Determination of the Hyperon Induced Polarization and Polarization–Transfer Coefficients for Quasi-Free Hyperon Photoproduction off the Bound Neutron NSTAR 2017 Colin Gleason University of South Carolina August 22, 2017 Work supported by NSF PHY-1505615 Colin Gleason (USC) August 22, 2017 1 / 25
Overview Motivation for studying K Λ photoproduction γ d → K 0 � Λ( p ) → p π + π − π − ( p ) Identification of the reaction of � Background subtraction Preliminary results Comparison of C x and P with current Bonn–Gatchina projections Comparison of C z with K + Λ Dependence on neutron momentum First interpretations with Legendre polynomial fits Summary Colin Gleason (USC) August 22, 2017 2 / 25
γ d → K 0 � Motivation for � Λ( p ) Majority of data is π N scattering or final states with π ’s Some resonances couple weakly to these channels while having significant branching ratios to K Λ ⋆ ✐ Most strangeness data from free proton ⋆ γ p → K + Λ moving + from ∗∗ to ∗ ∗ ∗ ⋆ ❢ N (1900) 3 2 γ n → K 0 Λ sensitive to − and ∗ ∗ N (2120) 3 2 ⋆ − ∗ ∗ ∗ N (1875) 3 2 How do data from the proton and bound neutron compare to each other? C. Patrignani et al. (Particle Data Group), Chin. Phys. C, 40, 100001 (2016). Colin Gleason (USC) August 22, 2017 3 / 25
Polarization Observables in K Λ Photoproduction The 4 complex scattering amplitudes can define 16 polarization observables Unpolarized Cross Section σ 0 Single P Σ T Beam-Recoil C x C z O x O z Target-Recoil T x T z L x L z Beam-Target E F G H The full scattering amplitude can be determined by carefully choosing 8 observables. d σ d Ω = σ 0 [1 − P lin Σ cos 2 φ − α cos θ x ( P lin O x sin 2 φ + P circ C x ) − α cos θ y ( − P + P lin T cos 2 φ ) − α cos θ z ( P lin O z sin 2 φ + P circ C z )] Colin Gleason (USC) August 22, 2017 4 / 25
γ N → K � Defining Kinematics: � Λ Two different coordinate systems for KY photoproduction: Observables are K 0 dependent on E γ x x 0 z 0 K 0 → π + π − and θ CM K y 0 y z The coordinate γ θ CM n K 0 systems define x, y, z θ x , θ y , θ z p θ Polarization of Λ ~ depends on Λ choice of axes Λ π − Colin Gleason (USC) August 22, 2017 5 / 25
Previous Studies of K 0 Λ Photoproduction γ d → K 0 � Compton d σ d Ω for � Λ( p ) (under review 2017) Bonn–Gatchina multi–channel fit for γ d → π − p ( p ), π − p → γ n , γ d → π 0 n ( p ), γ d → η n ( p ), γ d → K + Σ − “Both solutions seem to describe γ d → K + Σ − ( p ) and γ d → K 0 Λ( p ) reasonable well.” Need to include polarization observables in the fits to (a) resolve this ambiguity or (b) provide a new solution! arXiv:1706.04748 [nucl-ex] Colin Gleason (USC) August 22, 2017 6 / 25
Experimental Facility Hall–B (1997-2012) at Thomas Jefferson National Accelerator Facility (JLab) CEBAF Large Polarized electron beam Acceptance E e =2.0 and 2.6 GeV Spectrometer (CLAS) Target- p, d (polarized or Photon tagger unpolarized) E γ = E e − E e 0 ≈ 0 . 95 − 0 . 2 E e B.A. Mecking et al., Nucl. Instr. and Meth. A 503, 513 (2003) Colin Gleason (USC) August 22, 2017 7 / 25
Experimental Facility Identification of K 0 and Λ: M ( π + π − ) and M ( p π − ) M ( π + π − ) = � (˜ p π + + ˜ p π − ) 2 Fit peak with Gaussian, ± 4 σ cut 40000 M K 0 = 0 . 497 GeV/ c 2 µ = 0 . 498 GeV/ c 2 Counts/MeV 20000 0 0.4 0.45 0.5 0.55 0.6 2 Mass(GeV/c ) Colin Gleason (USC) August 22, 2017 8 / 25
Experimental Facility Identification of K 0 and Λ: M ( π + π − ) and M ( p π − ) M ( π + π − ) = � � (˜ p π + + ˜ p π − ) 2 M ( p π − ) = (˜ p p + ˜ p π − ) 2 Fit peak with Gaussian, ± 4 σ cut Fit peak with Gaussian, ± 4 σ cut 40000 M K 0 = 0 . 497 GeV/ c 2 M Λ = 1 . 115 GeV/ c 2 µ = 0 . 498 GeV/ c 2 µ = 1 . 116 GeV/ c 2 20000 Counts/(25 MeV) Counts/MeV 20000 10000 0 0 0.4 0.45 0.5 0.55 0.6 1.1 1.11 1.12 1.13 1.14 1.15 2 2 Mass(GeV/c ) Mass(GeV/c ) Colin Gleason (USC) August 22, 2017 8 / 25
Experimental Facility Identification of K 0 and Λ: M ( π + π − ) and M ( p π − ) M ( π + π − ) = � � (˜ p π + + ˜ p π − ) 2 M ( p π − ) = (˜ p p + ˜ p π − ) 2 Fit peak with Gaussian, ± 4 σ cut Fit peak with Gaussian, ± 4 σ cut 40000 M K 0 = 0 . 497 GeV/ c 2 M Λ = 1 . 115 GeV/ c 2 µ = 0 . 498 GeV/ c 2 µ = 1 . 116 GeV/ c 2 20000 Counts/(25 MeV) Counts/MeV 20000 10000 0 0 0.4 0.45 0.5 0.55 0.6 1.1 1.11 1.12 1.13 1.14 1.15 2 2 Mass(GeV/c ) Mass(GeV/c ) p X < 0 . 2 GeV/ c cut used to select the quasi–free events Colin Gleason (USC) August 22, 2017 8 / 25
Experimental Facility Background Channels: γ d → K 0 Λ( X ) Non–resonant, unpolarized γ d → π + π − p π − ( p ) Higher mass channels: γ d → K 0 Σ 0 ( p ) → K 0 Λ( γ p ) → π + π − p π − ( γ p ) γ d → K 0 Σ ⋆ 0 ( p ) → K 0 Λ( π 0 p ) → π + π − p π − ( π 0 p ) γ d → K ⋆ (892)Λ( p ) → K 0 Λ( π 0 p ) → π + π − p π − ( π 0 p ) 24000 22000 20000 20000 X = p 18000 Counts/(2.5 MeV/ c 2 ) 16000 � p Λ ) 2 M X = (˜ p γ + ˜ p d − ˜ p K 0 − ˜ 14000 X = γ p 12000 10000 8000 X = π 0 p 6000 4000 2000 0 0.8 0.85 0 . 9 0.9 0.95 1 . 0 1 1.05 1 . 1 1.1 1.15 1 . 2 1.2 1.25 1.3 M X (GeV/ c 2 ) 2 M (GeV/c ) X Colin Gleason (USC) August 22, 2017 9 / 25
Experimental Facility Background Channels: γ d → K 0 Λ( X ) Non–resonant, unpolarized γ d → π + π − p π − ( p ) Higher mass channels: γ d → K 0 Σ 0 ( p ) → K 0 Λ( γ p ) → π + π − p π − ( γ p ) γ d → K 0 Σ ⋆ 0 ( p ) → K 0 Λ( π 0 p ) → π + π − p π − ( π 0 p ) γ d → K ⋆ (892)Λ( p ) → K 0 Λ( π 0 p ) → π + π − p π − ( π 0 p ) 24000 22000 20000 20000 X = p 18000 Counts/(2.5 MeV/ c 2 ) 16000 � p Λ ) 2 M X = (˜ p γ + ˜ p d − ˜ p K 0 − ˜ 14000 Need a realistic event generator X = γ p 12000 and simulations to separate 10000 signal from background 8000 X = π 0 p 6000 4000 2000 0 0.8 0.85 0 . 9 0.9 0.95 1 . 0 1 1.05 1 . 1 1.1 1.15 1 . 2 1.2 1.25 1.3 M X (GeV/ c 2 ) 2 M (GeV/c ) X Colin Gleason (USC) August 22, 2017 9 / 25
Experimental Facility Background Subtraction Determine background free observable from ratios of background to signal events Done by fitting M X with the simulated background channels Colin Gleason (USC) August 22, 2017 10 / 25
Experimental Facility Background Subtraction Determine background free observable from ratios of background to signal events Done by fitting M X with the simulated background channels 1 Fit Double Gaus+background histograms to data 2 Use fit parameters to normalize background 3 Calculate background to total ratios: r B and r unpol 25000 Fit 20000 π π π + - - p Scaled 0 Σ 0 K Scaled 15000 0 Σ *0 K +K(892) Scaled 10000 5000 0 0.8 0.85 0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.3 2 M (GeV/c ) X Colin Gleason (USC) August 22, 2017 10 / 25
Experimental Facility Background Subtraction Determine background free observable from ratios of background to signal events Done by fitting M X with the simulated background channels 1 Fit Double Gaus+background histograms to data 2 Use fit parameters to normalize background A “total” observable can 3 Calculate background to total ratios: r B and r unpol be extracted for regions 1 and 2 The total observable is 25000 corrected using the Fit background to total ratios 20000 π π π + - - p Scaled i , r unpol ( r B ) to get a 0 Σ 0 K Scaled i “signal” (background free) 15000 0 Σ *0 K +K(892) Scaled observable C S x = 10000 r B 1 C T x , 2 − r B 2 C T 1 2 x , 1 1 r unpol + r unpol r B 1 − r B 2 − r B r B 5000 2 1 2 0 0.8 0.85 0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.3 2 M (GeV/c ) X 1: signal dominated, 2: K 0 Σ 0 dominated Colin Gleason (USC) August 22, 2017 10 / 25
Experimental Facility γ to � C x –Polarization Transfer from � Λ Along x–axis Two BonnGa solutions from fits to K 0 Λ cross–sections E <1.0 1.0<E <1.1 1.1<E <1.2 1.2<E <1.3 γ γ γ γ Preliminary Preliminary Preliminary Preliminary Same resonances included, two sets of parameters give reasonable fit to 1.3<E <1.4 1.4<E <1.5 1.5<E <1.6 1.6<E <1.7 γ γ γ γ Preliminary Preliminary Preliminary Preliminary γ d → K + Σ − ( p ) and K 0 Λ( p ) BonnGa provided me 1.7<E <1.8 1.8<E <1.9 1.9<E <2.0 2.0<E <2.1 γ γ γ γ with the two solution’s Preliminary Preliminary Preliminary Preliminary projected onto C x , C z , P No K 0 Λ polarization 2.1<E <2.2 2.2<E <2.3 2.3<E <2.4 E >2.4 γ γ γ γ 1 Preliminary Preliminary Preliminary Preliminary observables included C 0 in fits x -1 Potential impact: θ CM -1 0 1 cos resolution of current 0 K ambiguity, or lead to new results Colin Gleason (USC) August 22, 2017 11 / 25
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