• Figure 1: An illustration of proposed interpretations for how quarks fit together to form the X(3872) state.

  • Figure 2: The J/ψ π+π invariant-mass spectrum. The more prominent structure is from the well-known charmonium state ψ(2S). A strong signal of several thousands of X(3872) above the background is also observed (the inset shows a zoom).

  • Figure 3: The measured prompt X(3872) production rates as a function of transverse momentum (points). The lines indicate a theoretical prediction.

CMS measures production rates up to high transverse momenta

A decade after its unexpected discovery, the properties of the X(3872) exotic resonance are still under intense scrutiny. This state was first seen in 2003 by the Belle experiment and then quickly confirmed by BaBar, CDF and D0. It was observed to decay into a J/ψ and two charged pions immediately, suggesting that it could be a new, excited charmonium state, binding a charm quark with its own anti-quark. On the other hand, its mass, 3872 MeV, is very close to the sum of the masses of the D0 and anti-D0* mesons; decays to a D0 and an anti-D0* were observed, giving rise to two other explanations for what the mysterious X could be: a loosely-bound “molecule” of D0 and anti-D0* mesons, or a “tetra-quark” binding a di-quark and a di-antiquark (Figure 1). Measurements, most notably the recent ones by the LHCb experiment, led to the conclusion that the X(3872) has the quantum numbers 1++, making the charmonium-state hypothesis rather unlikely.

While Belle and BaBar only observe the X(3872) resonance in decays of B hadrons, at the Tevatron and LHC, we can also see the same resonance in “prompt” processes, where it is created directly through the strong interactions of the colliding quarks and gluons. Initial theoretical studies concluded that the measured prompt-production rate at the Tevatron was too large by orders of magnitude for the X(3872) to be a weakly bound charm-meson molecule. However, more detailed theoretical studies have since shown that re-scattering effects — additional interactions between the D mesons in the final state — could lead to significantly enhanced X(3872) production rates. Contributions from re-scattering could be significant if the relative momenta of the D mesons are small, and at large transverse momenta (pT, no contribution is expected. Therefore, measuring the pT-dependence of the X(3872) production rate could give insights about the validity of the “charm-meson molecule” hypothesis.

The CMS experiment studied a sample of several thousand X(3872) candidates decaying into a J/ψ and two charged pions (Figure 2), collected in 2011 using proton-proton collisions at a centre-of-mass energy of 7 TeV. From this X(3872) sample, CMS measured several of its properties with higher precision than ever before:

  • The X(3872) is more copiously produced through “prompt” processes and only 26% of the production rate is observed from decays of B hadrons.
  • A measurement of the mass spectrum of the pion pairs produced in X(3872) decays indicates that the decay into the two charged pions proceeds via an intermediate ρ0 state.
  • The “prompt” X(3872) production rates’ measurements are presented for the first time as a function of transverse momentum (pT), up to unprecedentedly high values of pT. Comparisons of the measured rate for X(3872) production with predictions, including re-scattering effects, show that the prediction is actually larger than the measured rate (Figure 3), but the dependence on transverse momentum is reasonably well modelled.

Whether the molecule hypothesis is valid or not remains an open issue, but the data collected will undoubtedly further our understanding of the mysterious X(3872) resonance by helping to improve theoretical models. With these results and others, such as the observation of the as-yet-unexplained J/ψφ peaks, CMS continues to make important contributions to clarifying the nature of the new and unpredicted exotic charm states.

— Submitted by Alessandra Fanfani and Andreas Meyer