S. Niccolai, IPN Orsay CLAS12 Workshop, Genova, 2/27/08

Conseil ScientifiqueIPN Orsay, 12/11/09

INFN Frascati, INFN Genova,

IPN Orsay,LPSC Grenoble

SPhN Saclay University of Glasgow

Deeply Virtual Compton Scatteringon the neutron at JLab with CLAS12

• GPDs and nDVCS

• JLab-Hall A measurement

• Neutron kinematics for nDVCS

• Central Neutron Detector (CND) for CLAS12

• Simulations: expected performances of CND

• Technical challenges

• Ongoing R&D in Orsay

• Cost estimate

M. Guidal, S. Niccolai, S. Pisano(groupe PHEN/JLab)

B. Genolini, T. Nguyen Trung, J. Pouthas (R&D)

Deeply Virtual Compton Scattering and GPDs

e’t

(Q2)

eL*

x+ξ x-ξ

H, H, E, E (x,ξ,t)~~

p p’

« Handbag » factorization validin the Bjorken regime:

high Q2 , (fixed xB), t<<Q2

• Q2= - (e-e’)2

• xB = Q2/2M=Ee-Ee’

• x+ξ, x-ξ longitudinal momentum fractions• t = (p-p’)2

• xB/(2-xB)

0,x ),( Ex q21 Hxdx qJG =

21J q

1

1)0 ,, (

Quark angular momentum (Ji’s sum rule)

X. Ji, Phy.Rev.Lett.78,610(1997)

Vector: H (x,ξ,t)

Tensor: E (x,ξ,t)

Axial-Vector: H (x,ξ,t)

Pseudoscalar: E (x,ξ,t)

~

~

conserve nucleon helicity

flip nucleon helicity

«3D» quark/gluonimage of

the nucleon

H(x,0,0) = q(x)

H(x,0,0) = Δq(x) ~

Extracting GPDs from DVCS spin observables

LU ~ sin Im{F1H + (F1+F2)H +kF2E}d~

Polarized beam, unpolarized proton target:

Unpolarized beam, longitudinal proton target:

UL ~ sinIm{F1H+(F1+F2)(H + … }d~

= xB/(2-xB)k=-t/4M2

Hn, Hn, En

Kinematically suppressed

Hp, Hp

~

A =

=

~

leptonic planehadronic

planep’

e’

e

LU ~ sin Im{F1H + (F1+F2)H - kF2E}d~Polarized beam, unpolarized neutron target:

Suppressed because F1(t) is small

Suppressed because of cancellation between PPD’s of u and d quarks

Hp, Hp, Ep

~

nDVCS gives access to E, the least known and

least constrained GPD that appears in Ji’s sum ruleHp(ξ, ξ, t) = 4/9 Hu(ξ, ξ, t) + 1/9 Hd(ξ, ξ, t)

Hn(ξ, ξ, t) = 1/9 Hu(ξ, ξ, t) + 4/9 Hd(ξ, ξ, t)

Unpolarized beam, transverse proton target:

UT ~ sinIm{k(F2H – F1E) + ….. }d Hp, Ep

Measuredor planned

at JLabin Hall B

Ju=.3, Jd=.1

Ju=.8, Jd=.1

Ju=.5, Jd=.1

= 60°xB = 0.2Q2 = 2 GeV2

t = -0.2 GeV2

Beam-spin asymmetry for DVCS: sensitivity to Ju,d

VGG Model(calculations by M. Guidal)

DVCS on the proton

Ju=.3, Jd=.8

Ju=.3, Jd=-.5

Ee = 11 GeV

= 60°xB = 0.17Q2 = 2 GeV2

t = -0.4 GeV2

Beam-spin asymmetry for DVCS: sensitivity to Ju,d

The asymmetry for nDVCS is:• very sensitive to Ju, Jd • can be as big as for the protondepending on the kinematics and on Ju, Jd

→ wide coverage needed

VGG Model(calculations by M. Guidal)

DVCS on the neutron Ju=.3, Jd=.1

Ju=.8, Jd=.1

Ju=.5, Jd=.1

Ju=.3, Jd=.8

Ju=.3, Jd=-.5

Ee = 11 GeV

First measurement of nDVCS: Hall A

Ee= 5.75 GeV/c Pe = 75 %L = 4 ·1037 cm-2 · s-1/nucleon

Q2 = 1.9 GeV2

xB = 0.360.1 GeV2 < -t < 0.5 GeV2

HRS

Electromagnetic Calorimeter (PbF2)

LH2 / LD2 target

e’

e

deedneenpeepXeeD ),(),(),(),(

Subtraction of quasi-elastic proton contribution deduced from H2 data convoluted with initial motion of the nucleon

Analysis done in the impulse approximation:Active nucleon identified

via missing mass

Twist-2

M. Mazouz et al., PRL 99 (2007) 242501

nDVCS in Hall A: results

S. Ahmad et al., PR D75 (2007) 094003

VGG, PR D60 (1999) 094017

M. Mazouz et al., PRL 99 (2007) 242501

Q2 = 1.9 GeV2 - xB = 0.36

Im(CIn) compatible with zero (→ too high xB?)

Strong correlation between Im[CId] and Im[CI

n]Big statistical and systematic uncertainties

(mostly coming from H2 and 0 subtraction)

Model dependentextraction of Ju and Jd

F. Cano, B. Pire, Eur. Phys. J. A19 (2004) 423

CEBAF @ 12 GeVAdd new hallAdd new hall

CEBAF@12 GeV: large Q2, xB

~2012

Low thresholdCerenkov Counter

Forward Time-of-Flight

Hall B @12 GeV: CLAS12

High thresholdCerenkov Counter

Drift Chambers

Forward ElectromagneticCalorimeter

PreshowerCalorimeter

Inner ElectromagneticCalorimeter

Central detector

Design luminosity ~ 1035 cm-2s-1

Concurrent measurement of deeply virtual exclusive,

semi-inclusive, and inclusive processes

Q2 > 2.5 GeV2

Central Detector (CD): 40°<<135°

Forward Detector: 5°<<40°

nDVCS with CLAS12: kinematics

More than 80% of the neutrons have >40°→ Neutron detector in the CD is needed!

DVCS/Bethe-Heitler event generatorwith Fermi motion, Ee = 11 GeV (Grenoble)

Physics and CLAS12 acceptance cuts applied:

W > 2 GeV2, Q2 >1 GeV2, –t < 1.2 GeV2

5° < e < 40°, 5° < < 40°

<pn>~ 0.4 GeV/c

ed→e’n(p)

Detected in forward CLAS

Detected inFEC, IC

Not detected

PID (n or ?), p, angles to identify the final state

CD

In the hypothesis of absence of FSI:pμ

p = pμp’ → kinematics are complete

detecting e’, n (p,,),

pμe + pμ

n + pμp = pμ

e′ + pμn′ + pμ

p′ + pμ

FSI effects can be estimated measuringen, ep, edon deuteron in CLAS12(same experiment)

• limited space available (~10 cm thickness)→ limited neutron detection efficiency→ no space for light guides→ compact readout needed• strong magnetic field (~5 T)→ magnetic field insensitive photodetectors (SiPMs or Micro-channel plate PMTs)

CTOF can also be used for neutron detection Central Tracker can work as a veto for charged particles

CND

CTOF CentralTracker

CND: constraints & design

Detector design under study:scintillator barrel

MC simulations done for: efficiency PID angular resolutions reconstruction algorithms background studies

Simulation of the CNDGeometry:• Simulation done with Gemc (GEANT4)• Includes the full CD• 4 radial layers (or 3, if MCP-PMTs are used)• 30 azimuthal layers (can still be optimized)• each bar is a trapezoid (matches CTOF)• inner r = 28.5 cm, outer R = 38.1 cm

Reconstruction: Good hit: first with Edep > threshold

TOF = (t1+t2)/2, with

t2(1) = tofGEANT+ tsmear+ (l/2 ± z)/veff

tsmear = Gaussian with = 0/√Edep (MeV)

0 = 200 ps·MeV ½ → σ ~ 130 ps for MIPs β = L/T·c, L = √h2+z2 , h = distance betweenvertex and hit position, assuming it at mid-layer θ = acos (z/L), z = ½ veff (t1-t2) Birks effect not included (will be added in Gemc) Cut on TOF>5ns to remove events produced in the magnet and rescattering back in the CND

z

y

x

CND: efficiency, PID, resolution

pn= 0.1 - 1.0 GeV/c= 50°-90°, = 0°

Efficiency: Nrec/Ngen

Nrec= # events with Edep>Ethr.

Efficiency ~ 10-16% for a threshold of 5 MeVand pn = 0.2 - 1 GeV/c

Layer 1 Layer 2

Layer 3 Layer 4

distributions (for each layer) for:• neutrons with pn = 0.4 GeV/c• neutrons with pn = 0.6 GeV/c• neutrons with pn = 1 GeV/c• photons with E = 1 GeV/c (assuming equal yields for n and )

n/ misidentificationfor pn ≥ 1 GeV/c

“Spectator” cut

p/p ~ 5%~ 1.5°

nDVCS with CLAS12 + CND: expected count rates

< (°)> σ(nb GeV 4) N

16 0.01794 5354

42 0.00627 1873

74 0.00276 824

104 0.00174 520

134 0.00137 410

165 0.00127 379

195 0.00126 377

225 0.00140 417

256 0.00172 513

286 0.00279 835

317 0.00616 1838

347 0.0182 5432

t = 0.2 GeV2 Q2 =0.55 GeV2

xB = 0.05 = 30°

• L = 1035cm-2s-1

• Time = 80 days

• Racc= bin-by-bin acceptance

• Eeff = 15% neutron detector efficiency (CND+CTOF+FD)

N = ∆t ∆Q2 ∆x ∆ L Time Racc Eeff

Count rates computed with nDVCS+BHevent generator + CLAS12 acceptance

(LPSC Grenoble)

<t> ≈ -0.4 GeV2

<Q2> ≈ 2GeV2

<x> ≈ 0.17

→ N = 1%- 5%

Electromagnetic background

Electromagnetic background rates and spectra for the

CND have been studied with Gemc (R. De Vita, INFNGE):

• The background on the CND produced by the beam through electromagnetic interaction in the target consists of neutrals (most likely photons)

• Total rate ~2 GHz at luminosity of 1035 cm-2·s-1

• Maximum rate on a single paddle ~ 22 MHz (1.5 MHz for Edep>100KeV)

This background can be reconstructed as a neutron:with a 5 MeV energy threshold the rate is ~ 3 KHzFor these “fake” neutrons <0.1-0.2 → pn < 0.2 GeV/c

The actual contamination will depend on the hadronic rate in the forward part of CLAS12 (at 1 KHz, the rate of fake events is 0.4 Hz)

, for Edep>5 MeV

Technical challenge: TOF resolution & B=5T

SiPM - PROS:

• Insensitive to magnetic field• High gain (106)• Good intrinsic timing resolution (30 ps/pixel)• Good single photoelectron resolution

SiPM - CONS:

• Very small active surface (1-3 mm2)

→ small amount of light collected (TOF~1/√Nphel)

• Noise

SiPM

APD – PROS:

• insensitive to magnetic field• bigger surface than SiPM → more light collected

APD – CONS:

• low gain at room temperature• timing resolution?

MCP-PMT – PROS:

• resistant to magnetic field ~1T• big surface• timing resolution ~ordinary PMT

MCP-PMT – CONS:

• behavior at 5T not yet studied• high cost (2K euros/PMT)• lifetime?

MCP-PMT

MCP-PMT

Measure TOF resolution with 2 standard PMTs Substitute PMT at one end with one SiPM, one APD

Test of MCP-PMTs Redo the same measurements with extruded scintillator (FNAL) + WLS fiber

(Kuraray) + SiPM (used in IC hodoscope @CLAS, ~ x5 more γ’s/mm2) • Test of MCP-PMTs in magnetic field (Saclay, mid November)

Tests on photodetectors with cosmic rays at Orsay

“Trigger” PMTs (Photonis XP2020)

Scintillator bar (BC408)80cm x 4 cm x 3 cm

“Trigger” scintillators(BC408) 1cm thick

“Reference PMT”Photonis XP20D0

Results from Orsay’s test bench

σ2test =1/2 (σ2

test,trig + σ2test,ref − σ2

ref,trig) TestRef

Trig Test = 1 SiPM Hamamatsu (MPPC 3x3mm2)• rise time ~5 ns (> capacitance)• more noise than 1x1 mm2

Test = 1 APD Hamamatsu (10x10 mm2 ) • TOF ~ 1.4 ns• high noise, high rise time

Test = 1 MCP-PMT Photonis/DEP (two MCPs)• TOF ~ 130 ps• will be tested in B field at Saclay (end of November)

Thi Nguyen TrungBernard Genolini

S. PisanoJ. Pouthas

Test = PMT• TOF < 90 ps• nphe ~1600

Single pe Test = 1 SiPM Hamamatsu (MPPC 1x1 mm2)• TOF ~ 1.8 ns • rise time ~ 1 ns• nphe ~1

Test = 1 MPPC 1x1mm2

Extruded scint. + WLS fiber• TOF ~ 1.4 ns• WLS -> Width ~ 15 ns

w

h

l

Scintillator (BC408)

z

y

x

Readout (MCP PMT)

Readout (MCP PMT)

Prototype (0 layer)

2 layers

Scint. 1

Scint. 2

3 layersScint. 2Scint. 1

Scint. 3

Simulations with Litrani

Pulse shapesRelative light yields

Simulations on time resolutions with MCP-PMT

Time resolution along the scintillator length

Relative light yield taken onthe first part of the signals (4ns)

Comparisonof time resolutions

for asame amount of light created

130 ps

Adjusted on thePrototype measurements

Ongoing: Introduction of the energy deposited by neutronsConversion in light (including Birks’ law)

Simulations on time resolutions with MCP-PMT

Plan for the next months

R & D:

• Measurements of time resolution with MCP-PMTs in magnetic field

(Saclay, end of November)

• Construction of prototype (one bin, 3 radial layers + MCP-PMTs)

• Measurements with prototype in neutron beam?

Simulation:

• Optimization of number of bins (signal/background studies with nDVCS/nep0

generator, Litrani simulations for time resolution vs. paddle size)

• Inclusion of Birks effect and new estimation of efficiency and resolutions

• Studies on electromagnetic background to estimate radiation damage on MCP-PMTs,

possible adoption of metal shield between CTOF and CND

• Using scintillator as detector material, detection of nDVCS recoil neutrons with ~10-15% of efficiency and n/ separation for p < 1 GeV/c seems possible from simulations, provided to have ~130 ps of TOF resolution for MIPs

• The strong magnetic field of the CD and the limited space available call for magnetic-fieldinsensitive and compact photodetectors: MCP-PMTs seem the best candidate, but their timing performances in a high magnetic field still need to be tested

• CTOF and neutron detector could coexist in one detector, whose first layer can be usedas TOF for charged particles when there’s a track in the central tracker, while the fullsystem can be used as neutron detector when there are no tracks in the tracker.

Conclusions and outlook• nDVCS is a key reaction for the GPD experimental program: measuring its beam-spin asymmetry can give access to E and therefore to the quark orbital angular momentum (via the Ji’s sum rule)• A large kinematical coverage is necessary to sample the phase-space, as the BSA is expected to vary strongly → CLAS12 at upgraded JLab is the ideal facility

• The detection of the recoil neutron is very important to ensure exclusivity, reduce background and keep systematic uncertainties under control• The nDVCS recoil neutrons are mostly going at large angles (n>40°), therefore a neutron detector should be added to the CLAS12 Central Detector, using the available spaceLoI submitted to JLab’s PAC34 (January 2009)

strongly encouraged to submit full proposal