Some readers are asking who wrote the articles in this blog during the conference… That’s right, I never took the time to present myself. My name is Tommaso Dorigo and I have taken the responsibility of broadcasting the highlights from the Neutrino Telescopes conference since a few years ago.
I am not a neutrino physicist so I think I am in the right position to transmit the essential information, automatically filtering out the too technical details and the notions way above my head – which are likely also out of reach of the general outside reader. I am a collider physicist working for the CMS collaboration at CERN; I run a blog since 2005, A Quantum Diaries Survivor, where I try to do physics outreach, with highs and lows.
Through many years of blogging I have understood that this kind of work is surely unrewarding, certainly time-consuming, but also useful and important to bridge the gap with the general public. There is a large potential audience of laypersons who read about science and want to keep up to date with the developments in basic scientific research.
So that’s it – if you want to keep following high-energy physics news, my blog (linked above) is a good option. Or wait for two years and come back here for more interesting news about neutrinos!
The XVI edition of Neutrino Telescopes is over and it is the time for some summing up – which I feel completely unsuited to do, as I was just an observer. My field is high-energy collider physics, and neutrino physics has become a very different thing since the discovery of neutral currents 42 years ago. Anyway, here are a few random thoughts.
First of all, I am really envious of the degree of activity, the large number of interesting experiments, the many complementary sides that can be taken to attack the problem of understanding the riddles posed by the physics of neutrinos. Collider physics pales in comparison, with only few experiments really capable of producing breakthroughs. The reason of this difference is that in trying to break the Standard Model and finding new physics beyond it, collider physicists are condemned to look only in one direction, that of the smallest distance scales and highest collision energies. In contrast, the field of neutrino physics is still in its infancy: after the discovery of neutrino oscillations in 1998 there is still a lot to learn, and many parameters to measure. Those parameters are tightly interconnected by the theory, but surprisingly decoupled in the experimentally accessible ways to determine them.
Experimental techniques are also quite varied as the source of neutrinos is not one and the same: we can study reactor (anti)neutrinos, solar neutrinos, cosmic-ray-originated atmospheric neutrinos; or we can produce them with accelerators, and there too, one may distinguish short and long-baseline setups, which are sensitive to different propagation properties of the neutrino beams; and there one can study appearance or disappearance of neutrino flavors. But one can also study geo-neutrinos, learning about the core of our planet; or astrophysical sources of neutrinos, cosmogenic neutrinos. And one can detect supernova explosions with the neutrino fluxes. It is astounding how rich is the physics program and how varied the implications of measurements of the properties of just one particle -well, one kind of particle, but actually three, or more of them.
Another reason to be enthusiastic is the incredibly large scale of some of the experiments – IceCube, Pingu, Km3Net, and Juno are some notable examples. These are real telescopes looking at the cosmos, the sun, and our planet as well. It is remarkable how we can exploit the ice of the south polar cap as a detector, or the water of the marine depths. Equally awesome is to know how detectors weighing just a few kilograms can compete in determining crucial properties of neutrinos: these are the solid-state experiments looking for neutrinoless double-beta decays. Of course, you do not need large dimensions when your process of interest is neutrinoless!
It is also quite interesting to remain in touch with the development of neutrino studies because of the tight interconnections with cosmology: dark matter searches use partly the same experimental techniques, and are now getting sensitive to the region where we should really start to detect a signal from weakly-interacting massive particles. If we see no signal in the next generation of searches, we may have to revise our general understanding of the universe, as the “cosmic miracle” of a weak neutral particle produced in the big bang with just the right abundance and mass to explain dark matter away, ending up in the exact mass region of the electroweak scale, will cease to provide a motivation for that explanation. And of course, cosmogenic neutrinos may provide further evidence of our model of the cosmos, or create new questions.
Overall, it was a lot of fun to listen to the many talks – all of very high quality, thanks to the careful job of the organizing committee. The latter must also be acknowledged for flawlessly organizing a very pleasant week, with a splendid venue and excellent food, and with a successful poster session. I am sorry to say that my only contribution to the conference, this blog, did not shine as much, despite my efforts. The problem, as I see it, is that the blog was not kept in high regard by the local organizers, so it remained “on the side”. I believe a conference blog requires more effort from more parties, more commitment and advertisement, if it is to become a very useful addition to an already well-oiled and working conference. Do not get me wrong: I am overall satisfied by the number of visitors we had last week (over 500 per day) – by themselves, the numbers justify the effort; the conference proceedings will never get as many readers. However, there was no real effort of making the online discussions a virtual place where to keep in touch with the external world. This is probably due to the fact that online communication is still not kept in high regard by our community. This has to change, I think, or the gap between hard science and the rest of the world will grow bigger and basic research will stop looking like a good idea. I hope I’m wrong…
Yesterday evening a Poster session was held at Neutrino Telescopes XVI. A total of 26 posters were on display – many of which have been described in the previous days in this blog. A committee chaired by Deborah Harris has selected the three best posters, whose authors receive awards and a prize offered by the conference sponsor, Hamamatsu:
1) iPad Air 2 (wi-fi + Cellular 16GB)
2) Samsung Galaxy Note 3 Smartphone
3) Samsung l9515 Galaxy S4 Smartphone, 16 GB
And the winners are:
3. M. Roda
2. G. Pizzigoni
1. S. Bordoni
The 3-flavor oscillation resonance in the Earth for 3-10 GeV neutrinos is usable to determine the sign of delta_m^2 (see Fig.1, right – the survival probability is shown as a function of energy for atmospheric neutrinos). To do that, neutrino telescopes need to distinguish neutrinos from antineutrinos as they have unequal fluxes and cross sections in the atmosphere, which result in percent-level differences in count rates.
One may measure the zenith angle and the energy of upgoing atmospheric GeV-scale neutrinos precisely, and count muon and electron channel events separately, to give an improved precision on the parameter of atmospheric neutrino oscillation.
The resonance signature below 20 GeV requires a very dense detector. Orca will have a few percent of the volume of the km3net but a much denser array of detector elements.
The detector has been shown in simulations to be able to detect very clearly electron-neutrino charged current interactions, reconstructing the full cherenkov ring of electrons. The figure illustrates the pattern of detected photons for events with a vertex at a distance of 20 and 50 meters. The median zenith angle resolution is of 5 to 10 degrees.
For a calculation of the sensitivity to the mass hierarchy, they have used a likelihood ratio technique, fitting together the oscillation parameters and the flux normailzation. They included in the study a 3-flagour earth matter oscillation, track versus shower event classification, response matrices for detector efficiency obtained from Monte Carlo simulations, and event contaminations to their signals. The result is shown in the figure below, which is computed for delta_cp=0 (NH=normal hierarchy of neutrino masses; IH=inverted hierarchy).
It appears that already for three years of operation a more than three-standard-deviation sensitivity can be obtained.
The morning session at NeuTel today is the last one of the conference. The first few talks have concerned the biggest neutrino telescopes in operation or construction: IceCube, Km3Net, Antares, and Pingu. Earlier posts here have covered some of these facilities, so here I will get away with just posting a few random illustrations.
The first is a picture of an optical module of Km3Net: a sphere containing many photomultiplier tubes looking in all directions. Going to small PMTs is cost-effective and increases the granularity of the detector – plus I think these are quite cool objects.
The third picture below is a more serious graph, showing the sensitivity that Pingu could have to detect tau neutrino appearance. With just months of data an observation could be in the bag!
The tau neutrino signal could be detected thanks to its characteristic distortion of the cascade distribution, and extracted by a statistical analysis. To explain in detail what the graph shows, first note that the vertical axis is the neutrino energy, and the horizontal axis is the cosine of zenith angle. The different colours indicate regions of equal figure of merit, the background-subtracted signal divided by the square root of the total number of background events. I will mention that such is not a very good estimator for a statistical significance, especially in situations of small number of events; but the speaker does indicate in the slide that 5-sigma are at easy reach.
Gerda (Fig.1) is a Germanium detector array, an ultra-low-background experiment looking for 0v2b decay of 76Ge, which occurs at a Q-value of 2039 keV. To observe that monochromatic line the key is of course to reduce backgrounds as much as possible.
The Gerda collaboration is an European group mainly from Italy, Germany and Russia. The detector has an outer water tank shielding from neutron contamination and providing a cherenkov muon veto, which also benefits from plastic scintillators on top of the cryostat. Inside the tank the germanium detectors are immersed in liquid argon.
To maximize the sensitivity to the half-life, one sees that the measurement depends inversely on the square root of background level and energy resolution, and proportionally to the square root of active target mass and measuring time.
Seven events are observed in the signal region in their data taking (see Fig.2), consistent with background-only hypothesis. They extract a lower limit on the half-life of 0v2b decay of Germanium at 2.1×10^25 years at 90%CL.
In phase-II Gerda will be upgraded to increase the total mass to 20 kg, reducing backgrounds by a factor of 10.
The half-life for processes not included in the standard model giving rise to neutrinoless double beta decay can be written as the project of a phase space factor depending on atomic physics, a matrix element squared for nuclear physics effects, and a factor enshrining the physics beyond the standard model, belonging to the realm of particle physics. The nuclear matrix element in turn contains a factor g_A^2.
The g_A factor used in most results for calculations of the neutrinoless double beta decay is 1.269. But g_A is renormalized in models of nuclei, so one can define an effective value of this parameter. This turns out to be in the range 0.5-0.6 in one analysis, or 0.7-0.8 in another.
Since the axial vector coupling constant g_A appears to the second power in the nuclear matrix element, the half-life depends on it with the fourth power. Therefore, the results for the expected half-lifes should be multiplied by a factor ranging from 6 to 34, and hence the limits on the average neutrino mass should be increased by a factor of 2.5 to 6, making it impossible to reach in the near future even the inverted hierarchy region with experimental measurements.
Possibilities to escape this negative conclusions may be:
1) if neutrino masses are degenerate and large – but this comes in tension with the cosmological bound on the sum of neutrino masses, which is of 0.23 eV from 2015 Planck data.
2) if both light and heavy neutrino exchange contribute simultaneously and are of the same order of magnitude, interfering constructively. This possibility, however, requires a fine tuning which looks quite unlikely.
3) Other scenarios (like Majoron emission or sterile neutrinos) must then be considered. The speaker discussed these in some detail -interested readers should refer to his slides, available in the conference web site.