Observation of forward neutron multiplicity dependence of dimuon acoplanarity in ultraperipheral PbPb collisions at $\sqrt{s_\mathrm{NN}} =$ 5.02 TeV

The CMS collaboration
Phys.Rev.Lett. 127 (2021) 122001, 2021.

Abstract (data abstract)
The first measurement of the dependence of $\gamma\gamma \to \mu^{+}\mu^{-}$ production on the multiplicity of neutrons emitted very close to the beam direction in ultraperipheral heavy ion collisions is reported. Data for lead-lead interactions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV, with an integrated luminosity of approximately 1.5 nb$^{-1}$, were collected using the CMS detector at the LHC. The azimuthal correlations between the two muons in the invariant mass region $8 < m_{\mu\mu} < 60$ GeV are extracted for events including zero, one, or at least two neutrons detected in the forward pseudorapidity range $|{\eta}| > 8.3$. The back-to-back correlation structure from leading-order photon-photon scattering is found to be significantly broader for events with a larger number of emitted neutrons from each nucleus, corresponding to interactions with a smaller impact parameter. This observation provides a data-driven demonstration that the average transverse momentum of photons emitted from relativistic heavy ions has an impact parameter dependence. These results provide new constraints on models of photon-induced interactions in ultraperipheral collisions. They also provide a baseline to search for possible final-state effects on lepton pairs caused by traversing a quark-gluon plasma produced in hadronic heavy ion collisions.

  • Table 1

    Data from Figure 1 (top left -- 0n0n).

    10.17182/hepdata.95233.v1/t1

    Neutron multiplicity dependence of acoplanarity ($\alpha$) from process $\gamma\gamma$ to $\mu^+\mu^-$ in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV.

  • Table 2

    Data from Figure 1 (top middle -- 0n1n).

    10.17182/hepdata.95233.v1/t2

    Neutron multiplicity dependence of acoplanarity ($\alpha$) from process $\gamma\gamma$ to $\mu^+\mu^-$ in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV.

  • Table 3

    Data from Figure 1 (top right -- 0nXn).

    10.17182/hepdata.95233.v1/t3

    Neutron multiplicity dependence of acoplanarity ($\alpha$) from process $\gamma\gamma$ to $\mu^+\mu^-$ in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV.

  • Table 4

    Data from Figure 1 (bottom left -- 1n1n).

    10.17182/hepdata.95233.v1/t4

    Neutron multiplicity dependence of acoplanarity ($\alpha$) from process $\gamma\gamma$ to $\mu^+\mu^-$ in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV.

  • Table 5

    Data from Figure 1 (bottom middle -- 1nXn).

    10.17182/hepdata.95233.v1/t5

    Neutron multiplicity dependence of acoplanarity ($\alpha$) from process $\gamma\gamma$ to $\mu^+\mu^-$ in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV.

  • Table 6

    Data from Figure 1 (bottom right -- XnXn).

    10.17182/hepdata.95233.v1/t6

    Neutron multiplicity dependence of acoplanarity ($\alpha$) from process $\gamma\gamma$ to $\mu^+\mu^-$ in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV.

  • Table 7

    Data from Figure 2 (upper).

    10.17182/hepdata.95233.v1/t7

    Neutron multiplicity dependence of average core acoplanarity ($<\alpha^{core}>$) distributions from process $\gamma\gamma$ to $\mu^+\mu^-$ in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV.

  • Table 8

    Data from Figure 2 (lower).

    10.17182/hepdata.95233.v1/t8

    Neutron multiplicity dependence of average invariant mass ($$) from process $\gamma\gamma$ to $\mu^+\mu^-$ in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV.

  • Table 9

    Data from Figure A2 (left -- larger n hemisphere in 0n1n) in supplemental materials.

    10.17182/hepdata.95233.v1/t9

    Acoplanarity ($\alpha$) distributions from process $\gamma\gamma$ to $\mu^+\mu^-$ for 0n1n class in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV. Dimuon rapidity is...

  • Table 10

    Data from Figure A2 (middle -- larger n hemisphere in 0nXn) in supplemental materials.

    10.17182/hepdata.95233.v1/t10

    Acoplanarity ($\alpha$) distributions from process $\gamma\gamma$ to $\mu^+\mu^-$ for 0nXn class in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV. Dimuon rapidity is...

  • Table 11

    Data from Figure A2 (right -- larger n hemisphere in 1nXn) in supplemental materials.

    10.17182/hepdata.95233.v1/t11

    Acoplanarity ($\alpha$) distributions from process $\gamma\gamma$ to $\mu^+\mu^-$ for 1nXn class in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV. Dimuon rapidity is...

  • Table 12

    Data from Figure A2 (left -- smaller n hemisphere in 0n1n) in supplemental materials.

    10.17182/hepdata.95233.v1/t12

    Acoplanarity ($\alpha$) distributions from process $\gamma\gamma$ to $\mu^+\mu^-$ for 0n1n class in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV. Dimuon rapidity is...

  • Table 13

    Data from Figure A2 (middle -- smaller n hemisphere in 0nXn) in supplemental materials.

    10.17182/hepdata.95233.v1/t13

    Acoplanarity ($\alpha$) distributions from process $\gamma\gamma$ to $\mu^+\mu^-$ for 0nXn class in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV. Dimuon rapidity is...

  • Table 14

    Data from Figure A2 (right -- smaller n hemisphere in 1nXn) in supplemental materials.

    10.17182/hepdata.95233.v1/t14

    Acoplanarity ($\alpha$) distributions from process $\gamma\gamma$ to $\mu^+\mu^-$ for 1nXn class in ultraperipheral PbPb at $\sqrt{s_{NN}}=5.02$ TeV. Dimuon rapidity is...

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