Jet Measurements with Neutral and Di-jet Triggers in Central Au+Au Collisions at \sqrt{{s}_{NN}} = 200 GeV with STAR
NNuclear Physics A 00 (2018) 1–4
NuclearPhysics A / locate / procedia Jet Measurements with Neutral and Di-jet Triggers in CentralAu + Au Collisions at √ s N N =
200 GeV with STAR
Nihar Ranjan Sahoo (for the STAR Collaboration)
Cyclotron Institute, Texas A & M University, USAEmail id: [email protected]
Abstract
We present two measurements related to jet production in p + p and central Au + Au collisions at √ s NN =
200 GeV.Firstly, a study of semi-inclusive charged recoil jets coincident with high-p T direct-photon and neutral pions, and sec-ondly, the hadron correlations with respect to reconstructed di-jet triggers. Indication of medium e ff ects is observed bycomparing γ + jet and π + jet measurements. The di-jet + hadron study shows signs that the medium-induced modifica-tions of an imbalanced set of di-jets with “hard cores” primarily a ff ect the recoil jet. Keywords: direct-photon, jets, di-jet, correlations, Quark Gluon Plasma
1. Introduction
Jets and their modifications due to partonic energy loss provide a powerful tool to study the propertiesof the Quark Gluon Plasm (QGP) created in ultra-relativistic heavy-ion collisions. In order to study partonenergy loss, we exploit the fact that di ff erent measurements have di ff erent trigger biases and explore thee ff ect of the medium on jets for di ff erent types of triggers: i) γ dir vs. π and ii) di-jet triggers. The γ dir triggersdo not su ff er from a surface bias and carry approximately the initial energy of the recoil parton [1, 2]; whilecomparing γ dir and π triggers o ff er the additional benefit of probing the color-factor dependence of therecoil parton energy loss.Recent A J measurements at STAR [3] found a sample of di-jets selected with “hard cores”, i.e. onlyusing constituents with p T > / c, that was imbalanced in central Au + Au collisions compared to a p + preference. This lost energy was recovered in soft constituents (p T > / c) with signs of broadening ofthe jet structure within a jet radius of R = .
4. Charged hadron correlations with these di-jets enable us toinvestigate the redistribution of energy within the medium in more detail.The STAR detector system provides full 2 π azimuthal coverage and pseudorapidity range within | η | < .
0. The Time Projection Chamber (TPC) is used as the charged-particle tracking device [4]. The BarrelElectromagnetic Calorimeter (BEMC) [5] is used for triggering and measuring the neutral trigger (photonor π ) and the neutral energy of a jet. Events were selected by an online high tower (HT) trigger, whichrequired an uncorrected transverse tower energy of E T > a r X i v : . [ nu c l - e x ] A p r N. R. Sahoo for the STAR Collaboration / Nuclear Physics A 00 (2018) 1–4 - ) [ G e V / c ] j e t η d r e c o , c h T , j e t / ( dp j e t s d N t r i g / N − − − − − Au+Au 200 GeV+jet π ,R= 0.3 T anti-k < 30 GeV/c trigT SEMEnorm. ME
STAR Preliminary [GeV/c] reco,chT,jet p20 − − SE / M E [GeV/c] chT,jet p0 5 10 15 20 25 30 35 - ) [ G e V / c ] j e t η d c h T , j e t / ( dp j e t s N d t r i g / N − − − − − − Au+Au 200 GeV,R= 0.3 T anti-k < 30 GeV/c trigT ± h +Jet π STAR Preliminary
Fig. 1. Left panel: The p reco , chT , jet distribution of π + jet in 0%-10% central Au + Au collisions at √ s NN =
200 GeV. SE (star marker),ME (black shaded region) and norm. ME (blue shaded region) represent same event, mixed event, and the normalization region,respectively. Right panel: The corrected p chT , jet distribution of both π + jet and h ± + jet [8] in 0%-10% central Au + Au collisions at √ s NN =
200 GeV. The shaded bands represent the systematic uncertainty. [GeV/c] chT,jet p + J e t γ + J e t π − ] dir γ / π PYTHIA[ , p+p purity 40%] rich γ / π PYTHIA[ , Au+Au purity 70%] rich γ / π PYTHIA[ [At particle level] - ) [ G e V / c ] j e t η d r e c o , c h T , j e t / ( dp j e t s d N t r i g / N − − − − − Au+Au 200 GeV,R= 0.3 T anti-k < 30 GeV/c trigT +Jet rich γ +Jet π STAR Preliminary [GeV/c] reco,chT,jet p20 − − + J e t r i c h γ + J e t π − Fig. 2. Left panel: Ratio of recoil yields for π + jet and γ + jet as a function of p chT , jet at particle level in the PYTHIA simulation. Solid,dotted and broken lines represent 100%, 70% (in Au + Au [7]) and 40% (in p + p [7]) purity level in direct-photon sample in the PYTHIAsimulation, respectively. Right panel: The p reco , chT , jet distributions of π + jet (star marker) and γ rich + jet (circle marker) in 0%-10% centralAu + Au collisions at √ s NN =
200 GeV.
2. Semi-inclusive neutral-trigger recoil jets
The charged recoil jets are reconstructed using the anti- k T algorithm from the FastJet package [6] witha jet resolution parameter of R = .
3, using charged tracks with p T > / c. The charged recoil jetsare selected within the azimuthal angle between the neutral trigger and recoil jets satisfying ∆ φ ∈ [ π , π ].The direct-photon hadron correlation analysis [7] provides a technique to trigger on direct photons using thetransverse shower profile (TSP) method which helps to discriminate between γ rich (enriched sample of directphoton) and π . The uncorrelated background jets are subtracted using a mixed-event (ME) method [8].The distribution of reconstructed charged recoil jet transverse momentum (p reco , chT , jet ) of π trigger is shownon the left side of Fig. 1. The raw reconstructed jet energy for each jet candidate i is corrected for theestimated average background energy contained within the jet. Here, p reco , iT , jet = p raw , iT , jet − ρ · A ijet , where p raw , iT , jet is the p T of i th jet candidate measured by the jet reconstruction algorithm, A ijet is the area of the jet and thebackground energy density, ρ ≡ median[ p raw , iT , jet A ijet ] (using the k T algorithm). At high jet p T , the correlated recoilsignal dominates the background. The integral of the ME jet p T distribution is normalised within the purelycombinatorial region with p reco , chT , jet < T distribution is . R. Sahoo for the STAR Collaboration / Nuclear Physics A 00 (2018) 1–4 η ∆ − − φ ∆ − c oun t s STAR Preliminary < 2.0 GeV/c assocT
Fig. 3. Left: 2D correlations after mixed event correction. Middle: Trigger jet yields in ∆ η for Au + Au (circle) and p + p (box). Right:Recoil jet yields in ∆ φ for Au + Au (circle) and p + p (box). The bands represent the systematic uncertainty. obtained by subtracting the normalised ME from SE. This subtracted jet p T distribution is then correctedby an unfolding procedure (adopting the singular value decomposition method [9]) for detector e ffi ciencyand p T -smearing due to background e ff ects. The right panel of Fig. 1 shows a comparison between thereconstructed charged recoil jet spectra triggered by π and h ± [8] after corrections. The spectra showagreement within systematic uncertainty. Additional measurements are ongoing to reduce the systematicand statistical uncertainty.A comparison between π + jet and γ + jet distributions is performed at the particle level PYTHIA [10]simulation as shown in Fig. 2. In this simulation, the purities of direct photons and π are varied from theideal to estimate the influence of impurity. When assuming 100% direct-photon and π purity, it is foundthat the ratio of the jet yield recoiling from a π compared to a direct-photon increases as a function of p chT , jet (above p chT , jet >
10 GeV / c): a trigger π carries on an average of 85% of total energy of its parent parton [7]in contrast to the direct-photon trigger. In our data, we estimate the direct-photon purity to be 70% [7] inAu + Au collisions (40% in p + p collisions) and hence we denote the trigger sample as “ γ rich ”. For lower γ dir purities, the enhancement is reduced (due to the contamination in the population) but still present. Themeasured yield ratio, however, is consistent with unity with a large statistical uncertainty in 0%-10% centralAu + Au collisions as shown on the right side of Fig. 2. This di ff erence could be due to the medium e ff ect;larger statistics and complete corrections, such as detector e ffi ciency and background fluctuations e ff ects,are needed to draw a strong conclusion.
3. Di-jet + hadron correlations The di-jet selection for this analysis is similar to that done in the STAR A J analysis [3], requiring apair of back-to-back “hard core” di-jets such that p leadT > . / c and p subleadT > . / c . Theback-to-back jets are reconstructed with a hard constituent cut of p constT > . / c and R = . k T algorithm from the FastJet package [6]. Events were selected to have an online calorimeterdeposition of E T > E T > + Au andp + p, the lower tracking e ffi ciency in central Au + Au events must be taken into account [3]. The p + p jetenergy scale was corrected by randomly discarding charged tracks before jet finding with a probability of 1- ( (cid:15) Au + Au ( η, p T ) /(cid:15) p + p ( η, p T )), where (cid:15) is the measured TPC e ffi ciency for that collision system. To accountfor background fluctuations, the p + p data are embedded into minimum bias, 0%-20% central Au + Au eventsfor jet-finding.Correlations are measured for both trigger and recoil jets with all charged tracks, in both ∆ η = η jet − η const and ∆ φ = φ jet − φ const as a function of p constT . Single particle tracking e ffi ciency was corrected to particlelevel using (cid:15) Au + Au ( η, p T ) and (cid:15) p + p ( η, p T ) in central Au + Au and p + p collisions, respectively. To correct pair-acceptance e ff ects, the mixed event technique is used. The raw signal correlations are divided by the mixedevent distribution in p constT bins; an example can be seen in the left panel of Fig. 3. The distribution is flat outto large ∆ η on the near side, has a strong near-side peak around the trigger jet axis, and an away side ridgearound ∆ φ ≈ π .Yields are calculated in ∆ η and ∆ φ by subtracting the background and integrating over bins. ∆ η back-ground subtraction is performed by fitting with a gaussian + constant and subtracting the constant. ∆ φ N. R. Sahoo for the STAR Collaboration / Nuclear Physics A 00 (2018) 1–4 background subtraction must account for long range ∆ η -independent correlations between the jet axis andthe underlying event. It is assumed that any flow ( v n ) correlations are independent of ∆ η . The side band sub-traction method is used, where the region 0 . < | ∆ η | < .
0, properly scaled, is used to subtract backgroundand any potential correlation with the underlying event from the signal region defined by | ∆ η | < . ∆ φ and ∆ η projections are consistent within uncertainty. The trigger jet yield in ∆ η shows no statistically significant modification at any p constT (middle panel of Fig. 3), implying a strongsurface bias, as expected from requiring a high energy trigger hadron. On the other hand, recoil jets showhint of increased correlated yield with respect to p + p at low p constT . Since the initial jet-finding is donewith p constT > . / c , it is by construction that modification is observed mainly below the 2.0 GeV / c threshold. This is also consistent with the interpretation of the A J results [3], where balance was restoredwhen constituents below 2.0 GeV / c were included. It is observed in the A J measurement that a large partof the di-jet population in Au + Au is balanced (p + p-like). As the correlations integrate over all ∆ A J = A hard J − A full J , this can lead to a reduction in the yield di ff erences between Au + Au and p + p.
4. Summary and outlook
Comparison between π + jet and h ± + jet distributions shows agreement within the systematic uncertain-ties. Larger statistics and final corrections are needed to draw strong conclusions about the potential mediume ff ects when comparing γ + jet and π + jet measurements in Au + Au collisions at RHIC. The di-jet + hadroncorrelations study supports the picture that the selected di-jet sample with hard cores is mostly tangentialto the fireball, and that while the signal is diluted by unmodified jets, modification primarily a ff ects therecoil jet. An increase in statistics from newer data sets (from Au + Au collisions measured by STAR inyears 2014 and 2016) will allow for precise measurements of jets recoiling o ff a high-p T neutral particle,as well as enable di ff erential studies of di-jet imbalance and potential path length control via “jet geometryengineering”. Acknowledgments
This work was supported by the US DOE under the grant DE-FG02-07ER41485 and DE-FG02-92ER-40713.
References [1] X.-N. Wang, Z. Huang, and I. Sarcevic, Phys. Rev. Lett. , 231 (1996).[2] X. -N. Wang and Y. Zhu, Phys. Rev. Lett. , 062301 (2013).[3] L. Adamczyk et al. [STAR Collaboration], arXiv:1609.03878 [nucl-ex].[4] M. Anderson et al. , Nucl. Instrum. Meth. A 499 , 659 (2003).[5] M. Beddo et al. , Nucl. Instrum. Meth.