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Cen Zhang

Publications and source records attributed to Cen Zhang.

At least 55 records · Page 3Linked to original sources
arXiv2019

A Monte Carlo global analysis of the Standard Model Effective Field Theory: the top quark sector

We present a novel framework for carrying out global analyses of the Standard Model Effective Field Theory (SMEFT) at dimension-six: SMEFiT. This approach is based on the Monte Carlo replica method for deriving a faithful estimate of the experimental and theoretical uncertainties and enables one to construct the probability distribution in the space of the SMEFT degrees of freedom. As a proof of concept of the SMEFiT methodology, we present a first study of the constraints on the SMEFT provided by top quark production measurements from the LHC. Our analysis includes more than 30 independent measurements from 10 different processes at 8 and 13 TeV such as inclusive top-quark pair and single-top production and the associated production of top quarks with weak vector bosons and the Higgs boson. State-of-the-art theoretical calculations are adopted both for the Standard Model and for the SMEFT contributions, where in the latter case NLO QCD corrections are included for the majority of processes. We derive bounds for the 34 degrees of freedom relevant for the interpretation of the LHC top quark data and compare these bounds with previously reported constraints. Our study illustrates the significant potential of LHC precision measurements to constrain physics beyond the Standard Model in a model-independent way, and paves the way towards a global analysis of the SMEFT.

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arXiv2019

Positivity constraints on aQGC: carving out the physical parameter space

Searching for deviations in quartic gauge boson couplings (QGCs) is one of the main goals of the electroweak program at the LHC. We consider positivity bounds adapted to the Standard Model, and show that a set of positivity constraints on 18 anomalous QGC couplings can be derived, by requiring that the vector boson scattering amplitudes of specific channels and polarisations satisfy the fundamental principles of quantum field theory. We explicitly solve the positivity inequalities to remove their dependence on the polarisations of the external particles, and obtain 19 linear inequalities, 3 quadratic inequalities, and 1 quartic inequality that only involve the QGC parameters and the weak angle. These inequalities constrain the possible directions in which deviations from the standard QGC can occur, and can be used to guide future experimental searches. We study the morphology of the positivity bounds in the parameter space, and find that the allowed parameter space is carved out by the intersection of pyramids, prisms, and (approximately) cones. Altogether, they reduce the volume of the allowed parameter space to only 2.1% of the total. We also show the bounds for some benchmark cases, where one, two, or three operators, respectively, are turned on at a time, so as to facilitate a quick comparison with the experimental results.

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arXiv2018

Learning to pinpoint effective operators at the LHC: a study of the $t\bar{t}b\bar{b}$ signature

In the context of the Standard Model effective field theory (SMEFT), we study the LHC sensitivity to four fermion operators involving heavy quarks by employing cross section measurements in the $t\bar{t}b\bar{b}$ final state. Starting from the measurement of total rates, we progressively exploit kinematical information and machine learning techniques to optimize the projected sensitivity at the end of Run III. Indeed, in final states with high multiplicity containing inter-correlated kinematical information, multi-variate methods provide a robust way of isolating the regions of phase space where the SMEFT contribution is enhanced. We also show that training for multiple output classes allows for the discrimination between operators mediating the production of tops in different helicity states. Our projected sensitivities not only constrain a host of new directions in the SMEFT parameter space but also improve on existing limits demonstrating that, on one hand, $t\bar{t}b\bar{b}$ production is an indispensable component in a future global fit for top quark interactions in the SMEFT, and on the other, multi-class machine learning algorithms can be a valuable tool for interpreting LHC data in this framework.

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arXiv2018

Learning to pinpoint effective operators at the LHC: a study of the $t\bar{t}b\bar{b}$ signature

In the context of the Standard Model effective field theory (SMEFT), we study the LHC sensitivity to four fermion operators involving heavy quarks by employing cross section measurements in the $t\bar{t}b\bar{b}$ final state. Starting from the measurement of total rates, we progressively exploit kinematical information and machine learning techniques to optimize the projected sensitivity at the end of Run III. Indeed, in final states with high multiplicity containing inter-correlated kinematical information, multi-variate methods provide a robust way of isolating the regions of phase space where the SMEFT contribution is enhanced. We also show that training for multiple output classes allows for the discrimination between operators mediating the production of tops in different helicity states. Our projected sensitivities not only constrain a host of new directions in the SMEFT parameter space but also improve on existing limits demonstrating that, on one hand, $t\bar{t}b\bar{b}$ production is an indispensable component in a future global fit for top quark interactions in the SMEFT, and on the other, multi-class machine learning algorithms can be a valuable tool for interpreting LHC data in this framework.

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arXiv2018

Dimension-six electroweak top-loop effects in Higgs production and decay

We study the next-to-leading order electroweak corrections to Higgs processes from dimension-six top-quark operators in the Standard Model Effective Field Theory approach. We consider the major production channels, including $WH$, $ZH$, and VBF production at the LHC, and $ZH$, VBF production at future lepton colliders, as well as the major decay channels including $H\to γγ, γZ, Wlν,Zll,b\bar b,μμ,ττ$. The results show that within the current constraints, top-quark operators can shift the signal strength of the loop-induced processes, i.e. $H\to γγ,γZ$, by factors of $\sim\mathcal{O}(1)-\mathcal{O}(10)$, and that of the tree-level processes, i.e. all remaining production and decay channels, by $\sim5-10\%$ at the LHC, and up to $\sim15\%$ at future lepton colliders. This implies that essentially all Higgs channels have started to become sensitive to top-quark couplings, and in particular, Higgs observables at high luminosity LHC as well as future lepton colliders, even below the $t\bar t$ threshold, will improve our knowledge of top-quark couplings. We derive the sensitivities of Higgs measurements to top-quark operators at the high luminosity LHC, using projections for both inclusive and differential measurements. We conclude that treating the dimension-six top-quark sector and the Higgs/electroweak sector separately may not continue to be a good strategy. A global analysis combining Higgs and top-quark measurements is desirable, and our calculation and implementation provide an automatic and realistic simulation tool for this purpose.

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arXiv2018

Global and optimal probes for the top-quark effective field theory at future lepton colliders

We study the sensitivity to physics beyond the standard model of precise top-quark pair production measurements at future lepton colliders. A global effective-field-theory approach is employed, including all dimension-six operators of the Warsaw basis which involve a top-quark and give rise to tree-level amplitudes that interfere with standard-model $e^+e^-\to t\,\bar t\to bW^+\bar bW^-$ ones in the limit of vanishing $b$-quark mass. Four-fermion and CP-violating contributions are taken into account. Circular-collider-, ILC- and CLIC-like benchmark run scenarios are examined. We compare the constraining power of various observables to a set of statistically optimal ones which maximally exploit the information contained in the fully differential $bW^+\bar bW^-$ distribution. The enhanced sensitivity gained on the linear contributions of dimension-six operators leads to bounds that are insensitive to quadratic ones. Even with statistically optimal observables, two centre-of-mass energies are required for constraining simultaneously two- and four-fermion operators. The impact of the centre-of-mass energy lever arm is discussed, that of beam polarization as well. A realistic estimate of the precision that can be achieved in ILC- and CLIC-like operating scenarios yields individual limits on the electroweak couplings of the top quark that are one to three orders of magnitude better than constraints set with Tevatron and LHC run I data, and three to two hundred times better than the most optimistic projections made for the high-luminosity phase of the LHC. Clean global constraints can moreover be obtained at lepton colliders, robustly covering the multidimensional effective-field-theory space with minimal model dependence.

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arXiv2018

Single-top associated production with a $Z$ or $H$ boson at the LHC: the SMEFT interpretation

At the LHC, top quarks can be produced singly with a sizeable rate via electroweak interactions. This process probes a limited set of top-quark electroweak couplings, i.e. , the same entering the top-quark decay, yet at higher scales and with a different sensitivity. Requiring the production of a $Z$ or $H$ boson in association with single-top significantly extends the sensitivity of this process to new physics, opening up the unique possibility of testing top-Higgs, top-gauge, triple gauge, gauge-Higgs interactions without being dominated by QCD interactions. We consider $tZj$ and $tHj$ production at the LHC, providing predictions at next-to-leading accuracy in QCD in the framework of the standard model effective field theory, including all relevant operators up to dimension six. We perform the first complete study of the sensitivity to new interactions of these processes, highlighting the interplay and complementarity among $tj$, $tZj$ and $tHj$ in simultaneously constraining top-quark, triple gauge, and gauge-Higgs interactions in the current and future runs at the LHC.

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arXiv2017

Scalar production and decay to top quarks including interference effects at NLO in QCD in an EFT approach

Scalar and pseudo-scalar resonances decaying to top quarks are common predictions in several scenarios beyond the standard model (SM) and are extensively searched for by LHC experiments. Challenges on the experimental side require optimising the strategy based on accurate predictions. Firstly, QCD corrections are known to be large both for the SM QCD background and for the pure signal scalar production. Secondly, leading order and approximate next-to-leading order (NLO) calculations indicate that the interference between signal and background is large and drastically changes the lineshape of the signal, from a simple peak to a peak-dip structure. Therefore, a robust prediction of this interference at NLO accuracy in QCD is necessary to ensure that higher-order corrections do not alter the lineshapes. We compute the exact NLO corrections, assuming a point-like coupling between the scalar and the gluons and consistently embedding the calculation in an effective field theory within an automated framework, and present results for a representative set of beyond the SM benchmarks. The results can be further matched to parton shower simulation, providing more realistic predictions. We find that NLO corrections are important and lead to a significant reduction of the uncertainties. We also discuss how our computation can be used to improve the predictions for physics scenarios where the gluon-scalar loop is resolved and the effective approach is less applicable.

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arXiv2017

Top Quark Properties

Recently, experimental collaborations have reported $\mathcal{O}(10)$ upper limits on the signal strength of four-top production at the LHC, which already leads to competitive constraints on certain type of top-quark operators. On the other hand, developments in the $b$-jet charge tagging algorithms open up new possibilities for the direct measurement of the top-quark width. In this talk, we briefly discuss some recent studies in these two directions.

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arXiv2017

Constraining $qqtt$ operators from four-top production: a case for enhanced EFT sensitivity

Recently, experimental collaborations have reported $\mathcal{O}(10)$ upper limits on the signal strength of four-top production at the LHC. Surprisingly, we find that the constraining power of four-top production on the $qqtt$ type of operators is already competitive with the measurements of top-pair production, even though the precision level of the latter is more than two orders of magnitude better. This is explained by the enhanced sensitivity of the four-top cross section to $qqtt$ operators, due to multiple insertion of operators in the squared amplitude, and to the large threshold energy of four-top production. We point out that even though the dominant contribution beyond the standard model comes from the $\mathcal{O}(C^4/Λ^8)$ terms, the effective field theory expansion remains valid for a wide range of underlying theories. Considering the possible improvements of this measurement with higher integrated luminosity, we believe that this process will become even more crucial for probing and testing the standard model deviations in the top-quark sector, and will eventually provide valuable information about the top-quark properties, leading to significant improvements in precision top physics.

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arXiv2017

Theoretical aspects of the study of top quark properties

We review some recent theoretical progresses towards the determination of the top-quark couplings beyond the standard model. We briefly introduce the global effective field theory approach to the top-quark production and decay processes, and discuss the most useful observables to constrain the deviations. Recent improvements with a focus on QCD corrections and corresponding tools are also discussed.

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arXiv2017

Asymmetric Dark Matter in Extended Exo-Higgs Scenarios

The exo-Higgs model can accommodate a successful baryogenesis mechanism that closely mirrors electroweak baryogenesis in the Standard Model, but avoids its shortcomings. We extend the exo-Higgs model by the addition of a singlet complex scalar $χ$. In our model, $χ$ can be a viable asymmetric dark matter (ADM) candidate. We predict the mass of the ADM particle to be $m_χ\approx1.3\, \textrm{GeV}$. The leptophilic couplings of $χ$ can provide for efficient annihilation of the ADM pairs. We also discuss the LHC signals of our scenario, and in particular the production and decays of exo-leptons which would lead to "lepton pair plus missing energy" final states. Our model typically predicts potentially detectable gravitational waves originating from the assumed strong first order phase transition at a temperature of $\sim$ TeV. If the model is further extended to include new heavy vector-like fermions, {\it e.g.} from an ultraviolet extension, $χ$ couplings could explain the $\sim 3.5σ$ muon $g-2$ anomaly.

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arXiv2017

Probing the top-quark width using the charge identification of $b$ jets

We propose a new method for measuring the top-quark width based on the on-/off-shell ratio of $b$-charge asymmetry in $pp\to Wbj$ production at the LHC. The charge asymmetry removes virtually all backgrounds and related uncertainties, while remaining systematic and theoretical uncertainties can be taken under control by the ratio of cross sections. Limited only by statistical error, in an optimistic scenario, we find that our approach leads to good precision at high integrated luminosity, at a few hundred MeV assuming 300-3000 fb$^{-1}$ at the LHC. The approach directly probes the total width, in such a way that model-dependence can be minimized. It is complementary to existing cross section measurements which always leave a degeneracy between the total rate and the branching ratio, and provides valuable information about the properties of the top quark. The proposal opens up new opportunities for precision top measurements using a $b$-charge identification algorithm.

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arXiv2016

NLO predictions for SMEFT in the top-quark sector

Predictions for the Standard Model Effective Field Theory at the next-to-leading order accuracy in QCD, including parton-shower effects, have started to become available in the MadGraph5_aMC@NLO framework. In this talk we summarize some recent results for $t\bar t$, single top, $t\bar tZ/γ$, and $t\bar tH$ production channels at dimension six.

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arXiv2016

Higgs production in association with a top-antitop pair in the Standard Model Effective Field Theory at NLO in QCD

We present the results of the computation of the next-to-leading order QCD corrections to the production cross section of a Higgs boson in association with a top-antitop pair at the LHC, including the three relevant dimension-six operators ($O_{t φ}, O_{φG}, O_{tG}$) of the standard model effective field theory. These operators also contribute to the production of Higgs bosons in loop-induced processes at the LHC, such as inclusive Higgs, $Hj$ and $HH$ production, and modify the Higgs decay branching ratios for which we also provide predictions. We perform a detailed study of the cross sections and their uncertainties at the total as well as differential level and of the structure of the effective field theory at NLO including renormalisation group effects. Finally, we show how the combination of information coming from measurements of these production processes will allow to constrain the three operators at the current and future LHC runs. Our results lead to a significant improvement of the accuracy and precision of the deviations expected from higher-dimensional operators in the SM in both the top-quark and the Higgs-boson sectors and provide a necessary ingredient for performing a global EFT fit to the LHC data at NLO accuracy.

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arXiv2016

Exo-Higgs at 750 GeV and Genesis of Baryons

We propose that the diphoton excess at 750 GeV reported by ATLAS and CMS is due to the decay of an ${\it exo-Higgs}$ scalar $η$ associated with the breaking of a new $SU(2)_e$ symmetry, dubbed ${\it exo-spin}$. New fermions, ${\it exo-quarks}$ and ${\it exo-leptons}$, get TeV-scale masses through Yukawa couplings with $η$ and generate its couplings to gluons and photons at 1-loop. The matter content of our model yields a $B-L$ anomaly under $SU(2)_e$, whose breaking we assume entails a first order phase transition. A non-trivial $B-L$ asymmetry may therefore be generated in the early universe, potentially providing a baryogenesis mechanism through the Standard Model (SM) sphaleron processes. The spontaneous breaking of $SU(2)_e$ can in principle directly lead to electroweak symmetry breaking, thereby accounting for the proximity of the mass scales of the SM Higgs and the exo-Higgs. Our model can be distinguished from those comprising a singlet scalar and vector fermions by the discovery of TeV scale exo-vector bosons, corresponding to the broken $SU(2)_e$ generators, at the LHC.

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arXiv2016

Implications of a Light "Dark Higgs" Solution to the $g_μ-2$ Discrepancy

A light scalar $ϕ$ with mass $\lesssim 1$ GeV and muonic coupling $\mathcal{O}(10^{-3})$ would explain the 3.5 $σ$ discrepancy between the Standard Model (SM) muon $g-2$ prediction and experiment. Such a scalar can be associated with a light remnant of the Higgs mechanism in the "dark" sector. We suggest $ϕ\to l^+l^-$ bump hunting in $μ\to eν\barνϕ$, $μ^-p\toν_μnϕ$ (muon capture), and $K^\pm\to μ^\pmνϕ$ decays as direct probes of this scenario. In a general setup, a potentially observable muon electric dipole moment $\lesssim 10^{-23}\ e \cdot\textrm{cm}$ and lepton flavor violating decays $τ\toμ(e) ϕ$ or $μ\to e ϕ$ can also arise. Depending on parameters, a deviation in BR($H\toμ^+μ^-$) from SM expectations, due to Higgs coupling misalignment, can result. We illustrate how the requisite interactions can be mediated by weak scale vector-like leptons that typically lie within the reach of future LHC measurements.

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arXiv2016

Single Top Production at Next-to-Leading Order in the Standard Model Effective Field Theory

Single top production processes at hadron colliders provide information on the relation between the top quark and the electroweak sector of the standard model. We compute the next-to-leading order QCD corrections to the three main production channels: $t$-channel, $s$-channel and $tW$ associated production, in the standard model including operators up to dimension-six. The calculation can be matched to parton shower programs and can therefore be directly used in experimental analyses. The QCD corrections are found to significantly impact the extraction of the current limits on the operators, because both of an improved accuracy and a better precision of the theoretical predictions. In addition, the distributions of some of the key discriminating observables are modified in a nontrivial way, which could change the interpretation of measurements in terms of UV complete models.

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