Charged Current Physics Sub-Group

The focus of this sub-group is the evaluation of the merits of polarized positron beams for electroweak physics, most likely accessible at EIC energies. Charged $W^{\pm }$ currents (CC) differentiate electron and positron beams as essentially different experimental probes, able to uniquely isolate positively or negatively charged quarks. CC DIS access combinations of quark flavors different from those measured with purely electromagnetic DIS, providing an alternative and novel source of information about PDFs, particularly for the unpolarized and polarized strange and to some extent charm PDFs. For example, the availability of polarized electron and positron beams would provide the necessary tools to measure the difference between the strange and anti-strange quark distributions as well as to investigate the isovector EMC effect.

1. Structure Functions with Charged and Neutral Current

a) The flavor separation of the pion and kaon structure could be achieved by comparing the difference between electron and positron interactions involving the Sullivan process with neutral and charged currents.

b) Neutral current.  The xF3 nucleon structure function, which is charge-conjugation odd and mostly dominated by the γZ interference contribution, will be directly sensitive to valence quark distributions. 

c) The charged-current deep inelastic scattering (DIS) cross section measurements provide possibly the most direct information on the flavor dependence of quark and anti-quark distributions. Depending on the charge of the exchanged W boson, the charged current process will be sensitive to either up-type or down-type flavors.

d) The charm and anticharm production in charged current DIS offers the best way to obtain information on strangeness in the nucleon, and the availability of polarized positron and electron beams would provide the necessary tools to extract strange and anti-strange distributions unambiguously.

e) The production of Ds+ mesons in diffractive charged current DIS  could provide information on the gluon structure of  the diffraction mechanism in QCD.

2. Electroweak form factors.

a) In connection with the study of axial form factors measured with neutrino scattering, reactions like p(e,n)nu with the neutrino being reconstructed by missing mass would bring new information. However n + neutrino is a challenging final state to reconstruct. With a positron beam, d(e+,pp)nubar might be much more feasible.  It requires detecting 2 low-momentum protons (and nothing else), and while there are issues to be worked out, it looks like the proposed ALERT detector with CLAS12 might be ideal.

3. Physics beyond the Standard Model.

 a) Right-handed W-boson exchange:  The Standard Model does not predict right-handed charged currents, so that the cross section for electron (positron)-proton charged current DIS with helicity +1(-1) is expected to be 0. Measuring the beam longitudinal polarization sensitivity of the total charged current cross section allows one to set limits on the right-handed W-boson exchange.

 b)  Other physics  beyond the Standard Model.
     1.  A longitudinally polarized positron beam also offers sensitivity for squark production in R-parity violating SUSY models, where only left- (right-) handed electrons (positrons) contribute. 
     2.  Different lepton beams and polarizations will allow selective increase in the sensitivity for different leptoquark  types.
     3.  Excited leptons require chiral  couplings between ordinary left(right)-handed and excited right(left)-handed (anti)leptons.

Link :

https://wiki.jlab.org/pwgwiki/index.php/JPos17_Charged_Current_Physics_Session