Kaladi Babu
Oklahoma State University

Zurab Berezhiani
University of L'Aquila and INFN

Yuri Kamyshkov
University of Tennessee

Boris Kerbikov
ITEP, Lebedev, MIPT

Program Coordinator:
Kimberlee Choe
(206) 685-3509

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INT Workshop INT-17-69W

Neutron-Antineutron Oscillations: Appearance, Disappearance, and Baryogenesis
October 23 - 27, 2017

Structure of the Workshop:

The Workshop will begin on Monday, October 23, and end on Friday, October 27, 2017. The suggested arrival date is Sunday, October 22, and the departure date might be Saturday October 28. We plan to have invited as well as contributed talks. The current plan is to have about 6 talks per day of 30 minute duration and leave some fair time (15 minutes after each talk) for discussions. One of the afternoons might be dedicated to original shorter talks (20 mins + 10 mins) that should provide an opportunity for talks by young researchers and graduate students. We might also reserve a block of time in the afternoons for additional key discussions. We would like to invite a critical discussion of all theoretical ideas and concepts of workshop topics, and their consistency between particle physics, nuclear physics and cosmology. Also, existing experimental measurements and indications should be critically examined between theory and experiment. The organizing committee will be developing a schedule flexible enough to include all invited and contributed talks. We encourage participation especially by junior scientists.

Example of the day schedule

09:00 am – 09:45 am Talk 1 (30 mins + 15 mins)
09:45 am – 10:30 am Talk 2 (30 mins + 15 mins)
10:30 am – 11:00 am Coffee Break
11:00 am – 11:45 am Talk 3 (30 mins + 15 mins)
11:45 am – 12:30 pm Talk 4 (30 mins + 15 mins)
12:30 am – 03:30 pm Lunch/Discussion Break
03:30 pm – 04:00 pm Coffee Break
04:00 pm – 04:45 pm Talk 5 (30 mins + 15 mins)
04:45 pm – 05:30 pm Talk 6 (30 mins + 15 mins)

We encourage potential participants to submit applications (including title and abstract of you plan to contribute) as soon as possible since the accommodation resources might be limited. For the full consideration, please apply before May 31, 2017.

A workshop registration fee will apply. Amount to be determined. The registration fee includes participation in the workshop, lectures, and coffee breaks.

Physics Overview:

Neutron is a perfect laboratory for the search of physics beyond the Standard Model. Ettore Majorana first conjectured in 1937 that the neutron might be its own antiparticle. Although neutron and antineutron are well known to be different particles interacting differently with matter, the spontaneous transformation of neutron to antineutron or matter to antimatter remains an intriguing possibility in particle and nuclear physics. Such transformation, leading to the appearance of antineutron from neutron, would violate baryon number by two units and thus might serve as one of the three components of Sakharov’s conditions required for the explanation of baryon asymmetry of the universe. Transformations with ΔB = 2 are alternative and complementary to ΔB = 1 ("proton decay"). They appear naturally in the GUT models with broken quark-lepton symmetry and accompany the seesaw mechanism needed for small neutrino masses. The new physics will require violation of BL symmetry that is conserved in the Standard Model; it will include ΔL = 2 (Majorana neutrino) and ΔB = 2 (n ® n transformation) as two sides of the coin. Since non-perturbative sphaleron mechanism of the Standard Model destroys any baryon asymmetry that was created obeying the selection rule Δ(BL) = 0, a violation of (B - L) is an essential feature of physics beyond the Standard Model that is relevant for baryogenesis. Thus, transformation n ® n, if it exists, could offer a mechanism of baryogenesis, which is an alternative to the popular leptogenesis mechanism. Existing models of Post-Sphaleron Baryogenesis (PSB) predict n ® n transformations with probabilities that can be probed in the next-generation n ® n search experiments. Presently there is an international "NNbar collaboration" which plans for an n ® n search experiment at the European Spallation Source (ESS), the construction of which in Lund (Sweden) will be finished in 2019. The goal of the NNbar collaboration is to reach with free neutron cold beam experiment the sensitivity of antineutron appearance probability at a level >1000 times higher than the present level set by the reactor experiment at Grenoble more than two decades ago. In this experiment, PSB models can be tested for a significant domain of the phase space. Also, searches of n ® n transformation inside nuclei might be possible in the large next-generation liquid argon detector DUNE, presently under construction in South Dakota. Experimental sensitivity reach of the n ® n intranuclear search in DUNE will strongly depend on the possibility of atmospheric neutrino background suppression, an issue that is not yet fully understood by the community. The n ® n appearance probability of both types as measured with free neutrons and extracted from intranuclear transformations (after unfolding nuclear suppression effects) still might be different, thus indicating another layer of new physics BSM.

Neutron also provides a clean laboratory to study the validity of fundamental symmetries such as Lorentz invariance, CP, CPT and the equivalence principle. In some cases, the discovery of n ® n transformations with free neutrons would set the best limits on possible deviations from these principles. Currently there are vigorous discussions on these issues in the community.

The nature of Dark Matter (DM) is presently unknown. All evidences for existence of DM are coming from cosmological observations of the effects of its gravitational interactions and from the cosmic microwave background asymmetry measurements. Direct detection searches for heavy DM particles as motivated by the supersymmetric theories have been unsuccessful so far. Also, no supersymmetric DM candidate particles have been found at the LHC. Alternatively, DM can be made of light particles as indicated by several controversial direct detection experiments. It is possible that DM sector might consist of a "mirror copy" of the Standard Model (SM), an idea originated and developed starting with the work of Lee and Yang of 1956. In principle, the phenomenology of DM as mirror matter is well defined by the similarity of interactions within the mirror SM' and the ordinary SM. Thus, mirror dark matter would consist mostly of mirror hydrogen and mirror helium with possible non-controversial cosmology of stellar and galactic formation. An intriguing possibility that the mirror sector is made of mirror anti-matter is being discussed, allowing for natural and simultaneous co-baryogenesis of both asymmetric sectors. One of the remarkable conjectures of the last decade was that the transformations between neutral particles of ordinary and mirror matter might exist connecting two worlds through BSM mechanisms. One amazing possibility is that neutron n can oscillate into mirror neutron n′ with the oscillation time of ~ tens seconds that is not excluded from the existing observations. Oscillations n ® n′ would lead to disappearance of the neutrons from ordinary world with apparent violation of baryon number and energy. In the theoretical BSM mechanism describing n ® n′ a new heavy Majorana fermion and a color-triplet di-quark scalar (the latter might be observable at LHC) are introduced. In the n ® n′ oscillation, the combined baryon number B + B′ might be conserved, therefore this process is not heavily suppressed. Similar mechanism would lead also to the higher order process of neutron to antineutron transformation that violates ΔB by 2 unit and suppressed by large mass scale. Thus, n ® n in these schemes would be less probable process than n ® n′. The idea of n ® n′ oscillation has inspired several initial experimental searches with ultra-cold neutrons (UCN) in Europe. The results of these experiments, which are somewhat controversial, have not yet been completely understood by the community, but they do not exclude the n ® n′ oscillation interpretation with oscillation time in the tens seconds range. Still another disturbing experimental indication is the significant difference of the neutron lifetime measurements in appearance and disappearance modes. This difference also can be attributed to n ® n′ oscillation effect present in the UCN experiments. The claim of n ® n′ oscillation can be tested by independent experiment employing regeneration method. Such a new experiment is presently being pursued in US by the nn′ collaboration. Neutrons in the cold beam will oscillate to the mirror state in vacuum inside the laboratory magnetic field that will be varied in magnitude and direction to find a resonance with the unknown mirror magnetic field. Neutron beam will then be absorbed by a beam dump, but mirror neutron will not be absorbed and will oscillate under the same magnetic field in vacuum to return back to the state of neutron. The latter will be detected in the low background environment giving regeneration signal with resonance dependence on the laboratory magnetic field. More collaborative work between theorists and experimenters would be needed to understand current results, as well as to propose, pursue, and interpret further experiments.