The Standard Model, a quantum gauge theory describing the strong, weak and electromagnetic interactions of quarks and leptons, represents a remarkable synthesis of several decades of experiments in particle physics. Its success in explaining experimental observations goes well beyond the original expectations, but despite this the Standard Model is far from solving all open issues in particle physics. For instance, how can we explain the asymmetry between matter and antimatter that is observed in the Universe? Why is each fermion forming ordinary matter replicated in two more (and only two) copies, leading to three families of quarks and leptons? Why are their masses so disparate, spanning eleven orders of magnitude? What is the dynamical origin of the observed mixing structure among these families? Flavour physics, the phenomenology and theory of the six types (or flavours) of quarks and leptons, their interactions and mixings, addresses all these fundamental questions on the nature of matter that lie hidden within the Standard Model and that only a more fundamental theory can answer.
The discovery of tiny neutrino masses has already revealed physics beyond the Standard Model at a very high mass scale, which roughly coincides with the energy at which the three gauge couplings of the Standard Model approximately unify. These energies are orders of magnitude higher than the scale of electroweak symmetry breaking. We would not be able to understand this hierarchy unless new physics already shows up at scales below 1 TeV and stabilises the electroweak symmetry breaking scale. Indeed the major experimental programme around the Large Hadron Collider (LHC) at CERN has the goal of probing the TeV scale directly and detecting new particles with masses in that energy region. A complementary approach to the search for new physics is to push the high-precision frontier further – this is done at low-energy by means of precision experiments in flavour physics. They also probe the TeV scale and above, albeit indirectly, and are a key ingredient in discovering the structure of New Physics.
The complementarity of the two approaches lies in the fact that determining the spectrum of the new theory (what is best done at the high-energy frontier) is not the only information we need: we also need to know the flavour structure of this new theory and determine the couplings and mixing patterns of the new particles. This can only be done at the high-precision frontier, with a rich lowenergy experimental programme which must be carried out in parallel even in the LHC and later International Linear Collider (ILC) eras.
LHC is expected to discover new particles that are essential to the stabilisation of the electroweak symmetry breaking scale. Flavour physics gives key information about the couplings and mixings of these new particles. It can further reveal or constrain extended Higgs sectors with enhanced couplings to down-type fermions, and assist in refining the search strategies for these bosons. If nature has chosen to solve the gauge hierarchy problem through a new strongly interacting scalar sector, an understanding of this requires the longstanding expertise of flavour theorists in exploring non-perturbative strong dynamics.
Experiments in flavour physics are currently running in many laboratories around the world. A renaissance in kaon physics has flourished, led by KTeV, NA48 and KLOE, with high-precision measurements of many kaon observables, such as lifetimes, branching ratios of many decay modes, including very rare ones and the tiny CP-violation parameters. The charm-quark sector is being extensively studied by CLEO-c and more results for D-mesons and charmonium spectroscopy are coming from BaBar, BELLE and Fermilab. This has led to the discovery of new potentially exotic states and a precision investigation of the charmonium spectrum. The remarkable first results of the BES experiment point the way to dramatic improvements to be achieved at BEPCII, a new tau-charm factory starting in 2008. Finally, B-physics has been thoroughly investigated in recent years: the two B-factories BaBar and BELLE have collected a wealth of high-precision data of which only a limited fraction has been analysed so far. Proposed upgrades to super-B factories should further increase the accuracy and scope of these results. Experiments based on hadronic facilities, CDF, D0 and specially LHCb, are expected to complement these results with the observation of many beautiful hadrons not produced at B-factories.
This surge of high-precision experimental activity must be matched with a theoretical apparatus of equal sharpness in order to determine Standard Model parameters and reveal discrepancies caused by New Physics. In most data the desired short-distance information on the quark dynamics is clouded by the strong interaction, which binds quarks into hadrons. The understanding of the corresponding theory, Quantum Chromodynamics (QCD), in the non-perturbative domain is challenging, but can nevertheless be achieved through various sophisticated techniques like dispersion relations, 1/Nc expansion and sum rules. In combination with the latter, two outstanding methods can bridge the gap between theory and experiment in an accurate and controllable manner, because of their systematic character: lattice gauge theory and effective field theories.
Lattice gauge theory solves QCD numerically by replacing the continuous space-time by a discrete lattice. Conceptual and technical challenges arise, because the volume used in the simulations is necessarily limited, realistically light quark masses are hard to simulate and approaching the continuum limit is expensive in computer time. Very recent innovations in hardware, algorithms and discretisation procedures have lead to a substantial progress on these issues and, most importantly, have enabled simulations with dynamical quarks, bringing lattice simulations much closer to the continuum physics measured in experiments. Effective field theories employ systematic expansions of QCD around certain symmetry limits. For example, in the limit of massless quarks, QCD becomes chirally symmetric; the corresponding effective theory, Chiral Perturbation Theory (ChPT), exploits the phenomenological consequences of this symmetry in a rigorous way. Heavy quark systems instead permit the use of Heavy-Quark Effective Theory (HQET), which uses the opposite limit of an infinitely heavy quark as a starting point. The latter approach has also been adapted to a variety of different physical situations, such as quarkonium dynamics and B-decays (Non-Relativistic QCD, vNRQCD and pNRQCD, Soft-Collinear Effective Theory SCET).
Lattice gauge theory and effective field theories are complementary systematic approaches built on quantum field theory: lattice results can be used to compute hadronic quantities from first principles and effective field theories incorporate the consequences of symmetries in simple analytic formulae. Both fields are now mature – they have reached a stage where they can profit best from their mutual interaction to match the accuracy of the data in flavour physics.
The discoveries brought by LHC will lead to new challenges for flavour physics. The knowledge and competences acquired in dedicated experiments, lattice simulations and effective field theories are essential to achieve an appropriate understanding of the flavour dynamics, but none of these fields can encompass the whole problem alone. It is necessary, as well as timely, to create a European forum where these scientific communities can share their abilities by joining forces: FLAVIAnet.
-- FlaviaNet - 15 May 2007