Simonetta Gentile

1
Physics at hadron collider with Atlas 2nd
lecture
Simonetta Gentile Università
di Roma La Sapienza, INFN on behalf of Atlas
Collaboration
2
Outline

• Introduction to Hadron Collider Physics
• LHC and ATLAS detector
• Test of Standard Model at LHC
• Parton distribution function
• QCD jet physics
• Electroweak physics (Z/W bosons)
• Top physics
• Search for Higgs boson
• Supersymmetry
• Conclusions

1st
2nd
3rd
4th
3
Cross Section of Various SM Processes

• The LHC uniquely combines the
• two most important virtues of
• HEP experiments
• High energy 14 TeV
• and high luminosity
• 1033 1034/cm2/s

4
K.Jacobs
5
Detector performance requirements

• Lepton measurement pT ? GeV ? 5 TeV
• ( b ? ?X, W/Z)

• Mass resolution (m 100 GeV)
• ? 1 (H ? gg, 4?)
• ? 10 (W ? jj, H ? bb )

• Calorimeter coverage ? lt 5
• (ETmiss, forward jet tag)
  

• Particle identification e, ?, ?, b

6
Three crucial parameters for precise
measurements

• Absolute luminosity goal lt 5
• Main tools machine, optical theorem, rate
  of
• known processes (W, Z, QED pp ? pp ??)

• ? energy scale goal 1 most
  cases
• 0.2
  W mass
• Main tool large statistics of Z ? ?? (close
  to mW , mH)
• 
  1 event/l/s at low L

• jet energy scale goal 1 (mtop, SUSY)
• Main tools Z1jet (Z ? ??) , W ?jj from top
  decay
• 
  10-1 events/s at low L

7
LEP

• Mw is an important parameter
• in precison test of SM
• MW80.425 0.034 GeV.
• 2007 Mw 80 20 MeV
• (Tevatron Run II)

Improvement at LHC requires Control systematic
better 10-4 level
8
W mass measurements motivation

• mw mt are fundamental parameters of Standard
  Model there are well defined relations between
  mw,mt,mH.
• Dependance on top and Higgs mass via loop
  corrections

• a electromagnetic constant
• Measured in atomic transitions,
• ee- machines
• GF Fermi constant measured in muon decay
• S in q w measured at LEP/SLC
• D r radiative corrections

• GF, a, sin q w are known with high precision
  precise measurements
• of W mass an top-quark mass constrains Higgs
  boson mass

• To match precision of top mass measurement
  of 2 GeV ?MW15 MeV DmW
  0.7 x 10-2 Dmt

9
W production process
50 times larger statistics than at
Tevatron 6000 times larger statistics than
at LEP
10
Method of mass measurements
Since pLn not known (only pTn can be
measured through ETmiss), measure transverse
mass, i.e. invariant of ?? perpendicular to the
beam mTW distribution is sensitive to mW
? fit experimental distributions with SM
prediction (Monte Carlo simulation) for
different values of mW ? find mW which
best fits data
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W mass
MC thruth
Estimated with W recoil

• Isolated lepton PTgt25 GeV
• ETmissgt25 GeV
• No high pt jet ETlt20 GeV
• W recoil lt 20 GeV

Full sim.
MTW (MeV)
? Sensitivity to MW through falling edge
c2 (data-MC)
? Compare data with Z0 tuned MC samples where
input MW varies in 80-81 GeV by 1 MeV steps ?
Minimize c2(data-MC) 2 MeV statistical precision
Input MW (GeV)
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W mass

• Uncertainties

• Come mainly from capability of Monte Carlo
  prediction to reproduce real life
• detector performance energy resolution, energy
  scale, etc.
• physics pTW, ?W,, backgrounds, etc.

Dominant error (today at Tevatron, most
likely also at LHC) knowledge of lepton energy
scale of the detector if measurement of lepton
energy wrong by 1, then measured mW wrong by
1

• Trailing edge of distribution is sensitive to
  W-mass
• Detector resolution smears the trailing edge of
  mT distribution

13
MW
Source CDF,runIb PRD64,052001 ATLAS 10 fb-1 Comments
Lepton E,p scale 75 15 B at 0.1, align. 1mm, tracker material to 1
PDF 15 10
Rad. decays 11 lt10 Improved theory calc.
W width 10 7 DGW30 MeV (Run II)
Recoil model 37 5 Scales with Z stat
pTW 15 5 Use pTZ as reference
Background 5 5
E resolution 25 5
Pile-up, UE - ?? Measured in Z events
Stat?syst 113 ? 25 W?e n
TOTAL 89 ? 20 W?e n W?m n
Most serious challenge

• Take advantage from
• large statistics
• Z ? ee?, ???
• Combine channels
• experiments
• ? ?mW ? 15 MeV

14
Calibration of the detector energy scale

• E measured 100.000 GeV for all calorimeter
  cells ?
• 
  perfect calibration
• To mesaure Mw to 20 MeV need a enegy scale to
  0.2 ,
• ( Eelectr 100 GeV then 99.98 GeV lt E
  measured lt 100.02 GeV )

15
Calibrations strategy.

• Calorimeter modules are calibrated with test beam
  of known energy
• In Atlas calorimeter sits behind Inner Detector
• electrons lose energy in material in front of
  calorimeter (inner detector)
• calibration in situ
  using physics sample

Z ? ee- with the constrain m ee mz
known to 10-5

• same strategy for muon spectrometer, using Z
  ? mm-

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Drell-Yan Lepton-Pair Production

• Total cross section
• pdf
• search for Z?, extra dim. , ...
• Much higher mass reach as
• compared to Tevatron

Z pole
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Drell-Yan Lepton-Pair Production

Zpole
Asymmetry ? sin2?W
Controlled at required level For the significance
of measurement
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Di - Boson production
Measuring Triple Gauge Couplings (TGC) Testing
gauge boson self couplings to SM
Charged Neutral TGSs

• WWg WWZ vertices exist
• ? 5 parameters
• in SM g1Z,kg, kZ 1lg, lZ 0
• ZZg , ZZZ do NOT exist in SM
• 12 couplings parameters
• hiV,fiV (Vg,Z)

• Some anomalous contributions ( ?-type)
  increase with s ?
• high sensitivity at LHC
• Sensitivity from
• -- cross-section (mainly ?-type)
  and pT measurements
• -- angular distributions (mainly
  k-type)

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WW? Couplings
Charged TGSs
Test CP conserving anomalous couplings at the
WW? vertex ?? and ?

• W? final states
• W ? e? and ??
• pT spectrum of bosons

Wg
Sensitivity to anomalous couplings from high end
of the pT spectrum
ATLAS 30 fb-1
pTg (GeV)
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ZZ? Couplings

• PTZ distribution

ATLAS 30 fb-1
WZ
pTZ (GeV)
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Sensitivity to WW? Couplings
Charged TGSs

• At LHC limits depend on energy scale ?
• Large improvement wrt LEP
• in particular on ? due to higher
• energy

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Triple Gauge Couplings
Neutral TGSs

• ZZZ vertex doesnt exist in SM
• gZZ vertex does exist in SM
• Analysis search for ZZ ? 4 leptons (e, µ)
• Main background
• - real ZZ events (s12pb)
• - Zjet
• Sensitivity 710-4
• (100fb-1 and ?FF6 TeV )

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ZZ? Couplings
Neutral TGSs
Example Couplings at the ZZ? vertex hi?

• Z? final states
• Z ? ee and ??
• pT spectrum of photons or Z and mT(???)

Zg
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Triple-Boson Production
Events for 100 fb-1 (mH 200 GeV) Produced (no cuts,no BR) Selected (leptons, pTgt20 GeV, ? lt 3)
pp ? WWW (3 ?s) 31925 180
pp ? WWZ (2 ?s) 20915 32
pp ? ZZW 6378 2.7
pp ? ZZZ 4883 0.6
pp ? W?? best channel for analysis best channel for analysis
Sensitive to quartic gauge boson couplings
(QGC)
30 W?? signal events in 30 fb-1
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Triple gauge couplings

• SM allowed charged TGC in WZ, Wg with 30 fb-1
• 1000 WZ (Wg) selected with S/B 17 (2)
• 5 parameters for anomalous contributions
• scale with vs for g1Z,ks and s for ?s
• Measurements still dominated by statistics, but
  improve LEP/Tevatron results by 2-10
• in SM g1Z,kg, kZ 1lg, lZ 0

ATLAS 95 CL (stat syst)
Dg1Z ? 0.010 ? 0.006
DkZ ? 0.12 ? 0.02
?Z ? 0.007 ? 0.003
Dkg ? 0.07 ? 0.01
?? ? 0.003 ? 0.001

• SM forbidden neutral TGC in ZZ, Zg with 100 fb-1
• 12 parameters, scales with s3/2 or s5/2
• Measurements completely dominated by statistics,
  but improve LEP/Tevatron limits by 103-105

ATLAS 95 CL stat
f 4 ,5 7 10-4
h 1, 3 3 10-4
h 2, 4 7 10-7
Z,g
Z,g
Z,g

• Quartic Gauge boson Coupling in Wgg can be
  probed with 100 fb-1

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Status of SM model

• High precision measurements? Test of Standard
  Model
• 1000 data points combined in 17 observables
  calculated in SM

TOP MASS

• a em(precision 3 10-9) (critical part Da had)
• GF (precision 9 10-6) (?MW)
• Mz(precision 2 10-5)from line-shape
• as(Mz) precision 2 10-2 hadronic observable

Mtop and MHiggs
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Measurement of the Top Mass Motivation
1-?r ? (1- ??)(1-?rW)
?rW ? (mt2-mb2)
?
Top mass from Tevatron (2005)
?rW ? log mH
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Combination of Measurements
Only best analysis from each decay mode, each
experiment.
Year Mtop GEV MHiggs GEV
2003 174.35.1 lt 219
2004 178.04.3 lt 251
2005 (june) 174.33.4 lt 208
2005 (july) 172.72.9 ?
Expected precision in 2007 at Tevatron 1GeV
EPS95 KojiSato
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Top Physics
Inverse ratio of production mechanism as compared
to Tevatron

• Top decay ? 100 t ? bW
• Other rare SM decays
• CKM suppressed t ? sW, dW 10-3 10-4 level
• t?bWZ O(10-6)
• difficult, but since mt ? mbmWmZ
  sensitive to mt
• non-SM decays, e.g. t ? bH

30
Top Decays

• the tt pair cross section is 600 pb
• Br (t?Wb) 100
• no top hadronization

• Di-lepton channel
• Both Ws decay via W?? n (? e or m 5)
• Lepton-jet channel
• One W decays via W?? n (? e or m 30)
• All hadronics
• Both W decay via W? qq (44)

tt final states (LHC,10 fb-1)
Signature Leptons Missing
transverse energy b-jets

• Full hadronic (2.6M) 6 jets
• Semileptonic (1.7M) ? n 4jets
• Dileptonic (0.3M) 2 ? 2n 2jets

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Tagging b-quarks
Soft lepton tag
Silicon vertex tag
displaced tracks
Search for non-isolated soft lepton in a jet
B mesons travel 3mm before decaying search
for secondary vertex
32
Top Mass from Semi-Leptonic Events

• Easiest channel tt ? bb qq l?
• Large branching ratio
• Easy to select

tt ? bb qq ?? events from ATLAS
33
Measurement of mtop

• Selection
• Require at least one e or µ
• PT gt 20 GeV/c in central detector
• 2 jets
• 2 b-jets
• Efficiency 65
• Systematics
• Dominant Final-state radiation
• jet energy calibration 1
• especially b-jet calibration

34
Top Mass from Semi-Leptonic Events
Reconstruct mt from hadronic W decay Constrain
two light quark jets to mW
70 top purity - efficiency 1.2

• Isolated lepton PTgt20 GeV
• ETmissgt20 GeV
• 4 jets with ETgt40 GeV DR0.4
• gt1 b-jet (?b?60, ruds?102, rc?101)

Background lt2 W/Zjets, WW/ZZ/WZ
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MTop from leptonjet SN-ATLAS-2004-040

• Golden channel
• BR 30 and clean trigger from isolated lepton
• Important to tag the b-jets
• enormously reduces background (physics and
  combinatorial)
• Hadronic side W from jet pair with closest
  invariant mass to MW
• Require MW-Mjjlt20 GeV
• Ligth jet calibrated with Mw constraint
• Assign a b-jet to the W to reconstruct Mtop
• Leptonic side Using remaining ? b-jet, the
  leptonic part is reconstructed
• m ? b -ltmjjbgt lt 35 GeV
• Kinematic fit to the t t hypothesis, using MW
  constraints

• Br(tt?bbjj ? ?)30for electron muon

• Isolated lepton PTgt20 GeV
• ETmissgt20 GeV
• 4 jets with ETgt40 GeV DR0.4
• gt1 b-jet (?b?60, ruds?102, rc?101)

36
Top Mass from Semi-Leptonic Events

• Linear with input Mtop
• Largely independent on Top PT

37
Top mass systematics

• Systematics from b-jet scale

Source ATLAS 10 fb-1
b-jet scale (1) 0.7
Final State Radiation 0.5
Light jet scale (1) 0.2
b-quark fragmentation 0.1
Initial State Radiation 0.1
Combinatorial bkg 0.1
TOTAL Stat ? Syst 0.9

• 3.5 million semileptonic events in 10 fb-1
• (first year of LHC operation)
• Error on mt ? ? 1 2 GeV
• Dominated by
• Jet energy scale (b-jets)
• Final state radiation