arXiv subjects

Sunghoon Jung

Publications and source records attributed to Sunghoon Jung.

52 records · Page 3Linked to original sources

arXiv2014

Implications of an axino LSP for naturalness

Both the naturalness of the electroweak symmetry breaking and the resolution of the strong CP problem may require a small Higgsino mass $μ$ generated by a realization of the DFSZ axion model. Assuming the axino is the lightest supersymmetric particle, we study its implications on $μ$ and the axion scale. Copiously produced light Higgsinos at collider (effectively only neutral NLSP pairs) eventually decay to axinos leaving prompt multi-leptons or displaced vertices which are being looked for at the LHC. We use latest LHC7+8 results to derive current limits on $μ$ and the axion scale. Various Higgsino-axino phenomenology is illustrated by comparing with a standard case without lightest axinos as well as with a more general case with additional light gauginos in the spectrum.

hep-ph↗

arXiv2014

Stop and Sbottom LSP with R-parity Violation

Considering a third-generation squark as the lightest supersymmetric particle (LSP), we investigate R-parity violating collider signatures with bilinear LH or trilinear LQD operators that may contribute to observed neutrino masses and mixings. Reinterpreting the LHC 7+8 TeV results of SUSY and leptoquark searches, we find that third-generation squark LSPs decaying to first- or second-generation leptons are generally excluded up to at least about 660 GeV at 95%C.L.. One notable feature of many models is that sbottoms can decay to top quarks and charged leptons that lead to a broader invariant mass spectrum and weaker collider constraints. More dedicated searches with $b$-taggings or top reconstructions are thus encouraged. Finally, we discuss that the recently observed excesses in the CMS leptoquark search can be accommodated by the decay of sbottom LSPs in the LQD$_{113+131}$ model.

hep-ph↗

arXiv2014

Resolving the existence of Higgsinos in the LHC inverse problem

The LHC inverse problem is infamously challenging when neutralinos and charginos are heavy and pure and other superparticles are decoupled. This limit is becoming more relevant to particle physics nowadays. Fortunately, in this limit, Higgsinos produce a distinctive signature if they are the LSPs or NLSPs. The identifying signature is the presence of equal numbers of Z bosons and Higgs bosons in NLSP productions and subsequent decays at hadron colliders. The signature is derived from the Goldstone equivalence theorem by which partial widths into Z and Higgs bosons are inherently related and from the fact that Higgsinos consist of two indistinguishable neutralinos. Thus it is valid in general for many supersymmetry models; exceptions may happen when Higgsino NLSPs decay to weakly coupled LSPs such as axinos or gravitinos.

hep-ph↗

arXiv2013

Gaugino physics of split supersymmetry spectrum at the LHC and future proton colliders

Discovery of the Higgs boson and lack of discovery of superpartners in the first run at LHC are both predictions of split supersymmetry with thermal dark matter. We discuss what it would take to find gluinos at hadron supercolliders, including the LHC at 14 TeV center of mass energy, and future pp colliders at 100 TeV and 200 TeV. We generalize the discussion by re-expressing the search capacity in terms of gluino to lightest superpartner mass ratio, and apply results to other scenarios, such as gauge mediation and mirage mediation.

hep-ph↗

arXiv2013

Radiative generation of the Higgs potential

We consider the minimal extension of the Standard Model with a generalized B-L gauge symmetry U(1)_X for generating the Higgs potential radiatively. Assuming that the full scalar potential vanishes at the vacuum instability scale, we achieve the goal in terms of two free parameters, the X gauge coupling and the right-handed neutrino Yukawa coupling. The X gauge symmetry is broken spontaneously by the Coleman-Weinberg mechanism while the scale symmetry breakdown induces electroweak symmetry breaking through the radiative generation of appropriate scalar quartic couplings. We show that there is a reasonable parameter space that is consistent with a correct electroweak symmetry breaking and the observed Higgs mass.

hep-ph↗

arXiv2013

Very Light Charginos and Higgs Decays

We explore modifications to the loop-induced Higgs couplings hγγ and hγZ from light charginos in the Minimal Supersymmetric Standard Model. When the lightest chargino mass is above the naive LEP bound of order 100 GeV the effects are modest, with deviations in the decay branching ratios typically less than 15% from the Standard Model predictions. However, if the charginos are lighter than 100 GeV, more dramatic alterations to these couplings are possible as a consequence of the rise of the one loop form factor. For example, the diphoton signal strength can be enhanced by as much as 70% compared to the Standard Model value. We scrutinize in detail the existing LEP, Tevatron, and LHC searches and present a scenario in which a very light chargino with a mass as light as half the Higgs mass is hidden at LEP and is allowed by all direct collider constraints and electroweak precision tests. The scenario has a sneutrino LSP with a macroscopic decay length of order 10-100 cm. We outline potential search strategies to test this scenario at the LHC.

hep-ph↗

arXiv2013

Implications of LHC data on 125GeV Higgs-like boson for the Standard Model and its various extensions

Recent data on 125 GeV Higgs-like boson at the LHC starts to constrain the electroweak symmetry breaking sector of the SM and its various extensions. If one imposes the local gauge symmetry of the Standard Model (SM) ($SU(3)_c \times SU(2)_L \times U(1)_Y$) to the SM and any possible new physics scenarios, the SM Higgs properties will be modified by intrinsically two different ways: by new physics either coupling directly to the SM Higgs boson $h$, or affecting indirectly the SM Higgs properties through the mixing of $h$ with a SM singlet scalar $s$. The models of two Higgs doublet, extra sequential and mirror fermions belong to the first category, whereas the models with a hidden sector dark matter, extra vector-like fermions and new charged vector bosons, which can enhance the diphoton rate of the SM Higgs-like resonance, belong to the second category. We perform a global fit to data in terms of the effective Lagrangian description of two interaction eigenstates of scalar bosons, a SM Higgs and a singlet scalar, and their mixing. This framework is more suitable to study singlet-extended scenarios discussed above compared to other approaches based on the Lagrangian of mass eigenstates. With fairly model-independent assumptions, the effective Lagrangian contains at most four free parameters still encompassing the majority of models in the literature. Interestingly, the SM gives the best fit if all data from ATLAS and CMS are used, whereas various singlet extensions can fit better to individual ATLAS or CMS data. Without further assumptions, an upper bound on the total width (or, non-standard branching ratio) is generically obtained. Furthermore, global fit based on our parameterization can be used to probe interactions of the singlet scalar if the singlet resides below $2m_W$.

hep-ph↗

arXiv2013

Cornering electroweakinos at the LHC

Squeezed supersymmetric spectra are challenging for the LHC searches based on a sizable missing energy and hard visible particles. One such scenario consists of chargino/second-lightest neutralino NLSPs and a lightest neutralino LSP with a relatively small mass gap (10-50GeV). In this note, we explore search strategies to better probe this parameter space. We focus on the three-lepton plus missing energy channel arising from the chargino/second-lightest neutralino associated production, and we investigate the role of a relatively hard initial state radiation (ISR) jet. In addition to typical kinematic variables, such as the minimum lepton pair invariant mass, we propose an angular separation variable and two ratio variables which capitalize on the main kinematic features; leptons stay relatively soft under the boost from ISR and a sizable missing energy arises only in tight correlation with the ISR boost. With 300/fb of data at the 14 TeV LHC, we expect to probe electroweakinos up to 320 (700) GeV with 30GeV mass-gap and up to 220 (620) GeV with 20GeV mass-gap if gauginos decay via gauge bosons (light sleptons). We emphasize that the three-lepton channel is technically challenging for mass-gap below 12GeV.

hep-ph↗

arXiv2012

Singlet Assisted Vacuum Stability and the Higgs to Diphoton Rate

Light, electrically charged vector-like `leptons' with order one Yukawa couplings can enhance the Higgs to diphoton decay rate, as is suggested by measurements of the signal strength μ_{γγ} by ATLAS and CMS. However, the large Yukawa interactions tend to drive the Higgs quartic coupling negative at a low scale Λ< 10 TeV, and as such new physics is required to stabilize the electroweak vacuum. A plausible option, which does not rely on supersymmetry, is that the Higgs in fact has a much larger tree level quartic coupling than in the Standard Model, which is possible with an extended scalar sector. We investigate in detail the minimal model with a new real singlet scalar which condenses and mixes with the Higgs. The mechanism is very efficient when the singlet is heavier than the weak scale m_s ~ O(TeV), in which case the vacuum can be stable and the couplings can be perturbative up to scales Λ~ 10^9 GeV while μ_{γγ} = 1.7. On the other hand, for a singlet at the weak scale, a substantial shift in the Higgs quartic coupling requires sizable Higgs-singlet mixing. Such mixing can be utilized to further enhance μ_{γγ} through Yukawa couplings of the singlet to the vector-like leptons. In this case, for an enhancement μ_{γγ} \sim 1.7 it is possible to obtain a stable vacuum up to scales Λ~ 1000 TeV. The singlet can have significant mixing with the Higgs particle, allowing for its resonant production via gluon fusion at the LHC, and can be searched for through Higgs like decays and decays to vector-like leptons, which can lead to multi-lepton final states. We also comment on UV extensions of the minimal model.

hep-ph↗

arXiv2011

Top asymmetry and the search for a light hadronic resonance in association with single top

The exchange of a light $t$-channel flavor-changing gauge boson, $\Vp$, with mass $\sim m_{top}$ remains a leading explanation for the anomalous forward backward asymmetry in top quark production at the Tevatron. Unlike other ideas, including heavier $t$-channel mediators, the light $\Vp$ model is not easily seen in the $\mtt$ distribution. We advocate a more promising strategy. While current analyses at hadron colliders may not be sensitive, we propose searching for a $jj$ resonance in association with single top that may allow discovery in existing data. Deviations in the lepton charge asymmetry in this sample should also be present.

hep-ph↗

arXiv2011

Top quark asymmetry and dijet resonances

CDF recently reported an anomaly in the $m_{jj}$ distribution of dijet events produced in association with a $W$ boson. If this anomaly is associated with a new flavor conserving vector resonance, $V$, one might have expected to observe effects in the analogous $m_{jj}$ distribution produced in association with a $γ$. No such excess is observed. A single $u-t-V$ flavor changing coupling, however, can contribute to the $m_{jj}$ anomaly while being consistent with other resonance searches. Furthermore, it gives a potential explanation of the observed forward-backward asymmetry in top quark production.

hep-ph↗

arXiv2011

Top quark asymmetry from a non-Abelian horizontal symmetry

Motivated by the persistence of a large measured top quark forward-backward asymmetry at the Tevatron, we examine a model of non-Abelian flavor gauge symmetry. The exchange of the gauge bosons in the $t$-channel can give a large asymmetry due to the forward Rutherford scattering peak. We address generic constraints on non-Abelian $t$-channel physics models including flavor diagonal resonances and potentially dangerous contributions to inclusive top pair cross sections. We caution on the general difficulty of comparing theoretical predictions for top quark signals to the existing experimental results due to potentially important acceptance effects. The first signature at the Large Hadron Collider can be a large inclusive top pair cross section, or like-sign dilepton events, although the latter signal is much smaller than in Abelian models. Deviations of the invariant mass distributions at the LHC will also be promising signatures. A more direct consistency check of the Tevatron asymmetry through the LHC asymmetry is more likely to be relevant at a later stage.

hep-ph↗

arXiv2010

Low-scale warped extra dimension and its predilection for multiple top quarks

Within warped extra dimension models that explain flavor through geometry, flavor changing neutral current constraints generally force the Kaluza-Klein scale to be above many TeV. This creates tension with a natural electroweak scale. On the other hand, a much lower scale compatible with precision electroweak and flavor changing neutral current constraints is allowed if we decouple the Kaluza-Klein states of Standard Model gauge bosons from light fermions $c_{\rm light}\simeq c_b\simeq 0.5$ bulk mass parameters). The main signature for this approach is four top quark production via the Kaluza-Klein excitations' strong coupling to top quarks. We study single lepton, like-sign dilepton, and trilepton observables of four-top events at the Large Hadron Collider. The like-sign dilepton signature typically has the largest discovery potential for a strongly coupled right-handed top case ($M_{KK} \sim 2-2.5 \TeV$), while single lepton is the better when the left-handed top couples most strongly ($M_{KK} \sim 2 \TeV$). We also describe challenging lepton-jet collimation issues in the like-sign dilepton and trilepton channels. An alternative single lepton observable is considered which takes advantage of the many bottom quarks in the final state. Although searches of other particles may compete, we find that four top production via Kaluza-Klein gluons is most promising in a large region of this parameter space.

hep-ph↗

arXiv2009

Comparison of electric dipole moments and the Large Hadron Collider for probing CP violation in triple boson vertices

CP violation from physics beyond the Standard Model may reside in triple boson vertices of the electroweak theory. We review the effective theory description and discuss how CP violating contributions to these vertices might be discerned by electric dipole moments (EDM) or diboson production at the Large Hadron Collider (LHC). Despite triple boson CP violating interactions entering EDMs only at the two-loop level, we find that EDM experiments are generally more powerful than the diboson processes. To give example to these general considerations we perform the comparison between EDMs and collider observables within supersymmetric theories that have heavy sfermions, such that substantive EDMs at the one-loop level are disallowed. EDMs generally remain more powerful probes, and next-generation EDM experiments may surpass even the most optimistic assumptions for LHC sensitivities.

hep-ph↗

arXiv2009

Top quark forward-backward asymmetry from new t-channel physics

Motivated by recent measurements of the top quark forward-backward asymmetry at the Tevatron, we study how t-channel new physics can contribute to a large value. We concentrate on a theory with an abelian gauge boson possessing flavor changing couplings between up and top quarks, but satisfies flavor physics constraints. Collider constraints are strong, but can be consistent with the aid of small flavor diagonal couplings. We find that M_Z' ~ 160 GeV can yield a total lab-frame asymmetry of ~18% without being in conflict with other observables. There are implications for future collider searches, including exotic top quark decays, like-sign top quark production, and detailed measurements of the top production cross section. An alternate model with a gauged non-Abelian flavor symmetry would have similar phenomenology, but lacks the like-sign top signal.

hep-ph↗

arXiv2008

Higgs boson decays to four fermions through an abelian hidden sector

We consider a generic abelian hidden sector that couples to the Standard Model only through gauge-invariant renormalizable operators. This allows the exotic Higgs boson to mix with the Standard Model Higgs boson, and the exotic abelian gauge boson to mix with the Standard Model hypercharge gauge boson. One immediate consequence of spontaneous breaking of the hidden sector gauge group is the possible decay of the lightest Higgs boson into four fermions through intermediate exotic gauge bosons. We study the implications of this decay for Higgs boson phenomenology at the Fermilab Tevatron Collider and the CERN Large Hadron Collider. Our emphasis is on the four lepton final state.

hep-ph↗