Why are the constituents of the proton confined?
All of the phase transitions that we are able to observe so far on earth involve the electromagnetic interaction, for example, the melting of ice, the formation of steam, or the magnetization of metals. We are seeking to see, for the first time on earth, an entirely new form of phase transition, that involving the Strong Interaction or quantum chromodynamics (QCD) in which a gas of quarks and gluons called a “Quark-Gluon Plasma” (QGP) condenses to form protons, neutrons and other hadrons. This is a remarkable transition because we believe that it also is the transition which gives rise to the protons and neutrons that make up most of the matter we see here on earth. Another curious thing occurs as protons and neutrons condense out of the hot vacuum that makes up the QGP that is presumably created in the most violent collisions.
Shortly after the big bang the universe went through a period of fantastic expansion, dubbed inflation. What could cause such a cataclysmic event? Just as the explosive power of turning water into steam powered much of the industrial revolution, it is believed that a phase transition of some sort – no one is sure what – powered inflation.
Our group is part of the PHENIX collaboration, which is one of the two major detectors at RHIC. This detector is optimized for the study of electrons, muons, and photons. It also has an extensive capability of measuring hadronic signatures of the Quark-Gluon Plasma.
RHIC can also collide polarized protons in addition to heavy-ions. The UC Riverside spin and heavy-ion physics groups work together closely.
Where does the spin of the proton come from?
By some mysterious mechanism, having to do with the complex behavior of QCD, the gluons which have one unit of spin and quarks which have 1/2 a unit of spin, bind together to form something that has exactly 1/2 of a unit of "spin".
Spin is a property of particles as fundamental as charge and mass. The spin of the proton was first determined in 1927, yet we still do not know what makes up the spin of the proton. It was believed that the spin was carried by the quarks that make up the proton. However, experiments in the 1980’s led to the startling discovery that quarks contribute very little to the proton spin, setting off the “proton spin crisis”. It is now theorized that the spin is carried by gluons, which hold the proton together. Spin measurements have historically yielded surprising results and are a stringent test to theories as spin is an intrinsically relativistic and quantum mechanical aspect of particle interactions.
The spin physics program at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory offers the first chance to test this hypothesis experimentally. RHIC can collide polarized protons in addition to heavy-ions (see the seperate UC Riverside heavy-ion physics page). The UC Riverside spin and heavy-ion physics groups work together closely.
Our group is part of the PHENIX collaboration, which is one of the two major detectors at RHIC. This detector is originally optimized for heavy-ion collisions, but the UC Riverside group has helped improve its capabilities for he spin physics program by developing the triggers necessary to take advantage of the high luminosities available in p+p collisions at RHIC. Ken Barish is a convenor of the PHENIX spin physics working group.