nDVCS measurement with
BoNuS RTPC
M. Osipenko
December 2, 2009,
CLAS12 Central Detector
Collaboration meeting
Two Alternatives
Neutron detector
BoNuS detector
e  D( p  n)  e    n  ( p) e  D( p  n)  e    p  (n)
Neutron is detected in range:
polar angles θ from 35 to 145º,
full azimuthal angle φ coverage,
3-momentum pn=0.3-1 GeV/c,
resolution Δθn=1.5º
resolution Δφn=12º
resolution momentum Δpn/pn=5%
Proton is detected in range:
polar angles θ from 35 to 145º,
full azimuthal angle φ coverage,
3-momentum pp=70-200 MeV/c,
resolution Δθp=3º
resolution Δφp=1.5º
resolution momentum Δpp/pp=few %
y
x
z
2
In OPE approximation:
Kinematics
 V (q)  D( D)   (q)  n( Pn)  p( Pp )
D  Pp  Pn
Impulse Approximation:
Pp2  M p2 Pn2  M n2 Pp  Pp
Pn   Pp
Pn  Pn  (q  q) - neutron detector
Pn  Pn  (q  q) and Pn   Pp - BoNuS detector
Beyond Impulse Approximation:
Pp2  M p2 Pn2  M n2 Pp  Pp
Q 2  q 2
t  (q  q)
- unaffected
2
- unaffected, but resolution is worse than
q
x
-affected
2qPn

2
f
e’
e
leptonic
planephoton
plane
p’
t   Pn  Pn 
- affected, frame
dependent
2
3
ISI & FSI
Main effect: obtained DVCS
cross section is on the offshell neutron, region of
large-x is critical.
H , E ( Pn2  M n2 , x, Q 2 , t )
Main effect: mixing of
different physical kinematics
in each measured point, region
of low-t is critical.
x 2
H
,
E
(
, Q , t ) f ( )d

4

Hardware & FSI
Both setups allow to suppress FSI via
kinematic cuts, provided that neutron is
fully reconstructed (momentum and
angles).

Pp
D
BoNuS
nDVCS
Inclusive BoNuS FSI
FSI is small when
Ciofi degli Atti and Kopeliovich, Eur. Phys. J. A17(2003)133
 0
Pp3  0
p  
Pp  0   
1
2
5
Resolutions in IA


M n2  t  2 Pn (q  q)
- neutron detector
M n2  t  2 Pn (q  q)
- BoNuS detector
Pn   Pp
Neglecting both nucleon momenta with respect to masses and assuming
struck neutron going forward one obtains:
M n2 ~ q Pn ~ 0.02 q
M n2 ~ q  Pp cos  qp  Pp sin  qp  qp  ~ 0.001 q


M n2
~ 20
2
M n
BoNuS gives better resolution on missing mass.
The calculation likely overestimates the ratio of resolutions, but the
conclusion sounds sensible.
6
MC simulations
Naïve geometrical simulations were performed (no efficiency or CLAS
acceptance). The physics model is approximated as a simple function
factorized in 4 independent dependencies:
1) 1/y in the range y=0.1-1
2) 1/Q4 – in the range Q2=1-4 GeV2
3) ebt with b=5 GeV-2 in range from tmin to 4 GeV2
4) Flat φ distibution from 0 to 2
5) Fermi motion with kF=120 MeV Fermi gas model
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MC
8
BoNuS
proton
long
target
Cuts
20 cm long
10 cm Rin
8% effect
nDVCS
neutron
66 cm long
R=33 cm
θmin=45º
?
12% effect
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Results
We are interested in ratio of yields:
BoNuS
N pD
N
nDVCS
nD

LpD ApD E pD
LnD AnD EnD
LnD  1035 cm 2 s 1 - standard CLAS12 luminosity
old
pD
L
2 1
 0.5 10 cm s
34
34
2 1
Lnew

2

10
cm
s
pD
EnD  10%
E pD  100%
AnD  62%
ApD  34%
N
BoNuS  old
pD
nDVCS
nD
N
5 10  0.34 1
 35
 0.27
10  0.62  0.1
33
BoNuS  new
N pD
nDVCS
N nD
Assume 160 nA
beam current,
target thickness is
12.6 mg/cm2 (20
cm x 7 bar
pressure D gas)
2 1034  0.34 1
 35
 1.1
10  0.62  0.1
EG6 run on ~20 mg/cm2 target at 130nA with DAQ rate 2.5 kHz.
DAQ rate limiting BoNuS RTPC was not estimated here (2 kHz for above
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conditions is mentioned in NIM A592 for 6 GeV beam energy).
Summary
1.
Detection of neutron or spectator proton are equivalent as far Impulse
Approximation is concerned,
2. Detection of spectator proton allows to suppress possible FSI effects
by the angular cut (with relative loss of statistics),
3. The expected yields of good events for these two scenarios are similar
in the ORDER OF MAGNITUDE ESTIMATES.
Desirable Improvements
1.
2.
3.
4.
5.
6.
Physical cross section in the Monte Carlo model, better momentum
distribution in deuteron,
Realistic CLAS12 acceptance for e- and ,
Z-vertex distribution for long target in BoNuS case,
Final setup of nDVCS option,
Realistic resolutions for both detectors,
Physical background to estimate losses in channel identification cuts.
11
The Tile Neutron Detector
Light is collected at
the back with a
large R optic fibre
The geometry has
been implemented
in Geant4
Neutron
incoming
direction
12
The Geant4 Simulations
Scintillator without
reflective wrapping
Scintillator with
reflective wrapping
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The Geant4 Simulations
With 0.9 reflectivity
1.5% photons reach
the optic fibre
Optic fibre
transmission not yet
implemented
Timing has not yet
studied
Considering 5 MeV
threshold, one may
expect
50000/2x0.015x0.3x0.2
=20 photoelectrons
(assuming 30% of
photons arriving to the
fiber entrance window
at any angle are
transmitted to PMT)
Timing resolution ~
1ns/Sqrt(20)~250ps
For Pn=550 MeV/c bc equal
to velocity of light in
plastic, and therefore
indetermination in the
interaction point cancels
out.
14