Urs Wenger

Research

Why does the cup stay on the table? And why are we not falling through the ground to the centre of the world? -- Ever since I was a little boy I have been fascinated by the fact that behind the macroscopic world as we know and see it, there is a hidden microscopic world full of strange and intriguing phenomena involving mysterious particles.

Today I find it rather remarkable that within the so-called Standard Model of Elementary Particle Physics all the observed microscopic forces between the fundamental particles can be accurately described using only a couple of parameters and a handful of dynamical degrees of freedom. The Standard Model provides an intriguing and compelling theoretical framework using local quantum gauge field theories and the single unifying principle of local gauge invariance to describe simultaneously the electroweak and the strong force. The degrees of freedom are the fermionic matter fields (different flavours of quarks and leptons), the Higgs fields, and the bosonic gauge fields (photon, W,Z-bosons and gluons) mediating the interactions between them.

It is both astonishing and contenting that the fundamental laws within the mathematical framework appear to be very simple yet the resulting consequences produce a world rich of complicated and interesting phenomena. For example through the dynamical Higgs mechanism the W,Z-bosons acquire a mass via the Higgs condensate and at energies below a couple of 100 GeV they can no longer be excited from the vacuum and consequently the corresponding weak interactions freeze out. We are then left with only the electro-magnetic interactions between the quarks, leptons and the photon described by Quantumelectrodynamics and the strong interactions between the quarks and the gluons described by Quantumchromodynamics (QCD). The parameters of the latter theory are the masses of the six quark flavours and the dimensionless gauge coupling. In the chiral limit, where the masses of the lightest three quarks are sent to zero and the remaining masses to infinity, the theory becomes a theoreticians paradise since we are left with the gauge coupling as the only parameter of the theory. Moreover, it turns out that this coupling depends on the scale at which the theory is observed, and through dimensional transmutation an intrinsic characteristic scale is generated so that all dimensional quantities can be unambiguously predicted in terms of one characteristic scale, e.g. the mass of the proton. I find it still both breathtaking and exciting that nature can be reduced, at low energies, to such a simple and beautiful quantum field theory.

QCD takes a special role in the Standard Model also from another viewpoint: due to the non-Abelian nature of its gauge group SU(3) the gluons are self-interacting through their colour (or chromo-electric) charge and as a consequence the strong interaction becomes stronger at low energies where it exhibits a variety of phenomena that can not be described in the framework of perturbation theory. Many of these non-perturbative aspects of the theory are still poorly understood, like for example colour confinement, the fact that only composite colour-neutral particles, so-called hadrons, can be observed in nature, but not the quarks and gluons themselves. Another non-perturbative aspect of QCD is the spontaneous breaking of chiral symmetry leading to the spectrum of the lightest hadrons, the pions, observed in nature. Yet another striking non-perturbative phenomenon is the restoration of chiral symmetry at the deconfinement temperature where ordinary hadronic matter transforms itself into a quark-gluon-plasma as is beautifully demonstrated by heavy-ion collision experiments.

It is clear that a thorough qualitative and quantitative understanding of all these non-perturbative aspects of QCD is of outmost importance for the justification of QCD as the fundamental theory describing the strong interactions and for the justification of the Standard Model in general. One of the main goals of my research is to provide exactly such an understanding of low-energy QCD via computer simulations -- precise ab-initio calculations of the non-perturbative properties of QCD in the chiral limit will enable a further confirmation of the theoretical framework and also provide valuable hints towards new physics beyond the Standard Model as it might be detected by the Large Hadron Collider at CERN.

Further Topics