OPERA neutrino anomaly

OPERA neutrino anomaly

The OPERA neutrino anomaly is the detection of apparently faster-than-light neutrinos by the OPERA experiment in September 2011. The detection is anomalous because speeds exceeding that of light in a vacuum are generally thought to violate special relativity,[1][2][3][4] a prevailing theory of physics. How to explain the results is debated, with potentially far-reaching implications for our understanding of physics.[5]

The muon neutrinos in the experiment were sourced at CERN on the Franco-Swiss border and detected at LNGS in Gran Sasso, Italy. OPERA researchers used common-view GPS to measure the times and places at which the neutrinos were emitted and detected. As computed, the neutrinos' average time of flight turned out to be less than what light would need to travel the same distance in a vacuum. The OPERA team repeated the measurement with much shorter pulses in a two-week span up to 6 November, which helped measure travel time of each detected neutrino separately, eliminating some sources of error.[6]

Dario Autiero, who leads the OPERA team's analysis of the superluminal result, has stated that further scrutiny and independent tests are necessary to definitely confirm or refute the results.[7] Independent tests by other collaborations are under way.

Contents

Detection

CNGS: Underground structures at CERN
OPERA time measurement schematic
CERN SPS/CNGS time measuring system

In March 2011, scientists of the OPERA collaboration discovered their data suggested neutrinos travelled faster than light, from CERN in Geneva to the OPERA detector, arriving earlier by 60 ns. After six months of cross checking,[8] on 23 September 2011 they announced that 17 GeV neutrinos had been observed travelling at faster-than-light speed. Similar results were obtained using 28 GeV neutrinos, which were observed to test energy dependence. The particles were measured arriving at the detector approximately 1/40,000 times faster than light; their superluminal excess speed as a fraction of lightspeed was (2.48 ± 0.28 (stat.) ± 0.30 (sys.))×10−5 (approximately 1 in 40,000), with a significance of 6.0 sigma (or 99.9999998 %, if the distribution is normal). This measure included estimates for both statistical and instrumentation error, though it may have left additional sources of systematic error unaccounted for.[9][10] For particle physics experiments involving collision data, the standard baseline for a discovery announcement is 5-sigma significance.[11]

The preprint of the research stated "[the observed] deviation of the neutrino velocity from c would be a striking result pointing to new physics in the neutrino sector" and referred to the "early arrival time of CNGS muon neutrinos" as an "anomaly."[10] OPERA collaboration scientist and spokesperson Antonio Ereditato explained that the OPERA team had "not found any instrumental effect that could explain the result of the measurement."[12] James Gillies, a spokesperson for CERN, said on 22 September that the scientists were "inviting the broader physics community to look at what they [had] done and really scrutinize it in great detail, and ideally for someone elsewhere in the world to repeat the measurements."[13]

The measurement

The OPERA team relied on the already available CERN Neutrinos to Gran Sasso beam for the measurement.[14] Distance was measured by accurately fixing the source and detector points on a global coordinate system (ETRF2000). CERN surveyors used GPS to measure the source location. On the detector side, the OPERA team worked with a geodesy group from the University of Rome to locate the detector's center with GPS and standard topographical techniques. To topographically link the surface GPS location to the underground detector traffic had to be partially stopped on the access road to the lab. Combining the two location measurements, the researchers calculated the distance[14] to an accuracy of 20 cm within the 730 km path.[15]

CERN's beams department engineers worked with the OPERA team to provide a time measurement between the source at CERN and a point just before the OPERA detector's electronics, using accurate GPS receivers. At Gran Sasso, the GPS receiver was on the surface, and an 8 km fiber cable linked it to the underground detector. Special techniques were used to measure the length of the fiber and its consequent delay, required as part of the overall calculation. Researchers from OPERA measured the remaining delays and calibrations not included in the CERN calculation.[14] To sharpen resolution from the standard GPS 100 ns to the 1 ns range metrology labs achieve, they used Septentrio’s precise PolaRx2eTR GPS timing receiver which allowed measurement of the time offset between an atomic clock and each of the Global Navigation Satellite System satellite clocks, along with time calibration procedures which allowed for common-view time transfer.[16] The equipment was taken to the Swiss Metrology Institute (METAS) for calibration. In addition, highly stable cesium clocks were installed both at LNGS and CERN to cross-check GPS timing and to increase its precision. After OPERA noted the superluminal result, the time calibration was rechecked both by a CERN engineer and the German Institute of Metrology (PTB).[14] Time-of-flight was eventually measured to an accuracy of 10 nanoseconds.[12][17][18]

Internal replication

Two facets of the result particularly criticized by the neutrino community were the GPS synchronization system and the profile of the proton beam spill that generated neutrinos.[8] OPERA scientists repeated the measurement over the same baseline without any assumptions about the details of neutrino production during the spill, such as energy distribution or production rate, by using a new CERN beam[19] which provided proton pulses of 3 nanoseconds each with upto 524 nanosecond gaps.[12] Beam time was allocated from 21 October to 7 November 2011.[19] Without using the earlier statistical computation, the researchers measured twenty detection events indicating neutrinos had travelled faster than light by 60 ns, with 10 ns uncertainty. The error bounds for the original superluminal speed fraction were tightened further to (2.37 ± 0.32 (stat.) +0.34/ −0.24 (sys.))×10−5. The new significance level became 6.2 sigma.[10] The collaboration has submitted its results for peer review to the Journal of High Energy Physics.[6][7]

Previous measurements

Main article: Neutrino#Speed

Previous experiments have not detected statistically significant deviations of neutrino speeds from the speed of light; for instance, observations from the SN 1987A supernova restricted 10 MeV anti-neutrino speed to less than 20 parts per billion (ppb) over lightspeed.[20] However, SN1987A neutrinos differ from the ones detected by OPERA in lepton number (anti-neutrino as against neutrino), flavor (electron versus muon neutrino), energy, and medium of travel (interstellar space versus solid rock).[21] Fermilab's MINOS collaboration (2007) reported results measuring the flight-time of 3 GeV neutrinos yielding a speed 1.000051±29 c, thus faster than that of light at 68% confidence level. However, at 99% confidence level including all margins of error, a range between 0.999976 c (slower than light) to 1.000126 c (faster than light) was given; thus within the margins of error the result was consistent with both slower and faster than light speed, although the central value was higher than the speed of light.[22][23]

Reception by the physics community

Nobel laureates Steven Weinberg,[24] and George Smoot,[25] as well as other physicists not affiliated with the experiment, including Martin Rees,[24] Lawrence Krauss,[24] and Michio Kaku,[26] expressed skepticism about the accuracy of the original experiment on the basis that the results challenged a long-held theory consistent with the results of many other experiments. Even after OPERA's replication, most in the field disbelieve the light-speed limit has been truly broken.[1][27] Nevertheless, Ereditato, the OPERA spokesperson, states no one has an explanation that invalidates the experiment's results.[28]

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.[10]

More than 80 papers discussing the experiment have been posted on the arXiv website for electronic preprints. Most try to explain the anomaly theoretically, while a small minority claim the experiment has problems.[5] CERN theorists held a workshop on October 14 to discuss the OPERA results. Ignatios Antoniadis, head of CERN’s theory group, states the view of CERN theorists was that "there [was] no consistent theoretical model that [could] accommodate the measurement."[29]

Cohen-Glashow effect

Andrew Cohen and Sheldon Lee Glashow have predicted superluminal neutrinos would radiate electrons and positrons and lose energy through vacuum Cherenkov effects. This energy attrition is seen neither in the OPERA experiment, nor in the colocated ICARUS experiment using the same CNGS beam as OPERA. This is considered by Cohen and Glashow as a significant challenge to the superluminal interpretation of the OPERA data.[30][31] Autiero has responded that Cohen and Glashow's premises may not be universally valid.[5] Sergio Bertolucci, director of research at CERN, contends if every new measurement is interpreted with older theories, a new theory is impossible.[32] Cohen and Glashow themselves note, toward the end of their paper, that the energy-loss mechanism they posit may not happen if light itself travels faster than its nominal value in vacuum, for the energy domain considered by the OPERA experiment.[30]

Giacomo Cacciapaglia, a theoretical physicist at King's College London, has suggested that the neutrinos might take "shortcuts" through extra dimensions, bypassing the Cohen-Glashow effect. Jorge Páramos, a theoretical physicist at the Higher Technical Institute in Lisbon, counters that tinkering with the current theory in this way is difficult.[5]

Discussions within the OPERA collaboration

Around 15 of the almost 195 researchers in the OPERA collaboration had not signed on to the first preprint of the paper, since they thought the publication was premature and further experimental checks were required.[33][34][35]

OPERA spokesperson Ereditato says that all the researchers have signed the final submission to JHEP.[27] Other sources had previously reported that some of the original dissenters had signed that paper, while others who had signed the first paper removed their names from the final one.[6][35] The current dissenters in the collaboration are concerned the time window for neutrino detection was initially assumed to be 10 ns but revealed by Dario Autiero to be 50 ns after the tests were done. While the discrepancy does not affect the test results, the dissenters consider this poor experimental procedure. They are also unhappy that only a small fraction of the analysis, carried out by Autiero, has been independently checked by others, leaving open the possibility of errors in the analysis.[6]

Independent replication

The key test of the result is expected to be replication by an independent experiment.[1] Following OPERA's and CERN's request, both Fermilab and the T2K experiment have announced they intend to test the OPERA result in coming months.[36] Fermilab has stated that the detectors for the MINOS project are being upgraded, and new results are not expected until at least 2012.[37] A result based on already recorded data collected in the last five years is expected to be available in mid to end 2012. Fermilab scientists hope to closely analyze the errors in their timing system and bound them to achieve a precision of 15 ns, enough to confirm or refute the OPERA early-arrival central value of about 60 ns.[38] The Borexino and ICARUS experiments (both located at Gran Sasso) will begin independent checks of OPERA's results in 2012.[32] OPERA itself will cross check clock synchronization between CERN and LNGS, perhaps by using an optical fiber. The researchers will also continue to take data into 2012, expecting to collect around a hundred neutrino events in a couple of months, to improve the accuracy of the results.[38]

See also

References

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