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The Need for a “Strong Force”

The Need for a “Strong Force”. Why do protons stay together in the nucleus, despite the fact that they have the same electric charge?.  They should repel since they are like charge. Why do protons and neutrons in the nucleus bind together?.

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The Need for a “Strong Force”

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  1. The Need for a “Strong Force” Why do protons stay together in the nucleus, despite the fact that they have the same electric charge?  They should repel since they are like charge Why do protons and neutrons in the nucleus bind together?  Since the neutron is electrically neutral, there shouldbe no EM binding between protons and neutrons.

  2. Announcements This Friday and then Monday I will begin reviewing what we havelearned up to now.This will also serve to focus you on the important concepts.I will also hand out a review sheet next Monday which summarizeswhat the exam will cover.Friday the 26th will be devoted to a pure Q&A session.

  3. The Strong Force • For the EM interactions, we learned that:The photonmediates the interaction between objects which carry electrical charge • For the Strong Interactions, we conjecture that:A force carrier, called the gluonmediates the interaction between objects which carry color charge(that is, the quarks, and …gluons !). • The most striking difference between the gluon and thephoton is:The gluon carries color charge, but the photondoes notcarry electric charge. Gluons can interact with other gluons !!!!

  4. Comparison Strong and EM force

  5. Color Charge of Quarks • Recall, we stated, without much explanation, that quarks come in3 colors. • “color charge”  strong-force as “electrical charge”  EM force. • Experiments show that there are 3 colors; not 2, not 4, but 3. • Again, this does not mean that if you could see quarks, you would see them as being colored. This “color” that we refer to isan “intrinsic property” and color is just a nice way to visualize it.

  6. BARYONS q1 q2 RED + BLUE + GREEN = “WHITE” or “COLORLESS” q3 MESONS q q q GREEN + ANTIGREEN = “COLORLESS”RED + ANTIRED = “COLORLESS” BLUE + ANTIBLUE = “COLORLESS” Color of Hadrons (II)

  7. Color of Gluons Each of the 8 color combinationshave a “color” and an “anti-color” When quarks interact, they “exchange” color charge. Don’tworryaboutwhat thismeans Quark 1 Quark 2

  8. Color & the Strong Force

  9. Emission of Gluon InitiallyAfter gluon emission RED RED-ANTIGREEN + GREEN (quark) (gluon) (quark) Re-absorption of Gluon Before gluon absorptionAfter gluon absorption RED-ANTIGREEN + GREEN  RED (gluon) (quark) (quark) Flow of Color Charge

  10. Color Exchange Quarks interact by the exchange of a gluon.Since gluons carry color charge, it is fair to say that the interaction between quarks results in the exchange of color charge (or just color) !

  11. q e+ This cannot happen, becausethe gluon does not interact withobjects unless they have color charge! g e - Gluons – Important Points • Gluons are the “force carrier” of the strong force. • They only interact with object which have color, or color charge. • Therefore, gluons cannot interact with leptonsbecause leptons do not have color charge !

  12. Feynman Diagrams forthe Strong Interaction • As before, we can draw Feynman diagrams to represent the strong interactions between quarks. • The method is more or less analogous to the case of EM interactions. • When drawing Feynman diagrams, we don’t show theflow of color charge (oh goody). It’s understood to be occurring. • Let’s look at a few Feynman diagrams…

  13. Feynman Diagrams (Quark Scattering) q Quark-antiquarkAnnihilation g q Quark-quarkScattering Could also beQuark-antiquarkScatteringorAntiquark-antiquarkScattering q q Position g q q time

  14. u d u u d u Where do we get quark and antiquarks from? Quarks PROTON And, antiquarks?… ANTIPROTON

  15. This can’t happen because the photononly interacts withelectrically chargedobjects ! g g g g g Flashback to EM Interactions Recall that photons do not interact with each other.Why? Because photons only interact with objects which have electric charge, and photons do not have electric charge !

  16. BUT GLUONS DO !!! Gluons carry the “charge” of the strong force, which is “color charge”, or just “color” !

  17. Gluon-gluon Scattering Gluon-gluon Fusion g g g g g g g g g g Ok, so here’s where it gets hairy! Since gluons carry “color charge”, they can interact with each other !(Photons can’t do that)

  18. Quark-AntiquarkAnnihilation Quark-gluon Scattering q g q q g g g g q g And quark-gluon interactions as well! Since both quarks and gluons have color, they can interact witheach other !!!

  19. Proton u u d ~10-15 [m] Where do the gluons come from ? • The gluons are all overinside hadrons!! • In fact there are a lot more than shown here !!! • Notice sizes here: In fact quarks are < 1/1000th of the size of the proton, so they are still too big in this picture ! • Even protons and neutronsare mostly empty space !!!

  20. Confinement Since the strong force increases as quarks move apart, they can only get so far…The quarks are confined together inside hadrons.Hadron jail !

  21. Hadrons! As quarks move apart, the potentialenergy associated with the “spring”increases, until its large enough, toconvert into mass energy (qq pairs) K- s K+ u u d p- d d d d p0 d Hadronization In this way, you can see that quarksare always confined inside hadrons(that’s CONFINEMENT) !

  22. What holds the nucleus together? The strong force ! • Inside the nucleus, the attractive strong force is stronger thanthe repulsive electromagnetic force. • Protons and neutrons both “experience” the strong force. • The actual binding that occurs between proton-proton andproton-neutron is the residual of the strong interaction.

  23. Food for thought Recall: Mass of Proton ~ 938 [MeV/c2] Proton constituents: 2 up quarks: 2 * (5 [MeV/c2]) = 10 [MeV/c2] 1 down quark: 1 * 10 [MeV/c2] = 10 [MeV/c2] Total quark mass in proton: ~ 20 [MeV/c2] Where’s all the rest of the mass ????? It’s incorporated in the binding energy associated with the gluons !  ~98% of our mass comes from glue-ons !!!!

  24. Summary (I) • The property which gives rise to the strong force is “color charge” • There are 3 types of colors, RED, GREEN and BLUE. • Quarks have color charge, and interact via the mediator of thestrong force, the gluon. • The gluon is massless like the photon, but differs dramaticallyin that: • It has color charge • It’s force acts over a very short range (inside the nucleus)

  25. Summary (II) • Because gluons carry color charge, they can interact among themselves. • Quarks and gluons are confined inside hadrons because of the nature of the strong force. • Only ~50% of a proton’s energy is carried by the quarks. Theremaining 50% is carried by gluons. • We learn about the strong force by hadron-hadron scatteringexperiments.

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