The Tevatron Run II will provide CDF and D0 with a large dataset of ppbar
interactions collected at sqrt(s)=2 TeV. We discuss the opportunity for the two
experiments to improve the understanding of electrowek and top physics in the
first years of data taking (Run IIa,2fb-1) in view of the upgrades of the
detectors. We also discuss the prospectives for a Higgs discovery at the
Tevatron in view of the Run IIb data taking period which will deliver an
additional of about 13fb-1 to each experimentComment: 5 pages, 4+1 figure, to appear in proceedings of the EPS HEP 2001
  Conference held in Budapest. To appear in JHEP Proceeding

Chiarelli, Giorgio

English

arXiv

The Tevatron Run II will provide CDF and D0 with a large dataset of p{bar p} interactions collected at {radical}s = 2 TeV. They discuss the opportunity for the two experiments to improve the understanding of electroweak and top physics in the first years of data taking (Run IIa, 2fb{sup -1}) in view of the upgrades of the detectors. They also discuss the prospectives for a Higgs discovery at the Tevatron in view of the Run IIb data taking period which will deliver an additional of about 13 fb{sup -1} to each experiment

Chiarelli, G.

UNT (University of North Texas) Digital Library

ElectroWeak symmetry breaking in Tevatron Run II

The Tevatron Run II will provide CDF and D0 with a large dataset of ppbar interactions collected at sqrt(s)=2 TeV. We discuss the opportunity for the two experiments to improve the understanding of electrowek and top physics in the first years of data taking (Run IIa,2fb-1) in view of the upgrades of the detectors. We also discuss the prospectives for a Higgs discovery at the Tevatron in view of the Run IIb data taking period which will deliver an additional of about 13fb-1 to each experimen

Chiarelli, G

CERN Document Server

 hep2001International Europhysics Conference on HEPPROCEEDINGSElectroWeak Symmetry Breaking in Tevatron Run IIGiorgio Chiarelli for the CDF and D0 Collaborations INFN,Sez.di Pisa, S.Piero a Grado(Pisa), Italy, Via Livornese 1291, 56010E-mail: giorgio.chiarelli@pi.infn.itAbstract: The Tevatron Run II will provide CDF and D0 with a large dataset of ppinteractions collected atps = 2TeV. We discuss the opportunity for the two experimentsto improve the understanding of electroweak and top physics in the first years of datataking (Run IIa, 2fb−1) in view of the upgrades of the detectors. We also discuss theprospectives for a Higgs discovery at the Tevatron in view of the Run IIb data takingperiod which will deliver an additional of about 13 fb−1 to each experiment.1. Run II of the Tevatron ColliderThe successful Run I of the Tevatron in 1992-1996 led to the upgrade of the acceleratorcomplex. The new run, started in March 2001, is known as Run II and is expected tocollect 2 fb−1 in a first phase (Run IIa) and, after another set of improvements to machineand detectors, gather additional 13 fb−1 or so in what is known as Run IIb. The net resultsof the upgrades are an increase in energy (fromps =1.8 to 2 TeV) and in instantaneousluminosity (from a typical 1031 in Run I up to 51032cm−2s−1 in Run IIb). The averagenumber of interactions per crossing is kept low by decreasing the inter bunch distance from3.5 s of Run I down to 396 ns and which will eventually become 132 ns.In order to match the technical challenges posed by those changes, as well as to ex-ploit the physics capabilities of the Tevatron, both CDF and D0 underwent a series ofupgrades [1, 2]. Here we just mention a few of them, which are of special interest to theelectroweak physics. CDF completely redesigned its front-end electronics and DAQ systemto match the 132 ns interbunch. The trigger was also completely rebuilt, online trackingfor hight Pt tracks was moved to the first level trigger, while a new special set of processors,providing the experiment with a second level trigger on tracks displaced from the primaryvertex (the Silicon Vertex Tracker), was constructed. Finally the whole tracking systemwas completely rebuilt. It is now made of a new silicon tracker (7 layers providing spacepoints for jj <2 at 2  R  28cm) and a new central drift chamber with a stronger 3D∗Speaker. hep2001International Europhysics Conference on HEPGiorgio Chiarelli for the CDF and D0 Collaborationsreconstruction capability. Just to mention a few figure of merits, this system will allowtracking up to jj <2 (therefore doubling the Run I coverage), and will increase the btagging efficiency in top events to 65 % per jet. The D0 upgrade was even more substantialas a solenoid, providing a 2 T magnetic field, was added. Futhermore the tracking sys-tem was rebuilt with a silicon vertex detector and a fiber tracker to fully exploit this newsituation (figure 1). The D0 trigger, front end and DAQ systems were also redesigned tocope with the decreased bunch spacing. In the end the two experiments will have similarperformances in terms of hermeticity and tracking capabilities.To put things in perspective, compared to=1.0=2.0=2.5SolenoidCentral PreshowerFiber TrackerSilicon TrackerForward PreshowerLuminosityMonitorCalorimeter50 cmFigure 1: D0 upgradeRun I the overall acceptance for W,Z events willbe doubled and, for events containing a vector bo-son pair, it will be tripled.2. Gauge Boson PhysicsRun IIa with its 2 fb−1 of data, will provide eachexperiment with over 3106 W’s (e and  chan-nel), therefore allowing a precise determination of many EW parameters. One of the mostimportant results will be a better determination of the W mass. In Run I CDF measuredMW with an accuracy of 79 MeV, whereas D0’s final accuracy was 84 MeV, bringing thetotal uncertainty for this measurement (Tevatron only) to ’ 60 MeV, with statistical andsystematics on equal footing. However, since most of the systematic uncertainty dependsupon the size of control samples, we will greatly benefit from the significantly larger dataset. As an example we show (table 1) the contributions, essentially all scaling with lumi-nosity, for the CDF W!  Run I measurement [6].The biggest component (momentum scale) is ob-Source uncert.(MeV/c2)Fit statistics 100Momentum scale 85Recoil model 35Background 25Mom.resolution 20Selection bias 18Table 1: Run I MW uncertaintiestained by fitting all the l+l− invariant mass spectrum,while W asymmetry is used to constrain PDF and theZ Pt spectrum is the input to the W Pt spectrum. With2 fb−1 we expect to reduce the overall uncertainty to 40MeV/c2, with 30 MeV/c2 as a possible target.The study of W and Z couplings will benefit fromthe wider acceptances and the new features of the twodetectors. The anomalous couplings in the ZV (V=Z,γ)vertex are parameterized in terms of h30 and h40. Anyvalue different from zero of those couplings would signalnew physics. In Run I D0 set a 95 % C.L.limit for h40 at about 0.3 and in Run IIa weexpect to reach a limit to better than 0.03 by using several hundreds of reconstructed Zγ! ee()γ events and a few ZZ ! 4l detected. In the W sector we expect to collectabout 100 events WW ! ll and about 30 events WZ ! lll. Scaling from Run Ianalysis, we expect to set limit for anomalous couplings, parameterized as ∆k and  to−0:12 < ∆k < 0:18 for  =0 and jj < 0:09 for ∆k = 0. One of the most important checksof the SM, still to be performed, is the detection of the zero amplitude in the Wγ ! γl– 2 – hep2001International Europhysics Conference on HEPGiorgio Chiarelli for the CDF and D0 Collaborationsprocess. Due to specific SM cancellations, a dip appears in the angular distribution of thefinal states (l,γ). CDF saw hints of this effect in Run I and we expect to collect about 3000Wγ ! lγ in Run IIa, therefore being able to isolate this process.3. Top Physics in Run IIRight after the discovery of the top quark, both experiments swiftly moved on to measure itsproperties (cross section, mass and couplings). Some of those measurements were alreadysystematic-limited at the end of Run I. We expect most of those systematics to scale withstatistics as they are determined by the size of control samples and therefore studies ofthe sixth quark will greatly benefit from the increase of luminosity and in c.o.m. energy.Raising the energy fromps = 1:8 to 2 TeV, the production cross section increases by 40%,as the gg scattering acquires a more prominent role.Table 2 shows the number of top eventsChannel CDF/D0Dilepton(e,) 155Dilepton() 19lepton+  3jets 1520lepton+  4 jets 1200lepton+  3jets+1btag 990lepton+  3jets+2btag 240Table 2: Yield of tt events in 2fb−1expected in 2 fb−1 per experiment. To putthings in perspective, CDF collected 9 candi-dates dilepton events in Run I, and 76 wereused in the l+4j sample (the most importantone to determine Mtop) to reconstruct the topmass. The larger statistics will be obtainedthanks to a wider acceptance and better track-ing capability. The improved b-tagging effi-ciency (for example at D0 b will be about 60% for b-jets with Pt >40 GeV/c) will greatly improve, besides the determination of Mtop,the measurement of the cross section and the study of the Wtb vertex.The measurement of Mtop is dominated by system- Source GeV/c2Jet En. scale 4.4(2.2)ISR and FSR 1.8(1.0)background 1.3(0.5)b-tag bias 0.4PDF 0.3Total 4.9(2.5)Table 3: Run I systematics ofCDF Mtop measurement.In paren-thesis expectations in 2 fb−1.atics, with the biggest one being the jet energy scale.The best way to tackle this problem is to identify a sig-nal on which calibrate the energy scale. In Run I CDFwas able to reconstruct a W ! jj peak in its tiny ttdouble b-tagged sample and to detect the Z ! bb signalin its dijet sample. Both experiments plan to do evenbetter in Run II by using a dedicated trigger to selectZ ! bb events. In this way the two vector bosons willbe used to set the jet energy scale and to check b-specificcorrections. Given that, we expect the uncertainty tobe reduced by a factor 2 (see table 3 for CDF estimatesof expected systematics in Run IIa; D0’s number are similar). Together with the precisedetermination of MW this will set stringent limit on the Higgs mass (see figure 2).Another topic in top physics which will be explored in Run II is the precision deter-mination of the top cross section, which is currently a ’ 25% measurement and which willbe determined to better than 10% at the end of Run IIa. The comparison between col-lider data and Monte Carlo will improve with the availability of larger samples of b tagged– 3 – hep2001International Europhysics Conference on HEPGiorgio Chiarelli for the CDF and D0 Collaborationsevents, the wider acceptance and unprecedented MC samples both in level of detail andsize. Studies of the Wtb vertex in Run I were performed at CDF. Both top helicity andratio of branching fractions R = t!Wbt!Wq =jVtbj2jVtbj2+jVtdj2+jVtsj2 were measured [3, 4]. CDFmeasurement of R to 26 % allowed an80.180.280.380.480.580.6130 140 150 160 170 180 190 200Mtop (GeV/c2)MW (GeV/c2)1002505001000Higgs Mass (GeV/c2 )RUN-IIMW-Mtop contours : 68% CLFigure 2: SM Higgs as a function of MW andMtop. Run IIa (2 fb−1), one experiment only.indirect determination of the CKM matrixelement jVtbj to ’ 15%. In Run II we expectto reduce the uncertainty on R to 6 % with acorresponding uncertainty on jVtbj to about3%. A direct determination of jVtbj (15%accuracy) can be obtained by identificationof the single top production process, whichhowever stands as an elusive process. Theunderstading of forward tracking and tag-ging will play a decisive role in this search.4. Higgs Searches in Run IIOver the last two years a lot of effort wasdedicated to better understand the chances to detect a SM Higgs particle at the Tevatron.Emphasis was set on the low mass (< 130 GeV/c2) region, where the Higgs decays almostcompletely into bb pairs. Although (figure 3) the dominant production process is the directproduction through gg fusion, the large QCD background renders this channel unfeasible.The associate production V H (V =W;Z), where√s = 2 TeVMt = 175 GeVCTEQ4Mgg→Hqq→Hqq qq_’→HWqq_→HZgg,qq_→Htt_gg,qq_→Hbb_MH [GeV]10-410-310-210-111010 280 100 120 140 160 180 200σ(pp_→H+X) [pb]σ(pp_ →H+X) [pb]Figure 3: SM Higgs productiona leptonic decay of the V would provide a clean trig-ger and bring to a favourable S/B ratio, is morepromising. A detailed study, based on a mix ofMonte Carlo paremeterization, Run I data analy-sis and comparison with simulation of the CDF andD0 detector, was performed in the last three years.It is not possible here to describe all the studiesperformed[5], the summary is provided in tables 4and 5 where the event yield in 15 fb−1 is shownfor the WH and ZH case (bb background not shownbut included in the significance). It is important tokeep in mind that all, but the single top, backgroundprocesses were already seen or measured in Run I.Therefore the calculation of the signal includes allknown effects. The studies of the ”high mass” region focused on the gg ! H !W W  !l+l− channel where the price paid for the low cross section is offset by the very smallbackground. An additional step was to add the associate production of H and W or Z:pp! W (Z)H ! lljjX. By vetoing on b-tagged jets (produced in tt events), the back-grounds are reduced to a manageable level. The final outcome of these studies is shown infigure 4. The 95 % C.L. limit as well as the 3 evidence and 5 discovery curves are shown– 4 – hep2001International Europhysics Conference on HEPGiorgio Chiarelli for the CDF and D0 CollaborationsMH (GeV/c2) 110 120 130Signal 75 60 45Wbb 435 375 285WZ 90 60 30tt 225 300 330single top 105 135 135S/pB 2.6 2.0 1.6MH (GeV/c2) 110 120 130Signal 69 46 31Zbb 84 69 52.Wbb 100 81 63ZZ 43.5 3 0.0tt 70.5 64.5 52.5single top 79.5 70.5 57.S/pB 2.4 2.0 1.5Table 4: Higgs yield in WH channel Table 5: Higgs yield in ZH channelas a function of the Higgs mass and of the integrated luminosity. The two experimentshave been combined. There is a window of opportunity in the low and intermediate Higgsmass region, if the Tevatron can provide an integrated luminosity in excess of 10-15 fb−1before the start of the LHC. In order to do so the Beams Division of Fermilab initiated anintensive R&D program to demonstrate the feasibility of delivering about 4 fb−1 per year.At the same time both CDF and D0started an intensive period of stud-Figure 4: SM Higgs expectations at the Tevatronies to understand which parts of theirdetectors need to be replaced to sur-vive luminosities more than 7 timeslarger than originally planned.5. ConclusionIn the next years of Run II we expectto gather more than 2 fb−1 per ex-periment. Thanks to the very largestatistics precise determinations of themasses of the top quark and of the Wvector boson will be possible. This will allow to set stringent limit on the Higgs mass.Furthermore, as the Tevatron collider will be upgraded to deliver ’ 4 fb−1 per year, thereis a chance to isolate the Higgs particle if its mass is low enough.References[1] F.Abe et al., CDFII Technical Design Report, FERMILAB-96-390-E.[2] M.Abolins et al., The D0 Upgrade, FERMILAB-96-357-E.[3] T.Affolder et al., Phys. Rev. Lett. 86 (2001) 3233.[4] B.Abbot et al., Phys. Rev. Lett. 85 (2000) 256.[5] M.Carena et al. Report of the Run II Higgs Working Group, hep-ph/0010338[6] T.Affolder et al. Phys. Rev. D 64 (2001) 052001– 5 –

ElectroWeak Symmetry Breaking in Tevatron Run II

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