The T2K experiment is a long baseline neutrino oscillation experiment. Its main goal is to measure the last unknown lepton sector mixing angle θ13θ13 by observing νeνe appearance in a νμνμ beam. It also aims to make a precision measurement of the known oscillation parameters, View the MathML sourceΔm232 and sin22θ23sin22θ23, via νμνμ disappearance studies. Other goals of the experiment include various neutrino cross-section measurements and sterile neutrino searches. The experiment uses an intense proton beam generated by the J-PARC accelerator in Tokai, Japan, and is composed of a neutrino beamline, a near detector complex (ND280), and a far detector (Super-Kamiokande) located 295 km away from J-PARC. This paper provides a comprehensive review of the instrumentation aspect of the T2K experiment and a summary of the vital information for each subsystem

The detection of a sterile neutrino could constitute the first observation of a particle that could have been produced before Big-Bang Nucleosynthesis (BBN), and could provide information about the yet untested pre-BBN era. The cosmological evolution in this era could be drastically different than typically assumed, in what constitutes the standard cosmology, as happens in a variety of motivated particle models. In this work we assess the sensitivity to different pre-BBN cosmologies in which entropy is conserved of 0.01 eV to 1 MeV mass sterile neutrinos produced in the early Universe via resonant active-sterile oscillations, which requires a large lepton asymmetry. We identify mass ranges where it is possible to have two populations of the same sterile neutrino, one with a colder and one with a hotter momentum spectra, which is in principle an observable effect. Furthermore, we show the regions in mass and mixing where fully resonant production (i.e. simultaneously coherent and adiabatic) can occur. We find that in all the cosmologies we consider, including the standard one, fully resonantly produced sterile neutrinos in the eV-mass range can evade all cosmological constraints.In the absence of a large lepton asymmetry the oscillations are non-resonant and sterile neutrinos are produced via the Dodelson-Widrow (DW) mechanism [45]. In the standard cosmology this mechanism results in a Fermi-Dirac relic momentum distribution of sterile neutrinos, with a reduced magnitude with respect to active neutrinos. Several studies have already been carried out on non-resonantly produced sterile neutrinos in different pre-BBN cosmologies ). This work is complementary to these previous studies in that we concentrate on the production of sterile neutrinos via resonant active-sterile oscillations, an often considered production mechanism that requires a significant lepton asymmetry in active neutrinos (this is the Shi-Fuller mechanism [52], see also Ref. [53,54]). In this case, sterile neutrinos are produced with a colder momentum distribution (i.e. with a lower average momentum) that is different from a Fermi-Dirac spectrum, even in the standard cosmology. In particular, we study the effect of different cosmologies on resonantly produced sterile neutrinos with mass 10 −2 eV < m s < 1 MeV. Since the production rate is usually not fast enough for sterile neutrinos to equilibrate, the final relic abundance and spectrum are fixed by freeze-in.We will analyze resonant sterile neutrino production within several example cosmological models in which entropy is conserved, characterized by the magnitude and temperature dependence of the Hubble expansion rate H in the non-standard cosmological phase. If H is larger than it would be in the standard cosmology, the resonant production of sterile neutrinos during this phase is suppressed with respect to standard production, and if H is lower, the production is enhanced. If the production is fully resonant, which requires adiabaticity and coherence at the resonance, the relic sterile neutrino ...

A laboratory detection of a sterile neutrino could provide the first indication of the evolution of the Universe before Big-Bang Nucleosynthesis (BBN), an epoch yet untested. Such "visible" sterile neutrinos are observable in upcoming experiments such as KATRIN/TRISTAN and HUNTER in the keV mass range and PTOLEMY and others in the eV mass range. A set of standard assumptions is typically made about cosmology before the temperature of the Universe was 5 MeV. However, non-standard pre-BBN cosmologies based on alternative assumptions could arise in motivated theoretical models and are equally in agreement with all existing data. We revisit the production of sterile neutrinos of mass 0.01 eV to 1 MeV in two examples of such models: scalar-tensor and low reheating temperature pre-BBN cosmologies. In both of them, the putative 3.5 keV X-ray signal line corresponds to a sterile neutrino with a mixing large enough to be tested in upcoming laboratory experiments. Additionally, the cosmological/astrophysical upper limits on active-sterile neutrino mixings are significantly weaker than in the standard pre-BBN cosmology, which shows that these limits are not robust. For example, in the scalar-tensor case the potential signal regions implied by the LSND and MiniBooNE short-baseline as well as the DANSS and NEOS reactor experiments are entirely not bound by cosmological restrictions. Our work highlights that sterile neutrinos may constitute a sensitive probe of the pre-BBN epoch.The cosmological evolution of the Universe before its temperature was T = 5 MeV is unknown because we have not detected so far any remnant from it. The earliest cosmological remnants are so far the light nuclei produced during Big Bang Nucleosynthesis (BBN) and if the highest temperature of the radiation-domination epoch in which BBN happened was just 5 MeV, BBN and all the subsequent evolution of the Universe would be unchanged [1][2][3][4][5][6]. Assumptions are made about cosmology to compute the relic abundance and momentum distributions of dark matter (DM) particles which are produced in this pre-BBN epoch. The standard assumptions are that the Universe was radiation-dominated, that only Standard Model (SM) particles are present and that no extra entropy in matter and radiation is produced. We call this set of assumptions the standard pre-BBN cosmology, which is an extension at higher temperatures of the standard cosmology we know at lower temperatures, T < 5 MeV. However, cosmologies based on alternative assumptions are equally in agreement with all existing data. The pre-BBN cosmological evolution could drastically differ from standard in some well motivated theoretical models, e.g. some based on moduli, extra dimensions or quintessence. A non-standard cosmological evolution, consistent with all existing bounds, could drastically affect the properties of any relics produced before the temperature of the Universe was 5 MeV. Detecting any relics sensitive to the pre-BBN cosmological history will open a new window into this yet unexplored epoch. Th...

Black holes formed in the early universe, prior to the formation of stars, can exist as dark matter and also contribute to the black hole merger events observed in gravitational waves. We set a new limit on the abundance of primordial black holes (PBHs) by considering interactions of PBHs with the interstellar medium, which result in the heating of gas. We examine generic heating mechanisms, including emission from the accretion disk, dynamical friction, and disk outflows. Using the data from the Leo T dwarf galaxy, we set a new cosmology-independent limit on the abundance of PBHs in the mass range , relevant for the recently detected gravitational-wave signals from intermediate-mass BHs.

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