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Barion Form Factors at FAIR

This article discusses the definitions and properties of nucleon time-like form factors, explores the open questions surrounding them, and highlights the experimental challenges in measuring them accurately.

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Barion Form Factors at FAIR

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  1. Barion Form Factors at FAIR P.Gianotti INFN Laoratori Nazionali Frascati

  2. Form Factors: definitions Space Like Time Like q2<0 0. 4MN2 q2>0 Annihilation or reversed channel Electron scattering

  3. Form Factors: definitions In the approximation of single gexchange ds/dW [ |GM(q2)|2(1+cos2q)+4mN2/q2 |GE(q2)|2 sin2q)] • Sachs and Pauli/Dirac FF • in the Space-Like region  Fourier transform of charge and magnetization distributions • At threshold (t=q2/4MN2=1) |GE| = |GM| • F1 and F2 are analytic functions of q2 real in Space-Like, butcomplex in Time-Like. • Time-Like GE and GM FF areanalytic prolongations of Space-Like non spin-flip (GE) and spin flip (GM) form factors. GE = F1 + t F2 GM = F1 + F2

  4. Why measuring nucleon TL FF? • FF are fundamental quantities of the nucleon (m, m, …) • Understanding the Nucleon structure • Comparison with Space-Like data that seems to show the non validity of the Born approximation • Testing the hypothesis GE/GM= 1 • Test different theoretical approaches

  5. Form Factors: open questions Perturbative QCD (q2 ∞) F1(q2)  q-4F2(q2)  q-6 q2F2/F1  constant exp data seems to indicate qF2/F1  constant limq2→∞ |GM|time like = |GM|space like exp data gives |GM|time like ≈ 2 |GM|space like

  6. SL FF present situation Data on the ratio GE/GM for the proton including the older Rosenbluth separation data (crosses), most recent JLab Rosenbluth separation data (filled circles), and polarization transfer data (triangles) New Jlab data have been obtained with the recoil polarization method stating that: PRL 94 142301 (2005) The old assumption seems no more valid

  7. FF TL: data extraction • |GE| and |GM| in the Time-Like region can be determined by the reactions pp↔e+e- • Presently statistics is limited  no real separation |GE|/ |GM| • |GM| extracted assuming |GE| = |GM| (true at threshold) • GE in Time-Like region is today unknown • Recent data from BaBar extraction of the ratio R = |GE|/ |GM| through the ISR method (e+e- gpp) (q2<7 (GeV/c)2)

  8. FF Time-Like: angular distribution In the approximation of single gexchange ds/dW [ |GM(q2)|2(1+cos2q)+4mN2/q2 |GE(q2)|2 sin2q)] is cross section isotropic? • With PANDA we will have access to almost total angular range • Direct access to |GM(q2)| and |GE(q2)| • The sensitivity to |GE(q2)| decreases while energy increase (4mN2/q2 = 0.25 at 10 GeV, and only 0.14 at 15 GeV) • 2g: odd cosqcontributions in the cross-section. Are these an explanation? (PL B659 197)

  9. FF Time-Like: present data A new bulk of data up to 20 GeV came from BaBar using the technique of ISR |GM|2 10-1 10-2 For E2 < 2.1 GeV2 the ratio |GE|/ |GM|has been found >1 in disagreement with previous LEAR data |GEp|/|GMp| 4 5 6 7 8 9 10 20 E (GeV) Ebeam (GeV) PRD 73 012005

  10. LEAR data p p → e+ e- BaBar data e+ e- → gp p Angular distributions Dashed: |GM| Dot-dashed: |GE| Mpp=1.92GeV/c2 Mpp=1.94GeV/c2 Mpp≈1.99 GeV/c2 Mpp≈1.91 GeV/c2 Mpp=1.96 GeV/c2 Mpp=1.99GeV/c2 Mpp≈2.06 GeV/c2 Mpp≈2.15 GeV/c2 Dashed: |GE|/|GM| free Mpp=2.05 GeV/c2 Solid: |GE|=|GM| Both data sets have big errors! Mpp≈2.30 GeV/c2 Mpp≈2.70 GeV/c2 PLB 559 20 PRD 73 012005

  11. Facility for Antiproton and Ion Research Primary Beams • 2(4)x1013/s 30 GeV protons Secondary Beams • Antiproton production target • Antiprotons 3 - 30 GeV HESR High resolution mode • dp/p ~ 10-5 (electron cooling) • Luminosity = 1031 cm-2 s-1 Storage and Cooler Rings • 1011 stored and cooled 0.8 - 14.5 GeV antiprotons High luminosity mode 100 m • Luminosity = 2 x 1032 cm-2 s-1 • dp/p ~ 10-4 (stochastic cooling)

  12. PANDA: the detector target calorimeter Forward Spectrometer Solenoid DIRC Forward tracking STT/TPC MVD

  13. PANDA: Physics program • Charmonium and open charm spectroscopy; • Charmed hybrids and glueballs: • Meson mass modification in the nuclear matter; • Double hypernuclei produced via Ξ-baryon capture; • Wide angle compton scattering; • Drell-Yan reactions; • Baryon-Antibaryon production; • CP-Violation (Λ,D). http://www-panda.gsi.de

  14. 0.1% 1% 10% Time-Like FF : world data |GM|2 10-1 10-2 in e+e- VEPP L=1032 Frascati L=2. 1032? BEPC/BES L=1033 Present typical errors 5% 20% 50% PANDA range : 5 to 30 (GeV/c)2 at L = 2. 1032 PANDA errors

  15. Plans for the TL FF • Separate measurement of |GE| et |GM| • Precisions on the ratio R=|GE|/|GM| and sR • DR/R <1% at low Q2 • DR/R = 10% at Q2=10 (GeV/c)2 • Separation possible up to Q2=15 (GeV/c)2 • Test of the 1 g hypothesis (symmetry of the angular distribution) • Measure |GM| up to Q2=25 (GeV/c)2 • Possibility to measure the phase difference j(GE) - j(GM) in case of transverse polarization pp↑↔e+e- • Double spin asymmetry will allow the independent measurements of GE and GM PAX program

  16. Experimental challenge 106 times higher background Thanks to PANDA PID capabilities we expect to reach a pion rejection power at the level of 10-8 ≈106 2 μb/8pb at q2=9 (GeV /c)2 Cherenkov radiation: above 1 GeV Radiators: quartz, aerogel, C4F10 Energy loss: below 1 GeV Tracking devices: best accuracy with a TPC Electromagnetic processes: EMC for e/g Time of flight Problem: no start detector, huge material

  17. Beam PANDA PID To allow electron-pion separation at 10-8 level PANDA must have excellent PID capability MVD PID capability Space resolution 50μm Low material 5% X/X0 dE/dx capability High rate 107ev/s

  18. PANDA tracking system Space resolution: r< 150µm z ~ few mm Momentum resolution: pt / pt ~ 1.2 % Material budget: X/X0 ~ 1.0-1.3% dE/dx capability Two options Straw Tube Tracker Time Projection Chamber

  19. dE/dxperformances, stt vs tpc supposing the particles are hitting 22 tubes Gaussian distribution ( ~ 6 %): probability of misidentified pion 3% at 1 GeV/c , 10% at 6 GeV/c stt tpc [Panda TPR]

  20. DIRC Concept Detection of Internally Reflected Cherenkov light • Different Cherenkov angles give different • reflection angles PANDA DIRC similar to BaBar • 96 Fused silica bars, 2.6m length • Water tank & 7000 PMTs • Alternative readout: (x,y,t), mirrors Schwiening Schwiening

  21. 1.0 GeV/c e  Electromagnetic Calorimeters Forward Endcap 4000 PWO crystals High occupancy in center Readout LA APD or vacuum triodes Backward Endcap 800 Crystals Worse resolution due to service lines of trackers Needed for hermeticity Barrel Calorimeter 11000 PWO Crystals LA APD readout σ(E)/E~1.5%/√E + const. Forward Shashlyk (after dipole magnet) 350 channels Readout via PMTs σ(E)/E~4%/√E + const. Alternative Designs: Spiral Shashlyk Segmented composite Shashlyk-Sandwich for for (e/h/µ) Energy deposit in EMC

  22. Pion rejection is OK

  23. en 10 7 s Nb of counts for pp→e+e- 6 104 counts Elab=1 GeV Elab=2.5 GeV Elab=5 GeVElab=10 GeV 106 counts 3000 counts s=q2=5.4 (GeV/c)2q2= 8.2 (GeV/c)2q2=12.9 (GeV/c)2q2=22.3 (GeV/c)2 Ntot=106Ntot=66000Ntot=2750Ntot=82 DR=0.6%DR=3%DR=25%DR= 100% cos(cm) FF Time-Like : GE et GM ~100 days, L = 2. 1032 cm-2s-1 ds/dW [ |GM(q2)|2(1+cos2q)+4mN2/q2 |GE(q2)|2 sin2q)] - - Nb of counts for pp→e+e- Nb of counts for pp→e+e- 10 B. Ramstein J. Van de Wiele 6 104 counts 5 106 counts 3000 counts 80 counts Q²=22.3 (GeV/c)² cos R=|GE|/|GM| EM working group

  24. Summary - electromagnetic form factors: fundamental property of nucleon - poorly known in time-like region - PANDA offers a unique possibility to measure in time-like domain up to q2 = 20 GeV2 - crucial: high luminosity & excellent particle ID

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