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September 23, 2011

Possibilities around the faster than light CERN neutrino experiment

Scientists at the European Organization for Nuclear Research (CERN) say they have measured tiny subatomic particles traveling faster than light (about 1.000025 times faster or 299,799,953 m/s. instead of 299,792,458 m/s).

Quantum Diaries has a live blog for the CERN EP Seminar “New results from OPERA on neutrino properties“, presented by Dario Autiero.

Arxiv - Measurement of the neutrino velocity with the OPERA detector in the CNGS beam (24 pages)

The OPERA neutrino experiment at the underground Gran Sasso Laboratory has measured the velocity of neutrinos from the CERN CNGS beam over a baseline of about 730 km with much higher accuracy than previous studies conducted with accelerator neutrinos. The measurement is based on high-statistics data taken by OPERA in the years 2009, 2010 and 2011. Dedicated upgrades of the CNGS timing system and of the OPERA detector, as well as a high precision geodesy campaign for the measurement of the neutrino baseline, allowed reaching comparable systematic and statistical accuracies. An early arrival time of CNGS muon neutrinos with respect to the one computed assuming the speed of light in vacuum of (60.7 plus or minus 6.9 (stat.) plus or minus 7.4 (sys.)) ns was measured. This anomaly corresponds to a relative difference of the muon neutrino velocity with respect to the speed of light (v-c)/c = (2.48 plus or minus 0.28 (stat.) plus or minus 0.30 (sys.)) X 10^-5.

If other labs can reproduce the effect (and no systemic error is found), physicists envision one of two far-reaching outcomes.

1) the CERN team's results could bolster quantum theories of gravity – the last of nature's four fundamental forces scientists are trying to fit under the umbrella of quantum physics. Theories of quantum gravity suggest that at sufficiently high energies, particles can appear to travel faster than light because they traverse extra dimensions of space.

One example is string theory, which posits a universe of many more dimensions than the four humans experience.

"If you have a theory in which there is more than one way to get from A to B, maybe you can have a shortcut and have the appearance of traveling faster than the speed of light," says Stephen Parke, who heads the theoretical physics department at the Fermi National Accelerator Laboratory in Batavia, Ill.

2) A pillar of modern physics – Einstein's theory of special relativity, in which the speed of light is a particle's absolute speed limit – could take its first serious hit. Perhaps not flat wrong, but only a piece of a more complete picture.




Fermilab could provide a reality check on the OPERA results in about three years.

Confirmation – or a "maybe not" – could come sooner from Japan, which has a similar, though shorter, long-baseline neutrino experiment. Even though the facility was affected by this year's tsunami-generating earthquake, researchers suggest that the lab's archives may hold the data needed to compare with OPERA's.

If that's the case, a second opinion on particles that appear to outrace light might be only months away.

The total statistics used for the analysis reported in this paper is of 16111 events detected in OPERA, corresponding to about 10^20 protons on target collected during the 2009, 2010 and 2011 CNGS runs. This allowed estimating δt with a small statistical uncertainty, presently comparable to the total systematic uncertainty.

The point where the parent meson produces a neutrino in the decay tunnel is unknown.
However, this introduces a negligible inaccuracy in the neutrino time of flight measurement, because the produced mesons are also travelling with nearly the speed of light.

With a full GEANT simulation of external events it is shown that ignoring the position of the interaction point in the rock introduces a bias smaller than 2 ns with respect to those events occurring in the target (internal events), provided that external interactions are selected by requiring identified muons in OPERA

A key feature of the neutrino velocity measurement is the accuracy of the relative time tagging at CERN and at the OPERA detector. The standard GPS receivers formerly installed at CERN and LNGS would feature an insufficient ~100 ns accuracy for the TOFν measurement. Thus, in 2008, two identical systems, composed of a GPS receiver for time-transfer applications Septentrio PolaRx2e operating in “common-view” mode and a Cs atomic clock Symmetricom Cs4000, were installed at CERN and LNGS. They were calibrated by the Swiss Metrology Institute (METAS) and established a permanent time link between two reference points (tCERN and tLNGS) of the timing chains of CERN and OPERA at the nanosecond level. The difference between the time base of the CERN and OPERA PolaRx2e receivers was measured to be (2.3 ± 0.9) ns. This correction was taken into account in the application of the time link.

The relative positions of the elements of the CNGS beam line are known with millimetre accuracy. When these coordinates are transformed into the global geodesy reference frame ETRF2000 by relating them to external GPS benchmarks, they are known within 2 cm accuracy.

The high-accuracy time-transfer GPS receiver allows to continuously monitor tiny
movements of the Earth’s crust, such as continental drift that shows up as a smooth variation of less than 1 cm/year, and the detection of slightly larger effects due to earthquakes.

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