Electroweak and top mass studies for the LHC

Craig Buttar

Sheffield University

Cosner’s House meeting

SM model physics at LHC

• W-mass

• Top mass

• Single top production

• TGCs

• MB+UE

LHC numbers

Process σ (nb) Ns-1 Events/year

(L = 10 fb-1)

W → eν 15 15 ~ 108

Z → e+ e― 1.5 1.5 ~ 107

tt 0.8 0.8 ~ 107

Inclusive jets

pT > 200 GeV

100 100 ~ 109

Statistics vs systematicsthrow out 90% of events and still have enough for precision measurements !

Typical SM processes

1 low luminosity year=10fb-1

ATLAS: Design and Performance

Magnetic Field2T solenoid plus air core toroid

Inner Detector/pT ~ 0.05% pT(GeV) (+) 0.1% Tracking in range || < 2.5

EM Calorimetry /E ~ 10% / √E(GeV) (+) 1%Fine granularity up to || < 2.5

Hadronic Calorimetry/E ~ 50% / √E(GeV) (+) 3%

Muon Spectrometer/pT ~ 2-7 %Covers < 2.7

Precision physics in ||<2.5

The CMS DetectorInner Detector:Silicon pixels and strips

Preshower:Lead and silicon strips

EM Calorimeter:Lead Tungstate

Hadron Calorimeters:Barrel & Endcap: Cu/Scintillating sheets

Forward:Steel and Quartz fibre

Muon Spectrometer:Drift tubes, cathode strip chambers and resistive

platechambers

Magnet: 4T Solenoid

%2)(

%5

GeVEE

%5)(

%65

GeVEE

W-massCuts:

Isolated charged lepton pT > 25 GeV || < 2.4

Missing transverse energy ETMiss > 25 GeV

No jets with pT > 30 GeV

Recoil < 20GeV

Sources of Uncertainty:• Statistical uncertainty

pp W + X = 30 nb (l= e,)

W ll 3 x 108 events

< 2MeV for 10 fb-1• Systematic Error

Detector performance

Physics

Relies on good modelling of detector and physicsin Monte Carlo

For EW fits: tW mM 2107.0

W-massSource MW

(MeV)

Statistics 2

Lepton E-p scale 15

Lepton E/p resolution 5

Recoil model 5

Lepton identification 5

pTW 5

Parton distribution functions

10

W width 7

Radiative decays 10

Background 5

Total 25

1 year, 1 lepton species: 25 MeV

Combining lepton channels: 20 MeV

Combining experiments: 15 MeVcf Tevatron ~25-30MeV combined ~2fb-1

PtW < 20 GeV

CDF

mass shift ~10MeV due to HO QED and QCD Use Z ll control sample

Top Mass• Together with MW helps to

constrain the SM Higgs mass

• tt production: main background to new physics processes: production and decay of Higgs bosons and SUSY particles

• Top events used to calibrate the calorimeter jet scale

• Precision measurements in the top sector provide information of the fermion mass hierarchy

At low luminosity:

Semi-leptonic: best channel for top massmeasurement (pure hadronic channel can also

be used)

Error dominated by systematic errors:• Jet energy scale• Final state gluon radiation

tt leptonic decays (t bW)

Single lepton

W l, W jj

29.6 %

2.5 x 106 events

Di-lepton

W l, W l4.9 %

400,000 events

eventsxpbttppNLO6108833)( -

qqbblWbbWtt

Top masssemi-leptonic decay

Source mt

Statistics 0.1GeV

b fragmentation 0.1GeV

ISR 0.1GeV

FSR 1.0GeV

Background 0.1GeV

Light q jet energy calibration 0.2GeV

b quark jet energy calibration 0.7GeV

Total ~1.3GeV

ATLAS estimates of systematicsUse Z/g-j, W(tt)jj, Z-b control samples

60k events/10fb-1

Reducing effect of FSR

PDGW

fittop

=M

M

jj l

jjb l b

m m

m m

Source mt

Statistics 0.1GeV

b fragmentation 0.1GeV

ISR 0.1GeV

FSR 0.5GeV

Background 0.1GeV

Light q jet energy calibration 0.2GeV

b quark jet energy calibration 0.7GeV

Total ~0.9GeV

Use kinematic fit

High-pt top

Mt

ISR 0.7

FSR 0.1

B-fragmentation 0.3

UE estimate 1.3

Mass scale calibration

0.9

1.8GeV

Reconstruct top decay in largecone directly from calorimetercells

Sensitive to the underlying event

Requires rescaling of the mass sample of ~4k events/10fb-1

Top mass via J/CMS A.Kharchilava Phys. Lett. B476 (2000) 73

Reconstruct M(J/l) Relies on simulationto determine Mt M(J/+l)

Top summary

Mt GeV Dominant error

Semileptonic (kinematic fit) 0.9-1.3 FSR+b-energy scale

Semileptonic high pt 1.8 Mass-scale+UE

Di-leptons 1.7 b-quark fragmentation

Multijet 3.0 FSR

J/ 1.4 Statistics 500fb-1+b-quark fragmentation

Range of top-quark mas measurements with different systematic errorsMt ~2GeV seems feasibleMt~3GeV with 2fb-1 at Run-II

EW single top quark production

Wq

q’

bt

Wq

q’

t

b

b

g

g

W

t

b bb

q

q’

tW

-

- -

W-gluon fusion

Wt processW* process

σWg~ 250 pb

σWt~ 60 pbσW*~ 10 pb

• Probe the t-W-b vertex

• Background: tt, Wbb, Wjj

- -

• Directly measurement (only) of •the CKM matrix element Vtb at ATLAS •(assumes CKM unitarity)

( lower theoretical uncertainties! )

for each process: σ ∝ |Vtb|2

Process S/B S/√B ∆Vtb/ Vtb – statistical

∆Vtb/ Vtb – theory

W-gluon 4.9 239 0.51% 7.5%

Wt 0.24 25 2.2% 9.5%

W* 0.55 22 2.8% 3.8%

L = 30 fb-1 Systematic errors: b-jet tagging, luminosity (∆L ~ 5 – 10%), theoretical (dominate Vtb measurements!).

• New physics: heavy vector boson W’

• Source of high polarized tops!

The self -couplings between the electroweak gauge bosons are specified by the SU(2)L × U(1)Y gauge symmetry of the Standard Model

Measurements of the gauge couplings therefore:

• Provide a test of this non-Abelian structure

the SM TGCs WWg and WWZ have been beautifully confirmed at LEP.

But also, probe for possible new physics Anomalous triple (or quartic) gauge couplings

• The most general Lorentz invariant parametrisation of WWV with V=Z,g is governed by 14 couplings, 7 for each vertex.

EM gauge invariance, C, P and CP conservation: g1Z, Z, Z and , ,

* In the SM, g1Z = = Z =1 and = Z = 0

usually quote the deviations from the SM: g1Z, Z,Z and , (=0 in SM)

* At LHC, greater sensitivity due to higher luminosity and higher centre-of-mass energy

Gauge Couplings: Phenomenology

TGCs: Measurement • Any ATGC contribution to some process gives a quadratic increase in the

cross-section with the anomalous parameter can set limits on ATGC parameters by comparing observed and expected

event rates

• Method is sensitive to overall normalisation hence systematic errors in, e.g. luminosity, and gives no information about where any AQGC contribution originates

Better to use a fit to the spectrum of some observable using a MC prediction

Example 1: Measure possible anomalous contribution to WW in W production

q

q q

q q

q

W W

W

W

• Consider pp→W with W→l, l = e,

Maximum likelihood method applied to the pT spectrum of offers good sensitivity to possible anomalous couplings and

TGCs Example 1: WW

Selection:

PT > 100GeVPTl > 25 GeVPT

miss > 25 GeVRl > 1

Expect ~3000 events in 30fb-1

(as plotted)

• sensitivity is in high pT tail (where backgrounds are small)

W result

Predicted 95% CL for Tevatron Predicted CL for LHC 30fb-1

• Can also measure ATGC contribution to WWZ through pp→WZ Maximum likelihood method applied to pT spectrum of the Z offers good

sensitivity to couplings g1Z,Z and Z

TGCs Example 2: WWZ

Selection:

3 leptons with pT > 25GeV

(One pair should be of same flavour, opposite sign and satisfy |mll-mZ| < 10 GeV)

PTmiss > 25GeV

Expect ~1200 events in 30fb-1

ATLAS: Precision Reach and Couplings

Triple Gauge Couplings: Precision

• Table shows expected 95% CLs on individual couplings in 30fb-1 (three years of low luminosity running)

• Both systematics and statistics limited since the sensitivity in the tails of the distributions.

• ~Order of magnitude improvement over LEP limits.

LEP2 TevatronLHC

κγ 0.05 0.1 0.08

λγ0.025 0.1 3•10-3

g1z 0.025 0.01

Z - 0.1

Z - .007

Underlying event

• Underlying event associated with ‘hard’-scatters

• important for energy corrections, central jet veto

High PT scatter

Beam remnants

ISR

Core x2 default

Default Pt-min=1.9

Increasing core size

Default

Transverse <Nch> vs jet pT

Tuning Pythia to CDF run-I analysis

Minimum bias Underlying event

MSUB(94)=1 (D=0)

MSUB(95)=1 (D=1)

MSUB(95)=1 (D=1)

MSTP(51)=7 (D=7)

MSTP(51)=7 (D=7)

MSTP(81) = 1 (D=1)

MSTP(81) = 1 (D=1)

MSTP(82) = 4 (D=1)

MSTP(82) = 4 (D=1)

PARP(82) =1.8 (D=1.9)

PARP(82) =1.8 (D=1.9)

PARP(84) = 0.5 (D=0.2)

PARP(84) = 0.5 (D=0.2)

PARP(90)=0.16

(D=0.16)

PARP(90)=0.16

(D=0.16)

π0, K0s and Λ0

stable

(D=decay’s on!)

MC distributions

corrected.

Non-diff. + d.diff.

CTEQ 5L

Double Gaussia

n

Exclude 8% of chd. tracks

Primary vertex

“D” = PYTHIA’s default

Required to compare to data

PYTHIA Tuning (AM Tune)

Multiple interactions

PT0

Core size

PT0 energy dependence

LHC predictions

Tevatron

● CDF 1.8 TeV

PYTHIA6.214 - tuned

dN/dη (η=0)

Nch jet-pt=20GeV

1.8TeV (pp) 4.1 2.3

14TeV (pp) 7.0 7.0

increase ~x1.8 ~x3

~80%~200%

LHC prediction

TevatronPYTHIA6.214 - tuned

● CDF 1.8 TeV

( )( ) 4.4

1.72.6( )

( )

particle

particle

UE LHCMB LHC

UE CDFMB CDF

LHC

Central-jet veto:

Cut non-tag jets

in |η|<3.2

PT>20GeVModel CJV efficiency Significance

Default pythia 82% 8.1

Default DG 71% 7.5

AM tuning 76% 7.6

Paper 86% 8.2

Pythia 6.214ATLFAST 602

e- channel onlyMH=160GeV

Tagging jet

Tagging jet

H

W

WZ/WZ/W

Effect of underlying event on central jet vetoin vector boson fusion

Summary and conclusions

• Top and W-MassMW Tevatron~30MeV LHC~20GeVMt Tevaton~3GeV LHC~2GeVLarge statistics at LHC allow for better control of systematics through control samplesTevatron will be essential for developing MCs with higher order corrections for precision measurements

• Single top should be observed• TGC limits should improve• Tuning of MCs to underlying event data from the Tevatron ensures

development of robust analysis and reconstruction code

Top Mass Measurements

Predicted error on the top mass measurement from the semi-leptonic channel of 1.3 GeV

(Di-leptonic channel: 2 GeV)

Pt-min is ~1.9GeV default value

ATLAS: Precision Reach and Couplings

Neutral Triple Gauge CouplingsNot Present in the Standard Model

• Possible anomalous Z/ZZ contribution to pp→Z

Couplings specified by 4 parameters:

fiV with i=4,5 and V=Z,

• Possible anomalous ZZ/ZZZ contribution to pp→ZZ:

Couplings specified by 8 parameters:

hiV with i=1…4 and V=Z,

Example: Fit to the pT spectrum of the

=> Again, the sensitivity is in the tail of the distribution