Measurement of Neutrino Oscillation by the K2K Experiment

Abstract

We present measurements of nu_mu disappearance in K2K, the KEK to Kamioka long-baseline neutrino oscillation experiment. One hundred and twelve beam-originated neutrino events are observed in the fiducial volume of Super-Kamiokande with an expectation of 158.1^{+9.2}_{-8.6} events without oscillation. A distortion of the energy spectrum is also seen in 58 single-ring muon-like events with reconstructed energies. The probability that the observations are explained by the expectation for no neutrino oscillation is 0.0015% (4.3sigma). In a two flavor oscillation scenario, the allowed Delta m^2 region at sin^2(2theta) is between 1.9 and 3.5 x 10^{-3} eV^2 at the 90% C.L. with a best-fit value of 2.8 x 10^{-3} eV^2.

Figures (48)

• Figure 1: A schematic view of the KEK-PS and neutrino beam line and the location of beam line components. The EP1 neutrino beam line leads protons through a distance of 400 m from the EP1-A extraction point to the target station via the straight and arc sections. The characters "C" and "S" in the figure show the locations of the CT and SPIC installations, respectively. The lower-left inset is a magnified view of the target station. The production target and a set of horn magnets are located in the target station. A pion monitor was installed on two occasions downstream the horn magnets.
• Figure 2: Schematic view of the two horn magnets. An electrical current of 250 kA is supplied to both horns, creating a toroidal magnetic field inside the horns. The production target, an aluminum rod of 66 cm in length and 3 cm in diameter, is embedded inside the first horn magnet, which also plays the role of inner conductor of the horn. The second horn is located 10.5 m downstream of the first horn.
• Figure 3: A schematic view inside the muon-pit. An ionization chamber (ICH) and an array of silicon pad detectors (SPD) are located inside the muon-pit.
• Figure 4: The number of protons delivered to the production target in the period from March 1999 to November 2004. The horizontal axis corresponds to the date. The upper figure shows the total number of protons on target (POT) accumulated since March 1999, and the lower figure shows the POT per spill averaged in a day. In total, $104.90\times10^{18}$ protons were delivered during the entire period including beam commissioning and tuning periods.
• Figure 5: Stability of the center of muon profile measured by the ionization chamber (ICH) in MUMON. The upper figure shows the profile center of the horizontal direction and the lower figure shows that of the vertical direction. In each figure, the beam direction to SK measured by GPS and $\pm1~{\rm mrad}$ off the center are indicated by horizontal lines. The data shown here are after good beam selection.