Measurement of the Hard photodisintegration of a proton-pair in the 3He nucleus.

Running now in JLab, hall A

E03-101:GamPP n

p

p

Ishay Pomerantz, Tel-Aviv University

Hall A Collaboration Meeting 2007 2

E03-101:GamPP

Hard photodisintegration: A process in which a high energy photon is absorbed by

a nucleon pair, resulting in two nucleons with large transverse momenta.

n

p

p

1-5 GeV

3He

1.5-2.5 GeV/c

”Spectator” neutron

1.5-2.5 GeV/c

γ

_ 3 He _ p _high pt__ p _high p

t__ n_slow_

n

p

p

Hall A Collaboration Meeting 2007 3

E03-101:GamPP

Motivation: Significant time has been devoted by JLab to investigate

the hard photodisintegration of the Deuteron. The breakup of a pp pair is a direct continuation of this study.

Deuteron results indicates that the quarks are the relevant degrees of freedom in describing these interactions, yet pQCD fails to describe them. Several quark models have been formulated, and their validity requires testing.

n

p

p

Hall A Collaboration Meeting 2007 4

E03-101:GamPP

Two possible mechanisms can explain the high Pt:

Breaking a transverse compact object

(Initial state)

Hard rescattering

(Final state)

n

p

p

Hall A Collaboration Meeting 2007 5

E03-101:GamPP

Breaking a transverse compact object: The high Pt Results from initial state correlation. A very minute part of the pair wave function.

n

p

p

p

p

γ

QGS

Quark Gluon String ModelV. Yu Grishina et al. EUR.J.Phys.A10,355(2000)

p

γ

p

RNA

Reduced Nuclear AmplitudesS.J. Brodsky, J.R. Hiller,Phys. Rev. C 28 (1983) 475

Hall A Collaboration Meeting 2007 6

E03-101:GamPP

Hard rescattering: One proton absorbs the photon, and then interacts with

the other member of the pair. Also a rare case (large c.m. scattering angle).

n

p

p

HRM

γ

ψpp

p

p

T PP_ PP

Hard Rescattering ModelL.L. Frankfurt, G.A Miller,M.M. Sargasian, M.I Strikman , Phys. Rev. Lett.84,3045(2000)

Hall A Collaboration Meeting 2007 7

E03-101:GamPP

Experimental setup

n

p

p

Electrons3He

Radiator Target

HRS

HRS

Photons

Protons

Hall A Collaboration Meeting 2007 8

E03-101:GamPP

The radiator

n

p

p

Hall A Collaboration Meeting 2007 9

E03-101:GamPP

Planned measurements: Scaling of the cross section with s Oscillations The light cone momentum distribution of the spectator

neutron Polarization transfer

n

p

p

Hall A Collaboration Meeting 2007 10

E03-101:GamPP

Scaling of the cross section with s: constituent counting rule predicts:

For the case of:

N =1(ɣ)+6(pp)+3(p)+3(p)-2=11

n

p

p

d_dt AB _ CD

_ S_ _N=n

A+n

B+n

C+n

D-2_

f _ts

_

_ pp _ p_highpt _ _ p_high pt _

Hall A Collaboration Meeting 2007 11

E03-101:GamPP

Oscillations: A unique feature of hard rescattering

n

p

p

d_dt

__ pp _ pp__d_dt

_ pp _ pp_

HRM

γ

ψpp

p

p

T PP_ PP

Hall A Collaboration Meeting 2007 12

E03-101:GamPP

The light cone momentum distribution of the spectator neutron:

The width of the distribution The asymmetry around

αn=1 indicates the s

dependence.

n

p

p

_ n=AEn_ pnz

EA_ pAz

~En_ pnz

mnCompact object

Rescattering

Hall A Collaboration Meeting 2007 13

E03-101:GamPP

Polarization transfer:

At the moment, doesn't

seem feasible because

of low rates at high

energy and priority

considerations.

n

p

p

Deu

tero

n da

ta

X. Jiang et al. Phys. Rev. Lett. 98, 182302 (2007)

Hall A Collaboration Meeting 2007 14

E03-101:GamPP

Offline analysis: Full kinematic reconstruction. Application of cuts. Comparison of the various reconstructed kinematic

variables with a simulation. Inspection of the Events/Unit charge ratio. Extraction of the scaled cross section and αn.

n

p

p

Hall A Collaboration Meeting 2007 15

E03-101:GamPP

Full kinematic reconstruction.

n

p

p

Hall A Collaboration Meeting 2007 16

E03-101:GamPP

Full kinematic reconstruction.

n

p

p

Neutron momentum Reaction point Photon energy

Gev/c Gevm

Hall A Collaboration Meeting 2007 17

E03-101:GamPP

Applying cuts on: Coincidence events. Detector acceptance. Reaction point within the

target. An energy bin of 200MeV

from the photon end point.

n

p

p

Gev

Photon energy at beam energy 2.1GeV

EndPoint

Hall A Collaboration Meeting 2007 18

E03-101:GamPP

Comparison of the various reconstructed kinematic variables with a simulation.

Simulation is used to evaluate coincidence efficiency

n

p

p

Red – Generated events(scaled) Black- events that fall within acceptance

Gev/c Gev/c m

Hall A Collaboration Meeting 2007 19

E03-101:GamPP

Events/Unit charge as a function of the run no.

n

p

p

With radiator

W/o radiator

Eγ=3.1GeV

Hall A Collaboration Meeting 2007 20

E03-101:GamPP

Inspection of the No. events/charge ratio. Oops – a pressure drop !

n

p

p

IOC reboot

Fixed

Hall A Collaboration Meeting 2007 21

E03-101:GamPP

Extraction of the scaled cross section and αn.

n

p

p

Hall A Collaboration Meeting 2007 22

E03-101:GamPP

Very Preliminary (online) results:

Points at 1.1 GeV 2.1GeV 3.1GeV 4.1GeV

n

p

p

Hall A Collaboration Meeting 2007 23

E03-101:GamPP

Very Preliminary (online) results:

Possible interpretation

n

p

p

?

Hall A Collaboration Meeting 2007 24

E03-101:GamPP

Plans for next week 1.65 GeV 2.4 GeV 4.72 GeV Maybe 0.8 GeV

n

p

p

Hall A Collaboration Meeting 2007 25

E03-101:GamPP

Summary In Hard photodisintegration two protons are found to have

large transverse momentum. The goal of the experiment is to find the origin of this

momentum. We look at features of the cross section, and the light cone

momentum distribution to establish an explanation. We found high cross section point at 2.1GeV that we'll

further investigate, and we plan to continue to 4.7GeV.

n

p

p