Measurements of neutrino speed

Measurements of neutrino speed are performed as tests of special relativity and to determine the mass of neutrinos. It is investigated whether light and neutrinos are sent simultaneously from a distant astronomical radiation source, at the same time arrive at the Earth. Terrestrial methods are to determine the neutrino velocity by a time of flight measurement using synchronized clocks, or to compare their speed with the other particles.

According to the special theory of relativity the neutrino speed is slightly slower than the speed of light. Previous measurements give an upper limit for deviations from 10-9, or about one- billionth the speed of light. This is within the measurement accuracy in accordance with the prediction of the theory of relativity.

• 7.1 Borexino
• 7.2 LVD
• 7.3 ICARUS
• 7.4 OPERA

• 8.1 Old timing system
• 8.2 New timing system

Overview

For a long time it was assumed in the Standard Model of particle physics that neutrinos are massless. Then they would have to move according to the special theory of relativity the speed of light. But since the discovery of neutrino oscillations is assumed that they have mass and therefore are slightly slower than light, because otherwise their relativistic energy would be infinite. This energy is given by

V with the neutrino velocity and the velocity of light c. The neutrino mass m is estimated to be less than 2 eV / c ² and may be smaller than 0.2 eV / c ². Only at low neutrino energies, a clearly measurable deviation of the speed would result (figure and table to the right, counting for 0.2 eV).

However Previous experiments used neutrino energies above 10 MeV. The predicted for these energy ranges of the special theory of relativity velocity differences can not be determined, therefore, to the current accuracy of the time measurement. That still experiments are carried out depends on the theoretical possibility of violations of Lorentz invariance, a fundamental property of the special theory of relativity, together. These are motivated by speculative variants of quantum gravity, which significantly larger deviations of the speed of light could be possible (see Modern tests of Lorentz invariance ). Besides flight measurements, this also allows the indirect determination of the neutrino velocity by analysis of possible Lorentz -violating effects.

Fermilab ( 1970 )

Fermilab led in the 1970s by terrestrial measurements, in which the speed of muons was compared with that of neutrinos and anti-neutrinos ( with energies between 30 and 200 GeV ). To measure the Fermilab narrow band neutrino beam was used. Take 400 - GeV protons onto a target, whereupon secondary beams from pions and kaons produced. In a 345 -meter-long evacuated decay tube then disintegrate into neutrinos and muons. The remaining hadrons are stopped by a secondary absorber, so that only the neutrinos and some high-energy muons penetrate the 500 meter long earth and steel plate to get to the particle.

Since the protons are transferred in bundles of a nanosecond duration and a distance of 18.73 ns, the rate of muons and neutrinos could be determined, because a speed difference would firstly lead to a stretching of the neutrino beam and secondly to a shift of the entire neutrino time spectrum. First, the rates of muons and neutrinos were compared. Later antineutrinos were considered. There was within the measurement accuracy, no deviation of the speed of light, the relative uncertainty was

An energy dependence of the neutrino velocity also could not be determined in this measurement accuracy.

Supernova 1987A

The most precise accordance with the speed of light could be 1987, as determined by observations of antineutrinos with energy from 7.5 to 35 MeV, which were formed during the supernova 1987A at a distance of about 160,000 light years. The few hours to the neutrinos arrived before the light, corresponding to a relative deviation of

But are attributed to the interaction- poor neutrinos could cross the field of supernova unhindered, while the light took longer for it.

MINOS (2007)

The first measurement of the absolute transit time of 3 GeV neutrinos was performed by the MINOS group ( 2007) of Fermilab for a distance of 734 km. To generate the neutrinos ( Numi beam) used the MINOS Fermilab Main Injector, which 120 GeV protons were shot in five to six bundles per phase extraction on a graphite target. The resulting mesons fell into a 675 meter long decay tunnel into muon neutrinos ( 93%) and Myonantineutrinos ( 6%). The arrival time was determined by comparing the arrival times at the near and far detector of MINOS. The clocks of both stations were synchronized by GPS with each other.

There was an early neutrino arrival of about 126 ns. In the uncertainty of systematic errors, the two fiber optic connections for transmission of time signals between the GPS receivers at the surface and the underground laboratories dominate. Based on the distance between the two detectors results in an apparent superluminal velocity with a relative deviation (68 % confidence interval), which was not significant at 1.8 σ. 5σ would be required for recognition as a scientific discovery.

In contrast to the 99% confidence interval is given by this experiment, a relative velocity deviation from

So that the result is compatible with sub- light speed.

OPERA (2011, 2012)

OPERA neutrino anomaly (2011)

The OPERA group led 2009-2011 flight measurements with 17- GeV muon neutrinos ( CNGS ) by. The measurement was made at a distance of about 730 km between a target at the Super Proton Synchrotron at CERN, where pions and kaons arise which decompose partially into muons and muon neutrinos, and the OPERA neutrino detector at LNGS. To synchronize the clocks and determining the exact distance GPS was used, where fiber cables have been used by about 8 km in length for signal transmission in the OPERA detector. The temporal distribution of 10.5 microseconds long proton pulses was compared through statistical means with about 16000 neutrino events detected. It was found that the neutrinos arrived at about 61 ns earlier at the detector than would have been expected with the speed of light. The anomaly appeared significantly associated with 6 σ, the error analysis, however, was designated as provisional.

To exclude some statistical error, OPERA led in October and November 2011 by a measurement under different conditions. The proton pulses were ns ns divided into short bursts of 3 at a distance of 524, so that each neutrino event could be associated with a bundle. The measurement of 20 neutrino events resulted in a premature arrival of about 62 ns, consistent with the previous results. In addition, updated OPERA early neutrino arrival in accordance with the major statistical analysis of September to about 57 ns. The authors stated that the deviation of 6.2 σ fraud, which would be significant. However, they added that they wanted to draw from the results of any further conclusions and it was necessary to continue the search for yet unknown systematic errors.

A number of explanations and reviews in arXiv preliminary publications were ( which, however, no precise assessment subject ) on this topic. Some of these have now been published in peer-reviewed journals. A significant objection to the OPERA result was published by Andrew G. Cohen and Sheldon Lee Glashow. The authors apply the vacuum - Cherenkov effect, which would have to occur in Lorentz -violating theories which allow superluminal, on neutrinos. They predict the production of electron-positron pairs, so that the neutrinos would lose in a short time considerable energy. Was not observed from the adjacent ICARUS group.

In February and March 2012 was found, however, that further tests have revealed two sources of error: either a faulty fiber optic cable connection between a GPS receiver and a computer card, and on the other hand, an oscillator that was used to measure the neutrino events during the GPS synchronization with a time stamp to be provided. The error function in the opposite direction. For further investigations, a comparison of the arrival of cosmic muons at the OPERA detector and the adjacent LVD detector that a time change for the period 2008 to 2011 compared to 2007 to 2008 and 2011 occurred until 2012. It was caused by the cable fault, so for early neutrino arrival of -60 ns now about 73 ns must be added. The opposed oscillator error was determined to be about -15 ns. Thus, these two problems have been confirmed as the cause of the OPERA anomaly of about -60 ns.

Final

In July 2012, the OPERA group released a new analysis of data from 2009 to 2011, in which the errors found were considered. There were new limits on flight time differences of

And upper limits for speed differences of

The new analysis of the bundled pulses of October and November 2011 revealed

All these results are consistent with the speed of light, wherein the limit is 10-6 of magnitude more precise than previous terrestrial flight measurements.

ICARUS (2012 )

Even before the OPERA group had corrected their original measurements, the ICARUS group published its own measurement of neutrino speed in March 2012. The ICARUS detector is as OPERA also in the LNGS. Thereby partially the same equipment was used for the external timing, whereas the internal time measurement was independent. ICARUS examined the neutrinos same proton pulses, which were also used by OPERA October-November 2011 so 3 -ns proton pulses with a spacing of 524 ns. This seven neutrino events were observed that could be directly linked to the respective proton pulse. The upper limit for the difference between the measured arrival time and those that can be expected at the speed of light is,

Within the measurement accuracy so match occurs with the speed of light.

LNGS (2012 )

In May 2012, CNGS neutrinos were again sent to Gran Sasso of CERN. The LNGS experiments Borexino, OPERA, ICARUS, and LVD began the data analysis of the neutrino events, with fit was with the speed of light. The 17- GeV muon neutrinos consisted of four pulses per beam extraction were separated by ≈ 300 ns. The pulses were in turn divided into 16 bundles at a distance of ≈ 100 ns, the beam width was ≈ 2 ns.

Borexino

The Borexino group analyzed the data from the measurements of the bundled CNGS beam from October to November 2011 and May 2012. From the data of 2011, they were able to evaluate 36 neutrino events and obtained an upper limit on flight time differences between light and neutrinos

For the analysis of data from 2012, they improved their measuring devices by installing a new trigger system and a Rubidiumuhr, which was coupled to a geodetic GPS receiver. Together with LVD and ICARUS, they conducted an independent, precise geodesy measurement. For the final analysis 62 neutrino events could be used, which resulted in more than a total flight time differences

Corresponding to the upper limit for the speed differences of

LVD

The LVD Group analyzed the first bundled CNGS beam from October to November 2011. They evaluated 32 neutrino events and were given an upper limit on flight time differences between light and neutrinos

During the measurements in May 2012, she used the external equipment that was developed by the Borexino group, and the values ​​determined by LVD and Borexino ICARUS geodesy data. They also improved their scintillation counter and the trigger. It was 48 neutrino events ( with energies greater than 50 MeV, where the average neutrino energy 17 GeV was ) for the analysis are used, with an upper limit on flight time differences:

And for differences in speed

ICARUS

After analyzing the bundled CNGS rays from October to November 2011 (see # ICARUS (2012 ) above) published the ICARUS group, the analysis of measurements from May They improved both their internal timekeeping, as well as between CERN and LNGS, used the geodetic measurements along with Borexino and LVD, and used Borexinos LNGS - time system. It could be evaluated 25 neutrino events, with an upper limit on flight time differences between neutrinos and light from

According to differences in speed of

Neutrino speeds that exceed the speed of light by more than (95 % confidence interval) are excluded.

OPERA

After correcting the original OPERA results also published the measurements of May 2012. To evaluate the neutrino events, four different methods of analysis and another, independent timing system were used. They revealed that a limit on flight time differences between light and muon neutrinos ( 48 to 59 neutrino events depending on the method of analysis ) of

And between light and Antimyonneutrinos (3 neutrino events) of

Consistent with the speed of light in the range of

MINOS (2012 )

Old timing system

Parallel to the LNGS measurements also led the MINOS preliminary measurements from 2007 on. This neutrino events were evaluated over seven years. In addition, the GPS timing system has been improved, the delays in the electronic components better taken into account and an upgrade of timekeeping equipment carried. The analysis of the 10 microsecond pulses neutrino each containing 5-6 bundle was carried out on two ways: First, the data of the detector are further away (as in the 2007 measurement) generally out of which the first detector is determined statistically. It was determined the following limit for time differences between light and neutrinos:

In the second method, the data of the individual neutrino beam itself were used. There was:

Neutrino speed and the speed of light so agree within the measurement accuracy.

New timing system

To further increase the precision has been developed a new timing system. For example, the "resistive wall Current Monitor" ( RWCM ) for measuring the time distribution of the protons, atomic clocks CS-, dual -frequency GPS receiver, and the auxiliary detector is installed for measurement of the detector latencies. For the analysis of the events with specific neutrino 10us pulses were connected and a likelihood analysis to be created. Then the probability values ​​of different events were combined. The result:

And consequently

More precise measurements to be 2013/14 performed with the improved MINOS detector.

Indirect provisions of the neutrino velocity

Lorentz Hurtful models such as the standard model extension also allow the indirect determination of differences between the speed of light and the neutrino velocity by their energy and the decay rates of other particles are investigated. So should emit so-called vacuum - Cherenkov radiation superluminal neutrino. This can be achieved much more precise limits, for example by Borriello et al. (2013 ):

For more such tests, see Modern tests of Lorentz invariance # speed.