Scientific Highlights

Network activities are organised through three large scale projects (LSPs), in turn divided into working groups (WGs). Here we give scientific highlights for each of the LSPs and WGs, referring to milestones (MS) from the schedule.

LSP1: Numerical Techniques

Development of faster computers with consequent new codes and improved algorithms is necessary to speed up the generation of gauge field configurations including light dynamical quarks needed to calculate phenomenological quantities in LSP2. Highlights of this work in year 2 are:

WG1: Algorithms

A conclusive comparison of efficient multiboson algorithms vs. the hybrid Monte Carlo method (Wuppertal-Orsay) was carried out, a paper is in preparation (MS 2). The exploration of finite-size scaling techniques has been continued and first results have been presented. The truncated eigenmode approximation, replacing the stochastic estimator technique, has been improved , and first results by partially quenched methods have been achieved (reaching MS 4). Network members led discussions at the International lattice conference (LAT02) in Boston in June on sharing QCD data in a transparent way on an international scale . This will be carried further at a meeting in Edinburgh in December.

WG2: Codes for state-of-the-art machines

The ApeNext and QCDOC software projects are proceeding well. QCDOC work continues on two software strands, building on low-level routines of common origin. Highly-portable software has been developed for quenched Ginsparg-Wilson fermions. Dynamical fermion configuration-generation code is being developed in parallel with physics measurement code towards MS 5. The ApeNext project has completed chip design and is working on compiler optimisation. Code performance on clusters has been significantly improved .

LSP2: Phenomenology

The calculation of quantities relevant to experimental particle physics is the main core of our work. This year has seen significant improvements in several calculations and the first results with 3 flavours of dynamical quarks.

WG3: Light hadron structure

The excited nucleon spectrum has been computed for quenched and unquenched Wilson fermions , (Edinburgh,NIC). The dynamical nucleon mass shows significant finite size effects on present volumes. A first extrapolation to the infinite volume has been performed using chiral perturbation theory . The masses of the light quarks have now also been determined completely non-perturbatively using the RI-MOM scheme , (Orsay,Rome 1). The strange quark mass has been determined with 2+1 flavours of dynamical (improved staggered) quarks (Edinburgh,Glasgow). The nucleon structure functions and form factors are presently being calculated in N_f=2 dynamical QCD.

WG4: Kaon physics

The goals set by WG4 for the end of year 2 (MS12) have been reached. The general study of finite volume effects in K → ππ decays was completed, with a demonstration that i) the Lellouch-Lüscher relation can be extended to all two-pion states with an s-wave component and ii) the relation between finite and infinite volume matrix elements is valid also if energy and momentum is inserted at the weak vertex. The first stage of a major project to compute the one-loop chiral corrections for K → ππ decays at general kinematics has been completed for Δ I=3/2 decays. The corresponding project for ΔI = 1/2 decays has been started. , , (Rome 1, Southampton). The B_K parameter has been computed (i) with Wilson fermions using two methods and continuum extrapolations; (ii) with overlap fermions in an exploratory study. Moreover its step scaling function has been computed non-perturbatively in a wide range of scales. An interesting theoretical development related to the Δ I = 1/2 rule consists in the determination of the renormalisation properties of the tmQCD formalism for such decays. , (Rome 2)

WG5: Heavy flavour physics

The charm quark's mass has been computed in quenched QCD, (DESY-Zeuthen) . A novel method has been introduced for calculating f_B on the lattice, using the dependence of finite size effects upon the heavy quark mass (Rome2) , . New results have been obtained for the B → light vector semi-leptonic decays, allowing estimates of the V_cs and V_ub CKM-matrix elements (Orsay,Rome1,Southampton) . The renormalisation and subtraction constants for the four fermion ΔF=2 operators, have been computed non-perturbatively and preliminary results given for the B-Bbar mixing parameter (Rome1) . The lifetime of beauty hadrons has been studied (Rome1) , including charm-quark mass corrections (Rome1, Southampton) . The first lattice determination of the g_DD^*π coupling constant has been obtained (Orsay, Roma1) , . New studies of heavy quark physics with 3 flavours of dynamical (improved staggered) quarks have shown results which for the first time give a single value of the lattice spacing across a range of hadron masses, necessary to obtain physical results (Glasgow, Edinburgh) , . Network members initiated plans to set up a Lattice Working Group to provide `world-averages' of lattice results relevant to studies of the CKM matrix . An initial meeting was held at CERN and further discussions at LAT02. It will now be carried further by network members and others.

LSP3: Theoretical Developments

Theoretical developments allow improved methodologies for lattice calculations and better understanding of the continuum and chiral limits taking lattice QCD to the real world.

WG6: Chiral symmetry/actions/improvement

The usefulness of improved staggered quark actions was investigated perturbatively (Glasgow,Edinburgh). The parameterized fixed-point action which has good chiral properties and reduced cut-off effects was tested further in , and the results of the first large scale simulation on light hadron spectroscopy with the fixed-point and another chirally improved action was presented in , (Bern,NIC). Twisted mass QCD has a rather promising approach towards chiral symmetry as discussed in , (DESY-Zeuthen). Chiral symmetry, at least in quenched QCD, seems to be under control (Orsay,NIC). Pure gauge theories give an excellent tool to test theoretical ideas, such as universality (DESY-Zeuthen). The role of the centre of the gauge group on confinement was also investigated . The QCD string excitation spectrum was studied and found to be in good agreement to the free bosonic string in , (Bern). At short distances the two spectra are qualitatively different, which is somewhat puzzling.

WG7: Renormalisation, matching and α_s

The renormalisation of the static axial current was computed non-perturbatively in the quenched approximation in two different regularisations. It was used to determine the B-meson decay constant to lowest order in 1/m_b and to extrapolate it to the continuum (DESY-Zeuthen, Southampton). In order to prepare the ground for future computations the scale r₀/a was obtained for very small lattice spacings (Rome 2). The gauge-dependence of RI/MOM renormalisation constants was found to be very weak , (Rome 1, Rome 2). For N_f=2 dynamical fermions, a first determination of the renormalisation of the local vector current used the correct isospin charge of the nucleon as renormalisation condition (NIC, Edinburgh). The running of the N_f=2 coupling was studied further on smaller lattice spacings . The scale dependence of the quark mass has also been explored and first promising results are available (DESY-Zeuthen). The subject was reviewed at LAT02 by the co-ordinator of WG7 .