results and the exciting future of 3 He targets

31 

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(1)

Gordon D. Cates University of Virginia

Users Group Meeting - 8 June, 2010

Results from the Hall A GEn experiment

Looking forward to 12 GeV - what high luminosity polarized 3He targets make possible: GEn, A1n,

and more.

G

E

n

results and the exciting

(2)

One of JLab’s most important discoveries is

the high Q

2

behavior of G

Ep

/G

Mp

(3)

One of JLab’s most important discoveries is

the high Q

2

behavior of G

Ep

/G

Mp

World data including JLab. Instead of remaining roughly flat, GEp/GMp was observed to reduce almost linearly with Q2

(4)

One of JLab’s most important discoveries is

the high Q

2

behavior of G

Ep

/G

Mp

Forced a reconsideration of the nucleon

wavefunction.

Quark orbital angular

momentum appears to be a necessary ingredient to explain the result.

Theoretical work to understand the proton results make predictions for the neutron.

(5)

Single-spin asymmetries in SIDIS

(an over-simplified view)

y z x e n al p n or d a h e n al p n ot p el l l S Ph Ph φh φS

A

U T

(

φ

h

,

φ

S

) =

A

CollinsU T

sin(

φ

h

+

φ

S

) +

A

SiversU T

sin(

φ

h

φ

S

)

(6)

Single-spin asymmetries in SIDIS

(an over-simplified view)

A nonzero Sivers amplitude can be interpreted as evidence of the dynamic importance of

quark orbital angular momentum.

y z x e n al p n or d a h e n al p n ot p el l l S Ph Ph φh φS

A

U T

(

φ

h

,

φ

S

) =

A

CollinsU T

sin(

φ

h

+

φ

S

) +

A

SiversU T

sin(

φ

h

φ

S

)

(7)

Spin asymmetry A

1n

in deep inelastic scattering

A

n

1

=

d

d

σ

σ

1/2

d

σ

3/2

1/2

+

d

σ

3/2

pQCD predictions

incorporating hadron helicity conservation don’t agree with the data.

Relativistic Constituent Quark Models that implicitly include quark orbital angular

momentum provide better fit.

Data on A1n after 12 GeV

upgrade will provide valuable insight

(8)

Method for measuring G

En

in E02-013

Here a, b and c are solely functions of kinematic factors (and not θ* or Φ*)

Measure coincidences using reaction:

Align polarization roughly perpendicular to q.

Asymmetry

GEn/GMn

3

He(

!

!

e, e

!

n

)

Aphys = A + A" = a · (GE/GM) sinθ∗ cosφ∗

(GE/GM)2 + c

+ b · cos θ∗

(9)

Experimental setup for E02-013

to measure G

En

in Hall A at JLab

Polarized

3He target neutron arm

Beam

(10)

The electron arm - BigBite Spectrometer

Data prior to JLab

Open-geometry single-dipole spectrometer. BigBite provides 75 msr solid angle, a momentum bite of ΔQ2/Q2 0.1, and excellent statistics.

• Calorimeter provided BigBite trigger.

• 15 planes of drift chambers provided tracking.

• Front plane operated at a singles rate of around 20 MHz !

(11)

Neutron arm - BigHAND

One of our most powerful cuts: he missing-momentum

component, pperp, between

q (as defined by the electron) and the direction

of the detected hadron.

Based on time-of-flight with 0.40 ns

time resolution

Flight distance around 10 m.

Acceptance matched to BigBite

1.6 x 5 m2 active area.

6-7 layers (~ 250 bars).

2 veto layers

Operation at effective luminosity of

3x1037 cm-2 s-1

(12)

Polarized

3

He target was based on

spin-exchange optical pumping

Important innovation was using a

hybrid mixture of K and Rb to achieve better efficiency

Polarizations of ~50% were

achieved over most of the running!!

pumping chamber

target chamber

(13)

GEN required a novel implementation of

3

He

spin-exchange technology

Data prior to JLab

• Needed to be extremely close to the open-geometry BigBite magnet.

• Needed magnetic field inhomogeneities no worse than around 10 mG/cm

• “Iron Box” magnet design permitted target/BigBite distance of ~ 1m.

(14)

Coincidence events for 1.7 (GeV/c)

2

Data prior to JLab

Coincidence events, identified as neutrons, plotted as a function of both qperp and W where again qperp = q tan(θqh),

and θqh is the angle between the direction of the detected

hadron and the direction of q.

(15)

Coincidence events for 3.4 (GeV/c)

2

Cuts include missing mass< 2 GeV, and pparallel < 400 MeV

Coincidence events, identified as neutrons, plotted as a function of both qperp and W where again qperp = q tan(θqh),

and θqh is the angle between the direction of the detected

(16)

W distributions with successive cuts

Black: raw spectrum Red: qperp cut

Blue: qperp, ToF and missing mass cuts

Quasi-elastic cleanly emerges.

(17)

Dealing with inelastics

A Monte Carlo based using MAID was implemented to correct for a small contamination of inelastics in our final event sample.

(18)

W (GeV) 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 Asymmetry -0.06 -0.04 -0.02 0 0.02 0.04 0.06

Statistics (Arbitrary Units)

Data Data Asymmetry MC Asymmetry < 0.45 GeV/c miss, , 0.15 < p 2 = 3.4 GeV 2 Asymmetry Comparison - Q W (GeV) 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 Asymmetry -0.06 -0.04 -0.02 0 0.02 0.04 0.06

Statistics (Arbitrary Units)

Data Data Asymmetry MC Asymmetry < 0.15 GeV/c miss, , 0.0 < p 2 = 3.4 GeV 2 Asymmetry Comparison - Q

The measured asymmetries of the inelastic

contamination is very similar to the

quasi-elastic asymmetry

• The asymmetry for quasi-elastics and elastics is nearly the same.

• The p⊥ cut can be used to vary the contribution from each (quasi-elastic/inelastic)

in order to de-convolve the effect of one from the other.

• We are in the process of recovering additional statistics from higher pseudo-W.

Data

Cuts for nearly pure quasi-elastics

p⊥ < 0.15 GeV

Cuts for nearly pure inelastics

0.15 GeV < p⊥ < 0.45 GeV Data

(19)

W (GeV) 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 Asymmetry -0.06 -0.04 -0.02 0 0.02 0.04 0.06

Statistics (Arbitrary Units)

Data Data Asymmetry MC Asymmetry < 0.45 GeV/c miss, , 0.15 < p 2 = 3.4 GeV 2 Asymmetry Comparison - Q W (GeV) 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 Asymmetry -0.06 -0.04 -0.02 0 0.02 0.04 0.06

Statistics (Arbitrary Units)

Data Data Asymmetry MC Asymmetry < 0.15 GeV/c miss, , 0.0 < p 2 = 3.4 GeV 2 Asymmetry Comparison - Q

The measured asymmetries of the inelastic

contamination is very similar to the

quasi-elastic asymmetry

• The asymmetry for quasi-elastics and elastics is nearly the same.

• The p⊥ cut can be used to vary the contribution from each (quasi-elastic/inelastic)

in order to de-convolve the effect of one from the other.

• We are in the process of recovering additional statistics from higher pseudo-W.

Data

Cuts for nearly pure quasi-elastics

p⊥ < 0.15 GeV

Cuts for nearly pure inelastics

0.15 GeV < p⊥ < 0.45 GeV Data

(20)

The Hall A GEN results on G

En

/G

Mn

Below expectation from ln2(Q2/Λ2)/Q2 scaling for F

2/F1 .

Below light-front cloudy bag model of Gerry Miller.

Good agreement with DSE/Faddeev equation approach from Argonne.

Already, GEN-I is impacting our understanding of nucleon structure.

] 2 [GeV 2 Q n M /G n E G n µ 0.0 0.2 0.4 0.6 0.8 RCQM GPD VMD Faddeev&DSE = 150 MeV ! , 1 /F 2 F = 300 MeV ! , 1 /F 2 F 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

(21)

10−2 10−1 100 101 102 p2 [GeV2] 10−3 10−2 10−1 100 101 M(p 2 ) [GeV] b−quark c−quark s−quark u,d−quark chiral limit M2(p2) = p2

The mass of the constituent quarks is dynamically

generated using the Dyson-Schwinger equations.

Three constituent quarks then serve as the degrees of freedom for a calculation involving a Faddeev equation.

Diquark-coupling is included.

While still a model, the calculation has features that move toward a true analytical approach.

(22)

At high 10 GeV2 DSE predictions could be

definitively tested

2 in GeV 2 Q 0 5 10 n M G/ n E G n M 0.0 0.5 1.0 VMD - E. Lomon (2002) RCQM - G. Miller (2002) DSE - C. Roberts (2009)

- Schiavilla & Sick 20 d(e,e’d) T = 300 MeV , , 2 )/Q 2 , / 2 (Q 2 ln t 1 /F 2 F Galster fit (1971) Madey, Hall C E02-013

(23)

Is there any way to reach

(24)

The “polarizing power” of

3

He targets at JLab

represents untapped potential

Spins polarized per second weighted by polarization squared. GDH

A1n GEn(1)

(25)

Alkali-hybrid cells with spectrally

narrowed high-power diode-laser arrays

(26)

Spins are being polarized so quickly that diffusion

limits polarization in the target chamber

With depolarization from the electron beam, the “polarization

gradient” is even more severe.

pumping chamber target chamber 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 5 10 15 20 25 30 Time (hours) 3 He Polarization (%)

Brady Spinup Aug 2008

Pumping Chamber Target Chamber

(27)

Convection-driven gas-flow test

• Heater drives convection

• Zapper coil depolarizes slug of gas

• Coils #1 - #4 monitor passage of depolarized slug.

• Heater temperature can be changed to adjust speed of gas.

• Plot shows speeds up to 80 cm/min.

AFP Corrected NMR Si gnal (mV ) 28.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0

Time since Zap (s) 280.0

20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 220.0 240.0 260.0 Coil 1 Coil 2 Coil 3 Coil 4 20 40 60 80 100 120 140 160 −10 0 10 20 30 40 50 60 70 80

90 Convection Test November 2008

Heated Transfer Tube Temperature (C)

3He Velocity in Target Chamber(cm/min)

A*(273 + T 273 + B1 1 ) A = (−7.67+ − .54)*104 K cm/min B = (24.48 + − .47) K

(28)

Very-high-luminosity polarized

3

He target

• Large pumping chamber provides ample reservoir of polarized spins to replenish the effects of intense electron beam.

• Convection-driven gas flow insures mixing times of minutes or less.

• Metal target cell (gold coated) ensures the target can physically tolerate the beam.

• Keeping the target cell in vacuum

ensures the detectors see a manageable overall luminosity.

• Separating the pumping chamber and the target chamber by arbitrary distances greatly simplifies the magnetic field.

60 cm glass

gold-plated metal

Combination oven and “magnet box”

Scattering chamber provides vacuum around target cell Pumping

chamber Laser light for

optical pumping

target chamber

Long blue parallel wires will provide transverse magnetic field with adequate homogeneity. Field can be

mani-pulated independently from the pumping chamber Heater on

transfer tube drives convection

(29)

Summary

The JLab Hall A GEn more than doubles the Q2 range over which GEn is

known.

The new data provide support for some of the new theoretical

understanding of nucleon structure, and perhaps provide intriguing hints regarding how we should refine our understanding.

The next-generation 3He target will provide a large increase in

luminosity, providing exciting new experimental possibilities

-

GEN -II with measurements up to 10 GeV2

-

A1n at high x

(30)
(31)

Error Budget

Figure

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References

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