Standard Model at LHC

Standard Model at LHCEric COGNERAS, LPC Clermont-Ferrand

On behalf of the ATLAS and CMS collaborations

HEP-MAD 07, Antananarivo, Madagascar2007, september 14

Motivation• SM successfully tested at current energy• LHC prospect physics at higher energy :

– Able to search directly beyond SM– But also precise measurements of SM

parameters• Goal for SM physics:

– Deeper understand the SM with :• Precision EW measurements (MW, sin2W ,…)• Top physics (mass,cross section,… measurement)

– Indirect estimate the Higgs mass• Using MW, mTop, sin2W precise measurement

– Search for deviation from SM

14/09/2007 Eric COGNERAS - Standard Model @ LHC 2

uu cc ttdd ss bbeeee

µµµµ

gg

WWZZ

Experimental FrameworkThe LHC

– CERN (Geneva)– pp collision @ √s = 14 TeV (10 × Tevatron)– Luminosity :

• Low luminosity phase L1033 cm-2s-1 (2008-2009?) [10 fb-1/year : 10× TV]• High luminosity phase L1034 cm-2s-1 (2010?-X) [100 fb-1/year : 100×TV]


ATLAS and CMS general purpose physicsLHCb b physicsALICE heavy ion physics

Experiments

EW precision measurement

Experimental Framework


• Tracking (||<2.5, B=2T) :– Si pixels and strips– Transition Radiation Detector (e/ separation)

• Calorimetry (||<5) :– EM : Pb/LAr with Accordeon shape– HAD : Fe/scintillator (central), Cu-W/Lar (forward)

• MuonSpectrometer (||<2.7) :– air-core toroids with muon chambers

• Tracking (||<2.5, B=4T) :– Si pixel and strips

• Calorimetry (||<5) :– EM : PbWO4 crystals– HAD : brass/scintillator (central,end-cap), Fe/Quartz (forward)

• MuonSpectrometer (||<2.5) :– return yoke of solenoid instrumented with muon chambers

p-p collisionsCross section and Event rate


Process (nb)

Evt/y (Low L:10 fb-1)

Minimum Bias 108 1015

Inclusive jets (pT>200 GeV)

100 109

W→l (l=e,µ) 30 108

Z→e+e- 1.5 107

tt 0.83 106

bb 5.105 10121 or 2 orders of magnitude larger than Tevatron

LHC is a W, Z, top factory :• small statistical errors in precision measurements• large samples for studies of systematic effects (calibration and syst. controls)Hadron collider problem :

PDF to be better known

x2x1

SM physics @ LHC• W/ Z physics (precision EW measurement)

– Parameters related to indirect MH measurement : • W mass and width• sin²W

– Constraints on the PDF• W/Z inclusive cross section as well as W/Z+jets• W rapidity

– Measurement of gauge boson pair production• Triple Gauge Boson Coupling

• Top physics– Parameters related to indirect MH measurement :

• Top mass/cross section– Deeper understanding of Top quark :

• Top spin correlation, probe of the Wtb vertex, single Top cross section, Top charge• QCD (→ see Albert talk on tuesday)• Higgs boson direct search• B physics• …

14/09/2007 6Eric COGNERAS - Standard Model @ LHC

SM physics at LHC covers a large number

of topics just focus on few items in this talk

Precision EW Measurement

MW measurement• LHC expects :MW < 15 MeV

• W channel : same as the Tevatron

using W→l decayBut LHC statistics is higher (60M recons. W→l @ low Lumi [10×

TV])

• Several methods available: R=MT

W / MTZ spectrum Small syst, sample /10

pTl spectrum pile-up , theo. know. of PT

W MT

W high stat, pile-up


Event selection:

• 1 isolated lepton (e,µ) pT> 25 GeV in ||<2.4

• missing ET > 25 GeV

•No jet with pT> 30 GeV

•Recoil |u|< 20 GeV

q

q’


MW measurement


,)cos1(2 , elppM lT

lT

TW ,

Use Transverse mass to

cope with unmeasured pL

pT from pT

l & recoil

Use the knowledge on the Z boson to constrain the W mass

pTl spectrum shape is sensitive

to MW

pT not necessary

, ,l Z l WZT T

W

MP P

M

W mass : fit exp. shape to MC sample with different Values of MWW mass : fit exp. shape to MC sample with different Values of MW


MW measurement : systematics


LHC goals : Wl : one lepton species, low L , per experiment, after 1 year

Combining channels and ATLAS/CMS exp., should reach MW 15 MeV

source CDF RunII, combined channel LHC

200 pb-1 60Mevts, 10fb-1

Statistics 0 MeV <2 MeV

Lepton scale 17 MeV 15 MeV

Energy resolution 3 MeV 5 MeV

Recoil model 12 MeV 5 MeV

Lepton id. --------- 5 MeV

PTW 3 MeV 5 MeV

PDF 11 MeV 10 MeV

W width --------- 7 MeV

Radiative decays 11 MeV <10 MEV

Background 0 MeV 5 MeV

TOTAL 26 MeV <25 MeV

stat. error : negligible

syst. error : •MC modelling of phys.•detector responses

PTW spectrum

PDF

W width

W rad. decaysBackground

Zll

LHC data

Zll,R,Tevatron

Theo. calculation

MC

physics

detectorlepton E&p scale

Lepton resolution

Recoil

Zll, E/p for e

Zll, E/p for e

Zll


Determination of sin²W


Parity violation in neutral current Asymmetry in the angular distribution of leptons from Z decay(-dependence of cross-section)

All eventsEvents with quark direction correctly estimated

Use AFB in p-p→Z/*→l+l-

At the Z pole, AFB comes from the interference of vector and axial component of the coupling

Where a, b calculated to NLO QED and QCD Assumption for p-p colliders : the quark direction is

the same as the boost of the Z Correct for large di-lepton rapidities Only EM calorimeters provide the required

large η-coverage (Z→e+e-)

2( sin )FB WA b a

Test of the Standard Model

(universality)


Determination of sin²W

• Results


Only Forward Electrons

All Events

Event selection:

• pTe > 20 GeV

•85.2 < Mee < 97.2 GeV

y cuts – e+e- (|y(Z)| > 1)

∆AFB

(Stat)∆sin2θW

(Stat)|y( l1,2 )| < 2.5 3.0 x 10-4 4.0 x 10-4

|y( l1 )| < 2.5 + |y( l2 )| < 4.9

2.3 x 10-4 1.4 x 10-4

L = 100 fb-1

• Current error on world average 1.6x10-4

• need small systematic error :• PDF uncertainty,• precise knowledge of lepton acceptance and efficiency• effects of higher order QCD

Sensitivity increases with forward electrons


Triple gauge boson couplingProbe the non abelian structure of SM

• 14 possible WW and WWZ coupling• Use 5 independent, CP conserving, EM gauge invariance preserving

couplings : g1Z, , Z, , Z

– At SM tree level, g1Z==Z=1 and =Z =0

– and Z grow with s big advantage for LHC

– = -1, g1Z

= g1Z -1, Z = Z -1 grow with s


•LO Feynman diagram :V1, V2, V3 = Z, W, → WW, ZW, W •Diboson final states have predictable production and manifest the

gauge boson coupling•In SM, only charged coupling WW and WWZ are allowed

q

q’s-channel

TGC manifest in :•Cross section enhancement•High pT(V=W, Z, )•Production angle


Triple gauge boson coupling


Exemple of WZ production• Use only leptonic final state• Event selection:

• Exactly 3 leptons with pT>25 GeV• At least one pair of leptons with same flavour and opposite charge and |mll-MZ|<10 GeV• …

SM

g1Z

= 0.05

Coupling Present Value LHC Sensitivity (95% CL, 30 fb-1, 1 exp)

g1Z 0.005-0.011

0.03-0.076

Z 0.06-0.12

0.0023-0.0035

Z 0.0055-0.0073

022.0019.0016.0

044.0045.0027.0

020.0021.0028.0

061.0064.0076.0

063.0061.0088.0

QCD-oriented Measurement

Measurement of W/Z cross sections

• Results (L=1 fb-1 [~600k Z→, ~6M W→])


Systematics come mainly from theory(acceptance+PDF uncertainty).At a later stage these processes can be used as

luminosity monitor (Error on luminosity: ~5%)

Z

Ldt

XWZpptotal

BackgroundCandidates )f-(1N

)/(

Estimation of the cross section

(Z + X) = 1160 ± 1.5 (stat) ± 27 (syst) 116 (lumi) pb(W + X) = 14700 ± 6 (stat) ± 485 (syst) 1470 (lumi)

pb Already dominated by systematics

Event selection: Z : 2 isolated µ, pT

µ > 20 GeV, ||<2, 84<Mµµ<99 GeV,... W : 1 isolated µ, pT

µ > 25 GeV, ||<2, 40<MT(µ,ETMiss)<200GeV,...

W

CERN/LHCC 2006-021CMS TDR 8.2

QCD-oriented Measurement

Constraints on PDF using W rapidity distributions


eWud

eWdu

CTEQ61 MRST02 ZEUS02

CTEQ61 MRST02 ZEUS02

e- rapidity e+ rapidity

GeneratedGenerated

y

d(W

e)/

dy

y

d(W

e)/

dy

Reconstructed Reconstructed

•At LHC, experimental uncertainty is dominated by systematics (large event production)

•The theoritical uncertainties are dominated by PDFs Exp. uncertainty sufficiently small to

distinguish between different PDF sets

PDF error sensitive to W→e rapidity distribution

e rapidity spectrum shape sensitive to gluon shape parameter (valence quark density)

Probe low-x gluon PDF at Q²=MW2

PDF uncertainties only slightly degraded after detector simulation and selection cuts

Top Quark Physics• Direct discovery in 1995 (Fermilab)

• Completes the 3 family structure of the SM• Very high mass 175 GeV


had = QCD-1 >> decay

No Top Hadron : Opportunity to measure parameters of a free quark

• Production at LHC:• tt production

• single top production

+(90%) (10%)

t-channel

Wt-channel

W* (s-channel)

~ 250 pb

~70pb

~ 10 pb

~ 833100pb @ 14TeV (NLO)

8M evts/y @ Low Lum

t Y

X

t b

W+

l

Production cross-section

Resonance production

Production kinematics

Top Spin Polarization

Top Mass W helicity

|Vtb|

Branching Ratios

Rare/non SM Decays

Anomalous Couplings

CP violationTop Spin

Top Charge

Top Width

_

Golden Channel

tt Physics• Decay : Determined by decay of Ws


2 b-jets

pT>20 (25) GeV

pT>20 GeV

≥4 jets (R=0.4) pT>40 GeV t (had)

t (lep)

W (had)

• Selection :• Only e/µ events Trigger• large event yield• small backbround

• Fully hadronic channel (44 %)• Di-leptonic channel (5 %)• Semi-leptonic channel (30 % no )

tt PhysicsTop quark mass measurement (Invariant mass spectrum of

reconstructed Had. Top : most straightforward technique)


Selection efficiency : =5.3 %

Event topology: 3 jets with highest ∑ pT

Mjjj (GeV)

L=100 pb-1

(1 day @ 1033 cm-2s-1)

Signal (MC@NLO)

W+n jets (Alpgen) + combinatorial

Full simulationEven

ts

(ATL-PHYS-PUB-2005-024)

Without b-tagging (early data) With b-tagging

S/B=O(100)

Very small SM Bck Top signal

W+jets background

Top mass (GeV)N

um

ber

of

Even

ts

L=O(100) pb-1

(few days @ 1033 cm-2s-1)

In this way, m(top) ~1.3 GeV When Kinematical Fit (using the leptonic

side) is used, m(top) ~1 GeV

Selection efficiency : =1-2%

tt Physicstt cross section


CERN/LHCC 2006-021CMS TDR 8.2

reco = 5% (S/B=5.5)

• Di-leptonic channel (ttbb llll):L=10fb-1

tt/tt= 11%(syst)±0.9%(stat)±3%(lum)

With Tau leptons (ttbb ll, hadrons):tt/tt= 16%(syst)±1.3%(stat)±3%

(lum)reco=2%, S/B~1, -tag=30% ]

• Fully Hadronic channel:

tt/tt= 20%(syst)±3%(stat)±5%(lum)

L=1fb-1

reco=2%, S/B<1/9 (QCD)

• Semi-leptonic channel (ttbbqql):

tt/tt= 9.7%(syst)±0.4%(stat)±3%(lum)

L=10fb-1

reco= 6.3%

tt Physics


Top polarizationTest the tbW decay vertexMeasure W polarization (F0, FL, FR) through lepton angular distribution in W cm system:

Semilep. +

Dileptonic

• Syst ( Eb-jet,mtop,FSR )• F0 F0 ~ 2% ; FR ~ 0.01

0.000(mb=0)

0.297

0.703

SM

0.003 0.024

FL

0.003 0.012

FR

0.004 0.015

F0

Error (±stat ±syst) (Mt=175 GeV)

(Eur.Phys.J.C44S2 2005 13-33)

L=10fb-1

cos(l*)

(1/

)d/d

cos(

l*)

single Top PhysicsProduction cross section


Common feature:

1 lepton, pT>25GeV/cHigh Missing ET

2 jets (at least 1 b-jet)

(ATL-PHYS-PUB-2007-005)

t-channel/= 8%(syst)±2.7%(stat)±5%(lum)Wt-channel/= 23.9%(syst)±8.8%(stat)±9.9%(MC)s-channel/= 31%(syst)±18%(stat)±5%(lum)

L=10fb-1

L=30fb-1

(CERN/LHCC 2006-021, CMS TDR 8.2)

Separate Channels by (Nj,Nb) in final state:

t-channel:

Stat: 7000 events (S/B=3)

(Nj=2,Nb=1)

Wt-channel:

Stat: ~1% (S/B=15%)(Nj=3,Nb=1)

s-channel:

Stat: 1200 events for tb (S/B=10%)(Nj=2,Nb=2)

/= 12%(syst)±1%(stat)±5%(lum)

/= 14%(syst)±1.5%(stat)±5%(lum)

/= 16%(syst)±12%(stat)±5%(lum)

Conclusion• Precision measurements are possible with hadron collider, as

demonstrated by the Tevatron• LHC will prospect higher energy physics

Detector to be understood with early dataHigher luminosityBetter S/B ratio

• LHC goals are ambitious– MW < 15 MeV– mtop < 1 GeV– sin2W ~ 10-4

But reachable …

as soon as the detectors performances and the systematics will be understood


Back-up Slides


MW measurement


Simple and powerful in principle : consider pTl

spectrum correlation between Z and W decay

CMS NOTE 2006/061

stat. error negligible (~2 MeV)

BUT need to predict the spectrum precisely !

tt PhysicsKinematical Fit (use leptonic side)

– Minimization of a ² function with constraints on W and Top masses

Reduces FSR systematics Cleaner event sample (using a cut on ²) measure mtop as a function of ²


uncertainty m(top) [GeV]Hadronic Top

m(top) [GeV]Kinematical Fit

light-jet nergy scale (1%) 0.2 0.2

b-jet nergy scale (1%) 0.7 0.7

FSR 1 0.5

TOTAL 1.3 0.9

2 2 22

2

, , ,

2 2

h

meas fit meas fit meas fit meas fiti i i i i i i i

i i ii jets i jets lepton i x y zE i

fitPDG PDGjjb Topjj W l W

W W t

E E P P

M MM M M M

2 2

l

fitjjb Top

t

M M

tt PhysicsTop spin correlation


(tLtL) + (tRtR) - (tLtR) - (tRtL)

(tLtL) + (tRtR) + (tLtR) + (tRtL)A=

A(LO)A(NLO)

0.3190.326

Although t and t are produced unpolarized their spins are correlated

SM:

Testing the tt Production cross-section

l+,t

lqq

t

Other angular distributions:

AD(LO)

AD(NLO)

-0.217-0.237

SM: X=spin analysing power of X

1 dN 1N dcos2

( 1 – ADXX

´cos)

=

tt PhysicsTop spin correlation


A) Spin correlations and angular distributions:

l,tl,q

t

q

B) Spin Asymmetries can also be used (X-check)

(for L=10fb-1 precision A,AD below 10% )(hep-ex/0605190, subm. to Eur.Phys.J.C)

(Eur.Phys.J.C44S2 2005 13-33)• Semileptonic + Dileptonic• Syst (Eb-jet,mtop,FSR)• ~4% precision-0.29

0.42

SMMtt<550 GeV

0.008 0.010AD

0.014 0.023A

Error (±stat ±syst)

L=10fb-1

(CERN/LHCC 2006-021, CMS TDR 8.2)

A

N(cos) -

N(cos) N(cos) +

N(cos)

= - ADXx´

1

2Ãxx´

tt PhysicsProbe the Wtb vertex


B) Anomalous Couplings in the tbW decay

Angular Asymmetries: AFB, A+ and A-

AFBA+

A-

cos(cos(ll*)*)

AFB [t=0] A± [t= (22/3-1)]±±

SM(LO):SM(LO):

(PRD67 (2003) 014009, mb≠0)

tt PhysicsProbe the Wtb vertex


1 Results:

B) Anomalous Couplings in the tbW decay

SM(LO): L=0.423R=0.0005 (mb≠0)

Standard Model at LHC

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