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Jim Hirschauer | USLHC | USA

View Blog | Read Bio

But what are quarks made of?

The pressure is on.  I have read and enjoyed the US LHC blogs off and on for the past couple years, and so I was thrilled to be offered a chance to join the ranks of these entertaining and informative writers.  Now that it comes time for my first post, I admit that I am wracked with anxiety.  Whatever academic writing skills I may possess will be of little use to me here, right?

Abstract: A new LHC blogger is introduced.  His research is described …

See? It doesn’t work.  So I suppose that, to get over my anxiety related to this first post, I will stick to a subject that I know very well: my own research!  Here it goes …

The remarkable success of the LHC and the experiments that reside on its ring [including my experiment, the Compact Muon Solenoid (CMS)] have made this an exciting time to be at CERN.  I have had the opportunity to help lead an exceptional group of researchers in a study of the early CMS data; this work has resulted in one of the first CMS publications based on data from 7 TeV proton-proton collisions.

To introduce this research, I’ll start with a little history:

We humans have been searching for the smallest unit of matter for a long time.  About 2500 years ago, Democritus proposed that all matter is made of tiny, indivisible (“atomos”) entities.  Unfortunately, Democritus was way ahead of his time, and even 2300 years after his hypothesis, we still did not know whether atoms really existed.  Finally, around 1800, Dalton and others realized that the elements combine in only certain proportions implying that there is a fundamental unit of each element; i.e., each element is made up of atoms.  Dalton’s atomic theory was a great advance, but it didn’t explain why there are so many (about 50, at the time) different elements.  The human tendency to categorize when presented with variety brought us Mendeleev’s Periodic Table of the Elements:

The Periodic Table of Chemical Elements


The fact that the elements fit nicely into a table based on their weights and chemical properties suggested that the elemental atoms are actually just different combinations of even smaller entities.  Only a few decades after Mendeleev presented his table, humans observed these sub-atomic entities when Thomson discovered the electron (1897), Rutherford the atomic nucleus (1910), and Chadwick the neutron (1932).

Soon after the discovery of the neutron, discoveries of particles that didn’t fit into our simple atomic model (e.g. pion, kaon, Lambda) hinted that a revision of that model was needed.  In the 1960’s, Gell-Mann suggested that these new particles, as well as protons and neutrons, were actually entries in another periodic table which he called the “Eightfold Way.”

The baryon octet of Gell-Mann's Eightfold Way.


Just as we now understand the diverse elements to be combinations of only three particles (protons, neutrons, and electrons), the Eightfold Way explained protons, neutrons, kaons, pions, etc. as combinations of particles that we now call quarks.  Only five years after Gell-Mann proposed his theory, these quarks were observed at the Stanford Linear Accelerator Center.

And this is where it stands today.  As far as we know, quarks are indivisible; i.e., quarks are the smallest unit matter in the nucleus.  But wait!  We do observe there to be six quarks arranged in three generations:

I know what you’re thinking:  But this is another table!  This looks just like the Periodic Table or the Eightfold Way!   Isn’t this therefore a hint that even quarks (and leptons) are made up of something smaller still?

That is certainly a very reasonable guess, but only experiment can tell us for sure, and unfortunately, it gets progressively more difficult to see these small particles: roughly speaking, the atom is one million times smaller than a human hair, and the proton is 100,000 time smaller than the atom.   Our current understanding is that the quark is a point-like particle with no spatial extent!

My recent research focuses on searching for evidence that quarks are made up of even smaller stuff by probing these tiny distance scales.   The unprecedented energy of the LHC allows us to probe smaller distances than ever before: about 1/20,000 the size of the proton.   In my next post, I’ll describe how we actually do this and tell you what we have found.