CP Violation in Elementary Particle Physics

Building Blockes of Matter in Elementary Particle Physics Samo Stanič, University of Nova Gorica
It's always hard to decide where exactly to start a story, since it's not clear where is the real beginning. Let us start this one with a somewhat longer introduction, and one of the place where we could start is in ancient Greece. The Greeks figured out the matter should be build of small, indivisible blocks, called atoms (athomos means indivisible in Greek). Nothing much happened until 19th century when a Russian chemist Mendeleev compiled a table of all possible types of atoms he could find in nature, the so called periodic table of elements. There are some 100 elements in there and as far as chemistry is concerned, anything can be cooked out of these. Whatever chemical reaction takes place, these atoms are connected in this and that way, but they are still the same atoms and the number of each kind stays the same. But this is far from where the story ends, this is where we could just as well make a beginning. They found out that these atoms are actually made of a positively charged (like + on the battery) and very heavy (99.99% or more of the total atom mass) atom core and negatively charged and very light particles called electrons somewhere around the core. They found out that only these electrons play a role in chemical reactions, while the atom cores stay as they are. How did they figure all this out? Among others, there were these famous experiments where they bombarded atoms of various elements (actually, they bombarded thin foils of certain material) with into matter penetrating rays (like x-rays, beta rays, alpha rays) where an extraordinary discovery emerged. The found out that almost all projectiles fly right through the foil, but some of them bounce right back at a sharp angle, up to 180 degrees. This led to a conclusion that the whole mass of the atom is concentrated in a small but heavy lump at the centre (the nucleus) and only those projectiles that hit the nucleus bounce back. The others simply fly through and are not affected much by the fly-weight electrons. Let us just mention that it is of course possible to change an atom core - by bombarding it with the right type of particles and with the right energy. We are talking about nuclear reactions here, an Alchimist's dream come true - it is now possible to transmutate led into gold, for example (though very far from economical) or to glue 2 hydrogen atom nuclea into a helium nuclei. The last one is called fusion and is the process that makes our sun and all other stars tick, since a huge amount of energy is produced in it. We can make it too, but can not really control it, and its huge power is unfortunately used for destruction only at the moment (hydrogen bomb). Well, even this is not the beginning yet. By bombarding foils by more and more energetic particles it was found out that the nuclei themself are made of two types of building blocks, positively charged protons and neutrons which have no electric charge. These in turn were also found to be composites of more elementary building blocks, called quarks (the name actually comes from Joyce's Finnegan's wake, 3 quarks for master Mark, as a proton or a neutron is each made of 3 quarks). The quarks are elementary and indivisible particles and the basic blocks of matter as we see it today. Let us go back to electrons, which we in our eagerness to discover all secrets of the atom core let lie by the road for a while. They were also found to be elementary and indivisible particles, though very light. Togeather with their two heavier brothers the mu and the tau particle, which apart for larger mass behave the same as electrons, form the other group of elementary particles, called leptons (leptos means light in Greek). The family of leptons includes also another type of particles, called neutrinos. There are three types of neutrinos, one being related to electrons, one to muons and one to tau particles. For now, let me just say that their mass is so small that it was in fact not yet measured (much much smaller than electron mass which is small itself), however, latest research makes us believe that their masses are not zero after all. They are not fiction, even if we can't measure their masses we can detect them, and they will play an important role in our story later on. So, to summarize, these are elementary particles of the modern physics: Leptons: e (electron) mu (muon) tau (tau lepton) nu_e (el. neutrino) nu_mu (mu. neutrino) nu_tau (ta. neutrino) (e, mu and tau have an electric charge -1, while neutrinos have no electric charge) Quarks: u (up) c (charm) t (top) d (down) s (strange) b (beauty) (in the case of quarks, physicists gave them rather poetic names, the t quark was also originally named "truth" but is now called "top". u, c, t have charge 2/3, and d, s, b have charge -1/3. So, a proton which is charged +1 is made of u,u,d and a neutron (no charge) of u,d,d) Anything we can find in nature is made out of these quarks; also things that we can at present not find in nature but we can produce in laboratories. The leptons do not combine into larger objects, but are always by themselves. From the particles in the table above, only the electron is something we can encounter in daily life, either bound to some atom in matter or even free in a ray that flies from the back of your TV set and lights up your screen. So, what now? These particles are not clicked together like some LEGO blocks, but interact among each other four different types of processes. The first one is gravity: there is gravitational force between any two massive particles, no matter how light they might be. Direction of the force is always such that it pulls one particle towards the other. In the world of elementary particles it is negligible, since other forces among these particles are millions of times stronger. The next one we should mention is electromagnetism. It works only among particles with charge, so it doesn't work for neutrinos, it works for e,mu,tau and all the quarks. Particles with same charges repel and particles with opposite charges attract. It is much much stronger than gravity. We can say that the force is mediated between two particles by a certain mediator. Picture yourself in a small boat on a pond, and me in another one close by, both boats being at rest. If I throw a heavy ball from my boat to you in yours, the boats will drift apart a little. You could say that the ball was a "mediator" the the force between the two boats. Particles that mediate electro-magnetic force are called photons, and are known as light, radio-waves, x-rays, etc. in daily life (yes, it's all the same thing!) OK then, a proton is made of two +2/3 quarks and one -1/3 quark, so the electro-magnetism is pushing at least two of them apart, how come they stay together? Because of the so called strong interaction, which is much much stronger than electro-magnetism, and works only among quarks. It is in fact so strong, that one can in no way have a single quark - pulling one away from another requires so much energy, that from the energy invested itself a new quark pair is born, one glued to each of the original pair we were pulling apart. This is also something we can experimentally see, and the fact itself that we can is extraordinary! The strong interaction among quarks is mediated by particles called gluons. Last but not least, there is a so called weak force, which works among all leptons and all quarks, that is among all elementary particles. It is weak, as the name says, weaker than strong interaction or electro-magnetic, but it can make for example a neutron disintegrate and a proton, an electron and a neutrino is born in the process. It is mediated by the so called weak bosons, very heavy "balls" called W⁺ (positive charge), W^- (negative) and Z⁰ (neutral). They were all very precisely measured in many experiments. However, there are aspects to the weak interaction that are not yet fully understood and could have deepest impact on our understanding of the basic processes of nature. CP Violation in Elementary Particle Physics

Building Blockes of Matter in Elementary Particle Physics

Samo Stanič, University of Nova Gorica

It's always hard to decide where exactly to start a story, since it's not clear where is the real beginning. Let us start this one with a somewhat longer introduction, and one of the place where we could start is in ancient Greece. The Greeks figured out the matter should be build of small, indivisible blocks, called atoms (athomos means indivisible in Greek). Nothing much happened until 19th century when a Russian chemist Mendeleev compiled a table of all possible types of atoms he could find in nature, the so called periodic table of elements. There are some 100 elements in there and as far as chemistry is concerned, anything can be cooked out of these. Whatever chemical reaction takes place, these atoms are connected in this and that way, but they are still the same atoms and the number of each kind stays the same.

But this is far from where the story ends, this is where we could just as well make a beginning. They found out that these atoms are actually made of a positively charged (like + on the battery) and very heavy (99.99% or more of the total atom mass) atom core and negatively charged and very light particles called electrons somewhere around the core. They found out that only these electrons play a role in chemical reactions, while the atom cores stay as they are. How did they figure all this out? Among others, there were these famous experiments where they bombarded atoms of various elements (actually, they bombarded thin foils of certain material) with into matter penetrating rays (like x-rays, beta rays, alpha rays) where an extraordinary discovery emerged. The found out that almost all projectiles fly right through the foil, but some of them bounce right back at a sharp angle, up to 180 degrees. This led to a conclusion that the whole mass of the atom is concentrated in a small but heavy lump at the centre (the nucleus) and only those projectiles that hit the nucleus bounce back. The others simply fly through and are not affected much by the fly-weight electrons. Let us just mention that it is of course possible to change an atom core - by bombarding it with the right type of particles and with the right energy. We are talking about nuclear reactions here, an Alchimist's dream come true - it is now possible to transmutate led into gold, for example (though very far from economical) or to glue 2 hydrogen atom nuclea into a helium nuclei. The last one is called fusion and is the process that makes our sun and all other stars tick, since a huge amount of energy is produced in it. We can make it too, but can not really control it, and its huge power is unfortunately used for destruction only at the moment (hydrogen bomb).

Well, even this is not the beginning yet. By bombarding foils by more and more energetic particles it was found out that the nuclei themself are made of two types of building blocks, positively charged protons and neutrons which have no electric charge. These in turn were also found to be composites of more elementary building blocks, called quarks (the name actually comes from Joyce's Finnegan's wake, 3 quarks for master Mark, as a proton or a neutron is each made of 3 quarks). The quarks are elementary and indivisible particles and the basic blocks of matter as we see it today. Let us go back to electrons, which we in our eagerness to discover all secrets of the atom core let lie by the road for a while. They were also found to be elementary and indivisible particles, though very light. Togeather with their two heavier brothers the mu and the tau particle, which apart for larger mass behave the same as electrons, form the other group of elementary particles, called leptons (leptos means light in Greek). The family of leptons includes also another type of particles, called neutrinos. There are three types of neutrinos, one being related to electrons, one to muons and one to tau particles. For now, let me just say that their mass is so small that it was in fact not yet measured (much much smaller than electron mass which is small itself), however, latest research makes us believe that their masses are not zero after all. They are not fiction, even if we can't measure their masses we can detect them, and they will play an important role in our story later on. So, to summarize, these are elementary particles of the modern physics:

Leptons:
e (electron) mu (muon) tau (tau lepton)
nu_e (el. neutrino) nu_mu (mu. neutrino) nu_tau (ta. neutrino)
(e, mu and tau have an electric charge -1, while neutrinos have no electric charge)

Quarks:
u (up) c (charm) t (top)
d (down) s (strange) b (beauty)
(in the case of quarks, physicists gave them rather poetic names, the t quark was also originally named "truth" but is now called "top". u, c, t have charge 2/3, and d, s, b have charge -1/3. So, a proton which is charged +1 is made of u,u,d and a neutron (no charge) of u,d,d)

Anything we can find in nature is made out of these quarks; also things that we can at present not find in nature but we can produce in laboratories. The leptons do not combine into larger objects, but are always by themselves. From the particles in the table above, only the electron is something we can encounter in daily life, either bound to some atom in matter or even free in a ray that flies from the back of your TV set and lights up your screen.

So, what now? These particles are not clicked together like some LEGO blocks, but interact among each other four different types of processes. The first one is gravity: there is gravitational force between any two massive particles, no matter how light they might be. Direction of the force is always such that it pulls one particle towards the other. In the world of elementary particles it is negligible, since other forces among these particles are millions of times stronger. The next one we should mention is electromagnetism. It works only among particles with charge, so it doesn't work for neutrinos, it works for e,mu,tau and all the quarks. Particles with same charges repel and particles with opposite charges attract. It is much much stronger than gravity. We can say that the force is mediated between two particles by a certain mediator. Picture yourself in a small boat on a pond, and me in another one close by, both boats being at rest. If I throw a heavy ball from my boat to you in yours, the boats will drift apart a little. You could say that the ball was a "mediator" the the force between the two boats. Particles that mediate electro-magnetic force are called photons, and are known as light, radio-waves, x-rays, etc. in daily life (yes, it's all the same thing!)
OK then, a proton is made of two +2/3 quarks and one -1/3 quark, so the electro-magnetism is pushing at least two of them apart, how come they stay together? Because of the so called strong interaction, which is *much much* stronger than electro-magnetism, and works only among quarks. It is in fact so strong, that one can in no way have a single quark - pulling one away from another requires so much energy, that from the energy invested itself a new quark pair is born, one glued to each of the original pair we were pulling apart. This is also something we can experimentally see, and the fact itself that we can is extraordinary! The strong interaction among quarks is mediated by particles called gluons. Last but not least, there is a so called weak force, which works among all leptons and all quarks, that is among all elementary particles. It is weak, as the name says, weaker than strong interaction or electro-magnetic, but it can make for example a neutron disintegrate and a proton, an electron and a neutrino is born in the process. It is mediated by the so called weak bosons, very heavy "balls" called W⁺ (positive charge), W^- (negative) and Z⁰ (neutral). They were all very precisely measured in many experiments. However, there are aspects to the weak interaction that are not yet fully understood and could have deepest impact on our understanding of the basic processes of nature.

CP Violation in Elementary Particle Physics

2004/10/26 Samo Stanic