Question: In particle physics, a massless particle is an elementary particle whose invariant mass is zero, the discovery of this phenomenon, known as neutrino oscillation, led to which Japanese physicist, known for neutrino experiments at the Kamiokande and its successor, Super-Kamiokande, shared the 2015 Nobel prize in physics?
Context: Inverse beta decay, commonly abbreviated to IBD, is a nuclear reaction involving electron antineutrino scattering off a proton, creating a positron and a neutron.  This process is commonly used in the detection of electron antineutrinos in neutrino detectors, such as the first detection of antineutrinos in the Cowan–Reines neutrino experiment, or in neutrino experiments such as KamLAND, Borexino, and SAGE (Soviet–American Gallium Experiment).  It is an essential process to experiments involving low energy neutrinos (< 60 MeV) such as those studying neutrino oscillation, reactor neutrinos, sterile neutrinos, and geoneutrinos.  The IBD reaction can only be used to detect antineutrinos (rather than normal matter neutrinos, such as from the sun) due to lepton conservation. Takaaki Kajita (梶田 隆章 , Kajita Takaaki ) is a Japanese physicist, known for neutrino experiments at the Kamiokande and its successor, Super-Kamiokande.  In 2015, he was awarded the Nobel Prize in Physics jointly with Canadian physicist Arthur B. McDonald. In particle physics, a massless particle is an elementary particle whose invariant mass is zero.  The two known massless particles are both gauge bosons: the photon (carrier of electromagnetism) and the gluon (carrier of the strong force).  However, gluons are never observed as free particles, since they are confined within hadrons. Neutrinos were originally thought to be massless.  However, because neutrinos change flavor as they travel, at least two of the types of neutrinos must have mass.  The discovery of this phenomenon, known as neutrino oscillation, led to Canadian scientist Arthur B. McDonald and Japanese scientist Takaaki Kajita sharing the 2015 Nobel prize in physics. In quantum field theory, specifically the theory of renormalization, the bare mass of an elementary particle is the limit of its mass as the scale of distance approaches zero or, equivalently, as the energy of a particle collision approaches infinity.  It differs from the invariant mass as usually understood because the latter includes the 'clothing' of the particle by pairs of virtual particles that are temporarily created by the force-fields around the particle.  In some versions of field theory, the bare mass of some particles may be plus or minus infinity.  In the theory of the electroweak interaction using the Higgs boson, all particles have a bare mass of zero. The Kamioka Observatory, Institute for Cosmic Ray Research (神岡宇宙素粒子研究施設 , Kamioka Uchū Soryūshi Kenkyū Shisetsu ) is a neutrino and gravitational waves laboratory located underground in the Mozumi Mine of the Kamioka Mining and Smelting Co. near the Kamioka section of the city of Hida in Gifu Prefecture, Japan.  A set of groundbreaking neutrino experiments have taken place at the observatory over the past two decades.  All of the experiments have been very large and have contributed substantially to the advancement of particle physics, in particular to the study of neutrino astronomy and neutrino oscillation. Lorentz-violating neutrino oscillation refers to the quantum phenomenon of neutrino oscillations described in a framework that allows the breakdown of Lorentz invariance.  Today, neutrino oscillation or change of one type of neutrino into another is an experimentally verified fact; however, the details of the underlying theory responsible for these processes remain an open issue and an active field of study.  The conventional model of neutrino oscillations assumes that neutrinos are massive, which provides a successful description of a wide variety of experiments; however, there are a few oscillation signals that cannot be accommodated within this model, which motivates the study of other descriptions.  In a theory with Lorentz violation neutrinos can oscillate with and without masses and many other novel effects described below appear.  The generalization of the theory by incorporating Lorentz violation has shown to provide alternative scenarios to explain all the established experimental data through the construction of global models. The Liquid Scintillator Neutrino Detector (LSND) was a scintillation counter at Los Alamos National Laboratory that measured the number of neutrinos being produced by an accelerator neutrino source.  The LSND project was created to look for evidence of neutrino oscillation, and its results conflict with the standard model expectation of only three neutrino flavors, when considered in the context of other solar and atmospheric neutrino oscillation experiments.  Cosmological data bound the mass of the sterile neutrino to m < 0.26eV (0.44eV) at 95% (99.9%) confidence limit, excluding at high significance the sterile neutrino hypothesis as an explanation of the LSND anomaly.  The controversial LSND result was tested by the MiniBooNE experiment at Fermilab, which refuted a simple 2-neutrino oscillation interpretation of the LSND result. Main injector neutrino oscillation search (MINOS) was a particle physics experiment designed to study the phenomena of neutrino oscillations, first discovered by a Super-Kamiokande (Super-K) experiment in 1998.  Neutrinos produced by the NuMI ("Neutrinos at Main Injector") beamline at Fermilab near Chicago are observed at two detectors, one very close to where the beam is produced (the "near detector"), and another much larger detector 735 km away in northern Minnesota (the "far detector"). Astroparticle physics, also called particle astrophysics, is a branch of particle physics that studies elementary particles of astronomical origin and their relation to astrophysics and cosmology.  It is a relatively new field of research emerging at the intersection of particle physics, astronomy, astrophysics, detector physics, relativity, solid state physics, and cosmology.  Partly motivated by the discovery of neutrino oscillation, the field has undergone rapid development, both theoretically and experimentally, since the early 2000s. Kam-Biu Luk (, born 1953) is a professor of physics, with a focus on particle physics, at UC Berkeley and a senior faculty member in the Lawrence Berkeley National Laboratory's physics division.  Luk has conducted research on neutrino oscillation and CP violation.  Luk and his collaborator Yifang Wang were awarded the 2014 Panofsky Prize “for their leadership of the Daya Bay experiment, which produced the first definitive measurement of θ angle of the neutrino mixing matrix.”  His work on neutrino oscillation also received 2016 Breakthrough Prize in Fundamental Physics shared with other teams.  Luk is a fellow of the American Physical Society.
Answer: Takaaki Kajita