arXiv subjects

Robert Ziegler

Publications and source records attributed to Robert Ziegler.

At least 55 records · Page 3Linked to original sources

arXiv2016

SUSY Meets Her Twin

We investigate the general structure of mirror symmetry breaking in the Twin Higgs scenario. We show, using the IR effective theory, that a significant gain in fine tuning can be achieved if the symmetry is broken hardly. We emphasize that weakly coupled UV completions can naturally accommodate this scenario. We analyze SUSY UV completions and present a simple Twin SUSY model with a tuning of around 10% and colored superpartners as heavy as 2 TeV. The collider signatures of general Twin SUSY models are discussed with a focus on the extended Higgs sectors.

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arXiv2016

Prompt Signals and Displaced Vertices in Sparticle Searches for Next-to-Minimal Gauge Mediated Supersymmetric Models

We study the LHC phenomenology of the next-to-minimal model of gauge-mediated supersymmetry breaking (NMGMSB), both for Run I and Run II. The Higgs phenomenology of the model is consistent with observations: a 125 GeV Standard Model-like Higgs which mixes with singlet-like state of mass around 90 GeV that provides a 2$σ$ excess at LEP II. The model possesses regions of parameter space where a longer-lived lightest neutralino decays in the detector into a gravitino and a $b-$jet pair or a tau pair. We investigate current lower bounds on sparticle masses and the discovery potential of the model, both via conventional sparticle searches and via searches for displaced vertices. The strongest bound from searches for promptly decaying sparticles yields a lower limit on the gluino mass of 1080 GeV. An analysis of 100 fb$^{-1}$ from Run II, on the other hand, is expected to be sensitive up to 1900 GeV. The displaced vertex searches from Run I suffer from a very low signal efficiency, mainly due to the presence of $b-$quarks in the final state. We show how the displaced vertex cuts might be relaxed in order to improve signal efficiency, while simultaneous prompt objects can be used to cut down background. We find that a combined search strategy with both prompt and displaced cuts potentially has a far better sensitivity to this model than either set alone, motivating a fully fledged experimental study.

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arXiv2016

Diphoton Signals from Colorless Hidden Quarkonia

We show that quarkonia-like states of a hidden SU(N) gauge group can account for the 750 GeV diphoton excess observed by ATLAS and CMS, even with constituents carrying standard model hypercharge only. The required hypercharge is modest, varying between about 1.3-1.6 for strong SU(N) coupling, to 2-3 for weak SU(N) coupling, for N=3, 4. This scenario predicts a variety of diphoton and multi-photon resonances, as well as photons from continuum pair production, and possibly exotic decays into standard model fermions, with no multi-jet resonances.

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arXiv2015

Lepton Flavor Non-Universality in B-meson Decays from a U(2) Flavor Model

We address the recent anomalies in semi-leptonic $B$-meson decays using a model of fermion masses based on the $U(2)$ flavor symmetry. The new contributions to $b \to s \ell \ell$ transitions arise due to a tree-level exchange of a $Z^\prime$ vector boson gauging a $U(1)$ subgroup of the flavor symmetry. They are controlled by a single parameter and are approximately aligned to the Standard Model prediction, with constructive interference in the $e$-channel and destructive interference in the $μ$-channel. The current experimental data on semi-leptonic $B$-meson decays can be very well reproduced without violating existing constraints from flavor violation in the quark and lepton sectors. Our model will be tested by new measurements of $b \to s \ell \ell$ transitions and also by future electroweak precision tests, direct $Z^\prime$ searches, and $μ$-$e$ conversion in nuclei.

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arXiv2015

Gauge Mediation in the NMSSM with a Light Singlet: Sparticles within the Reach of LHC Run II

Relatively light stops in gauge mediation models are usually made compatible with the Higgs mass of 125 GeV by introducing direct Higgs-messenger couplings. We show that such couplings are not necessary in a simple and predictive model that combines minimal gauge mediation and the next-to-minimal supersymmetric standard model (NMSSM). We show that one can obtain a 125 GeV Standard Model-like Higgs boson with stops as light as 1.1 TeV, thanks to the mixing of the Higgs with a singlet state at ${\cal O}(90-100)$ GeV that can explain the LEP excess. In this scenario the singlet-higgs-higgs superfields coupling $λ$ is small and $\tanβ$ large. Sparticle searches at the LHC may come with additional $b-$jets or taus and may involve displaced vertices. The sparticle production cross-section at the 13 TeV LHC can be ${\mathcal O}(10-100)$ fb, leading to great prospects for discovery in the early phase of LHC Run II.

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arXiv2015

Universal Constraints on Low-Energy Flavour Models

It is pointed out that in a general class of flavour models one can identify certain universally present FCNC operators, induced by the exchange of heavy flavour messengers. Their coefficients depend on the rotation angles that connect flavour and fermion mass basis. The lower bounds on the messenger scale are derived using updated experimental constraints on the FCNC operators. The obtained bounds are different for different operators and in addition they depend on the chosen set of rotations. Given the sensitivity expected in the forthcoming experiments, the present analysis suggests interesting room for discovering new physics. As the highlights emerge the leptonic processes, $μ\rightarrow eγ$, $μ\rightarrow eee$ and $μ\rightarrow e$ conversion in nuclei.

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arXiv2014

Lepton Flavor Violation in Flavored Gauge Mediation

We study the anatomy and phenomenology of Lepton Flavor Violation (LFV) in the context of Flavored Gauge Mediation (FGM). Within FGM, the messenger sector couples directly to the MSSM matter fields with couplings controlled by the same dynamics that explains the hierarchies in the SM Yukawas. Although the pattern of flavor violation depends on the particular underlying flavor model, FGM provides a built-in flavor suppression similar to wave function renormalization or SUSY Partial Compositeness. Moreover, in contrast to these models, there is an additional suppression of left-right (LR) flavor transitions by third-generation Yukawas that in particular provides an extra protection against flavor-blind phases. We exploit the consequences of this setup for lepton flavor phenomenology, assuming that the new couplings are controlled by simple U(1) flavor models that have been proposed to accommodate large neutrino mixing angles. Remarkably, it turns out that in the context of FGM these models can pass the impressive constraints from LFV processes and leptonic EDMs even for light superpartners, therefore offering the possibility of resolving the longstanding muon g-2 anomaly.

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arXiv2014

A Realistic Pattern of Lepton Mixing and Masses from S4 and CP

We present a supersymmetric model with the flavour symmetry S4 x Z3 and a CP symmetry which are broken to a Z3 subgroup of the flavour symmetry in the charged lepton sector and to Z2 x CP (x Z3) in the neutrino one at leading order. This model implements an approach, capable of predicting lepton mixing angles and Dirac as well as Majorana phases in terms of one free parameter. This parameter, directly related to the size of the reactor mixing angle theta_{13}, can be naturally of the correct order in our model. Atmospheric mixing is maximal, while sin^2 theta_{12} is larger than 1/3. All three phases are predicted: the Dirac phase is maximal, whereas the two Majorana phases are trivial. The neutrino mass matrix contains only three real parameters at leading order and neutrino masses effectively only depend on two of them. As a consequence, they have to be normally ordered and the absolute neutrino mass scale and the sum of the neutrino masses are predicted. The vacuum of the flavons can be correctly aligned. We study subleading corrections to the leading order results and show that they are small.

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arXiv2014

Linking Natural Supersymmetry to Flavour Physics

With the aim of linking natural supersymmetry to flavour physics, a model is proposed based on a family symmetry G \times U(1), where G is a discrete nonabelian subgroup of SU(2), with both F-term and (abelian) D-term supersymmetry breaking. A good fit to the fermion masses and mixing is obtained with the same U(1) charges for the left- and right- handed quarks of the first two families and the right-handed bottom quark, and with zero charge for the left-handed top-bottom doublet and the the right handed top. The model shows an interesting indirect correlation between the correct prediction for the V_{ub}/V_{cb} ratio and large right-handed rotations in the (s,b) sector, required to diagonalise the Yukawa matrix. For the squarks, one obtains almost degenerate first two generations. The main source of the FCNC and CP violation effects is the splitting between the first two families and the right-handed sbottom determined by the relative size of F-term and D-term supersymmetry breaking. The presence of the large right-handed rotation implies that the bounds on the masses of the first two families of squarks and the right handed sbottom are in a few to a few tens TeV range. The picture that emerges is light stops and left handed sbottom and much heavier other squarks.

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arXiv2013

Lepton Mixing Parameters from Discrete and CP Symmetries

We consider a scenario with three Majorana neutrinos in which a discrete, finite flavour group G_f is combined with a generalized CP transformation. We derive conditions for consistently defining such a setup. We show that in general lepton mixing angles and CP phases (Dirac as well as Majorana) only depend on one single parameter theta which can take values between 0 and pi, if the residual symmetries are G_e contained in G_f in the charged lepton and G_nu=Z_2 x CP in the neutrino sector. We perform a comprehensive study for G_f=S_4 and find five cases which are phenomenologically interesting. They naturally lead to a non-zero reactor mixing angle and all mixing parameters are strongly correlated. Some of the patterns predict maximal atmospheric mixing and maximal Dirac phase, while others predict trivial Dirac and Majorana phases.

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arXiv2013

Gauge Mediation beyond Minimal Flavor Violation

We study a minimal modification of Gauge Mediation in which the messenger sector couples directly to the MSSM matter fields. These couplings are controlled by the same dynamics that explain the flavor hierarchies, and therefore are parametrically as small as the Yukawas. This setup gives rise to an interesting SUSY spectrum that is calculable in terms of a single new parameter. Due to large A-terms, the model can easily accommodate a 126 GeV Higgs with a relatively light SUSY spectrum. The flavor structure depends on the particular underlying flavor model, but flavor-violating effects arise dominantly in the up-sector and are strongly suppressed in Delta F =2 observables. This strong suppression is reminiscent of what happens in the case of wave function renormalization or Partial Compositeness, despite the underlying flavor model can be a simple U(1) flavor model (which in the context of Gravity Mediation suffers from strong Delta F =2 constraints). This structure allows to account for the recent observation of direct CP violation in D-meson decays.

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arXiv2013

Particle-Antiparticle Mixing, CP Violation and Rare K and B Decays in a Minimal Theory of Fermion Masses

We present a detailed study of Delta F=2 observables and of rare K^+(K_L) and B_{s,d} meson decays in a "Minimal Theory of Fermion Masses" (MTFM). In this theory Yukawa couplings are generated through the mixing with heavy vectorlike (VF) fermions. This implies corrections to the SM quark couplings to W, Z and Higgs so that FCNC processes receive contributions from tree level Z and Higgs exchanges and W bosons couple to right-handed quarks. In a particular version of this model in which the Yukawa matrix lambda^D in the heavy down fermion sector is unitary, lambda^U =1 and M = M_{VF} is fixed, only three real and positive definite parameters describe New Physics (NP) contributions to all Delta F=2 and Delta F=1 observables in K and B_{s,d} systems once the known quark masses and the CKM matrix are correctly reproduced. For M>1 TeV NP contributions to B_{s,d}^0- bar{B}_{s,d}^0 mixings are found to be very small. While in principle NP contributions to epsilon_K and Delta F=1 processes could be large, the correlation between epsilon_K and K_L->mu^+ mu^- eliminates basically NP contributions to epsilon_K and right-handed current contributions to Delta F=1 FCNC observables. We find CMFV structure in B_{s,d} decays with BR(B_{s,d}->mu^+ mu^-) uniquely enhanced for M=3 TeV by at least 35% and almost up to a factor of two over their SM values. Also BR(K^+->pi^+ bar{nu} nu) and BR(K_L->pi^0 bar{nu} nu)$ are uniquely enhanced by similar amount but the correlation between them differs from the CMFV one. We emphasize various correlations between K and B_{s,d} decays that could test this scenario. The model favours gamma=68°, |V_{ub}|=0.0037, S_{psi K_S}=0.72, S_{psi phi}=0.04 and 4.2*10^-9 mu^+ mu^-)<5.0*10^-9 for M=3-4 TeV.

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arXiv2012

The Messenger Sector of SUSY Flavour Models and Radiative Breaking of Flavour Universality

The flavour messenger sectors and their impact on the soft SUSY breaking terms are investigated in SUSY flavour models. In the case when the flavour scale M is below the SUSY breaking mediation scale M_S, the universality of soft terms, even if assumed at M_S, is radiatively broken. We estimate this effect in a broad class of models. In the CKM basis that effect gives flavour off-diagonal soft masses comparable to the tree-level estimate based on the flavour symmetry.

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arXiv2011

Extended Tree-Level Gauge Mediation

Tree-level gauge mediation (TGM) is a scenario of SUSY breaking in which the tree-level exchange of heavy (possibly GUT) vector fields generates flavor-universal sfermion masses. In this work we extend this framework to the case of E_6 that is the natural extension of the minimal case studied so far. Despite the number of possible E_6 subgroups containing G_SM is large (we list all rank 6 subgroups), there are only three different cases corresponding to the number of vector messengers. As a robust prediction we find that sfermion masses are SU(5) invariant at the GUT scale, even if the gauge group does not contain SU(5). If SUSY breaking is mediated purely by the U(1) generator that commutes with SO(10) we obtain universal sfermion masses and thus can derive the CMSSM boundary conditions in a novel scenario.

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arXiv2011

FCNC Effects in a Minimal Theory of Fermion Masses

As a minimal theory of fermion masses we extend the SM by heavy vectorlike fermions, with flavor-anarchical Yukawa couplings, that mix with chiral fermions such that small SM Yukawa couplings arise from small mixing angles. This model can be regarded as an effective description of the fermionic sector of a large class of existing flavor models and thus might serve as a useful reference frame for a further understanding of flavor hierarchies in the SM. Already such a minimal framework gives rise to FCNC effects through exchange of massive SM bosons whose couplings to the light fermions get modified by the mixing. We derive general formulae for these corrections and discuss the bounds on the heavy fermion masses. Particularly stringent bounds, in a few TeV range, come from the corrections to the Z couplings.

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arXiv2010

mu-tau Symmetry and Charged Lepton Mass Hierarchy in a Supersymmetric D4 Model

In this note we discuss a supersymmetric (SUSY) D4 x Z5 model which leads to vanishing reactor mixing angle theta_13=0 and maximal atmospheric mixing theta_23=pi/4 in the lepton sector at leading order (LO), due to the preservation of non-trivial distinct D4 subgroups in the charged lepton and neutrino sectors, respectively. The solar mixing angle theta_12 remains undetermined and is expected to be of order one. Since right-handed charged leptons transform as singlets under D4, the charged lepton mass hierarchy can be naturally accounted for. The model predicts inverted mass hierarchy for neutrinos. Additionally, we show that, unlike in most of the other models of this type, all vacuum expectation values (VEVs) of gauge singlets (flavons) can be fixed through mass parameters of the superpotential. Next-to-leading order (NLO) corrections to lepton masses and mixings are calculated and shown to be under control, especially the corrections to theta_23=pi/4 and theta_13=0 are of the order of the generic expansion parameter epsilon = 0.04 and arise dominantly from the charged lepton sector.

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arXiv2009

General Aspects of Tree Level Gauge Mediation

Tree level gauge mediation (TGM) may be considered as the simplest way to communicate supersymmetry breaking: through the tree level renormalizable exchange of heavy gauge messengers. We study its general structure, in particular the general form of tree level sfermion masses and of one loop, but enhanced, gaugino masses. This allows us to set up general guidelines for model building and to identify the hypotheses underlying the phenomenological predictions. In the context of models based on the "minimal" gauge group SO(10), we show that only two "pure" embeddings of the MSSM fields are possible using $d< 120$ representations, each of them leading to specific predictions for the ratios of family universal sfermion masses at the GUT scale, $m^2_{\bar{5}} = 2 m^2_{10}$ or $m^2_{\bar{5}} = (3/4) m^2_{10}$ (in SU(5) notation). These ratios are determined by group factors and are peculiar enough to make this scheme testable at the LHC. We also discuss three possible approaches to the $μ$-problem, one of them distinctive of TGM.

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arXiv2009

Tree Level Gauge Mediation

We propose a new scheme in which supersymmetry breaking is communicated to the MSSM sfermions by GUT gauge interactions at the tree level. The (positive) contribution of MSSM fields to $\text{Str}(\mathcal{M}^2)$ is automatically compensated by a (negative) contribution from heavy fields. Sfermion masses are flavour universal, thus solving the supersymmetric flavour problem. In the simplest SO(10) embedding, the ratio of different sfermion masses is predicted and differs from mSugra and other schemes, thus making this framework testable at the LHC. Gaugino masses are generated at the loop level but enhanced by model dependent factors.

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