Top, Higgs, Diboson and Electroweak Fit to the Standard Model Effective Field Theory

The Standard Model Effective Field Theory (SMEFT) provides a powerful theoretical framework for searching for subtle deviations from the Standard Model. In this talk, we present the results from a global fit of the dimension-6 operators of the SMEFT to a combination of Higgs, top, diboson and electroweak precision observables. SMEFT constraints driven by data from the top quark sector are highlighted. We explore the interplay between the top and Higgs sectors, emphasising the need for a global approach to constraining the SMEFT.


Introduction
Experimental probes of new physics beyond the Standard Model (BSM) have been taken to an unprecedented level by data from the Large Hadron Collider (LHC). In particular Run II has provided many new precision measurements, for example differential measurements of top quark pair production [1,2]. Although there is no direct evidence for new physics within this data, the abundance of precision and high energy information provides an excellent opportunity for performing indirect searches for new physics, making us of the framework of the Standard Model Effective Field Theory (SMEFT) in this endeavour [3][4][5][6][7][8][9]. In this talk, we present the results of a global analysis of data from the top, Higgs, diboson and electroweak precision sectors [3].
The global fit is performed using the Fitmaker code * , employing the method of least-squares to constrain the dimension-6 operators of the Warsaw basis [10]. A 'top-specific' flavour symmetry is applied to the dimension-6 operators [11], reducing them to a set of 34 operators. The symmetry, denoted by SU (2) relaxes the assumption of flavour universality for operators involving the top quark. Under this symmetry four-fermion operators involving the top quark are allowed, as well as chirality-flipping interactions such as O tG , O tB and O tH , for example. Theory predictions in the SMEFT are calculated at linear order in the dimension-6 operators and to leading order in QCD using SMEFTsim [12] and SMEFT@NLO [13]. These operators are fit to a total of 341 datapoints, including Simplified Template Cross Section measurements of the Higgs sector [14], top pair production invariant mass distributions [1], top pair production charge asymmetry measurements [2] and measurements of the W boson polarisation in top quark decays [15]. For more details of the data and the technical settings, we refer to our main work, Ref. [3].   are stable under the removal of the Higgs and diboson data, although small changes are observed in the constraints on the coefficients C tH and C tG , indicating that the inclusion of Higgs data is advantageous in constraining these operators. Similarly, by comparing the red and orange constraints we see the impact of removing data from the top quark sector. In this case the constraints on C HG , C G and C tH widen, indicating that these coefficients benefit from the inclusion of top quark data in the marginalised fit. These subtle hints at an interplay between the top and Higgs sectors will be further explored in the next section. The extent to which data from the top sector constrains the SMEFT coefficients can be seen from a principal component analysis (PCA) of the global fit, displayed in Figure 2. Each row of the central panel represents an eigenvector, and the operator composition of each eigenvector is shown by the coloured squares. The lower limit at 95% CL on the new physics scale Λ associated to each eigenvector is given in the left panel, while the right panel indicates the relative constraining power of each dataset. We observe a number of strong constraints on the SMEFT originating from top quark data. The eigenvector predominantly composed of the coefficient C tG is constrained to Λ > 2 TeV, with top pair production and tt + V data providing the  dominant constraining power. Two rows above this, we see that the four-fermion operator O 1,3

Global SMEFT fit
Qq is similarly constrained to Λ > 2 TeV, in this case by single-top data, and a few rows below this the O tW eigenvector is constrained to Λ > 1 TeV by W boson polarisation measurements in top quark decays. In contrast, in the lower right hand side of the PCA, we see that eigenvectors composed of the four-fermion operators are more weakly constrained by tt data to Λ 300 GeV. In this case the validity of the SMEFT expansion is not guaranteed, and we expect that the quadratic contributions from dimension-6 operators will become important, as seen in [4]. However, we emphasise that these constraints result from a marginalised fit in which all 34 coefficients are allowed to float. Realistic BSM scenarios may generate a subset of these operators, resulting in a correspondingly higher new physics scale.

Top-Higgs interplay
Next, we further investigate the level of interplay between top and Higgs data in the global fit. In Figure 3 we display the results of a fit to the coefficients relevant to Higgs processes, {C H , C HG , C HW , C HB , C tH , C bH , C µH , C τ H }, as well as C tG and C G . The latter contribute to both top pair production as well as to the gluon gluon fusion mode of Higgs and Higgs + jet production. Constraints at 95% CL are displayed for the pairs of operator coefficients shown in each panel, obtained from a marginalisation over the remaining operators in the fit.  Figure 3: Constraints at 95% CL resulting from a fit of the coefficients {C G , C tG , C H , C HG , C HW , C HB , C tH , C bH , C µH , C τ H } to Higgs data excluding ttH, Higgs data including ttH, and Higgs & top data. In the latter case we assess the impact of switching on the coefficients of the four-fermion operators in the fit.
By comparing the constraints obtained from Higgs data with and without the inclusion of ttH data, in green and yellow respectively, we see that a flat direction between C tH and C HG is removed by the inclusion of the measurement of the top Yukawa, improving the sensitivity to both C tH and C HG as a result. A marked improvement in this sensitivity is seen when all top quark data is included in the fit, as shown by the comparison between green and purple ellipses. The top quark data is able to provide strong constraints on C tG and C G , allowing C HG to be further constrained by Higgs gluon gluon fusion production data. The result is a vast decrease in the area of the ellipses allowed at 95% CL, as well as a supression of correlations between operators. However, we note that top quark data is modified by SMEFT operators other than C G and C tG , in particular four-fermion operators, and that setting their contribution to zero may not be physically well-motivated in all BSM scenarios. The blue constraints show the impact of allowing these coefficients to vary in the fit, and we find that our conclusions are unchanged: the constraints from the combination of Higgs and top data are improved relative to those from Higgs data alone.

Conclusions
Searches for new physics within the SMEFT framework benefit from the approach of a global fit. In this talk we have presented the results of a global analysis of data from the Higgs, top, diboson and electroweak sectors, constraining 34 operators of the dimension-6 SMEFT. We observe some strong constraints originating from the top sector, and the need for a global fit is emphasised by the observation of an interplay between the top and Higgs sectors.