CMS-HIN-18-008

CMS-HIN-18-008 ; CERN-EP-2021-100
Two-particle azimuthal correlations in $\gamma$ p interactions using pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 8.16 TeV
CMS Collaboration
28 April 2022
Phys. Lett. B 844 (2023) 137905
Abstract: The first measurements of the Fourier coefficients ( ${V_{n\Delta}}$ ) of the azimuthal distributions of charged hadrons emitted from photon-proton ( $\gamma$ p) interactions are presented. The data are extracted from 68.8 nb $^{-1}$ of ultra-peripheral proton-lead (pPb) collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 8.16 TeV using the CMS detector. The high energy lead ions produce a flux of photons that can interact with the oncoming proton. This $\gamma$ p system provides a set of unique initial conditions with multiplicity lower than in photon-lead collisions but comparable to recent electron-positron and electron-proton data. The ${V_{n\Delta}}$ coefficients are presented in ranges of event multiplicity and transverse momentum ( ${p_{\mathrm{T}}}$ ) and are compared to corresponding hadronic minimum bias pPb results. For a given multiplicity range, the mean ${p_{\mathrm{T}}}$ of charged particles is smaller in $\gamma$ p than in pPb collisions. For both the $\gamma$ p and pPb samples, ${{V_{1\Delta}}}$ is negative, ${{V_{2\Delta}}}$ is positive, and ${{V_{3\Delta}}}$ consistent with 0. For each multiplicity and ${p_{\mathrm{T}}}$ range, ${{V_{2\Delta}}}$ is larger for $\gamma$ p events. The $\gamma$ p data are consistent with model predictions that have no collective effects thus suggesting the absence of collectivity in the $\gamma$ p system over the multiplicity range explored in this work.
Links: e-print arXiv:2204.13486 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The ${N_{\text {trk}}}$ spectra for $\gamma$ p and MB samples. The simulated $\gamma$ p distribution has been normalized to the same event yield of the data $\gamma$ p enhanced sample.
png pdf	Figure 2: Two-dimensional (left) and one-dimensional (right) correlation plots for $\gamma$ p-enhanced (upper) and MB (lower) events for 0.3 $< {p_{\mathrm {T}}} <$ 3.0 GeV/ $c$ and 2 $\leq {N_{\text {trk}}} <$ 35. For the two-dimensional distributions, the jet peak centered at ${{\Delta \eta}} = {{\Delta \phi}} =$ 0 is truncated to increase visibility. The rapidity gap requirement for the $\gamma$ p-enhanced sample limits the ${\| \Delta \eta \|}$ range to ${\| \Delta \eta \|} <$ 2.5. The one-dimensional ${{\Delta \phi}}$ distributions are symmetrized by construction around ${{\Delta \phi}} =$ 0 and $\pi$ . The Fourier coefficients, ${V_{n\Delta}}$ in the right column are fit over the ${{\Delta \phi}}$ range [0, $\pi$ ]. Points outside this range are shown as open circles and are obtained by symmetrization of those in [0, $\pi$ ]. Statistical error bars are shown for both one-dimensional distributions.
png pdf	Figure 3: Dependence of ${V_{n\Delta}}$ on ${N_{\text {trk}}}$ for $\gamma$ p and MB events for two different ${p_{\mathrm {T}}}$ ranges. Systematic uncertainties are shown by the shaded bars in the two panels. The 2 $\le {N_{\text {trk}}} <$ 5, 5 $\le {N_{\text {trk}}} <$ 10, 10 $\le {N_{\text {trk}}} <$ 35 are used for the lower ${p_{\mathrm {T}}}$ range and 2 $\leq {N_{\text {trk}}} <$ 5 and 5 $\leq {N_{\text {trk}}} <$ 35 for the higher range. The points are placed at the mean value of the corresponding ${N_{\text {trk}}}$ range. Lines indicate the prediction for $\gamma$ p events from PYTHIA8.
png pdf	Figure 4: Single-particle azimuthal anisotropy ${v_2}$ versus ${N_{\text {trk}}}$ for $\gamma$ p-enhanced and pPb samples in two ${p_{\mathrm {T}}}$ regions. Systematic uncertainties are shown by the shaded bars in the two panels. Predictions from the PYTHIA8 and HIJING generators are shown for the $\gamma$ p and MB pPb samples respectively. For the $\gamma$ p events, same ${N_{\text {trk}}}$ bin arrangement as in Figure 3 is kept while for pPb the bins [2, 5), [5, 10), [10, 15) and [15, 20) are used.

Tables
png pdf	Table 1: Mean ${N_{\text {trk}}}$ for the $\gamma$ p-enhanced and the MB data sets for five classes of ${N_{\text {trk}}}$ . Statistical uncertainties are negligible.
png pdf	Table 2: The ${V_{n\Delta}}$ coefficients for $\gamma$ p-enhanced events, as functions of ${p_{\mathrm {T}}}$ and ${N_{\text {trk}}}$ . Statistical and systematic uncertainties are added in quadrature.

Summary

For the first time, the study of long-range particle correlations has been extended to photon-proton (

$\gamma$ p) interactions. This study used proton-lead (pPb) collisions at

${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 8.16 TeV recorded with the CMS detector. The two-particle

${V_{n\Delta}}$ Fourier coefficients and corresponding single-particle

${v_2}$ azimuthal anisotropies are reported as functions of the multiplicity of charged hadrons (

${N_{\text{trk}}}$ ) for two transverse momentum (

${p_{\mathrm{T}}}$ ) ranges. For the

$\gamma$ p sample, the largest observed multiplicity was

${N_{\text{trk}}} \sim$ 3. The mean

${p_{\mathrm{T}}}$ of charged particles is smaller in the

$\gamma$ p sample than for pPb collisions within the same multiplicity range. No evidence for a long-range near-side ridge-like structure was found for either the

$\gamma$ p or hadronic minimum bias pPb (MB) samples within this multiplicity range. In all

${N_{\text{trk}}}$ and

${p_{\mathrm{T}}}$ ranges,

${{V_{1\Delta}}}$ is negative,

${{V_{2\Delta}}}$ is positive with a smaller magnitude than

${{V_{1\Delta}}}$ , and

${{V_{3\Delta}}}$ is consistent with zero. The magnitudes of both

${{V_{1\Delta}}}$ and

${{V_{2\Delta}}}$ increase with

${p_{\mathrm{T}}}$ . This increase has also been seen in electron-proton collisions. At a given

${p_{\mathrm{T}}}$ and track multiplicity,

${v_2}$ is larger for

$\gamma$ p-enhanced events than for MB pPb interactions. Predictions from the {PYTHIA}8 and HIJING models describe well the

$\gamma$ p and pPb MB data at low

${p_{\mathrm{T}}}$ , but slightly overestimate the data at higher

${p_{\mathrm{T}}}$ . Since these models do not have collective effects, these data suggest the absence of collectivity in the

$\gamma$ p system over the multiplicity range explored in this work.

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