Recasting Experimental Constraints on Relativistic Magnetic Monopoles

TL;DR

The paper addresses how to translate velocity-dependent flux limits for relativistic magnetic monopoles into mass-dependent constraints by explicitly modeling their acceleration in Galactic and intergalactic magnetic fields and their deceleration in Earth. By incorporating up-to-date cosmic-field parameters and backreaction effects, the authors recast MACRO, IceCube, Auger, and CTAO limits (as well as Parker and cosmological bounds) into $m$-dependent flux limits and identify regimes where intergalactic magnetic fields play a detectable role. They show that terrestrial experiments already probe IGMF-accelerated monopoles for a broad mass range and that future facilities like CTAO can fill gaps and potentially reveal information about IGMFs through monopole signals. The work emphasizes monopoles as cosmic magnetic-field messengers and outlines avenues for improved GMF/IGMF modeling and experimental strategies to exploit magnetic-field physics for fundamental constraints on new physics.

Abstract

Magnetic monopoles with masses up to $10^{14}$ GeV can be accelerated to relativistic velocities in Galactic and intergalactic magnetic fields. The cosmic flux of relativistic monopoles is constrained by various experiments, with the limits given as functions of the monopole velocity (Lorentz factor) at the detectors. The velocity, however, is usually treated as a free parameter due to the ambiguity in the computation of the acceleration before the monopoles arrive at Earth. We explicitly evaluate the velocity by exploiting recent studies on cosmic magnetic fields and the monopole acceleration therein, to recast experimental limits in terms of the mass of monopoles. By applying our method to various terrestrial experiments, including the Pierre Auger Observatory, IceCube, MACRO, and the upcoming Cherenkov Telescope Array Observatory, as well as to astrophysical constraints, we report limits on the flux of monopoles for a wide range of monopole masses. We also highlight the role of monopoles as messengers of cosmic magnetic fields, and discuss the possibility of using monopole experiments to probe intergalactic magnetic fields.

Figures (9)

• Figure 1: Parameter space of IGMF strength and coherence length. The gray region is excluded by various constraints Durrer:2013pga. The red region shows where IGMFs have negligible effects on the MM velocity at Earth. The blue stars indicate benchmark values that are used in \ref{['fig:gammabetam']}.
• Figure 2: Product of the velocity and Lorentz factor of MMs at Earth, as a function of mass. The MM charge is fixed to $g = g_{\mathrm{D}}$. The IGMF strength and coherence length are varied in each panel. In (a) the IGMF is too weak to affect the MM velocity. In (b)--(d) the IGMFs can predominantly accelerate the MMs, depending on the MM flux. The horizontal black dotted lines indicate the sensitivity thresholds for IceCube, IACTs, and Auger.
• Figure 3: Relation between IGMF strength and kinetic energy of MMs that arrive at Earth with relativistic velocities. The MM charge is fixed to $g = g_{\mathrm{D}}$, and the line colors denote different values of the MM flux. Solid lines are for $\lambda_{\rm I} \gtrsim 1/H_0$, while dashed lines are for $\lambda_{\rm I} = 1\, \mathrm{Mpc}$. The vertical segment where the lines overlap represents the minimum energy acquired in GMFs, and a departure from it signals extra acceleration in IGMFs. One can infer the IGMF strength and coherence length by measuring the MM flux and kinetic energy. The gray region is excluded by constraints from CMB.
• Figure 4: Experimental upper limits on the singly charged ($g=g_{\mathrm{D}}$) MM flux in terms of the MM velocity, at the detector. Shown are limits from MACRO MACRO:2002kki (blue), IceCube IceCube:2021eye (purple), RICE Hogan:2008sx (green), Auger PierreAuger:2016imq (brown), and a preliminary study Spengler:2011 for H.E.S.S. (magenta).
• Figure 5: Upper limits on the singly charged ($g=g_{\mathrm{D}}$) MM flux on Earth, in terms of the MM mass. Shown are results from MACRO (blue), IceCube (purple), Auger (brown), expected sensitivity for CTAO (magenta), Galactic Parker bound (red), seed Parker bound (pink), cosmological limit from comparison with the average dark matter density in the universe (gray), and the mass lower limit from MoEDAL (orange). The Auger and seed Parker limits depend on IGMFs, whose field strength is varied. See the text for details.