Beyond the God Particle (35 page)

In order for this to work, somehow the heavy elements must become
liberated from the cores of the super-massive protostars in which they were formed. Indeed, the nuclear furnace interiors of these monstrous stars eventually poison themselves. Filling with iron, the most stable atomic nucleus, they can no longer burn by nuclear fusion. The protostars then begin to collapse. Commanded by gravity, they cave inward upon themselves. No longer opposing gravity with the intense radiation of their nuclear engines, a sudden and rapid change occurs deep within their cores. There, the atoms of iron, supporting the entire weight of the massive hulk against the collapse by gravity, like the hull of a sinking submarine, give way and implode. The iron atoms are squeezed, subject to enormous pressure and density. This instantaneously creates a new state of matter, never before present in the universe—solid neutrons.

We've come a long way from Democritus, and we've learned that atoms consist of
electrons
, outwardly orbiting the compact nucleus that defines the center of the atom. The nucleus is made of
protons
and
neutrons
. When a protostar reaches its last stage of collapse, the electrons and protons in its core are squeezed together, merging within one another. A new set of physical processes, normally silently lurking in the background shadows of the everyday world around us, suddenly jumps to the fore. These are the
weak interactions
, the lowly radioactive decays that were not previously observed until Henri Becquerel's work in the 1890s, and they quickly convert the squeezed protons and electrons into neutrons. This produces, as a by-product, an explosive blast outward of elementary particles, the
neutrinos
. The dominant process of the weak interactions that destroys the monstrous protostars takes the form:

or, “proton plus electron converts to neutron plus electron-neutrino.” It's just beta decay slightly rearranged.

At the instant of the collapse of the core of a protostar, the weak interactions have stolen the show. The innermost core of the star is compressed into a ball of pure neutron matter, extremely compact, perhaps only ten miles in diameter, and yet as massive as our sun, but a trillion times more dense. The neutrinos, however, stream frantically outward from the core. As the neutrinos burst forth, they exert extreme pressure on the dense, hulking
outer parts of the protostar—
the outer shell of the star explodes
. This marks a
supernova
—the most intense and spectacular explosion to occur in the universe, after the big bang.
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It is remarkable and ironic that this ferocious “mother of all explosions” involves the lowly neutrino, an elementary particle that would seem otherwise to be the most inert and inconspicuous of all particles. Out blast the neutrinos, taking with them all of the outer matter of the star, and all of the newly synthesized elements, producing a brilliant flash of light, many thousands of times brighter than all of the stars shining within a single galaxy. The outer shell of the body of the protostar, containing all the elements from hydrogen to iron, is blown out into space, making a gigantic cloud, or “nebula” from which future and second-generation stars and solar systems (and us) will form. A dense spinning neutron star, or perhaps a black hole, is left behind. This is the tiny remnant of the pure neutron core of the protostar that was blown inward in the mighty supernova explosion, a few miles in diameter, spinning on its axis faster than once per second, but with a mass greater than that of our own sun.

Over time, the nebulae of gas and dust and debris, now containing the heavy elements—the cindered remains of the many deceased protostars in their violent fates—accumulated and encircled the galaxies. This gave the galaxies a new and grandiose shape: that of gossamer spirals with their outreaching and enveloping spiral arms. In the outer spirals of the galaxies were born the offspring of the protostars, the second generation of smaller, yellowish stars, like our sun, together with the comets, asteroids, moons, and planets. These were composed of the gas and the rocky and metallic remains of the protostars.
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The existence of everyday matter, the existence of the planets and the world we inhabit today, the existence of life and
our very existence
owes to the violent annihilation of these anonymous protostars that died in the ferocious oblivion of their supernovae, billions of years ago. All of our “everyday matter” was cooked together within these monstrous conflagrations. This process of heavy-element formation is ongoing throughout the universe even today. Many smaller large blue giant stars exist today, shining with the light of the fusion of almost pure hydrogen and helium, dwelling within the inner recesses at the centers of galaxies, detonating from time to time. In otherwise dim and distant galaxies millions of light-years
away, the supernovae light them up for a moment, flashing in the dark, distant universe like fireflies in the night. And some stars within our own galaxy, and not too distant from Earth, perhaps the unstable and dying Eta Carinae (eta kar-in-i), will one day brighten our own sky with their cataclysmic finales.
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THE PARTICLES WITH THE SMALLEST MASSES

Neutrinos hold a certain fascination because they are so weakly coupled to matter that they are very hard to detect, particularly at low energies. They are only detectable through the weak interactions. There are more than a hundred trillion neutrinos passing through your body every second, mainly from the sun. The sun emits neutrinos copiously as they are associated with the nuclear fusion processes that generate sunshine and the synthesis of atomic elements. These neutrinos pass freely through the earth, so your neutrino bath is harmless and continuous, and doesn't depend (much) upon day or night, whether the sun is up or has set.

Neutrinos come in three types, or “flavors.” These are called “electron neutrino,” “muon neutrino,” and “tau neutrino” (sometimes we just call them 1, 2, and 3). They are named for their closest relatives in the weak interactions, the electron, the muon, and the tau leptons. This pairing of charged lepton (e.g., the electron) with its neutrino (electron neutrino) is part of the symmetry of the Standard Model (see
Appendix
).

For many years it was thought that neutrinos were massless particles, that they only come in a left-handed variety (and their antiparticle would therefore be right-handed), and that they do not couple to the Higgs boson. If particles are massless, then there is no L-R-L-R march through space-time. Massless particles are either pure L or pure R and always travel at the speed of light. However, in the past few decades, we have learned that neutrinos do, in fact, have miniscule masses, but they are masses that are extremely hard to measure, and hence they are very feebly coupled to the Higgs boson. To this day the neutrino masses have been detected but not precisely measured. However, neutrinos also exhibit a dramatic phenomenon associated with their masses: they “oscillate,” between their various flavors (electron, muon, or tau) as they propagate through space.
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The neutrino masses, just like those of the other leptons or quarks, involve the Higgs field filling all of space. As you have learned, this leads to a “forced march”—the familiar L-R-L-R—where now the L and R are two distinct neutrino “chiralities” of “left” (L) and “right” (R). Each time a neutrino takes a step in the march, however, it very slightly changes its “flavor” identity. That is, if we go through one complete cycle, L-R-L, an L muon neutrino will end up as mostly an L muon neutrino, but it will pick up a little bit of electron neutrino or tau neutrino. So after many such steps the identity of the original muon neutrino has changed, and it has accumulated a significant probability of becoming a muon neutrino or a tau neutrino. (Likewise, if we “launch” an electron neutrino or a tau neutrino it, too, will similarly change identity.)

It would be as if every time your pet hamster took a step in his hamster wheel, he acquired a miniscule quantum probability of being a mouse. After many rotations of the hamster wheel you might find that he had morphed completely into a mouse. After some more running, perhaps he morphs into a rat. No doubt, you would become quite curious about this phenomenon, as have physicists become fascinated with neutrinos.

And, of course, this raises various questions: Does a mouse also morph back into a hamster? What does a rat morph into, a hedge fund manager? (Sorry, we couldn't resist that one.) Are there possible morphs other than mice, rats, and hamsters? Do things work in reverse as they do forward in time (time-reversal invariance)? Do left-handed hamsters behave the same way as right-handed ones? You can easily see the large multiplicity of questions we are interested in with regard to neutrinos.

In studying the weak interactions, we encountered the L neutrino (or R antineutrino). Only the L neutrino “feels” the W bosons and participates in weak interactions. Only later, with very sensitive experiments did we encounter neutrino mass, and therefore the march of the L into the R neutrino through the interaction with the Higgs boson. But—wait a minute—for such a long time we thought neutrinos were massless, so we didn't ever have to worry about the R neutrinos. Now we find neutrinos have masses, so if we make an L neutrino in a weak interaction, then what is the R neutrino into which it steps in the mass march?

Here is a mystery involving neutrinos that we don't encounter with electrons, or muons, or quarks. It's a little tricky—we have to do some
bookkeeping—but it isn't too hard, so hang in there. We've learned that the phenomenon of mass always requires an L-R-L-R march, whereby, e.g., an L electron converts to an R electron, which converts back to an L electron, and so on. The L and R electrons are two different particles that become independent of each other if we turn off the effects of mass (which is what happens as they approach the speed of light, from a stationary observer's perspective). Furthermore, both the L and R electrons must have the exact same electric charge of –1 since electric charge is conserved.
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Because of the foundational constraint—the conservation of electric charge—we wouldn't dare hypothesize that the R electron is just the anti–L electron, that is, the R positron in disguise. Indeed, the anti–L electron, or R positron, has right-handed chirality (its spin is counter-aligned with its velocity at the speed of light—it's the absence of L in the vacuum, hence R). But the R positron has an electric charge of +1, so it cannot participate in a L-R-L-R…march, since the electric charge would then oscillate in time: (–1)…(+1)…(–1)…(+1)…hopelessly violating the conservation of electric charge. No way! The L and R electrons, and also the L and R positrons, are all distinct particles—the lowly electron involves a total of 4 different particles, or 4 components; the same is true for the other charged leptons, muon and tau, and for quarks as well. This 4-component system of a spin-1/2 particle is called a “Dirac particle.”

However, neutrinos are different than quarks or leptons—they are electrically neutral—they have
zero electric charge
. It therefore becomes thinkable that an L neutrino can actually flip into
its own
anti–L neutrino, which has an R chirality. The point is that for neutrinos,
because they have no electric charge
, the one-step L-R flip can, theoretically, also
flip (particle) into (antiparticle)
. It is as though the hamster at every step on his wheel flips into an anti-hamster and subsequently back again (and then there are also the mixings of flavor, the slight probability of flipping into an anti-mouse and an anti-rat and so on). A particle that flips into its antiparticle when it does the L-R step is called a “Majorana particle” (my-hor-AH-na). It's actually doing the “L-anti-L” step.
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This is
the neutrino mystery
: we don't know if the masses of neutrinos are of the “Dirac form” (requiring 4 distinct components: L, R, anti-L, anti-R) or the “Majorana form” (requiring only L and anti-L). There may indeed be an independent R neutrino, and also the anti–R neutrino. The L-R-L-R
march involves the flipping of an L neutrino, which we can produce in a weak interaction, into a “sterile” neutrino, R, which we only see because of the mass. In that case, neutrino masses are just like those of charged particles, and they are then of the Dirac form. However, it is entirely possible, and in fact very likely from a theoretical perspective, that an L electron-neutrino flips into its own anti–L electron-neutrino—the antiparticle is then the R state, and the mass is of the Majorana form.

How can we tell if neutrinos have Dirac masses or Majorana masses? This is a really big question, one we hope to develop the tools to answer in the future. It may only be answered by seeing a particular, previously unseen, ultra-rare nuclear process called “neutrinoless double-beta decay.” Unfortunately, for lack of space we have to send you to another source to read more about this phenomenon.
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