Neutralino-chargino pair production at NLO + NLL with resummation-
improved parton density functions for LHC Run II
J. Fiaschi
*
and M. Klasen
†
Institut für Theoretische Physik, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Straße 9,
D-48149 Münster, Germany
(Received 17 July 2018; published 12 September 2018)
We make use of recently released parton den sity functions (PDFs) with threshold-resummation
improvement to consistently calculate theoretical predictions for neutralino and chargino pair production
at next-to-leading order and next-to-leading logarithmic accuracy. The updated cross sections have been
computed for experimentally relevant Higgsino and gaugino search channels at the ongoing Run II of the
LHC. A factorization method is applied to exploit the smaller PDF uncertainty of the global PDF sets and to
avoid complications arising in the refitting of threshold-resummation improved PDF replicas in Mellin
space. The reduction of the scale uncertainty due to the resummation is, however, explicitly taken into
account. As expected, the resummat ion contributions in the PDF fits partially compensate the cross section
enhancements induced by those in the partonic matrix elements.
DOI: 10.1103/PhysRevD.98.055014
I. INTRODUCTION
The minimal supersymmetric standard model (MSSM) is
a theoretically and phenomenologically well-motivated
extension of the Standard Model (SM) of particle physics
[1,2]. It predicts, in particular, fermionic partners of the
neutral and charged gauge and Higgs bosons called gauginos
and Higgsinos, whose lightest neutral mass eigenstate, the
lightest neutralino, is one of the best studied dark matter
candidates [3–6]. Heavier neutralinos and charginos decay
typically into multilepton final states and missing transverse
momentum. Searches for gaugino- [7,8] or Higgsino-like
particles [9,10] are important physics goals at the LHC. They
are often carried out in the framework of simplified models
[11,12]. Care must, howev er, be taken that the theoretical
assumptions are not overly simplified [13].
Experimental measurements of supersymmetric (SUSY)
production cross sections at the ongoing Run II of the LHC
require precise theoretical calculations at the level of
next-to-leading order (NLO) QCD and beyond [14–23].
In the perturbative expansion, logarithmically enhanced
terms appear beyond leading order in the strong coupling
constant α
s
, whose contributions can be sizeable close to
production threshold or at small transverse momentum of
the produced SUSY particle pair. Their effect on neutralino,
chargino [24–29], slepton [30–37], squark, gluino [38–41],
stop [42,43] and also new gauge boson production [44–47]
has been taken into account to all orders with resummation
techniques to next-to-leading logarithmic (NLL) accuracy
and beyond, and the results for the electroweak production
channels have been made publicly available with the code
RESUMMINO [48]. Parton showers also partially resum
logarithmically enhanced contributions, and complemen-
tary NLO calculations with parton showers have been
performed for squark [49,50], slepton [51] and gaugino pair
production [52,53]. The effect of higher order QCD
corrections is generally to enhance the theoretical estima-
tions for the cross sections, while on the other hand they
reduce the dependence of the results on the choice of the
unphysical renormalization and factorization scales.
A consistent prediction of hadronic cross sections would
in principle require the same precision in the calculation of
the partonic matrix elements of the specific process as in
the determination of the parton density functions (PDFs).
For this purpose some examples of threshold-resummation
improved PDFs have been recently released. In this work
we will make use of the NNPDF30_nlo_disdytop and
NNPDF30_nll_disdytop PDF sets [36], which have been
obtained by the NNPDF collaboration with partonic matrix
elements computed, respectively, at NLO and NLO þ NLL
in the fits of experimental data. Unfortunately, NLO þ NLL
accuracy is only readily available for a rather small subset
of relevant processes. Therefore, only part of the available
experimental data can enter in a consistent NLO þ NLL fit.
In particular, the resummed PDF fits rely only on data from
deep-inelastic scattering (DIS), the Drell-Yan (DY) process
*
fiaschi@uni-muenster.de
†
michael.klasen@uni-muenster.de
Published by the American Physical Society under the terms of
the Creative Commons Attribution 4.0 International license.
Further distribution of this work must maintain attribution to
the author(s) and the publis hed article’s title, jou rnal citation,
and DOI. Funded by SCOAP
3
.
PHYSICAL REVIEW D 98, 055014 (2018)
2470-0010=2018=98(5)=055014(8) 055014-1 Published by the American Physical Society