The scientific collaboration that operates the LHCb experiment, located near Geneva in the European Particle Physics Laboratory (CERN) and of which Spanish scientists are part, has detected for the first time the phenomenon known as ‘CP violation’ in the disintegrations of the particle called meson D0.
The discovery was presented this Thursday at the annual conference of physicists in Moriond (France) and at a seminar at CERN, which has issued a statement indicating that this advance will become part of the textbooks of particle physics.
“The result is a milestone in the history of particle physics. Since the discovery of meson D more than 40 years ago it was suspected that CP violation also occurred in this system, but only now, using the data set collected by the experiment, the LHCb collaboration has been able to observe this effect”, points out CERN’s Director of Research and Computing, Eckhard Elsen.
The term CP refers to the transformation that changes a particle with the specular image of its antiparticle. Weak interaction, one of the four fundamental forces of nature described in the standard particle physics model, produces a difference in the behavior of some particles and their antimatter equivalents, a difference known as a CP violation.
The effect was first observed in the 1960s at the Brookhaven laboratory (USA) on particles known as K mesons, which contain a strange quark. In 2001, the SLAC (USA) and KEK (Japan) laboratories also observed the phenomenon in neutral B mesons, which contain a bottom quark. These findings were worth two Nobel Prizes in Physics in 1980 and 2008, respectively.
The CP symmetry transformation changes a particle with the specular image of its antiparticle. The LHCb collaboration has observed the decomposition of this symmetry into the disintegrations of meson D (illustrated by the large sphere on the right) and its antimatter counterpart, anti-D (large sphere on the left), into other particles (smaller spheres). The degree of decomposition was deduced from the difference in the number of disintegrations in each case (vertical bars, for illustration only). (Photo: CERN)
The CP violation is an essential characteristic in our universe, necessary to provoke the process that, after the Big Bang, established the abundance of matter over antimatter that we observe today. The type of CP violation observed so far in standard model interactions, however, is too small to account for the current imbalance between matter and antimatter, suggesting the existence of undiscovered sources of CP violation.
Meson D is made of a quark charm and an antiquark up. Until now, the CP violation had been observed only in particles containing strange or bottom quarks. The findings carried out by the LHCb experiment have confirmed the pattern described in the standard model by the so-called ‘Cabibbo-Kobayashi-Maskawa Mixing Matrix (CKM)’, which describes how the different types of quarks, the ‘bricks’ that make up visible matter, transform into each other through weak interaction.
The deep origin of the CKM matrix, and the search for additional sources and new manifestations of CP violation, are among the great issues of particle physics. The discovery of CP violation in mesons D is the first evidence of this asymmetry in quark charm, adding new elements to the exploration of these issues.
To observe this CP asymmetry, the researchers of the LHCb experiment used the complete set of data provided by the large hadron collider (LHC) delivered to the LHCb experiment between 2011 and 2018 to analyze the disintegrations of meson D and its antiparticle in kaones or pions.
“Searching for these two disintegration products in our unprecedented sample of D particles allowed us the sensitivity required to measure the tiny amount of CP violation expected for this type of phenomena. The magnitude of the effect is obtained by the difference between the disintegrations of particles D and anti-D,” explains Giovanni Passaleva, spokesman for the LHCb collaboration.
The result has a statistical significance of 5.3 standard deviations, which exceeds the threshold of 5 standard deviations used in particle physics to proclaim a discovery. This measure will stimulate new theoretical work to evaluate its impact on the description of the CKM matrix of the CP violation in the standard model, and will open new avenues to look for possible new sources of CP violation using ‘enchanted’ particles (with quarks charm).