Elong-15-01-22

Theory Report on Azz (By C. Weiss)

This Letter of Intent describes a measurement of the tensor–polarized asymmetry Azz in electron scattering on polarized deuterium in the quasi-elastic region, at values of x = 0.8 − 1.75 (x is the equivalent Bjorken variable at the nucleon level) and Q2 = 1 − 2 GeV2. The aim is to determine with this observable the S/D wave ratio in the deuteron wave function at large relative momenta k > 300 MeV, which is important for understanding the NN interaction at short distances and the properties of the dominant pn short–range correlations in heavier nuclei. The same tensor–polarized asymmetry was/will be measured in elastic scattering (deuteron form factor) and deep–inelastic scattering (structure function b1); the proposed measurement in quasi-elastic scattering would fill the gap and study this observable in the region where it is most directly related to the short-range NN interaction. The tensor asymmetry at large recoil momenta also serves as a sensitive test of “relativistic effects” in the treatment of deuteron structure, which are an important aspect of the overall theoretical framework and the object of ongoing studies. A unique feature of the measurement proposed here is that it selects small-size configurations in the deuteron both through the tensor asymmetry (D–state) and the choice of kinematics (x > 1), amplifying the overall effect. The use of x > 1 for selecting small-size NN configurations has been demonstrated in previous studies of deep-inelastic structure.

The measurement proposed here arises from a well-developed context, presents a clear objective, and enjoys strong theory support. It would further explore the nature of short-range pn correlations in nuclei, the discovery of which has been one of the most important results of the JLab 6 GeV nuclear program. Development of a full proposal should be encouraged.

Technical Report on Azz

This experiment utilizes the same apparatus and techniques as the conditionally approved b1 experiment C13-12-011. The comments in the TAC report for that experiment also apply to this experiment.

The requirement to understand and mitigate time-dependent systematic effects may be less as the asymmetry Azz, at least for x>1, is expected to be larger than for b1. However, measuring with <math>\Delta A_{zz} < 0.10</math> still requires a systematic control of the raw asymmetry to better than 1%. This is still challenging with a target polarization that is cycled on and off about once a day. Furthermore, at x>1, short range structure enhances inclusive cross sections in nuclei relative to deuterium. This will reduce the dilution factor for x>1 measurements, reducing the raw asymmetries to levels where understanding and controlling systematic errors will still be important.

PAC42 Report on Azz

Motivation

This Letter of Intent is to express interest in measurement of the tensor asymmetry Azz in electron scattering on tensor-polarized deuterium in the quasi-elastic region for 0.8 < x_b < 1.75. The tensor asymmetry Azz is sensitive to the short-range part of the NN interaction and the deuteron wave function. The combination of the selected kinematic range and sensitivity to tensor interactions will explore the short-range nature of the NN interaction. One of the specific aims is to determine the S/D-wave ratio in the deuteron wave function at large relative momenta k > 300 MeV. In addition, the tensor asymmetries at large recoil momenta will serve as a sensitive test of “relativistic effects” in the treatment of deuteron structure, which are an important aspect of the overall theoretical framework. The letter cites recent calculations by M. Sargsian for Azz in the x > 1 range using virtual-nucleon and light-cone methods. These approaches show significant differences that can be discriminated experimentally at the 3 – 6 σ level. The letter also describes interest from several theoretical groups with interest on making tests of short-range NN interactions.

Measurement and Feasibility

Electron scattering off tensor-polarized deuterium would be measured in the quasi-elastic region using the Hall C HMS and SHMS spectrometers. This proposal would use the same setup as the C1-approved experiment E12-13-011, which is to measure the deuteron tensor structure function b1. The C1-approval is subject to demonstration that 35% tensor polarization is possible (Ellie: The requirement is actually 30%). The expected asymmetry is larger in the case of this measurement, but we anticipate that a similar requirement would apply. It is anticipated that a full proposal would be for 39 days, which would include 30 days for three different Q2 values and 9.1 additional days of overhead.

Issues

A significant amount of beam time will be required for this measurement. A full proposal will need a detail discussion of expected systematic and statistical errors similar to what is in the letter that carefully justifies the requested time. The proposal should also demonstrate what sensitivity they will have to NN interaction models, such as the 6-quark model, final state interaction models, and NN interaction models, mentioned in the proposal. It will also be important to discuss how the results will distinguish between effects from the NN-interaction, the treatment of these interactions at high virtuality, and the intrinsic deuteron wave function.

Recommendation

Proceed to proposal addressing the issues noted above.