Physicists have been intrigued by χc1(3872), also known as X(3872), since its discovery two decades ago. They have been exploring whether it is a conventional charmonium state composed of two quarks or an exotic particle made up of four quarks. The LHCb collaboration at CERN’s Large Hadron Collider (LHC) set out to find the answer.
Artist's impression of a tetraquark, made up of two charm quarks and an up and down antiquark. Image courtesy of CERN.
In the quark model of particle physics, there are heavy particles (composed of three quarks), mesons (consisting of quark-antiquark pairs), and exotic particles (comprising an unusual number of quarks).
To determine the composition of χc1(3872), physicists must measure properties like mass and quantum numbers.
According to theory, χc1(3872) could be a standard charmonium state made of a charm quark and an anticharm quark, or it could be an exotic particle consisting of four quarks.
These exotic particles could be tightly bound tetraquarks, molecular states, cc-gluon hybrid states, vector glueballs, or a combination of various possibilities.
Recent measurements by LHCb physicists revealed that its quantum number is 1++, and in 2020 they obtained precise data on the particle’s width (lifetime) and mass.
They also examined low-energy scattering parameters.
Their findings indicated that the mass of χc1(3872) is slightly less than the combined masses of the D0 and D*0 mesons.
These results have sparked debate within the theoretical community, with some proposing that χc1(3872) is a molecular state made up of spatially separated D0 and D*0 mesons.
However, this hypothesis faces challenges, as physicists anticipate molecular matter to be suppressed in hadron-hadron collisions, yet significant amounts of χc1(3872) are produced.
Other theorists suggest that the particle contains “compact” components, indicating a smaller size and potentially consisting of tightly bound charmonium or tetraquarks.
One method to uncover the composition of χc1(3872) is to calculate the branching ratio, which involves the probabilities of decay into different lighter particles.
By comparing the decay into a photon of the excited charmonium state, physicists can gain insights into the nature of the particle.
A key theoretical indicator is a non-zero ratio, suggesting the presence of compact components and countering a purely molecular model.
Using data from LHC Run 1 and Run 2, LHCb scientists found significant ratios beyond six standard deviations, ruling out a pure D0D*0 molecular hypothesis for χc1(3872).
Instead, the results support various predictions based on alternative hypotheses for the structure of χc1(3872, such as a mix of conventional (compact) charmonium, tetraquarks, light quarks, or molecules with a substantial compact core element.
Thus, the findings provide compelling evidence in favor of a χc1(3872) structure including a compact component.
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R. Aiji others (LHCb Collaboration). 2024. Probing the properties of the χc1(3872) state using radiative decay. arXiv: 2406.17006
This article is based on the original release from CERN.
Source: www.sci.news
