Search

The decay lifetime of free neutrons (∼880 s) is an important parameter of the weak interaction and for Big Bang Nucleosynthesis. However, results of measurements currently show discrepancies depending on the method used. As most experiments nowadays employ ultra cold neutrons, we have developed a new cold-beam experiment which we perform at the Japan Proton Accelerator Research Complex. As a special feature, a polarized neutron beam is bunched by a spin flip chopper. A time projection chamber operated with He and CO2 gas, including a well-controlled amount of 3He, is used for detection of the beta-decays and simultaneous determination of the beam intensity. Using the data between 2014 and 2016, we evaluated our first, preliminary result of the neutron lifetime as 896 ± 10(stat.) −10+14(sys.) s. We plan several upgrades to achieve our precision goal of 1 s.

The helium standard of Japan, referred to as HESJ, is an interlaboratory standard for the 3He/4He ratio. While the ratio of 3He and 4He of the HESJ was previously determined by a relative comparison to atmospheric helium, the absolute value of the 3He/4He ratio of the HESJ has not been directly determined yet. Therefore, it relies on the early measurements of that of atmospheric helium. The accuracy of the absolute 3He/4He ratios of the atmosphere and other working standards including HESJ is crucial in some applications of helium isotopes, such as tritium‐3He dating, surface‐exposure age determination based on cosmogenic 3He, and the accurate measurement of the neutron lifetime. In this work, new control samples of helium gases with 3He/4He ratios of 14, 28, and 42 ppm were fabricated with accuracy of 0.25–0.38% using a gas‐handling system for a neutron lifetime experiment at Japan Proton Accelerator Research Complex. The relative 3He/4He ratios of these samples and the HESJ were measured using a magnetic‐sector type, single‐focusing, noble gas mass spectrometer with a double collector system. As a result, the absolute 3He/4He ratio of the HESJ was determined as 27.36 ± 0.11 ppm. The atmospheric 3He/4He ratio was determined as 1.340 ± 0.006 ppm, based on this work. Plain Language Summary: Helium has two isotopes, 3He and 4He. The two isotopes have different origins. 3He in the Earth is a vestige of that which was originally trapped during the Earth's formation, whereas 4He generated mainly by radioactive decays of uranium and thorium has been added to primordial 4He, which means that the ratio of the two isotopes of helium can be used as a powerful tracer in geochemistry and cosmochemistry. In this work, we accurately measured the 3He/4He ratio of a standard calibrator by using an apparatus for a particle physics experiment and determined its value with an accuracy of 0.4%. By using the present result, the 3He/4He ratio of atmospheric helium was determined with best the accuracy ever than that before. Key Points: The absolute 3He/4He ratio of the helium standard of Japan (HESJ) was determined with accuracy of 0.40%The atmospheric 3He/4He ratio was determined as 1.340 ± 0.006 ppm [ABSTRACT FROM AUTHOR]

It has been widely known that bino-like dark matter in the supersymmetric (SUSY) theories in general suffers from over-production. The situation can be drastically improved if gluinos have a mass slightly heavier than the bino dark matter as they reduce the dark matter abundance through coannihilation. In this work, we consider such a bino–gluino coannihilation scenario in high-scale SUSY models, which can be actually realized when the squark-mass scale is less than 100–1000 TeV. We study the prospects for exploring this bino–gluino coannihilation scenario at the LHC. We show that the searches for long-lived colored particles with displaced vertices or large energy loss offer a strong tool to test this scenario in collider experiments. [ABSTRACT FROM AUTHOR]

Higgsino in supersymmetric standard models is known to be a promising candidate for dark matter in the Universe. Its phenomenological property is strongly affected by the gaugino fraction in the Higgsino-like state. If this is sizable, in other words, if gaugino masses are less than O ( 10 ) TeV, we may probe the Higgsino dark matter in future non-accelerator experiments such as dark matter direct searches and measurements of electric dipole moments. On the other hand, if gauginos are much heavier, then it is hard to search for Higgsino in these experiments. In this case, due to a lack of gaugino components, the mass difference between the neutral and charged Higgsinos is uniquely determined by electroweak interactions to be around 350 MeV, which makes the heavier charged state rather long-lived, with a decay length of about 1 cm. In this letter, we argue that a charged particle with a flight length of O ( 1 ) cm can be probed in disappearing-track searches if we require only two hits in the pixel detector. Even in this case, we can reduce background events with the help of the displaced-vertex reconstruction technique. We study the prospects of this search strategy at the LHC and future colliders for the Higgsino dark matter scenario. It is found that an almost pure Higgsino is indeed within the reach of the future 33 TeV collider experiments. We then discuss that the interplay among collider and non-accelerator experiments plays a crucial role in testing the Higgsino dark matter scenarios. Our strategy for disappearing-track searches can also enlarge the discovery potential of pure wino dark matter as well as other electroweak-charged dark matter candidates. [ABSTRACT FROM AUTHOR]

Particles with a sub-millimeter decay length appear in many models of physics beyond the Standard Model. However, their longevity has been often ignored in their LHC searches and they have been regarded as promptly-decaying particles. In this letter, we show that, by requiring displaced vertices on top of the event selection criteria used in the ordinary search strategies for promptly-decaying particles, we can considerably extend the LHC reach for particles with a decay length of ≳ 100 μm . We discuss a way of reconstructing sub-millimeter displaced vertices by exploiting the same technique used for the primary vertex reconstruction on the assumption that the metastable particles are always pair-produced and their decay products contain high- p T jets. We show that, by applying a cut based on displaced vertices on top of standard kinematical cuts for the search of new particles, the LHC reach can be significantly extended if the decay length is ≳ 100 μm . In addition, we may measure the lifetime of the target particle through the reconstruction of displaced vertices, which plays an important role in understanding the new physics behind the metastable particles. [ABSTRACT FROM AUTHOR]