Particle Physics Basics

Here are some articles on particle physics, ranging from very non-technical summaries to more organized presentations for those who want to learn more.  (Much more to come.)

25 thoughts on “Particle Physics Basics”

  1. I am sorry that I do not have a comment but rather as question: what happens when a neutron encounters a positron? This question is a surogate for any dissimilar anti-particle/particle encounter. The usual discussion seems to focus only or an anti-X/X encounter which almost always results in annililation with the concommittant creation of amoung other things a high energy photon plus perhaps a neutrino or an anti-neutrino.

    • There are many confusions about antiparticles. You cannot look at an object and say whether it is a particle or an anti-particle; you can only say X is an anti-particle of Y, and so Y is an anti-particle of X. You can’t say Y is the particle and X is the anti-particle, or vice versa.

      So there’s no essential difference between particle-particle encounters and particle-antiparticle encounters, unless the two interacting objects are each other’s antiparticles — and even then, whether something happens or not, and what happens, depends on the types of interactions that nature allows.

      For example, both electron collisions with neutrons and positron collisions with neutrons will typically lead to scattering of the two particles off each other, and at higher energy will lead to the breaking apart of the proton with the formation of multiple hadrons. The only difference is that the weak nuclear force will allow for the rare process

      positron + neutron –> anti-neutrino + proton

      which has no counterpart for electron + neutron. But this is VERY rare. And also, there are similar processes in which the neutron turns into a slightly more massive hadron

      positron + neutron –> anti-neutrino + Delta-plus hadron

      electron + neutron –> neutrino + Delta-minus hadron

      and both of these are allowed.

      So the qualitative differences are very small, especially at high energy, with only rare processes being important.

      However, electron + electron is very different from electron + positron. That’s because an electron and a positron, each other’s anti-particles, can turn into a wide variety of objects, because of the details of the types of allowed interactions; see

  2. Professor Strassler, You confirmed my preconceived notions about these sorts of things. I suppose that in the rare instance of a neutron reaching the end of its 15 minute “life time” at which point it decays into a proton, an electron and (what is it) an anti-neutrino, the electron and the positron might somehow interact to cause their mutual anillation. What I was not certain of when I asked this question is the possibiltiy of some electroweak interaction during and encounter of anti-X and Y might cause something like anillation or whatever. By the way I will be getting back to you someday soon with even more arcane questions. I am currently recovering from back surgery and can not spend a great deal of time at this before my wife begins to complain (read holler and scream). Thank you very much for your quick response. Wayne

  3. Professor Strassler, There is, out on the internet a UCSD website dealing with “Big Bang” basics that seems to imply that in the early phases of the universe, when it was radiation dominated, pairs of photons were capable of interacting and (occasionally)creating particle/anti-particle pairs. The exact nature of these photons nor their interaction was not well described. I am imagining they must each have identical or nearly identical energy, each of which must be greater than or equal the inherent mass of the particles being created out of the photons. First of all, is this an accurate description of how particles came into to being and if so, what is the nature of the photon/photon interaction? Is this where the Higgs’ boson comes in, which is to say does the Higgs boson facilitate this interaction.

  4. Is it possible that a photon, as a particle, has *some* mass?–Perhaps below a certain threshold mass–that would still allow it to travel at the speed of light and so not contradict it’s “zero” rest mass designation? I’m a writer trying to explain the photon in an understandable way to laypersons? Thanks.

    • In principle, yes, this is an experimental question. It could be that the photon could have a tiny mass. The photon’s mass cannot be larger [from the observation of certain black holes (and by a slightly complicated argument)] than about one trillionth of one trillionth of a hundredth of the electron’s mass. That’s small enough that any photon that you could ever observe, emitted in any earthly physical process, would travel so close to c (the universal speed limit) that you would not be able to measure its velocity is not being equal to c. So there would be no revisions to any earthly experiment, or corresponding theory, if the photon has a tiny mass. Specifically there would be no currently measurable effect on particle physics experiments.

      Conversely, there is no evidence in *favor* of the photon having a small mass.

  5. Don Lincoln recently wrote a Scientific American piece outlining the arguments for and against the idea that many of the currently defined elementary particles are actually made of smaller particles called preons. My understanding is that if this theory is true, particles like quarks may represent a hierarchy of non-elementary particles that can be organized in a manner similar to how atoms are arranged in a periodic table. My understanding from reading this article is that this idea has been well debated for some time in the literature but yet I have never seen this discussed in the “lay” discussions on particle physics. I also understand that current and future CERN experiments to establish if some of the current particles can be shown to have a non-zero size is key to supporting the preon theory. I also understand is that if they exist– preons can be explained in part by string theory. Is this something you can comment on and discuss?

    • The idea that the known particles are made of still more elementary ones is a very common one; there are many many versions of the basic idea, and absolutely no evidence in favor of any of them, so I wouldn’t know which one to describe to you. At this point all data from the Large Hadron Collider is consistent with all of the known particles being smaller in size than 10^(-18) meters or so. Clearly if we see deviations from the predictions of the Standard Model we’ll have to look at whether structure inside of the known particles might be responsible.

      There’s no logical connection of this idea with string theory; you can have either one without the other, or both.

  6. Let me agree with Matt. The direct experimental evidence for preons is exactly zero, a point which I emphasized in the Scientific American article. However, the existence of particle generations is not explained and preons is but one of the ideas floating around.

    Certainly, the LHC continues to look for evidence that the quarks and leptons are not pointlike. And one must follow one’s intuition when searching for something new…even if that intuition can sometimes fail you.

    While I personally think that something like preons will some day be found, I have not fallen into the trap of believing what I think. There is an answer and some day the universe will tell us what it is.

  7. Don and Matt
    Thanks so much for responding. I wanted to say I really enjoyed your Scientific American article. I do appreciate the fact there has been no experimental evidence to date to support the preon concept. However, I realize in physics perhaps more than in other areas of science –a theory based on mathematics/modeling precedes experimental evidence to provide support for a scientific concept. While I really enjoyed reading the article, my initial comment was based on my lack of mathematical and physics expertise, and curiosity if Matt (or any others who read this board) would comment or provide an opinion on how “supportive” the mathematical models (e.g. Harari and Shupe or any others) reinforce the general concept of preons and a higher level of particle structure.

    In addition, in my original comment– I believe I may have mis-stated the point I wanted to mention about string theory and preons. In the Scientific American article, there was speculation or an idea (one of several) that strings could make up preons or even pre-preons. Again, just an idea I found to be quite interesting.

  8. The mathematical models are generally primitive. But, then again, so was the original quark model. The models are typically based on symmetries, which just means that you can come up with a set of preons, with specific properties and rules, and then generate the existing particles. The question of how the preons are tied together is frequently left unsolved. On the other hand, quantum chromodynamics (the theory of how quarks are bound together) came after the quark idea. QCD became useful about a decade after the quark hypothesis. It has been longer for preons, but there is less guiding data as well.

    Don’t believe in preons. Don’t believe in strings. Take them for what they are…cool ideas that may be right.

    Or not.

  9. Hello, professor.I am a curious nine year old whose goal is to become a theoretical physicist like you. I really like your explanations. Knowing this, could you please explain the math behind the Heisenberg Uncertainty Theory? It would mean a lot to me.

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  14. Matt, About a year ago I wrote asking what happens when a positron encounters a neutron. Your answer confirmed my preconceived notion: nothing out of the ordinary. Here I am asking what happens when a positron encounters a hydrogen atom consisting of the two matter particles – a proton and an electron. Will this too be an unremarkable encounter or might, on occasion, the positron annihilate the orbital electron leaving, the proton to speed off in a direction very nearly opposite to that of the 1.02 MeV gamma ray?

    • I am not a professor but I think I can answer that question. Yes, the electron will annihilate with the positron and not with the proton. This is because positrons and protons are positively charged and like charges repel. However, the resulting gamma rays don’t necessarily collide with the proton unless it was very, very lucky (or unlucky, depending on how you like to view it). So yes, “on occasion” the proton will be affected by one of the two gamma rays but because the nucleus is tens of thousands of times smaller than the atom itself, so it is very unlikely to happen.

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