Collision of polarized proton
In 2007 Alan. D. Krisch published an article  which described the results of a series of experiments that were conducted in the University of Michigan since the seventies. In these experiments Krisch’s team created polarized proton beams which collide with polarized protons in hydrogen atoms.
The team found that the interaction of protons with parallel spins were much stronger than the interaction of protons with opposite spins. 
This phenomenon seems to contradict QCD  and Krisch reported that “people were totally astounded” . Glashaw called these results as “The thorn in the side of QCD” . In a lecture in 2005 Stanley Brodsky called this phenomenon “one of the unsolved problems in hadronic physics”.
Similar phenomenon is well-known in the case of collisions between electrons, as it stems from Rosenbluth Formula. This formula works because the force between electric charges is weaker with the increase of the distance between the particles. It is unclear how this formula can be applied to protons, because in the case of strong interactions, according to QCD there is asymptotic freedom which suggests that the force between strong charges becomes stronger with the increase of the distance between the particles.
Research topic 1: Find explanation to this phenomenon within the scope of QCD.
Research topic 2: Find explanation to this phenomenon outside the scope of QCD.
Remark: The Rosenbluth formula describes electromagnetic interactions of an electron scattered on a nucleon. This formula shows that the relative size of the spin-dependent magnetic term increases with energy. Contrary to QCD, this effect shows another similarity between electromagnetic scattering effects and strong interaction effects of polarized proton-proton scattering.
 Alan D. Krisch, Hard collisions of spinning protons: Past, present and future, The European Physical Journal A 31, 417-423 (2007)
 Proton spin surprise, Science News, vol 112, 1977. p.196.