Recent results of pion and kaon photoproduction at SPring-8/LEPS Oct / 9 / 2018 RCNP Osaka University / Nagoya University Hideki Kohri 1
SPring-8 beamline map LEPS2 LEPS beamlines 2
LEPS facility constructed in 2000 Collision 8 GeV electron Recoil electron SPring-8 Electron tagging SR ring Laser hutch Laser light Inverse Compton 20m g - ray Photon beam 10m E g = 1.5-2.95 GeV Energy 0 Intensity ~10 6 g /s Polarization linearly and circularly polarized beams. Experiment hutch P~90% at the maximum photon energy. 3
LEPS detector (optimized to detect f decaying to K + K - ) Aerogel TOF Cerenkov Dipole Magnet wall (n=1.03) (0.7 T) K + Start counter Target K - LH 2 , LD 2 g MWDC 3 1m MWDC 1 MWDC 2 Silicon Vertex 4 Detector
We are analyzing the data taken for K *0 S + photoproduction in 2007 -> K + p - 5 S.H. Hwang Ph.D. thesis Pusan National University (2012)
One of our physics objectives is to understand how hadrons are produced I want to obtain unified understanding of various qq productions. (1) g p -> p + n reaction dd production in the final state (2) g p -> K + L and K + S 0 reactions ss production in the final state (3) g p -> p - D ++ reaction uu production in the final state (4) g p -> p + D 0 reaction dd production in the final state 6
Another physics objective Missing resonance search Quark models predict more nucleon resonances than observed experimentally. Such missing nucleon resonances may be coupled to other channels than p N. 7 Simon Capstick and W. Robers Phys. Rev. D 58 074011 (1998)
Reaction mechanisms of KY photoproduction s channel Strong when a meson is t channel produced at forward angles. u channel Very weak when a meson is produced at forward angles. 8
Measurements of photon-beam asymmetry We used linearly polarized photon beams. Photon-beam asymmetry ( S ) is sensitive to reaction mechanisms. S (1) g p -> p + n reaction Positive p < r Natural parity exchange (2) g p -> K + L and K + S 0 reaction Positive K < K * Natural parity exchange (3) g p -> p - D ++ reaction Negative p > r Unnatural parity exchange (4) g p -> p + D 0 reaction Positive p < r Natural parity exchange 9
(1) Differential cross section and photon-beam asymmetry for the g p -> p + n reaction at forward p + angles at E g =1.5-2.95 GeV H. Kohri, S.Y. Wang, S.H. Shiu, W.C. Chang, Y. Yanai et al. LEPS Collaboration Abstract Published in Phys. Rev. C 015205 (2018) on the 22 nd of Jan. 10
Missing mass p ( g , p + ) X Neutron peaks are separately observed for 0.6 < cos q < 0.966. Positron mis-identification produces background between n and D 0 for 0.966 < cos q < 1. 11
Differential cross sections for g p -> p + n Forward peaking cross sections are observed. t-channel reaction is found to be dominant. 12
Differential cross sections for g p -> p + n d s /dcos q decreases as E g increases for 0.6 < cos q < 0.9. The energy dependence of E g < 2.2 GeV is different for 0.9 < cos q < 1. This energy dependence might be due to N * or D * , as reported by the DESY group. Good agreement with CLAS( □ ) and DESY( ▲ ) data. (2009) (1996, 1997) 13
Ratio (N V – N H ) / (N V + N H ) p + prefers to scatter at f p angles perpendicular to the polarization plane. Photon-beam asymmetries for g p -> p + n are found to be positive. 14
Photon-beam asymmetry S for g p -> p + n First photon-beam asymmetry data for E g > 1.9 GeV. Positive asymmetries are basically explained by r -meson exchange in the t-channel. SLAC data (1979) E g =3.4 GeV E g =16 GeV PLB 400 (1997) 6 15
(2) Photoproduction of L and S 0 hyperons off protons with linearly polarized photons at E g =1.5-3 GeV S.H. Shiu, H. Kohri, W.C. Chang et al. LEPS Collaboration Abstract Published in Phys. Rev. C 97 015208 (2018) on the 31 st of Jan. 16
Missing mass p ( g , K + ) X 2018 data E g =1.5-3.0 GeV 2006 data E g =1.5-2.4 GeV 17
Background p + D 0 (1232) produces a peak at around 1.15 GeV if kaon mass is used in missing mass calculation g + p → p + + X n D 0 18 Missing mass (GeV) calculated by pion mass
Differential cross sections for g p -> K + L and K + S 0 First cross section data for LEPS at 2.4 < E g < 3 GeV. K + L cross sections are larger than K + S 0 cross sections. No evident structure due to N * or D * . 19
Ratio (nN V – N H ) / (nN V + N H ) K + prefers to scatter at f K angles perpendicular to the polarization plane. Photon-beam asymmetries are found to be positive for both the reactions, suggesting dominance of K * -exchange. 20
Photon-beam asymmetry for K + L and K + S 0 First photon-beam asymmetries data for E g > 2.4 GeV. The asymmetries increase gradually as E g increases for both the reactions. K * -exchange contribution becomes larger. SLAC data 21 (1979)
(3) Differential cross section and photon-beam asymmetry for the g p -> p - D ++ (1232) reaction at forward p - angles for E g =1.5-2.95 GeV H. Kohri, S.H. Shiu, W.C. Chang, Y. Yanai, et al. LEPS Collaboration Abstract Published in Phys. Rev. Lett. 120 202004 (2018) on the 18 th of May. 22
Missing mass p ( g , p - ) X All BG D 2 p / r D All BG 2 p / r 3 p 3 p 23
Differential cross sections for g p -> p - D ++ First high-statistics cross section data. d s /dcos q decreases as E g increases. Strong forward peaking ( p -exchange). Theoretical calculations by S.i. Nam well reproduce the data by optimizing the cutoff mass parameter from 450 to 500 MeV. The energy dependence of E g < 1.8 GeV cannot be reproduced for cos q > 0.9. N * or D * ? 24
Comparison of d s /dcos q between p - D ++ ( ● ) and p + n( ■ ) Strong forward peaking cross sections suggest t-channel reaction is dominant. 25
Ratio (N V – N H ) / (N V + N H ) p - prefers to scatter at f p angles parallel to the polarization plane. Asymmetries are found to be negative in most of LEPS kinematical regions. 26
Photon-beam asymmetry for g p -> p - D ++ First asymmetry data for 1.5 < E g < 2.8 GeV. Asymmetries are found to be negative for most of LEPS kinematical regions, suggesting p -exchange dominance . Theoretical calculations by S.i. Nam well reproduce negative asymmetries for cos q > 0.933. The calculations cannot reproduce the data for cos q < 0.9. Additional unnatural parity exchange is needed. 27
Why only p - D ++ reaction favors light meson exchange ? S Negative Positive (1) g p -> p + n p(140 MeV ) or r(770 MeV ) spin 0 spin 1 dd production (2) g p -> K + L K(490 MeV ) or K * (890 MeV ) spin 0 spin 1 ss production (3) g p -> p - D ++ p(140 MeV ) or r(770 MeV ) spin 0 spin 1 uu production Other pseudoscalar meson photoproduction such as p + D 0 , p 0 p, h p has positive S . Heavy meson exchange is dominant. 28
Yukawa theory Uncertainty principle Strong interaction between nucleons is mediated by mesons . D E D t ~ ℏ Meson Energy conservation violates in a short time. A particle flying 1 fm with c has a mass, Proton Neutron M ～ ℏ c / ( c D t ) = ~200 MeV fm / 1 fm . Meson Light p meson can reach long distance. Interactions with heavy r, K * mesons are Neutron Proton limited in short distances. 29
My personal interpretation for these results p + g We detected mesons at forward angles. r - meson Small momentum transfer suggests exchange (1) reactions occur near the surface n p of the proton. (1) p + n reaction exchanges u quark with d. g K + (2) K + L reaction exchanges u quark with s. K * - meson (3) p - D ++ reaction exchanges d quark with u. exchange L (2) p p meson exchange is long ranged. d quark may live in the central region g p - of the proton ? p- meson The difference in the asymmetries exchange D ++ (3) p may suggest differences of u and d quark positions ? 30
d-quark staying in the central region of the proton Proton in diquark model u and d quarks are coupled to spin=0. Good diquark. p + Proton in pion cloud model n 4 1 1 p + p 0 p 0 : : p p n 31
Interpretation based on Vector-Meson-Dominance model uu production in the intermediate state dd production in the intermediate state r, w r, w Quark exchange Annihilation p + n r, w r, w Quark exchange Annihilation p - D ++ 32
Summary We took high momentum charged pion data for the first time in 2007. It enables us to study uu, dd, and ss productions and we want to obtain unified understanding of these qq productions. (1) g p -> p + n reaction Published in Phys. Rev. C on Jan/22/2018 dd production (2) g p -> K + L and K + S 0 reaction Published in Phys. Rev. C on Jan/31/2018 ss production (3) g p -> p - D ++ reaction Published in Phys. Rev. Lett. on May/18/2018 uu production (4) g p -> p + D 0 reaction Physics paper is prepared. dd production 33
Missing mass p ( g , , p ) X Missing mass spectrum is fitted with relativistic Breit-Wigner shape for D , 2p / r , 3p , and e - or e + curves. p ( g , p - )X p ( g , p + ) X 0.7<cos q p <1 0.7<cos q p <1 E g =1.5-2.95 GeV E g =1.5-2.95 GeV 34
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