cattering S ompton C irtual V Dhallaweb.jlab.org/experiment/DVCS/documents/talk/CEH_jan... · 2007....

Deeply Virtual Compton Scattering

on the neutron in .

Dr. Malek MAZOUZ

Ph.D. Defense, Grenoble

8 December 2006GPDs

+x x

t

Physics case

n-DVCS experimental setup

Analysis method

Results and conclusions

C. Hyde-Wright,

Hall A

Collaboration

Meeting

How to access GPDs: DVCS

222

2222

)'(

)'(

Qppt

MkkqQ

==

kk’

q’

GPDsp p’


is the simplest hard exclusive

process involving GPDs

pQCD factorization

theorem

Perturbative description

(High Q virtual photon)

Non perturbative description by

Generalized Parton Distributions

Bjorken

regime

2 2

2 2

2

x

B

B

B

Q Qx

pq M

x

x

= =

=

± = fraction of longitudinal

momentum

Handbag diagram

)0,,(xH

x


1

1

( , , )( , , ) ...P i

GPD x tGd PDx x

xt

+

± = +±

±=

The GPDs enter the DVCS amplitude as an integral over x :

Real part

Imaginary partDVCS amplitude

Expression of the cross-section difference

{ }25 5

232 12 2

1 2

1 2

( , , )1( , , )

2sin( ) sin(2 ) sin( )

( ) ( )

DVCSB I I

B

B e B e

x Q td dx Q t s

dQ dx dtd d dQ dx dtd ds s

P P= + +

r s

{ })()2(81

FII CyKys m=

{ } { }2 ( , , ) ( , )m ,qqq

qH te H t=H

2 25 5 2 m( .BH DVCS DVCS DVCSd ó d ó T T ) T T+

r sr s

GPDs

If handbag

dominance

CI (H , ˜ H ,E) = F1(t)H( ,t)+ GM (t) ˜ H ( ,t)+t

4M 2F1(t)E( ,t)

( ) ( )

( )

1 1 2 22( ) ( ) ( ) ( )

2 4

m 0.03 0.01 0. 3

m

1

B

B

I

I x tF t F t F t F t

M

C

Cx

= + +

= +

%HH E

-0.07-0.17-0.04-0.910.3

Neutron Target

2 ( )n

F t 1 ( )n

F t ( )1 2( ) ( ) /(2 )n n B BF t F t x x+ 2 2( / 4 ) ( )n

t M F tt

1.73-0.070.81neutron

Target H %H E(Goeke, Polyakov

and Vanderhaeghen)

Model:2 2

2

2 GeV

0.3

0.3 GeV

B

Q

x

t

=

=

=

CI (H , ˜ H ,E) = F1(t)H( ,t)+ GM (t) ˜ H ( ,t)+t

4M 2F1(t)E( ,t)

H

n-DVCS experiment

19.32

19.32

e

(deg)

2.95

2.95

Pe(Gev/c)

18.25

18.25

- *(deg)

240001.914.22

43651.914.22

Q

(GeV )

s

(GeV )

An exploratory experiment was performed at JLab Hall A on hydrogen target

and deuterium target with high luminosity (4.1037 cm-2 s-1) and exclusivity.

Goal : Measure the n-DVCS polarized cross-section difference

which is mostly sensitive to GPD E (less constrained!)

E03-106 (n-DVCS) followed directly the p-DVCS experiment and was

finished in December 2004 (started in November).

Ldt (fb-1)xBj=0.364

Hydrogen

Deuterium

Requires good experimental resolutionSmall cross-sections

Two scintillator layers:

-1st layer: 28 scintillators, 9 different

shapes

-2nd layer: 29 scintillators, 10 different

shapes

Proton array

Proton tagger : neutron-proton discrimination

Tagger

Proton tagger

Scintillator S1

Wire chamber H

Wire chamber M

Wire chamber B

Prototype

Scintillator S2

Calorimeter in theblack box

(132 PbF2 blocks)

Proton

Array

(100 blocks)

Proton

Tagger

(57 paddles)

4.1037

cm-2.s-1

Calorimeter energy calibration

bloc number0 20 40 60 80 100 120

gai

n v

aria

tio

n (

%)

0

10

20

30

40

50

60

70

80

90

2 elastic runs H(e,e’p) to calibrate the calorimeter

Achieved resolution :( )

2.4% at 4.2 GeV ; 2 mmE

xE

= =

Variation of calibration coefficients

during the experiment due to

radiation damage.

Ca

libra

tio

n v

ari

atio

n (

%)

Calorimeter block number

Solution : extrapolation of elastic

coefficients assuming a linearity

between the received radiation

dose and the gain variation

H(e,e’ )p and D(e,e’ )X data measured “before” and “after”


We have 2 independent methods to check and correct the calorimeter calibration

1st method : missing mass of D(e,e’ -)X reaction

Mp2

By selecting n(e,e’ -)p events,

one can predict the energy

deposit in the calorimeter using

only the cluster position.

a minimisation between the

measured and the predicted

energy gives a better

calibration.

2


2nd method : Invariant mass of 2 detected photons in the calorimeter ( 0)

0 invariant mass position

check the quality of the

previous calibration for

each calorimeter region.

Corrections of the previous

calibration are possible.

Differences between the results of the 2 methods introduce a

systematic error of 1% on the calorimeter calibration.

invariant mass (GeV)0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24

Nb

of C

ount

s

0

2000

4000

6000

8000

10000

12000

14000

16000

= 8.5 MeVσ

mass0π

Nb

of

co

un

ts

Invariant mass (GeV)

Triple coincidence analysis

Identification of n-DVCS events with the recoil detectors is impossible because

of the high background rate.

Many Proton Array blocks contain signals on time for each event .

Proton Array and Tagger (hardware) work properly during the experiment, but :

Accidental subtraction is made for p-DVCS events and gives stable beam

spin asymmetry results. The same subtraction method gives incoherent

results for neutrons.

Other major difficulties of this analysis:

proton-neutron conversion in the tagger shielding.

Not enough statistics to subtract this contamination correctly

The triple coincidence statistics of n-DVCS is at least a factor 20 lower

than the available statistics in the double coincidence analysis.

Double coincidence analysis

eD e X eH e X

accidentals accidentals

( , ' ) ( , ' ) ( , ' ) ( , ' )D e e X p e e p n e e n d e e d= + + +K

p-DVCS

events

n-DVCS

events

d-DVCS

events

Mesons

production

Mx2 cut = (MN+M )

2 Mx2 cut = (MN+M )

2


1) Normalize Hydrogen and Deuterium data to the same luminosity



2) The missing mass cut must be applied identically in both cases

- Hydrogen data and Deuterium data must have the same calibration

- Hydrogen data and Deuterium data must have the same resolution


block number20 40 60 80 100 120

inva

riant

mas

s re

solu

tion

(MeV

)

6

7

8

9

10

11

12

13

cinematique 2

cinematique 4

Re

so

lutio

n o

f 0 in

v.

ma

ss (

Me

V)

Calorimeter block number

Hydrogen

Deuterium

invariant mass (GeV)0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24

Nb

of

Co

un

ts

0

2000

4000

6000

8000

10000

12000

14000

16000

= 8.5 MeVσ

mass0π

Nb

of

co

un

ts

Invariant mass (GeV)

?






- Add nucleon Fermi momentum in deuteron to Hydrogen events







3) Remove the contamination of 0 electroproduction under the missing

mass cut.

0 to subtract

0 contamination subtraction

Mx2 cut =(Mp+M )

2

Hydrogen data







3) Remove the contamination of 0 electroproduction under the missing

mass cut.

Unfortunately, the high trigger threshold during Deuterium runs did

not allow to record enough 0 events.

But :0

0

( )0.95 0.06

( )

e e Xsys

e e X

d

p= ± ±

In our kinematics 0 come essentially

from proton in the deuterium

No 0 subtraction needed for neutron and coherent deuteron


)2 (GeV2xM0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Nb

of

cou

nts

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

cinematique 2cinematique 4

cin.4 - cin.2simulation

Hydrogen data

Deuterium data

Deuterium- Hydrogen

simulation

Nb o

f C

ounts

MX2 (GeV2)

Mx2 cut

n-DVCS ?

d-DVCS ?


(rad)φ0 1 2 3 4 5 6

- - N+ N

-1000

-500

0

500

1000N+ - N-

(rad)


MC Simulation

MC Simulation

Extraction results

Exploration of small –t regions in future experiments might be interesting

Large error bars (statistics + systematics)

d-DVCS extraction results

Extraction results

n-DVCS extraction results

Neutron contribution is small and close to zero

Results can constrain GPD models (and therefore GPD E)

Systematic errors

of models are not

shown

n-DVCS experiment results

Summary

- n-DVCS is mostly sensitive to GPD E : the least constrained

GPD and which is important to access quarks orbital momentum

via Ji’s sum rule.

Our experiment is exploratory and is dedicated to n-DVCS.

To minimize systematic errors, we must have the same calorimeter

properties (calibration, resolution) between Hydrogen and Deuterium data.

n-DVCS and d-DVCS contributions are obtained after a subtraction of

Hydrogen data from Deuterium data.

The experimental separation between n-DVCS and d-DVCS is plausible due to the

different kinematics. The missing mass method is used for this purpose.

Outlook

Future experiments in Hall A (6 GeV) to study p-DVCS and n-DVCS

For n-DVCS : Alternate Hydrogen and Deuterium data taking to

minimize systematic errors.

Modify the acquisition system (trigger) to record enough 0s

for accurate subtraction of the contamination.

Future experiments in CLAS (6 GeV) and JLab (12 GeV) to study

DVCS and mesons production and many reactions involving GPDs.

VGG parametrisation of GPDs

( ) ( )11

1 1

( ,( , ), , ) qq qF tx

d d x x DH x t+

= +

D-term

Non-factorized t dependenceVanderhaeghen, Guichon, Guidal,

Goeke, Polyakov, Radyushkin, Weiss …

( ) ( )

( )( )( )

( )

( )

2 2

2 12

'

1 2

1

12 2

2 1 1

( , , ) ,

,

b

q

t

bb

F

hb

qh

b

t

++

=

+=

+

Double distribution :

Profile function :

Parton distribution

( )for , the spin-flip parton densities is used :G PD qE e

Modelled using Ju and Jd as free parameters

-2' 0.8 GeV for quarks=

0 electroproduction on the neutron

Pierre Guichon, private communication (2006)

( ) 0 3( , ) ,3 NT N T T i T+= + +

Amplitude of pion electroproduction :

is the pion isospin

nucleon isospin matrix

0 electroproduction amplitude ( =3) is given by :

( )

( )

0

0

2 1,3

3 3

1 2,3

3 3

T T T

T

p

T Tn

u d

u d

+

+

= + +

= +

Polarized parton distributions in the proton

( ) ( )( )

,3 ,3 3 31.15

,3

/

/2

dpT

d

nT u

T up +

+ +


One can predict for each (e, ) event the Proton Array block where the

missing nucleon is supposed to be (assuming DVCS event).


PA energy cut (MeV)0 10 20 30 40 50 60

rela

tive

asy

mm

etry

(%

)-60

-50

-40

-30

-20

-10

0

10

20 1 predicted block

9 predicted blocks

25 predicted blocks

PA energy cut (MeV)0 10 20 30 40 50 60

rela

tive

asy

mm

etry

(%

)

-5

0

5

10

15

20 1 predicted block

9 predicted blocks

25 predicted blocks

Re

lative

asym

me

try (

%)

Re

lative

asym

me

try (

%)

PA energy cut (MeV)

PA energy cut (MeV)

After accidentals subtraction

neutrons selection

protons selection

-proton-neutron conversion in

the tagger shielding

- accidentals subtraction

problem for neutrons

p-DVCS events (from LD2

target) asymmetry is stable

hEntries 41118Mean -1.57RMS 6.12

time (ns)-30 -25 -20 -15 -10 -5 0 5 10 15 200

500

1000

1500

2000

2500

3000

3500

4000

hEntries 41118Mean -1.57RMS 6.12

= 1. nsσ

Time spectrum in the tagger(no Proton Array cuts)

0 contamination subtraction

One needs to do a 0 subtraction if the only (e, ) system is used

to select DVCS events.

Symmetric decay: two distinct photons are detected

in the calorimeter No contamination

Asymmetric decay: 1 photon carries most of the 0

energy contamination because DVCS-like event.

0.070.130.330.380.7

0.070.170.420.540.5

0.060.240.560.820.3

0.040.380.811.340.1

Proton Target

Proton

2 ( )p

F t 1 ( )p

F t ( )1 2( ) ( ) /(2 )p p B BF t F t x x+ 2 2( / 4 ) ( )p

t M F tt

t=-0.3

0.980.701.13Proton

Target H %H E

Goeke, Polyakov and Vanderhaeghen

Model:

2 22 GeV

0.3

0.3

B

Q

x

t

=

=

=

( )1 1 2 22( ) ( ) ( ) ( )2 4B

B

x tA F t F t F t F t

x M= + + %HHE

( )1 1 2 22( ) ( ) ( ) ( )2 4

0.34 0.17 + 0.06

B

B

x tA F t F t F t F t

x M

A

= + +

= +

%HHE

DVCS polarized cross-sections


We have 2 independent methods to check and correct

the calorimeter calibration

1st method : missing mass of D(e,e’ -)X reaction

By selecting n(e,e’ -)p events, one can

predict the energy deposit in the

calorimeter using only the cluster position.

a minimisation between the

measured and the predicted

energy gives a better calibration.

2

2nd method : Invariant mass of 2 detected photons in the calorimeter ( 0)

0 invariant mass position check the

quality of the previous calibration for

each calorimeter region.

Corrections of the previous

calibration are possible.

Differences between the results of the 2 methods introduce a

systematic error of 1% on the calorimeter calibration.

eD e X

p-DVCS and

n-DVCS

MN2

MN2 +t/2

d-DVCS

Analysis method

0e X XeD e

Contamination by

Mx2 cut = (MN+M )

2

Date post:	27-Jan-2021
Category:	Documents
Upload:	others
View:	0 times
Download:	0 times

cattering S ompton C irtual V Dhallaweb.jlab.org/experiment/DVCS/documents/talk/CEH_jan... · 2007....

Documents