Exclusive r0 Production at COMPASS
Etienne Burtin, CEA-Saclay
on behalf of the COMPASS Collaboration
-
Physics motivations
Experimental setup
Data analysis
Preliminary results
Baryons 2004, Palaiseau
October 28, 2004
Physics motivations
m’
m
g*
N
r0
pp+
N’
•
Diffractive processes : Pomerons or Reggeons
•
r0 Production : test of s-Channel Helicity
Conservation (SCHC)
•
Hard regime : Generalized Parton Distributions
COMPASS experimental set-up
~ 250 physicists from 28 institutes
use 160 GeV/c polarised m
beam from the CERN SPS
m filter
Trackers:
Si,SciFi,Micromegas,
GEM,DC,Straw,MWPC
Magnets
Trigger
hodoscopes
Polarised
target
H&E Cals
RICH
COMPASS @ Baryons 2004
K. Kurek : DG from high pT
Y. Bedfer : Spin physics results
R. Webb : Collins and Sivers asym.
Selection of the sample
m’
m
• Topology
N
– 1 incident and 1 scattered m
– only 2 hadrons of opposite charge
– vertex in the 6LiD polarized target
g*
r0
pp+
N’
• Kinematical conditions:
– n > 30 GeV
– Em’ > 20 GeV
– Q² > 0.01 GeV²
Focus on Incoherent Exclusive r0 production...
Incoherent exclusive r0 production
Mpp
Emiss
pt ²
Assuming both hadrons
are p:
0.5 < Mpp< 1 GeV
Background processes
rejected :
Exclusivity of the reaction
Emiss=(M²X-M²N)/2MN
-2.5 < Emiss < 2.5 GeV
Incoherent production:
0.15 < pt²< 0.5 GeV²
scattering off a
quasi-free nucleon
6LiD
Kinematical domain of final sample
after
event selection
10-4
10-3
10-2
10-1
Q² [GeV²]
2002 : this analysis
~ 700,000 evts (whole range)
~ 20,000 evts (Q²>1GeV²)
1
10
High statistics from
g-production to hard regime
2003 and 2004:
- trigger coverage improved
- enlarged acceptance at high Q² (up to 25 GeV)
- ~ 4 times more beam time
r0 angular distributions
Definitions :
in g*-p CM frame
p’
m’
g*
m
F
p
f
r
p-
q
p+
in r rest frame
The angular distribution
W(cosq,f,F)
depends on the r0 spin density matrix elements
 23 (15) measurable with polarised (unpol.) beam
This analysis :
one-dimension distributions
 test of s-Channel Helicity Conservation SCHC
studied by NMC,Zeus,H1,Hermes,E665
Determination of the Acceptance
• Use DIPSI event generator of exclusive meson production
developed for ZEUS.
• Fed through the GEANT-based simulation and reconstruction
programs of COMPASS.
• Same cuts as real data applied.
• Calculation of the acceptance
• Estimation of the effect of smearing and binning
– reject Q²<0.01 GeV² data due to high smearing
Angular distributions
0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q²
Preliminary :
- Corrected
for Acceptance
- Background
not subtracted
Statistical error only, limited by MC (not last Q² bin)
Measurement of r
Distribution :
3
W(cos q )  (1 - r0004 ) + (3r0004 - 1)cos 2θ
4


Spin density matrix elements:
T01 + (ε + δ) T00
2
r0004 
2
N T (1 + (ε + δ)R)
R  σ L /σ T
2
2
N T  T11 + T-11 + T01
Tlg are helicity amplitudes
meson photon
2
04
00
Determination of Rr° =sL/sT
If SCHC holds :
only T00≠0
T11≠0
Then :
2002
r0004
σL
1
R

σ T (ε + δ) 1 - r0004
2003
Impact on GPD :
sL is dominant at high Q²
factorisation only valid for sL
Measurement of r
Distribution :
W (f ) 
1
[ 1 - 2r1-041 cos 2f
2p
+ 2Im r13-1Pm 1 -  2 sin 2f ]
beam polarisation
if SCHC holds:

)
Re (T11T-*11 ) - (ε + δ) T10T-*10
r 
0
N T (1 + (ε + δ)R)
04
1-1
Im r13-1  ...  0
04
1-1
and Im r
3
1-1
Measurement of r 11-1
Distribution, if SCHC holds:

1
W (ψ ) 
1 + 2ε  r 11-1 cos 2ψ
2π
with

 f -F
if in addition, natural parity
exchange in the t-channel holds:
r
1
1-1

1
 1 - r 04
00
2
)
Angular distributions (2)
0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q²
Conclusions and perspectives
• Incoherent Exclusive r0 production channel under investigation:
– high statistics from photo-production to the hard regime
– larger acceptance at high Q² in 2003 and 2004 (up to 25 GeV²)
• Preliminary results on Spin Density Matrix Elements:
–
–
–
–
only statistical errors were shown (dominated by MC)
in agreement with other experiments
weak violation of SCHC observed
systematics to be evaluated especially on background
• Spin asymmetries for longitudinally and transversally polarised target
will also be determined.
• Exclusive production of f and J/Y also observed and will be studied
r0 mass distribution
Söding parametrization:
mpp mr (mpp )
2
dN
 A. 2
+ B + f PS
2
dmpp
mpp - mr + im r (mpp )
3
 q  mr
(mpp )  r  
 q0  mpp
No acceptance
corrections yet !
(subtracted)