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Karol Lang: Selected Topics from Minos

March 12, 2013

Karol explained at the start of his presentation that he focuses on results that came out recently. Minos is a two-detector experiment running on the long-baseline beam from Fermilab to the Soudan mine in Minnesota. There are two magnetized detectors allowing projection from near to far side. The intensity used to be the highest in the world (320 kW beam) before CERN stripped it of the primacy (the speaker attempted to say it was the highest, but was corrected by a person from the audience).

In 7 years they got 15.6×10^20 protons on target. The two detectors are tracking detectors using WLS fibers in scintillator strips, and the topology of the events is easy: long muons for charged current, or nuclear recoils for neutral current events. Charged-current electron-neutrino events are harder to classify.

The disappearance of neutrinos from beam and atmospheric events is studied in datasets of several thousand events. The results all together can fit to a model when nus and anti-nus oscillate the same way (two-parameter fit) they get a result of dm^2=2.41+-0.10 *10^-3 eV^2, and sin^2(2theta)=0.95+-0.036. This means an indication of non-maximal mixing: they also sort-of “exclude” sin^2=1 at a bit less than 2-sigma. The fit has 15 components, and systematics are still dominating the measurement.

They also perform a 4-parameter fit, where they let the delta-m’s and angles to be different for nu’s and anti-nu’s. They can compare the fit dm^2, and this is zero within error bars.

They also do a search for electron neutrino appearance. Since they cannot distinguish between electron neutrino and antineutrino events, they combine the two. The oscillation of muon neutrinos into electron neutrinos can be studied because electron-neutrino events can be tagged by using a discriminant variable which has a very good separation. They plot energy spectra in bins of the variable which distinguishes the signal from backgrounds, and they seem to obtain some mild indication of a signal in the high-S/N bins. Assuming the normal hierarchy, they obtain measurements of sin^2(theta_23)*sin^2(2*theta_13)=0.051+0.038-0.030. Assuming inverted hierarchy they constrain the quantity to 0.093+0.054-0.049.

They can use reactor theta_13 information with their electron-neutrino results to place constraints on the octant theta_23 versus delta. However the results are not yet conclusive here. Combining data from different sources is of course necessary in these studies.

In the future they are going to develop a three-flavour mixing framework, studying their sensitivity. They expect slight improvements in the sensitivity.

Finally, a recent result which is “obligatory” is the time of flight measurement of neutrinos, after the now-retracted claim of Opera. They determine the difference in arrival time with respect to expectation for speed-of-light travel to be -2.4 +-0.1 +-2.6 nanoseconds. The systematics dominate the uncertainty, mainly due to the inertial survey at the far detector in Soudan.

Lang mentioned at the end that they plan to update Minos to Minos+, which will have new target and horns, more beam power (700 kW), and 6×10^20 POT per year. The possible physics goals achievable in such a scenario are several. They expect to collect about 3000 charged-current muon neutrinos per year.

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