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Baryon Structure Using Dynamical QCD Simulations With Physical Values of the Light, Strange and Charm Quark Masses

Principal Investigator: Constantia Alexandrou, University of Cyprus (Cyprus)
HPC Platform: JUQUEEN of JSC

The goal of this project is to understand the properties of elementary particles such as the proton, which forms most of the ordinary matter around us. The fundamental theory that determines the mass and structure of such particles is the strong interaction, one of the four forces known to us, the others being electromagnetism, the weak interaction, and gravity.

Quantum ChromoDynamics (QCD) is the underlying theory of the strong interactions that describe a wide range of complex processes in the universe. Among them are the fusion and fission processes that power the sun and the other stars, the formation and explosion of stars and the state of matter at the birth of the universe.

QCD is a remarkable theory that differs from the other three fundamental interactions. The basic building blocks of the theory are the quarks that are always confined inside the hadrons. The proton is a baryon made of three light quarks as shown in the picture below. However, these quarks cannot be liberated and be observed freely, a property characteristic only of QCD and known as confinement. Furthermore, unlike a basket of apples where its weight is the sum of the weights of each apple and the empty basket, the weight or mass of the proton is almost exclusively determined by the interactions among the three quarks rather than their mass which is approximately 200 times less than that of the proton.

Clearly, to describe the properties of the proton one needs very different methods as compared to describing other physical systems such as for example the binding energy of the hydrogen atom. Thus, the challenge is to provide a quantitative description of strongly interacting particles using the underlying theory of QCD. This is done by defining the continuous equations of QCD on a discrete four-dimensional mesh or lattice and simulating it on the largest supercomputers available. This formulation, first written down by Nobel Prize winner K. Wilson, is known as lattice QCD and enables us to systematically study the properties of these particles numerically.

Baryon Structure Using Dynamical QCD Simulations With Physical Values of the Light, Strange and Charm Quark Masses

The biggest challenge in lattice QCD is to simulate the theory at physical values of the light quark masses. In addition to light quarks that make ordinary matter there are heavier quarks. The so called strange and charm quarks are the next heavier ones and particles containing these quarks are known as strange and charmed hadrons, respectively. Our collaboration, known as the European Twisted Mass Collaboration (ETMC), uses an improved formulation as compared to the original one by Wilson and is able to simulate QCD with physical values for the mass of the strange and charm quarks with the light quarks producing 15% heavier protons as in nature. These values of the light quark mass are close enough to the physical values to allow us to extrapolate and compare to the properties measured in experiment. Such experiments are being carried out at accelerator facilities such as the Jefferson Laboratory in the US or the Mainz facility in Germany. As an example of our results, we show the mass of baryons that can be created in the laboratory such as the Ω discovered 50 years ago at Brookhaven National laboratory in the US (see Fig.1 ), the mass of more recently discovered charmed baryons (see Fig. 2) or the prediction for the mass of Ωccc that is yet to be discovered (see Fig. 3).

Knowing the mass of a particle we can study its structure by a an external probe. Using the axial current as a probe we can evaluate the axial charge of the particle. We show in Fig.4 δSU(3), the SU(3) breaking parameter that it is zero if the mass of light quarks u and d and the strange are equal. In this case we have also compute the value using recent simulations with light quarks with physical values.

Baryon Structure Using Dynamical QCD Simulations With Physical Values of the Light, Strange and Charm Quark MassesFig.1: The masses of the octet and decuplet baryons. The horizontal lines are the experiemtnal vlaues. The filled red symbols are the masses computed using PRACE reources. Thye are compared to the results obtained using different discretization schemes by other lattice QCD collaborations.
Copyright: University of Cyprus and The Cyprus Institute

Baryon Structure Using Dynamical QCD Simulations With Physical Values of the Light, Strange and Charm Quark MassesFig. 2: Results on the mass of the charmed spin ½ baryons obtianed by the ETMC using PRACE computational resources. They are compared to results from other collaborations. The results are submitted for publication, see C. Alexandrou et al., arXiv:1406.4310
Copyright: University of Cyprus and The Cyprus Institute

Baryon Structure Using Dynamical QCD Simulations With Physical Values of the Light, Strange and Charm Quark MassesFig. 3: Results on the mass of the charmed spin -3/2 baryons obtianed by the ETMC using PRACE computational resources. Thye are compared to results from other collaborations. The results are submitted for publication, see C. Alexandrou et al., arXiv:1406.4310.
Copyright: University of Cyprus and The Cyprus Institute

Baryon Structure Using Dynamical QCD Simulations With Physical Values of the Light, Strange and Charm Quark MassesCopyright: University of Cyprus and The Cyprus Institute

Baryon Structure Using Dynamical QCD Simulations With Physical Values of the Light, Strange and Charm Quark MassesCopyright: University of Cyprus and The Cyprus Institute

Scientific team: C. Alexandrou, M. Constantinou, V. Drach, K. Hadjiyiannakou, K. Jansen, C. Kallidonis, G. Koutsou

Computational resources: State-of-the-art lattice QCD simulations rely on access to Tier-0 supercomputers. None of these results would have been obtained without access to JUQUEEN resources. We would like to take this opportunity to thank the Jülich Supercomputing Centre for the very good management of the machine which has enabled us to do internationally competitive research.

The project was made possible through the Partnership for Advanced Computing in Europe, PRACE.

Prof. Constantia Alexandrou
University of Cyprus
Department of Physics, Faculty of Pure and Applied Sciences
P.O.Box 20537, 1678 Nicosia/Cyprus
e-mail: alexand@ucy.ac.cy

September 2014