Event mixing does not reproduce single-particle acceptance convolutions for nonuniform pseudorapidity distributions
We point out that the mixed-event method for two-particle acceptance correction, widely used in particle correlation measurements at the BNL Realtivistic Heavy Ion Collider and the CERN Large Hadron Collider (LHC), is wrong in cases where the single-particle pseudorapidity distribution is significantly nonuniform. The correct acceptance should be the convolution of two single-particle efficiency × acceptance functions. The error of the mixed-event method, which guarantees a uniform ∆η two-particle combinatorial density, is, however, small in correlation analyses where the two particles are integrated over an extended pseudorapidity η range. With one particle fixed in η and the right acceptance correction, the background-subtracted correlated pair density may reveal not only a short-range but also a long-range ∆η dependence. This has important physics implication and may provide crucial information to disentangle physics mechanisms for the recently observed long-range ridge correlation in asymmetric proton-lead collisions at the LHC.