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Electroweak physics at LHC. Marina Cobal, University of Udine ‘IFAE’, Torino Apr 14 - Apr 16, 2004. Outline. Introduction W mass measurement Top mass measurement EW single top quark production: measurement of V tb . A FB asymmetry in dilepton production: sin 2 θ eff lept (M Z 2 ) . - PowerPoint PPT Presentation
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1
Electroweak physics at LHC
Marina Cobal, University of Udine‘IFAE’, Torino Apr 14 - Apr 16, 2004
P 2
Outline
Introduction
W mass measurement
Top mass measurement
EW single top quark production: measurement of Vtb.
AFB asymmetry in dilepton production: sin2θefflept(MZ
2).
Conclusion.
P 3
TOTEM
ALICE : heavy ions,p-ions
ATLAS and CMS :pp, general purpose
LHCb : pp, B-physics
27 km ring 1232 dipoles B=8.3 T
LHC and experiments
pp (mainly) at s = 14 TeV Startup in April 2007
Initial/low lumi L~1033 cm-2 s-1
2 minimum bias/x-ing “Tevatron-like” environment
10 fb-1 /year Design/high lumi L=1034 cm-2 s-1
after ~ 3 years ~ 20 minimum bias/x-ing
fast ( 50 ns) radhard detect 100 fb-1 /year
P 4 ATLAS cavern
Huge activities in construction of both CMS and ATLAS
CMS yoke
P 5
What we know
No observable directly related to mNo observable directly related to mHH. Dependence through radiative corrections. Dependence through radiative corrections.
Example: mExample: mWW=m=mWW((mmtt22,,log(mlog(mHH))))
SM(17 parameters)
mfermions (9)
mbosons (2)
VCKM (4)GF (1)
mH
predictions
from n, decays
not discovered yet
from directmeasurements
from meson decays
(down to 0.1% level)
t t m mtt22 H H ln(m ln(mHH/m/mWW))
By making precision measurements (already interesting per se):By making precision measurements (already interesting per se):• • one can get information on the missing parameter mone can get information on the missing parameter mHH• • one can test the validity of the Standard Modelone can test the validity of the Standard Model
P 6
Where we are
At the Tevatron run II: At the Tevatron run II: mmt t 2.5 GeV ; 2.5 GeV ; mmW W 30 MeV 30 MeV mmHH/m/mH H 35% 35%
What can we expect/should be our goal at LHC?What can we expect/should be our goal at LHC?
Uncertainties on mUncertainties on mtt, m, mWW are the dominating ones in the ew fit are the dominating ones in the ew fit
Each “observable” Each “observable” (A(Aqq, A, All, ,
RRqq, R, Rll, , , m, sin, m, sinWW,…) ,…) can can
be calculated as a function be calculated as a function of: of: hadhad, , ss(m(mZZ), m), mZZ, m, mtt,,mmHH
P 7
Predictions
LHC
TeVatron1034
1033
<1032
Luminosity
[cm-2s-1]
100
10
0.3
∫L
[fb-1/y]
14 LHC (high lum)
14 LHC (low lum)
2 TeVatron
√s
[TeV]
processprocess (pb)(pb) Events/sEvents/s Events/yEvents/y
bbbb 55101088 101066 10101313
ZZeeee 1.51.5101033 ~3~3 101077
WWℓℓℓ=e,μℓ=e,μ 33101044 ~60~60 101088
WWWWeeXX 66 1010-2-2 101055
tttt 830830 ~1.7~1.7 101077
HH(700 GeV/c(700 GeV/c22)) 11 221010-3-3 101044
--
--
P 8
• mtop and MW: equal weight in the EW fit if MW 0.007 mtop
at LHC: mtop 2 GeV gives the precision
• Methods: same as at the Tevatron, with Wl decays
but LHC statistics is higher
Wl 60 M reconstructed @ low L (x10 RunII)
PTl spectrum
R=MTW / MT
Z spectrum
MTW
MW measurement at the LHC
year 2004: MW 30 MeV , year 2007: MW < 30 MeV (Lep2+Tevatron)
MW : 15 MeV
Pile-up , theo. know. of PTW
Small syst, sample /10
Best choice for low lum phase
upp
T
l
T
(missing p(missing pTT))
e
Wbeam line
u
P 9
W mass : fit exp. shape to MC sample with different Values of MW
stat. error negligible
syst. error: MC modelling of the physics
and the detector responses
Mw measurement: Transverse Mass
,)cos1(2 , elppM l
P
TlT
TW
missT
Use Transverse mass to
cope with unmeasured PL
PTW
spectrumPDFW widthW rad. decaysBackground
ZllLHC dataZll,R,CDF/D0Theory, MC
Lep. E/p Lep. res.Recoil
Zll, E/p for e
Zll, E/p for e
Zll
physics detector
P 10
Advantages of LHC
Profit from Tevatron experience:
Many potential hidden sources of systematics from the detector description in the simulations (+pile-up, + radiation)
use data samples for in-situ calibrations !
Advantages of the LHC:
Huge statistics of signal & control samples like Z leptonic decays:
Larger production cross-section for the control process Larger production cross-section for the control process ZZ(e/(e/)(e/)(e/) (6 millions/y/exp)) (6 millions/y/exp)
granularity + acceptance + resolution are bettergranularity + acceptance + resolution are better
Error from control samples at the LHC < than at the TevatronError from control samples at the LHC < than at the Tevatron
P 11
Detector systematics
0.02% on lepton E/p scale precision needed0.02% on lepton E/p scale precision needed
ZZee, Zee, Z mass reconstruction mass reconstruction leptonic decays of leptonic decays of , J/, J/
~15 MeV/y-e-l can be reached~15 MeV/y-e-l can be reached
… … and ~1-2% uncertainty in E+p resolution and ~1-2% uncertainty in E+p resolution
Z width reconstructionZ width reconstruction beam tests data beam tests data
~5 MeV/y-e-l can be reached~5 MeV/y-e-l can be reached
Recoil modelling (UE + detector)Recoil modelling (UE + detector)
determine response/resolution of the recoil (+UE) from Zdetermine response/resolution of the recoil (+UE) from Z data data ~5 MeV/y-e-l can be reached~5 MeV/y-e-l can be reached
low extrapolation error Zlow extrapolation error ZWW
P 12
Physics systematics
ppTT(W)(W) use puse pTT(Z) from Z(Z) from Z, use p, use pTT(W)/p(W)/pTT(Z) to model MC.(Z) to model MC.
~5 MeV/y-e-l can be reached~5 MeV/y-e-l can be reached
Uncertainties on the parton density functionsUncertainties on the parton density functions Compare different models, constrain pdfs through Z+W dataCompare different models, constrain pdfs through Z+W data
~10 MeV/y-e-l can be reached~10 MeV/y-e-l can be reached
W width W width use measurement of use measurement of
<10 MeV/y-e-l can be reached<10 MeV/y-e-l can be reached
Radiative decaysRadiative decays constrained through Wconstrained through W? ?
<10 MeV/y-e-l, theory work needed<10 MeV/y-e-l, theory work needed
BackgroundBackground checks on Z and Wchecks on Z and W (composition similar to Tevatron)(composition similar to Tevatron)
<5 MeV/y-e-l should be reached<5 MeV/y-e-l should be reached
)(
)(
ZBR
WBRR
Z
W
Implicit SM assumption
to determine W From LEP
From theory
P 13
Mw measurement: errors
MW , Wl one lepton species low lum , per exp., per yearsource CDF RunIb LHC
30K evts,84pb-1 60M evts,10fb-1
Statistics
Lepton scale
Energy resolution
Recoil model
Lepton id.
PTW
W width
Radiative decays
Background
65 MeV
75 MeV
25 MeV
37 MeV
----------
15 MeV
15 MeV
----------
20 MeV
5 MeV
<2 MeV
15 MeV
5 MeV
5 MeV
5 MeV
5 MeV
10 MeV 7 MeV
<10 MeV
5 MeV
TOTAL 92 MeV < 25 MeV
Combining channels and exp., should reach MW 15 MeV
Much better!
Internal calibration from Internal calibration from Z data mainly. Z data mainly. Need excellent control of Need excellent control of energy flow+ p scaleenergy flow+ p scale
P 14
Mtop measurement: production
Cross section determined to NLO precision Total NLO(tt) = 834 ± 100 pb Largest uncertainty from scale variation
Compare to other production processes:
Top production cross section approximately 100x Tevatron
Opposite @ FNAL
32121 10~ ; ˆ xxxsxs
~90% gg~10% qqProcess N/s N/year
Total collected before start LHC
W e 15 108 104 LEP / 107 FNAL
Z ee 1.5 107 107 LEP
tt 1 107 104 Tevatron
bb 106 1012-13 109 Belle/BaBar ?
H (130) 0.02 105 ?
LHC is a top factory!
Low lumi
P 15
MTop from lepton+jet
Br(ttbbjjl)=30%for electron + muon
Golden channel Clean trigger from isolated lepton
The reconstruction starts with the W mass: different ways to pair the right jets
to form the W jet energies calibrated using mW
Important to tag the b-jets: enormously reduces background
(physics and combinatorial) clean up the reconstruction
Lepton side
Hadron side
Typical selection efficiency: ~5-10%:
•Isolated lepton PT>20 GeV
•ETmiss>20 GeV
•4 jets with ET>40 GeV
•>1 b-jet (b40%, uds10-3,
c10-2)
Background: <2%
W/Z+jets, WW/ZZ/WZ
P 16
Lepton + jet: reconstruct top
Hadronic side W from jet pair with closest invariant mass to MW
Require |MW-Mjj|<20 GeV
Assign a b-jet to the W to reconstruct Mtop
Kinematic fit Using remaining l+b-jet, the leptonic part is
reconstructed |mlb -<mjjb>| < 35 GeV Kinematic fit to the tt hypothesis,
using MW constraints
j1
j2
b-jet
tW-mass Selection efficiency 5-10%
Borjanovic et al., SN-ATLAS-2004-040
P 17
Mtop systematics
Method works: Linear with input Mtop
Largely independent on Top PT
Biggest uncertainties: Jet energy calibration FSR: ‘out of cone’ give
large variations in mass B-fragmentation
Verified with detailed detector simulation and realistic calibration
Source of uncertainty
Hadronic Mtop (GeV)
Fitted Mtop (GeV)
Light jet scale 0.9 0.2
b-jet scale 0.7 0.7
b-quark fragm 0.1 0.1
ISR 0.1 0.1
FSR 1.9 0.5
Comb bkg 0.4 0.1
Total 2.3 0.9
Challenge:
determine the mass of the top around 1 GeV accuracy in one year of LHC
P 18 Alternative mass determination
Select high PT back-to-back top events: Hemisphere separation
(bckgnd reduction, much less combinatorial) Higher probability for jet overlapping
Use the events where both W’s decay leptonically (Br~5%) Much cleaner environment Less information available from two ’s
Use events where both W’s decay hadronically (Br~45%) Difficult ‘jet’ environment Select PT>200 GeV
Mtop
Various methods all have different systematics
P 19
Use exclusive b-decays with high mass products (J/) Higher correlation with Mtop Clean reconstruction (background free) BR(ttqqb+J/) 5 10-5 ~ 30% 103 ev./100 fb-1
(need high lumi)
Mtop from J/
Different systematics (almost no sensitivity to FSR)
Uncertainty on the b-quark fragmentation function becomes the dominant error
M(J/+l) Mtop
M(J/+l)
MlJ/
P 20
Commissioning the detectors Determination MTop in initial
phase Use ‘Golden plated’ lepton+jet
Selection: Isolated lepton with PT>20 GeV Exactly 4 jets (R=0.4) with
PT>40 GeV Reconstruction:
Select 3 jets with maximal resulting PT
Signal can be improved by kinematic constrained fit Assuming MW1
=MW2 and MT1
=MT2
PeriodStat Mtop (GeV)
Stat /
1 year 0.1 0.2%
1 month 0.2 0.4%
1 week 0.4 2.5%No background
included
Calibrating detector in comissioning phase
Assume pessimistic scenario:
-) No b-tagging
-) No jet calibration
-) But: Good lepton identification
P 21
Commissioning the detectors
Signal plus background at initial phase of LHC
Most important background for top: W+4 jets Leptonic decay of W, with 4 extra ‘light’ jets
Alpgen, Monte Carlo has ‘hard’ matrix element for 4 extra jets(not available in Pythia/Herwig)
ALPGEN:
W+4 extra light jets
Jet: PT>10, ||<2.5, R>0.4
No lepton cuts
Effective : ~2400 pb
With extreme simple selection and reconstruction the top-peak should be visible at LHC
L = 150 pb-1
(2/3 days low lumi)
measure top mass (to 5-7 GeV) give feedback on detector performance
P 22
Single top production
Direct determination of the tWb vertex (=Vtb)
Discriminants:- Jet multiplicity (higher for Wt)
- More than one b-jet (increase W* signal over W- gluon fusion)- 2-jets mass distribution (mjj ~ mW for the Wt signal only)
Three production mechanisms:
Main Background [xBR(W→ℓ), ℓ=e,μ]: tt σ=833 pb [ 246 pb] Wbb σ=300 pb [ 66.7 pb] Wjj σ=18·103 pb [4·103 pb]
Wg fusion: 245±27 pbS.Willenbrock et al., Phys.Rev.D56, 5919
Wt: 62.2 pbA.Belyaev, E.Boos, Phys.Rev.D63, 034012-3. 7
+16.6 W* 10.2±0.7 pbM.Smith et al., Phys.Rev.D54, 6696
Wg [54.2 pb]
Wt [17.8 pb]
W* [2.2 pb]
1) Determination of Vtb
2) Independent mass measurement
P 23
Signal unambiguous, after 30 fb-1:
Complementary methods to extract Vtb
With 30 fb-1 of data, Vtb can be determined to %-level or better(experimentally)
Single top results
Detector performance critical to observe signal Fake lepton rate b and fake rate id Reconstruction and vetoing of
low energy jets Identification of forward jets
Each of the processes have different systematic errors for Vtb and are sensitive to different new physics heavy W’ increase in the
s-channel W* FCNC gu t increase in the
W-gluon fusion channel
ProcessVtb
(stat)Vtb
(theory)
Wg fusion 0.4% 6%
Wt 1.4% 6%
W* 2.7% 5%
Process Signal Bckgnd S/B
Wg fusion 27k 8.5k 3.1
Wt 6.8k 30k 0.22
W* 1.1k 2.4k 0.46
P 24 Drell-Yan production at the LHC
Goals:• Measure pdf• Determine parton lumi• Measure sin2W
P 25
From AFB in p-pZ, l+ l-
at the Z pole, AFB comes from interference
of the V and A components of the couplings
[hep/ph9707301]
At pp collider, forward direction not naturally defined BUT
- Always from sea softer: direction given by the sign of y(ll)
- Asymmetry increase with y(ll)
sin2leff measurement
q
AFB = b { a - sin2θefflept( MZ
2 ) }
a and b calculated to NLO in QED and QCD.
• sin2θefflept is one of the fundamental parameters of the SM!
• precise determination will constrain the Higgs mass and check consistency of the SM.
P 26
• Main sys. effect: uncertainty on pdf, lepton acceptance (~0.1%), radiative correction
To reach World Average precision, need to use full calorimeter coverage (e channel only) - Z + - ||<2.
- Z e+ e- ||<2.5 jet rejection factor 1000
- Z e+ e- 2.5<||<5 jet rejection factor 10 - 100
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 10-4
L = 100 fb-1
( statistical ) ( statistical )
Can be further improved: combine channels/experiments.
[%]
sin2leff measurement
P 27
Conclusions:
LHC will allow precision measurements: unexplored kinematic regions, high-statistics (W, Z, b, t factory);
MW can be measured with a precision of 15 MeV (combinig e/μ and ATLAS
+ CMS);
Mtop: ~ 1 GeV (combined with ∆mW~15 MeV, constrains MH to ~ 25%);
EW single top production: direct measurement of Vtb; measurement of top
polarization (Wg with statistical precision of ~ 1.6%);
sin2θefflept( MZ
2 ) can be determined with statistical precision of 1.4x10-4
(competitive to lepton collider measurements!)
P 28
lepton E,p scale
goal ~0.02% mandatory for MW
• knowledge of ID material to 1%• precise alignment (<1m)• precise magnetic fields maps (<0.1%)• differents sub-detectors concerned
jet energy scale (light jets / b jets)
goal ~1% mandatory for Mtop
coverage ||<2.5 ||<5 e, , jets
ATLAS & CMS: crucial parameters
ATLAS
CMS
Stan Bentvelsen Moriond-QCD 2004 29
Backup
P 30
MW & Mtop measurement -
2 fb-1 of RunII(per experiment)
10 fb-1 of LHC(per experiment)
From D. Glenzinski (CDF), ICHEP2002
P 31
tt Cross Section at 14TeV
total Td dp
NNLO: uncertainty from scale (mt/2, 2mt) < 3% !!!
(N.Kidonakis, hep-ph/0401147)
P 32
Consequences from Mtop
Assuming total uncertainty on W-mass of 15 MeV Combined LHC prospect Very challenging measurement!
Repeat the Electro-Weak fit changing the uncertainties: Mtop=1 GeV MW =15 MeV Same central values
SM constraints on MHiggs:
Summer 2003 values:Chances to rule out SM!58
3791 Hm ((mmHH/m/mH H 53%)53%)
221863 Hm ((mmHH/m/mH H 32%)32%)
Thanks to M.Grunewald
direct
EXCLU
DED
P 33
High Pt sample
The high pT selected sample deserves independent analysis: Hemisphere separation (bckgnd reduction, much less combinatorial) Higher probability for jet overlapping
Use all clusters in a large cone R=[0.8-1.2] around the reconstructed top- direction Less prone to QCD, FSR,
calibration UE can be subtracted
j1
j2
b-jet
t
Statistics seems OK and syst. under control
R
Mtop Mtop
P 34
Top mass from hadronic decay
Use events where both W’s decay hadronically (Br~45%) Difficult ‘jet’ environment
(QCD, Pt>100) ~ 1.73 mb (signal) ~ 370 pb
Perform kinematic fit on whole event b-jet to W assignment for combination
that minimize top mass difference Increase S/B:
Require pT(tops)>200 GeV
Source of uncertainty
Hadronic Mtop
(GeV)
Statistics 0.2
Light jet scale 0.8
b-jet scale 0.7
b-quark fragm 0.3
ISR 0.4
FSR 2.8
Total 3.0
3300 events selected:
(tt) = 0.63 %
(QCD)= 2·10-5 %
S/B = 18
Selection
6 jets (R=0.4), Pt>40 GeV
2 b-tagged jets
Note: Event shape variables like HT, A, S, C, etc not effective at LHC (contrast to Tevatron)
P 35
Alternative methods
Continuous jet algorithm Reduce dependence on MC Reduce jet scale uncertainty
Repeat analysis for many cone sizes R
Sum all determined top mass:robust estimator top-mass
Determining Mtop from (tt)? huge statistics, totally different systematics
But: Theory uncertainty on the pdfs kills the idea 10% th. uncertainty mt 4 GeV Constraining the pdf would be very precious… (up to a few % might not be a dream !!!)
Luminosity uncertainty then plays the game (5%?)
Luminosity uncertainty then plays the game (5%?)
P 36Triple 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 params: g1
Z, κγ, κZ, λγ and λγ
(SM values are equal to g1Z =
κγ = κZ = 1 and λγ = λγ = 0,
tree level).
• Anomalous contribution to TGC is enhanced at high √s (increase of production cross-section).
• 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 TGC vertex
W γ/Z
L = 30 fb-1 L = 30 fb-1
∆g1Z = 0.05
λ1 = 0.01
▓ SM▓ SM
Parameters Statistical (95% C.L.)
Systematic (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*|