DR ROSS YOUNG
Particle prediction
with precision
Particle physicists use the Standard Model to
help explain the interactions of the basic building
blocks of matter. Here, Dr Ross Young talks
about his efforts to improve the model for a
clearer picture of the universe
electromagnetism, the weak nuclear
force and the strong nuclear force. While
we are yet to have a complete quantum
understanding of gravity, the latter three
forces are understood to be governed by a
quantum field theory whose interactions
are dictated by the principle of local
gauge invariance – the particular internal
orientation of a system at a given point in
space cannot instantaneously affect any
distant points in space. This builds on the
familiar idea from Einstein’s relativity that no
information can propagate faster than the
speed of light.
How did your academic career guide you to
your current research endeavours?
I completed my PhD at the University of Adelaide
in 2004, working on theoretical aspects of
the strong interaction. This was followed by
postdoctoral appointments at the Jefferson
Lab and Argonne National Laboratory in the
US. This was a fantastic opportunity to develop
a broad perspective on the leading-edge
challenges within the field and I was strongly
influenced by the chance to engage closely with
experimental programmes.
I enjoy working on projects where the
experimental and theoretical advances
complement one another to improve our
understanding of the Universe. One of my present
research activities is focused on the precision
tests of the Standard Model and the search for
new physics.
What are the fundamental forces that describe
the interactions of matter? How is particle
physics seeking to understand these forces?
There are currently understood to be four
fundamental forces of nature: gravity,
The Standard Model of particle physics
describes the elementary field content of
the quantum theory, together with the
corresponding symmetry properties of each
field. One ongoing topic among the diverse
array of particle physics research is testing
to what degree nature obeys the prescription
dictated by the Standard Model.
The strong force is the least well tested
component of the Standard Model and is
governed by the fundamental theory of
quantum chromodynamics (QCD). Can
you discuss QCD?
The strong force of nature arises out of one
of the fundamental symmetries encoded
within the Standard Model, referred to as
quantum chromodynamics. As a gauge
theory, electromagnetism is encoded within
the theory of quantum electrodynamics
(QED) which describes the interactions of
charged particles with the photons acting
as the force carrier. QCD is a similar theory
which describes the interactions of quarks.
Quarks can carry one of three types of
charge, conventionally labelled by colour.
The interactions of these colour charges are
mediated by the corresponding force carriers,
called gluons. Unlike the electromagnetic
case, where the photon is neutrally charged,
gluons also carry a colour charge and hence
gluons will interact directly with other gluons.
It is this distinguishing difference between
electromagnetism and QCD that makes
theoretical calculations in QCD significantly
more challenging.
How are you and your collaborators
aiming to increase the precision of
theoretical predictions of QCD?
One of the most powerful tools for
computing the observable properties of
QCD has been through large scale numerical
simulations called lattice QCD. A technical
limitation of modern simulations of lattice
QCD is that most neglect the influence of
electromagnetism, meaning the current level
of agreement with experiment is typically
limited to about 2 per cent. While neglecting
the influence of electromagnetism is a very
good first approximation, we generally
expect these effects to contribute at the
level of about 1 per cent. With colleagues in
Australia, the UK, Germany and Japan, we
are developing new lattice simulations that
include the effects of electromagnetism which
will enable a new level of precision in the
strong force sector of the Standard Model.
What are your future research goals?
I continue to be enthused by the prospect
of future precision studies of QCD enabling
new tests of the Standard Model. The main
concern is to identify key observables that
can be reliably constrained by theory, with
corresponding experimental measurements
at a precision that is sensitive to the
influence of new forces. Away from the
precision frontier, simulations in lattice
QCD are just now making advances to
study the properties of short-lived particles
and thereby resolve, for instance, the
excitation spectrum of the proton. As the
analogue of doing atomic spectroscopy,
the pursuit of the spectrum of QCD poses a
fascinating challenge.
WWW.RESEARCHMEDIA.EU
37
DR ROSS YOUNG
Supercomputing the
weak and the strange
Australian researchers at the University of Adelaide are
collaborating in large international efforts to push the frontiers of
new physics and improve our understanding of the quantum world
ENCAPSULATING THE ELECTROMAGNETIC
and the strong and weak nuclear forces governing
the dynamics of subatomic particles, the Standard
Model serves particle physicists extremely well
despite its inability to properly account for gravity.
Attempts to explain phenomena that do not fit
into existing theories have led to intense searches
beyond the Standard Model to form a field aptly
termed new physics. At the cutting edge of these
investigations into the dynamics of subatomic
particles are two complementary strategies; the
construction of particle accelerators, an activity
now famous the world over due to the European
Organisation for Nuclear Research’s (CERN’s)
Large Hadron Collider (LHC) proving the existence
of the Higgs boson in 2012, and the decidedly
less ostentatious activity of performing precision
measurements at more modest energies.
Whereas the former are used to directly produce
new particles, deviations from the predictions
made by precision measurements are potential
signposts for forces as yet unidentified in nature.
Currently holding an Australian Research Council
(ARC) Future Fellowship at the University of
Adelaide, Dr Ross Young is involved in several
international collaborations probing the dark
corners of the Standard Model’s least tested
component: the strong nuclear force. A fuller
understanding of quantum chromodynamics
(QCD), the theory that aims to explain the strong
nuclear force, is essential for resolving a more
fundamental picture of the Standard Model as it
underpins the interactions between the building
blocks of all atomic nuclei. The tangible world is
38INTERNATIONAL INNOVATION
made up of molecules which consist of atoms.
Atoms are in turn made up of electrons, neutrons
and protons whose elementary foundations are
subatomic particles called quarks and gluons, the
carriers of the strong nuclear force. Governed by
this force, the interactions between quarks and
gluons are integral to reaching a more complete
understanding of the theory of QCD.
STRANGE BEHAVIOUR
Resolving the theory of the strong nuclear force
is no simple task. The large international effort
in which Young is involved attempts to compute
the theory’s observable properties through a
technique called lattice QCD, the only known
way to study QCD with systematically controlled
approximations. By putting space time on a
4D lattice this technique allows Young to run
simulations that solve quantum field equations
numerically. It’s an incredibly complex process
made all the more difficult by the gluons’ ability to
interact with each other as well as quarks. Aided
by the supercomputers available at the National
Computational Infrastructure (NCI) in Australia
and further resources in Europe and Japan these
simulations take some serious number crunching.
Despite the huge advances contributing to the
increased agreement between the theory and
the experiments in QCD, the 1 to 2 per cent level
so far achieved is dwarfed by the agreement
to 10 significant figures in certain quantum
electrodynamics (QED) tests. By including the
effects of electromagnetism, a factor usually
left out of the simulations, Young aims to
substantially improve the level of agreement
of QCD calculations and help compute the
dynamical QED effects associated with the
vacuum of QCD.
Even in the absence of QED effects, in order to
help paint a clearer picture of the QCD vacuum
Young examines strange quarks, an ephemeral
species of quark typically born of high-energy
particle collisions, as are top, bottom and charm
quarks. The much lighter up and down quarks that
make up the last of the six types are the essential
ingredients of most atomic nuclei, yet less than
1 per cent of the mass of a proton, for instance,
is attributable to the mass of these quarks. The
majority of their mass is actually produced by
the gluon and quark interactions that underpin
the strong nuclear force. It is for this reason that
Numerical simulation of the energy density fluctuations of the
QCD vacuum. Professor DB Leinweber, University of Adelaide.
What are quarks?
INTELLIGENCE
• The building blocks of protons, neutrons and
all atomic nuclei
INTERPLAY OF THE FORCES OF
NATURE: ELECTROWEAK AND
STRONG INTERACTION
•
Quarks are categorised in terms of six
species – up, down, strange, charm, bottom
and top
OBJECTIVES
• To improve theoretical techniques to
determine the predictions of quantum
chromodynamics (QCD) and hence improve
knowledge of the fundamental origins of
the strong nuclear force
•O
nly the up and down are necessary to build
protons and neutrons, with the ensemble of
heavy partners typically only produced in
particle accelerators
• To pursue key precision observables which
could reveal the signature of new physics
• The lightest of the heavy partners is the
strange quark and hence it is expected to
play the most significant role in the structure
of the proton
Artist impression of quarks making up a proton in the QCD
vacuum. The white line depicts an electron bouncing off
a strange quark by exchange of a photon. Professor DB
Leinweber, Univeristy of Adelaide.
Young’s investigations look beyond up and down
quarks to the properties of their strange relatives.
These theoretical investigations have studied
various aspects of the strange quark’s role in
nucleon structure, including the contribution to
the nucleon mass.
POLARISED PERFORMANCE
With no known experimental technique to
determine the strange quark component of
the nucleon mass, an inventive approach
by the teams at Massachusetts Institute
of Technology (MIT)-Bates, USA, Thomas
Jefferson National Accelerator Facility
(TJNAF), USA, and Mainz Microtron (MAMI),
Germany, have studied another perspective of
strange quarks in proton structure. By studying
the weak nuclear force between electrons and
protons, these experiments have measured
the strange quark charge distribution in the
proton. For a general scattering event between
an electron and a proton the influence of the
weak interaction is essentially invisible. Only
through the sensitivity to parity of the weak
force can this interaction be isolated.
The experiments are configured such that the
incident electron beam has a rapidly changing
polarisation that sees the spin of the electrons
alternated from left-hand to right-hand at up
to a thousand times a second.
Electromagnetism’s insensitivity to parity
means it interacts with both spin configurations
in the same manner, whereas the weak
KEY COLLABORATORS
Professor Anthony Thomas, University of
Adelaide, Australia
force has a subtle preference for scattering
left-hand electrons rather than right-hand
ones. “With high-precision instrumentation
and large statistical counts, this left–right
asymmetry has been measured at about one
part in a million,” reports Young. Taking over
15 years to arrive at their results, the teams
have reported that in contrast to suggestions
from early predictions, the strange quark has
little effect on the size, shape or magnetic
strength of a proton. This small influence
of strange quarks is in excellent agreement
with numerical calculations in lattice QCD by
Young and his collaborators.
MODEL REFINEMENT
With the strange quarks now resolved, these
results have provided the physicists with the
first ever chance to directly measure how
strongly a proton interacts via the weak
force and test it against the predictions of
the Standard Model. This is the goal of the
Q-weak experiment at TJNAF, which collected
data between 2011-12. Originally proposed in
2002, only 4 per cent of the vast store of data
produced by Q-weak has yet been analysed,
though the full set may be completed by early
2015. As it stands, the value yielded for the
proton’s weak charge is in good agreement
with the prediction of the Standard Model and
though much analysis remains, the results have
already provided constraints for investigations
being carried out at the LHC. Indeed a class
of new particles at the teraelectronvolt (TeV)
energy scale has already been ruled out.
Q-weak Collaboration, Thomas Jefferson
National Accelerator Facility, Virginia, USA
FUNDING
Australian Research Council (ARC)
CONTACT
Dr Ross Young
ARC Future Fellow
CSSM and CoEPP
School of Chemistry and Physics
University of Adelaide
South Australia, 5005
Australia
T +61 8 8313 3542
E [email protected]
www.adelaide.edu.au/directory/ross.young
ROSS YOUNG is an ARC Future Fellow;
Chief Investigator at the ARC’s Centre
of Excellence for Particle Physics at the
Terascale (CoEPP); and a faculty member
at the University of Adelaide since 2010.
Previously he held postdoctoral positions at
the Thomas Jefferson National Accelerator
Facility (TJNAF) and Argonne National
Laboratory in the US. Young completed his
PhD in theoretical physics at the University
of Adelaide in 2004.
It is an exciting time for new physics. The
results of the Q-weak experiment are expected
to provide a means of rigorously testing the
Standard Model, the tool through which we
attempt to explain the most fundamental
aspects of the universe. With the analysis of
the full dataset anticipated by 2015, Young is
enthusiastic about the potential implications
for new physics and beyond: “There is scope
for the increased precision to measure
the signature of a new force of nature, or
if agreement with the Standard Model is
maintained, place ever more stringent limits
of the types of new particles”.
WWW.RESEARCHMEDIA.EU
39