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Ijaz Ahmed National Centre for Physics, Islamabad
Top Anti-Top pair Production, Decays and Branching ratios
at LHC
Ijaz Ahmed National Centre for Physics, Islamabad
outlines
IntroductionWhy top quarkTop production and Decay diagramsDecay modes of Top quarkBranching Ratios + Events SelectionBackground ProcessesSpin Correlation
Ijaz Ahmed National Centre for Physics, Islamabad
Introduction Top quark---the heaviest quark Discovery 1995 at Tevatron (ppbar collider), CDF+D0 Pole mass--(174 ± 5.1) GeV Decay width ---1.4 GeV Life time ~ 10-25s <<< time for depolarization t--Wb BR(~100 %) Spin and parity----JP(SM) = 1/2+
Weak iso-spin eigen values =I3 = +1/2 ttbar Production cross-section at LHC = 830 pb (NLO) Integrated luminosity = 10 fb-1
ttbar events to be generated = 10 x 1015 x 830 x 10-12 = 8.3 M/year
Ijaz Ahmed National Centre for Physics, Islamabad
Need of Top Quark
Cancellation of triangle anomalyProbe the t--->Wb vertex (Vtb)
Contribution to W and Higgs boson propagator
Partner required for b quark
Ijaz Ahmed National Centre for Physics, Islamabad
Leading order ttbar pair production channel
Quark annihilation
Gluon fusion
Ijaz Ahmed National Centre for Physics, Islamabad
Top quark yields
Top quark in pairs Exclusively decay into W and b Events depend on W decay modes Leptons plus jets: one W decays to jets and other
into leptons (30%) Hadronic Decay: both W’s decay to jets (65%) Di-leptons: both W’s decay into leptons plus
neutrinos (~ 5%)
Ijaz Ahmed National Centre for Physics, Islamabad
Top Decay modes
ll = e,
Decay mode Branching ratio
tt→W+W-bb→bbqqqq 36/81
tt→W+Wbb→bbqq'e 12/81
tt→W+Wbb→bbqq‘ 12/81
tt→W+Wbb→bbqq‘ 12/81
tt→W+Wbb→ebb 2/81
tt→W+Wbb→ebb 2/81
tt→W+Wbb→bb 2/81
tt→W+Wbb→eebb 1/81
tt→W+Wbb→bb 1/81
tt→W+Wbb→bb 1/81
In Standard Model : t W+b
Nttbar = Lumi x ttbar
Nttbar = X events =>
Lumi x (ttbar x BR)
ud, us, ub
cd, cs, cb
td, ts, tb
Ijaz Ahmed National Centre for Physics, Islamabad
Branching ratio of W decays
W+/W-
Decay modes
W+→cs,ud
(6/9)
W+→ee
(1/9)
W+→(1/9)
W+→(1/9)
W-→cs,ud (6/9)
36/81 6/81 6/81 6/81
W-→e-(1/9)
6/81 1/81 1/81 1/81
W-→
6/81 1/81 1/81 1/81
W-→(1/9)
6/81 1/81 1/81 1/81
Ijaz Ahmed National Centre for Physics, Islamabad
1. Semi leptonic decay mode
Branching ratio = 12/81 = 14.8 % (1.21 M/year)2. PP→tt→bbqq'.......(4 jets++Emiss ) Branching ratio = 12/81 = 14.8% (1.21 M/year)
..................................................................................................... Total Branching ratios of Semi Leptonic decays = 29.6 % Total semi-leptonic events to be generated = 8.2x29.6/100(M) = 2.5 M/year
t t Main Reaction (tt→W+W-bb→bb(qq')(l) Medium sized branching ratio with the managable background1. PP→tt→bbqq'ee.......(4 jets+e+Emiss )
jets
W b
W b
jets
l+
jets q q
jets
Ijaz Ahmed National Centre for Physics, Islamabad
Some useful relations
Rapidity: Y = 1/2*log[(EW + PW)/(EW PW)]
Pseudorapidty: = -log(tan/2) Tan = Py / Px
Pt = sqrt (Px2 + Py
2)
Absolute Momentum = sqrt(Px2 + Py
2 + Pz2 )
Invariant mass = mW2 = sqrt [(El + EPl + P
P1P2(1 cos12)
Jet cone radius = sqrt [(
Angular distribution = cosPz / Pt
Ijaz Ahmed National Centre for Physics, Islamabad
Event Selection Criteria Three methods to measure the mass of top quark Three jets invariant mass of the hadronic top decay The entire ttbar system is fully exploited to determine the top quark mass from a
kinematics fit. Using kinematics fit, but jets are reconstructed using a continuous algorithms
At least 1 lepton with || < 2.4 Exactly 1 lepton with Pt(l) > 20 GeV Missing Et > 20 GeV Lepton isolation R = 0.3 < 0.1 At least 4 jets reconstructed with a cone size (Rcone = 0.4) 4 jets with Et > 40 GeV At least two jets to be tagged as b jets Total Et > 450 GeV Exactly 2 b jets with Et > 50 GeV 60 < reconst (MW) < 100 GeV Transverse mass mt (Wlep) < 100 GeV Rec. top mass difference |mt - mt| < 25 GeV R = 0.7 Pt (jjb > 250 GeV After selection cuts S/B ~ 78 (87,000 events)
Ijaz Ahmed National Centre for Physics, Islamabad
Background Processes
ttbar-l +jets = 2.2x106pb W+jets→l+jets Dominant Background
S/B = 18.6 => 7.8x103 pb (1658/year) Z+jets→l+l-+jets = 1.2x103 pb (232/year) WW→l+jets = 17.1 pb (10/year) WZ→l+jets = 3.41 pb (8/year) ZZ→l+l-+jets = 9.21 pb (14/year) ...................................................................................... Total BG events (1922/year) At production level S/B = 10-5
Ijaz Ahmed National Centre for Physics, Islamabad
2. Purely Leptonic Decays
Main Reaction (tt--->W+W-bb--->bb(l)(l) pp--->tt--->bbeeee.......(4jets+2e+Emiss ) Branching Ratio = 1/81 =1.23 %
(100,000/year) pp--->tt--->bbee.......(4jets+2e+Emiss ) Branching Ratio = 2/81 = 2.46 %
(200000/year) pp--->tt--->bb.......(4jets+2+Emiss ) Branching Ratio = 1/81 = 1.23 %
(100,000/year) .....................................................................................................
............ Total BR for Leptonic Decay = 4.9 %
(400,000/year)
Ijaz Ahmed National Centre for Physics, Islamabad
Detection of leptons and signatures
Two opposite sign leptons with |h| < 2.5 Pt(l1) > 35 GeV Pt(l2) > 25 GeV Emiss > 40 GeV |Mll-MZ| > 10 GeV M2
W = (l1 + 1)2 , M2W = (l2 + )2
M2top = (l1 + b1 + 1)2 , M2
top = (l2 + b2+ )2
Two b-jets with Pt > 25 GeV (S/B = 10) , || < 2.4, Rcons= 0.4 For two neutrinos "neutrino weighting" technique is used Neutrino rapidities, top mass, charged lepton and b-quark momenta,
system can be solved for transverse and longitudinal momentum components of neutrino
After event selection 80000 signal events are left
t tW+ b
W- b
l+
l-
jets
jets
Ijaz Ahmed National Centre for Physics, Islamabad
b-tagging
How to distinguish a b jet from a lighter quark jet:
– can contain low-p leptons from b c l (BR=10% per lepton) B hadrons have lifetimes long enough so
thatthey can travel several mm before decaying b-jet particles come from a displaced
vertex
Ijaz Ahmed National Centre for Physics, Islamabad
Background Processes
Dilepton decays have low statistics bbbar→l+jets WW+jets→(2l)(2)+jets Background is small mainly dominated by Z decays to
leptons Leptons misidentification increases Drell-Yan processes associated with jets Z→ t+t- (associated with jets) WW + jets Backgrounds easier to eliminate than in all-hadronic mode
because of lepton tag. Most promising decay mode for search
Ijaz Ahmed National Centre for Physics, Islamabad
3. Purly Hadronic DecaysMain Reaction (tt→W+W-bb→bb(jj)(jj) (370 pb)
pp→tt→bbudud............(2 bjets+4 quark jets) Branching Ratio = 9/81 (911,000/year) pp→tt→bbusus.............(2 bjets+4 quark jets) Branching Ratio = 9/81 (911,000/year) pp→tt→bbubub...........(2 bjets+4 quark jets) Branching Ratio = 9/81 (911,000/year) pp→tt→bbcdcd............(2 bjets+4 quark jets) Branching Ratio = 9/81 (911,000/year) pp→tt→bbcscs.............(2 bjets+4 quark jets) Branching Ratio = 9/81 (911,000/year) pp→tt→bbcbcb...........(2 bjets+4 quark jets) Branching Ratio =9/81 (911,000/year) .......................................................................................................... Total BR of purely Hadronic decays = 9/81*6 = 66 %
(5.41M/year)
Ijaz Ahmed National Centre for Physics, Islamabad
Event Selection Criteria
Multi jet trigger threshold ~ 4 jets Events are selected by requiring at least six or more jets with Pt = 40
GeV, and at least two of them are tagged as b-jets Jets are required to satisfy || < 3 (|| < 2.5 for b-jet candidates) Jets are reconstructed using a fixed cone algorithm with R = 0.4 Sum of the transverse momenta of the jets is required to be greater
than 200 GeV At least one b-tagging is required using secondary vertices Tagging required efficiency 60% with at least 100 rejection against
prompt jets ttbar signal efficiency for these cuts should be 19.3 % Only 0.29 % of QCD multi-jets events should be survived For QCD multi-jet cross-section of 1.4 *10-3 mb and Pt > 100 GeV, S/B
~ 1/57
Ijaz Ahmed National Centre for Physics, Islamabad
Background Processes
(qiqj ---> qiqj, qig ---> qig, gg ---> gg, qqbar---> gg, gg ---> qqbar,
qiqibar---> qjqjbar) (pp 6 jets) 103 (pp t t b b + 4 light-quark jets)
Difficult to distinguish between light jets and b jetsKinematics cut techniqueHigh statistics requiredLeast interesting decay modeLarge QCD multi-jet events
Ijaz Ahmed National Centre for Physics, Islamabad
Comment
All jets in the final state create difficulty in triggering. QCD background is generated with a pt cut on the hard
scattering process above 100 GeV, resulting cross-section 1.73 mb.
The requirement of having at lest two b-tagged jets in the final stat helps in rejecting a large part of the physical background, but also reduces considerable the signal sample. The fraction of signal events with at least two b-tagged jets is three times smaller than the fraction with at least one b-tagged jet. Requiring only one b-tagged jet would decrease the S/B ratio from 78 to 28, which would be still acceptable.
Ijaz Ahmed National Centre for Physics, Islamabad
Systematic Uncertainties in Top Mass
Main contributions Jet energy scaleISR and FSRMC generatorMethod for mass fittingModel for background
Ijaz Ahmed National Centre for Physics, Islamabad
Theoretical uncertainties in Top Mass
Renormalization scale < 10 MeV
(30-150 GeV)Strong coupling constant < 75 MeVMS bar ~ ± 12 MeV
Which implies that at LHC accuracy in top mass will be of the order of 1 GeV
Ijaz Ahmed National Centre for Physics, Islamabad
Spin Correlation in ttbar production Top decay width = t = 1.4 GeV QCD Hadronization scale = qcd = 0.22 GeV. Time scale for depolarization of top spin
= mt / 2qcd >> 1/t ~ 10-24 s
Spin correlation in decay products of tt systems is interesting for several reasons.
It provides probe of a quark that is at least free of confinement of effects.
Since life time of top quark is proportional to CKM matrix element |Vtb|2 , so observation of spin correlation would yield, information about lower limit of |Vtb| with out assuming that there are three generation of quarks.
Charged leptons +weak isospin quarks are sensitive to the initial polarization
*d2/d(cosd(coscoscos
Ijaz Ahmed National Centre for Physics, Islamabad
Top decay diagram
W+
W-b
bbar
t tbar
e-
e+
Jet1
Jet2Strong
couplingCKM
Weak coupling
Ijaz Ahmed National Centre for Physics, Islamabad
PYTHIA (Some results of ttbar pair production)
1000 events of ttbar pairs producedSigma (gluon fusion) = 435 pb (Sigma (quark annihilation) = 80 pbTotal cross-section = 515 pb 10,000 events of ttbar pairs producedSigma (gluon fusion) = 450 pb Sigma (quark annihilation) = 67 pbTotal cross-section = 518 pb