As Futura explained a few months ago, this year Cern is celebrating its 70th anniversary of descent into the world of the infinitely small with the microscopesmicroscopes high-energy particle accelerators equipped with detectors. It is a question of probing the mysteries of forces and matter, particularly during the Big BangBig Bang.
Following the discovery of quarks almost 60 years ago, followed shortly after by that of electroweak theory and quantum chromodynamics, physicistsphysicists understood that about a millionth of a second after the Big Bang, the plasma of quarks and gluonsgluonssometimes called quagma, which partly composed the contents of theUniverseUniverse primordial, must have condensed into drops of liquidliquid hadronic, that is to say protonsprotons and neutronsneutrons with a bath of others hadronshadrons transient and very unstable like the mesonsmesons and hyperons.
The physicists of CernCern know how to recreate this event with collisions between ionsions heavy objects accelerated by the Large Hadron Collider (LHCLHC). The products of these collisions and the associated phenomena are studied using Alice, one of the large giant detectors of the LHC. For years, researchers have been hunting for forms of matter there. exoticexotic called “hypernuclei”, as well as their corresponding antinuclei, but which exist fleetingly once synthesized, which explains why we do not usually see this form of matter around us. They hope to better understand not only the foundations of hadronic matter, but also to find clues to explain the enigma of cosmological antimatter.
Alice studies quark and gluon plasma, a state of matter believed to have existed just after the Big Bang. © RTS, ARTE GEIE and Cern
The heaviest antihypernucleus known
We know in fact that according to the physiquephysique known high energies, as many particles of matter asantimatterantimatter twins must have existed during the Big Bang to end up annihilating each other by giving photonsphotons. Clearly, this is not the case and it seems that a new physics made it possible to produce a little more matter than antimatter, but which one?
The members of the Alice collaboration have posted online, on arXiv, an article accompanied by a press release from Cern to announce that they had succeeded in producing and uncovering new hypernuclei and antihypernuclei. But what is it exactly?
To find out, let’s go back a little. It was at the very beginning of the 1930s that Werner Heisenberg proposed the modern concept of the core of atomsatoms composed of protons and neutrons. We then only knew these particles and electronselectrons and their antiparticulesantiparticules during this decade, which saw Robert Oppenheimer and his students lay the foundations of the theory of neutron starsneutron stars and black holesblack holes.
However, we began to suspect the existence of neutrinosneutrinos and massive cousins of photons “gluing” together protons and neutrons in nuclei, namely Yukawa pions. It was only from the 1940s to the 1960s that a tidal wavetidal wave new subatomic particles will arise and lead to the modern image of matter with leptonsleptons and hadrons.
Hadrons, leptons, mésons, hypérons… and all that
Let us also remember that it was the Russian physicist Lev Okun who, in 1962, proposed calling hadrons the set of particles sensitive to the strong nuclear forces between protons and neutrons. From the Greek word hadroswhich means more or less wide and heavy, he opposed the name lepton, from the Greek λεπτός / leptos (“light”), small and light, used to describe electrons and neutrinos. This choice was logical since a proton, like a neutron, is almost 2,000 times heavier than an electron and considerably heavier than neutrinos.
The hadronic particles of massesmasses intermediate between those of electrons and protons were called mesons, another word which comes from Greek, here medium which means “the middle, the right measure”.
Hadronic particles as heavy as, or even heavier than, protons and neutrons are called baryonsbaryonsalways because of the Greek and in this case baryswhich means “heavy”.
The hadron zoo was explained during the 1960s and 1970s with the development and discovery of quark theory and it became clear that mesons were pairs of quarks and antiquarks, while baryons were triplets . Among these triplets, some were discovered in cosmic rayscosmic rays and they are heavier than protons and neutrons, which is why they were logically called hyperons.
The term was coined by French physicist Louis Leprince-Ringuet in 1953 and first announced at the cosmic ray conference in Bagnères-de-Bigorre. Remarkably, as early as 1952, the physicists Danysz and Pniewski also discovered in cosmic rays a class of nuclei called hypernuclei or hyperfragments. They had shown that these were nuclei in which a proton or a neutron had been replaced by a hyperon.
A Mendeleev table for hypernuclei
Hypernuclei and hyperons have been studied since then, and it is known that hyperons are unstable particles that contain at least one strange quarkstrange quarkbut no beautiful quark or charmed quarkcharmed quark. Created fleetingly in an accelerator, these particles should exist inside neutron stars, where a plasma of quarks and gluons can also exist.
During collisions carried out in the context of nuclear physics or particle physics, hyperons can also be created. Since they are sensitive to interactions fortesinteractions fortesthey can be incorporated into a nucleus to give hypernuclei, characterized by a number of electric charges Z, but also another quantum numberquantum number S, strangeness, carried by the strange quark present in the hyperon.
We had therefore detected exotic nuclei of this kind for a long time, extending in a certain way the Mendeleev’s paintingMendeleev’s painting.
In fact, as Futura explained in a previous article, on August 23, 2023, during the European Physical Society conference on high energy physics (EPS-HEP), the LHCb collaboration, using another giant detector with the LHC, announced its first observation of hypertritons and antihypertritons.
Hypertritons get their name from the fact that they are analogues of isotopesisotopes of thehydrogenhydrogen what are the nuclei of tritiumtritiumthat is to say with one proton and two neutrons. But in the present case, one of the neutrons or antineutrons is replaced by a hyperon or an antihyperon Λ.
Antihyperhelium-4 based on antilambda
We can now understand what the Alice collaboration announced at the LHC, that is to say the very first observation of antihyperhelium-4 nuclei, which is composed of two antiprotonsantiprotonsan antineutron and an antilambda. It is also the heaviest antimatter hypernucleus ever observed at the LHC.
The Cern press release explains that the discovery is based on “ crash data leadlead-lead taken in 2018 at an energy of 5.02 teraelectronvolts (TeV) for each pair of nucleonsnucleons in collision (protons and neutrons). Using a machine learning technique that outperforms conventional hypernucleus search techniques, Alice researchers examined the data to detect signals from hyperhydrogen-4 (composed of an antiproton, two antineutrons, and an antilambda) , hyperhelium-4 and their antimatter partners. Candidates for (anti)hyperhydrogen-4 were identified by searching for the core of (anti)héliumhélium-4 and the charged pion into which it decays, while candidates for (anti)hyperhelium-4 were identified via its decay into an (anti)helium-3 nucleus, an (anti)proton and a charged pawn ».
Cern finally explains that “ The researchers also determined the antiparticle/particle yield ratios for the two hypernuclei and found that they agreed with unity, taking into account experimental uncertainties. This match is consistent with Alice’s observations of equal production of matter and antimatter at LHC energies and adds to ongoing research into matter-antimatter imbalance in the Universe. ».
