Moskov Amaryan Old Dominion University On behalf of the CLAS Collaboration

Transcription

1 High-t Exclusive 0 Photoproduction Moskov Amaryan Old Dominion University On behalf of the CLAS Collaboration Exclusive Meson Production and Short-Range Hadron Structure Workshop Jefferson Lab, Newport News January 23,

2 Outline -Introduction -Dalitz decay and Photon Conversion -CLAS Data analysis -Exclusive photoproduction cross section of neutral pions on hydrogen target in CLAS -Conclusions 2

3 Introduction In general pion photoproduction cross section in terms of amplitudes could be presented as: d d = q < 2 F 1 > 2 k For given isospin configuration there are four amplitudes: F = i F 1 + q (k ) qk F 2 + i kq qk F 3 + i qq q 2 F 4 G.F. Chew, M.L. Goldberger, F.E. Low, and Y. Nambu, Phys.Rev. 106, 1345 (1957) called CGNL amplitudes At low energies these amplitudes could be expanded in terms of linear combinations of associated Legendre polynomials and partial waves. 3

4 At higher energies number of relevant waves increases and it becomes very difficult to perform expansion of amplitudes. In this case one can use crossed-channel Regge expansion. 0,,b,h p p Regge trajectories contribute only to a particular combinations of invariant amplitudes. A i 4

5 The combination of CGNL amplitudes with good quantum numbers for neutral pion production are: F 1 = A 1 2mA 4, F 2 = A 1 + ta 2, F 3 =2mA 1 ta 4, F 4 = A 3. The vector The axial-vector (!, ) trajectories contribute to F 1,F 3. (b, h) trajectories contribute to F 2,F 4. In the leading Reggeon exchange approximation 1 e i F i (s, t) = i (t) ( ) 2 sin s 1 is Regge trajectory. The vector and axial-vector trajectories are: V (t) =0.9(t m 2 )+1, A (t) =0.7(t m 2 b 1 )+1. 5

6 In these terms the differential cross section is represented as: d dt (s, t) = 1 32 apple F3 2 t F 1 2 4m 2 t + F 2 2 t F 4 2 This model was applied by V. Mathieu and A. Szczepaniak to describe neutral pion photoproduction cross section at high energies (to be published). Regge exchange model was used by M. Guidal, J.-M. Laget and M. Vanderhaeghen in PLB 400, 6 (1997) to describe pion photoproduction.! Later Regge model of GLV was modified in J.M. Laget, PLB 695, 199 (2011) and includes pion rescattering as well as u-channel exchange. 6

7 Experiment Photoproduction cross section in the reaction + p! p 0! pe + e was measured in CLAS using data from g12 run period. It is reported in M.Kunkel s Ph.D. Thesis. Electron Beam Energy Electron Beam Current Electron Beam Polarization TABLE 5: Running conditions for g GeV na (production) & 24 na(single-prong) Circular Radiator Material/Density Au / 646µg/cm 2 Radiator Thickness Radius of Photon collimator Photon Beam Energy Range Target Shell Material Target Length/Diameter 6.4 mm GeV Kapton 40 cm/4 cm Target Inside Material `H 2 Target Position Target Polarization -90 cm from CLAS center None Torus Magnetic Current 1 2 B max =1930A 7

8 0 Decay Modes y Modes µ 1 p 0! M Pp (Br = 98.8%) 2 k (a) Pp M 1/q 2 k u (p e, s µ ) 0! e+ e (Br = 1.2%) Dalitz decay v (p+, s+ ) (b) Feynman diagram 0 two photon decay (a). we Feynman From the oftrigger point of view choose 0 diagram of Dalitz decay (b). Dalitz decay mode. The lepton pairs could be produced from the conversion! 98.8% in LH target as well. From MC simulation the final sample 0! e+ e 1.2% contains roughly equal number of events coming from We choose e+ e for cleaner signal and trigger Dalitz decay and pair production in LH target. 0 7 / 43 8

9 CLAS Detector CLAS Detector 9

10 Event Selection -Event are selected with + p! pq + q (X) final state -Identify pe + e (X) final state using CC and EC and TOF -Use kinematic fit to identify missing photon -Use kinematic fit to cut out + p! p + (X) 10

11 Missing energy vs Missing Mass 1 π 0 E beam < 3.6 GeV - π + π ) [GeV] - e + M E (pe π 0 E beam 3.6 GeV - π + π (p) [GeV ] M E ( p! pe + e ) vs. M 2 x( p! px). The horizontal red dashed-dotted line depicts the 75 MeV cut used in this analysis. The vertical red dashed-dotted line depicts boundary of single 11 0 to + production. M x 0

12 Reconstructed peak of 0! e + e 3 10 E beam < 3.6 GeV π 0 Yield: Background: 5000 Range: ± 2.5 σ 2 Mean: ± 1.27e-05 GeV 2 σ: 5.47e-03 ± 1.22e-05 GeV S = 9.87e-01 S+B E beam 3.6 GeV - π + π 8 Yield: π 0 Background: 750 Range: ± 2.5 σ 2 Mean: ± 3.99e-05 GeV 2 σ: 6.49e-03 ± 3.22e-05 GeV S = 9.79e-01 S+B (p) [GeV ] - π + π M x 12

13 New (preliminary) CLAS and World Data FIG. 3: (Color online) π 0 photoproduction cross section, dσ/dt, off the proton at E = MeV (W = MeV) versus t, t is the squared four-momentum transfer between the incident photon and E, while the center-ofmass total energy is indicated by W.Redsolid(bluedashed)linescorrespondtotheSAIDKU14(DU13 [7]) solution, which terminates at W = 2.5 GeV.Black dotted lines give the BG BnGa[8]predictions. It terminates at W =2.75GeV.MagentalongdashedlinesgiveBG2015fit(includednewCLASdata). Experimentaldataarefromthe current (red filled circles), CLAS [9] (black open circles), GRAAL [10] (magenta open triangles), LEPS [11] (blue plus), CB-ELSA [12] (green crosses). The plotted points from previously published experimental data above E = 2 GeV [13] (black filled squares) are those data points within E = ±3 MeVofphotonenergyinlaboratorysystemindicatedon each panel. 13

14 New (preliminary) CLAS and World Data (cont.) 7 14

15 New (preliminary) CLAS and World Data (cont.) MS BG old BG new New state: JP=13/2+ M=2560 MeV =220 MeV Laget 15

16 New CLAS and World Data (cont.) 16

17 New CLAS and World Data (cont.) 17

18 New CLAS and World Data (cont.) 18

19 Handbag Model q q = q k j = k j 2 k j = k j + 2 P. Kroll et al. p = p 2 p = p + 2 Applied to WACS is very successful Underestimates neutral pion cross section significantly 19

20 s 7 Scaling in + p! p 0 Laget MS CLAS g12 Kroll 20

21 Scaling: Future Prospects MAMI-B CLAS g1c SAID CLAS g12 Hall C projected 21