Particles of matter attract each other

Experiments at the Jülich accelerator COZY could explain the effect when neutrons and protons collide

WASA detector

Jülich - An international collaboration with the particle accelerator COZY at Forschungszentrum Jülich has indirectly detected an exotic particle made up of six quarks. The emergence of the "multiquark hadron" could also be the reason for the so-called ABC effect, which has already occurred in other particle collisions and the cause of which has not been clarified for around 50 years.

Hadrons are particles that are composed of quarks and antiquarks. The standard model of particle physics distinguishes between mesons, which consist of a quark and an antiquark, and baryons, which include protons and neutrons and which are made up of three quarks. Some physicists predict the existence of other, more complex particles that the Standard Model would allow: exotic hadrons or multiquark hadrons made up of six quarks.

An international collaboration of more than 120 scientists from six countries has now found new evidence of such a particle with the Swedish WASA detector at the COZY particle accelerator. They made protons and neutrons collide; when a proton and a neutron fuse, a deuterium nucleus is created, the nucleus of heavy hydrogen. The decay products of the reaction are photons that are registered by the detector. In some of the experiments, the researchers received an unusually large number of decay products. They assume that a multiquark hadron was created during the collisions for a duration of one hundred trillionth of a second - i.e. 10 to the power of 23 seconds. In this short period of time, light can only cross a small atomic nucleus. Such extremely short-lived particles are also known as resonances.

"In our experiments, we were able to investigate the reaction for the first time over the entire energy range with an unprecedented level of precision," says Hans Ströher from the Institute for Nuclear Physics at Forschungszentrum Jülich. With the particularly uniform proton beam from the COZY accelerator and the five-meter-long WASA detector, the researchers achieved a very high level of accuracy in their measurements. Only in this way could they draw conclusions about the properties of the resonances, which could no longer be explained by other theories than the origin of the multiquark hadron.

As early as 1960, scientists had observed an unusually high rate of decay products when light atomic nuclei collided. For this "ABC effect" - named after the physicists Alexander Abashian, Norman E. Booth and Kenneth M. Crowe - there has so far been no conclusive explanation. In the years to come, the researchers in Jülich want to test whether the resonance they have observed also occurs in the event of collisions with elastic collisions in which no fusion reaction takes place.