p( ,n      reaction measured with the Crystal Ball at MAMI Dan Watts, Derek Glazier University of Edinburgh Richard Codling, John Annand University of Glasgow Crystal Ball Collaboration meeting, Mainz, 2007

Why measure p( ,n    ’  Independent test of theoretical treatment of reaction amplitudes and rescattering effects in radiative  photoproduction  radiated from  + lines (rather than proton lines as in p  0  ’) – brem production has different strength/angular behaviour Give additional sensitivity to MDM? Blue lines :  + p → n +  + +  'Black lines :  + p → p +  0 +  '

Theoretical predictions p( ,n  +  ’) Predictions presently available in unitary model (and  EFT presently in development) Main features: 1) Cross sections ~5x larger than p( ,p  0  ’) 2) Linear asymmetries large and positive 3) Sensitivity to MDM marginal (in sampled kinematics) 4) But helicity asymmetry shows promise as complimentary determination of MDM Tree level Unitary model

 + detection in the Crystal Ball: Achieving good energy determination Utility of Crystal Ball for   detection well understood but    energy determination unexplored Expect some challenges: 1) Separation from proton/electron events 2) Hadronic/nuclear interactions 3) Unstable decay products } GEANT simulation to indicate CB response Particle-ID detector ●       (~26 ns) e  e  (~2  s) Michel spectrum of e+ energies

● Use shower shape to help identify event types ● Reject many of , NI events with simple restriction on N cryst <=4 Good Event Muon decay event Nuclear interaction Geantsimulation:  + shower shapes

Geant simulation: 150 MeV  + signals in the CB Counts Energy contained in cluster (GeV) Counts Energy contained in cluster (GeV) Split off clusters Muon decay Hadronic interactions No shower size restriction<=4 crystals in the shower

p( ,n  +  ’) : Outline of data analysis Accept events with: 1  +, 2 neutral clusters in CB/TAPS 1  +, 1 neutron TAPS, 1 other neutral p( ,n  +  ’) total 4-mom kinematic fit (CL>10 -1 ) If two neutrals assume either is photon or neutron, analyse both combinations Reject events with: 2 neutrals pass M  0 kinematic fit (CL>10 -3 ) - p  0,n  +  0 M  + miss = Mn Kin. Fit (CL>10 -3 ) - n  + n  + Total 4 momentum fit (CL>10 -2 ) - n  +  + shower condition <=4 crystals Data used in next plots: all MDM data at E e =885 MeV July/Sep/Jan Total p( ,n  +  ’) events – 70,000

p( ,n  +  ’) : Simulation data Run event generators through Monte Carlo of CB/TAPS Predicted energy deposits smeared according to observed experimental energy resolutions Event generators: p( ,n  +  p( ,n  +  - split off clusters from n/  + p( ,n  +  0  – Missed/combined  from  0 decay All phase space distributions at the moment!’) :

p( ,n  +  ’) : Analysis results N.B. Kinematic cuts to reject background relaxed in these plots!! Experiment Simulated n  +  Simulated n  + Simulated n  o   mass of the M   mass of the system recoiling from the pion minus the neutron mass M 

p( ,n  +  ’) : Analysis results Experiment Simulated n  +  Simulated n  + Simulated n  o 

p( ,n  +  ’) : Linear asymmetry E  = 360 ± 20 MeV    CM  = 0 o -70 o  CM  = 70 o -110 o  CM  = 110 o -180 o  = MeV  = MeV  = MeV

p( ,n  +  ’) : Linear asymmetry E  =420 ± 20 MeV  = MeV  = MeV  = MeV  CM  = 0 o -70 o  CM  = 70 o -110 o  CM  = 110 o -180 o  

E  = 320 ±20 MeVE  = 360 ±20 MeV E  = 420 ±20 MeV  o   (CM) < 110 o Linear Asymmetry p( ,n  +  ’) : Analysis results (Linear Asymmetry) Unitary model (   =2) Unitary model normalised to agree in soft photon limit Rescattering not included

E  = 320 ±20 MeVE  = 360 ±20 MeV E  = 420 ±20 MeV  o   (CM) < 70 o Linear Asymmetry p( ,n  +  ’) : Analysis results (Linear Asymmetry) Unitary model (   =2) Unitary model normalised to agree in soft photon limit Rescattering not included

E  = 320 ±20 MeVE  = 360 ±20 MeV E  = 420 ±20 MeV  o   (CM) < 180 o Linear Asymmetry p( ,n  +  ’) : Analysis results (Linear Asymmetry) Unitary model (   =2) Unitary model normalised to agree in soft photon limit Rescattering not included

p( ,n  +  ’) : Helicity dependence E  =420 ± 20 MeV    CM  = 0 o -70 o   CM  = 70 o -110 o    CM  = 110 o -180 o  = MeV  = MeV  = MeV  in CM frame z =  beam y =  x  beam

 = MeV  = MeV  = MeV    CM  = 0 o -70 o    CM  = 70 o -110 o   CM  = 110 o -180 o p( ,n  +  ’) : Helicity dependence E  =460 ± 20 MeV

  CM  = 0 o -70 o   CM  = 70 o -110 o   CM  = 110 o -180 o  = MeV  = MeV  = MeV p( ,n  +  ’) : Helicity dependence E  =620 ± 20 MeV

p( ,n  +  ’) : Analysis results (Helicity dependence) Helicity shows sin (  dependence Assumption: Fit distributions with sin(  ) - extract amplitude to give helicity asymmetry at phi =90 o

p( ,n  +  ’) : Analysis results (Helicity dependence) Unitary model   = 1   = 3   = 5 Experimental data: E  = 420±20 MeV All   (CM)   (CM) = 90 o  CM  = 110 o -180 o  CM  = 70 o -110 o  CM  = 0 o -70 o Unitary model integrated over appropriate  (CM) ranges (at fixed   (CM) = 90 o )  cir c 

p( ,n  +  ’) : Analysis results (Helicity dependence) Unitary model   = 1   = 3   = 5 Experimental data: E  = 470±20 MeV All   (CM)   (CM) = 90 o  CM  = 110 o -180 o  CM  = 70 o -110 o  CM  = 0 o -70 o Unitary model integrated over appropriate  (CM) ranges (at fixed   (CM) = 90 o )   cir c 

Summary We see a promisingly clean p( ,n  +  ’) signal Extracted linear polarisation observables will give important constraints on the theoretical modelling of radiative pion photoproduction Helicity asymmetry may show promising additional route to gain sensitivity to MDM - future dedicated beamtime ? Need to pass theoretical predictions through detector acceptance before publication (Unitary, CEFT?)

p( ,n  +  ’) : Analysis results E  = 470±20 MeV   (CM) = 90±?? o   (CM) = 90 o Unitary model   = 1   = 3   = 5  CM  = 0 o -70 o  CM  = 70 o -110 o  CM  = 110 o -180 o Unitary model integrated over appropriate  (CM) ranges

p( ,n  +  ’) : Analysis results Only keep data which have overall p( ,n  +  ’) 4-momentum with confidence level > 0.1 All plots: E  = 400 ± 20 MeV

Importance of MDM determination of   (1232) Present knowledge

Outline ● Motivation ● Count rate estimate ●   n (Deuterium data) ●  + detection – preliminary analysis of experimental data

Count rate estimate ● Detection efficiencies   + ~25%  n ~30%   ~90% (p  0    0 ~85%  p ~70%   ~90% ) ● Electron count rate 5x10 5 s -1 MeV -1 ● Tagging efficiency ~50% ● Tagged photon flux 2.5x10 5  s -1 MeV -1 ● 5cm long proton target 2.1x10 23 cm -2 ● Data acquisition live time ~70% ● d  /dE  ~0.5 nb/MeV ● Total count rate ~0.7x10 5 events (with  '= MeV E g = MeV)

p( ,n  +  ’) : Analysis results (Helicity dependence) Unitary model   = 1   = 3   = 5 E  = 420±20 MeV   (CM) = 90 ±?? o   (CM) = 90 o  CM  = 110 o -180 o  CM  = 70 o -110 o  CM  = 0 o -70 o Unitary model integrated over appropriate  (CM) ranges

 + detection in the Crystal Ball : Tracker & Particle-ID detector  ~ 1.5 o  ~ 1.3 o Two cylindrical wire chambers 480 anode wires, 320 strips 2mm thick EJ204 scintillator 320mm

p( ,n  +  ’) : Analysis results E  (MeV)  (barns)*10 -6 Acceptance x10 -3 E  (MeV) Acceptance Acceptance x10 -3

CB – data analysis parameters ● Threshold for cluster finding = 5 MeV ● Individual crystal threshold given by TDC (~1.5 MeV). ● Do not include clusters near to edge of CB -   deg ● Require PID hit within  =±10 deg of cluster centre ● 2-D region cut on plot of PID energy versus CB cluster energy Energy of cluster in CB(MeV) Energy deposited in PID Pion cut Protons

MWPC & Particle-ID in situ

p( ,n  +  ’) : Analysis results E  = 470±20 MeV   (CM) = 90±?? o   (CM) = 90 o Unitary model   = 1   = 3   = 5  CM  = 0 o -70 o  CM  = 70 o -110 o  CM  = 110 o -180 o Unitary model integrated over appropriate  (CM) ranges

 + - Selection of energy tagged events ● Use two-body kinematics  + p → n +  + ● Select n and  + events back-to- back in phi plane ● Calculate  + energy from pion angle  and E  ●  Note that wire chamber tracking NOT included – uncertainty from reaction vertex

Good angular and energy resolution, close to 4  acceptance Setup at MAMI Tracker & Particle-ID    GeV)  ~41cm ~25cm          sin 

Preliminary  + signals ●  E  calculated – E  Measured ● No restriction on shower size

Preliminary  + signals ●  E  calculated – E  Measured ● 4 or less crystals in the  + shower

Preliminary  + signals ●  E  calculated – E  Measured ● 2 or less crystals in the  + shower

Energy resolution ●  Includes uncertainties in reaction vertex, energy loss … as well as intrinsic CB resolution

Fraction with good energy determination ●  Look at fraction of events within

Conclusions ●  + p → n +  + events identified ● Energy tagged  + events indicate CB gives reasonable energy signal ● MWPC software now implemented – further studies ● Develop improved shower shape algorithm which exploits correlation of energy deposits and shape in pion induced shower. ● Look at sampling after pulse - see time dependence of positron decays?

Magnetic moment of the  + via the  + p n +  + +  ' reaction Daniel Watts – University of Edinburgh Ph.D student Richard Codling – University of Glasgow p n ++

Preliminary  + signals in CB ●  Plot E  calculated - E  Measured ● Shift of peak - energy losses? ● Simple shower shape restrictions give noticeable effect on response shape ● Development of better shower algorithms underway No. cryst <4No. cryst < Michel spectrum

 + - Comparison of calculated and measured energies ● Rough tagger random subtraction included ● All angles summed over

Incident  + energy (GeV) Highest cluster energy (GeV) No restriction on shower size Ncryst<3 & no neighbours  + decay Nuclear interaction Geant simulation:  + signals in the CB

Theoretical background ● m - quark spins & currents. ● Test validity of theoretical hadron description in NPQCD ● Long lived particles - precession in B-field ● Short lived - Radiative decay ● Pioneered in p + +p D ++ D ++ g ' ● - proof of principle g +p D + D + g ' p p 0 Energy s pp+pp+ T heory m D + / m N LQCD QCDSR Latt XPT RQM 2.38 NQM 2.73 XQSM 2.19 XB 0.75

Theoretical Background ● Reaction has important background terms ● Different for p p 0 and n p + final states ● Simultaneous measurement also tests p N rescattering D terms Born terms Black lines : g + p ->p + p 0 + g ' Blue lines : g + p ->n + p + + g ' w exchange

Theoretical model ● Effective lagrangian ● Integral s : sensitivity to m D + ● Kinematics can suppress brem. ● Simultaneous unitarised description

Experiment ● CB : 672 * 0.5m NaI TAPS : 540 * 0.25m BaF 2 ● Tracker: MWPC ● PID: 2mm plastic scint. Barrel ● >1 cluster trigger: Measure g + p ->n + p + + g ' and g + p ->p + p 0 + g ' (Expt. A2-1-02) simultaneously.

Neutron detection ● Neutron detection capabilities of CB established (BNL-AGS) p - p p 0 n ● e n ~10-40% ● Dq n < 10 o ; Df n < 20 o Stanislaus, Koetke et. al., NIM A (2001)

p + decay ● p + m + + n m (~26 ns) e + n e n m (~2 m s) ● NaI: t ~1 m s t r ~ 0.1 m s Energy of positron (MeV) 50 0 e+e+ nene nmnm Michel spectrum No. of counts e + n e n m (~2 m s)

 + signals in Crystal Ball ● 150 MeV  + - isotropic ● Spectra sensitive to time over which energy deposits are recorded ● See signal at T p but with background Michel spectrum t~ infinite Energy deposited in Ball (GeV) Nuclear intn.  + absorbed Nuclear intn. t< 1 m s !!

Neutron detection in the CB Neutron kinetic energy (MeV) Detection efficiency Neutron  difference (deg)  ~5 o

E  = 320 ±20 MeVE  = 360 ±20 MeV E  = 420 ±20 MeV  o   (CM) < 70 o  o   (CM) < 110 o  o   (CM) < 180 o Linear Asymmetry p( ,n  +  ’) : Analysis results (Linear Asymmetry)

p + signals in CB ● Simple cut on shower size. N cryst (HE clust) <3 & No neighbouring clusters ● Get peak with manageable background! ● Eff ~25% at 100MeV

Summary ● Simultaneous measurement of n p + g ' with p p 0 g ' improves confidence in model dependent extraction of m D + ● Measurement requires no extra beam time ● Establishing p + detection capabilities of CB - opens perspectives for other future measurements