FOCUS particle physics IMAGE COURTESY OF BERKELEY LABS ►The color charge acts such all matter must be locally white, binding quarks into combinations that preserve this colorlessness and preventing the sustained existence of free quarks. IMAGE COURTESY OF GETTY IMAGES ►This image, taken near RHIC at Brookhaven Laboratory depicts a subset of the STAR collaboration. Any angular momentum in the system will be passed on to the particles it generates, whose spins are aligned with the whirling structures from which they came. On average, those spins will indicate the net angular momentum created by the system. As Yale’s Professor Caines, spokesperson for the collaboration described, “when you collide them just a little bit edge on edge you set it spinning, and that’s basically what we measured, how fast it’s spinning collectively. We can detect that angular momentum.” The experiment used two instruments for this measurement. The first of these was the Time Project Chamber, which is filled with gas that surrounds the collision and allows scientists to track the particles released from the collision and therefore generated by the QGP. This detector measures protons created by the plasma. The second instrument, the Beam-Beam Counters, measures deflections in the paths of particles which whiz by one another and indicates the magnitude and direction of angular momentum in each collision. Correlating this measured angular momentum with aligned direction of the detected protons, STAR scientists searched for preference in direction that will indicate the vorticity of the system. This measurement indicates that the QGP is the most vortical fluid known, a measured value surprising in its magnitude. The vorticity of the QGP exceeded all predicted values – whirling faster than tornados, Jupiter’s red spot, and previous vorticity record-holder superfluid helium. This high vorticity is allowed to perpetuate by the low viscosity of the fluid – since high resistance to flow would dampen whirls prior to measurement. Vorticity is a property of the flow – it’s not intrinsic to the medium, but results from the medium and its motion. But since we know the conditions that produced the flow inside of the collider, it’s possible to gain useful information about the fluid properties. Knowing the specifics of the collision that generated it allows us to firm up our theories of how the QGP interacts and responds to forces. Changes in temperature and pressure induce phase transitions. Think briefly of ice: increasing the temperature will turn it to water, then to vapor. At 4 trillion degrees Celcius, the quark gluon plasma is very hot, which is why the strong mutual interactions of its constituent particles is puzzling. The QGP behaves more like a fluid than like a gas. Instead of expanding out evenly in all directions, it’s preferential in its flow. Is it a relic of the strong force, which governs interactions between its constituent parts? Or perhaps the extreme pressure of the cosmic soup? In future experiments, the STAR collaboration hopes to deduce the equation of state for the quark gluon plasma to resolve these questions and test theories about the strong force. This vorticity measurement will aid in improving theories of the plasma and how it affected the properties of the early universe. Most importantly, spinning charges produce magnetic fields, making this measurement an important first step in making the first measurements of the magnetic properties of the QGP, an important step in better understanding the weird physics surrounding this hot, flowy, whirling soup. Although the significance of such fundamental science isn’t always immediately clear, Yale’s Li Yi, a scientist with the collaboration sees this as exciting infinite possibility. “Maybe it sounds like science fiction, but in all of the modern world most of the technology is based on QED (quantum electro dynamics), our theory of the electromagnetic force. For example, we transfer the messages and communications through electromagnetic waves - this is all through the QED because we really understand what we can do. QCD we don’t know exactly how it works - or what we could use it for.” The STAR Collaboration’s measurements of the QGP are among the most important measurements for formulating and testing this theory. ABOUT THE AUTHOR SOPHIA SANCHEZ-MAES SOPHIA SANCHEZ-MAES is a junior Physics and Astrophysics double major in Timothy Dwight college. She works with Professor Jun Korenaga on studying the origin processes of plate tectonics, and topics in complex systems. THE AUTHOR WOULD LIKE TO THANK Professor Helen Caines and Dr. Li Yi for their incredible knowledge and willingness to share it. FURTHER READING The Star Collaboration. (2017). Global Λ hyperon polarization in nuclear collisions. Nature. Retrieved October 1, 2017. 14 Yale Scientific Magazine October 2017 www.yalescientific.org
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