Single W and single gamma productions which are sensitive to the trilinear
gauge coupling WWgamma have been studied at LEP. It isshown that single W
production has particular sensitivity to the "anomalous" magnetic moment Kgamma
of the W boson, complementary to WW production at LEP and Wgamma production at
hadron colliders. The invisible decay of W boson has been searched and the
limit on the invisible decay width of 27 MeV at 95% C.L. has been obtained.Comment: 5 pages, 10 figures, talk given at the XXIX International Conference
  on High Energy Physics, July 23-29, 1998, Vancouver, Canad

Tanaka, R.

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Single W production at Lep-2

Single W and single gamma productions which are sensitive to the trilinear gauge coupling WWgamma have been studied at LEP. It is shown that single W production has particular sensitivity to the "anomalous" magnetic moment Kgamma of the W boson, complementary to WW production at LEP and Wgamma production at hadron colliders. The invisible decay of W boson has been searched and the limit on the invisible decay width of 27 MeV at 95% C.L. has been obtained

Tanaka, R

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ECOLEPOLYTECHNIQUEIN2P3-CNRSX-LPNHE/98-08Single W Production at LEP2R. TanakaLPNHE, Ecole Polytechnique, F-91128 Palaiseau CEDEX, FRANCETalk given at the XXIX International Conferenceon High Energy Physics, July 23-29, 1998, Vancouver, CANADA.SINGLE W PRODUCTION AT LEP2R. TANAKALPNHE, Ecole Polytechnique, Route de Saclay, F-91128 Palaiseau CEDEX, FRANCEE-mail: Reisaburo.Tanaka@in2p3.frSingle W and single gamma productions which are sensitive to the trilinear gauge coupling WWγ have been studied at LEP. It isshown that single W production has particular sensitivity to the ‘anomalous’ magnetic moment γ of the W boson, complementaryto WW production at LEP and Wγ production at hadron colliders. The invisible decay of W boson has been searched and the limiton the invisible decay width of 27 MeV at 95 % C.L. has been obtained.1 IntroductionThe existence of the trilinear gauge couplings (TGC) isthe direct consequence of the non-Abelian SU(2) U(1)gauge theory which has not been studied in detail. Pre-cise measurements of these couplings make it possible totest the standard model. Any deviation from the stan-dard model would indicate the new physics. There are27 parameters of couplings in the eective Lagrangian1.By requiring C- and P-invariance, also gγ1 = 1 by elec-tromagnetic gauge invariance, the number of parametersreduces to 5: gZ1  gZ1 − 1; γ  γ − 1; Z Z − 1; γ and Z where all these parameters are van-ishing in the standard model. For W+ boson, these pa-rameters can be related to the electromagnetic charge:eW = e gγ1 , and the static moments2 as, magneticdipole moment: W = e2mW (gγ1 + γ + γ), and elec-tric quadrupole moment: QW = − em2W (γ − γ), andalso those associated with the weak boson Z.At LEP2, it became, for the rst time in the e+e−collider, to perform the direct measurement of TGC. Wpair production plays a principal role to study WWγand WWZ couplings 3. However, these two couplingscannot be separated each other. Single W production,e+e− ! eW 4;5, or single gamma production, e+e− !γ 6, can be used to test the WWγ coupling. At hadroncolliders, Wγ production has been studied to probe theWWγ vertex 7;8;9 where the form factor  is introducedto assure the unitarity 10. The TGC limits derived atLEP are insensitive to the form factor scale and power.To relate WWZ and WWg couplings, SU(2)U(1) con-straints, gZ1 = Z + γ tan2 W and Z = γ , areimposed.In the search for supersymmetric particles, charginopair production (e+e− ! +−) where charginos decaypredominantly into sneutrinos and leptons, it is experi-mentally dicult if the mass dierence between charginoand sneutrino is small. This is due to the huge back-grounds from two photon process. Therefore the searchfor single W events in e+e− ! W+W− process whereone W boson decays to undetected chargino and neu-tralino and the other W to the standard model particleshas been proposed11. A search for this scenario has beenperformed by ALEPH.All results presented in this paper are preliminaryexcept L3 results.2 eW Production2.1 Sensitivity to TGC (WWγ)The single W production, e+e− ! eW, is the standardmodel process 12 as shown in Fig.1. The total cross sec-tion is eW = 0:6 pb at the centre-of-mass energy of183 GeV which is much smaller than WW productionWW = 15:7 pb. Contributions from Z boson exchangediagrams are negligible at LEP energies. Thus this pro-cess oers almost pure sensitivity to the WWγ coupling5.The sensitivity for TGC parameters for the fourfermion process of e+e− ! e−eud is shown in Fig. 2.The total cross section has been calculated with theSU(2)xU(1) constraints. While WW production crosssection is minimum at gZ1 = γ = γ = 0, eWproduction cross section is minimum at γ = −1 andγ = 0. It can be seen that single W production is sensi-tive to γ , while it has the modest sensitivity to γ . Thisshould be compared to the Wγ production at hadron col-liders7;8;9 which is sensitive to γ or to b ! sγ 13;14 whichis sensitive to γ in the WWγ vertex.e-e+W +e-ν eγWe-e+W +e-ν eγeproduced by GRACEFIGFigure 1: Feynman diagrams for e+e− → e−eW+.100.511.522.53-2.5 0 2.5σ (pb)∆g1Z00.511.522.53-2.5 0 2.5∆κγWW+eνW+..WW00.511.522.53-2.5 0 2.5λγExcaliburgrc4fFigure 2: The total cross section for e+e− → e−eud¯ as functionsof 3 coupling parameters. The lower curves show the cross sectionsfor W pair production alone, and the upper curves are for all fourfermion diagrams. For one plot, the other two parameters are fixedto the standard model values. Closed points indicate the standardmodel prediction.a) W ! ee b) W ! DE L PH I R u n : E v t :B e am :DAS :P r o c :S c a n :9 1 . 6  GeV 1 4 - J u l - 1 9 9 82 1 - Oc t - 1 9 9 70 5 : 1 8 : 2 91 4 - J u l - 1 9 9 879938 13671Ta n+DSTDALI_E2     ECM=172.3 Pch=48.1 Efl=51.2 Ewi=.663 Eha=5.22  single_m                                                   Nch=1     EV1=.990 EV2=.650 EV3=.005 ThT=2.04           96−10−31 19:52           Detb=  E1FFFFRun=42503   Evt=3942    ALEPH.45Gev EC5.2Gev HCYX hist.of BA.+E.C.0|−500cm             500cm|X 0|−500cm             500cm|Y c) W !  d) W ! qq0Run #    688101    Event #  1741Transverse Imbalance : Longitudinal Imbalance : Thrust : Major : Minor : Event DAQ Time :  Total Energy :   28.47 GeV .8213    -.5313     .9780  .0468  .0000   971026  143011OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO 9xyz Ru n : e v e n t  7 6 4 8 :  1 2 6 9 7 E b e am  8 6 . 1 5 2  V t x  ( - 0 . 0 5 ,  0 . 0 5 , - 0 . 9 8 )                                            C t r k ( N=  3 7  S ump =  6 5 . 6 )  E c a l ( N=  4 7  S umE=  4 4 . 3 )Hc a l ( N= 1 7  S umE=  2 0 . 4 )  Mu o n ( N=  0 )  Emis=  79.9YXZFigure 3: Candidate events for e+e− → eW observed by a) DEL-PHI, b) ALEPH, c) L3 and d) OPAL experiments.2.2 Single W signalEvent characteristics of the single W production are asfollows. Due to its small momentum transfer, the out-going electron escapes in the beam direction. In theanalysis, it is required that the electron to be un-tagged.This is important to suppress the contribution from Wpair production. The associated neutrino may carry alarge transverse momentum, thus the signature of sin-gle W production is characterized by the large missingmomentum. For leptonic decay channel of W boson(W ! l), an isolated high Pt lepton with energy atabout 40 GeV is the signal. The dominant backgroundsare from ll (eeZ) processes. For hadronic W decaychannel (W ! qq0), the signature is the acoplanar twojets whose invariant mass is equal to W mass. The mainbackground is W pair production (WW ! qq0). If carries away large fraction of energy,  becomes invis-ible. It is practically impossible to distinguish this casefrom the single W production, thus becoming irreduciblebackgrounds.The denitions of the single W signal are dierentamong LEP experiments. The ALEPH collaboration, forexample, denes the signal as 19: e < 34 mrad;E‘ > 20 GeV and j cos ‘j < 0:95 for W ! l;Mqq′ > 60 GeV=c2 for W ! qq0;where e is the polar angle of the scattered electron, E‘and ‘ are the energy and polar angle of leptons fromthe W decay, respectively. Mqq′ is the invariant massof the quark pair. These cuts on W decay nal statesare necessary to remove the non-resonant four fermionbackgrounds. The selected evens are displayed in Fig. 3for four W decay modes.Monte Carlo generators of GRC4F 15, EXCAL-IBUR 16 and DELTGC 17 have been used to simulatethe eW production.2.3 Total cross sectionThe summary of analyzed data and observed number ofevents is given in Table 1. In addition to W decay toelectron or muon, ALEPH and L3 collaborations havealso analyzed the tau decay channel. ALEPH 19 hasmeasured the total cross section of eW production at183 GeV as eW = 0:40  0:17(stat:)  0:04(syst:) pbwhere the standard model predicts 0:41 pb. L3 21 hasalso measured the cross section at 183GeV as eW =0:62 +0:19−0:18(stat:)0:04(syst:)pb where 0:50pb is expectedfrom the standard model. All these results are consis-tent with the standard model expectation. In Fig. 4, thecross section as a function of the centre-of-mass energyis shown as measured by L3 experiment.2Table 1: Summary of single W measurement for leptonic and hadronic channels. Nobs is the number of selected data events. NSM andNeW are the expected number of total events (signal plus backgrounds) and eW signal events, respectively.ECMS Lumi. W ! l W ! qq0(GeV) (pb−1) Nobs NSM (NeW) Nobs NSM (NeW)ALEPH 19 161-183 78.9 11 11.1 (7.3) 21 21.5 (8.8)DELPHI 20 161-183 73.0 9 5.4 (5.2) 44 52.6 (19.9)L3 21 130-183 88.5 12 10.2 (6.0) 109 103.3 (14.7)OPAL 22 161-172 20.3 2 2.0 (0.8) 4 2.5 (1.3)L3DataGRC4FEXCALIBUR√s (GeV)σWeν (pb)00.511.5120 140 160 180 200Figure 4: The measured cross section of eW production as a func-tion of the centre-of-mass energy by L3.2.4 Limits on TGCSince WW backgrounds in hadronic W decay channel areirreducible (S/N=1/1 at best) and the 2/3 of W’s decayhadronically, the pure sensitivity to WWγ vertex of eWproduction is lost. This is because W pair productioncontains both WWZ and WWγ vertices that cannot beseparated. One is therefore obliged to either a) x the ir-reducible WW backgrounds in hadronic W decay as thestandard model (ALEPH, OPAL), or b) vary the WWbackgrounds simultaneously according to TGC values as-suming SU(2)U(1) constraints (DELPHI, L3). The for-mer takes the conservative approach, and the latter ben-ets the information contained in WW backgrounds.The sensitivity to γ of single W production whichis superior to WW production is demonstrated in Fig. 5.However there are two minima at γ = 0 (the standardmodel) and at -2 for single W alone due to the fact thattotal cross section has the same value at these points.This double minima structure can be solved by combin-ing it with the results from single gamma and/or WWDELPHI00.511.522.533.544.55-2 -1.5 -1 -0.5 0 0.5 1 1.5 2∆κγ-log LWW-> jjjjWW -> lνjjsingle Wsingle γFigure 5: The log-likelihood functions on ∆γ parameter measuredby DELPHI. The results from W pair production (hadronic, semi-leptonic), single W and single gamma are shown separately.productions.In Table 2, the limits on TGC parameters aresummarized. The event yields have been analyzed byBayesian approach (ALEPH) or by maximum likelihoodts to event rate (OPAL) and to kinematical distribu-tions (DELPHI, L3). One should note that the resultson γ obtained by DELPHI and L3 experiments benetinformation contained in W pair production. The intrin-Table 2: The 95%C.L. limits on TGC couplings. Note that L3 gives95%C.L. limits with 2 parameter fit, and the other experiments give1 parameter fit result fixing the rest as the standard model values.ALEPH −2:6 < γ < 0:5 −1:6 < γ < 1:6DELPHI −0:4 < γ < 0:9 −1:5 < γ < 1:5L3 −0:46 < γ < 0:57 −0:86 < γ < 0:75OPAL −3:6 < γ < 1:6 −3:1 < γ < 3:13sic sensitivity of single W production alone at the currentstatistics of LEP is jγ j < 0:5 (γ = 0) and jγ j < 1:6(γ = 0) at 95%C.L. provided that the double minimastructure for γ is resolved.3 γ ProductionAmongst various physics opportunities such as countingthe number of light neutrino species, the process e+e− !γ is also sensitive to WWγ coupling 6. There are threetypes of diagrams which contribute to the γ nal stateas shown in Fig. 6. The rst diagram is the radiativereturn to Z by emitting hard photon, the second one ist-ch W boson exchange, and the last one is W bosonfusion type which contains a WWγ vertex. The photonin the radiative return process has energy peaked at xγ =Eγ=Ebeam = 0:74 at 183 GeV. Monte Carlo programsbased on KORALZ 18 and DELTGC 17 are used.e-e+νeνeγZe-e+νeνeγWe-e+νeνeγproduced by GRACEFIGWWFigure 6: Feynman diagrams for e+e− → e¯eγ.Isolated photons have been searched in the analy-sis. It is found that the data are in good agreementwith the standard model expectation. When extract-ing coupling parameters with the maximum likelihoodmethod, the total yield of observed events, the energyand angular distributions are used as shown in Fig. 7.The photon energy region of xγ 2 [0:67; 0:76] is notused in ALEPH’s analysis. ALEPH 23 has obtained thetted results of γ = 0:05+1:2−1:1  0:3 (γ = 0) andγ = −0:05+1:6−1:5  0:3 (γ = 0), where the rst error isstatistical and the second is systematic. DELPHI 20 per-forms the binned likelihood t to the whole photon en-ergy spectrum, and gets γ = 0:00+1:01−1:010:36 (γ = 0)and γ = 0:72+1:12−1:12  0:36 (γ = 0). Both results areconsistent with the coupling parameters equal to zero.The sensitivity of γ to TGC parameters is 2 3 times weaker than that of eW, but nevertheless, itcontributes to solve the double minima structure for γin eW production as discussed above.4 Invisible W DecayThe ALEPH collaboration19 has performed the search forthe invisible W decay in e+e− ! W+W−. The mixed05101520253035400 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1DATAXeγNevt05101520253035400 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9DATA|cosθ|γNevtFigure 7: ALEPH measurements on a) photon energy (normalizedto the beam energy) and b) angular distribution of photons for ¯γproduction at 183 GeV.supersymmetric/standard model decay has been studied.One W boson decays to chargino and neutralino, followedby the chargino decay to sneutrino and lepton. The otherW boson decays to the standard model particles. Thewhole decay cascade can be illustrated as,e+ e− ! W W,! SM ,!  ,! ‘ ˜,!  :The supersymmetric decay of W boson becomes practi-cally invisible if the mass dierence between the charginoand the sneutrino (M  m± −m˜) is less than about3 GeV=c2. However this process still can be tagged by theW decay to the standard model particles. Three eventtopologies of the nal state, single lepton (e=), acopla-nar lepton pair (one is soft) and hadrons (missing massequal to W mass) have been studied.No excess of the signal has been observed and the re-sults are consistent with the standard model expectation.The limits at 95% C.L. on the W boson supersymmetricbranching ratio have been obtained as:Bsusy (M  0 GeV=c2) < 1:3 %;Bsusy (M = 3 GeV=c2) < 1:9 %;assuming B( ! ‘~) = 100% and m± = 45 GeV=c2.Degenerate (M  0 GeV=c2) case gives the quasimodel-independent limit on the invisible W decay widthvia direct search. The result translates as Γ(W ! inv) <27 MeV at 95% C.L..45 ConclusionSingle W production has been studied at LEP. The pro-duction cross section is consistent with the standardmodel expectation. It has been shown that eW pro-duction is sensitive to the WWγ coupling, in particu-lar to γ . However, the irreducible WW background inhadronic decay channel of eW does not allow the clearseparation of WWγ and WWZ couplings. Single gammaproduction has also been studied. No deviation from thestandard model is found.Search for invisible W decay has been performed byALEPH and the stringent limit on invisible W decaywidth of 27 MeV has been obtained at 95 % C.L..The current status and the future perspective on theWWγ coupling measurement are summarized in Table 3.One sees that the Wγ production at Tevatron and eWproduction at LEP provide complementary informationon TGC. It is anticipated that eW production at LEPhas the sensitivity of jγ j  0:1 with 500 pb−1 dataat higher energies. In future, one may combine leptonicdecay channel of eW and γ alone that are purelysensitive to the WWγ coupling. It is expected that theuse of kinematical information and the spin analysis willfurther improve the limits.Table 3: The current and future TGC limits at 95%C.L. par singleexperiment.Tevatron 9 Wγ 93 pb−1 jγ j < 0:9 jγ j < 0:3LEP eW 80 pb−1 jγ j < 0:5 jγ j < 1:6LEP2000 eW 500 pb−1 jγ j < 0:1 jγ j < 0:6AcknowledgementsThe author would like to thank following single W’ersand single γ’ers at LEP for discussions and correspon-dences, J. Boucrot, J.B. Hansen, A. Jacholkowska andD. Zerwas (ALEPH), C. Matteuzzi, R.L. Sekulin andO. Yushchenko (DELPHI), P. de Jong and A. Kounine(L3), G. Bella and M. Verzocchi (OPAL).References1. K. Hagiwara, K. Hikasa, R.D. Peccei and D. Zep-penfeld, Nucl. Phys. B 282, 253 (1987), K. Gae-mers and G. Gounaris, Z. Phys. C 1, 259 (1979).2. H. Aronson, Phys. Rev. 186, 1434 (1969),K.J. Kim and Y.-S. Tsai, Phys. Rev. D 7, 3710(1973).3. 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Single W Production at LEP2

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