CMS-PAS-HIN-18-008

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.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PLB.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	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.
png pdf	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, $\pi$ ].
png pdf	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.
png pdf	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
png pdf	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.
png pdf	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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