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.

Anyway, physicists in the sixties were learning this fact, and certainly didn't see this decay that they expected. It's absence was added to their list of mysteries. In 1970, Sheldon Glashow, John Iliopoulos, and Luciano Maiani theorized that if there was a fourth quark that paired up with the strange quark, we wouldn't expect to see this neutral flavor-changing decay. Of course, having a fourth quark means there should be a whole new group of potential particles in the particle zoo, which no one was really interested in looking for at the time.

In 1974, two groups of researchers (at Standford Linear Accelerator Center and at Brookhaven National Lab) were each probing a new energy region and saw a resonance. What does that mean? Take an unstable particle that decays into a pair of leptons. An experimenter can look through their data and pair up leptons that were generated at the same time, have opposite charge, and whatever other requirements they want. The energies and momenta of the pair can be added, producing the energy and momentum of whatever the pair came from. Now, most of these pairs came from random combinations and the sum will yield a random number.

But, if the pair resulted from the decay of a specific particle, the addition will give you the mass of the mother particle. Amongst all the random combinations, you will get an excess with that specific mass. This is called a resonance, and is one of the classic ways of discovering a new particle.

This plot shows real data taken by the CDF experiment at Fermilab of their J/Psi peaks.

So the two groups saw a new particle, realized they had seen the same particle, and announced their discovery jointly. The particle they found was the J/Psi (one letter for each group that discovered it), a meson made of a charm with anti-charm quark in one particular state. The discovery was awarded the 1976 Nobel prize, and ushered in a huge change in our understanding of particle physics that helped develop the Standard Model as we have it today.