ΔVCS Beam Spin Asymmetries with CLAS:Update

Transcription

1 ΔVCS Beam Spin Asymmetries with CLAS:Update Brahim Moreno e p e' Δ γ CLAS collaboration meeting

2 ΔVCS Beam Spin Asymmetries with CLAS: Update ΔVCS analyses reminder Double p 0 background study p p 0 background study ΔVCS: results Conclusions and outlook

3 In the Bjorken limit: ΔVCS and GPDs ep e Δ γ -N-Δ transitions at a partonic level -large Nc approimation: ΔVCS can be described hard part in terms of quark GPDs soft part L.L. Frankfurt M. V. Polyakov M. Strikman M.Vanderhaeghen Phys. Rev. Lett. 84: (000) Handbag diagram P.A.M Guichon L.Mossé M.Vanderhaeghen Phys. Rev. D (003) N-Δ GPDs : same information as nucleon GPDs H H H C C M E 1 C C3 C 4 N-Δ GPDs

4 ΔVCS BH BH σ( ep eδ γ) Δ nπ Interference between ΔVCS and Bethe-Heitler Δ o pπ P.A.M Guichon L.Mossé M.Vanderhaeghen Phys. Rev. D (003) ΔVCS (π ): ep eδ γ en π γ ΔVCS (π o ): ep eδ γ ep π 0 γ Eperimental data is integrated Calculated Beam Spin Asymmetry over this whole range

5 1 ΔVCS (π ): event selection ep eδ γ en π γ ALL final state particles detected data before cuts data after cuts Projected missing momentum 1 Missing energy ep en π γx 3 4 Missing mass 3 4

6 ALL final state particles detected Missing energy ΔVCS (π o ): event selection Projected missing momentum 1 ep ep eδ γ epγγγx ep π γ Missing mass data before cuts data after cuts Sideband method used to subtract combinatorial background under π o mass peak

7 ΔVCS : background subtraction ΔVCS (π 0 ) ΔVCS (π ) FINAL State (S) BACKGROUND (B) ep epπ o γ ep enπ γ ep ep epπ enπ o π π o o epπ enπ o γ (γ ) γ (γ ) background calculated and then subtracted using MC simulations and eclusive double pion production DATA same procedure used for both ΔVCS channel

8 ΔVCS raw asymmetries ep epπ o γx ep en π γx A A F (degrees) F (degrees) In the Δ region error bars: statistical only

9 Double p 0 production analysis ep epπ 0 π 0 ΔVCS (π 0 ) background

10 p 0 pair reconstruction Events with eactly 1 electron 1 proton and 4 photons Eclusive events E Photons ordered in decreasing energy γ1 γ γ 3 γ 4 E E E combination 1 combination first photon pair: p 0 1 γ 1 γ γ 1 γ 3 second photon pair: p 0 γ 3 γ 4 γ γ 4 combination 3 γ 1 γ 4 γ γ 3 i i IM γγ ( 1) IM γγ ( ) Invariant mass of the first photon pair for combination i Invariant mass of the second photon pair for combination i

11 IM i i 1 = IM = mπ Ideal combination: γγ ( ) γγ ( ) 0 IM γγ ( ) D i = ( i ()) i m 0 IM γγ 1 m 0 IM γγ ( ) π ( ) π IM i γγ ( ) 0 m π D i Identified best combination = the one closest to the ideal case (smallest D i ) 0 m π IM i γγ ( 1) IM γγ () 1 χ = ( 1) ( i m IM ) γγ π ( 1) ( ) ( i m ) 0 IM γγ 0 σ IM i γγ π σ IM i γγ ( ) c minimized to select the best combination Takes into account resolution

12 Eample Combination 3 IM i = 3 3 γγ () 1 ( GeV ) IM i = ( ) γγ ( GeV ) Identified p 0 p 0 candidates IM γγ ( 1) ( GeV ) ( ) ( GeV ) IM γγ

13 Double p 0 event selection 1 Missing mass Missing energy Eclusive events ep data after cuts mc data after cuts (phase space) M ep4 γ X 1 Eep 4γ X 3 Missing 4 mass Transverse missing momentum M e 4γ X P T ep 4γ X M 0 ep π M ep π 0 1 X X 3 4

14 Validity of phase space MC model ep data after cuts mc data after cuts Good agreement between Data/MC Validate the MC model for double p 0 production

15 p p 0 production analysis ep 0 ep enπ π enρ enπ 0 π ΔVCS (π ) background

16 Missing mass p p 0 event selection 1 Missing energy Eclusive events ep data after cuts mc data after cuts (phase space r ) M en π γγ X 1 Een π γγx 3 4 Missing Transverse mass missing momentum M e γγ X P T en π γγ X M en γγ X enπ X π M 3 4

17 Validity of p p 0 MC model ep data after cuts mc data after cuts MC = phase space r Good agreement between Data/MC Validate the MC model for p p 0 production

18 ΔVCS results ΔVCS (π 0 ): ΔVCS (π ): ep epπ o γ ep enπ γ in the Δ region

19 Contamination R= percentage of ΔVCS data samples estimated to be background ΔVCS (π 0 ) contamination 0 π 0 ep epπ ep ΔVCS (π ) contamination 0 ep enπ π enρ enπ 0 π 30% on average 45% on average F (degrees) F (degrees)

20 ΔVCS Beam Spin Asymmetries ΔVCS (π 0 ) ΔVCS (π ) F (degrees) F (degrees) error bars: statistical only

21 Conclusions Pioneering investigation for ΔVCS asymmetries First asymmetry results obtained for both ΔVCS channel BSA show a clear F dependency which may indicate that interference between ΔVCS and Bethe-Heitler was observed Limited statitics : more data needed for an etensive ΔVCS study Outlook Systematic uncertainties currently being evaluated PhD epected to be defended in June 009 Analysis note to be written

22 Complementary slides

23 DVCS asymmetry and GPDs e e' Polarized beam unpolarized target: Unpolarized beam longitudinaly polarized target: γ p' leptonic plane hadronic plane H H ~ E H H ~ Unpolarized beam transversaly polarized target: Beam charge asymmetry: H E H H ~ E

24 N-Δ GPDs Sum rules : ) ( ) ( * 1 1 Δ = Δ C E M C E M G dh ξ ) ( ) ( Δ = Δ A C dc ξ ) ( ) ( Δ = Δ A C dc ξ ) ( ) ( Δ = Δ A C dc ξ ) ( ) ( Δ = Δ A C dc ξ C E M H H H Relevant N-Δ GPDs for ΔVCS process: In the large Nc limit ( ) ( ) ( ) [ ] t E t E H t H d u M M 3 ξ ξ ξ = ( ) ( ) ( ) ( ) [ ] t H t H t C t C d u ~ ~ ξ ξ ξ ξ = ( ) ( ) ( ) ( ) [ ] t E t E t C t C d u ~ ~ 4 3 ξ ξ ξ ξ =

25 ΔVCS (π o ): π o reconstruction ΔVCS (π 0 ): ep o epπ γ ep3γ Which photons are likely to come from π o decay? Phase space generator 8% events under 0 GeV DVCS photon = most energetic one E γ DVCS E π γ 0 ( GeV)

26 Cuts optimization method Study statistical effect of evolving cuts on each event selection variables: -ΔVCS MC -Double π o MC Establish the optimal values Cut position R = N N after cut before cut Varying cut position R( cut value)

27 Eample for events below the cut for events above the cut

28 ep ep eδ γ epπ γ 0 epω epπ γ Omega contamination 0 Invariant mass π 0 γ

29 6σ ΔVCS (π o ): combinatorial background IM γγ 3σ 3σ Sideband method SB Background Signal m Δ A B

30 ΔVCS : background subtraction ΔVCS (π 0 ) ΔVCS (π ) FINAL State (S) BACKGROUND (B) ep epπ o γ ep enπ γ ep ep epπ enπ o π π o o epπ enπ o γ (γ ) γ (γ ) SB S B N = N N enπ γx enπ γ 1γ enπ π o 1γ N 0 π π 0 ep enπ π 1γ Acc 0 π π γ N 0 π π 0 ep enπ π γ Acc 0 π π single photon detected all photons detected N 1γ γ π π π γ π γ Acc 0 0 en = N en X Nen π π γ Acc 0 π π S SB B same procedure used for both ΔVCS channel

31 Eample Combination 3 IM i = 3 3 γγ () 1 ( GeV ) IM i = ( ) γγ ( GeV ) Selection cuts applied Identified p 0 p 0 candidates IM γγ ( 1) ( GeV ) ( ) ( GeV ) IM γγ