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Jim Hirschauer | USLHC | USA

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But what are quarks made of? Part 2

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!


21 Responses to “But what are quarks made of? Part 2”

  1. Lamont says:

    “if quarks are composite particles, colliding quarks will exit the collision at larger angles.”

    is this fundamentally the same kind of physics principle involved in the Geiger-Marsden-Rutherford discovery of the atom?

    • Lamont, this is an excellent question. In some ways this statement (and our experiment) is related to Rutherford scattering, in which gold atoms were bombarded with helium nuclei. In our experiment we are essentially bombarding quarks with other quarks. Rutherford et al. observed that most helium nuclei passed through the gold with very little deflection, and we also observe that most quarks pass by each other with little deflection. Rutherford discovered the nucleus by observing that some helium nuclei were deflected at large angles; we could discover quark compositeness by observing more large angle deflection than predicted by a theory of elementary quarks.

      Of course, there are many differences, too. Rutherford was observing a classically described process in which a positively charged helium nucleus was deflected by a positively charged gold nucleus via the well-known electromagnetic force. We are searching for quarks interacting via a new, currently unknown force. The description of this new force, which we use to calculate expected large angle deflection, is necessarily based on a non-classical (quantum) theory.

      Thanks for the great question! -Jim

  2. Hi…

    Its…Correct To Take Quarks As An Elementary Particle Temporarily While Analyzing Collision Jet Data In An Incredible Measurement, I Believe LHC Researcher Will Face An Exciting Shockwave Impact ^_^

    • Hi, Fujimia. As I mentioned above, our results are consistent with an elementary quark and a quark made up of particles interacting at very small distance scales (20,000 times smaller than the proton). So it is certainly OK to continue treating the quark as an elementary particle in our theories. However, if we find evidence that quarks are not elementary, we would certainly revise our theories. Thanks for reading. -Jim

    • Hi, Richard. Thanks very much for the nice write-up of my post on your blog. Thanks also for spreading the word regarding the importance of basic research. I’ll be sure to stop by your site frequently. Keep up the good work! -Jim

  3. I’d like to ask, might the determination of Quarks composition depend on factors that areyet unknown?
    Could Jet directions indicators be influenced by the TeV and elements being used?
    I wonder if the trajectories are entirely as a result of the collision or, perhaps a ‘deliberate’ path in the weird Quantum world?

    • Hi, Anthony. This is another excellent question. In my post, I talk about the comparison of measurement with calculation. You are wondering if other aspects of the measurement or calculation, in addition to the quark composition, can affect their comparison. The answer is “yes, but we think that we understand how large those other affects can be.”

      There are essentially two things that affect the measured jet direction (or any other quantity) in any experiment: the laws of physics and the measurement method. You are asking about the second item. The most important aspect of our job, as experimental physicists, is to understand how our methods affect our measurements. We perform extensive analysis of our methods to make sure we understand all sources of uncertainty in our measurements. Similarly, we work hard to quantify the uncertainty in our calculation. Finally, we account for these uncertainties when perform the measurement/calculation comparison. -Jim

  4. Jim-

    Looks like you (or someone else looking at your above post) checked out the post at ScienceSprings. If so, thanks, I appreciate it.

    ScienceSprings now has its own Twitter page


    The coolest thing was that the very first follower was

    Thanks again.


  5. Björn says:

    Hi Jim

    Thanks for the nice blogpost.
    If the quarks have zero size, how can they then collide?

    Best Regards


    • Hi, Bjorn. This is a tricky question. The word “collide” brings to mind a macroscopic process of two objects (a foot and a football, for instance) striking each other. However, on the microscopic level, when a foot strikes a ball, it is really the electrons on the outside of the foot repelling the electrons on the outside of the ball via the electromagnetic force. The electrons, which also have zero size, do not ever “touch” each other; indeed as you point out, the concept of “touch” breaks down for particles with zero size. In other words, the electrons interact via the electromagnetic force by exchanging a particle called a photon. Similarly, zero-sized quarks in the LHC “collide” with each other via the strong force by exchanging a particle called a gluon. This is what we mean by “collide” on the microscopic level. I hope this helps explain a very abstract idea. -Jim

  6. Don M. says:

    Hi Jim. Thanks for your response on my last question on Part 1. Are “jets” the only conceivable way to determine (i.e. prove) that quarks are “composites”? It seems to me to be a to abstract way for a “scientific proof”, we all know that “political pollsters” are not always right(just had to throw that in, I don’t like them no matter how good they are). Question – Is the non-classical(quantum) theory – “quantum gravity”?

    Thanks, Don Meares

  7. Lamont says:

    Jim: thanks.

    I hadn’t even considered that sub-quark-sub-quark scattering would also involve an as-yet-unknown force as well…

    Hopefully the LHC finds something fairly unexpected like this… It’ll be a little boring if all we get is a Higgs particle and some jet quenching… I want some supersymmetry, some technicolor, or some composite quarks, please!

  8. I really like reading your site, the posts are always really helpful and the way you lay everything out makes it simple to understand. I do not ordinarily comment but I decided I should at least thank you for doing an extremely good job. Thank you and keep up the great work.

  9. Henry Pieszko says:

    Hi….this is probably a very stupid question, but if the quarks interact via gluons, then all particles interact with the Higgs field via the Higgs boson, and therefore there must be literally trillions of those flying around. Why then are they so hard to detect?

  10. bormpromy says:

    Хороший у вас блог! удачи в развитии

  11. Ronald Schleyer says:

    Concerning “substructure” of so-called quarks (which really are triadic fields in self-relation), your physicist bloggers should ask themselves what actual necessity (from an absolute point of view) is there for anything smaller?

    The point is that what has been found is fully sufficient to describe the “ordinary” operations of the World in all their beauty and complexity. If YOU were the Creator, why bother with more? Isn’t this enough? Yes, it is!

    The chase after “strings” all all this is approaching the ridiculous, and millions of people in the world are aware that particle physics has taken almost a perverse turn, running away from what Mankind knows as important.

    If physicists would actually study the nucleonic micromachines as they operate at ordinary temperatures we would get much farther down the road in understanding WHAT TRULY IS and how it works from the inner to outer reality, which one suspects is a relation of life to life.


  12. Tom Heinrichs says:

    Why is it that the question: What are quarks made of, what is it actually that is vibrating, what is the substance of these particles, is never brought up? OK , I am not an expert, but it must be something that constitutes these particles, and if it is “pure energy”. What is sciences answer to this literally fundamental question?

  13. It is now 2012– what has the evidence at the HCL showning regarding quarks as being “elemtary particles?” From what I have heard, is there have been billions of collisions. What is the data showing?

  14. danielle says:

    I love your posts, I have been searching the internet looking for something like this. i notice that you havent posted in about two years!?!? why. Please dont leave us at home that are hungry for knowledge hanging.



  15. Michael Herman says:

    I believe that if Quarks have mass, they are not an elementary particle. An elementary particle(OK…. wave/whatever) should have no mass. Mass is the residual effect/echo of energy in its pure form accelerated to the speed of light or beyond, Thus explaining why c is squared in the specific theory of relativity…. That has been a mystery even to Einstein himself and why he could never create a general theory of relativity that encompasses Gravity. So by this hypothesis, anything that has mass or gravitational effects CANNOT be elemental. Looking at energy as something that could be accelerated to c or beyond into strings, creating the effect of gravity which we observe might lead to the discovery of the general theory of relativity which escaped Einstein.

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