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| CMS-PAS-HIN-18-008 | ||
| Search for elliptic azimuthal anisotropies in $\gamma$p interactions within ultra-peripheral pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}= $ 8.16 TeV | ||
| CMS Collaboration | ||
| July 2020 | ||
| Abstract: We present a first measurement of two-particle angular correlations for charged hadrons emitted from photon-proton, $\gamma$p, interactions over a wide range of pseudorapidity and full azimuth. The $\gamma$p events were produced within ultra-peripheral pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}= $ 8.16 TeV and were selected by requiring a large rapidity gap in the lead-going direction and no neutron emission from the lead nucleus. The results are compared to a sample of minimum-bias pPb events with same multiplicity. The observed azimuthal correlations at large relative pseudorapidity are used to extract the first, second and third-order two-particle anisotropy harmonics, ${{V_{1\Delta}}}$, ${{V_{2\Delta}}}$, and ${{V_{3\Delta}}}$ as a function of track multiplicity and transverse momentum ${p_{\mathrm{T}}}$. For both the $\gamma$p and minimum-bias hadronic pPb samples, significant negative ${{V_{1\Delta}}}$ and positive ${{V_{2\Delta}}}$ values are observed, while the ${{V_{3\Delta}}}$ values are consistent with zero. The single particle second-order harmonic ${v_2}({p_{\mathrm{T}}})$ is larger for $\gamma$p events than for minimum-bias hadronic pPb collisions at the same multiplicity. | ||
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These preliminary results are superseded in this paper, Submitted to PLB. |
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| Figures | |
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Figure 1:
${N_{\text {trk}}}$ spectra for $\gamma$p and minimum-bias samples. The shaded bands indicate the three ${N_{\text {trk}}}$ categories used in the analysis. |
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Figure 2:
Two-dimensional (left) and one dimensional (right) correlation plots for $\gamma$p enhanced (top) and minimum-bias events (bottom) for 0.3 $ < {p^{\text {trig}}_{\mathrm {T}}}$, ${p^{\text {assoc}}_{\mathrm {T}}} < $ 3.0 GeV/$c$ and 2 $ < {N_{\text {trk}}} < $ 35. For the two dimensional distributions the jet peak centered at $ {{\Delta \eta}} = {{\Delta \phi}} = $ 0 has been 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 $ {{\Delta \phi}} = \pi $, so all the data are contained in [0, $\pi $] and are averaged over $ {| {{\Delta \eta}} |} > $ 2. The Fourier coefficients, $ {{V_{n\Delta}}}$ in the right column are fitted over the $ {{\Delta \phi}} $ range [0, $\pi $]. Points outside this range are shown with blue color and are obtained by symmetrization of those in [0, $\pi $]. |
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Figure 3:
Magnitude of measured ${V_{n\Delta}}$ coefficients for $\gamma$p enhanced events for different classes of ${N_{\text {trk}}}$ and two different ${p_{\mathrm {T}}}$ ranges. For both samples the limits on ${p^{\text {trig}}_{\mathrm {T}}}$ and ${p^{\text {assoc}}_{\mathrm {T}}}$ are the same. Systematic uncertainties are shown by the shaded bars in the two panels. The shaded bands show the ${N_{\text {trk}}}$ regions used for each of the samples. The $\gamma$p enhanced points are placed at the mean value of the corresponding ${N_{\text {trk}}}$ range. |
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Figure 4:
Single-particle azimuthal anisotropy ${v_2}$ versus ${N_{\text {trk}}}$ for $\gamma$p enhanced and minimum-bias samples in two ${p_{\mathrm {T}}}$ regions. Systematic uncertainties are shown by the shaded bars in the two panels. The shaded bands show the ${N_{\text {trk}}}$ regions used for each of the samples. The $\gamma$p enhanced points are placed at the mean ${N_{\text {trk}}}$ of the 2 $ < {N_{\text {trk}}} < $ 5, 5 $ < {N_{\text {trk}}} \leq $ 10 and 10 $ < {N_{\text {trk}}} \leq $ 35 samples. |
| Tables | |
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Table 1:
Mean of ${N_{\text {trk}}}$ for the $\gamma$p enhanced and the minimum-bias data sets for the three classes of ${N_{\text {trk}}}$. The statistical uncertainties are negligible. |
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Table 2:
Measured ${V_{n\Delta}}$ coefficients for $\gamma$p enhanced events as a function of ${p_{\mathrm {T}}}$ and ${N_{\text {trk}}}$. For both samples the limits on ${p^{\text {trig}}_{\mathrm {T}}}$ and ${p^{\text {assoc}}_{\mathrm {T}}}$ are the same. The statistical and systematic uncertainties are added in quadrature. |
| Summary |
| In summary, we have studied long-range single-particle azimuthal anisotropies in ultraperipheral pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 8.16 TeV. A sample of $\gamma$p events is selected by requiring an asymmetric distribution of energy in the forward and backward calorimeters, a large rapidity gap in the lead-going direction and no neutron emission from the lead nucleus. Previous studies suggest that this sample is dominated by $\gamma$p events with some contribution from diffractive ${{\!I\!P{\mathrm{p}}}}$ events. The ${V_{n\Delta}}$ Fourier coefficients of the azimuthal distributions are measured via long-range ($|{{{\Delta\eta}} }| > $ 2) two-particle correlations as a function of event multiplicity and for two ${p_{\mathrm{T}}}$ ranges. The ${{V_{2\Delta}}}$ coefficient is positive while ${{V_{1\Delta}}}$ is negative, suggesting a strong effect of jet-like correlations. The single particle flow coefficient $v_2({p_{\mathrm{T}}})$ increases with ${p_{\mathrm{T}}}$ and is larger for $\gamma$p-enhanced events than for minimum-bias collisions of comparable multiplicity. These results extend the search for collectivity in small systems to $\gamma$p events. |
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