brian a. cole columbia university on behalf of the atlas collaboration december 2, 2010
DESCRIPTION
Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions with the ATLAS Detector. Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010. The paper: arXiv:1011.6182. STAR. Paper submitted on Nov 25, accepted by PRL. - PowerPoint PPT PresentationTRANSCRIPT
![Page 1: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/1.jpg)
Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions
with the ATLAS Detector
Brian A. ColeColumbia University
on behalf of the ATLAS Collaboration
December 2, 2010
![Page 2: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/2.jpg)
2
The paper: arXiv:1011.6182
•Paper submitted on Nov 25, accepted by PRL
Indirect jet quenching @ RHIC Direct quenching @ LHC?
STARSTAR
![Page 3: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/3.jpg)
3
The beginning ...
3
J.D. Bjorken, "Energy Loss of Energetic Partons in Quark-Gluon Plasma: Possible extinction of high-pT Jets in Hadron-Hadron Collisions", FERMILAB-PUB-82-059-T.
![Page 4: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/4.jpg)
4
The beginning … (2)
![Page 5: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/5.jpg)
Jet Quenching
•Key question:– How do parton
showers in hot medium (quark gluon plasma) differ from those in vacuum?
From “Jet Quenching in Heavy Ion Collisions”, U. Wiedemann, arXiv:0908.2306
![Page 6: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/6.jpg)
6
“Baseline”: jets in p-p
Leading jet : pT= 670 GeV, η = 1.9, φ = -0.5 Sub-leading jet: pT = 610 GeV, η = -1.6, φ = 2.8
![Page 7: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/7.jpg)
7
Heavy ion collision in ATLAS
![Page 8: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/8.jpg)
8
ATLAS luminosity, data-taking
•ATLAS luminosity profile vs day
Data-taking efficiency > 95%
•Paper used runs corresponding to 1.7 μb-1
(Nov 8 - 17) Fraction of data passing data quality selection
![Page 9: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/9.jpg)
9
Minimum-bias, centrality•Triggers: minimum-bias trigger scintillators, ZDC•Characterize centrality by percentiles of total cross-section using forward calorimeter (FCal) ΣET
(3.2 < |η| < 4.9)
b
![Page 10: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/10.jpg)
10
Measurements over Δη = 9.8
More central
![Page 11: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/11.jpg)
11
Tracking + Calorimetry: Z →e+e-
![Page 12: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/12.jpg)
12
Muons: Z →μ+μ-
![Page 13: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/13.jpg)
Pb+Pb Jet Measurements
in ATLAS
![Page 14: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/14.jpg)
14
A (more) symmetric dijet event
Peripheral, symmetric dijet event
![Page 15: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/15.jpg)
15
An asymmetric event
More central, asymmetric dijet event
![Page 16: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/16.jpg)
16
Another asymmetric event
Even more central collision, more asymmetric dijet
![Page 17: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/17.jpg)
17
Yet another asymmetric event
Central event, with split dijet + additional activity
![Page 18: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/18.jpg)
18
Jet reconstruction (1)
•Perform anti-kt reconstruction prior to any background subtraction– R = 0.4 for main analysis– R = 0.2, 0.6 for cross-check (+ physics)
•Input: Δη x Δφ = 0.1 x 0.1 towers 18
Use anti-kt clustering algorithm
cone-like butinfrared and collinear safe
Cacciari, M., Salam, G. P. and Soyez, G., The anti-kt jet clustering algorithm, Journal of High Energy Physics, 2008, 063
![Page 19: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/19.jpg)
19
Jet reconstruction (2)•Take maximum advantage of ATLAS segmentation– Underlying event estimated and subtracted for each
longitudinal layer and for 100 slices of Δη = 0.1
– ρ is energy density estimated event-by-eventFrom average over 0 < φ < 2π
•Avoid biasing ρ due to jets– Using anti-kt jets:
Exclude cells from ρ if
– Cross check
Sliding Window algorithm
•NO jet removal on basis of D, or any other quantity
0.1x0.1towers
![Page 20: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/20.jpg)
20
Dijet event before & after
•Before subtraction– ΣET in ΔηxΔφ =
0.1x0.1 towers
•After subtraction, underlying event at zero
•Event structure, topology unchanged by subtraction.
![Page 21: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/21.jpg)
21
Dijet analysis•Use R = 0.4 anti-kt jets
– calibrated using energy density cell weighting
•Select events with leading jet, ET1 > 100 GeV, |η| < 2.8 1693 events after cuts in 1.7 μb-1
•Sub-leading: highest E T jet in opposite hemisphere, Δφ > π/2 with ET2 > 25 GeV, |η| < 2.8
5% of selected have no sub-leading jet
•Introduce new variable to quantify dijetimbalance – Not used before in jet quenching literature:
Asymmetry:
•Robust variable:– Residual subtraction errors cancel in numerator
– Absolute jet energy scale errors cancel in ratio.
21
![Page 22: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/22.jpg)
22
Dijets: comparison to p+p, HIJING + PYTHIA
•Pb+Pb di-jet asymmetry (AJ), acoplanarity (Δφ)– Compare to p+p data– And PYTHIA (7 TeV) dijet events embedded in HIJING
No HIJING quenching, flow added in afterburner
•Data agrees with p+p, MC in peripheral Pb+Pb.22
Peripheral40-100%
![Page 23: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/23.jpg)
23
Pb+Pb, 40-100% - Peripheral
![Page 24: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/24.jpg)
24
Pb+Pb 20-40% - semi-central
![Page 25: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/25.jpg)
25
Pb+Pb, 10-20% - more central
![Page 26: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/26.jpg)
26
Pb+Pb, 0-10% - central
![Page 27: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/27.jpg)
27
Full centrality range: paper plots
•For more central collisions, see:– Reduced fraction of jets with small asymmetry– Increased fraction of jets with large asymmetry
For all centralities, Δφ strongly peaked at πPossible small broadening in central collisions
![Page 28: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/28.jpg)
28
Cross checks
•We have carried out a number of cross checks to test for detector or analysis sources of asymmetry.–A partial list to be shown below:
• Problems in calorimeter• Background subtraction• Different jet sizes• Jet shape• Energy loss to muons• Missing ET
• Many more, subset in backup
![Page 29: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/29.jpg)
29
Position Dependence
•Di-jet pairs distributed ~ uniformly throughout the detector.
Does not include |η| < 2.8 cut applied in analysis
Leading Jet (Jet 1) 2nd jet
![Page 30: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/30.jpg)
30
η distributions of leading, sub-leading jets
•η distributions of leading and sub-leading jets– For events with large asymmetry (AJ > 0.4)– And small asymmetry (AJ < 0.4)
Distributions show no dependence on asymmetry
•For matching to p-p and for this analysis, |η| < 2.8 cut applied to leading, sub-leading jets.
![Page 31: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/31.jpg)
31
Asymmetry vs φ
•No indication of position dependence in asymmetry distribution.
31
![Page 32: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/32.jpg)
32
Data-driven check on subtraction
•Evaluate jet edge ET in region 0.2 < r < 0.4– More susceptible to background subtraction errors.
Expect to be worse in central collisions
•No centrality dependence except where the asymmetry effects are largest.
0.2
0.4
r
![Page 33: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/33.jpg)
33
Cross-check: cone size dependence
•Compare results for 3 different jet radii– Asymmetry increases for R = 0.2
Opposite trend from what would be expected if asymmetry were due to background problems
![Page 34: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/34.jpg)
34
Calculate core/total ratio for leading, second jets in
Compare to HIJING + PYTHIA (7 TeV) MC
Jet shapes
•Peripheral events agree with Monte-Carlo– Monte Carlo and data agree well for leading jet– Systematic decrease of sub-leading Ψ(0.2) in more
central collisionsBut: beware, sub-leading jets are softer.
![Page 35: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/35.jpg)
35
Energy, pT flow analysis
•Independent check without jet algorithm and no subtraction– Asymmetry seen in
calorimeter data– And also in tracks
Calorimeter
ET, offset subtracted
Tracks, pT > 4 GeV
![Page 36: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/36.jpg)
36
Check for muons in jet events
•No indication of energetic muons associated with large-asymmetry events.
Muon pT spectrum in events withET > 100 GeV jets
Asymmetry distribution for all events and events with muons of different pT
![Page 37: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/37.jpg)
37
Missing ET : resolution
•Performance of calorimeter in Pb-Pb as characterized by missing ET resolution – Dependence on ΣET as expected – Consistent with extrapolation of p-p
•No anomalous missing ET
ATLAS Preliminary
![Page 38: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/38.jpg)
38
Summary
•We have reported the first observation of large dijet asymmetries in Pb+Pb collisions
Asymmetry increases in more central collisions.Not observed in p-p collisions
•Dijet Δφ distribution remains peaked near Δφ = π for all centralities.
Beginning of an exciting physics program ...
Many thanks to the LHC team!
![Page 39: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/39.jpg)
Backup
![Page 40: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/40.jpg)
40
Cross-check: subtraction method
•Compare asymmetry distribution using cross-check subtraction (black) with default (red)
Same behavior.
![Page 41: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/41.jpg)
41
Inclusive jet spectrum
•Uncorrected jet ET spectrum in minimum-bias Pb+Pb, R = 0.4, anti-kt.
![Page 42: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/42.jpg)
42
Event Quality Check
•No indication of background events in 100 GeV jet sample.
Comparison of MBTS timedifference for min-bias events with good vertex prior to ± 3 ns cut with events containing 100 GeV jet
Comparison of vertex z distribution between min-bias events and events containing 100 GeV jet
![Page 43: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/43.jpg)
43
Subtraction bias on asymmetry?
•Plot the mean energy subtracted from leading jet as a function of asymmetry in different centrality bins
No subtraction bias on asymmetry
![Page 44: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/44.jpg)
44
Subtraction bias on asymmetry (2)?
•Plot the mean energy subtracted from sub-leading jet as a function of asymmetry in different centrality bins
No subtraction bias on asymmetry
![Page 45: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/45.jpg)
45
MC - data matching: UE fluctuations (2)
45
![Page 46: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/46.jpg)
46
Track jet results
•Track jets are reconstructed by applying the
anti-kT algorithm to Inner Detector tracks
•Track selection– Underlying event suppression: pT > 4 GeV
– Tracking efficiency and purity: |d0| < 0.2 mm |z0sinθ| < 0.3 mm2 pixel detector hits, 8 SCT hits
•Produce 4-vectors using pion mass for tracks•Apply anti-kt algorithm with R = 0.4•No underlying event subtraction:
suppressed by pT cut
![Page 47: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/47.jpg)
47
Independent check on calo jet ET scale
•Perform separate track jet reconstruction– R = 0.4, pT > 4 GeV
With this cut, no underlying event– Compare track jet and calorimeter jet pT scales
For different centralities and p-p
•Good agreement between centralities and p-p
![Page 48: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/48.jpg)
48
Radius Comparison, central 0-20%
•AJ increases R = 0.2, decreases for R = 0.6– Opposite trend from what would be expected if
asymmetry were due to background problems
•Same conclusion as with min-bias.
![Page 49: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/49.jpg)
49
Check Jet <ET> Vs Centrality
•<ET> of “leading” jets stable vs centrality– Partly a selection bias.
•<ET> of second jet decreases in central collisions Important consistency check
Simply calculate <ET> for leading, second jets in skimmed events
No, leading, second correlation
Compare to HIJING + PYTHIA MC
49
![Page 50: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/50.jpg)
50
Three iconic results from RHIC
Single hadron suppression,no photon suppression*
Single electron suppression(c, b semi-leptonic decays)
Di-jet “disappearance” via 2-hadron correlations.
![Page 51: Brian A. Cole Columbia University on behalf of the ATLAS Collaboration December 2, 2010](https://reader035.vdocuments.us/reader035/viewer/2022062803/56814685550346895db3a570/html5/thumbnails/51.jpg)
51
Jet Reconstruction Efficiency
•Anti-kt R = 0.4, jet reconstruction efficiency– truth match ΔR < 0.2