arXiv subjects

Myeonghun Park

Publications and source records attributed to Myeonghun Park.

At least 55 records · Page 3Linked to original sources

arXiv2016

Beyond the Dark matter effective field theory and a simplified model approach at colliders

Direct detection of and LHC search for the singlet fermion dark matter (SFDM) model with Higgs portal interaction are considered in a renormalizable model where the full Standard Model (SM) gauge symmetry is imposed by introducing a singlet scalar messenger. In this model, direct detection is described by an effective operator m_q \bar{q} q \barχ χas usual, but the full amplitude for monojet + \not E_T involves two intermediate scalar propagators, which cannot be seen within the effective field theory (EFT) or in the simplified model without the full SM gauge symmetry. We derive the collider bounds from the ATLAS monojet + \not E_T as well as the CMS t\bar{t} + \not E_T data, finding out that the bounds and the interpretation of the results are completely different from those obtained within the EFT or simplified models. It is pointed out that it is important to respect unitarity, renormalizability and local gauge invariance of the SM.

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arXiv2016

The diphoton resonance as a gravity mediator of dark matter

We consider the possibility of interpreting the recently reported diphoton excess at 750 GeV as a spin-two massive particle (such as a Kaluza-Klein graviton in warped extra-dimensions) which serves as a mediator to Dark Matter via its gravitational couplings to the dark sector and to the Standard Model (SM). We model non-universal couplings of the resonance to gauge bosons in the SM and to Dark Matter as a function on their localization in the extra dimension. We find that scalar, fermion or vector dark matter can saturate the dark matter relic density by the annihilation of dark matter into a pair of the SM particles or heavy resonances, in agreement with the diphoton resonance signal strength. We check the compatibility of our hypothesis with other searches for the KK graviton. We show that the invisible decay rate of the resonance into a pair of dark matter is subdominant in the region of the correct relic density, hence leading to no constraints from the mono-jet bound at 8 TeV via the gluon coupling. We also discuss the kinematic features of the decay products of a KK graviton to distinguish the KK graviton from the SM backgrounds or a scalar particle interpretation of the diphoton resonance.

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arXiv2016

OPTIMASS: A Package for the Minimization of Kinematic Mass Functions with Constraints

Reconstructed mass variables, such as $M_2$, $M_{2C}$, $M_T^\star$, and $M_{T2}^W$, play an essential role in searches for new physics at hadron colliders. The calculation of these variables generally involves constrained minimization in a large parameter space, which is numerically challenging. We provide a C++ code, OPTIMASS, which interfaces with the MINUIT library to perform this constrained minimization using the Augmented Lagrangian Method. The code can be applied to arbitrarily general event topologies and thus allows the user to significantly extend the existing set of kinematic variables. We describe this code and its physics motivation, and demonstrate its use in the analysis of the fully leptonic decay of pair-produced top quarks using the $M_2$ variables.

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arXiv2015

Using sorted invariant mass variables to evade combinatorial ambiguities in cascade decays

The classic method for mass determination in a SUSY-like cascade decay chain relies on measurements of the kinematic endpoints in the invariant mass distributions of suitable collections of visible decay products. However, the procedure is complicated by combinatorial ambiguities: e.g., the visible final state particles may be indistinguishable (as in the case of QCD jets), or one may not know the exact order in which they are emitted along the decay chain. In order to avoid such combinatorial ambiguities, we propose to treat the final state particles fully democratically and consider the sorted set of the invariant masses of all possible partitions of the visible particles in the decay chain. In particular, for a decay to N visible particles, one considers the sorted sets of all possible n-body invariant mass combinations (2 <= n <= N) and determines the kinematic endpoint m_(n,r)^max of the distribution of the r-th largest n-body invariant mass m_(n,r) for each possible value of n and r. For the classic example of a squark decay in supersymmetry, we provide analytical formulas for the interpretation of these endpoints in terms of the underlying physical masses. We point out that these measurements can be used to determine the structure of the decay topology, e.g., the number and position of intermediate on-shell resonances.

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arXiv2015

Invisible dark gauge boson search in top decays using a kinematic method

We discuss the discovery potential of a dark force carrier ($Z'$) of very light mass, $m_{Z'} \lesssim {\cal O}(1-10)$ GeV, at hadron colliders via rare top quark decays, especially when it decays invisibly in typical search schemes. We emphasize that the top sector is promising for the discovery of new particles because top quark pairs are copiously produced at the Large Hadron Collider. The signal process is initiated by a rare top decay into a bottom quark and a charged Higgs boson ($H^\pm$) decaying subsequently into a $W$ and one or multiple $Z'$s. The light $Z'$ can be invisible in collider searches in various scenarios, and it would be hard to distinguish the relevant collider signature from the regular $t\bar{t}$ process in the Standard Model. We suggest a search strategy using the recently proposed on-shell constrained $M_2$ variables. Our signal process is featured by an $\textit{asymmetric}$ event topology, while the $t\bar{t}$ is $\textit{symmetric}$. The essence behind the strategy is to evoke some contradiction in the relevant observables by applying the kinematic variables designed under the assumption of the $t\bar{t}$ event topology. To see the viability of the proposed technique, we perform Monte Carlo simulations including realistic effects such as cuts, backgrounds, detector resolution, and so on at the LHC of $\sqrt{s}=14$ TeV.

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arXiv2015

Improving the sensitivity of stop searches with on-shell constrained invariant mass variables

The search for light stops is of paramount importance, both in general as a promising path to the discovery of beyond the standard model physics and more specifically as a way of evaluating the success of the naturalness paradigm. While the LHC experiments have ruled out much of the relevant parameter space, there are "stop gaps", i.e., values of sparticle masses for which existing LHC analyses have relatively little sensitivity to light stops. We point out that techniques involving on-shell constrained M_2 variables can do much to enhance sensitivity in this region and hence help close the stop gaps. We demonstrate the use of these variables for several benchmark points and describe the effect of realistic complications, such as detector effects and combinatorial backgrounds, in order to provide a useful toolkit for light stop searches in particular, and new physics searches at the LHC in general.

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arXiv2015

$\mathcal{O}$(1) GeV dark matter in SUSY and a very light pseudoscalar at the LHC

We analyze the prospects of the detection of a $\mathcal{O}(1)$ GeV neutralino dark matter, $\widetildeχ^0_{1}$, in the Next-to-Minimal Supersymmetric Standard Model at the 14 TeV LHC. We perform dedicated scans of the relevant parameter space of the model and find a large number of points where the thermal relic abundance due to such a dark matter is consistent with the PLANCK measurement. We note that this dark matter is highly singlino-dominated and is always accompanied by a pseudoscalar, $A_1$, with a mass around twice its own, which is responsible for its resonant annihilation. For two benchmark points from our scan, we then carry out a detector-level signal-to-background analysis of the pair production of a heavier higgsino neutralino and a chargino. The higgsino thus produced decays into the dark matter and either the $Z$ boson or the $A_1$. For the $Z$-associated production of $\widetildeχ^0_{1}$, we investigate the scope of the trilepton search channel. For the $A_1$-associated production mode, in order to identify the two collimated muons coming from the decay of the $A_1$, we employ an unconventional method of clustering them together into one jet-like object. Using this method, for certain parameter space configurations, a much larger sensitivity can be obtained at the 14 TeV LHC for the $A_1\widetildeχ^0_{1}$ channel compared to the $Z\widetildeχ^0_{1}$ channel, with an integrated luminosity of 300 fb$^{-1}$.

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arXiv2015

Accessing the core of naturalness, nearly degenerate higgsinos, at the LHC

The presence of two light higgsinos nearly degenerate in mass is one of the important characteristics of suspersymmetric models meeting the naturalness criteria. Probing such higgsinos at the LHC is very challenging, in particular when the mass-splitting between them is less than 5 GeV. In this study, we analyze such a degenerate higgsino scenario by exploiting the high collinearity between the two muons which originate from the decay of the heavier higgsino into the lighter one and which are accompanied by a high-$p_T$ QCD jet. Using our method, we can achieve a statistical significance $\sim 2.9\,σ$ as well as a $S/B \sim 17\%$ with an integrated luminosity of 3000 fb$^{-1}$ at the 14 TeV LHC, for the pair production of higgsinos with masses 124 GeV and 120 GeV. A good sensitivity can be achieved even for a smaller mass-splitting when the higgsinos are lighter.

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arXiv2015

The Hierarchy Solution to the LHC Inverse Problem

Supersymmetric (SUSY) models, even those described by relatively few parameters, generically allow many possible SUSY particle (sparticle) mass hierarchies. As the sparticle mass hierarchy determines, to a great extent, the collider phenomenology of a model, the enumeration of these hierarchies is of the utmost importance. We therefore provide a readily generalizable procedure for determining the number of sparticle mass hierarchies in a given SUSY model. As an application, we analyze the gravity-mediated SUSY breaking scenario with various combinations of GUT-scale boundary conditions involving different levels of universality among the gaugino and scalar masses. For each of the eight considered models, we provide the complete list of forbidden hierarchies in a compact form. Our main result is that the complete (typically rather large) set of forbidden hierarchies among the eight sparticles considered in this analysis can be fully specified by just a few forbidden relations involving much smaller subsets of sparticles.

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arXiv2015

On-shell constrained $M_2$ variables with applications to mass measurements and topology disambiguation

We consider a class of on-shell constrained mass variables that are 3+1 dimensional generalizations of the Cambridge $M_{T2}$ variable and that automatically incorporate various assumptions about the underlying event topology. The presence of additional on-shell constraints causes their kinematic distributions to exhibit sharper endpoints than the usual $M_{T2}$ distribution. We study the mathematical properties of these new variables, e.g., the uniqueness of the solution selected by the minimization over the invisible particle 4-momenta. We then use this solution to reconstruct the masses of various particles along the decay chain. We propose several tests for validating the assumed event topology in missing energy events from new physics. The tests are able to determine: 1) whether the decays in the event are two-body or three-body, 2) if the decay is two-body, whether the intermediate resonances in the two decay chains are the same, and 3) the exact sequence in which the visible particles are emitted from each decay chain.

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arXiv2015

Supersymmetric Higgs-portal and X-ray lines

We consider a Dirac singlet fermion as thermal dark matter for explaining the X-ray line in the context of a supersymmetric Higgs-portal model or a generalized Dirac NMSSM. The Dirac singlet fermion gets a mass splitting due to their Yukawa couplings to two Higgs doublets and their superpartners, Higgsinos, after electroweak symmetry breaking. We show that a correct relic density can be obtained from thermal freeze-out, due to the co-annihilation with Higgsinos for the same Yukawa couplings. We discuss the phenomenology of the Higgsinos in this model such as displaced vertices at the LHC.

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arXiv2014

Exploring Theory Space with Monte Carlo Reweighting

Theories of new physics often involve a large number of unknown parameters which need to be scanned. Additionally, a putative signal in a particular channel may be due to a variety of distinct models of new physics. This makes experimental attempts to constrain the parameter space of motivated new physics models with a high degree of generality quite challenging. We describe how the reweighting of events may allow this challenge to be met, as fully simulated Monte Carlo samples generated for arbitrary benchmark models can be effectively re-used. In particular, we suggest procedures that allow more efficient collaboration between theorists and experimentalists in exploring large theory parameter spaces in a rigorous way at the LHC.

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arXiv2014

Charged Higgs Probes of Dark Bosons at the LHC

A very light (GeV scale) dark gauge boson ($Z'$) is a recently highlighted hypothetical particle that can address some astrophysical anomalies as well as the $3.6 σ$ deviation in the muon $g$-2 measurement. We suggest top quark decays as a venue to search for light dark force carriers at the LHC. Such $Z'$s can be easily boosted, and they can decay into highly collimated leptons (lepton-jet) with large branching ratio. We investigate a scenario where a top quark decays to $b W$ accompanied by one or multiple dark force carriers and find that such a scenario could be easily probed at the early stage of LHC Run 2.

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arXiv2014

Probing resonance decays to two visible and multiple invisible particles

We consider the decay of a generic resonance to two visible particles and any number of invisible particles. We show that the shape of the invariant mass distribution of the two visible particles is sensitive to both the mass spectrum of the new particles, as well as the decay topology. We provide the analytical formulas describing the invariant mass shapes for the nine simplest topologies (with up to two invisible particles in the final state). Any such distribution can be simply categorized by its endpoint, peak location and curvature, which are typically sufficient to discriminate among the competing topologies. In each case, we list the effective mass parameters which can be measured by experiment. In certain cases, the invariant mass shape is sufficient to completely determine the new particle mass spectrum, including the overall mass scale.

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arXiv2014

Dark decay of the top quark

We suggest top quark decays as a venue to search for light dark force carriers. The top quark is the heaviest particle in the standard model whose decays are relatively poorly measured, allowing sufficient room for exotic decay modes from new physics. A very light (GeV scale) dark gauge boson (Z') is a recently highlighted hypothetical particle that can address some astrophysical anomalies as well as the 3.6 sigma deviation in the muon g-2 measurement. We present and study a possible scenario that top quark decays as t -> b W + Z's. This is the same as the dominant top quark decay (t -> b W) accompanied by one or multiple dark force carriers. The Z' can be easily boosted, and it can decay into highly collimated leptons (lepton-jet) with large branching ratio. We discuss the implications for the Large Hadron Collider experiments including the analysis based on the lepton-jets.

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arXiv2014

Beyond Geolocating: Constraining Higher Dimensional Operators in $H \to 4\ell$ with Off-Shell Production and More

We extend the study of Higgs boson couplings in the "golden" $gg\to H \to ZZ^\ast \to 4\ell$ channel in two important respects. First, we demonstrate the importance of off-shell Higgs boson production ($gg\to H^\ast \to ZZ \to 4\ell$) in determining which operators contribute to the $HZZ$ vertex. Second, we include the five operators of lowest non-trivial dimension, including the $Z_μZ^μ\Box H$ and $H Z_μ\Box Z^μ$ operators that are often neglected. We point out that the former operator can be severely constrained by the measurement of the off-shell $H^\ast \to ZZ$ rate and/or unitarity considerations. We provide analytic expressions for the off-peak cross-sections in the presence of these five operators. On-shell, the $Z_μZ^μ\Box H$ operator is indistinguishable from its Standard Model counterpart $H Z_μZ^μ$, while the $H Z_μ\Box Z^μ$ operator can be probed, in particular, by the $Z^\ast$ invariant mass distribution.

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arXiv2014

Gravity-mediated (or Composite) Dark Matter

Dark matter could have an electroweak origin, yet communicate with the visible sector exclusively through gravitational interactions. In a set-up addressing the hierarchy problem, we propose a new dark matter scenario where gravitational mediators, arising from the compactification of extra-dimensions, are responsible for dark matter interactions and its relic abundance in the Universe. We write an explicit example of this mechanism in warped extra-dimensions and work out its constraints. We also develop a dual picture of the model, based on a four-dimensional scenario with partial compositeness. We show that Gravity-mediated Dark Matter is equivalent to a mechanism of generating viable dark matter scenarios in a strongly-coupled, near-conformal theory, such as in composite Higgs models.

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arXiv2014

Gravity-mediated (or Composite) Dark Matter Confronts Astrophysical Data

We consider the astrophysical bounds on a new form of dark matter, the so called Gravity-mediated Dark Matter. In this scenario, dark matter communicates with us through a mediator sector composed of gravitational resonances, namely a new scalar (radion) and a massive spin-two resonance (massive graviton). We consider specific models motivated by natural electroweak symmetry breaking or weak-scale dark matter in the context of models in warped extra-dimensions and their composite duals. The main Dark Matter annihilation mechanism is due to the interactions of KK gravitons to gauge bosons that propagate in bulk. We impose the bounds on monochromatic or continuum photons from Fermi-LAT and HESS. We also explore scenarios in which the Fermi gamma-ray line could be a manifestation of Gravity-mediated Dark Matter.

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