Electroweak physics at the LHC with ATLAS APS April 2003 Meeting – Philadelphia, PA Arthur M....
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Transcript of 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
EW physics at ATLASA. M. Moraes
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
EW physics at ATLASA. M. Moraes
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
EW physics at ATLASA. M. Moraes
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
EW physics at ATLASA. M. Moraes
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
EW physics at ATLASA. M. Moraes
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
EW physics at ATLASA. M. Moraes
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 physics at ATLASA. M. Moraes
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
EW physics at ATLASA. M. Moraes
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
EW physics at ATLASA. M. Moraes
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
EW physics at ATLASA. M. Moraes
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!