Constraining BSM Physics at the LHC: Four top final states with NLO accuracy in perturbative QCD

TL;DR

The paper addresses the need for precise Standard Model predictions in four-top production, a key channel for probing new physics at the LHC. It delivers a full NLO QCD calculation for pp -> tttt + X using the Helac-NLO framework, comparing fixed and dynamical scales and providing integrated and differential results. The authors find a moderate NLO correction (~27%) with reduced scale uncertainties, but show that fixed-scale predictions distort differential shapes, whereas a dynamical HT/4 scale yields flatter, more stable K-factors across observables. They estimate a SM cross section around 17 fb at 14 TeV with manageable PDF uncertainties, underscoring the importance of NLO accuracy for interpreting potential BSM signals in multi-top final states.

Abstract

Many theories, from Supersymmetry to models of Strong Electroweak Symmetry Breaking, look at the production of four top quarks as an interesting channel to evidentiate signals of new physics beyond the Standard Model. The production of four-top final states requires large partonic energies, above the 4mt threshold, that are available at the CERN Large Hadron Collider and will become more and more accessible with increasing energy and luminosity of the proton beams. A good theoretical control on the Standard Model background is a fundamental prerequisite for a correct interpretation of the possible signals of new physics that may arise in this channel. In this paper we report on the calculation of the next-to-leading order QCD corrections to the Standard Model process pp -> tttt + X. As it is customary for such studies, we present results for both integrated and differential cross sections. A judicious choice of a dynamical scale allows us to obtain nearly constant K-factors in most distributions.

Figures (14)

• Figure 1: A representative set of Feynman diagrams contributing to the $pp \to t\bar{t}t\bar{t}$ process at ${\cal O}(\alpha_s^4)$. Double lines correspond to top quarks, single lines to light quarks and wiggly ones to gluons.
• Figure 2: A representative set of pentagon and hexagon diagrams for the $pp \to t\bar{t}t\bar{t} + X$ process at NLO QCD. Double lines correspond to top quarks, single lines to light quarks and wiggly ones to gluons.
• Figure 3: A representative set of Feynman diagrams contributing to the real emission corrections to the $pp \to t\bar{t}t\bar{t} + X$ process at ${\cal O}(\alpha_s^5)$. Double lines correspond to top quarks, single lines to light quarks and wiggly ones to gluons.
• Figure 4: Scale dependence of the LO cross section with the individual contributions of the partonic channels (left panel) and scale dependence of the LO and NLO cross sections (right panel) for the $pp\rightarrow t\bar{t} t\bar{t} ~+ X$ process at the LHC for $\sqrt{s}=14$ TeV. The scale is set to a common value $\mu_R=\mu_F= \xi \cdot \mu_0$ where $\mu_0= 2m_t$.
• Figure 5: Averaged differential cross section distributions as a function of transverse momentum of the $t\bar{t}$ pair (upper-left panel) and the top quark (upper-right panel) for $pp \to t \bar{t} t \bar{t} + X$ production at the LHC with $\sqrt{s}= 14 ~\textnormal{TeV}$. Also shown is the differential cross section distribution as a function of the total transverse energy of the system (lower panel). The dash-dotted (blue) curve corresponds to the LO, whereas the solid (red) one to the NLO result. The scale choice is $\mu_F = \mu_R =\mu_0= 2 m_t$. The uncertainty bands depict scale variation. The lower panels display the differential $\cal K$ factor.