News about ghost particles

This model of the GERDA experiment shows the onion-like structure which suppresses interfering signals from the environment. The germanium diodes in the center of the cryostat filled with liquid argon (–186°C) are to a larger scale. Credit: MPI for Nuclear Physics

This model of the GERDA experiment shows the onion-like structure which suppresses interfering signals from the environment. The germanium diodes in the center of the cryostat filled with liquid argon (–186°C) are to a larger scale. Credit: MPI for Nuclear Physics

(Phys.org) —Neutrinos are the most elusive particles having extremely weak interactions with all other particles. They have rather unusual properties and are even expected to be identical with their own antiparticles. So far this property is, however, not experimentally verified even though many studies of neutrinos over the last 60 years have already boosted our understanding of elementary particle physics.

Now scientists of the GERDA collaboration obtained new strong limits for the so-called neutrino-less double beta decay, which tests if neutrinos are their own antiparticles. The result rules out an earlier claim and has various important implications for cosmology, astrophysics and particle physics and it adds information about neutrino masses.
Besides photons, neutrinos are the most abundant particles in the Universe. They are often called `ghost particles’, because they interact extremely weakly with matter. They are therefore an invisible, but very important component of the Universe, which could carry altogether as much mass as all other known forms of matter, albeit traveling almost at the speed of light over fantastic distances. Their tiny masses have also important consequences for the structures in the Universe and they are the driving element in the explosion of Supernovae. But their most remarkable and important property has been proposed by Ettore Majorana in the 1930ies: Unlike all other particles that form the known matter around us, neutrinos may be their own antiparticles.

The GERDA (GERmanium Detector Array) experiment, which is operated at the Laboratori Nazionali del Gran Sasso underground laboratory of the Istituto Nazionale di Fisica Nucleare in Italy, is aiming to resolve the question whether neutrinos are in fact their own antiparticles, and to determine their mass. GERDA looks for so-called double beta decay processes in the germanium isotope Ge-76 with and without the emission of neutrinos, the latter being a consequence of the Majorana properties. In normal beta decay, a neutron inside a nucleus decays to a proton, an electron and an antineutrino. For nuclei like Ge-76, normal beta decay is energetically forbidden, but the simultaneous conversion of two neutrons with the emission of two antineutrinos is possible and has been measured by GERDA recently with unprecedented precision. This is one of the rarest decays ever observed with a half-life of about 2*1021 years, which is about 100 billion times longer than the age of the Universe. If neutrinos are Majorana particles, neutrino-less double beta decay should also occur at an even lower rate. In this case, the antineutrino from one beta decay is absorbed as neutrino by the second beta-decaying neutron, which becomes possible if neutrinos are their own antiparticle.

Read more at: http://phys.org/news/2013-07-news-ghost-particles.html#jCp

High Energy Neutrinos from Space

Artist’s drawing of the IceCube detector

Thomas K. Gaisser
This paper reviews the status of the search for high-energy neutrinos from astrophysical sources. Results from large neutrino telescopes in water (Antares, Baikal) and ice (IceCube) are discussed as well as observations from the surface with Auger and from high altitude with ANITA. Comments on IceTop, the surface component of IceCube are also included…..
Read more: http://arxiv.org/pdf/pdf

Solar Neutrinos in 2011

The Borexino detector. It was designed to detect sub-MeV solar neutrinos. It features a high light-yield, ultra-pure liquid scintillator target. A non-scintillating buffer region serves as shielding for external γ -rays. Its location at a deep underground site and its muon veto suppress cosmic backgrounds


Alvaro Chavarria
I give an overview of the recent developments in the solar neutrino field.
I focus on the Borexino detector, which has uncovered the solar neutrino spectrum below 5 MeV, providing new tests and confirmation for solar neutrino oscillations. I report on the updated measurements of the 8B solar neutrino flux by water Cherenkov and organic scintillator detectors.
I review the precision measurement of the 7Be solar neutrino flux by Borexino and the search for its day-night asymmetry.
I present Borexino’s latest result on the study of pep and CNO neutrinos. Finally, I discuss the outstanding questions in the field and future solar neutrino experiments.
Read more: http://arxiv.org/pdf

Neutrino watch

… Speed claim baffles CERN theoryfest

Typical event recorded in ICARUS. Evidence for a pair of γ’s from a π_o (tracks 16a and 16b) with a momentum of 912 MeV/c pointing at the primary vertex, showing the typical behavior of γ conversions in the TPCN LAr Imaging chamber.

Even a meeting of elite minds at Europe’s top particle physics lab couldn’t do it: reconciling neutrinos that appear to break the cosmic speed limit with the laws of physics is still beyond us. However, a paper on the speeding neutrinos has been accepted for publication and the first preliminary results from a comparable experiment are out.

“For the moment, there is no explanation that works,” says physicist Ignatios Antoniadis, who helped to organise the meeting at CERN near Geneva, Switzerland, last Friday. It was three weeks to the day after physicists in the OPERA collaboration at Gran Sasso, Italy, announced that neutrinos travelling from CERN had apparently moved faster than light.

Frantic calculation, speculation and debate have followed in the wake of the announcement. The meeting’s goal was to “review the situation and discuss whether it is possible [that neutrinos broke the speed of light]” , says Antoniadis.

The biggest challenge yet to the OPERA result comes from Nobel laureate Sheldon Glashow and his Boston University colleague Andrew Cohen in a paper posted online a few weeks ago.

First publication

Physical Review Letters has agreed to publish the paper, making it the first scientific journal to accept work on the OPERA result.

In the paper, Glashow and Cohen point out that if neutrinos can travel faster than light, then when they do so they should sometimes radiate an electron paired with its antimatter equivalent – a positron – through a process called Cerenkov radiation, which is analogous to a sonic boom. Each electron-positron pair should carry away a large chunk of the neutrinos’ energy: Cohen and Glashow calculated that at the end of the experiment, the neutrinos should have had energies no higher than about 12 gigaelectronvolts. But OPERA saw plenty of neutrinos with energies upwards of 40 GeV.

“It doesn’t correspond to the energies measured at all,” says CERN physicist Christophe Grojean.

Another strike against the speedy neutrinos comes from the fact that neutrinos are linked to certain other particles – electrons, muons and tau particles – via the weak nuclear force. Because of that link, neutrinos can’t travel faster than light unless electrons do too – although electrons needn’t travel as fast as the neutrinos.

Speedy electrons

CERN physicist Gian Giudice, who spoke at the seminar, and colleagues looked into what would happen if electrons travelled faster than light by one part in 100,000,000, a speed consistent with the OPERA neutrino measurement. Such speedy electrons should emit a cone of Cerenkov radiation in empty space – but previous experiments show that they don’t.

The only way out, theorists at the meeting decided, was to break another supposedly fundamental law of nature – the conservation of energy. But that suggestion seems even more ludicrous than breaking the speed of light.

“At the moment, there is no concrete model that really avoids all these theoretical constraints,” Grojean says. “That’s why it’s so interesting. We cannot explain it in terms of known physics.”

Despite the care the OPERA researchers took to rule out errors in the measurement, that possibility remains. Another unpublished paper on the arxiv.org physics preprint server has attracted attention with its explanation. Ronald van Elburg at the University of Groningen in the Netherlands has calculated that special relativity could have messed up the synchronisation of the clocks at CERN and Gran Sasso. This would make neutrinos appear to arrive 64 nanoseconds early – almost exactly what the OPERA experiment observed.

Icarus test

If this argument holds up, rather than breaking Einstein’s theory of special relativity, the faster-than-light neutrinos would actually end up reaffirming it. But it’s unclear whether the result has legs. “In general, the feeling of theorists is that one should repeat the experiment,” Antoniadis says.

CERN plans to provide a new neutrino beam to do this. Meanwhile, the first glimpses from another detector at the Gran Sasso laboratory don’t look good for the faster-than-light hypothesis. An experiment there called ICARUS (Imaging Cosmic And Rare Underground Signals) has been catching neutrinos travelling from CERN since last year. The 100 or so it has seen do not seem to travel faster than light. ICARUS also doesn’t see any evidence of the Cerenkov-like radiation Glashow and Cohen predicted.

The case is far from closed, however. “For the moment, we don’t have an answer,” Antoniadis says. “That doesn’t mean an answer doesn’t exist.”

References: Glashow and Cohen: arxiv.org/abs/1109.6562; van Elburg: arxiv.org/abs/1110.2685; ICARUS:arxiv.org/abs/1110.3763
http://www.newscientist.com

ICARUS Refutes Opera’s Superluminal Neutrinos

a picture of ICARUS in the LNGS cavern

The saga of the superluminal neutrinos took a dramatic turn today, with the publication of a very simple yet definitive study by ICARUS, another neutrino experiment at the Gran Sasso Laboratories, who has looked at the neutrinos shot from CERN since 2010.

The ICARUS team jumped on the chance to test the Opera result based on the article recently published by Cohen and Glashow. The latter argue that superluminal neutrinos should lose energy through  neutral-current weak-interaction radiation -the analogue of Cherenkov radiation for a neutral particle. Given a neutrino moving at a speed v>c as the one measured by Opera, and given the distance traveled to the Gran Sasso cavern, one can relatively easily compute the energy spectrum of observable neutrinos at the cavern, given the production energy spectrum.

The physics is a bit more complicated than I summarized it in the paragraph above, but really, you need not squeeze your brains: there is nothing much to know. What is important is that there is a clean and simple relationship between the superluminal speed and the rate of decrease of the neutrino energy. Neutrinos at CERN are produced with an average energy of 28.2 GeV, and neutrinos at the receiving end – the LNGS where Opera and ICARUS both sit – should have an average energy of only 12.1 GeV for neutrinos detected via charged-current interaction.

Incidentally, a charged-current neutrino interaction occurs when the neutrino “exchanges” a unit of electric charge, along with weak quantum numbers, with a nucleus. The neutrino thus turns into a muon, while the nucleus breaks apart in a shower of light hadrons. The muon is then very easy to detect and measure.

I can imagine the ICARUS team brainstorming all together at a meeting. Everybody brings about their favourite objections to the timing measurement of Opera. Some argue whether they can redo the Opera measurement. Others pass along the tray of donuts. Then somebody brings up the Cohen-Glashow paper: “Look, it is quite easy: we take neutrino interactions, measure their energy, and compare with various hypotheses for the superluminal speed. All based on known physics and hard facts. Can we do it ? Can we ? OMG wait… We have already those neutrino interactions!”

So off they go, and do their homework. And a very good homework it is: in less than three weeks from the appearance of the Cohen-Glashow paper -yesterday evening-, they publish a preprint. Kudos to them for their speed and focus. True, ICARUS is not flooded with neutrino statistics these days -I could not help chuckling at their honest but a bit vintage description of why they lost this or that event, ending up with a statistics of less than 100 interactions (OPERA has 16000, although they’ve run for much longer so far). But those less-than-100 neutrinos do kick ass.

In fact, what do they find ?

http://www.science20.com

Faster-Than-Light Neutrino Puzzle Claimed Solved by Special Relativity

The relativistic motion of clocks on board GPS satellites exactly accounts for the superluminal effect, says physicist
It’s now been three weeks since the extraordinary news that neutrinos travelling between France and Italy had been clocked moving faster than light. The experiment, known as OPERA, found that the particles produced at CERN near Geneva arrived at the Gran Sasso Laboratory in Italy some 60 nanoseconds earlier than the speed of light allows.

The result has sent a ripple of excitement through the physics community. Since then, more than 80 papers have appeared on the arXiv attempting to debunk or explain the effect. It’s fair to say, however, that the general feeling is that the OPERA team must have overlooked something.

Today, Ronald van Elburg at the University of Groningen in the Netherlands makes a convincing argument that he has found the error.

First, let’s review the experiment, which is simple in concept: a measurement of distance and time.

Results of the OPERA experiment

The distance is straightforward. The location of neutrino production at CERN is fairly easy to measure using GPS. The position of the Gran Sasso Laboratory is harder to pin down because it sits under a kilometre-high mountain. Nevertheless, the OPERA team says it has nailed the distance of 730 km to within 20 cm or so.

The time of neutrino flight is harder to measure. The OPERA team says it can accurately gauge the instant when the neutrinos are created and the instant they are detected using clocks at each end.

But the tricky part is keeping the clocks at either end exactly synchronised. The team does this using GPS satellites, which each broadcast a highly accurate time signal from orbit some 20,000km overhead. That introduces a number of extra complications which the team has to take into account, such as the time of travel of the GPS signals to the ground.

But van Elburg says there is one effect that the OPERA team seems to have overlooked: the relativistic motion of the GPS clocks.

It’s easy to think that the motion of the satellites is irrelevant. After all, the radio waves carrying the time signal must travel at the speed of light, regardless of the satellites’ speed.

But there is an additional subtlety. Although the speed of light is does not depend on the the frame of reference, the time of flight does. In this case, there are two frames of reference: the experiment on the ground and the clocks in orbit. If these are moving relative to each other, then this needs to be factored in.

So what is the satellites’ motion with respect to the OPERA experiment? These probes orbit from West to East in a plane inclined at 55 degrees to the equator. Significantly, that’s roughly in line with the neutrino flight path. Their relative motion is then easy to calculate.

So from the point of view of a clock on board a GPS satellite, the positions of the neutrino source and detector are changing. “From the perspective of the clock, the detector is moving towards the source and consequently the distance travelled by the particles as observed from the clock is shorter,” says van Elburg.

By this he means shorter than the distance measured in the reference frame on the ground.

The OPERA team overlooks this because it thinks of the clocks as on the ground not in orbit.

How big is this effect? Van Elburg calculates that it should cause the neutrinos to arrive 32 nanoseconds early. But this must be doubled because the same error occurs at each end of the experiment. So the total correction is 64 nanoseconds, almost exactly what the OPERA team observes.

That’s impressive but it’s not to say the problem is done and dusted. Peer review is an essential part of the scientific process and this argument must hold its own under scrutiny from the community at large and the OPERA team in particular.

If it stands up, this episode will be laden with irony. Far from breaking Einstein’s theory of relatively, the faster-than-light measurement will turn out to be another confirmation of it.

Ref: arxiv.org/abs/1110.2685: Times Of Flight Between A Source And A Detector Observed From A GPS Satellite

http://www.technologyreview.com/blog/arxiv/27260/