Measurements of Inclusive Lambda-bar Production with Large x_f at the SPS-Collider

TL;DR

This paper reports measurements of anti-Lambda production at the SPS collider, comparing results with previous experiments and Monte Carlo models, revealing discrepancies in baryon fragmentation predictions.

Contribution

First measurements of anti-Lambda production at large x_f at SPS energies, highlighting model inadequacies in baryon fragmentation functions.

Findings

Measured cross sections are consistent with earlier ISR results.

Discrepancies between data and PYTHIA/JETSET predictions at 0.6 < x_f < 0.9.

Anti-Lambda polarization measured as 6 +- 12%, consistent with previous data.

Abstract

We report results of inclusive measurements of anti-Lambda, produced in the forward direction at the SPS with sqrt(s) = 630 GeV, using the UA8 small angle Roman Pot spectrometers. These measurements cover the range in Feynman-x_f and transverse momentum, 0.6 < x_f < 1.0 and 0.4 < p_t < 0.7 GeV, respectively. Within a systematic uncertainty of +-20% on the absolute cross section measurements, the results are indistinguishable from those made by some of us in two earlier experiments at the CERN Intersecting Storage Rings, with energies, sqrt(s) = 53 and 62 GeV. In the x_f-range, 0.6-0.9, the absolute cross sections are lower by a factor of 2 to 3 than the predictions of the Lund model as embodied in the PYTHIA 5.6 and JETSET 7.3 Monte Carlo programs, indicating inadequacies in knowledge of the baryon fragmentation function. For the largest x_f-range, 0.9-1.0, the measurements agree with the Monte Carlo predictions. We have measured the average anti-Lambda polarization for our events and find (6 +- 12%), consistent with previous measurements at the ISR in the present region of x_f - p_t.