Post on 28-Mar-2015
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