1. W/Z/g + Jets at ATLAS Jimmy Proudfoot Argonne National Laboratory On behalf of the ATLAS Collaboration
LO -> NLO -> NNLO -> Experiment
Standard Model Test and Background to New Physics
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

2. The ATLAS Calorimeter System
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

3. Calorimeter Response GOAL: sE/E = 50%/√E + 3%
Dead Material Map
Tile Sampling Fraction vs h
Phi Modulation from Accordian Structure
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

4. This technology has 2 specific technical issues: pileup and noise
Electromagnetic Calorimeter and the Endcap Hadron Calorimeters use Liquid Argon for Ionization Measurement
Ionisation signal is sampled: 25ns, 12 bit ADC, 3 gains
Physics data: Usually 5 samples (6 samples in TB)
Calibration signal: Up to 32 samples (to determine waveform)
This technology has 2 specific technical issues: pileup and noise
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

5. Weight Cell/Layer Energies to Optimise Linearity and resolution
EM Scale
“Simple” weighting scheme using depth dependent weights
Other approaches also work well and the merits of the different schemes are being discussed
The Ultimate Test will be with DATA
Weighted
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

6. J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007
W/Z/g + Jets Overview
W+Jets:
Only studied at present as background to other physics (TTbar, Single Top, SUSY
Z + Jets:
SM Test of pQCD in Z+Njets
“Data-Based” calibration of physics backgrounds from Z decays such as Z->nn for SUSY and Z-> BBbar for Higgs , and Ws by inference
Jet energy scale calibration
g + Jets:
Only studied at present in the context of jet energy scale calibration
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

7. Guidance from the Tevatron
SM Tests of W+Jets
Spectra are sensitive to choice of renormalization and factorization scales
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

8. More Guidance from the TeVatron
Backgrounds to Z+Jets do not show the same increase as a function of Jet Pt and the number of jets - at least for a suitable event selection (DR between leptons and jets)
BUT - this is hard to establish for the LHC until we have DATA
W + Jets
Increasing and significant rate of QCD and Top backgrounds to W+Jets
Top Background in W+Jets will be even worse at the LHC
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

9. MadEvent: LHC != Scaled TeVatron
qg=>Zqg ~0.75stot
qg=>Zqg ~0.26stot
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

10. Z+ nJets Use to establish the reliability of our understanding W+Jets
LO <=> NLO <=NNLO
Basis for determining SM background to searches
Bootstrap the light jet energy scale calibration
Use to measure the b pdf in the proton
Use to establish b jet energy scale
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

11. Standard Model Backgrounds to New Physics A “Shopping” List
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

12. Top - Without B-tagging
W-BBbar/ W+Jets x-section ratio => the irreducible background
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

13. H production in VBF: mH = 160 GeV
Rejection factor ~120
Tag Jets (WW* Analysis):
Pt > 40 GeV/c, Pt > 20 GeV/c
Dhtags > 3.8
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

14. H production in VBF: mH = 120 GeV
Rejection factor ~70
Tag Jets (tt Analysis):
Pt > 50 GeV/c, Pt > 30 GeV/c
Dhtags > 4.4
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

15. J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007
Background To SUSY
Black: ISAJET Red: PYTHIA
Susy Background:
Z(->nn) +jet
Effective Mass distribution for No-Leptons Mode after standard event selection M(g)≈M(q)≈1TeV
Susy Atlas meetings
T.S.S.Asai U. of Tokyo
Event Topology
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

16. Estimating SM Background to SUSY
Assume shape of BG in control region (MT<100 ) is shape of BG in signal region (MT>100 ) → Just need to scale with # events
Assume No SUSY in control region (MT<100) Et
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

17. 1D Method: Use MC Driven by Data
MT<80GeV
MT>80GeV
ETmiss>250GeV
Signal side
Calibration
side
Obtain an absolute normalization of W+jets on the calibration side from data by means of (Z+jets, and/or g+jets?) calibration.
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

18. Results for Simple 2D Method
TTbar
W+Jets (ALPGEN)
SUSY
No clear SUSY-free region from which to extract the shape of the background which then can be extrapolated into the SUSY region
BUT
Maybe can use a more sophisticated approach which will probably require a better understanding of the physics of the backgrounds
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

19. J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007
B Quark pdf from Z+B
LHC is ~5% of entire Z production
Spread of existing pdf’s gives a 10% spread in the prediction of the H cross section
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

20. J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007
B-Tagging
Life time of a bottom hadron is about t ~ 1.5 ps long enought to permit to a hadron of 30 GeV of energy to do a distance of L ~ 3 mm before decaying
BTagging Efficiency 59.5%
Purity 60.7%
Based on TeVatron and LEP experience expect to determine tagging efficiency from B-enriched samples to a precision of ~5%
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

21. Some MC Tools in Use At ATLAS
AcerMC: Zbb~, tt~, single top, tt~bb~, Wbb~
Alpgen (+ MLM matching): W+jets, Z+jets, QCD multijets..
HERWIG: QCD multijets, Drell-Yan, SUSY MadEvent: Z/W+jets...
tt~, Drell-Yan, boson pair production
Pythia: QCD multijets, B-physics, Higgs production...
Sherpa: W+jets/Z+jets...
WINHAC: W production and decay
AND Since we have no DATA for pp at 14 TeV
We can only compare Monte Carlo to Monte Carlo !
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

22. Particle Content of Z/g + Jets
Z+Jets
Pythia:
Large event-to-event fluctuations dominated by low Pt particles - mostly central
g+Jets
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

23. Jets In Z + Jets Events Jet Pt >10GeV Large Pt Dependence
Large dependence on hadronization and fragmentation model (not a surprise)
Many soft Jets
Tails and Mean Multiplicity are Sensitive to Underlying Event
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

24. A Monte Carlo Study of Z-Jet Pt Balancing in ATLAS
There are two key issues:
Correlation between the jets
Jet reconstruction efficiency
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

25. Jet Energy Calibration
Ave. Pt Balance for all Jets above 1 GeV
Want to decouple as much as possible physics effects from detector effects
=> but particle Pt and h cuts alone give a net imbalance
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

26. Another Generator Study
Average Pt Balance for Jets above 10 GeV
Back-to-Back Cut on Leading Jet relative to Z
Back-to-Back Cut on Leading Jet relative to Z
AND
No Second Jet with Pt>10GeV
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

27. Some Important Z/W + nJets Features for LHC Physics
Compare Z’s to W’s using Monte Carlo to establish level to which Z’s (in data) can be used to establish W backgrounds to new physics and precision with which we understand variables to be used in kinematic cuts e.g.:
(Z/W+{n+1}Jet) / (Z/W+{n}Jet) cross section ratio
Jet Pt for Leading, next-to-leading jet..
dR between the two leading jets
Jet rapidity
Z + bb (vital for low mass Higgs searches)
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

28. Jets in Z events versus Jets in W Events
Compare ratio of (N(Jets) >= i)/(N(Jets) = 1) for W + n Jets and Zee + n Jets
ALPGEN
W + n Jets
Zee + n Jets
Same ratio
(With Some Precision)
Next step : compare different MC generators (with new samples) to get an idea of systematic uncertainties on shape and normalization factor.
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

29. Compare Shape of Meff in Z + n Jets vs W + n Jets
ALPGEN
Considering all Z samples :
Z + 0, 1, 2, 3, 4, 5 jets
Considering only Z samples :
Z + 2, 3, 4, 5 jets as for W
1 <= n jets <= 3, W
>= 4 jets, W
>= 4 jets, Z
1 <= n jets <= 3, Z
Normalization : all shapes a normalize to 1. !
Shapes from W and Z productions are the same (if considering same samples)
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

30. Jet Pt Distributions in Z+Jets
ATLFAST Jet Parameterization
Leading jet is harder in ALPGEN than in Pythia Z+q/g, although both have same matrix element
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

31. Jet Angular Correlations
Z+Jets
Larger Angle between two leading jets in ALPGEN than in Pythia
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

32. Summary and Conclusions
W’s, Z’s and g’s may be “just” Standard Model Physics
BUT
They aren’t simple
They aren’t dull
Even here we may see surprises when we final see pp collisions at 14 TeV
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

33. J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007
Backup
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

34. J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007
LHC vs Tevatron
J.Campbell et al.
Phys.Rev.D69:074021,2004
Cross Section (pb)
TEVATRON
LHC
Processes
ZQ inclusive
13.40.9 0.8 0.8
6.83
49.2
13.8
89.7
Zj inclusive
The measurement of Z+b should be more interesting at LHC than at Tevatron:
Signal cross-section larger (x80), and more luminosity
Relative background contribution smaller (x5)
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

35. J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007
BTagging
All Jets
30 fb-1
b jet
other
# events
176642
204265
B Jets
BTagging Efficiency 59.5%
Purity 60.7%
Soft Muon Tagging
All Muons
30 fb-1
b jet
other
# events
22630
68088
B Muons
Soft MuonTagging Efficiency 7.2%
Purity 37.2%
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

36. A Monte Carlo Study of Z-Jet Pt Balancing in ATLAS
A Tale of Two Jets
A Monte Carlo Study of Z-Jet Pt Balancing in ATLAS
Z + 1 Jet
No Additional Jet with Et>10GeV
df (Jet,Z)>2.88radians
|h| (Jet)<2.6
PtZ> 27.5GeV
Z + 2 Jets
No Third Jet with Et>10GeV
df (J1+J2,Z)>2.88radians
|h| (J1 & J2)<2.6
PtZ> 40.0GeV
J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007