Color-Reconnection Effects on the nMPI-Nch Conditional Multiplicity Fluctuations in PYTHIA 8 pp Collisions

Abstract

We examine conditional charged-multiplicity fluctuations at fixed multiparton interaction (MPI) count in retained PYTHIA proton-proton samples at centre-of-mass energies of 2.76, 7, and 13 TeV. Charged particles satisfy |eta| < 0.8 and 0.15 < pT < 20 GeV/c. In the historical colour-reconnection (CR)-labelled baseline samples, the CR-ON conditional mean is smaller by a larger fraction than the standard deviation: the absolute width is smaller, while the coefficient of variation is larger in every retained matched MPI bin. Effective lifetime/decay settings and their per-run correspondence are unverified for this baseline, so these comparisons do not establish that CR alone causes the observed differences or their energy dependence. A separately documented 13 TeV comparison using the Compact Muon Solenoid (CMS) CP5 tune, with equal non-CR physics settings and particle data across the ON/OFF pair, exhibits the same absolute-versus-relative width ordering. Retained independent Monash and A14 samples also show this pattern, subject to historical production-record limitations. A comparison with mean-transverse-momentum data from A Large Ion Collider Experiment (ALICE) and simulated activity estimators provides final-state context within the stated particle-definition and provenance limits.