YSM Issue 90.4

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