Exploring Cosmic Strings: Observable Effects and Cosmological Constraints.
Abstract: Observation of cosmic (super)strings can serve as a useful hint to
understand the fundamental theories of physics, such as grand unified theories (GUTs)
and/or superstring theory. In this regard, I present new mechanisms to produce particles
from cosmic (super)strings, and discuss their cosmological and observational effects in
this dissertation. The first chapter is devoted to a ... read morereview of the standard cosmology,
cosmic (super)strings and cosmic rays. The second chapter discusses the cosmological
effects of moduli. Moduli are relatively light, weakly coupled scalar fields, predicted in
supersymmetric particle theories including string theory. They can be emitted from cosmic
(super)string loops in the early universe. Abundance of such moduli is constrained by
diffuse gamma ray background, dark matter, and primordial element abundances. These
constraints put an upper bound on the string tension as strong as G μ ≤
10-28 for a wide range of modulus mass m. If the modulus
coupling constant is stronger than gravitational strength, modulus radiation can be the
dominant energy loss mechanism for the loops. Furthermore, modulus lifetimes become shorter
for stronger coupling. Hence, the constraints on string tension G μ and modulus mass
m are significantly relaxed for strongly coupled moduli predicted in superstring theory.
Thermal production of these particles and their possible effects are also considered. In
the third chapter, moduli emitted from cosmic string cusps are studied. Highly boosted
modulus bursts emanating from cusps subsequently decay into gluons and generate hadronic
cascades which in turn produce large number of neutrinos. For reasonable values of the
modulus mass and coupling constant, observable ultra high energy neutrino fluxes can be
produced for a wide range of string tension G&mu. The fourth chapter discusses cosmic
rays produced by the charged particles ejected from cusps of superconducting cosmic
strings. In many particle physics theories, cosmic strings respond to external magnetic
fields, e.g., in clusters of galaxies, and develop currents. Observable UHE neutrino fluxes
can be achieved for a range of symmetry breaking scale of strings. In this model, neutrinos
with E ≥ 1011 GeV are expected in correlation with
clusters of galaxies. Another unique signature of the model is simultaneous appearance of
several neutrino-produced showers in the field of view of very large detectors, such as
JEM-EUSO. The flux of UHE protons from cusps may account for a large fraction of the
observed events at the highest energies. The dissertation ends with overall
Thesis (Ph.D.)--Tufts University, 2011.
Submitted to the Dept. of Physics.
Advisor: Alexander Vilenkin.
Committee: Jose Blanco-Pillado, Ken Olum, Krzysztof Sliwa, and Tanmay Vachaspati.
Keywords: Theoretical Physics, Particle Physics, and Astrophysics.read less