Measurement of visible cross sections in proton-lead collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV in van der Meer scans with the ALICE detector

TL;DR

This paper reports the ALICE vdM-scan-based measurement of visible cross sections for two reference processes (V0 and T0) in proton–lead collisions at $\sqrt{s_{NN}}=5.02$ TeV, with separate results for the $p$--$Pb$ and $Pb$--$p$ configurations due to asymmetric detector acceptances. The analysis combines a robust vdM framework with corrections for background, pileup, luminosity decay, and beam dynamics, and compares two independent methods (fit and numerical integration) to extract the effective beam widths used in luminosity calculations. The measured cross sections are used to derive integrated luminosities and, via ZDC-based ratios, a configuration-independent cross section for a third reference process, $\sigma_{ZDC}=2.22$ b. The study achieves total systematic uncertainties around 3.2–3.7% and provides cross-checks between V0-, T0-, and ZDC-based luminosities, strengthening the precision of luminosity calibration for ALICE $p$--$Pb$ data. These results underpin accurate normalization in heavy-ion measurements and validate the vdM approach in asymmetric collision systems.

Abstract

In 2013, the Large Hadron Collider provided proton-lead and lead-proton collisions at the center-of-mass energy per nucleon pair $\sqrt{s_{\rm{NN}}}=5.02$ TeV. Van der Meer scans were performed for both configurations of colliding beams, and the cross section was measured for two reference processes, based on particle detection by the T0 and V0 detectors, with pseudo-rapidity coverage $4.6<η< 4.9$, $-3.3<η<-3.0$ and $2.8<η< 5.1$, $-3.7<η<-1.7$, respectively. Given the asymmetric detector acceptance, the cross section was measured separately for the two configurations. The measured visible cross sections are used to calculate the integrated luminosity of the proton-lead and lead-proton data samples, and to indirectly measure the cross section for a third, configuration-independent, reference process, based on neutron detection by the Zero Degree Calorimeters.

Figures (11)

• Figure 1: (Colour online) Bunch intensities $N_1$ and $N_2$ for all colliding bunches, for an arbitrary timestamp during the p--Pb (left) and Pb--p (right) scan sessions.
• Figure 2: (Colour online) Raw rate of the T0 (left) and V0 (right) process for a typical colliding bunch pair, as a function of time, during the p--Pb scan session. In each plot, the first (second) bell-shaped structure corresponds to the beam separation in the horizontal (vertical) direction being varied from negative to positive values. The third (fourth) bell-shaped structure corresponds to the beam separation in the horizontal (vertical) direction being varied from positive to negative values.
• Figure 3: (Colour online) Correlation between the sum and difference of arrival times (relative to the bunch crossing) on the two V0 arrays. The left plot was obtained at zero beam separation; the right plot was obtained at a beam separation of 0.12 mm, roughly corresponding to five times the RMS of the beam profile. Events lying inside the area within the continuous lines are flagged as beam-beam interactions.
• Figure 4: (Colour online) Background- and pileup-corrected head-on rates of the T0 (left) and V0 (right) reference process as a function of time for one interacting bunch crossing in the p--Pb scan session. The solid red curve is an exponential fit to the data points.
• Figure 5: (Colour online) Background (R$_{\rm{BB}}$/R$_{\rm{raw}}$), pileup (R$_{\rm{PU}}$/R$_{\rm{BB}}$) and luminosity decay (R$_{\rm{DC}}$/R$_{\rm{PU}}$) correction factors to the T0 (left) and V0 (right) rates as a function of the beam separation for one typical pair of colliding bunches during the first p--Pb vertical scan. Due to the different size of the background correction factor for T0 and V0, the two figures have different vertical scales.