The facts: 1 - Dark matter
A number of experimental observations of the cosmos have clarified in the past few decades that the Universe is much more massive than the shining of its stars would imply. Matter in the Universe resides in galaxies made primarily of billions of stars. Stars radiate out energy due to the fusion processes at their interior. More matter produces more fusion, and a part of the resulting energy can be detected as visible light. We can thus have a hunch of the amount of matter that exists in a galaxy by measuring the light its stars emit.
However, through detailed measurements of the rotation velocity of stars around the center of the galaxy they belong to, we know that galaxies are six times more massive than we can compute from light emission. We are led to conclude that there is a large amount of matter that does not shine: dark matter is the name we unimaginatively gave to it. What is this dark matter made of? Thousands of particle physicists and astrophysicists are attacking this question by performing experiments at colliders, detectors underground, and astronomical measurements.
There are literally dozens of ideas of what dark matter is made of. The most widely accepted model implies that it is made of massive, neutral, stable, weakly interacting particles. These may have been produced in the Big Bang and still hang around today, gravitationally lingering around galaxies. As the Earth travels with our solar system around the center of the galaxy, we may thus be exposed to a wind of these dark matter particles. They do not do much to us, as they are weakly interacting; but occasionally, they might produce collisions with ordinary matter, which we may be able to detect in very sensitive devices located deep underground, where the noise from the rain of cosmic rays is tamed by kilometers of rock.
2 - Dama-Libra
Dama-Libra is a small experiment located in the underground laboratories of Gran Sasso, in central Italy. It is just one of a large number of underground detectors that passively wait for particles to interact in their sensitive volume. However, there are a number of different technologies deployed to detect the tiny signal of a dark matter interaction, and Dama-Libra has its own strong points. In addition, it has collected a large amount of data by operating for over a decade now. I wrote about this experiment in detail here a few months ago.
Unlike all other experiments, Dama-Libra has consistently produced a signal which may be due to the interaction of dark matter particles in the detector. The observed effect is systematical in nature: the statistical precision of the effect is extremely high. What is observed is an annual modulation in the rate of detected interactions: just what one would expect if there were a wind of particles that crossed our planet at higher speed when we travel against it, and at lower speed six months later (when, due to the rotation of the Earth around the Sun, our planet's speed with respect to the center of the galaxy is slower). The graph below shows the detected interactions as a function of time in the detector. The effect cannot be questioned, but is it really due to dark matter?
The problem with the Dama-Libra signal is that, if interpreted as originated by the most en-vogue kinds of dark matter particles, it cannot be true: it would have been seen by other experiments with different technologies and even higher sensitivity to it. So, unfortunately for the Dama-Libra researchers, no Nobel prize is in sight yet. However, while a number of proposed mundane explanations have been put forth to explain the annual modulation effect, none of them is convincing: the signal may not be due to the dark matter we thought of, but it cannot be due to anything else either! This is therefore a quite nagging unsolved riddle.
3 - Quark Nuggets
No, this is not properly a fact - Axion Quark Nuggets (AQN) have not been proven to exist. Yet the article by Zhitnitsky I quoted above brings this hypothetical things to the fore, as the originators of the Dama-Libra effect. Let us see what they are.
The accepted paradigm for how our Universe was created is that all matter was generated in a Big Bang, some 14 billion years ago. From a soup of incredibly hot plasma expanding at the speed of light, matter emerged in the form we know by organizing itself into baryons (the protons and neutrons that make up atomic nuclei), electrons, photons. Light atoms were created in the cooling off of the plasma; heavier elements were instead synthetized much later, in the core of stars.
One problem with the above picture is that we are not sure we understand how a universe made of baryons can be originated out of pure energy: all the reactions we know which create a baryon also create its anti-matter counterpart, so either the Universe contains an equal amount of anti-baryons, or there has been a baryon number violating reaction which produced this asymmetry. We tend to believe that the latter is the case, as we do not observe large amounts of antimatter around us.
A model which may explain the asymmetry posits that along with baryons, the Big Bang created quark nuggets made of matter and ones made of antimatter, in a non equal amount. Quark nuggets contain large numbers of nuclear matter, and so may have huge baryonic number. In such a model the total number of baryons minus antibaryons in the Universe is zero, but ordinary matter is made only of baryons, while the excess antibaryons reside in quark nuggets.
Quark Nuggets and Dama-Libra
If quark nuggets exist, they could in principle explain the dark matter puzzle. What's more, they would be hard to detect with dark matter search experiments, because, being so heavy, there is the need to hypothesize a much smaller number of these things around in order to balance the observed matter / dark matter ratio in galaxies. But what would quark nuggets do to Dama-Libra?
The article explains that a wind of quark nuggets would produce a path of annihilations with ordinary matter as these bodies cross our Earth. These annihilations would in turn produce neutrinos, and the neutrinos would be able to produce neutron spallation from heavy nuclei. There would be, as a result, a flux of neutrons that would have an annual modulation with the same characteristics expected for dark matter interaction in our detectors. These neutrons, according to the calculations in the paper, might be all what is needed to explain the observed Dama-Libra oscillation in the count rate. Dama-Libra might be observing a dark matter signal after all, but it would be an indirect signal due to the complex chain of reactions originated by the annihilation path of quark nuggets!
A number of objections might arise in the head of the most informed among us. One is, why does Dama detect this and other detectors do not? The energy of the involved reactions might explain this neatly. In fact, the underlying theory fixes the maximum energy of neutrinos from the annihilation processes. This sharp cutoff would both make the Dama-Libra neutron-induced signal reside below 6 keV (where it is indeed observed), and make the neutrino flux a stealth one, undetectable by existing large underground neutrino telescopes. Other objections are discussed in detail in the article, but I imagine this level of detail is unnecessary here. It is interesting to note that the author claims quark nuggets could explain also other small anomalies observed in astrophysical observations, and that the model appears to be compatible with all existing measurements. In addition, the model is easily testable!
In summary, the paper offers a fresh new look at two unsolved mysteries: the dark matter puzzle, and the Dama-Libra anomaly. Given the importance of this topic, the hypothesis of quark nuggets cannot be brushed off and requires experimental investigation... And if this interpretation turned out to be true, the theory would deserve not one, but two Nobel prizes!
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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”. You can get a copy of the book on Amazon.
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