Measurement of charged jet production cross sections and nuclear modification in p-Pb collisions at $\sqrt{s_\rm{NN}} = 5.02$ TeV

TL;DR

This study measures charged jet production in p-Pb collisions at √sNN = 5.02 TeV with the ALICE detector, using anti-kT jets for R = 0.2 and 0.4. A NLO-based pp reference is constructed by scaling 7 TeV results, and the nuclear modification factor RpPb is evaluated, finding compatibility with unity across the studied pT range (20–120 GeV/c). The analysis also probes jet structure via the cross-section ratio of the two R-values, finding agreement with pp data and with PYTHIA/NLO predictions, indicating no significant modification of jet fragmentation in p-Pb. Overall, results support the absence of strong cold nuclear matter effects on high-pT jet production at this energy and validate pQCD-based expectations for p-Pb collisions.

Abstract

Charged jet production cross sections in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV measured with the ALICE detector at the LHC are presented. Using the anti-$k_{\rm T}$ algorithm, jets have been reconstructed in the central rapidity region from charged particles with resolution parameters $R = 0.2$ and $R = 0.4$. The reconstructed jets have been corrected for detector effects and the underlying event background. To calculate the nuclear modification factor, $R_{\rm pPb}$, of charged jets in p-Pb collisions, a pp reference was constructed by scaling previously measured charged jet spectra at $\sqrt{s} = 7$ TeV. In the transverse momentum range $20 \le p_{\rm T,ch\ jet} \le 120$ GeV/$c$, $R_{\rm pPb}$ is found to be consistent with unity, indicating the absence of strong nuclear matter effects on jet production. Major modifications to the radial jet structure are probed via the ratio of jet production cross sections reconstructed with the two different resolution parameters. This ratio is found to be similar to the measurement in pp collisions at $\sqrt{s} = 7$ TeV and to the expectations from PYTHIA pp simulations and NLO pQCD calculations at $\sqrt{s_{\rm NN}} = 5.02$ TeV.

Figures (6)

• Figure 1: (Color online) Left: Probability distribution of the event-by-event transverse momentum background density (see Eq. \ref{['def:CMSBackground']}). The mean and variance for two event classes are indicated in the figure. $p_\mathrm{T,\,ch\;jet}^\mathrm{raw}$ represents uncorrected jet $p_\textnormal{T}$. Right: Probability distribution of background fluctuations calculated with the random cone approach and defined via Eq. \ref{['def:DeltaPt']} (resolution parameter $R=0.4$).
• Figure 2: (Color online) Left: Projection of the combined unfolding matrix for jets with particle-level momentum $45 < p_\mathrm{T,\,ch\;jet}^\mathrm{part} < 50$ GeV/$c$. The matrix is obtained from the combination of the detector response and background fluctuation matrices, which are also shown as projections (see text for details). Right: Probability distribution of the relative difference between particle-level (generated true) and detector-level charged jet transverse momentum for jets with different momenta. The effect of background fluctuations is included for the jets reconstructed at detector level. Characteristic values of the distributions are summarized in Table \ref{['tab:residuals']}.
• Figure 3: (Color online) Top panel: $p_\textnormal{T}$-differential production cross section of charged jet production in $\textnormal{p--Pb}$ collisions at $5.02$ TeV for $R=0.4$. Bottom panel: Ratio of data and NLO pQCD calculations. The global uncertainty from the measurement of the visible cross section of $3.5\%$ is not shown. The uncertainties on the pQCD calculation are only shown in the ratio plot as dashed lines. The pQCD calculations take into account the rapidity shift of the nucleon-nucleon center-of-mass system in $\textnormal{p--Pb}$ with a boosted parton system.
• Figure 4: (Color online) Top panel: $p_\textnormal{T}$-differential production cross section of charged jet production in $\textnormal{p--Pb}$ collisions at $5.02$ TeV for $R=0.2$. Bottom panel: Ratio of data and NLO pQCD calculations. The global uncertainty from the measurement of the visible cross section of $3.5\%$ is not shown. The uncertainties on the pQCD calculation are only shown in the ratio plot as dashed lines. The pQCD calculations take into account the rapidity shift of the nucleon-nucleon center-of-mass system in $\textnormal{p--Pb}$ with a boosted parton system.
• Figure 5: (Color online) Nuclear modification factors $R_\mathrm{pPb}$ of charged jets for $R=0.2$ (left) and $R=0.4$ (right). The combined global normalization uncertainty from $\left<T_\mathrm{pPb}\right>$, the correction to NSD events, the measured pp cross section, and the reference scaling is depicted by the box around unity.