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Pion Polarizabilities at COMPASS: Experimental Results and Theoretical Predictions

This paper discusses the measurement of pion polarizabilities using the COMPASS spectrometer. Experimental results are compared to theoretical predictions from Chiral Perturbation Theory and other models. The Primakoff reaction and radiative pion photoproduction are also studied. Preliminary results are presented.

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Pion Polarizabilities at COMPASS: Experimental Results and Theoretical Predictions

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  1. First results on pion polarizabilitiesat COMPASS Daniele Panzieri Università del Piemonte Orientale & INFN Torino for the COMPASS collaboration

  2. Czech Republic, Finland, France, Germany, India, Israel, Italy, Japan, Poland, Portugal, Russia Bielefeld, Bochum, Bonn, Burdwan, Calcutta, CERN, Dubna, Erlangen, Freiburg, Heidelberg, Helsinki, Lisbon, Mainz, Miyazaky, Moscow, Munich, Nagoya, Prague, Protvino, Saclay, Tel Aviv, Torino, Trieste, Warsaw 28 Institutions, ~ 230 physicists COmmon Muon and Proton Apparatusfor Structureand Spectroscopy EINN07 - Milos 12-15 september 2007

  3. HADRON program • Pion polarizabilities • Chiral anomaly • Charm Barions • Glueballs and exotic search • MUON program • ΔG/G • Structure functions • Exclusive production of vector meson • Λ-physics • Transversity LHC SPS N EINN07 - Milos 12-15 september 2007

  4. The Spectrometer • m/h beam • high beam intensity • broad kinematical range • large angular acceptance • Two stage spectrometer • Large angle spectrometer (SM1) • Small angle spectrometer (SM2) • tracking, calorimetry, PID • SciFi Straws • Silicon SDC • Micromegas MWPC • GEMs W45 EINN07 - Milos 12-15 september 2007

  5. Hadron-beam Run 2004 • Low beam intensity: 2 ·106 p-/spill • Beam time: 10 days • Different targets: 1.6 - (2+1) - 3 mm Pb, 7 mm Cu, 23 mm C • Saturated trigger rate (40-50k/spill) EINN07 - Milos 12-15 september 2007

  6. qq ap Pion polarizabilities The polarizability (electric  and magnetic ) relates the average dipole (electric p and magnetic ) moment to an external electromagnetic field, characterizing the rigidity of the quark-antiquark system Theoretical predictions The pion polarizabilities has been described in the framework of Chiral Perturbation Theory (cPT) cPT (2 loops) ap+bp= 0.16·10-4 fm3 ; ap-bp= (5.7 ± 1.0)·10-4 fm3 Other theoretical models: QCMap+bp=0.23·10-4 fm3 ;ap-bp= 7.05 ·10-4 fm3 QCD sum rulesap= (5.6 ± 0.5)·10-4 fm3 Dispersion Sum Rulesap+bp= (0.166 ± 0.024)·10-4 fm3 ; ap-bp= (13.60 ± 2.15)·10-4 fm3 EINN07 - Milos 12-15 september 2007

  7. Radiative pion photoproduction Radiative pion scattering Photon-photon collision Experimental methods EINN07 - Milos 12-15 september 2007

  8. Experimental values p-+ Z p-+ Z + g SIGMA-AYAKS (Serpukhov) ap = 6.8 ± 1.4stat± 1.4syst (for ap+bp = 0) g +pg+ p++ n Lebedev ap= 20 ± 12stat A2 (MAMI) ap- bp= 11.6 ± 1.5stat ± 3.0syst ± 0.5mod g+ gp+ + p- MARK II ap = 2.2 ± 1.6stat+syst PLUTO ap = 19.1 ± 4.8stat± 5.7syst DM1 ap = 17.2 ± 4.6stat DM1 ap = 26.3 ± 7.4stat The experiments are affected by a large statistical and/or systematic errors All values in: 10-4 fm3 EINN07 - Milos 12-15 september 2007

  9. ’ g g* απ, βπ independently  Z,A βπ The Primakoff Reaction p- + Z p- + Z + g Inverse kinematics for the Compton scattering: γ*π→ γ π’ In the incoming pion rest frame (Anti-laboratory system) ω is the energy of the virtual photon in the alab. Assuming (απ+ βπ) = 0 in the Laboratory system: Q2max depends on analysis cut Eγis the energy of the real photon EINN07 - Milos 12-15 september 2007

  10. Trigger • About 10 days of data taking (pilot run); integrated beam flux is 1011 pions • Beam: 190 GeV/c;~106π/s, 4.8 s / 16 s spill structure 190 GeV/c; ~108μ/s • 2 triggers: • VetoxHodoscopehit x Ecal2 (E > 50 GeV) x Hcal (E > 18 GeV) (primakoff1) • VetoxEcal2 (E> 100 GeV) (primakoff2) EINN07 - Milos 12-15 september 2007

  11. p - + Pb p -+ Pb +g γ ECal2 ECal2 Typical reconstructed event EINN07 - Milos 12-15 september 2007

  12. total energy Event Selection • The current analysis is based on: • 3 days of data taking • Pb 2+1 mm target • primakoff2 trigger π + γ in the final state primary vertex near the target nominal position invariant mass cut Mπγ < 3.75 mπ |E γ+Eπ - Ebeam| < 25 GeV PT > 45 MeV/c 0.5 < Eγ/Ebeam< 0.9 Q2 < 7.5·10-3 (GeV/c)2 EINN07 - Milos 12-15 september 2007

  13. Analysis procedure use pointlike reference particle within the same setup (m) Q2 (GeV/c)2 EINN07 - Milos 12-15 september 2007

  14. Background corrections Kaon peak • ρ production: π- + Z→ρ- + Z→π- +Z + γ + γsuppressed by Mπγ cut • e-→e- + γ (0.1% of e- in hadron beam)suppressed by PT cut • µ-→µ- + γ (0.1% of µ- in hadron beam) σsyst • diffractive process subtracted • kaon decay: k-→π-π0→π- + γ + γsubtractedwith empty target (~4% of K- in hadron beam) EINN07 - Milos 12-15 september 2007

  15. Primakoff analysis Q2 distribution for different target material Z2 dependency Good agreement of the Z2 dependency for the Primakoff cross-section for a large Z range EINN07 - Milos 12-15 september 2007

  16. COMPASS preliminary results MC simulation • POLARIS generator for Primakoff πγ and µγ events. • COMPASS simulation based on GEANT3 w = Eg/Ebeam The acceptance behaviour is similar for muon and pion EINN07 - Milos 12-15 september 2007

  17. COMPASS preliminary results Preliminary estimation of the Radiative Corrections work still ongoing • Vacuum polarization • Compton vertex • Multiple photon exchange • Screening by atomic electrons The correction for pion polarizability is about: 0.6 x 10-4 fm3 EINN07 - Milos 12-15 september 2007

  18. COMPASS preliminary results Pion Muon EINN07 - Milos 12-15 september 2007

  19. COMPASS preliminary results Pion Muon bp= (-2.5 ±1.7stat) · 10-4 fm3 bm= (-0.2 ±0.5stat) · 10-4 fm3 EINN07 - Milos 12-15 september 2007

  20. COMPASS preliminary results Systematic uncertainties ap = - bp = 2.5 ± 1.7stat ± 0.6syst· 10-4 fm3 EINN07 - Milos 12-15 september 2007

  21. COMPASS preliminary results p-+ Z p-+ Z + g COMPASS ap = 2.5 ± 1.7stat± 0.6syst (for ap+ bp = 0) SIGMA-AYAKS (Serpukov) ap = 6.8 ± 1.4stat± 1.4syst (for ap+bp = 0) g +pg+ p++ n Lebedev ap= 20 ± 12stat A2 (MAMI) ap- bp= 11.6 ± 1.5stat ± 3.0syst ± 0.5mod g+ gp+ + p- MARK II ap = 2.2 ± 1.6stat+syst PLUTO ap = 19.1 ± 4.8stat± 5.7syst DM1 ap = 17.2 ± 4.6stat DM1 ap = 26.3 ± 7.4stat All values in: 10-4 fm3 EINN07 - Milos 12-15 september 2007

  22. Conclusions Preliminary result of the measurement of pion polarizabilities at COMPASS, under the approximation ap + bp = 0is: ap = - bp = 2.5 ± 1.7stat ± 0.6syst· 10-4 fm3 Present analysis is based on only 3 days of data taking and is at the level of the previous measurements Systematic uncertainties are well understood and could be further improved with a better statistic with muon beam Work is ongoing to extract separately apandbp EINN07 - Milos 12-15 september 2007

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