Recent advances in observational astronomy and the discovery of 125-GeV Higgs boson have brought paradigm shifts on the potential connections between new fundamental particles and our understanding of their impact on the early universe and its evolution. With the content of the universe well known from astrophysical observations, a key aspect is that 27% of the universe appears to consist of dark matter. If current theories are correct, the particle physics candidate for this matter may well be observed in ongoing direct and/or indirect dark matter detection experiments or at the LHC. In addition, about 69% of the universe, the dark energy, still remains a significant mystery that major theoretical attempts are trying to understand. Continua a leggere 9th International Conference on Interconnections between Particle Physics and Cosmology
Grazie ad uno studio recente guidato da un gruppo di fisici inglesi e giapponesi è stato possibile confermare che le particelle subatomiche, denominate neutrini, possiedono una nuova identità. Questi risultati, che danno inoltre credito agli esperimenti realizzati in Giappone presso il rivelatore T2K che ha lo scopo di studiare l’oscillazione dei neutrini, potrebbero un giorno aiutare gli scienziati a capire come mai l’Universo contiene in gran parte materia e solo ‘poche tracce’ di antimateria (vedasi Enigmi Astrofisici).
Alfons Weber, a professor of Physics at STFC and the University of Oxford is one of many scientists in the UK working on T2K. He explains: “The UK particle physics community was one of the driving forces behind this experiment. We not only provided part of the detector that characterises the beam, but also designed the target that produces the neutrinos in the first place. The long years of hard work have now come to fruition. Our findings now open the possibility to study this process for neutrinos and their antimatter partners, the anti-neutrinos. A difference in the rate of electron or anti-electron neutrino being produced may lead us to understand why there is so much more matter than antimatter in the Universe. The neutrino may be the very reason we are here“.
In 2011, the T2K collaboration announced the first indication of this process. Now with 3.5 times more data and a significance of 7.5 sigma, this behaviour is firmly established and can now be called a discovery.
There are three types, or ‘flavours,’ of neutrinos, one paired with the electron (called the electron neutrino), and two more paired with the electron’s heavier cousins, the muon and tau leptons. These different flavours of neutrinos can spontaneously change into each other, a phenomenon called neutrino oscillations. Observations have previously been made of a number of different types of oscillations, however the T2K results are the first discovery of the appearance of electron neutrinos in a beam of muon neutrinos, and it is this kind of oscillation which is the key to making measurements to distinguish the oscillations of neutrinos and anti-neutrinos. To explore the neutrinos’ oscillations, the T2K experiment fired a beam of neutrinos from the J-PARC laboratory at Tokai Village on the eastern coast of Japan, and detected them at the Super-Kamiokande neutrino detector, 295 km away in the mountains of the north-western part of the country. Here, the scientists looked to see if the neutrinos at the end of the beam matched those emitted at the start.
They found 22.5 neutrinos appearing in the beam of muon neutrinos, where if there were no oscillations they only expected to see an average of 6.4. This indicates the discovery of the new type of oscillation.
Now the team must make more accurate measurements of this new oscillation, and then run their experiment with an anti-neutrino beam to see if the results change. Professor Dave Wark of the Science and Technology Facilities Council (STFC) and Oxford University, leads the UK’s involvement in the international experiment. He said: “It’s a joy to see T2K deliver the science we designed it for. I have been working on this for more than a decade, and what these results tell us is that we have more than another decade of work ahead of us. We have seen a new way for neutrinos to change, and now we have to find out if neutrinos and anti-neutrinos do it the same way. If they don’t, it may be a clue to help solve the mystery of where the matter in the Universe came from in the first place. Surely answering that is worth a couple of decades of work!“.
Ogni secondo, trilioni di particelle chiamate neutrini attraversano il nostro corpo. Queste particelle elusive hanno una massa così piccola che non è stata ancora misurata. Inoltre, esse interagiscono debolmente con la materia che è quasi impossibile rivelarle e ciò rende molto complicato studiare le loro proprietà.
Since arriving at MIT in 2005, Joseph Formaggio, an associate professor of physics, has sought new ways to measure the mass of neutrinos. Nailing down that value, and answering questions such as whether neutrinos are identical to antineutrinos, could help scientists refine the Standard Model of particle physics, which outlines the 16 types of subatomic particles (including the three neutrinos) that physicists have identified. Those discoveries could also shed light on why there is more matter than antimatter in the Universe, even though they were formed in equal amounts during the Big Bang. “There are big questions that we still haven’t answered, all centered around this little particle. It’s not just measuring some numbers; it’s really about understanding the nature of the equation that explains particle physics. That’s really exciting”, Formaggio says. Formaggio, the only child of Italian immigrants, was the first in his family to attend college. Born in New York City, he spent part of his childhood in Sicily, his parents’ homeland, before returning to New York. From an early age, he was interested in science, especially physics and math. At Yale University, he studied physics but was also interested in creative writing. The summer after his freshman year, in search of a summer job, he “called every publishing house in New York City, all of which resoundingly rejected me”, he says. However, his call to the Yale physics department yielded an immediate offer to work with a group that was doing research at the Collider Detector at Fermilab. That led to a senior thesis characterizing the excited states of the upsilon particle, which had recently been discovered. As a student, Formaggio was drawn to both particle physics and astrophysics. At Columbia University, where he earned his PhD, he started working in an astrophysics group that was studying dark matter. Neutrinos were then thought to be a prime candidate for dark matter, and the mysterious particles intrigued Formaggio. He eventually joined a neutrino research group at Columbia, which included Janet Conrad, a professor who is now at MIT. While a postdoc at the University of Washington, Formaggio participated in experiments at the Sudbury Neutrino Observatory (SNO), located in a Canadian nickel mine some 6,800 feet underground. Those were the first experiments to show definitively that neutrinos have mass, albeit a very tiny mass. Until then, “there were definitely hints that neutrinos undergo this process called oscillation where they transmute from one type to another, which is a signature for mass, but all the evidence was sort of murky and not quite definitive”, Formaggio says. The SNO experiments revealed that there are three “flavors” of neutrino that can morph from one to the other. Those experiments “basically put the nail in the coffin and said that neutrinos change flavors, so they must have mass”, Formaggio says. “It was a big paradigm shift in thinking about neutrinos, because the Standard Model of particle physics wants neutrinos to be massless, and the fact that they’re not means we don’t understand it at some very deep level”. Another possible discovery that could throw a wrench into the Standard Model is the existence of a fourth type of neutrino (post). There have been hints of such a particle but no definitive observation yet. “If you put in four neutrinos, the Standard Model is done”, Formaggio says, “but we’re not there yet”.
In his current work, Formaggio is focused on trying to measure the mass of neutrinos.
In one approach, he is working with an international team on a detector called KATRIN, located in a small town in southwest Germany. This detector, about the size of a large hangar, is filled with tritium, an unstable radioactive isotope. When tritium decays, it produces neutrinos and electrons. By measuring the energy of the electron released during the decay, physicists hope to be able to calculate the mass of the neutrino, an approach based on Einstein’s E=mc2 equation. “Because energy is conserved, if you know how much you started out with and how much the electron took away, you can figure out how much the neutrino weighs”, Formaggio says. “It’s a very hard measurement but I like it because the experiment is a giant electromagnetic problem”. The KATRIN detector is under construction and scheduled to begin taking data within the next two years. Formaggio is also developing another tritium detector, known as Project 8, which uses the radio frequency of electrons to measure their energies. Formaggio hopes that one day, tritium-based detectors could be used to find neutrinos still lingering from the Big Bang, which would require even larger quantities of tritium. “There are many holy grails in physics, and finding those neutrinos is definitely one of them. People look at the light from the Big Bang, but that’s actually closer to 300,000 years old, or thereabouts. Neutrinos from the Big Bang have been around since the first second of the Universe”, Formaggio says.
- Sterile-neutrino hunt gathers pace at Gran Sasso (physicsworld.com)
- Particle Pals: Neutrino Experiment Shows Protons and Neutrons Pairing Up (scientificamerican.com)
- Lab plans meetings for neutrino experiment (rapidcityjournal.com)
- What are neutrinos, and how do they come from beyond our solar system? (io9.com)