Gluons as a source of left-right asymmetries

Researchers from Groningen (RUG) and Amsterdam (VU) nationally collaborating within Nikhef have put forward a new explanation for the differences between left and right that arise in collisions between protons whenever one of the protons has its rotation direction (spin) orthogonal to the direction of motion. In a paper published this week (online) in Physical Review Letters, they link these differences and previously proposed explanations to a single correlation in the strong gluonic fields inside the proton. Such a unifying picture can be used as a new viewpoint for understanding the structure of spin-polarized protons and relating this to the outcomes of experiments that investigate that structure.

More than 25 years ago it became apparent that protons that are spinning around a direction orthogonal to their direction of motion lead to large left-right asymmetries observed in the distribution of particles produced in collisions involving spin-polarized protons, even at high collision energies. It has posed a formidable challenge to theoretically explain the origin of this asymmetry. A large variety of mechanisms has been put forward over the years, but in the new publication in Physical Review Letters it is demonstrated that these mechanisms all have the same origin in high-energy collisions where the force particles, the gluons, inside protons dominate. This description can be tested in the coming years in experiments at Brookhaven National Laboratory in the United States and also possibly in future experiments at CERN or a new Electron-Ion Collider for which there are plans in the United States.

This research has been carried out by Daniël Boer (Van Swinderen Institute, University of Groningen) in collaboration with Miguel Garcia Echevarria, Jian Zhou and Piet Mulders (Nikhef and Vrije Universiteit Amsterdam)

Contact: Prof.dr. Daniël Boer, (050) 363 36 56

Reference: ‘Single Spin Asymmetries from a single Wilson loop’ , by Daniël Boer, Miguel G. Echevarria, Piet J. Mulders, Jian Zhou

Online version of Physical Review Letters of March 21, 2016