Measurement of the centrality dependence of J/ψ yields and observation of Z production in lead-lead collisions with the ATLAS detector at the LHC

TL;DR

This ATLAS study measures centrality-dependent yields of J/ψ and Z bosons in Pb+Pb collisions at √sNN = 2.76 TeV via μ⁺μ⁻ decays. J/ψ yields show a centrality-driven suppression not explained by naive N_coll scaling, while Z yields are consistent with scaling but statistics are limited. The analysis relies on MC-driven efficiency corrections, background subtraction, and a Glauber-model determination of N_coll, highlighting systematic uncertainties and cross-checks. These are the first PbPb results for both J/ψ and Z at the LHC, establishing ATLAS's capability to use quarkonia and electroweak bosons as probes of the hot, dense medium created in heavy-ion collisions.

Abstract

Using the ATLAS detector, a centrality-dependent suppression has been observed in the yield of J/ψ mesons produced in the collisions of lead ions at the Large Hadron Collider. In a sample of minimum-bias lead-lead collisions at a nucleon-nucleon centre of mass energy \surd sNN = 2.76 TeV, corresponding to an integrated luminosity of about 6.7 μb^{-1}, J/ψ mesons are reconstructed via their decays to μ+μ- pairs. The measured J/ψ yield, normalized to the number of binary nucleon-nucleon collisions, is found to significantly decrease from peripheral to central collisions. The centrality dependence is found to be qualitatively similar to the trends observed at previous, lower energy experiments. The same sample is used to reconstruct Z bosons in the μ+μ- final state, and a total of 38 candidates are selected in the mass window of 66 to 116 GeV. The relative Z yields as a function of centrality are also presented, although no conclusion can be inferred about their scaling with the number of binary collisions, because of limited statistics. This analysis provides the first results on J/ψ and Z production in lead-lead collisions at the LHC.

Figures (4)

• Figure 1: (top row) The number of Pixel (left) and SCT (right) hits on tracks for data (points with errors) and MC (histogram) for two different centrality bins: 0-10% (open/dotted) and 40-80% (closed/solid). (bottom row) The average number of Pixel (left) and SCT (right) hits as a function of $\eta$ for MC and data in the same two centrality bins.
• Figure 2: Oppositely-charged di--muon invariant mass spectra in the four considered centrality bins from most peripheral (40-80%) to most central (0-10%). The $J/\psi$ yields in each centrality bin are obtained using a sideband technique. The fits shown here are used as a cross check.
• Figure 3: (left) Relative $J/\psi$ yield as a function of centrality normalized to the most peripheral bin (black dots with errors). The expected relative yields from the (normalized) number of binary collisions ($R_{\mathrm{coll}}$) are also shown (boxes, reflecting 1$\sigma$ systematic uncertainties). (right) Value of $R_{cp}$, as described in the text, as a function of centrality. The statistical errors are shown as vertical bars while the grey boxes also include the combined systematic errors. The darker box indicates that the 40-80% bin is used to set the scale for all bins, but the uncertainties in this bin are not propagated into the more central ones.
• Figure 4: The di--muon invariant mass (left) after the selection described in the text. The value of $R_{cp}$ (right) computed with the 38 selected $Z$ candidates. The statistical errors are shown as vertical bars while the grey boxes also include the combined systematic errors. The darker box indicates that the 40-80% bin is used to set the scale for all bins, but the uncertainties in this bin are not propagated into the more central ones.