1. Gomel School of Physics 2005
Physics at hadron collider
with Atlas
3rd lecture
Simonetta Gentile
Università di Roma La Sapienza, INFN
on behalf of Atlas Collaboration
Simonetta Gentile
Gomel School of Physics 2005

2. Gomel School of Physics 2005
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
Simonetta Gentile
Gomel School of Physics 2005

3. Gomel School of Physics 2005
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
Low luminosity phase
1033/cm2/s = 1/nb/s
approximately
200 W-bosons
50 Z-bosons
1 tt-pair
will be produced per second!
Simonetta Gentile
Gomel School of Physics 2005

4. Gomel School of Physics 2005
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 ) (critical part Da had)
GF (precision ) (→MW)
Mz(precision )from line-shape
as(Mz) precision hadronic observable
Mtop and MHiggs
Simonetta Gentile
Gomel School of Physics 2005

5. Gomel School of Physics 2005
Top Mass from Semi-Leptonic Events j1 j2
b-jet t W
Reconstruct mt from hadronic W decay
Constrain two light quark jets to mW
70% top purity - efficiency 1.2 %
Atlas
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
Linear with input Mtop
Largely independent on Top PT
Simonetta Gentile
Gomel School of Physics 2005

6. Gomel School of Physics 2005
Top Mass from Other Channels
Di-lepton events:
BR  5%
low background
but two neutrinos in final state
mt   1.7 GeV
Atlas
Fully hadronic events:
BR  45%
difficult jet enviroment
mt   3 GeV
Correlation di-lepton mass to top mass from ATLAS (Bentvelsen talk)
high pT sample is for semi-leptonic, right?
select high pT of top quarks to
increase signal to background
Simonetta Gentile
Gomel School of Physics 2005

7. Gomel School of Physics 2005
Top Mass from J/ channel
Method:
Partial reconstruction of top J/ + lepton
The lepton from W decay is rather sensitive to mt
BR(ttWb+qqJ/) 
efficiency (e ~ 30%)
Low background
indipendent of jet energy scale
limited by b fragmentation & needs high luminosity
L =100fb-1
Estimated ultimate precision:
mt  1 GeV
Simonetta Gentile
Gomel School of Physics 2005

8. Gomel School of Physics 2005
Top mass from J/
M J/
Atlas
Different systematics (almost no sensitivity to FSR)
Uncertainty on the b-quark fragmentation function becomes the dominant error
Atlas
M(J/+ )
M(J/)
Pttop
Simonetta Gentile
Gomel School of Physics 2005

9. Gomel School of Physics 2005
W Polarization
Massive gauge bosons have three polarization states
At LEP in e+e  W+W :
determine W helicity from lepton (quark) decay angle in W rest frame 
(1  cos )2 transverse
sin2 longitudinal 
Fraction of longitudinal W
in e+e  W+W
0.218  0.031
SM: 0.24
Tevatron:
Longitudinal W in top decays
0.91  CDF
0.56  D0
SM: 0.7
Simonetta Gentile
Gomel School of Physics 2005

10. W Polarization in Top Decays
Use tt events to study the Lorentz structure of tt decays.
P ossible W configuration in top decays.
Standard Model
Within SM (V-A coupling) only 2 configuration are allowed
Lepton kinematical distribution are rather sensitive to W boson helicity
Simonetta Gentile
Gomel School of Physics 2005

11. Gomel School of Physics 2005
W Polarization in Top Decays
A sensitive W spin analyser in top decays is the angle q*
between the charged lepton in W rest frame and W in top rest frame
cos q*
cos q*
Simonetta Gentile
Gomel School of Physics 2005

12. Gomel School of Physics 2005
W Polarization in Top Decays
F0=0.699 ± 0.005
FL=0.299 ± 0.003
FR=0.002 ± 0.003 SM
ATLAS
(±stat ±syst) F0
0.703
  0.015 F-
0.297
  0.024 F+
0.000
  0.012
ATLAS 10 fb-1
Atlas
Systematic dominated by b-jet scale.
Precision on fraction of long.polarization W~ 0.03 after 10fb-1
Simonetta Gentile
Gomel School of Physics 2005

13. Use double leptonic decays Gomel School of Physics 2005
tt Spin Correlation
Very short lifetime, no top bound states
Spin info not diluted by hadron formation
Measure the correlation through the angular distribution of daughter of articles in top rest frame
Distinguishes between
quark annihilation
A =
and gluon fusion
A =
Predicted value A=0.31
Use double leptonic decays
tt  bb l l
Simonetta Gentile
Gomel School of Physics 2005

14. tt-correlation dileptonic events
Double leptonic decays
tt  bb l l
e+/+
top
 *
θ*  angle of charged lepton in top (or antitop) rest frame
<Cos θ *+  · Cos θ * - >
<Cos θ *+  · Cos θ *-  >
With
helicity
correlation
Without
helicity
correlation
Dilepton and -Semileptonic events analysis with same power to probe SM at 5s in 10fb-1
Simonetta Gentile
Gomel School of Physics 2005

15. Gomel School of Physics 2005
Single Top Production
Production mechanisms and cross sections:
 10 pb
 60 pb
 245 pb 1 2 3
direct measurement of Vtb
( observable by Tevatron in Run II )
tt final states (LHC,10 fb-1)
W→en,mn
process 1 (0.5M) :  + n + 4jets
process 2 (0.12M) :  + n + 3ets
process 3 (0.02M) : 2 + n + 2jets
Simonetta Gentile
Gomel School of Physics 2005

16. Gomel School of Physics 2005
Single Top Production
Production mechanisms and cross sections:
 10 pb
 60 pb
 245 pb
Process
Vtb (stat)
Vtb (theory) 1
0.4% 6% 2
1.4% 3
2.7% 5%
Selection:
t  bW  b e ()
b-jet + high pT lepton
reconstruction of top mass
Background from tt
signal to bkgd : 1
experimental determination of Vtb
to percent level (with 30 fb-1)
Simonetta Gentile
Gomel School of Physics 2005

17. Radiative top production
Determination of Top Charge
Top charge:
Qt = +2/3 not yet established t  W+b
Qt = -4/3 not yet excluded t  W–b
Methods:
jet charge determination. Measure the lepton charge from W decays and distinguish b-quark from anti-b with jet charge determination
ttg events . Final state photon
radiation would be more likely
if |Q| is larger.
cross section proportional to Q2top.
Radiative top production
Radiative top decay
Simonetta Gentile
Gomel School of Physics 2005

18. Gomel School of Physics 2005
Determination of Top Charge
Determine charge from rate of radiative tt events
pT spectrum of photons for 10 fb-1:
Atlas
Qt = +2/3
Qt = -4/3
Q=2/3
Q=-4/3
pptt
101 ± 10
295 ± 17
pptt ; tWb
6.2 ± 2.5
2.4 ± 1.5
Total background
38 ± 6
events
pT()
Simonetta Gentile
Gomel School of Physics 2005

19. Top Charge Determination ATL-PHYS-2003-035
Determine charge of b-jet and combine with lepton
Use di-lepton sample Investigate ‘wrong’ combination b-jet charge and lepton charge
Effective separation b and b-bar possible in first year LHC
Study systematics in progress
Simonetta Gentile
Gomel School of Physics 2005

20. Gomel School of Physics 2005
Measurement of tt cross section
Total cross section:
At 14 TeV interesting in itself
Sensitive to top mass tt  1/mt2
Differential cross sections:
d/dpT checks pdf
d/d checks pdf
d/dmtt sensitive to production of heavy object
decaying to top-pairs Xtt
Atlas
xBR required for a discovery
Atlas
1.6 TeV resonance
830fb
30 fb-1
σxBR [fb]
300 fb-1
Mtt
mtt [GeV/c2]
1TeV
Simonetta Gentile
Gomel School of Physics 2005
A resonance could be discovered if sxBr >830 fb

21. Gomel School of Physics 2005
Summary & Conclusions
SM physics at the LHC with ATLAS
Very important in initial phase
to check detector
to check generators (pdf)
to prepare discoveries
Large potential for precision measurements
large cross sections
precision limited by systematics
use as many different strategies as possible
Credits: Marina Cobal, Matt Dobbs, Fabiola Gianotti, Alexander Oh, Dominique Pallin,Sergey Slabospitsky
Simonetta Gentile
Gomel School of Physics 2005

22. Gomel School of Physics 2005
Standard Model
Higgs Search
Simonetta Gentile
Gomel School of Physics 2005

23. Combination of Measurements
Only best analysis from each decay mode, each experiment.
Year
Mtop
[GEV]
MHiggs
2003
174.3±5.1
< 219
2004
178.0±4.3
< 251
2005 (june)
174.3±3.4
< 208
2005 (july)
172.7±2.9 ?
Expected precision in 2007 at Tevatron:
± ~1GeV
EPS95 KojiSato
Simonetta Gentile
Gomel School of Physics 2005

24. Gomel School of Physics 2005
SM Higgs search at Tevatron
S.Lammel, Fermilab,LP05
New Tevatron results
LP05
Simonetta Gentile
Gomel School of Physics 2005

25. Higgs Production Mechanism@ LHC Gomel School of Physics 2005
Simonetta Gentile
Gomel School of Physics 2005
4 production mechanisms key to measure H-boson parameters

26. Standard Model Higgs production
Direct production
gg fusion
gg fusion dominant
Associated production
tt H, WH, ZH :
1-10% of gg fusion
isolated lepton from W decay
reconstruct top-quarks
WW/ZZ fusion
VectorBosonFusion 20% of gg at 120 GeV
2 large h
Simonetta Gentile
Gomel School of Physics 2005

27. Properties of Higgs boson
The Higgs boson couples to particles proportionally their mass
→ preferred decays in heaviest particle allowed.
W+, Z, t,b,c, t+ …. g H
W-, Z, t,b,c, t- …. g
Lepton & photons are essential final state against QCD background
bb decay mode only possible in associated production tt…
Simonetta Gentile
Gomel School of Physics 2005

28. Higgs Discovery Channels at LHC Gomel School of Physics 2005
Dominant BR for mH<2mZ:
 (H  bb)  20 pb;
 (bb)  500 b
for m(H) = 120 GeV
no hope to trigger
or extract fully
hadronic final states
look for final
states with , 
( = e, )
m(H) > 2 mZ :
H  ZZ  4
qqH  ZZ    *
qqH  ZZ   jj *
qqH  WW jj * * for mH > 300 GeV forward jet tag
Low mass region: m(H) < 2 mZ :
H   : small BR, but best resolution
H  bb : good BR, extract backg ttH, WH
H  ZZ*  4
H  WW*   or jj : via VBFand direct production
H   : via VBF
Simonetta Gentile
Gomel School of Physics 2005

29. Gomel School of Physics 2005
Discovery in a channel
Particle decaying in X → mm
Signal significance
Ns= number of signal events
Nb= number of background events
peak
In the signal region
mmm
Error on number of background events
(for large statistics, for low Poisson statistics)
S > 5 : signal larger 5 times error on background.
Gaussian probability that background fluctuates up more
than 5 s is 10-7→ DISCOVERY
Simonetta Gentile
Gomel School of Physics 2005

30. Gomel School of Physics 2005
Considerations
Detector resolution
If the detector resolution became worst sm . To keep the same number of signal events, Ns ,the region has to be 2 times larger
to keep the same number of events.
If background flat →→ Nb ,increases ~ 2
decrease
An high resolution detector has better chance for a discovery
If GH << s m (if the width particle X is broad of detector resolution this comment is not important)
Remind : mH = 100 GeV → GH ~ GeV
mH = 200 GeV → GH ~ 1 GeV
mH = 600 GeV → GH ~ 100 GeV GH ~ m3
Simonetta Gentile
Gomel School of Physics 2005

31. Gomel School of Physics 2005
Considerations
Luminosity
Ns ~ L
Nb ~ L
S ~√ L
Simonetta Gentile
Gomel School of Physics 2005

32. Gomel School of Physics 2005
The Challenge
Theoretical limits:
M(Higgs) < 1 TeV
LEP
Experimental limits:
M(Higgs) > GeV
Indirect constraints:
M(Higgs) < 208 GeV
Atlas
Safe channels:
H→ gg
H→ZZ→    
here discovery “easy”
with H  4
Simonetta Gentile
Gomel School of Physics 2005

33. Gomel School of Physics 2005
Prospects for Standard Model Higgs searches
Discovery: several complementary channels.
some channels with very exclusive
topologies (large bgd. suppression).
Coupling 30fb-1.
Mass, width (direct and indirect).
Detector performance is crucial: b-tag, / , E-resolution,
g/j separation, ETmiss resolution, forward jet tags, central jet-veto,
t-reconstruction.
LEP direct MH>114.4 CL
EW fit MH<211 CL
Atlas
Expected results discussed at Integrated Luminosity of
10fb-1 30fb fb-1.
Simonetta Gentile
Gomel School of Physics 2005

34. Gomel School of Physics 2005
SM-like Higgs searches early reach
Low mass
mH ~ 115 GeV
LEP : mH > GeV
observation of all channels important to extract convincing signal 4-5  significance in first year
( Several channels give ~ 2 s significance 10 fb-1 )
H   relies only on electromagnetic calorimeter, (constant term < 0.7 %)
Atlas
Simonetta Gentile
Gomel School of Physics 2005

35. Gomel School of Physics 2005
MH= 115 GeV
H   b
ttH  tt bb  b bjj bb H 
qqH  qq
H   ttH  ttbb qqH  qq
( + -had)
S ~ 10
B ~ 10
S/B
Signif
CLb
Total S/ B for 10 fb-1 and complete detector: ~ 4.2 
Simonetta Gentile
Gomel School of Physics 2005

36. Gomel School of Physics 2005
SM-like Higgs searches early reach
Inter mass
mH ~ 130 GeV
observation of all channels important to extract convincing signal 6  significance in first year ( Several channels give
<3 s significance 10 fb-1 )
4 complementary channels  robustness
 H  4 : low rate but very clean (large S/B, narrow mass peak)
< 3 significance per channel
(except qqWW counting channel)
Atlas
Simonetta Gentile
Gomel School of Physics 2005

37. Gomel School of Physics 2005
MH= 130 GeV
Intemediate mass
10 fb-1
complete detector
H   qqH  qq H  4 qqH  qqWW
( + -had)
S ~ ~
B ~ <
S/B ~ ~ 1
Signif ~
CLb
Total S/B for 10 fb-1 and complete detector:
~ 6.5 
Simonetta Gentile
Gomel School of Physics 2005

38. H -> ZZ* ->4 leptons Gomel School of Physics 2005
Light Higgs Search:
H -> ZZ* ->4 leptons
s Br = 5.7 fb (mH =100 GeV)
Example of analysis:
PT of 2 most energetic m > 20 GeV
PT of 2 less energetic m > 7 GeV
|η|<2.5 (barrel)
M(  ) ~ Mz
M(  ) < Mz
Invariant mass constraints
background
L =100 fb-1
Simonetta Gentile
Gomel School of Physics 2005

39. H -> ZZ* ->4 leptons Gomel School of Physics 2005
Light Higgs Search:
H -> ZZ* ->4 leptons
background
Top production
tt →W b W b →n cn n cn
s Br ≈ 1300 fb
W b W b
Associated production of Zbb
Zbb → c n cn
s Br(Z→mm) ≈ 22.8pb
To reject the background:
1.leptons from b-quarks decays are not isolated
2. don’t originated from primary vertex (tb ~1.5 ps)
ZZ background
after isolation cut continuum
s Br(Z→bb) Br(Z→mm) ≈ 0.15pb
Discovery potential in mass range from ~ 130 GeV to 600 GeV
Simonetta Gentile
Gomel School of Physics 2005

40. Gomel School of Physics 2005
Light Higgs
Search:
H -> ZZ* -> 4m
100fb–1
required lumi for discovery
100fb–1
Simonetta Gentile
Gomel School of Physics 2005

41. Gomel School of Physics 2005
H→gg
Simonetta Gentile
Gomel School of Physics 2005

42. Light Higgs Search: gg, ttH channels Gomel School of Physics 2005
100fb–1
Significance: 2.8 to 4.3 for 100 fb-1
Signal significance (5) :
mH < 120 GeV needs 100 fb–1
Simonetta Gentile
Gomel School of Physics 2005

43. VBF Higgs boson production at low mass Gomel School of Physics 2005
Distinctive Signature of:
- two high PT forward jets
- little jet activity in the central region
 Jet Veto
Rapidity distribution of tag jets
VBF Higgs events vs. tt-background
Channels studied:
qqH   qqWW*  qq  n  n 
qqH  qq t t   qq  n n  n n
 qq  n n had n
Higgs tt
h jet
Hard leptons with distinctive kinematics;
Full H->tt reconstruction possible
Simonetta Gentile
Gomel School of Physics 2005

44. Gomel School of Physics 2005
Forward jet tagging
Rapidity distribution of tag jets
VBF Higgs events vs. tt-background
Rapidity separation
Simonetta Gentile
Gomel School of Physics 2005

45. Gomel School of Physics 2005
Background
el.weak background:
WW jj production Z + 2 jets
QCD backgrounds:
tt production Z + 2 jets
Background rejection:
qqH   qqWW*  qq l n l n
Lepton PT cuts and tag jet requirements
(Dh, PT)
Require large mass of tag jet system
Jet veto
Lepton angular and mass cuts
Higgs boson (mH = 160 GeV)
tt background
g* / Z + jets
el.weak WW jj
Simonetta Gentile
Gomel School of Physics 2005

46. Gomel School of Physics 2005
qq→qq W W*→qq  n  n 
Transverse distribution excess
On the background tt -production
Simonetta Gentile
Gomel School of Physics 2005

47. Gomel School of Physics 2005
qq→qq W W*→qq  n  n 
K.Jacobs
Azimuthal difference
Between the two leptons
Simonetta Gentile
Gomel School of Physics 2005

48. mT = 2pT ETmiss(1-cosf) Gomel School of Physics 2005
H  WW*  
mT = 2pT ETmiss(1-cosf)
Observe excess in transverse
mass distribution
S ~ events for 10fb-1
B ~ events p H W l n n l
Two isolated leptons
Two forward tag jets
Central jet veto: pT<20 GeV
lepton angular correlation
Simonetta Gentile
Gomel School of Physics 2005

49. Gomel School of Physics 2005
Vector Boson Fusion
These studies demonstrate that vector boson fusion channels may be accessible in the low mass region already in the first year. 10 5 1
mHiggs (GeV)
200
Signal significance
ATLAS
qqH  qq
qqH  qqWW
All channels
For 10 fb-1 in ATLAS:
5 s significance for
120  mH  190 GeV
Results are conservative:
K-factor not included
very simple analyses used
Simonetta Gentile
Gomel School of Physics 2005

50. Gomel School of Physics 2005
Low mass remarks
The 3 channels are complementary  robustness:
different production and decay modes
different backgrounds
different detector/performance
requirements:
-- ECAL crucial for H  
(in particular response uniformity) :
/m ~ 1% needed
-- b-tagging crucial for ttH :
4 b-tagged jets needed to reduce combinatorics
-- efficient jet reconstruction over || < 5
crucial for qqH  qq :
forward jet tag and central jet veto
needed against background
Note :
all require “low” trigger thresholds.
e.g. ttH analysis cuts :
pT () > 20 GeV,
pT (jets) > GeV
Simonetta Gentile
Gomel School of Physics 2005

51. Gomel School of Physics 2005
200 GeV<MH<600 GeV
Large mass
10 fb-1
complete detector
200 GeV < m(Higgs) < 600 GeV:
- discovery in H  ZZ  +-+-
background smaller than signal,
Higgs natural width larger than experimental resolution (mHiggs > 300 GeV)
- confirmation in H  ZZ  +- jj channel
H  ZZ  +-+-
m(Higgs) > 600 GeV:
4 lepton channel statistically limited
H  ZZ  +- nn
H  ZZ  +- jj , H  WW  +n jj (150 times larger BR than 4l channel)
Event signature: high pT lepton, two high pT jets
Combination of analyses allows Higgs discovery in full mass range
Simonetta Gentile
Gomel School of Physics 2005

52. Gomel School of Physics 2005
Higgs signal in ATLAS
LHC can probe entire set of ”allowed” Higgs mass values (100 GeV – 1 TeV)
at least 2 channels for most of range
Atlas
Full mass range can be covered
After few years at low luminosity
30 fb-1
Simonetta Gentile
Gomel School of Physics 2005

53. Gomel School of Physics 2005
Discovery potential
All channels together
For most of the mass range at least two channels available
Good sensitivity over the full mass range from
~ 100 GeV to ~ 1TeV
mH<180 GeV: several complementary
channels (gg, ttbb, 2ETmiss , 3ETmiss, 4 , tt)
mH>180 GeV: easy with gold-plated H  ZZ*  4
Challenging channels (multijets…, WH) not included
Simonetta Gentile
Gomel School of Physics 2005

54. Gomel School of Physics 2005
Standard Model Higgs
Once Higgs boson has been discovered
measure its properties:
Simonetta Gentile
Gomel School of Physics 2005

55. Gomel School of Physics 2005
Measurement of the Higgs boson mass
No theoretical error
Dominant systematic
uncertainty: g /, E scale.
Assumed 1‰
Goal ‰
Scale from Z   (close to light Higgs)
Assumed 1% jets
Resolution for
g g &   1.5 GeV/c2
bb 15 GeV/c2
At large masses decreasing
precision due to large GH
Dominated from
ZZ→4  and H → g g
Higgs boson mass can be measured with a precision of 0.1% over a large mass range (130 - ~450 GeV / c2)
Simonetta Gentile
Gomel School of Physics 2005

56. Measurement of Branching Ratios Gomel School of Physics 2005
Measurement of relative branching ratios
Fitting Br(H->XX) (assume Higgs production works as in SM)
Simonetta Gentile
Gomel School of Physics 2005

57. Gomel School of Physics 2005
Simonetta Gentile
Gomel School of Physics 2005

58. Measurement of Higgs Boson Couplings Gomel School of Physics 2005
Global likelihood-fit (at each possible Higgs boson mass)
Input: measured rates, separated for the various production modes
Output: Higgs boson couplings, normalized to the WW-coupling
Relative couplings can be measured
with a precision of 10-20% (for 300 fb-1)
Simonetta Gentile
Gomel School of Physics 2005

59. Gomel School of Physics 2005
Conclusions on the SM Higgs
mH > GeV
Inclusion of Vector Boson Fusion channels improves SM Higgs discovery potential:
~10 fb-1 needed for 5s discovery over the full (interesting) mass range
30 fb-1 more robust result
here discovery “easy”
with H  4
Simonetta Gentile
Gomel School of Physics 2005