The Strong Interaction

1. The Strong Interaction Michael Mattern

2. Michael Mattern Contents • The fundamental forces • History • The needof a strong force • The Theroryfrom Yukawa • The pionas the mediator • QCD • Quantum chromodynamics (QCD) • SU(3) and color blindness • Color chargeofparticles • Flow ofcolorcharge • QED vs. QCD • Confinement • Asymptoticfreedom • Nuclearforce

3. Michael Mattern The fundamental forces

4. Michael Mattern History: The need of a strong force • Classical period (1897-1932): discovery of protons, neutrons in a compact cluster (the nucleus/core) and electrons around in a shell • Problem: Why do positive charged protons & electrically neutral neutrons bind together in the nucleus of an atom? • Electromagnetic  protons should repel one another violently, no force on neutrons • Gravity  weaker than electromagnetic force •  need of another strong force

5. History: The therory Michael Mattern • 1934: first significant theory of the strong force proposed by Yukawa • Proton and neutron are attracted by some sort of properly quantized field • Exchange particle 300 times heavier than electron and a sixth of a proton Yukawa (Source: Wikipedia) • Particle called meson because of it´s middle-weight • 1947: Powell discovered two middle-weight particles in cosmic rays (pion and muon) • The true Yukawa meson is the pion

6. History Michael Mattern Source: David Griffiths (2008) Introduction to Elementary Particles, Second Edition.

7. Michael Mattern History: The pion as the mediator • in 1947 it seemed that the job of elementary particle physics was essentially done • The proposed theory allows easier calculations and more descriptive representations, but it is valid only within a limited energy range • During the 1950´s discovery of a large and ever-growing number of particles called hadrons • It seemed that this large number of particles could not all be fundamental

8. Michael Mattern History: QCD • First, the particles were classified by charge and isospin • 1953: classification according to strangeness • 1961: hadrons were sorted into groups having similar properties and masses using the eightfold way • 1963: explaining the structure of the groups by the existence of three flavors of smaller particles inside the hadrons: the quarks •  In 1965: proposal that quarks have an additional degree of freedom (the color charge) and that quarks might interact via an octet of vector gauge bosons: the gluons

9. Michael Mattern History: Summary • Quarks are fundamental particles • Hadrons made of quarks • Baryons (made of three quarks)  for example proton and neutron • Mesons (made of one quark and one antiquark)  for example pion • Hadrons held together by the strong force • Gluons mediator of the strong force • Quarks are color “charged” (equivalent to the electrical charge)

10. Michael Mattern History: QCD • Free quark searches consistently failed  • Gell-Mann thought that quarks were just mathematical constructs and strong interactions could not be fully described by quantum field theory • Feynman argued that high energy experiments showed quarks are real particles • Difference caused split in the physics community • 1969 S-matrix  Theorie • 1973 Discovery of the asymptotic freedom •  rehabilitating of the quantum field theory

11. Michael Mattern Quantum chromodynamics (QCD) • It is the study of the SU(3) Yang–Mills theory • Is a quantum field theory of a special kind called a non-abelian gauge theory, consisting of a 'color field' mediated by a set of exchange particles (the gluons) • Describing the interactions between color charged particles (quarks and gluons) which make up hadrons • Two strange properties: • Confinement • Asymptotic freedom

12. Michael Mattern green blue SU(3) and color blindness red • Color charge is associated with the Strong Interaction • But: the properties of the strong interaction do not depend on the color of the quark • Color is associated with some abstract space Rotations in this space changes the color of the quarks • Strong Interaction is “color-blind”  symmetry space • The rotations (“symmetry transformations”) are mathematically equivalent to rotations in three complex dimensions

13. Michael Mattern Color charge • There are three color values (found experimentally) they were assigned as (redgreenandblue) • quarks carry SU(3)-color charge  • also antiquarks and anitcolors  • unlike Electromagnetism, we find that the mediator of the strong force (the gluon) also carries color charge • Gluons carry a color and a anticolor q q q q q q g g

14. Michael Mattern Color of hadrons • Baryons (red + blue + green = white or colorless) • Mesons green + antigreen = colorless red + antired = colorless blue + antiblue = colorless q q q q q q q q q

15. Michael Mattern Color of gluons • It seems to give 9 types of gluon: redanti-redredanti-blueredanti-green blueanti-redblueanti-blueblueanti-green greenanti-redgreenanti-bluegreenanti-green • Just 8 gluons  no redanti-red g g g

16. Michael Mattern Flow of color charge • Emission of a gluon: red  red anti-blue + blue • Re-absorption of a gluon: q q q q g g

17. Michael Mattern Color flow inside hadrons Source: http://en.wikipedia.org/wiki/Color_charge

18. Michael Mattern QED vs. QCD

19. Michael Mattern QED vs. QCD (feynman diagrams) • Normal particle (quark): • Antiparticle (antiquark): • Interaction by gluon: Analogous to photon exchange of QED 3-gluon vertex 4-gluon vertex

20. Michael Mattern QED vs. QCD (feynman diagrams) The first vertex produces the following set of combinations: Source: http://teachers.web.cern.ch

21. Michael Mattern QED vs. QCD Summary • QCD more difficult because: • Additional vertices (gluon-gluon interaction) • Coupling constant of QED is 1/137  this small value limits the sum of more and more complicated feyman diagrams • In QCD coupling constant is close to 1  effects of simple and complicated diagrams are both strong • No separated quarks in QCD  difficult calculations

22. Michael Mattern Confinement • Phenomenon that color charged particles cannot be separated  direct observation impossible • Quarks areonlyseenconfined in colorlesscombinations • Strong force gets stronger with the distance • Whathappensifwetryto separate a quark-aniquark-pair • Gluon-tube between quarks elongates • Strong force gets stronger with the distance • As soonasthereisenoughenergyinside thesystem a newquark-antiquark pair will becreated Source: http://www.rug.nl/

23. Michael Mattern Hadronization Source: http://en.wikipedia.org/

24. Michael Mattern Asymptotic freedom • Phenomenon that strong force gets weaker with decreasing distance • Asymptotically approaches zero for close confinement (or high energy) • Quarks in close confinement can move “freely”  quark gluon plasma • Described qualitatively as resulting from the penetration of the gluon cloud surrounding the quarks

25. Michael Mattern Running coupling constant of QCD strength 1 distance 0 radius of a proton

26. Michael Mattern But what holds the nucleus together? • Before the introduction of the quark model, the strong interaction was the force between the nucleons • Now it is the force acts on quarks and gluons (or QCD) • We know that hadrons does not have a color charge (so they do not exchange gluons)  there must be an other explanation • Today we understand the problem as a residual effect of the even more powerful strong interaction • We call it nuclear force

27. Michael Mattern Nuclear force • Like Yukawa proposed, the mediator of the nuclear force is the meson • Today we know that this mesons which are transmitted between hadrons are combinations of quarks and gluons • Strength and range of the nuclear force limit the maximum size of the nucleus (because of the short range of the nuclear force and the infinite range of the elecromagnetic force, after a certain number of nucleons, the elecromagnetic force dominates)

28. Michael Mattern Nuclear force Source: http://en.wikipedia.org/

29. Michael Mattern Thank you for your attention