Top Production Cross Section from CDF
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Transcript of Top Production Cross Section from CDF
Top Production Cross Section from CDF
Anyes TaffardUniversity of Illinois
On behalf of CDF Collaboration
32nd International Conference on High Energy Physics
2Why Study Top Quark Physics ? Experimentally, still know very little about the top
quark Run I: ~110 pb-1, ~100 top candidates
o Overall consistency with SM prediction, statistic limited Only know fermion with mass near electroweak scale
Theoretical models proposed to solve the problems of the SM often have top playing a leading role:
o SUSY: Large top mass causes EWSBo In many dynamical symmetry breaking models, top interactions are
modified (e.g technicolor) Probe for physics beyond the SM
o Non SM production (Xtt, Xll+jets+ET)o Non-SM decay: Heavy enough to decay to exotic particles (tXb)
On-sheel charged Higgs, SUSY
Signal of today, background of tomorrow
3Tevatron & CDF Proton-antiproton collisions
s=1.96 TeV Main injector
150 GeV proton storage ring
•New DAQ•New Track Trigger
•New Silicon ||<2•Improved b-tagging
•New Drift Chamber•New Plug Calorimeter
•Increase acceptance•Upgrade Muon Detectors
4Tevatron & CDF: LuminosityRecord Peak Luminosity: July 16, 2004: 10.3 x 1031 cm-2sec-1
With mixed pbar (recycler & accumulator)
Acquired luminosity in 2004 already surpassed 2003 totalCDF ~450 pb–1 total on tape
In this talk:160 pb-1 <Ldt <200 pb-1
days
CD
F ac
quire
d Lu
min
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(pb-1
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Initi
al L
umin
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(E30
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Store Number
2002 2003 2004
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Pair Production 85% qq, 15 % gg (fractions reversed @
LHC) Central, spherical events Large transverse energy
Cross section increases 30% with Tevatron s increase to 1.96 TeV
Single top production is a factor of 2 smaller
BR(tWb) 100 % Both W’s decay via Wl (l = e or ; 5%)
o final state l l bb : dilepton One W decays via Wl (l = e or ; 30%)
o final state l qq bb : lepton+jets Both W decays via Wqq (44%)
o final state: qq qq bb: all hadronic
Top Quark Production & Decay
e-e (1/ 81)mu-mu (1/ 81)tau-tau (1/ 81)e -mu (2/ 81)e -tau (2/ 81)mu-tau (2/ 81)e+jets (12/ 81)mu+jets (12/ 81)tau+jets (12/ 81)jets (36/ 81)
Bonciani et al., Nucl. Phys. B529, 424 (1998)Kidonakis and Vogt, Phys. Rev. D68, 114014
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Measuring tt Cross Section Starting point for all top physics
Events triggered on one high momentum lepton or multi-jets Optimized event selection for top physics and new physics Define top sub-samples by counting lepton and jets
2 jets in dilepton channel 3 jets in l+jets channel 6 jets in all hadronic channel
Efficiency & Acceptance: Measured from MC (Pythia) & Data
Background estimate:most challenging part
Improve S:B withb-tagging
7Dilepton Channel Small sample, but highest S/B Backgrounds:
Z/* l+l- WW, WZ, ZZ W+jets (fake leptons)
Background can be furtherreduced with an HT cut.Ht: Scalar summed ET of jets, leptons, and missing ET
8Counting Experiments
Higher purity, lower statistical significance
Lower Purity, higher acceptance (~20% from )
1st Run II paperCombined Result: hep-ex/0404036
Standard Run I method (ee,,e)Looser selection: e/ + track
With higher statistics in Run II observe good agreement with SM
13 candidates: 1 ee, 3 9 e
9Inclusive Dilepton Analysis No cuts other than two identified leptons
If same flavor, Z e+e-/+- dominates: require significant Etmiss
Advantages: increase acceptance Goal: search for new physics Fit data to tt,WW, Z +- contribution in 2D (Et
miss, Njet) plane
Top Cross Section
WW Cross Section
CDF RunII Preliminary
J.M.Campbell and R.K.Ellis, Phys.Rev.D60 113006 (1999)
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Larger sample due to BR, but less pure
Improve S:B by requiring b-tagging 2nd vertex semileptonic decay
L+jets Channel
Candidate Event Display
3 Jets
11Using Vertex Tagging B decay signature: displaced
vertex Long life time c ~ 450 m Travels Lxy~3mm before decay
Top event tagging efficiency: 52% False tag rate per QCD jets: 0.5%
@least 1 b-tag & HT>200 GeV
12Using Double b-Tag Double b-tag event essential for top mass
measurement: reduces combinatorics Improve b-tagger:
Increase per jet tagging: 10.8 % 12.0% False tag rate: x 3.4
Significance up by 18%: tt expectation increases from 8.7 13.0 3x more background
Loose SecVtxStandard SecVtx
13Using Soft Lepton Tag Tagging B may decay semileptonically Leptons id challenges:
softer spectrum than leptons from W/Z non-isolated (cannot use calorimetry information)
Id low pT muon Background dominated by fake tag
o Punch though, decay in flight
Top event tagging efficiency: 15% False tag rate (QCD jets): 3%
14Using Kinematics Fits Determined signal fraction using kinematics
shape
Fit NN output (7 kin. var.) Fit leading jet ET, require @ least 1 b-tag
15All Hadronic Channel Final State: 4 jets from W, 2 b-jets (data sample: multi-jets
trigger) Large statistics, but huge QCD background:
S:B1:2500
Increased S:B by requiring: ET>320GeV Topological cuts (aplanarity, centrality)
o Event selection efficiency 6.2%o S:B 1:24
@ least 1 b-tag jet: S:B 1:4
Measure Xs with 6 jets & 1 b-tag jet
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Probe EW coupling, direct determination of Vtb
Sensitive to new physics t-channel: anomalous couplings, FCNC s-channel: new charged gauge boson
Single Top
t-channel production (Wg fusion) = 1.98 pb
s-channel production (W*)
= 0.88 pbB.W. Harris et al.Strategy:
Isolate W+exactly 2 jets & 1 b-tag jetNon-top 89%: W+jets (62%), false tags (25%), QCD (10%) Top: 11%
Likelihood Fit to Ht (combined): to discover Likelihood Fit to Q* (t-channel) : to see new physics
o Q of lepton, of light quark jet
17Single Top cont.
CDF Run II Limits at 95% C.L.:Combined: 17.8 pb (β95=6.2/4.8)t-channel: 10.1 pb (β95=5.1/5.6)s-channel: 13.6 pb (β95=15.4/13.7)(Format: β95=observed/expected)
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18Summary x2 Run I dataset
Observed consistent with SM prediction @ mtop=175 GeV/c2
1st paper out, more coming soon. Near future: already x2 more data on tape