Electroweak physics at the
LHC with ATLAS
Arthur M. Moraes
University of Sheffield, UK
(on behalf of the ATLAS collaboration)
APS April 2003 Meeting – Philadelphia, PA
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(MZ2).

EW single top quark production: direct measurement of Vtb.

Triple gauge boson couplings (TGC).

Conclusion.
A. M. Moraes
EW physics at ATLAS
APS April 8, 2003
LHC (Large Hadron Collider):
• p-p collisions at √s = 14 TeV
• bunch crossing every 25 ns
(40 MHz)
 low-luminosity: L ≈ 2 x 1033cm-2s-1
(L ≈ 20 fb-1/year)
 high-luminosity: L ≈ 1034cm-2s-1
(L ≈ 100 fb-1/year)
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 GeV
100
~ 109
A. M. Moraes
W, Z, top, b, … factory!
• Large statistics → small statistical error!
• Huge discovery potential for new physics:
Higgs and Supersymmetry.
EW physics at ATLAS
APS April 8, 2003
ATLAS: A Toroidal LHC AparatuS
7,000 tons
• Multi-purpose detector
coverage 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.
~22m
• Calorimetry
highly granular LAr EM calorimeter
(| η | < 3.2);
hadron calorimeter – scintillator tile
(| η | < 4.9).
~44m
• Muon Spectrometer
Lepton energy scale: precision of 0.02% ( Z → ll)
air-core toroid system
(| η | < 2.7).
Jet energy scale: precision of 1% ( W → jj; Z ( ll) + jets)
Absolute luminosity: precision ≤ 5% ( machine, optical
theorem, rate of known processes)
A. M. Moraes
EW physics at ATLAS
APS April 8, 2003
W mass measurement
• W mass is one of the fundamental parameters
of the SM (αQED, GF, sinθW)

GF
1
2 sin W 1  R 
• Precise measurements will constrain the mass
of the SM Higgs or the h boson of the MSSM;
• At the time of the LHC start-up the W mass will
be known with a precision of about 30 MeV
(LEP2 + Tevatron)
MW (GeV)
MW 
MW = 80.446 ± 0.040 GeV (LEP2 – PDG)
experimental lower
bound on MH
• Equal weights in a χ2 test:
M W  0.7  10 2 mt
At the LHC ∆mt~ 2 GeV
current
LHC
MH (GeV)
MW should be known with a precision of
about 15 MeV (combining e/μ and CMS data).
(achievable during the low-luminosity phase at ATLAS)
• constrains MH to ~25% .
A. M. Moraes
EW physics at ATLAS
APS April 8, 2003
W mass measurement
Sources of uncertainty:
• Statistical uncertainty: < 2 MeV for L ≈ 10 fb-1
pp  W  X
W  l l
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.
• Lepton energy and momentum scale:
~0.1% at Tevatron
~0.02% at LHC – ATLAS (tuned to Z  l  l  , l=e, μ)
 Detector resolution + pile-up will
smear significantly the transverse mass
distribution.
(method limited to the low-luminosity phase!)
• p.d.f.’s & radiative corrections: improve
theoretical calculations!
A. M. Moraes
EW physics at ATLAS
APS April 8, 2003
Top mass
SM dominant decay
• Together with MW, mt helps to constrain the SM Higgs mass.
-
mt = 175.3 ± 4.4 GeV (global fit – PDG)
• tt production is expected to be the main background to new
physics processes: production and decay of Higgs bosons and
SUSY particles.
• Precision measurements in the top sector are important
to get more clues on the origin of the fermion mass hierarchy.
-
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)
• Top events will be used to calibrate the calorimeter jet scale
(W→jj from t→bW).
 NLO pp  t t = 833 pb at LHC
> 8x106 events at (L ≈ 10 fb-1)

-

gg → tt (~ 90%)
qq → tt (~ 10%)
(~7 pb at Tevatron)
Best channel for mt measurement will
be tt→WWbb→(lν)(jj)bb (mt = mjjb )
∆mt ~ 2 GeV
L ≈ 10 fb-1
∆mt at LHC will be dominated by systematic errors!
• Jet energy scale:
~3% at Tevatron
~1% at LHC – ATLAS (including W→jj from t→bW)
• Final state gluon radiation (~1%)
A. M. Moraes
EW physics at ATLAS
APS April 8, 2003
Determination of sin2θefflept(MZ2 )
[%]
• 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.
AFB = b { a - sin2θefflept( MZ2 ) }
a and b calculated to NLO in
QED and QCD.
σ ( Z → l +l - ) ~ 1.5 nb (for either e or μ)
y cuts – e+e-
∆AFB
∆sin2θefflept
( | y(Z) | > 1 )
( statistical )
( statistical )
| 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 10-4
A. M. Moraes
L = 100 fb-1
• Main systematic effect: uncertainty on the
p.d.f.’s, lepton acceptance (~0.1%), radiative
correction calculations.
sin2θefflept( MZ2 ) = 0.23126 ± 1.7 x 10-4 (global fit PDG)
Can be further improved:
combine channels/experiments.
EW physics at ATLAS
APS April 8, 2003
EW single top quark production
( not yet observed! )
q’
q
q’
q
W
b
t
b
b
W
g
t
-
g
W-gluon fusion
σWg~ 250 pb
b
q
W
b
t
W
-
q’
t
b
W* process
σW*~ 10 pb
Wt process
σWt~ 60 pb
( lower theoretical
uncertainties! )
for each process: σ ∝ |Vtb|2
• Probe the t-W-b vertex
Process
S/B
S/√B
∆Vtb/ Vtb –
statistical
∆Vtb/ Vtb –
theory
• Directly measurement (only) of the CKM matrix
element Vtb at ATLAS (assumes CKM unitarity)
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%
• New physics: heavy vector boson W’
• Source of high polarized tops!
• Background: tt, Wbb, Wjj
A. M. Moraes
Systematic errors: b-jet tagging, luminosity
(∆L ~ 5 – 10%), theoretical (dominate Vtb
measurements!).
EW physics at ATLAS
L = 30 fb-1
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).
W
• 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.
W
L = 30 fb-1
γ/Z
L = 30 fb-1
▓ SM
• This sector of the SM is often
described by 5 parameters: g1Z, κγ, κZ,
λγ and λγ, (SM values are equal to g1Z =
κγ = κZ = 1 and λγ = λγ = 0, at the tree
level).
TGC
vertex
λ1 = 0.01
▓ SM
∆g1Z = 0.05
Gauge, C and P invariance
• Anomalous contribution to TGC is enhanced at high
√s (increase of production cross-section).
A. M. Moraes
EW physics at ATLAS
APS April 8, 2003
• Variables:
Wγ: (mWγ , |ηγ*| ) and
(pTγ,
WZ: (mWZ , |ηZ*| ) and
(pTZ,
θ* )
θ* )
sensitive to high-energy
behaviour: mWV, pTV
sensitive to angular
information: |ηV*|, θ*
• SM: vanishing helicity at low |η|
Non-standard TGC: partially eliminates `zero radiation’
Parameters
Statistical
Systematic
(at 95% C.L.)
(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.
L = 30 fb-1
Using max-Likelihood fit to
mWV |ηV*|
• Theoretical uncertainties: p.d.f.’s & higher order corrections
A. M. Moraes
EW physics at ATLAS
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( MZ2 ) 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!
A. M. Moraes
EW physics at ATLAS
APS April 8, 2003