Transcript Polarized 3 He

Measurement of doublepolarized asymmetries in quasielastic processes 3He(e,e’d) and
3He(e,e’p)
Miha Mihovilovič
For the E05-102 Collaboration
Why study 3He?
- Proton is well known and its properties are precisely measured.
- Neutron is relatively poorly understood. Only loose constraints
on the charge, magnetism and spin distribution
Problem: direct measurements not possible, no neutron
target.
Solution: indirect measurements using appropriate
targets:
1.) Deuteron where neutron behaves almost as a free particle due
to the small binding energy (~2MeV)
2.) Polarized 3He used as effective polarized neutron target.
!!!
3He
as effective n0 target
Precision of this approximation depends
on the understanding of the structure
of the 3He.
3He
is a calculable nuclear system,
where theoretical predictions of its
nuclear structure can be compared
with data to an increasingly accurate
degree.
Precise test of understanding of the
3He structure, nuclear forces between
nucleons, FSI, MEC.
Understand the structure of the n0
Error budget for An1:
aside statistical error,
leading source of error is
due to the uncertainty
of the polarization of
the proton and neutron
in polarized 3He.
3He
ground-state wave-function
S
- A bound state of p,p,n: S = ½, T = ½ (MT = +½)
- Calculations predict three dominant components:
1.) Spatially symmetric state S (90%):
Protons are in spin-singlet state. 3He spin
is dominated by spin of n. Therefore 3He
can be used as an effective n target.
p
S’
p
n
3.) Mixed symmetry state S’ (2%): Arises from
differences between T=0 and T=1 forces and hence
reflects (spin-isospin)-space correlations.
p
(L=0)
p
2.) State D (8%): Nucleon spins oriented
in opposite to the 3He nuclear spin.
(L=0)
Generated by tensor component of NN force.
n
D
p
n
p
(L=2)
Ax,Az Asymmetry Measurement
Experiment E05-102 at Jefferson Lab
- Understanding the role of the D and S' states in 3He is a very
important aspect of the few-body theory.
- Observables sensitive to the S' state constitute a stringent
test of the theory. Among them are also asymmetries Ax and Az.
- For polarized beam and polarized target, the cross-section for
the 3He(e,e’d) is:
The (±; ±) signs represent
the beam helicities and the
projections of the target spin.
- Measuring asymmetries saves a lot of problems, because “all”
the problems with normalization of cross-section disappear.
!
Thomas Jefferson National Accelerator Facility
- CEBAF center at JLab
was built to investigate
the structure of nuclei
and hadrons at
intermediate energies
and underlying
fundamental forces.
A
B
C
6 GeV polarized continuous beam with
currents up to 100uA is delivered to
three experimental Halls A, B and C.
Experimental Setup in Hall A
Experiment E05-102 in Hall-A
Incident polarized
electron
Beam Helicity
θL
θq
γ*
p
n
p
Ax
Az
p
n
p
Detected Deuterons
and Protons
Polarized 3He Target
Polarized 3He Cell
12C
Optics Target
Oven with
Mirrors
Beam Direction
Large Coil
Vertical Coil
Small Coil
Polarization of the 3He Target
• Five High-Power IR-Diode lasers
(~30W) are used to polarize the
target in all three directions
• Optical table with lenses, mirrors, λshifters is used to properly guide light
from optical fibers to the target.
High Resolution Spectrometers
Detector package
Dipole
2 Quadrupoles
Particle Track
Quadrupole
HRS - Detector Package
Electronics
Open Detector-Hut
VDCs
BigBite Spectrometer
- Single normal-conducting
dipole magnet spectrometer
- Combines a large solid
angle with a large momentum
acceptance.
- Two MWDCs for tracking;
Each MWDC consists of 6
wire planes u,u’,v,v’,x,x’
-Two Scintillation planes
E/dE for particle
identification and Energy
determination
Analysis of Measured Data
- Experiment was done in June 2009. We accumulated ~15C and
collected 7TB of data.
- Now Analysis is now underway. At the moment we are doing
calibration of BigBite. After the calibration the asymmetry
determination will folow.
BigBite Optics Calibration
- Purpose of optics calibration is to determine target variables
(yTg, φTg, θTg, δTg) from focal plane variables (xFp, yFp, θFp, φFp).
- There are many parameterizations possible. We use polynomials:
What are the theoretical expectations?
- The role of S’ is most evident in region of small recoil momenta
where Ax is large. Az is close to zero at small pr.
- Much stronger variation of Az at high pr. This behavior is
governed by the D-state.
D
S’
D
2H
S
Results from NIKHEF
D
p
p
n
(L=0)
n
(L=2)
- Deuteron is S=1 particle made
of two S=1/2 nucleons (p,n)
-Sign-change of the Asymmetry
is a clear sign that D-state
component becomes important
in the nucleus at high pmiss.
-This was observed in NIKHEF
in the experiment with 2H
polarized target
Conclusions
- Experiments with polarized target and polarized beam are
important step forward in the experimental nuclear physics.
- Asymmetries give an insight to the properties of the nucleons that
were not measurable with unpolarized experiments.
- Many 3He experiments were already done.
- Why is experiment E05-102 so special?
1.) Double polarized experiment (3He, e )
2.) Measured all three (p,d,n) channels at same Q2 with ω
covering the whole q-e peak and more.
3.) Measured asymmetries as function of pmiss.
It is the first, where D-wave and S’-wave contributions to 3He will be
inspected in detail in order to understand Spin, Iso-Spin structure
of Nuclei, MEC, FSI. Very important for all further experiments on
3He.
Thank you for listening!