The Particle MASSquerade
Clutching her invitation, Alice climbed up the broad stone steps which led to the tall polished door. She could not remember how she had come to be there, though she remembered being given the invitation. "So I expect this is probably the right place for the MASSquerade, whatever that may be," she said to herself encouragingly. "I always seem to end up somehow where people want me to be."
She stopped outside the door and examined it. Its paint was very smooth and glossy, deep red in color. It had a shiny brass doorknob and an equally shiny brass knocker in the shape of a grotesque face. It was also closed and locked. Cheerful candlelight streamed from the keyhole and Alice could hear loud music being played within.
How was she to enter? The answer seemed obvious enough, so she firmly grasped the knocker and hammered loudly.
"Ow! Do you mind!" exclaimed an anguished voice from close at hand. Literally at hand, in fact. Alice stared in surprise at the door, to meet the furious glare of an irate door knocker.
"That was my nose!" it exclaimed indignantly. "What do you want anyway?"
"I am really sorry," said Alice, "but I thought that, as you are a door knocker, I might use you to knock at the door. How am I to get in if I do not knock?" she asked plaintively.
"There's no use in knocking," said the knocker huffily. "They are making such a noise inside no one could possibly hear you." And certainly there was a great deal of noise going on within: a buzz of conversation, a louder voice speaking above the rest, but still not quite audible through the door, and, above it all, the sound of the music.
"How am Ito get in then?" asked Alice, in some frustration.
"Are you to get in at all?" said the door knocker. "That's the first question, you know."
It was, no doubt, but Alice did not like to be told so. "It is really dreadful," she muttered to herself, "the way everyone will argue so." Raising her voice she addressed the knocker, though she felt a little self-conscious in talking to a door knocker at all. "I have an invitation," she said, holding it up in front of his face.
"So I see," replied the knocker. "That is an invitation to the Particle MASSquerade, which is a function for particles only. Are you a particle?"
"I am sure that I don't know," declared Alice. "I did not think that I was, but with all the things that have happened to me, I am beginning to feel that I must be."
"Well, let me see if you meet the requirements," said the knocker, rather more agreeably now that its nose felt recovered. "Let me just look at my notes for a moment." Alice did not see how a door knocker could keep notes, let alone look at them, but after a short pause the knocker continued. "Ah, yes. Here we are. The list of specifications to define a particle."
"One," it read out. "Whenever you are observed, are you invariably observed in a reasonably well-defined position?"
"Yes, I think so, as far as I know," answered Alice.
"That's fine," said the knocker encouragingly.
"Two. Do you have a unique and well-defined mass―apart from the normal fluctuations, of course."
"Well, yes. My weight has not changed very much for some time." That was what Alice believed, at any rate.
"Good, that is a very important requirement. All the different particles have their particular masses. It is one of their most distinctive features and very useful when it comes to telling one particle from another." Alice was rather taken with the notion that people might be identified by weighing them rather than looking at their faces, but she realized that particles did not on the whole have anything very definite in the way of faces.
"Three. Are you stable?"
"I beg your pardon?" said Alice, feeling distinctly affronted.
"I said, 'Are you stable?' It is a simple enough question. Or at least it ought to be: The requirement has become increasingly blurred recently. It used to mean quite simply, 'Do you decay to something else?' If you were likely to decay at any time in the future, then you were unstable, and that was that. But that wasn't good enough! People started to say, 'We cannot be sure that anything lives forever, so a distinct state that exists for a long enough time can be classed as a particle.' Then the question is, 'What counts as long enough?' Is it years, or seconds, or what? At the moment they accept lifetimes of less than a hundred million millionth of a second as being stable," he finished disgustedly. "So, I must now ask you: Do you expect to survive for longer than a hundred million millionth of a second?"
"Oh yes, I should think so," answered Alice confidently.
"Good, then I can count you as a stable particle. You had better go inside. You may not have anything better to do than stand about out here, but I have," grumbled the door knocker. There was a click and the door swung open. Alice lost no time in passing through it.
Inside she walked through an elegant entrance hall, with pale paneled walls, chandeliers, and alcoves containing statues. As they were all statues of notable particles, it was rather difficult for Alice to make out much detail. She thought it was rather clever the way the sculptor had managed to make the features of a statue appear so vague and unlocalized. In fact, to the uninitiated, they looked much like shapeless pieces of stone.
Beyond the entrance hall Alice entered a large room, which seemed to be a main ballroom or salon. It was lit by ornamental chandeliers hanging from the ceiling, but somehow they did not give much light and the room was mostly in shadow. The shadows were made more intense by contrast with a few bright spotlights which spun around the room. One came to rest in a circle of light immediately in front of Alice. Into the center of this circle leaped a figure clad much like the joker in a pack of cards. His ridiculously cut costume was gaily striped in red, blue, and green. On closer examination, Alice saw that it was also striped in antired, antiblue, and antigreen. Alice had never seen such colors before. (Unfortunately this book does not have colored illustrations, so you cannot see what these colors look like.) His fantastic appearance was completed by a mask, which was set in an unbelievably wide permanent smile.
He addressed Alice. "Bon soir, mademoiselle. Guten Abend, Fraulein. Good evening, young lady. Willkommen. Bienvenue. Welcome. Welcome to the MASSquerade."
"Thank you," replied Alice. "But who are you and what is a MASSquerade?"
"I am the Master of Ceremonies for this MASSquerade," he replied, "which is the Masked Dance of the Particles. An evening of Revelry and Revelation. An Exploration of the Mystery behind the Mask. The particles all come here to whirl about in joyous dance and, at suitable occasions, they unmask. Your mask, if I may say so, is particularly inspired," he added.
"I am not wearing a mask," said Alice coldly.
"Ah, but can you be certain of that? We all wear masks of some sort. Why, tonight we have had two unmaskings already."
"I do not see how that can be," challenged Alice. "You can only unmask once. You are either wearing a mask or you are not, surely."
"Why, it depends how many masks you wear. Particles can wear many masks. At the beginning of the evening we had a group of atoms, and then these unmasked to reveal themselves as a crowd of electrons and a number of nuclei. Later in the evening we had a further time of unmasking and the nuclei shed their disguise to show that they were actually neutrons and protons, with a sprinkling of pions among them. I confidently anticipate that there will be further revelations before the night is over.
"But now," he cried out in a voice suddenly loud enough to carry throughout the whole room, "on with the festivities! Mesdames et Messieurs, Damen and Herren, Ladies and Gentlemen, I pray you to step lively in a Collider-Dance."
There was a bustle of movement and Alice saw that the assembled particles were beginning to circulate around the room. She could not truly say that they were dancing, but they were certainly going around and around, with ever-increasing speed. The main problem was that there seemed to be no general agreement on the direction in which they were to circulate, so some went round one way and some the other.
Faster and faster the circulating bunches of particles rushed through one another. Before long the inevitable happened and two particles collided with a great crash. Alice looked over in concern to see if they had been hurt in the collision. She could not really determine whether they had been hurt, but they were certainly not the same after their interaction. She saw several small pions rush away from the collision, which she did not believe had been there before, and the colliding particles themselves were changed into something quite new. They were larger and somewhat more exotic particles than they had been-definitely they were not the same.
The dance continued and there were further collisions, more and more as time went on. With every one which took place, relatively familiar nuclear particles changed to something new and strange. Soon there was a bewildering variety of different particles present-far more types than Alice had seen before or than she had imagined to exist.
"A marvelous sight, is it not?" inquired a voice by Alice's ear. It was the Master of Ceremonies, his grinning mask a mere arm's length away. "Such a fine hadronic assembly of particulate revelers. Such a splendor of baryonic variety. Why, I do believe that there are now no two of them the same!"
Alice did not understand many of the words he had used and felt that it was wisest not to ask about them. She wanted to know, as simply as possible, just what had been happening. "Where have all these new types of particles come from?" she asked.
"They have been created in the collisions, of course. As you saw, the particles were all circulating very quickly indeed, so they each had a large amount of kinetic energy. When they collided, this kinetic energy could be converted to rest mass energy, so that particles of higher mass could be created. In the different collisions which took place, different particles were produced. Each one has its own distinctive rest mass which serves conveniently to identify it, though there are also other, more subtle, differences. I expect that by now there are no two strongly interacting particles present here which have the same mass. That is what happens at a MASSquerade."
Once again his voice became loud as he addressed the whole room. "The dance is now finished. Please assemble in your appropriate multiplets."
At his request the assembled particles began to gather together into separate little clumps, scattered around the room. Alice saw that mostly they gathered into groups of eight particles, six arranged in the form of a hexagon around the outside and two together in the middle. A few groups contained ten particles in a triangular layout which had four of the particles spaced across its base.
"There you see the particles gathered into their symmetry groups," the Master of Ceremonies said quietly to Alice. "These groups are collections of particles which all have the same values for some property, such as spin. You can see that there is a striking regularity in all the different groupings. This provides an indication of an underlying similarity beneath the skin, or rather beneath the mask. You may recognize some of the members of that nearest group," he added.
Alice looked at the eight particles nearby and saw that the two on the top edge of the six-sided pattern were a proton and a neutron. The others, however, were unknown to her.
"That is a group of baryons which all have a spin of one-half," she was told. That meant nothing at all to Alice, but for the moment she was quite prepared to believe it.
"The neutron and proton I believe you have already met. In the next row you have the sigma particle, which can manifest with both positive and negative electric charge and also with no charge at all. It consequently appears as if it were three different particles. In the center of the pattern you have the lambda, which is a single particle with no charge. These are all strange particles," he added.
See end-of-chapter note 1
"They all seem very strange to me," agreed Alice, as she came over to look at them more closely.
"No, no. Strangeness is simply a property possessed by certain particles and which happens to have been given the name of strangeness. Rather like electric charge, you know―except that it is totally different," he added unhelpfully. "The remaining two particles are both the Cascade. It comes in two different charge states, so there are two of it," he explained. "It is doubly strange, of course."
"Of course," echoed poor Alice.
"And now the time is upon us," he called out suddenly, speaking loudly and clearly so that his voice carried through the whole room. "Now is the time for the final unmasking of the evening. Mesdames et Messieurs, Damen and Herren, Ladies and Gentlemen, I bid you all. . . unmask!"
Just how it was done Alice could never quite decide, but all around her the aspect of the particles was changed. She looked at the particle nearest to her, which was the one the Master of Ceremonies had called the lambda. It no longer looked like a particle, but like a sort of bag, within which she could see three shapes. She came closer, to try to make them out more clearly, and felt herself being pulled within the enclosure. She tried to pull away, but despite her efforts she found herself sucked in.
Once inside, Alice found that there was not sufficient room for her to stand. She tried kneeling down on the floor, but the container still pressed in on her so closely that she tried lying down with one elbow on the floor and the other arm curled around her head.
In this awkward position she looked around and stared at the three small figures which she had dimly glimpsed from outside. Now that she could see them, she observed that they were different from any of the particles which she had so far encountered. Each one was colored in a distinctive shade. One was red, one was green, and one was blue. She noted that they were attached to one another with lengths of multicolored cable of some sort. It was variously striped in the three colors, together with the three anticolors which she had seen on the costume of the Master of Ceremonies.
Alice was so engrossed in studying these odd new particles that she was quite startled to hear a voice coming from one of them.
"If you think we are moving pictures," he said, "you ought to pay, you know. Moving pictures are not made to be looked at for nothing. No how!"
"Contrariwise," he added, "if you think that we are alive, you ought to say hello and shake hands."
"I am sorry," exclaimed Alice contritely as, with some difficulty, she held out her hand. She was not quite sure how it happened, but somehow she found that, instead of a hand, she was holding the large rubber bulb of an old motor horn. When she pressed it there was a loud honking noise.
"Well, who are you then?" she asked, a little irritated by this foolery.
"We need no introduction, so I shall perform it. We are the Three Quark Brothers," answered the spokesperson (spokesparticle?), wiggling heavy eyebrows at her. "I am Uppo, this is Downo, and that is Strangeo over there." Uppo was green, Downo red, and Strangeo blue.
"I hope you do not mind if I join you," said Alice, trying to make light of her awkward position.
"Why? We are not going to come apart," answered Uppo and they all laughed uproariously.
Alice was not amused; she had not found the joke very funny. In fact, on further consideration she was not sure that she had found it at all humorous. She looked at the three brothers in irritation and was struck by the fact that now Uppo was red and Downo was green.
"You have changed color," she announced in a tone that was almost accusing.
"Naturally," replied Uppo calmly, "we are usually off color. When I started I was quite green, then I felt a little blue, but now I am beginning to see red. You know that particles which have electric charge exchange photons?" he said abruptly.
"Yes, I was told about that," replied Alice.
"Well, we Quarks are colorful characters. We stick together by exchanging gluons. Through thick and thin, or rather through red, green, and blue. The gluons stick around when they see the color of our money; they monitor our color. Particles which have color all exchange gluons. The gluons hold them together in much the same way as photons do for particles with charge."
"But why do you change color?" asked Alice. "Charged particles do not change their electric charge when they exchange photons."
"No, but photons do not carry charge. There is no charge on a photon, which is why they are so popular. Gluons do carry color. When a colored gluon escapes from its source, then that hue is transferred to the Quark that catches it. It is a regular who's hue, I can tell you." As Uppo was speaking, Downo changed his hue to blue, and Strangeo became red, his curly hair taking on a particularly vivid shade.
See end-of-chapter note 2
Uppo indicated Strangeo. "There," he said, "that is a source of a different color!
"It is because we have such colorful gluons that we can never be separated. One for all and all for nothing. United we stand and divided we remain inseparable."
"I am afraid that I do not see what you mean at all," protested Alice.
"Well, we all know that opposite electric charges attract, but you can separate particles which are suffering from that sort of attraction. They are held together by photon exchange, but the photons have no charge."
"If there is-a no charge on photons then-a they free. They go wherever they want," said Downo suddenly.
"Right, because photons have no charge they are free, free to spread out as far as they want. They do not exchange other photons between themselves."
"If there is-a no change and no charge, then there is-a no transaction," added Downo. "These photons, they no do-a business together."
"Without charge the virtual photons have no business with one another, so they do not attract one another. No one gets a charge out of them. So they just spread out all over the place. The farther apart the source charges get, the more place there is for the photons to spread out over. The photons are spread out thin. They have a thin time of it, with less momentum to transfer."
"Last job, I get-a transfer," cut in Downo helpfully. "They say they going to give-a me a little momentum, but all they give-a me was the boot."
"And you felt the force of their argument," replied Uppo. "But with less momentum to give, the force gets weaker. You pull charges far apart, they lose touch, the attraction gets weaker and weaker, and eventually they get so out of touch that they don't even remember to write. Give them enough energy and you can pull them anywhere. They can get so far apart there is no attraction left to speak of. The charges are then quite independent. I expect you know what someone means by an 'independent charge,' or what I charge someone with independent means, for that matter?" he added.
"But enough about electric charges, we are here to talk about Quark charges."
"What's a Quark charge?" asked Alice curiously, always anxious to get as much clear as she could manage.
"Double rate on weekends and for up-Quarks," answered Downo. "But we-a very cheap. Our charge only a third of other particles' charge."
"One thing I do not understand," said Alice to Downo. (That was an understatement, as there were many things that she did not understand by now.) "Why do you try to talk as if you were Italian? I do not believe that you are."
"It is because he is a fermion," replied Uppo on his behalf. "Enrico Fermi was Italian."
"But aren't you all fermions?" protested Alice.
"Certainly, one for all and all for Pauli. Which nobody can deny." All three Quarks stood to attention and saluted.
"We are one group indivisible. A Quark cannot escape from inside a proton or from any other particle. This is all because of the red, green, and blue. There's Old Glory for you."
"Pardon me," began Alice.
"Gesundheit!" answered Uppo, but Alice continued determinedly.
"I don't know what you mean by glory."
"Of course you don't-until I tell you. I meant, 'There's a nice knockdown argument for you!"'
"But glory doesn't mean that!" protested Alice.
"When I use a word, it means just what I choose it to mean, neither more nor less. The question is, which is to be master-that's all. But it is another question with gluons," he added gloomily. "There is no mastering them, they never let go-not like the photons. The trouble is that the gluons are all colored. And color creates gluons like charge creates photons, so all the gluons emit other gluons, and those gluons emit more gluons. You start with just one or two, and you end up with hundreds. It's like having the wife's family stay. And because they are all exchanging gluons, they all stick together, just like the wife's family. Instead of spreading out in a wide fuzzy cloud like photons, they bunch up to form the tight, colored strings of virtual gluons that you see here. Because they are bunched up they are not free to spread out like the photons. There is no such thing as a free bunch."
"When one Quark moves away he soon comes to the end of his tether. If we have more energy, the gluons will give us more rope, but we are still hanging on the end of it. However far we roam, the gluon attraction pulls us home. We cannot break free, but we can still escape with a little help from our friends."
At that singularly appropriate moment, a photon of very high energy crashed into the little group of Quarks. Alice had had no warning as she had not seen it coming. In fact, as she now realized, photons move so fast that she had never yet seen one coming before it arrived. This photon collided with Strangeo, exciting him to a quite manic frenzy, and he rushed off, honking loudly on a horn. Behind him his tethering rope stretched out farther and farther. Alice could see that, however far it stretched, the rope was not becoming in any way thinner or weaker. It was obvious that it could go on stretching indefinitely and that the escaping Quark would soon run out of energy, with no chance of breaking free. But no sooner had Alice reached this conclusion than ... the rope broke!
Where a moment before there had been one long and steadily lengthening cord which was soaking up all the energy that the photon had delivered, now there were two very short lengths with a large and steadily widening gap between them. On either side of this break had appeared a new Quark, each anchoring one of the fractured ends of the ropes. On the end of the rope which was still attached to the two Quarks who had remained with Alice was a Quark who looked exactly like Downo, apart from being a different color. The rapidly receding Strangeo was trailing his own short length of rope, to which was attached a reversed version of Downo. This Alice assumed correctly to be an anti-Quark. "Whatever has happened?" asked Alice in some confusion.
"You have just seen a Quark escape with the help of friends in low places. In the vacuum, in fact, and you can't get much lower than that. You cannot detach a gluon rope once it has seen the color of a Quark, so we have to fool it with something which looks just like a Quark."
"And what is that?" asked Alice.
"Another Quark, of course. When the gluon string has stretched long enough so that it now contains enough energy to create the rest masses of two Quarks, then we cut the string and work the switch. One end gets a new Quark, the other Not."
"There is-a knot in the string?" asked Downo (one of the Downos).
"That's right, there's a Quark at one end, Not-a-Quark at the other."
"What's a Not-a-Quark?" asked Alice.
"An anti-Quark. And if you believe that, you should see my uncle. Part of the original string has vanished rapidly into the distance, carrying off the energy and connecting the absent Strangeo to the new antiQuark. So, you see, absence makes the part go yonder."
"He may have escaped, but he still isn't free," protested Alice.
"With a bound he was free. He is free of us now, but he is still bound. With his anti-Quark he is bound into a boson. That's like a pion, but pions can be deceiving and in this case they have formed a kaon instead. You do not see a free Quark-or even free a Quark sea, but that's another kettle of fish."
"Are the fish in-a the Quark sea?" asked Downo.
"No, there's nothing fishy about the Quark sea. Its sole purpose is to hold virtual Quark-Antiquark pairs."
"The sole I understand, and the porpoise I understand, but why the pears in-a the sea?" argued Downo.
"Forget the sea," replied Uppo, "or we will all be at sea. The point is that you will never find a Quark on his own."
See end-of-chapter note 3
"Does that mean that you have to stay here forever with no chance of a change?" asked Alice in concern.
"Oh, we can have a change all right. They say a change is as good as a rest, but I feel quite at liberty to discuss the weak interaction."
"I heard that mentioned when I was visiting the Nucleus. I believe it had something to do with beta decay of nuclei, whatever that may be."
"It is the same thing. In fact it is a far, far beta thing. What happens is that a neutron inside the Nucleus changes into a proton and an electron, together with another particle called a neutrino. This neutrino has no charge, no mass, and no strong interaction. It doesn't do much of any thing really, like most of the folk I know. Anyway, that's the story we tell. What really happens is that a down Quark inside the neutron changes to an up Quark, an electron, and a neutrino. When the down Quark changes to an up Quark, then everything is on the up. It puts the charge up, the neutron becomes a proton, and there you are. Hang around, and you might get lucky."
Hardly had he spoken when, by a most convenient coincidence, one of the two Downos became blurred and began to change and lose his identity. After a fleeting moment of transition, Downo was no longer there and in his place stood a duplicate of Uppo. As he moved aside, Alice saw an electron rush away from the same spot. This was followed by yet another particle. Alice caught only the briefest glimpse of this one, something barely perceived and very hard to see at all. This she assumed to have been the neutrino, performing its usual role of ignoring and being ignored by everything and everybody.
The group of three Quarks now consisted of one Downo and two identical Uppos. Identical, that is, but for the fact that one was currently green and the other blue. "My," said Alice. "That was a most remarkable thing altogether."
Obediently the two Uppos replied, in perfect unison, "That was a most remarkable thing."
"But what can you expect," they added, "when the particles exchanged in an interaction have an electric charge? Photons don't have a charge, but this isn't the Charge of the Light Brigade. When a source emits one of these charged particles, it has to share the charge. There are no fluctuations allowed there, you know. When the particle's electric charge has changed then it counts as a different particle. You must have heard of charge accounts. That is how we Quarks get to change," they added.
"But where does the electron come from?" asked Alice, who felt that the explanation was a little lacking.
"The particles exchanged in the weak interaction are called W," began Uppo rather inconsequentially.
"What?" responded Alice, temporarily forgetting her manners. "Not 'What,' just W. It is not much of a name, but it is all they have, poor things. There are two of them, you know: One is W Plus, and one is W Minus. No one has ever asked them what the W stands for," he finished thoughtfully. "Anyhow," he continued, "these W's, as their friends call them, are very friendly types. They will mix with anyone. They interact with both leptons and hadrons, with electrons as well as strongly interacting particles. So when a down Quark decides it is time to change into an up Quark, it gets all charged up. The electric charge of the Quark has increased, so it gives out a W Minus particle to balance the books. This W in turn plays it by the book and interacts with a passing neutrino, which has no electric charge at all, turning it into an electron, which does have an electric charge. The electron finds itself in company with a lot of strongly interacting particles, where it has no right to be, and leaves as rapidly as it can."
See end-of-chapter note 4
"But where does the W find a neutrino which it can change into an electron?" asked Alice in some puzzlement. "I didn't think there had been a neutrino there before. I thought that it was emitted after the decay, along with the electron."
"Ah, that is where it fools you. You thought it should be there before, but instead it was there after. You are expecting it to arrive from the past, so it sneaks up on you, back from the future, and still arrives just when it is needed. Of course, because it came from the future, it is still around afterward, on its way to arrive. In this way it gets to be both the neutrino converted by the W and the one emitted after the decay. That cuts down on the overheads."
"But how can it arrive from the future?" asked Alice. As she spoke she had a distinct feeling that she already knew the answer to this question.
"It is an antineutrino, of course. One of my favorite anti's. Every particle has its antiparticle, which travels backward in time and so is opposite in every way. That's the great principle of antiparticles-' Whatever it is, I'm agin it."'
"And is there no way any of you can ever get free?" asked Alice, to be quite sure on this point.
"No, no way at all," they assured her.
"Does that mean that I cannot escape either?" asked Alice in dismay, as she did not really wish to be trapped with them forever. "Not at all. You have no color so the gluons don't hold you. You are one of the most colorless people we have ever met, so there is nothing to keep you; you can leave whenever you wish. We won't even notice. You can get up and walk away. Just don't forget to leave a tip."
This sounded much too simple, but Alice tried it anyway. She stood up and found that indeed there had been nothing to prevent her from leaving the group at any time. She stretched after her cramped confinement in such a small space, looked around her, and found that she was standing face to mask with the Master of Ceremonies. His grinning mask was just a few feet from her face. She stared at him, hypnotized by his wide, fixed grin and the dark eyeholes above. Deep within their black depths where his eyes should have been, she thought she could see an intense blue spark, like a distant star on a clear, frosty night.
"And how did you enjoy your meeting with the Quarks?" he asked her merrily.
"It was very interesting," she replied truthfully. "They were most colorful characters, but I did find them rather changeable.
"Was that the last unmasking that will take place tonight," Alice continued, "or are there further layers to be stripped away before I can see what is really there?"
"Who can say?" he replied. "How can you ever know if you are finally looking on the naked face of Nature or if you are simply looking at yet another mask? Tonight however there is but one more unmasking to come. I have yet to remove my own mask."
As he was speaking, the bright spotlight which had followed him all through the evening began to dim, and the light from the chandeliers overhead became even fainter than it had been before. As the light died, the Master of Ceremonies lifted both hands to his face and slowly pulled off his mask.
In the rapidly fading light Alice looked at the face behind the mask. She could see nothing but a smooth oval, a total blank with no features of any sort discernible. She stared in astonishment at this enigmatic visage, and, as the last gleam of light died, she saw the mask wink at her.
Notes
1. The protons and neutrons which inhabit the nucleus (known collectively as nucleons) are examples of strongly interacting particles, also known as hadrons. There are many other hadrons, though not all particles interact strongly. The class of particles which are known as leptons do not feel the strong interaction at all. Electrons belong to this class and so are not bound inside the nucleus together with the nucleons. They are aware of the nucleus only as a positive electric charge which holds them loosely bound within the atom.
Experiments in high-energy physics have discovered hundreds of strongly interacting particles. This situation presents a fairly familiar scenario in physics. Whenever a class contains a very large number of members, they usually turn out to be built up as composites of something more basic. The various chemical compounds identified are all composed of atoms. There are 92 naturally occurring varieties of atoms that are stable, and they are all composed of electrons arranged in differing numbers around a central nucleus. Nuclei in turn are composed of neutrons and protons bound by the ex-change of pions. These are mentioned in the previous chapter. Now the neutron and proton are found to be just two members of a class with hundreds of others: Κ, ρ, ω, Λ, Σ, Ξ, Ω, Δ, and so on. These particles have now been shown all to be composed of quarks.
2. Quarks are held together by forces like, but yet unlike, the electrical interaction. These forces do not act on electrical charge, but on something else which is called color charge or just color. This has nothing to do with color as we normally understand it; it is just a name which has been given to something completely new. The fact that the word color is already in use is perhaps unfortunate, though it is not the first time that a word has had two different meanings.
The interaction between two electrically charged particles is due to the exchange of virtual photons. The interaction between quarks is caused by the exchange of a new class of particles which have been named gluons. There are differences between the interactions: Electrical charges come in only two forms, positive and negative, or charge and anticharge. The photons which are exchanged between electrical charges are themselves electrically neutral; they carry no charge and so do not emit more virtual photons in their own right.
The gluons exchanged between quarks are emitted by a form of charge carried by the quarks, but completely different from the normal electrical charge. It is called color charge, though it has nothing whatsoever to do with the colors that we can see. While there is only one form of electrical charge, together with its opposite, or anticharge, there are three different forms of color charge, given the names blue, green, and red. Again it should be stressed that these names are just conventions and have nothing to do with ordinary color. Associated with each color charge there is an anticolor, and there are two ways of producing color-neutral objects. With electrical charge you can only produce an electrically neutral object by combining charge and anticharge (positive and negative charge). There are two ways to produce color-neutral particles: a combination of color and anticolor (as in bosons) or a combination of all three colors of quarks (as in fermions).
3. When particles are bound together by the electrical interaction, the potential energy in the binding decreases rapidly as they move far apart. If a particle is given enough energy, it can break free completely, as a rocket which has reached escape velocity has then enough energy to break free of the earth's potential. When a gluon string has already been stretched, however, it takes just as much energy to stretch it a little farther as it did initially. It is like stretching an elastic string; it does not get any easier the farther you stretch it. It is also like an elastic string in that, when you stretch it, it can break.
The gluon string is capable of absorbing more and more energy as the quarks separate and the string stretches. Eventually the energy in the string is more than is necessary to create a quark-antiquark pair. The string breaks and its broken ends are terminated on the color charges of the new quark and antiquark. In place of the original bound system of three quarks there are now two separate systems, one of three quarks and one of a quark and an antiquark. Instead of releasing a free quark the energy has created a new particle, a boson. This will always happen and free quarks are never produced.
4. Though the quarks cannot escape from the "particles" within which they are bound, they can change from one type to another. This is caused by a peculiar process called the weak interaction. The weak interaction is a broad-minded process which will interact with virtually everything. The electromagnetic interaction affects only particles that have electric charge. The strong interaction affects only the strongly interacting particles (or hadrons) and not leptons. The weak interaction will affect them all, though the effect is rather slow and weak because it is a weak interaction.
The weak interaction is peculiar in that it can change quarks. It can change either a down quark or a strange quark to an up quark. In the process, the electric charge of the quark is changed, with the surplus charge carried off by the "W boson," the type of particle which is exchanged in the weak interaction. This charge may then be handed over to newly created leptons, an electron and a massless electrically neutral lepton known as an antineutrino. This happens in the process of nuclear β decay, in which a radioactive nucleus emits a fast electron. This process had been known for many years, but was odd in that it was quite clear that there were not any electrons available within the nucleus to be so emitted. The electron is created in the decay process and, as it is not bound, leaves the nucleus immediately.