ijaz ahmednational centre for physics, islamabad top anti-top pair production, decays and branching...

Ijaz Ahmed National Centre for Physics, Islamabad

Top Anti-Top pair Production, Decays and Branching ratios

at LHC


outlines

IntroductionWhy top quarkTop production and Decay diagramsDecay modes of Top quarkBranching Ratios + Events SelectionBackground ProcessesSpin Correlation


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


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


Leading order ttbar pair production channel

Quark annihilation

Gluon fusion


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%)


Top Anti-Top Decay modes


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



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


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


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


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


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)


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


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)


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


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



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


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)


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



(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


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.


Overview of Branching ratios


Systematic Uncertainties in Top Mass

Main contributions Jet energy scaleISR and FSRMC generatorMethod for mass fittingModel for background


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


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


Top decay diagram

W+

W-b

bbar

t tbar

e-

e+

Jet1

Jet2Strong

couplingCKM

Weak coupling


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

ijaz ahmednational centre for physics, islamabad top anti-top pair production, decays and branching...

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