Fig. 1 What OPERA saw. Leftmost is the proton beam from the CERN SPS accelerator. It passes the beam current transformer (BCT), hits the target, creating first, pions and then, somewhere in the decay tunnel, neutrinos. The red lines are the Cern Neutrinos to Gran Sasso (CNGS) beam to the LNGS lab where the OPERA detector is. The proton beam is timed at the BCT. The left waveform is the measured distribution of protons, and the right that of the detected OPERA neutrinos. The shift is the neutrino travel time. Distance traveled is roughly 731 km. At the top are the GPS satellites providing a common clock to both sites, making time comparison possible. Only the PolaRx GPS receiver is above-ground, and fiber cables bring the time underground.

The faster-than-light neutrino anomaly is the detection by the OPERA experiment of subatomic particles, neutrinos, that appear to travel faster than light. This result was publicly announced in September 2011 with the stated intention of promoting further inquiry and debate, and some potential measurement errors are now being investigated. It is considered anomalous since speeds higher than that of light in a vacuum are generally thought to violate special relativity, a cornerstone of modern understanding of physics for over a century.^[1][2]

The experiment created a form of neutrinos, muon neutrinos, at CERN's older SPS accelerator, on the Franco–Swiss border, and detected them at the LNGS lab in Gran Sasso, Italy. OPERA researchers used common-view GPS, derived from standard GPS, to measure the times and place coordinates at which the neutrinos were created 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. In a two-week span up to November 6, the OPERA team repeated the measurement with a different way of generating neutrinos, which helped measure travel time of each detected neutrino separately. This eliminated some possible errors related to matching detected neutrinos to their creation time.^[3]

The OPERA collaboration has stated in their initial press release that further scrutiny and independent tests are necessary to definitely confirm or refute the results.^[4] Independent tests by other collaborations are under way.

In February 2012, the OPERA collaboration confirmed a ScienceInsider report on two possible sources of errors that could significantly affect the reported result.^[5] One was a crystal oscillator timestamping events.^[6] The other was a fiber connection, which, if loose, would have washed out the faster-than-light effect. Another experimental run is planned after the CERN beam is switched on again in March 2012.

[edit] Detection

[edit] First results

In a March 2011 analysis of their data, scientists of the OPERA collaboration found evidence that neutrinos they produced at CERN in Geneva and recorded at the OPERA detector at Gran Sasso, Italy, had traveled faster than light. The neutrinos were calculated to have arrived approximately 60.7 nanoseconds (60.7 billionths of a second) sooner than light would have if traversing the same distance in a vacuum. After six months of cross checking, on September 23, 2011, the researchers announced that neutrinos had been observed traveling at faster-than-light speed.^[7] Similar results were obtained using higher-energy (28 GeV) neutrinos, which were observed to check if neutrinos' velocity depended on their energy. The particles were measured arriving at the detector faster than light by approximately one part per 40,000, with a 0.2-in-a-million chance of being wrong, if the error were distributed as a bell curve (significance of six sigma). This measure included estimates for both errors in measuring and errors from the statistical procedure used. It is, however, a measure of precision, not accuracy, which could be influenced by elements such as incorrect computations or wrong readouts of instruments.^[8][9] For particle physics experiments involving collision data, the standard for a discovery announcement is a five-sigma error limit, looser than the observed six-sigma limit.^[10]

The preprint of the research stated "[the observed] deviation of the neutrino velocity from c [speed of light in vacuum] 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".^[11] OPERA spokesperson Antonio Ereditato explained that the OPERA team had "not found any instrumental effect that could explain the result of the measurement".^[4] James Gillies, a spokesperson for CERN, said on September 22 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".^[12]

[edit] Internal replication

Fig. 2 Analysis of the internal replication in November. Distribution of the early-arrival values for each detected neutrino with bunched-beam rerun. The mean value is indicated by the red line and the blue band.

In November, OPERA published refined results where they noted their chances of being wrong as even less, thus tightening their error bounds. Neutrinos arrived approximately 57.8 ns earlier than if they had traveled at light-speed, giving a relative speed difference of approximately one part per 42,000 against that of light. The new significance level became 6.2 sigma.^[13] The collaboration has submitted its results for peer-reviewed publication to the Journal of High Energy Physics.^[14][15]

In the same paper, the OPERA collaboration also published the results of a repeat experiment running from October 21, 2011 to November 7, 2011. They detected twenty neutrinos consistently indicating an early neutrino arrival of approximately 62.1 ns, in agreement with the result of the main analysis.^[16]

[edit] Possible measurement errors

In February 2012, the OPERA collaboration announced two possible sources of error that could have significantly influenced the results.^[4][6]

If a fiber link from a GPS receiver to the OPERA master clock were loose, the delay through the fiber would increase and wrongly enhance the faster-than-light effect. A small team of OPERA researchers tried loosening the fiber link, pulling at the coaxial connector plug, and checked if that would cause the anomaly.^[17] They found for dimmer light pulses the connection, if loose, would lengthen the delay up to 100 nanoseconds; if the cable were tilted from the ideal position only parts of the signal would be received.^[18] Whether the connection was actually loose during the experiment is unknown.^[19] The fiber connection delay had been measured in 2008 by the project lead for the faster-than-light result, Dario Autiero. Some researchers had suggested remeasuring before the JHEP submission in November, but the OPERA scientific management had not agreed.^[17]

The second error, dealing with an oscillator used to timestamp GPS synchronization events, lengthens the reported flight-time of neutrinos and wrongly weakens the apparent faster-than-light effect. The component has been operating outside its specifications.^[19][20]

The first effect is considered larger than the second one.^[20][21] Together, these effects might have caused the anomaly of 60ns,^[18] explaining the apparent faster-than-light result.^[20] The OPERA collaboration will further analyze the existing data to check if the errors were active during the experiment, and check the effects directly when a bunched beam is available after March 2012.^[20][22]

[edit] The measurement

The OPERA experiment was designed to capture how neutrinos switch between different identities, but Autiero realized the equipment could be used to precisely measure neutrino speed too.^[23] A tentative result from the MINOS experiment at Fermilab showing neutrinos traveling faster than light lent credence to the idea.^[24] The principle of the OPERA neutrino velocity experiment was to compare travel time of neutrinos against travel time of light. The neutrinos in the experiment emerged at CERN and flew to the OPERA detector. The researchers divided this distance by the speed of light in vacuum to predict what the neutrino travel time should be. They compared this expected value to the measured commute time.^[25]

[edit] Overview

The OPERA team used an already existing beam of neutrinos traveling continuously from CERN to LNGS, the CERN Neutrinos to Gran Sasso beam, for the measurement. Measuring speed meant measuring the distance traveled by the neutrinos from their source to where they were detected, and the time taken by them to travel this length. The source at CERN was more than 730 kilometres (450 mi) away from the detector at LNGS (Gran Sasso). The experiment was tricky because there was no way to time an individual neutrino, necessitating more complex steps. As shown in Fig. 1, CERN generates neutrinos by slamming protons, in pulses of length 10.5 microseconds (10.5 millionths of a second), into a graphite target to produce intermediate particles, which decay into neutrinos. OPERA researchers measured the protons as they passed a section called the beam current transducer (BCT) and took the transducer's position as the neutrinos' starting point. The protons did not actually create neutrinos for another kilometer, but because both protons and the intermediate particles moved almost at light speed, the error from the assumption was acceptably low.

The clocks at CERN and LNGS had to be in sync, and for this the researchers used high-quality GPS receivers, backed up with atomic clocks, at both places. This system timestamped both the proton pulse and the detected neutrinos to a claimed accuracy of 2.3 nanoseconds. But the timestamp could not be read like a clock. At CERN, the GPS signal came only to a receiver at a central control room, and had to be routed with cables and electronics to the computer in the neutrino-beam control room which recorded the proton pulse measurement (Fig. 3). The delay of this equipment was 10,085 nanoseconds and this value had to be added to the time stamp. The data from the transducer arrived at the computer with a 580 nanoseconds delay, and this value had to be subtracted from the time stamp. To get all the corrections right, physicists had to measure exact lengths of the cables and the latencies of the electronic devices. On the detector side, neutrinos were detected by the charge they induced, not by the light they generated, and this involved cables and electronics as part of the timing chain. Fig. 4 shows the corrections applied on the OPERA detector side.

Since neutrinos could not be accurately tracked to the specific protons producing them, an averaging method had to be used. The researchers added up the measured proton pulses to get an average distribution in time of the individual protons in a pulse. The time at which neutrinos were detected at Gran Sasso was plotted to produce another distribution. The two distributions were expected to have similar shapes, but be separated by 2.4 milliseconds (2.4 thousandths of a second), the time it takes to travel the distance at light speed. The experimenters used an algorithm, maximum likelihood, to search for the time shift that best made the two distributions to coincide. The shift so calculated, the statistically measured neutrino arrival time, was approximately 60 nanoseconds shorter than the 2.4 milliseconds neutrinos would have taken if they traveled just at light speed. In a later experiment, the proton pulse width was shortened to 3 nanoseconds, and this helped the scientists to narrow the generation time of each detected neutrino to that range.^[26]

[edit] Measuring distance

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 Sapienza University of Rome to locate the detector's center with GPS and standard map-making techniques. To link the surface GPS location to the coordinates of 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,^[27] to an accuracy of 20 cm within the 730 km path.^[28]

[edit] Measuring trip time

The travel time of the neutrinos had to be measured by tracking the time they were created, and the time they were detected, and using a common clock to ensure the times were in sync. As Fig. 1 shows, the time measuring system included the neutrino source at CERN, the detector at LNGS (Gran Sasso), and a satellite element common to both. The common clock was the time signal from multiple GPS satellites visible from both CERN and LNGS. CERN's beams-department engineers worked with the OPERA team to provide a travel time measurement between the source at CERN and a point just before the OPERA detector's electronics, using accurate GPS receivers. This included timing the proton beams' interactions at CERN, and timing the creation of intermediate particles eventually decaying into neutrinos (see Fig. 3).

Fig. 3 CERN SPS/CNGS time measuring system. Protons circulate in the SPS till kicked by a signal to the beam current transformer (BCT) and on to the target. The BCT is the origin for the measurement. Both the kicker signal and the proton flux in the BCT get to the waveform digitizer (WFD), the first through the Control Timing Receiver (CTRI). The WFD records the proton distribution. The common CNGS/LNGS clock comes from GPS via the PolaRx receiver and the central CTRI, where the CERN UTC and General Machine Timing (GMT) also arrive. The difference between the two references is recorded. The marker x ± y indicates an 'x' nanosecond delay with a 'y' ns error bound.

Fig. 4 OPERA time measuring system at LNGS: various delays of the timing chain, and the standard deviations of the error. The top half of the picture is the common GPS clock system (PolaRx2e is the GPS receiver), and the bottom half is the underground detector. Fiber cables bring the GPS clock underneath. The underground detector consists of the blocks from the tt-strip to the FPGA. Errors for each component are shown as x ± y, where x is the delay caused by the component in transmitting time information, and y is the expected bound on that delay.

Timing systems at the two ends of the OPERA experiment

Researchers from OPERA measured the remaining delays and calibrations not included in the CERN calculation: those shown in Fig. 4. The neutrinos were detected in an underground lab, but the common clock from the GPS satellites was visible only above ground level. The clock value noted above-ground had to be transmitted to the underground detector with an 8 km fiber cable. The delays associated with this transfer of time had to be accounted for in the calculation. How much the error could vary (the standard deviation of the errors) mattered to the analysis, and had to be calculated for each part of the timing chain separately. Special techniques were used to measure the length of the fiber and its consequent delay, required as part of the overall calculation.^[27]

In addition, to sharpen resolution from the standard GPS 100 nanoseconds to the 1 nanosecond range metrology labs achieve, OPERA researchers used Septentrio’s precise PolaRx2eTR GPS timing receiver,^[29] along with consistency checks across clocks (time calibration procedures) which allowed for common-view time transfer. The PolaRx2eTR allowed measurement of the time offset between an atomic clock and each of the Global Navigation Satellite System satellite clocks. For calibration, the equipment was taken to the Swiss Metrology Institute (METAS).^[27] 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 found the superluminal result, the time calibration was rechecked both by a CERN engineer and the German Institute of Metrology (PTB).^[27] Time-of-flight was eventually measured to an accuracy of 10 nanoseconds.^[4][30] As shown in Table 1, the final error bound was derived by combining the variance of the error for the individual parts.

[edit] Measurement uncertainties

The measurement uncertainty of the November main analysis includes systematic and statistical error bounds (Table 1).

Note: Errors were combined by squaring them, summing them, and then taking the square root.

[edit] The analysis

The OPERA team has analyzed the results in different ways and using different experimental methods. Following the initial main analysis released in September, three further analyses were made public in November. In the main November analysis, all the existing data were reanalyzed to allow adjustments for other factors, such as the Sagnac effect in which the Earth's rotation affects the distance traveled by the neutrinos. Then an alternative analysis adopted a different model for the matching of the neutrinos to their creation time. The third analysis of November focused on a different experimental setup ('the rerun') which changed the way the neutrinos were created.

In the initial setup, every detected neutrino would have been produced sometime in a 10,500 nanoseconds (10.5 microseconds) range, since this was the duration of the proton beam spill generating the neutrinos. It was not possible to isolate neutrino production time further within the spill. Therefore, in their main statistical analyses, the OPERA group generated a model of the proton waveforms at CERN, took the various waveforms together, and plotted the chance of neutrinos being emitted at various times (the global probability density function of the neutrino emission times). They then compared this plot against a plot of the arrival times of the 15,223 detected neutrinos. This comparison indicated neutrinos had arrived at the detector 57.8 nanoseconds faster than if they had been traveling at the speed of light in vacuum. An alternative analysis in which each detected neutrino was checked against the waveform of its associated proton spill (instead of against the global probability density function) led to a compatible result of approximately 54.5 nanoseconds.^[32]

The November main analysis, which showed an early arrival time of 57.8 nanoseconds (see Table 2), was conducted blind to avoid observer bias, whereby those running the analysis might inadvertently fine-tune the result toward expected values. To this end, old and incomplete values for distances and delays from the year 2006 were initially adopted. With the final correction needed not yet known, the intermediate expected result was also an unknown. Analysis of the measurement data under those 'blind' conditions gave an early neutrino arrival of 1043.4 nanoseconds. Afterward, the data were analyzed again taking into consideration the complete and actual sources of errors. If neutrino and light speed were the same, a subtraction value of 1043.4 nanoseconds should have been obtained for the correction. However, the actual subtraction value amounted to only 985.6 nanoseconds, corresponding to an arrival time 57.8 nanoseconds earlier than expected.^[33]

Two facets of the result came under particular scrutiny within the neutrino community: the GPS synchronization system, and the profile of the proton beam spill that generated neutrinos.^[34] The second concern was addressed in the November rerun: for this analysis, OPERA scientists repeated the measurement over the same baseline using a new CERN proton beam which circumvented the need to make any assumptions about the details of neutrino production during the beam activation, such as energy distribution or production rate. This beam provided proton pulses of 3 nanoseconds each with up to 524 nanosecond gaps. This meant a detected neutrino could be tracked uniquely to its generating 3 nanoseconds pulse, and hence its start and end travel times could be directly noted. Thus, the neutrino's speed could now be calculated without having to resort to statistical inference.^[4]

[edit] Overview of results

In addition to the four analyses mentioned earlier—September main analysis, November main analysis, alternative analysis, and the rerun analysis—the OPERA team also split the data by neutrino energy and reported the results for each set of the September and November main analyses (Table 3). The rerun analysis had too few neutrinos to consider splitting the set further.

Table 3. Results of the September and November analyses according to neutrino energy

Early arrival (blind analysis)	Corrections	Neutrino energy	Early arrival	Speed difference	Absolute speed difference
September (16111 neutrino events from 2009–2011)
Main analysis
1048.5 ± 6.9 (stat.) ns	-987.8 ns	17 GeV	(60.7 ± 6.9 (stat.) ± 7.4 (sys.)) ns	(2.48 ± 0.28 (stat.) ± 0.30 (sys.))×10−5	∼7.4 km/s
Analysis of energy dependence
		13.9 GeV	(53.1 ± 18.8 (stat.) ± 7.4 (sys.)) ns	In agreement with September main analysis; no significant energy dependence[35]
		42.9 GeV	(67.1 ± 18.2 (stat.) ± 7.4 (sys.)) ns
		28.1 GeV (average)	(60.3 ± 13.1 (stat.) ± 7.4 (sys.)) ns
November (15223 neutrino events from 2009–2011)
Main analysis
1043.4 ± 7.8 (stat.) ns	-985.6 ns	17 GeV	(57.8 ± 7.8 (stat.) (sys.)) ns	(2.37 ± 0.32 (stat.)  (sys.))×10−5	∼7.1 km/s
Alternative analysis
1040.1 ± 5.0 (stat.) ns	-985.6 ns	17 GeV	(54.5 ± 5.0 (stat.) (sys.)) ns	In agreement with November main analysis
Analysis of energy dependence
		13.8 GeV	(54.7 ± 18.4 (stat.) (sys.)) ns	In agreement with November main analysis; no significant energy dependence[36]
		40.7 GeV	(68.1 ± 19.1 (stat.) (sys.)) ns
		28.2 GeV (average)	(61.1 ± 13.2 (stat.) (sys.)) ns
CNGS bunched beam rerun (20 neutrino events from October 22 to November 6, 2011)
Rerun analysis
			(62.1 ± 3.7 (stat.)) ns	In agreement with November main analysis

Note: (54.7 ± 18.4 (stat.) (sys.)) ns, as an example, indicates a measured average value of 54.7 with an expected error of 18.4, higher or lower, due to the statistical technique used, and an error 7.3 to the higher side and 6.9 to the lower side from inaccuracies in the measuring equipment and methods.

[edit] Reception by the physics community

Nobel laureates Steven Weinberg,^[37] George Smoot III, and Carlo Rubbia,^[38] and other physicists not affiliated with the experiment, including Michio Kaku,^[39] 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.^[40] Nevertheless, Ereditato, the OPERA spokesperson, states no one has an explanation that invalidates the experiment's results.^[41]

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.^[42]

[edit] Previous measurements and considerations

Previous experiments of neutrino speed play a role in the reception of the OPERA result by the physics community. Those experiments did not detect statistically significant deviations of neutrino speeds from the speed of light. For instance, Astronomer Royal Martin Rees and theoretical physicists Lawrence Krauss^[43] and Stephen Hawking^[44] state neutrinos from the SN 1987A supernova explosion arrived almost at the same time as light, indicating no faster-than-light neutrino speed. John Ellis, theoretical physicist at CERN, believes it difficult to reconcile the OPERA results with the SN 1987A observations.^[45] Observations of this supernova restricted 10 MeV anti-neutrino speed to less than 20 parts per billion (ppb) over lightspeed.^[46][47] This is one of the reasons most physicists suspect the OPERA team has made an error.^[48] However, SN 1987A neutrinos differ from the ones detected by OPERA in lepton number (antineutrino versus neutrino), flavor (electron neutrino versus muon neutrino), energy, and medium of travel (interstellar space versus solid rock).^[49]

In 2007 Fermilab's MINOS collaboration reported measuring the flight-time of lower-energy neutrinos (3 Gev neutrinos) as an average 51 ppm over speed of light, with a range of 22 ppm to 80 ppm at a 68% confidence level. However, at a 99% confidence level, while the average was still 51 ppm, the range broadened to 24 ppm below light speed to 126 ppm above it. The result was seen as inconclusive on whether neutrinos went faster than light did in a vacuum.^[45][46] For details, see Neutrino speed. In addition, both MINOS and MiniBooNE, experiments at Fermilab, have reported hints of possible CPT symmetry and Lorentz violation in neutrino and anti-neutrino oscillations, CPT and Lorentz invariance being two principles related to the light-speed limit. These hints might be consistent with the Lorentz violations indicated by the faster-than-light speed of neutrinos.^[50][51] For details, see Neutrino anomalies.

The idea of faster-than-light neutrinos has been around for at least 25 years,^[50] while theories of superluminal particles called tachyons are even older. For instance, speculative tachyon models were proposed by Olexa-Myron Bilaniuk, George Sudarshan, and Gerald Feinberg in the 1960s, and were considered as being in agreement with special relativity ("Lorentz invariant"), even though the theory of their interactions remained not fully known. Tachyons are faster at lower energies.^[52] However, per Jean Alexandre, John Ellis, and Nick E. Mavromatos, since the higher-energy OPERA neutrinos are apparently faster than the lower-energy SN 1987A ones, they are not conventionally tachyonic.^[53]

[edit] Comparison with Lorentz-violating models

Contrary to the conventional Lorentz-invariant tachyonic models described above, some frameworks allowing superluminal particles such as neutrinos were proposed by slightly modifying the standard model of particle physics, a model which provides a unified description of many elementary particles and forces. These modifications involved small violations of the principles of special relativity ("Lorentz violations"). Many such Lorentz-violating frameworks have been proposed and used to analyze experiments even before the OPERA result, one example the model of Sidney Coleman and Nobel laureate Sheldon Lee Glashow, and another the Standard-Model Extension.^[54] The predictions of some such models are now being compared against the OPERA result.

A successful theory explaining the OPERA result is expected to be able to explain the SN 1987A data, the very small deviations possible over light speed (tight bounds on Lorentz violation) for charged particles such as electrons, and the chance, as explained below, of intense neutrino decays predicted by many models which include violations of standard relativity.^[55]

[edit] Cohen–Glashow effect

Andrew Cohen and Glashow have predicted that superluminal neutrinos would radiate electrons and positrons and lose energy through vacuum Cherenkov effects, where a particle traveling faster than light decays continuously into other slower particles.^[56] However, this energy attrition is absent both in the OPERA experiment and in the colocated ICARUS experiment, which uses the same CNGS beam as OPERA.^[1] This discrepancy was seen by Cohen and Glashow to present "a significant challenge to the superluminal interpretation of the OPERA data".^[56] Their analysis rests on neutrinos being faster than the real speed of light, and on the usual linear conservation laws for energy and momentum, two fundamental physics quantities preserved in interactions.^[57] 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, with photons as energetic as the OPERA neutrinos. Cohen also states that while the paper refutes some theories trying to explain the OPERA results, it does not prove OPERA definitively wrong.^[58]

In response, Autiero has stated that the Cohen–Glashow premises may not be universally valid.^[1] Sergio Bertolucci, director of research at CERN, contends that if every new measurement is interpreted with older theories, a new theory is impossible.^[59] 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.^[1]

Hooman Davoudiasl and Thomas Rizzo have suggested that superluminal neutrinos could be detected (assuming the Cohen–Glashow theory holds) by checking for traces of neutrino decay in the Large Hadron Collider, a particle accelerator at CERN, within data sets already recorded by ATLAS and CMS, both experiments at CERN.^[60] While agreeing in principle, Cohen and Glashow consider the effort needed for such an analysis to be somewhat prohibitive; they also point out that a null result might just mean their theory is faulty.^[61]

[edit] Pion decay issues

Ramanath Cowsik, Shmuel Nussinov and Utpal Sarkar, using a framework for Lorentz violation similar to that of Cohen and Glashow, note that at the high energies of the OPERA experiment, pions, the intermediate particles which eventually turn into neutrinos, will not do so for a longer period of time. Hence the net energy of the produced neutrino would be less than what is actually seen. They consider the observation of even-higher-energy neutrinos at the IceCube Neutrino Observatory to be proof high-energy pions do decay per the standard ideas of physics, preserving energy and momentum, making superluminal neutrinos hard to produce in "the present framework of physics".^[62] Sarkar, however, notes there are models possible (neutrino-condensate dark energy) where neutrinos can hop through at effective superluminal speed, bypassing the above pion-decay constraint.^[63] Nussinov states the only way to accommodate such a measurement inconsistent with basic physics would be to have the neutrinos break physics rules (perhaps by going to other dimensions or through some such unusual mechanism).^[64]

[edit] 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.^[65] The experiment's advisory board, however, felt it their duty to open the results to public scrutiny.^[23] OPERA spokesperson Ereditato says that all the researchers have signed the final submission to the Journal of High-Energy Physics,^[66] but others indicate only most have.^[23] Other sources had previously reported that some of the original dissenters had signed the final paper, whereas others who had signed the first paper removed their names from the final one.^[14][67]

The current dissenters in the collaboration are concerned the time window for neutrino detection, the spread of the histograms in Fig. 2, was initially assumed to be 10 ns but revealed by 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.^[14]

OPERA team member Luca Stanco of the National Institute of Nuclear Physics (INFN) in Italy considers the OPERA results an embarrassment. He believes that is the consensus of the physics community since the results fit no known physics framework, and they urgently need to be independently confirmed.^[64]

[edit] Independent replication

Replication by a separate lab would be the key test of the result.^[68] OPERA and CERN advanced two requests: to Fermilab, a US lab which has both a neutrino generator and a detector; and to the T2K experiment, a neutrino generator and detector in Japan. Although both initially agreed to test the OPERA result,^[69] T2K is now unsure of its commitment.^[70] Five efforts are currently underway to test the OPERA results.^[71] Of these, Fermilab has stated that the detectors for the MINOS project are being upgraded,^[72] and new results are not expected until at least 2012. A result based on data collected over the last five years is expected to be available sometime toward the end of 2012. Fermilab scientists hope to closely analyze and place bounds on the errors in their timing system to achieve a precision of 15 nanoseconds, enough to confirm or refute the OPERA result.^[73] The lab's director Pier Oddone states MINOS may make some measurements before March 2012, when the proton beam making the neutrinos is scheduled to be shut down for a year. More thorough measurements will be made in 2013 and a full analysis done in 2014.^[74] New sensitive equipment has been installed and data will be collected with it in April 2012, and analyzed by the end of the year.^[71] The cost is expected to be roughly half-a-million dollars.^[75]

The Borexino and ICARUS experiments (both located at Gran Sasso) will begin checks of OPERA's results in 2012.^[76] 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 detect and time around a hundred neutrinos more in a couple of months, to improve the accuracy of the results.^[77] The CNGS beam, now shut down, will restart mid-March 2012.^[78] OPERA, in addition to detecting neutrino interactions at the scintillation counters,^[79] is gearing up to time neutrino interactions with its next-in-detection-stage resistive plate chamber (RPC) detectors as well.^[78][80] In parallel, CERN scientists will use diamond radiation-detectors to measure muons, an intermediate particle produced along with the neutrinos, after the point where neutrinos are produced.^[78]

[edit] See also

• List of unsolved problems in physics

[edit] Notes

[edit] References

[edit] Further reading

[edit] Peer reviewed publications

2011 publications[show] Steven S. Gubser (October 15, 2011). "Superluminal neutrinos and extra dimensions: Constraints from the null energy condition". Physics Letters B 705 (3): 279. arXiv:1109.5687. Bibcode 2011PhLB..705..279G. doi:10.1016/j.physletb.2011.10.028.  Jean Alexandre (October 20, 2011). "Lifshitz-type quantum field theories in particle physics". International Journal of Modern Physics A 26 (26): 4523. arXiv:1109.5629. Bibcode 2011IJMPA..26.4523A. doi:10.1142/S0217751X11054656.  Pablo Alejandro Sanchez, Mariano Anabitarte, Mauricio Bellini (October 25, 2011). "Dirac equation for massive neutrinos in a Schwarzschild–de Sitter spacetime from a 5D vacuum". Physics Letters B 705 (5): 535. arXiv:1110.1300. Bibcode 2011PhLB..705..535S. doi:10.1016/j.physletb.2011.10.058.  F.R. Klinkhamer, G.E. Volovik (November 10, 2011). "Superluminal neutrino and spontaneous breaking of Lorentz invariance". Pisma Zh.Eksp.Teor.Fiz 94: 731. arXiv:1109.6624. Bibcode 2011arXiv1109.6624K.  M V Berry, N Brunner, S Popescu, P Shukla (November 11, 2011). "Can apparent superluminal neutrino speeds be explained as a quantum weak measurement?". Journal of Physics A: Mathematical and Theoretical 44 (49): 492001. arXiv:1110.2832. Bibcode 2011JPhA...44W2001B. doi:10.1088/1751-8113/44/49/492001. http://iopscience.iop.org/1751-8121/44/49/492001/.  P N Bhat, S Guiriec (November 12, 2012). "An overview of the current understanding of Gamma-ray Bursts in the Fermi era". Bulletin of the Astronomical Society of India 39 (3): 471. arXiv:1111.4909. Bibcode 2011BASI...39..471B. http://www.ncra.tifr.res.in/~basi/toc11September.htm.  Altschul, Brett (November 14, 2011). "Consequences of neutrino Lorentz violation for leptonic meson decays". Physical Review D 84 (9): 091902. arXiv:1110.2123. Bibcode 2011PhRvD..84i1902A. doi:10.1103/PhysRevD.84.091902.  D. Autiero, P. Migliozzi and A. Russo (November 14, 2011). "The neutrino velocity anomaly as an explanation of the missing observation of neutrinos in coincidence with GRB". Journal of Cosmology and Astroparticle Physics 11 (026). arXiv:1109.5378. Bibcode 2011JCAP...11..026A. doi:10.1088/1475-7516/2011/11/026.  Nojiri, Shin’ichi; Odintsov, Sergei D. (November 16, 2011). "Could dynamical Lorentz symmetry breaking induce the superluminal neutrinos?". European Physical Journal C 71 (11): 1801. arXiv:1110.0889. Bibcode 2011EPJC...71.1801N. doi:10.1140/epjc/s10052-011-1801-4.  Cacciapaglia, Giacomo; Deandrea, Aldo; Panizzi, Luca (November 28, 2011). "Superluminal neutrinos in long baseline experiments and SN1987a". Journal of High Energy Physics 2011 (11): 137. arXiv:1109.4980. Bibcode 2011JHEP...11..137C. doi:10.1007/JHEP11(2011)137. http://www.springerlink.com/content/5684752812200w82/.  Rupert Leitner, Michal Malinsk, Bedrich Roskovec and He Zhang (December 1, 2011). "Nonstandard antineutrino interactions at Daya Bay". Journal of High Energy Physics 12 (001). arXiv:1105.5580. Bibcode 2011JHEP...12..001L. doi:10.1007/JHEP12(2011)001. http://www.springerlink.com/content/rk5425648554k57l/.  Xue, She-Sheng (December 6, 2011). "Do high-energy neutrinos travel faster than photons in a discrete space–time?". Physics Letters B 706 (2): 213–218. arXiv:1110.1317. Bibcode 2011PhLB..706..213X. doi:10.1016/j.physletb.2011.11.001.  Bi, Xiao-Jun; Yin, Peng-Fei; Yu, Zhao-Huan; Yuan, Qiang (December 8, 2011). "Constraints and tests of the OPERA superluminal neutrinos". Physical Review Letters 107 (24): 241802. arXiv:1109.6667. Bibcode 2011PhRvL.107x1802B. doi:10.1103/PhysRevLett.107.241802.  Marija Dimitrijevich, Larisa Jonke (December 21, 2011). "A twisted look on kappa-Minkowski: U(1) gauge theory". Journal of High Energy Physics 2011 (12): 80. arXiv:1107.3475. Bibcode 2011JHEP...12..080D. doi:10.1007/JHEP12(2011)080.  Oda, Ichiro; Taira, Hajime (December 21, 2011). "Superluminal Neutrinos from Gauge Field". Modern Physics Letters A 26 (39): 2917-2921. arXiv:1110.0931. Bibcode 2011MPLA...26.2917O. doi:10.1142/S0217732311037297.

January 2012 publications[show] R. A. Konoplya (January 5, 2012). "Superluminal neutrinos and the tachyon's stability in the rotating Universe". Physics Letters B 706 (4-5): 451. arXiv:1109.6215. Bibcode 2012PhLB..706..451K. doi:10.1016/j.physletb.2011.11.037.  Jean Alexandre, John Ellis, Nick E. Mavromatos (January 5, 2012). "On the possibility of superluminal neutrino propagation". Physics Letters B 706 (4-5): 456. arXiv:1109.6296. Bibcode 2012PhLB..706..456A. doi:10.1016/j.physletb.2011.11.038.  Ram Brustein, Dmitri Semikoz (January 5, 2012). "Apparent superluminal propagation caused by nonlinear coherent interactions in matter". Physics Letters B 706 (4-5): 462. arXiv:1110.0762. Bibcode 2012PhLB..706..462B. doi:10.1016/j.physletb.2011.11.039.  C. A. G. Almeida, M. A. Anacleto, F. A. Brito, E. Passos (January 2012). "Superluminal neutrinos from Lorentz-violating dimension-5 operators". European Physical Journal C 72 (1): 1855. arXiv:1111.0093. doi:10.1140/epjc/s10052-011-1855-3.  Garattini, Remo; Mandanici, Gianluca (January 6, 2012). "Particle propagation and effective space-time in gravity's rainbow". Physical Review D 85 (2): 023507. arXiv:1109.6563. Bibcode 2012PhRvD..85b3507G. doi:10.1103/PhysRevD.85.023507.  M V Berry (January 10, 2012). "Causal wave propagation for relativistic massive particles: physical asymptotics in action". European Journal of Physics 33 (2): 279. Bibcode 2012EJPh...33..279B. doi:10.1088/0143-0807/33/2/279.  Winter, W. (January 17, 2012). "Constraints on the interpretation of the superluminal motion of neutrinos at OPERA". Physical Review D 85 (1): 017301. Bibcode 2012EJPh...33..279B. doi:10.1088/0143-0807/33/2/279.  Drago, Alessandro; Masina, Isabella; Pagliara, Giuseppe (January 17, 2012). "The Hypothesis of Superluminal Neutrinos: comparing OPERA with other Data". Europhysics Letters 97 (2): 21002. arXiv:1109.5917. Bibcode 2012EL.....9721002D. doi:10.1209/0295-5075/97/21002.  Bo-Qiang Ma (January 19, 2012). "The phantom of the OPERA: Superluminal neutrinos". Modern Physics Letters A 27: 1230005. arXiv:1111.7050. doi:10.1142/S0217732312300054.  A. Cutolo (January 20, 2012). "The superluminal limit as a consequence of the classical causality principle". European Physics Journal Plus 127: 3. Bibcode 2012EPJP..127....3C. doi:10.1140/epjp/i2012-12003-5.  F. R. Klinkhamer (January 23, 2012). "Superluminal neutrino, flavor, and relativity". Physical Review D 85: 016011. arXiv:1110.2146. Bibcode 2012PhRvD..85a6011K. doi:10.1103/PhysRevD.85.016011.  Massimo Mannarelli, Manimala Mitra, Francesco Lorenzo Villante, Francesco Vissani (January 25, 2012). "Non-standard neutrino propagation and pion decay". JHEP 2012 (1): 136. arXiv:1112.0169. doi:10.1007/JHEP01(2012)136.  Winter, Walter (January 27, 2012). "Interpretation of neutrino flux limits from neutrino telescopes on the Hillas plot". Physical Review D 85 (2): 023013. arXiv:1103.4266. Bibcode 2012PhRvD..85b3013W. doi:10.1103/PhysRevD.85.023013.  Lacki, Brian C. (January 31, 2012). "Olber's paradox for superluminal neutrinos: constraining extreme neutrino speeds at TeV–ZeV energies with the diffuse neutrino background". Journal of Cosmology and Astroparticle Physics (054): 054. arXiv:1111.3045. Bibcode 2012JCAP...01..054L. doi:10.1088/1475-7516/2012/01/054.  Amelino-Camelia, Giovanni; Gubitosi, Giulia; Loret, Niccoló; Mercati, Flavio; Rosati, Giacomo; Lipari, Paolo (January 31, 2012). "OPERA-reassessing data on the energy dependence of the speed of neutrinos". International Journal of Modern Physics D 20 (14): 2623-2640. arXiv:1109.5172. doi:10.1142/S0218271811020780.

• Bezrukov, Fedor; Lee, Hyun Min (February 6, 2012). "Model dependence of the bremsstrahlung effects from the superluminal neutrino at OPERA". Physical Review D 85 (3): 031901. arXiv:1112.1299. Bibcode 2012PhRvD..85c1901B. doi:10.1103/PhysRevD.85.031901. 

• Tomaschitz, R. (February 6, 2012). "Neutrino currents in the absolute spacetime: Relating the refractive index of the aether to the OPERA excess velocity". Europhysics Letters 97 (3): 39003. doi:10.1209/0295-5075/97/39003. 

• D. Marfatia, H. Pas, S. Pakvasa, T. J. Weiler (February 7, 2012). "A model of superluminal neutrinos". Physics Letters B 707 (5): 553. arXiv:1112.0527. Bibcode 2012PhLB..707..553M. doi:10.1016/j.physletb.2012.01.028. 

• Claudio Germana (February 7, 2012). "Are OPERA neutrinos faster than light because of non-inertial reference frames?". Astronomy & Astrophysics 538. arXiv:1201.4147. Bibcode 2012A&A...538L...5G. doi:10.1051/0004-6361/201118745. 

• Jian Tang and Walter Winter (February 10, 2012). "Requirements for a new detector at the south pole receiving an accelerator neutrino beam". Journal of High Energy Physics 2012 (2): 28. arXiv:1110.5908. doi:10.1007/JHEP02(2012)028. 

• Huo, Yunjie; Li, Tianjun; Liao, Yi; Nanopoulos, Dimitri V.; Qi, Yonghui (February 10, 2012). "Constraints on neutrino velocities revisited". Physical Review D 85 (3): 034022. arXiv:1112.0264. doi:10.1103/PhysRevD.85.034022. 

• Jentschura, U. D.; Wundt, B. J. (February 15, 2012). "Localizability of Tachyonic Particles and Neutrinoless Double Beta Decay". European Physical Journal C 72: 1894. arXiv:1201.0359. doi:10.1140/epjc/s10052-012-1894-4. 

• B. Alles (February 21, 2012). "Relativity accommodates superluminal mean velocities". Physical Review D 85 (4): 047501. arXiv:1111.0805. doi:10.1103/PhysRevD.85.047501. 

[edit] External links

• BBC documentary on the OPERA result
• OPERA Main Page
• CNGS Neutrino beam at CERN
• OPERA publications
• Webcast of OPERA neutrino anomaly presentation by Dario Autiero
• Works which cite the OPERA result
• Resource list of The Net Advance of Physics
• Another summary of OPERA-related arXiv papers
• CERN OPERA neutrinos travel faster that light, September 22, 2011, YouTube
• The Neutrinos CERN interview.faster than light Einstein might have been wrong?! HD, YouTube, September 23, 2011
• CERN/LNGS researchers' discussion of how to check the time transfer