Letter of Intent for a Drell-Yan Experiment with a Polarized Proton Target

Geesaman, D.¹  Reimer, P.¹  Brown, C.²  Christian, D.³ 

[1]Argonne National Lab. (ANL), Argonne, IL (United States)

[2]Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)

[3]Fermi National

DOI  :  10.2172/1296770
RP-ID  :  FERMILAB-PROPOSAL--1039
PID  :  OSTI ID: 1296770
Others  :  Other: 1309534

美国|英语

来源: SciTech Connect

【 摘 要 】

It is well known that the proton is a spin-1/2 particle, but how the constituents (quarks and gluons) assemble to this quantized spin is still a mystery. There is a worldwide effort to map out the individual contributions to the proton spin. It is established that the quark spins contribute around 30%, while the gluon intrinsic angular momentum is still under active investigation at the Relativistic Heavy Ion Collider. Fully resolving the proton spin puzzle requires information on the orbital angular momentum (OAM) of both quarks and gluons. Recent studies have shown that the so-called transverse momentum-dependent parton distribution functions (TMDs) can inform us about the OAM of the partons. One of the most important TMDs, and the main focus of this LOI, is the so-called Sivers function. To summarize, we propose to make the first measurement of the Sivers function of sea quarks, which is expected to be non-zero if the sea quarks contribute orbital angular momentum to the proton spin, as expected from the pion cloud model which also partially explains the E866 results. Thus, we will be able to deduce whether or not sea quark orbital motion contributes significantly to the proton spin. Specifically, we will determine the contribution from the anti-up quarks, with Bjorken-x in the range of ~ 0.1 to 0.5. Drell-Yan production off a polarized proton target has never been measured and is complementary to the recently approved (stage-1) experiment E1027 at Fermilab, which will measure the Sivers function of the valence quarks using a polarized proton beam on an unpolarized proton target. If the measured sea quark Sivers function is non-zero, we will also determine its sign.