A review of the measurements of hadron, flavour-tagged and lepton-pair
production cross-sections and lepton-pair forward-backward asymmetries
performed by the four LEP experiments ALEPH, DELPHI, L3 and OPAL at energies
between 130 and 183 GeV is given. All 183 GeV results are preliminary. The
searches by the four collaborations for new physics phenomena like contact
interactions and compositeness, exchange of R-parity violating sneutrinos or
squarks, leptoquarks or additional heavy gauge bosons Z' are summarized. No
evidence for deviations from the Standard Model expectations is found and new
or improved limits are derived.Comment: 6 pages, 3 figures, 2 tables, presented at the XXXIInd Rencontres de
  Moriond, Electroweak Interactions and Unified Theories, Les Arcs, France,
  March 14-21, 199

Bourilkov, Dimitri

English

arXiv

A review of the measurements of hadron, flavour-tagged and lepton-pair production cross-sections and lepton-pair forward-backward asymmetries performed by the four LEP experiments ALEPH, DELPHI, L3 and OPAL at energies between 130 and 183 GeV is given. All 183 GeV results are preliminary. The searches by the four collaborations for new physics phenomena like contact interactions and compositeness, exchange of R-parity violating sneutrinos or squarks, leptoquarks or additional heavy gauge bosons Z' are summarized. No evidence for deviations from the Standard Model expectations is found and new or improved limits are derived.A review of the measurements of hadron, flavour-tagged and lepton-pair production cross-sections and lepton-pair forward-backward asymmetries performed by the four LEP experiments ALEPH, DELPHI, L3 and OPAL at energies between 130 and 183 GeV is given. All 183 GeV results are preliminary. The searches by the four collaborations for new physics phenomena like contact interactions and compositeness, exchange of R-parity violating sneutrinos or squarks, leptoquarks or additional heavy gauge bosons Z' are summarized. No evidence for deviations from the Standard Model expectations is found and new or improved limits are derived

CERN Document Server

ETHZ-IPP PR-98-02June 23, 1998FERMION-PAIR PRODUCTION ABOVE THE ZAND SEARCH FOR NEW PHENOMENADimitri BourilkovInstitute for Particle Physics (IPP), ETH Zurich,CH-8093 Zurich, SwitzerlandAbstractA review of the measurements of hadron, avour-tagged and lepton-pair productioncross-sections and lepton-pair forward-backward asymmetries performed by the four LEPexperiments ALEPH, DELPHI, L3 and OPAL at energies between 130 and 183 GeVis given. All 183 GeV results are preliminary. The searches by the four collaborationsfor new physics phenomena like contact interactions and compositeness, exchange of R-parity violating sneutrinos or squarks, leptoquarks or additional heavy gauge bosons Z0are summarized. No evidence for deviations from the Standard Model expectations isfound and new or improved limits are derived.Presented at the XXXIInd Rencontres de Moriond, Electroweak Interactions and UniedTheories, Les Arcs, Savoie, France, March 14-21, 1998.e-mail: Dimitri.Bourilkov@cern.chFermion-pair Production above the ZThe successful running of LEP in 1995-1997 at centre{of{mass energies well above the Z reso-nance allows to search for new physics beyond the Standard Model (SM). Any signicant de-viation from the SM predictions in the electron-positron annihilation into fermion-pairs wouldherald the presence of new phenomena. The dierential cross section for fermion-pair produc-tion is described by the  or Z exchange amplitudes in the s-channel, and for e+e nal statesalso in the t-channeldd= js+ Zs+ elec  (t+ Zt) + New Physics ?!j2(1)The presence of additional amplitudes will signal new physics eects and can be observedthrough modications in the measured dierential cross section. In the helicity amplitudesformalismAefLR(RL)(s) = (ASMLR(RL)(s) +ANPLR(RL)(s))ts(2)AefLL(RR)(s) = (ASMLL(RR)(s) +ANPLL(RR)(s))us(3)where ASM(ANP) are the SM (or New Phenomena) amplitudes, L (or R) denote the helicitiesof the ingoing and outgoing fermion, s, t, u are the Mandelstam variables. For a generic newinteraction with coupling g and typical energy (or mass) scale  the dierential cross sectioncan be decomposed as followsdd= SM(s; t) +g22CInterference(s; t) +g44CNew Phenomena(s; t) (4)If the new phenomena are strong enough (like resonance formation), they can be observeddirectly through the CNew Phenomenaterm. Otherwise, the interference with the SM amplitudescan lead to observable (but smaller) eects. If no deviations from the SM are found, we canconstrain g=.Measurements of Cross Sections and Asymmetries at LEP2All four LEP experiments have presented results at centre{of{mass energies 130{172 GeV. All183 GeV results are preliminary [1]. The collected luminosity ranges from 6 pb 1at 130 GeVto 55 pb 1at 183 GeV. Up to 172 GeV the statistical errors are dominant. The 183 GeV datamark the point where systematic eects gain in importance.The event selection is an extension of selections on the Z peak with some important newfeatures: two distinctive event classes depending on the eective CMS energyps0:{ inclusive events -ps0=ps > 0:1 (`return to the Z'){ non-radiative events -ps0=ps > 0:85   0:90 cross section  10 2of Z peak values{ higher non-annihilation background (beam-wall/gas, cosmic muons){ higher 2- background ( e+e  ! e+e ff) 4-fermion `background' (W+W , ZZ, Z e+e , We, ...)1Figure 1: Total cross section measurements from ALEPH, including 183 GeV data.00.10.20.30.40.50.60.70.80.9150 100 15011010 210 3-0.5 0 0.5-1-0.8-0.6-0.4-0.200.20.40.60.8150 100 15002468-1 -0.5 0 0.5 102468-1 -0.5 0 0.5 1√s /GeVAsymmetryOPALpreliminary(a) e+e- (c) e+e-183 GeVcosθdσ/dcosθ / pb√s /GeVAsymmetryOPALpreliminary(b) µ+µ-,τ+τ-s′/s > 0.01s′/s > 0.7225(d) µ+µ-183 GeVcosθdσ/dcosθ / pb(e) τ+τ-183 GeVcosθdσ/dcosθ / pbFigure 2: Dierential cross section measurements and leptonic forward-backward asymmetriesfrom OPAL, including 183 GeV data.2Cross sections and asymmetries are measured for the inclusive and the non-radiative events.The latter sample is the main search eld for new physics.ps0is calculated from the initialstate radiation photon(s) if seen in the detector or from the angles of the outgoing fermions orkinematic ts. The denition ofps0is not unique due to interference between initial- and nal-state radiation. One solution (favoured by OPAL) is to correct the measured cross sectionsto `no interference'. The corrections can reach  1-2 % forps0=ps > 0:85 and rely on thetheoretical estimate calculated only to O(). A second option is an inclusive approach, whereps0is dened as the eective mass of the outgoing fermion pair.The preliminary results of all four experiments at 183 GeV show good agreement with theSM predictions, as seen in Figure 1 and Figure 2. Applying avour tagging for the hadronsample, ALEPH and OPAL measure the fraction of b-quarks Rbup to 183 GeV. DELPHImeasures Rb, Rcand Rudsup to 172 GeV.Searches for New PhenomenaThe four-fermion contact interaction (CI) oers a general framework for a new interaction withcoupling g and typical energy scale  ps. By convention it is assumed thatg24= 1 andjijj  1 (i; j = L;R), where ijare the parameters dening the helicity structure of the newinteraction. The corresponding amplitudes have the form [2]ACIij(s) = ijs12(5) is the electromagnetic ne structure constant. The models considered here are dened inTable 1: Models of contact interaction considered. The parameters ijdene the helicityamplitudes, Aij, which are active. Also shown are the amplitudes for the exchange of possiblenew particles, where the CI gives the limiting case.Model LLRRLRRLqLepto-qLL 1 0 0 0d-type S0(gL)RR 0 1 0 0LR 0 0 1 0 ?u-typeS1=2RL 0 0 0 1 ?VV 1 1 1 1AA 1 1 1 1Table 1. In the same table the amplitudes contributing in the specic cases of new particleexchange, considered further, are highlighted.The four collaborations express the results of their ts as 95 % CL lower limits on the scale for e+e , + , + , l+l (leptons), qq, uu, dd (one up or down avour is aected),avour-tagged qq, ff (all fermions combined) nal states. The limits +( ) correspond tothe upper (lower) sign combination in Table 1. ALEPH, L3 and OPAL have updated theirresults using 183 GeV data, as shown in Table 2. Only the combined l+l and qq results aregiven, when available.The fermion-pair cross section can be sensitive to exchange of single supersymmetric par-ticles [3] if R-parity is violated (6 Rp). The most general superpotential even for a minimal3Table 2: Limits on +( ) in TeV at 95 % CL for l+l and qq nal states.l+l ALEPH DELPHI L3 OPAL + + + +LL 5.4 6.1 4.4 3.8 4.1 5.8 5.3 5.2RR 5.2 5.9 4.2 3.7 4.0 5.6 5.1 5.0LR 5.2 6.5 4.7 4.1 5.4 3.9 5.2 5.6VV 9.4 11.8 8.2 7.1 8.5 8.5 9.3 9.6AA 9.0 8.2 6.1 6.2 5.3 9.3 8.3 7.7qq ALEPH DELPHI L3 OPAL + + + +LL 2.7 3.9 tag 2.5 3.8 2.8 4.4RR 3.6 2.9 3.5 2.8 3.9 3.0LR 3.4 3.1 3.2 3.0 3.6 3.3RL 4.3 2.4 4.3 2.3 4.9 2.5VV 5.2 4.0 5.0 3.8 5.7 4.1AA 3.7 5.6 3.5 5.6 3.8 6.3supersymmetric model contains interactions violating the lepton number L and coupling spar-ticles to lepton-lepton or lepton-quark pairsW6R= ijkLiLLjLEkR+ 0ijkLiLQjLDkR+ ::: (6)where (L,Q) are left-handed doublets of leptons, quarks, (E,D) are right-handed singlets ofcharged leptons, down-type quarks and ijk are generation indices. These terms introduce 9 and 27 0independent Yukawa couplings (as ijk6= 0 only for i 6= j). Lower energy measure-ments place (in some cases severe) constrains on the possible values of  and 0as a functionof the sparticle mass. We will assume that one (or two)  are much stronger than the others inthe searches that follow. For heavy sparticle masses the LEP2 data reaches higher sensitivityin some cases.The most exciting possibility is resonance formation of single sneutrinosif the sneutrinomass is close to the centre{of{mass energy. Then large eects can occur. Also t- or u-channelexchange of singleor squarkqis possible in 6 Rptheories. L3, OPAL and DELPHI havesearched for the most dramatic s-channel eects involving. In Figure 3 the L3 results including183 GeV data are shown for e+e and + nal states. The t- or u-channelqexchange leadsto similar eects as the exchange of leptoquarks, which appear in many unication theories.The CDF and D collaborations exclude scalar leptoquarks with mass below 225 GeV andbranching (LQ! lq) = 1. At LEP2 higher masses can be probed if the coupling is sizeable.From the many possible leptoquark states two are of special interest, as they give limits alsofor squarks: S0(L) - limit on 01jk, (j = 1; 2) - nal state u  type, (k = 1; 2; 3) - exchanged~dR  typeS1=2- limit on 01jk, (j = 1; 2; 3) - exchanged ~uL  type, (k = 1; 2; 3) - nal state d  typeL3 and OPAL have updated their limits using 183 GeV data. For an exchanged mass of 200GeV typical 95 % CL upper limits for the couplings g or 0are 0.25 for S0(L) and 0.65 forS1=2.A complimentary search for an additional heavy gauge boson Z0is performed by the L3 andDELPHI collaborations. For specic models of the Z0couplings exclusion limits in the plane of4Figure 3: Limits on the couplings 121, 131, 232as function of the sneutrino mass at 95 % CL.the Z0mass and the mixing angle Mwith the SM Z boson are derived. For most of the modelstypical values are jMj < 0:003 and mZ0> 300 GeV.OutlookThe production of fermion-pairs above the Z resonance is a precise testing ground, where theStandard Model can be probed to the largest momentum transfers, and a lively hunting eldfor new particles in the several hundred GeV mass range, which couple to lepton-lepton orlepton-quark pairs. In the four-fermion contact interaction framework the measurements aresensitive to scales approaching 10 TeV.In summary: the Standard Model describes all fermion-pair production measurements up to 183 GeV no hint for new phenomena so far new or improved limits obtained: contact interactions,/q, leptoquarks, Z0 the LEP2 discovery potential is large; the LEP community is looking forward to moredata at higher energiesReferences[1] Contributions to the 1998 winter conferences by the four LEP collaborations (from thehome pages of the experiments).[2] H. Kroha, Phys.Rev., D46(1992)58.[3] J. Kalinowski, R. Ruckl, H. Spiesberger and P. Zerwas, Zeit. fur Phys., C74(1997)595.J. Kalinowski, R. Ruckl, H. Spiesberger and P. Zerwas, Phys.Lett., B406(1997)314.5

Fermion-pair Production above the Z and Search for New Phenomena

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