d/u RAtios and A=3 EMC Effect in Deep Inelastic Electron Scattering Off the Tritium and Helium MirrOr Nuclei

This is an updated proposal for the twice conditionally approved E12-010-103 JLab Experiment. It was originally proposed, as PR12-06-118 in 2006, and reviewed by PAC30, which considered “the physics goals of this experiment as highlights of the 12 GeV physics program.” The condition for (full) approval imposed by PAC30 was a “Management review of the safety aspects of the Tritium target.” A review of a conceptual target design was conducted in June 2010, which found “no show stoppers” for further development of a low activity tritium target at Jefferson Lab. The outcome of the review allowed the resubmission of the proposal to PAC36, which, per preliminary PAC-closeout communication, reaffirmed the scientific importance of the experiment but imposed a full detector design for the Super BigBite Spectrometer as a condition for (full) approval. We propose to perform deep inelastic electron scattering off the H and He mirror nuclei with the 11 GeV upgraded beam of Jefferson Lab. The experiment will measure the EMC effect for H and He and determine the ratio of the neutron to proton inelastic structure functions, F n 2 /F p 2 , and the ratio of the down to up quark distributions in the nucleon, d/u, at medium and large Bjorken x. It will use a room-temperature, moderate-pressure H, He and deuterium gas target system, and the Hall A BigBite (instead of the Super BigBite) and the Left High Resolution Spectrometers. The tested and successfully operated BigBite Spectrometer will be used in its standard electron detection configuration with modifications to its Cherenkov counter. The required beam time is 42 days at a beam current of 25 μA. The F n 2 /F p 2 ratio will be extracted from the inelastic cross section ratio of the two nuclei by exploiting their mirror symmetry with a minimal theoretical correction. The F n 2 /F p 2 ratio is expected to be almost free of nuclear effects, which introduce a significant uncertainty in its extraction from deep inelastic scattering off the proton and deuteron. The results are expected to test perturbative and non-perturbative mechanisms of spin-flavor symmetry breaking in the nucleon, and constrain the structure function parametrizations needed for the interpretation of high energy hadron collider and neutrino oscillations data. The precision of the expected data for the ratio of the EMC effects for H and He will offer a unique opportunity to test theoretical calculations of the EMC effect and will provide critical experimental input for the establishment of a unique canonical model for the explanation of its dynamical origin.