Today (March 26, 2015) at 15:25 the J-PARC accelerator achieved the milestone of delivering 1×1021 protons on target to T2K. Many thanks are due to the J-PARC directorate and the staff members for their outstanding work to make this possible !
The protons are produced by J-PARC in Tokai on the east coast of Japan. They are made to collide with a graphite target, with one of the products of these collisions being charged pions. These pions pass through a series of three magnetic horns, which are able to preferentially focus either positively charged or negatively charged pions in the direction of T2K’s two detectors. Positively charged pions quickly decay to muon neutrinos and antimuons, while negatively charged pions decay to muon antineutrinos and muons. This means that T2K can choose whether its beam is composed mostly of muon neutrinos or mostly of muon antineutrinos by changing the direction of the magnetic horn current (the muons or antimuons and any remaining pions are stopped by a second layer of graphite at ~100m downstream of the target).
The number of protons on target delivered by J-PARC to T2K is a measure of how much data T2K has collected. T2K started taking physics data in January 2010, and collected data in neutrino beam mode until May 2014. Since that time, T2K has taken data in antineutrino beam mode. The figure of 1×1021 is the total number of protons on target delivered to T2K between January 2010 and today in both modes.