ALICE finds that charm hadronization differs


This text is based on a news by the European research organisation CERN

New measurements by the ALICE collaboration show that the way charm quarks form hadrons in proton-proton collisions differs significantly from expectations based on electron collider measurements. The ALICE research department at GSI was substantially involved in the measurement and evaluation of the results.

Quarks are among the elementary particles of the Standard Model of Particle Physics. Besides up and down quarks, which are the basic building blocks of ordinary matter in the Universe, four other quark flavors exist and are also abundantly produced in collisions at particle accelerators like the CERN Large Hadron Collider. Quarks are not observed in isolation due to a fundamental aspect of the strong interaction, known as color charge confinement. Confinement requires particles that carry the charge of the strong interaction, called color, to form states that are color-neutral. This in turn forces quarks to undergo a process of hadronization, i.e. to form hadrons, which are composite particles mostly made of a quark and an antiquark (mesons) or of three quarks (baryons). The only exception is the heaviest quark, the top, which decays before it has time to hadronize.

At particle accelerators, quarks with a large mass, such as the charm quark, are produced only in the initial interactions between the colliding particles. Depending on the type of beam used, these can be electron-positron, electron-proton or proton-proton collisions (as at the LHC). The subsequent hadronization of charm quarks into mesons (D0, D+, Ds) or baryons (Λc, Ξc, …) occurs on a long space-time scale and was considered to be universal - that is, independent of the species of the colliding particles - until the recent findings by the ALICE collaboration.

The large data samples collected during Run 2 of the LHC allowed ALICE to count the vast majority of charm quarks produced in the proton-proton collisions by reconstructing the decays of all charm meson species and of the most abundant charm baryons (Λc and Ξc). The charm quarks were found to form baryons almost 40% of the time, which is four times more often than what was expected based on measurements previously made at colliders with electron beams (e+e- and ep in the figure below).

“Coordinated by Dr. Andrea Dubla, our local ALICE group at GSI has produced and published many of these results. This also involved the use of a software framework for decay reconstruction developed for the FAIR experiment for studies of compressed nuclear matter CBM and now shared between CBM and ALICE,” explains Professor Silvia Masciocchi, head of the ALICE department at GSI. “The study of heavy quarks is one of the main focuses of our ALICE research at GSI, and we are very pleased that our long-standing efforts have now contributed to such impressive results. Our research also profits strongly from the postdoctoral research fellowship program HGF-GSI-OCPC, which allows us to attract and recruit excellent researchers from universities in China. This opens up exciting perspectives for the future.”

The measurements show that the process of color-charge confinement and hadron formation is still a poorly understood aspect of the strong interaction. Current theoretical explanations of baryon enhancement include the combination of multiple quarks produced in proton-proton collisions and new mechanisms in the neutralisation of the color charge. Additional measurements during the next run of the LHC will allow these theories to be scrutinized and further our knowledge of the strong interaction. (CERN/CP)

Further information