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:
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.)
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!