Gauge-boson Physics
at the LHC
Hadron Collider Physics 2004
Michigan State U.
Matt Dobbs
Lawrence Berkeley Laboratory, USA
Physics Division
Outline

LHC Physics Environment


Precision Gauge Boson Physics

W-mass
AFB and sin2θW

Triple Gauge-boson Couplings


ATLAS and CMS Detectors
Di-bosons

Tri-bosons

Challenges ahead:


Monte Carlo Tools
Experimental measurements – PDFs, energy scales, etc.
Matt Dobbs
Hadron Collider Physics 2004
2


14 TeV proton-proton
collisions
broad-band q & g collider,

Large Hadron
Collider

Low L→ 2x1033/cm2/s


scales →few TeV
precision physics
High L →1034 /cm2/s
(~23 interactions/crossing)
Process
Z→e+e-
W→e±v
t anti-t
ET>100 GeV
Jets
Events/
10 fb-1
Tevatron
2 fb-1
~107-8
~108-9
~107
105-6
5x103
1010
106-7
300 fb-1 in ≤ 10 years
106-7
Matt Dobbs
Hadron Collider Physics 2004
3
The ATLAS Detector
Inner Detector
s
 0.05% PT ( GeV )  0.01
PT
Tracking in range |h| < 2.5
Silicon Pixels, Strips & TRT
EM Calorimetry
s
10%

E
E (GeV )
Fine granularity up to |h| < 2.5
Pb/LAr Accordian
Hadronic Calorimetry
s
50%
 0.03
E
E (GeV )
Barrel: Fe/Scintillating tiles
Endcaps: Cu & W / LArFine
Muon Spectrometer:
s/pT ~ 7 % at 1 TeV
Covers |h| < 2.7
Magnet
2T solenoid plus air core toroid

Matt Dobbs
Hadron Collider Physics 2004
4
The CMS Detector
Inner Detector:
s
 0.015% PT ( GeV )  0.005
PT
Silicon pixels and strips
Preshower:
Lead and silicon strips
EM Calorimeter:
Lead Tungstate
s
E

2  5%
 2%
E (GeV )
Hadron Calorimeters:
Barrel & Endcap:
Cu/Scintillating sheets
s
E

65%
 5%
E (GeV )
Forward:
Steel and Quartz fibre
Muon Spectrometer:
s/pT ~5% at 1 TeV(combined)
Drift tubes, cathode strip
chambers and resistive plate
chambers
One Magnet: 4T Solenoid
Matt Dobbs
Hadron Collider Physics 2004
5
W-Mass
Physics Division
Mass(W)

MW 
2
electroweak fit
EM
2GF
1


sin W 1  
radiative
correction
s











M top , ln M Higgs


M W 
2
0.007 M top

1.5 GeV at LHC
require  MW  15 MeV
(LEP2: 42 MeV,
Tev RunI: 59 MeV)
such that MW is not the
dominant error in EW fit.
constrains MHiggs &
consistency check
Matt Dobbs
Hadron Collider Physics 2004
7
Measuring Mass(W)

Measured with MTRAN of Leptonic Channels
M TW  2 pTl pT (1  cos  )

MTRAN very sensitive to
detector effects
vs.
PT(W)=0
Finite PT(W)

PTl± very sensitive to
higher order corrections
+ detector effects
Baur, hep-ph/0304266
Matt Dobbs
Hadron Collider Physics 2004
8
Measuring Mass(W)
Source
CDF Run Ib
ATLAS or CMS W→ l ν , one lepton species
30K evts, 84 pb-1
60M evts, 10fb-1
Statistics
65 MeV
< 2 MeV
Lepton scale
75 MeV
15 MeV most serious challenge
Energy resolution
25 MeV
5 MeV known to 1.5% from Z peak
Recoil model
33 MeV
5 MeV scales with Z statistics
W width
10 MeV
7 MeV ∆ГW≈30 MeV (Run II)
PDF
15 MeV
Radiative
decays
20 MeV
PT(W)
Background
TOTAL


45 MeV
10 MeV
<10 MeV (improved Theory calc)
5 MeV PT(Z) from data,
PT(W)/ PT(Z) from theory
5 MeV
5 MeV
113 MeV
≤ 25MeV
Per expt, per lepton species
Combining channels and CMS data, expect ΔMW ≈ 15 MeV
expect improvements from using MTRAN(W)/MTRAN(Z)
Matt Dobbs
Hadron Collider Physics 2004
9
W-mass: Challenges
0.03% knowledge of lepton energy
scale
calibrate with 6 million Zl+l- events






(CDF/D0 achieved 1% despite small Z
samples)


80
0.01
0.1
but MC model of ppZll is further
behind in some cases (no multi-photon
corrections)
Zll ≠ Wlν
Theory modeling of radiative decays
and recoil
1
Error on Lepton Scale/%
CTEQ6 ERROR PDFs
RMS = 9.8 MeV
new LHC-HERA workshop
mitigate some theory errors by using
W/Z ratio methods

80.5
active program for measuring PDFs at
LHC from Day 1


tracker material to 1%
overall alignment to 1 μm
B-field knowledge to 0.1%
muon E-loss to ¼%
Well constrained PDFs

81
20
15
10
5
MeV

Shift on W mass = 550 MeV / 1%
Mass GeV

0
600
-5
605
610
615
620
625
630
635
640
PDF
-10
-15
-20
ATLAS: C. Marques, Lisbon
Matt Dobbs
Hadron Collider Physics 2004
10
The State of the Art Today
QCD
EW
Style
RESBOS-A
resummed
PT(W)
Final State
NLO(αQED)
Distr. only
WGRAD2
none
complete
NLO(αQED)
Distr. only
MC@NLO
NLO(αS) + all none
orders parton
shower
Event
Generator
(some -1 evts)
& other codes too!..
• confused? a word from our sponsors…
Matt Dobbs
Hadron Collider Physics 2004
11
Matt Dobbs
Hadron Collider Physics 2004
12
Matt Dobbs
Hadron Collider Physics 2004
13
Weinberg Angle:
Physics Division
2
sin θW
Measuring sin2θW with AFB
A FB 

At the
s F s B
s F s B

 b a  sin  eff
2
pp Tevatron, defining
lept
(M Z )

known to NLO in EW, QCD
(effects can be as large as 30%)
AFB is easy.
eθFB
Z°/γ
AntiProton
Beam
Proton Beam
e+
AntiProton
Beam

Proton Beam
But for symmetric proton-proton beams (LHC), there
is no asymmetry WRT the beams.
Matt Dobbs
Hadron Collider Physics 2004
15
Measuring

2
sin θ
W
with AFB
Instead, we “sign” the forward direction by the l+l- boost.
Z°/γ
Proton Beam


Proton Beam
Measure asymmetry in charged lepton direction WRT CMS
boost direction
Asymmetry increases at high Y(l+l-)
Matt Dobbs
Hadron Collider Physics 2004
16
Measuring
2
sin θ
W
ATLAS-PHYS-2000-018
with AFB
CMS IN 2000/35
Cuts
AFB (%)
Δ AFB (%)
Δ sin2θeff(MZ)
Both e±, |η|<2.5
0.774
0.020
One e±, |η|<2.5
1.98
0.018
0.00066
0.00014
other e±,|η|<4.9
for comparison, Δ sin2θeff=0.00053 combining 4 LEP
expts and e,μ,τ channels [CERN-EP/2001-098]

Statistical precision using 100 fb-1

Performance issue:


increasing forward lepton tagging acceptance greatly improves measurement
Systematic PDF uncertainty is most challenging.
Matt Dobbs
Hadron Collider Physics 2004
17
Triple Gauge-boson
Couplings
Physics Division
Probing the
Triple Gauge-boson Couplings

non-abelian SU(2)L×U(1) Y
gauge group (foundation of SM!)
 WWγ

WWZ couplings
most-general C & P conserving WWZ,WWγ
vertices are specified by just 5 parameters:
g1Z , Δκ Z , Δκ  Z , 

 
 ZERO in the S.M.
grow likeŝ
big advantage for LHC
grow like ŝ
 model independent parameterization
Probe

tool: sensitive to low energy remnants of new physics
operating at a higher scale
complement to direct searches
Matt Dobbs
Hadron Collider Physics 2004
19
95% C.L. for Wγ


binned max. likelihood fit to
PT(V) distribution
sensitivity comes from a few
events in the high PT(V) tail
ATLAS
Matt Dobbs
Hadron Collider Physics 2004
20
TGC Limits vs. Integrated Luminosity
ATLAS
95% C.L., 30 fb-1, Syst. Incl.
-0.0035 < λγ < +0.0035
-0.0073 < λZ < +0.0073
-0.075 < Δκγ < +0.076
-0.11 < ΔκZ < +0.12
-0.0086 < Δg1Z < 0.011
systematic contribution
typically
order of magnitude better than LEP/TeVa [O(.10-.20), 95% C.L.]
•Statistics will dominate LHC measurements (except for Δ g1)
sensitivity derived from a few events in the high PT(V) tail
•Dominant systematics are theoretical:
neglected higher orders and pdf’s
Matt Dobbs
Hadron Collider Physics 2004
21
Limits vs. Form Factor Scale



ATLAS
new form factor
strategy is introduced
rather than imposing
an arbitrary form
factor in the model,
the limits are
reported as a
function of a mass
scale cutoff
Matt Dobbs
Hadron Collider Physics 2004
22
Neutral TGC’s

no tree level neutral
couplings in SM
g1Z , Δκ Z , Δκ  Z , 

 
grow like ŝ
h1,3f 4,5

grow like ŝ

grow like ŝ
Zγ
h 2,4

3
grow like ŝ
5
typically 3-5 orders of
magnitude improvement in
limits at LHC over LEP.
Matt Dobbs
Hadron Collider Physics 2004
23
The State of the Art Today
NLO(QCD) lepton anomalous style
corr.
TGC’s
Baur et. al.
yes
φ-slicing
almost yes
distributions
only
Dixon/Kunst yes
/Signer
subtract
yes
yes
distributions
only
MCFM
yes
subtract
yes
no
distributions
only
MC@NLO
yes +
parton
shower(!)
no(!)
no(!)
event
generator
(some -1 evts)
& other codes too!..
Matt Dobbs
Hadron Collider Physics 2004
24
Tri-boson Production
(MHIGGS=200 GeV)
no branching
pure leptons,
ratios, no cuts PT > 20 GeV , |η| < 3
pp WWW (difficult, 3 ν’s)
31925
180
ppWWZ
20915
32
ppZZW
6378
2.7
ppZZZ
4883
0.6
(difficult, 2 ν’s)
ppWγγ, preferred due to thresholds and BR’s.

van der Bij, Ghinculov
hep-ph/9909409
Events for 100 fb-1
“gold-plated” channels
would require full LHC
data set
sensitive to quartic gauge-boson couplings (QGC’s)
Matt Dobbs
Hadron Collider Physics 2004
25
Tri-boson Production

pp Wγγ




σ x BR(Wl,ν)
√s > MW production threshold
σ = 1.96 fb (μ±,e± after efficiency, detector effects)
WWγγ couplings
Eboli, Gonzalez-Garcia,
Lietti, Phys Lett D63, 2001
ATL-PHYS 2003-051
W2GRAD
(Baur, Stelzer)
Matt Dobbs
Hadron Collider Physics 2004
26
Conclusions




CMS & ATLAS are under construction.
LHC physics potential includes competitive
precision electroweak measurements: sin2θW,
mass(W)
Order of magnitude and better improvement in
anomalous TGC limits  precision arena for
diboson production
Challenges include:


Detector performance: lepton energy scale, forward
tagging
More precise measurement of PDFs


no good prediction of LHC precision exists.
Theory: next-generation codes need QCD + QED
Matt Dobbs
Hadron Collider Physics 2004
27