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The OPERA neutrino anomaly is a discrepancy detected by the OPERA experiment in Italy, between the measured speed of neutrinos and the maximum speed predicted by special relativity. The neutrinos were sourced at CERN on the Franco-Swiss border and detected at LNGS in Gran Sasso, Italy.

Detection

On 23 September 2011, the OPERA Collaboration announced that 17 GeV neutrinos had been observed travelling from CERN in Geneva to the OPERA detector at faster-than-light speed. They relied on the already available CERN Neutrinos to Gran Sasso beam for the measurement. Similar results were obtained using 28 GeV neutrinos, which were observed to test energy dependence. The particles were measured arriving at the detector by a factor of (v − c)/c = (2.48 ± 0.28 (stat.) ± 0.30 (sys.))×10 (approximately 1 in 40,000) prior to the time expected if they were travelling at lightspeed, with a significance of 6.0 sigma (or 99.9999998%, if the distribution is normal). This measure includes estimates for both statistical and instrumentation error, though it may leave additional sources of systematic error unaccounted for. For particle physics experiments involving collision data, the standard baseline for a discovery announcement is 5-sigma significance.

OPERA collaboration scientist and spokesperson Antonio Ereditato explained that the OPERA team has "not found any instrumental effect that could explain the result of the measurement." James Gillies, a spokesman for CERN said on 22 September that the scientists were "inviting the broader physics community to look at what they done and really scrutinize it in great detail, and ideally for someone elsewhere in the world to repeat the measurements."

Reception by the physics community

Nobel laureates Steven Weinberg, and George Smoot, as well as other physicists not affiliated with the experiment, including Martin Rees, Lawrence Krauss, and Michio Kaku, have expressed skepticism about the result's accuracy on the basis that it challenges a long-held theory consistent with the results of many other experiments.

Physicists affiliated with the experiment have refrained from interpreting the result, stating in their paper:

Despite the large significance of the measurement reported here and the stability of the analysis, the potentially great impact of the result motivates the continuation of our studies in order to investigate possible still unknown systematic effects that could explain the observed anomaly. We deliberately do not attempt any theoretical or phenomenological interpretation of the results.

Up to half of the members of the OPERA project are opposed to immediately publishing the result in a peer-reviewed journal. Laura Patrizii, who leads OPERA's Bologna group, characterizes the objections as:

It is not that people think there is a mistake that is being hidden. But since something going faster than light would kill modern physics as we know it, some researchers would feel more at ease with these independent checks.

According to CERN theoretical physicists, as of 19 October 2011, the anomaly cannot be explained in terms of known physics.

Debates on the experiment

Previous experiments

Previous experiments have not detected statistically significant faster-than-light neutrino motion; for instance, in 2007 Fermilab's MINOS collaboration reported results measuring the flight-time of 3-GeV neutrinos yielding a speed exceeding that of light by 1.8 sigma. Those measurements were considered statistically consistent with neutrinos traveling at lightspeed.

Per Frank Wilczek, OPERA results contradict a previous measurement of the velocity of 10-MeV anti-neutrinos from the SN 1987A supernova. Had anti-neutrinos emitted by SN 1987A been travelling with the speed reported by the OPERA experiment, they would have reached Earth almost four years before light from the event, but the detectors registered them arriving almost at the same time as light. However, the measurement from SN 1987A cannot immediately be seen as contradicting the measurement from the OPERA experiment, as it is not currently known how neutrino velocity may depend on particle energy, distance traveled, or other factors.

Energy decay through pair bremsstrahlung

Andrew Cohen and Nobel laureate Sheldon Glashow have authored a paper, accepted in Physical Review Letters, arguing that, according to the Standard Model, extended by including Lorentz invariance-breaking terms compatible with particle physics theory, superluminal neutrinos should produce virtual electron-positron pairs, causing them to lose energy by a process analogous to Cherenkov radiation. Since no such energy spectrum shift was observed at LNGS, they conclude that their results pose a "significant challenge" to the OPERA results. Their theory and analysis impose a stringent requirement of 14 parts per billion (ppb) to 0.375 ppb for neutrino velocity over speed of light, in energy ranges from 1GeV to 1TeV and 16TeV to 100TeV. Scientists of the ICARUS experiment, co-located with the OPERA experiment and relying on the same-length CNGS beam, have detected no Cherenkov radiation analogue. They state this implies the neutrinos would be travelling no faster than 40 ppb above light speed.

Professor Robert Ehrlich has suggested that neutrino speed variations could account for this inconsistency, and that the energy detected at LNGS would be just a sample of what left the source.

Neutrino beam instability

Fluctuations in the beam of neutrinos could increase the uncertainty in their time of flight enough to produce the faster-than-light result. Other preprint papers have argued that if the neutrinos undetected at the target are biased as a function of time, an apparent superluminal speed might result. Specifically, if the drop fraction either increases or decreases monotonically as a function of time, such a wrong result might show up. The papers do not specify any physical process or noise source which could lead to such a skew, however. Qualifying and restricting these objections, Walter Winter shows "possible smearing effects in the proton waveform or in the extrapolation from neutrino to proton waveform do not change the OPERA results qualitatively."

Relativistic motion of the GPS satellites

Ronald van Elburg, an AI researcher at the University of Groningen with a PhD in physics, has suggested the 64-nanosecond difference could be caused by relativistic effects on the GPS satellites used to time the event, although the OPERA team claimed to have allowed for such effects, and relativistic effects are corrected for in normal GPS receiver operation. Van Elburg's calculation has not been independently confirmed.

Replication

Following OPERA and CERN's request for confirmation, both Fermilab and the T2K experiment have announced they intend to test the OPERA result in coming months. Fermilab has stated that the detectors for the MINOS project are being upgraded, and new results are not expected until at least 2012. A result based on already recorded data should be available in 4 to 6 months. But this result can only possibly refute OPERA's original result; it cannot confirm faster-than-light speeds because the accuracy would not be high enough.

Autiero, Migliozzi and Russo have suggested using the IceCube experiment to search for neutrinos in the over 100 GeV range, from a Gamma-ray burst (GRB), to indirectly support or directly contradict the OPERA results. They suggest absence of neutrinos from GRBs can be explained by assuming the neutrinos travelled superluminally, while presence of the predicted neutrinos would imply they did not.

CERN plans to provide a new neutrino beam to repeat the experiment.

References

1. ^ "OPERA experiment reports anomaly in flight time of neutrinos from CERN to Gran Sasso" (Press release). CERN. 23 September 2011. Archived from the original on 24 September 2011. Retrieved 24 September 2011. {{cite press release}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
2. ^ T. Adam et al. (OPERA collaboration) (22 September 2011). "Measurement of the neutrino velocity with the OPERA detector in the CNGS beam". arXiv:1109.4897 .
3. Giulia Brunetti. "Neutrino velocity measurement with the OPERA experiment in the CNGS beam" (PDF). Retrieved 8 October 2011.
4. C. Seife (2000). "CERN's Gamble Shows Perils, Rewards of Playing the Odds". Science. 289 (5488): 2260. doi:10.1126/science.289.5488.2260.
5. F. Jordans, S. Borenstein (22 September 2011). "Roll over Einstein: Law of physics challenged (Update 3)". PhysOrg. Retrieved 24 September 2011.
6. ^ John Matson (26 September 2011). "Faster-Than-Light Neutrinos? Physics Luminaries Voice Doubts". Scientific American. Retrieved 9 October 2011.
7. "U.S. Scientists to Test Findings that Neutrinos Defied Physics' Basic Tenet". International Business Times. 28 September 2011. Retrieved 9 October 2011.
8. Jordans, Frank; Borenstein, Seth (24 September 2011). "Challenging Einstein is usually a losing venture". AP/Yahoo News. Retrieved 26 September 2011.{{cite web}}: CS1 maint: multiple names: authors list (link)
9. ^ Edwin Cartlidge (7 October 2011). "'Tension' emerges within OPERA collaboration". physicsworld.com.
10. ^ Lisa Grossman (19 October 2011). "Neutrino watch: Speed claim baffles CERN theoryfest". NewScientist: Physics & Math. Retrieved 19 October 2011.
11. ^ "OPERA experiment reports anomaly in flight time of neutrinos". Fermilab Today. Fermilab. Retrieved 24 September 2011.
12. D. Overbye (22 September 2011). "Tiny neutrinos may have broken cosmic speed limit". New York Times. That group found, although with less precision, that the neutrino speeds were consistent with the speed of light.
13. Stephanie Holden (14 October 2011). "Not so fast, neutrinos". Retrieved 15 October 2011. {{cite news}}: Unknown parameter |publication= ignored (help)
14. "Faster-than-light travel discovered? Slow down, folks | Bad Astronomy | Discover Magazine". Blogs.discovermagazine.com. Retrieved 25 September 2011.
15. "Naughty 'Faster Than Light' Neutrinos a Reality?". Discovery News. Retrieved 25 September 2011.
16. S. Coleman and S. L. Glashow, High-Energy Tests of Lorentz Invariance, Phys. Rev. D 59, 116008 (1999) preprint
17. A. G. Cohen; S. L. Glashow (2011). "New Constraints on Neutrino Velocities". arXiv:1109.6562 ..
18. Clay Dillow (3 October 2011). "Physicists Say Speed-of-Light-Breaking Neutrinos Would've Lost Their Energy Along the Way". Retrieved 9 October 2011. {{cite web}}: Unknown parameter |publication= ignored (help)
19. ICARUS Collaboration (17 October 2011). "A search for the analogue to Cherenkov radiation by high energy neutrinos at superluminal speeds in ICARUS". arXiv:1110.3763. {{cite arXiv}}: Unknown parameter |accessdate= ignored (help)
20. R. Ehrlich (2011-10-06). "Resolving 8 OPERA problems with superluminal neutrinos". arXiv:1110.0736v2 . {{cite arXiv}}: Unknown parameter |url= ignored (help)
21. ^ "The Net Advance of Physics: Superluminal neutrinos". Retrieved 19 October 2011.
22. Eugenie Samuel Reich (5 October 2011). "Faster-than-light neutrinos face time trial". Retrieved 9 October 2011. {{cite web}}: Unknown parameter |publication= ignored (help)
23. Boguslaw Broda (4 October 2011). "A classical model explaining the OPERA velocity paradox". arXiv:1110.0644 .
24. Robert Alicki (26 September 2011). "A possible statistical mechanism of anomalous neutrino velocity in OPERA experiment?". arXiv:1109.5727 .
25. Walter Winter (5 October 2011). "How large is the fraction of superluminal neutrinos at OPERA?". arXiv:1110.0424v2 .
26. Richard Chirgwin (18 October 2011). "OPERA review serves up a feast for physics geeks". Retrieved 19 October 2011. {{cite web}}: Unknown parameter |publication= ignored (help)
27. Phil Plait (15 October 2011). "Followup: FTL neutrinos explained? Not so fast, folks". Discover Magazine. Retrieved 15 October 2011.
28. Ronald A.J. van Elburg (12 October 2011). "Times Of Flight Between A Source And A Detector Observed From A GPS Satellite". arXiv:1110.2685v1.
29. Damon Poeter (15 October 2011). "Boo! Hiss! Dutch Scientist Rains on Faster-Than-Light Neutrino Parade". PC Magazine. Retrieved 15 October 2011.
30. Misra, Pratap; Enge, Per (2006). Global Positioning System: Signals, Measurements, and Performance. Ganga-Jamuna Press. p. 115. ISBN 0-9709544-1-7.
31. E. Brown, A. Khan (23 September 2011). "Faster than light? CERN findings bewilder scientists". Los Angeles Times. Retrieved 24 September 2011. MINOS scientists may perform experiments of their own in as soon as six months, said particle physicist and MINOS co-spokesperson Jenny Thomas. Plans to test the CERN results in Japan's multinational T2K (Tokai-to-Kamioka) experiment are in the works, said neutrino physicist and T2K spokesman Chang Kee Jung.
32. Franzen, Carl (2011-09-26). "Was Einstein Wrong? U.S. Accelerator Lab To Test CERN's Results | TPM Idea Lab". TPMIdeaLab. Retrieved 5 October 2011. We should have a result in 4-6 months as the data is already taken. We just have to measure some of our delays more carefully
33. "Tension emerges within OPERA collaboration". 2011-10-7. Retrieved 8 October 2011. . . . new data collected with the upgraded detector combined with a better analysis of existing data could allow MINOS to largely rule out the OPERA result within the next four to six months (but not to rule it in, given that this would require a higher level of accuracy). {{cite web}}: Check date values in: |date= (help)
34. D. Autiero, P. Migliozzi, A. Russo (25 September 2011). "The neutrino velocity anomaly as an explanation of the missing observation of neutrinos in coincidence with GRB". arXiv:1109.5378v1.{{cite arXiv}}: CS1 maint: multiple names: authors list (link)

External links

• OPERA Main Page
• CNGS Neutrino beam at CERN
• OPERA publications
• Webcast of OPERA neutrino anomaly presentation by Dario Autiero
• arXiv preprints about OPERA

• Unsolved problems in physics
• 2011 in science