Monday, May 28, 2018

A Look at the Top Quark

All the known matter in the universe is composed of two classes of particles: quarks and leptons. There are six types, or flavors, of quarks and six types of leptons. The figure to the left shows these fundamental particles. Three of the leptons, the electron (e), the muon (μ), and the tau lepton (τ) have an electrical charge that is a factor of 1 that of a proton, and three of the leptons, called the electron neutrino (νe), the muon neutrino (νμ), and the tau neutrino (ντ) have zero electrical charge. Quarks are named (in order of increasing mass) up (u), down (d), strange (s), charm (c), bottom (b), and top (t). The up, charm, and top quark have a charge that is +2/3 that of a proton, and the down, strange, and bottom quarks have a charge that is 1/3 that of a proton.

Therefore, in an atom composed of a nucleus surrounded by electrons, the electrons are fundamental particles, which means they are not composed of anything smaller as far as we know. But the nucleus is composed of neutrons and protons, which are themselves composed of quarks. At a very basic level a proton is made up two up quarks and a down quark with electric charge +2/3 + 2/3  1/3 = 1 while a neutron is made up of one up quark and two down quarks with an electric charge of +2/3  1/3  1/3 = 0. The two quarks and two leptons in the first column in the figure are called the first generation of particles, the second column is the second generation of particles, and the third column is the third generation of particles. Most of all the matter we know of is made of the first generation of particles since atoms are made of neutrons and protons and electrons with the neutrons and protons made of up and down quarks.

Quarks are very bizarre in many ways. First, as already indicated, they have a charge that is a fraction of the proton's charge rather than an integer value of the proton charge. Second, as far as we know they have no size since our theoretical idea of fundamental particles is that they don't have any size. Third, they can not be isolated to exist by themselves. Though leptons, like an electron, can be found by themselves quarks are always bound together in groups of three called a baryon (protons and neutrons are examples of baryons), or bound together in a quark-antiquark pair called a meson. (Every particle in the opening figure has an antiparticle partner with the same mass but opposite charge. The antiparticle of the electron is a positron with the same mass as an electron but an electric charge of +1. The other antiparticles are just named with an anti as an anti-up anti-quark with the same mass as an up quark but an electric charge of 2/3 rather than +2/3).

The top quark is the heaviest fundamental particle so far ever discovered. It has a mass equal to about 184 times that of a single proton. This is an unbelievably large mass for a fundamental particle. It is approximately the mass of a single tungsten nucleus which has an isotope composed of 552 up and down quarks.  In other words, one top quark has the same mass as about 550 up and down quarks! When a top quark is created in the laboratory it only exists for about 5 × 1025 seconds before it decays to other particles. This is such a short length of time that it doesn't have time to actually bind together with other quarks. Thus, it is the only known quark that exists apart from other quarks, (though admittedly it doesn't exist very long). The large mass of the top quark along with the large mass of the Higgs Boson plays a role in allowing our universe to have a stable or meta-stable vacuum. That is, the masses of these two very heavy particles are correctly tuned to allow our universe to exist over a long period of time.

In a previous blog post I discussed virtual particles and described how top quarks produced virtually inside protons actually fine-tune the mass of the proton to give its life-permitting value. Let me repeat some of what I said in that entry.

If the mass of the proton were to change slightly there would be many life-destroying consequences. Stars like our sun that burn stably for a long time would not be possible. If the proton's mass were to change slightly neutron decay would be altered and similar changes would radically affect chemistry and biology. Very rarely inside the proton, a top quark and a top-antiquark will momentarily be created and destroyed as virtual particles. Its like two pickup trucks being created and destroyed inside your bicycle while you're riding it. So ultimately, the mass of the proton is partially determined by the mass of the virtual top quarks that are momentarily created then destroyed inside the proton. Consequently, if the mass of the top quark were to change by a few percent, virtual top quarks in the proton would change the mass of the proton as well, ultimately leading to a universe in which life could not exist. So the exact mass of the top quark and its virtual existence inside the proton is required for our existence in this universe.

The Large Hadron Collider at CERN is a top quark factory. It creates over 100 top or anti-top quarks every minute. These particles are created from the energy of the colliding protons. Einstein's famous equation E = mc2 means that mass is one of many forms of energy. Since energy can be transformed from one form to another, in our particle colliders  some of the kinetic energy (energy of motion) of the colliding protons transforms into mass energy and thus creates very heavy particles such as the top quark. (In chemistry class students are sometimes taught that mass can't be created or destroyed but, in fact, mass can be created and destroyed easily as energy is transformed from one form into another.) One of the most active areas of research at CERN is studying the properties of the top quark. Because of its unusually large mass, physicists expect that careful measurements of the properties of the top quark may provide deep insight into some of the mysteries of nature.

Other than some extreme fine tuning required for our existence, I don't know that we necessarily gain any specific insight into the character of God by studying the top quark. Yet as a scientist who is also a Christian I constantly marvel at the unexpected and fascinating workings of the universe. I expect God's creation to show his character. The work of art shows the soul of the artist. The intricacies of how nature work are always amazing to behold and much more ingenious and expansive that I could ever imagine. This reminds me that the creator behind it all is also more ingenious and expansive that I could ever imagine.

The grandeur of the universe is not only revealed in the amazing things we have discovered about the subatomic world including the top quark. As the first sentence in this blog implies, the quarks and leptons we know about make up only a small fraction of the energy density of the universe. The rest of the universe is made up apparently of dark matter and dark energy. Those entities will be the subject of the next blog post.

International headlines announcing the discovery of the top quark at Fermilab in 1995

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