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1 Nihar Sahoo, Texas A&M University
Recent Jet Measurements in Heavy-ion Collisions at STAR
in a Nutshell
Nihar Sahoo Texas A&M University
2017 RHIC & AGS Annual Users’ Meeting
• Jet-like correlation
• γ+hadron and π0+hadron [PLB 760 (2016) 689]
• Semi-inclusive recoil charged jet • h±+jet [arXiv:1702.01108] • γ+jet and π0+jet [ongoing work: preliminary results:QM2017]
• Di-jet • AJ measurement (momentum imbalance) [accepted in PRL, arXiv:1609.03878] • Di-jet+hadron correlations [preliminary results:QM2017] • Di-jet zg (Splitting function) [ongoing work: preliminary results:QM2017 and
HP2016]
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng
2017 2
Recent jet measurements in Heavy-Ion collisions
• Jet-like correlation • γ+hadron and π0+hadron [PLB 760 (2016) 689]
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng 2017 3
4
γ-hadrons and π0-hadrons correlations
• On ave. γ-‐triggered parton (jet) loses less energy than that of π0-‐trigger • due to path length (surface biased of π0-‐trigger)
• γ-‐triggered parton (jet) loses less energy than π0-‐trigger recoil
• due to color factor ( CA/CF = 9/4) (dominant γ producDon: qg -‐>qγ)
Hadron (π0) Ideal jet/ Direct-‐photon
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng 2017
YaJEM: T. Renk arXiv:1212.0646
STAR: PLB 760 (2016) 689
Jet-like correlation: γ+hadron and π0+hadron
• Large suppression at high zT and pTassoc
• Within large uncertainty, no clear signature of color factor and path length dependence of energy loss
• Precision measurements, like γ+jet and π0+jet, are needed for better understanding (different method and also higher statistics)
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng 2017 5
IAA(x) =Y
Au+Au(x)
Y
p+p(x)
=pTassoc/pTtrig
W. Chen, et al. arxiv:1704.03648 Coupled linear Boltzmann Transport and 3+1D hydro (CoLBT-hydro)
Jet-like correlation: γ+hadron
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng 2017 6
• enhancement of so] hadrons due to jet-‐induced medium excitaDon
• medium enhances producDon of so] parDcles in parton fragmentaDon, relaDve to pp, preferenDally at large angles
Wei Chen, QM2017 talk
Similar effect from low to high pTγ ?
ξ
ξ
Hard so] Hard so]
pTγ: 5-‐7 GeV/c
pTγ: 12-‐20 GeV/c
pTγ: 9-‐12 GeV/c
pTγ: 7-‐9 GeV/c
• Semi-inclusive recoil charged jet • h±+jet [arXiv:1702.01108]
• γ+jet and π0+jet [ongoing work: preliminary results:QM2017]
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng 2017 7
Trigger particle
Recoil jet(s) search area
Courtesy of Alex Schmah
1
Nhtrig
dNjet
dpT,jet=
1
�pp!h+X
d�pp!h+jet+X
dpT,jet
Semi-inclusive yield of jets* recoiling from a high pT hadron trigger
Measured Calculable in fixed-‐order pQCD
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng 2017 8
Semi-‐inclusive hadron+jet
• Use all jet candidates on the other azimuthal hemisphere within +/- 45 degree • no fragmentation bias on recoil side
• Mixed event correction for uncorrelated background jets in HIC • allow to reach low jet pT
[arXiv:1702.01108]
• Energy transported to larger jet R è interaction of jet with the medium • Medium-induced energy transport to large anglesèlesser at RHIC than LHC
Note: PYTHIA p+p in ALICE vs. Peripheral Au+Au in STAR
ALICE: JHEP09(2015)170
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng 2017 9
h+Jet yield suppression [h+jet: arXiv:1702.01108]
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Lin Chen, et. al, arXiv:1607.01932
Transverse Momentum Broadening in HIC
• For a quark jet: the amount of medium-induced pT broadening describes the Au+Au central collisions from RHIC data.(Using global χ2 analysis) • Higher statistics and low beam energy (e.g, 62.4 GeV) helpful for
better insight of pT broadening
< p2? >⇠ 13 GeV2
STAR: h+jet, π0+hadron PHENIX: charge dihadrons
Vacuum so] gluon radiaDon + medium induced pT broadening
π0+jet vs. h±+jet
• Applying correction due to detector and background fluctuations effects
• Singular value decomposition (SVD) method for unfolding
• Taking into account systematic
effects, π0 +jet and h±+jet show agreement within uncertainties
• Any new information ? • Icp, IAA, • Different hadron trigger
What about comparison between γ+jet and π0 +jet ?
(h±+jet: arXiv:1702.01108)
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng 2017 11
(Ongoing analysis)
N. Sahoo[QM2017 talk]
π – π/4
π + π/4
π0/ γ
Trigger Tower
Recoil Jet
• Charged jet reconstruction (using FastJet3.0.6)
• kt algorithm for background subtraction • Anti-kt algorithm for jet reconstruction
• π0/ϒ discrimination in heavy-ion experiment • STAR barrel electromagnetic calorimeter
(BEMC) and shower maximum detector (BSMD)
• Transverse shower profile (TSP) method
�� 2 [3⇡
4,5⇡
4]
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng 2017 12
Semi-‐inclusive γ+jet and π0+jet
13
Purity of γdir : p+p ~40% Au+Au (0-10%) ~70%
• PYTHIA predicts larger jet yield for π0 trigger than γdir (π0 carries ~85% energy of near side parton, STAR:PLB 760 (2016) 689)
• In p+p, reasonable agreement with standalone PYTHIA considering purity of γdir
p+p
[STAR:PLB 760 (2016) 689]
Raw Jet pT without background and detector effect correction
π0 +jet vs. γ+jet
STAR preliminary
N. Sahoo[QM2017 talk]
[GeV/c]chT,jet
p5 10 15 20 25 30
+Jet
γ+J
et0 π
1−10
1
10
]dir
γ/0πPYTHIA[, Au+Au purity 70%]
richγ/0πPYTHIA[
, p+p purity 40%]rich
γ/0πPYTHIA[
[At particle level]
p+p
Au+Au
(Ongoing analysis)
PYTHIA expectaLon(for different purity of γdir)
Nihar Sahoo, RHIC&AGS Users' meeDng 2017
14
π0 +jet vs. γ+jet
• Medium effect ? (Color factor and/or path length effect) • To extract medium effect for π0 +jet vs. γ+jet, need full corrections,
detailed study and large statistics
Au+Au
[GeV/c]chT,jet
p5 10 15 20 25 30
+Jet
γ+J
et0 π
1−10
1
10
]dir
γ/0πPYTHIA[, Au+Au purity 70%]
richγ/0πPYTHIA[
, p+p purity 40%]rich
γ/0πPYTHIA[
[At particle level]
STAR preliminary
(Ongoing analysis)
N. Sahoo[QM2017 talk]
p+p Reference
Raw Jet pT without background and detector effect correction
Purity of γdir : p+p ~40% Au+Au (0-10%) ~70%
PYTHIA expectaLon(for different purity of γdir)
[STAR:PLB 760 (2016) 689]
Nihar Sahoo, RHIC&AGS Users' meeDng 2017
p+p
Au+Au
• Di-jet • AJ measurements [accepted in PRL, arXiv:1609.03878]
• Di-jet+hadron correlation [preliminary results:QM2017]
• Di-jet zg (Groomed Momentum Sharing zg)
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng 2017 15
R = 0.4
• Stronger di-jet Imbalance in AuAu for pTCut > 2 GeV for hard core selection
• But pp balance recovered for pTCut > 0.2 GeV for R=0.4 (Not for R=0.2!)
Di-jet imbalance
Charged + neutral constituents “Hard Core:” High Tower (HT) trigger ET>5.4 GeV pTCut = 2 GeV/c
How do we understand the redistribuDon of energy within each jets ?
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng 2017 16
Accepted by PRL arXiv:1609.03878
φ∆0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8
φ∆
dN/d
Dije
ts1/
N
1−
0
1
2
3
4
5AuAu HT 0-20%p+p HTtracking unc. Au+Autracking unc. p+pJES unc.
< 2.0assocT
1.0 < p|<0.45η∆|
recoil jet
STAR Preliminary
φ∆0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8
φ∆
dN/d
Dije
ts1/
N
1−
0
1
2
3
4
5AuAu HT 0-20%p+p HTtracking unc. Au+Autracking unc. p+pJES unc.
< 2.0assocT
1.0 < p|<0.45η∆|
trigger jet
STAR Preliminary
Trigger jet
Recoil jet
Trigger jet
Recoil jet
Updated Hard Core selection: pTCut = 2 GeV/c and Jet pTLead>20 GeV/c Jet pTSubLead>10 GeV/c Trigger Jet contains HT with ET>6 GeV
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng 2017 17
Dijet+hadron correlation
Similar observaDon in Δη correlaDon.
Δφ = φtrig – φhadron Δφ = φrecoil – φhadron Nick Elsey QM2017 poster
• Trigger jet +hadron: no significant difference (Surface biased) • Recoil jet +hadron: hints of excess yields at low hadron pT < 2 GeV/c • Expecting excess yields in Au+Au than p+p, but hints of balanced dijets
( because no AJ cut is used)
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng 2017 18
Updated Hard Core selection: pTCut = 2 GeV/c and Jet pTLead>20 GeV/c Jet pTSubLead>10 GeV/c Trigger Jet contains HT with ET>6 GeV
Jet yields in Au+Au and p+p collisions Trigger jet
Recoil jet
Trigger jet
Recoil jet
N. Sahoo [QM2017 talk] Nick Elsey QM2017 poster
Associated hadron pT
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng 2017 19
Jet grooming Removes so] divergences and uncorrelated backgrounds in a jet
Marta Verweij QM2017 talk
Shared momentum fraction (zg)
No flavor and (weak) jet pT dependence
Soft drop condition z > zcut θβ
Momentum fraction carried by the subleading branch
Di-jet zg in Au+Au and p+p
• No significant splitting modification on near- or away-side (with hard-core jet selection but AJ is modified!)
• Contrast to LHC (Note:No dijet selection)
gz0.1 0.2 0.3 0.4 0.5 0.6
Au+
Au
/ p+p
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2 = 10-20 GeV/cRecoil
TRecoil Jet, p
Au+Au MB⊕Au+Au HT / p+p HT
STAR Preliminary
gz0.1 0.2 0.3 0.4 0.5 0.6
Au+
Au
/ p+p
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2 = 20-30 GeV/cTrig
TTrigger Jet, p
Au+Au MB⊕Au+Au HT / p+p HT
STAR Preliminary
Trigger Recoil
20
K. Kauder[QM2017 talk]
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng 2017
CMS preliminary
20/06/17 Nihar Sahoo, RHIC&AGS Users' meeDng 2017 21
What we learned so far ?
• Jet-like correlation (γ+hadron and π0+hadron) • Precision measurements to understand color factor and path
length effect • CoLBT-Hydro: jet-induced medium excitation and soft particle
emission at large opening angle • Extracting of medium induced pT broadening
• Semi-inclusive recoil charged jet (h±+jet /γ+jet ) • Energy transported to larger jet cone R due to interaction of jet
with the medium • Lesser medium-induced energy transport to large angles at RHIC
than LHC (Lower RHIC energy, i.e. 62.4 GeV, may give more info) • π0+jet vs. γ+jet: hints of medium effect ? Ongoing work…
• Di-jet • “Lost” energy contained within R=0.4 • Splitting via zg not modified Stay tuned….
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Back Up
23 Nihar Sahoo, RHIC&AGS Users' meeDng 2017 20/06/17
Au+Au p+p Raw Jet pT without background and detector effect correction
π0 +jet vs. γ+jet
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