[Paper Review] First experimental search for production of magnetic monopoles via the Schwinger mechanism

B. S. Acharya, Jean Alexandre+65 more|arXiv (Cornell University)|Jun 22, 2021

Particle physics theoretical and experimental studies24 references4 citations

TL;DR

This paper presents the first experimental search for magnetic monopoles (MMs) produced via the Schwinger mechanism in strong magnetic fields, using Pb-Pb collisions at the LHC's MoEDAL experiment. It excludes MMs with Dirac charges 1≤g≤3gD and masses up to 75 GeV/c², setting the first collider-based lower mass limit for finite-size monopoles and extending previous bounds significantly.

ABSTRACT

Schwinger showed that electrically-charged particles can be produced in a strong electric field by quantum tunnelling through the Coulomb barrier. By electromagnetic duality, if magnetic monopoles (MMs) exist, they would be produced by the same mechanism in a sufficiently strong magnetic field. Unique advantages of the Schwinger mechanism are that its rate can be calculated using semiclassical techniques without relying on perturbation theory, and the finite MM size and strong MM-photon coupling are expected to enhance their production. Pb-Pb heavy-ion collisions at the LHC produce the strongest known magnetic fields in the current Universe, and this article presents the first search for MM production by the Schwinger mechanism. It was conducted by the MoEDAL experiment during the 5.02 TeV/nucleon heavy-ion run at the LHC in November 2018, during which the MoEDAL trapping detectors (MMTs) were exposed to 0.235 nb$^{-1}$ of Pb-Pb collisions. The MMTs were scanned for the presence of magnetic charge using a SQUID magnetometer. MMs with Dirac charges 1$g_D$ $\leq$ $g$ $\leq$ 3$g_D$ and masses up to 75 GeV/c$^2$ were excluded by the analysis. This provides the first lower mass limit for finite-size MMs from a collider search and significantly extends previous mass bounds.

Motivation & Objective

• To search for magnetic monopoles produced via the Schwinger mechanism in strong magnetic fields generated during Pb-Pb heavy-ion collisions at the LHC.
• To test the theoretical prediction that magnetic monopoles, if they exist, would be produced by quantum tunnelling in strong magnetic fields analogous to electric particle production in strong electric fields.
• To set the first collider-based mass exclusion limit for finite-size magnetic monopoles using non-perturbative Schwinger mechanism calculations.
• To overcome the limitations of perturbative cross-section estimates for monopole production in elementary particle collisions by exploiting the non-perturbative nature of the Schwinger mechanism.
• To use MoEDAL’s trapping detectors (MMTs) and SQUID magnetometry to detect magnetic charge, enabling sensitivity to monopoles with Dirac charges up to 5gD.

Proposed method

• Utilized the MoEDAL experiment’s trapping detectors (MMTs) made of soft magnetic material to capture magnetic monopoles produced in Pb-Pb collisions at 5.02 TeV/nucleon.
• Employed a SQUID magnetometer to scan the MMTs for persistent magnetic flux, which would indicate the presence of magnetic charge.
• Calculated the expected monopole production rate using the Schwinger mechanism in the context of finite-size monopoles, incorporating non-perturbative semiclassical techniques.
• Modeled monopole production in the Fokker-Planck approximation (FPA) for monopoles with Dirac charges from 1gD to 5gD and masses from 50 to 85 GeV/c².
• Accounted for detector efficiency, material budget, and systematic uncertainties in the trapping rate estimation, with error propagation based on experimental and theoretical inputs.
• Performed repeated SQUID scans on MMT samples, applying statistical analysis to identify spurious flux jumps and estimate false negative probabilities.

Experimental results

Research questions

• RQ1 Can magnetic monopoles be produced via the Schwinger mechanism in the strong magnetic fields generated in Pb-Pb collisions at the LHC?
• RQ2 What is the sensitivity of the MoEDAL experiment to finite-size magnetic monopoles with Dirac charges 1gD ≤ g ≤ 5gD using the Schwinger mechanism?
• RQ3 What mass limits can be set for magnetic monopoles based on the absence of detected magnetic charge in the MMTs after exposure to 0.235 nb⁻¹ of Pb-Pb collisions?
• RQ4 How do systematic uncertainties in material budget and detector efficiency affect the exclusion limits for monopole production?
• RQ5 Can the Schwinger mechanism provide a viable, non-perturbative alternative to Drell-Yan or photon-photon production for detecting composite magnetic monopoles at colliders?

Key findings

• The MoEDAL experiment excluded magnetic monopoles with Dirac charges 1gD ≤ g ≤ 3gD and masses up to 75 GeV/c² at more than 99.75% confidence level.
• The analysis set the first collider-based lower mass limit for finite-size magnetic monopoles, significantly extending previous bounds from perturbative production models.
• For monopoles with 1gD charge, the expected trapping rate in the MMTs ranged from 1.97×10² to 1.50×10³ per minute for masses between 50 and 100 GeV/c², depending on the model.
• The false negative probability for detecting a monopole with |g| ≥ 0.5gD was estimated at less than 0.2%, confirming high sensitivity of the SQUID scanning system.
• Systematic uncertainties in the material budget dominated the error budget, with the 95% confidence level exclusion region shown in blue in Extended Data Figure 4.
• No magnetic charge signal was observed across 129 MMT samples, and any outliers were attributed to spurious flux jumps from ferromagnetic impurities or environmental noise.