Helicity Asymmetry E for γ p → π 0 p from JLAB CLAS g9a/FROST dataset with application of Machine Learning Chan Kim Igor Strakovsky, William Briscoe, Stuart Fegan The George Washington University APS Division of Nuclear Physics October 15, 2019 1 / 43
Overview 1 Motivation 2 Event Selection 3 ML: Target Classification 4 ML: Hydrogen Contamination on Carbon 5 Helicity Asymmetry E 6 Next Steps 2 / 43
Motivation Baryon Spectroscopy Baryon Spectroscopy Baryon Spectroscopy is the study of excited nucleon states. Excitation Different quark models have different degrees of freedom, causing different predictions of resonance states & parameters of resonances (mass, width, etc). 3 / 43
Motivation Thomas Jefferson National Accelerator Facility (JLab) JLab Continuous e − Beam Accelerator (6 Gev, before upgrade to 12 GeV) Electron Beam Energy (GeV) Photon Beam Polarization # of Events (M) Observable 1.645 Circular ∼ 1000 E 2.478 Circular ∼ 2000 E 2.751 Linear ∼ 1000 G 3.538 Linear ∼ 2000 G 4.599 Linear ∼ 3000 G Hall B g9a/FROST run from 12/2007 ∼ 2/2008 4 / 43
Motivation CLAS g9a/FROST Experiment CLAS g9a/FROST Experiment ◦ Bremsstrahlung radiation (gold foil or thin diamond) → real polarized photon ◦ Dynamic Nulcear Polarization → polarized targets ◦ g9a/FROST - Circularly polarized photons with E γ ≈ 0 . 4 − 2 . 4 GeV and longitudinally polarized proton target ◦ 8 observables at fixed ( E γ , θ ) → 4 helicity amplitudes → Resonances (PWA) UP T and UP R UP T and P R P T and UP R P T and P R d σ UP B P T T x ′ , T z ′ , L x ′ , L z ′ d Ω LP B − Σ O x ′ , ( − T ) , O z ′ H , ( − P ) , − G CP B − C x ′ , − C z ′ F , − E UP , P , LP , CP , B , T , R denote unpolarized, polarized, linearly polarized, circularly polarized, beam, target, and recoil, respectively. 5 / 43
Motivation Helicity Asymmetry E Helicity Asymmetry E ◦ Double polarization observable E is the helicity asymmetry of the cross section: E = σ 3 / 2 − σ 1 / 2 for 3 2 & 1 are total helicty states σ 3 / 2 + σ 1 / 2 2 d σ ◦ d Ω of polarized beam & polarized target for E (theo. & exp.): � d σ � d σ N 1 � = d σ 0 � 2 , 3 2 d Ω (1 ∓ ( P z P λ ) 1 2 E ) = 2 , 3 d Ω d Ω A · F · ρ · ∆ x i 2 , 3 1 1 2 , 3 2 2 ◦ E is measured via: D f = dilution factor � � N 3 P z = Polarization of target in ˆ z − N 1 � � � � 1 1 E = 2 2 + N 1 D f P z P λ N 3 P λ = Polarization of beam 2 2 N 3 2 = # of events 2 , 1 6 / 43
Motivation Butanol & Carbon Targets Butanol & Carbon Targets ◦ Butanol target ( C 4 H 9 OH ) consists of polarized hydrogen (free-nucleons) & unpolarized carbon and oxygen (bound-nucleons) ◦ Fermi motion of bound-nucleons → negative missing mass M π 0 ◦ Carbon target consists of unpolarized bound-nucleon ◦ Scale carbon target events & subtract from butanol target events 7 / 43
Motivation ML Objectives: Target Selection & Ice on Carbon ML Objectives: Target Selection & Ice on Carbon ◦ Target Selection - Events with z-vertex ∈ [2, 5]cm, uncertain whether γ hit Butanol or Carbon ◦ Ice on Carbon - Carbon events (bound-nucleon) expected to have broader m 2 π 0 peak due to Fermi motion. - Sharp peak (free-nucleon) observed in the Carbon target region. 8 / 43
Event Selection Event Selection Event Selections (a) Proton selection (b) Radial vertex selection (c) Z-vertex selection (d) Fiducial selection (e) TOF paddles (f) M 2 X ( E γ , m pi , E pf , p γ , p p 2 ) 9 / 43
ML: Target Classification Neural Network Training Flowchart m φ 1 T φ 2 E β φ 3 B m 2 Event φ 4 C T’ Loss fn π 0 P p � φ 5 . . . . . . Loss score W (2) z φ 6 W (1) Optimizer Weight update 10 / 43
ML: Target Classification Training Data Selection ◦ Randomly select events with z-vertex position in close proximity of each targets - Butanol ∈ [-3.3, 3.3]cm - Carbon ∈ [5.5, 7.0]cm - Polythene ∈ [15.5, 17.0]cm 11 / 43
ML: Target Classification Result on Target Selection ◦ Classified Carbon events from Butanol in z-vertex ∈ [2.5, 4.5]cm ◦ Some Carbon events in Polythene regions & Polythene events in Butanol region. 12 / 43
ML: Hydrogen Contamination on Carbon Training Data for Hydrogen Contamination ◦ Tight cut on the m 2 ◦ Randomly select events within three π 0 peak on g9a-Carbon data (or MC sim) as ice criterion: - Bound-nucleon (fermi p) - Classified as carbon events in → broader m 2 distribution previous target classification - Sharper peaks from free-nucleon distribution (ice) & Broad background from - Missing mass squared / ∈ [ − σ, σ ] bound-nucleon (carbon) - Z-vertex position ∈ [5 . 5 , 6 . 5] 13 / 43
ML: Hydrogen Contamination on Carbon Final Result of ML: ICE vs CARBON [Result from USC for γ p → π + n ] ◦ Classified ice events from Carbon target in z-vertex ∈ [6.0, 7.5]cm ◦ It is likely that ice was formed in 20 K heat shield in between Carbon and Polythene targets. 14 / 43
Helicity Asymmetry E N C 4 H 9 OH Scale Factor ( ) & Dilution Factor N C ◦ Sector dependence only evident in low Energy: E γ ∼ [0 , 0 . 45] GeV ◦ As E γ ↑ , more interactions in butanol target than carbon 74 ∼ � total nucleon in butanol = 10 free H in butanol low lim = = 0 . 135 ◦ D f � N B , f N B , tot ∼ = 1 − s ( E γ ) × N C ( E γ ,θ cm ) ◦ D f ( E γ , θ cm ) = N B , tot ( E γ ,θ cm ) 15 / 43
Helicity Asymmetry E Preliminary: Helicity Asymmetry E � � N 3 − N 1 � � � � ◦ E = 1 1 2 2 D f P γ P T N 3 + N 1 2 2 ◦ Result of ∼ 30% of JLab CLAS g9a experiment data ◦ Measured E comparison to SAID Partial Wave Analysis predictions 16 / 43
Next Steps Next Steps ◦ Process all g9a data for full statistics ◦ Quantify uncertainties in neural network training - Bayesian Neural Network - probability distribution to weights and biases while training - Compute purity of the training data used for uncertainty ◦ Energy loss correction ◦ Systematic Error studies ◦ Measured E into world database → more constrains on reaction amplitude Acknowledgements This work was performed with support from US DOE DE-SC001658, The George Washington University. 17 / 43
Next Steps Backup Slides 18 / 43
Next Steps Constituent Quark Models and LQCD Backup: Constituent Quark Models & LQCD Predictions of Non-Strange Baryon Resonances Constituent Quark Model Lattice QCD Constituent Quark Models predicted states: 64 N ∗ & 22 ∆ ∗ Experimentally confirmed state: 26 N ∗ & 22 ∆ ∗ 19 / 43
Next Steps Polarized Photon Beam Backup: Hall B Photon Tagger Bremsstrahlung radiation due to slowing of electrons by EM field of radiator (gold foil or thinyo diamond) p → π 0 p by E γ = E 0 − E e Determine incoming photon energy of � γ� g9a/FROST - circularly polarized photons with E γ ≈ 0 . 4 ∼ 2 . 4 GeV Tagger was built by the GWU, CUA, & ASU nuclear physics group 20 / 43
Next Steps Polarized Photon Beam Backup: Circularly Polarized Photon Beam Linearly Circularly Bremsstrahlung Polarized Polarized Electron Beam Photon Beam Polarization transfer: 4 x − x 2 P ( γ ) = P ( e ) 4 − 4 x + 3 x 2 x = k photon energy = E 0 incident electron energy H. Olsen and L.C. Maximon, Phys. Rev. 114, 887 (1959) 21 / 43
Next Steps Frozen Spin Target Backup: Frozen Spin Target C. Keith et al. Nucl Instrum Meth A 684, 27 (2012) 22 / 43
Next Steps Frozen Spin Target Backup: CLAS g9a/FROST Data p → π 0 p events Select only � γ� p → π 0 p resonance channels � γ� Appropriate enegy bins - include all resonances ( ≤ 1500 MeV) 23 / 43
Next Steps Frozen Spin Target π 0 photoproduction p → π 0 p : From T Matrix to Helicity Amplitudes of � γ� � q m s ′ | T | k m s λ � = � m s ′ | J | m s � · ǫ λ ( k ) H i ( θ ) ≡ � λ 2 | J | λ 1 � 4 Complex Helicity Amplitudes: � +3 � � +1 � � � +1 � � +1 � � � � � � H 1 ( θ ) = � J H 2 ( θ ) = � J � � � � 2 2 2 2 � � � � � � � � +3 � − 1 +1 � − 1 � � � � H 3 ( θ ) = � J H 4 ( θ ) = � J � � � � 2 2 2 2 24 / 43
Next Steps Frozen Spin Target Backup: Complete Experiment - 8 Polarization Observables Polarizable: incoming photons, target & recoiling nucleons 8 well chosen observables at fixed E γ & angle → 4 helicity amplitudes UP T and UP R UP T and P R P T and UP R P T and P R d σ UP B P T T x ′ , T z ′ , L x ′ , L z ′ d Ω − Σ O x ′ , ( − T ) , O z ′ H , ( − P ) , − G LP B CP B − C x ′ , − C z ′ F , − E UP , P , LP , CP , B , T , R denote unpolarized, polarized, linearly polarized, circularly polarized, beam, target, and recoil, respectively. Helicity asymmetry E related to other observables via Fierz identities: E 2 + F 2 + G 2 + H 2 = 1 + P 2 − Σ 2 − T 2 FG − EH = P − Σ T . . . 25 / 43
Recommend
More recommend
