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1. Experimental Approach to Search for Free Neutron-Antineutron Oscillations Based on Coherent Neutron and Antineutron Mirror Reflection

   https://doi.org/10.1103/PhysRevLett.122.221802  

   Nesvizhevsky, V. V.; Gudkov, V.; Protasov, K. V.; Snow, W. M.; Voronin, A. Yu. (June 2019, Physical Review Letters)
2. The effect of collisions with the wall on neutron-antineutron transitions

   https://doi.org/10.1016/j.physletb.2019.06.041  

   Kerbikov, B.O. (August 2019, Physics Letters B)
3. Neutron-Antineutron Oscillation Search using a 0.37 Megaton·Year Exposure of Super-Kamiokande

   Super-Kamiokande, Collaboration (January 2021, Physical review)

   We performed a n − ¯n oscillation search with SK-I-IV data using a multivariate analysis. Compared to previous results [15], the updated final state interaction model predicts fewer pions and less separation between signal and neutrino backgrounds. With the advanced MVA method and the inclusion of multiple new variables, the sensitivity of this analysis is still greatly enhanced. For the 0.37 megaton-year exposure at SK, we observed 11 events with an expected background of 9.3  2.7 events. There is no statistically significant excess of data events, so a lower limit on the neutron lifetime is set at 3.6 × 1032 years at 90% C.L., corresponding to a lower limit on the neutron-antineutron oscillation time in 16O of τn→¯n > 4.7 × 108 s. This is the world’s most stringent limit on neutron-antineutron oscillation so far, with 90% improvement from the previous best limit [15], and is reaching the predicted parameter space of some theoretical models. 

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4. A new approach to search for free neutron-antineutron oscillations using coherent neutron propagation in gas

   https://doi.org/10.1016/j.physletb.2020.135636  

   Gudkov, V.; Nesvizhevsky, V.V.; Protasov, K.V.; Snow, W.M.; Voronin, A.Yu. (September 2020, Physics Letters B)

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5. First application of a liquid argon time projection chamber for the search for intranuclear neutron-antineutron transitions and annihilation in ⁴⁰ Ar using the MicroBooNE detector

   https://doi.org/10.1088/1748-0221/19/07/P07032  

   Abratenko, P; Alterkait, O; Andrade_Aldana, D; Arellano, L; Asaadi, J; Ashkenazi, A; Balasubramanian, S; Baller, B; Barr, G; Barrow, D; et al (July 2024, Journal of Instrumentation)

   Abstract We present a novel methodology to search for intranuclear neutron-antineutron transition (n⟶n̅) followed byn̅-nucleon annihilation within an⁴⁰Ar nucleus, using the MicroBooNE liquid argon time projection chamber (LArTPC) detector. A discovery of n⟶n̅transition or a new best limit on the lifetime of this process would either constitute physics beyond the Standard Model or greatly constrain theories of baryogenesis, respectively. The approach presented in this paper makes use of deep learning methods to select n⟶n̅events based on their unique features and differentiate them from cosmogenic backgrounds. The achieved signal and background efficiencies are (70.22 ± 6.04)% and (0.0020 ± 0.0003)%, respectively. A demonstration of a search is performed with a data set corresponding to an exposure of 3.32 ×10²⁶neutron-years, and where the background rate is constrained through direct measurement, assuming the presence of a negligible signal. With this approach, no excess of events over the background prediction is observed, setting a demonstrative lower bound on the n⟶n̅lifetime in⁴⁰Ar of τ_m≳ 1.1×10²⁶years, and on the free n⟶n̅transition time of τ_n⟶n̅≳ 2.6×10⁵s, each at the 90% confidence level. This analysis represents a first-ever proof-of-principle demonstration of the ability to search for this rare process in LArTPCs with high efficiency and low background. 

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