ELEMENTARY PARTICLE PHYSICS

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.

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 Ω_ccc 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 δ_SU(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.