Réunion de groupe ILC - LAL - 22/02/2011

Top studies

Philippe DoubletThibault Frisson, Roman Pöschl,

François Richard

Introduction : why top ?

• Top : – known particle,– heaviest SM fermion (EWSB connections) extra-dimensions– detector challenges b-tagging, jets, lepton tracking…

t b W+

b l+v = 1b jet + 1 top-like-sign lepton

ILC, 500 GeV

We study : semileptonic top decays, i.e. ttbWbWb(lv)b(qq) Signature = 4 jets (2 b) + 1 lepton

Direct decay : no hadronisation

b qq = 1b jet + 2 light jets from a W

A hint of theory : extra-dimensions

• Higgs on IR brane for gauge hierarchy problem• SM fermions have different locations along the 5th dimension• Fermion-boson couplings = wavefunctions overlaps• Overlaps leptons – Higgs in the 5th dimension generate good

Yukawa couplings with O(1) localisation parameters

tR0e0

Aµ0 H

y (5th dim.)0 πR

UV brane(MPl)

IR brane(TeV)

bulk

Aµ1

u0

Aim : measure tR and tL couplings• L/R couplings affected by localisation on the brane via Z-Z’ mixing

Δg(tR)/g(tR)

Δg(tL)/g(tL)

SM 0,0Djouadi

-34%,-1%Hosotani+18%,-7%

Gherghetta -20%,-20%

Carena0,-20%

Observables connected to g(tR) and g(tL) ?

ALR and AFB

ALR and AFB

• Definitions : ALR = (polar. flip)

AFB = (direction)

Error : ΔA =

• Conclusion : only need to count top events for ALR

• Corollary : Reconstruct at most 1 top direction for AFB

• tt{bW}{bW}{b(lv)}{b(qq)} : reconstruct the hadronic top

)()(

)()(

RtopLtop

RtopLtop

eNeN

eNeN

)0(cos)0(cos

)0(cos)0(cos

ttopttop

ttopttop

NN

NN

N

A LRFB2

)(1

Semileptonic channel• Clean and easy signature : 1 lepton + at least 1 b jet– Remark : lepton = e/µ and τe/µ adds statistics

• Backgrounds ?– Worst : ZWW (Zbb) but under control (MC level)– Hadronic top (bbqqqq): needs to be done– All the rest should be fine

σ(tt) ≈ 600 fb at 500 GeV(≈ 180 fb for SL top into e and µ)

ZWW (Zbb) ≈ 8 fb

ZWW/top separation at MC level

Lepton finding : method1. Topology forced to 4 jets the lepton is embedded in one jet2. Check all leptons in jets (lepton id.)3. Ours is either leading or isolated in its closest jet (cuts)

Case 1 : leading Case 2 : isolated

Lepton finding : efficiencies and purities

Tracking inefficiencies :• Muon = 94.4 % - 92.6 % = 1.8 % (|cos θ|>0.97)• Electron = 93.5 % - 87.7 % = 5.8 % (TPC disk,|cos θ|>0.97)

Particle Efficiency Bad lepton PurityMuon 93.6% 2.9% 97.1%Electron 89.7% 3.9% 96.1%

Efficiencies : angular and energetic

• Effiencies under control : – Tracking worse at large angles and in the TPC disk– Leptons with small energies are suppressed by isolation cuts

B-tagging on top events

• We require to find 1 b jet, highest btag > 0.5• One b found comes with a second b

Jet with the highest B tag among the 4

Real B jet

Light jet (negligible)

Btag > 0.5 :Purity = 98%Efficiency = 91%

0.5Excellent angular coverage (for the best B jet)

Cross-section and ALR

• σ = N/(εL), L = 500fb-1

• σ(ttSL)unpol. = 159.4 fb, Δσ/σ = 0.37% (stat.)– Whizard : σ(ttSL)unpol. = 159.6 fb (-0.1%)– P(e+e-)= (±30%, ±80%) Δσ/σ = 0.28%/0.42% (stat.)

• ALR = 0.435, ΔALR/ALR = 0.54% (stat.)– ALR = 0.37 expected… Whizard problem ?– However, interest lies in relative uncertainty– P(e+e-)= (±30%, ±80%) ΔALR/ALR = 0.69% (stat.)

A look at reconstructed tops

• 2 b jets + 2 light jets ( = W )

• Keep top closest to :– Etop = 250 GeV and Mtop = 172 GeV

Only 2 possible combinations for the top = (b1W) or (b2W)

Reconstructed mass without effort (not our goal), just a cross-check

An idea of top direction… (Pe- = +100%)

ALR = 0.367 – 0.41 expected… Whizardise…

Réunion de groupe ILC - LAL - 22/02/2011

Conclusion and perspectives

• Measurement of top asymetries in the semileptonic channel– Lepton and first B jet very efficient and pure– τ channel helps with statistics– Check full hadronic top

• Need to look further into top reconstruction• Use angular distributions of leptons for CPV and

complementary studies