Question: The Canadian astrophysicist born in 1943 won the 2015 Nobel prize in physics for his discovery of what phenomenon? 
Context: The Nobel Prize in Physics (Swedish: "Nobelpriset i fysik" ) is a yearly award given by the Royal Swedish Academy of Sciences for those who conferred the most outstanding contributions for mankind in the field of physics.  It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others being the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and Nobel Prize in Physiology or Medicine. Aage Niels Bohr (] ; 19 June 1922 – 8 September 2009) was a Danish nuclear physicist who shared the Nobel Prize in Physics in 1975 with Ben Mottelson and James Rainwater "for the discovery of the connection between collective motion and particle motion in atomic nuclei and the development of the theory of the structure of the atomic nucleus based on this connection".  Starting from Rainwater's concept of an irregular-shaped liquid drop model of the nucleus, Bohr and Mottelson developed a detailed theory that was in close agreement with experiments.  Since his father, Niels Bohr, had won the prize in 1922, he and his father were one of the six pairs of fathers and sons who have both won the Nobel Prize and one of the four pairs who have both won the Nobel Prize in Physics. 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. SPEAR (originally Stanford Positron Electron Asymmetric Rings, now simply a name) was a collider at the SLAC National Accelerator Laboratory.  It began running in 1972, colliding electrons and positrons with an energy of .  During the 1970s, experiments at the accelerator played a key role in particle physics research, including the discovery of the meson (awarded the 1976 Nobel Prize in physics), many charmonium states, and the discovery of the tau (awarded the 1995 Nobel Prize in physics). Calvin Barth Netterfield (born 29 April 1968), known as Barth Netterfield, is a Canadian astrophysicist, and a Professor in the Department of Astronomy and the Department of Physics at the University of Toronto.  He is a leading expert in the development of balloon-borne telescopes.  These are astrophysical experiments that are lifted into the stratosphere by high-altitude balloons where they conduct observations that would be hindered by atmospheric interference if done on the ground.  Netterfield is primarily known for his work in observational cosmology, specifically in developing instrumentation to observe the cosmic microwave background (CMB) radiation.  Most notably, he was a key member of the instrument team for BOOMERANG, the experiment that made one of the first accurate determinations of the age, geometry, and mass-energy content of the universe.  More recently, he has delved into the field of submillimetre astronomy and the physics of star formation, through his involvement with the BLAST telescope.  Netterfield was featured prominently in BLAST! , a documentary film about the 2005 and 2006 flights of BLAST from Sweden and Antarctica. Tu Youyou (; born 30 December 1930) is a Chinese pharmaceutical chemist and educator.  She is best known for discovering artemisinin (also known as "qinghaosu") and dihydroartemisinin, used to treat malaria, and which has saved millions of lives.  Tu Youyou's discovery of artemisinin and its treatment of malaria is regarded as a significant breakthrough in 20th century tropical medicine and an important health improvement for people of tropical developing countries in South Asia, Africa, and South America.  For her work, Tu received the 2011 Lasker Award in clinical medicine and the 2015 Nobel Prize in Physiology or Medicine jointly with William C. Campbell and Satoshi Ōmura.  Tu is the first Chinese Nobel laureate in physiology or medicine and the first female citizen of the People's Republic of China to receive a Nobel Prize in any category, as well as the first Chinese person to receive the Lasker Award.  Tu Youyou was born and educated and carried out research exclusively in China. Since 1901, the Nobel Prize in Literature (Swedish: "Nobelpriset i litteratur" ) has been awarded annually to an author from any country who has, in the words of the will of Alfred Nobel, produced "in the field of literature the most outstanding work in an ideal direction" (original Swedish: "den som inom litteraturen har producerat det mest framstående verket i en idealisk riktning").  Though individual works are sometimes cited as being particularly noteworthy, here "work" refers to an author's work as a whole.  The Swedish Academy decides who, if anyone, will receive the prize in any given year.  The academy announces the name of the chosen laureate in early October.  It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895; the others are the Nobel Prize in Chemistry, Nobel Prize in Physics, Nobel Peace Prize, and Nobel Prize in Physiology or Medicine. Arthur Bruce McDonald, {'1': ", '2': ", '3': ", '4': "} (born August 29, 1943) is a Canadian astrophysicist.  McDonald is the director of the Sudbury Neutrino Observatory Institute and holds the Gordon and Patricia Gray Chair in Particle Astrophysics at Queen's University in Kingston, Ontario.  He was awarded the 2015 Nobel Prize in Physics jointly with Japanese physicist Takaaki Kajita. The Nobel Prize in Physiology or Medicine (Swedish: "Nobelpriset i fysiologi eller medicin" ) is awarded annually by the Swedish Karolinska Institute to scientists and doctors in the various fields of physiology or medicine.  It is one of the five Nobel Prizes established by the 1895 will of Alfred Nobel (who died in 1896), awarded for outstanding contributions in chemistry, physics, literature, peace, and physiology or medicine.  As dictated by Nobel's will, the award is administered by the Nobel Foundation and awarded by a committee that consists of five members and an executive secretary elected by the Karolinska Institute.  While commonly referred to as the Nobel Prize in Medicine, Nobel specifically stated that the prize be awarded for "physiology or medicine" in his will.  Because of this, the prize can be awarded in a broader range of fields.  The first Nobel Prize in Physiology or Medicine was awarded in 1901 to Emil Adolf von Behring, of Germany.  Each recipient receives a medal, a diploma and a monetary award that has varied throughout the years.  In 1901, von Behring received 150,782 SEK, which is equal to 7,731,004 SEK in December 2008.  In 2013, the prize was awarded to James E. Rothman, Randy W. Schekman and Thomas C. Südhof; they were recognised "after discovering how cells precisely transport material".  The award is presented in Stockholm at an annual ceremony on December 10, the anniversary of Nobel's death. This is a list of Danish Nobel laureates.  Since the Nobel Prize was established per the will of Swedish inventor Alfred Nobel in 1895, 12 of the prize winners have been from Denmark.  The first Danish Nobel laureate was Niels Ryberg Finsen, who won a Nobel Prize for medicine in 1903 for his work in using light therapy to treat diseases.  The most recent Danish Nobel Prize winner was Jens Skou who won the prize in chemistry for his discovery over the enzyme, Na+/K+-ATPase in 1997.  To date, of the 13 Nobel Prizes won by Danish people, 5 have been for medicine, 3 have been for physics, 3 have been for literature, 1 has been for chemistry and one has been for peace.
Answer: neutrino oscillation