CGC/saturation approach: re-visiting the problem of odd harmonics in angular correlations

TL;DR

This paper revisits the problem of odd harmonic generation in angular correlations, showing how event selection based on multiplicity affects azimuthal symmetry and harmonic suppression, especially in the context of saturation physics at different energies.

Contribution

It demonstrates the impact of multiplicity-based event selection on azimuthal symmetry violation and odd harmonic suppression within the CGC/saturation framework, considering different dipole sizes and energies.

Findings

Violation of azimuthal symmetry can be large for high multiplicity events.

Odd harmonics are suppressed for large dipoles when $Q^2 < Q_s^2$, but not for small dipoles.

Event selection influences the observed harmonic structure in angular correlations.

Abstract

In this paper we demonstrate that the selection of events with different multiplicities of produced particles, leads to the violation of the azimuthal angular symmetry, $\phi \to \pi - \phi$. We find for LHC and lower energies, that this violation can be so large for the events with multiplicities $n \geq 2 \bar{n}$, where $\bar{n}$ is the mean multiplicity, that it leads to almostno suppression of $v_n$, with odd $n$. However, this can only occur if the typical size of the dipole in DIS with a nuclear target is small, or $Q^2 \,>\,Q^2_s\Lb A, Y_{\rm min},b\Rb$, where $Q_s$ is the saturation momentum of the nucleus at $Y = Y_{\rm min}$. In the case of large sizes of dipoles, when $Q^2 \,<\,Q^2_s\Lb A, Y_{\rm min},b\Rb$, we show that $v_n =0$ for odd $n$. Hadron-nucleus scattering is discussed.