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Hard exclusive processes at EIC

Hard exclusive processes at EIC. - experimental aspects -. Andrzej Sandacz. Sołtan Institute for Nuclear Studies, Warsaw. Introduction. DVCS – from Central Detector only. DVCS – including fast ‘recoil’ proton. Exclusive Meson production. Tagging spectator protons for deuteron beam.

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Hard exclusive processes at EIC

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  1. Hard exclusive processes at EIC - experimental aspects - Andrzej Sandacz Sołtan Institute for Nuclear Studies, Warsaw • Introduction • DVCS – from Central Detector only • DVCS – including fast ‘recoil’ proton • ExclusiveMeson production • Tagging spectator protons for deuteron beam • Conclusions workshop on ‘Hard Exclusive Process at JLab 12 GeV and a Future EIC’ University of Maryland College Park, October 29-30, 2006

  2. Deeply Virtual Compton Scattering ep→epg The same final state in DVCS and Bethe-Heitler interferenceI interference + structure of azimuthal distributions a powerful tool to disantangle leading- and higher-twist effects and extract DVCS amplitudes including their phases up to twist-3 BMK (2002) P1 (Φ), P2 (Φ) BH propagators harmonics with twist-2 DVCS amplitudes (related to GPDs) c0DVCS, c1I, s1Iand c0I(the last one Q suppresed)

  3. Towards extraction of full set of GPDs Aim for DVCS - go beyond measurements of unpolarized cross sections, access interference term and exploit azimuthal angle dependence example: method proposed by Belitsky, Mueller, Kirchner (2002) • measuree p → e p γ cross sections both for e+ and e- for unpolarized, longitudinally and transversely polarized protons • σ+(φ) – σ-(φ) IΛ(φ) Λ = {unp, LP, TnP, TsP} σ+(φ) + σ-(φ) – 2 σBH(φ) 2 σDVCS,Λ(φ) • from φ-dependence of IΛ’s extract 8 leading-twist harmonics cI1,Λ sI1,Λ • from these determine all 4 DVCS amplitudes (including their phases) which depend on GPDs • another 8 leading-twist harmonics cDVCS0,Λ and cI0,Λcross check in experiment asymmetries simpler than cross sections, but extraction of DVCS amplitudes more involved

  4. Available measurements of asymmetries for DVCS lepton charge or single spin asymetries at moderate and large xB HERMES and JLAB results • beam-charge asymmetry AC(φ) • beam-spin asymmetry ALU(φ) • longitudinal target-spin asymmetry AUL(φ) • transverse target-spin asymmetry AUT(φ,φs) F1and F2 are Dirac and Pauli proton form factors

  5. Unpolarized cross sections for DVCS cross section σDVCS averaged over φ for unpolarised protons H1 and ZEUS Hsea, Hg at small xB ( < 0.01) DIS 2006

  6. 2° <θe’< 178° 2° < θγ < 178° Ee’ > Emin GeVEγ > 0.5 GeV Emin = 2 GeV (HE) or 1 GeV (LE) 1 < Q2 < 50 GeV2 1 < Q2 < 50 GeV2 10 < W < 90 GeV 0.05 < |t| < 1.0 GeV2 0.05 < |t| < 1.0 GeV2 A simulation of DVCS at eRHIC HE setup: e+/-(10 GeV) + p (250 GeV) L = 4.4 · 1032 cm-2s-1 38 pb-1/day LE setup: e+/-( 5 GeV) + p ( 50 GeV) L = 1.5 · 1032 cm-2s-1 13 pb-1/day diam. of the pipe - 20 cm, space for Central Detector: ≈ +/- 280 cm from IP acceptance of Central Detector(improved ZDR)2° <θlab< 178° acceptance simulated by kinematical cuts event generator: FFS (1998) parameterization with R=0.5, η = 0.4 and b = 6.2 GeV-2 DVCS + BH + INT cross section kinematical smearing: parameterization of resolutions of H1 (SPACAL, LArCal) + ZEUS (θγ, φγ) + expected for LHC (θe’, φe’) LEsetup HEsetup due to acceptance and to ‘reasonably’ balance DVCS vs. BH following kinematical range chosen 2.5 < W < 28 GeV

  7. Distributions of events within acceptance of Central Detector HEsetup DVCS BH Number of event [arbitrary units] log10(xBj) Q2[GeV2] W [GeV] LEsetup W [GeV] Q2[GeV2] log10(xBj) Complementarity of HE and LE setups for covered W and xBj ranges good coverage for3 < W < 90 GeVand 1.5 · 10-4 < xBj < 0.2

  8. Q2 > 1 GeV2 Q2 > 1 GeV2 2.5 < W < 28 GeV 10 < W < 95 GeV 0.05 < |t| < 1.0 GeV2 0.05 < |t| < 1.0 GeV2 Precision of DVCS unpolarized cross sections at eRHIC (1) eRHIC HEsetup eRHICLEsetup σ(ep→epγ) = 173 pb σ(ep→epγ) = 292 pb Assume 2 weeks for each setup Lint = 180 pb-1 Lint = 530 pb-1 Reconstructed: ≈ 133 000 events ≈ 28 000 events Events divided into 6x6 bins of Q2 and W for each setup Lint = 530 pb-1 <W> = 37 GeV Δσ /σ For one out of 6 W intervals: 30 < W < 45 GeV Q2[GeV2]

  9. eRHIC HEsetup σ(γ*p →γ p) [nb] Lint = 530 pb-1 (2 weeks) <W> = 37 GeV Q2[GeV2] Precision of DVCS unpolarized cross sections at eRHIC (2) For one out of 6 W intervals (30 < W < 45 GeV) • eRHIC measurements of cross section will provide significant constraints

  10. Precision of DVCS unpolarized cross sections at eRHIC (3) For one out of 6 Q2 intervals (8 < Q2 < 15 GeV2) σ(γ*p →γ p) [nb] <Q2> = 10.4 GeV2 W[GeV] • EIC measurements of cross section will provide significant constraints also significantly extend the range towards small W

  11. Dependence on azimuthal angle φ for (DVCS+BH+INT) Determination of smearing and acceptance as a function of φ crucial for the Fourier analysis, asymmetry also for φ-integrated cross sections RMS = 15º HE setup Nb of events [arb. units] Acceptance φgen[º] (φrec-φgen) [º] An example: Lepton charge asymmetry precision at eRHIC Lint = 530 pb-1 divided in half betweene+ande- cross section in 6x6 bins of Q2 and W

  12. model ofBelitsky, Mueller, Kirchner(2002) forGPDs at small xB parameters of sea-quark sector fixed using H1 DVCS data (PL B517 (2001)) except magnetic momentκsea(-3 < κsea< 2),whichenters Ji’s sum rule for Jq BMK use ‘improved’ charge asymmetries CoAunpc(1) and CoAunps(1) W= 75 GeV , -t = 0.1 GeV2 Q2 =4.5 GeV2 , -t = 0.1 GeV2 Q2 [GeV2] W[GeV] Lepton charge asymmetry precision at eRHIC κsea= 2 κsea= -3 κsea= -3 κsea= 2 • measurements of asymmetries at EIC sensitive tool to validate models of GPDs

  13. Detection of scattered fast protons (‘recoils’) • Aim: clean subsample of exclusive events => control of effects of DD in main sample Since scattering angles of fast protons are small they stay within the beampipe and follow trajectories determined by magnetic fileds of accelarator Note different θrec scales for HE and LE setups HE LE Nb of events [arb. units] θr[rad] θr[rad] HE LE pr[GeV] pr[GeV]

  14. = A method for detection of recoil protons Beam transport matrix (or horizontally) 10 cm at the detector at the IP Elements of TM depend on distance L from IP and on δ= (pr –pb)/pb σx ≈ σy ≈ 30 μm Requirements • Distance from the nominal beam orbit > 12 σ beam envelope • High sensitivity to the angles at the IP • No strong dependence of TM elements on δ

  15. Beams characteristics and transport Considered option: linac-ring for 10 GeV e + 250 GeV p transport program written by Christoph Montag (CAD-BNL) protons ε*= 9.5 nm β*x/y = 0.26 m σ0x/y=50 μmσ0θx/y= 191 μrad electrons ε*= 2.5 nm β*x/y = 1 m σ0x/y=50 μmσ0θx/y= 50 μrad 12 σ beam envelope RP 1 @ 23.3 m RP 2 @ 57.4 m Distance from beam orbit [m] L[m]

  16. Transverse coordinates of recoil protons at positions of RP’s L = 23.3 m L = 57.4 m yD [m] all yD [m] in acceptance of RP xD [m] xD [m]

  17. ‘reasonable’ acceptance |t| > 0.35 GeV2 for RP 1 |t| > 0.15 GeV2 for RP 2 Full acceptance including Roman Pots RP 1 @ L = 23.3 m RP 2 @ L = 57.4 m generated accepted (CD + RP) Nb of events [arb. units] -t [GeV2] -t [GeV2] Acceptance average acceptance for 0.05 < |t| < 1.0 GeV2 12% for RP 1 25% for RP 2 -t [GeV2] -t [GeV2] For RP 2 range of t with reasonable acceptance wider ,but …

  18. θ*x ≈xD / Lxeff θ*y ≈yD / Lyeff Determination of recoil angles θand φat the IP TM elements relevant for determination of θ*xandθ*y Leff [m] a11 or a33 pr [GeV] pr [GeV] • Effect of transverse smearing of IP ( ≈ 70 μm) small at RP’s because of small a11and a33 θ*and φ of recoil at IP • With RP1significantly higher sensitivity to angle at IP because of larger Leff

  19. Resolution for reconstructed recoil and tagging of exclusive events Full simulation incl. smearing of CD and RP, size of IP and angular beam divergence RP 2 RP 1 RP 1 RMS RMS 21 μrad 0.046 No measurement ofpr Nb of events [arb. units] trrec ≈ - (pbeam·θrrec)2 RMS 124 μrad (θrrec-θrgen) [rad] (θrrec-θrgen) [rad] (trrec – tgen) [GeV2] RP 1 RMS RMS RMS 5.7 º 2.8 º 6.4 º Nb of events [arb. units] (φeγrec-φeγgen) [º] (φrrec-φrgen) [º] (φeγrec-φrrec) [º]

  20. Conclusions for resolution and tagging recoil protons • Detection of fast recoil protons possible at moderate |t| (above ≈ 0.3 GeV2) • Good precision of reconstructed angles at IP for recoils • Accuracy of t derived from recoil limited by beam angular divergence and unmeasured recoil momentum • A possible method to tag exclusive process by correlation of azimuthal angles • Extension of |t| range (down to ≈ 0.12 GeV2) with detected recoil possible but with poor precision of recoil angles

  21. Exclusive production of mesons at eRHIC Results of previous simultations of ρ0 andJ/ψexclusive production at eRHIC shown at ‘Current and Future Directions at RHIC’ - 2002 ρ0production at large Q2 Main ingredients of those simulations Lint = 330 pb-1 e(10 GeV) + p(250 GeV) Lint = 330 pb-1 Detector angular acceptance between ZDR (± 1m) and ‘updated ZDR’ (± 3m) Ranges of W and xbj for hard production similar as shown for DVCS Nb of accepted events ≈ 650 000

  22. Exclusive production of mesons at eRHIC (2) J/ψphoto-production J/ψproduction at large Q2 Nb of accepted events ≈ 5200 Nb of accepted events ≈ 67 000

  23. Tagging spectator protons from deteron beam Assumed settings of magnets for 250 GeV deuteron beam in deuteron rest frame each component with Gaussian distribution σ = 35 MeV spectator protons traced down to RP1 or RP2 Results below – for RP1 all in acceptance of RP in acceptance of RP yD [m] Nb of events Nb of events efficiency ≈ 0.96 yD [m] xD [m] xD [m] • Tagging ofproton spectators feasible, with high efficiency

  24. Summary for DVCS at eRHIC • Wide kinematical range, overlap with HERA and COMPASS 1.5 ·10-4 < xB < 0.2- sensitivity to quarks (mostly u+ubar) and gluons 1 < Q2 < 50 GeV2 - sensitivity to QCD evolution • DVCS cross sections - significant improvement of precision wrt HERA • Intereference with BH - pioneering measurements for a collider powerfull tool to study DVCS amplitudes full exploratory potential, if e+ and e- available as well as longitudinaly and transversely polarized protons • Feasibility of using RP detectors at range of moderate to large t for selection of exclusive events

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