dvcs cross section measurements at...

DVCS cross section measurements at JLab

Carlos Munoz Camacho,for the JLab Hall A Collaboration and the DVCS Collaboration

Los Alamos National Laboratory

Hard Exclusive Processes at JLab 12 GeV and a Future EICOct 29-30, 2006

Carlos Munoz Camacho, for the Jefferson Lab Hall A Collaboration and the DVCS Collaboration Oct 29, 2006

DVCS in Jefferson Lab Hall A 1

E00-110 kinematics

Plastic scintillator array

LH2 targete

Beam

PbF2 Electromagnetic

calorimeter

HRS

p

e

γ

Kin Q2 xB θγ∗ s(GeV2) (deg.) (GeV2)

1 1.5 0.36 22.3 3.5

2 1.9 0.36 18.3 4.2

3 2.3 0.36 14.8 4.9

I Measurement of both helicity-dependent andhelicity-independent DVCS cross sections independently

I Q2−dependence of helicity-dependent cross section

Goal:Test twist-2 dominance of DVCS at moderate Q2 ∼ 2 GeV2.

. . . and if so, access combinations of GPDs.



Helicity-dependent and helicity-independent cross sections

Accurate determination of φγγ dependence of dΣ =σ→−σ←

2and dσ =

σ→+σ←

2Q2 = 2.3 GeV2, 〈t〉 = −0.28 GeV2, xBj = 0.36

=m CI,exp(F)

=m CI(Feff)

Total fit

<e CI,exp(F)

<e [CI + ∆CI ]exp(F)

<e CI(Feff)

Bethe-Heitler

Total fit

ff 1 freeparametereach

9>>=>>;1 freeparametereach

fixed

dσ: - rich and complex φγγ structure beyond BH- interesting & complementary GPD information (real part)



Q2−dependence: twist-2 dominance

0.4 < −t < 0.12 (GeV2)

=m CI(F)

)2 (GeV2Q1.4 1.6 1.8 2 2.2 2.4

0

0.5

1

1.5

2

2.5

3

3.5

4

=m CI(Feff)

)2 (GeV2Q1.4 1.6 1.8 2 2.2 2.4

0

0.5

1

1.5

2

2.5

3

3.5

4

I No Q2−dependence in =m CI(F) within 3% statistical error bars

I Sets an upper limit for twist-4 and higher ≤ 10%



GPD linear combinations and integrals

)2

< t > (GeV-0.3 -0.25 -0.2 -0.15

-3

-2

-1

0

1

2

3

4

5

2=1.5 GeV2 QIIm C2=1.9 GeV2 QIIm C2=2.3 GeV2 QIIm C

(VGG)IIm C

2=2.3 GeV2 QIRe C2=2.3 GeV2) QI C∆+I- Re (C

(VGG)I Re C

) (VGG)I C∆+I- Re (C

I Possible (likely) contribution of DVCS2 terms in these interference results



DVCS2 contribution

1.- Helicity-correlated cross section: =maginary part

d5Σd5Φ

=12

[d5σ+

d5Φ+

d5σ−

d5Φ

]=

sin(φγγ)Γ=1 =mhCI(F)

i− sin(2φγγ)Γ=2 =m

hCI(Feff)

i| {z }

Interference BH-DVCS

+ sin (φγγ)Γ=1 ηs1=mhCDVCS(Feff,F∗)

i| {z }

|DVCS|2 (twist-3)

I Different φγγ dependence of Twist-2 & Twist-3 interference terms:⇒ independent determination

I sinφγγΓ=1 term determines observable =m[CI,exp(F)

]:

=m[CI,exp(F)

]= =m

[CI(F)

]+〈ηs1〉=m

[CDVCS(Feff,F∗)

]|〈ηs1〉|E00−110 < 0.01



DVCS2 contribution

d5σ

d5Φ=

12

[d5σ+

d5Φ+

d5σ−

d5Φ

]=

d5σ(|BH|2)d5Φ︸︷︷︸

Known from FF

+ Γ η CDVCS(F ,F∗)︸︷︷︸|DVCS|2 (twist-2)

+

(Γ<0 − cos(φγγ)Γ<1 )<e[CI(F)

]+ Γ<0,∆<e

[CI + ∆CI

](F) + cos(2φγγ)Γ<2 <e

[CI(Feff)

]︸︷︷︸Interference BH-DVCS

I <e[CI, exp(F)

]= <e

[CI(F)

]+ 〈ηc1〉 CDVCS(F ,F∗)

I <e[CI, exp + ∆CI, exp

](F) = <e

[CI + ∆CI

](F) + 〈η0〉 CDVCS(F ,F∗)

|〈η0,c1〉|E00−110 < 0.05

ηc1 and η0 depend on beam energy ! =⇒Carlos Munoz Camacho, for the Jefferson Lab Hall A Collaboration and the DVCS Collaboration Oct 29, 2006


DVCS2: proposed separation (with JLab 6 GeV!)

Q2 = 1.9 GeV2, xB = 0.36, s = 4.9 GeV2

(GeV)beamE4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 6 6.2

6 G

eV C

/ Cδ

-1

-0.8

-0.6

-0.4

-0.2

-0

0.2

0.4

0.6

0.8

1 -14.4 -15.3 -16.8 -18.2 -18.7BH (at 180 deg)

(twist-2)2DVCS (twist-3)2DVCS

Inteference (twist-2)Inteference (twist-3)DIS

Unpolarized cross section

(GeV)beamE4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 6 6.2

6 G

eV C

/ Cδ

-1

-0.8

-0.6

-0.4

-0.2

-0

0.2

0.4

0.6

0.8

1 (twist-3)2DVCS

Inteference (twist-2)

Inteference (twist-3)

DIS

-14.4 -15.3 -16.8 -18.2 -18.7

Helicity-dependent cross section (deg) *γθ

Possible DVCS2 separation by changing beam energy



JLab 12 GeV proposal PR12-06-114 (J. Roche, C. Hyde-Wright, B. Michel, C.M.C et al.) – Approved by PAC-30

Kinematic coverage

JLab12 with 3, 4, 5 pass beam(6.6, 8.8, 11.0 GeV beam energy)

Bjx0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

)2 (G

eV2

Q

0

5

10

2<4 GeV2W

11 GeV≤beam

Unphysical with E

= 6.6 GeVbeamE

= 8.8 GeVbeamE

= 11.0 GeVbeamE

= 5.75 GeVbeamE

DVCS measurements in Hall A/JLab

Beam time (days)Q2 xBj

(GeV2) 0.36 0.50 0.603.0 34.0 24.55 13.1 54.8 46.3 47.2 75.1 136.0 167.7 139.0 20

Total 6 20 62

1 GeV2 range in tmin − t

88 days250k events/setting




Experimental configuration (e p → e γ X)




Missing mass resolution

(Cf. Table V for all kinematic settings)

6.6 GeV setting 11 GeV setting

E00-110 This proposal

Very similar M2X resolution ⇒ same exclusivity with e γ detection only.




Cross sections

I Model byVanderhaeghen, Guichon& Guidal (VGG), withfactorized t−dependence

I 250k events/setting or40k events per t−bin

I Similar statisticalaccuracy as E00-110

Helicity-independent cross sections (pb/GeV4)

6.6 GeV setting Q2 = 3.0 GeV2, xBj = 0.36

Helicity-dependent cross sections (pb/GeV4)




Cross sectionsHelicity-independent cross sections (pb/GeV4)

8.8GeV setting Q2 = 4.8 GeV2, xBj = 0.50 11 GeV setting Q2 = 9.0 GeV2, xBj = 0.60

Helicity-dependent cross sections (pb/GeV4)




Systematic errors

Type Relative errors (%)E00-110 proposed

Luminosity target length and beam charge 1 1HRS-Calorimeter Drift chamber multi-tracks 1.5 1

Acceptance 2 2Trigger dead-time 0.1 0.1

DVCS selection π0 subtraction 3 1e(p,e’γ)πN contamination 2 3radiative corrections 2 1

Total cross section sum 4.9 4.1

Beam Polarization ∆P/P 2 1Total cross section difference 5.3 4.2



Backup slides



Missing mass squared e p → e γ X (E00-110)

)2 (GeV2XM

0 0.5 1 1.5 2 2.5)2 (GeV2

XM0 0.5 1 1.5 2 2.5

0

1000

2000

3000

4000

5000 Raw

contribution0π

subtracted0π cut2

XM

Obtained from) DATAγ γH(e,e

)2 (GeV2XM

0 0.5 1 1.5 2 2.5)2 (GeV2

XM0 0.5 1 1.5 2 2.5

0

1000

2000

3000

4000

5000 Xγe p -> e pγe p -> e

Inclusive contamination

p) scaledγH(e,e

cut2XM

Competing channels:

I π0 electroproduction: e p → e p π0X → e p γ γ X

I Associated DVCS: e p → e N π γ

I Non-resonant: e p → e N π γ M2X > (M + mπ0)2

I Resonant: e p → e (∆ or N∗)γ m2∆ = 1.52 GeV2



π0 substraction (π0 → γγ)

decaySymmetric

Pion center−of−mass Laboratory frame

θ

Direction of the boost

Direction of the boostAsymmetricdecay

I Symmetric decay: minimum angle in lab of 4.4◦ for Emaxπ0 = 3.5 GeV

⇒ Clusters separation

I Asymmetric decay: sometimes only 1-cluster⇒ Mistaken for DVCS event



π0 substraction (π0 → γγ)

decaySymmetric

Pion center−of−mass Laboratory frame

θ

Direction of the boost

Direction of the boostAsymmetricdecay

Substraction procedure:

1. Compute kinematics of each detected π0 (2 clusters in calorimeter).

2. Randomize the decay : sample cos θ randomly between [-1,1] a bignumber of times (∼ 5000).

3. Compute the ratio of 2-cluster/1-cluster events generated by this π0

(∼ 30% in average).

Repeating this procedure for each detected π0 provides an automatic

normalization of the contamination as a function of Q2, t, ϕ, . . .



π0 subtraction results for different (t, φγγ) bins

)2 (GeV2XM

0 0.5 1 1.5 2 2.5)2 (GeV2

XM0 0.5 1 1.5 2 2.5

-200

0

200

400

600

800

1000

1200

1400

cut2XM

2<t>=-0.33 GeV

)2 (GeV2XM

0 0.5 1 1.5 2 2.5)2 (GeV2

XM0 0.5 1 1.5 2 2.5

-200

0

200

400

600

800

1000

1200

1400

cut2XM

2<t>=-0.28 GeV

)2 (GeV2XM

0 0.5 1 1.5 2 2.5)2 (GeV2

XM0 0.5 1 1.5 2 2.5

-200

0

200

400

600

800

1000

1200

1400

cut2XM

2<t>=-0.23 GeV

)2 (GeV2XM

0 0.5 1 1.5 2 2.5)2 (GeV2

XM0 0.5 1 1.5 2 2.5

-200

0

200

400

600

800

1000

1200

1400

cut2XM

2<t>=-0.17 GeV




0 0.5 1 1.5 2 2.50 0.5 1 1.5 2 2.5-200

0

200

400

600

800

1000

1200

1400

cut2XM

2<t>=-0.33 GeV

0 0.5 1 1.5 2 2.50 0.5 1 1.5 2 2.5-200

0

200

400

600

800

1000

1200

1400

cut2XM

2<t>=-0.28 GeV

0 0.5 1 1.5 2 2.50 0.5 1 1.5 2 2.5-200

0

200

400

600

800

1000

1200

1400

cut2XM

2<t>=-0.23 GeV

0 0.5 1 1.5 2 2.50 0.5 1 1.5 2 2.5-200

0

200

400

600

800

1000

1200

1400

cut2XM

2<t>=-0.17 GeV




)2 (GeV2XM

0 0.5 1 1.5 2 2.5)2 (GeV2

XM0 0.5 1 1.5 2 2.5

-200

-100

0

100

200

300

400

<43φ, 317<2<t>=-0.23 GeV <43φ, 317<2<t>=-0.23 GeV

)2 (GeV2XM

0 0.5 1 1.5 2 2.5)2 (GeV2

XM0 0.5 1 1.5 2 2.5

-200

-100

0

100

200

300

400

<130φ, 43<2<t>=-0.23 GeV <130φ, 43<2<t>=-0.23 GeV

)2 (GeV2XM

0 0.5 1 1.5 2 2.5)2 (GeV2

XM0 0.5 1 1.5 2 2.5

-200

-100

0

100

200

300

400

<216φ, 130<2<t>=-0.23 GeV <216φ, 130<2<t>=-0.23 GeV

)2 (GeV2XM

0 0.5 1 1.5 2 2.5)2 (GeV2

XM0 0.5 1 1.5 2 2.5

-200

-100

0

100

200

300

400

<302φ, 216<2<t>=-0.23 GeV <302φ, 216<2<t>=-0.23 GeV




0 0.5 1 1.5 2 2.50 0.5 1 1.5 2 2.5-200

-100

0

100

200

300

400

<43φ, 317<2<t>=-0.23 GeV <43φ, 317<2<t>=-0.23 GeV

0 0.5 1 1.5 2 2.50 0.5 1 1.5 2 2.5-200

-100

0

100

200

300

400

<130φ, 43<2<t>=-0.23 GeV <130φ, 43<2<t>=-0.23 GeV

0 0.5 1 1.5 2 2.50 0.5 1 1.5 2 2.5-200

-100

0

100

200

300

400

<302φ, 216<2<t>=-0.23 GeV <302φ, 216<2<t>=-0.23 GeV



π0 subtraction vs. θlab

(deg)labθ-24 -22 -20 -18 -16 -14 -12 -10 -80

200

400

600

800

1000

1200

2<t>=-0.37 GeV

(deg)labθ-24 -22 -20 -18 -16 -14 -12 -10 -80

200

400

600

800

1000

1200

1400

1600

2<t>=-0.33 GeV

(deg)labθ-24 -22 -20 -18 -16 -14 -12 -10 -80

200

400

600

800

1000

1200

1400

1600

1800

2<t>=-0.28 GeV

(deg)labθ-24 -22 -20 -18 -16 -14 -12 -10 -80

500

1000

1500

2000

2500

2<t>=-0.23 GeV

(deg)labθ-24 -22 -20 -18 -16 -14 -12 -10 -80

500

1000

1500

2000

2500

3000

3500

2<t>=-0.17 GeV



GPDs from cross sections vs. GPDs from asymmetries

)2< t > (GeV-0.3 -0.25 -0.2 -0.150

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5IE00-110 Im C

)]2)2∈(2-y)(1+B/(8 K xBH0[c(E00-110)LUA>φ<sin

(VGG)IIm C



GPDs from cross sections vs. GPDs from asymmetries

Even within a model (VGG). . . :

-t(GeV2) =mCI 〈sinφ〉[cBH0 /8KxB(2− y)(1 + ε2)2] Error

0.19 7.22 4.65 36%

0.30 4.47 2.40 46%

0.46 2.22 0.92 59%



Cross sections for each t bin (〈Q2〉=2.3 GeV2)

(deg)γγϕ0 90 180 270 360

0

0.01

0.02

0.03 2<t>=-0.33 GeV

(deg)γγϕ0 90 180 270 360

0

0.01

0.02

0.03 2<t>=-0.28 GeV

(deg)γγϕ0 90 180 270 360

0

0.01

0.02

0.03 2<t>=-0.23 GeV

(deg)γγϕ0 90 180 270 360

0

0.01

0.02

0.03 2<t>=-0.17 GeV

)IIm (C)eff

IIm (C

E00-110Fit

σ1-

)4) (nb/GeVγγϕdtdBdx2dQ

-σ4d - γγϕdtdBdx2dQ

+σ4d( 21

(deg)γγϕ0 90 180 270 360

0

0.02

0.06

0.08

0.1

0.12 2<t>=-0.33 GeV

(deg)γγϕ0 90 180 270 360

0

0.02

0.04

0.06

0.08

0.1

0.12 2<t>=-0.28 GeV

(deg)γγϕ0 90 180 270 360

0

0.02

0.06

0.08

0.1

0.12 2<t>=-0.23 GeV

(deg)γγϕ0 90 180 270 360

0

0.02

0.04

0.06

0.08

0.1

0.12 2<t>=-0.17 GeV

BH)IRe (C

)IC∆+ IRe (C)eff

IRe (C

E00-110Fit

σ1-

)4 (nb/GeVγγϕdtdBdx2dQ

σ4d



Counts

(deg.)ϕ0 50 100 150 200 250 300 350

(deg.)ϕ0 50 100 150 200 250 300 350

-+N+

N

-1000

-800

-600

-400

-200

0

200

400

600

0.12±): -2.16 unpIRe(C

0.08±): 0.47 unpIC∆+unp

IRe(C

0.45±): -0.56 effIRe(C

/n=1.872χTotal Fit:

2, <t>=-0.37 GeV2>=2.3 GeV2

(deg.)ϕ0 50 100 150 200 250 300 350

(deg.)ϕ0 50 100 150 200 250 300 350

-+N+

N

-1000

-800

-600

-400

-200

0

200

400

600

800

0.12±): -2.48 unpIRe(C

0.09±): -0.05 unpIC∆+unp

IRe(C 0.53±): -1.46 eff

IRe(C


2, <t>=-0.33 GeV2>=2.3 GeV2

(deg.)ϕ0 50 100 150 200 250 300 350

(deg.)ϕ0 50 100 150 200 250 300 350

-+N+

N

-1000

-500

0

500

1000

0.12±): -2.08 unpIRe(C

0.08±): 0.56 unpIC∆+unp

IRe(C

0.60±): 0.61 effIRe(C


2, <t>=-0.28 GeV2>=2.3 GeV2

(deg.)ϕ0 50 100 150 200 250 300 350

(deg.)ϕ0 50 100 150 200 250 300 350

-+N+

N

-1000

-500

0

500

1000

0.12±): -1.80 unpIRe(C

0.08±): 1.29 unpIC∆+unp

IRe(C

0.79±): 1.02 effIRe(C


2, <t>=-0.23 GeV2>=2.3 GeV2

(deg.)ϕ0 50 100 150 200 250 300 350

(deg.)ϕ0 50 100 150 200 250 300 350

-+N+

N

-1000

-500

0

500

1000

1500

0.15±): -0.81 unpIRe(C

0.09±): 2.19 unpIC∆+unp

IRe(C

1.45±): 3.40 effIRe(C


2, <t>=-0.17 GeV2>=2.3 GeV2



Calorimeter radiation damage

E00-110 experience:

I Dose dominated by e and π0 above 15◦ and Møller below 10◦

I Dose grows a factor 5 from 11.5◦ to 7.5◦

I 20% gain loss without loss in M2X/energy resolution

New experiment strategies:

I Minimum angle of the closest block: 7◦

I Luminosity equal to the peak luminosity in E00-110 taking into

account the distance to the target: L = 4 · 1037(D/110 cm)2 cm−2s−1

I Blue light curing (MAMI-A4): 17 h to cure a transparency loss of 25%

Curing every 6th day of running at the minimum angle

Total of 12 curing days for 88 beam days



Physics reach

1. Q2 variation:I 2:1 range at each xBjI Accurate measurement of twist-2 dominance

2. xBj variation (ξ dependence):I Precision data on variation of t−dependence with xI Study of transverse correlations

3. t variation:I 5 bins in 0 < tmin − t < 1 GeV2

I Fourier-conjugate to the spatial distributions of quarks asa function of their momentum fraction x

4. π0 electroproduction cross section:I Dominance of Twist-2 (isolation of leading twist)I Sensitive to nucleon GPDs H and E (× the π DA)



π0 electroproduction: σL + σT/ε

At leading twist:

dσL

dt=

12Γ

∑hN ,hN′

|ML(λM = 0, h′N , hN )|2 ∝ 1Q6

σT ∝1

Q8

ML ∝[ ∫ 1

0dz

φπ(z)z

] ∫ 1

−1dx

[1

x− ξ+

1x + ξ

]×

{Γ1Hπ0+Γ2Eπ0

}

Different quark weights: flavor separation of GPDs

|π0〉 = 1√2{|uu〉 − |dd〉} Hπ0 =

1√2

{23Hu +

13Hd

}|p〉 = |uud〉 HDV CS =

49Hu +

19Hd



Upgrades (from E00-110)

1. Expanded PbF2 calorimeter: 11×12 + 76 blocks.

I Higher acceptance for π0 measurements/subtraction.I Increased t−acceptance: ∆(tmin − t) = 1 GeV2.

2. Electronics:

I ARS system (as E00-110) + Upgraded calorimeter trigger(2 thresholds to increase ep → epπ0 statistics).

I FPGA & VME upgrades to increase livetime & bandwidth.

3. No proton detection: calorimeter can handle 4× E00-110 rate

4. Flared beam pipe to minimize secondary background incalorimeter.(Background dominated by Møller and π0 → γγ from target)



Model prediction for Q2, xBj and t dependenciesI Sample of statistical &

systematic errors on

coefficients

I Total of 55 data points

I Full t−dependence at each

(Q2,xBj) point

)2 (GeV2Q2 3 4 5 6 7 8 9

IIm

C

0

0.5

1

1.5

2

2.5

3=0.36Bx=0.50Bx=0.60Bx

Bjx0.3 0.35 0.4 0.45 0.5 0.55 0.6

-0.5

0

0.5

1

1.5

2

2.5 (F)IIm C

(F)IRe C

] (F)I C∆+IRe [C

)2-t (GeVmint0 0.1 0.2 0.3 0.4 0.5

IIm

C

0

1

2

3

4

5

6=0.36Bx=0.50Bx=0.60Bx



Summary

I Absolute DVCS cross sections in almost the completekinematic region of JLab12

I Precision determination of Twist-2 and Twist-3 observables:ξ, t (and Q2) scan

I π0 electroproduction cross sections

. . . using the successful experimental technique of E00-110 (nucl-ex/0607029)

This experiment requires 88 days of beam + 12 days for calorimeter curing

Future extentions: DVCS on the neutron, recoil polarimetry. . .



dvcs cross section measurements at...

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