LHCb: The differences between antimatter and matter are becoming clearer

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We exist. Why, if just after the Big Bang, equal numbers of matter and antimatter particles were created, which should immediately turn into energy in the processes of annihilation? Some subtle differences between the matter and antimatter must have accounted for the survival of the matter that made up our world. We have known some of them for years. Now an international team of physicists, working as part of the LHCb experiment at the European nuclear facility CERN, has made another building block to this complex puzzle, seemingly small but of great importance.

The recipe for obtaining antimatter is theoretically very simple. We take a particle of ordinary matter, change its charge to the opposite, mirror it – done! Thus, it can be said that particles and antiparticles are symmetrical in relation to the combination of charge change (C) and mirror image (P). At the same time, we know that our universe began with the Big Bang. If we understand well what then and a little later happened, equal numbers of particles and antiparticles must have arisen from the gigantic energy densities after the Big Bang. In contact with each other, they should annihilate and they did annihilate, turning into electromagnetic radiation. However, for some as yet unclear reason, the balance sheet of this scenario is wrong: after these violent processes of creation and annihilation, more matter is left than antimatter. It is matter that mainly exists in today’s universe, we and everything we know is made of it. Antimatter, antiparticles, are quite rare.

Scientists believe that the explanation for this mystery – the mystery of our existence – is a minimal „imperfection” in the symmetry between matter and antimatter, causing some particle processes to run slightly differently from those involving their counterparts with opposite sign and parity. They call it CP symmetry breaking, and they try to observe the elementary processes by which this asymmetry could be seen, examined closely, and as a result, know its nature. Mesons are an attractive object for such research.

Mesons are unstable particles, but live long enough for their properties to be studied. The first mesons (pions) were discovered in the first half of the 20th century in cosmic rays, and their existence Was theoretically postulated earlier. Today we know that they are composed of a pair of a quark and an antiquark, and since we have 6 types of quarks (physicists talk about six smells, which has absolutely nothing to do with the olfactory sensations of everyday life!), there are really many combinations leading to different mesons. Of course, every meson also has its antiparticle, in which the relevant quarks and antiquarks are turned into their antiparticles.

The breaking of CP symmetry in the decays of neutral kaons, i. e. mesons with a quark and an upper and a strange antiquark, Was observed over half a century ago and Was awarded the Nobel Prize. Twenty years ago, symmetry breaking Was also noticed among mesons containing a beautiful quark. The mystery Was, among others, the mesons with the charm quark, which in terms of mass is between the strange and the beautiful quark.

„Until now, we have not been able to measure directly and with the appropriate precision how the breaking of CP symmetry manifests itself in the decays of charm mesons. The results of the analyzes presented at the recently completed ICHEP conference of high energy physics in Bologna perfectly fill this gap”, says Prof. dr hab. Wojciech Wiślicki from the National Center for Nuclear Research (NCBJ).

The latest results are the result of detailed analyzes of about fifty million cases with the decay of D0 charm mesons into positively and negatively charged kaons. These decays were recorded over several years during proton collisions in the LHCb detector operating at the Large Hadron Collider at the CERN facility near Geneva. Part of the calculations related to data processing were carried out in the Świerk IT Center.

„In fact, breaking the space-charge symmetry in the decays of charm mesons Was first noticed in the LHCb experiment three years ago” – explains PhD. Artur Ukleja (NCBJ). „Contrary to the current measurement, the measurement at that time did not suggest so clearly possible different matter-antimatter asymmetries between the decays of charm mesons into kaon-antikkaon pairs and decays into plumb-antipion pairs”.

The latest results of experiments in the LHCb detector significantly complement our understanding of the difference between matter and antimatter. The Standard Model imposes restrictions on breaking the symmetry between matter and antimatter. If too many such phenomena were observed, the result would be inconsistent with the predictions of the Standard Model, signaling the existence of a new physics.

„The data presented in Bologna show that breaking the CP symmetry rarely occurs in the case of charm mesons” – concludes Prof. Wiślicki. „This is exactly what the theorists expected. Thus, the Standard Model, used to describe elementary particles and their interactions, has once again proved its strength. And this is extremely intriguing because we know for sure that he does not describe reality completely."

Unfortunately, the discrepancy between the amount of matter observed in the universe and the predictions of our cosmological models remains apparent even with the most recent results. Apparently, we still don’t know something very significant about antimatter. More clues to unravel this mystery may be seen in the next stage of particle collisions in the LHC accelerator, which has just begun.

We also recommend information in English:

on the LHCb experiment page –

https: //lhcb-outreach. web. cern. ch/2022/07/13/the-first-evidence-for-cp-v …

in the CERNCOURIER magazine –

https: //cerncourier. com/a/lhcb-digs-deeper-in-cp-violating-charm-decays/


Our matter consists mainly of protons and neutrons bound in atomic nuclei that are surrounded by electrons. Unlike electrons, protons and neutrons are not elementary particles, but extremely dynamic clumps of quark triplets: a proton of two up and one down quarks, a neutron of two down and one up quarks. In the Standard Model describing the structure of matter and its interactions, there are three families of quarks. In order of increasing masses, they are: upper and lower, strange and alluring, and beautiful and real. The electrons together with their associated electron neutrinos form one of the three families of leptons. The muons and muon neutrinos are more massive, the most massive – tau and tau neutrinos. All matter particles have antimatter counterparts in the Standard Model.

Grafika pokazująca w sposób artystyczny symetrię CP