Physics Letters B 803 (2020) 135305
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
HERA data and collinearly-improved BK dynamics
B. Ducloué
a,b
, E. Iancu
a,∗
, G. Soyez
a
, D.N. Triantafyllopoulos
c
a
Institut de physique théorique, Université Paris Saclay, CNRS, CEA, F-91191 Gif-sur-Yvette, France
b
Higgs Centre for Theoretical Physics, University of Edinburgh, Peter Guthrie Tait Road, Edinburgh EH9 3FD, UK
c
European Centre for Theoretical Studies in Nuclear Physics and Related Areas (ECT*) and Fondazione Bruno Kessler,
Strada delle Tabarelle 286, I-38123 Villazzano, TN, Italy
a r t i c l e i n f o a b s t r a c t
Article history:
Received 3 January 2020
Accepted 13 February 2020
Available online 17 February 2020
Editor: A. Ringwald
Keywords:
QCD
Parton saturation
Deep inelastic scattering
Within the framework of the dipole factorisation, we use a recent collinearly-improved version of the
Balitsky-Kovchegov equation to fit the HERA data for inclusive deep inelastic scattering at small Bjorken x.
The equation includes an all-order resummation of double and single transverse logarithms and running
coupling corrections. Compared to similar equations previously proposed in the literature, this work
makes a direct use of Bjorken x as the rapidity scale for the evolution variable. We obtain excellent fits
for reasonable values for the four fit parameters. We find that the fit quality improves when including
resummation effects and a physically-motivated initial condition. In particular, the resummation of the
DGLAP-like single transverse logarithms has a sizeable impact and allows one to extend the fit up to
relatively large photon virtuality Q
2
.
© 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
Derived via systematic approximations within perturbative QCD, the Colour Glass Condensate (CGC) effective theory [1–5]is a powerful
framework for computing high-energy processes in the presence of non-linear effects associated with high parton densities. There are
intense ongoing efforts towards extending this effective theory to next-to-leading order (NLO) accuracy, as required by realistic applications
to phenomenology [6–19]. These efforts refer both to the Balitsky-JIMWLK equations [20–26], which govern the high-energy evolution of
the scattering amplitudes, and to the impact factors, which represent cross-sections at relatively low energy. The first NLO results, obtained
more than a decade ago [6–8], refer to the Balitsky-Kovchegov (BK) equation [20,27]. The latter is a non-linear equation emerging from
the B-JIMWLK hierarchy in the limit of a large number of colours (N
c
→∞). It describes the evolution of the elastic scattering amplitude
between a colour dipole and a dense hadronic target. Via appropriate factorisation schemes, like the “dipole factorisation” or the “hybrid
factorisation” [28], the BK equation also governs the high-energy evolution of the cross-sections for processes of phenomenological interest,
like deep inelastic scattering (DIS) at small Bjorken x, or forward particle production in proton-nucleus collisions.
A few years after the full NLO BK equation was first presented [8], its numerical study in [29]showed that it is unstable, as anticipated
in [30,31]. Similar problems had been identified, and cured [32–37], for the NLO version of the BFKL equation [38–40]— the linearised
version of the BK equation, valid for weak scattering. The origin of this difficulty has been clearly identified, both numerically [29] and
conceptually [31,41]: it is associated with large and negative NLO corrections enhanced by a double “anti-collinear” logarithm, generated
by integrating out gluon emissions with small transverse momenta (see Sect. 2). Such double-logarithmic corrections spoil the convergence
of the fixed-order perturbative expansion of the high-energy evolution, unless they are properly resummed to all orders. Refs. [31,41]
proposed
two different strategies for resumming this series of double-logarithmic corrections to all orders.
At a first sight, these strategies seemed to be successful, leading to stable evolution equations [41,42] and allowing for good fits
to the small-x HERA data [43,44]. However, a recent study [45]revealed some inconsistencies in the original analyses in [31,41,43,44].
In particular, there was a confusion concerning the meaning of the rapidity variable which plays the role of the evolution time. The
variable which a priori enters the perturbative calculations at NLO [8] and in the resummed equations proposed in [31,41]is the rapidity
Y ≡ ln(s/Q
2
0
) of the dipole projectile, with s the centre-of-mass energy squared and Q
0
a typical transverse momentum scale for the
*
Corresponding author.
E-mail addresses: bertrand.ducloue@ed.ac.uk (B. Ducloué),
edmond.iancu@ipht.fr (E. Iancu),
gregory.soyez@ipht.fr (G. Soyez),
trianta@ectstar.eu (D.N. Triantafyllopoulos).
https://doi.org/10.1016/j.physletb.2020.135305
0370-2693/© 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.
剩余10页未读，继续阅读
资源评论
