Dark matter: latest news from the axion hunt at CERN

CERN does not chase dark matter particles only by colliding proton beams produced by the LHC in its giant detectors. It also has a sort of telescope allowing it to track down a particular type of dark matter that the Sun could produce or which could be found in the halo around our Galaxy. A new assessment of this hunt with what are called axions has just been published.

The standard cosmological modelstandard cosmological model explaining the formation of galaxies and the large structures that bring them together, and even more the cosmic radiationcosmic radiation from Big BangBig Bangstill largely requires, to be understood, the existence of new particles never before seen directly in laboratories on Earth.

There are several possible theories beyond the standard modelstandard model from physiquephysique known particles that naturally contain particles of black matterblack matterparticles that interact with normal matter only through the force of gravitationgravitation essentially, and absolutely not, or very little, with light. We once thought that the neutrinosneutrinos known could do the trick, because precisely neutral and sensitive at most to gravitation and weak nuclear forceweak nuclear forcebut they are both far too light and too few in number (much more than the photonsphotons fossil radiation) to play the expected role of dark matter particles.

Theorists had a lot of hope with the so-called supersymmetric theories that contained heavy particles that could ideally fit. The most credible variants of these theories have unfortunately been largely refuted by experiments carried out in CernCern with the LHC for a decade, except to give them characteristics in massemasseand in the ability to interact with the very particular normal matter, in a corner of the space of the possible parameters attributable to the particles of these theories.

After the supersymmetric theories, the theorists considered second very light particles and less exoticexotic which are called axionsaxions. These particles must in particular be produced in large numbers both by the furnace of the Big Bang and by that of the heart of the SoleilSoleiland it is for this reason that CERN has been conducting research for years with CastCast(telescopetelescope for CERN’s solar axions).

Particles converted into X-rays in magnetic fields

The instrument contains magnets superconductorssuperconductors cooled to low temperatures to produce a strong magnetic field. The axion theory predicts that, in such a field, part of the axions can be converted into X-raysX-rays (the reverse process is also possible) according to a variant of what is known in nuclear physics with the mesonsmesons pi, the pions, under the name of Primakoff effect.

Here too, the mass of the axions and their probability of converting into X photons in a magnetic field are unknown and it is therefore necessary to methodically explore the space of these parameters to exclude certain regions.

The members of the Cast collaboration played this game by pointing their instrument in the direction of the Sun, but as they explain in a recent article published in Nature Communicationsthey were able to modify the axion telescope to hunt directly those of the Big Bang which must be, if they exist, in an almost spherical halo of dark matter in which would bathe the Milky WayMilky Way.

A press release from CERN explains what the physicists did:For its new study, the Cast team installed a resonator, made up of four cavities, inside one of the two tubes of the experiment’s magnet, to constitute an axion detector which this time seeks axions in the dark matter “halo” of the Milky Way; this axion haloscope was named Cast-Capp.

In a strong magnetic field, such as that provided by the magnet in the Cast experiment, axions should turn into photons. An axion haloscope resonator is something of a radioradio that researchers can tune in order to find the frequencyfrequency of these photons from axions. However, since the frequency of the axions’ “radio set” is unknown, scientists must slowly scan a range of frequencies to try to identify the frequency of the signal emitted by the axions.».

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No signal that could betray the axions was recorded in the range explored for 4,124 hours, from September 12, 2019 to June 21, 2021, with frequencies between 4.774 and 5.434 GHz, which corresponds to axions whose mass is between 19.74 and 22.47 microelectronvolts. Recall that the mass of a proton is approximately one gigaelectron voltsgigaelectron volts.

If axions exist, at least a little more is known now about what they are NOT, and so the hunt will continue.

Dark matter to solve a riddle of strong nuclear forces

Finally, let us recall some additional explanations already given by Futura about the theory behind these hypothetical particles and the reasons for their introduction.

The standard model of elementary particleselementary particles predicts a very low value of the electric dipole moment of the’electronelectron (the equivalent of magnetic momentmagnetic moment of a bar magnet with two magnetic polesmagnetic poles but with two opposite electric charges), so small that it is not yet within the reach of the experiments intended to measure it. Some theories, beyond the standard model, on the other hand predict a larger value, and this is why the quest to measure the electric dipole moment of the electron is a possible research path to discover new physics.

Conversely, the standard model, more precisely the equationsequations of QCD, the theory of strong nuclear forces, allows a very high value for the dipole moment of the neutron, in contradiction with the experiments which attribute none to it. The most commonly accepted explanation today again involves new physics.

In 1977, Roberto Peccei and Helen Quinn hypothesized that the term in the Standard Model equations responsible for causing a moment dipolairemoment dipolaire for the neutronneutronwas eliminated by the existence of a new scalar field (a cousin of that of bosonboson of Brout-Englert-Higgs) as a factor in front of this term, because the value of this field after the Big Bang would have become zero. This term was also responsible for phenomena violating the CP symmetry in the context of the quantum chromodynamicsquantum chromodynamics which, again, were not observed experimentally. As these two false predictions of the standard model “tainted” it, the Nobel Prize in physics Frank Wilczek gave the name axion to the particle associated with the scalar field of Peccei and Quinn, in reference to a brand of washing powder.

Dark matter driven out by an Earth-Sun-supermassive black hole alignment

Article of Laurent SaccoLaurent Sacco published on December 22, 2016

CERN physicists hunt dark matter particles called axions other than with the LHCLHC taking advantage of an annual astronomical alignment of the Sun, Earth and Sagittarius A*, the supermassive hole at the center of our Milky Way. They even took the opportunity to look for other particles, related to thedark energydark energy.

While Ligo went back to hunting for the gravitational waves produced by the fusionfusion from stellar black holesstellar black holes or neutron star collisions, Cern’s “Axion Solar Telescope” (Cast) went hunting for dark matter and dark energy by taking advantage of an astronomical event that occurred on December 18 2016. On this date, the Earth, the Sun and the supermassive black holesupermassive black hole center of our Milky Way are aligned and we can then benefit from a prediction of the theory of the general relativitygeneral relativity d’EinsteinEinstein : the effect of gravitational lensgravitational lens.

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Among the most serious candidates for the title of particles of black matter figure the axion. In recent years, the existence of cousin particles, which can explain the nature of dark energy, has also been proposed by specialists in astroparticulesastroparticules. They called them chameleon particles. Axions and chameleons are likely to manifest via an equivalent of the Primakoff effect (see article below). These particles can be born from photons in a magnetic field and, conversely, disintegrate there giving light.

A gravitational lens that amplifies a billion times

Axions and chameleons, if they exist, could be produced in large quantities in the environment of supermassive black holes like Sagittarius A*at the center of our GalaxyGalaxy. Calculations show that our Sun, with its field of gravitygravitycould act as a magnifying glass, concentrating and amplifying a billion times the flow of axions or chameleons coming from this starstar compact. During such an alignment, an instrument like Cast should therefore detect them. But the measurement is not so easy because the Sun is already supposed to produce axions and it is precisely this flux that Cast must measure. It is therefore necessary to find a characteristic allowing to distinguish the two sources.

The detection logic remains pretty much the same. Cast is indeed a prototype of magnetsmagnets dipoles originally developed for the LHC. In its magnetic field, the hypothetical flux of axions or chameleons amplified by the Sun should produce a very characteristic X-ray flux too. The researchers also set out to try to detect the direct interaction of chameleon particles with matter. The undertaking is difficult because the forces exerted are very weak and this is why it was necessary to build a special detector, a sensorsensor of optomechanical force called KWISP (forKinetic Weakly Interacting Slim Particle detector).

The Cast experiment began analyzing the data collected on December 18. The first results, however, suggest that the researchers failed to observe new particles from the black holeblack hole. The team will now begin preparing for next year’s roster, which will also include the LuneLune.

According to Konstantin Zioutas, spokesperson for the experiment,“the experiment went very well and the discussions about the first online data have already started”. However, it seems that the first results are negative. Researchers are not discouraged, however, and during a next alignment in 2017, the Moon should also enter the dance.

Observing quasars across the Sun!

Article by Laurent Sacco published on 06/23/2007

Malcolm Fairbairn, un physicistphysicist of CERN, has just proposed to observe the light coming from the quasarsquasars… through the Sun! Neither he nor his Russian and German colleagues have lost their minds by publishing an article in which they study the possibility of detecting gamma raysgamma rays, very energetic, through the Sun with the help of the Fermi satellite. Our star could indeed be transparent for this type of radiation, if the hypothetical particle proposed by the Nobel Prize Franck Wilczekthe axion, is indeed part of the zoo of elementary particles.

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Recall that the axion is a very light particle, weakly coupling with matter, which was proposed in the 1970s to solve certain phenomenological problems in the theory of strong interactions, QCD. It is a good candidate for being an important component of dark matter, but its implications in the domains of astroparticles are also numerous. Scientists have been hunting for it for a long time already and our Sun itself should produce a significant amount of it.

When a magnetic field is strong enough, a photon with enough energy can be converted into an axion and vice versa. One idea proposed to produce and detect them is to send a ray laserlaser in an area with a strong magnetic field just in front of a murmur. Some of the photons would then change into axions which, due to their weak coupling with matter, will cross the wall without any problem to penetrate just after in a second region which also has an intense magnetic field. A reverse conversion process would occur and the laser would therefore illuminate an area behind the wall!

More generally, a laser beam passing through a magnetic field would see some of its characteristics altered. Thus, in the experience PVLASPVLAS in Italy, a change in the polarization of photons emitted by a laser source was detected. This experiment is in agreement with the predictions giving a certain coupling constant of the axion with the electromagnetic fieldelectromagnetic field. The problem is that, if we admit the existence of this particle with this coupling, we come into conflict with the data on dark matter. If the latter were really composed of this particle, it would couple with ordinary matter much more strongly than what is observed. Worse, the Cast axion telescope, from CERN, should have detected those coming from the Sun a long time ago!

The principle of the experiment is the same as that indicated above. Fifteen years ago, at Brookhaven, the physicist Pierre Sikivie had already proposed using the Primakoff effect of converting photons into axions, by diffusiondiffusion in the magnetic field of the solar plasma, in the opposite direction. In a terrestrial experiment using the magnetic field produced by a powerful electromagnet, X-ray photons would be produced from the solar axions.Nothing was observed by Fermi, in contradiction to the conversion rate predicted from the PVLAS data. So one of the experiments is probably wrong assuming axions are real.

One way to decide is then to observe the very energetic gamma photons coming from a quasar located 4 billionlight yearslight years of the Sun, 3C279. By penetrating inside the Sun, these photons will be partly converted into axions which will be able to pass through our star and be transformed back into gamma photons when it leaves. During the’occultationoccultation of this quasar by the Sun, some gamma photons, which the Sun rarely produces energetically, can then be detected by the satellite Fermi which will soon be launched.

Malcolm Fairbairn has already carried out this test using data concerning 3C279 recorded in 1991 by EGRET. Unfortunately, these are too poor to come to a conclusion one way or the other. The next alignment between Earth, Sun and 3C279 is scheduled for October 2007.

Fermi will still be on the ground, but the gamma-ray telescopes in orbit may be sensitive enough to come to a firm conclusion. Unless the ALPS experiment, from the DESY laboratory in Hamburg and designed to verify PVLAS, does not already settle the question during this summer!



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