The Quark Structure of the Nucleon Inti Lehmann

The Quark Structure of the Nucleon Inti Lehmann & Ralf Kaiser University of Glasgow Cosener’s House Meeting 23/05/2007 • Nucleon Structure • Generalised Parton Distributions • What are GPDs? • Deeply Virtual Compton Scattering • GPD Experiments • HERMES at DESY • CLAS at JLAB • PANDA at FAIR I. Lehmann & R. Kaiser Nucleon Structure

Nucleon Structure n n n Proton consists of 3 quarks (Gell. Mann, Zweig 1964). . . and gluons and sea quarks (QCD) Partons (Feynman/Bjorken) identified with quarks and gluons and verified in scattering experiments Proton has spin 1/2, and so do the quarks 2004 Nobel Prize for Gross, Wilczek, Politzer I. Lehmann & R. Kaiser Nucleon Structure 2

Form Factors n n Form Factors are measured in elastic scattering experiments Transverse Structure of the proton Fourier Transform of the form factor gives charge distribution Example: q n Electric form factor of the neutron GEn. Result shows evidence for pion cloud. Glasgow: q Form factor measurements at MAMI and JLAB I. Lehmann & R. Kaiser Nucleon Structure 3

Parton Distribution Functions n n n Structure functions and parton distribution functions are measured in deep inelastic scattering Longitudinal Structure of the proton Example: Measurement of Structure Function F 2 from H 1 and ZEUS at DESY. q n The number of quarks and gluons seen increases dramatically for small momentum fraction x. Glasgow: q Polarised quark distributions with HERMES at DESY. I. Lehmann & R. Kaiser Nucleon Structure 4

Comparison Form Factor I. Lehmann & R. Kaiser Parton Distribution Function Nucleon Structure 5

Generalised Parton Distributions Form Factor I. Lehmann & R. Kaiser Parton Distribution Function Nucleon Structure Generalised Parton Distribution 6

Generalised Parton Distributions I. Lehmann & R. Kaiser Nucleon Structure 7

Hadron Tomography n The Fourier transform of GPDs at x=0 leads to a 3 dimensional picture of the nucleon: q longitudinal momentum fraction and transverse impact parameter space. I. Lehmann & R. Kaiser Nucleon Structure 8

Hadron Tomography n GPD Model restricted by form factor data exists: [P. Kroll, hep-ph/0612026, 4. Dec. 2006] n u-quark (left) and d-quark (right) density in impact parameter plane. Proton polarised in x-direction I. Lehmann & R. Kaiser Nucleon Structure 9

GPDs and the Spin Puzzle Ji Sum Rule: Measure GPDs to determine Lq ! I. Lehmann & R. Kaiser Nucleon Structure 10

How to Measure GPDs DVCS n n Deeply Virtual Compton Scattering DVCS: cleanest process to measure GPDs Experimentally: q n Measurements of several asymmetries Main current and future experiments: q HERMES and JLAB, especially with 12 Ge. V upgrade, perhaps COMPASS I. Lehmann & R. Kaiser Nucleon Structure 11

Kinematical Coverage of DVCS Experiments n HERA collider experiments H 1 and ZEUS have small skewedness n Fixed target experiments are crucial to explore GPDs ! I. Lehmann & R. Kaiser Nucleon Structure 12

DVCS Asymmetries - First Measurements n n n First measurements of DVCS asymmetries: Beam-spin asymmetry by HERMES and CLAS, both published in PRL 87(2001). Glasgow group involved in both of them. HERMES 27. 5 Ge. V, beam polarisation ~55%, recoil proton not detected CLAS (JLab Hall B) 4. 25 Ge. V, beam polarisation ~70%, produced photon not detected I. Lehmann & R. Kaiser Nucleon Structure 13

HERMES at DESY Exclusive measurements (e. g. DVCS) with Forward Spectrometer and Recoil Detector since 2006 I. Lehmann & R. Kaiser Nucleon Structure 14

HERMES Recoil Detector n n Challenging Detectors Successfully commissioned Project Leader R. Kaiser, Glasgow I. Lehmann & R. Kaiser Nucleon Structure 15

HERMES with Recoil Detector n n n Remove background from associated BH/DVCS with intermediate D-production and from semi-inclusive processes improve t-resolution at small t (with Sidetector) About the same statistics in 1 year as in the 10 years before Background Reduction from. Nucleon 17% to ~2% Structure I. Lehmann & R. Kaiser 16

CLAS at JLAB n n CEBAF Large Acceptance Spectrometer at JLab in Hall B Toroidal Magnet Electron and tagged real photon beams Large collaboration of almost 200 members I. Lehmann & R. Kaiser Beam Nucleon Structure 17

CLAS : High Statistics Beam Spin Asymmetry n n Experiment E 01 - 113, preliminary data, still unpublished All three final state particles (electron, photon, proton) detected Statistics allows 3 -d binning in x, Q 2 and t First glimpse at what future JLab experiments will be able to do preliminary I. Lehmann & R. Kaiser Nucleon Structure 18

Ju + Jd Limits from HERMES and JLab n n n JLab Hall A E 03 -106 neutron beam-spin asymmetry using unpolarised LD 2 target HERMES transverse target spin asymmetry on proton target Combination gives (model-dependent) constraint on Ju+Jd I. Lehmann & R. Kaiser Nucleon Structure 19

CLAS 12 at JLAB n n I. Lehmann & R. Kaiser From 2013 high statistics measurements at 11 Ge. V with upgraded CLAS 12 detector Extraction of GPD H from BSA Nucleon Structure 20

DVCS at PANDA at FAIR n n PANDA will have a 15 Ge. V anti-proton beam on a proton target. It therefore can measure the ‘cross channel’ or ‘timelike’ version of the DVCS process, that depends on the same GPDs More precisely on Generalised Distribution Amplitudes, introduced by M. Diehl et. al. to describe the inverse process [PRL. 81: 1782 (1998)]. I. Lehmann & R. Kaiser Nucleon Structure 21

Outlook n Current measurements by CLAS and HERMES: q n Future experiments at JLAB and FAIR: q n n n Lots of data is coming; analysis work still do be done Hall A, CLAS 12 and PANDA, respectively The Glasgow NP group is unique in its access to all key GPD data worldwide and is playing a leading role in Europe in coordinating GPD research Framework 6 Joint Research Activity coordinated by R. Kaiser, Glasgow (gpd. gla. ac. uk) Framework 7 Network in preparation, global fit to all existing data (R. Kaiser) I. Lehmann & R. Kaiser Nucleon Structure 22

Supplements I. Lehmann & R. Kaiser Nucleon Structure 23

Deeply Virtual Compton Scattering exclusive meson production PDF in DIS I. Lehmann & R. Kaiser GPD s Crossed Compton Scattering Wide Angle Compton Scattering - pp annihilation Form Factors orbital angular momentum hadron tomography Nucleon Structure 24

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