In the energy region of LEP2 and above, four fermion final states in e+e-
collisions can be produced by Feynman diagrams involving two, one or zero
resonant bosons...Comment: 4 pages, 2 figures, presented at LEPTRE, XIII Convegno sulla Fisica
  al LEP, ROma, 200

Valassi, A.

English

arXiv

In the energy region of LEP2 and above, four fermion final states in e+e- collisions can be produced by Feynman diagrams involving two, one or zero resonant bosons...In the energy region of LEP2 and above, four fermion final states in e+e- collisions can be produced by Feynman diagrams involving two, one or zero resonant bosons..

Valassi, Andrea

CERN Document Server

arXiv:hep-ex/0106073v1  20 Jun 2001MEASUREMENTS OFSINGLE W PRODUCTION AT LEP2Andrea ValassiCERN, EP Division, 1211 Geneva 23, SwitzerlandIn the energy region of Lep2 and above, four fermion final states in e+e− collisionscan be produced by Feynman diagrams involving two, one, or zero resonant bosons.The four fermion process e+e− → eνf f¯ ′ is an interesting reaction because it involvest-channel diagrams, whose contribution is enhanced when the electron is scattered atvery low angles and escapes experimental detection along the beam pipe. Under theseconditions, the process is commonly referred to as single W production, e+e− → eνW,as it is dominated by diagrams involving one single resonant heavy W boson.The interest of the experimental study of single W production at Lep2 is twofold.First, this process is a powerful probe of possible physics beyond the Standard Model,thanks to its strong sensitivity to anomalous triple gauge boson couplings (TGCs) [1].Secondly, the experimental signature of single W events, with large missing energy andtransverse momentum due to the undetected electron and neutrino, is also characteristicin the searches for new particles, and its measurement provides a useful check of thevalidity of the background estimation for these analyses.Measurements of single W production cross sectionsSingle W cross sections at Lep2 have been measured by the four Lep experimentsat energies between 130 and 207 GeV [2], in both the hadronic and leptonic decaychannels of the W boson. Hadronic single W events, characterised by two acoplanarjets of high invariant mass accompanied by large transverse missing energy, are typicallyselected with 40% efficiency and 40% purity, with large WW → qq¯τν and ZZ→ qq¯νν¯backgrounds. Leptonic single W events, with a single isolated electron or muon (or thinjet for τ decays) of high pT , are typically selected with 60% efficiency and 60% purity,the largest backgrounds being Zee or radiative Z events with Z→ νν¯, as well as radiativeBhabha events. At 189 GeV, 60 (30) events per experiment are typically accepted bythe hadronic (leptonic) analyses. Both measurements are limited by statistical errors.The combined values of the single W cross sections measured by the four Lepexperiments between 183 and 207 GeV [2] are compared in Figure 1 to several theoreticalTo appear in Proceedings of LEPTRE, XIII Convegno sulla Fisica al LEP, Roma (2001) 100.250.50.751160 170 180 190 200 210Ecm [GeV]s(e+e– → enW) [pb]LEP Preliminary±5.0% uncertaintygrc4fWPHACT IFL/a (t)WTO EFLW → qq ’00.511.5160 170 180 190 200 210Ecm [GeV]s(e+e– → enW) [pb]LEP Preliminary±5.0% uncertaintygrc4fWPHACT IFL/a (t)W → ff-’Figure 1: Lep measurements of the hadronic (left) and total (right) single W produc-tion cross section, compared to the predictions of WTO, WPHACT and grc4f [3]. Theshaded areas represent the ±5% uncertainty on the predictions [4].predictions [3, 4]. No deviation of the measurements from the expectations is observed.The values in the figures represent single W cross sections according to the common Lepdefinition [2], where single W production is considered as the complete t-channel subsetof Feynman diagrams contributing to eνe f¯f′ final states, with additional kinematic cutsto exclude the regions of phase space dominated by multiperipheral diagrams.Sensitivity to triple gauge boson couplingsTriple gauge boson couplings, usually described in terms of the three parameters ∆gZ1 ,∆κγ and λγ [5], are measured at Lep2 [6] primarily from the analysis of W pair events,e+e− → W+W−. Since this process is sensitive to both the WWZ and the WWγ ver-tices, it is difficult to disentangle the two effects and large correlations exist betweenthe fitted values of the couplings from these analyses. Single W production at Lep2is especially interesting because it is sensitive to the WWγ vertex alone [1], the con-tribution of the WWZ vertex being suppressed by the t-channel exchange of a Z, thusproviding useful constraints complementary to those from W pair events.Measurements of ∆κγ and λγ from the analysis of single W events have beenperformed by Aleph at 161–202, Delphi at 189–202, L3 at 130–202 and Opal at189 GeV [7]. TGCs are extracted via maximum likelihood fits to total event ratesand differential distributions, most of the sensitivity coming from event rates. As ex-pected [1], for a given integrated luminosity, the sensitivity of single W production to∆κγ is comparable to that obtained from W pair events, in spite of the much lower crosssection. This can be seen, for instance, in Figure 2, where the Aleph measurements ofλγ and ∆κγ at energies up to 202 GeV [8] from W pair, single W and single γ events arecompared and combined. As the figure also shows, W pair events have a considerablyhigher sensitivity to λγ compared to single W events. Combined Lep results for TGCsfrom W pair, single γ and single W events have been presented in Ref. [6].To appear in Proceedings of LEPTRE, XIII Convegno sulla Fisica al LEP, Roma (2001) 200.511.522.53-0.5 -0.25 0 0.25 0.5Dkg- DLog(L)00.511.522.53-0.5 -0.25 0 0.25 0.5lg- DLog(L)ALEPH preliminary ALEPH preliminaryFigure 2: Aleph preliminary measurements of λγ (left) and ∆κγ (right) at energies upto 202 GeV [8], from W pair (dashed red), single γ (dotted blue) and single W events(dashed-dotted purple) and their combination (solid black).Conclusions and acknowledgementsSingle W production has been studied by the four Lep experiments at all Lep2 energiesbetween 130 and 207 GeV. The process is sensitive to anomalous triple gauge bosoncouplings, especially to the WWγ coupling ∆κγ . No deviation of the measured crosssections from the theoretical predictions of the Standard Model has been observed.I would like to thank my colleagues from the Lep experiments and the Lep WWWorking Groups for many useful discussions. I also warmly thank the organisers of thisconference for their kind hospitality.References[1] T. Tsukamoto and Y. Kurihara, Phys. Lett. B389 (1996) 162.[2] Lep WWWork. Group, Lepewwg/Xsec/2001–01 (Winter ’01), and refs. therein.[3] G. Passarino, Nucl. Phys. B578 (2000) 3; E. Accomando et al., Comp. Phys.Comm. 99 (1999) 270; Y. Kurihara et al., Prog. Theor. Phys. 103 (2000) 1199.[4] M. Gru¨newald et al., in “Reports on precision calculations for LEP2 physics”,eds. S. Jadach et al., CERN 2000–009; E. Accomando, these proceedings.[5] G. Gounaris et al., in “Physics at LEP2”, eds. G. Altarelli et al., CERN 96–01.[6] Lep TGC Work. Group, Lepewwg/TGC/2000–02 (Summer ’00); F. Terranova,these proceedings.[7] Aleph 2000–054 Conf 2000–036 (Summer ’00); Delphi 2000-146 Conf 445 (Sum-mer ’00); L3 Note 2518 (Winter ’00); Opal PN427 (March ’00).[8] Aleph 2000–055 Conf 2000–037 (Summer ’00).To appear in Proceedings of LEPTRE, XIII Convegno sulla Fisica al LEP, Roma (2001) 3

Measurements of Single W Production at LEP2

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