Recently viewed 0 Save Search. Your current browser may not support copying via this button. Subscriber sign in You could not be signed in, please check and try again. Username Please enter your Username. Password Please enter your Password. Forgot password? Don't have an account? Sign in via your Institution. You could not be signed in, please check and try again. Dirac see Figure 1 developed a highly successful relativistic quantum theory that laid the foundations of quantum electrodynamics QED.
His theory, for example, explained electron spin and magnetic moment in a natural way. By , Dirac, along with Oppenheimer, realized this was a prediction of positively charged electrons or positrons. In , American physicist Carl Anderson discovered the positron in cosmic ray studies. In , Yukawa predicted pions as the carriers of the strong nuclear force, and they were eventually discovered.
Muons were discovered in cosmic ray experiments in , and they seemed to be heavy, unstable versions of electrons and positrons. After World War II, accelerators energetic enough to create these particles were built. Not only were predicted and known particles created, but many unexpected particles were observed.
But patterns were observed in the particle zoo that led to simplifying ideas such as quarks, as we shall soon see. Figure 1. The positron was only the first example of antimatter. Every particle in nature has an antimatter counterpart, although some particles, like the photon, are their own antiparticles. Antimatter has charge opposite to that of matter for example, the positron is positive while the electron is negative but is nearly identical otherwise, having the same mass, intrinsic spin, half-life, and so on.
When a particle and its antimatter counterpart interact, they annihilate one another, usually totally converting their masses to pure energy in the form of photons as seen in Figure 2. Neutral particles, such as neutrons, have neutral antimatter counterparts, which also annihilate when they interact. Certain neutral particles are their own antiparticle and live correspondingly short lives. Without exception, nature is symmetric—all particles have antimatter counterparts.
For example, antiprotons and antineutrons were first created in accelerator experiments in and the antiproton is negative.
Antihydrogen atoms, consisting of an antiproton and antielectron, were observed in at CERN, too. However, particles of the same charge repel each other, so the more particles that are contained in a trap, the more energy is needed to power the magnetic field that contains them. It is not currently possible to store a significant quantity of antiprotons. At any rate, we now see that negative charge is associated with both low-mass electrons and high-mass particles antiprotons and the apparent asymmetry is not there.
But this knowledge does raise another question—why is there such a predominance of matter and so little antimatter? Possible explanations emerge later in this and the next module. Particles can also be revealingly grouped according to what forces they feel between them.
All particles even those that are massless are affected by gravity, since gravity affects the space and time in which particles exist. All charged particles are affected by the electromagnetic force, as are neutral particles that have an internal distribution of charge such as the neutron with its magnetic moment. Special names are given to particles that feel the strong and weak nuclear forces.
Hadrons are particles that feel the strong nuclear force, whereas leptons are particles that do not. The proton, neutron, and the pions are examples of hadrons.
The electron, positron, muons, and neutrinos are examples of leptons, the name meaning low mass. Leptons feel the weak nuclear force. In fact, all particles feel the weak nuclear force. This means that hadrons are distinguished by being able to feel both the strong and weak nuclear forces.
Table 1 lists the characteristics of some of the most important subatomic particles, including the directly observed carrier particles for the electromagnetic and weak nuclear forces, all leptons, and some hadrons. Several hints related to an underlying substructure emerge from an examination of these particle characteristics. Note that the carrier particles are called gauge bosons. Fermions obey the Pauli exclusion principle whereas bosons do not.
All the known and conjectured carrier particles are bosons. Figure 2. When a particle encounters its antiparticle, they annihilate, often producing pure energy in the form of photons. In this case, an electron and a positron convert all their mass into two identical energy rays, which move away in opposite directions to keep total momentum zero as it was before.
Similar annihilations occur for other combinations of a particle with its antiparticle, sometimes producing more particles while obeying all conservation laws. All known leptons are listed in the table given above. There are only six leptons and their antiparticles , and they seem to be fundamental in that they have no apparent underlying structure.
In this video, we will learn how to determine the strangeness of composite particles and sets of particles and whether given interactions conserve strangeness. In the world of high-energy physics, this term has a very specific meaning.
When we first hear this term, strangeness, we might think of the six different types of quark and recall that strange is one of those types. And indeed, the strangeness of a given particle does have to do with how many strange quarks it possesses.
And yet, historically, strangeness was a concept in physics even before quarks had been discovered. As researchers studied high-energy collisions amongst particles that were very unusual, that is, strange, they noticed that in some of these interactions, there seemed to be a quantity that was conserved, much like energy or charge is conserved. This quantity was called the strangeness of a nuclear interaction, and it referred to the collective strangeness of the particles involved.
By way of example, consider this interaction of subatomic particles. This first particle here, the one with a K and a zero in its superscript, is called a kaon. In this overall interaction then, we have a neutral kaon combining with a proton to yield a charged kaon and a neutron. Now, earlier, we mentioned that this term strangeness was originally thought to be a conserved quantity like, say, electric charge. If we looked at this nuclear interaction from the perspective of electric charge, we would see that the neutral kaon has a net charge of zero.
The proton has a relative charge of positive one. Then, on the other side, our charged kaon has a charge of positive one, and a neutron has a charge of zero.
Just as we can analyze nuclear equations for their relative charge, we can do the same thing for their strangeness. Now, if we look at the quarks that make up a neutral kaon, those are the down quark and the strange antiquark. Recall we said earlier that strangeness is connected with strange quarks. The fact that the strangeness of a strange quark is equal to negative one while that of a strange antiquark is positive one comes down to historical reasons.
For our purposes, the important thing is to remember these relations. Any time we see a strange quark in a particle, we know that contributes a strangeness of negative one, while every strange antiquark contributes a strangeness of positive one. And this brings us back to the quarks that make up our neutral kaon: a down quark and a strange antiquark. But the strange antiquark, on the other hand, does. We see, according to our rule, that it gives a strangeness of positive one to this particle.
So the strangeness of a neutral kaon is positive one. A proton is made up of two up quarks and one down quark. None of these contribute anything to the strangeness of a proton. So overall, its strangeness is zero. Then, if we go to the other side of our interaction, to our charged kaon, this is made of an up quark and a strange antiquark, where, as always, the only contributor to the strangeness of the particle comes either from a strange quark or a strange antiquark.
Our rule tells us that every strange antiquark contributes positive one, which means that the overall strangeness of this charged kaon is positive one. And then, for our neutron, this is made of an up quark, a down quark, and another down quark, none of which contribute to the strangeness of the particle.
So its overall strangeness is zero. Looking at this interaction as an equation, we see that the strangeness on the left adds up to one and the total on the right is the same. In this interaction then, strangeness is conserved just like electric charge. While electric charge, as far as we know, is always conserved in nuclear interactions, strangeness is not.
When it is, as it is in this case, that tells us something about the interaction taking place. Recall that the four fundamental forces are called the strong force, the electromagnetic force, the weak force, and gravity.
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