25 Days of Particles: Day 25

Photon

Classification: boson
Fundamental: yes
Mass: massless, as far as we can tell
Interactions: electromagnetic
Spin: 1
Lifetime: stable

To conclude our Christmas tour of particles, there is one more force carrier we must meet. The strong force has its gluons, the weak force has its Ws and Zs, gravity even theoretically has its gravitons, and the electromagnetic force has its photons. These massless packets of energy interact with all charged matter and carry around electromagnetic fields. Since they are massless, photons are very easy for matter particles to emit and absorb, which is why all charged matter always interacts electromagnetically with its surroundings. Since photons are not self-interacting, they can travel away from the particles that create them, allowing the electromagnetic force to be observed both micro- and macroscopically. This makes photons the only vector boson that physicists can detect directly.

In everyday occurrences, photons are what makes up light, or all forms of electromagnetic radiation. The visible light we see by, the infrared heat we feel, the microwave radiation we use to heat food, the ultraviolet radiation that gives us sunburns, all are from photons. The apparent difference in behavior is due to the different amounts of energy carried by the photons involved.

25 Days of Particles: Day 24

Graviton

Classification: boson
Fundamental: yes (we think)
Mass: Unknown, but it should be massless
Interactions: gravity
Spin: 2
Lifetime: Unknown

Today's particle was theoretically predicted because physicists like symmetries and things to behave in the same ways. All of the other forces, when crafted into modern, quantum field theory terms, have vector bosons zipping around as the force carriers. The strong force has the gluon, the weak force has the W and Z bosons, and so on. So gravity, when it finally gets made into a quantum field theory like everything else is, should have a boson, and that boson has been dubbed the graviton.

We know it should be massless, because the force of gravity travels a very long way and lighter things travel farther than heavier things. It should be spin 2, because the math says so (gravity comes from a second-rank stress-energy tensor, which provides the necessary components for a spin-2 boson). We also are not going to be directly detecting a graviton any time soon. First, the mathematical theory necessary for a quantum theory of gravity isn't behaving very well right now; our best efforts yield theories that predict infinite probabilities and other impossibilities. Second, gravitons don't interact with matter much, which is why gravity is extremely weak for particles and only really noticeable when large quantities of matter are clumped together. These means that we can't build a detector that can detect gravitons in a reasonable amount of time.

Instead, physicists hunt for gravity waves, or coherent states of many gravitons. The LIGO and VIRGO experiments are already on the hunt. While they won't detect the particles, they can possibly tell us more about how gravity behaves for quantum things, which would be awesome. Most of what we know about gravity now comes from the work of exactly two people: Isaac Newton and Albert Einstein. It isn't the easiest subject to tackle.

25 Days of Particles: Day 23

Magnetic Monopole

Sorry, but I can't give you a list of even potential properties for this particle. It's defined by exactly one characteristic, that of having only one magnetic pole.

As everyone who's played around with a few magnets knows, magnets have ends that repel each other and ends that attract each other. These attracting/repelling regions are the poles. Every magnet has two, a positive one and a negative one, and cutting a magnet in half is going to yield two smaller magnets that still each have two poles. Likes repel and opposites attract.

But magnetism is just part of electromagnetism, and it's caused by the movement of charged particles. With electricity, you also get positive and negative stuff, with the unique element that you can separate the two, forming an electric monopole. In fact, that's really easy to do. An electron is an electric monopole, contentedly going through life with just one of kind of charge. But magnetic monopoles, particles that have only one end of the magnet, either positive or negative, don't exist, or at least have never before been observed.

So why is that weird? Why should we want magnetic monopoles to exist? Back in 1931, Paul Dirac was hard at work creating a quantum theory of electromagnetism and discovered that if even a single magnetic monopole exists somewhere in the universe, it would explain why all electric charges everywhere are quantized the way they are. It is a very, very good thing that electric charges cancel to neutral the way they do; otherwise the universe would be a rather inhospitable, lightening filled sort of place. So every since then, the hunt has been on to find a particle that contains only one magnetic pole.

So far, no conclusive evidence has been found. It is currently thought that any magnetic monopoles must have masses greater than about 600000 MeV, or beyond the reach of previous experiments.

And yes, we use that explanation a lot. So far, that is the typical explanation when we think we should see a particle and it hasn't been there.

25 Days of Particles: Day 22

S-Top

Classification: boson
Fundamental: yes (we think)
Mass: Unknown, but heavy enough we haven't seen it yet
Interactions: same as top quark
Spin: 1
Lifetime: Unknown

There is an art to creating elegant theories in physics, an art I don't really understand very well. But I've been told it is built a lot of symmetries and conversation laws. Conversation laws describe those fundamental quantities that aren't changed by all the monkeying around of particle interactions. Symmetries describe how apparently different things act in the same way. Symmetries can seem pretty basic, such as that the laws of physics should behave the same every where in the universe. But they can also have amazing effects.

We've already met one symmetry, a symmetry of charge. The electron and positron are exactly the same except for their charge, and every particle has an antimatter partner with opposite charge. The idea of symmetries between particles has been used again, and in the 1970s a symmetry between bosons and fermions was proposed.

It seems like a crazy idea, but supersymmetry or SUSY as it was called had a few things to theoretically recommend it. Supersymmetric theories help the Standard Model mathematically behave better by canceling out divergences. SUSY theories can unify the strong force with the electroweak force, which has been a goal of theoretical physicists since they successfully showed the electromagentic and weak forces were really high and low masses parts of the same force. Finally, virtually all SUSY theories predict many neutral stable particles that could be dark matter WIMPS.

25 Days of Particles: Day 21

Dark Matter

Classification: Unknown
Fundamental: yes (we think)
Mass: Unknown
Interactions: Gravity and maybe weak
Spin: Unknown
Lifetime: Unknown

If the point of particle physics is to answer the question "What is the universe made of?", this next "particle" is embarrassing evidence that we aren't doing all that great a job.

The story begins in 1933, with Fritz Zwicky and galaxy rotational curves. He was measuring the speed of stars in galaxies and found they were moving very quickly. It takes a lot of mass to produce the gravity necessary to hold quickly moving objects, so Zwicky used the measured light from the stars closer to the center of the galaxy to estimate how much mass was holding the fast stars into the galaxy. There was not enough visible mass to hold the outer stars into the galaxy, not at the speeds they were traveling. There should have been stars flying out of galaxies at high speeds, instead of obeying some mysterious force that kept them in their orbits.

The foremost theory was that there some additional matter that wasn't giving off light, and that matter was producing the extra gravity needed. It was called appropriately dark matter, so that everyone knew which mystery was being referred to. There were also ideas that the effect could be due to our theory of gravity being wrong or something like that.

25 Days of Particles: Day 20

Higgs Boson

Classification: boson
Fundamental: yes (we think)
Mass: Unknown, but most likely heavier than 114000 MeV
Interactions: weak
Spin: 0
Lifetime: Unknown

We're now going to change gears just a bit and take a few days to discuss particles that physicists wish would show up for the holidays. These are the theoretical particles, or particles that we have good reason to think actually exist, but that nobody's observed yet.

Let's take the Standard Model. When it was getting pulled together in the 1960s, trying to include mass was a bit of a problem. Mass terms could only be allowed if the masses were zero; otherwise, the Standard Model would predict that physical laws were different in different parts of the universe or other impossibilities. But the particle masses are not zero, not by a long shot. So to make the math behave, several physicists, Peter Higgs amongst them, developed a mechanism to include mass terms.

It required postulating that the universe was full of a field that interacted with all matter. Particles that interacted more strongly with the field were the more massive particles. But the field also produced a new particle, a scalar neutral boson which was called the Higgs boson.

The Higgs boson has been a big deal since it was theorized. It is the only remaining Standard Model particle to be discovered. Also, since the boson is related to how everything gets mass, it effects all particles, including all the ones we haven't discovered yet. Any theory that tries to explain how the universe was formed or anything like that has to establish how that would change. This is why the Higgs boson is hugely important and has garnered so many dramatic nicknames.

To be fair, some of those nicknames are not so flattering. See, physicists have been hunting for the Higgs boson for going on fifty years now, and haven't found it. The Large Electron-Positron collider looked for it, the Tevatron is looking for it, and the Large Hadron Collider is getting in on the search. We know there must be some evidence, in the form of particles or particle behavior, that explains how particles get mass. It might be the Higgs boson, or it might be something else. But there's got to be something, and we'd really like to find it and solve the puzzle. We've been hunting for a long time.

25 Days of Particles: Day 19

Z boson

Classification: boson
Fundamental: yes
Mass: 91000 MeV
Interactions: weak
Spin: 1
Lifetime: 3e-25s

When the electromagnetic and weak forces were unified into electroweak theory, two force carriers were predicted. These were called the W and Z bosons, and they were discovered at the Super Proton Synchotron at CERN in 1983, thus establishing the standard model on strong experimental basis.

The Z boson interacts with particle, anti-particle pairs. It can decay into any particle that weighs less than half of its own weight. Since the Z is quite heavy, it can decay into everything except pairs of top quarks. In fact, the Z boson has been hugely useful in experimental particle physics. Since pairs of electrons or muons from Z boson decays are fairly easy to find in collisions, physicists use them to test properties of the detectors themselves. The most common method is called a tag and probe, when one lepton is tagged, and the quality of the second lepton is examined to see how good a job we did at identifying the lepton.

The Z boson also has helped put limits on the number of particle generations. The Z boson decays into pairs of neutrinos, which we can't detect. But we can establish how many Z bosons should be produced, and measure the rest of its decays, and use that to measure what fraction of decays we're missing and estimate the number of types of neutrinos. If there was a fourth family of particles with a light neutrino, the Z would decay into it and we would notice.

Actually, from Z decays we measure that there are just under three types of neutrinos. The "just under" accounts for neutrino mixing. So, if there is a fourth family of particles, its neutrino is heavy enough that the Z can't decay into it, and that would make a fourth family heavy indeed.

25 Days of Particles: Day 18

W Boson

Classification: boson
Fundamental: yes
Mass: 80400 MeV
Interactions: electromagnetic (+1 or -1) and weak
Spin: 1
Lifetime: 3e-25s

Having met all the known matter particles, let's turn to how they talk to each other, or the four forces. The force of gravity had been identified as such by Sir Isaac Newton in the seventeen century, and electromagnetism was the cutting edge of physics during the nineteenth. The discovery of radioactive decays required a new force, which was called the weak force because it happened so infrequently, and the discovery of all the positively charged protons in the atomic nucleus required a strong force.

During the 1950s and 1960s, many theoretical physicists focused on developing quantum theories of these forces. When they were done, they had managed not only to recast three of the forces into quantum terms, but they had also managed to unify the electromagnetic and weak forces into one mathematical element. These new quantum theories make up what we now have as the Standard Model, and predicted two new heavy gauge bosons that carried the weak part of the electroweak force. In particular, the decays were ascribed to the activity of the charged W boson.

25 Days of Particles: Day 17

Top Quark

Classification: quark, fermion
Fundamental: yes
Family: third
Mass: 172000 MeV
Interactions: Weak, Electromagnetic (with charge 2/3), Strong, Gravity
Spin: 1/2
Lifetime: ~5e-25 s

When the bottom quark was discovered in 1977, it seemed almost a given that it would have a quark partner. Down had up, strange had charm, and so bottom should have top. The Standard Model needed a full set of six quarks to mathematically behave itself, keeping observables finite and probabilities less than one, that sort of thing. Almost all of the properties of the top quark could be predicted, including how it could be produced at any of the experiments of the day.

What wasn't known was the mass of the new quark. The Standard Model has never been able to predict the masses of the matter particles; those have to be determined by experiment. More massive particles require more powerful experiments to produce. The top quark had to be more massive than all the other quarks; otherwise it would have been discovered by then. So the hunt was on to detect mesons containing the top quark.

25 Days of Particles: Day 16

Bottom Quark

Classification: quark, fermion
Fundamental: yes
Family: third
Mass: 4190 - 4670 MeV
Interactions: Weak, Electromagnetic (with charge -1/3), Strong, Gravity
Spin: 1/2
Lifetime: depends on meson, but often ~1e-12 s

The bottom, or beauty, or just b, quark is the down-type member of the third family. It was hypothesized in the mid-1970s before being discovered, which has become to typical method for particle discovery. By that time, with two families already known, postulating a third wasn't particularly radical. Also, having three generations of quarks allowed for a good mathematical theory describing CP violation, or when processes that we would expect to be symmetric (perhaps between matter and antimatter) turn out not to be symmetric. This can be mathematically described by the mixing between quarks, but only if there are three generations of them.

Of course, then you also have to have the b quark's partner. But the b is lighter and was discovered first, so I'm talking about it first.
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Many of the mesons that contain b quarks display CP violation in their decays, making them excellent ways to study CP violation and its possible effect on the balance of matter and antimatter in the universe. Several experiments such as Babar (from b-b-bar, and yes their slogan involved an elephant) and Belle specialized in producing and studying b mesons for exactly that reason.

The b quark also plays in an important role in many other physics processes. Since the b is fairly heavy, heavier things tend to decay into it. Also, b hadrons have a long lifetime compared to other hadrons, which means the b quarks created by the heavier things tend to travel away from where they were created before decaying. The decay of the b quark produces a jet of hadrons. So physicists studying this can find jets that originate a distance away from the main particle interaction, and be fairly confident that those jets are from b quarks and not from the other, lighter quarks. This is called b-tagging, and it gives physicists another way to either select interesting particle events from the data or reject things that they don't want to study. So even those of us who don't like to study jets salute those brave persons who look at the dozens of particles in a jet that splatter in the detector and try to sort out where they came from. 'Tis a difficult, difficult task.

25 Days of Particles: Day 15

Muon/Tau Neutrinos

Classification: lepton, fermion
Fundamental: yes
Family: second and third
Mass: < 0.000002 MeV
Interactions: Weak, Gravity (barely)
Spin: 1/2
Lifetime: stable

Today we are almost going to revisit a particle we met earlier. If you'll recall, the electron had a little buddy, the electron neutrino. This little neutral particle was appeared with electrons in weak decays and was one of the four members of the first family. The second and third families have their neutrino members as well, the muon and tau neutrinos. These neutrinos only appear in weak reactions with muons and tau leptons, respectively.

Now, neutrinos are not the easiest particles to detect and count, but it became apparent in the 1960s that some electron neutrinos were missing. One of the major sources of neutrinos in our solar system is from fusion reactions in our sun, which primarily produce electron neutrinos. The volume of electron neutrinos the sun should produce can be calculated, and experiments sensitive to electron neutrinos set about to measure that volume. They came to the conclusion that only between 1/3 and 1/2 of the expected number of neutrinos were making it to Earth.

This leaves two possible scenarios. Either we over-estimated how many neutrinos the sun makes, or something we don't know about is happening to the neutrinos in transit. What exactly was going on remained a mystery for over thirty years.

25 Days of Particles: Day 14

Tau

Classification: lepton, fermion
Fundamental: yes
Family: third
Mass: 1777 MeV
Interactions: Electromagnetic (charge -1), Weak, Gravity
Spin: 1/2
Lifetime: 2.9e-13 s

Next up in our tour of the particle physics world is the tau lepton. This is the third family's charged lepton, the correspondent to the electron and muon. The tau is also the most massive particle we've met to date. The tau lepton all by its fundamental self is about as heavy as a helium atom.

It's a fact of nature that objects tend to fall into the lowest energy state they can get to. In particle physics, this takes the form of heavy stuff decaying into light stuff. Heavy particles tend to decay really quickly, and if they don't, it normally means the decay is forbidden in someway. Tau leptons decay quickly enough that they are almost never seen in a particle detector. When particle physicists want to study taus, they need to search for their decay products. This was how the tau lepton was discovered in the first place. Physicists at SLAC (the Stanford Linear Accelerator Center) observed events containing an electron, a muon with the opposite charge from the electron, and at least two invisible particles in their electron-positron collisions. They knew there needed to be more than one invisible particle to make conservation of energy and momentum work out. It took several years to establish exactly what the physicists were seeing, since the decays of charmed mesons can produce a similar signature at a similar energy. While the tau was first observed in the mid-1970s, the Nobel prize in physics for its discovery was awarded to Martin Perl in 1995.

It's short lifetime distinguishes the tau from the other leptons for all experimental purposes. We never see it directly, and the tau lepton is the only charged lepton heavy enough to decay into quarks. Those quarks form jets, and it tends to be difficult to figure out what process a jet comes from. For these reasons, the tau tends to be more of an object to be searched for than a tool that can be used to discover new particles.

25 Days of Particles: Day 13

Charm Quark

Classification: quark, fermion
Fundamental: yes
Family: second
Mass: 1270 MeV
Interactions: Electromagnetic (charge +2/3), Strong, Weak, Gravity
Spin: 1/2
Lifetime: unstable, but lifetime depends on baryon or meson

In a fit of girlishness, I just had to include the picture above. Isn't the charm quark cute, with his little rose? This is one of the plush toys of the Particle Zoo. I would get myself one for Christmas, if I could ever make up my mind which one I would want to get first.

When the quark model was first proposed, it involved only three flavors of quark, the up, down, and strange. This was before the family structure of particles was really known, so physicists weren't looking for new quarks. However, these three quarks had a little theoretical problem. The strange quark is unstable, and for the electromagnetic force identical to the down quark. So a strange quark should be able to decay into a down quark without a change in electric charge. Such a decay is called a flavor-changing neutral current.

However, flavor-changing neutral currents don't exist. This was one of two facts I had to memorize during my first particle physics course and recite to myself while I was doing my homework. There are no flavor-changing neutral currents.

25 Days of Particles: Day 12

Muon

Classification: fermion, lepton
Fundamental: yes
Family: second
Mass: 105.7 MeV
Interactions: Electromagnetic (charge -1), Strong, Weak, Gravity
Spin: 1/2
Lifetime: 2.2e-6 s

To meet and appreciate today's guest, we need to jump back in time a bit. In 1936, Carl D. Anderson and Seth Neddermeyer were studying cosmic radiation and observed negatively charged particles that had more mass than electrons but less than protons. They dubbed these particles mesotrons, using the Greek prefix 'meso' meaning 'mid.' But this was at the beginning of the discovery of the particle zoo, and physicists soon found many other mid-range mass particles that also got called mesons. The mesotron got named the mu meson to tell all the mesons apart.

But the mu mesons decayed differently than the other mesons, producing two neutrinos instead of just one, and all the other mesons turned out to be made of quarks. Particles containing two quarks were officially named mesons, and since the mu particles were not made of quarks, they were officially named muons.

25 Days of Particles: Day 11

Gluon

Classification: boson
Fundamental: yes
Mass: 0
Interactions: Strong
Spin: 1
Lifetime: stable

So, we have a bunch of particles that spend their merry lives zipping around and interacting with each other. But what does it mean for particles to interact? In our current theories of particle physics, we understand that particles can absorb or emit force-carrying particles, which changes the charges and spins and momentum of the particles involved. Each type of interaction has its own force carrier(s), and each particle will only interact with specific force carriers. These force carriers exist in matter but don't make up matter, and all the known force carriers have spin of one. This makes them vector bosons (with non-zero integer spin) instead of fermions with spin-1/2 like all the matter particles.

To illustrate how important the force carriers can be, consider the proton. The proton has a mass of 938.3 MeV. We know that the proton is made up of three quarks, two ups and a down. It's a little tricky to establish quarks masses, but the highest estimates for the masses of these quarks yields a sum of 12.4 MeV. So where does the other 925.9 MeV of mass come from?

25 Days of Particles: Day 10

Strange Quark

Classification: quark, fermion
Fundamental: yes
Family: second
Mass: ~101 MeV
Interactions: Electromagnetic (charge -1/3), Strong, Weak, Gravity
Spin: 1/2
Lifetime: unstable, but lifetime depends on baryon or meson

If you remember back to our discussion of the discovery of the kaon, you remember that one of the odd things about kaons is that they needed a new quantum number to describe them. This quantum number was called strangeness, and when the quark description of hadrons was developed, physicists figured out that strangeness meant the particle contained a specific quark. That quark inherited the name of the strange quark and particles that contain strange quarks are called strange particles. To put the kaons into context this way:

K+ = up quark + anti-strange quark
K- = strange quark + anti-down quark
K_0 = a linear combination of strange + anti-down and down + anti-strange quarks

Now, it may seem that the strange quark's properties seem awfully familiar. We've met a quark with electric charge -1/3 and spin of 1/2 before; those are the down quark's properties as well. Down quarks and strange quarks have all the same quantum numbers and can interact in all the same ways, so in the mathematical theories one could almost wonder if they were truly different particles.

But only almost wonder, because the strange quark is about twenty times more massive than the down quark. That is a lot of extra mass.

25 Days of Particles: Day 9

Down Quark

Classification: quark, fermion
Fundamental: yes
Family: first
Mass: 4.1 - 5.8 MeV
Interactions: Electromagnetic (charge -1/3), Strong, Weak, Gravity
Spin: 1/2
Lifetime: dependent on meson/baryon containing down quark

The up quark's buddy in just about everything is the down quark. He's just a little bit heavier, and he has a charge of -1/3. Like the electron and electron neutrino are paired together in all weak interactions, the up and down quarks show up together when the weak force is involved. These four particles (electron, electron neutrino, up quark, down quark) make up the first family of particles. Together, the up and down quarks can take credit for making up most of the matter of our universe. To revisit some particles we've already met:

a proton = two up quarks + a down quark
a neutron = an up quark + two down quarks
pi+ = an up quark + an anti-down quark
pi- = an anti-up quark + a down quark
pi_0 = a linear combination of up + anti-up and down + anti-down

Now you begin to see why quarks made keeping track of the particle zoo much simpler. You dream up every possible combination of two or three quarks, and there is an observable particle corresponding to that combination. Since you know the properties of the quarks, you can make really good estimations of what the properties of the observable particle should be.

But why only two or three? Why do we need to have those numbers of quarks? It has to do with how the strong force behaves. Like the electromagnetic force has a charge associated with it, the strong force has charges, too. However, the electromagnetic force has one type of charge and its opposite, while the strong force has three types of charge and their opposites. For lack of better names, these charges got labeled red, blue, and green, or the color charges. Their opposites are called anti-red, anti-blue, or anti-green.

25 Days of Particles: Day 8

Up Quark

Classification: fermion, quark
Fundamental: yes
Family: first
Mass: 1.7-3.3 MeV
Interactions: Electromagnetic (charge +2/3), Strong, Weak, Gravity
Spin: 1/2
Lifetime: stable

So we have now met several prominent members of the particle zoo, and there are several more that had also been discovered by the 1950s. The total was up to dozens of particles, all of which appeared to be fundamental, all with unique sets of quantum numbers. They could be classified by their quantum numbers, but all in all it was turning into a bit of a headache.

Between 1961 and 1964, several physicists independently proposed methods of classifying many of the known particles as clusters of smaller particles. These smaller particles had the properties of spin and electric charge, and the properties of particles like the proton or the kaon was the sum of the properties of its constituents. This enabled the dozens of known particles to be classified much more simply, and even enabled their (very) different masses to be related to one another via the masses of the constituents.

Murray Gell-Mann named this smaller particles quarks. The pronunciation was inspired by the quacking sounds of ducks, and the spelling came from a James Joyce's book, "Finnegans Wake."

25 Days of Particles: Day 7

Kaon

Classification: boson, meson
Fundamental: no
Mass: 493.7 MeV (K+ and K-), 497.6 MeV (K_short and K_long)
Interactions: Electromagnetic (for pi+ and pi-; not for pi_0), Strong, Weak, Gravity
Spin: 0
Lifetime: 1.23e-8 s (K+ and K-), 0.89e-10 s (K_short), 5.12e-8 (K_long)

During the late 1930s and 1940s, the discovery of new particles was a fairly regular occurrence. The phis and etas and rhos all showed up and were identified, what is now called the particle zoo was starting to get populated, and physicists were running out of Greek letters. In 1947, when the first evidence of the kaons was published, it seems like they could have gotten lost in the shuffle.

They didn't, because kaons have a whole lot to teach physicists about particles. They were important subjects to study in the 40s, and they still are today.

When the kaons were discovered, particle physicists classified particles according to their quantum numbers. Unique particles had to have a unique set of values for their quantum numbers, and this allowed the particles to be grouped according to their properties. But then the kaons came along and complicated things, because they needed a new quantum number to be described. When the weak force was used to produce or decay kaons, they went about it pretty slowly; when strong force interactions (such as those between pions and protons) were invoked, kaons showed up and vanished more rapidly. This was finally explained by introducing a new quantum that the strong force conserved and the weak force didn't. This quantum number was called "strangeness," which gives you a hint of what physicists thought of it at the time.

25 Days of Particles: Day 6

Pion

Classification: boson, meson
Fundamental: no
Mass: 139.6 MeV (pi+ and pi-), 135.0 (pi_0)
Interactions: Electromagnetic (for pi+ and pi-; not for pi_0), Strong, Weak, Gravity
Spin: 0
Lifetime: 2.6e-8 s (pi+ and pi-), 8.4e-17 s (pi_0)

We now have a nice picture of atomic structure, with protons and neutrons clumped together in the nucleus at the center of the atom and the electrons zipping around the outer edge, and this nice picture had issues from its inception. We'll skip over the question of how the negatively charged electron doesn't just crash straight into the positively charged nucleus; this is the question that quantum mechanics was invented to answer. Instead we'll turn to a similar problem with the nucleus itself.

How do a bunch of positively charged protons stay packed tightly into the nucleus? All that positive charge should repel so strongly that the nucleus couldn't hold itself together. The answer that isn't really an answer is that there must be some other, nuclear force strong enough to over-power the electromagnetic repulsion. This is a statement of fact, not an explanation, and physicists immediately began working to figure out what that force was. In 1935, Hideki Yukawa published his theory that there was an intermediate mass particle, called a meson, that carried the strong nuclear force between protons and neutrons.

Wait, hold on, particles carrying forces?

Yup. In particle physics, we think of some particles as being what makes up matter. These are the fermions with half-integer spins like protons and electrons and neutrons. Other particles are force carriers; they are emitted or absorbed by particles, and the emission or absorption is where the interaction happens. These guys are the bosons with integer spins. So Yukawa looked at how big atomic nuclei could get before they became unstable, concluded that was the distance a force carrier could travel, and used that to predict the mass of this meson. It, or rather they, were discovered in 1947 and got the final name of pions.

There are three pions, as they come in positively charged, negatively charged, and neutral varieties. Yukawa wasn't entirely right about the pions being the strong force carriers, either, though this was a step in the right direction and pions play an important role. Their discovery marks the beginning of the particle zoo, the ever-expanding family of particles that continues to grow to this day, and led to a much more complete picture of all the strongly interacting particles pull it off.

25 Days of Particles: Day 5

Positron

Classification: lepton, fermion
Fundamental: yes
Family: anti-first
Mass: 0.51 MeV = 9.11e-31 kg
Interactions: Electromagnetic (charge +1), Weak, Gravity
Spin: 1/2

Having started with the most familiar particles, we're now going to branch into those slightly less well known. Well, maybe not so unknown to science fiction aficionados, and increasingly to those who've read a certain popular writer of thrillers with a historical bent.

In 1928, Paul Dirac published a paper introducing the Dirac equation, a mathematical way of describing particles that combined quantum mechanics, special relativity, and the then-new concept of particle spin. In particular, Dirac used his new equation to describe and explain some features of how electrons in atoms behave. This solution allowed two possible solutions, a positive energy/negative charge one and a negative energy/positive charge one.

The positive energy/negative charge solution was easily identified as the electron, but Dirac didn't know what to make of the other possible solution. In quantum mechanics, anything that can happen does with some probability, so Dirac couldn't just toss the possible solution as impossible because it looked funny. So there should be some particle running around with the same mass and spin as the electron, but with a positive charge. There was some thought that this positive particle was the proton, that had somehow managed to pull a trick not shown in the math and become 1000 times heavier than the electron, but that was discounted as impossible. If the proton and electron were really opposites like this, hydrogen atoms would self-destruct. In 1931, Dirac published a paper stating that a "anti-electron" should exist.

25 Days of Particles: Day 4

Electron Neutrino

Classification: lepton, fermion
Fundamental: yes
Family: first
Mass: < 0.000002 MeV
Interactions: Weak, Gravity (barely)
Spin: 1/2

Once upon a time in 1930, Wolfgang Pauli was studying beta decay and discovered a problem. In beta decay, a nucleus suddenly acquires an extra proton and spits out an electron (we now know this is from a neutron decaying, but the neutron hadn't been discovered quite yet). But Pauli noticed a problem with this decay. If you start with a stationary nucleus and it suddenly fires an electron off in one direction, something's got to recoil in the other direction. This is required by the laws of conversation of momentum and conversation of energy and conservation of angular momentum. Stuff's got to recoil.

The problem was that there wasn't any recoil.

Or rather, that there wasn't enough, and to complicate matters further the missing amount of recoil wasn't always the same. Still, physicists are strongly attached to their conservation laws, and Pauli made the logical guess that something was recoiling, but that he couldn't detect it. That meant the particle had to be neutral. He dubbed this hypothetical particle the neutron. We can probably blame Pauli for starting the trend that continues to this day of naming particles way, way before we ever actually see them in experiments.

Of course, at about the same time, a much more massive neutral particle was also getting discovered, and it also got named the neutron. Enrico Fermi resolved the issue by naming the smaller of the two the neutrino.

25 Days of Particles: Day 3

Neutron

Classification: hadron, baryon, fermion
Fundamental: no
Mass: 939.6 MeV = 1.67e-27 kg
Interactions: Weak, Nuclear/Strong, Gravity (all due to constituents)
Spin: 1/2

Paired up with the protons in the atomic nucleus is the neutron. It has about the same mass as a proton, but lacks any electric charge. This makes the neutron effectively invisible to most research techniques. Physicists like to use electric and magnetic fields to pull particles out of the atoms they come in and to move the freed particles around, but only electrically charged particles pay any attention to these fields. The neutral ones continue on their way until they smash into enough stuff to loose all their energy or until they trip over something they can interact with via the weak force. While you can use both of these effects to "find" neutrons, neutrons are more able to ignore these than charged particles can ignore electric fields.

This made discovering the neutron more difficult than discovering either the electron or proton. With the discovery of the proton and successful measurement of its charge and mass, a new puzzle popped up in describing what was in the atomic nucleus. Typically, an atom with atomic number X contains X protons and X electrons, but the mass of that nucleus corresponds to the mass of 2X protons. So everyone knew there had to be some additional mass in the nucleus that wasn't contributing to the overall charge. Rutherford, after discovering the proton, postulated that there could be some particles without electric charge in the nucleus, but couldn't prove it because he couldn't detect such particles. The other prevailing theory was that there were additional protons and electrons in the nucleus that were canceling out each other's electric charge.

But that led to another problem for the atomic researchers of the time. Particles have spin, and when you combine them into atoms, atoms have the spin that comes from vector-adding the component spins, and atomic can be measured. The nitrogen-14 nucleus had a measured spin of 1, which must be the net spin of all its constituent particles. But if that nucleus contains 7 protons producing the correct charge and another 7 protons plus 7 electrons to produce the right mass, that gives you 21 spin-1/2 particles. Spins can only be either up or down, and there is no way to vector-add 21 halves to get one.

25 Days of Particles: Day 2

Proton

Classification: hadron, baryon, fermion
Fundamental: no
Mass: 938.3 MeV = 1.67e-27 kg
Interactions: Electromagnetic (charge +1), Weak, Nuclear/Strong, Gravity (all due to constituents)
Spin: 1/2

Our tour continues with the proton, the positively charged component of the atom. Unlike the highly mobile electron, the proton more or less stays put in the nucleus of the atom. The attraction between the positively charged protons in the nucleus and the negatively charged electrons is what holds the electrons in the atom. The proton share space in the nucleus with the neutron. Most atoms have equal numbers of the two, but hydrogen (the lightest element) has only a single proton in its nucleus in the most common stable isotope. Very heavy elements tend to have more neutrons than protons. It takes a lot of energy to remove a proton from the nucleus, making them the most noticeable and permanent feature of the atom. The atomic number used to classify elements is the number of protons found in the nucleus.

Since protons are so difficult to pull out of atoms, they were discovered to be particles over fifteen years later than electrons. At the same time that it was discovered you could pull negatively charged stuff out of metals, it was discovered that you could pull positively charged stuff. However, the positive charges did not have a constant charge to mass ratio, unlike the well-behaved electrons. Thomson developed the idea that the newly discovered electrons in atoms were scattered through a cloud of diffuse positive charge. In 1911, Ernest Rutherford discovered that atoms had a hard nucleus in the middle using a scattering experiment: he shot helium nuclei at gold atoms, and they bounced off. In the thinking of Thomson's model, that was like firing a gun at a tissue and having the bullet bounce back at you. Less than ten years later, Rutherford found he could kick hydrogen nuclei out of nitrogen atoms, implying that the hydrogen nucleus was a constituent of the nitrogen nucleus. He dubbed this particle the proton.

But wait! How do a bunch of positively charged particles stay bunched up together in the nucleus? This puzzling question led to the conclusion that there were more forces in the world than gravity and electromagnetism, and eventually led to the discovery that protons are not fundamental. They are made up of smaller parts called quarks. The interactions of protons we see can be traced to what the quarks inside the protons are doing. However, it is physically impossible to break a proton into its component quarks.

Protons also currently hold the special distinction of being the particles to smash in most of the world's highest energy accelerators and colliders. HERA, part of the DESY facilities in Germany, collides protons with either electrons or positrons to explore proton structure. The Tevatron at Fermilab in the US has been colliding protons with antiprotons for almost thirty years. The Large Hadron Collider here at CERN collides two beams of protons to hunt for never-before-seen particles. A salute to the proton, for leading the way into new knowledge at accelerators around the world!

25 Days of Particles: Day 1

Electron

Classification: lepton, fermion
Fundamental: yes
Family: First
Mass: 0.51 MeV = 9.11e-31 kg
Interactions: Electromagnetic (charge -1), Weak, Gravity
Spin: 1/2

Our Christmas tour of the world of particles starts with the electron. This is the negatively charged particle that zips around the perimeter of atoms, and is responsible for virtually all atomic and molecular interactions. Electricity is the flow of electrons through some material. Pretty much life and science as we know them are dependent on these little guys zipping around. It doesn't take a lot of energy to remove electrons from atoms, so they move around matter easily and interact with each other a lot. In particle physics, they can actually be a little annoying to work with. They readily interact with your detector, so you find them easily, but they readily interact with your detector, so they are going to make a mess splashing energy around as they plow into it and get stuck there.

Electrons are examples of leptons, those fundamental particles that can be separated from all other particles. They were the first sub-atomic particles to be discovered. During the nineteenth century, research into electricity and magnetism were cutting edge in physics, and several experiments had found that heating up metals caused them to spit out negatively charged bits of stuff. They even proved the stuff cast its own shadow. Early theories postulated that this strange material was negatively charged atoms or some previously unknown fourth state of matter. In 1896, J. J. Thomson and his colleagues began studying these 'cathode rays' and successfully measured their charge to mass ratio. They showed that these rays were intrinsically charged, and that you got the same rays regardless of what type of metal you heated up, and even that you got the same rays from non-metallic materials. They hypothesized that these rays were a type of matter found inside atoms, the first known sub-atomic particles, and dubbed them electrons. Contemporary experiments in radioactivity showed that these electrons were also emitted from radioactive materials without any interference from anyone, strengthening the proof that these electrons were parts of atoms. Thomson received the 1906 Nobel prize in physics for his work in discovering electrons.

Today, the electron is the most studied of the all the particles; physicists and chemists know it and the systems it appears in best. We use them for everything from making light bulbs glow to taking pictures of things too small for light to see to shooting them around giant particle accelerators. You can thank the mighty electron for both the lovely Christmas lights and for every time you get a static-electric shock when taking off your coat. It's all thanks to him.