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Jim Hirschauer | USLHC | USA

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But what are quarks made of? Part 2

Friday, December 3rd, 2010

Hello, again.  Thanks for all the excellent comments on my last post.

In my last post I explained that our current theory makes the assumption, which has not been experimentally verified, that quarks are indivisible, point-like particles (“elementary” particles).   I also discussed why it is reasonable to think that quarks might actually be made up of even smaller, currently unknown particles.   If this is the case, we would call quarks “composite” particles instead of “elementary” particles.

In this post I describe how we search for evidence that quarks are composite particles.  (Sometimes we phrase this as a “search for quark compositeness.”)   The distance scales in which we are interested (50,000 times smaller than the proton) are far too small to be probed with even a very powerful microscope, so we have to take a route that will probably seem indirect:

1. First we pick a physical quantity that we are able to measure with our particle detector and that is related to the composition of the quark – let’s call it Q.  Then we measure Q using our experiment.  Because of the inherent statistical nature of any experiment, there is some uncertainty on our measured value of Q. (This is the same type of “plus or minus” uncertainty that political pollsters quote when they predict that 50% plus or minus 3% of voters will choose Barack Obama, for instance, in an election.)

2. Then we also calculate Q making various assumptions about the composition of quarks.  We calculate Q assuming that our current theory of elementary quarks is correct, and we also calculate Q assuming that quarks are composite particles.

3. Finally, we compare the measured and calculated values of Q.  If we find that the measured Q is inconsistent with a certain calculated value of Q, we conclude that the assumptions used in that calculation are incorrect.  However, because of the statistical uncertainty on the measurement, our measured Q might be consistent with more than one of the calculated values of Q.  (If the election mentioned above ultimately showed that 47% or 50% or 52% of voters actually chose Obama, we would say that these three results are all consistent with the poll’s prediction of 50% plus or minus 3%.)  More on this possibility below …

So far this discussion has been very abstract.  To make things more concrete, I will describe the actual physical quantity Q that we use to search for quark compositeness.  Before I do, I need to explain what happens in a proton-proton collision in the Large Hadron Collider:  When two protons smash into each other at 99.999999% of the speed of light, a lot different things can happen, but a very frequent occurrence is a single quark from one proton colliding with a single quark from the other proton like this:

You can see that the quarks that collide exit the collision at a large angle, while the quarks that did not collide zip right past each other.  Because of the laws of physics governing the behavior of quarks, the quarks that collided instantly turn into showers of other particles such as pions, protons, neutrons, etc.  We call these showers of particles “jets.”   With our particle detector we measure the energy and direction of these jets.  Here is a picture of a two-jet collision event that was actually recorded by our detector:

Two jets (green) exiting the collision point (yellow dot). The measured energy of the jets is represented by the red and blue wedges. In this view, you are looking down the barrel of the detector; i.e., one proton would have been traveling into your computer screen and the other proton out of your screen.

That’s all the background you need for me to tell you about the physical quantity Q that we use to determine whether quarks are composite particles.   The quantity Q is the direction of these jets.   As I mentioned above, for the jet direction to be useful in determining quark composition, the direction of the jets must depend on quark composition, and indeed, according to calculations, if quarks really are made up of smaller particles, the angle at which the colliding quarks exit the collision will tend to be larger than if quarks are really elementary particles.

Now let’s go back to the three step method for searching for quark compositeness that I described above:

1. The first step was to measure the quantity Q:   Well, to measure the average direction the jets, we simply count the number of proton-proton collisions that produce jets in the center of our detector (red-shaded region in the diagram below).    (We actually measure the ratio of the number of central jets to the number of non-central events, but that is just a detail.)

Diagram of CMS particle detector as viewed from the side. The protons enter from the left and right and collide in the center. The blue shapes represent the detector components that measure the energy and direction of jets. The red-shaded region denotes the central region of the detector.

2. The second step was to calculate Q under several assumptions:   I already mentioned that, if quarks are composite particles, colliding quarks will exit the collision at larger angles.  For this reason, the number of central jets calculated assuming  composite quarks is larger than the number calculated assuming elementary quarks.

3. The last step was to compare the measured and calculated values of Q:   We take our measured number and compare it with our calculations.  If we were to find too many central jets,  we could conclude that our measurement is inconsistent with our calculations performed under the assumption that quarks are elementary particles.  This would be evidence that quarks are made up of smaller, unknown particles.  (It could also just be evidence that our calculation is wrong, and so this is something that we cross check thoroughly.)

The actual results:  Unfortunately, the new LHC measurements of jet directions from the CMS and ATLAS experiments do not find evidence that quarks are composite particles.   Just as in the above example of the political poll results that are consistent with several election outcomes, the LHC results are consistent with both elementary quarks and quarks made up of particles interacting at distances as large as 1/20,000 of the proton radius.  This means that, while we confirm the current theory of elementary quarks to be correct at distances 10,000 times smaller than the proton, we cannot conclude that quarks are certainly elementary particles, because our measurements are also consistent with quarks made up of particles that interact at distances 20,000 (or 50k or 100k)  times smaller than the proton.

Fortunately, as we record and analyze more data, the statistical uncertainty on our measurement of the jet directions will get smaller, and we will be able to probe distance scales even smaller than 1/20,000 of the proton radius.  Stayed tuned for more results from the LHC and more blog posts from me!


But what are quarks made of?

Thursday, November 18th, 2010

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.