It appears that the faster-than-light neutrino results, announced last September by the OPERA collaboration in Italy, was due to a mistake after all. A bad connection between a GPS unit and a computer may be to blame.
Physicists had detected neutrinos travelling from the CERN laboratory in Geneva to the Gran Sasso laboratory near L’Aquila that appeared to make the trip in about 60 nanoseconds less than light speed. Many other physicists suspected that the result was due to some kind of error, given that it seems at odds with Einstein’s special theory of relativity, which says nothing can travel faster than the speed of light. That theory has been vindicated by many experiments over the decades.
According to sources familiar with the experiment, the 60 nanoseconds discrepancy appears to come from a bad connection between a fiber optic cable that connects to the GPS receiver used to correct the timing of the neutrinos’ flight and an electronic card in a computer. After tightening the connection and then measuring the time it takes data to travel the length of the fiber, researchers found that the data arrive 60 nanoseconds earlier than assumed. Since this time is subtracted from the overall time of flight, it appears to explain the early arrival of the neutrinos. New data, however, will be needed to confirm this hypothesis.
The claim by a team of researchers in Italy that neutrinos can travel faster than the speed of light will require extra checks before being submitted to a peer-reviewed journal. That is the position of a number of researchers in the OPERA collaboration, which announced on 23 September that it had observed superluminal neutrinos travelling from the CERN particle-physics lab near Geneva to the Gran Sasso underground lab in central Italy.
The announcement made headlines around the world, since it appears to contradict Einstein's special theory of relativity. However, not everyone within OPERA was happy to release the results publicly, with several of the 30 group leaders within the 160-strong collaboration being opposed to the release of a paper on the arXiv preprint server and the accompanying seminars and press release without further tests of possible systematic errors being carried out. Now, a larger fraction of the group leaders is concerned about the paper being submitted to a research journal. One member of OPERA, who does not wish to be named, says there is a "lot of tension" within the collaboration and that up to half of the members are opposed to an immediate submission.
Precision measurements
Neutrinos are produced by accelerating protons at CERN's Super Proton Synchrotron accelerator and colliding proton bunches 10 µs in length into a graphite target, generating mesons that in turn decay into neutrinos. The 1300-tonne OPERA detector, which has been running since June 2008, measures the properties of muon neutrinos as they travel 730 km through the Earth's crust from CERN to Gran Sasso.
The experiment was originally designed to study the oscillation of muon neutrinos into tau neutrinos, but following tentative results in 2007 from the MINOS experiment in the US that showed neutrinos appearing to travel faster than light, researchers realized accurate velocity measurements could also be carried out with OPERA. Researchers installed atomic clocks at both ends of the neutrino beam to establish exactly when the neutrinos are created and detected, and used GPS-based measurements to precisely measure the length of the baseline – the velocity being derived by dividing the baseline by the time of flight.
Collecting more than 16,000 events between 2009 and 2011, the OPERA collaboration calculated that muon neutrinos arrive on average 60.7 ns earlier than they would have done had they travelled at the speed of light, which corresponds to a fractional increase over light speed of 25 parts in a million. Having accounted for a host of possible systematic errors, including uncertainties relating to the precise moment of creation and detection of the neutrinos plus errors introduced by cabling and clock synchronization, the researchers arrived at a total systematic error of 7.4 ns, comparable with the statistical error of 6.9 ns.
The OPERA collaboration calculated a confidence level of "6σ", or a one in a billion chance the result was a statistical fluke, and this persuaded most of the collaboration that the result was solid enough to publish. However, some members were worried that unknown sources of systematic error might potentially destroy the confidence level. They argued that before making an announcement, further checks should be carried out – a process that could take several months.
One such check regards the timing of the neutrinos' arrival at Gran Sasso, and involves carrying out an analysis of timing data collected by monitoring the charge, rather than the light, generated by particles passing through the detector. This analysis relies on a very precise and painstaking measurement of the length of the cabling used to collect the timing data, in order to isolate any systematic errors that may be present within the electronics or other parts of the timing system.
Another independent check involves the statistical analysis of the data collected by OPERA. The researchers are not able to track, and therefore time, individual neutrinos as they travel from Geneva to Gran Sasso, but instead they measure the temporal distribution of the protons within each bunch just before the protons hit the graphite target and then compare this with the distribution of the corresponding neutrinos as they are detected in OPERA – with the temporal offset between the two revealing the time of flight. Some members of the collaboration argue that this offsetting procedure needs to be carried out independently, in order to be sure that the temporal profile of the neutrinos leaving CERN can be inferred accurately from that of the protons that produced them.
Heated debate
Discussions about whether or not the collaboration was ready to publish took place in early September. As these discussions were quite animated, the decision was put to a vote, with collaboration spokesperson Antonio Ereditato from the University of Bern proposing that initially the research be published on arXiv while at the same time being presented in a series of scientific seminars, before later being submitted to a peer-reviewed journal. This strategy received a majority, but not a unanimous, vote. It was then left to individual researchers to sign the arXiv paper, with about 10 senior members out of a total of 170 people (including some non-official members) deciding not to do so.
There are so many things that people outside can't check. It is these things that we have to do before publishing
Caren Hagner, Hamburg University
Ereditato says that the collaboration will continue to carry out checks but will do so in parallel with the journal submission. He maintains that no-one outside the collaboration, either at the seminars or via e-mail, has yet presented "smoking guns against what we have seen" and adds that "as experimentalists we have done everything we can". However, Caren Hagner, leader of the OPERA group at Hamburg University and one of the people whose name does not appear on the arXiv paper, believes that the collaboration should carry out the extra checks before submitting the paper for peer review. "Many of the collaboration are convinced that if a mistake is subsequently found it, won't be down to OPERA," she says. "But I am not really convinced. There are so many things that people outside can't check. It is these things that we have to do before publishing."
Laura Patrizii, who is leader of OPERA's Bologna group and who did sign the preprint, clarifies the motivation of the dissenters. "It is not that people think there is a mistake that is being hidden," she says. "But since something going faster than light would kill modern physics as we know it, some researchers would feel more at ease with these independent checks."
Looking to the outside
In addition to the checks that can be carried out within the collaboration, there are also some additional checks that CERN could perform, such as using detectors downstream of the graphite target to provide a better estimate of the profile of the departing neutrinos. The MINOS experiment is also currently improving its cabling and electronics, and collaboration co-spokesperson Jenny Thomas from University College London says that new data collected with the upgraded detector combined with a better analysis of existing data could allow MINOS to largely rule out the OPERA result within the next four to six months (but not to rule it in, given that this would require a higher level of accuracy).
Giovanni Amelino-Camelia, a theoretical physicist at the University of Rome "La Sapienza", believes that a confirmed OPERA result would lead to a "revolution" within physics. But he thinks that this confirmation is unlikely, pointing out that in the history of physics there have been many experimental "alarms" suggestive of a revolution but that only a small fraction of these, such as the Michelson–Morley experiment, have been confirmed.
With OPERA in the spotlight, collaboration members also disagree about their future research programme. Luca Stanco, leader of the OPERA group from the University of Padova and one of the people who did not sign the preprint, believes that the priority now should be further investigation of the superluminal-neutrino result, rather than neutrino oscillations. Ereditato, however, says that even though the collaboration will pursue superluminal research, "the main focus will continue to be oscillations".
About the author
Edwin Cartlidge is a science writer based in Rome
中微子地下直线穿行距离732公里,根据中微子在CERN的发射时间和在Gran Sasso 接受
时间,就可求出中微子的穿行速度。该实验团队宣称,中微子的穿行速度比标准的光速
(299792458 米/秒)大 0.0025%。昨晚,他们的论文已在网上贴出(见arXiv:
1109.4897v1),题目是“Measurement of the neutrino velocity with the OPERA
detector”,作者共174人,来自大小诸国。在大国中,有法,意,日,德,俄,没有美
,英,中。
直到写此文的时候,同行对CERN – Gran Sasso 结果的态度,似乎持保留或观望的居多
(当然,我是在美国的统计)。
Gran Sasso 实验室由意大利修公路的公司承建。意大利的高速公路,山洞极多。给实验
室挖几个山洞,不在话下。听说,贵州的高速公路也是意大利公司承建,因为山洞极多
。
2006 年,当CERN – Gran Sasso 中微子实验在建设时,我又去过 Gran Sasso (图
3)。从照片可以看到山洞实验大厅的规模,CERN-Gran Sasso 中微子实验的终端探测器
OPERA (The Oscillation Project with Emulsion-tRacking Apparatus),即建于此
大厅里。OPERA这个名字是着意取的,不懂一点威尔弟,普契尼,大概很难混进OPERA 团
队。
Gran Sasso 是风景旅游胜地(图4)。但在 Gran Sasso实验室里工作,却极为单调
烦闷,因为环境孤立。只有在孤立安静的环境中才有可能探测到中微子的微弱信号。在
Gran Sasso实验室,常常雇人来做单调烦闷的工作,但工资不菲。九十年代时,
Gran Sasso实验室一直雇有这类中国雇员。他们大多是中国科学院的年轻研究人员。现
在几乎没有了,或雇不到了,因为中国科学院的工资也不菲了。