Everyday matter is made up of protons, neutrons, electrons, and something called electron neutrinos. These particles interact with each other via one or more of four forces: gravity, electromagnetic (which is the unified force of electricity and magnetism), strong nuclear, and weak nuclear. Almost all of everyday life could be explained pretty well with just this short list of four particles and four forces.
But science is unreasonable. It is not content to deal with reasonably good explanations of just the easily accessible but pushes its explorations to the limits of accuracy and precision. In particular, it probes those particles and forces in more detail in order to understand them better and in the process we have discovered a whole slew of other particles that we were unaware of before. These are either extremely short-lived, or cannot be isolated because they are always ‘hidden’ inside other particles, or both, which is why we were not aware of their existence for so long.
In order to explain this large menagerie of particles that now run into well over a hundred, we have had to change and expand our understanding of what constitutes elementary particles and how they interact, and the resulting theory that we have now arrived at is what is called the Standard Model.
The idea of mass will play an important role in this series of posts and it is usually expressed in units of energy, ever since Einstein showed that mass (m) and energy (E) are convertible using E=mc2, where c is the speed of light.
The basic unit of energy used in particle physics is called the electron-volt (eV). But because we will be dealing with a huge range of energies, we also use derivatives of this basic unit (like we do with derivatives of the meter such as millimeters, centimeters, and kilometers) such as thousands of eV (KeV), millions of eV (MeV), billions of eV (GeV), and trillions of eV (TeV). For example, the electron has a mass of about 511,000 eV or 0.511 MeV, while the proton is 938.272 MeV and the neutron is 939.566 MeV. In the world of elementary particle physics, 1 TeV is considered a huge amount of energy, although in terms of everyday life it is tiny, corresponding to the amount of energy used by a 60-watt light bulb in about one-billionth (10-9) of a second.
When dealing with a composite object made up of smaller entities, the mass of the composite includes not only the masses of the components but also the interaction energies between the components. So for example, if you have two masses connected by a spring, and you squeeze the masses together by compressing the spring, the total mass is not just the mass of the two objects plus the spring. It will also include the energy stored in the compressed spring. In everyday life, this is a very small effect so we ignore it, but it is the basis for the enormous energies that are released in nuclear reactions. This is why the mass of a hydrogen atom (938.738 MeV) is not exactly the sum of the mass of a proton (938.272 MeV) plus the mass on an electron (0.511 MeV), but slightly less, since some energy is released when the proton and electron come together to form an atom.
In developing the Standard Model of particle physics, we have come to realize that protons and neutrons and many other particles are no longer ‘elementary’ (in that they have no substructure) but are comprised of yet smaller entities called quarks. These quarks are believed to be elementary and have the peculiar property that they are found in nature only in combinations with other quarks inside other non-elementary particles (like protons and neutrons) and never as isolated single entities. Hence they are quite elusive and hard to detect.
So how does one determine the mass of an entity that is part of a bigger entity and cannot be separated from it? It would be like trying to measure just the mass of your arm without separating it from the rest of your body. It cannot be done directly but has to be done using a theoretical framework. In the case of quarks, the mass is inserted as a parameter into a theory and the value that gives the best results with some observable quantity is then taken as the mass. But this is always a theory-dependent value. In the case of quarks, the most commonly used masses in particle data tables are the so-called ‘current quark’ masses that are obtained using lattice gauge theory.
The Standard Model postulates the existence of six kinds of quarks with the following names and approximate masses:
up=2 MeV; down=5 MeV; strange=100 MeV; charm=1 GeV; bottom=4 GeV; top=172 GeV
We also have a further six elementary particles known as leptons consisting of three negatively charged ones (electron=0.5 MeV; muon=106 MeV; tau=1.78 GeV) and three electrically neutral ones (electron neutrino, muon neutrino, and tau neutrino). The three neutrino masses are believed to be non-zero but so small that we have not been able to measure them as yet and can effectively treat them as zero. Of these, the electron is a ‘stable’ particle meaning that it can exist alone and if isolated will live forever without decaying into other particles. The muon and tau are short-lived particles that are produced in reactions but decay soon afterwards into more stable particles like the electron and neutrinos. The neutrinos interact so weakly with matter that they are extremely hard to detect.
The number of fundamental forces has remained unchanged at four but we have deepened our understanding of them so that they are now described as being generated by the exchange of different kinds of elementary particles called ‘gauge bosons’. So the gravity force between two particles occurs when they exchange a particle called the graviton; the electromagnetic force is mediated by the photon; the strong nuclear force by the gluon; and the weak nuclear force by three particles known as the positively charged W+, the negatively charged W-, and the electrically neutral Z.
The masses of these force carriers are:
graviton=0; photon =0; gluon=0; W+= W-=80.4 GeV; and Z=91.2 GeV
The graviton and the photon are also stable particles but the graviton is so weakly interacting that it only plays an important role when interacting with massive objects like planets and stars. For the world of particle physics, we can pretty much ignore the graviton because its effects are negligible. The gluons, just like the quarks, are never found in isolation but always inside non-elementary particles like the proton and neutron. The W+, W-, and Z are very short-lived particles, quickly decaying into other particles.
Next: The basic elements of the Standard Model