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| CMS-HIN-19-014 ; CERN-EP-2020-196 | ||
| Observation of forward neutron multiplicity dependence of dimuon acoplanarity in ultraperipheral PbPb collisions at $\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 10 November 2020 | ||
| Phys. Rev. Lett. 127 (2021) 122001 | ||
| 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 {\smash [b]{s_{_{\mathrm {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. | ||
| Links: e-print arXiv:2011.05239 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
Neutron multiplicity dependence of acoplanarity distributions from $ {\gamma \gamma \to \mu^{+} \mu^{-}} $ for $ {p_{\mathrm {T}}} ^{\mu} > $ 3.5 GeV, $ {| \eta ^{\mu} |} < $ 2.4, $ {| y^{\mu \mu} |} < $ 2.4, and 8 $ < m_{\mu \mu} < $ 60 GeV in ultraperipheral 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 and dotted lines indicate the core and tail contributions, respectively, found using a fit to Eq. (1). The vertical lines on data points depict the statistical uncertainties, while the systematic uncertainties and horizontal bin widths are shown as gray boxes. |
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Figure 2:
Neutron multiplicity dependence of $ {< \alpha ^\text {core} >} $ (upper) and $ {< m_{\mu \mu} >} $ (lower) of $\mu^{+} \mu^{-} $ pairs in ultraperipheral PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The vertical lines on data points depict the statistical uncertainties, while the systematic uncertainties of the data are shown as shaded areas. In the upper plot, the dashed-dot (red) line shows the STARlight prediction, and the dotted (black) line corresponds to the leading-order QED calculation of Ref. [43]. |
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Figure 3:
A direct comparison between data and model calculations for the $\alpha $ distributions with $\alpha < 0.008$. The dot-dashed line shows the STARlight prediction and the dashed line corresponds to the leading-order QED calculation of Ref. [48]. The dot-dot-dashed and dotted lines indicate the core and tail contributions, respectively. The vertical lines and shaded areas represent the statistical and systematic uncertainties, respectively. |
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Figure A1:
The left panel shows the correlation between energy distributions of the Minus and Plus ZDC detectors (one entry per event), while the right panel shows a multi-Gaussian function fit to the Minus ZDC energy distribution. |
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Figure A2:
Acoplanarity distributions of $ {\gamma \gamma \to \mu^{+} \mu^{-}} $ events for three different neutron multiplicity classes with asymmetric neutron numbers. The solid red (open blue) symbols correspond to events where the dimuon rapidity is in the hemisphere containing larger (smaller) neutron multiplicity. The vertical lines on data points depict the statistical uncertainties while the systematic uncertainties are shown as shaded areas. |
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Figure A3:
The efficiency corrected invariant mass distribution of muon pairs in inclusive ultraperipheral Pb-Pb collisions, for the kinematic range $ {p_{\mathrm {T}}} ^{\mu} > $ 3.5 GeV, $ {| \eta ^{\mu} |} < $ 2.4, and $ {| y^{\mu \mu} |} < $ 2.4. The result of the fit to the data is shown as solid blue line. The yields of muon pairs from $ {\gamma \gamma} $ scattering in the $\Upsilon $ mass region are shown as dashed red line. The separate yields for each $\Upsilon $ state are shown as dotted violet lines. |
| Summary |
| In summary, the first measurements of $\gamma\gamma \to \mu^{+}\mu^{-}$ production as a function of forward neutron multiplicity in ultraperipheral lead-lead collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV are reported. A significant broadening of back-to-back azimuthal correlations is seen, with respect to the leading-order $\gamma\gamma \to \mu^{+}\mu^{-}$ process, for increasing multiplicities of emitted forward neutrons. This observed trend is qualitatively reproduced by a leading-order quantum electrodynamics calculation, demonstrating the importance of an impact parameter dependent photon $p_{\mathrm{T}}$. A similar trend of increasing average invariant mass of muon pairs with neutron multiplicity is also observed. These measurements provide the first experimental demonstration that the initial energy and transverse momentum of photons exchanged in ultraperipheral heavy ion collisions depend on the impact parameter of the interaction. These results call for theoretical efforts to improve the precision in modeling photon-induced interactions. Future searches for electromagnetic interactions of leptons inside the quark-gluon plasma created in heavy ion collisions should incorporate a baseline where the initial broadening effects presented in this Letter are properly taken into account. |
| Additional Figures | |
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Additional Figure 1:
Neutron multiplicity integrated acoplanarity distribution from $ \gamma\gamma \to \mu^{+}\mu^{-} $ for $ p_{\mathrm{T}}^{\mu} > $ 3.5 GeV, $ |\eta^{\mu}| < $ 2.4, $ |y^{\mu\mu}| < $ 2.4, and 8 $ < m_{\mu\mu} < $ 60 GeV in ultraperipheral Pb-Pb 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 and dotted lines indicate the core and tail contributions, respectively, found using a fit to Eq. (1). The vertical lines on data points depict the statistical uncertainties, while the systematic uncertainties and horizontal bin widths are shown as gray boxes. |
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Additional Figure 2:
Distribution of $ \langle \alpha^\text{core} \rangle $ of $ \mu^{+}\mu^{-} $ pairs in $ \gamma\gamma \to \mu^{+}\mu^{-} $ events for three different neutron multiplicity classes with asymmetric neutron numbers. The solid red (open blue) circles correspond to events where the dimuon rapidity is in the hemisphere containing larger (smaller) neutron multiplicity. 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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Additional Figure 3:
Neutron multiplicity dependence of $ \langle \alpha^\text{core} \rangle $ of $ \mu^{+}\mu^{-} $ pairs in ultraperipheral Pb-Pb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV. The vertical lines on data points depict the statistical uncertainties, while the systematic uncertainties of the data are shown as shaded areas. The dot-dashed line shows the STARLIGHT prediction, and the dashed line corresponds to the QED calculation of Ref. [48] incorporating Sudakov radiative corrections in addition. |
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Compact Muon Solenoid LHC, CERN |
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