Effects of kinematic cuts on net-electric charge fluctuations

The effects of kinematic cuts on electric charge fluctuations in a gas of charged particles are discussed. We consider a very transparent example of an ideal pion gas with quantum statistics, which can be viewed as a multi-component gas of Boltzmann particles with different charges and masses. Cumulants of net-electric charge fluctuations $\chi_n^Q$ are calculated in a static and expanding medium with flow parameters adjusted to the experimental data. We show that the transverse momentum cut, $p_{t_\text{min}}\leq p_t\leq p_{t_\text{max}}$, weakens the effects of Bose statistics, i.e. contributions of effectively multi-charged states to higher order moments. Consequently, cuts in $p_t$ modify the experimentally measured cumulants and their ratios. We discuss the influence of kinematic cuts on the ratio of mean and variance of electric charge fluctuations in a hadron resonance gas, in the light of recent data of the STAR and PHENIX Collaborations. We find that the different momentum cuts of $p_{t_\text{min}}=0.2$~GeV (STAR) and $p_{t_\text{min}}=0.3$~GeV (PHENIX) are responsible for more than 30\% of the difference between these two data sets. We argue that the $p_t$ cuts imposed on charged particles will influence the normalized kurtosis {$\kappa_Q\sigma_Q^2 = \chi_4^Q/\chi_2^Q$} of the electric charge fluctuations. In particular, the reduction of $\kappa_Q\sigma_Q^2$ with increasing $p_{t_\text{min}}$ will lead to differences between PHENIX and STAR data of ${\cal O}(6\%)$ which currently are buried under large statistical and systematic errors. We furthermore introduce the relation between momentum cut-off and finite volume effects, which is of relevance for the comparison between experimental data and lattice QCD calculations.