Short-lived exotic: For years, physicists have been puzzling over the nature of an exotic particle made of quarks and antiquarks that was discovered in a number of particle collisions. Now, for the first time, it has been possible to detect this particle, christened X(3872), in quark-gluon plasma – the original state of the cosmos briefly generated in the accelerator. This now opens up the possibility of clarifying the structure and characteristics of this particle, as the researchers report.
Quarks are fundamental building blocks of matter – in matter, two or three quarks are connected to each other via gluons. But in the meantime, physicists have discovered a whole range of other exotic quark combinations in collisions in particle accelerators, including particles made up of four, five or even six quarks. For some of them, however, it is still completely unclear how the quarks are grouped in them and what their nature is.
Mystery of the X particle
One of these puzzle particles is X(3872), a particle that first appeared in electron-positron collisions in the Belle experiment in Japan in 2003. It was later also detected in proton collisions in the Large Hadron Collider (LHC) at the CERN research center. The problem, however, is that this X-particle was so rare and short-lived that its structure is still unclear. The particle mass of 3,872 megaelectronvolts and the decay behavior also allow several possibilities for the internal structure of X(3872).
It could be a tetraquark made up of two quarks and two antiquarks or a loosely bound “molecule” made up of a D0 meson and its antiparticle. A D0-Meson consists of a heavy charm quark and an anti-up quark. However, it is also conceivable that the X particle is a new, excited “charmonium” – a combination of a charm quark and an anti-charm quark.
Searching for clues in quark-gluon plasma
In order to find out more about the nature of the mysterious C particle, physicists from the CMS collaboration have now chosen a different approach: They have tried to produce and detect X(3872) even in high-energy collisions of lead nuclei in the LHC. So much energy is released during these collisions that for tiny fractions of a second a quark-gluon plasma is created inside the accelerator – the high-energy, super-liquid state of matter that prevailed immediately after the Big Bang.
“There are so many quarks and gluons present in these plasmas that this should increase the production of X-particles,” says Yen-Jie Lee of the Massachusetts Institute of Technology (MIT). “However, their detection was considered too difficult because too many other particles are created in this primordial quark soup.” To solve this problem, the research team trained an adaptive algorithm to recognize the decay signatures of X(3872) and left the program then go for an LHC dataset of around 13 billion lead collisions.
With success: the physicists managed to identify the traces of around 100 particles of type X(3872) among the billions of other particle signatures. They manifested themselves as measurable “bumps” in the mass curve of the particles produced in the lead collisions. “It’s almost unbelievable that we actually managed to fish out these 100 particles from such a large data set,” says Lee.
This is the first time that the mysterious X(3872) particle has been detected in a quark-gluon plasma. The significance of this evidence is 4.2 standard deviations, the team reports. “Now we want to use the quark-gluon plasma to find out more about the internal structure of the X particle,” says Lee. So far, however, the data are not sufficient to clearly delimit the structure of the X particle. They leave both variants – the tetraquark or a D0-Meson pair to.
“Just the Beginning”
“But that’s just the beginning,” Lee said. “Now we have shown that we can find the signal of X(3872). In the years to come we will then be able to obtain much more data and thus learn more about the inner structure of the particle.” This will also contribute to expanding our knowledge of the types of particles that existed in the early days of the universe. (Physical Review Letters, 2022; doi: 10.1103/PhysRevLett.128.032001)
Quelle: Massachusetts Institute of Technology (MIT)