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S. Pokorski

Publications and source records attributed to S. Pokorski.

At least 55 records · Page 3Linked to original sources
arXiv1996

ALEPH Four-Jet Excess, $R_b$ and $R-$Parity Violation

We review briefly the indications for some relatively light superpartners based on the ~$R_b$ ~anomaly and discuss the dependence of the potential increase in $R_b$ on the assumption about ~$R-$parity (non)conservation. We point out that the exotic 4-jet events reported by ALEPH may constitute a signal for supersymmetry with such a light spectrum and with explicitly broken $R-$parity. A parton level simulation shows that production of a pair of light charginos with their subsequent baryon-number violating decays (either through a stop or through a neutralino) could possibly give rise to this excess. The decay $χ^- \ra \tilde{t}_R^\ast b \ra d s b$ with $m_{χ^-_1} \sim 60 \gev$ and $m_{\tilde t} \sim 52 \gev$ leads to signatures very close to the experimental observations.

hep-ph↗
arXiv1996

Abelian Flavour Symmetries in Supersymmetric Models

We propose a theory of flavour based on abelian horizontal gauge symmetries and modular invariances. We construct explicit supergravity models where the scale of the horizontal $U(1)$ symmetry breaking is fixed by the Green-Schwarz mechanism for anomaly cancellation. The supersymmetric spectrum is obtained in terms of the $U(1)$ charges which are determined by the Yukawa matrices.

hep-ph↗
arXiv1996

Chargino Mass and $R_b$ Anomaly

We re-examine the possible magnitude of the supersymmetric contribution to $R_b$ with imposed all available phenomenological constraints and demanding good quality of the global fit to the precision electroweak data. For low $\tanβ$ we find a new region of the parameter space, with $M_2\approx |μ|$ and $μ<0$ where $R_b$ remains large, $\sim0.2180$ even for the lighter chargino as heavy as $90-100$ GeV. It is an interesting mixture of the up-higgsino and gaugino. The rôle of various phenomenological constraints is discussed in analitic form and importance of small but non-negligible left-right mixing in the stop sector is emphasized in this context. The large $\tanβ$ option for enhancement of $R_b$ is also reviewed. The available data do not rule out this scenario.

hep-ph↗
arXiv1996

Light Neutralinos as Dark Matter in the Unconstrained Minimal Supersymmetric Standard Model

The allowed parameter space for the lightest neutralino as the dark matter is explored using the Minimal Supersymmetric Standard Model as the low-energy effective theory without further theoretical constraints such as GUT. Selecting values of the parameters which are in agreement with present experimental limits and applying the additional requirement that the lightest neutralino be in a cosmologically interesting range, we give limits on the neutralino mass and composition. A similar analysis is also performed implementing the grand unification constraints. The elastic scattering cross section of the selected neutralinos on $^{27}$Al and on other materials for dark matter experiments is discussed.

hep-ph↗
arXiv1996

Searches For New Physics

Review of prospects for discovery of new physics signals at LEP2. The areas covered include SUSY, exotic fermions, BESS models, leptoquarks, virtual effects and CP violating observables.

hep-ph↗
arXiv1995

Precision Tests of the MSSM

The LEP data and the measurement of $M_W$ confirm the electro weak symmetry breaking by the Higgs mechanism with (2--3)\% accuracy. For the Higgs boson mass, fits to the present data (without the SLD result) give $M_h=121^{+207}_{-118}$ GeV and $M_h<800$ GeV at 95\% C.L. We analyze those fits from the point of view of the impact of different measurements on the final result for $M_h$. All the relevant observables can be grouped into two categories: those which correlate $M_h$ with $m_t$ and those which constrain the range of $m_t$. Using this classification we discuss potential improvements in the limits on $M_h$, anticipating improvements in the precision of the data. Also the sensitivity of the limits on $M_h$ to the value of $α_s(M_Z)$ is discussed in a toy model which reproduces all the results of the SM but the $Z^0\rightarrow\bar bb$ width is increased by hand so that $R_b=0.22$ and, in consequence, the fitted value of $α_s(M_Z)\approx0.11$ (qualitatively similar situation occurs in the Minimal Supersymmetric Standard Model with light chargino and right--handed top squark).

hep-ph↗
arXiv1995

Gauge Coupling Unification in GUT and String Models

The results for the running of the gauge couplings in the MSSM are up-dated by proper inclusion of all low scale effects. They are presented as predictions for the strong coupling constant in the scenario with only two parameters at the GUT scale ~ ($α_{U}$ ~and ~$M_{U}$) ~and as a mismatch of the couplings at the scales ~$\sim 3 \times 10^{16}$ GeV ~and ~$4\times10^{17}$ GeV, when all three couplings are taken as the experimental input.

hep-ph↗
arXiv1995

Precision tests of the MSSM

We present the results of a first global fit to the electroweak observables in the MSSM. The best fit selects either very low or very large values of ~$\tanβ$ ~and, correspondingly, chargino (higgsino--like) and stop or the ~$CP-$odd Higgs boson are within the reach of LEP 2. Moreover, the best fit gives ~ $α_s(M_Z)=0.118^{+0.005}_{-0.010}$, ~which is lower than the one obtained from the SM fits. The overall fit is excellent ~ ($χ^2=7.2$ ~for 15 d.o.f. as compared to ~$χ^2=11$ ~in the SM). Those results follow from the fact that in the MSSM one can increase the value of ~ $R_b\equivΓ_{Z^0\to\bar bb}/Γ_{Z^0\to hadrons}$ ~ {\it without} modyfying the SM predictions for other observables.

hep-ph↗
arXiv1995

Yukawa matrices from a spontaneously broken abelian symmetry

We classify all the phenomenologically viable fermion mass matrices coming from a spontaneously broken abelian symmetry $ U(1)_X, $ with one and two additional chiral fields of opposite charges $ X=\pm 1. $ We find that the non-trivial Kähler metric can fill zeroes of the fermion mass matrices up to phenomenologically interesting values. A general anomaly analysis shows that for one additional chiral field the only way to achieve anomaly cancellation is by use of the Green-Schwarz mechanism. For two additional fields with $X=\pm 1$ and negative charge differences in the lepton sector the anomalies can however be directly put to zero. This case gives a unique prediction for the ratio of the two Higgs scalars of MSSM, ${\rm tg} β\sim { m_t \over m_b} (sin θ_c)^ 2$, where $θ_c$ is the Cabibbo angle.

hep-ph↗
arXiv1994

Constraints on the Minimal SUSY SO(10) Model from Cosmology and the $b\to s γ$ Decay

It is shown that the minimal supersymmetric SO(10) model with electroweak radiative breaking and universal soft mass terms at the GUT scale is strongly disfavoured by the combination of constraints from the $b\to s γ$ decay and the condition $Ωh^2 < 1$ for the lightest (stable) neutralino. The constraints are, however, easily satisfied for certain class of supersymmetric SO(10) models with non--universal scalar masses which gives small supersymmetric corrections to the bottom quark mass and light higgsino--like neutralinos.

hep-ph↗
arXiv1994

$\sin^2θ_W(M_Z)$ ~in the MSSM and Unification of Couplings

We calculate ~$\sin^2θ(M_Z)$ ~in the MSSM in terms of ~ $α_{EM}$, ~$G_F$, ~$m_t$, ~$\tanβ$ ~and SUSY mass parameters with the same accuracy as the present calculations of ~$\sin^2θ(M_Z)$ ~in the SM. We compare the results with the standard leading logarithmic approximation used for SUSY threshold corrections and find important differences in the case of light sparticles. We give approximate formulae connecting coupling constants in the SM and in the MSSM and comment on process dependence of such formulae. The obtained values of the MSSM couplings ~$α_i(M_Z)$ ~are used to investigate gauge coupling unification in the minimal SUSY ~SU(5) ~model. Our non-logarithmic corrections lower the predicted value of the Higgs triplet mass. The interplay between the supersymmetric and GUT thresholds in achieving unification for the coupling constants in the range of the experimentally acceptable values is quantified.

hep-ph↗
arXiv1994

Constraints on non-universal soft terms from flavor changing neutral currents

The smallness of flavor changing neutral currents constrains the soft parameter space of supersymmetric extensions of the Standard Model. These low energy constraints are translated to the soft parameter space generated at some high energy scale $\Mgut$. For gaugino masses larger than the scalar masses and non-universal $A$-terms the constraints are significantly diluted at $\Mgut$ and do allow for the possibility of non-universal scalar masses. The strongest constraints arise in the slepton sector of the theory.

hep-ph↗
arXiv1994

Electroweak Symmetry Breaking with Non-Universal Scalar Soft Terms and Large $\tanβ$ Solutions

We discuss radiative electroweak symmetry breaking with non-universal scalar masses at the GUT scale. Large $\tanβ$ solutions are investigated in detail and it is shown that qualitatively new (as compared to the universal case) solutions exist, with much less correlation between soft terms. We identify two classes of non-universalities which give solutions with $A_o\simeq B_o\simeq M_{1/2}\simeq O(M_Z)$, $μ\simeq O(0.5 m_o)$, $m_o>>M_Z$ and $A_o\simeq B_o\simeq M_{1/2} \simeqμ\simeq O(M_Z)$, $m_o\ge M_Z$, respectively. In each case, after imposing gauge and Yukawa coupling unification, we discuss the predictions for $m_t$, $m_b$ and the spectrum of supersymmetric particles. One striking consequence is the possibility of light charginos and neutralinos which in the second option can be higgsino-like. Cosmological constraint on the relic abundance of the lightest neutralino is also included.

hep-ph↗
arXiv1994

Electroweak Symmetry Breaking and Bottom-Top Yukawa Unification

The condition of unification of gauge couplings in the minimal supersymmetric standard model provides successful predictions for the weak mixing angle as a function of the strong gauge coupling and the supersymmetric threshold scale. In addition, in some scenarios, e.g.\ in the minimal SO(10) model, the tau lepton and the bottom and top quark Yukawa couplings unify at the grand unification scale. The condition of Yukawa unification leads naturally to large values of $\tanβ$, implying a proper top quark--bottom quark mass hierarchy. In this work, we investigate the feasibility of unification of the Yukawa couplings, in the framework of the minimal supersymmetric standard model with (assumed) universal mass parameters at the unification scale and with radiative breaking of the electroweak symmetry. We show that strong correlations between the parameters $μ_0$ and $M_{1/2}$ appear within this scheme. These correlations have relevant implications for the sparticle spectrum, which presents several characteristic features. In addition, we show that due to large corrections to the running bottom quark mass induced through the supersymmetry breaking sector of the theory, the predicted top quark mass and $\tanβ$ values are significantly lower than those previously estimated in the literature.

hep-ph↗
arXiv1993

Theoretical Uncertainties and Phenomenological Aspects of $B \rightarrow X_s γ$ Decay

We analyze uncertainties in the theoretical prediction for the inclusive branching ratio $BR[B \rightarrow X_s γ]$. We find that the dominant uncertainty in the leading order expression comes from its $μ$-dependence. We discuss a next-to-leading order calculation of $B \rightarrow X_s γ$ in general terms and check to what extent the $μ$-dependence can be reduced in such a calculation. We present constraints on the Standard and Two-Higgs-Doublet Model parameters coming from the measurement of $b \rightarrow s γ$ decay. The current theoretical uncertainties do not allow one to definitively restrict the Standard Model parameters much beyond the limits coming from other experiments. The bounds on the Two-Higgs-Doublet Model remain very strong, though significantly weaker than the ones present in the recent literature. In the Two-Higgs-Doublet Model case, the $b \rightarrow s γ$, $Z \rightarrow b \bar{b}$ and $b \rightarrow c τ\barν_τ$ processes are enough to give the most restrictive bounds in the $M_{H^{\pm}}-tan β$ plane.

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