Exclusive  production
with muons
0
'

*
p
 0
p'
Massimo Bettinelli
(LMU, Muenchen)
On Behalf of the
COMPASS Collaboration
Photoproduction
E
• Aston et al., CERN SPS (Phys. Lett.
92B (1980)), E =20-70 GeV, assuming
s-channel helicity conservation
(SCHC)
b1 (1235) , J PC  1 
 mixture of
and a dominant  ' (1250) ,
1
• Not supported by results for linearly
polarized photons:
– Atkinson et al., CERN SPS (Nucl. Phys.,
B243 (1984), < E >~30 GeV
– Brau et al., SLAC (Phys. Rev. D37 (1988)),
E = 20 GeV
non-SCHC production,
b1 dominant
Annihilation
pn    0, Crystal Barrel (Nucl. Phys. A740 (2004))
suggest 3  ' states at:
– 1200 MeV,
– 1450 MeV,
– 1700 MeV,

3
decay dominant
(non - )
“
“
“
“

Interpretation
Reviewed by Donnachie and Kalashnikova, HADRON 01,
 
including results from e e and  -decay
Dominant 3 P0 model decay
13 D1
mixed
hybrid
23 S1
13 S1
1700
1450
1250
770
1
a1
h1
a1

NOT PDG accepted
COMPASS experimental setup
~ 250 physicists from 28 institutes
use 160 GeV/c polarised muon
beam from the CERN SPS
 filter
Trackers:
Si, SciFi, Micromegas,
GEM, DC, Straw, MWPC
Magnets
Trigger
hodoscopes
Polarised
Target (Li6 D)
ECAL2+
HCAL2
RICH
Event selection
• 8 weeks in 2004 - longitudinal target
polarization
• best primary vertex with identified
incoming beam track and scattered
muon + 2 oppositely charged tracks
• no RICH information used; pion mass
hypothesis
• 4 and only 4 ECAL2 clusters in event
(no associated charged tracks; cluster
energy E > 1 GeV)
• pi0 selection: 120  m( )  150 MeV
and    0.025 rad
• omega selection:
750  m(   0 )  815 MeV
• exclusivity cut:
-6 < missing energy < 4 GeV
Final sample:
0
943 events with ONE and only ONE

'




120  m( )  150

 
2
M miss
 M P2
Emiss 
2M P
Missing energy vs.  mass
0
Exclusive events
 invariant mass
for exclusive events
0
934 events
Acceptance variation < 20%
Mean value ~1250 MeV
Width ~300 MeV
   0 vs.    0 0 invariant mass
for exclusive events



p   p 
2
max
 is flat outside exclusivity peak
Background from 3 (non   )
< 12%
Dalitz plot and  distribution
Monte Carlo generated
  0
     decay
X
T1  T2
Q 3
T0 1
Y  ;
Q 3
Q  T0  T1  T2 ;
pion kinetic
energies
T0 , T1 , T2
  2
p1  p2
   2
p1  p2 MAX
P

  0

(
J

1
)


  :
For
intensity ~  .
Kinematic distributions
for final sample
Q 2  0.1 GeV 2
E *  90 GeV
Momentum-transfer-squared distribution (not released): diffractive (exponential) shape
Decay angular distributions
*

(in p overall c.m. system)




 0
in


n

 0 c.m.

Omega decay plane in
omega c.m.
Assumption:
SCHC (s-channel helicity conservation)
b1 , 1 I ( ) ~ 1  x cos 2 
 ' , 1 I ( ) ~ 1  cos2 
I ( ) ~ sin 2 
I ( ) ~ 1  cos 2 
x  ( D / S amplitude ratio ) 2  0.07
0
MC simulation for     
0


• Estimated acceptance 0.5% total (geom. x interaction x
reconstruction x selection);
10% ECAL geometry only
•  dependence strongly,  dependence only weakly affected
Theta distribution (SCHC assumed)


1
1
•
1 cos 2  and sin 2  shape for 1 and 1 , resp., roughly
maintained

• Not released data resemble 1 case
Correlation with  scattering plane
• SCHC: W ( )  1   cos 2
(1)

•  = angle of “spin analyzer” a vs.  scattering plane
•  = linear
 polarization0of  *
• a  n  p for ( )1
 
p 
•
“ (  )  (770)
(   )  (770)
muoproduction at COMPASS
Longitudinal photon contribution
~0.1 at Q 2  0.1 GeV 2,
increasing with Q 2
cos 2
  
• Non released data for a  n  p show same
modulation, but with smaller amplitude
(1) valid for transverse photons (helicity  1 ),
see Schilling, Wolf, Nucl. Phys. B61 (1973) 381 and quoted photoproduction experiments
Conclusions
•

0
production with ~90 GeV virtual photons
observed in  scattering
• broad (~300 MeV) peak at m~1250 MeV, in
agreement with previous photoproduction
experiments
• preliminary results on angular correlations
indicate dominant J PC  1  contribution – if
SCHC holds
• appreciable increase of statistics expected with
2006 and 2007 data