‘Exotic vector-like pair’ of color-triplet scalars

Abstract

We propose a minimal extension of Standard Model, generating a Majorana mass for neutron, connected with a mechanism of Post-Sphaleron Baryogenesis. We consider an 'exotic vector-like pair' of color-triplet scalars, an extra Majorana fermion ψ, and a scalar field φ, giving mass to ψ. The vector-like pair is defined 'exotic' because of a peculiar mass term of the color-triplet scalars, violating Baryon number as ∆B = 1. Such a mass term could be generated by exotic instantons in a class of string-inspired completions of the Standard Model: open (un-)oriented strings attached between D-brane stacks and Euclidean D-branes. A Post-Sphaleron Baryogenesis is realized through φ-decays into six quarks (antiquarks), or through ψ-decays into three quarks (antiquarks). This model suggests some intriguing B-violating signatures, testable in the next future, in NeutronAntineutron physics and LHC. We also discuss limits from FCNC. Sterile fermion can also be light as 1 − 100 GeV. In this case, the sterile fermion could be (meta)-stable and n −n oscillation can be indirectly generated by two n − ψ, ψ −n oscillations, without needing of an effective Majorana mass for neutron. Majorana fermion ψ can be a good candidate for WIMP-like dark matter.

Un-oriented quiver theories for Majorana neutrons

“…These effects can also dynamically propagate from the string scale to much lower energies, as shown in [1]. 2 The possibility of an effective Majorana mass term for the neutron was firstly proposed by Majorana himself in [31]. Such a mass term could induce neutron-antineutron transitions, violating Baryon number, contrary to the predictions of the Standard Model [36][37][38].…”

“…More precisely, M 0 is replaced by the mass parameter of the lightest mass state, be it a fermion ψ D ,C as in figure 1, or a scalar φ D ,C as in figure 2. The scalars φ D ,C have in general a non-diagonal mass matrix [2] of the form…”

“…This drastically changes the constraints on the other parameters: for M 0 = 1 − 10 TeV, a light ψ of µ = 1 ÷ 100 GeV would be enough! The phenomenology of (i) is discussed in [2]. In [2], a toy-model was shown in which the so called X , Y are nothing but φ D ,C respectively.…”

“…The phenomenology of (i) is discussed in [2]. In [2], a toy-model was shown in which the so called X , Y are nothing but φ D ,C respectively. There are some differences not allowing a perfect identification X = φ D and Y = φ C .…”

“…Recently we have proposed the possibility that a Majorana mass term for the neutron could be indirectly generated by non-perturbative quantum gravity effects present in string theory: the exotic instantons [1,2]. 1 In theories with open and un-oriented strings, instantons have a simple geometrical interpretation: they are nothing but Euclidean D-branes (a.k.a.…”