Special Research Centre for the Subatomic Structure of Matter

The Special Research Centre for the Subatomic Structure of Matter (CSSM) carries out research in theoretical nuclear and particle physics.

Our aim is to make advances in our understanding of the structure of subatomic matter and establish a sound and consistent theoretical framework. We integrate theory and experiment to allow a variety of attacks on the problem so that progress can be made more rapidly than by a set of isolated projects.

The centre has an emphasis on the strong interactions and their importance in determining the nature of matter. The strong interactions are described by the quantum field theory - quantum chromodynamics (QCD) - which governs the interactions between quarks and gluons.

We are working to clarify further the behaviour of the microscopic world through a variety of complementary techniques, including:

  • numerical simulations of fundamental quantum theories (such as QCD);
  • effective field theory calculations; and
  • building models that capture the essential degrees of freedom in complex systems (such as the quark-meson coupling model).

Research strengths  People / Publications  Study with us 

 

Visualisations of theoretical physics

Empty space is not empty... it's filled with the quark and gluon fields of Quantum Chromodynamics (QCD), the quantum field theory of the strong interactions governing the structure of subatomic matter.

Explore more visualisations of quantum chromodynamics on our YouTube channel.

CSSM news

Research funding to boost Australia's capacity in nuclear theory

New research project funding for CSSM's Professor Tony Thomas

CSSM seminar history

A summary of seminars hosted by CSSM and CoEPP.

Scientists explore mine over matter

Scientists are placing giant crystals in an old gold mine more than 1km underground, in order to find elusive dark-matter particles.

more...

Our research

CSSM is at the forefront of advances in our understanding of the structure of hadronic matter. Hadronic matter makes up almost the entire mass of the tangible universe, from the protons and neutrons that are within the nuclei inside atoms and molecules, to neutron stars.

Unravelling the structure and interactions of the strongly interacting particles (known generically as hadrons) is one of the remaining great challenges in physics.

Research strengths

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  • Measuring the weak charge of the proton

    Measuring the weak charge of the proton

    With the announcement of the detection of the Higgs boson in 2012, observation of the full suite of particles predicted by the Standard Model finally was complete. However, despite that success, the Standard Model does have its limitations. Various phenomena, including gravity, dark matter, dark energy, and the matter/anti-matter asymmetry, remain unexplained by the Standard Model. Thus, the search for Beyond-the-Standard-Model physics (BSM) continues.

    One search method concerns the production of new particles by high-energy accelerators; any such particles, by definition not predicted by the Standard Model, will be indicative of the presence of BSM physics. This is a direct method of exploration which, thus far, has not resulted in any new particles.

    Another approach is to use some indirect method - look for something which, whilst based on observations of SM particles or other observable phenomena, allows one to make predictions relevant to BSM physics.

    The reduced asymmetry 𝐴𝐞𝐩/𝐴0=𝑄𝐩𝐰+𝑄2𝐵(𝑄2,𝜽=0) versus Q

    The reduced asymmetry𝐴𝐞𝐩/𝐴0=𝑄𝐩𝐰+𝑄2𝐵(𝑄2,𝜽=0) versus Q2
    Full caption

    As part of the Jefferson Lab Qweak collaboration, the CSSM's Associate Professor Ross Young has contributed to the first successful high-precision measurement of the weak charge of the proton. The proton's weak charge determines how strongly a proton will interact with other particles via the electro-weak force, one of the four fundamental forces within the Standard Model framework.

    The very precise experimental value obtained for the proton's weak charge allows researchers to place very strong constraints on certain types of interactions that are not described by the Standard Model, thus providing a predictive capability within the context of as-yet unobserved BSM phenomena, particularly towards higher particle masses.

    The work is described fully in Nature, and in a more condensed manner on the Faculty of Sciences news blog.

  • Novel constraint on dark matter hypothesis

    Novel constraint on dark matter hypothesis

    For some years there has been a 3.5 standard deviation discrepancy between two methods of measuring the lifetime of the neutron which, in free space, is a little over 14 minutes. In an exciting paper earlier this year, Fornal and Grinstein proposed that the explanation might lie with dark matter, with only one of the methods of measurement sensitive to a hidden decay mode where a neutron decays to dark matter plus something.

    Nuclear constraints mean that the dark matter mass would have to be within roughly one percent of the mass of the neutron. This proposal was particularly timely, as it was followed closely by an article in Nature suggesting the need for a dark matter particle with a mass below twice the neutron mass.

    CSSM - Implications of neutron star properties for the existence of light dark matter

    Comparison of the mass versus radius curves resulting from the solution of the TOV equations for the case where only nucleons are allowed and where dark matter is included. The reduction in the maximum possible mass is dramatic, from around 2.2 M to just 0.7 M. Read the article

    A collaboration involving staff of CSSM (Theo Motta and Anthony Thomas) and the CEA in France (Pierre Guichon) realised that such a particle may well have consequences for the properties of neutron stars, as neutrons high in the Fermi sea in the dense matter at the core of a neutron star would decay to dark matter particles with much lower energy and pressure.

    Indeed, their calculations showed that (as illustrated in the figure below), for weakly interacting dark matter, this decay process would dramatically reduce the maximum mass of a neutron star, to just 0.7 solar masses, which is way below the range of masses of observed neutron stars.

    As a consequence one is led to reject the hypothesis that the neutron has this hidden decay mode.

  • Coulomb sum rule

    Coulomb sum rule

    For more than 80 years, the atomic nucleus has been viewed as a weakly-bound collection of protons and neutrons.

    However, with the discovery that the proton and neutron are themselves composed of more fundamental particles known as quarks, scientists have wondered whether their quark structure might change when the protons and neutrons are located inside a nucleus as compared with when they are unbound.

    CSSM - Coulomb sum rule

    This figure illustrates the Coulomb sum rule for nuclear matter with a density consistent with that of 12C (carbon) and 208Pb (lead).

    It illustrates the role of relativistic effects but most importantly, at large three-momentum transfer (larger |q| values), a reduction of up to 20% in C(|q|) (red solid curve compared with the green dash-dot) resulting from the in-medium modification of the proton electric form factor.

    It has been suggested that such a change might be crucial in understanding nuclear binding. As well as exploring the consequences of this idea for the properties of atomic nuclei, it is vital to search for experimental tests.

    Bentz, Cloët and Thomas have predicted a remarkable signal of this anticipated change in structure. If confirmed by an experiment performed at the Thomas Jefferson National Accelerator facility in Virginia (USA), which is expected to announce results soon, this would herald a new paradigm for the structure of atomic nuclei.

  • Baryon's innards have molecular structure

    Baryon's innards have molecular structure

    In its excited state, the Lambda baryon behaves like a molecule, according to new lattice QCD simulations of the particle’s magnetic structure carried out at the CSSM.

    Graduates of Particle Physics 101 know that baryons are made of three quarks. An excited state of the Lambda baryon, the Λ(1405) appears to defy this simple description: the particle behaves like a 'molecule' made of a quark pair in the form of a meson and a quark triplet in the form of a baryon. This picture, which can’t be explained by the standard quark model, has been debated for almost fifty years. Now the molecular picture has been resolved via first principles calculations by theorists at the University of Adelaide.

    What’s puzzling about the Lambda baryon - a bound state of an up, down, and strange quark - is that much less energy is needed to excite it than is expected for three bound quarks.

    The molecular picture explains the difference by assuming that the excitation produces two compound particles: an antikaon (composed of a strange quark and an anti-up or anti-down quark) and a nucleon (a proton or neutron). This system of five quarks is more strongly bound than three bare quarks.

    In the past, theoretical calculations haven’t been able to convincingly predict the Λ(1405)’s internal structure because they approximated, via models, the quantum field theory that describes quarks.

    Lattice QCD Evidence that the Λ(1405) Resonance is an Antikaon-Nucleon Molecule

    Professor Derek Leinweber and his colleagues tackled the problem with lattice quantum chromodynamics (QCD), which uses supercomputers to simulate quark theory. The authors calculated the strange quark’s contribution to the Λ(1405) magnetic moment and found it was zero - the value expected if the strange quark is bound in an antikaon which, in turn, is bound to a nucleon.

    The finding is a strong sign that the structure of the Λ(1405) is dominated by an antikaon-nucleon molecular structure. In other configurations, the authors emphasise that the strange quark would have a non-zero magnetic moment.

    More information


    Image: A visualisation of the structure of the Lambda 1405 baryon resonance

    The energy density of a typical gluon field fluctuation found in the QCD vacuum is illustrated. Its lumpy structure, representative of the underlying instanton-like field configurations, attract the quarks of the Lambda 1405.

    Here the up, down and strange (u, d and s) valence quarks of the Lambda 1405 are complemented by an up-anti-up (u − u-bar) quark pair. A flux tube between the s and anti-u quarks indicates the formation of a K - meson while the remaining quarks (u, u, d) are bound to a proton with the Y-shape flux tube. Together, these form the molecular bound state of the Lambda 1405.

    An electron (solid line) scatters off the strange quark through the exchange of a photon (wavy line), providing information on the strange magnetic form factor of the Lambda 1405.

     

  • Structure of finite nuclei starting at the quark level

    Structure of finite nuclei starting at the quark level

    Traditional nuclear theory starts with non-relativistic two- and three-body forces, and concentrates on solving the many-body problem as accurately as possible. For medium- and heavy-weight nuclei, the most successful tool is the density functional approach, which is based upon purely phenomenological, density-dependent Skyrme forces.

    The Figure illustrates the predictions for superheavy nuclei, where the binding energies (which were not fitted) [upper panel] were reproduced with an accuracy of 0.1% and the deformations [lower panel] were also extremely well described.

    This figure illustrates the predictions for superheavy nuclei, where the binding energies (which were not fitted) [upper panel] were reproduced with an accuracy of 0.1% and the deformations [lower panel] were also extremely well described.

    Only recently has it been possible to go to a more fundamental level and derive a density-dependent effective force starting at the quark level.

    Now, for the first time, this derived force has been systematically applied to the properties of finite nuclei across the Periodic Table. With a far smaller number of parameters (the σ, ω and ρ couplings to the light quarks) for the effective force and the traditional two pairing parameters, the binding energies of a sample of more than one hundred nuclei across the entire Periodic Table were reproduced within 0.35%.

    Another notable successes of this initial study of the QMC Skyrme force was the prediction of shape co-existence between oblate, spherical and prolate shapes in Zr, with N=60 being a critical transition point. It also predicted a double quadrupole-octupole phase transition in the Ra-Th region.

    We expect to see many applications and developments of this model in the next few years. This is especially interesting in the light of the number of new rare ion facilities worldwide.

  • Visualisations of quantum chromodynamics

    Visualisations of quantum chromodynamics

    As a key component of the Standard Model of the Universe, Quantum Chromodynamics (QCD) describes the interactions between quarks and gluons as they compose particles such as the proton or neutron.

    The CSSM has developed collection of visualisations of QCD developed by Professor Derek Leinweber, available on our YouTube channel. State of the art order a4-improved lattice operators are used in creating the animations, including the three-loop improved lattice gauge action and the five-loop improved lattice field strength tensor.

    The animations illustrate the typical four-dimensional structure of gluon-field configurations averaged over in describing the vacuum properties of QCD. The volume of the box is 2.4 by 2.4 by 3.6 fm, big enough to hold a couple of protons. Contrary to the concept of an empty vacuum, QCD induces chromo-electric and chromo-magnetic fields throughout space-time in its lowest energy state.

    After a few sweeps of smoothing the gluon field (50 sweeps of APE smearing), a lumpy structure reminiscent of a lava lamp is revealed. This is the QCD Lava Lamp. The action density s similar to an energy density, while the topological charge density is a measure of the winding of the gluon field lines in the QCD vacuum.

    The first animation was featured in Professor Frank Wilczek's 2004 Nobel Prize Lecture.

  • Quantum chromodynamics and high performance computing

    Quantum chromodynamics (QCD) and high performance computing (HCP)

    Professor Derek Leinweber discusses lattice QCD and the origin of mass, and the importance of world-class HPC facilities.

  • CSSM computing facilities

    CSSM computing facilities

    Visualising the massive amounts of scientific data generated in supercomputer simulations has provided many new insights into the manner in which QCD constructs the world around us.

    'Isaac' is a dedicated resource for the CSSM Lattice group, and is a cluster consisting of eleven nodes, each of which contains 16 Xeon E5 cores and four GTX Titan GPU's. In total, this yields 176 Xeon E5 cores and 44 GTX Titan GPU's.

    The maximum performance on the whole cluster is about 43 Tflops sustained.

 

People and publications

CSSM is home to an energetic team of academic, research and support staff, postgraduate and honours students and local and international visitors and collaborators. To ensure a coherent research program of the highest quality, we continue to attract researchers in the field from around the world.

These related sites have formal agreements of cooperation with the CSSM.

Institution Location
Abdus Salam International Centre for Theoretical Physics Trieste, Italy
Argonne National Laboratory Chicago, USA
Bonn University Bonn, Germany
Chinese Academy of Sciences Beijing, China
Commissariat à l'Energie Atomique Paris, France
European Centre for Theoretical Studies in Nuclear Physics Trento, Italy
Forschungszentrum Jülich (KFA) Jülich, Germany
Indiana University Bloomington, USA
Institute for Nuclear Theory Seattle, USA
Instituto de Fisica Teorica, Universidade Estadual Paulista São Paulo, Brasil
Joint Institute for Nuclear Research Dubna, Russia
MESON (Medium Energy Science Open Network) Osaka, Japan
Osaka University Osaka, Japan
Svedberg Laboratory (TSL) Uppsala, Sweden
Technische Universität München Munich, Germany
Thomas Jefferson National Accelerator Facility (TJNAF) Newport News, USA
TRIUMF Vancouver, Canada
University of Arizona Tucson, USA
Université Blaise Pascal Aubières, France
University of Erlangen Nuremberg, Germany
University of L'Aquila L'Aquila, Italy
Universität Tübingen Tübingen, Germany