The Discovery of the Quark Mac Mestayer, Jlab the discovery of the nucleus - “Rutherford scattering” – method: measure scattering rates vs. angle the discovery of quarks – evidence that the proton is not a ‘point’ particle – evidence for charged “partons” inside the proton – properties ( frac. charge, spin, momentum ) the continuing search – details of quark-pair creation the discovery of the nucleus - “Rutherford scattering” – method: measure scattering rates vs. angle the discovery of quarks – evidence that the proton is not a ‘point’ particle – evidence for charged “partons” inside the proton – properties ( frac. charge, spin, momentum ) the continuing search – details of quark-pair creation April 30, 20101Quarks: search for the smallest detectors

Atomic structure (1897) electron discovered  how is it arranged with the positive charge? (1902) Lord Kelvin - “raisin pudding” model  electrons are ‘raisins’ embedded in a positive ‘pudding’ (1907) at University of Manchester; use  -particles as a beam Rutherford, Geiger, Marsden: (professor) (post-doc) (undergrad) (1897) electron discovered  how is it arranged with the positive charge? (1902) Lord Kelvin - “raisin pudding” model  electrons are ‘raisins’ embedded in a positive ‘pudding’ (1907) at University of Manchester; use  -particles as a beam Rutherford, Geiger, Marsden: (professor) (post-doc) (undergrad) April 30, 20103Quarks: search for the smallest but- a few at large-angle ! ‘backscatters’ due to small, heavy nucleus Hans Geiger Ernest Rutherford Ernest Marsden

relation between rates and angle April 30, Quarks: search for the smallest More area for small-angle scattering  higher rates “beams-eye” view side-view  scattering angle

The “Rutherford scattering”* experiment * done by Geiger and Marsden Rutherford did calculations like orbital mechanics ; using 1/r 2 electrostatic forces and a massive charged center. Knowing the charge of the nucleus and the alpha particle, he estimated that the nucleus was smaller than cm. April 30, 2010Quarks: search for the smallest5

Electron Scattering - Bigger & Better (1950’s) Cornell & Stanford Univ’s built electron accelerators to study the structure of the nucleus, and even of the proton. Electron scattering from Hydrogen  deviation from 1 / sin 4 (  /2)  proton is NOT a point particle  radius (proton) ~ cm (1950’s) Cornell & Stanford Univ’s built electron accelerators to study the structure of the nucleus, and even of the proton. Electron scattering from Hydrogen  deviation from 1 / sin 4 (  /2)  proton is NOT a point particle  radius (proton) ~ cm April 30, 20106Quarks: search for the smallest 1 m.

Proton has a finite size April 30, 20107Quarks: search for the smallest Electron scattering from proton, Hofstadter, McAllister (1955) Experimentalists defer to future theory, BUT make a conjecture ! … that they are measuring the proton’s size; ~ cm radius … and Coulomb’s law holds. a two-page paper ! Robert Hofstadter

Elastic  inelastic scattering April 30, 20108Quarks: search for the smallest If the object stays intact  elastic. one pool ball hitting another: elastic snow-ball striking the side of the house: inelastic eP  eP : elastic eP  eN  + : inelastic electron scattering  exchange of a photon If the object stays intact  elastic. one pool ball hitting another: elastic snow-ball striking the side of the house: inelastic eP  eP : elastic eP  eN  + : inelastic electron scattering  exchange of a photon Proton ++ Neutron electron photon electron

Momentum & energy transfer for elastic scattering April 30, 20109Quarks: search for the smallest Proton electron photon electron q P Relativistic equations for momentum and energy exchange from electron to photon to proton. 4-momentum transfer squared, Q 2, and energy transfer, are proportional Proton M (mass of the final state) P’ 4-momentum transfer squared, Q 2, and energy transfer, are NOT proportional W (mass of the final state) ++ Neutron Momentum & energy transfer for inelastic scattering

Inelastic scattering  elastic scattering from “parton” followed by “hadronization”  Q 2 now proportional to  again ! Deep inelastic scattering  “elastic scattering” (off partons) April 30, Quarks: search for the smallest Proton pion Neutron electron photon electron Excited State mass = W Proton electron photon electron Richard Feynman

“Elastic” scattering from a parton April 30, Quarks: search for the smallest Proton electron photon electron Excited State mass = W q xP P’ How is x defined? Proton’s structure: “structure function” F is the product of momentum distribution: f(x) charge (squared) of the component  Rate of interaction ~ F  F ~ q 2 * f(x) How is x defined? Proton’s structure: “structure function” F is the product of momentum distribution: f(x) charge (squared) of the component  Rate of interaction ~ F  F ~ q 2 * f(x)

“Bjorken scaling” April 30, Quarks: search for the smallest “scaling”  function of two variables becomes a function of their quotient probability of scatter = probability that parton has fraction (x ) of proton’s momentum times probability of interaction (charge 2 ) Richard Taylor James Bjorken

Evidence for “partons” Hypothesis: proton made of “parts” revealed in scattering experiments (like Rutherford’s discovery) carry a fraction (x) of the proton’s 4-momenta (p q = x P) assumed structure-less, so electron scatters elastically off partons functions of Q 2 and become function of x;  x = Q 2 / 2m cross-section depends only on the x-distribution and charge “Scaling” occurs whenever the cross-sections (for different Q 2 and ) becomes a function of their ratio, x, only. Hypothesis: proton made of “parts” revealed in scattering experiments (like Rutherford’s discovery) carry a fraction (x) of the proton’s 4-momenta (p q = x P) assumed structure-less, so electron scatters elastically off partons functions of Q 2 and become function of x;  x = Q 2 / 2m cross-section depends only on the x-distribution and charge “Scaling” occurs whenever the cross-sections (for different Q 2 and ) becomes a function of their ratio, x, only. April 30, Quarks: search for the smallest

Scaling seen  partons inside proton April 30, Quarks: search for the smallest F 2 plotted versus ratio of 2m  q  Many different energies and angles overplotted, but they lie on one curve if plotted versus  F (x) 1/x Jerry Friedman Henry Kendall Richard Taylor

angle of “jets”  quarks are spin 1/2 April 30, Quarks: search for the smallest Gail Hanson Marty Perl

Other properties of partons fractional charge momentum distribution fractional charge momentum distribution April 30, Quarks: search for the smallest connected, we measure q 2 * f(x) Original quark model of 1964, proposed that many of the new particles (basically excited protons) were composed of three quarks. The quarks have charge 2/3 or -1/3; e.g. the proton has charge 1 = 2/3 + 2/3 - 1/3; while the neutron has charge 0 = 2/3 -1/3 -1/3. For these charges the momentum fraction of the proton which is carried by the partons is only 50%. What carries the remainder? Murray Gell-Mann

Quarks discovered!! fractionally charged, spin ½ partons  Quarks are discovered … but many mysteries remained - what carries the rest of the proton’s momentum ? - does ‘scaling’ hold exactly ? - let’s see fractionally charged, spin ½ partons  Quarks are discovered … but many mysteries remained - what carries the rest of the proton’s momentum ? - does ‘scaling’ hold exactly ? - let’s see April 30, Quarks: search for the smallest

Pattern of scaling violation April 30, Quarks: search for the smallest Structure function is NOT a function of x only; depends on Q 2. Small-x values INCREASE with Q 2. Large-x values DECREASE with Q 2.  quarks are radiating energy ! (probability increases with Q 2 ) WHAT are they radiating ? -quanta of the strong color field GLUONS This pattern of scale-breaking can be calculated using QCD. F 2 (x,q 2 ) Q 2 (GeV 2 ) ‘lines’ at constant x

Evidence for QCD Missing momentum & pattern of scaling violation – Explained by “gluon radiation” – analogous to bremsstrahlung (“braking radiation”) How can electrons scatter from quarks elastically? – they act like free particles, but are bound in the proton ! Missing momentum & pattern of scaling violation – Explained by “gluon radiation” – analogous to bremsstrahlung (“braking radiation”) How can electrons scatter from quarks elastically? – they act like free particles, but are bound in the proton ! April 30, Quarks: search for the smallest If you probe the proton at small distances (high Q 2 ), the quark responds as if it is not bound (free), but as it pulls away to larger distances (fm’s), it feels the attractive force.

asymptotic freedom & QCD April 30, Quarks: search for the smallest “for the discovery of asymptotic freedom in the theory of the strong interaction” 2004 Nobel Prize in Physics David Politzer Frank Wilczek David Gross

A Modern Particle Detector CLAS detector: -magnetic spectrometer (curvature ~ 1/p) -drift chambers (tracking) -scintillators (timing) -calorimeters (energy, e/  ) -Cerenkov (e/  ) Fast: > 2000 evts/sec Large acceptance > 2  sr CLAS detector: -magnetic spectrometer (curvature ~ 1/p) -drift chambers (tracking) -scintillators (timing) -calorimeters (energy, e/  ) -Cerenkov (e/  ) Fast: > 2000 evts/sec Large acceptance > 2  sr April 30, Quarks: search for the smallest Bryce

First, we have to build them, ~1995 April 30, Quarks: search for the smallest

April 30, 2010 Quarks: search for the smallest Geiger counter: gas ionization by particles tube gas wire (at positive high-voltage, ~ 2000 V cosmic ray ~1 ionization/ 300 mm electrons / ionization ~ 100 electrons/cm 23

April 30, 2010 Quarks: search for the smallest “drifting” of the electrons wire at positive voltage electrons drift to the wire strike a molecule every 2  m velocity ~ 50  m/ns (max) 24

April 30, 2010 Quarks: search for the smallest how tracking works hit wires shown in yellow minimize rms between track and calculated distance 25 Georges Charpak

April 30, Quarks: search for the smallest simulated tracking

Quarks: what next? QCD: well-established as the theory of the strong interactions  forces between quarks BUT, it’s a strongly-interacting field theory  very difficult to SOLVE the equations INSTEAD, people GUESS solutions based on qualitative aspects of QCD … and work out the consequences. QCD: well-established as the theory of the strong interactions  forces between quarks BUT, it’s a strongly-interacting field theory  very difficult to SOLVE the equations INSTEAD, people GUESS solutions based on qualitative aspects of QCD … and work out the consequences. April 30, Quarks: search for the smallest

Gluons: the strong force-field April 30, Quarks: search for the smallest Because of self-interactions the field lines compress into a tube. The field energy grows linearly with separation  constant force ~ 1 GeV/fm

April 30, Quarks: search for the smallest Nathan Isgur

April 30, 2010Quarks: search for the smallest Analysis: Detect Electron Cerenkov with C 4 F 10 e.m. shower counter Identify Kaon & Proton time of flight: ~100 ps p/K separation to 2 GeV/c Missing-mass for  good resolution: 0.5% dp/p separate  from  0 e p  K+  experiment at CLAS 30

how to measure Lambda polarization Lambda is a spin ½ particle – decays to Proton (spin ½) and  - (spin 0) – two amplitudes: s-wave (L=0) and p-wave (L=1) – (A1+A2) 2 ~ (1.+  cos  )  = 0.61  measure the angular distribution of the decay proton relative to some axis and fit to (1. + P  cos  )  P is the polarization of the sample of Lambda’s Lambda is a spin ½ particle – decays to Proton (spin ½) and  - (spin 0) – two amplitudes: s-wave (L=0) and p-wave (L=1) – (A1+A2) 2 ~ (1.+  cos  )  = 0.61  measure the angular distribution of the decay proton relative to some axis and fit to (1. + P  cos  )  P is the polarization of the sample of Lambda’s April 30, Quarks: search for the smallest

April 30, 2010 Quarks: search for the smallest Simpler in quark picture ?  Polarization Transfer xyz system defined in electron plane z along  direction Polarization transfer near maximal along z ~ 75% ~0 along x direction Models are only “ok” but, not tuned sensitive to polarization Carman et al, PRL (2003) 32

Quark-pair creation ‘flux-tube’ broken by the creation of a q- q pair ! An ‘escaping’ quark always gets a partner anti-quark ! ‘flux-tube’ broken by the creation of a q- q pair ! An ‘escaping’ quark always gets a partner anti-quark ! April 30, Quarks: search for the smallest note spin correlation

Two model explanations … April 30, Quarks: search for the smallest Two views of how the  is polarized : top: u-quark polarized; sbar polarization selected opposite; s- sbar in spin-0 state bottom: s and s-bar polarized directly by photon Both can explain  polarization ! On-going studies to distinguish between the two models. Two views of how the  is polarized : top: u-quark polarized; sbar polarization selected opposite; s- sbar in spin-0 state bottom: s and s-bar polarized directly by photon Both can explain  polarization ! On-going studies to distinguish between the two models.

building the equipment ! April 30, Quarks: search for the smallest

it takes all types … April 30, Quarks: search for the smallest experimenters detector builders theorists

Summary: the discovery of the quark How the quark was discovered: scattering experiments: measure rate vs. angle, momentum elastic e-p cross-section deviates from 1/sin 4 (  /2)  proton has finite size inelastic e-p scattering shows ‘scaling’ behavior  “partons” in proton Development of the theory: QCD can explain the scattering data with fractionally charged, spin ½ quarks and a gluonic force-field. Questions remain*: dynamics of quark-pair creation… How the quark was discovered: scattering experiments: measure rate vs. angle, momentum elastic e-p cross-section deviates from 1/sin 4 (  /2)  proton has finite size inelastic e-p scattering shows ‘scaling’ behavior  “partons” in proton Development of the theory: QCD can explain the scattering data with fractionally charged, spin ½ quarks and a gluonic force-field. Questions remain*: dynamics of quark-pair creation… * “It does no harm to the mystery to understand a little about it.” - Richard Feynman April 30, Quarks: search for the smallest

relation between rates and angle April 30, Quarks: search for the smallest More area for small-angle scattering  higher rates

Two model explanations … April 30, Quarks: search for the smallest Two views of how the  is polarized : top: u-quark polarized; sbar polarization selected opposite; s- sbar in spin-0 state bottom: s and s-bar polarized directly by photon On-going studies to distinguish between the two models.  measure  polarization for production of K* +  final state Two views of how the  is polarized : top: u-quark polarized; sbar polarization selected opposite; s- sbar in spin-0 state bottom: s and s-bar polarized directly by photon On-going studies to distinguish between the two models.  measure  polarization for production of K* +  final state K* +

K+K+ susu  udSudS ss produced From flux-tube Quark Pair Creation Quark-pair creation: “kernel” of exclusive production What field couples to the q-q current? Quark-pair creation: “kernel” of exclusive production What field couples to the q-q current? susu sudsud K+K+  ss produced from photon Sept. 26, 2009Mac Mestayer 40Hadron Spectroscopy Meeting ++ d d N u u P 00 s-quark   K + final state d-quark  N  + final state u-quark  P  0 final state -measure ratio of rates -different ratios

Using Exclusive Production to Study Quark Pair Creation Lund model: successful phenomenology for hadron production; e.g. in e + e - reactions color flux-tube broken by qq production – production rate depends on constituent quark mass – : : ~ 1 : 1 : 0.2 Vector meson dominance: photon fluctuates into a virtual qq meson – production rate depends on quark charge – : : ~ 1: 0.25 : 0.25 Lund model: successful phenomenology for hadron production; e.g. in e + e - reactions color flux-tube broken by qq production – production rate depends on constituent quark mass – : : ~ 1 : 1 : 0.2 Vector meson dominance: photon fluctuates into a virtual qq meson – production rate depends on quark charge – : : ~ 1: 0.25 : 0.25 uudd ss uudd ss Sept. 26, 2009Mac Mestayer 41Hadron Spectroscopy Meeting

October 15, 2004 Spin2004 Mac Mestayer ,  0 Kaon Identification Hyperon Missing Mass Mass = P /  (GeV) Missing Mass (GeV) e p  e’ K + (X) Kaon candidates after timing cut