1 / 16

Exclusive production at HERMES

Exclusive production at HERMES. Cynthia Hadjidakis on behalf of the HERMES collaboration. Rencontres de Moriond, La Thuile March 28 th – April 4 th , 2003. Exclusive physics and Generalized Parton Distributions Compton scattering (DVCS) Vector meson production Pion production.

varick
Download Presentation

Exclusive production at HERMES

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Exclusive production at HERMES Cynthia Hadjidakis on behalf of the HERMES collaboration Rencontres de Moriond, La Thuile March 28th – April 4th, 2003 • Exclusive physics and Generalized Parton Distributions • Compton scattering (DVCS) • Vector meson production • Pion production

  2. Generalized Parton Distributions (GPDs) Q2>>, t<< g*L t p0, r0L, g ... - Műller(1994) - - Ji & Radyushkin(1996) - -2 x 4 GPDs defined for each quark flavour: HqHqconserve nucleon helicity EqEqflip nucleon helicity ~ x+x x-x ~ N N’ unpolarized polarized 3 variables: x, x, t x+x Longitudinal momentum fraction of the quark -2x Exchanged longitudinal momentum fraction t Squared momentum transfer GPDs = probability amplitude for N to emit a parton (x+x) and for N’ to absorb it (x-x)

  3. Limiting cases and sum rules 30%(DIS) Ji sum rules =1/2 DS+ D Lz ( H + E ) x dx =Jquark

  4. Generalized Parton Distributions (GPDs) • Quantum number of final state selects different GPDs Vector mesons (r, w, f) unpolarised GPDs H E Pseudoscalar mesons (p, h) polarised GPDs H E DVCS (g) depends on both unpolarised and polarised GPDs ~ ~ • Different combination of GPDs according to the reaction: • flavour decomposition ~ ~ ~ Hpp0: 2/3 Hu/p + 1/3 Hd/p Hpp+: Hu/p - Hd/p ~ ~ ~

  5. HERMES Spectrometer → → e+, e- 27.5 GeV Tracking: 57 tracking planes dP/P = 2 %, dq < 0.6 mrad Particle Identification: RICH, TRD, preshower, calorimeter lepton identification 99% p identification 98% for pp > 2 GeV

  6. Deep Virtual Compton Scattering: e p → e p g BH larger than DVCS for HERMES kinematics: DVCS Bethe-Heitler DVCS-BH interference leads to non-zero azimuthal asymmetry

  7. Azimuthal asymmetry DVCS-BH interference gives a non zero azimuthal asymmetry: g - Beam helicity asymmetry - Beam charge asymmetry

  8. Exclusivity for e p → e p g Missing Mass2 = ( p + g* – g )2 Missing Mass resolution : 0.8 GeV Exclusive region: Missing Mass < 1.7 GeV

  9. DVCS asymmetry measurement e p → e p g e+/- p → e p g PRL, 87 (2001), 182001 ‹xB›=0.11 ‹Q2›=2.6 GeV2 ‹-t›=0.27 GeV2 ALU = -0.230.04(stat)0.03(syst) AC=0.110.04(stat)0.03(syst) Expected cos F dependence sin F dependence • Kivel, Polyakov & Vanderhaeghen(2001) – Signal of DVCS process Can be described by GPD calculation

  10. HERMES upgrade: the recoil detector projection of the statistical accuracy of ALU for 2 fb-1 (1 year of data) Around the target: reconstruct the kinematics of the recoil particles

  11. Vector mesons g*L r0L,wL,fL p p e p → e r0 (p) p+p- Missing Mass2 = ( g*- p+ - p- )2 e p → e w (p) p+p- p0 e p → e f (p) K+K- DE = (M2X-M2p)/2Mp • Monte Carlo simulation of non-exclusive background • sL/ sT separation from decomposition of decay angular distributions

  12. Cross sections for r0, f W [GeV] GPD calculation: 2 gluon exchange mechanism quark exchange mechanism • Guichon, Guidal & Vanderhaeghen(2001) –

  13. Exclusivity for e p → e p+(n) Missing Mass2 = ( g* + p - p+ )2 Use of p- yield to subtract the non exclusive background p+ enhancement Exclusive neutron peak: position and width in agreement with MC based on GPD model - Mankiewicz, Piller & Radyushkin (1999) -

  14. Longitudinal target spin asymmetry e p → e p+ n AUL=-0.18± 0.05 ± 0.02 Polarized cross section: sS= [ST sL +SLsLT] AULsinF ‹xB›=0.15 ‹Q2›=2.2 GeV2 ‹-t›=0.46 GeV2 sLT suppressed by 1/Q but SL > ST=|S|sin qg Hermes kinematics: ST/|S|~0.17 AUL= ST/SAUT 2005: end of run with a tranverse polarized target

  15. Exclusive reaction at Hermes: ongoing analysis Run with transverse polarized target (2002-2005 ) - Goeke, Polyakov & Vanderhaeghen (2001) - - Frankfurt, Pobylitsa, Polyakov, Strikman (1999) - g*L p → p+ n TARGET SPIN ASYMMETRY Target spin asymmetry: most promising observable which allow to access J

  16. Summary and outlook • GPDs can be probed by hard exclusive meson production • Measurements at HERMES: DVCS: clear signal of the process in agreement with GPD Vector mesons: longitudinal cross sections measurement in agreement with GPD calculation Pion: large longitudinal target spin asymmetry • 2005: end of transverse target runs →transverse spin asymmetry accessible both for p+ and r0 • 2005: run with the recoil detector

More Related