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Switching Neutrinos Research in Progress Physics Fl avor- -Switching Neutrinos rof. Ewa Rondio from the National P Center for Nuclear Research (NCBJ) explains the nature of neutrinos, the measurements taken by the Super-Kamiokande detector, and the involvement of Polish scientists in the project. MIJAKOWSKI PIOTR ACADEMIA: What are neutrinos? What is the difference between the neutrinos that PROF. EWA RONDIO: Literally translated, the word come from space and the neutrinos created on Earth? “neutrino” means “little neutral one.” The name was Neutrinos arriving from space are almost as abundant first suggested when the existence of neutrinos was as photons in the cosmic background radiation, but proposed to patch up the law of conservation of en- their energies are so small that we’re unable to detect ergy. It had turned out that without postulating their them. We can’t see them, even though there are so existence we could not explain why the spectrum of many of them. We can only observe higher-energy energy in beta decay is continuous, whereas two-body neutrinos, for example those arriving from the Sun. decay should give a single constant value. Back then, At first glance, they do not differ in any way from that postulate was considered very risky, because it those made on Earth. However, they have somewhat was believed that such particles would be impossible different energies. Also, those arriving from space are to observe. Experimental physicists have nowadays predominantly neutrinos, whereas those we produce managed that, although it took them quite a long time. on Earth produce are chiefly antineutrinos. This is Neutrinos come in three types, called flavors. They because solar neutrinos are produced when light el- appear in interactions together with different charged ements are fused into heavier ones, whereas most of leptons, from which they derive their names. The elec- the neutrinos produced on Earth are produced in nu- tron neutrino, associated with the electron, and the clear reactors, in nuclear fission reactions triggered by muon neutrino, associated with the muon, have been neutrons. They release energy and produce unstable known for a long time. The tau neutrino, associated nuclei, the decays of which produce antineutrinos. with the heavy lepton tau, was discovered this century. These days, we use neutrinos as regular particles. We Does this mean that the National Center for Nuclear study the neutrinos coming from space, but we also Research in Świerk produces antineutrinos? make our own beams of neutrinos: controlled, artifi- Yes, but we conduct no research using antineutrinos. cial, created here on Earth. The trouble with all experiments involving neutrinos THE MAGAZINE OF THE PAS 46 2/46/2016 THE POLISH NEUTRINO GROUP is that they are extremely unwilling to interact. They formations depend on the neutrino’s energy and the The Super-Kamiokande pass through the whole Earth and most of them don’t distance it has travelled. Since this process is cyclical, detector – a water-filled even notice it. If we wanted to perform measurements which means that neutrinos of a specific type disap- cylinder with involving reactor neutrinos in Świerk, we would have pear and then appear again, the changes were called photomultipliers covering all the walls, located to wait for a very long time to get enough interactions. oscillations. In recognition of that discovery, the 2015 1,000 meters underground Nobel Prize in Physics went to Takaaki Kajita from the in the Kamioka mine, Japan Why did neutrino physics prove worthy of the Super-Kamiokande experiment, who studied atmo- Nobel Prize in 2015? spheric neutrinos, and Arthur B. McDonald from the In various experiments observing both neutrinos ar- Sudbury Neutrino Observatory (SNO) experiment, riving from the Sun and those that are produced in the who studied solar neutrinos and demonstrated that Earth’s atmosphere, the numbers of neutrinos of partic- neutrinos of a certain type disappeared while those ular types were not consistent with expectations. Models of a different type appeared. All those experiments were used to predict how many electron neutrinos showed that the overall count of all types of neutrinos the Sun should emit, but the measurements showed detected was as expected, even if some of a given flavor a deficit. In turn, experiments involving neutrinos that were missing. As for solar neutrinos, that was a break- were created in the atmosphere and reached the de- through; as long as the count was wrong only for solar tector from various directions showed that the largest neutrinos, it was said that our model of the Sun might deficit was observed for neutrinos coming from below, be inaccurate. Meanwhile, showing that the overall ones that had traveled all the way through the Earth. count of neutrinos remained unchanged offered tan- In order to explain those deficits, researchers hy- gible proof of the existence of oscillations. pothesized the existence of neutrino oscillations, Those studies were conducted at the turn of the postulating that neutrinos switched identities in mid- century: the Super-Kamiokande experiment in 1998, flight. A neutrino of a certain type sometimes stays the in which Takaaki Kajita led the group that examined same, but sometimes changes its flavor. Such trans- atmospheric neutrinos, was the first to present results THE MAGAZINE 47 OF THE PAS 2/46/2016 Research in Progress Physics at a conference, showing deficits of neutrinos that This is why we talk about two types of interactions: were coming in from below. The SNO experiment, charged-current interactions, in which neutrinos con- for which Arthur B. McDonald was one of the initi- vert into charged particles, and neutral-current inter- ators and project leaders, demonstrated in the early actions, in which neutrinos remain neutrinos. 2000s that the total count of neutrinos from the Sun is consistent with expectations. How are Polish researchers involved in these experiments? Where are these detectors located? Prof. Danuta Kiełczewska was involved in the Su- Such detectors are located underground in order to per-Kamiokande experiment from the outset. Also, isolate them from the background of charged parti- she had been involved in earlier Cherenkov exper- cles coming in from space, interacting and creating iments, including the projects involving the Ir- noise. The Super-Kamiokande detector – located in vine-Michigan-Brookhaven (IMB) detector. Together Kamioka, Japan – is a cylinder filled with ultra-pure with the Super-Kamiokande detector, the IMB de- water and located in a mine over 1,000 meters under- tector observed the signal of neutrinos from the ex- ground. The detector’s walls are covered with huge plosion of a supernova in 1987. Initially, Prof. Kiełcze- and extremely photogenic photomultipliers. wska was the only Pole there. We find neutrinos by observing the products of After returning to Poland, she was one of the cre- their interactions. These products are charged parti- ators of the Polish neutrino group. I think that is cles, which leave traces in the detector. We can mea- something we can be proud of. We currently have sure these traces and use these measurements to de- team of nearly 30 individuals who work closely with termine the energy of neutrinos, say something about one another in experimental neutrino physics. their properties. The Super-Kamiokande detector The group was formed in 2000 to take part in ex- shows us the tracks of charged particles that travel periments in which a neutrino beam was directed through the detector, provided that these particles from the European Organization for Nuclear Re- move sufficiently fast. search (CERN) to the Gran Sasso laboratory in Italy, Such detectors use the Cherenkov effect, or the fact where a huge detector called ICARUS was being built. that a particle that passes through a medium at a speed Instead of water, it used liquid argon to observe the greater than the speed of light in that medium emits traces left by charged particles. radiation. This is possible in a medium, because the The areas of study later expanded, because the ex- speed of the particle still doesn’t exceed the speed of periment at the Gran Sasso laboratory was delayed by light in a vacuum, which is the greatest possible speed. several years. We had a team ready to work, so we de- The detector used in the SNO experiment was also cided to join the experiment called T2K, or Tokai to a Cherenkov detector, but it used heavy water to make Kamioka. It started in 2009 and the first neutrinos in it easier to record interactions in which a neutrino re- a beam were observed in 2010. The accelerator that pre- mained a neutrino and dissociated deuterium as a re- pares the beam of neutrinos for us is located in Tokai, sult of transferring energy. Observing the products Japan. We direct the beam through the whole of the of such dissociation made it possible to count muon island of Honshu to Kamioka, where the underground neutrinos arriving from the Sun for the first time. Super-Kamiokande detector is located. It’s the same detector that earlier worked on atmospheric neutrinos. As for the T2K experiment, Polish groups partic- ipated in the construction of the part of the detector Polish researchers studying the “little neutral ones” that recorded the interactions near the source of the The history of Poland’s participation in neutrino oscillation experiments beam. They are actively involved in data analysis and started from Prof. Danuta Kiełczewska’s involvement in the IMB experiment, they are working to improve the models that describe which was searching for proton decay in which neutrino interactions were the most important background, The IMB experiment observed neutrinos neutrino interactions. from Supernova 1987A. In that experiment, Prof. Kiełczewska worked in The recipe for producing neutrino beams reflects Prof.
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