arXiv subjects

Jeonghyeon Song

Publications and source records attributed to Jeonghyeon Song.

At least 55 records · Page 3Linked to original sources

arXiv2012

Comprehensive study of two Higgs doublet model in light of the new boson with mass around 125 GeV

The recent discovery of a new boson of mass roughly 125 GeV has been reported by the ATLAS and CMS collaborations. Although its signals in various decay modes resemble those of the standard model (SM) Higgs boson, observed are the combinations of entangled information about the production, decay rates and total decay width of the new boson. In addition, some decay channels show non-negligible deviation from the SM expectation, such as the $2σ$ excess in the diphoton channel. In the four types (I, II, X and Y) of two Higgs doublet models, we perform the global $χ^2$ fit in three scenarios: (i) the new boson is the light CP-even Higgs boson $h^0$; (ii) it is the heavy CP-even Higgs boson $H^0$; (iii) the signals are from degenerate $h^0$ and the pseudoscalar $A^0$. Considering other phenomenological constraints such as flavor physics, electroweak precision data, and the LEP search for the Higgs boson, we find that the the first scenarios in Type II and Type Y models actually provide better or similarly good fit to the data than the SM. All the other models are excluded at 95% C.L..

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arXiv2012

Direct bound on the minimal Universal Extra Dimension model from the $t\bar{t}$ resonance search at the Tevatron

In the minimal Universal Extra Dimension (mUED) model, the second Kaluza-Klein (KK) gluon $g^{(2)}$ has loop-induced vertices with the standard model quarks, mediated by the first KK modes of the quark and the gluon/Higgs boson. With a top quark pair, this vertex is enhanced by the cooperation of the strong coupling of a gluon and the large Yukawa coupling of a top quark, leading to substantial branching ratio of $Br(g^{(2)} \to t \bar{t}) \approx 7%-8%$. As the $g^{(2)}$ coupling with two gluons appears via dimension-6 operator, $q\bar{q}\to g^{(2)}\to t\bar{t}$ is the golden mode for the mUED model. Hence the best channel is the $t\bar{t}$ resonance search in $p\bar{p}$ collisions. The recent Tevatron data at $\sqrt{s}=1.96$ TeV with an integrated luminosity of 8.7 fb$^{-1}$ are shown to give the first direct bound on the $g^{(2)}$ mass above 800 GeV. The implication and future prospect at the LHC are discussed also.

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arXiv2012

Probing axino LSP from diphotons events with large missing transverse energy

In a supersymmetry model with an axino as the lightest supersymmetric particle (LSP) and a Bino as the next LSP (NLSP), supersymmetry particles produced in pair at a high energy collider end up with including two Binos, followed by each Bino's decay into a photon and an axino. Final states are diphoton with large missing energy. We have comprehensively studied the implication of $γγ+$MET data from the ALEPH, CDF II, and recent ATLAS and CMS experiments. No excess over the standard model backgrounds can be explained in this model if the NLSP Bino decays outside the detector, which happens when the axino decay constant $f_a$ is large enough. The ALEPH and CDF II data put a very strong bound on $f_a$ for light Bino case with $m_{\tilde{B}} < 150$ GeV: the narrow hadronic axion window around $f_a \sim 10^6$ GeV is completely closed. In the case where the gluino mass is smaller than about 776 GeV, the recent ATLAS data limit $f_a > 10^5$ GeV for the Bino mass below 700 GeV. This is already stronger than the previous laboratory bounds.

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arXiv2012

Kinematic Cusps With Two Missing Particles I: Antler Decay Topology

The kinematics of a final state system with two invisible particles and two visible particles can develop cusped peak structures. This happens when the system has a fixed invariant mass (such as from a narrow resonant particle decay or with a fixed collision c.m. energy) and undergoes decays of two on-shell intermediate particles. Focusing on the "antler decay topology", we derive general analytic expressions for the invariant mass distribution and the kinematic cusp position. The sharp cusp peaks and the endpoint positions can help to determine the masses of the missing particles and the intermediate particles. We also consider transverse momentum variables and angular variables. In various distributions the kinematic cusp peaks are present and pronounced. We also study the effects on such kinematic cusp structures from realistic considerations including finite decay widths, the longitudinal boost of the system, and spin correlations.

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arXiv2012

Kinematic Cusps with Two Missing Particles II: Cascade Decay Topology

Three-step cascade decays into two invisible particles and two visible particles via two intermediate on-shell particles develop cusped peak structures in several kinematic distributions. We study the basic properties of the cusps and endpoints in various distributions and demonstrate that the masses of the missing particles and the intermediate particles can be determined by the cusp and endpoint positions. Effects from realistic considerations such as finite decay widths, longitudinal boost of the parent particle, and spin correlations are shown to be under control for the processes illustrated.

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arXiv2011

Implications on the mUED model from the early LHC data on $\ell+\met$ signal

Recently the ATLAS and CMS collaborations reported their search for a new heavy gauge boson $W'$ with one lepton plus missing transverse momentum. We find that $W^{(2)}$, the second Kaluza-Kelin (KK) state of the $W$ boson in the minimal Universal Extra Dimension (mUED) model, can be a good candidate for this signal, as its branching ratio into $\ellν$ is sizable. Moreover, nearly degenerate KK mass spectra in the mUED model yield generically very soft SM particle accompanying $W^{(2)}$ from the subsequent decays of the second KK quarks and gluons. In a hadron collider, this indirect $W^{(2)}$ production is difficult to distinguish from the $W^{(2)}$ single production. The involved strong interactions make it more important than the single production. The early LHC data on $\ell+\met$ signal for 1.1 fb$^{-1}$ integrated luminosity is shown insufficient to limit our model even with significant indirect production of $W^{(2)}$. However, the results show that the present LHC 5.6 fb$^{-1}$ data can cover most of the reasonable parameter space of the mUED model.

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arXiv2011

The Golden Mode for a Baryonic $Z'$ Boson at Hadronic Colliders: pp/ppbar -> WZ' -> l nu b b-bar

Associated production of a baryonic Z' boson with the W boson can account for the excess in Wjj production observed by the CDF collaboration at the Tevatron. We analyze other possible channels of this Z' at the Tevatron and at the LHC, including γZ' and Z Z' with the Z' -> jj. We show that the chances of confirming this baryonic Z' is better at the Tevatron than at the LHC because of the faster growing backgrounds at the LHC. Unfortunately the current systematic uncertainties of the order of 10% cannot yield any significant excess in both γZ' and Z Z' channels at the Tevatron and also at the LHC. Nevertheless the search using the b\bar b decay mode of Z' is much more feasible at the LHC, provided that the branching ratio B(Z' -> b\bar b) > 0.1. In particular, the W Z' -> l νb\bar b mode has a signal-to-background ratio larger than 1. Even with 1 fb^{-1} luminosity at the LHC it can lead to a high significance level. The WZ' -> l νb \bar b and γZ' -> γb \bar b are also highly observable at the Tevatron.

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arXiv2011

The second Kaluza-Klein neutral Higgs bosons in the minimal Universal Extra Dimension model

Loop-induced decay of a neutral Higgs boson into a pair of gluons or photons has great implications for the Higgs discovery at the LHC. If the Higgs boson is heavy with mass above $\sim 500\gev$, however, these radiative branching ratios are very suppressed in the standard model (SM), as the new decay channels are kinematically open. We note that these radiative decays can be sizable for the heavy CP-odd second Kaluza-Klein (KK) mode of the Higgs boson, $χ^\tw$, in the minimal universal extra dimension model: highly degenerate mass spectrum of the theory prohibits kinematically the dominant KK-number-conserving decays into the first KK modes of the $W$, $Z$ and top quark. We find that the CP-even decay of $h^{(2)} \to g g$ is absent at one-loop level since $h^{(2)}$ couples with different mass eigenstates of $t_{1,2}^\on$ while a gluon does with the same mass eigenstates. The $h^\tw$ production at the LHC is very suppressed. On the contrary, the process $ gg \to χ^\tw \to γγ$ in an optimal scenario can be observed with manageable SM backgrounds at the LHC.

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arXiv2011

Baryonic Z' Explanation for the CDF Wjj Excess

The latest CDF anomaly, the excess of dijet events in the invariant-mass window 120-160 GeV in associated production with a $W$ boson, can be explained by a baryonic $Z'$ model in which the $Z'$ boson has negligible couplings to leptons. Although this $Z'$ model is hardly subject to the Drell-Yan constraint from Tevatron, it is constrained by the dijet data from UA2 ($\sqrt{s} = 630$ GeV), and the precision measurements at LEP through the mixing with the SM $Z$ boson. We show that under these constraints this model can still explain the excess in the M_{jj} \sim 120 - 160 GeV window, as well as the claimed cross section $σ(W Z') \sim 4$ pb. Implications at the Tevatron would be the associated production of $γZ',\; Z Z'$, and $Z'Z'$ with the $Z' \to jj$. We show that with tightened jet cuts and improved systematic uncertainties both $γZ' \to γjj$ and $Z Z' \to \ell^+ \ell^- j j$ channels could be useful to probe this model at the Tevatron.

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arXiv2011

The CDF dijet excess and Z'_{cs} coupled to the second generation quarks

Recently the CDF collaboration has reported the excess in the dijet invariant-mass distribution of the Wjj events, corresponding to a significance of 3.2 standard deviations. Considering the lack of similar excesses in the $γjj$ and $Z jj$ events yet, we propose a new Z' model: Z' couples only to the second generation quarks. Single production of $\zsc$ as well as associated production with $W,γ, Z$ are mainly from the sea quarks. Only $W Z'$ production has additional contribution from one valence quark and one sea quark, which is allowed by CKM mixing. We found that if the new gauge coupling is large enough, marginally permitted by perturbativity, this new model can explain the observed CDF $Wjj$ anomaly as well as the lack of $\gm jj$ and $Z jj$ anomalies. Vanishing coupling of Z'-b-b protects this model from the constraint of p pbar ->WH -> lνb \bar{b}.

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arXiv2011

The B ---> π K puzzle and the Bulk Randall-Sundurm model

The recent measurements of the direct CP asymmetries ($A_{CP}$) in the penguin-dominated $B\to K π$ decays show some discrepancy from the standard model (SM) prediction. While $A_{CP}$ of $B^+ \to π^0 K^+$ and that of $B^0 \to π^- K^+$ in the naive estimate of the SM are expected to have very similar values, their experimental data are of the opposite sign and different magnitudes. We study the effects of the custodial bulk Randall-Sundrum model on this $A_{CP}$. In this model, the misalignment of the five-dimensional (5D) Yukawa interactions to the 5D bulk gauge interactions in flavor space leads to tree-level flavor-changing neutral current by the Kaluza-Klein gauge bosons. In a large portion of the parameter space of this model, the observed nonzero $A_{CP}(B^+ \to π^0 K^+) - A_{CP}(B^0 \to π^- K^+)$ can be explained only with low Kaluza-Klein mass scale $M_{KK}$ around 1 TeV. Rather extreme parameters is required to explain it with $M_{KK}\simeq 3\tev$. The new contributions to well-measured branching ratios of $B \to K π$ decays are also shown to be suppressed.

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arXiv2010

Kinematic Cusps: Determining the Missing Particle Mass at Colliders

In many extensions of the SM, neutral massive stable particles (dark matter candidates) are produced at colliders in pairs due to an exact symmetry called a "parity". These particles escape detection, rendering their mass measurement difficult. In the pair production of such particles via a specific ("antler") decay topology, kinematic cusp structures are present in the invariant mass and angular distributions of the observable particles. Together with the end-points, such cusps can be used to measure the missing particle mass and the intermediate particle mass in the decay chain. Our simulation of a benchmark scenario in a Z' supersymmetric model shows that the cusp feature survives under the consideration of detector simulation and the standard model backgrounds. This technique for determining missing particle masses should be invaluable in the search for new physics at the LHC and future lepton colliders.

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arXiv2010

Cosmic positron and antiproton constraints on the gauge-Higgs Dark Matter

We calculate the cosmic ray positron and antiproton spectra of a gauge-Higgs dark matter candidate in a warped five-dimensional $SO(5) \times U(1)$ gauge-Higgs unification model. The stability of the gauge-Higgs boson is guaranteed by the H parity under which only the Higgs boson is odd at low energy. The 4-point vertices of HHW^+W^- and HHZZ, allowed by H parity conservation, have the same magnitude as in the standard model, which yields efficient annihilation rate for $m_H > m_W$. The most dominant annihilation channel is $H H \to W^+ W^-$ followed by the subsequent decays of the $W$ bosons into positrons or quarks, which undergo fragmentation into antiproton. Comparing with the observed positron and antiproton spectra with the PAMALA and Fermi/LAT, we found that the Higgs boson mass cannot be larger than 90 GeV, in order not to overrun the observations. Together with the constraint on not overclosing the Universe, the valid range of the dark matter mass is restricted to 70-90 GeV.

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arXiv2010

Collider signatures of the Gauge-Higgs Dark Matter

A recently proposed SO(5)xU(1) gauge-Higgs unification model in the Randall-Sundrum warped space contains a very interesting dark matter candidate, the Higgs boson. As a part of the fifth component of a five-dimensional gauge field, the four-dimensional neutral Higgs boson has odd H parity while all the other standard model particles have even parity to all orders in perturbation theory. Based on H-parity conserving WWHH and ZZHH vertices, we investigate the collider signatures of Higgs dark matter production associated with a SM W or Z gauge boson at the LHC and the ILC. The final state consists of a W or a Z boson with large missing energy. We found that the level of the signal cross section is quite hopeless at the LHC while we may be able to identify it at the ILC with polarized electron beams.

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arXiv2009

Impact on the Light Higgsino-LSP Scenario from Physics beyond the Minimal Supersymmetric Standard Model

The modest addition of the dimension-5 term lambda(H_u.H_d)^2/M to the superpotential of the minimal supersymmetric standard model (MSSM) originated from physics beyond the MSSM (BMSSM) has a significant impact on the scenario of the Higgsino-dominated neutralino state being the lightest supersymmetric particle (LSP). It increases the mass difference between the LSP and the lighter chargino as well as that between the LSP and the second-lightest neutralino. This enhances the LHC discovery potential of the chargino and neutralino decays, producing more energetic charged leptons or pions than the decays without the BMSSM corrections. Furthermore, the coannihilation between the lighter chargino or second-lightest neutralino and the LSP is reduced substantially such that the LSP mass does not have to be very heavy. Consequently, an almost pure Higgsino LSP with its mass ~100 GeV in the BMSSM can account for all the relic density of cold dark matter in the Universe unless tan(beta) is too large.

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arXiv2008

Production and decays of the light pseudoscalar boson $η$ at the LHC in the simplest little Higgs model

In many extensions of the standard model, the Higgs sector often contains an additional pseudoscalar boson. A good example is the SU(3) simplest little Higgs model, which accommodates a light pseudoscalar boson $η$ with quite different characteristics from those in other multi-Higgs-doublet models. We study various phenomenological signatures of the $η$ at the LHC. In particular, we calculate in details both production and decays in the Drell-Yan type channel $q \bar{q} \to Z/Z' \to h η$, and in the associated production with a $t\bar t$ pair, $gg (q\bar q) \to t \bar t η$. We emphasize the $τ^+ τ^-$ decay mode of the $η$ boson when its mass is below the $b\bar b$ threshold. We show that $t\bar t η$ production is in fact large enough to give a sizable number of events while suppressing the backgrounds. We also comment on the direct gluon fusion process and the indirect decay from the heavy $T$ quark ($T \to t η$)

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arXiv2007

Custodial bulk Randall-Sundrum model and B->K* l+ l'-

The custodial Randall-Sundrum model based on SU(2)_L X SU(2)_R X U(1)_(B-L) generates new flavor-changing-neutral-current (FCNC) phenomena at tree level, mediated by Kaluza-Klein neutral gauge bosons. Based on two natural assumptions of universal 5D Yukawa couplings and no-cancellation in explaining the observed standard model fermion mixing matrices, we determine the bulk Dirac mass parameters. Phenomenological constraints from lepton-flavor-violations are also used to specify the model. From the comprehensive study of B->K* l+ l'-, we found that only the B->K*ee decay has sizable new physics effects. The zero value position of the forward-backward asymmetry in this model is also evaluated, with about 5% deviation from the SM result. Other effective observables are also suggested such as the ratio of two differential (or partially integrated) decay rates of B->K*ee and B->K*mu mu. For the first KK gauge boson mass of M_A^(1)=2-4 TeV, we can have about 10-20% deviation from the SM results.

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arXiv2007

Roles of Higgs decay into two pseudoscalar bosons in the search of intermediate-mass Higgs Boson

The dominance of $h\to ηη$ decay mode for the intermediate mass Higgs boson is highly motivated to solve the little hierarchy problem and to ease the tension with the precision data. However, the discovery modes for $m_h \alt 150$ GeV, $h \to γγ$ and $W/Z h \to (\ellν/\ell \bar \ell) (b\bar b)$, will be substantially affected. We show that $h \to ηη\to 4b$ is complementary and we can use this decay mode to detect the intermediate Higgs boson at the LHC, via $Wh$ and $Zh$ production. Requiring at least one charged lepton and 4 $B$-tags in the final state, we can identify a clean Higgs boson signal for $m_h \alt 150$ GeV with a high significance and with a full Higgs mass reconstruction. We use the next-to-minimal supersymmetric standard model and the simplest little Higgs model for illustration.

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