Fig. 1. The General view of that part CERN where the proton bunch is deduced from accelerator SPS and generates neutrino a bunch flying in a direction of laboratory Gran-Sasso. The image from discussed article
On Friday on September, 23rd in archive of electronic pre-prints there was article collaboration OPERA, devoted to direct measurement of speed of movement neutrino. Results sound sensationally: speed v has appeared slightly — but is statistically authentic! — there is more than velocity of light. Article collaboration contains the analysis of various sources of errors and uncertainties, however reaction of the overwhelming majority of physicists remains very sceptical first of all such result will not be co-ordinated with other experimental data on properties neutrino.
Experiment details
The idea of experiment (see OPERA experiment) is very simple. The nejtrinnyj bunch is born in CERNe, flies through the Earth to the Italian laboratory Gran-Sasso and passes there through special v detector OPERA. Neutrino very poorly co-operate with substance, but that their stream from CERNa is very great, some neutrino nevertheless face atoms in the detector. There they generate the cascade of the charged particles and by that leave in the detector the signal. Neutrino in CERNe are born not continuously, and "splashes" and if we know the birth moment neutrino and the moment of its absorption in the detector, and also distance between two laboratories, we can calculate speed of movement neutrino.
The distance between a source and the detector on a straight line makes about 730 km and it is measured with accuracy of 20 sm (the exact distance between reference points makes 730 534,61 ± 0,20 metres). However, the process leading to a birth neutrino, is not localised not so with such accuracy. In CERNe the bunch of protons of high energy takes off from accelerator SPS, is dumped on a graphite target and generates in it secondary particles, including mesons. They still fly forward with near light in the speed and hurriedly break up on muons with emission neutrino. Muons too break up and generate additional neutrino. Then all particles, except neutrino, are absorbed in thickness of substance, and those free reach a detecting place. The general scheme of this part of experiment is resulted on fig. 1.
All cascade leading to occurrence neutrino of a bunch, can be stretched on hundreds metres. However as all particles in this clot fly forward with near light in the speed, for time of detecting there is no difference, was born neutrino at once or through way kilometre (however is of great importance, when that initial proton which has led to a birth given neutrino, has taken off from the accelerator). As a result born neutrino by and large simply repeat a profile of an initial proton bunch. Therefore key parametre here is the time profile of a bunch of the protons which are taking off from the accelerator, in particular — its exact position of forward and back fronts, and this profile is measured with the good time permission (fig. 2 see).
Fig. 1. The General view of that part CERN where the proton bunch is deduced from accelerator SPS and generates neutrino a bunch flying in a direction of laboratory Gran-Sasso. The image from discussed article
Each session of dump of a proton bunch on a target (in English such session is called spill, «splash») lasts approximately 10 micro seconds and leads to a birth of huge number neutrino. However practically all of them fly by the Earth (and the detector) through without interaction. In the same rare occurences when the detector all the same registers neutrino, it is impossible to tell, during which moment during a 10-microsecond interval it has been let out. The analysis can be spent only statistically, that is to save up many cases of detecting neutrino and to construct their distribution from time to time concerning the moment of a reference mark for each session. In the detector that moment of time when the prearranged signal, moving with a velocity of light and radiated exactly at the moment of forward front of a proton bunch, reaches the detector is accepted to a reference mark. Exact measurement of this moment became possible thanks to synchronisation of hours in two laboratories with accuracy in some ns.
Fig. 3. Distribution of the moments of registration neutrino concerning a conditional reference mark. On a horizontal axis time in ns, on vertical — quantity neutrino events with such delay is shown. The red line shows a hypothetical "basic" signal. The image from discussed article
On fig. 3 the example of such distribution is shown. Black points are real neutrino the data registered by the detector and prosummarized on a great number of sessions. The red curve shows a conditional "basic" signal which would move with a velocity of light. It is visible that the data begins approximately on 1048,5 nanoseconds before a basic signal. It, however, yet does not mean that neutrino is valid for micro second advances light, and is only an occasion carefully to try on all the length long cables, speed of operation of equipment, times of a delay of electronics and so on . This recheck has been executed, and it has appeared that it displaces "the basic" moment on 988 nanoseconds. Thus, it turns out that neutrino the signal really overtakes basic, but only approximately on 60 ns. In recalculation for the speed neutrino it answers excess of a velocity of light approximately 0,0025 %.
The error of this measurement has been estimated by authors of the analysis in 10 ns that includes both statistical, and regular errors. Thus, authors assert that they "see" superlight movement neutrino at level of statistical reliability in six standard deviations.
Difference of results from expectations on six standard deviations is already enough great and is called in the physicist of elementary particles as a loud word "opening". However it is necessary to understand this number correctly: it only means that the probability of statistical fluctuation in the data is very small, but does not speak about that, the technique of data processing is how much reliable and how much well physics have considered all tool errors. Eventually , in the physicist of elementary particles there are many examples when unusual signals with exclusively big statistical reliability did not prove to be true other experiments.
To that contradict superlight neutrino?
Contrary to the widespread opinion, the special theory of a relativity does not forbid in itself existence of particles, moving with superlight speed. However for such particles (them generalized name «tachyons») the velocity of light too is a limit, but only from below — they cannot move more slowly it. Thus dependence of energy of particles on speed turns out return: the more energy, the more close speed tachyons to a velocity of light.
Much more serious problems begin in the quantum theory of a field. This theory comes in the stead of the quantum mechanics when it is a question about quantum particles with big energies. In this theory of a particle is not points, and, conditionally speaking, clots of a material field and to consider them separately from a field it is impossible. It appears that tachyons lower energy of a field so , do vacuum astable. To emptiness then more favourably spontaneously to be scattered on huge number of these particles and consequently to consider movement of one tachyon in usual empty space simply senselessly. It is possible to tell that tachyon is not a particle, and instability of vacuum.
In a case tahionov-fermionov the situation is slightly more difficult, but also there too there are the comparable difficulties, disturbing to creation self-co-ordinated tachyonoi the quantum theory of a field including the usual theory of a relativity.
However, it too not last word in the theory. The same as experimenters measure everything that gives in to measurement, theorists too check all possible hypothetical models which do not contradict the available data. In particular , there are theories in which it is supposed small, not noticed while a deviation from postulates of the theory of a relativity — for example, the velocity of light in itself can be a variable. Direct experimental support at such theories while is not present, but they while are not closed.
Under this short sketch of theoretical possibilities it is possible to bring such result: in spite of the fact that in some theoretical models movement with superlight speed probably, they remain exclusively hypothetical designs. All experimental data available for today are described by standard theories without superlight movement. Therefore if it has authentically proved to be true though for any particles, the quantum theory of a field should be altered cardinally.
Whether it is necessary to consider result OPERA in this sense as "first signs"? While is not present. Perhaps, the most important occasion to scepticism there is that fact that result OPERA will not be co-ordinated with other experimental data on neutrino.
First, during the well-known flash supernew SN1987A have been registered and neutrino which have come some hours prior to a light impulse. It does not mean that neutrino went faster light, and only reflects that fact that neutrino are radiated at earlier stage of a collapse of a kernel at flash supernew, than light. However time neutrino and light, having spent to ways of 170 thousand years, have not dispersed more, than at some o'clock, means, speeds at them are very close and differ no more than on milliard shares. Experiment OPERA shows stronger divergence in thousand times.
Here, of course, it is possible to tell that neutrino, born at flashes supernew, and neutrino from CERNa strongly differ on energy (some tens MeV in supernew and 10–40 GeV in described experiment), and speed neutrino changes depending on energy. But this change in this case works in "the wrong" party: after all the above energy tachyon the is closer their speed should to a velocity of light. Certainly, and here it is possible to think up any updating tachyon theories in which this dependence would be absolutely another, but in that case it is necessary to discuss "twice-hypothetical" model already.
Further, from set of experimental data on neutrino oscillations, received during the last years, follows that weights of all neutrino differ from each other only on shares an electron. If result OPERA to perceive as display of superlight movement neutrino then the size of a square of weight at least one neutrino will be an order – (100 MeV) 2 (the negative square of weight is and there is a mathematical display of that the particle is considered tachyon). Then it is necessary to recognise that all grades neutrino — тахионы and possess about such weight. On the other hand , direct measurement of weight neutrino in beta disintegration of kernels tritium shows that the weight neutrino (on the module) should not exceed 2an electron. Differently, all these given to co-ordinate with each other will not be possible.
The conclusion from here can be made such: the declared result collaboration OPERA is difficult for containing in any, even in the most exotic theoretical models.
What further?
In all big collaboration in the physicist of elementary particles normal practice is the situation when each concrete analysis is carried out by small group of participants, and only then results are taken out for the general discussion. In this case , apparently, this stage was too short therefore not all participants collaboration have agreed to substitute the signature under article (the complete list totals 216 participants of experiment, and the pre-print has only 174 authors). Therefore in the near future , most likely, inside collaboration the set of additional checks, and only will be spent after that article will be sent to the press.
Certainly, now it is possible to expect and a stream of theoretical articles with various exotic explanations of this result. However while the declared result will not be reliably rechecked, to consider as its full opening it is impossible.
Sources:
- OPERA Collaboration. Measurement of the neutrino velocity with the OPERA detector in the CNGS beam //an e-print arXiv:1109.4897 [hep-ex].
- Dario Autiero. New results from OPERA on neutrino properties, the report at a special seminar in CERN e on September, 23rd, 2011 .
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