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Acknowlegements: Betsy Beise, Abhay Deshpande, Naomi Makins, Kent Paschke, Bernd Surrow... www.particleadventure.org What is the Proton Made of? Krishna S. Kumar University of Massachusetts, Amherst, USA A H is to rica l Pe rs pe ct ive PANIC11 Pedagogical Lectures MIT, July 24 2011 1 Fundamental Questions What are we made of? What holds us together? An early attempt What is the Proton made of? Current understanding 2 Krishna Kumar, PANIC11, July 24 2011 Fundamental Particles & Fundamental Forces Visible matter made up of first generation quarks and leptons ee- strong interaction electromagnetic (EM) interaction Are there other forces? Is there a single superforce? Do quarks and leptons have structure? How do protons, neutrons and nuclei get built from quarks and gluons? … What is the Proton made of? 3 Krishna Kumar, PANIC11, July 24 2011 Atomic Substructure: Length Scales Helium Atom Alpha Particle What is the Proton made of? 4 Krishna Kumar, PANIC11, July 24 2011 Outline Rutherford Scattering and the Atomic Nucleus Electron Scattering and Nuclear Size Electron Scattering and Proton Size What is inside the Proton? Current Research What is the Proton made of? 5 Krishna Kumar, PANIC11, July 24 2011 Rutherford and the Atom Parity-Violating Electron Scattering 6 Krishna Kumar, Mainz, Feb. 25 2011 Rutherford Scattering ~ 1910 Thomson’s “plum pudding” model 0.1 nm Rutherford devised a scattering experiment involving a beam of alpha particles scattering off a gold foil An alternative hypothesis testable by experiment What is the Proton made of? 7 Krishna Kumar, PANIC11, July 24 2011 Rutherford Scattering ~ 1910 Thomson’s “plum pudding” model 0.1 nm Rutherford devised a scattering experiment involving a beam of alpha particles scattering off a gold foil An alternative Atom’s positive charge: static EM potential hypothesis testable by alpha particles & gold atom’s positive charge: experiment interaction governed by Maxwell’s equations What is the Proton made of? 7 Krishna Kumar, PANIC11, July 24 2011 Rutherford Scattering ~ 1910 Thomson’s “plum pudding” model 0.1 nm Rutherford devised a scattering experiment involving a beam of alpha particles scattering off a gold foil An alternative Atom’s positive charge: static EM potential hypothesis testable by alpha particles & gold atom’s positive charge: experiment interaction governed by Maxwell’s equations differential cross section � � dσ dΩ × ∆Ω # alphas scattered into solid angle per unit time Incident beam intensity What is the Proton made of? 7 Krishna Kumar, PANIC11, July 24 2011 Geiger-Marsden Data Differential Cross Section � � � �2 dσ Z1 Z2 α = 2 2 dΩ 2mv sin (θ/2) •Established that atoms have a tiny nuclear core ~ 10-14 m, much smaller than the atomic size of 10-10 m •Revolutionized experimentation: particle scattering as a microscope What is the Proton made of? 8 Krishna Kumar, PANIC11, July 24 2011 Electron Scattering and the Size of the Nucleus Parity-Violating Electron Scattering 9 Krishna Kumar, Mainz, Feb. 25 2011 A femtoscope Electron Scattering Electron interactions well understood At nuclear scales, particles behave mostly like waves Particle Accelerators Wave mechanics or quantum mechanics How to produce femtometer wavelengths in the laboratory? What is the Proton made of? 10 Krishna Kumar, PANIC11, July 24 2011 A femtoscope Electron Scattering Electron interactions well understood At nuclear scales, particles behave mostly like waves Particle Accelerators Wave mechanics or quantum mechanics How to produce femtometer wavelengths in the laboratory? e e pp p p Electron scattering: electromagnetic interaction, described as an exchange of a virtual photon. What is the Proton made of? 10 Krishna Kumar, PANIC11, July 24 2011 A femtoscope Electron Scattering Electron interactions well understood At nuclear scales, particles behave mostly like waves Particle Accelerators Wave mechanics or quantum mechanics How to produce femtometer wavelengths in the laboratory? e e pp p p Electron scattering: electromagnetic interaction, described as an exchange of a virtual photon. 4-momentum transfer θ q = −4 EE ʹ′ sin 2 2 € 2 Q2: -(4-momentum)2 of the virtual photon Increase momentum transfer -‐> shorter wavelength -‐> higher resolu6on -‐> smaller scales What is the Proton made of? 10 Krishna Kumar, PANIC11, July 24 2011 1930’s and 40’s Mott Scattering Quantum Electrodynamics µ Free fermions fields are solutions to the Dirac equation (iγ µ∂ − m)ψ = 0 Feynman Rules: emission and absorption of virtual photons by fermion electromagnetic current current γ € What is the Proton made of? 11 Krishna Kumar, PANIC11, July 24 2011 1930’s and 40’s Mott Scattering Quantum Electrodynamics µ Free fermions fields are solutions to the Dirac equation (iγ µ∂ − m)ψ = 0 Feynman Rules: emission and absorption of virtual photons by fermion electromagnetic current current γ € E’ E θ Heavy, spinless N nucleus What is the Proton made of? Differential Cross Section � dσ � 4Z 2 α2 E 2 dΩ 11 M ott = q4 Krishna Kumar, PANIC11, July 24 2011 1930’s and 40’s Mott Scattering Quantum Electrodynamics µ Free fermions fields are solutions to the Dirac equation (iγ µ∂ − m)ψ = 0 Feynman Rules: emission and absorption of virtual photons by fermion electromagnetic current current γ € E’ E θ Heavy, spinless N nucleus Differential Cross Section � dσ � 4Z 2 α2 E 2 dΩ M ott = q4 If Q sufficiently large, nuclear size modifies formula dσ = dΩ � dσ dΩ � Mott � �2 � F (q) � F (q) = The point-‐like sca:ering probability modified: account for Finite Target Extent with a “form factor” What is the Proton made of? 11 � eiqr ρ(r)d3 r Form factor is the Fourier transform of charge distribu6on Krishna Kumar, PANIC11, July 24 2011 Form Factors & Nuclear Size 208Pb 1 What is the Proton made of? 12 2 3 q (fm)-‐1 Krishna Kumar, PANIC11, July 24 2011 Form Factors & Nuclear Size 208Pb few 100 MV of acceleration 1 What is the Proton made of? 12 2 3 q (fm)-‐1 Krishna Kumar, PANIC11, July 24 2011 Electron Scattering and the Size of the Proton Parity-Violating Electron Scattering 13 Krishna Kumar, Mainz, Feb. 25 2011 Proton Form Factors e e p p p p For a point-like target, accounting for target recoil: Function of (E,θ). � � dσ dσ 2 = 1 + 2 τ tan (θ/2) dΩ dΩ Mott Cross-section for infinitely heavy, fundamental target What is the Proton made of? τ=Q2/4M2 14 Krishna Kumar, PANIC11, July 24 2011 Proton Form Factors e e p p p p For a point-like target, accounting for target recoil: Function of (E,θ). � � dσ dσ 2 = 1 + 2 τ tan (θ/2) dΩ dΩ Mott Cross-section for infinitely heavy, fundamental target τ=Q2/4M2 If proton is not point-like: The electric and magnetic form factors GE and GM parameterize the effect of proton structure. dσ dσ E� = dΩ dΩ Mott E � � 2 2 (GE + τ GM ) + 2 τ G2M tan2 (θ/2) 1+τ If the proton were like the electron: GE = 1 (proton charge) e e p p GM = 1 (magnetic moment = 1 Bohr magneton). What is the Proton made of? 14 Krishna Kumar, PANIC11, July 24 2011 1950’s Hofstadter Data Otto Stern (1932) measured the proton magnetic moment µp~ 2.5 µBohr (first indication that the proton was not a point-like particle, Nobel prize 1943) What is the Proton made of? 15 Krishna Kumar, PANIC11, July 24 2011 1950’s Hofstadter Data Precise e-p cross section measurements at various scattering angles Otto Stern (1932) measured the proton magnetic moment µp~ 2.5 µBohr (first indication that the proton was not a point-like particle, Nobel prize 1943) Stanford U. Mark III Accelerator McAllister and Hofstadter, Physical Review 102 (1956) 851. What is the Proton made of? 15 Krishna Kumar, PANIC11, July 24 2011 1950’s Hofstadter Data Precise e-p cross section measurements at various scattering angles Otto Stern (1932) measured the proton magnetic moment µp~ 2.5 µBohr (first indication that the proton was not a point-like particle, Nobel prize 1943) Stanford U. Mark III Accelerator McAllister and Hofstadter, Physical Review 102 (1956) 851. What is the Proton made of? 15 Krishna Kumar, PANIC11, July 24 2011 1950’s Hofstadter Data Precise e-p cross section measurements at various scattering angles Otto Stern (1932) measured the proton magnetic moment µp~ 2.5 µBohr (first indication that the proton was not a point-like particle, Nobel prize 1943) Stanford U. Mark III Accelerator McAllister and Hofstadter, Physical Review 102 (1956) 851. What is the Proton made of? 15 Krishna Kumar, PANIC11, July 24 2011 1950’s Hofstadter Data Precise e-p cross section measurements at various scattering angles Otto Stern (1932) measured the proton magnetic moment µp~ 2.5 µBohr (first indication that the proton was not a point-like particle, Nobel prize 1943) Stanford U. Mark III Accelerator McAllister and Hofstadter, Physical Review 102 (1956) 851. What is the Proton made of? 15 Krishna Kumar, PANIC11, July 24 2011 1950’s Hofstadter Data Precise e-p cross section measurements at various scattering angles Otto Stern (1932) measured the proton magnetic moment µp~ 2.5 µBohr (first indication that the proton was not a point-like particle, Nobel prize 1943) Stanford U. Mark III Accelerator McAllister and Hofstadter, Physical Review 102 (1956) 851. It isn’t Mott, nor Dirac, nor modified Dirac with GE=1 and GM=2.79… What is the Proton made of? 15 Krishna Kumar, PANIC11, July 24 2011 1950’s Hofstadter Data Precise e-p cross section measurements at various scattering angles Otto Stern (1932) measured the proton magnetic moment µp~ 2.5 µBohr (first indication that the proton was not a point-like particle, Nobel prize 1943) Stanford U. Mark III Accelerator McAllister and Hofstadter, Physical Review 102 (1956) 851. It isn’t Mott, nor Dirac, nor modified Dirac with GE=1 and GM=2.79… the proton has finite size! Robert Hofstadter – Noble Laureate 1961 What is the Proton made of? 15 Krishna Kumar, PANIC11, July 24 2011 What is inside the proton? Parity-Violating Electron Scattering 16 Krishna Kumar, Mainz, Feb. 25 2011 Early 60’s Quark Model Late 40’s to the late 70’s, explosion of new “particle” discoveries Made spectacular prediction for new bound state which was subsequently found with precisely the predicted properties Gell-Mann & co-workers: Quark hypothesis Explained known bound-states with 3 degrees of freedom: u, d and s quarks Example: u and d flavors: SU(2) symmetry Gell-Mann: 1969 Nobel Prize What is the Proton made of? 17 Krishna Kumar, PANIC11, July 24 2011 Deep Inelastic Scattering Late 1960’s e- eγ* p What is the Proton made of? X In the mid-60s, electron accelerators achieved electric field gradients that made 10 Billion Volts of acceleration possible •proton absorbs energy and breaks up •total observed energy no longer conserved •need to measure both scattering angle and scattered momentum 18 Krishna Kumar, PANIC11, July 24 2011 Deep Inelastic Scattering Late 1960’s e- eγ* p X In the mid-60s, electron accelerators achieved electric field gradients that made 10 Billion Volts of acceleration possible •proton absorbs energy and breaks up •total observed energy no longer conserved •need to measure both scattering angle and scattered momentum W1 and W2 are structure functions What is the Proton made of? x is fraction of proton momentum carried by struck fragment 18 Krishna Kumar, PANIC11, July 24 2011 Deep Inelastic Scattering Late 1960’s e- eγ* p X In the mid-60s, electron accelerators achieved electric field gradients that made 10 Billion Volts of acceleration possible •proton absorbs energy and breaks up •total observed energy no longer conserved •need to measure both scattering angle and scattered momentum W1 and W2 are structure functions x is fraction of proton momentum carried by struck fragment SLAC: Stanford Linear Accelerator Center Electron Spectrometer Friedman, Kendall & Taylor: Nobel Prize 1990, Measurement at SLAC What is the Proton made of? 18 Krishna Kumar, PANIC11, July 24 2011 Scaling and Partons W = Mass of recoiling fragments As W increased, the structure function became surprisingly independent of Q2 electrons are hitting structureless objects that have negligible size! discovery of proton constituents called partons: associate with quarks What is the Proton made of? 19 Krishna Kumar, PANIC11, July 24 2011 Quark Model & QCD Quantum Chromodynamics ✦QCD naturally introduces the concept of “color” charge: necessary ingredient of Quark Model ✦There are 3 types of “color”: red, green and blue ✦QCD bound states are “color neutral”: individual constituents cannot be observed (confinement) ✦The exchanged force carrier, the gluon, itself carries color; gluons can interact with gluons ✦The interaction strength decreases with distance (asymptotic freedom) Gross, Politzer, Wilzcek: Nobel Prize 2004 What is the Proton made of? 20 Krishna Kumar, PANIC11, July 24 2011 Scaling Violations & QCD 1980’s & 90’s 10-18 m What is the Proton made of? 21 Krishna Kumar, PANIC11, July 24 2011 Scaling Violations & QCD 1980’s & 90’s 10-18 m What is the Proton made of? 21 Krishna Kumar, PANIC11, July 24 2011 Scaling Violations & QCD 1980’s & 90’s 10-18 m What is the Proton made of? 21 Krishna Kumar, PANIC11, July 24 2011 Current Research Parity-Violating Electron Scattering 22 Krishna Kumar, Mainz, Feb. 25 2011 Some Open Questions ✦Basic bulk properties of the proton are not easily calculable from first principles QCD • theoretical progress ongoing from Lattice QCD • Ongoing research to obtain new, precise data on structure of protons; and associated theoretical research to interpret data ★ electron, muon and proton beams in many different accelerator facilities ✦Several predicted quark bound states have not yet been found • In particular, do quark-gluon or pure gluon states exist? What is the Proton made of? 23 Krishna Kumar, PANIC11, July 24 2011 What carries the Spin of the Proton? ✦ Spin: Fundamental quantum-mechanical property of elementary particles; no classical analog ✦ all matter particles have two substates; moving particles can have spin aligned or anti-aligned ✦ Proton has spin-1/2: how is that built up by the constituents (quarks, gluons and orbital motion)? quark contribution is only 25% ± 10%; where is the rest? must be gluons or orbital motion What is the Proton made of? 24 Krishna Kumar, PANIC11, July 24 2011 Do Gluons and Sea Quarks affect Proton Bulk Properties? For example: u, d & s quarks and anti-quarks are known to appear and disappear in the proton curve from precise data collected over past 20 years This can be studied by teasing out the small component of the weak force between electrons and protons Quantitative understanding leads to deeper knowledge about the dynamics of QCD at long distance scales A complete separation of the u,d & s contributions to the charge, magnetism and spin of the proton is ongoing with precise experiments What is the Proton made of? 25 Krishna Kumar, PANIC11, July 24 2011 Take-Away Message Saclay, France NIKHEF, Netherlands MIT-Bates, USA SLAC, USA DESY, Germany CERN, Switzerland Jefferson Lab, USA Mainz, Germany RHIC-Spin, USA J-PARC, Japan ✦ Quark Model and QCD provide qualitative understanding of the structure of the proton ✦ Theoretical and experimental research focus on quantitative understanding to gain further insights into the dynamics of QCD at the full relevant range of length scales ✦ Newer, improved accelerator facilities play an essential role: a new polarized electron light-ion collider (EIC) is motivated in the future What is the Proton made of? 26 Krishna Kumar, PANIC11, July 24 2011 Related Talks at PANIC11 ✦ Morning Plenary Sessions • • • Radial Distribution of the Proton and its Constituents (Haiyan Gao) • Probing Hadron Structure and New Physics with Parity-Violating Electron Scattering (Jeff Martin) • • Light Baryon Spectroscopy (Volker Crede) • Future Directions in Accelerator-Based HEP & NP Facilities (Thomas Roser) The Quark-Gluon Structure of the Nucleon (Naomi Makins) Glimpsing the Fly in the Cathedral: Marking the First Centennial of the Description of the Atomic Nucleus (Brian Cathcart) Recent Results in Particle and Nuclear Physics from Lattice QCD (Tetsuo Hatsuda) ✦ Public Lectures by Brian Cathcart and Jerome Friedman What is the Proton made of? 27 Krishna Kumar, PANIC11, July 24 2011
