1. Transversal Target Asymmetries in Threshold
p0 Photoproduction
Peter-Bernd Otte, Annual SFB School
Boppard, October 2014

2. Photoinduced Reactions on Protons
Introduction
Photoinduced Reactions on Protons
Produktion verstehen von \gamma p  X oder \pi+ oder \pi0:
 Meine Analyse: „Bereich der Schwelle ganz genau untersuchen“
MAMI geht bis sqrt(s) = 2,09 GeV, d.h. alles wird abgedeckt.
s = 2*m_p+E_g+m_p**2
Schwelle bei (ein- und auslaufendes Proton hat Impuls = 0):
2E_Schwelle*m_p = m_pi^2 + 2*m_p*m_pi
Ethr = 145MeV (Eg)
MAMI energy range

3. Pion Photoproduction Introduction S- und P-wave multipole amplitudes
Models: large differences, although they all describe the existing (published) data
Determination of multipole amplitudes  model independent

4. Mainz Microtron 1) Experimental Setup cw electron accelerator sources:
100% duty factor
sources:
unpol.: Imax = 100µA
pol.: Imax = 20µA with Pe≈80%
MAMI-B/C:
MeV DE = 13keV
MeV DE = 110keV
RTM = Race Track Mikrotron (Mehrfaches Durchlaufen einer Beschleunigungseinheit)
HDSM = Harmonic double sided Mikrotron
MAMI C seit 2006

5. Photon beam 1) Experimental Setup
Photon tagging spectrometer („Glasgow-Mainz-Tagger“):
e- Bremsstrahlung
momentum determination of scattered e-
Eg = E0-Ee
coverage: 7..93% E0
DE ≈ 1MeV
flux: 4*105 g/s/MeV
Polarised photons
e- long. pol.  circ. pol.
Crystal  lin. pol.
Helicity transfer e- -> g
Rückstoß auf Atomkern vernachlässigbar.
lin. pol. e-: 100µm Diamant
electrons E0
g beam

6. Target Cell 1) Experimental Setup Target cell Butanol C4H9OH
2 cm long and 2 cm diameter
or Carbon foam / LH2
Rückstoß auf Atomkern vernachlässigbar.
lin. pol. e-: 100µm Diamant
electrons E0
g beam

7. Detector System 1) Experimental Setup Target cell Butanol C4H9OH
4π photon spectrometer (n and charged particles as well)
Target cell
Butanol C4H9OH
2 cm long and 2 cm diameter
or Carbon foam / LH2
g beam

8. Detector System 1) Experimental Setup Particle ID (DE/E)
4π photon spectrometer (n and charged particles as well)
Particle ID (DE/E)
with thin plastic scintilators
20°<q<160° : barrel of 24x
2°<q<20° : 384x
g beam

9. Detector System 1) Experimental Setup optional: Threshold C Detector
4π photon spectrometer (n and charged particles as well)
optional:
Threshold C Detector
MWPC
2 cylindrical chambers
charged particles only
g beam

10. Detector System 1) Experimental Setup TAPS 366 BaF2 crystals (PMTs)
4π photon spectrometer (n and charged particles as well)
TAPS
366 BaF2 crystals (PMTs)
12 radiation lengths
1°<q<20° (3%)
self triggering
Crystal Ball
672 NaI(Tl) crystals (PMTs)
16 radiation lengths
20°<q<160° (94%)
s ≈ 2-3°
self triggering
g beam

11. 1) Experimental Setup
Detector System
photons
TAPS CB

12. Target: Mainz-Dubna Pol. Frozen Spin Target
1) Experimental Setup
Target: Mainz-Dubna Pol. Frozen Spin Target
Target:
See talk J. Linturi tomorrow!
„Dynamic Nucleon Polarisation“
polarise free electrons (radical)
transfer pol. to protons
P0 ≈ 90% (only H), P=P0e-t/t
3He/4He dilution cryostat
T = 26mK & B = 0.44T for t≈2*10³h
H spin:
transversal or
longitudinal
super conducting saddle coil
DNP: Dynamic Nucleon Polarisation („70GHz Mikrowellen System“):
Electrons polarised by microwaves and transfer their polarisation to the protons.
Polarising field B = 2.5 T
spin of nuclei = 0
background = quasi free and coherent \pi0 production

13. Target Material 1) Experimental Setup Unpolarised Polarisable Material
unpol. liquid Hydrogen (lH2)
@ low T: parahydrogen (spins: )
not polarisable
Polarisable Material
Butanol
(advantages: DNP, high P, large t, radiation hard, large d, high f)
only H polarised
unpol. BG
I(12C) = I(16O) = 0
DNP: Dynamic Nucleon Polarisation („70GHz Mikrowellen System“):
Electrons polarised by microwaves and transfer their polarisation to the protons.
Polarising field B = 2.5 T
spin of nuclei = 0
background = quasi free and coherent \pi0 production

14. Butanol Target: important properties
1) Experimental Setup
Butanol Target: important properties
Dilution Factor
Filling Factor
not trivial!
Best mess. method: melting  f=60(3)% for butanol
butanol balls
Effective d(q,E) necessary
Mandatory: decent statistics
 Does not work in threshold region

15. Photoinduced Reactions on Protons
2) Photo p Production
Photoinduced Reactions on Protons
D region
2nd and 3rd resonant region
threshold
Produktion verstehen von \gamma p  X oder \pi+ oder \pi0:
 Meine Analyse: „Bereich der Schwelle ganz genau untersuchen“
MAMI geht bis sqrt(s) = 2,09 GeV, d.h. alles wird abgedeckt.
s = 2*m_p+E_g+m_p**2
Schwelle bei (ein- und auslaufendes Proton hat Impuls = 0):
2E_Schwelle*m_p = m_pi^2 + 2*m_p*m_pi
Für die 2. und 3. Resonanz-Region werden bis hin zu F-Wellen von Relevanz.
Ethr = 145MeV (Eg)
MAMI energy range

16. Amplitudes in Pion Photo Production
2) Photo p Production
Amplitudes in Pion Photo Production
Spin observables Oi (q, W)
Legendre expansion up to Lmax
Legendre coefficients Aik bilinear combinations of multipoles, e.g.:
Omelaenko (1981): 5 observables necessary for complete experiment
Talk from
Y. Wunderlich
tomorrow

17. Threshold p0 production
2) Photo p Production
Threshold p0 production
Beam / Target Polarisation:
In threshold region (Eg= MeV):
PWA (L=1): only S- and P-wave amplitudes E0+ & M1+, M1-, E1+
near threshold assumption: E0+ complex, all other real
but Im(E0+) is fixed by Fermi Watson theorem (unitarity):
necessary: determine 4 real numbers from experiment
independent. meas. of Im(E0+) requires add. observable  “complete data base”
E0+ etc. are multipoles
\beta = cusp parameter, real value around 3
a = exchange scattering length
Q = Pion momentum

18. p0 threshold production
2) Photo p Production
p0 threshold production
2001: First measurements with TAPS (A. Schmidt et al., PRL2001, )
lin. pol. photons
unpol. H target
s0, but only one S point
Physical Review Letters
expand Observable in a cos(\theta) series for energy-angle separation
direct access to real part / imaginary part of E0+
also possible in treshold region: test dominance at D-Waves with F

19. p0 threshold production
2) Photo p Production
p0 threshold production
2001: First measurements with TAPS (A. Schmidt et al., PRL2001, )
lin. pol. photons
unpol. H target
s0, but only one S point
2008: high precision measurement s0, S with CB/TAPS (D. Hornidge et al., PRL2013, )
 Re(E0+, M1+, M1-, E1+)
example:
Physical Review Letters
expand Observable in a cos(\theta) series for energy-angle separation
direct access to real part / imaginary part of E0+
also possible in treshold region: test dominance at D-Waves with F

20. P-Waves Amplitudes 2) Photo p Production
deviations for large E  D res.
Messung stimmt überall gut mit der Theorie überein!
Geringer stat. Fehler
Lediglich für große E stimmt die Näherung für M1+ nicht mehr, da hier die Ausläufer der Delta-Res. greifen.

21. S wave amplitude 2) Photo p Production direct measurement via:
Fermi Watson theorem or measurement
(p+n thr.)
Direkte Messung von ImE0+ ab \pi+ Schwelle
unitarity cusp

22. Threshold p production
2) Photo p Production
Threshold p production
add measurement with unpol. photons & trans. pol. target: (2010, 2011)
Measurement of with T  b and known E0+
Direct measurement of Im(E0+)
Check consistency with S measurements (2008)
Physics Review Letters
Ladungsaustausch messbar
expand Observable in a cos(\theta) series for energy-angle separation
direct access to real part / imaginary part of E0+
also possible in treshold region: test dominance at D-Waves with F ?
 Test strong isospin breaking

23. Analysis: Definitions
2) Photo p Production
Analysis: Definitions
Relevant:
Determination of Obs.: asymmetry
Event by event selection into 2 bins: „+“ and „-“
for T: for F:
p spin (incoming) p0 g
Was ist der T? „Target Asymmetrie“  Wie wird T gemessen? Erklären!
Wechsel von pT (10-5Hz) nicht zwingend notwendig, nur zur Reduktion von falschen Asymmetrien durch eine unterschiedliche Detektor-Akzeptanz
(T & F gleichzeitig bestimmbar) p
for T’:
Physical results <>0 expected
=0 expected

24. Analysis: 3 Methods 2) Photo p Production
Objective: get T of pure pol. hydrogen
Diluted asymmetry on butanol target:
Methods:
determine (works only >220MeV)
denominator  normalise with simulation & f
denominator  normalise with unpol. Measurement & f
“Ingredients:”
butanol + carbon meas.
butanol + simulation
butanol + hydrogen
Dilution Factor: Fermi motion and FSI  need for eff. Dilution factor
 Same results expected…

25. Analysis details 3) Results Butanol Signal missing mass /MeV
2g  p0 reconstruction (no problem)
proton detection (problematic, does not emerge the target)
 Technique: Missing Mass
(half of the total data set, all energies and q)
Signal
Um zu zeigen, dass die Analyse die Reaktion identifizieren kann.
Kombinatorischer Untergrund bei der 2g-Masse zu sehen.
missing mass /MeV

26. Photoinduced Reactions on Protons: Measured
3) Results
Photoinduced Reactions on Protons: Measured
Threshold & D region
2nd and 3rd resonant region
Produktion verstehen von \gamma p  X oder \pi+ oder \pi0:
 Meine Analyse: „Bereich der Schwelle ganz genau untersuchen“
MAMI geht bis sqrt(s) = 2,09 GeV, d.h. alles wird abgedeckt.
s = 2*m_p+E_g+m_p**2
Schwelle bei (ein- und auslaufendes Proton hat Impuls = 0):
2E_Schwelle*m_p = m_pi^2 + 2*m_p*m_pi
S. Schumann
P. Barrientos
P. Otte
Analysis by:
V. Kashevarov
MAMI energy range

27. Status & Results for T and F
Black = But/Sim red = But/H green = But/C
Blue = MAID
All analyses completed
possible overview via Legendre Polynoms
330MeV
330MeV
𝐴⋅𝜎=𝜌⋅sin⁡(𝜃) 𝑝 0 + 𝑝 1 cos 𝜃 + 𝑝 cos 2 𝜃 −1 2 +…

28. Systematic uncertainties
3) Results
Systematic uncertainties
Sources, O(~%)
different analysis techniques (e.g. carbon subtraction)
Helicity dep. photon flux (A=0,005)
Detector asymmetries
Goodness of detectors maintenance
butanol target, contribution of I=1/2 atoms
C13 (1,1%) and O17 isotopes.
F4 in container
(He3 for cooling)
limits resolution
future investigations
Hierzu beantworte ich gerne am Ende Fragen.

29. 3) Results
Systematic Errors
ssys(T) = global Difference between analyses T’ ssys(F) = global Difference between analyses DA(beam flux) %
Error in g flux (tagger poblems)
3,0
Unstable detectors and electronics f
2,0 (?)
P(target)
2,0
P(beam)
2,7
total global sys. Error
(indep. of q, E)
5,1 (T)
5,8 (F)
depends on q, E

30. Thank you Summary / Outlook 4) Outlook Goal: model independent PWA
from threshold up to W=2GeV
Using techniques from Stahov?  talk tomorrow
Measured & analysed
Pion photoproduction: S, T, F
Actual work in group
measurements with “n”
longitudinal pol.: G, E (in analysis)
ditto:
other channels
Ideal: remeasure T
with all improvements we learned
Thank you
More channels like \eta 2\pi_0 werden untersucht