HC 209H: Discovery of Fundamental Particles and Interactions Sixth Lecture: Quarks and the Strong Interaction
Chris Potter
University of Oregon
HC 209H: Sixth Lecture – p.1/13 The Proton has Structure (1956)
Robert Hofstadter (1915-1990), an American physicist at Stanford, conducted experiments with a 188 MeV electrons produced by a linear accelerator. Hofstatder also first proposed building the 2 mile long Stanford linear accelerator, which was built forthwith. The experiment establishing proton structure was very much like the experiment of Rutherford, Geiger and Marsden establishing the compact nucleus of the atom. The idea is to shoot high energy (therefore short wavelength) particles at the target, and measure where they end up. “The root-mean square radii of charge and magnetic moment are each (0.74 ± 0.24) × 10−13cm.”
HC 209H: Sixth Lecture – p.2/13 Proliferation of Hadrons: 1947-1961
Hadrons (Greek hadrys, strong) are mesons (middleweight) plus baryons (heavyweight). More (and more) hadrons were discovered in cosmic rays and accelerators, 1947-1961. Recall, in cloud chambers: Momentum p = qBR, for charge q, magnetic field B and radius R Velocity v can be determined by energy loss (ionization and bremsstrahlung) Mass m = p/v in the nonrelativistic regime, m = E/γc2 in the relativistic regime
Mesons in cloud chambers and accelerators: me
Baryons in cloud chambers: m > mp Neutral Lambda baryon Λ0 → pπ− in cloud chambers (1951) Charged Sigma baryon Σ+ → pπ0 in cloud chambers (1953)
Mesons in accelerators: me HC 209H: Sixth Lecture – p.3/13 The Omega Ω, For Example (1964) A beam of 5 GeV K− mesons struck a hydrogen target in a bubble chamber at Brookhaven. HC 209H: Sixth Lecture – p.4/13 Known Particles in 1964 Some particles known after the discovery of ρ,η,ω (1961): Leptons Mesons Baryons Interaction e± π0, π± p,n γ νe ρ,ω,η Λ µ± K± Σ± strong? 0 0 νµ K Σ weak? The muon is a lepton - it does not feel the nuclear force: a muon behaves exactly as an electron does, except that it is more massive (mµ ≈ 212 × me). The muon also has it’s own neutrino νµ, distinct from the electron neutrino νe. The messenger particles of the strong and weak interactions were not yet understood. The strong messenger particle would come to be known as the gluon g. In addition to the proliferation of hadrons (mesons and baryons), in 1956 it was shown thatthe proton has structure. It is not fundamental. In 1964 particle physics was desperate for an organizing principle of hadrons, just as in 19th century chemistry was desperate for an organizing principle of elements. HC 209H: Sixth Lecture – p.5/13 Murray Gell-Mann and Quarks Murray Gell-Mann (1929-2019), an American physicist, provided that organizing principle - just as Dmitri Mendeleev provided it for elements. Gell-Mann defined a new physical property, strangeness S = −1, 0, +1: S Particle -1 K−, Σ− 0 π,p,n +1 K+, Σ+ Gell-Mann’s organizing principle was plotting strangeness S vs. electric charge q. Recall that Mendeleev’s was to plot valence vs. atomic weight. In 1964 Gell-Mann postulated three quarks to explain all known hadrons: up u, down d and strange s. Credit: Wikimedia Commons HC 209H: Sixth Lecture – p.6/13 The Eightfold Way: Meson and Baryon Octets Credit: NobelPrize.org Credit: Wikipedia HC 209H: Sixth Lecture – p.7/13 Quarks q, Mesons qq¯ and Baryons qqq Credit: Wikipedia Gell-Mann posited quarks: u,d,s. Quark is a nonsense word from Joyce’s Finnegan’s Wake: Three quarks for Muster Mark! Sure he hasn’t got much of a bark Mesons are a quark and antiquark, while baryons are three quarks: Pions are π0 = uu,¯ dd¯, π+ = ud¯, π− =ud ¯ . Strange Kaons are K0 = ds¯, K¯ 0 = ds¯ , K+ = us¯, K− =us ¯ . Protons are p = uud and neutrons are n = udd. Three more quarks for Muster Mark: Charm quark c in J/Ψ= cc¯ discovered at the Stanford Positron Electron Ring (1974) Bottom quark b in Υ= b¯b discovered at Fermilab E288 (1977) Top quark t in decay t → bW discovered at Fermilab Tevatron (1995) HC 209H: Sixth Lecture – p.8/13 Discovery of Partons in Protons: SLAC In 1964, a team of physicists from SLAC and MIT carried out electron-proton scattering experiments using linear accelerator electron beam with energies in the range 7-17 GeV. According to De Broglie formula, a particle with momentum p has an associated wavelength λ = h/p. Only short wavelengths will resolve small objects, so only high momentum particles can resolve the proton structure. Think of ocean waves striking a log. If the waves are far apart the log makes little impact on the waves. Closer together, they gives hints about the structure of the log. At right is a plot of the parton density functions for a proton. This is the probability for a parton to carry a fraction x of the proton’s momentum. Partons are...quarks! And gluons..... Credit: Particle Data Group HC 209H: Sixth Lecture – p.9/13 Gluon g is to QCD as Photon γ is to QCD In the modern theory of the electromagnetic interaction (QED), the photon γ is the messenger particle which communicates a force from one electrically charged object to another. In close analogy, the modern theory of the strong interaction (QCD) has a messenger particle called the gluon g. The analog of electric charge (+,-) in QCD is color charge (r, r,g,¯ g,b,¯ ¯b). Unlike photons, gluons carry color charge. At right is a gluon communicating the strong force from a b charged quark q1 to a r charged quark q2, carrying charge r¯b. According to QCD confinement, only colorless states rr¯, gg¯, b¯b and rgb, r¯g¯¯b are ever observed (in mesons and baryons), which is why single quarks are never seen. Credit: Worldwide Web and Google HC 209H: Sixth Lecture – p.10/13 Discovery of Charm Quark: SLAC The Stanford Positron Electron Accelerator Ring (SPEAR) is shown at right. SPEAR was a synchrotron with counter-rotating electron beams and positron beams: e+e− → cc¯. The bound state of the charm and anti-charm quarks, cc¯, was discovered in 1974 jointly by teams at Stanford (cc¯ = ψ) and Brookhaven (cc¯ = J). The J/ψ has a mass of about 3 GeV/c2 and a very short lifetime, and was discovered in its decay to lepton pairs, J/ψ → e+e−, µ+µ−. At right is a plot of cross section vs. energy for e+e− → hadrons. The ρ,ω mesons we have seen. The φ is ss¯, a strange and anti-strange quark bound state. Shown also are the cc¯ = J/ψ and b¯b =Υ. Credit: SLAC (top), PDG (bottom) HC 209H: Sixth Lecture – p.11/13 Discovery of Bottom and Top Quarks: Fermilab At right is Fermilab, near Batavia, Illinois. The Tevatron, a proton-antiproton (pp¯) synchrotron is the ring structure. The bottom quark b (also called beauty) was discovered in 1977 by a team led by Leon Lederman at Fermilab’s E288 experiment. The b was found in a bound state meson b¯b =Υ, initially called the “oops-Leon” due to initial missteps by Lederman. The top quark t (also called truth) was discovered in 1995 jointly by two teams at the Fermilab Tevatron, the DZero and CDF collaborations. The t was produced in the reaction pp¯ → tt¯and was found in its decay t → bW and its mass could be reconstructed (right). Credit: Fermilab, CDF Collaboration HC 209H: Sixth Lecture – p.12/13 Worksheet 6 Preparation The Quark Model Quarks q posited by Murray Gell-Mann were the flavors up, down and strange (u,d,s). Up quarks have electric charge +2/3, down and strange quarks have electric charge -1/3. Mesons qq¯ are a quark q and an antiquark q¯, not necessarily the same flavor. Baryons qqq are three quarks, not necessarily the same flavor. To determine all possible combinations of quark flavors for mesons, consider a table: q/q¯ u d s u¯ d¯ s¯ Quantum wavelength λ associated with a particle with momentum p: Luis deBroglie wavelength: λ = h/p where h is Planck’s constant, p is momentum. Notice that p = h/λ, and pc = hc/λ = 2π~c/λ. If mc ≪ p, then m is negligible compared to momentum so E ≈ pc = 2π~c/λ. Conversely, we also have λ = 2π~c/E. HC 209H: Sixth Lecture – p.13/13