Deeply Virtual Compton Scattering on the neutron

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1 Deeply Virtual Compton Scattering on the neutron Malek MAZOUZ For JLab Hall A & DVCS collaborations Physics case n-dvcs experimental setup Analysis method Results and conclusions Exclusive Reactions at High Momentum Transfer May 1 st 007

2 Deeply Virtual Compton Scattering GPDs give an access to quark angular momentum (Ji s sum rule) q q J q = ΔΣ q + Lq = xdx H ( x, ξ,0) + E ( x, ξ,0) 1 less constrained GPD No link to DIS DVCS is the simplest hard exclusive process involving GPDs k k p p GPDs q Factorization theorem in the Bjorken regime Q = q t = ( p p') = ( k k') = Δ << Q >> M Non perturbative description by GPDs

3 DVCS and Bethe-Heitler The total cross-section accesses the real part of DVCS and therefore an integral of GPDs over x The polarized cross-section difference accesses the Imaginary part of DVCS and therefore GPDs at x=±ξ r s 5 5 BH DVCS DVCS DVCS d σ d σ = I m( T. T ) + T T r s Purely real and fully calculable 5 5 B Small at JLab energies (twist-3 term) If handbag dominance r s d σ d σ =Γ( x, Q, t, ϕ) ImC sinϕ ( I ) I t C ( F) = F1() t H+ ξ ( F1() t + F() t ) H% F () t E 4M

4 Neutron Target Model: (Goeke, Polyakov and Vanderhaeghen) Target H %H E neutron Q x B = GeV = 0.3 t = 0.3 GeV x t I = () H+ () () H% F () t E ( C ) F t ( F t + F t ) m I B 1 1 xb 4M t n F t () 1 n () n n F t ( 1 ) F () t + F () t x /( x ) B B n ( t/4 M ) F ( t) Im B ( C ) ( ) I ( C ) I = x t F() t H + F() t + F () t H% F () t E 1 1 xb 4M I m =

5 n-dvcs experiment An exploratory experiment was performed at JLab Hall A on hydrogen target and deuterium target with high luminosity ( cm - s -1 ) and exclusivity. Goal : Measure the n-dvcs polarized cross-section difference which is mostly sensitive to GPD E (less constrained!) E (n-dvcs) followed directly E (p-dvcs) which shows strong indications of handbag dominance at Q about GeV. (C. Muñoz-Camacho et al., PRL 97 (006) 600.) x Bj =0.364 Hydrogen Deuterium s (GeV²) Q² (GeV²) P e (Gev/c) Θ e (deg) -Θ γ* (deg) Ldt (fb -1 )

6 Experimental apparatus Beam energy = 5.75 GeV Beam polarization = 75% Beam current = ~ 4 μa Luminosity = cm -.s -1 nucleon -1 Scattered Electron Left HRS Polarized Electron Beam LH / LD target γ DVCS events are identified with M X An experimental challenge -Specific Scattering Chamber - Čerenkov based Electromagnetic Calorimeter - Customized Electronics & Data Acquisition Electromagnetic Calorimeter

7 Analysis method ed eγ X (target mass = M N ) M x cut = (M N +M π ) p-dvcs and n-dvcs d-dvcs M N +t/ M N N + mesons (Resonnant or not) Contamination by ed 0 eπ X eγ X accidentals

8 Helicity signal and exclusivity After : -Normalizing H and D data to the same luminosity -Adding Fermi momentum to H data π d-dvcs π + + n-dvcs ϕ π S ( ) ( ) h = N N d N N dϕ 0 principle sources of systematic errors : -The contamination of π 0 electroproduction on the neutron (and deuteron). - The uncertainty on the relative calibration between H and D data

9 1 d r dσ Δ Extraction of observables s dσ = ϕ ϕ Δ ϕ ϕγγ Q dxbd d ed γγ dq dxbd d ed Exp Δ N ( i ) = N N + e i i e I exp I exp ( Cn ) sinϕ d xb e Δ m( Cd ) ( x, ϕ,, ϕ) m (, ϕ,, ϕ) sinϕ Γn B e Δ I + Γ I A. V. Belitsky, D. Muller, A. Kirchner, Nucl. Phys. B69, 33 (00). e with ( X ϕ ) i = bins in M,, t e MC ( I exp ) ( I exp Δ N ( i ) m.sin m ) e = L I Cn Γn ϕ Acc+ I Cd Γ.sin x i d ϕ Acc e x ie Luminosity MC sampling MC sampling MC includes real radiative corrections (external+internal) χ = i e Exp MC ΔN ( ie) ΔN ( ie) σ Exp ( ie ) Im Im ( I exp C ) n ( I exp C ) d

10 d-dvcs extraction results Extraction results PRELIMINARY F. Cano & B. Pire calculation Eur. Phys. J. A19, 43 (004). Deuteron moments compatible with zero at large -t Exploration of small t regions in future experiments is interesting

11 n-dvcs extraction results Extraction results PRELIMINARY VGG Code : M. Vanderhaeghen, P. Guichon and M. Guidal GPD model : LO/Regge/D-term=0 Goeke et al., Prog. Part. Nucl. Phys 47 (001), 401. Neutron contribution is small and compatible with zero Results can constrain GPD models (and therefore GPD E)

12 n-dvcs experiment results Systematic errors of models are not shown n-dvcs is sensitive to Jd p-dvcs is sensitive to Ju VGG Code GPD model : LO/Regge/D-term=0 Goeke et al., Prog. Part. Nucl. Phys 47 (001), 401. Complementarity between neutron and transversally polarized proton measurements

13 Summary and conclusion Our experiment is exploratory and is dedicated to n-dvcs. n-dvcs and d-dvcs contributions are obtained after a subtraction of Hydrogen data from Deuterium data (no recoil detectors needed). n-dvcs and d-dvcs polarized cross-sections difference are compatible with zero. Neutron results can constrain GPD models (GPD E parametrization) Neutron has a different flavor sensitivity to GPD E than transversally polarized proton. Neutron experiments are mandatory complements to proton ones. Re(DVCS) from unpolarized cross-section should be measured.

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15 Analysis method ed eγ X eh eγ X M x cut = (M N +M π ) M x cut = (M N +M π ) accidentals accidentals D(, e e' γ ) X = p(, e e' γ) p+ n(, e e' γ) n+ d(, e e' γ) d+k p-dvcs events n-dvcs events d-dvcs events Mesons production

16 Double coincidence analysis Nb of counts Nb of Counts Hydrogen cinematique data Deuterium cinematique data 4 Deuterium- cin.4 - cin. Hydrogen simulation M x cut (GeV M x ) M X (GeV )

17 Helicity signal and exclusivity π π + + S ( ) ( ) h = N N dϕ N N dϕ 0 π After : -Normalizing H and D data to the same luminosity -Adding Fermi momentum to H data d-dvcs n-dvcs principle sources of systematic errors : -The contamination of π 0 electroproduction on the neutron (and deuteron). - The uncertainty on the relative calibration between H and D data

18 π 0 contamination subtraction H data M x cut =(M p +M π ) π 0 to subtract Subtraction of π 0 contamination (1γ in the calorimeter) is obtained from a phase space simulation which weight is adjusted to the experimental π 0 cross section (γ in the calorimeter).

19 π 0 contamination subtraction Unfortunately, the high trigger threshold during Deuterium runs did not allow to record all exclusive π 0 events (M X <1.15 GeV ) H data But : according to the procedure of π 0 contamination subtraction, we must have : Actually, we find : 0 0 σ( edσ ( ene π Xe) π n) 0. < = < 0.5 ± 0.06 ± sys 0 σ( epσ ( epe π Xe ) π p) by comparing two samples of with M X <1.15 GeV high energy π 0 in each case

20 Exclusive π 0 asymmetry ed ep( n) π + ed en( p) π + ed edπ ep 0 epπ D data H data ( 0 0 n π + d π ) ( 0 S ) h ep epπ Sh e en e ed 0.5 < < 1 Well known from H data

21 sin(φ) and sin(φ) moments Results are coherent with the fit of a single sin(φ) contribution

22 Test of the handbag dominance : E p-dvcs experiment results C. Muňoz-Camacho et al., to appear in PRL (007) Twist- contribution dominates the total cross-section and the cross-section difference. No Q dependence of twist- and twist-3 terms Strong indications for handbag dominance

23 VGG parametrisation of GPDs Non-factorized t dependence 1 β 1 q q q x H ( x, ξ,) t = dβ dα δ ( x β αξ) F ( βα,,) t + θ ( ξ x) D ξ 1 1+ β Vanderhaeghen, Guichon, Guidal, Goeke, Polyakov, Radyushkin, Weiss Double distribution : α ' = GeV for quarks Profile function : F h q 1 ( βα,, t) = h, β α ' t β ( βα, ) = ( b ) 1 ( b ) ( β α ) q ( ) b+ 1 1 ( 1 β ) ( β ) α Γ + Γ + b+ 1 b D-term Parton distribution ( ) for G PD E, the spin-flip parton densities is used : e β Modelled using J u and J d as free parameters q

24 n-dvcs polarized cross-section difference

25 d-dvcs polarized cross-section difference Experimental results + Prediction from F. Cano and B. Pire. Eur. Phys. J. A19, 43 (004)

26 π 0 electroproduction on the neutron Pierre Guichon, private communication (006) Amplitude of pion electroproduction : α 0 ( ) T( N, α) = δ α,3 T + τnt + iε τ T + β 3αβ α is the pion isospin nucleon isospin matrix π 0 electroproduction amplitude (α=3) is given by : ( p,3) ( n,3) 1 u Δd Δu Δd = + Δ + T T T T + 0 = + T T T ( p ) T ( n ) T ( p,3),3 +,3 3+ 3Δd / Δu Δd / Δu Polarized parton distributions in the proton

27 Triple coincidence analysis Proton Array and Tagger (hardware) work properly during the experiment, but : Identification of n-dvcs events with the recoil detectors is impossible because of the high background rate. Many Proton Array blocks contain signals on time for each event. Accidental subtraction is made for p-dvcs events and gives stable beam spin asymmetry results. The same subtraction method gives incoherent results for neutrons. Other major difficulties of this analysis: proton-neutron conversion in the tagger shielding. Not enough statistics to subtract this contamination correctly The triple coincidence statistics of n-dvcs is at least a factor 0 lower than the available statistics in the double coincidence analysis.

28 Triple coincidence analysis One can predict for each (e,γ) event the Proton Array block where the missing nucleon is supposed to be (assuming DVCS event).

29 Triple coincidence analysis After accidentals subtraction -proton-neutron conversion in the tagger shielding - accidentals subtraction problem for neutrons relative asymmetry (%) Relative asymmetry (%) neutrons selection 0 1 predicted block 10 9 predicted blocks 0 5 predicted blocks p-dvcs events (from LD target) asymmetry is stable relative asymmetry (%) Relative asymmetry (%) PA energy cut (MeV) protons selection PA energy cut (MeV) 0 1 predicted block 15 9 predicted blocks 5 predicted blocks PA energy cut (MeV) PA energy cut (MeV)

30 Calorimeter energy calibration We have independent methods to check and correct the calorimeter calibration 1 st method : missing mass of D(e,e π - )X reaction M p By selecting n(e,e π-)p events, one can predict the energy deposit in the calorimeter using only the cluster position. χ a minimisation between the measured and the predicted energy gives a better calibration.

31 Calorimeter energy calibration nd method : Invariant mass of detected photons in the calorimeter (π 0 ) Nb Nb of counts Counts π 0 mass σ = 8.5 MeV π 0 invariant mass position check the quality of the previous calibration for each calorimeter region Corrections of the previous calibration are possible invariant mass (GeV) Invariant mass (GeV) Differences between the results of the methods introduce a systematic error of 1% on the calorimeter calibration.