Reference cross section measurements with ALICE in pp and Pb-Pb collisions at LHC

TL;DR

The paper reports ALICE measurements of reference trigger cross sections using van der Meer scans in pp (at 7 TeV and 2.76 TeV) and Pb--Pb (2.76 TeV) to calibrate luminosity for absolute cross sections. It details detector configurations (VZERO and ZDC), vdM methodology, and a comprehensive set of corrections (pile-up, luminosity decay, background/satellite, ghost charges, and length-scale) with a careful treatment of systematic uncertainties. Key results include σ_VBAND for pp(7 TeV)=54.34 mb and pp(2.76 TeV)=47.67 mb, σ_ZED for Pb--Pb=371.4 barn, and σ_VLN=4.10 barn, all corrected and validated across multiple scans. These reference cross sections underpin subsequent cross-section measurements in ALICE and validate the vdM approach for both pp and heavy-ion collisions.

Abstract

Cross sections of reference trigger processes were obtained based on beam property measurements in dedicated luminosity calibration experiments (van der Meer scans). These cross-sections are essential for absolute cross section determinations of physics processes. The reference cross sections are presented for pp collisions at two center of mass energies; 2.76 TeV and 7 TeV, and for Pb-Pb collisions at 2.76 TeV together with a discussion of the systematic uncertainty originating from beam intensity and rate measurement uncertainties.

Figures (8)

• Figure 1: Schematic view showing the approximate positions and shapes of the ALICE VZERO detectors (top) and ZDC detectors (bottom). Top panel: side view of the beam pipe at the ALICE IP. The locations of the VZERO arrays are indicated. Bottom panel: cross section view (perpendicular to the beam line), of the ZDC system.
• Figure 2: Interaction pattern during a vdM scan (Scan-V), as seen in ALICE. The histogram shows the total integrated VBAND trigger counts and the arrows show the scheduled time for collisions expected for the LHC filling scheme.
• Figure 3: Top three drawings show the processes ($m$-, $a$-, and $c$-processes) detected by different trigger conditions. Fourth drawing shows an example configuration for pile-up events where there is no $m$-process but one for each $a$- and $c$-processes thus fulfills $m$-process trigger condition. The last one is $e$ which means empty. Empty does not necessarily mean that there is no interaction.
• Figure 4: VZERO-A and VZERO-C timing difference with respect to the time when vertical scan has been performed. This is an example of Scan-V.
• Figure 5: Left: Beam separation dependent correction factors applied for Scan-V analysis for Scan-V vertical scan case case. Right: Comparison of rates before and after the corrections for Scan-II vertical scan case. Solid circles are before any corrections, and open boxes are after applying all the corrections. The Gaussian fit (red solid line) was performed only to guide eyes, and a circle indicates that there is large effect of satellite and background at large displacement.