Session W10: Beyond the Standard Model Physics I

Search for new physics using top quark pairs produced in association with a boosted Z or Higgs boson in effective field theory

A data sample containing top quark pairs produced in association with a boosted Z or Higgs boson is used to search for signs of new physics within the framework of effective field theory (EFT). The data correspond to an integrated luminosity of 138 fb^-1 of proton-proton collisions produced at a center-of-mass energy of 13 TeV at the LHC and collected by the CMS experiment. Selected collision events contain a single lepton and hadronic jets, including two identified with the decay of bottom quarks,

plus an additional large-radius jet with high transverse momentum (p_T) identified as a Z or Higgs boson candidate decaying to a bottom quark pair. Machine learning techniques are employed to discriminate ttZ and ttH events from background processes, dominated by tt+jets production. The signal strengths of boosted ttZ and ttH processes are measured, and upper limits are placed on the ttZ and ttH differential cross sections as functions of the Z or Higgs boson p_T. In addition, new physics effects are probed using a parameterization of the signal in terms of eight Wilson coefficients corresponding to EFT operators. The eight coefficients are fit simultaneously to the data. Limits on the values of the Wilson coefficients are presented, including five which strengthen constraints from measurements in other final states.   

Search for Heavy (pseudo)Higgs boson A/H produced in association with a top-antitop quark pair leading to the final state with four top quarks inρρ collisions at√s =13 TeV with the ATLAS detector

Four top-quark production, a rare process in the Standard Model (SM) with a cross-section around 12 fb, is one of the heaviest final states produced at the LHC, and it is naturally sensitive to physics beyond the Standard Model (BSM). A data excess is observed with twice of the expectation. A follow-up analysis is the search for Heavy (pseudo)Higgs boson A/H produced in association with a top-antitop quark pair leading to the final state with four top quarks. The data analyzed correspond to an integrated luminosity of 139 fb^-1. In this talk, the four top-quark decay final states containing either a pair of same-sign leptons or multi-lepton (SSML) are considered. To enhance the search sensitivity, a mass-parameterized BDT is introduced to discriminate the BSM signal against the irreducible SM four-top and other dominant SM backgrounds. Expected upper bounds on the production cross-section of A/H are derived in the mass range from 400 GeV to 1000 GeV.   

Search for a heavy resonance decaying to tH in the boosted all-hadronic final state

A search for a heavy resonance decaying to a top quark and a Higgs boson in the fully hadronic final state is presented. The analysis is performed using the full Run2 dataset recorded by the CMS experiment at the LHC. The search is focused on heavy resonances, where the decay products of each top quark or Higgs boson are expected to be reconstructed as a single, large-radius jet with a distinct substructure. The production of a vector-like top quark, T', is used as a benchmark when setting limits on the cross section for a heavy resonance decaying to a top quark and a Higgs boson.   

Searching Heavy Axion-like Particles at Future Muon Collider

Heavy axion-like particles (ALPs) with masses around or above the weak scale serve as an important new physics benchmark at colliders. In this talk, I will present a detailed analysis of projected sensitivities for such heavy ALPs coupled exclusively to electroweak gauge bosons at a future muon collider. Because of vector boson fusion (VBF) enhancement and low hadronic background, we find that a future high-energy muon collider has a powerful reach to such particles, demonstrating the potential of a muon collider as an electroweak boson collider. Furthermore, our study illustrates how the analysis on such VBF-enhanced signals can be carried out systematically, and similar strategies can be applied to searches for other W/Z-coupled heavy resonances.    

Search for dark mesons decaying to top and bottom quarks in 139 fb-1 of proton-proton collision data at √s = 13 TeV with the ATLAS detector at the LHC.

Extending the Standard Model (SM) with a new strongly coupled dark sector can generate models with both an experimentally accessible dark meson production mechanism and a viable dark matter candidate particle. A search for dark mesons is presented in an integrated luminosity of 139 fb⁻¹ of proton-proton collision data at a center of mass energy of 13 TeV with the ATLAS detector at the Large Hadron Collider (LHC). In this model, dark pions are pair-produced and then decay to top-antitop or top-antibottom quark pairs. This talk describes the strategy and expected sensitivity of the all-hadronic channel of this search, with a signature of six or more jets and no additional missing transverse energy. Exclusion limits from this analysis are expected to provide new dedicated constraints on dark meson parameter space, exceeding existing limits derived from the reinterpretation of prior analyses.   

Search for heavy fermionic partner of the top quark with charge 5/3 in the single leptonic channel using CMS Run 2 Data

With the discovery of the Higgs boson in 2012 by the CMS and ATLAS experiments, searches for new heavy particles have ensued in hope of solving the hierarchy problem. In this talk, I will be discussing the search for the X_5/3, a strongly interacting fermionic partner of the top quark with charge +5/3. Left-handed and right-handed coupling of the X_5/3 to W bosons are considered separately. The search is conducted using the CMS Run 2 datasets collected in 2017 and 2018. Data was collected with the CMS detector at a luminosity of 41.5 fb^-1 (59.97 fb^-1)  in 2017 (2018)  and a center-of-mass energy of sqrt(s) = 13 TeV. The search looks for events with pair production of an X5/3 and its antiparticle which subsequently decay to a top and W boson. To enhance signal separation, the search is constructed to only look for events where one W decays to a lepton and neutrino, while the other three W bosons decay hadronically. Limits on the cross section will be presented and compared to previous results.   

Track-Based Triggers for Unconventional Signatures at the HL-LHC

This talk presents the results of a study on HL-LHC opportunities for track-based triggers for long-lived particles and unconventional exotic signatures. Using a representative array of signatures, including highly ionizing tracks, displaced leptonic and hadronic decays, and soft unclustered energy patterns, this study provides an overview of how the parameters of tracker capabilities affect sensitivity. The goal is to provide an input for those designing these systems and the subsequent trigger menus to insure maximum sensitivity to a wide range of unconventional signatures.