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Register a new userExotic decay channels are not the cause of the neutron lifetime anomaly
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Since long neutron lifetimes measured with a beam of cold neutrons are significantly different from lifetimes measured with ultracold neutrons bottled in a trap. It is often speculated that this neutron anomaly is due to an exotic dark neutron decay channel of unknown origin. We show that this explanation of the neutron anomaly can be excluded with a high level of confidence when use is made of new data on neutron decay parameters. Furthermore, data from neutron decay now compare well with Ft data derived from nuclear b{eta} decays.
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Free neutrons have a measured lifetime of 880 s, but disagreement between existing laboratory measurements of ~10 s have persisted over many years. This uncertainty has implications for multiple physics disciplines, including standard-model particle physics and Big-Bang nucleosynthesis. Space-based neutron lifetime measurements have been shown to be feasible using existing data taken at Venus and the Moon, although the uncertainties for these measurements of tens of seconds prevent addressing the current lifetime discrepancy. We investigate the implementation of a dedicated space-based experiment that could provide a competitive and independent lifetime measurement. We considered a variety of scenarios, including measurements made from orbit about the Earth, Moon, and Venus, as well as on the surface of the Moon. For a standard-sized neutron detector, a measurement with three-second statistical precision can be obtained from Venus orbit in less than a day; a one-second statistical precision can be obtained from Venus orbit in less than a week. Similarly precise measurements in Earth orbit and on the lunar surface can be acquired in less than 40 days (three-second precision) and ~300 days (one-second precision). Systematic uncertainties that affect a space-based neutron lifetime measurement are investigated, and the feasibility of developing such an experiment is discussed.
The results of measurements performed using UCN storing method are in good agreement. The latest most accurate measurements of the neutron decay asymmetry and neutron lifetime measurements by storage method are in agreement within the Standard Model. However, there is a significant discrepancy at $3.6sigma$ (1% of decay probability) level with beam method experiment. This article discusses the possible causes of discrepancy in the measurements of the neutron lifetime with beam method experiment. The most probable cause, apparently, is the loss of protons in beam method experiment during storage in a magnetic trap due to charge exchange collisions of protons with the residual gas. The proton becomes neutral and leaves the trap, which leads to a decrease in the number of registered protons, i.e. to a decrease in the probability of neutron decay or to an increase in the measured neutron lifetime.
The review of experimental measurements of neutron lifetime is presented. Latest measurements with gravitational trap (PNPI NRC KI) and magnetic trap (LANL, USA) confirmed the result obtained by PNPI group in 2005. The results of measurements performed using UCN storing method are in good agreement; however, there is a significant discrepancy at 3.6{sigma} (1% of decay probability) level with beam method experiment. The latest most accurate measurements of the neutron decay asymmetry and neutron lifetime measurements by storage method are in agreement within the Standard Model. This article discusses the possible causes of discrepancy in the measurements of the neutron lifetime. The most probable cause, apparently, is the loss of protons in beam method experiment during storage in a magnetic trap due to charge exchange collisions of protons with the residual gas. The proton becomes neutral and leaves the trap, which leads to a decrease in the number of registered protons, i.e. to a decrease in the probability of neutron decay or to an increase in the measured neutron lifetime.
We report an improved measurement of the free neutron lifetime $tau_{n}$ using the UCN$tau$ apparatus at the Los Alamos Neutron Science Center. We counted a total of approximately $38times10^{6}$ surviving ultracold neutrons (UCN) after storing in UCN$tau$s magneto-gravitational trap over two data acquisition campaigns in 2017 and 2018. We extract $tau_{n}$ from three blinded, independent analyses by both pairing long and short storage-time runs to find a set of replicate $tau_{n}$ measurements and by performing a global likelihood fit to all data while self-consistently incorporating the $beta$-decay lifetime. Both techniques achieve consistent results and find a value $tau_{n}=877.75pm0.28_{text{ stat}}+0.22/-0.16_{text{ syst}}$~s. With this sensitivity, neutron lifetime experiments now directly address the impact of recent refinements in our understanding of the standard model for neutron decay.
We present the status of current US experimental efforts to measure the lifetime of the free neutron by the beam and bottle methods. BBN nucleosynthesis models require accurate measurements with 1 second uncertainties, which are currently feasible. For tests of physics beyond the standard model, future efforts will need to achieve uncertainties well below 1 second. We outline paths achieve both.
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