In-Medium Jet Modification Measured by PHENIX Via two-particle Correlations and High p T Hadrons in A+A Collisions
NNuclear Physics A 00 (2020) 1–5
NuclearPhysics A / locate / procedia XXVIIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions(Quark Matter 2019)
In-Medium Jet Modification Measured by PHENIX ViaTwo-Particle Correlations and High p T Hadrons in A + A Collisions
Anthony Hodges for the PHENIX Collaboration
Georgia State University
Abstract
The first evidence of jet quenching was observed at RHIC via suppression of single high p T hadron R AA and the dis-appearance of the away-side jet peak in two-particle correlations. Since then, hadron R AA and two-particle correlationscontinue to be useful probes of the QGP in heavy ion collisions, since the particles involved are fragments of the jetsproduced in the initial hard scattering. PHENIX recently improved the width measurements extracted from π -hadroncorrelations after removing the higher order flow terms in the underlying event subtraction. Measurements of the away-side jet correlated with high p T neutral pions show an increase in low p T particle production at wide angles consistentwith theoretical expectations for energy loss. The system size dependence of energy loss is further investigated atPHENIX by measuring the absolute yield and R AA for various hadron species at high p T in several collision systemsincluding U + U and Cu + Au . These proceedings will present the newest PHENIX R AA and two-particle correlationmeasurements and their role in our understanding of jet quenching and medium response in heavy ion collisions. Keywords:
Jets, Two-Particle Correlations, RAA, Hadrons, Heavy Ion Collisions, Quark Gluon Plasma
1. Introduction
High energy heavy ion collisions produced at the Relativistic Heavy Ion Collider (RHIC) and the LargeHadron Collider (LHC) provide a unique environment to study Quantum Chromodynamics (QCD). In heavyion collisions, a hot, dense state of matter, known as a Quark Gluon Plasma (QGP), is formed in whichquarks and gluons have their own degrees of freedom. After the initial hard scatterings in the early stages ofthe collision, the now deconfined quarks and gluons propagate through the QGP medium and lose energy.This results in a modification to the yields of several particle species relative to their yields in p + p collisions,with the notable exception of the direct photon, which escapes the plasma unmodified. One area of researchis quantifying how energy is dissipated through the QGP via either gluon radiation or by secondary partoniccollisions within the QGP. These proceedings detail new results on energy loss studies from PHENIX usingtwo techniques: measurements of the single particle R AA and two-particle correlations. a r X i v : . [ nu c l - e x ] M a y Anthony Hodges for the PHENIX Collaboration / Nuclear Physics A 00 (2020) 1–5
2. Single Particle R AA The R AA , or nuclear modification factor, of a given particle species quantifies the extent to which theyield of a given particle species in A + A collisions is modified relative to the yield in p + p scaled by thenumber of binary collisions. The definition of R AA is given in Eqn. (1). R AA = (cid:104) T AA (cid:105) d N AA / d p T d η d σ pp / d p T d η (1)PHENIX has measured the R AA for several meson species in Cu + Au at √ s NN =
200 GeV and U + U √ s NN =
192 GeV, with the U + U results shown in Fig. 1. The R AA is plotted on the y -axis as a function ofthe transverse p T on the x -axis. In both collision systems in the most central collisions, there is a separationbetween the φ and K ∗ R AA and the π and the η R AA at low p T . All species remain suppressed relative to the p + p baseline across all p T , however. This separation may be attributable to strangeness enhancement whichreflects the quark content of the species shown. The φ , for instance, is entirely composed of strange quarks( s ¯ s ), whereas the π ((( u ¯ u ) + ( d ¯ d )) / √
2) has no contribution to its makeup from strange quarks and, thus,would not see this e ff ect. The dominant source of systematic uncertainty (represented by the empty boxesaround the data points) is the uncertainty in the raw yield extraction and in the reconstruction e ffi ciency. Fig. 1: R AA for various meson species as a function of p T in U + U at 192 GeV collisions. One can see features of possiblestrangeness enhancement at low p T in the 0–20% centrality binwhich disappears at higher p T Fig. 2: Integrated R AA ’s for di ff erent meson species and colli-sion systems for p T > / c The p T integrated (cid:104) R AA (cid:105) across Au + Au , Cu + Au , and U + U as a function of the system size N part and for various mesons is shown in Fig. 2, with the (cid:104) R AA (cid:105) on the y -axis and the transverse p T on the x -axis.The di ff erent data points correspond to di ff erent particle species. When only looking at the high p T region( p T > / c), e ff ects due to strangeness enhancement disappear, and the integrated (cid:104) R AA (cid:105) for all mesonspecies across the di ff erent collision species follow the same trend. This shows that at high p T , energy lossis the dominant QGP-related e ff ect.
3. Two-Particle Correlations
Measurement of two-particle correlations allows one to study the modification of both the particle yieldand shape of recoil jets in A + A relative to a p + p baseline. In this method, we correlate all the chargedhadrons in an event to a high p T trigger particle (in this case, the π ) and measure the angular separationin the azimuthal direction, ∆ φ . This results in a distribution with two prominent peaks at ∆ φ = ∆ φ = π , which are called the near and away-side peaks, respectively. From here, subtraction of correlations nthony Hodges for the PHENIX Collaboration / Nuclear Physics A 00 (2020) 1–5 due to flow (in this analysis flow harmonics up to n = N t dN Pair d ∆ φ = N t N Pair (cid:15) (cid:82) ∆ φ dN PairSame / d ∆ φ dN PairMix / d ∆ φ − ξ (1 + (cid:88) n = (cid:104) v tn (cid:105)(cid:104) v an (cid:105) cos( n ∆ φ )) (2)Here, N t and N Pair refer to the number of trigger π ’s and correlated pairs, respectively. “Same” denotesa π -hadron pair that come from the same event, whereas pairs denoted “Mix” come from two separateevents and are used to correct for detector e ff ects via event mixing. (cid:15) is the charged hadron e ffi ciency, and ξ represents the magnitude of the correlated background due to flow, calculated using the absolute backgroundsubtraction method [1]. Lastly, v tn and v an represent the flow harmonic values for both the trigger and theassociated hadron, respectively. Both are taken from previous PHENIX analyses [2, 3].From the jet function, we extract the yield of the away-side jet peak by integrating the region | ∆ φ − π | < π/
2. We then take the integrated yields in A + A and p + p , Y AA and Y pp , respectively, andcalculate the I AA = Y AA / Y pp . Fig. 3 shows the I AA on the y -axis plotted as a function of the associate parti-cle p T on the x -axis in two di ff erent trigger p T ranges. The empty boxes around the data points in Fig. 3, 4,and 5 represent the systematic uncertainties and stem from the π combinatorial background, the estimationof the background level ξ , and from the flow harmonic measurements themselves from [2, 3]. Additionally,the blue box shown at unity in Fig. 3 and 5 represents a global scale uncertainty primarily coming fromthe charged hadron tracking e ffi ciency correction, (cid:15) . I AA is an important observable in two-particle corre-lations because it allows us to directly study modifications to the fragmentation function, D ( z ), due to thefact that the integrated yields are approximately equivalent to the fragmentation functions; that is to say: I AA = Y AA / Y pp ≈ D AA ( z ) / D pp ( z ). Fig. 3: I AA as a function of the associate particle p T in two trig-ger p T bins, and in 0–20% (black) and 20–40% (red) centralityclasses. Fig. 4: Extracted away-side Gaussian widths plotted as a func-tion of the associate partilce p T in 0–20% (black) and 20–40%(red) centrality classes. The blue points are the p + p baseline. The I AA measurement shown in Fig. 3 shows an enhancement in the yield of low p T associate particles,and a suppression in the yield of high p T associate particles. This trend is consistent across the two trigger p T bins shown and in the 0–20% and 20–40% centrality bins. Additionally, the width of the away-side jetpeak was measured by fitting it with a Gaussian and extracting the width, σ , as previously shown in [4]. Theresult of this measurement is shown in Fig. 4, with the jet width, σ , on the y -axis and the associate particle p T on the x -axis. The away-side widths in Au + Au collisions are broader than those in p + p at low associateparticle p T . At high p T , the Au + Au jet widths converge to the p + p baseline in both centrality bins.The broadening of the away-side at low p T seen in [4] coupled with the enhancement in the away-sideyield at low p T in Fig. 3 show us that the mechanism behind energy loss within the QGP is consistent withgluon bremsstrahlung being radiated at wide angles relative to the recoil parton trajectory. We extend thisanalysis further to track the lost energy in ∆ φ space. To do this, we plot the I AA in a given trigger p T binas a function of the separation angle ∆ φ over the range | ∆ φ − π | (cid:46) π/
3, encompassing the away-side jetpeak. The result is shown in Fig. 5. Each of the di ff erent sets of color points represents a di ff erent associateparticle p T range, with red being the lowest (0 . / c) and black being the highest (5–7 GeV / c). The I AA Anthony Hodges for the PHENIX Collaboration / Nuclear Physics A 00 (2020) 1–5 for the highest p T associate particles, (i.e. the 5–7 GeV / c and 3–5 GeV / c bins), is suppressed by nearly thesame amount across the ∆ φ range. As the p T of the associate particle decreases, however, the I AA begins totake on a ∆ φ dependence, showing enhancement at wide angles relative to the away-side jet peak ( ∆ φ = π ),as is seen in the 0 . / c and 1–2 GeV / c bins. Near ∆ φ ≈ π , however, the I AA for the 1–2 GeV / c binappears to be consistent with one, whereas the 0 . / c shows enhancement across almost all ∆ φ binson the away-side. Lastly, there appear to be no major changes in the measurement when looking at di ff erenttrigger p T bins. Fig. 5: I AA as a function of the separation angle ∆ φ in four trigger p T bins. Each color represents a di ff erent associate particle p T range. The filled points are calculated directly by dividing a given Au + Au jet function by the same bin in the p + p baseline. Thesepoints are then mirrored across ∆ φ = π , which is represented by the open points.
4. Conclusions
Single particle R AA measurements have been performed by PHENIX in A + A ( B ) collisions at √ s NN =
200 GeV and 193 GeV. At high p T , the integrated R AA ’s for several meson species were found to follow thesame trend as a function of N Part , showing that the modification in their yield is dependent only on N Part andnot the collision species nor meson species. This means that modifications to single particle yields at high p T are dominantly due to energy loss.Measurements of π -triggered two-particle correlations from PHENIX in Au + Au collisions at √ s NN =
200 GeV from RHIC Year-10 and 11 datasets show modification to the away-side jet peak’s width and yieldrelative to the p + p baseline. Extraction of the I AA and jet width σ show that, at low associate particle p T ,recoil jets in Au + Au are broader and have a larger particle yield than in p + p . Meanwhile, at high associateparticle p T , these jets are as collimated as their p + p counterparts, and their particle yield is suppressedrelative to the p + p baseline. Lastly, a new measurement, the I AA as a function of ∆ φ , shows that high p T associate particles are suppressed at the same level in ∆ φ space, whereas the enhancement of low p T associate particles has a ∆ φ dependence and is most prominent at large angles relative to the away-side jetpeak. The two-particle correlation studies presented in this analysis will be expanded upon by adding thestatistics of two of PHENIX’s largest Au + Au at √ s NN =
200 GeV datasets, Run 14 and Run 16, as well asincluding direct photon triggered correlations.
Acknowledgements
This work was supported by the National Science Foundation [Grant Number 1714801 & 1848162]
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