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Exclusive dimuon production in ultraperipheral Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV with ATLAS

The collaboration
CERN-EP-2020-138, 2020.

Abstract
Exclusive dimuon production in ultraperipheral collisions (UPC), resulting from photon-photon interactions in the strong electromagnetic fields of colliding high-energy lead nuclei, $\mathrm{PbPb}(\gamma\gamma) \rightarrow \mu^+\mu^- (\mathrm{Pb}^{(\star)}\mathrm{Pb}^{(\star)} )$, is studied using $\mathcal{L}_{\mathrm{int}} = 0.48$ nb$^{-1}$ of $\sqrt{s_\mathrm{NN}}=5.02$ TeV lead-lead collision data at the LHC with the ATLAS detector. Dimuon pairs are measured in the fiducial region $p_{\mathrm{T}\mu} > 4$ GeV, $|\eta_{\mu}| < 2.4$, invariant mass $m_{\mu\mu} > 10$ GeV, and $p_{\mathrm{T,\mu\mu}} < 2$ GeV. The primary background from single-dissociative processes is extracted from the data using a template fitting technique. Differential cross sections are presented as a function of $m_{\mu\mu}$, absolute pair rapidity ($|y_{\mu\mu}|$), scattering angle in the dimuon rest frame ($|\cos \vartheta^{\star}_{\mu\mu}|$) and the colliding photon energies. The total cross section of the UPC $\gamma \gamma \rightarrow \mu^{+}\mu^{-}$ process in the fiducial volume is measured to be $\sigma_{\mathrm{fid}}^{\mu\mu} = 34.1 \! \pm \! 0.3 \mathrm{(stat.)} \! \pm \! 0.7 \mathrm{(syst.)}$ $\mu\mathrm{b}$. Generally good agreement is found with calculations from STARlight, which incorporate the leading-order Breit-Wheeler process with no final-state effects, albeit differences between the measurements and theoretical expectations are observed. In particular, the measured cross sections at larger $|y_{\mu\mu}|$ are found to be about 10-20% larger in data than in the calculations, suggesting the presence of larger fluxes of photons in the initial state. Modification of the dimuon cross sections in the presence of forward and/or backward neutron production is also studied and is found to be associated with a harder incoming photon spectrum, consistent with expectations.

• #### Differential cross section for mass in 10 < |m_mumu| < 20 GeV

Data from arXiv:2011.12211 Figure 6a

10.17182/hepdata.104407.v1/t1

Differential UPC dimuon cross sections shown as a function of $|y_{\mu\mu}|$ in the interval $10 < |m_{\mu\mu}| < 20$ GeV.

• #### Differential cross section for mass in 20 < |m_mumu| < 40 GeV

Data from arXiv:2011.12211 Figure 6b

10.17182/hepdata.104407.v1/t2

Differential UPC dimuon cross sections shown as a function of $|y_{\mu\mu}|$ in the interval $20 < |m_{\mu\mu}| < 40$ GeV.

• #### Differential cross section for mass in 40 < |m_mumu| < 80 GeV

Data from arXiv:2011.12211 Figure 6c

10.17182/hepdata.104407.v1/t3

Differential UPC dimuon cross sections shown as a function of $|y_{\mu\mu}|$ in the interval $40 < |m_{\mu\mu}| < 80$ GeV.

• #### Data/STARlight ratio in 10 < |m_mumu| < 20 GeV

Data from arXiv:2011.12211 Figure 6d

10.17182/hepdata.104407.v1/t4

Ratio of differential UPC dimuon cross sections from data and STARlight shown as a function of $|y_{\mu\mu}|$ in the interval...

• #### Data/STARlight ratio in 20 < |m_mumu| < 40 GeV

Data from arXiv:2011.12211 Figure 6e

10.17182/hepdata.104407.v1/t5

Ratio of differential UPC dimuon cross sections from data and STARlight shown as a function of $|y_{\mu\mu}|$ in the interval...

• #### Data/STARlight ratio in 40 < |m_mumu| < 80 GeV

Data from arXiv:2011.12211 Figure 6f

10.17182/hepdata.104407.v1/t6

Ratio of differential UPC dimuon cross sections from data and STARlight 2.0 shown as a function of $|y_{\mu\mu}|$ in the...

• #### Differential cross section for mass in 0 < |y_mumu| < 0.8

Data from arXiv:2011.12211 Figure 7a

10.17182/hepdata.104407.v1/t7

Differential UPC dimuon cross sections shown as a function of $|m_{\mu\mu}|$ in the interval $0 < |y_{\mu\mu}| < 0.8$.

• #### Differential cross section for mass in 0.8 < |y_mumu| < 1.6

Data from arXiv:2011.12211 Figure 7b

10.17182/hepdata.104407.v1/t8

Differential UPC dimuon cross sections shown as a function of $|m_{\mu\mu}|$ in the interval $0.8 < |y_{\mu\mu}| < 1.6$.

• #### Differential cross section for mass in 1.6 < |y_mumu| < 2.4

Data from arXiv:2011.12211 Figure 7c

10.17182/hepdata.104407.v1/t9

Differential UPC dimuon cross sections shown as a function of $|m_{\mu\mu}|$ in the interval $1.6 < |y_{\mu\mu}| < 2.4$.

• #### Data/STARlight ratio in 0 < |y_mumu| < 0.8

Data from arXiv:2011.12211 Figure 7d

10.17182/hepdata.104407.v1/t10

Ratio of differential UPC dimuon cross sections from data and STARlight shown as a function of $|m_{\mu\mu}|$ in the interval...

• #### Data/STARlight ratio in 0.8 < |y_mumu| < 1.6

Data from arXiv:2011.12211 Figure 7e

10.17182/hepdata.104407.v1/t11

Ratio of differential UPC dimuon cross sections from data and STARlight shown as a function of $|m_{\mu\mu}|$ in the interval...

• #### Data/STARlight ratio in 1.6 < |y_mumu| < 2.4

Data from arXiv:2011.12211 Figure 7f

10.17182/hepdata.104407.v1/t12

Ratio of differential UPC dimuon cross sections from data and STARlight 2.0 shown as a function of $|m_{\mu\mu}|$ in the...

• #### Diff. cross section for |cos(theta*)| in 10 < |m_mumu| < 20 GeV

Data from arXiv:2011.12211 Figure 8a

10.17182/hepdata.104407.v1/t13

Differential UPC dimuon cross sections shown as a function of $|\cos\theta^{\star}_{\mu\mu}|$ in the interval $10 < |m_{\mu\mu}| < 20$ GeV.

• #### Diff. cross section for |cos(theta*)| in 20 < |m_mumu| < 40 GeV

Data from arXiv:2011.12211 Figure 8b

10.17182/hepdata.104407.v1/t14

Differential UPC dimuon cross sections shown as a function of $|\cos\theta^{\star}_{\mu\mu}|$ in the interval $20 < |m_{\mu\mu}| < 40$ GeV.

• #### Diff. cross section for |cos(theta*)| in 40 < |m_mumu| < 80 GeV

Data from arXiv:2011.12211 Figure 8c

10.17182/hepdata.104407.v1/t15

Differential UPC dimuon cross sections shown as a function of $|\cos\theta^{\star}_{\mu\mu}|$ in the interval $40 < |m_{\mu\mu}| < 80$ GeV.

• #### Data/STARlight ratio for |cos(theta*)| in 10 < |m_mumu| < 20 GeV

Data from arXiv:2011.12211 Figure 8d

10.17182/hepdata.104407.v1/t16

Ratio of differential UPC dimuon cross sections from data and STARlight shown as a function of $|\cos\theta^{\star}_{\mu\mu}|$ in the interval...

• #### Data/STARlight ratio for |cos(theta*)| in 20 < |m_mumu| < 40 GeV

Data from arXiv:2011.12211 Figure 8e

10.17182/hepdata.104407.v1/t17

Ratio of differential UPC dimuon cross sections from data and STARlight shown as a function of $|\cos\theta^{\star}_{\mu\mu}|$ in the interval...

• #### Data/STARlight ratio for |cos(theta*)| in 40 < |m_mumu| < 80 GeV

Data from arXiv:2011.12211 Figure 8f

10.17182/hepdata.104407.v1/t18

Ratio of differential UPC dimuon cross sections from data and STARlight 2.0 shown as a function of $|\cos\theta^{\star}_{\mu\mu}|$ in the...

• #### Diff. xsect for |cos(theta*)| in 10<|m_mumu|<20 GeV, |y|<0.8

Data from arXiv:2011.12211 Figure 9a

10.17182/hepdata.104407.v1/t19

Differential UPC dimuon cross sections shown as a function of $|\cos\theta^{\star}_{\mu\mu}|$ in the interval $10 < |m_{\mu\mu}| < 20$ GeV...

• #### Diff. xsect for |cos(theta*)| in 20<|m_mumu|<40 GeV, |y|<0.8

Data from arXiv:2011.12211 Figure 9b

10.17182/hepdata.104407.v1/t20

Differential UPC dimuon cross sections shown as a function of $|\cos\theta^{\star}_{\mu\mu}|$ in the interval $20 < |m_{\mu\mu}| < 40$ GeV...

• #### Diff. xsect for |cos(theta*)| in 40<|m_mumu|<80 GeV, |y|<0.8

Data from arXiv:2011.12211 Figure 9c

10.17182/hepdata.104407.v1/t21

Differential UPC dimuon cross sections shown as a function of $|\cos\theta^{\star}_{\mu\mu}|$ in the interval $40 < |m_{\mu\mu}| < 80$ GeV...

• #### Data/SL ratio for |cos(theta*)| in 10<|m_mumu|<20 GeV, |y|<0.8

Data from arXiv:2011.12211 Figure 7d

10.17182/hepdata.104407.v1/t22

Ratio of differential UPC dimuon cross sections from data and STARlight shown as a function of $|\cos\theta^{\star}_{\mu\mu}|$ in the interval...

• #### Data/SL ratio for |cos(theta*)| in 20<|m_mumu|<40 GeV, |y|<0.8

Data from arXiv:2011.12211 Figure 9e

10.17182/hepdata.104407.v1/t23