Post on 24-Aug-2020
Neutron DVCS
Carlos Munoz Camacho
IPN-Orsay, CNRS/IN2P3 (France)
Next generation of nuclear physics with JLab12 and EICFlorida International University
February 10–13, 2016
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 1 / 26
Introduction
Outline
Motivation of DVCS off the neutronand experimental challenges
Experimental program at Jefferson Lab
Recent (preliminary) results from Hall A
Outlook at JLab12
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 2 / 26
Introduction Motivation
Deeply Virtual Compton Scattering (DVCS): γ∗ p→ γ p
Handbag diagram
High Q2
Perturbative QCD
Non-perturbativeGPDs
Bjorken limit:
Q2 = −q2 → ∞ν → ∞
}xB =
Q2
2Mνfixed
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 3 / 26
Introduction Experiments
DVCS experimentally: interference with Bethe-Heitler (BH)
At leading twist:
d5→σ −d5 ←σ = =m (TBH · TDV CS)
d5→σ +d5
←σ = |BH|2 + <e (TBH · TDV CS) + |DV CS|2
T DV CS =
∫ +1
−1dx
H(x, ξ, t)
x− ξ + iε+ · · · =
P∫ +1
−1dxH(x, ξ, t)
x− ξ︸ ︷︷ ︸Access in helicity-independent cross section
− iπ H(x = ξ, ξ, t)︸ ︷︷ ︸Access in helicity-dependent cross-section
+ . . .
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 4 / 26
Introduction Experiments
Accessing different GDPs
Polarized beam, unpolarized target (BSA)
dσLU = sinφ · Im{F1H+ xB(F1 + F2)H − kF2E}dφ
Unpolarized beam, longitudinal target (lTSA)
dσUL = sinφ · Im{F1H+ xB(F1 + F2)(H+ xB/2E)− xBkF2E . . . }dφ
Polarized beam, longitudinal target (BlTSA)
dσLL = (A+B cosφ) · Re{F1H+ xB(F1 + F2)(H+ xB/2E) . . . }dφ
Unpolarized beam, transverse target (tTSA)
dσUT = cosφ · Im{k(F2H− F1E) + . . . }dφ
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 5 / 26
Introduction n-DVCS
Neutron DVCS
1 Flavor sensitivity of GPDs (when combined with proton DVCS)
ImH = −π(4
9Hu +
1
9Hd
)(Proton)
ImH = −π(1
9Hu +
4
9Hd
)(Neutron)
2 Enhanced sensitivity to GPD E (eg. with long. ~e off unpol. target)
LD2 target (Fn2 (t)� Fn
1 (t) !)
A = F1(t)H+xB
2− xB[F1(t) + F2(t)]H − t
4M2· F2(t) · E︸ ︷︷ ︸
Main contribution for neutron
H is expected small by compensation of u and d distributions in n
Contribution of the angular momentum of quarks to proton spin:
J =1
2
∫ 1
−1dxx[H(x, ξ, 0) + E(x, ξ, 0)]
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 6 / 26
Experimental results
The n-DVCS program at Jefferson Lab
Hall A:
E03-106: Beam helicity-dependent DVCS cross section
E08-025: Beam helicity-independent DVCS at 2 beam energies
Hall B:
E12-11-003: Beam spin asymmetries with CLAS12
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 7 / 26
Experimental results E03-106
Hall A DVCS experimental setup
100-channel scintillator array
High Resolution Spectrometer
132-block PbF2 electromagnetic calorimeter
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 8 / 26
Experimental results E03-106
Neutron detection (proton veto)
Charged particle veto in front of scintillator array:
Proton: signal in both detectors
Neutron: signal only in thick scintillator
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 9 / 26
Experimental results E03-106
Impulse approximation
D(~e, e′γ)X = d(~e, e′γ)d+ n(~e, e′γ)n+ p(~e, e′γ)p+ . . .
P (GeV)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.01
0.02
0.03
0.04
0.05Spectator nucleon
Recoil nucleon
N(P) dP
FSI between the np pair should be small
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 10 / 26
Experimental results E03-106
Neutron and coherent deuteron
M2X = (n+ q − q′)2 = M2
n +
(1− Mn
Md
)t 'M2
n +t
2
Coherent d separated using kinematical separation in the M2X spectrum
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 11 / 26
Experimental results E03-106
Missing mass & exclusivity
Exclusivity ensured by a cut at the π-production threshold
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 12 / 26
Experimental results E03-106
Helicity signal
Sh =
∫ π
0(N+ −N−) dφ−
∫ 2π
π(N+ −N−) dφ ,
)2 (GeV2XM
0 0.5 1 1.5 2 2.5 3
hS
0
500
1000
1500
2000
2500
3000
3500 data2D
data2H
p-DVCS simulation
cut2XM
)2 (GeV2M0 0.5 1 1.5 2 2.5 3
hS
-2000
-1000
0
1000
2000
d-DVCS
n-DVCS d-DVCS simulationn-DVCS simulation
n-DVCS + d-DVCS
No significant signal after LD2–LH2 subtraction
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 13 / 26
Experimental results E03-106
E03-106 results
)2t (GeV
-0.5 -0.45 -0.4 -0.35 -0.3 -0.25 -0.2 -0.15 -0.1
exp
)dI
Im(C
-4
-2
0
2
4
6This experiment
Cano & Pire calculation [29]
)2t (GeV
-0.5 -0.45 -0.4 -0.35 -0.3 -0.25 -0.2 -0.15 -0.1
exp
)nI
Im(C
-4
-3
-2
-1
0
1
2
3
=-0.4uJ=-0.6
dJ
=0.3uJ=0.2
dJ
=0.6uJ=0.8
dJ
This experiment
AHLT calculation [32]
VGG calculation [30]
Model-dependent sensitivity to Ju, Jd
uJ-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1
dJ-1
-0.8
-0.6
-0.4
-0.2
-0
0.2
0.4
0.6
0.8
1
HERMES Preliminaryp-DVCS
JLab Hall An-DVCS
Lattice QCDSF
0.14± = 0.18 5
uJ + dJ
Helicity-dependent nDVCS cross-section compatible with zero, BUTstill sets constraints to GPD combinations (and Ju, Jd models. . . )
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 14 / 26
Experimental results E03-106
E03-106: lessons learnt
LD2–LH2 subtraction very sensitive to calorimeter calibration (drifts)LD2 and LH2 running should be interleaved
Significant contamination from π0’s that was hard to subtracthigh γ threshold → reduced with improved trigger+electronics
Recoil particle detection/tagging extremely difficult at high luminosity
t (ns)0 20 40 60 80 100 120
t (ns)0 20 40 60 80 100 120
AR
S c
ha
nn
els
−350
−300
−250
−200
−150
−100
−50
0
50
Large background + low recoil momentum
Charge exchange reaction difficult tomeasure/estimate
No recoil detector anymore → less deadtime and higher statistics
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 15 / 26
Experimental results E08-025
E08-25 nDVCS experiment
Ran in 2010
No recoil detection → higher luminosity
Improved trigger and DAQ → reduced deadtime
Interleaved LD2 and LH2 running
Q2 = 1.75 GeV2, xB = 0.36 at Eb = 4.45 and Eb = 5.55 GeVC. Desnault’s PhD thesis
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 16 / 26
Experimental results E08-025
Analysis method
σ = |BH|2 + Γ0(xB , Q2, t, ϕ)1C0(ξ, t) + Γ1(xB , Q
2, t, ϕ) C1(ξ, t) cosϕ+
Γ2(xB , Q2, t, ϕ) C2(ξ, t) cos 2ϕ
ξ ' xB2− xB Ci(ξ, t): combinations of GPDs
NExp(ie) = Nie −NBHie
NMC(ie) = L[∑
i
Ci
∫x∈ie
Γi · cos (iϕ)⊗ Acc.︸ ︷︷ ︸MC sampling
]
MC includes real radiative corrections (both external and internal).
χ2 =∑ie
[NExp(ie)−NMC(ie)
]2[σExp(ie)]
2 ⇒{C0
C1, . . .
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 17 / 26
Experimental results E08-025
Rosenbluth-like separation of the DVCS cross section
σ(ep→ epγ) = |BH|2︸ ︷︷ ︸Known to ∼ 1%
+ I(BH ·DV CS)︸ ︷︷ ︸Linear combination of GPDs
+ |DV CS|2︸ ︷︷ ︸Bilinear combination of GPDs
I∝ 1/y3 = (k/ν)3,∣∣T DV CS∣∣2∝ 1/y2 = (k/ν)2
BKM-2010 – at leading twist → 7 independent GPD terms:{<e,=m
[CI , CI,V , CI,A
](F)
}, and CDV CS(F ,F∗).
ϕ-dependence provides 5 independent observables:
∼1, ∼ cosϕ,∼ sinϕ, ∼ cos(2ϕ),∼ sin(2ϕ)
The measurement of the cross section at two or more beam energies for exactly
the same Q2, xB , t kinematics, provides the additional information in order to
extract all leading twist observables independently.Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 18 / 26
Outlook Hall B E12-11-003
E12-11-003: DVCS sur le neutron avec CLAS12
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 19 / 26
Outlook Hall B E12-11-003
E12-11-003: projections
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 20 / 26
Outlook Future possibilities
Polarized 3He target
n lum. of 1036/cm2/s (14 atm×40 cm)
“Background” luminosity:
p in 3He + entrance/exit windows
1037/cm2 total luminosity
Polarization: 50%
Nuclear physics dilution factor 0.86(d-state)
-2.8% p polarization
Long. & Trans.
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 21 / 26
Outlook Future possibilities
3He target upgrade
Separate polarization and tgt volumes
Increase throughput by factor 10–100
Cool and/or compress 3He in targetarea by a factor of 10(10K at 10 atm×20 cm)
Rapid cycling of 3He through target
Reduce depolarization effect of tgtdensity, beam current, tgt wallsReplace thick glass with thinmetallic walls
Neutron luminosity of 1037/cm2/s
Proton luminosity 2 · 1037/cm2/sEndcaps ≤ 1037/cm2/s
Target polarization: 0.5·(0.86n−0.028p)
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 22 / 26
Outlook Future possibilities
DVCS on polarized neutron
~n(~e, e′γ) via 3 ~He(~e, e)X
Long or Trans normal polarization
Target single spin cross sections
dσ ∼ sinφ (twist-2): Im[BH·DVCS]
Unpolarized protons in 3He cancel
Target double spin
dσ ∼ c0 + c1 cosφ: Re[BH2+(BH·DVCS)+DVCS2]
Unpolarized protons cancel
Transverse sideways: sinφ −→ cosφ
All other “neutron” observables (total σ, beam-spin) have largeincoherent proton contributions
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 23 / 26
Outlook Future possibilities
Cross section projections (at 1037 cm−2s−1)
C. Hyde
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 24 / 26
Conclusion
Summary
Neutron DVCS is a required complement to the proton program:
Flavor separation of GPDs
Access to different combinations of GPDs
Very challenging experimentally:
Difficult to detect at high luminosities
Efficiencies, charge exchange, etc hard to estimate
Hall A program in Hall A provided some initial results and contraints
Approved program with CLAS12 off unpolarized neutrons
Possibilites of polarized nDVCS with high luminosity 3He target
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 25 / 26
Acknowledgements
This work was supported by:
IN2P3/CNRS
Office for Science & Technology at the Embassy of France (U.S.A.)
Carlos Munoz Camacho (IPN-Orsay) Neutron DVCS FIU, Feb 2016 26 / 26