Gamma Ray Signatures from Ordinary Cosmic Strings

Jun 19, 1992

Jane H. MacGibbon, Robert H. Brandenberger

I. INTRODUCTION

The search for direct evidence of cosmic strings has been a motivated aspect within the field of cosmology. There is a possibility that cosmic string theory provides a compelling narrative for the large-scale structure of the universe. Yet, it is only with the detection of direct string signatures that the theory's integrity will be ultimately validated. An interesting area of discussion is the detection of ultra-high energy (UHE) photon radiation specifically from the cusp regions of individual near-by, ordinary cosmic strings.

In earlier research, we pondered on the possibility of detecting UHE neutrino radiation from these cusps. We observed that the total energy in this region releases as highly energetic particles instantaneously. These particles then decay down to a superheavy fermion scale (Qf). Following this, we applied an extrapolation of the QCD multiplicity function to determine the unique energy distribution of subsequent particles

The energy range of UHE neutrino background from a distribution of strings lies below the present observational parameters. Nevertheless, these levels of cusp background increase due to a larger count of smaller loops, only to decrease later. Cusp neutrinos can be seen above a certain energy level or by detectors whose sensitivity matches the expected levels of background radiation.

II. PHOTON EMISSION FROM CUSP DECAYS

The photon emissions from the decay of cusps, when integrated over the strings in the Universe, should resemble the neutrino emission in terms of shape and magnitude. The particles with lower energies present today must have originated from strings at a higher redshift. But photons emitted with a particular energy will already have a pair production off the cosmic microwave and radio backgrounds.

III. DETECTION OF GAMMA RAY SIGNATURES

Current TeV detectors are very sensitive to a photon burst from strings. This is because an incoming photon has a greater opportunity to interact with the Earth’s atmosphere than an incoming neutrino. Current research calculates the maximum photon cusp radiation from individual or background strings and compares this with the detection capabilities of existing and future air shower array and Cerenkov telescopes. The findings conclude that searching for TeV gamma-ray bursts or a gamma-ray background from strings probably represents the most promising way of detecting cusp radiation from strings, if it occurs.

IV. INDEPENDENT CONSTRAINTS ON STRING MODELS

An additional motive for our work is the importance of looking for independent constraints on string models. More particle physics models predict strings with a smaller mass per unit length, which would be insignificant for structure formation but might have other observable consequences.

V. EMISSION FROM COSMIC STRINGS

Ordinary cosmic strings decay predominantly via gravitational radiation. There are also two mechanisms for particle emission from these ordinary strings. Both are suppressed heavily relative to gravitational radiation, except at the final stages of string life. One of the mechanisms involves the production of particle-antiparticle pairs within the string.

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