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 | |
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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. |
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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. |
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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. |
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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 | |
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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. |
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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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