nDVCS measurement with BoNuS RTPC
M. OsipenkoDecember 2, 2009,
CLAS12 Central DetectorCollaboration meeting
2
z
y
x
Two Alternatives( ) ( )e D p n e n p ( ) ( )e D p n e p n
Neutron detector BoNuS detector
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=1.5ºresolution 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=3ºresolution Δφp=1.5º
resolution momentum Δpp/pp=few %
3
Kinematics( ) ( ) ( ) ( ) ( )V n pq D D q n P p P
2 2p pP M
2( )t q q
p pP P
p nD P P Impulse Approximation:
- affected, frame dependent
-affected
2 2Q q
2
2 n
qx
qP
In OPE approximation:
- unaffected
Beyond Impulse Approximation:
2
n nt P P
2 2n nP M
( )n nP P q q ( )n nP P q q
- neutron detector
n pP P
and - BoNuS detector
n pP P
2 2p pP M 2 2
n nP M p pP P
- unaffected, but resolution is worse than
leptonic planephoton
planep’
e’
e
4
ISI & FSI
2 2 2, ( , , , )n nH E P M x Q t
Main effect: obtained DVCS cross section is on
the off-shell neutron, region of large-x is critical.
Main effect: mixing of different physical kinematics
in each measured point, region of low-t is critical.
2, ( , , ) ( )x
H E Q t f d
5
Hardware & FSI
10
2pP
3 0pP 0
BoNuSnDVCS
p
pP
D
Ciofi degli Atti and Kopeliovich, Eur. Phys. J. A17(2003)133
Inclusive BoNuS FSI
FSI is small when
Both setups allow to suppress FSI via kinematic cuts, provided that neutron is fully reconstructed (momentum and angles).
6
Resolutions in IA
2 ~ ~ 0.02n nM q P q
2
2~ 20n
n
M
M
2 2 ( )n nM t P q q
2 2 ( )n nM t P q q
- neutron detector
- BoNuS detector
2 ~ cos sin ~ 0.001n p qp p qp qpM q P P q
n pP P
Neglecting both nucleon momenta with respect to masses and assuming struck neutron going forward one obtains:
BoNuS gives better resolution on missing mass.
The calculation likely overestimates the ratio of resolutions, but the conclusion sounds sensible.
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MC simulationsNaïve 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=0.1-12) 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 25) Fermi motion with kF=120 MeV Fermi gas model
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CutsBoNuSproton
nDVCSneutron
20 cm long10 cm Rin
66 cm longR=33 cm
θmin=45º?
long target
12% effect
8% effect
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Results
35 2 110nDL cm s
100%pDE
34 2 10.5 10oldpDL cm s
10%nDE
BoNuSpD pD pD pD
nDVCSnD nD nD nD
N L A E
N L A E
We are interested in ratio of yields:
34 2 12 10newpDL cm s
- standard CLAS12 luminosity
62%nDA 34%pDA
33
35
5 10 0.34 10.27
10 0.62 0.1
BoNuS oldpD
nDVCSnD
N
N
34
35
2 10 0.34 11.1
10 0.62 0.1
BoNuS newpD
nDVCSnD
N
N
Assume 160 nA beam current,
target thickness is 12.6 mg/cm2 (20 cm x 7 bar pressure D gas)
DAQ rate limiting BoNuS RTPC was not estimated here (2 kHz for above conditions is mentioned in NIM A592 for 6 GeV beam energy).
EG6 run on ~20 mg/cm2 target at 130nA with DAQ rate 2.5 kHz.
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Summary1. 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 Improvements1. Physical cross section in the Monte Carlo model, better
momentum distribution in deuteron,2. Realistic CLAS12 acceptance for e- and ,3. Z-vertex distribution for long target in BoNuS case,4. Final setup of nDVCS option,5. Realistic resolutions for both detectors,6. Physical background to estimate losses in channel identification
cuts.
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The Tile Neutron DetectorThe Tile Neutron Detector
Neutron incoming direction
Light is collected at the back with a large R optic fibre
The geometry has been implemented in Geant4
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The Geant4 SimulationsThe Geant4 SimulationsScintillator without reflective wrapping
Scintillator with reflective wrapping
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The Geant4 SimulationsThe Geant4 SimulationsWith 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 c equal to velocity of light in plastic, and therefore indetermination in the
interaction point cancels out.
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