electroweak physics at the lhc with atlas aps april 2003 meeting – philadelphia, pa arthur m....

Electroweak physics at the LHC with ATLAS

APS April 2003 Meeting – Philadelphia, PA

Arthur M. Moraes

University of Sheffield, UK(on behalf of the ATLAS collaboration)

APS April 8, 2003

EW physics at ATLASA. M. Moraes

Outline LHC and ATLAS.

W mass measurement

Improvements in the measurements of the mass of the top quark

(mt).

AFB asymmetry in dilepton production: sin2θefflept(MZ

2).

EW single top quark production: direct measurement of Vtb.

Triple gauge boson couplings (TGC).

Conclusion.

APS April 8, 2003


LHC (Large Hadron Collider):

• p-p collisions at √s = 14 TeV

• bunch crossing every 25 ns (40 MHz)

• Huge discovery potential for new physics: Higgs and Supersymmetry.

• Large statistics → small statistical error!

Process σ (nb) Events/year (L = 10 fb-1)

W → eν 15 ~ 108

Z → e+ e― 1.5 ~ 107

tt 0.8 ~ 107

Inclusive jets

pT > 200 GeV100 ~ 109

low-luminosity: L ≈ 2 x 1033cm-2s-1 (L ≈ 20 fb-1/year)

high-luminosity: L ≈ 1034cm-2s-1

(L ≈ 100 fb-1/year)

W, Z, top, b, … factory!

-

APS April 8, 2003


ATLAS: A Toroidal LHC AparatuS

~44m

~22m

7,000 tons

A. M. Moraes

• Multi-purpose detectorcoverage up to |η|=5;design to operate at L= 1034cm-2s-1

• Inner Detector (tracker)Si pixel & strip detectors + TRT;2 T magnetic field;coverage up to |η|< 2.5.

• Calorimetryhighly granular LAr EM calorimeter(| η | < 3.2);hadron calorimeter – scintillator tile(| η | < 4.9).

• Muon Spectrometerair-core toroid system(| η | < 2.7).

Lepton energy scale: precision of 0.02% ( Z → ll)

Jet energy scale: precision of 1% ( W → jj; Z ( ll) + jets)

Absolute luminosity: precision ≤ 5% ( machine, optical theorem, rate of known processes)

APS April 8, 2003


W mass measurement• W mass is one of the fundamental parameters of the SM (αQED, GF, sinθW)

• Precise measurements will constrain the mass of the SM Higgs or the h boson of the MSSM;

• Equal weights in a χ2 test:

At the LHC ∆mt~ 2 GeV

• At the time of the LHC start-up the W mass will be known with a precision of about 30 MeV (LEP2 + Tevatron)

MH (GeV)M

W (

GeV

)

experimental lowerbound on MH

currentLHC

MW should be known with a precision of about 15 MeV (combining e/μ and CMS data).

RGM

WF

W

1sin

1

2

tW mM 2107.0

(achievable during the low-luminosity phase at ATLAS)

• constrains MH to ~25% .

MW = 80.446 ± 0.040 GeV (LEP2 – PDG)

APS April 8, 2003


W mass measurement

• Lepton energy and momentum scale: ~0.1% at Tevatron ~0.02% at LHC – ATLAS (tuned to , l=e, μ)

Sources of uncertainty:

• p.d.f.’s & radiative corrections: improvetheoretical calculations!

Detector resolution + pile-up will smear significantly the transverse mass distribution.(method limited to the low-luminosity phase!)

• Statistical uncertainty: < 2 MeV for L ≈ 10 fb-1

XWpp llW

σ = 30 nb (l=e,μ)

3x108 events (L ≈ 10 fb-1)

• Systematic error a) physics: W pt spectrum, structure functions, W width, radiative decays and background. b) detector performance: lepton scale, energy/momentum resolution and response to recoil.

llZ

APS April 8, 2003


Top mass

• tt production is expected to be the main background to new physics processes: production and decay of Higgs bosons and SUSY particles.

ttppNLO = 833 pb at LHC > 8x106 events at (L ≈ 10 fb-1)

• Together with MW, mt helps to constrain the SM Higgs mass.

-

• Precision measurements in the top sector are importantto get more clues on the origin of the fermion mass hierarchy.

• Top events will be used to calibrate the calorimeter jet scale (W→jj from t→bW).

tt leptonic decays (t→bW)

single lepton:

W→lν, W→jj

29.6%

(2.5 x 106 events)

di-lepton:

W→lν, W→lν

4.9%

(400,000 events)

L ≈ 10 fb-1

-

• Jet energy scale: ~3% at Tevatron~1% at LHC – ATLAS (including W→jj from t→bW)

∆mt at LHC will be dominated by systematic errors!

• Final state gluon radiation (~1%)

(~7 pb at Tevatron)

Best channel for mt measurement willbe tt→WWbb→(lν)(jj)bb

- - -

∆mt ~ 2 GeV

(mt = mjjb )

SM dominant decay

gg → tt (~ 90%)

qq → tt (~ 10%)

---

mt = 175.3 ± 4.4 GeV (global fit – PDG)

APS April 8, 2003


Determination of sin2θefflept(MZ

2 )

• Main systematic effect: uncertainty on the p.d.f.’s, lepton acceptance (~0.1%), radiative correction calculations.

• sin2θefflept is one of the fundamental

parameters of the SM!• precise determination will constrain the Higgs mass and check consistency of the SM.

• sin2θefflept

will be determined at the LHC by measuring AFB in dilepton production near the Z pole.

σ ( Z → l +l - ) ~ 1.5 nb (for either e or μ)

y cuts – e+e- ( | y(Z) | > 1 )

∆AFB ∆sin2θefflept

| y( l1,2 ) | < 2.5 3.03 x 10-4 4.0 x 10-4

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

2.29 x 10-4 1.41 x 10-4sin2θeff

lept( MZ2 ) = 0.23126 ± 1.7 x 10-4 (global fit PDG)

AFB = b { a - sin2θefflept( MZ

2 ) }

L = 100 fb-1

a and b calculated to NLO in QED and QCD.

( statistical ) ( statistical )

Can be further improved: combine channels/experiments.

[%]

APS April 8, 2003


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 - -

( not yet observed! )

• 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!

APS April 8, 2003


Triple gauge boson couplings • TGC of the type WWγ or WWZ provides a direct test of the non-Abelian structure of the SM (EW symmetry breaking).

• This sector of the SM is often described by 5 parameters: g1

Z, κγ, κZ, λγ and λγ, (SM values are equal to g1

Z = κγ = κZ = 1 and λγ = λγ = 0, at the tree level).

• Anomalous contribution to TGC is enhanced at high √s (increase of production cross-section).

W

W γ/Z

TGC vertex

• It may also indicate hints of new physics: new processes are expected to give anomalous contributions to the TGC.

• New physics could show up as deviations of these parameters from their SM values.

L = 30 fb-1 L = 30 fb-1

∆g1Z = 0.05

λ1 = 0.01

Gauge, C and P invariance

▓ SM ▓ SM

APS April 8, 2003


Parameters Statistical (at 95% C.L.)

Systematic (at 95% C.L.)

∆g1Z - 0.0064

+ 0.010±0.0058

∆κZ- 0.10+ 0.12

±0.024

λZ- 0.0065+ 0.0066

- 0.0032+ 0.0031

∆κγ- 0.073+ 0.076

- 0.015+ 0.0076

λγ±0.0033 ±0.0012

Systematic uncertainties:

• At the LHC, sensitivity to TGC is a combination of the very high energy and high luminosity.

• Uncertainties arising from low pT background will be quite small: anomalous TGC signature will be found at high pT.

• Theoretical uncertainties: p.d.f.’s & higher order corrections

L = 30 fb-1

• Variables:

Wγ: (mWγ , |ηγ*| ) and (pTγ, θ* )

WZ: (mWZ , |ηZ*| ) and (pTZ, θ* )

sensitive to high-energy behaviour: mWV, pT

V

sensitive to angular information: |ηV*|, θ*

• SM: vanishing helicity at low |η| Non-standard TGC: partially eliminates `zero radiation’

Using max-Likelihood fit to mWV |ηV*|

APS April 8, 2003


Conclusions: LHC will allow precision measurements: unexplored kinematic regions,

high-statistics (W, Z, b, t factory); ATLAS: valuable precision measurements of SM parameters; W mass can be measured with a precision of 15 MeV (combinig e/μ and

ATLAS + CMS);

Top mass: ~ 2 GeV (combined with ∆mW~15 MeV, constrains MH to ~ 25%);

sin2θefflept( MZ

2 ) can be determined with statistical precision of 1.4x10-4

(competitive to lepton collider measurements!)

EW single top production: direct measurement of Vtb; measurement of top

polarization (Wg with statistical precision of ~ 1.6%);

Sensitivity to anomalous TGC’s: indicative of new physics!

electroweak physics at the lhc with atlas aps april 2003 meeting – philadelphia, pa arthur m....

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