[Frost] Prospects for Double Pions in Deuterized Butanol

[Volker Crede]

Dear Eugene,

you suggested this morning the possibility to study double-pion production 
with a deuterized butanol target. I have attached some pictures from g9a 
using a lineary-polarized beam with a coherent edge at 1.3 GeV; only one 
target orientation has been used (L-+,<=) to make these distributions. For 
this polarization configuration, we have a total of 5 observables (all 
degrees of polarization are set to 1.0):

  I = I_0 ( ( 1 + P_z ) +
       sin [ (2 beta) (I_s + P^s_z) ] + cos [ (2 beta) (I_c + P^c_z) ] )

The picture 'I_s_energyIndex13.eps' shows (very, very preliminary) the 
combination of (I_s + P^s_z), i.e. the combination of the beam asymmetry 
I_s (that Chuck Hanretty has been extracting from g8b data) and the new
beam-target observable P^s_z. The photon energy is [1100, 1150] MeV; the 
observable is plotted versus phi*, which is the azimuthal angle of the pi+ 
in the rest frame of the two mesons. The different distributions show the 
binning in the corresponding cos(theta*) variable (pretty much the same 
thing that Chuck always shows). It starts out very flat, but polarization 
effects are clearly visible at larger values for cos(theta*).

The other two pictures show the missing proton peak integrated over all 
bins (only pi+ and pi- detected) as well as the lab_beta modulation for 
just 0.1 < cos(theta*) < 0.2 and the corresponding fourth data point in 
there ... a very fine binning.

These are distributions for double-polarization and with a pretty fine 
binning in three of the 5 independent variables. The statistics is very
good. No background subtraction has been performed and there is still a 
lot of background involved (of the order of 50%). The total cross section 
for two-pion production off the proton is of the order of 40-60 microb for 
this energy range; the cross sections off the neutron are about 60-70% of 
the proton cross sections ... still pretty big. Most important, the 
attached distributions are based on just 35 hours of data-taking ... less 
than two days. The total number of events for PARA is 179,647,134 and for 
PERP is 163,187,819.

If we decide to go with just 1.1 GeV or 0.9 GeV coherent-edge position, 
the count rates should even be better. This corresponds to the 1500-1700 
MeV mass region, very interesting to study for example N* decays into 
Delta pi, which are poorly understood for many states. Delta-pi decays in 
D-wave seem to be stronger or equal in strength to Delta-pi decays in 
S-wave ... not expected from naive phasespace arguments. This could be 
part of a physics motivation.

Best wishes

    Volker

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