Manifestations of string theory in astrophysical data and at the LHC

Abstract

AbstractWith the advent of the LHC and the continuing influx of cosmological data, phenomenological aspects of string theory have received renewed attention in recent years and many problems have been properly incorporated in this framework. For instance, recent theoretical considerations in string theory have applied a statistical approach to the enormous landscape of metastable vacua. The large number of vacua may shed some light on the cosmological constant problem. In addition, in string theory, attempts have been made to address the hierarchy problem within the context of the existence of large or warped internal dimensions transverse to a braneworld where we are confined, which lowers the effective scale of gravity to the TeV region. If this were the case, unseen dimensions of the space‐time can be at the border of the energy domain within reach of the next generation of particle accelerators.Although the picture of the landscape may be the key to the cosmological constant problem, it is well‐known that the compactification of a string background to a four dimensional solution undergoing accelerating expansion is difficult, which is described by the no‐go theorem of Maldacena‐Nuñez. In the first part of this paper, we investigate the cosmological content of the Salam‐Sezgin supergravity which circumvents one of the hypotheses of the no‐go theorem of Maldacena‐Nuñez and consequently can support a de Sitter phase when lifted to string theory. We find a solution to the field equations in qualitative agreement with the observed dark energy density. The carrier of the acceleration in the present de Sitter epoch is a quintessence field slowly rolling down its exponential potential. Intrinsic to this model, there is a second modulus, which is automatically stabilized and acts as a source of cold dark matter with a mass proportional to an exponential function of the quintessence field.In the second part, we explore a “new physics” signal at LHC, in the processes pp →γ + jet and pp→γ γ. In D‐brane quivers, there are one or more additional U(1) gauge symmetries, beyond the U(1)Y of the standard model, which follows from the property that the gauge group for open strings terminating on a stack of N identical D‐branes is U(N) rather than SU(N) for N > 2. (For N = 2 the gauge group can be Sp(1) rather than U(2).) With this property, the photon will participate with the SU(N) gauge boson in string tree level scattering processes which in the standard model occur only at one‐loop level. In order to evaluate this stringy correction, we considered the processes gg → g γ and gg →γγ, and found that cross section measurements of the process pp →γ + jet at the LHC will attain 5 σ discovery reach on low scale string models for Mstring as large as 4 TeV.