We discuss recent results in the electroweak sector at the Tevatron. We will focus on latest results in the measurement of diboson production cross sections and limits on anomalous triple gauge coupling. At first, we will consider purely leptonic decays. Then, we will describe the recent observations of diboson processes with jets in the final state

Catastini, P.

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DOI 10.1393/ncc/i2011-10721-5Colloquia: LaThuile10IL NUOVO CIMENTO Vol. 33 C, N. 5 Settembre-Ottobre 2010Electroweak Physics at the TevatronP. Catastini on behalf of the CDF and D0 CollaborationsFermilab - Batavia, IL, USA(ricevuto il 14 Settembre 2010; pubblicato online l’11 Gennaio 2011)Summary. — We discuss recent results in the electroweak sector at the Tevatron.We will focus on latest results in the measurement of diboson production crosssections and limits on anomalous triple gauge coupling. At ﬁrst, we will considerpurely leptonic decays. Then, we will describe the recent observations of dibosonprocesses with jets in the ﬁnal state.PACS 14.70.Fm – W bosons.PACS 14.80.Bn – Standard-model Higgs bosons.1. – IntroductionThe study of diboson production represents a relevant portion of the current Elec-troweak Physics program at the Tevatron. Dibosons are of great interest because theyprovide unique opportunities to test the Standard Model at the TeV scale and they are arelevant probe to new physics through deviations of Triple Gauge Couplings (TGCs) fromStandard Model (SM) predictions. Moreover, each combination of associated productionof W and Z boson decaying in any ﬁnal state has a counterpart search channel for theHiggs boson at the Tevatron. Worth mentioning: WW + WZ → lν + jj that sharesthe same topology of WH → lν + bb¯, the golden process for low mass Higgs searches;WW → lν + lν that is the dominant background for H → WW , the golden process forhigh mass Higgs searches.As a consequence, establishing diboson production, at ﬁrst in their leptonic decaysand more recently with jets in the ﬁnal state, represented an important milestone forthe development and the assessement of techniques used in Higgs boson searches at theTevatron. In this paper we examine diboson production in 1.96TeV pp¯ collision usingthe CDF and D0 detectors, compare this production to Standard Model predictions, andset limits on the strength of some anomalous couplings. Signals of the WW , WZ, ZZare searched through their leptonic and semi-leptonic ﬁnal states.c© Societa` Italiana di Fisica 247248 P. CATASTINI on behalf of the CDF and D0 COLLABORATIONS (GeV)TLeading Lepton p0 50 100 150Events / 12.5 GeV0102030DataWWll→*γZ/Events / 12.5 GeV -1DØ, 1 fbμμ+μ(a) ee+e0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1020406080100120 W+jetsγWWZZZ*γZ/ttWWDataNominal MCMatrix Element Likelihood Ratio (LRWW)Events / 0.04 Fitted TemplatesCDF Run II Preliminary -1 L = 3.6 fb∫Fig. 1. – (Left) Leading lepton PT distribution in WW → l+l−νν¯ candidate events at D0.(Right) Likelihood ratio used to extract the WW → l+l−νν¯ signal at CDF.2. – WW → lν + lνThe process WW → lν + lν is interesting for several reasons. Among them, we mightmention the measurement of WW cross section to test the SM prediction and the searchfor anomalous trilinear gauge couplings. In addition, it represents the main backgroundof the process H → WW , the golden process for high mass Higgs boson searches at theTevatron.DO, using a data sample of 1 fb−1, studied the process WW → l+l−νν¯ and measuredthe WW cross section to be σ(pp¯ → WW ) = 11.5 ± 2.1(stat + syst) ± 0.7(lum) pb, inagreement with the SM expectation of 12.0± 0.7 pb. In addition, D0 imposed also limitson TGC [1].CDF measured the WW production cross section in the two charged lepton (e or μ)and two neutrino ﬁnal state using an integrated luminosity of 3.6 fb−1 [2]. The WWsignal is separated from backgrounds using matrix element based likelihood ratios. TheWW cross section is extracted using a binned maximum likelihood method which bestﬁts LRWW signal and background shapes to data (see ﬁg. 1, right). The measured WWcross section is 12.1± 0.9(stat)+1.6−1.4(syst) pb. It is in good agreement with the StandardModel prediction and it represents the most precise measurement up to date. An updatedversion of this analysis sets limits on TGC [3].3. – ZZThe SM prediction for the total ZZ production cross section in pp¯ collisions at√s =1.9TeV is σ(ZZ) = 1.4±0.1 pb. Therefore, ZZ has the smallest SM diboson productioncross section. The requirement of leptonic Z boson decays provides a clear signaturewith extremely low background; on the other hand, it reduces the ZZ observable crosssection, making its measurement rather challenging.Using 1.7 fb−1 of data, D0 observed, for the ﬁrst time at a hadron collider, ZZ →ELECTROWEAK PHYSICS AT THE TEVATRON 249Four lepton invariant mass (GeV)0 100 200 300 400 500Events/50 GeV0.511.522.5Events/50 GeV 4e data dataμ4signalbackground-1DØ Run II, 1.7 fb]2Z [GeV/cT leading PllM20 40 60 80 100 120 140]2Z [GeV/cT subleading PllM20406080100120140 ZZ )+jetsγZ(Data CDF Run II Preliminary-1 L dt = 4.8 fb∫Fig. 2. – (Left) Four-lepton invariant mass distribution at D0. (Right) Mjj of the leading PT Zcandidate vs. the subleading one at CDF.l+l−l′+l′− (l, l′ = e or μ) production with a signiﬁcance of 5.3 standard deviations, ﬁg. 2(left). D0 also combined this channel with ZZ → l+l−νν¯, yielding a signiﬁcance of 5.7standard deviations and measured σ(ZZ) = 1.60± 0.63(stat)+0.16−0.17(syst) pb [4].Recently, CDF re-observed ZZ → l+l−l′+l′− with a signiﬁcance of 5.7 standard de-viations, using 4.8 fb−1 of data, ﬁg. 2 (right). CDF measured σ(ZZ) = 1.56+0.80−0.63(stat)±0.25(syst) pb [5]. D0 and CDF measurements are in agreement with the SM prediction.4. – Combined limits on anomalous trilinear gauge couplingsD0 combined diﬀerent diboson production and decay channels to set limits on WWγand WWZ trilinear gauge couplings [6]. Four channels were considered: WW + WZ →lν + jj, WW → l+l−νν¯, WZ → lν + l+l− and Wγ → lνγ.The corresponding results, shown in ﬁg. 3, set the most stringent limts to date on Wmagnetic dipole μW and quadrupole qW .5. – V V →ET + jjThe V V (V V = WW,WZ,ZZ) production and decay into hadronic ﬁnal states aretopologically similar to the VH production and decay which is the most promising Higgsdiscovery channel at low Higgs mass. Also, study of the diboson production is sensitiveto extra gauge couplings not present in the Standard Model.CDF observed diboson production with jets in the ﬁnal states for the ﬁrst time at ahadron collider using a sample of events with large ET (> 60GeV) and two jets withET above 25GeV [7]. Due to limited energy resolution it is impossible to distinguishbetween WW , WZ and ZZ events so what CDF measured was a sum of these processes.No requirement on the number of leptons in the event was applied, therefore the analysiswas also sensitive to lepton decays of the gauge bosons. The multijet QCD contribution,which is large in this channel, is heavily suppressed through novel algorithms related toET signiﬁcance. Diboson signal was extracted from the background using the invariant250 P. CATASTINI on behalf of the CDF and D0 COLLABORATIONSFig. 3. – Combined D0 limits on TGC.mass distribution of the two selected jets (ﬁg. 4). The extraction of the signal did notuse the theoretical calculation of the V+jets integral cross section and its invariant massshape was cross checked with γ+jets events from the data, hence considerably reducingthe systematic uncertainty on the shape of this main background. The ﬁnal dijet massﬁt was an unbinned extended maximum likelihood with jet energy scale, and the slopeand the normalization of the multijet background treated as nuisance parameters andallowed to ﬂoat in the ﬁt within their predetermined uncertainties. The EWK backgroundFig. 4. – V V → ET+jets search ﬁt result at CDF.ELECTROWEAK PHYSICS AT THE TEVATRON 251RF Output0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Events / 0.0405001000150020002500Events / 0.04 DataDiboson SignalW+jetsZ+jetsTopMultijet-1DØ, 1.1 fbDijet Mass (GeV)0 50 100 150 200 250 300Events / 10 GeV050010001500200025003000Events / 10 GeV DataDiboson SignalW+jetsZ+jetsTopMultijet-1DØ, 1.1 fbFig. 5. – WV → l ET + jj: (left) Random Forest discriminant ﬁt at D0. (right) dijet mass ﬁtat D0.and diboson signal normalization are also freely ﬂoating in the ﬁt with no constraints.Using a sample of 3.5 fb−1 of data, CDF observed 1516 ± 239(stat)± 144(syst) dibosonevents with a signiﬁcance of 5.3σ. The corresponding cross section was measured to beσ(pp¯ → V V + X) = 18.0 ± 2.8(stat) ± 2.4(syst) ± 1.1(lum) pb, in good agreement withthe Standard Model predictions.6. – WV → l ET + jjD0 and CDF utilize similar selections to reconstruct WV → l ET +jj with V = W,Z.At ﬁrst we look for an energetic lepton, electron or muon; we then require large ET andat least two additional jets in the event. Moreover, both CDF and D0 further selectevents with higher transverse mass of the lepton-ET system. The main background forthe process WV → l ET +jj is W +jets with much less, but not negligible, contributionsfrom tt¯, single t, Z + jets and multijet QCD background.D0 reported the ﬁrst evidence of WW + WZ production in lepton+jets ﬁnal stateswith a statistical signiﬁcance of 4.4 σ using 1.1 fb−1 of data [8]. Diboson signal wasseparated from backgrounds using a multivariate classiﬁer to combine information fromseveral kinematic variables. A Random Forest (RF) classiﬁer was built using thirteenkinematic variables characterized by separation between signal and at least one of thebackgrounds. The signal cross section is determined by a ﬁt to the RF distribution byvarying the diboson and the dominant background W+jet templates. Other backgroundcontributions are normalized to the SM predictions. D0 measured σ(WW + WZ) =20.2± 2.5(stat)± 3.6(syst)± 1.2(lum) pb from the ﬁt to the RF distribution, ﬁg. 5 (left).A consistent result was found by performing a ﬁt to the dijet invariant mass distributionas a cross check, ﬁg. 5 (right).CDF observed WW/WZ → l + νjj production and decay [9]. Two diﬀerent ap-proaches were used: the ﬁrst looks for a bump in the dijet mass distribution (Mjj), whilethe second uses Matrix Element computation (ME) to exploit additional kinematic in-formation. In 3.9 fb−1, the Mjj method result had a signiﬁcance of 4.6 σ, while the MEapproach resulted in the ﬁrst WW/WZ → lνjj observation, with a signiﬁcance of 5.4 σ.Both CDF analyses have been recently updated using additional data. The Mjjmethod now uses a data sample corresponding to approximately 4.3 fb−1 of integratedluminosity to reconstruct WW/WZ events [10]. The diboson signal is extracted from252 P. CATASTINI on behalf of the CDF and D0 COLLABORATIONSEvent Probability Discriminant0 0.2 0.4 0.6 0.8 1Events / 0.0510210310410Events / 0.05Events / 0.05WW+WZW+jetsNon-WZ+jetsTopData-1CDF Run II Preliminary, L=4.6 fb0.7 0.75 0.8 0.85 0.9 0.95 1050100150200Fig. 6. – WV → l ET + jj: (left) EPD discriminant ﬁt at CDF, (right) dijet mass ﬁt at CDF.the background using a χ2 ﬁt of the invariant mass distribution of the two leading jetseparately for the electron and muon samples. This simple method allows to search for asignal peak over a smooth background. The ﬁt (ﬁg. 6 right) estimates 1582±275(stat)±107(syst) WW + WZ → νjj events, corresponding to a statistical signiﬁcance of 5.2σ(5.1σ expected) and measured σ(WW + WZ) = 18.1± 3.3(stat)± 2.5(syst).The ME approach uses 4.6 fb−1 and takes advantage of a multivariate technique toexploit all the information in the event [11]. Event probability densities are calculatedunder the signal and background hypotheses using a set of measured variables of eachevent (the 4-vectors of the lepton and the two jets). The probability is constructedby integrating over the parton-level diﬀerential cross-section, which includes the matrixelement for the process, the parton distribution functions, and the detector resolutions.These probabilities are used to construct a discriminant variable for each event, referredto as the Event Probability Discriminant, or EPD. To quantify the WW + WZ contentin the data, a binned maximum likelihood ﬁt to the data was performed (ﬁg. 6 left) byﬁtting a linear combination of signal and background shapes of the event probabilitydiscriminant. CDF, using the ME method, measured σ(WW + WZ) = 16.5+3.3−3.0(stat +syst) pb, with a signiﬁcance of 5.4σ. The results from the two approaches at CDF are inagreement with each other and with the Standard Model prediction.7. – ConclusionsThe Electroweak Physics program at the Tevatron is producing exciting results in thediboson sector. Both CDF and D0 have well-established ﬁnal states with leptons andjets. These processes represent important tests of the Standard Model by measuring theproduction cross sections and setting limits on TGC. Moreover, dibosons represent animportant milestone for the development of techniques used in low-mass and high-massHiggs boson searches.ELECTROWEAK PHYSICS AT THE TEVATRON 253REFERENCES[1] Abazov V. M. et al., Phys. Rev. Lett., 103 (2009) 191801.[2] Aaltonen T. et al., arXiv:/0912.4500v1.[3] http://www-cdf.fnal.gov/physics/ewk/2009/wwtgc/index.html.[4] Abazov V. M. et al., Phys. Rev. Lett., 101 (2008) 171803.[5] http://www-cdf.fnal.gov/physics/ewk/2009/ZZllll/ZZWeb/cdf9910 ZZ 4leptonspublic.pdf.[6] Abazov V. M. et al., arXiv:/0907.4952.[7] Aaltonen T. et al., Phys. Rev. Lett., 103 (2009) 091803.[8] Abazov V. M. et al., Phys. Rev. Lett., 102 (2009) 161801.[9] Aaltonen T. et al., Phys. Rev. Lett., 104 (2010) 101801.[10] http://www-cdf.fnal.gov/physics/ewk/2010/WW WZ/index.html.[11] http://www-cdf.fnal.gov/physics/new/hdg//Results files/results/wwwz oct09/.

Electroweak Physics at the Tevatron

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Electroweak Physics at the Tevatron

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