December 16 2008 1

Extraction of exclusive neutral pions electro-production cross sections in

Hall A @ Jefferson Laboratory.

Eric FUCHEYPh.D Student @ Laboratoire de Physique

CorpusculaireUMR 6533 CNRS/IN2P3Université Blaise Pascal

Clermont-Ferrand

December 16 2008 2

Outline

• 1– Motivations• 2– 0 electroproduction formalismes

– Kinematical variables – Formalismes

• 3– Experimental layout• 4– Cross sections extraction

– Calibration– Extraction method– Selection cuts

• 5– Conclusion and perspectives

December 16 2008 3

Outline

• 1– Motivations• 2– 0 electroproduction formalismes

– Kinematical variables – Formalismes

• 3– Experimental layout• 4– Cross sections extraction

– Calibration– Extraction method– Selection cuts

• 5– Conclusion and perspectives

December 16 2008 4

0 interest

• DVCS is the smartest way to get access to GPD’s theoretically.

• In most of DVCS experiments, 0 production ( with ) is an important by-product (Pollution for DVCS analysis)

0epep 0

December 16 2008 5

Outline

• 1– Motivations• 2– 0 electroproduction formalismes

– Kinematical variables – Formalismes

• 3– Experimental layout• 4– Cross sections extraction

– Calibration– Extraction method– Selection cuts

• 5– Conclusion and perspectives

December 16 2008 6

Kinematic variables

• Virtuality:

• Momentum fraction:

• Quadri transfert to proton:

• *-azimuthal angle:

• *-p CM energy:

• Transverse momentum:

22 )'( kkQ

qp

QxBj

2

2

2)''( qkkt

)11

(22 Bj

p xQMW

* h

)sin( hhh pp

December 16 2008 7

Formalismes

• Drechsel-Tiator (Nucl.Part.Phys.18(1992)449-497):

with

and

• More « intuitive » formalisme

h

v

he d

d

ddEd

d

'

5

1

1'

2 22 Q

k

E

E

)2cos()cos()1(2 hh

TTLh

h

TLL

h

LL

h

T

h

v

d

d

d

d

d

d

d

d

d

d

h

TTh

h

TLL d

d

d

d

'' 1)sin()1(2

December 16 2008 8

Formalismes

• Levelt-Mulders (Phys.Rev.D.49(1994)96-113) :

• Show an explicit dependence in transverse momentum

21

2

//4

2

2

5

21

4H

E

yMxyHyx

p

E

Q

ME

ddzdydpdx

d BjBj

h

h

hhBj

42

2

32)2cos(

21

21

)cos(2

1

21

)2( H

yE

MxE

yMxy

Q

pH

yE

MxE

yMxy

yQ

ph

Bj

Bj

hh

Bj

Bj

h

December 16 2008 9

Formalismes: comparison

• Interest: Being able to get easily cross sections from Drechsel-Tiator as function of pion transverse momentum:

With :

1HM

S

dt

d T

1224

2

HMQ

SHQ

qS

dt

d L

34Hp

Q

qS

dt

dh

TL

54

' HpQ

qS

dt

dh

TL

42

4

1Hp

QS

dt

dh

TT

hCMCMEM

WkqS

116 4

December 16 2008 10

Outline

• 1– Motivations• 2– 0 electroproduction formalismes

– Kinematical variables – Formalismes

• 3- Experimental layout• 4– Extraction des section efficaces

– Calibration– Extraction method– Selection cuts

• 5– Conclusion and perspectives

December 16 2008 11

DVCS experiment

• Incident beam : E = 6 GeV, L ~ 1037cm-2s-1 during experiment– Scattered electron measured by HRS

– 2 photons issued from 0 measured by 132 PbF2 blocks calorimeter

– Recoil proton detected by proton array -> not used

December 16 2008 12

Outline

• 1– Motivations• 2– 0 electroproduction formalismes

– Kinematical variables – Formalismes

• 3– Experimental layout• 4– Cross sections extraction

– Calibration– Extraction method– Selection cuts

• 5– Conclusion and perspectives

December 16 2008 13

Calibration

• Goal : searching coefficients to apply to calorimeter blocks in order to:– bring their reconstructed missing mass

squared value to the proton missing mass squared.

– Bring then simulated missing mass mean and resolution to real ones

Mp2

December 16 2008 14

Calibration• Calibration has been performed for set of Kin3 0 data and

our set of Kin3 0 simulated.

December 16 2008 15

Extraction method

• Kinematical domains :– Kin2: xBj = 0.36, Q2 = 1.9

GeV2

– Kin3: xBj = 0.36, Q2 = 2.3 GeV2

• Analysis made according to tmin-t.

• Fitting real data with simulated to take into account of acceptance effect.

December 16 2008 16

Extraction method

Principle: compare expected number of counts for data

to expected number of counts for simulated

),( 22

dtddxdQig

dtddxdQ

dLdtN Bj

Bini Bj

iiBinR

),( 2

2

dtddxdQig

dtddxdQ

d

N

psfLdtN Bj

BiniBinBj

ii

GenBinS

No access

What we want to extract

December 16 2008 17

Selection cuts• Missing mass cut to ensure

exclusivity

• Each photon is in a calibrated block, and E > 1.0 GeV

• k’ = pHRS +/- 4.5% ; r-function > 0.005.

• -6.0 cm < zvtx < 7.5 cm.

22mMM pX

0epep Xeep

December 16 2008 18

Fit results• Fit has been performed for helicity-independent cross

section and helicity-dependent cross section

• However, still not able to extract cross sections (debugging…)

2/NDF = 2.77 2/NDF = 0.76

December 16 2008 19

Outline

• 1– Motivations• 2– 0 electroproduction formalismes

– Kinematical variables – Formalismes

• 3– Experimental layout• 4– Cross sections extraction

– Calibration– Extraction method– Selection cuts

• 5– Conclusion and perspectives

December 16 2008 20

4 – Conclusion / perspectives

• 0 electroproduction cross section extraction in Hall A is in progress.

• Calibration has been performed (see talk on calibration this afternoon)

• Once first results obtained: – Optimizing some selection cuts

– Study evolution in other variables: xBj, Q2, W