A state of matter called a quark–gluon plasma is produced in energetic collisions of heavy ions. The rotation of this plasma has been measured for the first time, providing insights into the physics of the strong nuclear force. See Letter p.62
The hottest and densest matter ever produced in a laboratory is formed in collisions of heavy ions travelling close to the speed of light. Such collisions take place at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, New York. A deeper understanding of the properties of this matter would reveal details about the evolution of the Universe shortly after the Big Bang — a period when space was filled with a plasma of elementary particles called quarks and gluons. On page 62, the STAR Collaboration1 at RHIC report the first measurement of the rotation of the quark–gluon plasma produced in heavy-ion collisions, providing crucial information for theoretical models that try to explain how such a plasma is formed.
Matter comes in different phases, regardless of whether it is ordinary matter such as water or, in the case of this study, nuclear matter. The types of transition between phases are summarized in a graph called a phase diagram. With regards to nuclear matter, physicists are interested in the transition between the quark–gluon-plasma phase and the phase in which the quarks and gluons are confined in hadrons (particles including protons and neutrons that interact through the strong nuclear force). In particular, they would like to know whether the transition is first-order — characterized by an abrupt change in thermodynamic properties.
One reason for this interest is that the transition from quark–gluon plasma to hadrons in the early Universe is also the process by which most of the mass in the Universe was generated. To address such fundamental questions about the phase diagram of strongly interacting matter, dedicated facilities and experimental programmes exist or are being planned2. These include the RHIC Beam Energy Scan programme and, in the future, experiments at the Nuclotron-based Ion Collider Facility (NICA) in Dubna, Russia, and the Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany.
At RHIC, the collisions between atomic nuclei rarely happen head-on. As a consequence, the resulting quark–gluon plasma rotates, with an angular momentum that is perpendicular to the plane of the collision (Fig. 1). The plasma's rotation is quantified by its vorticity — its tendency to rotate at a particular point. Water going down a drain acts in the same way. Although this feature of the quark–gluon plasma had been predicted theoretically3, it had not been observed4 until now.
The vorticity of the quark–gluon plasma cannot be detected directly. Instead, experimentalists must analyse the polarization of particles produced in the nuclear collisions. A particle is polarized if its internal angular momentum (spin) is aligned with the angular momentum of the plasma. Although some of the hadrons that are emitted from heavy-ion collisions have a spin that lines up with the vorticity of the plasma, this spin is difficult to detect. However, one type of hadron — the Λ hyperon — is self-analysing, meaning that one of its decay products (a proton) has a momentum that is preferentially in the direction of the hadron's spin.
The STAR Collaboration therefore focused on the emission of Λ hyperons from energetic heavy-ion collisions at RHIC, and measured the polarization of the hyperons' decay products. Using these measurements, the authors determined that the produced quark–gluon plasma rotates about 1022 times per second. When compared to other systems in nature, the plasma is the fastest-spinning fluid ever observed.
The authors' measurement is useful in at least two ways. First, to draw any conclusions from the observation of newly produced particles in the explosive process of a heavy-ion collision, an understanding of the dynamic evolution of the system is essential. The new result provides key information to advance theoretical models of the quark–gluon plasma5,6,7. Only by improving these models is it possible to learn more about the phase diagram of nuclear matter and, in particular, discover whether there is a first-order phase transition.
Second, the spins of Λ hyperons (and their antiparticles) can be anti-aligned (or aligned) with a magnetic field, if one is present. This is the mechanism that is used in MRI (magnetic resonance imaging) technology. In heavy-ion collisions, the charged nuclei can be regarded as a fast electric current, and therefore the build-up of an extremely large magnetic field is predicted8. This magnetic field and the possible observation of related phenomena (such as the chiral magnetic effect or the chiral vortical effect) are of great interest9. This is because they can be used to probe the fundamental topological properties of quantum chromodynamics — the quantum field theory that describes the behaviour and interactions of nuclear matter. The authors' measurement of polarized Λ hyperons provides an interesting prospect for constraining the magnitude of the magnetic field in heavy-ion collisions, once the precision of the measurement is improved to quantify the difference in spin alignment between Λ hyperons and their antiparticles.Footnote 1
The STAR Collaboration. Nature 548, 62–65 (2017).
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