High energy particles from monopoles connected by strings

Monopole-antimonopole pairs connected by strings and monopole-string networks with $N>\mathrm{}2\mathrm{}$ strings attached to each monopole can be formed at phase transitions in the early universe. In such hybrid defects, monopoles accelerate under the string tension and can reach ultrarelativistic Lorentz factors $\gamma \gg 1$. We study the radiation of gauge quanta by accelerating monopoles. For monopoles with a chromomagnetic charge, we also discuss the high-energy hadron production through emission of virtual gluons and their subsequent fragmentation into hadrons. The relevant parameter for gauge boson radiation is $M/a$ where $M$ is the boson mass and $a$ is the proper acceleration of the monopole. For $M\ll a$, the gauge bosons can be considered as massless and the typical energy of the emitted quanta is $E\sim \gamma a$. In the opposite limit $M\gg a$, the radiation power is exponentially suppressed and gauge quanta are emitted with a typical energy $E\sim \gamma M$ in a narrow range $\Delta E/E\sim {\mathrm{}(a/M)\mathrm{}}^{1/2}$. Cosmological monopole-string networks can produce photons and hadrons of extremely high energies. For a wide range of parameters these energies can be much greater than the Planck scale.