X(3872): an exotic combination of quarks?

According to the Standard Model of particles, quarks are the smallest building blocks of matter. So far, only quark-antiquark pairs (mesons) and quark triplets (baryons) have been observed. However, over the last few decades, some not-yet-understood states have started to appear in the particle zoo. Their nature is still unclear but the LHCb experiment has now made a big step towards understanding one of them: the X(3872).


A proton-lead ion collision, as observed by the LHCb detector during the 2013 data-taking period.

The X(3872) has not yet made headlines, but its existence is an intriguing mystery that scientists have been trying to elucidate over the past ten years since the particle was first observed by the Belle experiment. So far, its inner nature has remained unknown because of theoretical difficulties in cataloguing it as a quark-antiquark state in the so-called "charmonium" spectrum (as charm quarks are involved) and because of the experimental difficulties involved in catching it.

Thanks to increased statistics, a sophisticated angular analysis of the 2011 data-set has been performed, and even in the quite crowded environment of the LHC forward region, the LHCb collaboration successfully and unambiguously determined with very high significance (8 sigma) the quantum numbers of the X(3872). “The mass value and the quantum numbers we find cannot be easily explained by a simple combination of a quark and an antiquark,” says Pierluigi Campana, LHCb Spokesperson. Indeed, the LHCb result favours a more exotic nature for the X(3872), such as a bound state of several quarks (tetra-quark or D-D* molecule).

The X(3872) is not the only particle whose nature is unclear. Other examples are well known but still under scrutiny. “We will continue to work on these exotic states using the full 2011-12 statistics,” concludes Campana. “With this result we have given a further handle to theorists who are working on new models to explain the exotic nature of the X(3872).”

The moment is approaching for this particle to take on a less obscure name.

by Antonella Del Rosso