HERWIG: a Monte Carlo Program for QCD at LHC

Abstract

Monte Carlo programs could be used for the partial description of particle physics production at high energy. These particles are hadrons, such as proton, neutron, pion and many others. They are described by QCD (quantum chromodynamics). At perturbative level QCD is formulated as gauge interaction of quarks and gluons which have color number force. They produce hadrons as physical bound states, see for instance (1). At non perturbative level, controlled numerically by computer simulations, QCD produces physical hadrons. The mass of the proton is about 1.672 × 10 Š 27 kg ≃ 0.937 GeV. The FermiLab machine, see (2), has proton and antiproton running in the center of mass with an energy of 2 TeV. It was intensively operating in the last years. The FermiLab is located near Chicago. The LHC (large hadron collider) machine is located at CERN, see (3). The machine is made of two round circles which are 27 km long. Here particles are running near Geneva between France and Switzerland. The two protons are running here one against the other with an energy of 3.5 TeV c 2 so that the total mass of the colliding system is of 7 TeV. This machine started at the end of 2009. It operates until 2011 and then it will start again at a double of its present total colliding mass. The LHC total colliding energy discloses a key point of the Standard model for particle physics, the presence of the Higgs meson which is expected with a mass between hundred and hundred and fifty time the proton mass, i.e. MHiggs between 100 and 150 GeV. The Higgs meson is needed to give a simple description of the present Standard model. His absence should generate a substantial problem to this theory which unifies all elementary particle physics. A major point of the Higgs field is that it gives a mass to the weak boson Z and W ± . The above limit on Higgs mass are obtained from intense studies of present data on particle experiments at FermiLab, see (2), and at CERN, see (3). The Standard model is the present theory for elementary particles, except for gravitational forces. This theory includes QCD and involves electroweak particles such as leptons (electron, muon and tau) and corresponding neutrinos. It involves photons and massive electroweak bosons as Z and W ± . QCD is the theory one uses for hadron interactions. Hadrons are strong interaction particles such as barions (as proton, nucleon etc) and mesons (as pions etc.). By perturbative description these fields interact via a gauge Lagrangian. Fields are given by partons, i.e. quark, antiquark and gluon. Hadrons are made of bound states of these elementary fields. The perturbative formulation does not allow to generate hadrons which are color singlet states of partons (color is one of parton index). QCD hadrons are generated as bound states of quarks