https://doi.org/10.1140/epjc/s10052-016-4240-4
Regular Article - Theoretical Physics
The asymptotic behaviour of parton distributions at small and large x
1
The Higgs Centre for Theoretical Physics, University of Edinburgh, JCMB, KB, Mayfield Rd., Edinburgh, EH9 3JZ, Scotland
2
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, 1 Keble Road, Oxford, OX1 3NP, UK
* e-mail: emanuele.nocera@physics.ox.ac.uk
Received:
30
May
2016
Accepted:
28
June
2016
Published online:
8
July
2016
It has been argued from the earliest days of quantum chromodynamics that at asymptotically small values of x the parton distribution functions (PDFs) of the proton behave as 
, where the values of 
can be deduced from Regge theory, while at asymptotically large values of x the PDFs behave as 
, where the values of 
can be deduced from the Brodsky–Farrar quark counting rules. We critically examine these claims by extracting the exponents and from various global fits of parton distributions, analysing their scale dependence, and comparing their values to the naive expectations. We find that for valence distributions both Regge theory and counting rules are confirmed, at least within uncertainties, while for sea quarks and gluons the results are less conclusive. We also compare results from various PDF fits for the structure function ratio 
at large x, and caution against unrealistic uncertainty estimates due to overconstrained parametrisations.
© The Author(s), 2016
