Effects of azimuth-symmetric acceptance cutoffs on the measured asymmetry in unpolarized Drell-Yan fixed target experiments

TL;DR

This paper investigates how azimuth-symmetric acceptance cutoffs in unpolarized Drell-Yan experiments can artificially create asymmetries, affecting the measurement of the cos(2φ)-asymmetry and the extraction of the Boer-Mulders function.

Contribution

It demonstrates through simulations that azimuth-symmetric acceptance cutoffs can produce significant artificial asymmetries, highlighting the need for strict angular cutoffs for accurate measurements.

Findings

Acceptance cutoffs can produce up to 10% artificial asymmetries.

Artificial asymmetries can mimic physical effects, complicating analysis.

Strict angular cutoffs improve measurement accuracy without reducing statistical significance.

Abstract

Fixed-target unpolarized Drell-Yan experiments often feature an acceptance depending on the polar angle of the lepton tracks in the laboratory frame. Typically leptons are detected in a defined angular range, with a dead zone in the forward region. If the cutoffs imposed by the angular acceptance are independent of the azimuth, at first sight they do not appear dangerous for a measurement of the cos(2\phi)-asymmetry, relevant because of its association with the violation of the Lam-Tung rule and with the Boer-Mulders function. On the contrary, direct simulations show that up to 10 percent asymmetries are produced by these cutoffs. These artificial asymmetries present qualitative features that allow them to mimic the physical ones. They introduce some model-dependence in the measurements of the cos(2\phi)-asymmetry, since a precise reconstruction of the acceptance in the Collins-Soper frame requires a Monte Carlo simulation, that in turn requires some detailed physical input to generate event distributions. Although experiments in the eighties seem to have been aware of this problem, the possibility of using the Boer-Mulders function as an input parameter in the extraction of Transversity has much increased the requirements of precision on this measurement. Our simulations show that the safest approach to these measurements is a strong cutoff on the Collins-Soper polar angle. This reduces statistics, but does not necessarily decrease the precision in a measurement of the Boer-Mulders function.