The proton's gravitational form factors A, B, D and C-bar are defined by the matrix element of the QCD energy-momentum tensor, and the D-term - which encodes the internal shear-force and pressure distributions - is the last of these mechanical properties still unconstrained by experiment. Large-Nc QCD and the chiral quark-soliton model predict the D-term to be flavour-independent, so the strange-quark piece D_s may be roughly half of D_u and D_d even though strange quarks carry very little of the proton's momentum or spin; lattice QCD currently gives D_u = D_d = 0.56 against D_s(0) = -0.18 +/- 0.17, and a positive D_s would imply the sea quarks are mechanically unstable relative to the valence quarks. Exclusive near-threshold phi electroproduction is the only known observable sensitive to D_s: in the conformal partial-wave treatment of deeply virtual meson production, the j = 1 amplitude combination A_a + xi^2 D_a nearly cancels for u, d and gluons at xi ~ 0.4 while the strange A_s ~ 0.03 is tiny, leaving the cross section about four times more sensitive to D_s than to the gluon D-term. This Jefferson Lab Hall C proposal requests 35 days of 10.6 GeV beam at 75 microamps on a 10 cm liquid-hydrogen target, detecting the scattered electron in the SHMS at 13 degrees and the recoil proton in the HMS at 32 degrees, at mean Q^2 = 3.4 GeV^2 and W = 2.25 GeV, and reconstructing the phi by missing mass in H(e,e-prime p)X rather than by kaon detection - a choice that avoids kaon particle identification and branching-fraction losses at the cost of an irreducible continuum and eta-prime background only 62 MeV away.
We propose a measurement of exclusive electroproduction of $φ$ mesons near threshold in Hall C. We will measure the |t|-dependence of the exclusive $φ$ electroproduction cross section, which has rece…