CMS-PAS-HIN-19-014

CMS-PAS-HIN-19-014
Observation of forward neutron multiplicity dependence of dimuon acoplanarity in ultra-peripheral PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV
CMS Collaboration
June 2020

Abstract: The first measurement of forward neutron multiplicity dependence of $\gamma\gamma \rightarrow \mu^{+}\mu^{-}$ production in ultra-peripheral PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV is reported by the CMS experiment at the LHC. The azimuthal correlations between two muons in the invariant mass region 8 $ < M_{\mu\mu} < $ 60 GeV are extracted for events associated with zero, one or at least two neutrons produced in the very forward region. The strong back-to-back correlation structure from leading-order photon-photon scattering is significantly broadened for events producing a larger number of neutrons from each nucleus, corresponding to interactions having a smaller impact parameter. This observation demonstrates the transverse momentum and energy of photons emitted from relativistic ions have impact parameter dependence. These results constrain precision modeling of initial photon-induced interactions in ultra-peripheral collisions. They also provide a controllable baseline to search for possible final-state effects on lepton pairs resulting from the production of quark-gluon plasma in hadronic heavy ion collisions.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL. The superseded preliminary plots can be found here.

Figures	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: Neutron multiplicity dependence of $\alpha $ spectra from $\gamma \gamma \rightarrow \mu ^+\mu ^-$ within the CMS acceptance for 8 $ < M_{\mu \mu} < $ 60 GeV in PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}=$ 5.02 TeV. The $\alpha $ distributions are normalized to unit integral over their measured range. The dot-dot dashed line depicts the core contribution while the dotted line represents the tail contribution. The vertical lines on data points depict the statistical uncertainties, while the systematic uncertainties are shown as gray boxes.
png pdf	Figure 2: Neutron multiplicity dependence of $< \alpha ^{\rm core} > $ (a) and $< M_{\mu \mu} > $ of $\mu ^+\mu ^-$ pairs (b) within the CMS acceptance in the mass region 8 $ < M_{\mu \mu} < $ 60 GeV. The vertical lines on data points depict the statistical uncertainties, while the systematic uncertainties of the data are shown as shaded areas.

Summary

In summary, first measurements of $\gamma\gamma\rightarrow\mu^+\mu^-$ production as a function of forward neutron multiplicity in ultra-peripheral lead-lead collisions at nucleon-nucleon center-of-mass energy of 5.02 TeV are reported. Strong neutron multiplicity dependence of azimuthal correlations between muon pairs produced from $\gamma\gamma$ scattering is observed. A significant broadening of back-to-back azimuthal correlations from the leading-order $\gamma\gamma\rightarrow\mu^+\mu^-$ process is seen as the neutron multiplicity in the forward region increases. A similar trend of increasing average invariant mass of muon pairs with neutron multiplicity is also observed. These measurements demonstrate that initial photon energy and transverse momentum in photon-induced interactions of heavy ion collisions have impact parameter dependence. These results call for new theoretical efforts to improve the modeling of photon-induced interactions and make fundamental impacts in many aspects. For instance, future searches for quark-gluon-plasma induced electromagnetic interactions should carefully consider these initial broadening effects when forming a baseline model to compare against.

Additional Figures
png pdf	Additional Figure 1: (a) The Minus vs. Plus ZDC energy distributions. (b) Multi-Gaussian fit to Plus ZDC energy distribution. (c) Multi-Gaussian fit to Minus ZDC energy distribution.
png pdf	Additional Figure 2: The $\alpha $ spectra from $\gamma \gamma \rightarrow {\mu }^+{\mu }^-$ within the CMS acceptance for 8 $ < M_{{\mu} {\mu} } < $ 60 GeV in ultra-peripheral PbPb collisions at $ {\sqrt {\smash [b]{s_{_{{\mathrm {NN}}}}}}} = $ 5.02 TeV. The $\alpha $ distribution is normalized to unit integral over its measured range. The dot-dot dashed line depicts the core contribution while the dotted line represents the tail contribution. The vertical lines on data points depict the statistical uncertainties while the systematic uncertainties are shown as gray boxes.
png pdf	Additional Figure 3: Rapidity dependence of $\alpha $ spectra from $\gamma \gamma \rightarrow {\mu} ^+{\mu} ^-$ within the CMS acceptance in the mass region 8 $ < M_{{\mu }{\mu} } < $ 60 GeV for all analyzed neutron multiplicity classes. For each neutron multiplicity class, both $\alpha $ spectra are normalized by the total yields measured in this class. The vertical lines on data points depict the statistical uncertainties while the systematic uncertainties of the data are shown as shaded areas.
png pdf	Additional Figure 4: Rapidity dependence of $< \alpha ^{\rm core} > $ in neutron multiplicity classes with asymmetric neutron numbers. The vertical lines on data points depict the statistical uncertainties while the systematic uncertainties of the data are shown as shaded areas.

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