Today, at a CERN seminar, the LHCb collaboration reported precise measurements of the properties of the mysterious particle χc1(3872), also known as the X(3872), using radiative decays χc1(3872)→ψ(2S)γ and χc1(3872)→J/ψγ.

Since its discovery 20 years ago physicists have been fascinated by this particle trying to understand if it is a conventional charmonium state composed of c and c quarks or if it is an exotic particle composed of four quarks. An exotic particle of this type could be a tightly bound tetraquark, a molecular state, a cc gluon hybrid state, a vector glueball or a mixture of different possibilities. A very interesting feature of this particle is that its mass is remarkably close to the sum of the masses of the D0 and D*0 mesons.

The particle was originally discovered in B+ meson decays, specifically the process B+→X(3872)K+, by the BELLE collaboration in 2003. Its existence was later confirmed by the CDF, D0 and BaBar experiments. LHCb first reported studies of the X(3872) in the data sample taken in 2010 (see 27 October 2010 news) and later unambiguously determined its quantum numbers to be 1++. The determination of the quantum number led the Particle Data Group to change the name of this particle to χc1(3872).

LHCb announced in 2020 precise measurements of the width (related to its lifetime) and mass of the particle. The results showed that the mass of χc1(3872) is just a little smaller than the sum of the masses of D0 and D*0 mesons. The measured difference is only 70±120keV, fulfilling a necessary condition to interpret the χc1(3872) as a quasi-bound D0D*0 molecule. At the same time, LHCb physicists also measured the so-called “low-energy scattering parameters” (scattering length and effective range), and the reported values led to very intense and hot theoretical discussions. The theoretical community was divided between those who interpreted the result as a clear evidence for a “compact” component in χc1(3872), and those who argued that it nicely supported the molecular approach. The scattering parameters were also measured by the Belle and BESIII collaboration using different approaches.

The ratio R of χc1(3872)→ψ(2S)γ to χc1(3872)→J/ψγ branching fractions have been proposed as a tool to study the nature of the χc1(3872) particle. There is a clear theoretical signature: if the ratio R is non-vanishing, it is an evidence for some compact component (charmonium or tetraquark) in χc1(3872), disfavouring the pure moleculare model. The experimental situation has ben controversial and the aim of the LHCb measurement is to resolve this controversy.

The χc1(3872) mesons were identified using the beauty-particle decays B+→χc1(3872)K+. The mass of radiative decays clustered around the B+ mass, as shown in the image above for the χc1(3872)→ψ(2S)γ decay, the observation of which has been announced today by LHCb.

The complete data samples from Run 1 and Run 2 were used in the analysis. The value of the ratio R from Run-1 data is 2.5±0.52+0.20-0.23±0.06 and from Run-2 1.49±0.23+0.13-0.12±0.03. The combined value is R=1.67±0.21±0.12±0.04.

Today’s LHCb Run-1 and Run-2 results (in red) are compared to the BaBar, Belle and BES III results as well as to previous LHCb results in the plot on the left. The coloured band corresponds to the average of today’s LHCb results.

The large measured value of the R ratios is generally inconsistent with the calculations based on the pure D0D*0 molecular hypothesis for the χc1(3872) particle. On the contrary, it agrees with a wide range of predictions based on other hypotheses of the χc1(3872) structure, including conventional cc charmonium, ccqq tetraquark, and molecules mixed with a substantial compact component. In short, today’s result provides a strong argument in favour of a compact component in the χc1(3872) structure.

The χc1(3872) particle continues to fascinate the particle physics community. Recent developments and future plans will be discussed by theoretical and experimental physicists during the June 27 workshop “LHCb meets Theory: Probing the nature of the X(3872) state using radiative decays“.

Read more in the LHCb seminar presentation and in the forthcoming paper.