arXiv subjects

Thomas Dent

Publications and source records attributed to Thomas Dent.

At least 55 records · Page 3Linked to original sources

arXiv2007

Primordial nucleosynthesis as a probe of fundamental physics parameters

We analyze the effect of variation of fundamental couplings and mass scales on primordial nucleosynthesis in a systematic way. The first step establishes the response of primordial element abundances to the variation of a large number of nuclear physics parameters, including nuclear binding energies. We find a strong influence of the n-p mass difference (for the 4He abundance), of the nucleon mass (for deuterium) and of A=3,4,7 binding energies (for 3He, 6Li and 7Li). A second step relates the nuclear parameters to the parameters of the Standard Model of particle physics. The deuterium, and, above all, 7Li abundances depend strongly on the average light quark mass hat{m} \equiv (m_u+m_d)/2. We calculate the behaviour of abundances when variations of fundamental parameters obey relations arising from grand unification. We also discuss the possibility of a substantial shift in the lithium abundance while the deuterium and 4He abundances are only weakly affected.

astro-ph↗

arXiv2006

Varying "constants" in astrophysics and cosmology and ...

We review astrophysical, cosmological and terrestrial evidence for and against the constancy of fundamental parameters of particle physics, and discuss theoretical issues of unification and scalar-mediated forces, finding that the current rate of variation is bounded by limits on violations of the weak equivalence principle.

hep-ph↗

arXiv2006

Composition-dependent long range forces from varying m_p/m_e

We relate possible cosmological variations in the mass ratio mu \equiv m_p/m_e and the fine structure constant alpha to long-range composition-dependent forces mediated by a scalar field. The differential acceleration eta in Eotvos-type experiments is bounded below by 10^{-14}, except in cases where one or more scalar couplings vanish. We consider what values for these couplings could arise from unified theories. By considering the contribution of the scalar field to the cosmological energy density we use bounds on eta to put upper bounds on the current rate of change of mu and alpha.

hep-ph↗

arXiv2005

Non-thermal leptogenesis via direct inflaton decay without SU(2)(L) triplets

We present a non-thermal leptogenesis scenario following standard supersymmetric hybrid inflation, in the case where light neutrinos acquire mass via the usual seesaw mechanism and inflaton decay to heavy right-handed neutrino superfields is kinematically disallowed, or the right-handed neutrinos which can be decay products of the inflaton are unable to generate sufficient baryon asymmetry via their subsequent decay. The primordial lepton asymmetry is generated through the decay of the inflaton into light particles by the interference of one-loop diagrams with exchange of different right-handed neutrinos. The mechanism requires superpotential couplings explicitly violating a U(1) R-symmetry and R-parity. We take into account the constraints from neutrino masses and mixing and the preservation of the primordial asymmetry. We consider two models, one without and one with SU(2)(R) gauge symmetry. We show that the former is viable, whereas the latter is ruled out. Although the broken R-parity need not have currently observable low-energy signatures, some R-parity-violating slepton decays may be detectable in the future colliders.

hep-ph↗

arXiv2004

Fermion masses and proton decay in string-inspired SU(4)SU(2)^2U(1)_X

We present a supersymmetric model of fermion masses with SU(4)*SU(2)^2*U(1)_X gauge group with matter in fundamental and antisymmetric tensor representations only. The up, down, charged lepton and neutrino Yukawa matrices are distinguished by different Clebsch-Gordan coefficients due to contracting over SU(4) and SU(2)_R indices. We obtain a hierarchical light neutrino mass spectrum with bi-large mixing. The condition that anomalies be cancelled by a Green-Schwarz mechanism leads to fractional U(1)_X charges which exclude $B$ violation through dimension-4 and -5 operators.

hep-ph↗

arXiv2004

Leptogenesis through direct inflaton decay to light particles

We present a scenario of nonthermal leptogenesis following supersymmetric hybrid inflation, in the case where inflaton decay to both heavy right handed neutrino and SU(2)_L triplet superfields is kinematically disallowed. Lepton asymmetry is generated through the decay of the inflaton into light particles by the interference of one-loop diagrams with right handed neutrino and SU(2)_L triplet exchange respectively. We require superpotential couplings explicitly violating a U(1) R-symmetry and R-parity. However, the broken R-parity need not have currently observable low-energy signatures. Also, the lightest sparticle can be stable. Some R-parity violating slepton decays may, though, be detectable in the future colliders. We take into account the constraints from neutrino masses and mixing and the preservation of the primordial lepton asymmetry.

hep-ph↗

arXiv2003

Varying alpha, mass generation and extra dimensions

We consider variation of coupling strengths and mass ratios in and beyond the Standard Model, in the light of various mechanisms of mass generation. In four-dimensional unified models, variations in charged particle thresholds, light quark masses and the electron mass can completely alter the (testable) relation between Delta ln alpha and Delta ln mu, where mu equiv m_p/m_e. In extra-dimensional models where a compactification scale below the fundamental scale is varying, definite predictions may result even without unification; we examine some models with Scherk-Schwarz supersymmetry-breaking.

hep-ph↗

arXiv2003

Varying alpha, thresholds and extra dimensions

We consider variations of coupling strengths and mass ratios in and beyond the Standard Model, in the light of various mechanisms of mass generation. In four-dimensional unified models, heavy quark and superparticle thresholds and the electron mass can completely alter the (testable) relation between Delta ln alpha and Delta ln mu, where mu \equiv m_p/m_e. In extra-dimensional models where a compactification scale below the fundamental scale is varying, definite predictions may result even without unification; we examine some models with Scherk-Schwarz supersymmetry-breaking.

hep-ph↗

arXiv2003

Time-varying coupling strengths, nuclear forces and unification

We investigate the dependence of the nucleon-nucleon force in the deuteron system on the values of coupling strengths at high energy, which will in general depend on the geometry of extra dimensions. The stability of deuterium at all times after nucleosynthesis sets a bound on the time variation of the ratio of the QCD confinement scale to light quark masses. We discuss the relation between this ratio, which is exponentially sensitive to high-energy couplings, and fundamental parameters, in various classes of unified theory. The binding energy of the deuteron is also expected to have an important effect on nucleosynthesis and we quantify this effect.

hep-ph↗

arXiv2002

A realistic formulation of approximate CP

CP violation in the SM is naturally implemented as a small imaginary perturbation to real Yukawa couplings. For example, a large CP asymmetry in B_d decays can arise if the imaginary parts of quark mass matrices are of order 10^(-3)m_t,b or smaller. Applying the same principle of ``additive CP violation'' to soft SUSY-breaking terms, the electric dipole moments of the neutron and mercury atom are predicted near current experimental limits; for nonuniversal A-terms, EDM bounds can be satisfied given certain flavour structures. The proposal is conveniently formulated in a democratic basis, with Yukawas and soft terms of the form const. x (1+eps+i zeta) where eps<<1, zeta<~10^(-3), motivated by approximate permutation x CP symmetry.

hep-ph↗

arXiv2002

Modular symmetry, twisted sectors and flavour

We investigate the implications for fermion mass models in heterotic orbifolds of the modular symmetry mixing twisted states localized at different fixed points. We show that, unlike in the case of continuous gauge symmetries, the mass eigenstates do not mix under the symmetry; thus coupling constants in the low-energy theory are invariant functions of the moduli.

hep-ph↗

arXiv2002

CP from strings: ideas and problems

CP has a natural embedding in superstring models as a gauge symmetry involving inversion of the compactified space. Hence the source of CP violation could be geometrical. Such models face the problem of how to suppress contributions to fermion electric dipole moments from softly-broken supersymmetry, as well as other problems of string phenomenology. Stringy symmetries are useful in evaluating models, and rule out some scenarios.

hep-ph↗

arXiv2002

On the modular invariance of mass eigenstates and CP violation

We investigate the modular transformation properties of observable (light) fields in heterotic orbifolds, in the light of recent calculations of CP-violating quantities. Measurable quantities must be modular invariant functions of string moduli, even if the light fields are noninvariant. We show that physical invariance may arise by patching smooth functions that are separately noninvariant. CP violation for on the unit circle, which requires light and heavy states to mix under transformation, is allowed in principle, although the Jarlskog parameter J_CP(T) must be amended relative to previous results. However, a toy model of modular invariant mass terms indicates that the assumption underlying these results is unrealistic. In general the mass eigenstate basis is manifestly modular invariant and coupling constants are smooth invariant functions of T, thus CP is unbroken on the unit circle. We also discuss the status of CP-odd quantities when CP is a discrete gauge symmetry, and point out a link with baryogenesis.

hep-th↗

arXiv2002

Breaking CP and supersymmetry with orbifold moduli dynamics

We consider the stabilization of string moduli and resulting soft supersymmetry-breaking terms in heterotic string orbifolds. Among the results obtained are: formulae for the scalar interaction soft terms without integrating out the hidden sector gaugino condensate, which reduce to standard expressions in the usual "truncated" limit; an expression for the modular transformation of A-terms; a study of the minima of the scalar potential in the Kaehler modulus direction; and a discussion of the implications for CP violation phenomenology. Some closely related results have appeared in a recent paper of Khalil, Lebedev and Morris, namely the exact modular invariance of A-terms up to unitary mixing, and the existence of certain complex minima for the moduli.

hep-th↗

arXiv2002

Standard Model baryogenesis through four-fermion operators in braneworlds

We study a new baryogenesis scenario in a class of braneworld models with low fundamental scale, which typically have difficulty with baryogenesis. The scenario is characterized by its minimal nature: the field content is that of the Standard Model and all interactions consistent with the gauge symmetry are admitted. Baryon number is violated via a dimension-6 proton decay operator, suppressed today by the mechanism of quark-lepton separation in extra dimensions; we assume that this operator was unsuppressed in the early Universe due to a time-dependent quark-lepton separation. The source of CP violation is the CKM matrix, in combination with the dimension-6 operators. We find that almost independently of cosmology, sufficient baryogenesis is nearly impossible in such a scenario if the fundamental scale is above 100 TeV, as required by an unsuppressed neutron-antineutron oscillation operator. The only exception producing sufficient baryon asymmetry is a scenario involving out-of-equilibrium c quarks interacting with equilibrium b quarks.

hep-ph↗

arXiv2002

Baryogenesis with four-fermion operators in low-scale models

We describe a nonstandard proposal for baryogenesis in models with a low (O(10-100)TeV) fundamental scale. The scenario has Standard Model field content and enhanced baryon number violating operators deriving from time-dependent fermion localization in an extra dimension. The CKM matrix provides sufficient CP violation. The major constraints are the low reheating temperature and rate of perturbative B-violating reactions compared to the total entropy created. A sufficient baryon fraction may arise, but the cosmological evolution required is likely to be somewhat contrived. Based on work in collaboration with D.J.H. Chung.

hep-ph↗

arXiv2001

CP violation and modular symmetries

We reconsider the origin of CP violation in fundamental theory. Existing string models of spontaneous CP violation make ambiguous predictions, due to the arbitrariness of CP transformation and the apparent non-invariance of the results under duality. We find an unambiguous modular CP invariance condition, applicable to predictive models of spontaneous CP violation, which circumvents these problems; it strongly constrains CP violation by heterotic string moduli. The dilaton is also evaluated as a source of CP violation, but is likely experimentally excluded. We consider the prospects for explaining CP violation in strongly-coupled strings and brane worlds.

hep-ph↗

arXiv2000

CP violation and target-space modular invariance

We show that, in perturbative string models where the source of CP violation is a complex vacuum expectation value (v.e.v.) for one or more compactification moduli, CP is conserved if a CP transformation acting on the modulus values is an element of a target-space (self-)duality group. Where the duality group is SL(2,Z) the result confirms a conjecture of Bailin et al. that CP is conserved for v.e.v.'s of the T modulus on the boundary of the fundamental domain, and generalises Giedt's result on the removability of complex Yukawa couplings in such models. Our result applies to any model of spontaneous CP violation where the CP-odd scalar transforms under a symmetry that is not explicitly broken. We consider whether similar results could be obtained in ``brane worlds''.

hep-th↗