W and Z/γ ∗ boson production in association with a bottom-antibottom pair
Rikkert Frederix
4
6
Stefano Frixione
1
2
5
Valentin Hirschi
2
Fabio Maltoni
3
Roberto Pittau
0
4
5
Paolo Torrielli
2
0
Departamento de Fsica Teorica y del Cosmos y CAFPE, Universidad de Granada
, Granada,
Spain
1
On leave of absence from INFN, Sezione di Genova, Genoa,
Italy
2
ITPP, EPFL, CH-1015 Lausanne,
Switzerland
3
Centre for Cosmology, Particle Physics and Phenomenology (CP3), Universite catholique de Louvain
, B-1348 Louvain-la-Neuve,
Belgium
4
KITP,
University of California Santa Barbara
,
CA 93106-4030, U.S.A
5
PH Department, TH Unit
, CERN, CH-1211 Geneva 23,
Switzerland
6
Institut fur Theoretische Physik, Universitat Zurich
, Winterthurerstrasse 190, CH-8057 Zurich,
Switzerland
We present a study of bb and +bb production at hadron colliders. Our results, accurate to the next-to-leading order in QCD, are based on automatic matrix-element calculations performed by MadLoop and MadFKS, and are given at both the parton level, and after the matching with the Herwig event generator, achieved with aMC@NLO. We retain the complete dependence on the bottom-quark mass, and include exactly all spin correlations of final-state leptons. We discuss the cases of several observables at the LHC which highlight the importance of accurate simulations.
1 Introduction 2 3
Conclusions and outlook
The discovery and identification of new degrees of freedom and interactions at high-energy
colliders relies on the detailed understanding of Standard Model (SM) background
processes. Prominent among these is the production of electroweak bosons (W, Z) in
association with jets, of which one or more possibly contain bottom quarks. The prime example
is the observation of the top quark at the Fermilab Tevatron collider, produced either in
pairs [1, 2] or singly [3, 4], since both of these mechanisms typically lead to W plus b-jets
signatures. Other crucial examples involve the search for a SM Higgs in association with
vector bosons (W H/ZH), with the subsequent Higgs decay into a bb pair, a sought-for
discovery channel both at the Tevatron [5] and at the LHC [6, 7]. Finally, in models which
feature an extended Higgs sector, such as the MSSM or more generally a two-Higgs
doublet model, a typical Higgs discovery channel is through Hbb and Abb final states, with
an H/A + decay. In this case, the SM process +bb can provide an important
reference measurement.
Next-to-leading-order (NLO) QCD calculations for the production of a vector boson in
association with jets have by now quite a successful record. Accurate predictions for W plus
up to four light jets [813] and for Z plus up to three light jets [8, 9, 14, 15] have become
available in the past few years. Associated production with heavy quarks, and in particular
with bottom quarks, has been studied using various approximations. The W bb and Zbb
processes have been calculated at the NLO for the first time in refs. [16] and [17] respectively,
by setting the bottom-quark mass equal to zero. Such calculations can be used only for
observables that contain at least two b-jets. The same processes have been considered again
in refs. [1820], where a non-zero bottom-quark mass has been used; however, the matrix
elements still involved on-shell vector bosons, thereby neglecting spin correlations of the
leptons emerging from W and Z decays. In the case of W production, this limitation has been
recently lifted in ref. [21], which presents the NLO calculation for the leptonic process bb.
Other NLO calculations for final states with one b-jet, W b and Zb [22, 23], and one b-jet
plus a light jet, W bj and Zbj [24, 25], are also available in the five-flavour scheme. All such
calculations have played a role and/or have been extensively compared to the data collected
at the Tevatron [2629], and now start to be considered in LHC analyses as well [30, 31].
In this paper we present a calculation of bb production that includes NLO QCD
corrections (analogous to that of ref. [21]), and the first calculation at the NLO of +bb
production with massive bottom quarks; we retain the full spin correlations of the
finalstate leptons.1 Furthermore, we match both of these results to the Herwig event generator
by adopting the MC@NLO formalism [32]. Therefore, our results include all the relevant
features which are important in experimental analyses, and can be used in order to obtain
NLO predictions for a large class of observables, including those with zero, one and two
b-jets. All aspects of the calculations are fully automated and analogous to the calculation
recently appeared for Htt/Att production [33]. One-loop amplitudes are evaluated with
MadLoop [34], whose core is the OPP integrand reduction method [35] as implemented
in CutTools [36]. Real contributions and the corresponding phase-space subtractions,
achieved by means of the FKS formalism [37], as well as their combination with the
oneloop and Born results and their subsequent integration, are performed by MadFKS [38].
The MC@NLO matching is a (...truncated)