Ultra-Peripheral Collisions physics with ATLAS

Exclusive production of dilepton pairs, γγ → ℓℓ, ℓ = e, μ, τ, is studied using data from ultraperipheral collisions (UPC) of lead nuclei at sNN=5.02 TeV recorded by the ATLAS detector [1] at the LHC. The process of interest proceeds via photon-photon interactions in the strong electromagnetic fields of relativistic lead nuclei. In the case of the dielectron production, within experimental uncertainties the measured integrated cross section is in good agreement with the QED predictions from the Monte Carlo programs STARlight and SuperChic, confirming the broad features of the initial photon fluxes. The differential cross section shows systematic differences from these predictions which are more pronounced at high absolute rapidity, |yee | and scattering angle, |cosθ*|. In the dimuon production case, the measured cross sections at large absolute rapidity |yμμ | is 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. The γγ → ττ process is observed in the UPCs with a significance exceeding 5 standard deviations. This observation can be turned into a constraint on the τ-lepton anomalous magnetic moment. Measurement of light-by-light scattering data are also performed during the LHC Run 2. The diphoton invariant mass distribution is used to set limits on the production of axion-like particles. This result provides the most stringent limits to date on axion-like particle production for masses in the range 6-100 GeV.


Introduction
Collisions of ultrarelativistic heavy ions (HI) provide an opportunity to study not only the strong interactions between nucleons but also processes involving electromagnetic (EM) interactions.In the WeizsckerWilliams approach [2,3] also referred to as the equivalent photon approximation (EPA), the Lorentz-contracted EM fields act as a source of high-energy, nearly real photons.Therefore, the EM fields associated with the ultrarelativistic nuclei can be treated as fluxes of quasi-real photons according to EPA.The photon flux from each nucleus is enhanced by a factor of Z 2 , where Z is the atomic number.That results in a Z 4 enhancement of cross-sections.For lead (Z = 82), this Z 4 enhancement is 4.5×10 7 .Another advantage of studying photon-induced interactions in UPC HI is the low number of interactions per LHC bunch crossing.The mean number of simultaneous interactions is typically at the subpercent level.This provides a clean environment, facilitating the detection of the interaction products, and little contamination from unrelated interactions in the same crossing.Zero Degree Calorimeters (ZDCs) which are detectors offering a control over backgrounds and impact parameter dependence play a major role in this measurement.We will go through four different processes: exclusive γγ → ee[4]/µµ [5]/τ τ [6] productions and light-by-light scattering into two photons [7].The latter is considered a tool to search for Beyond Standard Model physics.

The ZDC detectors
The ATLAS detector at the LHC covers nearly the entire solid angle around the collision point.During heavy-ion running, two zero-degree calorimeters (ZDC) are installed at z = ±140 m, upstream and downstream of the interaction point, each covering approximately |η| > 8.3.They are well suited to measure neutral particles originating from the collision.Each selected exclusive dilepton event can be classified according to its forward neutron topology, utilizing the two ZDCs.The three primary topologies available for these events are: the most probable configuration is with no activity in either ZDC ("0 n 0 n "), the next most likely configuration is to observe one or more forward neutrons in one ZDC, and none in the other ("X n 0 n "), and finally, the rarest configuration is observing one or more forward neutrons in both ZDC arms ("X n X n ").Different selections on the ZDC topology probe different ranges of dilepton mass and impact parameters as photon fluxes vary [8].
3. Exclusive γγ → dielectron, dimuon and τ τ productions Among the possible set of photon-induced reactions, the exclusive production of dilepton pairs (electrons, muons [4, 5]) from photon-photon collisions is one of the cleanest elementary processes.This process is a non-resonant two-photon scattering to opposite-charge electron/muon pairs.The outgoing nuclei may be excited, for example via the giant dipole resonance.The most widely used models to describe these processes are STARlight [9] and SuperChic [10].Figure 1 shows the cross-section measured inclusively in ZDC "0 n 0 n " category Differential crosssections measured inclusively in ZDC "0 n 0 n " category as a function of |y ee | for data (dots) and MC predictions from STARlight [9] (solid blue) and SuperChic [10] (dashed red).as a function of the absolute rapidity of the ee system, |y ee |, corrected for the presence of additional neutrons.Figure 2 shows differential cross sections as a function of m µµ for the

Conclusions
ATLAS provides precision results on dilepton exclusive productions via γγ scattering in heavy ion collisions.The ZDC apparatus provides constraints for background and impact-parameter dependence.The exclusive dilepton studies suggest that large fluxes of photons are present in the initial state and consequently that final and initial state radiations need to be accounted for in the models to get a good description of data.A first measurement of exclusive τ τ production is observed with more than 5σ significance, from which a competitive confidence interval for the τ -lepton anomalous magnetic moment is obtained.Light-by-light measurements (γγ → γγ) have been performed with the full LHC Run 2 data and are used to set the most stringent limits on axion-like particles production in the mass range 6-100 GeV.

Figure 2 .
Figure 2. Differential cross sections shown as a function of m µµ for the rapidity bin |y µµ | < 0.8 (left) and as a function of |y µµ | for the mass interval 10 < m µµ < 20 GeV (right [5]).

Figure 4 .
Figure 4. Compilation of exclusion limits at 95% CL in the axion-like-particle-photon coupling (1/Λ a ) versus axion-like-particle-mass (m a ) plane obtained by different experiments in the mass range 1 GeV< m a < 120 GeV [7].