SMParametersInYourEverydayLife
The eighteen arbitrary parameters of the standard model in your everyday life https://cds.cern.ch/record/306051?ln=en
- Contrary to popular conception, the purpose of particle physics is to understand the everyday world.
- Current theory depends on eighteen parameters, which are a priori arbitrary.
- Until we can explain the origin of these parameters, we cannot say we truly understand why our everyday world is as it is.
- Every second, tens of cosmic-ray muons traverse your body,
- Nine of these set the masses of the quarks and charged leptons
- as long as we do not understand why the parameters have the values they do, our understanding of our physical environment will remain fundamentally incomplete: we shall be unable to explain at a fundamental level why there are even atoms and molecules.
- Our knowledge of fundamental particles and interactions is such that we can explain everything about our everyday world and we can explain nothing at all.
- you’d probably guess that a proton is heavier than a neutron, reasoning that there must be some extra Coulomb energy associated with the charge.
- the d quark is heavier than the u and the neutron has two d’s and one u, while the proton has two u’s and one d
- Turn up the mass of the u quark by 2.6 MeV and the proton would be heavier than the neutron by 1.3 MeV instead of the other way around.
- What is new is that these questions have passed from the realm of metaphysics or philosophy to that of science.
- Our current description of physical laws has two components. The first is a set of rules for calculating the probabilities of various processes occurring. The second is a list of eighteen parameters, masses and such, that need to be inserted into the calculations at appropriate moments.
- By modifying a parameter we obtain a perfectly good description of a world that might have been.
- 12 free parameters: the masses of the six quarks and the three charged leptons and three coupling constants. There are six more, four of which are related to Beta-decay.
- If V were real, it would be simply a 3D rotation matrix, and it could be parametrized by three Euler angles. The fourth free parameter makes it truly complex.
- Particle physics has developed through a tension between symmetry and symmetry breaking. First we uncover an approximate symmetry, then we try to learn why it fails to be a true symmetry. Isospin is an approximate symmetry, first recognized as the similarity between the neutron and proton, which form an iso-doublet. The symmetry is not exact. The proton and neutron masses are very similar but not identical. Clearly the symmetry is broken by electromagnetic forces, which influence the proton and neutron quite differently. We now understand that isospin symmetry is also broken by the difference in mass between the u quark and the d quark, a difference that is not the result of electromagnetism.
- Certainly this symmetry is broken, since the electron and neutrino are quite dissimilar. Indeed, if it were not broken, all the quarks and leptons, and the W and Z, too, would be massless.
- Usually a symmetry can be broken by introducing an interaction that directly contradicts the symmetry. It turns out that this kind of bias cannot be introduced for the electroweak interactions. Explicit symmetry breaking destroys the theory altogether
- We must be grateful—if supersymmetry there is—that the breaking of this symmetry left us with light, charged fermions, rather than light, charged bosons.
- The neutrino and the electron are really as different as they can be. How does that happen? Why do we have two very light quarks and one very light charged lepton? Why did electroweak symmetry breaking leave one symmetry unbroken, bequeathing us the photon? Why is there light, and why does matter take the form it does? These are the goals of particle physics: not to describe the collisions of highly relativistic protons, but to learn why our world has the shape and form it does. But to answer questions about the everyday world we need to observe phenom- ena that occur only at very high energies.
- That the u quark is slightly lighter than the d quark gives us a stable proton. That the electron’s mass is less than the mass difference between the neutron and proton guarantees that the hydrogen atom is stable against electron capture.
- the eighteen arbitrary parameters of the standard model are the recipe out of which the universe is made.
- Particle physicists construct accelerators and detectors not ultimately to find the t quark or the Higgs boson, but because that is the only way to learn why our everyday world is the way it is.