Electroweak Physics at the LHC Precision Measurements and New Physics
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Transcript of Electroweak Physics at the LHC Precision Measurements and New Physics
Electroweak Physics at the LHCElectroweak Physics at the LHCPrecision Measurements and New PhysicsPrecision Measurements and New Physics
PY898: Special Topics in LHC PhysicsPY898: Special Topics in LHC Physics
By Keith OtisBy Keith Otis
4/13/20094/13/2009
OutlineOutline
Electroweak ParametersElectroweak Parameters W-MassW-Mass Top-MassTop-Mass Electroweak Mixing AngleElectroweak Mixing Angle
Drell-YanDrell-Yan Forward-Backward Asymmerty in Z Decays (AForward-Backward Asymmerty in Z Decays (AFBFB)) Triple Gauge Boson CouplingsTriple Gauge Boson Couplings
Charged TGCsCharged TGCs Neutral TGCsNeutral TGCs Anomalous Quartic CouplingsAnomalous Quartic Couplings
Heavy LeptonsHeavy Leptons
Electroweak ParametersElectroweak Parameters The main parameters of EW theory are measured to very high The main parameters of EW theory are measured to very high
precision.precision. The mass of the W (MThe mass of the W (MWW) is known with an uncertainty of 0.03%) is known with an uncertainty of 0.03%
MMWW= 80.428 ± 0.039 GeV UA2/CDF/D0, 80.376 ± 0.033 GeV LEP 2= 80.428 ± 0.039 GeV UA2/CDF/D0, 80.376 ± 0.033 GeV LEP 2 SM predicts 80.375 ± 0.015 GeVSM predicts 80.375 ± 0.015 GeV
The uncertainty in the mass of the Top (mThe uncertainty in the mass of the Top (mtt) is 0.7%) is 0.7% mmtt= 170.9 ± 1.8 GeV= 170.9 ± 1.8 GeV SM predicts 171.1 ± 1.9 GeVSM predicts 171.1 ± 1.9 GeV
The uncertainty on the electroweak-mixing angle (The uncertainty on the electroweak-mixing angle (θθww) is 0.07%) is 0.07% cos(cos(θθww)= M)= Mww/M/Mzz
The LHC will be able to improve on these in a relatively short period The LHC will be able to improve on these in a relatively short period of time.of time.
Tevatron: 2 TeVTevatron: 2 TeV
LHC: 10-14TeVLHC: 10-14TeV
LHC @ 10LHC @ 103333 Luminosity Luminosity 150 Hz W150 Hz W 50 Hz z50 Hz z 1 Hz tT1 Hz tT
10 pb10 pb-1-1 of Luminocity of Luminocity 150k W→e150k W→eνν 15k Z→ee15k Z→ee 10k tT10k tT
W-MassW-Mass
W→W→llvv signature signature Isolated charged lepton pT > Isolated charged lepton pT >
25 GeV |25 GeV || < 2.4| < 2.4 Missing transverse energy Missing transverse energy
ETMiss > 25 GeVETMiss > 25 GeV No jets with pT > 30 GeVNo jets with pT > 30 GeV Recoil < 20GeVRecoil < 20GeV
For an integratedFor an integrated luminosity of 1 luminosity of 1 fbfb−1−1, 4 million events with , 4 million events with W→W→llvv (ℓ (ℓ = e or = e or μμ) decays are expected.) decays are expected.
)(
direct
indirect
Summer 2005 result
68% CL68% CL
W mass extractionW mass extraction
The W mass is extracted from the The W mass is extracted from the measured pmeasured pll
TT distribution or from the distribution or from the
Jacobian peak observed in the transverse Jacobian peak observed in the transverse mass of the lepton-neutrino system, Mmass of the lepton-neutrino system, MWW
TT..The W mass is obtained by comparing the The W mass is obtained by comparing the
measured distributions with template measured distributions with template distributions generated from data (Z distributions generated from data (Z events) or MC.events) or MC.
MC Template method
Source Source MMW W (MeV)(MeV) MMTTee(W)(W) PPTT(e)(e)
StatisticsStatistics 22 22
BackgroundBackground 55 55
Lepton E-p scaleLepton E-p scale 44(15)(15) 44(15)(15)
Lepton E/p resolutionLepton E/p resolution (5)(5) (5)(5)
Recoil modelRecoil model 55
Lepton identificationLepton identification 88 88
Total InstrumentalTotal Instrumental <20<20 <20<20
ppTTWW 55
Parton distribution Parton distribution functionsfunctions
33((10)10) 33((10)10)
W widthW width 11(7)(7) 11(7)(7)
Radiative decaysRadiative decays 1010 1010
Based on 10fb-1 of data corresponding to ~10M Wl
Fit MT(W) or pT (e) to Z0 tuned MC
Z-Samples play a crucial role in reducing systematics and theoretical uncertaintiesRequires further study
ATLAS
W-mass: ATLAS
Scaled observables
Use Z events as templatesUse Z events as templates Scaled observable using Scaled observable using
weighting fnweighting fn Morphing using kinematic Morphing using kinematic
transformationtransformation Limited by Z-statisticsLimited by Z-statistics
Detector and theoretical effects Detector and theoretical effects cancel at least partiallycancel at least partially
Pt(l) better than MT as it is not Pt(l) better than MT as it is not sensitive to Esensitive to ETT systematics systematics But needs PBut needs PTT(W) to be (W) to be
understoodunderstood
CMS
Scaled pScaled pTT(l ) (l )
WWee
““Morphed” mMorphed” mTT
WW
StatisticalStatistical 1515 1515
InstrumentalInstrumental <20<20 <30<30
PDFPDF <10<10 <10<10
WW <15<15 <10<10
PPTT(W)(W) 3030 ----
Systematic uncertainties (MeV) on W-mass for 10fb-1
PT(W) needs to reduced ET-systematics
W-mass: CMS
New Physics?New Physics?
tW MM 2107.0• For equal contribution to MH uncertainty:
• MW is a fundamental SM parameter linked to
the top, Higgs masses and sinW.
Mt < 2 GeV MW < 15
MeV
rGM
WF
EMW
1sin
1
2
Mar 06
LEP2+Tevatron MW=30MeVTevatron run2 (2fb-1) 30MeV combined
Can get MH/MH~30%Important cross-check with direct measurements
W-mass summaryW-mass summary
A number of methods have been studiedA number of methods have been studied Direct measurement of MDirect measurement of MTT p pTT(l)(l)
Z-events used to tune W MCZ-events used to tune W MC scaled observable pt(l), ‘morphing’scaled observable pt(l), ‘morphing’
Z-events used as a templateZ-events used as a template Systematics greatly improved using Z-samplesSystematics greatly improved using Z-samples All methods are giving All methods are giving MMww in range of 20MeV per channel in range of 20MeV per channel
per variable, so combined <15MeV per experiment seems per variable, so combined <15MeV per experiment seems to be achievable for 10fbto be achievable for 10fb-1-1
Need to understand correlationsNeed to understand correlations Main issues at EMain issues at ETT for M for MTT and P and PTT(W) for p(W) for ptt(l)(l) MMW W ~10MeV looks possible~10MeV looks possible Requires Requires MMtt<1GeV for EW fits<1GeV for EW fits
Top MassTop Mass
Top quark pairs, mainly produced via gluon fusion, yields a production cross-section of 833 pb, at next to leading order, 100 times higher than at Tevatron.
The "golden" channel is the semi-leptonic channel:tT→Wb+WB→ (lv)b+(jj)B
Top MassTop Mass Golden Channel event selection:Golden Channel event selection:
Isolated high pIsolated high pTT lepton, E lepton, EmissmissTT and at and at
least 4 jets, two of which are b-least 4 jets, two of which are b-tagged.tagged.
This gives a signal efficiency of ~5% This gives a signal efficiency of ~5% with a signal to background ratio of with a signal to background ratio of the order 10.the order 10.
Primary backgroundsPrimary backgrounds The main backgrounds are single top The main backgrounds are single top
events, mainly reduced by the4 jet events, mainly reduced by the4 jet cut, fully hadronic t T events, reduced cut, fully hadronic t T events, reduced by the lepton requirements, W+jet by the lepton requirements, W+jet and Z+jet events and Z+jet events
Finding the Top MassFinding the Top Mass
Reconstruction of the hadronic side of the decay is done Reconstruction of the hadronic side of the decay is done by minimization procedure.by minimization procedure. This minimization constrains the light jet pair mass to MThis minimization constrains the light jet pair mass to Mww, via , via
corrections to the light jet energies.corrections to the light jet energies. After trying all possible jet combinations the one minimizing the After trying all possible jet combinations the one minimizing the
χχ22 is kept. is kept. The b-jet closest to the hadronic W is associated to the chosen The b-jet closest to the hadronic W is associated to the chosen
pair.pair. The three jet invariant mass is then fitted with a Gaussian plus a The three jet invariant mass is then fitted with a Gaussian plus a
polynomialpolynomial The Result: MThe Result: Mtt= 175.0±0.2(stat.)±1.0(syst.) GeV, for an input = 175.0±0.2(stat.)±1.0(syst.) GeV, for an input
mass of 175 GeV and 1 fbmass of 175 GeV and 1 fb−1−1..
Drell-YanDrell-Yan
As discussed in As discussed in previous weeks this is previous weeks this is where a heavier where a heavier neutral gauge boson neutral gauge boson (Z’) would show up(Z’) would show up
AAFB FB of the leptons from of the leptons from
ZsZs
Drell YanDrell YanImportant benchmark process:• Measure cross-section• parton-parton luminosity functions• constrain PDFs• measure sin2W
• Deviations from SM
Need to define direction
At the Tevatron -- well defined
At LHC no asymmetry wrt beamAssume that there is a Q-qbar collision quark direction from y(ll)Requires measurement at high y(ll)
Determination of sinDetermination of sin22θθeffeffleptlept(M(MZZ
22 ))
AFB = b { a - sin2θefflept( MZ
2 ) } Measure Afb with leptons in Z0 DY events
Q q
q q
Can fit with Mt to constrain MHa, b calculated to NLO QED and QCD.
Determination of Determination of sinsin22θθeffeff
leptlept(M(MZZ22
))y cuts – ey cuts – e++ee--
((||y(y(ZZ))|| > 1) > 1)
ATLAS ∆AATLAS ∆AFBFB
(Stat)(Stat)
ATLAS ATLAS ∆sin∆sin22θθeffeffleptlept
(Stat)(Stat)
||y( y( ll1,21,2 ) )|| < 2.5 < 2.5 3.0 x 103.0 x 10-4-4 4.0 x 104.0 x 10-4-4
||y( y( ll11 ) )|| < 2.5 + < 2.5 +
||y( y( ll22 ) )|| < 4.9 < 4.92.3 x 102.3 x 10-4-4 1.4 x 101.4 x 10-4-4
Can be further improved by combining Z decay channels
[%]
Systematics: PDF, lepton acc. (~0.1%), radiative correction calculations
Current error on world average 1.6x10-4
sin2θeff =0.23153±0.00016
Associated Production of Gauge Associated Production of Gauge BosonsBosons
Triple gauge boson Triple gauge boson couplingscouplings
, ,, Z1 ZZg
s ~ grows
s ~ grows
SM gauge group SU(2)LxU(1)Y
WW and WWZ couplings(charged TGCs)
Couplings described by 5 independent parameters
s ~ grows
All are zero in SM
Any deviations is a signal of new physics
Anomalous couplings in Anomalous couplings in WWWW
Most sensitive measurement is looking for high pMost sensitive measurement is looking for high pTT Zs or Zs or ss
30fb-1
~3000 evts
ATLASCMS
Charged TGC predictionsCharged TGC predictions
95% CL 30fb95% CL 30fb-1-1 (inc syst) (inc syst)
-0.0035<-0.0035<<+0.0035<+0.0035
-0.0073<-0.0073<ZZ<+0.0073<+0.0073
-0.075<-0.075<<+0.076<+0.076
-0.11<-0.11<ZZ<+0.12<+0.12
-0..86<-0..86<gg11ZZ<+0.011<+0.011
Results expected to be ~x10 better than LEP/Tevatron
Results are statistics limited(except for g1
Z )
2,45,43,1 , hfh
2
3
s ~ grows
All are zero in SM
2
5
s ~ grows
Neutral TGCsNeutral TGCs
No tree level neutral couplings in SM
Leads to 3-5 order of magnitude improvement compared to LEP
95% CL 100fb95% CL 100fb-1-1 (inc syst) (inc syst)
-6.5x10-6.5x10-4 -4 <h<h3030ZZ<+6.4x10<+6.4x10-4-4
-1.8x10-1.8x10-6 -6 < h< h4040ZZ
< +1.7x10< +1.7x10-6-6
CMS
Quartic CouplingsQuartic Couplings
Anomalous Quartic Anomalous Quartic couplingscouplings
Look for W, low production threshold at Mw
S/B~1
ATLAS 30fb-1 e-~14 events(~x4 for l+/-)
Heavy LeptonsHeavy Leptons
““Evidence grows for charged heavy lepton Evidence grows for charged heavy lepton at 1.8-2.0 GeV”- Physics Today (1977)at 1.8-2.0 GeV”- Physics Today (1977)
Current limits: mCurrent limits: mL(±)L(±) >100.8GeV >100.8GeVNeutral Heavy Lepton Mass LimitsNeutral Heavy Lepton Mass Limits
Mass m> 45.0 GeV, 95% CL (Dirac)Mass m> 45.0 GeV, 95% CL (Dirac)Mass m> 39.5 GeV, 95% CL (Majorana)Mass m> 39.5 GeV, 95% CL (Majorana)
Heavy LeptonsHeavy Leptons Relic abundance of the leptons must not “over-close” the universe.Relic abundance of the leptons must not “over-close” the universe.
Can’t provide more than the critical energy density (10Can’t provide more than the critical energy density (10-5-5GeV cmGeV cm--
33)) A stable, charged lepton must have a low enough relic abundance A stable, charged lepton must have a low enough relic abundance
for it not to have been detected in searches for heavy isotopes in for it not to have been detected in searches for heavy isotopes in ordinary matterordinary matter
The mass and lifetime of the new leptons musts not be such that The mass and lifetime of the new leptons musts not be such that they would have been detected in a previous collider experiment.they would have been detected in a previous collider experiment.
There are no theoretical constraints found for lifetimes less that ~10There are no theoretical constraints found for lifetimes less that ~1066 s even for masses up to the TeV scale.s even for masses up to the TeV scale.
Only limits are the experimental onesOnly limits are the experimental ones
Heavy LeptonsHeavy Leptons
Where do we look for heavy Leptons?Where do we look for heavy Leptons?Drell-YanDrell-YanOther mechanismsOther mechanisms
pp→pp→γγ→γγ→LL++LL--
pp→Zpp→Zγ→γ→LL++LL--
Mechanisms for introducing new leptonsMechanisms for introducing new leptonsNew fermionic degrees of freedomNew fermionic degrees of freedom
Vector Singlet Model (VSM)Vector Singlet Model (VSM)Vector Doublet Model (VDM)Vector Doublet Model (VDM)Fermion-mirror-fermion Model (FMFM)Fermion-mirror-fermion Model (FMFM)
Heavy LeptonsHeavy Leptons
In these new models:In these new models:Exotic leptons mix with the standard leptons Exotic leptons mix with the standard leptons
through the standard weak vector bosons and through the standard weak vector bosons and according to the Lagrangiansaccording to the Lagrangians
LL±± Detection Detection
Time-of-FlightTime-of-FlightHeavy particlesHeavy particlesDetectable in both the central tracker and Detectable in both the central tracker and
muon chambersmuon chambersUse measured momentum and time delay to Use measured momentum and time delay to
reconstruct the massreconstruct the mass
LL±± Detection Detection
Imperfections in the time and momentum Imperfections in the time and momentum resolutions will cause a spread in the resolutions will cause a spread in the mass peakmass peak
Bunch crossing identificationBunch crossing identificationMuons from D-Y and heavy quark decaysMuons from D-Y and heavy quark decays
For a background signal to look like a heavy lepton neutral current For a background signal to look like a heavy lepton neutral current two opposite charge muons would have to be mis-identified at the two opposite charge muons would have to be mis-identified at the same time.same time.
Make pMake pTT cut at 50 GeV to eliminate heavy quark decays cut at 50 GeV to eliminate heavy quark decays
LL±± Detection Detection
Detection at the LHC is entirely cross Detection at the LHC is entirely cross section limited.section limited.
LL±± Detection Detection
Detection of up to 1TeV should be Detection of up to 1TeV should be possible a the LHCpossible a the LHC
Leptons vs. SleptonsLeptons vs. Sleptons
Study the angular distributionStudy the angular distribution
Leptons vs. SleptonsLeptons vs. Sleptons
Heavy Lepton SummaryHeavy Lepton Summary
Assuming standard model couplings and Assuming standard model couplings and long lifetime:long lifetime:We can detect heavy charged leptons in We can detect heavy charged leptons in
intermediate scale models up to 950 GeV with intermediate scale models up to 950 GeV with 100 fb100 fb-1-1
Above 580 GeV it’s hard to distinguish them Above 580 GeV it’s hard to distinguish them from scalar leptonsfrom scalar leptons
SummarySummary
The Electroweak sector, while one of the The Electroweak sector, while one of the better understood sectors of the SM, still better understood sectors of the SM, still holds important information and even holds important information and even some exciting new physics at the LHCsome exciting new physics at the LHC
Backup slides
2.5
Constraining gluon PDF with Ws
• Many W+Z measurements have pdf uncertainties• at LHC Q2~MZ
2 corresponds to sea-sea collisions depends on gluon from gqq• Need to improve understanding of gluon
ZEUS to MRST01 central value difference ~5%ZEUS to CTEQ6.1 central value difference ~3.5% (From LHAPDF eigenvectors)
W Rapidity Distributions for W Rapidity Distributions for different PDFsdifferent PDFs
CTEQ6.1M MRST02
GOAL: syst. exp. error ~3-5%
~ ±5.2% @y=0~ ±8.7% @y=0 ~ ±3.6% @ y=0
ZEUS-S
• At Detector level reflects generator level distributions ~8% PDF uncertainty at y=0 remains
CTEQ61
MRST01 ZEUS-S
CTEQ61
MRST01 ZEUS-S
e- rapidity e+ rapidity
Generator Level
ATLASDetector Levelwith sel. cuts
Error boxesare the Full PDF Uncertainties
Electron distributions
First measurements of W and ZFirst measurements of W and Z
W3.3%~ Z%3.2~
W and Z cross-sectionsFor ~1fb-1 data, systematics dominate
(CMS)tracker efficiency
Main theoretical contributionPT(W/Z) LO-NLO ~2%
ET
Initial luminosity uncertainty ~10%, reduced to 5%
ATLAS
-reconstruction efficiency from 20pb-1 Z
Barrel and endcapTo 0.5% in 0.2 bins
Minimum bias and Underlying Event
Tevatron
● CDF 1.8 TeV
PYTHIA6.214 - tuned dN/ddN/dηη ((ηη=0=0))
NNchch jet- jet-
pptt=20GeV=20GeV
1.8TeV (pp)1.8TeV (pp) 4.14.1 2.32.3
14TeV (pp)14TeV (pp) 7.07.0 7.07.0
increaseincrease ~x1.8~x1.8 ~x3~x3
~80%~200%
LHC prediction
Tevatron
PYTHIA6.214 - tuned
● CDF 1.8 TeV
MB onlyUE includes radiation and small impact parameter bias
LHC
First measurements at the LHC ?First measurements at the LHC ?Charged particle density at Charged particle density at = 0 = 0
(Only need central inner tracker and a few thousand pp events)
LHC?
• Min bias events are also crucial for intercalibration CMS require 18M events to intercalibrate ECAL in at 2%• ATLAS studies of use of MB events to study L1 trigger rates
Measuring the minimum bias events at ATLASMeasuring the minimum bias events at ATLAS
dNch/d
dNch/dpT
Black = Generated (Pythia6.2)Black = Generated (Pythia6.2)
Blue = TrkTrack: iPatRecBlue = TrkTrack: iPatRec
Red = TrkTrack: xKalmanRed = TrkTrack: xKalman
Only a fraction of tracks reconstructed,:Only a fraction of tracks reconstructed,:
limited rapidity coveragelimited rapidity coverage
Measure central plateauMeasure central plateau
can only reconstruct track pcan only reconstruct track pTT with with
good efficiency down to ~500MeV, but good efficiency down to ~500MeV, but most particles in min-bias events have most particles in min-bias events have ppTT < 500MeV < 500MeV
Hard extrapolation.Hard extrapolation. Reconstruct tracks Reconstruct tracks with:with:
1) pT>500MeV1) pT>500MeV 2) |d2) |d00| < 1mm| < 1mm 3) # B-layer hits >= 13) # B-layer hits >= 1 4) # precision hits >= 4) # precision hits >=
88
pT (MeV)
UE uncertaintiesUE uncertaintiesTra
nsvers
e <
Nch
g >
PYTHIA6.214 - tuned
PHOJET1.12
x 3
LHC
x1.5
Extrapolation of UE to LHC is unknownDepends on• Multiple interactions• Radiation• PDFs
CDF definition of UE
Ra
tio
<N
Tra
ckR
eco>
/<N
Tra
ckM
C>
Leading jet ET (GeV)
ReconstructinReconstructing the g the
underlying underlying eventevent
Njets > 1, |ηjet| < 2.5, ET
jet >10 GeV,
|ηtrack | < 2.5, pT
track > 1.0 GeV/c
ATLAS DC2 Simulated data
Analyse with first data(a la CDF)Need to ensure overlap between MB and jet triggerRequire ~20M MB events to get pt
jet~30GeV