Possible Origin Of The Neutrino Speed Anomaly Reported By OPERA

Recently the OPERA collaboration reported a measurement of a superluminal speed of muon neutrinos travelling through the Earth's crust between their production site at CERN and their detection site under Gran Sasso, ~730 km away. The measurement was based on the assumption that the pulse shape of the neutrinos from the decay of parent mesons produced in proton-target collisions is the same as that of the incident protons. Here we argue that the effective column density of the target along the beam direction decreases with time during the 10.5 microseconds duration of the proton pulse. This is because of the thermal expansion and expulsion of target material along the beam by the energy-momentum deposition during the 10.5 microseconds pulse. The progresive reduction in the effective column density during the pulse decreases the neutrino production rate per incident proton. It could have advanced the mean production time of the detected neutrinos relative to that calculated from the proton pulse-shape, by an amount comparable to the measured neutrino lead time (60.7+/-6.9(stat) +/-7.4(sys) ns. This explanation implies that the planned measurements by OPERA of the speed of neutrinos produced in much shorter (a few ns) pulses, should yield a speed consistent with the speed of light in free space.