YSM Issue 90.4

particle physics
FOCUS
distance, it gets increasingly strong, like a
spring that wishes to retract. Eventually, so
much energy is expended in pulling apart
this particle that another particle antiparticle
pair is pulled from the vacuum to preserve
net colorlessness. This means that it’s
impossible to detect free quarks. This might
be puzzling, since quarks in the QGP aren’t
bound in mesons or nucleons, but zooming
in anywhere in this soup, conditions are “locally”
colorless. Since scientists can’t detect
the quarks individually, the STAR collaboration
scientists instead detect the particles
created from the QGP as it cools, whose motion
is inherited from this fluid that formed
them.
It’s not “just a phase”
not surprising that the medium breaks other
records. These record-setting properties
include a low viscosity, a high temperature,
and a now legendary vorticity, or swirling
properties. These measurements not only
help us understand this unexpected liquid
phase, but also assist in our understanding of
quantum chromodynamics (QCD), a theory
which governs the interactions of the colored
charged particles (quarks and gluons)
which make up the QGP.
The force is strong with this one
Quarks are never observed in alone, but
are always bundled in pairs or groups of
three by gluons, which carry the strong
force. There are six types of quark, with
up and down quarks combining in groups
of three to create protons and neutrons
– the heaviest components of atoms. The
theory governing their interactions is
called quantum chromodynamics (QCD).
Quarks come in three states called ‘colors.’
The strong force, which governs the behavior
of quarks, is incredibly unusual.
Gravity, the force with which we’re most
familiar, only attracts objects toward each
www.yalescientific.org
IMAGE COURTESY OF WIKIPEDIA`
►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.
other. Electromagnetism has positive and
negative (anti-positive) charges – but like
gravity, their effect fades with the inverse
square of the distance. Quarks and gluons
have color-charge, which has nothing
to do with color but for convenient
metaphor. There are three colors (red,
green, and blue) and three anti-colors
(anti-red, anti-green, anti-blue) Any stable
state must be colorless – which can
only be achieved by mixing all the colors,
like in protons and neutrons (made up of
three quarks) – or by combining a color
and anti-color, like in particles called mesons
(which consist of a quark and an anti-quark).
Furthermore, mass and charge
are fundamental properties of particles,
whereas color charge can change via gluon
interactions. It is the strong force, and
the color charge that mediates it, that is
responsible for the interactions between
quarks and gluons. By understanding
the behavior of the QGP, scientists are
probing these fundamental forces and
properties.
Try pulling a meson apart and you’ll see
that the strong force operates differently
in another important way: with increasing
The QGP is a fluid, and is defined by superlatives.
At four trillion degrees Celsius,
tens of thousands of degrees hotter than a
supernova, it’s the hottest thing we know
in the universe. It also has the lowest viscosity,
or resistance to flow, of any known
fluid. Now, the STAR collaboration has also
made the first measurement of its vorticity,
or rotation.
When gold ions collide head-on, their
constituent particles combine to create the
QGP; but typically the collision between
ions is more glancing. In that case, region
of contact, like the center of a Venn diagram,
will combine to become QGP, while
the outside regions will continue speeding
away from each other. These glancing collisions
give the resulting soup a net angular
momentum, setting the QGP spinning.
Scientists in the STAR collaboration
aimed to detect this spin by detecting the
particles generated by the plasma as it cools.
October 2017
IMAGE COURTESY OF WIKIPEDIA
►The quark gluon plasma kicked off the start
of the radiation era of the universe’s history
from 10e-12 to 10e-6 seconds after the big
bang, right after cosmic inflation.
Yale Scientific Magazine
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