Exceeding the speed limit? Measuring neutrinos to the nanosecond

This graph shows the “bunch” formed from all neutrino events recorded at the MINOS far detector over the two-month run period. The blue distribution shows neutrinos that have traveled the 735 kilometers from Fermilab to Soudan and interacted in the middle of the MINOS detector. The open red distribution shows neutrinos that have interacted in the rock surrounding the detector and produced a muon, which has been detected in MINOS, arriving on average a nanosecond later as expected.

The fastest man on the planet is Jamaican sprinter Usain Bolt. To measure his speed, he races on an accurately measured track, with the starting pistol and photo-finish system synchronized to an accurate clock. In order to determine whether neutrinos are the fastest particles on the planet (and in response to the OPERA experiment’s subsequently corrected September 2011 measurement of the neutrino velocity at 0.002 percent faster than light), MINOS must accurately measure both the 735-kilometer distance between its two detectors, at Fermilab and in Soudan, Minnesota, and the time it takes for a neutrino to travel between them.

Measuring this time to an accuracy of a billionth of a second requires scrupulous care and attention to detail. Optic fibers change length with changing temperature, and thresholds in electronics drift over time and in response to environmental conditions. All delays must be measured redundantly, and preferably in situ, to eliminate the possibility of human or instrumental error. Atomic clocks by each detector serve as a time reference but are not accurate enough by themselves. The offset and drift between clocks at the two sites is measured with redundant GPS receivers—more sophisticated than those in cellphones, but operating on the same principles. Each of the 32 GPS satellites contains atomic clocks that are regularly synchronized with each other and with the master clock on the ground. By comparing the time signal received from at least four satellites, a GPS receiver can determine its position and time.

For neutrinos, there is an extra complication. We observe Usain Bolt by recording the light reflected from his body, which does not materially affect his progress. For neutrinos, however, the act of observation is also an act of destruction: Once it has interacted in the detector, the neutrino no longer exists. The neutrinos observed in the two MINOS detectors are not the same particles.

To determine the speed of the neutrino, we exploit the fact that the accelerator clusters protons together in short “bunches” in order to accelerate them. The neutrino beam inherits the timing distribution of the parent protons. We can’t tell whether an individual neutrino came from the beginning or the end of a bunch, but with enough neutrinos we can build up a picture of the shape of the bunch and determine its average timing.

The result? Neutrinos travel at the speed of light, to an accuracy of one part in a million. The speed of light remains the unchallenged world champion.

MINOS is grateful to the many people throughout Fermilab who helped us design and install a new timing system on very short notice in order to make this measurement. This measurement was made in collaboration with precise time experts from the United States Naval Observatory and the National Institute for Standards and Technology, to whom we are indebted for sharing their timing expertise with us.

—Phil Adamson

The particles represented here in red (associated with the red curve in the above graph) are expected to travel on average a foot farther than those represented in blue (represented in the blue curve above) since there’s a change of direction at the interaction in the rock. The red particles are seen to arrive a nanosecond later, as expected.