Precision measurement of the proton electric to magnetic ...



Thesis Proposal

Chris Crawford

November 14, 2001

Precision measurement of the proton electric to magnetic form factor ratio with BLAST

Spokespersons: H. Gao, J.R. Calarco, H. Kolster

Outline

1. Introduction and Motivation

2. Existing Measurements

3. The Proposed Experiment

a. Overview

b. Formalism

c. Requirements

d. Measurement

4. Conclusions

1. Introduction and Motivation

• The electromagnetic form factors are fundamental quantities related to the distribution of charge and magnetization within a nucleon.

• The electromagnetic form factor ratio is also important, as any Q2 dependence will suggest different charge and magnetization spatial distributions inside the nucleon.

o High Q2 (beginning at 0.5 GeV2 ) results from JLAB show a downward trend in this ratio for the proton.

o More recent higher Q2 data from JLAB show a continuing downward trend up to Q2 =5.6 GeV2.

o Low Q2 data from 0 – 1 GeV2 is the focus of this experiment, which is required to determine the turning point in the Q2 dependence of the ratio.

Theory

• Lattice calculations:

o expected in the near future for low Q2 (0 – 1 GeV2).

• Models with explicit meson degrees of freedom:

o Vector Meson Dominance (VMD), Höhler 1976. Photons couple to hadrons only via intermediate vector bosons.

o VMD and Chiral Perturbation Theory (ChPT), Mergell et al. 1996.

o Soliton Model, Holzworth 1996. This treats the Baryon as an extended object with coupling to vector mesons.

• QCD based Quark models:

o Constituent quarks, Frank et al. 1996. This model constructs valence quark wavefunctions and does not include the pion cloud or gluons.

o Cloudy bag, Lu et al. 1998. Photons couple with a valence quark core and meson fields.

[pic]

2. Measurement Techniques

• Rosenbluth separation

[pic]

o Plot of [pic] vs. tan2((/2) gives [pic] and [pic]

o Difficult at [pic] ([pic]), and low [pic] ([pic]).

• Polarization transfer measurements [pic] using a Focal Plane Polarimeter (FPP).

[pic]

[pic] and [pic] of the scattered proton were measured simultaneously within the FPP using scattering from [pic].

o Milbrath et al. (BATES), 1999. Also [pic]

o Jones et al. (JLAB), 2000.

o Dieterich et al. (MAMI, Mainz), 2001.

• Polarized beam and target measurement

o Well suited to BLAST

o Different systematics than above

o Insensitive to beam and target polariazation

o Our proposed experiment

Unpolarized Scattering and Polarization Transfer Data

[pic]

Polarization Transfer Data

[pic]

3. The Proposed Experiment

3a. Overview

• Will use the reaction [pic] to measure [pic]

at Q2 = 0.07 – 0.9 GeV 2 to unprecedented precision at beam energies 440 MeV and 880 MeV.

• Completely different experimental technique:

polarized H target instead of recoil proton polarimeter.

• Fits nicely between higher Q2 data at JLAB

and lower Q2 data from RPEX (future exp at BATES)

( [pic] from Q2 = 0.02 – 6 (GeV/c) 2.

• Approved at the 28th BATES PAC with the highest scientific priority.

3b. Formalism

• A longitudinally polarized electron scatters from a polarized proton in the lab frame.

[pic]

• The proton may be polarized in any direction described by [pic] and [pic] with respect to [pic].

• The cross section for polarized scattering

[pic]

where h is the helicity of the electron beam, [pic] is the unpolarized cross section and [pic] is the spin dependent part.

• Theoretical asymmetry:

[pic]

• Experimental asymmetry:

[pic]

• Ratio of [pic] and [pic] (same [pic]):

[pic]

[pic]

• We are therefore able to extract the form factor ratio independent of the beam and target polarizations to first order.

3c. Requirements

• Polarized electron beam stored in the ring (SHR)

o 60% polarization, 80μA average current

o Compton polarimeter

• Polarized internal hydrogen target

o Atomic Beam Source (ABS)

o Laser Driven Target (LDT)

• Symmetric Large Acceptance Detector (BLAST).

o Toroidal Magnetic Field

o Wire Chambers

o Timing Scintilators

o Cěrenkov

o Lead Glass Calorimeter

o Neutron Detectors

Polarized Beam

[pic]

• The Compton Polarimeter

[pic]

Polarized Target

• Atomic Beam Source

o Well established technology

o Can create pure spin states

o Polarization: P = 0.8

o Flux: ( = 7×1016 /s

o Thickness: t = 5.5×1013 /cm2

• Laser Driven Target

o Compact design

o Active pumping—higher flux

o Flux: ( = 2×1018 atoms/s

o Atomic fraction F = 0.6 (expected)

o Polarization P = 0.5 (expected)

Atomic Beam Source

[pic]

Atomic Beam Source

[pic]

Laser Driven Target

• Optical Pumping and Spin Exchange

[pic]

• Schematic of Target

[pic]

[pic]

• Dissociation Results:

• Polarization Results:

[pic]

BLAST Detector

[pic]

[pic]

Monte Carlo

[pic]

[pic]

3d. Measurement

|E(MeV) |[pic]min(deg) |[pic]max(deg) |[pic]min(deg) |[pic]max(deg) |

|440(c) |33.5 |82.5 |37.8 |66.2 |

|880(c) |26.5 |82.5 |30.5 |65.5 |

|880(s) | | |18.9 |30.5 |

|880(c)* |103.0 |113.0 |18.9 |22.3 |

(c) coincidence (s) proton singles

(c)* coincidence with extra detectors

• Beam time: 300 hrs (440 MeV), 900 hrs (880 MeV)

• Luminosity: L = 2.7×1031 s-1 cm-2

|E = 440 MeV | |E = 880 MeV |

| |rate |dR/R | | |Rate (Hz) |dR/R |

|(MeV2) |(Hz) |(%) | |(MeV2) | |(%) |

|0.07 |33.5 |0.46 | |0.21 |6.41 |0.40 |

|0.10 |8.5 |0.69 | |0.34 |1.67 |0.35 |

|0.13 |4.4 |1.47 | |0.47 |0.56 |0.44 |

|0.16 |2.4 |1.?? | |0.59 |0.23 |0.66 |

|0.20 |1.4 |1.70 | |0.69 |0.11 |1.17 |

|0.23 |0.9 |1.72 | |0.85 |0.08 |2.49 |

• Systematic Errors

o scattered electron energy

o scattered electron angle

o target spin direction

[pic]

4. Conclusions

• This experiment will be the first determination of the proton form factor ratio using the [pic] reaction.

• The proton form factor ratio will be measured from

Q2 = 0.07 to 0.9 (GeV/c) 2 to unprecedented precision and dominated by statistical error.

• This experiment is well suited for BLAST.

• In the future a 1.1GeV beam may be used with the same technique with BLAST to determine the proton form factor ratio to Q2 = 1.2 (GeV/c) 2.

Acknowledgements

• B. Clasie, J. Seely, H. Kolster, V. Ziskin,

M. Farkhondeh, B. Franklin, T. Smith

References

Proposal to the MIT-Bates PAC “Precision measurement of the proton electric to magnetic form factor ratio with BLAST”.

M. Jones et al., Phys. Rev. Lett. 84, 1398 (2000).

B. Milbrath et al., Phys. Rev. Lett. 80, 452 (1998), Phys. Rev. Lett. 82, 221 (E) (1999).

S. Dieterich et al., Phys. Lett. B500, 47 (2001).

T. W. Donnelly et al., Ann. Phys. 169, 247 (1986).

G. Höhler et al., Nucl. Phys. B114, 505 (1976).

P. Mergell et al., Nucl. Phys. A596, 367 (1996).

G. Holzworth, Z. Phys. A356, 339 (1996).

M. R. Frank et al., Phys. Rev. C54, 920 (1996).

D. H. Lu et al., Phys. Rev. C57, 2628 (1998).

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