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# Search for magnetic monopoles produced via the Schwinger mechanism

## 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 model3,4,5,6,7 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.

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## Data availability

All data used to produce the results of this work, along with the data points shown in the main figures of the paper are stored either on CERN lxplus server or on CERN’s GitLab. They are available upon request to the corresponding author without specific conditions. Source data are provided with this paper.

## Code availability

All code used to produce the results of this work, including code to perform statistical analysis and produce the figures, is stored on CERN’s GitLab server and is available upon request to the corresponding author without specific conditions.

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## Acknowledgements

We thank CERN for the LHC’s successful Run-2 operation, as well as the support staff from our institutions without whom MoEDAL could not be operated. We acknowledge the invaluable assistance of particular members of the LHCb Collaboration: G. Wilkinson, R. Lindner, E. Thomas and G. Corti. Computing support was provided by the GridPP Collaboration, in particular by the Queen Mary University of London and Liverpool grid sites. This work was supported by grant PP00P2 150583 of the Swiss NSF; by the UK Science and Technology Facilities Council via the grants ST/L000326/1, ST/L00044X/1, ST/N00101X/1, ST/P000258/1, ST/P000762/1, ST/T000732/1, ST/T000759/1 and ST/T000791/1; by the Generalitat Valenciana via a special grant for MoEDAL and via the projects PROMETEO-II/2017/033 and PROMETEO/2019/087; by MCIU/AEI/FEDER, UE via the grants FPA2016-77177-C2-1-P, FPA2017-85985-P, FPA2017-84543-P and PGC2018-094856-B-I00; by the Physics Department of King’s College London; by NSERC via a project grant; by the V-P Research of the University of Alberta (UofA); by the Provost of the UofA; by UEFISCDI (Romania); by the INFN (Italy); by the Estonian Research Council via a Mobilitas Pluss grant MOBTT5; by the Research Funds of the University of Helsinki; and by the NSF grant 2011214 to the University of Alabama MoEDAL group. A.R. was also supported by Institute for Particle Physics Phenomenology Associateship.

## Author information

Authors

### Contributions

The Monopole and Exotics Detector at the LHC was constructed and is maintained by the MoEDAL collaboration. A large number of authors contributed to the data processing, detector calibration and Monte Carlo simulations used in this work. The MoEDAL collaboration acknowledges the substantial contributions to this manuscript from A.U. and I.O. (simulation, statistical analysis, result plots, paper writing); O.G., D.L.-J.H. and A.R. (theoretical calculations, paper writing); and N.E.M. and J.P. (paper writing). The manuscript was reviewed and edited by the collaboration and all authors approved the final version of the manuscript.

### Corresponding author

Correspondence to I. Ostrovskiy.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

## Peer review information

Nature thanks Muneto Nitta, Steve Ahlen and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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## Extended data figures and tables

### Extended Data Fig. 1 Mean expected rate of Schwinger MMs (Rexp).

The mean expected rate of MMs with 1gD (left) and 2gD (right) magnetic charge in the MMT as a function of the MM mass in the FPA model. The black line corresponds to the default geometry. The grey region corresponds to the systematic error, which is dominated by the material budget. The 95% confidence level mass exclusion region is shown in blue.

### Extended Data Fig. 2 Transverse momentum distribution of Schwinger MMs.

The transverse momentum distribution for Schwinger MMs derived from the FPA, as a function of MM mass (M) plotted versus MM β.

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Acharya, B., Alexandre, J., Benes, P. et al. Search for magnetic monopoles produced via the Schwinger mechanism. Nature 602, 63–67 (2022). https://doi.org/10.1038/s41586-021-04298-1

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• DOI: https://doi.org/10.1038/s41586-021-04298-1

• ### Searches for magnetic monopoles

• Iulia Georgescu

Nature Reviews Physics (2022)