Degenerate Squarks Research Articles

Confronting the coloured sector of the MRSSM with LHC data

R-symmetry leads to a distinct low energy realisation of SUSY with a signicantly modified colour-charged sector featuring a Dirac gluino and scalar colour octets (sgluons). In the present work we recast results from LHC BSM searches to discuss the impact of R-symmetry on the squark and gluino mass limits. We work in the framework of the Minimal R-symmetric Supersymmetric Standard Model and take into account the NLO corrections to the squark production cross sections in the MRSSM that have become available recently. We find substantially weaker limits on squark masses compared to the MSSM: for simple scenarios with heavy gluinos and degenerate squarks, the MRSSM mass limit is {m}_{tilde{g}} > 1:7TeV, approximately 600 GeV lower than in the MSSM.

Confronting the Coloured Sector of the MRSSM with LHC Data

R-symmetry leads to a distinct low energy realisation of SUSY with a signicantly modified colour-charged sector featuring a Dirac gluino and scalar colour octets (sgluons). In the present work we recast results from LHC BSM searches to discuss the impact of R-symmetry on the squark and gluino mass limits. We work in the framework of the Minimal R-symmetric Supersymmetric Standard Model and take into account the NLO corrections to the squark production cross sections in the MRSSM that have become available recently. We find substantially weaker limits on squark masses compared to the MSSM: for simple scenarios with heavy gluinos and degenerate squarks, the MRSSM mass limit is $$ {m}_{\tilde{g}} $$ > 1:7TeV, approximately 600 GeV lower than in the MSSM.

Rare top-quark decays to Higgs boson in MSSM

In full one-loop generality and in next-to-leading order in QCD, we study rare top to Higgs boson flavour changing decay processes $t\to q h$ with $q=u,c$ quarks, in the general MSSM with R-parity conservation. Our primary goal is to search for enhanced effects on $Br(t\to q h)$ that could be visible at current and high luminosity LHC running. To this end, we perform an analytical expansion of the amplitude in terms of flavour changing squark mass insertions that treats both cases of hierarchical and degenerate squark masses in a unified way. We identify two enhanced effects allowed by various constraints: one from holomorphic trilinear soft SUSY breaking terms and/or right handed up squark mass insertions and another from non-holomorphic trilinear soft SUSY breaking terms and light Higgs boson masses. Interestingly, even with $\mathcal{O}(1)$ flavour violating effects in the, presently unconstrained, up-squark sector, SUSY effects on $Br(t\to q h)$ come out to be unobservable at LHC mainly due to leading order cancellations between penguin and self energy diagrams and the constraints from charge- and colour-breaking minima (CCB) of the MSSM vacuum. An exception to this conclusion may be effects arising from non-holomorphic soft SUSY breaking terms in the region where the CP-odd Higgs mass is smaller than the top-quark mass but this scenario is disfavoured by recent LHC searches. Our calculations for $t\to q h$ decay are made available in SUSY_FLAVOR numerical library.

Exact NLO strong interaction corrections to the ΔF= 2 effective Hamiltonian in the MSSM

We compute the order αs3 (next-to-leading) corrections to the Wilson coefficients of the ΔF = 2 effective Hamiltonian in the Minimal Supersymmetric Standard Model with completely arbitrary soft terms. These results are relevant for phenomenological studies of neutral meson mixings in the presence of large squark mass splittings, in particular in hierarchical scenarios of supersymmetry. These corrections achieve a considerable reduction of the uncertainty related to the matching scale ambiguity, and are therefore numerically relevant. We briefly analyze the effect of certain mass splittings on the size of the NLO corrections, and compare it with the case of almost degenerate squarks in the mass insertion approximation.

SUSY contributions to ϕBs from hierarchical sfermions

Hierarchical soft terms describe a class of supersymmetric theories in which the first two generations of squarks and sleptons are heavier than the rest of the supersymmetric spectrum. They make well-defined, interesting predictions as there are fewer free parameters than in the ordinary case of degenerate squarks and provide a characteristic correlation between ΔF = 1 and ΔF = 2 transitions. We study the constraints on the flavor-violating parameters and point out that values of the phase of Bs mixing larger than in the case of degenerate soft terms can be obtained.

Hierarchical soft terms and flavor physics

We study the framework of hierarchical soft terms, in which the first two generations of squarks and sleptons are heavier than the rest of the supersymmetric spectrum. This scheme gives distinctive predictions for the pattern of flavor violations, which we compare to the case of nearly degenerate squarks. Experiments in flavor physics have started to probe the most interesting parameter region, especially in b ↔ s transitions, where hierarchical soft terms can predict a phase of B s mixing much larger than in the Standard Model.

Neutralino annihilation toqq¯g

We compute the cross section for {chi}{chi}{yields}qqg at order {alpha}{sub s}{sup 2}/M{sub q-}t{sub ilde}{sup 6} arising from interference between the tree-level and loop-induced processes. This interference term is the same order in {alpha}{sub s} as {chi}{chi}{yields}gg; for mass degenerate squarks M{sub q-tildeR}=M{sub q-}t{sub ildeL}=M{sub q-tilde} we find v{sub rel}{sigma}{sub int}=[-2m{sub {chi}}{sup 2}/3M{sub q-tilde}{sup 2}]v{sub rel}{sigma}({chi}{chi}= {yields}gg)

Top-squark searches at the Fermilab Tevatron in models of low-energy supersymmetry breaking

We study the production and decays of top squarks at the Fermilab Tevatron collider in models of low-energy supersymmetry breaking. We consider the case where the lightest standard model (SM) superpartner is a light neutralino that predominantly decays into a photon and a light gravitino. Considering the lighter top squark to be the next-to-lightest standard model superpartner, we analyze top squark signatures associated with jets, photons and missing energy, which lead to signals naturally larger than the associated SM backgrounds. We consider both 2-body and 3-body decays of the top squarks and show that the reach of the Tevatron can be significantly larger than that expected within either the standard supergravity models or models of low-energy supersymmetry breaking in which the top squark is the lightest SM superpartner. For a modest projection of the final Tevatron luminosity, L⋍4fb -1 , top squark masses of order 300 GeV are accessible at the Tevatron collider in both 2-body and 3-body decay modes. We also consider the production and decay of ten degenerate squarks that are the supersymmetric partners of the five light quarks. In this case we find that common squark masses up to 360 GeV are easily accessible at the Tevatron collider, and that the reach increases further if the gluino is light.

D-term spectroscopy in realistic heterotic-string models

The emergence of free fermionic string models with solely the MSSM charged spectrum below the string scale provides further evidence to the assertion that the true string vacuum is connected to the Z_2 x Z_2 orbifold in the vicinity of the free fermionic point in the Narain moduli space. An important property of the Z_2 x Z_2 orbifold is the cyclic permutation symmetry between the three twisted sectors. If preserved in the three generations models the cyclic permutation symmetry results in a family universal anomalous U(1)_A, which is instrumental in explaining squark degeneracy, provided that the dominant component of supersymmetry breaking arises from the U(1)_A D-term. Interestingly, the contribution of the family--universal D_A-term to the squark masses may be intra-family non-universal, and may differ from the usual (universal) boundary conditions assumed in the MSSM. We contemplate how D_A--term spectroscopy may be instrumental in studying superstring models irrespective of our ignorance of the details of supersymmetry breaking. We examine the possible effect of the intra--family non--universality on the resulting SUSY spectrum and the values of the strong coupling, effective weak mixing angle and W-gauge boson mass, up to a two loop accuracy, in the two models (universal and non-universal). We find that non universality relaxes the constraint of color and charge breaking minima which appears in the universal case. In addition, it predicts a 3% smaller value of \alpha_s due to different threshold masses obtained in the latter scenario. Finally, we present the experimentally allowed predictions of the two models in an M_0 and M_{1/2} parameter space.

Phenomenological constraints on supersymmetric models with an anomalous U(1) flavor symmetry

We investigate supersymmetric models in which an anomalous U(1) X symmetry explains the Yukawa hierarchy, and the related D X -term plays a role in supersymmetry breaking. We use a bottom-up approach to model building. Phenomenological viability leads to a scenario with degenerate squark and slepton spectra and with heavy gauginos. Features of a Kähler potential that allows for such a scenario are described.

AnomalousU(1), holomorphy, supersymmetry breaking, and dilaton stabilization

We show that in models with family symmetries, the implementation of the alignment mechanism for the suppression of the flavor changing neutral currents requires specific mass matrices, necessarily with holomorphic zeros in the down quark sector. Holomorphic zeros open flat directions that spoil the uniqueness of the supersymmetric vacuum, making alignment models less attractive. We are thus unavoidably lead to the recently found, unique, anomalous $U(1)$ model without holomorphic zeros in the quark sector that can reproduce all the mass hierarchies, provided that $\mathrm{tan}\ensuremath{\beta}$ is of order one. To avoid undesired flavor changing neutral currents, we propose a supersymmetry breaking mechanism and a dilaton stabilization scenario that result in degenerate squarks at $M\ensuremath{\sim}{M}_{\mathrm{GUT}}$ and a calculable low energy spectrum. We present the numerical predictions of this model for the Higgs boson mass for different values of M and $\mathrm{tan}\ensuremath{\beta}$.

A family-universal anomalous U(1) in string models as the origin of supersymmetry breaking and squark degeneracy

Recently a promising mechanism for supersymmetry breaking that utilizes both an anomalous U(1) gauge symmetry and an effective mass term m ∼ 1 Tev of certain relevant fields has been proposed. In this paper we examine whether such a mechanism can emerge in superstring-derived free fermionic models. We observe that certain three-generation string solutions, though not all, lead to an anomalous U(1) which couples universally to all three families. The advantages of this three-family universality of U(1) A , compared to the two-family case, proposed in earlier works, in yielding squark degeneracy, while avoiding radiative breaking of color and charge, are noted. The root cause of the flavor universality of U(1) A is the cyclic permutation symmetry that characterizes the Z 2 × Z 2 orbifold compactification with standard embedding, realized in the free fermionic models by the NAHE set. It is shown that non-renormalizable terms which contain hidden-sector condensates, generate the required suppression of the relevant mass term m, compared to the Planck scale. While the D-term of the family-universal U(1) A leads to squark degeneracy, those of the family-dependent U(1)' s, remarkably enough, are found to vanish for the solutions considered, owing to minimization of the potential. Motivations are provided for the combined U(1) A -Dilaton SUSY breaking.

Strong CP and low-energy supersymmetry

A spontaneously-broken CP provides an alternative to the KM mechanism for CP violation with the advantage that the strong CP problem is solved. We consider, for such a model with a new gauged U(1), the incorporation of low-energy supersymmetry and find the constraints on alignment and squark degeneracy. The conclusion is that although the $\bar{\theta}$ constraints are much less severe than in other generic schemes with supersymmetry breaking and spontaneous CP violation, one restriction remains stronger than needed in the MSSM for suppression of FCNC.

Supersymmetric theory of flavor and R parity.

We construct a renormalizable, supersymmetric theory of flavor and $R$ parity based on the discrete flavor group $(S_3)^3$. The model can account for all the masses and mixing angles of the Standard Model, while maintaining sufficient squark degeneracy to circumvent the supersymmetric flavor problem. By starting with a simpler set of flavor symmetry breaking fields than we have suggested previously, we construct an economical Froggatt-Nielsen sector that generates the desired elements of the fermion Yukawa matrices. With the particle content above the flavor scale completely specified, we show that all renormalizable $R$-parity-violating interactions involving the ordinary matter fields are forbidden by the flavor symmetry. Thus, $R$ parity arises as an accidental symmetry in our model. Planck-suppressed operators that violate $R$ parity, if present, can be rendered harmless by taking the flavor scale to be $\lesssim 8 \times 10^{10}$ GeV.

Mass matrix models: the sequel

The smallness of the quark sector parameters and the hierarchy between them could be the result of a horizontal symmetry broken by a small parameter. Such an explicitly broken symmetry can arise from an exact symmetry which is spontaneously broken. Constraints on the scales of new physics arise from new flavor-changing interactions and from Landau poles, but do not exclude the possibility of observable signatures at the TeV scale. Such a horizontal symmetry could also lead to many interesting results: (i) quark—squark alignment that would suppress, without squark degeneracy, flavor-changing neutral currents induced by supersymmetric particles, (ii) exact relations between mass ratios and mixing angles, (iii) a solution of the μ-problem and (iv) a natural mechanism for obtaining hierarchy among various symmetry-breaking scales.

Flavor symmetries and the problem of squark degeneracy.

If supersymmetry exists at low energies, it is necessary to understand why the squark spectrum exhibits sufficient degeneracy to suppress flavor-changing neutral currents. In this article, we point out that gauged horizontal symmetries can yield realistic quark mass matrices, while at the same time giving just barely enough squark degeneracy to account for neutral K-meson phenomenology. This approach suggests likely patterns for squark masses, and indicates that there could be significant supersymmetric contributions to B-B\ifmmode\bar\else\textasciimacron\fi{} and D-D\ifmmode\bar\else\textasciimacron\fi{} mixing and CP violation in the K and B systems.

(S)quark masses and non-Abelian horizontal symmetries

We present a model of quark and squark masses which is based on a non-Abelian horizontal symmetry. It leads to order of magnitude relations between quark mass ratios and mixing angles and to the successful exact relation sin θ C = m d m s to better than 20% accuracy. The non-Abelian symmetry also ensures the necessary squark degeneracy to suppress FCNC mediated by loops with squarks and gluinos, in the neutral meson systems.

New theoretical lower bounds on the Higgs sector of minimal SUSY

We show the existence of a new theoretical one-loop lower bound on the lightest Higgs boson h of the minimal supersymmetric standard model. This bound restricts considerably the parameter space of the model for tan β<1 and allows for the pseudoscalar Higgs field A to be actually lighter than h in some regions of parameter space. The lower bound on the heavy Higgs field H is correspondingly increased. Our numerical results are obtained for the case of degenerate squark masses but can be readily generalized. We conclude that the one-loop analysis favors the embedding of the model in the SUSY GUTs scenario with radiative electroweak symmetry breaking.

Searches for supersymmetric particles produced in Z-boson decay.

We have searched for supersymmetric particles in 528 Z decays with the Mark II detector at the SLAC Linear Collider. We place 95%-confidence-level lower mass limits on degenerate squarks, nondegenerate up-type squarks, nondegenerate down-type squarks, charginos, pair-produced unstable neutralinos, and neutralinos from associated production. © 1990 The American Physical Society.