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Recently the OPERA collaboration reported a measurement of a superluminalspeed of muon neutrinos travelling through the Earth's crust between theirproduction site at CERN and their detection site under Gran Sasso, ~730 kmaway. The measurement was based on the assumption that the pulse shape of theneutrinos from the decay of parent mesons produced in proton-target collisionsis the same as that of the incident protons. Here we argue that the effectivecolumn density of the target along the beam direction decreases with timeduring the 10.5 microseconds duration of the proton pulse. This is because ofthe thermal expansion and expulsion of target material along the beam by theenergy-momentum deposition during the 10.5 microseconds pulse. The progresivereduction in the effective column density during the pulse decreases theneutrino production rate per incident proton. It could have advanced the meanproduction time of the detected neutrinos relative to that calculated from theproton 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 plannedmeasurements by OPERA of the speed of neutrinos produced in much shorter (a fewns) pulses, should yield a speed consistent with the speed of light in freespace.

Possible Origin Of The Neutrino Speed Anomaly Reported By OPERA