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 %

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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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MC

9

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.