Magnetic monopoles may be produced by the Schwinger effect in the strong magnetic fields of peripheral heavy-ion collisions. We review the form of the electromagnetic fields in such collisions and calculate from first principles the cross section for monopole pair production. Using the worldline instanton method, we work to all orders in the magnetic charge, and hence are not hampered by the breakdown of perturbation theory. Our result depends on the spacetime inhomogeneity through a single dimensionless parameter, the Keldysh parameter, which is independent of collision energy for a given monopole mass. For realistic heavy-ion collisions, the computational cost of the calculation becomes prohibitive and the finite size of the monopoles needs to be taken into account, and therefore our current results are not applicable to them. Nonetheless, our results show that the spacetime dependence enhances the production cross section and would therefore lead to stronger monopole mass bounds than in the constant-field case. * oliver.gould@helsinki.fi † d.ho17@imperial.ac.uk ‡ a.rajantie@imperial.ac.uk

Methods Monte Carlo simulation of the MoEDAL experimentThe MM simulation code is developed in Gauss 40 , which is the LHCb simulation framework that uses Geant4 as the simulation engine. MoE-DAL simulations use a dedicated Geant4 class that describes production and propagation of MMs 41 . The MM ionization energy losses, geometry and material content of the MoEDAL detector and its vicinity are modeled in the simulation. The MMTs are described in Geant4 as sensitive detectors and produce hits when MMs are trapped in them. These hits are recorded in simulation and analysed for calculating efficiency and the expected rate of MMs detection. A custom-made momentum distribution of MMs derived from Schwinger kinematics (equation ( 3)) is implemented and propagated through the MoEDAL geometry.

We show that in the SU(2) Georgi-Glashow model, 't Hooft-Polyakov monopoles are produced by a classical instability in magnetic fields above the Ambjørn-Olesen critical field, which coincides approximately with the field at which Schwinger pair production becomes unsuppressed. Below it, monopoles can be produced thermally, and we show that the rate is higher than for pointlike monopoles by calculating the sphaleron energy as a function of the magnetic field. The results can be applied to production of monopoles in heavy-ion collisions or in the early Universe.

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We present the results of an explicit numerical computation of a novel instanton in Georgi-Glashow SU(2) theory. The instanton is physically relevant as a mediator of Schwinger production of 't Hooft-Polyakov magnetic monopoles from strong magnetic fields. In weak fields, the pair production rate has previously been computed using the worldline approximation, which breaks down in strong fields due to the effects of finite monopole size. Using lattice field theory we have overcome this limit, including finite monopole size effects to all orders. We demonstrate that a full consideration of the internal monopole structure results in an enhancement to the pair production rate, and confirm earlier results that monopole production becomes classical at the Ambjørn-Olesen critical field strength.