# Measuring strong coupling constant

How can the strong coupling constant be measured?

I’ll try to explain one method that measures $${{\alpha }_{s}}$$ within ±8% at very high energies. Consider $$pp$$ or $$p\bar{p}$$ collisions with CM energy in the TeV range. What happens in the events of interest is that one “parton” from each of the nucleons participates in hard scattering via single-gluon exchange, and the rest of each nucleon proceeds essentially undisturbed, with a substantial fraction of the nucleon’s initial momentum and <1 GeV/c of deflection. You should see four distinct jets, two of them along the axis of collision and two at large angles. The experimental technique involves measuring the 4-momentum of each jet. The 4-momentum of each deflected parton can be inferred from the different between the momentum of the incoming nucleon and that of the undeflected jet. The 4-momentum of the gluon can in turn be inferred from the difference between the momentum of the deflected parton and that of the outgoing jet. Since the differential scattering cross-sections for $$qq$$ and $$q\bar{q}$$ are well known, aside from the coefficient, the measured statistics reveal $${{\alpha }_{s}}(t)$$. (Minor complication: In $$qq\to qq$$ scattering, there is a u-channel process that interferes with the t-channel process.)