A Few More Facts About Protons

[Reminder: I’ll be interviewed today at 5 p.m. Eastern time, at http://www.blogtalkradio.com/virtuallyspeaking/2012/02/15/matt-strassler-tom-levenson-virtually-speaking-science , which you can listen to either live or later.  My interviewer, Tom Levenson, is an eminent science journalist who has written fascinating and surprising books on Einstein and on Newton, among otherswon awards for his work on television (e.g. NOVA), has a great blog (and also posts here), and is a professor of science writing at MIT.  Should be fun!]

Since a number of readers were surprised to learn, from yesterday’s article about the benefits of increasing the energy of the protons at the Large Hadron Collider [LHC], that protons are very complicated and have a lot more in them than just two up quarks and a down quark, I thought I’d put up a plot or two that gives some indication of how particles are distributed inside a proton. Caution: the answers you get, and the physical intuition you obtain, depends in some subtle ways on exactly what you ask, so you should pay some attention to precisely which question I’m answering below. The details matter.

Two plots, differing only in the range for the vertical axis, showing the relative likelihood of striking a gluon or an up or down quark or antiquark carrying a fraction x of the proton's energy. At low x gluons dominate (and quarks and antiquarks become equally likely, and numerous, though far less so than gluons), while quarks dominate (but are very rare) at moderate x. Plotted using a Mathematica package (Trout and Olness, 2000) based on CTEQ5L results; somewhat out of date, but accurate enough for today's purposes.

The two plots in the Figure show exactly the same thing, just with a different vertical scale, so that certain things that are hard to see on one plot are clearer on the other. And what they show is this: if a proton is flying toward you in a Large Hadron Collider [LHC] proton beam, and you strike something inside that proton, how likely are you to have hit an up quark, or down quark, or gluon, or up antiquark, or down antiquark, that carries a fraction x of the proton’s energy? From these plots we can learn:

  • From the fact that the curves all grow very rapidly at small x (clearest in the lower plot), you learn that most particles in the proton carry much less than 10% (that is, x < 0.1) of the proton’s energy, and you are most likely to hit one of those, with lower energies being much more likely than higher ones. [10% isn’t small, by the way; in 2012 the LHC beams will have 4 TeV = 4000 GeV per proton, so 10% means 400 GeV — whereas to make a 124 GeV Higgs particle from two gluons you would only need 62 GeV per gluon.]
  • From the fact that (lower plot) the yellow curve is so much higher than the others, you learn that if you do hit something that carries less than 10% of the proton’s energy, then it is most likely a gluon; and quarks and antiquarks (the other curves) are about equally likely once you get below about 2% of the proton’s energy.
  • From the fact that (upper plot) the gluon curve dips below the quark curves at higher x, you learn that if you hit something with much above 20% (that is, x>0.2) of the proton’s energy —- which is very, very rare — it is most likely a quark, and about twice as likely to be an up quark as a down quark. [Here we see a remnant of the idea that “a proton is two up quarks and a down quark.”]
  • The curves all fall off very quickly as x increases; you are very unlikely indeed to hit anything with more than 50% of the proton’s energy.

These observations are reflected indirectly in Figure 1 of this article on what you gain in going from 7 TeV to 8 TeV at the LHC. Here are a couple of other things that are not obvious from the plots above:

  • Most of the energy of the proton is shared (roughly equally, for no special reason) among the small number of higher-energy quarks and the huge number of lower-energy gluons.
  • However the number of particles is dominated by the lower-energy gluons, with very low-energy quarks and antiquarks coming in next.

The number of quarks and antiquarks is huge… but: The total number of up quarks minus the total number of up antiquarks is 2, and the total number of down quarks minus the total number of down antiquarks is 1. And as we saw above, these excess quarks carry a significant fraction (but not the vast majority) of the energy of a proton as it is flying toward you. In these senses only, the proton is substantially made from two up quarks and a down quark.

By the way, all of this information was obtained from a fascinating combination of experiments (mostly scattering of electrons or neutrinos off of protons or off of the atomic nuclei of heavy hydrogen [“deuterium”, which contains one proton and one neutron]), assembled together using the detailed equations that describe the electromagnetic, strong nuclear and weak nuclear forces.  It’s a long story going back to the very late 1960s and early 1970s.  And it works beautifully when used to predict the phenomena observed in proton-proton or proton-antiproton colliders like the Tevatron and LHC.

49 responses to “A Few More Facts About Protons

  1. You just gave a large push to my view of the essence , there must by some quantum rules , criteria , something –never mind names— there must be something specifically designed to keep a permanent protonness for 10^32 years amid that tremendous activity , never once UDD was produced instead of UUD , please do not reject my view because of some strange names i give .
    That mechanism that keeps the UUD protonness intact is what i mean by the proton essence, ontologically there must be essences for every thing that determines its properties , specifications , criteria…..etc.
    Again i am not a physicist but at least i got the feel of the matter.

    • but UDD are produced they’re called neutrons in fact there are many different particles which have excesses which add up to different number of U, D, anti-U, anti D, so theres nothing really special about the proton.

  2. I was wondering if the large number of quarks in baryons is fixed or whether it varies from baryon to baryon?

    • Quark number isn’t a fixed quantity. The idea of a static baryon, or meson, only makes sense in the limit that they move at the speed c and relativistic time dilation freezes all action from *your* point of view (Feynman argued this before the advent of QCD in his parton model). Also, Matt didn’t mention anything about evolution equations, but those plots of “probability of hitting quark x or a gluon” depend on how close you look (quantified by a variable we call Q).

      What he mentions above about the number of up quarks minus the number of up anti-quarks always being 2 is the only thing like a fixed number of quarks. You also have d minus anti-d = 1, s minus anti-s = 0, etc. We call these the number of “valence” quarks. There is the possibility of more complex arrangements of valence quarks than the baryons with three and the mesons with two. For example, you could have a u u d u anti-u valence state (I’ve added a quark plus an anti-quark to conserve baryon number, also we need integer electric charge and color neutral states).

      There is a possibility that we’ve seen some mesons with four quarks rather than the usual two from Belle and BaBar called the X, Y, and Z states.

  3. Why don’t the gluons or quarks inside a proton hit each other and create Higgs particles or other particles sometimes?

  4. My friends ; this is something very very special , never ignore that being special is a proof that it is special never underestimate this.
    My dear friend matt. i am very happy as i see in the smallest the same riddle of the biggest….. is it not identical to the excess of matter over antimatter riddle?
    Harry ; very good reflection as it makes the special much more special

  5. Matt: “The number of quarks and antiquarks is huge… but: The total number of up quarks minus the total number of up antiquarks is 2, and the total number of down quarks minus the total number of down antiquarks is 1. And as we saw above, these excess quarks carry a significant fraction (but not the vast majority) of the energy of a proton as it is flying toward you. In these senses only, the proton is substantially made from two up quarks and a down quark.”

    Matt’s explanation is very clear for physicists. For a layman, I would like to try one analogy.

    A wax volcano lamp consists of three parts, a glass container, a type of liquid (such as colored water) and a chunk of soft wax. When the lamp is heated with a light bulb underneath (adding energy), the soft wax liquefies and forms all different patterns circulating in the glass container.

    1. The wax volcano lamp is one entity regardless of the zillion patterns inside when it is energized.

    2. When a bullet hits the energized lamp, its internal pattern does make the differences for the outcome. If it hits no wax, it will only be coated by liquids. If it hits a small ball of wax, it could be coated with wax. If it hits a big chunk of wax, it could be buried in that wax, etc..

    However, the wax lamp can work the way as above if and only if the liquid and the wax have a lower level of structure, such as the liquid molecules and the wax molecules. Without this lower level of structure, there is no way to form any internal pattern. That is, with this analogy, the quarks must have a lower level of structure too.

  6. Matt.
    Following the lead of the concept of the quantum higgs field could have been zero , then let me ask : is the relation SIGMA U-SIGMA U* =2 is a logical necessity that cannot be other wise ( same for D,D*=1) or it is an arbitrary rule ?which can be otherwise.
    In general you said that we cannot know why the world is described by the rules of Q.M. and as such do you agree that ( the law of physicodynamical incompleteness0 as per Dr. david abel holds true??? we just cannot explain physics from inside physics.

  7. Fascinating stuff, thanks for the article, I am learning new things with each new one! Question about the proton buildup containing quarks and antiquarks, are these ‘permanent’ or do their numbers (pairs?) fluctuate in and out of existence like virtual particles, but average out to a certain average for all protons? And is it correct to state that all visible matter (particles) consists of both matter and antimatter, but that the slight excess of regular matter gives it its properties, or would that be too simplified?
    If you have any recommendations regarding books on this matter (pun intented) that would be great

    • You’re asking good questions, but the answers are subtle. I have to think about how to answer them better in future. Here’s my best shot for now.

      Crudely: the answer to your first question is that the numbers are constantly fluctuating (for instance, a quark and an antiquark can run into each other and turn into two or three gluons, or two gluons can run into each other and turn into quark and an antiquark, perhaps with an extra gluon or two thrown in for good measure). I mentioned in response to an earlier comment a day or so ago that in a system as complicated as a proton the distinction between real and virtual particles starts to break down.

      And as far as an average — the best way to answer that is that any average property of protons that you can measure is the same for every proton, because all protons are identical, in that each one has exactly the same intrinsic properties as every other. [The statement of exact identity is not a theoretical argument; it is something with direct physical consequences for the structure of nuclei, which would be very different if protons were not identical.]

      I do intend to write an article about identity and complexity one of these days. It’s a very important feature of particle physics that all electrons are identical, all muons are identical, all protons are identical, etc. And it is a consequence of the fact that, for instance, electrons are ripples in an electron field, etc.

      Second question: this is tricky.

      In hydrogen itself, you can find virtual positrons (positron = anti-electron, the lightest anti-matter particle), so in this sense matter atoms contain some antimatter. But this is a bit silly to say, because in empty space you can find virtual positrons. You can find matter and antimatter everywhere; if you want to check that it’s there, just put enough energy in a small space [for instance, make two high-energy photons collide] and poof! out will come an electron and a positron.

      The proton is a little different, but only because it is so much more extreme than hydrogen. For the same reason as in hydrogen, there are virtual quarks and antiquarks inside the proton — just as there are in empty space. But there is so much energy trapped inside a proton that there is enough to make those virtual quarks and antiquarks almost real. HOWEVER. Define “real.” Real particles are the quantum analogue of nicely behaved waves that can travel long distances maintaining their shape. But inside a proton a particle cannot go very far before finding out about the proton’s boundary, or being struck by another quark or gluon or antiparticle. The effect of this is that even what you might be tempted to call real particles never get a chance to establish themselves as such. It’s not like hydrogen, where the life of the electron is a peaceful one. The particles inside protons are whizzing around and banging into each other and appearing and disappearing. There’s nothing that you can point to and say — ah, that particle is a real one. It’s too complicated a system.

      You might ask — well, what about those two extra up quarks and that one extra down quark? Are those real?

      Well, that is a good question indeed, and you might try to make sense of this up to a point. First you have to ask this: since all up quarks are identical, and since the environment inside the proton is so complicated, how are you going to establish which up quarks are the extras and which are not? You would pick out the ones with the highest energy, but over time they may lose their energy during collisions with gluons, or annihilate with an antiquark, leaving what you thought was an extra up quark as the highest energy one. So it can’t make permanent sense to say the extra up quarks and extra down quark are real while the others are not. That said, it is interesting to think about whether this could be done for a finite time period — I have never heard a suitable answer to this question, but perhaps one can be given. So here my own thinking is just a little murky.

      The proton is so messy, and the activities inside it are all happening so fast, that it isn’t entirely clear that thinking about this too hard is actually useful to your intuition. Some systems are just complicated, and simple pictures of them may not actually exist. What is remarkable is that experiments can be done to learn some of the proton’s properties, and that what we learn — which isn’t everything we’d want to know, but is still substantial — can then be used to make predictions of many of the most interesting things that happen in proton-proton collisions.

      • To follow up on the matter/anti-matter question – is the fact that the 3 matter quarks predominate related/connected/somehow responsible for the fact that matter dominates the universe? Popular cosmology books always says its a happy coincidence that there was just a touch more matter at the big bang or all would have annihilated; but this makes it seem like the reason might be more fundamental.

        • No — let’s not forget there are antiprotons as well as protons in nature, and they have the reverse situation. So the fact that protons are matter is logically balanced by the fact that antiprotons are antimatter.

          The issue with matter dominating the universe is, essentially, this question: why are there more protons than antiprotons (and more electrons than positrons=anti-electrons)?

      • Your explanation regarding whether the three “extra” quarks are real or not sounds analogous to some sort of Uncertainty Principle. It sounds as if the moment you pointed to one of the quarks that make a proton a proton and declared “That one is real.” it would disappear into the soup of particles and another “extra” quark of the same type would pop up somewhere else and zing off giggling to itself “I’m free, I’m free.” And the whole process would occur repeatedly at a furious pace. But your thought about whether a particular quark could be called “real” for a finite period caught my attention and makes me wonder if this is completely different from some version of the Uncertainty Principle.

  8. When you say that, “the number of quarks and antiquarks is huge”, can you say that although the number is huge there is exactly the same huge number of quarks and antiquarks in each proton, does it vary from proton to proton, or no one knows?

  9. Very interesting. One of my old science teachers started the first lesson of our graduation year by stating that this year we’d finally learn how everything really worked. This year we’d learn that much of what we’d learnt in previous years had been a lie. He went on to say that if we went on to university we’d find out that what we’d learnt this year would also have been a lie. A useful lie but a lie all the same. Although lie was not the right word, the right word was model.
    I never suspected my internal model of the proton was this incomplete. Exciting stuff.

    • It’s well put.

      Part of my goal with this website is to find ways to break through that cycle to the extent possible. Of course I can’t possibly explain all the technical details to non-experts. But having been fortunate to run through the educational process all the way to the very frontiers of human knowledge in this research field, I can now look back and see which of the many lies I was told weren’t really necessary — simplifications of the reality that both were deeply misleading and also hid things that, while certainly complicated, weren’t beyond my understanding at the time.

      • You do an awesome job. I thought I was as up to date as a layman could be, but this proton stuff is just awesome – thanks for skipping the less useful lies.

  10. Could you answer the question I asked on twitter about the difference between nucleons being bound = less energy/mass and quarks being bound = more energy/mass

    • The reason I haven’t answered is that the answer really deserves a full article, not a comment-sized reply. It really requires some work on my part to do it well, because it involves more than one conceptual step. But I PROMISE I will answer the question, and that you’ll appreciate the answer being longer-winded, with pictures, etc.

  11. NOW AS MATT. HOPES TO WRITE A PAPER ON ( COMPLEXITY AND IDENTITY) GENERATED BY HOW THE EXTREME COMPLEX TURBULENCE CAN STILL BE DESIGNED TO HAVE A PERMANENT FIXED IDENTITY/ESSENCE/…….NESS BE AWRE HERE MATT THAT. YOU ARE ENTERING THE REALM OF THEOLOGY AND DIVINE DESIGN WHICH IS THE ONLY LOGICAL EXPLANATION A RATIONAL MIND CAN ACCEPT FOR THE ORIGIN OF THE FORMAL DIRECTING ALL PHYSICAL ASPECTS.
    AGAIN I STRESS VERY MUCH THAT EVERY ONE READ ( THE LAW OF PHYSICODYNAMICAL INCOMPLETENESS –BY Dr. DAVID ABEL) WITH THE HIGHEST PROOF RANK THAT CAN BE CONCEIVED .
    I HOPE THAT BY SAYING THIS WE REACHED A VERY HIGH CONCLUSION OF ALL OUR DIALOGS.
    THANKS AND MAY GOD BLESS ALL WHO WONDER , FEEL GREAT AWE AND HUMILITY IN THE VAST RICHNESS OF GOD,s DESIGN AND WISDOM.
    aa. sh.

  12. I’m still a little hung up on this large number of quark-antiquark pairs in the proton. Isn’t the vacuum already filled with particle-antiparticle pairs constantly appearing and annihilating? And when, say, an electron and an atomic nucleus exchange a photon, does it not have a contribution where the photon splits into an electron-positron pair and then recombines? And yet your picture of an atom is simple, and your picture of a proton is filled with extra particles. I guess what I’m asking is: Are these extra quark-antiquark pairs *different* than the virtual particle-antiparticle pairs that are all over the place anyway (even in empty space)? And if so, how are they different?

    • There isn’t a sharp distinction.

      One issue is that although the hydrogen atom does have extra particles in it — photons and electrons and positrons — these are 0.2% effects (α/π, where α~1/137 is the electromagnetic coupling constant) or smaller. For the proton, these are 100% effects.

      A second issue is that there is plenty of energy around in a proton (hundreds of MeV) to make quarks which have masses of order a few MeV. The energies running around in hydrogen are 100,000 times too small to make real electron-positron pairs.

      It is not really even possible to make precise sense of what the particles are inside a proton; they are constantly being disturbed by other particles or are being annihilated. That is in constrast to the hydrogen atom, which has nothing so complex going on; it is well described by ordinary quantum mechanics, with any relativistic effects (including particle pair creation) being small corrections to the simple picture. You would have no hope of using simple quantum mechanics for the proton, where relativistic effects not only aren’t small, they are dominant.

      I don’t know how to be more precise than this. I don’t think you can be. Hydrogen is quantum mechanics plus small corrections; you can do most of the calculations in junior year and the rest early in grad school. For a proton you need a giant computer and there is no simple approximation scheme.

      • So to sum up, a hydrogen atom is well-approximated by just a proton and an electron, but a proton is not well-approximated by just two up quarks and a down quark. That makes sense. Thank you.

  13. Prof. Strassler,

    Perhaps a direct comparison of energy scales is beneficial in understanding why a Hydrogen atom is a significantly simpler structure than the proton. The ground state energy of the Hydrogen atom is approx 14 eV (abs. value), the proton mass is approx. 1 GeV and the QCD scale is approx. 200 MeV. Recalling that 1 MeV = 10^6 eV and 1 GeV = 10^9 eV, makes obvious to everyone why ordinary Quantum Mechanics is sufficient to describe the physics of the Hydrogen atom, whereas non-perturbative QFT (in particular Lattice QCD) are needed to account for the web of complex interactions “inside” a proton. Metaphorically speaking, it is like looking through an extremely powerful microscope that reveals a large “zoo” of quark-antiquark pairs and gluons buzzing around and coming into sharp focus…

  14. Maybe a simple question. At school we learn the electrons in a hydrogen atom form shells because they cannot occupy the same state.

    Is the same true for all these quarks inside the proton?

    • I’m afraid that protons are just a lot more complicated than anything we learn about in quantum mechanics class (or even particle physics class, if we take one.) Relativistic effects, and particle creation and annihilation, are minor issues for atoms and even for atomic nuclei; not so for the internals of a proton.

      • Thank you for the reply. Maybe I take from this that it is just too hard to picture what’s inside of a proton. At school we leant that one of the reasons for introducing colour was was to avoid the Pauli exclusion principal acting on similar quarks inside a nucleon. Which is why I was wondering about the picture you had in a recent blog post “whats-a-proton-anyway” where the nucleon contained zillions of quarks and anti-quarks. So maybe the Pauli principal does not apply to the stormy sea of virtual particles inside a nucleon, or maybe they form cooper pairs or something. Well I don’t know, it’s just I feel a proton should be one of the easiest things in nature to understand, but I’m just far too dumb to understand.

        • The proton is a tough nut to crack. I owe you a longer answer to this question; bug me if I forget to provide one. Too much going on right now.

          • I’m guessing it is the QCD condensate I need to read up on to understand the structure of a nucleon.

            Is it reasonable to think of the higgs field as a condensate?

          • I’m afraid that the QCD condensate (which you should read up on) won’t tell you much about nucleon structure, only about nucleon existence.

            The QCD condensate is a non-zero value for a composite spin-zero field made from a quark field and an antiquark field.

            The Higgs condensate is a non-zero value for an (apparently-)elementary spin-zero field, not made from anything that we know of. But technicolor is the idea that the Higgs field is composite in analogy with QCD.

            The Higgs and QCD condensates are remarkably similar not only in their rough structure but also in their precise quantum numbers. If there were no Higgs field in nature, the QCD condensate would still form (we think; but let me not go into this subtlety, which has to do with how QCD with six light quarks behaves) and would break electroweak symmetry just as the Higgs does, leading to (small) masses for the W and Z particles. [However the QCD condensate would not give mass to the electron, and all of the quarks would be very light, including even the top quark.] That’s why technicolor is such a great idea; it suggests that the Higgs condensate forms in a way which is analogous to how the QCD condensate forms. But technicolor appears to be very strongly disfavored by the observation of a light-weight Higgs-like particle, as well as by comparing precision measurements and calculations.

  15. Do protons spontaneously emerge into being when the random fluctuations in the fields of their “particle + disturbance” constituents achieve the configuration(quantity & distribution) within the bounded space defined by the volume of the proton? If so, is there a probability for this?

    • There is a conservation law: you cannot make a quark without making an anti-quark. So to make a proton (Which has three more quarks than antiquarks), you must make an anti-proton too.

      There is another conservation law: the energy you start with is the energy you end with. You cannot make a particle from nothing; even a proton at rest has mass-energy, and that energy had to come from somewhere. So to make a proton and an anti-proton, you must get some energy from somewhere. This is something the vacuum of space cannot do on its own.

      The vacuum can have “virtual protons and antiprotons.” But as I’ve emphasized in my articles, virtual particles aren’t particles at all, and they don’t behave like particles.
      http://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/ The configuration and distribution of quarks and gluons in a proton would not be achieved for long enough to even say what the configuration and distribution was.

  16. Thank you very much Professor. I think I get it. Although the energy in empty space is non-zero, it is a far cry from the energy required to make a particle pair, let alone a proton pair. Just one more thing. Is a proton considered high entropy because of the huge number of possible arrangements of the zillions of constituents racing around or low entropy because there are only on average just three quarks.

    • It’s a bit tricky: you don’t quite have it yet. The issue is that the energy density in the vacuum cannot be mobilized by the vacuum itself.

      Imagine a box; inside the box there is a certain amount of energy due to the vacuum itself. But the total energy inside the box with the proton inside it is larger than the total energy inside the box without the proton inside it. That energy has to come from somewhere.

      As for entropy: this is a tough question for me to answer, because I can see what you want to know, but the problem is that entropy is not defined in this way, so your question, as you’ve asked it, has no answer. What I can say is that a proton is extraordinarily complex, but that has nothing to do with its entropy, which strictly speaking is zero… because if you say “proton” I know *exactly* what you mean. To really do this right requires quantum mechanics; have you had any?

  17. No I don’t have much. I am retired fire fighter and just starting to learn these concepts. I got the job out of high school and made the decision to work instead of following academic pursuits. Something I regret in one way but not in another. I took some night school Physics and Math courses over the years, but life gets in the way. Now I have time to pursue. I have an old Physics text by Halliday and Resnick that I am going through,

    • Ok. Well, entropy is a tricky concept, and in quantum mechanics it gets trickier. The real point of entropy is to characterize how much you have left unspecified about a system, not the system’s complexity. There is often a correlation between these two things, because in complex systems we may leave many things unspecified; for example, if we have a thousand marbles of different colors, we might say “here is a jar of a thousand marbles, 250 of them red, 250 blue, 250 green and 250 orange”; but in saying this we have left many things unspecified, such as how those marbles are arranged inside the jar. There are many different possible jars which could be characterized in exactly the same way. The logarithm of the number of such jars would be the entropy.

      But when we say “proton”, everything about the object is specified. The proton may be complicated, but it is the unique object that we could call “proton”. So its entropy is the logarithm of 1, which is zero. (I’ve oversimplified just slightly; but no matter how you do it, a system that contains a proton and nothing else is a very low-entropy state.)

      On the other hand, if I said, “here is a box full of lots of quarks, antiquarks and gluons” and I didn’t tell you anything else, I would be giving you something very poorly specified, and therefore a very high entropy state.

      To understand why a proton is a unique object that should not be thought of as a random box full of quarks, antiquarks and gluons requires quantum mechanics. It’s not intuitively obvious. But a hint is that a random box with properties similar to a proton (with the same total electric charge and the same excess of three quarks over antiquarks) would always have energy larger than a proton. The proton is the unique lowest-energy state with these properties.

  18. You are a patient man Professor Strassler to try and explain these concepts to people with limited understanding like me and I thank you very much.
    I see researchers have determined that the charge of the electron is a perfect sphere to within a hair width to the size of our solar system. Is this in agreement with predictions of QCD on how the charge of quarks affect the distribution of charge of the electron? Or is this observation mean their is something “not quite right” with QCD or our understanding of the internal structure of the proton?

    • Hmm. I am not aware of any reason why you would put these two questions together.

      The electron’s charge is not very much related to issues involving quarks. Moreover, the size of the electron is smaller than anything we can observe, so what I think you really mean is that there is no sign of any deviation from a sphere, but also no sign of any deviation from a point.

      The proton, on the other hand, is a (relatively) large and complex object made from many other particles. So the issues involving the shape of the proton and its distribution of charge are completely different from those involving the electron.

      Can you explain a bit more what led you to ask these questions in the way that you did?

  19. In a lecture I watched online the lecturer said that the two up quarks(+2/3, + 2/3) and a down quark(-1/3) that make the positive charge of a proton, according to QCD, should cause a spherical deviation or lack of roundness of the electron charge. I was asking if the observed roundness or lack of spherical deviation in the electron charge was evidence that there was a problem with by QCD?

  20. I guess I misunderstood what was being said. The speaker (Ed Copeland) was talking about a dipole moment for the electron which was caused by quarks, but virtual quarks, and at 10^−38 e·cm. Which is a lot smaller than they were looking. I thought that them not finding a dipole moment meant that there was something wrong with QCD and the quarks in the proton. He did say it is bad news for SQCD as some aspects of supersymetry predict currently unseen particles that should cause a dipole moment at the sensitivity they were probing. Hope I got is right this time.

    • You’re still a little bit mixed up; let me try to set it straight.

      1) in the Standard Model, which includes QCD (the theory of quarks and gluons) the *prediction* for the electron’s electric dipole moment is 10^−38 e·cm. That is too small to measure. The best measurements are only able to exclude a dipole moment a hundred billion times larger. Therefore, not observing an electric dipole moment for the electron with current experiments is consistent with the Standard Model and poses no problems whatsoever for the theory, including its description of quarks and gluons.

      2) if the Standard Model is not complete and new particles must be added to it, these new particles (through quantum effects involving virtual particles) can cause a much larger electric dipole moment for the electron. The fact that no electric dipole moment has yet been observed implies that certain of the various speculative ideas about various new particles are wrong. But there are many, many variants of most of these ideas, including supersymmetry. Some of them predict a large electric dipole moment, but others do not. All we learn from current experiments is that some variants of these ideas can be discarded, but many still remain.

      So — no problems with the Standard Model; some powerful but incomplete constraints on speculative ideas that go beyond the Standard Model; and nothing definitive on its own. This is not unusual; typically several different measurements are needed for real insights to be obtained.

  21. The wiki page for partons suggests that for collisions involving a small amount of energy or momentum, then protons act like they only have 3 particles. But if the energy or momentum is very high, then protons act like they have many particles. Is this the result of the Uncertainty Principle? So the higher the momentum involved in the entire proton/proton collision, the less time the entire collision takes place, and the greater the effect virtual particles have?

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