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Generalized Parton Distributions @

Generalized Parton Distributions @. « Expression of Interest » SPSC-EOI-005  a complete proposal for autumn 2008. The main objectives for GPD at COMPASS The detectors and target to be implemented Roadmap for the realisation of this experimental program. GPD 2008 workshop,

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Generalized Parton Distributions @

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  1. Generalized Parton Distributions @ « Expression of Interest » SPSC-EOI-005  a complete proposal for autumn 2008 • The main objectives for GPD at COMPASS • The detectors and target to be implemented • Roadmap for the realisation of this experimental program GPD 2008 workshop, Trento, 13 June 2008 Nicole d’Hose, Saclay, CEA/DAPNIA On behalf of the COMPASS collaboration

  2. γ* γ Q2 hard x +ξ x -ξ soft GPDs p p’ t =Δ2 InDVCSandmeson production we measure Compton Form Factor For example at LO in S: DGLAP t, ξ~xBj/2 fixed q(x) DGLAP DGLAP ERBL

  3. At COMPASS with + and -  accessboth ImHand ReH μ μ + p p DVCS BH calculable  μ’ * θ μ  Known expression  p Pμ eμ eμ Pμ Twist-2 M11 >> Twist-3 M01 Twist-2 gluon M-11 Belitsky,Müller,Kirchner with DVCS + BH with polarized and charged leptons and unpolarized target dσ(μpμp) = dσBH + dσDVCSunpol + PμdσDVCSpol + eμ aBHRe ADVCS + eμ PμaBHIm ADVCS

  4. At COMPASS with + and -  accessboth ImHand ReH μ μ + p p DVCS BH calculable  μ’ * θ μ  p with DVCS + BH with polarized and charged leptons and unpolarized target dσ(μpμp) = dσBH + dσDVCSunpol + PμdσDVCSpol + eμ aBHRe ADVCS + eμ PμaBHIm ADVCS • cos nφsin nφ n=0,1,2,3 n=1,2

  5. At COMPASS with + and -  accessboth ImHand ReH μ μ + p p DVCS BH calculable  μ’ * θ μ  p with DVCS + BH with polarized and charged leptons and unpolarized target Analysis through dependence in  and Q2 Absolute cross section measurements

  6. Importance to measure both Im H and Re H • a dispersion relation • After a few GPD models for DVCS • (VGG (1998), Freund et al. (2003), Guzeyet al. (2006), …) • 2007-2008 many theoretical papers promising • Fitting procedure (Mueller) or GPD quintessence function (Polyakov): • based on dual parametrization • partial wave expansion with respect to the angular momentum • partial wave amplitude of mesonic exchanged in the t-channel •  Regge phenomenology is a guide line towards realistic GPD ansatze

  7. 3-dim picture of the partonic nucleon structure density of quark with longitudinal momentum fraction x and transerse distance b from the center of momentum z t=-2 x P b y x boost  independent of models • measurement of the transverse size via d/dt for DVCS or meson production IM(H), Re(H) with DVCS and Beam Charge and Spin Difference

  8. b or r 3-dim picture of the partonic nucleon structure density of quark with longitudinal momentum fraction x and transerse distance b from the center of momentum t=-2

  9. 2 Parametrizations of GPDs Factorization:H(x,ξ,t) ~ q(x) F(t) or Regge-motivated t-dependence: more realistic with x-t correlation it considers that fast partons in the small valence core and slow partons at larger distance (wider meson cloud) H(x,0,t) ~ q(x) e t <b2> <b2> ~ 8 α’ ln 1/x + 2 B + C (1-x)2 transverse extension of partons transverse size in hadronic collisionshas to be finite due to confinement (α’slope of Regge traject.) for valence quark α’ ~ 1 GeV-2 to reproduce FF  meson Regge traj. for gluon α’ ~ 0.164 GeV-2 (J/ at Q2=0) α’ ~ 0.02 GeV-2 (J/ at Q2=2-80 GeV2) << α’ ~ 0.25 GeV-2 forsoft Pomeron

  10. b or r 0.65 0.02 fm??? measurements of d/dt H1 - PLB659(2008) @ COMPASS at large distance : the gluon density generated by the pion cloud increase of the N transverse size for xBj < mπ/mp=0.14 (chiral dynamics prediction)

  11. Q2 7 6 5 4 3 2 0.05 0.1 0.2 xBj μ’  * Beam Charge and Spin Asymmetry at E = 100 GeV COMPASS prediction μ p  BC&SA 6 month data taking in 2010 250cm H2 target 25 %global efficiency

  12. μ’  * Beam Charge and Spin Asymmetry at E = 100 GeV COMPASS prediction μ p  BC&SA VGG: Double-Distribution in x, model 1:H(x,ξ,t) ~ q(x) F(t) model 2 and 2*: Regge-motivated t-dependence <b2> = α’ln 1/x H(x,0,t)=q(x)et <b2>= q(x)/xα’t α’val=0.8 GeV-2 α’val=1.1 GeV-2 Guzey and Teckentrup: Dual parametrization model 3: Regge-motivated t-dependence with α’val=1.1(1-x) GeV-2 α’sea=0.9 GeV-2 α’gluon=0.5 GeV-2

  13. q q p p t 2nd ultimate goal with GPD Contribution to the nucleon spin knowledge ½ = ½ ΔΣ+ ΔG + < Lzq > + < Lzg> the GPDs correlation between the 2 pieces of information: -distribution of longitudinal momentum carried by the partons -distribution in the transverse plane the GPDE is related to the angular momentum 2Jq =  x (Hq (x,ξ,0)+Eq (x,ξ,0)) dx  with a transversely polarized target DVCS et MV E

  14. How to get the GPD E ? With a transversely polarized target DVCS: Vector meson (M) production: • transversely polarized target inside the recoil detector  study for a second stage of the experiment

  15. L hard soft meson g* x + ξ x - ξ GPDs p p’ t =Δ2 Hard exclusive meson production at COMPASS Collins et al. (PRD56 1997): -factorization applies only for *L -probably at high Q2 Filter of GPDs: ,, production presently studied at COMPASS Vector Meson (,,…):H and E Pseudo-scalar Meson (,,…) : H and E ~ ~ Different flavor contents: Hρ0= 1/2 (2/3Hu + 1/3Hd + 3/8Hg) Hω= 1/2 (2/3Hu – 1/3Hd + 1/8Hg) H = -1/3Hs - 1/8Hg measurement of d /dt and ratio of meson production

  16. Competition in the world and COMPASS role E=190, 100GeV HERA Ix2 COMPASS at CERN-SPS High energy muon beam 100/190 GeV Unique availability of μ+ and μ- polar(μ+)=-0.80 polar(μ-)=+0.80 change once per day 2.108 μper SPS cycle if intensity  2, Q2 range up to 11 GeV2 after 2010, which improvements on the muon line ? Gluons valence quarks valence quarks and sea quarks and gluons COMPASS JLab 12 GeV, FAIR,… 2010 2015

  17. μ’ p’ μ Additional equipment to the COMPASS setup DVCSμp μ’p’ all COMPASS trackers: SciFi, Si, μΩ, Gem, DC, Straw, MWPC ECal1 + ECal2   10° 2.5m cryogenic target to be designed and built • - 2011: long H2 target • later: transversely polarized + additional calorimeter ECal0 at larger angle 4m long Recoil proton detector to insure exclusivity to be designed and built Nμ=2.108/SPScycle (duration 5.2s, each 16.8s) Possibility for an increase of intensity?

  18. Recoil detector + calorimetry

  19. Requirements for the recoil detector 1) Time of Flight measurement  t and  (sectorization) (ToF) < 300 ps P/P ~ 3 à 15 % t = (p-p’)²= 2m(m-Ep’) t/t ~2 P/P  10 bins in t from tmin to 1 GeV2 t is the Fourier conjugate of the impact parameter r t is the key of the measurement 315  12 ps have been achieved during the 2006 test intrinsic limit due to the thin layer A 2) Hermiticity + huge background + high counting rates Rejection of extra ° at large angle  study of rejection with angular and coplanarity constrains • if not, detection of extra ° at a reasonable cost

  20. Present 1m long Recoil Proton Detector in COMPASS for the hadron program (spectroscopy)

  21. Existing Calorimeters Q2 + 3m x 3m ECAL0 + 4m x 4m ECAL0 xbj Xbj-bins Calorimeter acceptance y > 0.05 E. Burtin

  22. Kinematical ranges for photons in ECALs A.Sandacz Max=60 GeV Max>100 GeV x<0.03 Min=5GeV Min=10GeV for /°separation E threshold = 1GeV = 2GeV

  23. Physical background to DVCS exclusive 0production in COMPASS data Source :Pythia 6.4generated DIS events One scattered muon One photon with E1 > 3 GeV -2.5 < Emiss < 2.5 GeV 0.03<xBj<0.3 and 0.1<y<0.9 No other photon in calorimeters: 1GeV < E>1 < 3 GeV One proton in recoil detector No charged tracks in spectrometer -2.5 2.5 GeV Emiss=(MX2-MP2)/2MP in this case DVCS is dominant • But ideal case • Only acceptance • No pile up • + angular correlation • + coplanarity E. Burtin Q2

  24. 2008: DVCS feasibility study in the COMPASS environment μ p  μ p in the RPD in ECAL1+2 • 2008: Request to the collaboration for switch-over • from pion beam to muon beam for ~5 days • Goal 1: a better knowledge of the + and - beams • Goal 2: a better knowledge of the RPD and its extrapolation to a larger size • hadrons(5107  / 10s (5 MHz)) : muons(25 108  / 5s (50 MHz)) • with the exclusive rho production (~1000 evts in 5 days) Goal 3: a better knowledge of the Calorimetry low photon energy threshold necessary for a good/°separation

  25. and if in 2009 (?) we get 30 days of dedicated beam to perform a « GPD » pilot run

  26. μ’  * “GPD” pilot run in 2009 1 month data taking 40cm H2 target 25 %global efficiency Q2 7 6 5 4 3 2 0.05 0.1 0.2 xBj θ at E = 100 GeV 6 month data taking in2010 250cm H2 target 25 %global efficiency μ p  Beam Charge and Spin Asymmetry 14000 evts 780 evts

  27. Conclusion & prospects • Possible physics ouput • Sensitivity to transverse size of partons inside the nucleon • Sensitivity to the GPD E and total angular momentum • Working on a variety of models toquantify the physics potentialof a GPD exp. at COMPASS • Experimental requirements • Recoil Detection • Good calorimetry and Extension at larger angle • Roadmap • A global COMPASS proposal for the period 2009-2015 including transversity, longitudinal, spectroscopy + GPD + Drell-Yan will be presented to SPSC at the end of the year 2008 • 2008-9: A small RPD and a liquid H2 target are available for the hadron program  tests of DVCS feasibility • from 2011: A complete GPD program at COMPASS with a long RPD + liquid H2 target (2011) + transversely polarized target (later) very favourable timing window before the availability of JLab 12 GeV, EIC, FAIR…

  28. μ pμ p  global efficiency of 25% ? Data taking SPS availability 0.80 Spectro availability 0.90 DAQ livetime 0.93 Inter run 0.99 Spill gate 0.96 Quality checks 0.92 Trigger Veto livetime 0.80 Trigger efficiency 0.80 Reconstruction Incident muon 0.93 Scattered muon 0.90  31% DVCS photon 0.90 would be a guess! Recoil proton 0.90 Control of absolute cross section within 5-10% : - determination of F2 (already done in NMC within 2%) - determination of Bethe-Heitler cross section (in 2009)

  29. Collimators 1 2 3 4 H V H V scrapers T6 primary Be target Compass target Be absorbers Protons 400 GeV Muon section 400m Hadron decay section 600m Naturally Polarized μ+andμ-beams • Solution proposed by Lau Gatignon: • To select Pπ=110GeV andPμ=100GeV • to maximise the muon flux • 2) To keep constant the collimator • settings which define • the π and μ momentum spreads • Pol μ+ = -0.8andPol μ- = +0.8 • Nμ+ ~ 1.6  Nμ- • 2.108μ-per SPS cycle (5.2s, 16.8s) Requirements for DVCS: -same energy -maximum intensity -opposite polarisation to a few % -switch: one per day 2.108 muons/spill 1.3 1013protons/spill

  30. Outer Layer Inner Layer Bi-1 Ai-1 Target Ai Bi+1 i Ai+1  Criteria for a good proton • geometrical configuration • Correlation energy loss and measured • Good timing and good vertex with the muon   online (2001) offline (2001) protons Zproton-Zmuon [cm] • Efficiency ~ 90% • Purity ~ 80% • Control with the present 1m long RPD

  31. Requirements for calorimetry Competing reactions to DVCS DVCS:μp  μp HEπ°P:μp  μpπ° ~    Dissociation of the proton: μp  μN*~ /10  Nπ DIS:μp μpX dominant with 1, 1π°, 2π°,η… Beam pile-up DVCS photon kinematics ECAL1 ECAL0 Q2=1 GeV2  E DVCS and E threshold for ° detection xBj=0.05 minEDVCS=10 GeV E thr. = 2 GeV in ECAL2 xBj=0.1 minEDVCS=5 GeV E thr. = 1 GeV in ECAL1

  32. 2008: DVCS feasibility study in the COMPASS environment exclusive 0production in COMPASS data at 160 GeV -2.5 2.5 GeV Emiss=(MX2-MP2)/2MP μ p  μ p in the RPD in ECAL1+2 2008: Request to the collaboration for switch-over from pion beam to muon beam for ~5 days Goal 1: a better knowledge of the + and - beams beam size (Si),momentum (BMS), beam halo (forward spectro, RPD) Goal 2: a better knowledge of the RPD and its extrapolation to a larger size comparison: hadrons (5107  / 10s (5 MHz)) : muons(25 108  / 5s (50 MHz)) - rates in PMT, multipl., waveforms using Rand., Incl., RPD triggers with the exclusive rho production (~1000 evts in 5 days) with Incl. trigger - efficiency, purity of the RPD - kinematical fit, azimuthal correlation - comparison t from recoil proton detector or from COMPASS spectro t = (p - p’)2 = (q - q’)2

  33. Goal 3: a better knowledge of the Calorimetry • ° detection: • as a function of E° and the E threshold energy • in ECAL2 (x_Bj ~0.05 and EDVCS> 10 GeV) • in ECAL1 (x_Bj ~0.1 and E DVCS> 5 GeV) • thresholds of 1 GeV in ECAL1 and 2 GeV in ECAL2 • should be necessary for a good /°separation • low noise level and pile up rejection (with the SADC) • study as a function of the beam intensit • necessity to measure with random trigger without/with beam • effect of the charged tracks given by the muons • selection of °exclusif with exclusivity constrains

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