Measurement of inclusive jet charged-particle fragmentation functions in Pb+Pb collisions at $\sqrt{s_{NN}} = 2.76$ TeV with the ATLAS detector

TL;DR

ATLAS measures charged-particle fragmentation functions inside jets in PbPb collisions at sqrt(s_NN)=2.76 TeV using anti-kt jets with R=0.2, 0.3, and 0.4. After subtracting the underlying event and unfolding detector effects with a robust SVD method, they extract D(z) and D(pT) across seven centrality bins and compare central to peripheral collisions. The data show enhanced very soft fragments (low z) and suppressed intermediate-z fragments in central events, with some high-z enhancement, indicating medium-induced modifications of parton showers. The results challenge simple radiative-energy-loss expectations and suggest that biases due to quenched jet energies and flavor composition play a role, providing important constraints for jet-quenching models.

Abstract

Measurements of charged-particle fragmentation functions of jets produced in ultra-relativistic nuclear collisions can provide insight into the modification of parton showers in the hot, dense medium created in the collisions. ATLAS has measured jets in $\sqrt{s_{NN}} = 2.76$ TeV Pb+Pb collisions at the LHC using a data set recorded in 2011 with an integrated luminosity of 0.14 nb$^{-1}$. Jets were reconstructed using the anti-$k_{t}$ algorithm with distance parameter values $R$ = 0.2, 0.3, and 0.4. Distributions of charged-particle transverse momentum and longitudinal momentum fraction are reported for seven bins in collision centrality for $R=0.4$ jets with $p_{T}^{\mathrm{jet}}> 100$ GeV. Commensurate minimum $p_{\mathrm{T}}$ values are used for the other radii. Ratios of fragment distributions in each centrality bin to those measured in the most peripheral bin are presented. These ratios show a reduction of fragment yield in central collisions relative to peripheral collisions at intermediate $z$ values, $0.04 \lesssim z \lesssim 0.2$ and an enhancement in fragment yield for $z \lesssim 0.04$. A smaller, less significant enhancement is observed at large $z$ and large $p_{\mathrm{T}}$ in central collisions.

Figures (8)

• Figure 1: Comparison between data and MC distributions for four different charged-particle reconstruction selection parameters. The distributions are shown for the 0--10% centrality bin and for charged-particle transverse momenta in the range $5 < \hbox{$p_{{\mathrm{T}}}^{\mathrm{ch}}$} < 7$Ge V. Top: average number of pixel (left) and SCT (right) hits per track. Bottom: distribution of track impact parameters with respect to the reconstructed primary vertex; both transverse, $d_0$ (left), and longitudinal, $z_0 \sin\theta$ (right), impact parameters are shown. Ratios of distributions in data to those in MC simulation are shown for each quantity.
• Figure 2: Charged-particle reconstruction efficiency as a function of truth $p_{\mathrm{T}}$, for 0--10% (red) and 60--80% (blue) centrality bins in the region $|\eta| < 1$ (top) and $1<|\eta|<2.5$ (bottom). The $p_{\mathrm{T}}$ values for the 0--10% points are shifted for clarity. The solid curves show parameterizations of efficiencies. The shaded bands show the systematic uncertainty in the parameterized efficiencies (see text).
• Figure 3: Measured and unfolded $D(z)$ distributions for $R = 0.4$ and $R= 0.2$ jets in central (0--10%) and peripheral (60--80%) collisions. Top left: $R = 0.4$$D^{\mathrm{meas}}(z)$ and $D(z)$ distributions, bottom left: ratios of measured to unfolded $R = 0.4$$D(z)$ distributions with the 0--10% shifted by $+1$ for clarity. Top middle and right: central-to-peripheral ratios of measured ($R_{D(z)}^{\mathrm{meas}}$) and unfolded ($R_{D(z)}$) distributions for $R = 0.4$ and $R= 0.2$, respectively. Bottom middle and right: ratio of $R_{D(z)}^{\mathrm{meas}}$ to $R_{D(z)}$ for $R = 0.4$ and $R= 0.2$, respectively.
• Figure 4: Measured and unfolded $D(p_{\mathrm{T}})$ distributions for $R = 0.4$ and $R= 0.2$ jets in central (0--10%) and peripheral (60--80%) collisions. Top left: $R = 0.4$$D^{\mathrm{meas}}(p_{\mathrm{T}})$ and $D(p_{\mathrm{T}})$ distributions, bottom left: ratios of measured to unfolded $R = 0.4$$D(p_{\mathrm{T}})$ distributions with the 0--10% shifted by $+1$ for clarity. Top middle and right: central-to-peripheral ratios of measured ($R_{D(p_{\mathrm{T}})}^{\mathrm{meas}}$) and unfolded ($R_{D(p_{\mathrm{T}})}$) distributions for $R = 0.4$ and $R= 0.2$, respectively. Bottom middle and right: ratio of $R_{D(p_{\mathrm{T}})}^{\mathrm{meas}}$ to $R_{D(p_{\mathrm{T}})}$ for $R = 0.4$ and $R= 0.2$, respectively.
• Figure 5: Unfolded $R = 0.4$ longitudinal charged particle fragmentation function, $D(z)$ and the charged particle transverse momentum distribution, $D(p_{\mathrm{T}})$, for the seven centrality bins included in this analysis. The statistical uncertainties are everywhere smaller than the points. The yellow shaded error bars indicate systematic uncertainties. Grey lines connecting the central values of distributions are to guide the eye.