Sunday, January 23, 2011

Our Universe Part 8: Hadron Epoch

During this epoch matter as we know it is on its way toward existence. There are no atoms yet, no electrons or even nuclei, but  hadrons, which are protons and neutrons, are forming. However, because almost all physical processes appear to be symmetrical in nature, particles of antimatter also form. Equal amounts of antimatter should annihilate matter as it appears. But it didn't, as evidenced by everything around us today. Why?

The universe, just a fraction of a second old now, operates under the four fundamental laws as we know them today. It is an expanding bubble of energetic quarks and gluons, some of which are just now settling into particles called hadrons, as the temperature cools to about 1013 K.

Hadrons are composite particles made up of quarks held together by gluons (gluons carry the strong force). There are six kinds of quarks. They all have mass, they all interact with all four fundamental forces, and they all have an antiparticle twin, but only two kinds, up and down quarks, are (generally) stable and make up protons (below left) and neutrons (below right).
Up quarks have a charge of +2/3 while down quarks have a charge of -1/3. This gives a proton a charge of +1 and a neutron zero charge. The other four kinds (called flavours) of quarks (called strange, charm, bottom and top) are all very unstable and much more massive. They do not make up stable matter. Instead they decay into up and down quarks.

Not all hadrons make up matter as we know it. Hadrons come in two general kinds: baryons, made up of three quarks of which the proton and the neutron are examples, and mesons, made up of one quark and one antiquark. An antiquark is an antimatter twin of a quark, and we will explore what antimatter is in a moment. For now, just know that antimatter and matter particles annihilate each other on contact. You might think that the two particles in a meson would instantly annihilate each other, but instead they orbit each other like binary stars, with the strong force keeping them together. Mesons are very unstable, and they only exist (very briefly) in extremely high-enegy environments like this epoch. 

Baryons, on the other hand, are generally stable. The appearance of baryons marks the beginning of matter in the universe. The mass of the proton and the neutron tell us something interesting about mass/energy equivalence. Protons, for example, composed of two up quarks (3.1 MeV/ceach) and a down quark (5.7 MeV/c2), are far more massive (0.938 MeV/c2) than the sum of the masses of their quarks. The kinetic and potential energies of the quarks is converted into mass in the proton.

About seven protons will be created for every neutron because protons have a lower mass/energy and so their production is favoured. Neutrons bound inside atoms are stable but free neutrons, as they are during this epoch, decay into more stable protons with a half-life of about ten minutes. The reason for this is that down quarks themselves are slightly unstable. They last about 900 seconds before they decay into up quarks (unless they are bound in atoms). You will find out in Photon Epoch article that almost all neutrons will be bound up in stable nuclei well before they have time to decay. 

There are many kinds of mesons, composed of various kinds of quarks combined with various kinds of antiquarks but they are all very unstable. In the universe today they are created only when cosmic rays interact with matter and inside powerful colliders. Pions, for example, are a subtype of meson that is composed of only up and down quarks. A π+ pion is made of an up quark and an anti-down quark, shown below. 
It lasts only 2.6 x 10-6 seconds before it decays into two new particles - an anti-muon and a muon neutrino. These two particles are examples of leptons, a family of particles that includes the electron. Anti-muons and their matter partners, muons, are very unstable themselves but muon neutrinos aren't. These particles are very interesting because they oscillate. The Sun, for example, radiates only electron neutrinos, but detectors on Earth pick up not only electron neutrinos but muon and tau neutrinos as well. This is because some electron neutrinos change into tau neutrinos and muon neutrinos along the way.

The current universe contains all flavours of neutrino. They are leptons like electrons but they have no charge, they might have no mass, and they do not make up atomic matter. At the low energy level of the current universe, only the up and down quarks, the neutrinos and the electron exist as particles of matter.
In this epoch, the electron has appeared but it is not stable at all yet. Electrons will first appear and remain when the universe cools to about 10 billion (1010) K. Electrons (eand their antimatter twins, positrons (e+), form when very energetic gamma photons (γreact with each other in an equilibrium reaction, shown below.

 γ γ ↔ e+ + e

At 1013 K, the universe is a soup of photons, positrons and electrons. When the universe is about 15 seconds old, it will be cool enough to favour positrons and electrons. Most of the positrons and electrons will annihilate, releasing gamma photons in the process, and temporarily reheat the universe. But one electron in every billion will survive annihilation to seed the universe with electrons. The electron story continues in the next article, Lepton Epoch.

For now, the antiquark's (and the electron's) appearance brings up a puzzling question to which there is no easy answer. Why is there matter left over in the universe today?

What is Antimatter?

Almost every subatomic particle has an antiparticle twin with the same mass but with opposite charge. The concept of antimatter came from Paul Dirac in the 1920's. He managed to integrate quantum physics with Einstein's theory of special relativity, an enormous breakthrough in its own right. Physicists could now describe particles, such as electrons, moving near the speed of light. Unexpectedly he discovered that his formulas worked equally well for electrons with a negative charge and "electrons" with a positive charge, so he suggested that each particle should have a twin of opposite charge, an antiparticle. Since then, these "electrons," called positrons, have been observed in particle accelerators. The positron, electron's antimatter twin is produced during some kinds of radioactive decay. The laws of physics are very symmetrical with regard to matter/antimatter: An antimatter Earth would be identical to our Earth. All the laws of physics in its universe would be the same and our matter would be its antimatter. The processes involved in the formation of our universe up to this epoch also seem to have proceeded symmetrically. So why isn't the mass of our universe half matter and half antimatter, or none at all?

The distinction between charged particles and their antimatter twins seems straightforward, but the situation becomes more complex when neutral antiparticles are explored. Some neutral particles are believed to be their own antimatter twins, for example photons. However, for some the distinction between a neutral particle and its antimatter twin can be subtle. An example is the neutrino. Antineutrinos can be distinguished from neutrinos and they have been observed. Both neutrinos and antineutrinos are neutrally charged, but they are opposite each other in a quality called helicity. This is what helicity means: All particles have momentum, even those without mass, and they also have a quantum spin value. All matter particles (called fermions) have a 1/2 spin. That means electrons, quarks, protons and neutrons all have a spin of 1/2. Force particles (called bosons), have a whole-interger spin of one, or two in the case of theoretical gravitons or zero, as the Higgs boson should have. Helicity is the projection of the particle's spin onto the direction of its momentum. Neutrinos have a left-handed helicity and antineutrinos have a right-handed helicity.

CP symmetry

To figure out why matter survived this epoch, well first focus our attention on the symmetry of the universe, in particular something called CP (charge/parity) symmetry. CP symmetry is the product of two symmetries: charge (which turns a particle into its antiparticle) and parity (which creates a mirror image of a physical system). Physicists expected all physical laws to preserve CP symmetry, and the universe to observe perfect CP symmetry. Yet, they now have very good evidence that the universe is only approximately CP-symmetrical. Somehow this symmetry is violated to create an imbalance of matter over antimatter from an initial condition of balance. Does that mean that the laws of physics must have acted differently on matter versus antimatter? It turns out that this is kind of the wrong question to ask. Physicists have known for a few decades that CP symmetry is in fact violated during the natural process of radioactive beta decay, and this is how it happens: When an unstable (radioactive) nucleus is placed in a magnetic field, electrons produced from its beta decay are preferentially emitted in the direction opposite to the aligned angular momentum of the nucleus. This work was done by Chen Ning Yang and Tsung-Dao Lee in the 1950's and it earned them the Nobel prize in Physics. The consequence of this is that our Earth is in fact distinguishable from its antimatter twin, at least in terms of its weak force, which carries out radioactive decay. That's a violation of parity.

Since then, physicists have refined CP symmetry into CPT symmetry. To get a better idea of this concept, try Charge, parity and Time Reversal Symmetry, in the teacher's guide to the nuclear wall chart site. The T stands for time symmetry. Time was included because, in the realm of quantum mechanics, physicists found that equations work just fine if time runs forward or backward (this goes against our experience in which we can easily tell if a film of an event, for example, is run backward or not; we experience time's apparent one-way arrow). The reason time is so critical here is that CPT symmetry can be preserved (the CP violation can be compensated for) if time is permitted to run in either direction. This idea is called retrocausality and it is built into the very foundation of a very successful theory called quantum electrodynamics (QED), which explains the bizarre quantum behaviour of light and matter. Try Richard Feynman's book, QED, in which he explains this rather spooky quantum world in which we live, and in which CPT symmetry is explained very well with the aid of diagrams. If you are curious about the nature of time itself, try my article, Time.

So what does CP symmetry-breaking have to do with more matter being created than antimatter? One candidate explanation has to do with the genesis of quarks. This story begins with a new particle called a kaon. This very unusual type of mesonic hadron can be positively, negatively or neutrally charged. A positive kaon decays into three mesonic particles called pions, each of which consists of a quark pair, in a process that involves both the strong force (g) and the weak force (W+), shown below.


One positively charged kaon (composed of an up quark and an antimatter strange quark) decays into three pions, two positively charged (both consist of an up quark and an antimatter down quark) and one negatively charged (a down quark and an antimatter up quark) in the following process: The strange antiquark of the kaon decays into an up antiquark, and emits a W+ (weak force) boson in the process. The W+ boson itself then decays into a down antiquark and an up quark. Each up quark then emits a gluon (strong force) as it decays into a down quark.

A negative kaon (the antiparticle) decays in a similar manner, with opposite charges on the particles. This quark decay process may seem complicated. The point I try to make here is that the charged kaons decay in a predictable manner that conserves charge. The decays of a neutral kaon and its antimatter twin are different. To conserve charge, they should both decay into two pions, rather than three, π+ and π-. This raises the question of how you prove there are is an antimatter neutral kaon. Couldn't they both be identical? Murray Gell-Mann and Kazuo Nishijima discovered that the neutral kaon could be distinguished from its antimatter partner in a way analogous to but different from neutrinos. One kaon has a quantum number, called strangeness, of +1 and its antipartner has a strangeness of -1. Keep in mind the + and - aren't charges here. Strangeness describes the decay of particles in strong and electromagnetic reactions over a period of time. Gell-mann's and Nishijima's theory meant that strangeness should be conserved in neutral kaon production but violated in the decay into the pair of pions. The -1 kaon would violate CP symmetry if it decayed into any π/π pair because any pair of pions will have two states of opposite strangeness, and this gives them together a strangeness of +1. This forces the -1 kaon to decay into three pions instead of two. This decay is much slower as a result because the mass/energy of the -1 kaon is just a tiny bit more than the mass/energy of its products. It's much more weakly favoured. What's really interesting here is that occasionally the -1 kaon actually does violate CP parity by decaying right into a π/π pair, about once every 500 decays. Christenson, Cronin, Fitch and Turlay discovered this violation in the 1960's.

Both the +1 kaon and its antimatter partner, the -1 kaon, naturally oscillate. There should be a mixture of equal parts of each particle in the universe during this epoch. The final decay product of the kaon is a stable lepton - an electron or a positron. The +1 kaon always decays into a an electron and the -1 kaon always decays into a positron. Because the decay of the -1 kaon is so much slower than the +1 kaon, electrons are created at a faster rate than positrons are.

Physicists now know of several examples of CP violation. The B meson is a another good example because it might explain why the universe experienced an excess of protons over antiprotons. The B meson contains a massive bottom quark, which makes it heavy and especially unstable. The bottom quark can decay into one of many different lighter quarks. Neutral B mesons can oscillate just like the neutral kaons do, and like the neutral kaon, these mesons show different rates of decay. In this case, the heavy bottom quark seems to decay preferentially into protons rather than antiprotons and this could account for some of the calculated mass of the universe today. This idea is currently being investigated. It is also explained well here, in the teacher's guide to the nuclear wall chart site. Also try the Cabibbo-Kobayashi-Maskawa matrix. These physicists won the Nobel Prize for this quark-mixing matrix theory, which explains CP violation. And finally, I recommend using this site as a reference for all of the subatomic particles I have mention in my articles, especially the unfamiliar unstable ones in this article.

These theories are theories in progress. None of them explains why there is so much more matter than antimatter in the universe today. For example, several particle accelerators have been designed to supply lots of B mesons. You need a lot of them in order to detect occasional CP violation out of all the possible decay products. Measurements from the SLAC accelerator in California predict one leftover proton for every 1018 protons created. but cosmologists place the ratio closer to one leftover proton in every 109 protons.

There is another possibility currently being investigated, that does not require CP violation. There might not be a large matter/antimatter asymmetry at all, Instead, these two kinds of matter could be widely separated from each other in large distinct regions of the current universe. From a distance, antimatter atoms would look the same as matter atoms. Only at the boundaries between pockets of these two kinds of matter would you have any evidence that they exist, this evidence being in the form of gamma photons emitted from annihilation. This then becomes a separation problem instead. No such boundaries have been detected so far but this theory comes with a tantalizing possibility: Antimatter could have a repulsive effect on matter, acting like a kind of antigravity, much like elusive dark energy does. It could be dark energy. Antimatter, if it exists in these pockets, might explain why the universe is expanding at an accelerating rate. The Alpha Magnetic Spectrometer attached to the international space station, is currently looking for positrons, a signature for antimatter in the universe. This 24-minute Cosmic Journeys video explores the mystery of antimatter and how the AMS may be able to detect its presence in the current universe:

Meanwhile, people have come up with ways to trap and use antimatter. Positrons, electron's antimatter twin, can be trapped for use in medical (PET) imaging, for example. These particles can be kept away from matter particles inside magnetic bottles because these particles are charged. Physicists have also managed to trap some antimatter atoms as well. Storing antimatter atoms is trickier because they are neutral. Scientists made a trap lined with magnetic "mirrors" for anti-hydrogen gas, by exploiting the very tiny magnetic moment specific to anti-hydrogen atoms (this is also explored in the video above). Anti-hydrogen gas could tell researchers a lot about gravity and about the origin of matter in the universe. 

During this epoch, the temperature continues to fall toward a point where matter/antimatter pairs are no longer produced. Many particles are annihilated, releasing a large amount of gamma radiation into the universe, but not all. The remaining quarks form protons and neutrons that will seed the universe with the beginnings of what will eventually become atomic matter. The annihilation process is all but over about one second after the Big Bang, when the next epoch, The Lepton Epoch, begins.

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