Searcharxiv⌕ Search

arXiv subjects

Jonathan L. Feng

Publications and source records attributed to Jonathan L. Feng.

At least 163 records · Page 9Linked to original sources
arXiv2001

Saltatory Relaxation of the Cosmological Constant

We modify and extend an earlier proposal by Brown and Teitelboim to relax the effective cosmological term by nucleation of branes coupled to a three-index gauge potential. Microscopic considerations from string/M theory suggest two major innovations in the framework. First, the dependence of brane properties on the compactification of extra dimensions may generate a very small quantized unit for jumps in the effective cosmological term. Second, internal degrees of freedom for multiply coincident branes may enhance tunneling rates by exponentially large density of states factors. These new features essentially alter the relaxation dynamics. By requiring stability on the scale of the lifetime of the universe, rather than absolute stability, we derive a non-trivial relation between the supersymmetry breaking scale and the value of the cosmological term. It is plausibly, though not certainly, satisfied in Nature.

hep-th↗
arXiv2001

Focus Point Supersymmetry: Proton Decay, Flavor and CP Violation, and the Higgs Boson Mass

In focus point supersymmetry, all squarks and sleptons, including those of the third generation, have multi-TeV masses without sacrificing naturalness. We examine the implications of this framework for low energy constraints and the light Higgs boson mass. Undesirable contributions to proton decay and electric dipole moments, generic in many supersymmetric models, are strongly suppressed. As a result, the prediction for alpha_s in simple grand unified theories is 3 to 5 standard deviations closer to the experimental value, and the allowed CP-violating phases are larger by one to two orders of magnitude. In addition, the very heavy top and bottom squarks of focus point supersymmetry naturally produce a Higgs boson mass at or above 115 GeV without requiring heavy gauginos. We conclude with an extended discussion of issues related to the definition of naturalness and comment on several other prescriptions given in the literature.

hep-ph↗
arXiv2001

Theoretical Overview: Motivations for Lepton Flavor Violation

In the coming years, experiments underway will increase the sensitivity to charged lepton flavor violation by four orders of magnitude. These experiments will stringently probe weak scale physics. I review the status of global symmetries in the standard model and present several well-motivated models that predict observable lepton flavor violation. Finally, I describe what we might learn from future experimental results, whether positive or null.

hep-ph↗
arXiv2000

Dark Matter Implications for Linear Colliders

The existence of dark matter is currently one of the strongest motivations for physics beyond the standard model. Its implications for future colliders are discussed. In the case of neutralino dark matter, cosmological bounds do not provide useful upper limits on superpartner masses. However, in simple models, cosmological considerations do imply that for supersymmetry to be observable at a 500 GeV linear collider, some signature of supersymmetry must appear before the LHC.

hep-ph↗
arXiv2000

Prospects for Indirect Detection of Neutralino Dark Matter

Dark matter candidates arising in models of particle physics incorporating weak scale supersymmetry may produce detectable signals through their annihilation into neutrinos, photons, or positrons. A large number of relevant experiments are planned or underway. The `logically possible' parameter space is unwieldy. By working in the framework of minimal supergravity, we can survey the implications of the experiments for each other, as well as for direct searches, collider searches, low-energy experiments, and naturalness in a transparent fashion. We find that a wide variety of experiments provide interesting probes. Particularly promising signals arise in the mixed gaugino-Higgsino region. This region is favored by low-energy particle physics constraints and arises naturally from minimal supergravity due to the focus point mechanism. Indirect dark matter searches and traditional particle searches are highly complementary. In cosmologically preferred models, if there are charged superpartners with masses below 250 GeV, then some signature of supersymmetry must appear before the LHC begins operation.

astro-ph↗
arXiv2000

Neutralino Dark Matter in Focus Point Supersymmetry

In recent work, it has been argued that multi-TeV masses for scalar superpartners are not unnatural. Indeed, they appear to have significant phenomenological virtues. Here we explore the implications of such `focus point' supersymmetry for the dark matter problem. We find that constraints on relic densities do not place upper bounds on neutralino or scalar masses. We demonstrate that, in the specific context of minimal supergravity, a cosmologically stable mixed gaugino-Higgsino state emerges as an excellent, robust dark matter candidate. We estimate that, over a wide range of the unknown parameters, the spin-independent proton-neutralino cross sections fall in the range accessible to planned search experiments.

hep-ph↗
arXiv2000

Naturalness Re-examined: Implications for Supersymmetry Searches

We discuss the origin of "focus points" in the scalar mass RGEs of the MSSM and their implications for collider searches. We present a new exact analytic solution to the homogeneous system of scalar mass RGEs in the MSSM for general $\tanβ$. This is then used to prove that the focus point for $m^2_{H_u}$ depends only on the value of the top Yukawa coupling at the {\em weak} scale (not its value at the GUT scale) and is independent of the bottom Yukawa coupling.

hep-ph↗
arXiv2000

Neutrino Parameters, Abelian Flavor Symmetries, and Charged Lepton Flavor Violation

Neutrino masses and mixings have important implications for models of fermion masses, and, most directly, for the charged lepton sector. We consider supersymmetric Abelian flavor models, where neutrino mass parameters are related to those of charged leptons and sleptons. We show that processes such as τto μγ, μto eγand μ-e conversion provide interesting probes. In particular, some existing models are excluded by current bounds, while many others predict rates within reach of proposed near future experiments. We also construct models in which the predicted rates for charged lepton flavor violation are below even the proposed experimental sensitivities, but argue that such models necessarily involve loss of predictive power.

hep-ph↗
arXiv1999

Superheavy Supersymmetry from Scalar Mass--A Parameter Fixed Points

In supersymmetric models, the well-known tension between naturalness and experimental constraints is relieved if the squarks and sleptons of the first two generations are superheavy, with masses of order 10 TeV, and all other superpartners are light, with masses of order 1 TeV. We show that even if all scalar masses and trilinear A parameters are of order 10 TeV at some high scale, a mass-squared hierarchy of order 400 may be generated dynamically through renormalization group evolution. The required high energy relations are consistent with grand unification, or, alternatively, may be realized in moduli-dominated supersymmetry-breaking scenarios.

hep-ph↗
arXiv1999

Focus Points and Naturalness in Supersymmetry

We analyze focus points in supersymmetric theories, where a parameter's renormalization group trajectories meet for a family of ultraviolet boundary conditions. We show that in a class of models including minimal supergravity, the up-type Higgs mass has a focus point at the weak scale, where its value is highly insensitive to the universal scalar mass. As a result, scalar masses as large as 2 to 3 TeV are consistent with naturalness, and {\em all} squarks, sleptons and heavy Higgs scalars may be beyond the discovery reaches of the Large Hadron Collider and proposed linear colliders. Gaugino and Higgsino masses are, however, still constrained to be near the weak scale. The focus point behavior is remarkably robust, holding for both moderate and large \tanβ, any weak scale gaugino masses and A parameters, variations in the top quark mass within experimental bounds, and for large variations in the boundary condition scale.

hep-ph↗
arXiv1999

Multi-TeV Scalars are Natural in Minimal Supergravity

For a top quark mass fixed to its measured value, we find natural regions of minimal supergravity parameter space where all squarks, sleptons, and heavy Higgs scalars have masses far above 1 TeV and are possibly beyond the reach of the Large Hadron Collider at CERN. This result is simply understood in terms of ``focus point'' renormalization group behavior and holds in any supergravity theory with a universal scalar mass that is large relative to other supersymmetry breaking parameters. We highlight the importance of the choice of fundamental parameters for this conclusion and for naturalness discussions in general.

hep-ph↗
arXiv1999

Supernatural Supersymmetry: Phenomenological Implications of Anomaly-Mediated Supersymmetry Breaking

We discuss the phenomenology of supersymmetric models in which supersymmetry breaking terms are induced by the super-Weyl anomaly. Such a scenario is envisioned to arise when supersymmetry breaking takes place in another world, i.e., on another brane. We review the anomaly-mediated framework and study in detail the minimal anomaly-mediated model parametrized by only 3+1 parameters: M_aux, m_0, \tanβ, and sign(μ). The renormalization group equations exhibit a novel "focus point" (as opposed to fixed point) behavior, which allows squark and slepton masses to be far above their usual naturalness bounds. We present the superparticle spectrum and highlight several implications for high energy colliders. Three lightest supersymmetric particle (LSP) candidates exist: the Wino, the stau, and the tau sneutrino. For the Wino LSP scenario, light Wino triplets with the smallest possible mass splittings are preferred; such Winos are within reach of Run II Tevatron searches. Finally, we study a variety of sensitive low energy probes, including b -> s gamma, the anomalous magnetic moment of the muon, and the electric dipole moments of the electron and neutron.

hep-ph↗
arXiv1999

Discovering Supersymmetry at the Tevatron in Wino LSP Scenarios

In supersymmetric models, Winos, partners of the SU(2) gauge bosons, may be the lightest supersymmetric particles (LSPs). For generic parameters, charged and neutral Winos are highly degenerate. Charged Winos travel macroscopic distances, but can decay to neutral Winos and extremely soft leptons or pions before reaching the muon chambers, thereby circumventing conventional trigger requirements based on energetic decay products or muon chamber hits. However, these charginos are detectable, and can be triggered on when produced in association with jets. In addition, we propose a new trigger for events with a high p_T track and low hadronic activity. For Tevatron Run II with luminosity 2 fb^-1, the proposed searches can discover Winos with masses up to 300 GeV and explore a substantial portion of the parameter space in sequestered sector models.

hep-ph↗
arXiv1999

Naturally Heavy Scalars in Supersymmetric Grand Unified Theories

The supersymmetric flavor, CP and Polonyi problems are hints that the fundamental scale of the soft supersymmetry breaking parameters may be above a TeV, in apparent conflict with naturalness. We consider the possibility that multi-TeV scalar masses are generated by Planck- or unification-scale physics, and find the conditions under which the masses of scalars with large Yukawa couplings are driven, radiatively and asymptotically, to the weak scale through renormalization group evolution. Light third generation scalars then satisfy naturalness, while first and second generation scalars remain heavy to satisfy experimental constraints. We find that this mechanism is beautifully realized in the context of grand unified theories. In particular, the existence of right-handed neutrinos plays an important role in allowing remarkably simple scenarios. For example, for SO(10) boundary conditions with the squared masses of Higgs scalars double those of sleptons and squarks, we find that the entire scalar mass scale may be increased to 4 TeV at the unification scale without sacrificing naturalness.

hep-ph↗
arXiv1999

Solving the Supersymmetric Flavor Problem with Radiatively Generated Mass Hierarchies

The supersymmetric flavor problem may be solved if the first and second generation scalars are heavy (with multi-TeV masses) and scalars with large Higgs couplings are light (with sub-TeV masses). We show that such an inverted spectrum may be generated radiatively; that is, from initial conditions where all scalar masses are multi-TeV at some high scale, those with large Higgs couplings may be driven asymptotically to the weak scale in the infra-red. The lightness of third generation scalars is therefore a direct consequence of the heaviness of third generation fermions, and fine-tuning is avoided even though the fundamental scale of the soft supersymmetry breaking parameters is multi-TeV. We investigate this possibility in the framework of the usual Yukawa quasi-fixed point solutions. The required high scale boundary conditions are found to be simple and highly predictive. This scenario also alleviates the supersymmetric CP and Polonyi problems.

hep-ph↗
arXiv1998

Tevatron Signatures of Long-lived Charged Sleptons in Gauge-Mediated Supersymmetry Breaking Models

In supersymmetric models with gauge-mediated supersymmetry breaking, charged sleptons are the next lightest supersymmetric particles and decay outside the detector for large regions of parameter space. In such scenarios, supersymmetry may be discovered by searches for a number of novel signals, including highly ionizing tracks from long-lived slow charged particles and excesses of multi-lepton signals. We consider this scenario in detail and find that the currently available Tevatron data probes regions of parameter space beyond the kinematic reach of LEP II. Future Tevatron runs with integrated luminosities of 2, 10, and 30 fb-1 probe right-handed slepton masses of 110, 180, and 230 GeV and Wino masses of 310, 370, and 420 GeV, respectively, greatly extending current search limits.

hep-ph↗
arXiv1998

Supersymmetry at Linear Colliders: The Importance of Being e-e-

Advantages of the e-e- option at linear colliders for the study of supersymmetry are highlighted. The fermion number violating process $e^- e^- \to \selectron^- \selectron^-$ provides unique opportunities for studies of slepton masses and flavor mixings. In particular, slepton mass measurements at the 100 MeV level through threshold scans of scalar pair production may be possible. Such measurements are over an order of magnitude better than those possible in e+e- mode, require far less integrated luminosity, and may lead to precise, model-independent measurements of $\tanβ$. Implications for studying gauginos and the importance of accurate beam polarimetry are also discussed.

hep-ph↗