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WW and WZ Production at the Tevatron

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W W AND W Z PRODUCTION AT THE TEVATRON

E. LIPELES

arXiv:hep-ex/0701038v1 19 Jan 2007

University of California, San Diego E-mail: lipeles@fnal.gov
This report summarizes recent measurements of the production properties of W W and W Z pairs of bosons at the Tevatron. This includes measurements of the cross-section and triple gauge couplings in the W W process and the ?rst evidence for W Z production. Keywords: Diboson; W W ; W Z

1. Introduction Boson pair production is one of the few processes that have signi?cant e?ects from triple boson vertices at tree level. These couplings are predicted in the standard model and are directly related to its gauge group structure. One of the goals of diboson measurements is to limit deviations from the standard model values of these triple gauge couplings (TGCs). Such deviations could be observed in either the cross-sections or in the kinematic distributions of the observed events. Possible causes of anomalous TGCs include new particles in loop diagrams.1 It is also possible for diboson ?nal states to receive contributions from the s-channel production of an as yet unobserved particle, most notably the standard model Higgs decaying to a pair of W bosons. This report summarizes recent measurements by the CDF and D? collaborations of W W and W Z production at the Tevatron. The Tevatron produces pp collision at 1.96 TeV center of mass energy. The dominant contributions to the cross-sections for W W and W Z production are the t-channel (and similar u-channel) process involving two instances of the well measured boson-quark couplings and the s-channel process involving triple gauge couplings, shown in Figure 1. The TGCs can in general be functions of √ the ? invariant mass of the ?nal state bosons s,

so measurements at the Tevatron compliment previous measurements at LEP because √ they probe larger values of s. Furthermore, ? the W Z ?nal state, which is not accessible in e+ e? collisions, isolates the W W Z coupling from W W γ.
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Fig. 1. Dominant diagrams in the production of boson pairs: (a) t-channel (b) s-channel.

Of the heavy diboson processes, W W production is the largest with a standard model next-to-leading order (NLO) prediction of σ(W W )N LO = 12.4 ± 0.8 pb , followed by W Z production with an NLO prediction of σ(W Z)N LO = 3.7 ± 0.1 pb.2 Sec1

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tion 2 of this report covers measurements of the W W cross-section; Section 3 describes measurements of the triple gauge couplings in W W production, and Section 4 presents the ?rst evidence for W Z production.

Table 1. Expected composition and observed yield for the CDF W W analysis. Mode Drell-Yan W +jets W Z + ZZ Wγ tt Sum Bkg WW Expected Data Events ± Stat ± Syst 11.8 11.0 7.9 6.8 0.2 ± ± ± ± ± 0.8 0.5 0.0 0.2 0.0 ± ± ± ± ± 3.1 3.2 0.8 1.4 0.0

2. W W cross-section Both CDF and D? have measured the W W production cross-section in the all leptonic ?nal state lνl′ ν, where l, l′ = e or ?. Although this is the lowest branching fraction ?nal state of W W decay (4.6%), it has signi?cantly lower backgrounds than the other ?nal states, which all involve hadronic jets. The presence of neutrinos is identi?ed as lack momentum balance in the plane transverse to the beam using the missing trans/ ?T verse energy variable E T ≡ i Ei ni , where ni is the transverse component of a unit ?T vector connecting the interaction point to a calorimeter cell i and Ei is the energy depo/ / sition in that cell. D? requires E T ≡ |E T | > 30(ee), 40(??), 20(e?) GeV while CDF re/ quires E T > 25 GeV for all ?nal states. / The primary backgrounds to the ll′ E T ?nal state are from W → lν with an associated jet or photon which is misidenti?ed as another lepton, Drell-Yan (Z/γ ? ) production of lepton pairs combined with large false / E T due to detector e?ects, tt → W W bb → ll′ ννbb, and other heavy dibosons, either W Z → lllν, with a lost lepton, or ZZ → llνν. The predicted composition of the selected sample in the CDF analysis is shown in Table 1. After selection of events with two lep/ tons, signi?cant E T and a jet veto to suppress the tt background, CDF observes 95 events in 825 pb?1 of data with an expected background of 37.8±0.9(stat.)±4.7(syst.)3 and D? observes 25 events in 224 ? 252 pb?1 of data with an expected background of 8.1±0.6(stat)± 0.6(sys)±0.5(lumi).4 These observations correspond to cross-sections of σ(W W ) = 13.6 ± 2.3(stat) ± 1.6(sys) ±

37.8 ± 0.9 ± 4.7 52.4 ± 0.1 ± 4.3 90.2± 0.9 ± 6.4 95

1.2(lum) pb (CDF) and σ(W W ) = 13.8+4.3 (stat)+1.2 (sys) ± 0.9(lum) pb (D? ), ?3.8 ?0.9 both of which are consistent with a NLO calculation of the standard model expectation. 3. Triple gauge couplings in W W and W Z The s-channel W W production process (shown in Figure 1b) has contributions from both W W γ and W W Z TGCs while the W Z process only gets a contribution from the W W Z vertex. D? has set limits on anomalous couplings in the W W → ll′ νν sample described above. CDF has recently probed the TGCs in the combination of W W and W Z modes using the lνjj ?nal state, where l = e or ?. While the background in the lνjj is dramatically larger than the purely leptonic ?nal states due to the pp → W +jets process, the branching fractions are ≈6.5 times higher for W W and ≈10 times higher for W Z. Because anomalous TGCs are expected to enhance the high W transverse momentum region, where the backgrounds from W +jets are smaller, the lνjj ?nal state can be sensitive to anomalous couplings even without observation of the standard model processes. The TGCs are parameterized by adding terms with variable coupling constants to the

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Leading Lepton - e? Channel (WWγ =WWZ, Λ = 2.0 TeV)
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Fig. 2. Distributions which are ?t for triple gauge couplings: (a) leading lepton pT in the ll′ νν ?nal state at D? and (b) leptonically decaying W pT in the lνjj ?nal state at CDF.

standard model Lagrangian
V LW W V = ig1 (W?ν W ? V ν ? W? Vν W ?ν ) iλV ? + iκV W? Wν V ?ν + 2 Wλ? Wν V νλ , MW Z where κγ = κZ = g1 = 1 and λZ = λγ = 0 in the standard model. These couplings can be related to the electric and magnetic dipole and quadrupole moments of the W and Z. In general the coupling constants α = Z κγ , κZ , g1 , λZ , and λγ can be functions of the √ ? invariant mass of the diboson pair s. As a simpli?cation, both experiments assume the functional form α(?) = α0 /(1+ s/(2 TeV)2 )2 , s ? which turns o? the couplings at very large s ? where the couplings would violate unitarity. In order to further simplify the coupling parameter space, the equal coupling scheme is used, ?κ ≡ κγ ?1 = κZ ?1 and λ ≡ λZ = λγ Z with g1 = 0. Other simpli?cations of the parameter space have also been studied. D? sets limits on the anomalous coupling constants using the ll′ νν sample by ?tting the leading lepton pT spectrum as shown in Figure 2a). The ?gure shows the e?ect on the shape of the spectrum due to anomalous couplings near the current bounds. CDF performs a similar ?t to the pT spectrum of the leptonically decaying W in the lνjj ?nal state (Figure 2b). The results of these ?ts are

?0.32 < ?κ < 0.45 and ?0.29 < λ < 0.30 (D?) and ?0.51 < ?κ < 0.44 and ?0.28 < λ < 0.28 (CDF).

4. Search for W Z production Both CDF and D? search for W Z in the lll′ν ?nal state (l, l′ = e or ?) which has a combined branching fraction of 1.8%, including contributions from τ → lνν where l = e or ?. The small standard model prediction for the W Z cross-section makes this a very low rate signal, so both experiments have optimized their lepton selection criteria to maximize e?ciency and acceptance. The dominant backgrounds in these searches are Z → ll with a jet or photon misidenti?ed as a lepton and ZZ → lll′ l′ where one lepton is not reconstructed. As shown in Figure 3, these backgrounds are both strongly suppressed by requiring the / event to have E T > 25 GeV (CDF), 20 GeV (D?). CDF observes 2 events expecting 3.72 ± 0.02(stat.) ± 0.15(syst.) W Z signal events and 0.92 ± 0.07(stat.) +0.16 (syst.) ?0.10 background events using 825 pb?1 of data. Based on this CDF sets an upper limit of σ(W Z) < 6.3 pb at 95% CL. D? observes 12 events with an expectation of 7.5 ± 1.2

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CDF II Preliminary (825 pb ) WZ signal backgrounds data
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gauge coupling measurements for the W W ?nal state are becoming increasingly precise, while the ?rst evidence for W Z production has just been seen. References
1. For a theoretical review of diboson physics at the Tevatron see: J. Ellison and J. Wudka, Ann. Rev. Nucl. Part. Sci. 48, 33 (1998) [arXiv:hep-ph/9804322]. 2. J. M. Campbell and R. K. Ellis, Phys. Rev. D 60, 113006 (1999) [arXiv:hep-ph/9905386]. 3. M. S. Neubauer [CDF Collaboration], arXiv:hep-ex/0605066. 4. V. M. Abazov et al. [D? Collaboration], Phys. Rev. Lett. 94, 151801 (2005), arXiv:hep-ex/0410066. 5. V. M. Abazov et al. [D? Collaboration], arXiv:hep-ex/0608011. 6. V. M. Abazov et al. [D? Collaboration], D? Note 5110-CONF (available at http://wwwd0.fnal.gov).

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/ Fig. 3. The E T distribution for W Z → lll′ ν can/ didate events at CDF showing the power of the E T requirement for suppressing backgrounds.

signal and 3.6 ± 0.2 background events using 800 pb?1 of data. This constitutes 3.3σ evidence for W Z production and corresponds to a cross-section of σ(W Z) = 4.0+1.9 pb, con?1.5 sistent with the standard model prediction. The distributions of the observed events in / the dilepton invariant mass versus E T plane show that signal events cluster near the Z / mass and values of E T consistent with expectation (Figure 4). 5. Summary With the increasingly large accumulated datasets at the Tevatron, CDF and D? are probing electroweak diboson production with new sensitivity. Cross-section and triple




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