Abstract
Electrically charged particles can be created by the decay of strong enough electric fields, a phenomenon known as the Schwinger mechanism1. By electromagnetic duality, a sufficiently strong magnetic field would similarly produce magnetic monopoles, if they exist2. Magnetic monopoles are hypothetical fundamental particles that are predicted by several theories beyond the standard model3-7 but have never been experimentally detected. Searching for the existence of magnetic monopoles via the Schwinger mechanism has not yet been attempted, but it is advantageous, owing to the possibility of calculating its rate through semi-classical techniques without perturbation theory, as well as that the production of the magnetic monopoles should be enhanced by their finite size8,9 and strong coupling to photons2,10. Here we present a search for magnetic monopole production by the Schwinger mechanism in Pb-Pb heavy ion collisions at the Large Hadron Collider, producing the strongest known magnetic fields in the current Universe11. It was conducted by the MoEDAL experiment, whose trapping detectors were exposed to 0.235 per nanobarn, or approximately 1.8 × 109, of Pb-Pb collisions with 5.02-teraelectronvolt center-of-mass energy per collision in November 2018. A superconducting quantum interference device (SQUID) magnetometer scanned the trapping detectors of MoEDAL for the presence of magnetic charge, which would induce a persistent current in the SQUID. Magnetic monopoles with integer Dirac charges of 1, 2 and 3 and masses up to 75 gigaelectronvolts per speed of light squared were excluded by the analysis at the 95% confidence level. This provides a lower mass limit for finite-size magnetic monopoles from a collider search and greatly extends previous mass bounds.
Highlights
All searches for the direct production of magnetic monopoles (MMs) at particle accelerators to date have focused on collisions of elementary particles such as electrons, or quarks in the case of hadron collisions, assuming production via fermion-antifermion annihilation or photonphoton collisions
Due to the coherence of the magnetic field, the potential exponential suppression by e−4/α is absent for MM production through the Schwinger mechanism
We have considered two approximate approaches to the calculation of the overall MM production cross section: the free-particle approximation (FPA) given by equation (2) in the Methods section and the locally-constant field approximation (LCFA) given by equation (1)
Summary
All searches for the direct production of MMs at particle accelerators to date have focused on collisions of elementary particles such as electrons, or quarks in the case of hadron collisions, assuming production via fermion-antifermion annihilation (the Drell-Yan mechanism) or photonphoton collisions. Schwinger production of MMs in a constant, or slowly-varying, magnetic field is well understood theoretically, and the production probability has been calculated accurately from first principles 2,11, the strong space and time dependence of the electromagnetic fields of LHC heavy-ion collisions present additional theoretical challenges. Progress on this front is made possible due to the large charges of heavy ions, as a consequence of which the resulting electromagnetic field can be approximated as a coherent classical field sourced by the ions’ charge distribution. The expression for this is given in equation (3) in the Methods section
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