The LHCb probes the properties of one of the strangest particles in physics

χ_c1(3872) is an intriguing particle. It was first discovered over 20 years ago in B⁺ mazoni is broken by the collaboration BELLE, KEK, Japan. Since then, the LHCb collaboration reported it in 2010 and measured some of its properties. But here’s the catch—physicists still don’t know what it’s actually made of.

In the quark model of particle physics, there are baryons (consisting of three quarks), mesons (consisting of a quark-antiquark pair), and exotic particles (consisting of an unusual number of quarks). To find out what χ_c1(3872) consists of, physicists must measure its properties, such as its mass or quantum number.

Theories suggest that χ_c1(3872) could be a conventional carmonium state, composed of charm and anticharm quarks, or an exotic particle composed of four quarks. An exotic particle of this type can be a tightly bound tetraquark, a molecular state, a cc-gluon hybrid state, a vector glueball, or a mixture of various possibilities.

Previously, the LHCb collaboration found its quantum number to be 1⁺⁺ and, in 2020, made precise measurements of the particle’s width (lifetime) and mass. The collaboration also measured what are known as its low-energy scattering parameters. The results showed that its mass is slightly less than the sum of the masses of D⁰ and D^*0 mesons.

After these results, the theoretical community was divided. Some argued that χ_c1(3872) was a molecular state composed of spatially separated D⁰ and D^*0 mesons. This molecular state would be much larger than typical particle size and more comparable to a heavy nucleus.

However, this argument runs into a problem, namely that physicists expect molecular objects to be crushed in hadron-hadron collisions, and χ_c1(3872) is abundantly produced. Other theorists interpreted the results as clear evidence that χ_c1(3872) has a “compact” component. This means that it is a much smaller particle, containing either a tightly bound carmonium or a tetraquark.

A way to help determine what χ_c1(3872) contains is to calculate the ratio between the decay probabilities into different lighter particles (branched fractions).

By comparing the rate at which it decays to either an excited carmonium state or a carmonium state and a photon, physicists can glean clues about what kind of particle it is. There is a clear theoretical signature: if the ratio does not vanish, it is evidence for some compact components in χ._c1(3872), disfavoring the pure molecular model.

Now, using the full LHC Run 1 and Run 2 data set, the LHCb collaboration has found that these ratios do not vanish, with significance exceeding six standard deviations. The letter is available at arXiv preprint server.

The large measured value of the ratios is inconsistent with expectations based on pure D⁰D^*0 molecular hypothesis for χ_c1(3872) particle.

Instead, it supports a wide range of predictions based on other hypotheses of χ_c1(3872) structure, including conventional (compact) carmonium, a compact tetraquark containing a charm quark, charm antiquark, light quark, and light antiquark, or a mixture of molecules with an essential component of the compact core. In short, the result provides a strong argument in favor of χ_c1(3872) structure containing a compact component.

X_c1(3872) particle continues to fascinate the particle physics community.