Analog SFF, March 2012 (14 page)

The tiny rest mass of neutrinos is puzzling. Why do the three neutrinos species, unlike their charged lepton brothers and their quark cousins, have rest masses that are nearly (but not quite) zero? The Standard Model of particle physics is silent on this question. Unlike the photon, which must have a zero mass because it is the mediating particle of the infinite-range electromagnetic force, there is no fundamental reason why neutrinos should be massless.

One way of measuring the e-neutrino rest mass does so indirectly by examining the energy spectrum of electrons produced in low-energy nuclear beta decays. The “end-point” or energy region where the most energetic electrons are found is most sensitive to the mass of the e-neutrino (or e-anti-neutrino, which should have identical mass). If the e-neutrino has zero mass, the energy region near the end-point smoothly merges into the baseline. But if the e-neutrino has a small mass, the distribution at the end-point is chopped off early, producing a “nose” with an abrupt edge at the end of the electron energy distribution.

It is not widely appreciated, but this end-point technique does not actually measure the mass of the neutrino. Because of the way that the neutrino mass affects the electron energy spectrum, the measured quantity is the
square
of the neutrino mass. Curiously, essentially all of the experimental determinations of neutrino mass have shown some significant probability of negative values for the measured mass-squared. However, the negative mass-squared region is not taken seriously by the physics community because the error bars are too large, and the implications of a negative mass-squared are too bizarre. A negative mass-squared would occur only if the e-neutrino had an
imaginary rest mass
.

Can a particle actually have an imaginary rest mass? In 1967, Gerald Feinberg of Columbia University suggested the existence of hypothetical imaginary-mass particles, which he christened “tachyons.” Feinberg's tachyons are particles that always travel at velocities greater than the speed of light. Instead of speeding up when they are given more kinetic energy, they slow down so that their speed moves closer to the velocity of light from the high side as they become more energetic. Feinberg argued that since there are no physical laws forbidding the existence of tachyons, they could exist and should be looked for. This prompted a number of experimental searches for tachyons. However, until OPERA, there has been no convincing evidence of their existence.

What did OPERA measure? The motivation for the experiment was to use the proton beam of the CERN SPS (Super Proton Synchrotron) to produce a beam of m-neutrinos and observe their oscillation into t-neutrinos as they traveled from the CERN site near Geneva to the underground Grand Sasso Laboratory in a highway tunnel through the mountains East of Rome. The detector consists of two “brick walls” of passive emulsion detector bricks backed by magnetic muon trackers that point to the bricks that should be examined. The trackers can identify energetic m leptons produced in m-neutrino interactions with matter, and back-track them to the emulsion bricks or rocks in which they were produced. So far, in three years of operation, OPERA has unambiguously identified one t lepton with their system (as compared to an expected number of 1.6), which presumably was produced by a t-neutrino that oscillated from their beam of m-neutrinos.

A byproduct of the OPERA experiment is that they detect thousands of m-neutrinos, and they can in principle determine the m-neutrino velocities by dividing the flight distance by the flight time. This, however, is not as easy as it sounds. The source and detectors are located 454 miles apart, and one must know the flight time and the separation distance to parts per million to make a significant speed measurement.

The measurement of the source-to-detector distance, to an accuracy of 20 cm, was accomplished using GPS techniques to precisely locate reference points at CERN and at the two ends of the 10-km-long Grand Sasso tunnel. The experimenters then transported these references into the laboratory inside the tunnel (blocking off one lane of automobile traffic in the Grand Sasso tunnel in the process). They thus measured the benchmark neutrino flight path to be 730534.61 m to an accuracy of 0.20 m. The precision of these distance measurements also permitted them to observe the effect of continental drift, changing the measured distances by about 1 cm per year, and they also saw a 10 cm shift from an earthquake in 2009 in the L'Aquina region of Italy. The tidal forces produced by the Moon and the Sun on the Earth are also a potential source of distance change, but fortunately the flight path from CERN to Grand Sasso is primarily North-South, and the tidal flexing of the Earth is primarily East-West, so that effect proved to be negligible.

The other issue is the precise determination of the neutrino flight time. The synchronization of clocks between CERN and Grand Sasso proved challenging, but was accomplished to a precision of a few nanoseconds through an elaborate scheme using GPS synchronizing of cesium clocks. The more serious problem was that the neutrinos were produced during the CERN SPS synchrotron acceleration cycle in two successive “beam spills,” each of which had a duration of about one microsecond. The uncertainty of which proton during the spill actually produced the detected neutrino made nanosecond time-of-flight timing difficult.

This problem was addressed by carefully recording the beam profile of each spill and correlating these with each neutrino detection. Fortunately, the two spills each had a distinctive fingerprint of intensity fluctuations, as well as distinctive leading and trailing edges, and these could be matched by maximum likelihood analysis to the time profiles of the 16,000 detected m-neutrinos.

The result was that the time of flight of the m-neutrinos could be determined to a statistical precision of about 6.9 nanoseconds (ns) and a systematic uncertainty of 7.4 ns, and has a value of 60.7 ns. This corresponds to a m-neutrino velocity that is 24.8 parts per million faster than the speed of light. This is a 6-standard-deviation (6s) effect, where 3s is generally considered to be the threshold of believability.

The OPERA group also performed one interesting refinement of their analysis. The average m-neutrino energy of their entire data set was 28.1 GeV, and gave a transit time of 60.7 ns, with a statistical uncertainty of 13 ns. They divided their data into two subsets of about equal statistics, one with an average m-neutrino energy of 13.9 GeV and the other 42.9 GeV and computed the transit time of each subset. The 13.9 GeV data set gave a transit time of 53.1 ns and the 42.9 GeV data set gave 67.1 ns, both with statistical uncertainties of about 18 ns. The difference in transit times shows no convincing energy dependence of the kind that would be expected for tachyonic particles, but at the 1-s level the lower energy neutrinos do show an indication of being slightly faster.

Are the m-neutrinos really traveling faster than the speed of light? The odds are against it. The usual outcome of such a spectacular physics result is that a flaw is found somewhere in the analysis, and the result is withdrawn. The OPERA result needs to be confirmed or contradicted. Fortunately, the MINOS (Main Injector Neutrino Oscillation Search) experiment at FermiLab did a very similar measurement. They also found a slight indication of FTL m-neutrinos, but their experimental precision was an order of magnitude poorer, and the superluminal result was taken as a statistical fluke. However, the MINOS precision can be improved by careful metrology to determine the distance from the FermiLab source site to the Soudan Mine detector location, and this is now being done. (We note that the Tevatron collider at FermiLab, the injector of which was used to make neutrinos for MINOS, is having its final run on September 30, 2011, just a few days after the OPERA announcement.) The Japanese also have a neutrino oscillation experiment using the Super-K detector that may be able to shed light on the OPERA result.

This is a science fiction magazine, and so I want to close by taking the OPERA result seriously and exploring its implications. Are m-neutrinos tachyons? Are all neutrinos tachyons? Superluminal m-neutrinos constitute a serious discrepancy in our supposed understanding of the way the universe works, and raise many questions. A new revolution of physics would be needed, throwing out many cherished ideas. It's encouraging to SF readers that something seems to be able to travel faster than light.

One can take the OPERA numbers at face value and calculate the imaginary rest mass that a tachyon should have, if its total energy was 28.1 GeV and its velocity was 1.0000248?c. A tachyon with these characteristics would have an imaginary rest mass of i?198 MeV. This is not a low-mass particle, and its numerical value is suspiciously close to the mass of the mu lepton (105.7 MeV). It would be interesting to know if the evidence for neutrino oscillations from SNO and Kamiokande could be reconciled with tachyonic neutrinos with imaginary masses so large. (I doubt it.)

Finally, let me mount my hobby horse and go for a short ride. My AV column in the October 1993
Analog,
at a time when the mass-squared of electron neutrinos was measuring negative, described a space drive using tachyons. The basic idea is that tachyonic particles can be made to have an infinite exhaust velocity and carry significant momentum but have zero total energy (i.e., no energy cost). They represent a “free” fuel that could be created in unlimited quantities and used for space propulsion without violating any physical laws. If the OPERA results hold up, it may be worth having a closer look at the idea of a tachyonic space drive.

AV Columns Online
: Electronic reprints of over 155 “The Alternate View” columns by John G. Cramer, previously published in
Analog,
are available online at: www.npl.washington.edu/av.

References:

"Measurement of the neutrino velocity with the OPERA detector in the CNGS beam,” The OPERA Collaboration, preprint arXiv.1109.4897 [hep-ex].

Copyright © 2011 by John G. Cramer

[Back to Table of Contents]

What you see is not necessarily what you get—in a wide range of situations

They only called me in an emergency. Although the government had a hazy definition of what constituted an emergency, by the time they brought me onto a job, people were usually screaming at each other, making rash decisions, panicking. I straightened them out. Sometimes I played Mr. Nice Guy, other times I was the anvil and hammer. Either way, it was all a deception. I straightened people out with lies. I used to chuckle about that, but it had gotten old after twelve years on the job. My whole life was a lie.

Despite my expectations, I walked into a quiet conference room. No screams, no pulling of hair, no gnashing of teeth. Many of the staff members at the small hospital, which was located in a thinly populated county in South Dakota, were busy elsewhere, going about their business. Of the ten people present, only two showed any signs of tension. Both were scientists. I'd studied their dossiers on the flight.

Poe Weffle, 28; a prodigy and a maverick. Already achieved tenure at Crestview University—an astonishing feat considering its faculty was full of Nobel Laureates. Weffle was a southerner, and I'd noted a trace of an accent while I listened to recordings of some of his lectures. He was tall, slender, and blond. He looked up at me serenely as I came into the room, but his nose wrinkled ever so slightly; I'd observed that very same expression in the recordings when a particularly dense student seemed to have trouble understanding the most basic of theories.

Clarissa Jardin, 46; a conventional, by-the-book scientist. Chaired the biology department of a local college. Jardin was from the area, born in Rapid City. She'd done well, but never moved up into the big leagues; for whatever reason, she remained a big fish in a small pond. She studied me intensely with dark brown eyes, and brushed a lock of graying hair off her broad, heavily lined forehead. I detected a brief frown.

"I'm Marlon Mathers,” I said, smiling. I'd decided on the name ten minutes ago. An instant after I spoke it, the spyware that monitored me transmitted the name to the central computers, which created the appropriate entry into the databases. Marlon Mathers had the same features I'd chosen for this assignment: dark hair, dark eyes, 35 years old, slightly crooked nose that he had broken while playing hockey in school. The other details would be filled in as I made them up. “I'm from the Bureau of Safety and Security.” I held up an identification card with a bar code that, if scanned, would now support my assertions. Nobody bothered to scan it so I put it away. “I understand you've got a security issue here."

You should be pleasant at first. If you began with fire and brimstone you usually only succeeded in burning up any chance of cooperation. I brought out the stick only when someone got out of line.

Poe Weffle got out of line right away. “I don't believe there's anything you can do. It's not a security issue."

An insinuation that I was superfluous couldn't be tolerated. “The President of the United States thinks it's a security issue. So does the director of the Bureau of Safety and Security, along with other high-level officials. I thank you for your input, Mr. Weffle, but as far as security is concerned, your opinion is irrelevant."

I expected Poe's face to grow red, maybe while he glared at me. But nothing like that happened. Instead his nose wrinkled a smidgen more. “That's not what I meant,” he said in a calm voice.

Clarissa also surprised me—she gave me a wry grin. “Poe thinks the bodies just got up and walked out of here,” she said.

I looked at Poe. “You're saying they weren't taken? They woke up on their own?"

"Not exactly. I'm not sure. They may have had help—a very interesting kind of help, although I'm not quite ready to hypothesize at the moment."

An embarrassed silence fell over the conference room. Clarissa shook her head and rolled her eyes.