Intra-jet correlations of high-pt hadrons from STAR

Systematic measurements of pseudorapidity (∆η) and azimuthal (∆φ) correlations between high-pt charged hadrons in √ sNN=200 GeV Au+Au collisions are presented. An enhancement of correlated yield at large ∆η on the near-side is observed. This effect persists up to trigger p t ∼ 9 GeV/c, indicating that it is associated with jet production. More detailed analysis suggests distinct short-range and long-range components in the correlation. Dihadron azimuthal correlation studies in nuclear collisions have shown that hard partons interact strongly with the matter that is generated and provide a sensitive probe of the medium [1, 2, 3]. Enhanced near-side (∆φ ∼ 0) correlated yield at large ∆η (the ridge) has been observed in measurements with trigger particles at intermediate pt (4 < p t < 6 GeV) [4, 5] and for dihadron pairs having pt < 2 GeV but no trigger requirement [6]. However, inclusive hadron production at pt <∼ 6 GeV/c exhibits large differences between nuclear collisions and more elementary collisions [7, 8]. It is therefore unclear from these existing measurements whether the ridge is associated with hard partonic scattering and jet production. In these proceedings we extend the near-side correlation measurement to p t ∼ 9 GeV/c, well into the kinematic region where inclusive hadron production is similar in nuclear and elementary collisions and where jet fragmentation is thought to dominate. We observe the persistence of the ridge effect to the highest measured trigger pt, suggesting that it is indeed associated with jet production. We further characterize the ridge, to gain insights into its origin. To illustrate the analysis method, Fig. 1 shows the ∆η × ∆φ distribution of hadrons with p t > 2 GeV, associated with trigger hadrons 3 2 GeV. Figure 2. Near-side yield of associated particles in ∆η and ∆φ with p t > 2 GeV as a function of Npart in Au+Au for 3 < p trig t < 4 GeV (details see text). To extract the ridge yield from dihadron measurements we project the two dimensional (∆η × ∆φ) correlation function (Fig. 1) onto ∆φ and ∆η in different ∆η × ∆φ regions. Three methods were used to characterize the small ∆η jet-like (J) and the large ∆η ridge-like (R) contributions to the near-side jet yield‡ in ∆η and ∆φ: • ∆φ(J + R) : Projecting onto ∆φ with the full experimental ∆η acceptance (|∆η| < 1.7 was used in this analysis) and subtracting the elliptic flow (v2) modulated background§. • ∆φ(J) : Subtracting the ∆φ projection for 0.7 <|∆η| < 1.4 from the ∆φ projection |∆η| ≤ 0.7 (near-side). • ∆η(J) : Projecting onto ∆η in a ∆φ window |∆φ| < 0.7 (near-side). A constant fit to the measurements was used to subtract the background. In Fig. 2 the near-side yield is shown as a function of the number of participants Npart for all three methods. The agreement of the measured jet-like yield between the ∆η(J) and ∆φ(J) method for all centrality bins, within the sensitivity of this analysis, supports the assumption that the ridge-like correlation is uniform in the ∆η acceptance. Further detailed studies of the ridge shape in the high statistics Au+Au central data set, especially at high p t , will be pursued. Note that the jet-like correlated yield is independent of centrality and agrees with the p+p reference measurements [5]. In contrast the ∆φ(J + R) yield shows a significant increase with centrality due to the inclusion of the correlated yield at large ∆η (ridge). For the purpose of this analysis one can define the (absolute) ridge yield = yield(∆φ(J +R))− yield(∆η(J))‖. The main systematic error is the uncertainty in the elliptic flow measurement for the ∆φ(J +R) method. The v2 value used in this analysis ‡ The yields are extracted from bin-counting in the interval |∆φ| = |∆η | < 1 § Not corrected for the finite ∆η pair-acceptance. ‖ The ridge yield depends on the ∆η integration window used in the ∆φ(J +R) method. Intra-jet correlations of high-pt hadrons from STAR 3 Figure 3. (Absolute) Ridge yield for different centralities as a function of p t for p assoc t > 2 GeV in Au+Au. Figure 4. (color online) Ridge/Jet-like yield (filled/open symbols) as function of p t for different p t in 0-10% central Au+Au collisions. As reference the inclusive spectrum (0-5% central Au+Au, stars) is also shown [10]. The lines represent exponential fits to the data. is the mean of the reaction plane (v2{RP}) and four-particle cumulant method (v2{4}) in Au+Au collisions [9]. The systematic uncertainties were estimated using v2{RP} as maximum and v2{4} as minimum v2 values (represented as lines in all figures). Fig. 3 shows that a significant (absolute) ridge yield persists up to the highest p t , with yield increasing with centrality. The finite ridge yield at p t up to 9 GeV, where parton fragmentation is expected to be the dominant hadron production mechanism [7, 8], indicates that the ridge is associated with jet production. To characterize in more detail the properties of particles associated to the ridge-like or jet-like near-side correlation we use the p t spectrum in different p trig t windows, as shown in Fig. 4. An exponential function dN dpt ∝ pte is fitted to the data to extract the inverse slope parameter T . A clear difference between the slopes of the jet-like yield, using the ∆η(J) method, and the ridge-like yield is seen: while the pt dependence of the ridge yield is similar to the inclusive particle production, the jet-like associated yield has a significantly harder pt-spectrum, increasing with p trig t , as expected from jet fragmentation. The slope of the ridge-like yield is largely independent of p t and only slightly harder than the inclusive spectrum with a slope difference ∆T ≈ 40-50 MeV. Fig. 5 a) shows the p t dependence of the near-side zT di-hadron fragmentation function (zT =p assoc t /p trig t ) in central Au+Au collisions (for details see [11]). Subtracting the ridge-like contributions, using the ∆φ(J) method, one observes a near-side fragmentation that is approximately independent of p t (Fig. 5 b)). The zT distributions in central Au+Au collisions after subtracting the ridge contribution are comparable to the d+Au reference measurements (Fig. 5 c)) in contrast to the non-ridge subtracted distributions [11]. Intra-jet correlations of high-pt hadrons from STAR 4 Figure 5. (color online) Nearside zT di-hadron fragmentation function for different p t in central Au+Au collisions before a) and after ridge subtraction b) as well as the ratio to d+Au reference measurements c) (see also [11]). These observations support the ansatz that the near-side ∆η × ∆φ correlation consists of two distinct components: a jet contribution, consistent with the p+p and d+Au dihadron reference measurements [4, 5], and the ridge contribution with properties similar to the medium. This could arise from partonic energy loss followed by fragmentation in vacuum, with the lost energy appearing dominantly in the ridge. Several models are qualitatively able to describe the presented phenomena: coupling of induced radiation to longitudinal flow [12], a combination of jet-quenching and strong radial flow [13] and recombination of locally thermal enhanced partons due to partonic energy loss in the recombination framework [14]. A comparison of quantitative theoretical calculations to the measurements are needed in order to understand the origin of the ridge.