement the searches at HERA 5 .  2.4 Extra Dimensions Recently, theories of quantum gravity in extra spatial dimensions 6 have been an elegant means to escape the hierarchy problem: Gravitons are considered to propagate in a compactified higher dimensional space while Standard Model particles exist in the usual 3+1 space-time dimensions. The Planck mass in the 'normal' 4 dimensions is related to a fundamental scale Mv � Aew in n+4 dimensions by M'j,1 = Rn M�+2 , where R is the compactification radius of extra dimensions. The exchange of gravitons could contribute to the processes e+e- --t ff, '"'f''/, ZZ, w+w- . The search for indirect effects of low scale gravity was performed by all LEP collaborations seperately 7 and has not yet been combined. The best sensitivity can be reached in the process e+e- --t e+e- . Including this channel, preliminary results on Ms oc Mv range between 0.98 TeV /c2 and 1 . 17 TeV /c2 for a theory parameter ,\ = -1  and between 0.84 TeV /c2 and 1 .06 TeV /c2 for ,\ = +l. Acknowledgments I thank my colleagues from the LEP collaborations and the LEPEWWG Fermion Pair Subgroup. References 1 .  LEPEWWG Fermion Pair Subgroup, LEP2FF/00-03 . 2. H. Kroha, Phys. Rev. 046 (1992) 451. 3. P. Langacker, R.W. Robinett and J.L. Rosner, Phys. Rev. 030 ( 1984) 1470; D. Londond and J .L. Rosner, Phys. Rev. 034 (1986) 1530; J .C. Pati and A. Salam, Phys. Rev. 010 (1974) 275; R.N. Mohapatra and J.C. Pati, Phys. Rev. 011 ( 1975) 566. 4 .  A. Djouadi et al. , Z. Phy C 46; B. Schrempp, Proceedings, Physics at Hera (Hamburg 1991) , eds. W. Buchmiiller and G. Ingelman; J. Kalinowski et al. , Z. Phys. C74 (1997) 595. 5 .  Hl Collab., C. Adloff et al. , Phys. Lett. B479 (2000) 358. 6 .  N. Arkani-Hamed, S. Dimopoulos, G. Dvali, Phys. Lett. B429 ( 1998) 263; I. Antoniadis et al., Phys. Lett. B436 (1998) 257; N. Arkani-Hamed, S. Dimopoulos, G. Dvali, Phys. Rev. 059 (1999) 86004. 7. ALEPH Collab., ALEPH 2000-047 (2000) ;  DELPHI Collab., DELPHI 2001-128 (2001) ; . L3 Collab., Note 2647 (2001) ; OPAL Collab. ,  OPAL Physics Note PN 471 (2001) .  

Search for exotic physics with four-fermion coupling at LEP

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