Transcript Bosons
The Standard Model The Standard Model combines the electromagnetic, weak, and strong forces (= interactions). boson fermion fermion Bosons with spin 1 communicate the force between fermions with spin ½. Glashow An extra feature is the Higgs boson with spin 0. Salam Weinberg Bosons Three sets of bosons mediate three interactions: Electromagnetic photon Weak Z, W Strong gluons The principle stays the same, only the players change: • Bosons communicate interactions between fermions. • Each interaction has its characteristic set of bosons. The Higgs boson • The Higgs boson is unique. There is no other fundamental particle with spin 0. • It was concocted by theorists to explain why the Z ,W bosons have mass while the photon and the gluons don’t. • It also describes the mass of fermions (electron, quarks).* • The Higgs particle was observed at the LHC in 2012. * However, the influence of the Higgs boson on the mass of humans, stars, and galaxies is negligible. These are made of atoms, whose mass is dominated by the mass of their nucleons. Those consist of quarks and gluons. The quark mass is < 2% of the nucleon mass (Slide 7) and the gluon mass is zero. Instead, the nucleon mass is dominated by the kinetic energy of quarks and gluons, converted to mass using E =m c2. How the Higgs field creates mass Mathematically , the mass comes from a clever manipulation of quantum fields. But it is hard to describe in plain words. The British Science Minister Waldegrave challenged physicists in 1993 to produce an explanation on a single sheet of paper. The result can be condensed into a few bullets: • The Higgs field fills space like molasses, attaching itself to particles. • When a particle is accelerated, it has to drag the Higgs field along. That reduces its acceleration a. • Newton’s F= ma tells us that a smaller acceleration a for the same force F implies a larger mass m. The Higgs field is different The Higgs field oscillates around a non-zero average. All other fields oscillate around zero , for example the electric and magnetic field of an electromagnetic wave. Higgs field All other fields 0 0 non-zero average Fermions Three generations, each consisting of lepton and quark pairs. light heavier heaviest ‘Who ordered that?’ (Isaac Rabi) Is there a 4th generation ? The green curve is calculated for 3 neutrino generations. It fits the data perfectly.The red curves for 2 and 4 generations don’t fit. Number of observed particles Are there more than three particle generations ? This question can be answered by measuring the number of particles that originate from the decay of a Z boson.The Z itself is produced by an electron-positron pair. Z boson Energy (GeV) Why are there three generations of fermions ? We are still clueless. Convergence of the three interactions The strength of an interaction is determined by its coupling constant . Inverse coupling constant Electromagnetic Coupling ‘constants’ are not constant. They change with energy. At energies approaching the Planck energy they seem to converge to a single, unified coupling constant. Weak This convergence is improved by introducing a new symmetry called supersymmetry, which predicts a new set of supersymmetric particles with masses >1TeV.The coupling constants converge to a value close to the number 1/8 . Strong 1 Energy (GeV) TeV Planck Energy Caution! This is an extrapolation over 13 orders of magnitude in energy. The anthropic principle The Standard Model does extremely well at predicting all kinds of measurements done by accelerators. But it is not able to calculate coupling constants. Some argue that coupling constants cannot be calculated: Different universes may have different coupling constants. Our universe has a particular set of coupling constants because otherwise humans would not exist and contemplate this question. For example, the Sun can’t be too hot or too cold for life. That would happen with an electromagnetic coupling constant slightly smaller or larger than 1/137. This is a slick excuse for our inability to predict the many numbers that the Standard Model cannot calculate ( 20).