From Science:

"... electrons also interact with the nuclei through the weak force, which violates parity and is not mirror symmetric. As a result, right-spinning and left-spinning electrons ricochet off the target differently, creating a slight asymmetry in their scattering pattern
...
according to the standard model, the right- and left-spinning quarks should interact slightly differently with an incoming electron, producing an additional asymmetry, or parity violation, when the spin of the incoming electrons is flipped. Now, Xiaochao Zheng, a nuclear physicist at the University of Virginia in Charlottesville, and colleagues have observed that smaller contribution, as they report today in Nature."

This work was done at the Thomas Jefferson National Accelerator Facility:
http://hallaweb.jlab.org/experiment/PVDIS/publications/index.html

For more see:

http://news.sciencemag.org/physics/2014/02/quarks-know-their-left-their-right

and from UVaToday:

https://news.virginia.edu/content/uva-led-study-puts-quarks-looking-glass

Nature article:

http://www.nature.com/nature/journal/v506/n7486/full/nature12964.html