Inelastic Electron Scattering Theory

Fred Gilman Bloom Symposium

March 18, 2016

Introduction • It is a little daunting talking about science

that goes back almost five decades, tied to such strong personal and family friendships.

• I will first discuss the broader scientific context, and SLAC in the 1960s and 1970s.

• Nevertheless, part of this talk will necessarily be more specific to my own and Elliott’s work from the late 1960s to the early 1970s.

• While we went on to other physics, the duality concept that we proposed has been re-energized with industrial strength.

Particle Physics Mid-1960s • It is difficult to appreciate today our lack of understanding of the zoo of particles and of the theory of weak and strong interactions. • At Caltech, where Elliott was a grad student and I a postdoc, the focus was on symmetries, SU(3), especially those of the electromagnetic and weak interaction currents. The fundamental representation of SU(3) is a triplet - what came to be called quarks - from which one could build the currents plus the baryons and mesons. • But quarks were never directly observed. What results derived from quarks were true? …false?

Strong Interactions • Much of experiment and theory was devoted to

thinking about bouncing protons (or mesons) off protons and strong interactions.

• Interesting insights were uncovered, including that non-diffractive 2 2 scattering amplitudes could be looked at equivalently in terms of being due to exchanges of particles or sums of direct channel resonances = “duality”

• Though much higher proton than electron beam energies were available, in hindsight we know that they still were not high enough to clearly show the quark constituents of hadrons.

Electromagnetic Interactions • With electrons scattering off protons, the

known electromagnetic interaction can be “factored out” to probe the interaction of a virtual photon (q = p – p′ ), with the nucleon (P).

• The physics is summarized in two structure functions W1 and W2 which depend on q2 and on s = (q + P )2. [ q∙P/MN = q0,Lab = E – E′ ]

• There were very different theoretical proposals: If “ɣ” acts ~ rho meson, “ɣ” + p ~ rho + proton, a hadronic collision; If “ɣ” probes point-like quarks no length scale in the process “scaling” = physics depends on ω = 2P∙q/q2.

Inelastic Electron Scattering The Heroic Process

SLAC • At SLAC (1967- ) the idea of a “quasi-elastic”

peak in electron-proton inelastic scattering due to electrons striking point quarks inside the proton made a prominent appearance in Bjorken’s 1967 Lepton-Photon Symposium talk at SLAC, including the idea of scaling. How big a “kick” in | q2 | is needed? 1 GeV2? 10 GeV2?

• Ans: ~1 GeV2. Nature was kind. At the 1969 Lepton-Photon Symp. scaling – a consequence of point-like quarks in the proton – was the centerpiece of Dick Taylor’s and my talks. The “ɣ” acting ~ rho meson was demolished.

1969 Lepton-Photon Symposium I Setting the Stage: Electron scattering on atoms, nuclei, & protons to probe the constituents. Observation of a quasi- elastic peak as incident electrons hit atomic electrons. Similar exp’ts for electrons on nuclei.

1969 Lepton-Photon Symposium II

SLAC-MIT: Scaling observed in the 6 and 10 degree data: a “universal scaling curve”

1969 Lepton-Photon Symposium III • I had been thinking about the behavior of resonances motivated by duality: Sum of resonances ↔ the non-diffractive component of “ɣ” + p. After my plenary talk, Gutbrod & Sakurai asked about the behavior of resonances at large q2. Answer: At a fixed mass & very large q2, the cross section falls quickly with q2 as ωResonance 1.

Bloom-Gilman Duality I • Getting to a quantitative understanding of the behavior of the resonances percolated while the experiments on deep inelastic electron scattering on deuterium were ongoing. • Using ω, the nucleon is fixed at ω = 1, while each resonance moves as q2 changes. But they are related ground and excited states. • All are on the same basis by using (EDB) the scaling variable ω′ = 1 + s/q2. The nucleon is at ω′ = 1 + M2/q2. The difference between ω′ and ω is M2/ q2 and 0 in the scaling limit of large q2.

Bloom-Gilman Duality II E. D. Bloom and F.J. Gilman, “Scaling, Duality, and the Behavior of Resonances in Inelastic Electron-Proton Scattering,” Phys. Rev. Lett. 25, 1140 (1970). INSPIRE: 553 Citations >500 Citations = “Renowned” E.D. Bloom and F. J. Gilman, Phys. Rev. D4, 290 (1971). INSPIRE: 440 Citations >250 Citations = “Famous”

Bloom-Gilman Duality III • The resonances follow in magnitude the

scaling curve fit to the data at high mass and q2. The nucleon and its resonant states are an intrinsic part of the scaling behavior, viewed as a sum of final hadronic states of the quarks, and dual to the view in terms of quarks.

• It’s the quarks, stupid. Simply different manifestations of the same physics.

Bloom-Gilman Duality IV • A dual view relating physics at the quark level

to that of final hadrons has found many other applications, e.g., the semileptonic weak decays of B mesons to get at the CKM matrix elements, the electron-positron annihilation cross sections to hadrons, Z decays, etc.

• With the development of QCD, the logarithmic corrections to scaling have been incorporated.

• The duality between quarks and hadrons has become so ingrained that at times its use is not justified or even noted.

It Was the Best of Times • SLAC became center-stage in particle physics. • From data on electron, muon, and neutrino

scattering experiments over a large kinematic range, the picture of quark constituents of the nucleon became conclusive.

• The theorists caught up, with gauge theories of the electroweak and then strong interactions (QCD), to give us “asymptotic freedom” and a framework for understanding scaling.

• By the mid-1970s, the Standard Model of particle physics was in place.

SLAC Theory Group 1969

• Many of us either followed the path to higher and higher beam energies or moved to non-accelerator branches of particle physics. A new electron-accelerator lab was built, the Thomas Jefferson National Accelerator Facility. It has undergone a recent upgrade and operates at energies not so different than the original SLAC. • After decades, an extensive experimental (at JLab in particular) and theoretical revival of work on Bloom-Gilman Duality has taken place under the title of Quark-Hadron Duality.

Transitions

Quark-Hadron Duality I • A extensive summary of the experimental

and theoretical situation up to 2005 is found in W. Melnitchouk, R. Ent, and C. Keppel, Phys. Rept. 406, 127-301 (2005).

• Work in the last decade is found in preprints and journal articles, as well as in many talks given at internal JLab workshops and external seminars and conference talks to the present day.

Quark-Hadron Duality II From E. Cristy, “Quark- Hadron Duality: New Results from JLab” Fermilab Seminar, 2010 “The Beginning”

Quark-Hadron Duality III From O. Lalakulich, “Quark hadron duality,” November 2005

Quark-Hadron Duality IV From C. Keppel, “Quark-Hadron Duality,” December 2015 Duality in F2 Revisited

Quark-Hadron Duality V From C. Keppel, “Quark-Hadron Duality,” December 2015 Duality in F1 and FL

Quark-Hadron Duality VI From I. Nicolescu, “Quark-Hadron Duality – Recent Jefferson Lab Results,” CIPANP2015, May 2015 Duality for F2 of the neutron

Quark-Hadron Duality VII From C. Keppel, “Quark-Hadron Duality,” December 2015 Duality for Spin- Dependent Structure Fcns.

Conclusion • Our work together has led to quite a scientific

saga that now spans decades. • It is still amazing to recall how much we did

not understand and how much fun it was to be a part of putting pieces of the puzzle together.

• Elliott, Thanks for your collaboration in those early days and the friendship of our families over the decades.