The XVII edition of the “Neutrino Telescopes” conference closed its works yesterday after a lively debate on the future of the neutrino physics program in Europe. Here I would like to give my own impressions of the picture that has been drawn this week of a lively field of research in fundamental physics – one that encompasses very naturally accelerator and particle physics, cosmology, astrophysics, nuclear physics, detector development, and a number of ancillary topics that range from geophysics to sea science.
I find it good that there be a number of competing initiatives attacking several nagging questions – the mass hierarchy of neutrinos being only the most evident example. What I find a bit annoying is that there is no “grand plan”. At a time when fundamental science faces big challenges from continuous reduction of funds (and we have not seen the US budget for science yet), it seems a bit futile to have many experiments fighting over the question of who will have a zero point something better sensitivity on this or that parameter in ten years.
Stated in another way, there is richness in a diverse program with many bright leaders taking arms simultaneously attacking the big problems that neutrinos have brought to our table in the course of the past 20 years or so, but there is also a feeling of suboptimality of the budget choices and of the knowledge return that this multi-pronged attack entails.
What Carlo Rubbia said yesterday during the discussion session, and what Luca Stanco also echoed (you can find some trace of it in the previous article here), are important questions. It seems that the global neutrino plan lacks coherence, and this harms everybody, especially people who might not have twenty years to get an answer to questions they want to know an answer to in their lifetime (and this includes me and you, of course). It may be a provocatory statement, but it is what goes through my mind at the moment, and I thought I had the right to express it in this less-than “institutional” piece here.
Apart from this critical consideration, as a HEP physicist I envy the large amount of important issues that the neutrino experiments have at arms’ reach in the next decade. The LHC, I am positive, will NOT discover any new beyond-the-standard-model physics. I was among the first to say this out loud, when I bet (and later won) $1000 on the statement, and I continue to say it with doubled conviction. And if the discovery that neutrinos are massive had an impact comparable to the rearrangement of the furniture in our living room -not a revolution of our understanding, that is, and not properly “new physics”- they are offering us more knowledge gain per buck than any other fundamental physics endeavour around.
That said, I leave this space, which will likely come back to life in a couple of years, hopefully with new breakthroughs!
Tommaso Dorigo is an experimental particle physicist, who works for the INFN at the University of Padova, and collaborates with the CMS experiment at the CERN LHC. He coordinates the European network AMVA4NewPhysics as well as research in accelerator-based physics for INFN-Padova, and is an editor of the journal Reviews in Physics. In 2016 Dorigo published the book “Anomaly! Collider physics and the quest for new phenomena at Fermilab”.
Kenneth Long introduced the open discussion session. He chairs the ICFA neutrino panel. They just finalized a neutrino roadmap, so he went through it. The programme must have discovery potential and deliver critical measurements in the short and medium term. And develop the capabilities required to outperform the present and next-generation experiments.
Among its assignments, ICFA should encourage timely consideration of the development of the hadro-production measurement programme; it should encourage nad promote a sustained effort towards reliable and precise calculations of neutrino-nucleus scattering processes. It should complete the present detector R&D programme and make provision for an appropriately resourced detector deveopment programme over the lifetime of the next generation experiments. Other assignments include a census of accelerator-based neutrino community, and a new 4th international meeting on large neutrino infrastructures.
Yifang Wang briefly discussed the unknowns of neutrinos. There are a number of them: the parameters we try to measure – the oscillation parameters, the CP phase, etcetera; and in general the unitarity of the matrix. There is the neutrino mass, the issue of Dirac vs Majorana, the existence of steriles, their magnetic moments. For some of the above items, we do not have reason to believe we can find a solution in the short term. He considered the present experiments, in particular JUNO, and made the point that there needs to be a global coordination to construct huge detectors at the scale of billions of dollars to increase our understanding and detect neutrino masses, double beta decay, and magnetic moments, or other important questions that at the point remain unaccessible.
Antonio Masiero discussed the role of multi-messenger in the APPEC (astroparticle physics european coordination) roadmap. The next generation of astroparticle physics research infrastructures require substantial capital investment. So there needs to be a resource aware strategy. European astroparticle physics does not profit from a natural and strong intergovernmental organization like CERN, ESO and ESA to drive the field. In 2001 APPEC was found to coordinate the field. The “C” in APPEC since 2012 means “consortium”. This has a profound meaning. APPEC must formulate and realize the european astroparticle physics strategy.
In 2008 the roadmap included the “magnificent 7”, HE gammas, neutrinos, and cosmics; gravitational waves, dark matter, neutrino mass, and neutrino mixing and proton decay. However at least four or five of these domains have not seen a signal yet. However the situation has now changed, as two of them have seen new signals: astrophysical sources of neutrinos and gravitational waves.
The new map, coming out now, and there are now 9 topics of relevance. Dark energy and the CMB have entered the club of astroparticle items to be followed by the european agencies for astroparticle physics.
Masiero explained that with its global partners and in consultation with the gravitational wave international committee, APPEC will define timelines for upgrades of existing as well as next-generation ground-based interferometers. APPEC strongluy supports further actions strenghtening the collaboration between gravitational-wave laboratories. For neutrinos, APPEC strongly supports the present range of direct neutrino-mass measurements and searches for neutrinoless double-beta decay. Guided by the results of running experiments and in consultation with its glibal partners, APPEC intends to converge on a roadmap for the next generation of experiments into neutrino mass and nature by 2020.
Note of the editor: It is interesting that while the 2009 report had Ursa Major in the cover, the 2016 report has the Leo constellation. Is this a message in itself ? Masiero explained that it was driven by going from 7 to 9 hot topics, as the number of stars in the constellations.
Masato Shiozawa explained what is the program leading to Hyper-Kamiokande, envisioned to start in 2026. It involves gradual upgrades to both the detector and the accelerator facility. Hyper-Kamiokande will have a mass of 190 ktons, impinged by a 1.3MW beam. It is a seamless program to get timely results, with a rich physics program and big chances of discoveries. For proton decay, HK will be able to reconstruct the proton mass and have zero background, so that one event would be a discovery, and this can push lifetime limits above 10^35. It will have sufficient angular sensitivity to point to SN sources within 1 degree, to alert other telescopes, up to Mega-parsec distances.
Soo-Bong Kim took a broad view of the neutrino science. He made the point that we need to study more SN burst and extend our search to more distant supernovas, as the number of burst is too rare otherwise. In Hyper-Kamiokande the sensitivity will extend to Megaparsecs away.
In Korea they are interested in having one of the Hyper-Kamiokande detectors there. They can receive the J-PARC beam 1100 km away. There are several candidate sites. They can have 1000m overburden, to be sensitive to low-energy physics.
When the floor was opened by the chairman Mezzetto, he invited Rubbia to start the discussion.
Rubbia said it’s a pity that we do not have a US view of the situation here. Therefore he will remain on the questions in Europe. In his view, there is a very serious problem there. It reminds him of something that already happened when CERN asked itself what to do after the PS. Every European machine was coming up with proposals for 40 GeV machines. Then CERN became the only European entity. Only Weisskopf could argue for that kind of solution. This points to all European countries working together for a single big project. With EPPAC we are missing a great opportunity to join forces.
Another point is HK versus DUNE. It is clear to Rubbia that there are various questions with DUNE. Will it compete time-wise with HK, which uses well-oiled technology. Also the question of funding is a key: how much money is there available to do DUNE ? INFN can give 20 million euros to DUNE, but that is just a drop in the sea. So the question is, do we need two experiments competing with each other, or is one enough ? Because the need for two experiments is not clear.
Mezzetto said that the absence of the US was his fault as he was unable to convince Nigel Lockyer to come. Similarly for the Russians.
Masiero said that the whole purpose of APPEC, and the reason why in 2001 the president and directors of the major agencies in Europe decided to sit together at a table was the need to coordinate a coherent action. Considering the US, Japan etcetera, there is one single source of funding, or two. In Europe there are many. Rubbia countered that there is CERN. Masiero explained that there is no natural center of mass for astroparticle physics. As each agengy has to answer its own ministers, and there is no common body in Europe for this kind of research, it becomes hard to find a common intent. For the future, to have the famous large experiments for dark matter or double-beta decay, it becomes necessary for the agencies to find an agreement.
As for the second question, Masiero reminded that the main source of funding is coming from outside Europe, so the effort is to see how to complement the action of other agencies and sources. At the moment it is not clear that one experiment can be enough. We have redundancy, but we are facing a formidable challenge from the experimental results we want to achieve.
In the last presentation of the 2017 conference, Grigory Domogatsky (right) described the Bajkal detector. It is 4km from the shore, constructed in 1998, the first one in a natural medium. It was built to investigate atmospheric neutrinos, search for diffuse flux of high-energy neutrinos, and for wimp annihilation in the Earth.
In 2003-2005 they upgraded it ot extend more strings at larger distance, to include a larger volume, and have a higher efficiency of detection.
Now they are doing another upgrade, for a gigaton volume detector, with an array spanning 1km in width and 6oo m in depth. Strings have 3 sections with 12 optical modules each.
The PMT used is from Hamamatsu, R7081-100, with a good (35%) quantum efficiency. The diameter is of 10 inches. The optical module includes the PMT and is embedded in a mu-metal cage, includes the electronics and is embedded in a pressure-resistance glass sphere. The main problem is the time synchronization. This is performed by LED beacon, with 6 blue LEDs pointing upward plus 6 pointing horizontally.
The speaker finally showed the “Dubna” cluster with an estimated energy of 1.9 PeV.
To conclude his presentation Domagatsky showed the cumulative number of clusters expected per year by the upgraded detector.
Mauro Taiuti (left) talked about the ARCA part of the Km3Net project, dedicated at astroparticle research in the high-energy part of the neutrino spectrum.
IceCube has indicated the path for neutrino astronomy. We learned a lot from it, and we look forward to improve the situation; water may provide a higher resolution to neutrino signals.
The main feature of the DOM modules is the capability to detect the direction of the incoming light. Each is provided with 31 3″ PMTs. Each detection unit has 18 DOM integrated on vertical strings deployed in the sea and anchored to the sea floor.
In phase 1, a preliminary production of a limited number of detection units, as a proof of feasibility and first science results, with 24 ARCA strings. This is fully funded. In phase 2, two blocks of ARCA have to be constructed, with which a program of studies of neutrino astronomy can be done. 1 block has been almost fully funded already. In phase 3, 6 blocks will be built, with which neutrino astronomy and galactic sources can be studied.
Some performance results were presented by the speaker for muon detection. The muon track covers a large volume in the detector, allowing for good angular and energy resolution. For the angle they expect to reach a 0.1 degrees for 10 TeV. The energy resolution should be 0.3 in logarithm of the muon energy.
Arca can detect events from electron neutrinos; one may still reconstruct the incoming angle, with worse resolution but still workable, 2 degrees for energies above 10 TeV. The energy resolution is better, about 10%.
Taiuti also showed the sensitivity to point-like galactic sources, with a sensitivity above 5 sigma with 5 years of observation data.
Paolo Lipari (left) gave a talk on the interpretation of the signals of astrophysical sources of neutrinos. Neutrinos have two purposes: they are messengers, which we can use to study the universe. And they are also probes, to infer the fundamental properties of the standard model and of fundamental laws. As messengers, they tell us about the sun, the Earth, and the cosmos, Supernovas. They span many orders of energy. This topic can be divided in many, as the energy range is very broad. The IceCube signal is from 10^14 to 10^16 eV. So actually the topic is the high-energy Universe. You have an ensemble of astrophysical objects that generate relativistic particles. Neutrinos must be understood as messengers, and together with cosmic rays, photons, and gravitational waves they can help us uncover deep mysteries.
The mechanism of creation is understood as acceleration of hadrons, that decay to the neutrinos; but neutral pions generate photons, and there you have a relation between the two fluxes. Then also leptonic fluxes are related to that. The concept is that you generate a population of protons, they interact in the medium and produce gammas and neutrinos. The spectra of neutrinos from pions end up being very similar in spectrum.
If the target is not a gas but a radiation field, the interaction is p-gamma and not p-p. So single pion production is dominated, and you have an excess of positive pions, which changes the ratio of photons, neutrinos, antineutrinos. If the energy increases you then find more photons. The spectral index of neutrinos reflects the ones of the progenitors. If you observe a source in photons you can then relate it to neutrinos and you can distinguish the mechanisms of creation.
The gamma-ray sky is composed of three componends: the galactic plane, a uniform flux, and some sources. Eighty percent of the photons at 1 GeV is concentrated at +-5 degrees around the galactic equator. This is measured by Fermi with a spectrum with a spectral index of 2.7. The flux is also concentrated in the galactic center. Then there is an ensemble of sources, more than 3000 found by Fermi. 440 of these are galactic. In the TeV range you see the same thing.
Then there is an extragalactic gamma ray flux. It is isotropic and with a power of E^-3. This is nearly certainly an effect of absorption, due to their impossibility to travel on extragalactic distances. And then there are sources: very interesting beasts of different kinds. Active galactic nuclei emanate jets from the center, with the brightest sources of extragalactic gamma rays. Our own galactic center is also active, with a black hole at the center that we are studying.
The extragalactic flux is composed of two parts: a resolved one, from bright sources. One is 3C454, then there are others, about 2000, and very likely there is then a half of the flux is from unresolved sources. So it is not surprising that if you see 54 neutrinos you do not see a point source – the brightest in photons is 1.8% of the total.
The speaker moved then to discuss the IceCube data, where the statistical significance of the new component is largely coming from the highest-energy events. There are events where the interaction of the neutrino is within the detector, and then there is the flux from muons. The two results seem to give two different power spectra, as discussed by the previous speaker. There is a lot of discussion on this topic with hundreds of papers published. Within systematics they are not so different, but the possibility of two components is intriguing.
A critical discussion of the result should address several questions. First, is the signal of astrophysical neutrinos real ? The answer is most certainly yes. The second is, can the signal be contaminated by a non negligible contribution of atmospheric neutrinos? If you consider the three components (conventional atmo neutrinos, prompt atmo neutrinos, and astrophysical ones), the real dangerous one is from prompt atmospheric neutrinos, from the decay of charm. It is a difficult problem. The perturbative way to produce charm is through gluon fusion diagrams. The cross section is measured, but the Ice Cube result uses this calculation as a model for charm production, fixing it. There are a number of papers that discuss this issue critically. Francis Halzen himself wrote a paper discussing the charm contribution, and it is interesting that he concludes that non-perturbative mechanisms can be a dominant source of neutrino production from charm. This contribution cannot accommodate the PeV flux, and it is not important in the entire energy range. However the non-perturbative effects are the most important ones.
There is a handle to try and distinguish the two effects. The speaker commented that we cannot entirely dismiss the possibility that a charm contribution distorts the spectrum. There have been discussions on measuring these effects at the LHC, to cast more light on the issue.
Another question is, does the IceCube signal have a Galactic component? If one looks at the map, it is difficult to establish by eye. In the literature there are attempts to model the signal as entirely due to galactic origin. They could be generated by dark matter in the galaxy. Another paper that assumes that the signal is entirely galactic is one that is inspired by the observations of the “fermi bubbles”, with large halos around the galaxy.
In this conference it has been suggested that a significant fraction of the signal comes from the disk. It would be very nice if IceCube itself could give an answer to this question. Several works discuss model with both a galactic and an extragalactic component. The speaker mentioned a few of these. The problem is that the galactic emission that we see seems to be lower than what we see.
To conclude, Lipari mentioned the interest of studying the extragalactic component lies in the fact that it could be coming from an ensemble of sources, and identifying these sources is the focus of the issue.
Christian Spiering (right) discussed some aspects of the searches with the IceCube detector, concentrating in particular on astrophysical neutrino studies. He showed two very energetic events (>500 TeV), which were nicknamed Ernie and Bert. They are above 1 PeV. Following that, there was an analysis for a high-energy neutrino flux. On the fourth year of data taking they took another high-energy event, “Big bird”, of 2 PeV.
For through-going muons, from six years of data (2009-2015) the muon energy spectra were shown to match the expectation from atmospheric source, with however some outliers. One was shown to have a deposited energy of 2.6+-0.3 PeV, with the most probable energy of 7 PeV. That is the highest-energy event in their sample.
The spectrum for astrophysical sources shows a power index of -2.13+-0.13. There is some tension between cascade and track analysis for the shape of the spectrum.
The flavor composition was studied. For a far-away astrophysical source, the normal assumption is that you get muons from pion decay. So you would have one electron neutrino per every two muon neutrinos at the source. If however the muon decay of pions were suppressed you would have a 0:1:0 ratio, while for neutron decay the ratio would be 1:0:0 at the source for the three flavours. The result is now an exclusion of the dominance of neutron decay sources.
A limit on cosmogenic and AGN neutrino fluxes has been extracted by looking at the extremely high energy tail of the spectrum, based on observing two events.
IceCube studied the Crab Nebula, which emits gamma rays. Assuming these come from pizeros, one can compute the flux of neutrinos from charged pions. This was constrained by the data. Constrains were also put on the flux from Blazars, and there is hope that one could detect neutrinos from these sources.
Neutrinos could also be observed in association with Gamma-Ray Bursts. They looked for coincidences with GRBs in their field of view among 506, and found one single low-significance coincidence, consistent with atmospheric background.
Awards and consumer electronic prizes were given to the three best posters. Chiara Sirignano, the chair of the poster committee, announced the winners before the morning coffee break.
The winning poster was the one presented by Jannick Hofestadt, titled “Event reconstruction in Km3Net-Orca“. A picture of the poster is shown below.
The second best poster was the one of Aurora Meroni, titled “Neutrino Masses and Ordering with Multimessenger Astronomy”
The third best poster was the one by Andrea Gallo Rosso, titled “Searching for Supernova Neutrinos with Dark Matter Detectors“.
A special mention was also given to Matteo Tenti for his poster titled “Determination of the Neutrino Mass Hierarchy with a New Statistical Method“.