A new hunt for dark matter passes under the mountain
Давид Д’Анджело не всегда интересовался тёмной материей, но теперь он попал на передний край охоты за наиболее неуловимой частицей во Вселенной
About an hour from Rome there is a dense cluster of mountains called Gran Sasso d'Italia . They are known for their natural beauty and attract tourists all year round, offering world-class ski resorts and hiking trails in the winter, as well as the opportunity to swim in the summer. For the 43-year-old Italian physicist David D'Angelo, these mountains are like a second home. Unlike most visitors to Gran Sasso, D'Angelo spends most of his time under the mountains, not on them.
There, in a space full of caves a thousand meters below the surface of the earth, D'Angelo is working on a new generation of experiments devoted to the hunt for dark matter particles - an exotic form of matter whose existence has been assumed for several decades, but has not yet been experimentally proven.
It is believed that dark matter makes up to 27% of the Universe, and the description of this elusive substance is one of the most acute problems of modern physics. Although D'Angelo is optimistic that a breakthrough will occur even during his lifetime - the previous generation of physicists thought the same way. In principle, there are good chances that the particles sought by D'Angelo do not exist at all. However, for physicists probing the fundamental nature of the Universe, the ability to spend an entire career in the “ghost hunt,” as D'Angelo says, is the price of advancing science.
What lies under the "great stone"?
In 1989, the Italian National Institute of Nuclear Physics opened the National Laboratory in Gran Sasso , the world's largest underground laboratory dedicated to astrophysics. The three underground halls of the Gran Sasso abounding in caves were specially built for physicists - a rather elegant setting for a research center. Most of the underground astrophysical laboratories, for example, SNOLAB , are spontaneously set up using old or working mines, and this fact limits the amount of time that can be spent in the laboratory and the types of equipment used.
Gran Sasso, located a kilometer under the ground in order to protect it from the noisy cosmic rays washing the planet, sheltered several experiments in particle physics, probing the foundations of the Universe. Over the past few years, D'Angelo has divided his schedule between the Borexino Observatory and a sodium iodide detector with active background screening (Sodium Iodide with Active Background Rejection Experiment, SABRE ), investigating solar neutrinos and dark matter, respectively. D'Angelo working with a prototype SABER
Over the past 100 years, the description of solar neutrinos and dark matter are considered the most important tasks of particle physics. Today the mystery of solar neutrinos has been solved, but these particles are still extremely interesting to physicists, because they give a lot of information about the nuclear fusion that takes place in our Sun and other stars. But the composition of dark matter is still considered one of the major issues of nuclear physics. Despite the completely different nature of these particles, the issues of their study are still related, since these particles can be detected only under conditions of minimal background radiation: thousands of meters under the ground.
“The mountains work like a shield, so if you are under them, you experience the so-called“ cosmic silence, ”said D'Angelo. “This is the most favorite part of my research: you go into a cave, you start working with a detector, and you try to understand the signals you see.”
After graduating from the institute, D'Angelo got a job at the Italian National Institute of Nuclear Physics, where his research focused on solar neutrinos, free-charge subatomic particles that appear as a result of nuclear synthesis on the Sun. For most of the four decades, solar neutrinos have been at the center of one of the biggest mysteries of astrophysics. The problem was that the instruments measuring the energy of solar neutrinos produced much less results than those predicted by the Standard Model.- The most accurate theory of fundamental particles in physics.
Given how accurate the standard model turned out to be in other aspects of cosmology, physicists did not want to make changes to it in order to take into account this discrepancy. One possible explanation was that physicists had made the wrong model of the Sun, and that it was necessary to make better measurements of pressure and temperature in its core. However, after a series of observations in the 60s and 70s, it turned out that, in general, the models of the Sun were made correctly, and then physicists turned to neutrinos for alternative explanations.
The Tale of Three Neutrinos
Since the Austrian physicist Wolfgang Pauli in the 1930s for the first time suggested the existence of neutrinos , they were constantly attracted to plug holes in theories. In the case of Pauli, the assumption of the existence of extremely light particles that have no charge was a “desperate tool” for explaining why the energy conservation law does not work during radioactive decay. Three years later, the Italian physicist Enrico Fermi gave the name of these hypothetical particles. He called them " neutrino, " which in Italian means "small neutrons."
A quarter of a century later after the assumption of Pauli, two American physicists reported on the first evidence of neutrino production in a nuclear reactor. In the following, in 1957, Bruno Maksimovich Pontecorvo, a physicist of Italian origin, who worked in the USSR, developed a theory of neutrino oscillations . At that time, neutrino properties were poorly studied, and Pontecorvo suggested that there are several types of neutrinos. In this case, he suggested, it was possible that neutrinos could change their types.
By 1975, Pontecorvo’s theory was proven. Three different types, or "flavor" of neutrinos, were discovered: electron, muon, and tau. It is also important that observations in an experiment in South Dakota showed that the Sun produces electron neutrinos. The only problem was that the experiment recorded fewer neutrinos than the Standard Model predicted.
Until the end of the 90s there was scant evidence that neutrinos could pass from one flavor to another. In 1998, a group of researchers working at the Japanese Super-Kamiokande Observatory observed oscillations of atmospheric neutrinos, mainly due to the interactions of photons with the atmosphere of the Earth. Three years later, the first direct evidence of solar neutrino oscillations was obtained at the Canadian Observatory in Sudbury (SNO).
This, to put it mildly, was a big event in cosmology. The mystery of the missing solar neutrinos was solved, or why in experiments about one third of the neutrinos flying from the Sun were observed, compared to the predictions of the Standard Model. If neutrinos can oscillate by changing their flavor, then neutrinos emitted by the core of the sun can already be of different types by the time they reach the Earth. Until the mid-80s, in most experiments on Earth, only electron neutrinos were searched for, which means that they missed two other flavors appearing along the way from the Sun to the Earth.
When the SNO was conceived in the 80s, it was designed so that it could detect all three types of neutrinos, and not just electronic ones. And this decision paid off. In 2015, the director of the experiments of Super-Kamiokande and SNO shared the Nobel Prizein physics for solving the riddle of missing solar neutrinos. Detector in Borexino Although the mystery of solar neutrinos has been solved, there is still much to be done in science to better understand them. Since 2007, the Borexino observatory at Gran Sasso has improved the measurement of solar neutrino oscillations, which gave physicists unprecedented information about nuclear fusion feeding the Sun. Outside, the observatory looks like a huge metal sphere, and inside - like technology coming from another planet. In the center of the sphere is, in fact, a huge transparent nylon bag with a diameter of 10 m and a thickness of half a millimeter. The bag contains liquid scintillator
, a chemical mixture that emits energy when neutrinos pass through it. This nylon sphere is suspended in a thousand tons of purified buffer fluid and surrounded by 2,200 sensors capable of detecting the energy emitted by electrons, which is released when neutrinos interact with a liquid scintillator. There is another buffer, consisting of 3000 tons of ultrapure water, providing additional protection for the detector. All this together provides the greatest protection of the observatory from ambient radiation among all the liquid scintillators in the world.
In the last decade of physics in Borexino - including D'Angelo, who joined the project in 2011 - use this unique observation devicefor low-energy solar neutrinos generated by proton collisions during nuclear fusion in the Sun’s core. Considering how difficult it is to detect these ultra-light particles that have no charge, which almost do not interact with matter, it would be almost impossible to detect low-energy solar neutrinos without such a sensitive machine. When SNO directly detected the first oscillations of solar neutrinos, he could only observe the most energetic solar neutrinos due to interference from background radiation. And it was only about 0.01%from the neutrinos emitted by the sun. Borexino's sensitivity allows him to observe solar neutrinos with energy, a whole order less than those found by SNO, which opens up the possibility of creating an incredibly refined model of solar processes and more exotic phenomena, like supernovae.
“It took physicists 40 years to understand solar neutrinos, and this was one of the most interesting mysteries of particle physics,” D'Angelo told me. “Something like what dark matter is now.”
Shedding light on dark matter
If neutrinos were a mysterious particle of the 20th century, then dark matter is a puzzle of our time. Just as Pauli proposed neutrino as a desperate means to explain why experiments seem to violate one of the most fundamental laws of nature, they suggested the existence of dark matter particles, since cosmological observations do not converge.
In the early 1930s, the American astronomer Fritz Zwicky studied the movements of several galaxies in the Volon Veronica cluster, a collection of more than 1000 galaxies located about 320 million light years from Earth. Using data published by Edwin Hubble , Zwicky calculated the mass of the entire galactic Volos cluster of Veronica. Having finished, he discovered something strange in the velocity dispersiongalaxies (statistical distribution of the velocity of a group of objects): the distribution of velocities is 12 times higher than the value calculated based on the amount of matter. In the Gran Sasso Laboratory This was an unexpected calculation and its importance did not escape Zwicky. "If this is confirmed, " he wrote , "we will get an amazing result, according to which dark matter will be much more than luminous." The idea that the Universe is mainly composed of invisible matter, at the time of Zwicky, seemed radical - it remains so today. However, the main difference is that today's astronomers have much more empirical evidence pointing to its existence. For the most part this can be attributed to Vera Rubin.
, an American astronomer, whose measurements of the rotation of galaxies in the 1960s and 70s eliminated all doubts about the existence of dark matter. Based on the measurements of Rubin and subsequent observations, physicists believe that dark matter makes up about 27% of the total matter of the Universe - about seven times more than the usual baryonic matter familiar to us. The main question - what does it consist of?
Since the moment of groundbreaking observations, Rubin has already proposed many candidates for the title of dark matter particles, but so far they all avoid detecting even the most sensitive instruments in the world. Partly because physicists are not quite sure what they are looking for. A small part of physicists generally believe that dark matter may not be particles, but may represent an exotic gravitational effect. This makes the development of experiments similar to searches in the parking lot at the stadium of a car, to which recently found keys fit. There is a chance that the car is in the parking lot, but you have to go around a lot of doors until you find it - if it is there at all.
Among the candidates for dark matter are subatomic particles with foolish names like axions , gravitino ,massive astrophysical compact halos (MACHO) and weakly interacting massive particles (WIMP). D'Angelo and his colleagues from Gran Sasso put on WIMP, which until recently were considered the leading candidates for dark energy.
However, over the past few years, physicists began to search for other possibilities, after some critical tests failed to confirm the existence of WIMP. WIMP is a class of hypothetical elementary particles that practically do not interact with ordinary baryonic matter and do not emit light, which makes them extremely difficult to detect. This problem is aggravated by the fact that no one is sure exactly how WIMP looks. Needless to say, it is very difficult to find something if you are not even sure what you are looking for.
So why do physicists think that WIMPs exist at all? In the 1970s, physicists conceived the Standard Model of particle physics, which claimed that everything in the Universe consisted of a small set of fundamental particles. The Standard Model perfectly explains almost everything that the Universe can give it, but it is still incomplete, since gravity does not enter there. In the 1980s, an SM expansion appeared under the name supersymmetry , according to which every fundamental particle of the CM should have a partner. These pairs are known as supersymmetric particles, and are used in theoretical explanations of various puzzles of SM physics, for example, the Higgs boson massand the existence of dark matter. Some of the most complex and expensive experiments of the world, for example, the Large Hadron Collider, were created in an attempt to open these supersymmetric partners, but so far their existence has not received any experimental evidence.
Many of the lightest particles proposed in the supersymmetric model are WIMP, and they have names such as gravitino, neytrino, and neutralino. Many physicists still consider the last of them to be the leading candidate for dark matter, and they think that it was formed in large quantities in the early Universe. The discovery of evidence for the presence of this ancient theoretical particle is the goal of many experiments with TM, including the one D'Angelo is working on at Gran Sasso.
D'Angelo told me that he became interested in dark matter a few years after he joined the Gran Sasso laboratory and began to contribute to the DarkSide experiment , which seemed to be a natural continuation of his work on solar neutrinos. DarkSide, in fact, is a huge tank filled with liquid argon, and equipped with incredibly sensitive sensors. If WIMPs exist, physicists believe that they will be able to detect them due to the ionization that appears due to their interaction with argon nuclei.
DarkSide has been going to Gran Sasso since 2013, and D'Angelo said that it will continue for several more years. However, he is now involved in another experiment with TM at Gran Sasso called SABER, which is also looking for direct evidence of the presence of TM particles based on light that appears when energy is released as a result of their collisions with sodium iodide crystals.
The SABER experiment device is specifically made similar to another experiment that has been going on at Gran Sasso since 1995, called DAMA. In 2003, the DAMA experiment began searching for seasonal fluctuations of dark matter particles, predicted in the 1980s as a consequence of the movement of the Earth and the Sun relative to the rest of the galaxy. The theory said that the relative speed of any dark matter particles found on Earth should reach a maximum in June and a minimum in December.
For almost 15 years, DAMA did registerseasonal fluctuations in the detectors that coincided with the theory and with the expected signature of TM particles. It looked like DAMA was the first experiment in the world to detect a particle of dark matter. But the problem was that DAMA could not completely exclude the possibility that the signature it found was related to some other seasonal fluctuation of the Earth, and not to changes in the flow of dark matter associated with the movement of the Earth around the Sun.
SABER should eliminate ambiguities in DAMA data. Once the hardware has eliminated all the shortcomings, the experiment at Gran Sasso will be half the SABER. The other half will be located in Australia, in the former gold mine. The presence of laboratories in the northern and southern hemisphere should help eliminate all the false positives associated with normal seasonal fluctuations. So far, the SABER detector is still in the state of a working prototype, and should begin observations in both hemispheres in the next few years.
The SABER experiment may also refute the best evidence available on the presence of TM particles found by physicists. But, as D'Angelo pointed out, this kind of disappointment is a fundamental part of science.
“I, of course, worry that we will not find any TM, and that we hunt for ghosts, but science is so arranged,” said D'Angelo. “Sometimes you spend several years looking for something, and then it doesn’t end up there, and you have to change the way you think about everything.”
For D'Angelo, probing the subatomic world through studies of neutrinos and dark matter in an Italian cave is his way of explicit communication with the Universe. “The finest elements of the Universe are associated with the most macroscopic phenomena, for example, with the expansion of the Universe,” said D'Angelo. “The infinitely small comes in contact with the infinitely large, and it seems to me amazing. The goal of physics, which I do - to go beyond the boundaries of human knowledge. "
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