Postsphaleron baryogenesis

Why very cold neutrons could be useful for neutron antineutron oscillation searches

This note is based on a talk given at the “Workshop on Very Cold and Ultra Cold Neutron Sources for ESS”. It presents several arguments in favor of using very cold neutrons (VCN) for neutron – antineutron ( n − n ‾) searches. It also proposes a scheme for the implementation of a solid-deuterium VCN converter with a fluorinated detonation nanodiamond (F-DND) reflector that is optimized for an ( n − n ‾) experiment with VCN. An analysis of the feasibility of such a source, as well as its effect on the ( n − n ‾) experiment are beyond the scope of this short note. They will, however, be pursued in the near future in a collaborative manner.

Topological Portals from Matter to Antimatter

We discuss possibilities of generating a Majorana mass for the neutron from topological quantum gravity effects which survive at mesoscopic scales from decoherence. We show how virtual micro-black hole (BH) pairs with skyrme/baryon hairs induce a neutron–antineutron transition which can be tested in next generation of experiments. Such effects do not destabilize the proton. We also discuss how BHs with mix ordinary and mirror baryon hairs can mediate neutron-mirror neutron mixings.

Status of the Design of an Annihilation Detector to Observe Neutron-Antineutron Conversions at the European Spallation Source

The goal of the HIBEAM/NNBAR program is to search for baryon number violation via the conversion or oscillation of neutrons into sterile neutrons and/or antineutrons at the European Spallation Source. A key experimental component of the program is the construction of an annihilation detector to directly observe the production of an antineutron following the oscillation. Design studies for the annihilation detector are presented. The predicted response of the detector models are studied using Geant4 simulations made with Monte Carlo simulations of the annihilation signal topology and cosmic ray backgrounds. Particle identification and sensitive discriminating observables, such as invariant mass and sphericity, are shown.

A Possible Neutron-Antineutron Oscillation Experiment at PF1B at the Institut Laue Langevin

We consider a possible neutron–antineutron (n−n¯) oscillation experiment at the PF1B instrument at Institut Laue Langevin. It can improve the best existing constraint on the transition rate and also allow the testing of the methods and instrumentation which would be needed for a later larger-scale experiment at ESS. The main gain factors over the most competitive experiment, performed earlier at PF1 instrument at ILL, are: a more intense neutron beam and a new operating mode based on coherent n and n¯ mirror reflections. The installation of such an experiment would need a temporary replacement of the existing ballistic neutron guide by a specially designed n/n¯ guide with a gradually increasing cross section and a specially selected coating as well as the development and construction of an advanced n¯ annihilation detector with a high efficiency and low background. The overall gain factor could reach up to an order of magnitude and depends on the chosen experiment configuration.

Future searches for free and bound n → n̅ transformations

Baryon number violation, a key, non-perturbative prediction of the Standard Model (SM) via electroweak instantons (sphalerons), has never been definitively observed. However, its relationship to baryogenesis is obscure, and, within the context of the SM, seems to require fine tuning and complex dynamics to occur mere instants after the chaos of the Big Bang began. Post-sphaleron baryogenesis (PSB), a SM extension first proposed by Babu et al. in 2006, seems to compellingly quell many of these theoretical conundrums while effectively predicting the baryon abundance, and simultaneously offering a tantalizing experimental observable: neutron–antineutron transformations (n → n̅). This rare event, a phenomena similar to meson oscillations, can be thought of as a form of dinucleon decay, and is hypothesized to occur for both the free and bound neutron; what's more, within the context of PSB, there exits an upper limit on the free neutron transformation rate. The subject of the relatedness of the free and bound rates promises a wealth of exciting nuclear and high-energy physics, and the complimentary nature of both types of experimental searches argues for their mutual necessity. In this paper, we briefly discuss the physics of the transformation, and our groups' plans to search for this critically important phenomena using both the free and bound neutron.

Implications of an Improved Neutron-Antineutron Oscillation Search for Baryogenesis: A Minimal Effective Theory Analysis

Future neutron-antineutron (n-n[over ¯]) oscillation experiments, such as at the European Spallation Source and the Deep Underground Neutrino Experiment, aim to find first evidence of baryon number violation. We investigate implications of an improved n-n[over ¯] oscillation search for baryogenesis via interactions of n-n[over ¯] mediators, parametrized by an effective field theory (EFT). We find that even in a minimal EFT setup there is overlap between the parameter space probed by n-n[over ¯] oscillation and the region that can realize the observed baryon asymmetry of the Universe. The mass scales of exotic new particles are in the tera-electron-volt-peta-electron-volt regime, inaccessible at the LHC or its envisioned upgrades. Given the innumerable high energy theories that can match, or resemble, the minimal EFT that we discuss, future n-n[over ¯] oscillation experiments could probe many viable theories of baryogenesis beyond the reach of other experiments.

Supersymmetric model for dark matter and baryogenesis motivated by the recent CDMS result

We discuss a supersymmetric model for cogenesis of dark and baryonic matter where the dark matter (DM) has mass in the 8-10 GeV range as indicated by several direct detection searches, including most recently the CDMS experiment with the desired cross section. The DM candidate is a real scalar field. Two key distinguishing features of the model are the following: (i) in contrast with the conventional weakly interacting massive particle dark matter scenarios where thermal freeze-out is responsible for the observed relic density, our model uses nonthermal production of dark matter after reheating of the Universe caused by moduli decay at temperatures below the QCD phase transition, a feature which alleviates the relic overabundance problem caused by small annihilation cross section of light DM particles and (ii) baryogenesis occurs also at similar low temperatures from the decay of TeV scale mediator particles arising from moduli decay. A possible test of this model is the existence of colored particles with TeV masses accessible at the LHC.

Unified TeV scale picture of baryogenesis and dark matter

We present a simple extension of the minimal supersymmetric standard model which provides a unified picture of cosmological baryon asymmetry and dark matter. Our model introduces a gauge singlet field N and a color triplet field X which couple to the right-handed quark fields. The out-of-equilibrium decay of the Majorana fermion N mediated by the exchange of the scalar field X generates adequate baryon asymmetry for MN approximately 100 GeV and MX approximately TeV. The scalar partner of N (denoted N1) is naturally the lightest SUSY particle as it has no gauge interactions and plays the role of dark matter. The model is experimentally testable in (i) neutron-antineutron oscillations with a transition time estimated to be around 10(10)sec, (ii) discovery of colored particles X at LHC with mass of order TeV, and (iii) direct dark matter detection with a predicted cross section in the observable range.