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EuCARD >> News >> Newsletters >> Issue 11 >> Article 1

A word about faster-than-light neutrinos

In September 2011 the headlines read "faster-than-light neutrinos" and "could Einstein be wrong". But what were the research results behind these claims, what did the OPERA collaboration actually find and what have been the reactions from the neutrino community. Vittorio Palladino, leader of EuCARD's NEu2012 neutrino network, looks at the findings behind the headlines. The neutrino's OPERA With construction dating back to 2000, the "CERN Neutrinos to Gran Sasso" (CNGS) collaboration have been sending neutrinos from CERN to Italy since 2006. Protons from CERN's Super Proton Synchrotron (SPS) hit a graphite block creating a cocktail of particles, including neutrinos that travel 732 km underground to reach the OPERA detector.

Schematic of the time of flight measurement. Image courtesy of OPERA (arXiv:1109.4897v2).

The unprecedented long distance coupled with today's frontier technologies allow the researchers to make improved measurements of accelerator neutrino velocity: the ratio of precision measurements of the CERN to Gran Sasso distance and of the time of flight of the neutrinos (TOFu).

In view of this, dedicated upgrades of the timing system for the time tagging and synchronization of the CNGS beam and the OPERA detector took place, leading to statistical and systematic uncertainties on TOFu being reduced to an estimated 8 nanoseconds (ns). All necessary ancillary time delay measurements were done with comparable or better accuracy. In addition, a high accuracy geodesy campaign allowed distance measurements to a precision of 20 cm. All measurements were certified by Swiss and Germany Metrology Institutes.

Using this state-of-the-art hardware, researchers noted that neutrinos appeared to arrive, on average, 58 ns earlier than light would. They measured a ratio greater than the speed of light c, by about 24 parts per million. With a seemingly earlier arrival time so much greater than the 8 ns uncertainties, the results were striking. 

The impact of such findings, if confirmed, would be so important that a mandatory word of caution and prudence comes directly from the authors, well aware that the observed anomaly may still just be an unknown systematic error, in spite of the robustness and significance of their analysis.

In addition, the astrophysical measurement obtained with the SN1987 supernova excluded a deviation from c greater than 2 parts per billion for that burst of 10 MeV or so neutrinos. The two measurements can both be right only if the neutrino speed were higher only for the more energetic, 30 GeV or so, CNGS neutrinos.

There are, in fact, important experimental issues. The time of flight that can be accurately measured is not the time of detection of the same neutrino at CERN and at Gran Sasso. It is the interval between the time of detection of protons at CERN (just ejected from the SPS, traversing their so called BCT detector, on their way to neutrino production further downstream) and the time of detection of a neutrino interaction in one (the TT target tracker) of OPERA’s detectors. This bias is negligible, however.

More concern, until very recently, came from the measured time of flight not being either the time between detection of a neutrino at Gran Sasso and of its parent proton at CERN. Exception made for a short two week special SPS “bunched beam” run in November 2011, where the time of the parent proton could be tagged at the few ns level at the price of much reduced statistics. For all the previous OPERA samples, it is the average time separating the two 10.5 ms wide distributions of the detection times of about 10²⁰ protons and of 15,223 of the neutrinos that they produced. This special run, a much better start-to-stop measurement, seems to confirm the result.

Let the investigation begin

OPERA submitted the results to the scrutiny of the scientific community. Valid suggestions for experimental checks have been and are being implemented or considered. The optimal scheduling in 2012 of normal and special running, where a rate of a few neutrino interactions per day seems possible, is meanwhile being defined. The CNGS will restart mid-March 2012.

An additional independent system of synchronization between CNGS and OPERA, based on dedicated 730-km long optical fibres, is being investigated. At CERN, the time distribution of the CNGS muons, downstream of neutrino production, will also be measured with new diamond detectors. OPERA is gearing up to time precisely neutrino interactions also with its RPC (resistive plate chambers) detectors.

MINOS at Fermilab, USA, is upgrading hardware for a similar competitive measurement. Other significantly different measurements are highly desirable and will certainly in the end be the key to progress. Were a systematic error found, one presently unknown obstacle to precise measurements will have preciously been removed.

The aim of OPERA

OPERA was not built to measure time-of-flight, but instead to be at the forefront of the study of neutrino oscillations. Its long baseline configuration was imposed by the long wavelength of the oscillation phenomena. Its main goal is detection of a few of the tau neutrinos (u_t) expected from transmutation in flight of CNGS muon neutrinos (u_m).

Its heart is the super-high-precision tracking detector called nuclear emulsions that can detect very short tracks, but is of no use to measure neutrino velocity. One landmark picture of u_t appearance event was announced in 2010, other candidates are under study, there should be 7 or 8 such events by the end of the experiment, if our present understanding of neutrino oscillation is correct.

OPERA and EuCARD The OPERA team is one of the main components of the European accelerator neutrino community and the flagship of the CNGS programme. Many members have relevant roles in the FP7 LAGUNA-LBNO and EUROnu Design Study and in the effort of the EuCARD NEu2012 network to formulate a coherent staged programme of neutrino oscillation experiments for Europe, an European neutrino road map to be submitted to the CERN Council strategy update process. Technically, OPERA owns the nuclear emulsion technology for ut detection that may well prove again instrumental in the study of neutrino transitions. It is not difficult to predict, however, that a vigorous accelerator European programme will also be a tool to study a wider range of properties of the still so mysterious neutrino. Complementary, possibly even more fundamental, research avenues will certainly be explored too. The CNGS OPERA joint enterprise has proved that once more.

Comparison of the measured neutrino interaction time distributions (data points) and the proton PDF (red line) for the two SPS extractions before (top) and after (bottom) correcting for δt (blind) resulting from the maximum likelihood analysis. Image courtesy of OPERA (arXiv:1109.4897v2).

- Vittorio Palladino, INFN, EuCARD-NEu2012 (WP3), Pasquale Migliozzi, vice-spokesman of OPERA and Kate Kahle, CERN, EuCARD-DCO (WP2).

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