Limiting soft particle emission in e+e-, hadronic and nuclear collisions

TL;DR

This paper demonstrates that soft particle emission in e+e-, hadronic, and nuclear collisions is governed by coherent soft gluon bremsstrahlung, with spectra becoming energy-independent at low momenta and proportional to color factors, indicating a universal QCD mechanism.

Contribution

It provides experimental evidence that soft particle spectra across different collision types are consistent with coherent gluon bremsstrahlung, highlighting a universal soft QCD process.

Findings

Soft particle spectra become energy-independent at low momenta.

Spectra are proportional to color factors C_A and C_F.

Data from pp and nuclear reactions align with e+e- results.

Abstract

In e+e- collisions the particle spectra at low momenta reflect the properties of the underlying "soft" QCD gluon bremsstrahlung: the particle density, in the limit p\to 0, becomes independent of the incoming energy \sqrt{s} and directly proportional to the colour factors C_A,C_F for primary gluons or quarks respectively. We find that experimental data from the pp and nuclear reactions reveal the same behaviour: in the limit p_T\to 0 the invariant particle spectra become independent of the collision energy, and their intensities in e+e-, pp and nuclear reactions are compatible with the expected colour factors C_F: C_A: (N_{part}/2) C_A for N_{part} nucleons, participating in the interaction. Coherent soft gluon bremsstrahlung is, therefore, suggested to be the dominant QCD mechanism for the soft particle production in all these reactions. These "soft" particles probe the very early stage of hadron formation in the collision. Future measurements at the LHC will provide crucial tests on the contributions from possible incoherent multi-component processes.