arXiv subjects

Hsin-Chia Cheng

Publications and source records attributed to Hsin-Chia Cheng.

At least 73 records · Page 4Linked to original sources

arXiv2001

GUT Breaking on the Lattice

We construct a supersymmetric grand unified model in the framework of a latticized extra dimension. The SU(5) symmetries on the lattice are broken by the vacuum expectation values of the link fields connecting adjacent SU(5) sites, leaving just the MSSM at low energies. Below the SU(5) breaking scale, the theory gives rise to a similar spectrum as in orbifold breaking of SU(5) symmetry in 5 dimensions, and shares many features with the latter scenario. We discuss gauge coupling unification and proton decay emphasizing the differences with respect to the usual grand unified theories. Our model may be viewed as an effective four dimensional description of the orbifold symmetry breaking in higher dimensions.

hep-ph↗

arXiv2001

Bounds on Universal Extra Dimensions

We show that the bound from the electroweak data on the size of extra dimensions accessible to all the standard model fields is rather loose. These "universal" extra dimensions could have a compactification scale as low as 300 GeV for one extra dimension. This is because the Kaluza-Klein number is conserved and thus the contributions to the electroweak observables arise only from loops. The main constraint comes from weak-isospin violation effects. We also compute the contributions to the S parameter and the $Zb\bar{b}$ vertex. The direct bound on the compactification scale is set by CDF and D0 in the few hundred GeV range, and the Run II of the Tevatron will either discover extra dimensions or else it could significantly raise the bound on the compactification scale. In the case of two universal extra dimensions, the current lower bound on the compactification scale depends logarithmically on the ultra-violet cutoff of the higher dimensional theory, but can be estimated to lie between 400 and 800 GeV. With three or more extra dimensions, the cutoff dependence may be too strong to allow an estimate.

hep-ph↗

arXiv2001

Dynamical Electroweak Breaking and Latticized Extra Dimensions

Using gauge invariant effective Lagrangians in 1+3 dimensions describing the Standard Model in 1+4 dimensions, we explore dynamical electroweak symmetry breaking. The Top Quark Seesaw model arises naturally, as well as the full CKM structure. We include a discussion of effects of warping, and indicate how other dynamical schemes may also be realized.

hep-ph↗

arXiv2001

Axions and a Gauged Peccei-Quinn Symmetry

The axion solution to the strong CP problem requires an anomalous global U(1) symmetry. We show that the existence of such a symmetry is a natural consequence of an extra dimension in which a gauged U(1) is spontaneously broken on one of two branes, leaving an accidental global symmetry on the other brane. Depending on where the standard model matter lives, the resulting axion can be either the DFSZ or hadronic type. Gaugino-mediated supersymmetry breaking fits comfortably in our framework. In addition, we present a model in which the supersymmetry-breaking and Peccei-Quinn breaking scales are naturally of the same size.

hep-ph↗

arXiv2001

The Standard Model in the Latticized Bulk

We construct the manifestly gauge invariant effective Lagrangian in 3+1 dimensions describing the Standard Model in 4+1 dimensions, following the transverse lattice technique. We incorporate split generation fermions and we explore naturalness for two Higgs configurations: a universal Higgs VEV, common to each transverse brane, and a local Higgs VEV centered on a single brane with discrete exponential attenuation to other branes, emulating the split-generation model. Extra dimensions, with explicit Higgs, do not ameliorate the naturalness problem.

hep-th↗

arXiv2000

Electroweak symmetry breaking and extra dimensions

The electroweak symmetry may be broken by a composite Higgs which arises naturally as a bound state of the top quark if the standard model gauge fields and fermions propagate in extra dimenions. The top quark mass and the Higgs mass can be predicted from the infrared fixed points of the renormalization group equations. The top quark mass is in good agreement with the experimental value, and the Higgs boson mass is predicted to be $\sim 200$ GeV. The bounds on the compactification scale can be quite low if all standard model fields propagate in the same extra dimensions due to the momentum conservation in extra dimensions. The current lower limits are about 300 GeV for one extra dimension and 400-800 GeV for two extra dimensions. The future collider experiments may either discover the Kaluza-Klein (KK) states of the standard model fields or raise their mass limits significantly. There may also be some other light bound states which could be observed at upcoming collider experiments.

hep-ph↗

arXiv2000

Self-Breaking of the Standard Model Gauge Symmetry

If the gauge fields of the Standard Model propagate in TeV-size extra dimensions, they rapidly become strongly coupled and can form scalar bound states of quarks and leptons. If the quarks and leptons of the third generation propagate in 6 or 8 dimensions, we argue that the most tightly bound scalar is a composite of top quarks, having the quantum numbers of the Higgs doublet and a large coupling to the top quark. In the case where the gauge bosons propagate in a bulk of a certain volume, this composite Higgs doublet can successfully trigger electroweak symmetry breaking. The mass of the top quark is correctly predicted to within 20%, without the need to add a fundamental Yukawa interaction, and the Higgs boson mass is predicted to lie in the range 165 - 230 GeV. In addition to the Higgs boson, there may be a few other scalar composites sufficiently light to be observed at upcoming collider experiments.

hep-ph↗

arXiv2000

Minimal electroweak symmetry breaking model in extra dimensions

We show that if the Standard Model gauge fields and fermions propagate in extra dimenions, a composite Higgs field with the correct quantum number can arise naturally as a bound state due to the strong gauge interactions in higher dimensions. The top quark mass and the Higgs mass can be predicted from the infrared fixed points of the renormalization group equations. The top quark mass is in good agreement with the experimental value, and the Higgs boson mass is predicted to be ~200 GeV. There may be some other light bound states which could be observed at upcoming collider experiments.

hep-ph↗

arXiv2000

Electroweak Symmetry Breaking and Extra Dimensions

Electroweak symmetry can be naturally broken by observed quark and gauge fields in various extra-dimensional configurations. No new {\it fundamental} fields are required below the quantum gravitational scale ($\sim$ 10 - 100 TeV). We examine schemes in which the QCD gauge group alone, in compact extra dimensions, forms a composite Higgs doublet out of (t,b)_L and a linear combination of the Kaluza-Klein modes of t_R. The effective theory at low energies is the Standard Model. The top-quark mass is controlled by the number of active t_R Kaluza-Klein modes below the string scale, and is in agreement with experiment.

hep-ph↗

arXiv2000

Electroweak symmetry breaking by extra dimensions

Electroweak symmetry breaking may be naturally induced by the observed quark and gauge fields in extra dimensions without a fundamental Higgs field. We show that a composite Higgs doublet can arise as a bound state of $(t, b)_L$ and a linear combination of the Kaluza-Klein states of $t_R$, due to QCD in extra dimensions. The top quark mass depends on the number of active $t_R$ Kaluza-Klein modes, and is consistent with the experimental value.

hep-ph↗

arXiv1999

New Higgs Signals from Flavor Physics in Large Extra Dimensions

In a realistic theory with large extra dimensions and TeV scale gravity, there are often other particles living in the extra dimensions in addition to gravitons. A well motivated candidate is the flavon in a flavor theory which explains the smallness of the fermion masses by the large volume of the extra dimensions. The flavon interactions involve the Higgs field and can therefore give rise to new mechanisms of Higgs boson production. We study these new Higgs signals, including both real flavon emission and virtual flavon exchange. They could provide powerful probes of the string scale which are comparable to the extensively studied graviton production, if the flavons and the graviton feel the same number of extra dimensions. If the flavons only live in a subspace of the extra dimensions, they can yield even stronger bounds and may generate spectacular displaced vertex signals. Our study also applies to a four dimensional flavor theory with light flavons as well as to any theory with bulk scalar fields coupled to the Higgs and fermions.

hep-ph↗

arXiv1999

Doublet-Triplet Splitting and Fermion Masses with Extra Dimensions

The pseudo-Goldstone boson mechanism for the ``doublet-triplet splitting'' problem of the grand unified theory can be naturally implemented in the scenario with extra dimensions and branes. The two SU(6) global symmetries of the Higgs sector are located on two separate branes while the SU(6) gauge symmetry is in the bulk. After including several vector-like fields in the bulk, and allowing the most general interactions with their natural strength (including the higher dimensional ones which may be generated by gravity) which are consistent with the geometry, a realistic pattern of the Standard Model fermion masses and mixings can be naturally obtained without any flavor symmetry. Neutrino masses and mixings required for the solar and atmospheric neutrino problems can also be accommodated. The geometry of extra dimensions and branes provides another way to realize the absence of certain interactions (as required in the pseudo-Goldstone boson mechanism) or the smallness of some couplings (e.g., the Yukawa couplings between the fermions and the Higgs bosons), in addition to the usual symmetry arguments.

hep-ph↗

arXiv1999

Gauge Coupling Unification with Extra Dimensions and Gravitational Scale Effects

We study gauge coupling unification in the presence of extra dimensions compactified at a few TeV. Achieving unification requires a large number of gauge boson Kaluza-Klein excitations lighter than the string scale, such that the higher-dimensional gauge couplings are O(1) in string scale units. Corrections to the gauge couplings from two or more loops are about 10% or larger, hence string (or M) theory is generally expected to be strongly coupled in TeV-scale extra-dimensional scenarios. Higher-dimensional operators induced by quantum gravitational effects can shift the gauge couplings by a few percent. These effects are sufficiently large that even the minimal Standard Model, or the MSSM, allow unification at a scale in the TeV range. The strongly coupled unified theory may induce dynamical electroweak symmetry breaking.

hep-ph↗

arXiv1998

Generic and Chiral Extensions of the Supersymmetric Standard Model

We construct extensions of the Standard Model in which the gauge symmetries and supersymmetry prevent the dangerously large effects that may potentially be induced in a supersymmetric standard model by Planck scale physics. These include baryon number violation, flavor changing neutral currents, the $μ$ term, and masses for singlet or vector-like fields under the Standard Model gauge group. For this purpose we introduce an extra non-anomalous $U(1)_μ$ gauge group. Dynamical supersymmetry breaking in a secluded sector triggers the breaking of the $U(1)_μ$ and generates soft masses for the superpartners via gauge mediation, with the scalars possibly receiving sizable contributions from the $U(1)_μ$ D-term. We find several classes of complete and calculable models, in which the messengers do not present cosmological problems and neutrino masses can also be accomodated. We derive the sparticle spectrum in these models and study the phenomenological consequences. We give an exhaustive list of the potential experimental signatures and discuss their observability in the upcoming Tevatron runs. One class of models exhibits interesting new discovery channels, namely $WW \not E_T$, $Wγ\not E_T$ and $WZ \not E_T$, which arise when the next-to-lightest supersymmetric particle is a short-lived $SU(2)_W$ neutralino.

hep-ph↗

arXiv1998

Duality in the Presence of Supersymmetry Breaking

We study Seiberg duality for N=1 supersymmetric QCD with soft supersymmetry-breaking terms. We generate the soft terms through gauge mediation by coupling two theories related by Seiberg duality to the same supersymmetry-breaking sector. In this way, we know what a supersymmetry-breaking perturbation in one theory maps into in its ``dual''. Assuming a canonical Kahler potential we calculate the soft terms induced in the magnetic theory and find that some of the scalars acquire negative masses squared. If duality is still good for small supersymmetry breaking, this may imply some specific symmetry breaking patterns for supersymmetric QCD with small soft supersymmetry-breaking masses, in the case that its dual theory is weakly coupled in the infrared. In the limit of large supersymmetry breaking, the electric theory becomes ordinary QCD. However, the resulting symmetry breaking in the magnetic theory is incompatible with that expected for QCD.

hep-th↗

arXiv1998

A Chiral Supersymmetric Standard Model

We propose a supersymmetric extension of the Standard Model with an extra U(1) gauge symmetry, so that all supersymmetric mass terms, including the $μ$-term, are forbidden by the gauge symmetries. Supersymmetry is broken dynamically which results in U(1) breaking and generation of realistic $μ$ term and soft breaking masses. The additional fields required to cancel the U(1) anomalies are identified with the messengers of supersymmetry breaking. The gaugino masses arise as in the usual gauge mediated scenario, while squarks and sleptons receive their masses from both the U(1) $D$-term and the two-loop gauge mediation contributions. The scale of supersymmetry breaking in this model can be below $10^6$ GeV, yielding collider signatures with decays to goldstinos inside the detector.

hep-ph↗

arXiv1998

Duality after Supersymmetry Breaking

Starting with two supersymmetric dual theories, we imagine adding a chiral perturbation that breaks supersymmetry dynamically. At low energy we then get two theories with soft supersymmetry-breaking terms that are generated dynamically. With a canonical Kahler potential, some of the scalars of the "magnetic" theory typically have negative mass-squared, and the vector-like symmetry is broken. Since for large supersymmetry breaking the "electric" theory becomes ordinary QCD, the two theories are then incompatible. For small supersymmetry breaking, if duality still holds, the magnetic theory analysis implies specific patterns of chiral symmetry breaking in supersymmetric QCD with small soft masses.

hep-th↗

arXiv1998

Precision Supersymmetry Measurements at the e^-e^- Collider

Measurements of supersymmetric particle couplings provide important verification of supersymmetry. If some of the superpartners are at the multi-TeV scale, they will escape direct detection at planned future colliders. However, such particles induce nondecoupling corrections in processes involving the accessible superparticles through violations of the supersymmetric equivalence between gauge boson and gaugino couplings. These violations are analogous to the oblique corrections in the electroweak sector of the standard model, and can be parametrized in terms of super-oblique parameters. The $e^- e^-$ collision mode of a future linear collider is shown to be an excellent environment for such high precision measurements of these SUSY parameters, which will provide an important probe of superparticles beyond reachable energies.

hep-ph↗