arXiv subjects

Seung J. Lee

Publications and source records attributed to Seung J. Lee.

At least 37 records · Page 2Linked to original sources

arXiv2021

Muon g-2 from Millicharged Hidden Confining Sector

We provide a novel explanation to the muon $g-2$ excess with new physics contributions at the two-loop level. In this scenario, light millicharged particles are introduced to modify the photon vacuum polarization that contributes to muon $g-2$ at one additional loop. The muon $g-2$ excess can be explained with the millicharged particle mass $m_χ$ around 10 MeV and the product of the multiplicity factor and millicharge squared of $N_χ\varepsilon^2 \sim 10^{-3}$. The minimal model faces severe constraints from direct searches at fixed-target experiments and astrophysical observables. However, if the millicharged particles are also charged under a hidden confining gauge group $SU(N_χ)$ with a confinement scale of MeV, hidden-sector hadrons are unstable and can decay into neutrinos, which makes this scenario consistent with existing constraints. This explanation can be well tested at low-energy lepton colliders such as BESIII and Belle II as well as other proposed fixed-target experiments.

hep-ph↗

arXiv2021

Global Electroweak Symmetric Vacuum

Although the Higgs potential in the Standard Model (SM) contains only a simple electroweak symmetry breaking vacuum in the small field region, additional metastable or global vacua could exist in models beyond the SM. In this paper, we study one intriguing scenario with an additional electroweak symmetric vacuum that could be the global one. For the thermal universe ending at the current metastable vacuum, the electroweak symmetry should stay non-restored at high temperatures. We realize the scenario in a model with Higgs-portal couplings to SM singlet scalars with approximately global O(N) symmetries with a large N. For a large portion of model parameter space, both the quantum and thermal tunneling rates are suppressed such that our current metastable vacuum is long-lived enough. Our scenario predicts order-one changes for the Higgs self-couplings and a large contribution to the signal of the off-shell Higgs invisible decay. It can be partly probed at the LHC Run 3 and well tested at the high luminosity LHC. We also discuss the subcritical (anti-de Sitter) bubbles from the thermal tunneling that could have a large population and interesting cosmological implications.

hep-ph↗

arXiv2021

Proceedings of the second MadAnalysis 5 workshop on LHC recasting in Korea

We document the activities performed during the second MadAnalysis 5 workshop on LHC recasting, that was organised in KIAS (Seoul, Korea) on February 12-20, 2020. We detail the implementation of 12 new ATLAS and CMS searches in the MadAnalysis 5 Public Analysis Database, and the associated validation procedures. Those searches probe the production of extra gauge and scalar/pseudoscalar bosons, supersymmetry, seesaw models and deviations from the Standard Model in four-top production.

hep-ph↗

arXiv2020

Reinterpretation of LHC Results for New Physics: Status and Recommendations after Run 2

We report on the status of efforts to improve the reinterpretation of searches and measurements at the LHC in terms of models for new physics, in the context of the LHC Reinterpretation Forum. We detail current experimental offerings in direct searches for new particles, measurements, technical implementations and Open Data, and provide a set of recommendations for further improving the presentation of LHC results in order to better enable reinterpretation in the future. We also provide a brief description of existing software reinterpretation frameworks and recent global analyses of new physics that make use of the current data.

hep-ph↗

arXiv2020

End-point resummation in squark decays

We study soft and collinear gluon emission in squark decays to quark--neutralino pair, at next-to-next-to-leading logarithmic (NNLL) accuracy in the end-point region, using Soft Collinear Effective Theory (SCET), and at next-to-leading (NLO) fixed order in the rest of the phase space. As a phenomenological case study we discuss the impact of radiative corrections on the simultaneous measurements of squark and neutralino masses at a linear $e^{+}e^{-}$ collider based on $\sqrt{s} = 3$ TeV Compact Linear Collider (CLIC). Since the majority of mass measurement techniques are based on edges in kinematic distributions, and these change appreciably when there is additional QCD radiation in the final state, the knowledge of higher-order QCD effects is required for precise mass determinations.

hep-ph↗

arXiv2020

Heavy dark matter through the dilaton portal

We re-examine current and future constraints on a heavy dilaton coupled to a simple dark sector consisting of a Majorana fermion or a Stückelberg vector field. We include three different treatments of dilaton-Higgs mixing, paying particular attention to a gauge-invariant formulation of the model. Moreover, we also invite readers to re-examine effective field theories of vector dark matter, which we show are missing important terms. Along with the latest Higgs coupling data, heavy scalar search results, and dark matter density/direct detection constraints, we study the LHC bounds on the model and estimate the prospects of dark matter production at the future HL-LHC and 100 TeV FCC colliders. We additionally compute novel perturbative unitarity constraints involving vector dark matter, dilaton and gluon scattering.

hep-ph↗

arXiv2019

Continuum Naturalness

We present a novel class of composite Higgs models in which the top and gauge partners responsible for cutting off the Higgs quadratic divergences form a continuum. The continuum states are characterized by their spectral densities, which should have a finite gap for realistic models. We present a concrete example based on a warped extra dimension with a linear dilaton, where this finite gap appears naturally. We derive the spectral densities in this model and calculate the full Higgs potential for a phenomenologically viable benchmark point, with percent level tuning. The continuum top and gauge partners in this model evade all resonance searches at the LHC and yield qualitatively different collider signals.

hep-ph↗

arXiv2019

Infrared Safety of a Neural-Net Top Tagging Algorithm

Neural network-based algorithms provide a promising approach to jet classification problems, such as boosted top jet tagging. To date, NN-based top taggers demonstrated excellent performance in Monte Carlo studies. In this paper, we construct a top-jet tagger based on a Convolutional Neural Network (CNN), and apply it to parton-level boosted top samples, with and without an additional gluon in the final state. We show that the jet observable defined by the CNN obeys the canonical definition of infrared safety: it is unaffected by the presence of the extra gluon, as long as it is soft or collinear with one of the quarks. Our results indicate that the CNN tagger is robust with respect to possible mis-modeling of soft and collinear final-state radiation by Monte Carlo generators.

hep-ph↗

arXiv2018

Probing New Physics by the Tail of the Off-shell Higgs in $V_LV_L$ Mode

Off-shell Higgs at the high mass tail may shed light on the underlying mechanism of the electroweak symmetry breaking. Due to the large cancellation in the standard model (SM) between the box and Higgs-mediated triangle diagrams, the $gg\to WW(ZZ)$ process in the SM is dominated by the $V_T V_T$ transverse-mode at the high mass tail. The cancellation does not necessarily hold, when there is a sufficiently large new physics contribution resulting in $V_LV_L$ longitudinal mode, which is commonly the case when the Higgs sector is modified. Thus the $V_LV_L$ final states in the high mass tail can be utilized as a sensitive probe for such models. In the paper we focus on a study of the $gg \to ZZ$ process in the fully leptonic decay modes, proposing to utilize the polarization modes of the off-shell Higgs to probe new physics, whose contribution mainly shows in the longitudinal mode. As examples, we analyze three different Higgs sector new physics cases (Higgs portal with a light scalar, a broad-width scalar that mixes with the Higgs, and quantum critical Higgs models), and demonstrate that the angular information relating to the polarization serves as very sensitive probe for such new physics.

hep-ph↗

arXiv2018

On the Validity of the Effective Potential and the Precision of Higgs Self Couplings

The global picture of the Higgs potential in the bottom-up approach is still unknown. A large deviation as big as O(1) fluctuations of the Higgs self couplings is still a viable option for the New Physics. An interesting New Physics scenario which can be linked to a large Higgs self coupling is the baryogenesis based on the strong first order phase transition. We revisit the strong first order phase transition in two classes of Beyond the Standard Models, namely the Higgs portal with the singlet scalar under the Standard Model gauge group with Z2 symmetry and the effective field theory approach with higher-dimensional operators. We numerically investigate a few important issues in the validity of the effective potential, caused by the breakdown of the high-temperature approximation, and in the criteria for the strong first order phase transition. We illustrate that these issues can lead to O(1) uncertainties in the precision of the Higgs self couplings, which are relevant when discussing sensitivity limits of different future colliders. We also find that the quartic coupling of the above two classes of scenarios compatible with the strong first order electroweak phase transition where the cubic coupling is not negligible, can achieve a $2σ$ sensitivity at the 100 TeV pp-collider. From this novel observation, we show that the correlation between the Higgs cubic coupling and the quartic coupling will be useful for differentiating various underlying New Physics scenarios and discuss its prospect for the future colliders. Throughout our numerical investigation, the contribution from Goldstone boson is not included.

hep-ph↗

arXiv2018

Proceedings of the first MadAnalysis 5 workshop on LHC recasting in Korea

We present the activities performed during the first MadAnalysis 5 workshop on LHC recasting that has been organized at High 1 (Gangwon privince, Korea) on August 20-27, 2017. This report includes details on the implementation in the MadAnalysis 5 framework of eight ATLAS and CMS analyses, as well as a description of the corresponding validation and the various issues that have been observed.

hep-ph↗

arXiv2017

Scrutinizing the Higgs quartic coupling at a future 100 TeV proton-proton collider with taus and b-jets

The Higgs potential consists of an unexplored territory in which the electroweak symmetry breaking is triggered, and it is moreover directly related to the nature of the electroweak phase transition. Measuring the Higgs boson cubic and quartic couplings, or getting equivalently information on the exact shape of the Higgs potential, is therefore an essential task. However, direct measurements beyond the cubic self-interaction of the Higgs boson consist of a huge challenge, even for a future proton-proton collider expected to operate at a center-of-mass energy of 100 TeV. We present a novel approach to extract model-independent constraints on the triple and quartic Higgs self-coupling by investigating triple Higgs-boson hadroproduction at a center-of-mass energy of 100 TeV, focusing on the $ττb \bar{b} b \bar{b}$ channel that was previously overlooked due to a supposedly too large background. It is thrown into sharp relief that the assist from transverse variables such as $m_{T2}$ and a boosted configuration ensures a high signal sensitivity. We derive the luminosities that would be required to constrain given deviations from the Standard Model in the Higgs self-interactions, showing for instance that a $2σ$ sensitivity could be achieved for an integrated luminosity of 30 ab$^{-1}$ when Standard Model properties are assumed. With the prospects of combining these findings with other triple-Higgs search channels, the Standard Model Higgs quartic coupling could in principle be reached with a significance beyond the $3σ$ level.

hep-ph↗

arXiv2017

LHC vector resonance searches in the $t\bar{t} Z$ final state

LHC searches for BSM resonances in $l^+ l^-, \, jj, \, t\bar{t}, γγ$ and $VV$ final states have so far not resulted in discovery of new physics. Current results set lower limits on mass scales of new physics resonances well into the $\mathcal{O}(1)$ TeV range, assuming that the new resonance decays dominantly to a pair of Standard Model particles. While the SM pair searches are a vital probe of possible new physics, it is important to re-examine the scope of new physics scenarios probed with such final states. Scenarios where new resonances decay dominantly to final states other than SM pairs, even though well theoretically motivated, lie beyond the scope of SM pair searches. In this paper we argue that LHC searches for (vector) resonances beyond two particle final states would be useful complementary probes of new physics scenarios. As an example, we consider a class of composite Higgs models, and identify specific model parameter points where the color singlet, electrically neutral vector resonance $ρ_0$ decays dominantly not to a pair of SM particles, but to a fermionic top partner $T_{f1}$ and a top quark, with $T_{f1} \rightarrow tZ$. We show that dominant decays of $ρ_0 \rightarrow T_{f1} t$ in the context of Composite Higgs models are possible even when the decay channel to a pair of $T_{f1}$ is kinematically open. Our analysis deals with scenarios where both $m_ρ$ and $m_{T_{f1}}$ are of $\mathcal{O}(1)$ TeV, leading to highly boosted $t\bar{t}Z$ final state topologies. We show that the particular composite Higgs scenario we consider is discoverable at the LHC13 with as little as 30 fb$^{-1}$, while being allowed by other existing experimental constraints.

hep-ph↗

arXiv2016

The Quantum Critical Higgs

The appearance of the light Higgs boson at the LHC is difficult to explain, particularly in light of naturalness arguments in quantum field theory. However light scalars can appear in condensed matter systems when parameters (like the amount of doping) are tuned to a critical point. At zero temperature these quantum critical points are directly analogous to the finely tuned standard model. In this paper we explore a class of models with a Higgs near a quantum critical point that exhibits non-mean-field behavior. We discuss the parametrization of the effects of a Higgs emerging from such a critical point in terms of form factors, and present two simple realistic scenarios based on either generalized free fields or a 5D dual in AdS space. For both of these models we consider the processes $gg\to ZZ$ and $gg\to hh$, which can be used to gain information about the Higgs scaling dimension and IR transition scale from the experimental data.

hep-ph↗

arXiv2016

Probing TeV scale Top-Philic Resonances with Boosted Top-Tagging at the High Luminosity LHC

We investigate the discovery potential of singly produced top-philic resonances at the high luminosity (HL) LHC in the four-top final state. Our analysis spans over the fully-hadronic, semi-leptonic, and same-sign dilepton channels where we present concrete search strategies adequate to a boosted kinematic regime and high jet-multiplicity environments. We utilize the Template Overlap Method (TOM) with newly developed template observables for tagging boosted top quarks, a large-radius jet variable $M_J$ and customized b-tagging tactics for background discrimination. Our results show that the same-sign dilepton channel gives the best sensitivity among the considered channels, with an improvement of significance up to 10%-20% when combined with boosted-top tagging. Both the fully-hadronic and semi-leptonic channels yield comparable discovery potential and contribute to further enhancements in the sensitivity by combining all channels. Finally, we show the sensitivity of a top-philic resonance at the LHC and HL-LHC by showing the $2σ$ exclusion limit and $5σ$ discovery reach, including a combination of all three channels.

hep-ph↗

arXiv2016

Hadronic Calorimeter Shower Size: Challenges and Opportunities for Jet Substructure in the Superboosted Regime

Hadrons have finite interaction size with dense material, a basic feature common to known forms of hadronic calorimeters (HCAL). We argue that substructure variables cannot use HCAL information to access the microscopic nature of jets much narrower than the hadronic shower size, which we call superboosted massive jets. It implies that roughly 15% of their transverse energy profile remains inaccessible due to the presence of long-lived neutral hadrons. This part of the jet substructure is also subject to order-one fluctuations. We demonstrate that the effects of the fluctuations are not reduced when a global correction to jet variables is applied. The above leads to fundamental limitations in the ability to extract intrinsic information from jets in the superboosted regime. The neutral fraction of a jet is correlated with its flavor. This leads to an interesting and possibly useful difference between superboosted W/Z/h/t jets and their corresponding backgrounds. The QCD jets that form the background to the signal superboosted jets might also be qualitatively different in their substructure as their mass might lie at or below the Sudakov mass peak. Finally, we introduce a set of zero-cone longitudinal jet substructure variables and show that while they carry information that might be useful in certain situations, they are not in general sensitive to the jet substructure.

hep-ph↗

arXiv2016

Probing Higgs self-interactions in proton-proton collisions at a center-of-mass energy of 100 TeV

We present a phenomenological study of triple-Higgs production in which we estimate the prospects for measuring the form of the Higgs potential at future circular collider projects. We analyze proton-proton collisions at a center-of-mass energy of 100 TeV and focus on two different signatures in which the final state is made of four b-jets and either a pair of photons or a pair of tau leptons. We study the resulting sensitivity on the Higgs cubic and quartic self-interactions and investigate how it depends on the b-tagging, tau-tagging and photon resolution performances of detectors that could be designed for these future machines. We then discuss possible luminosity goals for future 100 TeV collider projects that would allow for a measurement of the Higgs potential and its possible departures from the Standard Model expectation.

hep-ph↗

arXiv2016

WIMP Dark Matter in Composite Higgs Models and the Dilaton Portal

We study under which conditions a scalar particle is a viable WIMP Dark Matter candidate with Higgs and dilaton interactions. The theory is a composite Higgs model with top partial compositeness where both the Higgs and the Dark Matter candidate arise as pseudo Goldstone boson of the coset SO(6)/SO(5) from a new physics sector. We highlight the role of the dilaton in direct and indirect searches. We find that a Dark Matter particle with a mass around 200-400 GeV and a relatively light dilaton are a fair prediction of the model.

hep-ph↗