Status report on parity violation in the (1232) resonance

Transcription

1 Status report on parity violation in the (1232) resonance Luigi Capozza A4 Collaboration Institut für Kernphysik Johannes Gutenberg Universität Mainz Institutsseminar Luigi Capozza, Institutsseminar /38

2 Outline Theory Measurement principle Physical processes Detector response Some results Luigi Capozza, Institutsseminar /38

3 Outline Theory Measurement principle Physical processes Detector response Some results Luigi Capozza, Institutsseminar /38

4 Theory Parity violation asymmetry in ep enπ: At Tree level: A RL = σ R σ L = G F Q 2 σ R + σ L 2 2πα W EM : unpolarised electromagnetic W PV W EM W PV : helicity dependent interference of EM and NC transition amplitudes Flavor-SU(3) and isospin: J EM µ = Jµ EM (T = 1) + JEM µ (T = 0) Jµ NC = ξv T=1 Jµ EM (T = 1) + ξv T=0 Jµ EM (T = 0) + ξ (0) J NC 5µ = ξ T=1 A A (3) µ + ξ T=0 A A (8) µ + ξ (0) A A(s) µ V V(s) µ Luigi Capozza, Institutsseminar /38

5 Theory N transition: only isovector A RL = A res RL + A non-res RL Non resonant asymmetry A non-res RL : model dependent phenomenological effective interaction lagrangians: Luigi Capozza, Institutsseminar /38

6 Theory J EM µ = J EM µ (T = 1) + J EM µ (T = 0) J NC µ = ξv T=1 J EM µ (T = 1) + ξt=0 V J NC 5µ = ξ T=1 A A (3) µ + ξ T=0 A A (8) J EM µ µ + ξ(0) A A(s) µ (T = 0) + ξ(0) V V(s) µ Resonant asymmetry: A res RL = G F Q 2 4 [ g e A ξv T=1 + g e V 2πα ξt=1 A F(Q 2, s) ] Luigi Capozza, Institutsseminar /38

7 Theory Resonant amplitudes: (p ) JEM µ N(p) = ū λ (p ) D E (p ) Jµ NC + JNC 5µ N(p) = " ū λ (p ) Considering: isospin symmetry» C γ «3 M γν + Cγ 4 M 2 p ν + Cγ 5 M 2 pν (g λµ g ρν g λρ g µν)q ρ γ 5 u(p) C Z 3V M γν + CZ 4V M 2 p ν + CZ 5V M 2 pν C Z 3A M γν + CZ 4A M 2 p ν conservation of vector current spin and parity of the (1232) (J π = 3/2 + ) dominance of magnetic dipole amplitude! (g λµ g ρν g λρ g µν)q ρ γ 5 + C Z 6V g λµγ 5! (g λµ g ρν g λρ g µν)q ρ + C Z5A g λµ + C Z6A p λq µ # u(p) Luigi Capozza, Institutsseminar /38

8 Theory Resonant asymmetry: A res RL = G F Q 2 4 [ g e A ξv T=1 + g e V 2πα ξt=1 A F(Q 2, s) ] Axial response function of p transition: F(Q 2, s) = P CA 5 C V 3 [1 + W2 Q 2 M 2 C A ] 4 2M 2 C5 A Luigi Capozza, Institutsseminar /38

9 Outline Theory Measurement principle The A4 experiment Energy spectrum Problem: Handling the background Physical processes Detector response Some results Luigi Capozza, Institutsseminar /38

10 The A4 experiment longitudinally polarised electron beam unpolarised l-h 2 target counting of scattered particles measurement of scattered particle energy Luigi Capozza, Institutsseminar /38

11 A4 experimental setup MAMI E=855 MeV or E=570 MeV I=20 µ A, P= 80 % Compton laser back scatter polarimeter e - Hydrogen target e - e - e - Luminosity Transmission Compton Polarimeter Beam Dump PbF2 Calorimeter Luigi Capozza, Institutsseminar /38

12 A4 experimental setup Lead fluoride Cherenkov calorimeter: 1022 crystals 7 rings 146 frames θ (30, 40 ), ϕ (0, 2π) Readout electronics: sum of 9 neighbouring crystals Luigi Capozza, Institutsseminar /38

13 Energy spectrum counts 3 10 E (MeV) Elastic cut Elastic peak Elastic cut 80 peak πth W(MeV) ADC Channel Luigi Capozza, Institutsseminar /38

14 Extraction of the physical asymmetry A exp = N + ρ + N ρ N + ρ + + N ρ = P A phys + A inst extraction of counts normalisation on target density correction of helicity correlated instrumental effects Luigi Capozza, Institutsseminar /38

15 Problem: Handling the background First step: Identification of the contributing physical processes Estimation of their contribution to the spectrum Second step: Estimation of their asymmetry Calculation of a dilution factor Luigi Capozza, Institutsseminar /38

16 Knowledge of the background 3 Elastic cut Elastic cut physical processes inside the target + response of the apparatus }{{} Monte Carlo simulations counts E (MeV) peak πth W(MeV) Elastic peak ADC Channel Luigi Capozza, Institutsseminar /38

17 Knowledge of the background 3 Elastic cut Elastic cut physical processes inside the target + response of the apparatus }{{} Monte Carlo simulations counts E (MeV) peak πth W(MeV) Elastic peak ADC Channel Event generator GEANT4 simulations Luigi Capozza, Institutsseminar /38

18 Outline Theory Measurement principle Physical processes Elastic e-p scattering Energy straggling in the target Inelastic e-p scattering Detector response Some results Luigi Capozza, Institutsseminar /38

19 Event generator Variables to be generated: x : position of the scattering θ : polar scattering angle E : final electron energy Needed: ranges: (x min, x max ), Ω, (E min, E max) differential cross sections: dσ(x,θ, E ) Luigi Capozza, Institutsseminar /38

20 Elastic e-p scattering Rosenbluth cross section: d 2 σ dω Ros (E,θ) (dipole fit for G E and G M ) Radiative corrections to the elastic scattering: _ e _ e i p f p = _ e _ e i p f p + _ e _ e i p f p + _ e _ e i p f p + _ e _ e i p f p + _ e _ e i p f p Luigi Capozza, Institutsseminar /38

21 Radiative corrections to the elastic scattering Two kinematical regions: radiative tail from the elastic peak (E < E el E r) d 3 σ dωde (E, E, θ) tail elastic peak (E > E el E r) d 2 σ (E, θ) = (1 + δ( E r, E, θ)) d2 σ (E, θ) dω dω peak Ros Peaking approximation Mo and Tsai s formulae for δ and d3 σ dωde Luigi Capozza, Institutsseminar /38

22 Energy straggling in the target Energy losses given by: Radiation Collisions ] -1 [MeV Ie Mo and Tsai Landau Moller / E [MeV] Large energy losses mainly due to Bremsstrahlung Luigi Capozza, Institutsseminar /38

23 Straggling function Formula of Mo and Tsai: I e (E 0, E, t) = bt E 0 E [ E E ( E0 E E 0 ) ] 2 ( ln E ) bt 0 E for Bremsstrahlung using peaking approximation valid up to a cut E < E 0 E s for E > E 0 E s Je Es (E 0, t) = 1 E0 E s 0 de I e (E 0, E, t) Luigi Capozza, Institutsseminar /38

24 Generation of ep ep(γ) events Energy straggling + radiative corrections 4 kinematical regions 2500 E el 2000 E el - E r E E s E E 0 Luigi Capozza, Institutsseminar /38

25 Generation of ep ep(γ) events Region Cross section I d 2 σ dω d 2 σ = J e (E 0 ) I dω peak II d 3 σ dωde = I e (E 0, E) de II de d 2 σ dω (E) peak III d 3 σ dωde = J e III d 3 σ dωde (E 0 ) tail IV d 3 σ dωde = IV d 3 σ de I e (E 0, E) E min dωde (E) Emax tail Luigi Capozza, Institutsseminar /38

26 Inelastic e-p scattering Processes: e + p e + p + π 0 e + p e + n + π + Inclusive cross section: d 3 σ d 5 σ dω e de = dω π dω e de dω π nb MeVsr d σ dωde E [MeV] Luigi Capozza, Institutsseminar /38

27 Outline Theory Measurement principle Physical processes Detector response Simulation of the A4 detector Particle tracking with GEANT4 Production and detection of Cherenkov light Parameterisation of the photoelectron emission Some results Luigi Capozza, Institutsseminar /38

28 The A4 detector PbF 2 Cherenkov calorimeter l-h 2 target 1022 crystals ordered in 7 rings Luigi Capozza, Institutsseminar /38

29 The response of the A4 detector What happens between scattering and the energy spectrum? Passage through material layers Physics of the detector { Electromagnetic shower Cherenkov effect Luigi Capozza, Institutsseminar /38

30 Simulation of the A4 detector Definition of the detector geometry: Volumes (shape, dimentions, position) Materials (composition, ρ, Z, A) Luigi Capozza, Institutsseminar /38

31 Simulation of the A4 detector Definition of particles and processes γ: Compton scattering pair production photoelectric effect e and e + : ionisation Bremsstrahlung multiple scattering only for e + : annihilation Luigi Capozza, Institutsseminar /38

32 Particle tracking with GEANT4 processes cuts discrete (DP) σ s mean free path (MFP) materials continuous (CP) particle tracking stepping (step only 1 DP) 1. choose shortest MFP 2. calculate probabilities 3. choose DP by MC 4. sample position 5. calculate effect of CP s 6. sample final state of DP s Luigi Capozza, Institutsseminar /38

33 Production and detection of Cherenkov light More geometry and material properties: refractive indexes absorption lengths optical surfaces More particles and processes: optical photons Cherenkov effect absorption boundary process absorption length [m] refractive index Wavelenght [nm] tracking of optical photons Wavelenght [nm] Luigi Capozza, Institutsseminar /38

34 Production and detection of Cherenkov light Spectral sensitivity characteristic: input window photocathode sensitivity ma W sk e Quantum efficiency: ( 124 nm QE(λ) sk e (λ) W ) % λ ma Wavelenght [nm] Luigi Capozza, Institutsseminar /38

35 Parameterisation of the photoelectron emission Simulating the whole electromagnetic shower is possible Tracking all Cherenkov photons takes too long Parameterisation needed: deposited energy in one crystal Photoelectrons in the respective PMT Luigi Capozza, Institutsseminar /38

36 Parameterisation of the photoelectron emission Ansatz: gaussian fluctuations of N pe N pe E d [MeV] strong linear correlation r = mean N pe linear dependent on E d variance of N pe σ 2 N pe also linear in E d E d (MeV) Luigi Capozza, Institutsseminar /38

37 Outline Theory Measurement principle Physical processes Detector response Some results Luigi Capozza, Institutsseminar /38

38 Comparison with the experimental spectrum 1. Calibration : N pe ADC channel knowledge of offset and peak position linearity 2. Scaling factor ξ ξ = L σ t N evt L : luminosity (ρ I l) σ : total cross section t : run duration N evt : simulated events Luigi Capozza, Institutsseminar /38

39 Result for electrons Input information: from the spectrum itself: scattering processes position of the peak detector physics Luigi Capozza, Institutsseminar /38

40 Result for backward scattering 3 counts / background is dominant ADC channel Luigi Capozza, Institutsseminar /38

41 Result for backward scattering 3 counts / ADC channel background is dominant it is neutral particles γ s Luigi Capozza, Institutsseminar /38

42 Result for backward scattering 3 counts / 10 background is dominant ADC channel it is neutral particles γ s coincidence spectrum reproduced by simulation Luigi Capozza, Institutsseminar /38

43 Work in progress Contribution of γ s? (at backward angle important!) Processes e + p e + p + π 0 (e + p) + γ + γ e + p (e + p) + γ Detector response very similar to e conversion (dominant) Compton Luigi Capozza, Institutsseminar /38

44 Summary Parity violation in the (1232) interesting for hadron structure Possibility of measuring the PV asymmetry within the A4 experiment Large background: understanding of energy spectrum needed Study of scattering processes detector response Detector response under control Electron contribution well understood Working on contribution of γ s Luigi Capozza, Institutsseminar /38