1. Jet Correlations in ATLAS
RHIC Correlations and High-pT Measurements with ATLAS
Jet Correlations in ATLAS
Nathan Grau
Columbia University, Nevis Labs
High-pT Physics at the LHC
3/25/2007

2. High-pT Physics at the LHC
Outline
A very brief review of some key RHIC data on high-pT (> 5 GeV trigger, mostly) correlations
Questions rather than answers
Move on to possibly finding answers:
Need event-by-event data, not integral/average quantities
i.e. LHC jet studies at ATLAS
High-pT Physics at the LHC
3/25/2007

3. Jet Quenching at RHIC: Singles
Dramatic quenching relative to binary-scaled p+p is observed
How sensitive is RAA to the details of energy loss?
T. Renk
Hard Probes 2006
High-pT Physics at the LHC
3/25/2007

4. Jet Quenching at RHIC: Pairs
Why are high-pT two-particle away-side correlation yields suppressed but their shape is not broadened?
8 < pT(trig) < 15 GeV/c
8 < pT(trig) < 15 GeV/c
High-pT Physics at the LHC
3/25/2007

5. High-pT Physics at the LHC
IAA vs RAA
J. Jia, QM 2006
Cu+Cu
D. Magestro, QM 2005
Why is IAA ~ RAA?
High-pT Physics at the LHC
3/25/2007

6. IAA vs RAA : An Interesting Implication
Is this “factorized” e-loss?
RAA dominated by geometry, so is IAA?
Pair suppression
High-pT Physics at the LHC
3/25/2007

7. High-pT Physics at the LHC
IAA vs RAA : Is It True?
Chun Zhang, QM 2006
h-h Correlations from Au+Au 2-3 GeV/c trigger
DAA
00-20% : RAAtrig=0.60, RAAassoc=0.30
20-40% : RAAtrig=0.75, RAAassoc=0.45
60-92% : RAAtrig=0.90, RAAassoc=0.80
High-pT Physics at the LHC
3/25/2007

8. Jet Medium Interactions: The Ridge in STAR
J. Putschke, QM 2006
And it’s always there…
pt,assoc. > 2 GeV
STAR has measured a very
pronounced ridge structure
in Au+Au
High-pT Physics at the LHC
3/25/2007

9. Jet Medium Interactions: The Ridge in PHENIX
Chun Zhang, QM 2006
h-h Correlations from 0-20% Au+Au
Near Side
Seems at higher associated pT ridge even goes away?
Where is the ridge the 4-5 GeV/c bin?
High-pT Physics at the LHC
3/25/2007

10. Jet Medium Interactions: Near-side Energy Loss
H Pei, QM2006
~1/Ntrigg dN/djT jT
Increased yield in radiative tail of jT distribution measured in Cu+Cu
Not the ridge since this is hard (perturbatively calculable) radiation
Near side jet loses energy: not surface emission.
High-pT Physics at the LHC
3/25/2007

11. Jet Medium Interactions: The Mach Cone
J. Jia, QM % Au+Au x h-h
I think PHENIX has their v2
systematics under control.
Is this just momentum conservation?
Know v2 depending on trigger’s RP orientation.
J. Bielcikova et al, Phys. Rev. C69:021901, 2004
High-pT Physics at the LHC
3/25/2007

12. Jet Medium Interactions: Cone or Bent Jet
M. Horner, QM 2006
Two-particle can possibly rule out a bent-jet scenario: seems like dip is filled in by the jet.
But this is not event-by-event information
High-pT Physics at the LHC
3/25/2007

13. pT-Dependence of Jet Modification
How do di-hadrons vary with pT:
Conical  extinction  punch-through?
NG QM2006, p0-h in Au+Au
High-pT Physics at the LHC
3/25/2007

14. Summary of the Introduction
Discovery of jet quenching occurred at RHIC!
But that was in
What more can we say since then about
Medium properties?
Energy-dependence of energy loss?
Path-length dependence of energy loss?
To borrow a phrase from Mike: “We are still on a long learning curve.”
What is missing: A clear event view of a ridge/mach cone/broadened jet etc. a la Paris 1982
Where can we do this: at the LHC with ATLAS
High-pT Physics at the LHC
(and CMS and ALICE)
3/25/2007

15. High-pT Physics at the LHC
The ATLAS Detector
High-pT Physics at the LHC
3/25/2007

16. High-pT Physics at the LHC
The ATLAS Detector
Full azimuthal acceptance in all detectors
Unprecedented pseudorapidity coverage for A+A
High-pT Physics at the LHC
3/25/2007

17. Tracking with the Inner Detector
Inner detector has full azimuthal coverage within |h|<2.5 and consists of
Pixel detector
Silicon tracking detector
Transition radiation tracker
Results from p+p tracking algorithm optimized for HI environment.
Reconstructed tracks with |h|<1
Tracking down to 500 MeV.
Important for D(z) and v2.
High-pT Physics at the LHC
3/25/2007

18. High-pT Physics at the LHC
ATLAS Calorimetry
Hadronic Barrel
EM Barrel
Forward
EM EndCap
Hadronic EndCap
High-pT Physics at the LHC
3/25/2007

19. Longitudinal Segmentation: 3-d Jets
Sampling of a 100 GeV jet (no background) 1 2 3 4 5 6
Note the h-f
region is 0.8x0.8:
a typical jet size
Sampling aides in
energy resolution.
Important for EM
vs. hadronic
energy separation.
High-pT Physics at the LHC
3/25/2007

20. Approach to Cone Jet Reconstruction
Original cells
Cloned cells
Different background
Techniques are possible:
E-by-E cell level
E-by-E tower level
E-avg cell level
E-avg tower level
Iteration also possible.
Original towers
Background
subtraction
(exclude seeds)
Subtracted cells
Cells: single readout channel
Tower: 0.1x0.1 h-f sums of constituent cells
Need to understand the fluctuations in the different methods
Inherent limitations on jet measurements.
New towers
Use standard ATLAS code, except for
the background subtraction.
Reconstructed jets
High-pT Physics at the LHC
3/25/2007

21. Hard Radiation in p+p Collisions
Vacuum radiation is important
Changes “typical” R-cut in cone and kT algorithms, i.e. R<0.7
Splitting/Merging of proto-jets is important in p+p
Simulated pythia event with
hard scattering from GeV
High-pT Physics at the LHC
3/25/2007

22. Radiation From E-Loss: More Subjets
The distribution of radiated gluons related to
Hard radiation in the tail subjets
Requires a precise measure of the jet direction
High-pT Physics at the LHC
3/25/2007

23. Cone Jet Position ResolutionsDf Dh
GeV
35-70 GeV
GeV
Very good position resolution of jets
Position resolution not dominant error on jT
5% uncertainty in the angular measurement, pT is about 2%
High-pT Physics at the LHC
3/25/2007

24. High-pT Physics at the LHC
Energy Resolution
Energy Resolution of jets necessary for
Jet spectrum  show jet cross section binary scales
Fragmentation functions since error in z is dominated by error on jet ET
This is using the most brute force cell background subtraction
Improvements on the method are being made
Calibrations due to subtraction have not been applied. h
High-pT Physics at the LHC
3/25/2007

25. Utilizing the kT Algorithm
See W. Holzmann’s talk for all of the details
Cacciari and Salam: Fast kT
Not a new kT algorithm, just improved the algorithms speed
Fast enough to run on HI events before subtracting the background event!
Use background jets to discriminate real jets: i.e. their properties are different
High-pT Physics at the LHC
3/25/2007

26. “Fast” kT Finder: Discriminating Jets and Background
Real Jets appear
as narrow towers
“Fake” Jets
appear flat and broad
Each color is a different “jet” defined by the algorithm
“Real” jets and “Background” jets look different.
High-pT Physics at the LHC
3/25/2007

27. Advantages of kT Algorithm
Not a fixed cone size
Easier to handle the hard radiation from jets
No seeds are required
All the energy in a high multiplicity (central) heavy ion event results in a jet
Changed the problem of subtracting a background to discrimination of clustered energy
Could suffer less from background fluctuations
High-pT Physics at the LHC
3/25/2007

28. The More Perfect Way: g-jet
EM Layer 1 ET (GeV) 
Single f slice 0.1 rad
g-jet event
embedded
Jet
Back
ground
All too wide for single photons
Single f slice 0.1 rad
Jet
Background
Single f slice 0.1 rad
Barrel EMCal front layer finely segmented in h for vectoring H  gg events and p0 rejection.
Example of jet embedded in central b=2 fm HIJING event.
High-pT Physics at the LHC
3/25/2007

29. Separation from p0 Decay
Fractional energy outside core
Energy of a 2nd maximum
Cuts to discriminate g/p0 before isolation
Inverting these cuts results in p0 measurement
High-pT Physics at the LHC
3/25/2007

30. High-pT Physics at the LHC
g/p0 Separation
Rejection of p0 with appropriate cuts on previous variables
Efficiency in p+p is ~90%, flat with ET and h
Some rejection prior to isolation from jet!
High-pT Physics at the LHC
3/25/2007

31. High-pT Physics at the LHC
Summary & Conclusion
RHIC high-pT discoveries and measurements are interesting and the I don’t understand all of the measurements at the moment.
ATLAS will provide useful and necessary measurements complimentary to the RHIC program and extend these results and, ideally, help to answer the questions I have.
Single jet, di-jet, and g-jet
Unfortunately I could not cover all aspects of ATLAS high-pT program: heavy flavor bound states, heavy flavor energy loss, v2, etc.
Let’s have more than 1 ATLAS and 1 CMS talk next time?
High-pT Physics at the LHC
3/25/2007

32. ATLAS Heavy Ion Working Group
A. Ajitanand10, A. Angerami3, G. Atoian11, M. Baker1, P. Chung10, B. Cole3, R. Debbe1, A. Denisov5, J. Dolejsi2, N. Grau3, J. Hill7, W. Holzmann3, V. Issakov11, J. Jia10, H. Kasper11, R. Lacey10, A. Lebedev7, M. Leltchouk3, A. Moraes1, R. Nouicer1, A. Olszewski6, A. Poblaguev11, V. Pozdnyakov8, M. Rosati7, L. Rosselet4, M. Spousta2, P. Steinberg1, H. Takai1, S. Timoshenko9, B. Toczek6, A. Trzupek6, F. Videbaek1, S. White1, B. Wosiek6, K. Wozniak6, M. Zeller11
1 Brookhaven National Laboratory, USA
2 Charles University, Prague
3 Columbia Unversity, Nevis Laboratories, USA
4 University of Geneva, Switzerland
5 IHEP, Russia
6 IFJ PAN, Krakow, Poland
7 Iowa State University, USA
8 JINR, Dubna, Russia
9 MePHI, Moscow, Russia
10 Chemistry Department, Stony Brook University, USA
11 Yale University, USA
High-pT Physics at the LHC
3/25/2007

33. High-pT Physics at the LHC
Backup Slides
High-pT Physics at the LHC
3/25/2007

34. IAA from xE distributions in Cu+Cu
High-pT Physics at the LHC
3/25/2007

35. IAA from xE distributions in Cu+Cu
High-pT Physics at the LHC
3/25/2007

36. High-pT Physics at the LHC
Tracking to Lower pT
Minimum bias p+p
Preliminary
Work extending pT reach important for p+p and A+A.
dN/dh in both cases
v2 in A+A
Ongoing with high energy and heavy ion participation
Efficiency: red/black
Fake rate: red/green
High-pT Physics at the LHC
3/25/2007