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 rich and complex structure of the strongly interacting particles (known generically as hadrons) and their interactions is one of the remaining great challenges in physics.
The marvellous organizing principle for almost all of our understanding of modern physics is referred to as the Standard Model. This brings together in one elegant framework three of the four fundamental interactions in physics: the electromagnetic interaction, the weak interaction (responsible for radioactive decay), and the strong interaction (responsible for hadronic structure and interactions). The strong interaction is widely believed to be described by a theory known as Quantum Chromodynamics (QCD).
The fundamental constituents of the QCD description of hadronic matter are referred to as quarks (analogues of electrons) and gluons (analogues of photons, but self-interacting). Assuming that QCD is the correct theory for the strong interactions, we will use it to understand the observed structure of hadrons and hadronic matter and to predict important new features. Conversely, when pushed to its limits, QCD may eventually fail to predict the observed hadronic world. This possibility would constitute an extremely interesting outcome and would require major modifications to the Standard Model.
The essential aim of the Centre is to make major advances of international significance in our understanding of the structure of hadronic matter. With a variety of attacks on the problem being made within the same Centre, progress can be made more rapidly than by a set of isolated individuals or small groups. The cross-fertilisation of this environment offers great opportunities for major breakthroughs in our understanding.
In order to ensure a coherent research program of the highest quality we will continue to attract the very best researchers in the field from around the world for significant periods of time. Through a vigorous program of topical workshops, we involve a significant fraction of the Australian subatomic physics community in the research programs of the Centre. Not only does their involvement strengthen the research programs of the Centre, but also through the close contact with each other, the staff of the Centre and the visiting experts from overseas, their own productivity and enthusiasm are significantly enhanced.
- 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.
A collaboration involving staff of CSSM (Theo Motta and Anthony Thomas) and the CEA in France (Pierre Guichon) realized 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 (Ref. https://doi.org/10.1088/1361-6471/aab689 ). As a consequence one is led to reject the hypothesis that the neutron has this hidden decay mode.
- 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.
Indeed, 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. In a recent Physical Review Letter, Bentz, Cloët and Thomas 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.
The figure above (from Phys. Rev. Lett. 116 (2016) 032701) 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.
- 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, Australia.
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.
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.
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.
This research is published in Physical Review Letters as an
Lattice QCD Evidence that the Λ(1405) Resonance is an Antikaon-Nucleon Molecule
Jonathan M. M. Hall, Waseem Kamleh, Derek B. Leinweber, Benjamin J. Menadue, Benjamin J. Owen, Anthony W. Thomas, and Ross D. Young
Phys. Rev. Lett. 114, 132002 (2015)
Published April 1, 2015
For more details, please see: http://www.physics.adelaide.edu.au/theory/staff/leinweber/VisualQCD/Nobel/Lambda1405.html
- 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.
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 (P.A.M. Guichon et al., Nucl. Phys. A772 (2006) 1).
Now, for the first time, this derived force has been systematically applied to the properties of finite nuclei across the Periodic Table (J.R. Stone, P.A.M. Guichon, P.G. Reinhard and A.W. Thomas, Phys. Rev. Lett. 116 (2016) 092501). 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%.
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.
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.