Higgs probes of top quark contact interactions and their interplay with the Higgs self-coupling

Published for SISSA by Springer Received: February 16, 2022 Accepted: April 21, 2022 Published: May 17, 2022 Lina Alasfar,a Jorge de Blasb and Ramona Gröberc a Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D-12489 Berlin, Germany b CAFPE and Departamento de Física Teórica y del Cosmos, Universidad de Granada, Campus de Fuentenueva, E-18071 Granada, Spain c Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova and Istituto Nazionale di Fisica Nucleare, Sezione di Padova, Via F. Marzolo 8, I-35131 Padova, Italy E-mail: , , Abstract: We calculate the dominant contributions of third generation four-quark operators to single-Higgs production and decay. They enter via loop corrections to Higgs decays into gluons, photons and bb, and in Higgs production via gluon fusion and in association with top quark pairs. We show that these loop effects can, in some cases, lead to better constraints than those from fits to top quark data. Finally, we investigate whether these four-fermion operators can spoil the determination of the trilinear Higgs self-coupling from fits to single-Higgs data. Keywords: Higgs Properties, SMEFT ArXiv ePrint: 2202.02333 Open Access, c The Authors. Article funded by SCOAP3 . https://doi.org/10.1007/JHEP05(2022)111 JHEP05(2022)111 Higgs probes of top quark contact interactions and their interplay with the Higgs self-coupling Contents 1 2 Notation 3 3 Contribution of four-quark operators to Higgs production and decay 3.1 Higgs coupling to gluons and photons 3.2 Higgs decay to bottom quarks 3.3 Associated production of a Higgs boson with top quarks 3.4 Results 4 4 8 9 12 4 Fit to Higgs observables 4.1 Fit methodology 4.2 Fit to LHC run-II data 4.3 Prospects for HL-LHC 14 15 17 20 5 Summary and discussion 23 A Numerical input 26 B Two parameter fits 28 1 Introduction The precise determination of the Higgs boson properties is one of the main focus of the Large Hadron Collider (LHC) physics programme. Within the current experimental precision, the measurements of the Higgs couplings so far appear to be in agreement with the Standard Model (SM) predictions within an accuracy of, typically, ten percent [1, 2]. In many beyond the SM (BSM) scenarios, however, it is expected that new physics will introduce modifications in the Higgs properties. If the new BSM degrees of freedom are much heavier than the electroweak scale, a general description of potential new physics effects can be formulated in the language of an effective field theory (EFT). One possibility of such a parameterization is the so-called Standard Model EFT (SMEFT), in which new physics effects are given in terms of higher-dimensional operators involving only SM fields and that also respect the SM gauge symmetries. The dominant effects on Higgs physics, electroweak physics and top quark physics stem from dimension-six operators, suppressed by the new physics scale Λ. This approach is justified in the limit in which energy scales E  Λ are probed. In this paper we will consider a small subset of these operators, namely four-fermion operators of the third generation quarks. A direct measurement of the four-top quark oper√ ators requires the production of four top quarks. At the LHC, for s = 13 TeV, and within –1– JHEP05(2022)111 1 Introduction 1 We note that a CMS combination from different LHC runs [5], though having lower signal significance, shows agreement with the SM prediction. 2 Alternatively, other indirect probes of four-top quark interactions that have been proposed include top quark pair production [11], ∆F = 2 flavour processes [14] and electroweak precision data [12, 13]. The latter mostly leads to bounds on operators that can be constrained only weakly from Higgs data. –2– JHEP05(2022)111 the SM, this is a rather rare process, with a cross section of about 12 fb including NLO QCD and NLO electroweak (EW) corrections [3]. This is due to the large phase space required for the production of four on-shell top quarks. First experimental measurements [4] indicate a slightly higher cross section than the SM prediction.1 Though four-top production gives direct access to four-top operators, the main effect comes from O(1/Λ4 ) terms when computing the matrix element squared [6], questioning whether one should neglect, in general, the effects of dimension-eight operators in the calculation of the amplitudes. At any rate, current experimental bounds on the four-top operators are rather weak. A significant improvement in constraining power would be expected, however, at a future 100 TeV pp collider, due to the growth with the energy of the diagrams involving four-top operators [7]. The situation is rather similar for the operators leading to ttbb contact interactions. They can be measured directly in ttbb production, see [8, 9] for experimental √ analyses at s = 13 TeV, but also leading to rather weak limits in SMEFT fits [6, 10]. Given the rather weak “direct” bounds on the tttt and ttbb contact interactions, here we will discuss alternative probes, showing how these interactions can be constrained indirectly via their contributions to single-Higgs observables.2 These operators generate contributions to the effective couplings of the Higgs to gluons and photons via two-loop diagrams. At the one-loop level, they also modify associated production of a Higgs boson with top quarks and, in the case of ttbb operators, also the Higgs decay to bottom quarks. While the leading log results can be easily included by renormalisation group operator mixing effects [15–17], in this paper we will compute also the finite terms and show that they can be numerically important. In addition, we will study the interplay between the extraction of the Higgs selfcoupling measurement from single-Higgs production and decay and the four-fermion operators. It was previously proposed that competitive limits to the ones from Higgs pair production on the trilinear Higgs self-coupling can be set using single-Higgs data [18–25]. A global fit including all operators entering in Higgs production and decay at tree-level plus the loop-modifications via the trilinear Higgs self-coupling has been performed in [26]. Searches for modifications of the trilinear Higgs self-coupling via single-Higgs production have been presented by the ATLAS [27] and CMS [28] collaboration. Using the example of the four-quark operators, we will show that there are other weakly constrained dimensionsix operators, that enter at the loop level, that should be included in such analyses as they have a non-trivial interplay with the trilinear Higgs self-coupling extraction from singleHiggs measurements. We will hence perform a series of combined fits of these four-fermion operators together with the operator modifying the trilinear Higgs self-coupling. While our study does not consider a global fit to all operators entering Higgs data, the results of our computations can be easily used in globa (...truncated)