arXiv subjects

Konstantin T. Matchev

Publications and source records attributed to Konstantin T. Matchev.

143 records · Page 8Linked to original sources

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

Discovering a Light Higgs Boson with Light

We evaluate the prospects for detecting a non-standard light Higgs boson with a significant branching ratio to two photons, in Run II of the Fermilab Tevatron. We derive the reach for several channels: $2γ$ inclusive, $2γ+1$ jet and $2γ+2$ jets. We present the expected Run II limits on the branching ratio of $h\toγγ$ as a function of the Higgs mass, for the case of ``bosonic'', as well as ``topcolor'' Higgs bosons.

hep-ex↗

arXiv1999

Tau Jet Signals for Supersymmetry at the Tevatron

We present a more detailed account of our study (hep-ph/9903238) for the supersymmetry reach of the Tevatron in channels with isolated leptons and identified tau jets. We review the theoretical motivations for expecting such signatures, and describe the relevant parameter space in the minimal supergravity and the minimal gauge-mediated models. With explicit Monte Carlo simulations we then show that for certain parameter ranges, channels with two leptons and one tau jet offer a better reach in Run II than the clean trilepton signal. We emphasize than improving on tau ID is an important prerequisite for successful searches in multiple tau jet channels. Finally, we discuss some triggering issues.

hep-ex↗

arXiv1999

Stop and Sbottom Searches in Run II of the Fermilab Tevatron

We estimate the Tevatron Run II potential for top and bottom squark searches. We find an impressive reach in several of the possible discovery channels. We also study some new channels which may arise in non-conventional supersymmetry models. In each case we rely on a detailed Monte Carlo simulation of the collider events and the CDF detector performance in Run I.

hep-ph↗

arXiv1999

Supersymmetry Signatures with Tau Jets at the Tevatron

We study the supersymmetry reach of the Tevatron in channels containing both isolated leptons and identified tau jets. In the most challenging case, where the branching ratios of gauginos to taus dominate, we find that searches for two leptons, a tau jet and a large amount of missing transverse energy have a much better reach than the classic trilepton signature. With total integrated luminosity of ${\rm L} \gsim 4 {\rm fb}^{-1}$, the Tevatron will start extending the expected LEP-II reach for supersymmetry.

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

Higgs and Z-boson Signatures of Supersymmetry

In supersymmetric theories of nature the Higgsino fermionic superpartner of the Higgs boson can arise as the lightest standard model superpartner depending on the couplings between the Higgs and supersymmetry breaking sectors. In this letter the production and decay of Higgsino pairs to the Goldstone fermion of supersymmetry breaking and the Higgs boson, h, or gauge bosons, Z or $γ$ are considered. Relatively clean di-boson final states, hh, $h γ$, hZ, $Z γ$, or ZZ, with a large amount of missing energy result. The latter channels provide novel discovery modes for supersymmetry at high energy colliders since events with Z bosons are generally rejected in supersymmetry searches. In addition, final states with real Higgs bosons can potentially provide efficient channels to discover and study a Higgs signal at the Fermilab Tevatron Run II.

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

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

New backgrounds in Trilepton, Dilepton and Dilepton plus Tau Jet SUSY Signals at the Tevatron

We determine the Tevatron's reach in supersymmetric parameter space in trilepton, like-sign dilepton, and dilepton plus tau-jet channels, taking all relevant backgrounds into account. We show results for the minimal supergravity model. With a standard set of cuts we find that the previously unaccounted for $Wγ^\ast$ background is larger than all other backgrounds combined. We include cuts on the dilepton invariant mass and the $W$-boson transverse mass to reduce the $Wγ^\ast$ background to a reasonable level. We optimize cuts at each point in supersymmetry parameter space in order to maximize signal-to-noise.

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

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

New Limits on the SUSY Higgs Boson Mass

We present new upper limits on the light Higgs boson mass mh in supersymmetric models. We consider two gravity-mediated models (with and without universal scalar masses) and two gauge-mediated models (with a 5+5 or 10+10 messenger sector). We impose standard phenomenological constraints, as well as SU(5) Yukawa coupling unification. Requiring that the bottom and tau Yukawa couplings meet at the unification scale to within 15%, we find the upper limit mh<114 GeV in the universal supergravity model. This reverts to the usual upper bound of 125 GeV with a particular nonuniversality in the scalar spectrum. In the 5+5 gauge-mediated model we find mh<97 GeV for small tan beta and mh = 116 GeV for large tan beta, and in the 10+10 model we find mh<94 GeV. We discuss the implications for upcoming searches at LEP-II and the Tevatron.

hep-ph↗

arXiv1997

QCD Corrections to Flavor-Changing Neutral Currents in the Supersymmetric Standard Model

We compute the leading QCD corrections to K-Kbar mixing in the supersymmetric standard model with general soft supersymmetry-breaking parameters. We construct the ΔS=2 effective Lagrangian for three hierarchies of supersymmetric particle masses, namely, when the gluino mass is comparable to, much greater than, or much less than the masses of the first two generation squarks. We find that the QCD corrections tighten the limits on squark mass splittings by more than a factor of two.

hep-ph↗

arXiv1997

Precision Corrections in the Minimal Supersymmetric Standard Model

In this paper we compute one-loop corrections to masses and couplings in the minimal supersymmetric standard model. We present explicit formulae for the complete corrections and a set of compact approximations which hold over the unified parameter space associated with radiative electroweak symmetry breaking. We illustrate the importance of the corrections and the accuracy of our approximations by scanning over the parameter space. We calculate the supersymmetric one-loop corrections to the W-boson mass, the effective weak mixing angle, and the quark and lepton masses, and discuss implications for gauge and Yukawa coupling unification. We also compute the one-loop corrections to the entire superpartner and Higgs-boson mass spectrum. We find significant corrections over much of the parameter space, and illustrate that our approximations are good to O(1%) for many of the superparticle masses.

hep-ph↗

arXiv1997

Gauge and Yukawa Unification in Models with Gauge-Mediated Supersymmetry Breaking

We examine gauge and Yukawa coupling unification in models with gauge-mediated supersymmetry breaking. We work consistently to two-loop order, and include all weak, messenger and unification-scale threshold corrections. We find that successful unification requires unification-scale threshold corrections that are in conflict with the minimal SU(5) model, but are consistent with the modified missing doublet SU(5) model for small tan beta, and large tan beta with mu>0.

hep-ph↗

arXiv1996

Weak-scale phenomenology of models with gauge-mediated supersymmetry breaking

We study in some detail the spectral phenomenology of models in which supersymmetry is dynamically broken and transmitted to the supersymmetric partners of the quarks, leptons and gauge bosons, and the Higgs bosons themselves, via the usual gauge interactions. We elucidate the parameter space of what we consider to be the minimal model, and explore the regions which give rise to consistent radiative electroweak symmetry breaking. We include the weak-scale threshold corrections, and show how they considerably reduce the scale dependence of the results. We examine the sensitivity of our results to unknown higher-order messenger-sector corrections. We compute the superpartner spectrum across the entire parameter space, and compare it to that of the minimal supergravity-inspired model. We delineate the regions where the lightest neutralino or tau slepton is the next-to-lightest supersymmetric particle, and compute the lifetime and branching ratios of the NLSP. In contrast to the minimal supergravity-inspired model, we find that the lightest neutralino can have a large Higgsino component, of order 50%. Nevertheless, the neutralino branching fraction to the gravitino and the light Higgs boson remains small, < 10^{-4}, so the observation of such a decay would point to a non-minimal Higgs sector.

hep-ph↗