Topping-up multilepton plus b-jets anomalies at the LHC with a Z′ boson

Abstract

During the last years ATLAS and CMS have reported a number of slight to mild discrepancies in signatures of multileptons plus b-jets in analyses such as $$ t\overline{t}H $$ t t ¯ H , $$ t\overline{t}{W}^{\pm } $$ t t ¯ W ± , $$ t\overline{t}Z $$ t t ¯ Z and $$ t\overline{t}t\overline{t} $$ t t ¯ t t ¯ . Among them, a recent ATLAS result on $$ t\overline{t}H $$ t t ¯ H production has also reported an excess in the charge asymmetry in the same-sign dilepton channel with two or more b-tagged jets. Motivated by these tantalizing discrepancies, we study a phenomenological New Physics model consisting of a Z′ boson that couples to up-type quarks via right-handed currents: $$ {t}_R{\gamma}^{\mu }{\overline{t}}_R $$ t R γ μ t ¯ R , $$ {t}_R{\gamma}^{\mu }{\overline{c}}_R $$ t R γ μ c ¯ R , and $$ {t}_R{\gamma}^{\mu }{\overline{u}}_R $$ t R γ μ u ¯ R . The latter vertex allows to translate the charge asymmetry at the LHC initial state protons to a final state with top quarks which, decaying to a positive lepton and a b-jet, provides a crucial contribution to some of the observed discrepancies. Through an analysis at a detector level, we select the region in parameter space of our model that best reproduces the data in the aforementioned $$ t\overline{t}H $$ t t ¯ H study, and in a recent ATLAS $$ t\overline{t}t\overline{t} $$ t t ¯ t t ¯ search. We find that our model provides a better fit to the experimental data than the Standard Model for a New Physics scale of approximately ∼500 GeV, and with a hierarchical coupling of the Z′ boson that favours the top quark and the presence of FCNC currents. In order to estimate the LHC sensitivity to this signal, we design a broadband search featuring many kinematic regions with different signal-to-background ratio, and perform a global analysis. We also define signal-enhanced regions and study observables that could further distinguish signal from background. We find that the region in parameter space of our model that best fits the analysed data could be probed with a significance exceeding 3 standard deviations with just the full Run-2 dataset.

Top-philic heavy resonances in four-top final states and their EFT interpretation

“…This allows for a direct comparison of the constraints derived from the results of the LHC in the simplified model approach, with those derived in the corresponding EFT limit. Our work is also motivated by recent studies [42,46] that suggest that tensions in ATLAS t th and t tt t observations with respect to the SM predictions could be an indication of the existence of a top-philic new boson with a mass of O(100) GeV.…”

Top-philic heavy resonances in four-top final states and their EFT interpretation

“…This allows for a direct comparison of the constraints derived from the results of the LHC in the simplified model approach, with those derived in the corresponding EFT limit. Our work is also motivated by recent studies [46,50] that suggest that tensions in ATLAS tth and tttt observations with respect to the SM predictions could be an indication of the existence of a top-philic new boson with a mass of O(100) GeV.…”

“…Compared to refs. [4,8,50], we use a 'vectorial' interaction of the top-philic state with the top quark, so that the cross section predictions do not exhibit any chirality-flip enhancement at low mass. While the vector model with purely chiral interactions and additional flavourviolating interactions of ref.…”

“…While the vector model with purely chiral interactions and additional flavourviolating interactions of ref. [50] allows for a good fit to ATLAS data, we have not directly examined this option in our study, any flavour-violating coupling being absent from our beyond the SM parametrisation.…”

Multi-lepton probes of new physics and lepton-universality in top-quark interactions

“…Furthermore, the tri-and four-leptons channels were recently analysed in searches for the classic t tZ, t tW [104,105] and t tH [106,107] top signals, as well as the interesting 4tops (pp → tt tt ) signal [108,109]. They are also useful for generic NP searches [110,111] and, although they are flavor-blind, they can also be very effectively used to search for FCNC physics in the top sector [50,56].…”