arXiv subjects

Borut Bajc

Publications and source records attributed to Borut Bajc.

At least 37 records · Page 2Linked to original sources

arXiv2015

Resurrecting the minimal renormalizable supersymmetric SU(5) model

It is a well-known fact that the minimal renormalizable supersymmetric SU(5) model is ruled out assuming superpartner masses of the order of a few TeV. Giving up this constraint and assuming only SU(5) boundary conditions for the soft terms, we find that the model is still alive. The viable region of the parameter space typically features superpartner masses of order $10^2$ to $10^4$ TeV, with tan beta values between 2 and 5, but lighter spectra with single states around 10 TeV are also possible. The main constraints come from proton decay, the Higgs mass, the requirement of the SU(5) spectrum being reasonably below the Planck scale, and the lifetime of the universe. A generic feature of the model is metastability of the electroweak vacuum. In the absence of a suitable dark matter particle in the neutralino sector, a light (order GeV or smaller) gravitino is a natural candidate.

hep-ph↗

arXiv2015

R-parity violation in SU(5)

We show that judiciously chosen R-parity violating terms in the minimal renormalizable supersymmetric SU(5) are able to correct all the phenomenologically wrong mass relations between down quarks and charged leptons. The model can accommodate neutrino masses as well. One of the most striking consequences is a large mixing between the electron and the Higgsino. We show that this can still be in accord with data in some regions of the parameter space and possibly falsified in future experiments.

hep-ph↗

arXiv2015

A Minimal Supersymmetric $\mathrm{E}_6$ Unified Theory

We show explicitly that supersymmetric $E_6$ Grand Unified Theory with a Higgs sector consisting of $\{27+\bar{27}+351'+\bar{351'}+78\}$ fields provides a realistic scenario for symmetry breaking and fermion mass generation. While gauge symmetry breaking can be achieved without the $78$ field, its presence is critical for a successful doublet-triplet mass splitting. The Yukawa sector of the model consists of only two symmetric matrices describing all of quark, lepton and neutrino masses and mixings. The fermion mass matrices are computed at low energy and a fit to the second and third generation masses and mixings is performed. We find a good numerical fit to the low-energy data. Thus, this model, having $11$ superpotential parameters, alongside the two symmetric Yukawa matrices, seems to be the best realistic candidate for a minimal renormalizable supersymmetric $E_6$ unified theory.

hep-ph↗

arXiv2014

Towards the minimal renormalizable supersymmetric $E_6$ model

We find an explicit renormalizable supersymmetric $E_6$ model with all the ingredients for being realistic. It consists of the Higgs sector $351'+\overline{351'}+27+\overline{27}$, which breaks $E_6$ directly to the Standard Model gauge group. Three copies of $27$ dimensional representations then describe the matter sector, while an extra $27+\overline{27}$ pair is needed to successfully split the Standard Model Higgs doublet from the heavy Higgs triplet. We perform the analysis of the vacuum structure and the Yukawa sector of this model, as well as compute contributions to proton decay. Also, we show why some other simpler $E_6$ models fail to be realistic at the renormalizable level.

hep-ph↗

arXiv2013

A realistic renormalizable supersymmetric E6 model

A complete realistic model based on the supersymmetric version of $E_6$ is presented. It consists of three copies of matter 27, and a Higgs sector made of $2\times(27+\bar{27})+351'+\bar{351'}$ representations. An analytic solution to the equations of motion is found which spontaneously breaks the gauge group into the Standard Model. The light fermion mass matrices are written down explicitly as non-linear functions of three Yukawa matrices. This contribution is based on Ref. [1].

hep-ph↗

arXiv2013

Challenging the minimal supersymmetric SU(5) model

We review the main constraints on the parameter space of the minimal renormalizable supersymmetric SU(5) grand unified theory. They consist of the Higgs mass, proton decay, electroweak symmetry breaking and fermion masses. Superpartner masses are constrained both from below and from above, giving hope for confirming or definitely ruling out the theory in the future. This contribution is based on Ref. [1].

hep-ph↗

arXiv2013

Scalar potentials, propagators and global symmetries in AdS/CFT

We study the transition of a scalar field in a fixed $AdS_{d+1}$ background between an extremum and a minimum of a potential. We first prove that two conditions must be met for the solution to exist. First, the potential involved cannot be generic, i.e. a fine-tuning of their parameters is mandatory. Second, at least in some region its second derivative must have a negative upper limit which depends only on the dimensionality $d$. We then calculate the boundary propagator for small momenta in two different ways: first in a WKB approximation, and second with the usual matching method, generalizing the known calculation to arbitrary order. Finally, we study a system with spontaneously broken non-Abelian global symmetry, and show in the holographic language why the Goldstone modes appear.

hep-th↗

arXiv2013

On the matching method and the Goldstone theorem in holography

We study the transition of a scalar field in a fixed $AdS_{d+1}$ background between an extremum and a minimum of a potential. We compute analytically the solution to the perturbation equation for the vev deformation case by generalizing the usual matching method to higher orders and find the propagator of the boundary theory operator defined through the AdS-CFT correspondence. We show that, contrary to what happens at the leading order of the matching method, the next-to-leading order presents a simple pole at $q^2=0$ in accordance with the Goldstone theorem applied to a spontaneously broken dilatation invariance.

hep-th↗

arXiv2012

Supersymmetry breaking induced by radiative corrections

We show that simultaneous gauge and supersymmetry breaking can be induced by radiative corrections, a la Coleman-Weinberg. When a certain correlation among the superpotential parameters is present, a local supersymmetry-breaking minimum is found in the effective potential of a gauge non-singlet field, in a region where the tree-level potential is almost flat. Supersymmetry breaking is then transmitted to the MSSM through gauge and chiral messenger loops, thus avoiding the suppression of gaugino masses characteristic of direct gauge mediation models. The use of a single field ensures that no dangerous tachyonic scalar masses are generated at the one-loop level. We illustrate this mechanism with an explicit example based on an SU(5) model with a single adjoint. An interesting feature of the scenario is that the GUT scale is increased with respect to standard unification, thus allowing for a larger colour Higgs triplet mass, as preferred by the experimental lower bound on the proton lifetime.

hep-ph↗

arXiv2012

Spontaneous breaking of a discrete symmetry and holography

We present an exactly solvable model of a scalar field in an AdS$_{d+1}$ like background interpolating between a $Z_2$ preserving and a $Z_2$ breaking minima of the potential. We define its holographic dual through the AdS/CFT dictionary and argue that at zero temperature the $d-$dimensional strongly coupled system on the boundary of AdS$_{d+1}$ exhibits a phase with a spontaneously broken discrete symmetry. In the presence of a black hole in the bulk ($T\neq 0$) we find that, although the metastable phase is present, the discrete symmetry gets restored. We compute exactly the lowest order boundary correlation functions in the spontaneously broken phase at T=0, finding out a pole of the propagator for zero momenta that signals the presence of a massless mode.

hep-th↗

arXiv2011

Problems with False Vacua in Supersymmetric Theories

It has been suggested recently that in a consistent theory any Minkowski vacuum must be exactly stable. As a result, a large class of theories that in ordinary treatment would appear sufficiently long-lived, in reality make no sense. In particular, this applies to supersymmetric models in which global supersymmetry is broken in a false vacuum. We show that in any such theory the dynamics of supersymmetry breaking cannot be decoupled from the Planck scale physics. This finding poses an obvious challenge for the idea of low-scale metastable (for example gauge) mediation.

hep-th↗

arXiv2011

Classicalization via Path Integral

Recently, it was suggested that a large class of non-renormalizable theories may need no UV completion. By analogy with gravity where classical black holes are expected to be created in high-energy scatterings, it is conjectured that similar classical solutions, so-called classicalons, should occur. In this way the theory protects itself against non-unitarity, for instead of probing small distances at high energies one enters a classical regime. An effective theory of Goldstone bosons provides and example in which the size of classicalons grows with energy, and the high energy scattering is cut-off by small momenta, inversely proportional to the classicalon size. In this note we offer an alternative, path integral discussion of this important result.

hep-ph↗

arXiv2010

MSSM in view of PAMELA and Fermi-LAT

We take the MSSM as a complete theory of low energy phenomena, including neutrino masses and mixings. This immediately implies that the gravitino is the only possible dark matter candidate. We study the implications of the astrophysical experiments such as PAMELA and Fermi-LAT, on this scenario. The theory can account for both the realistic neutrino masses and mixings, and the PAMELA data as long as the slepton masses lie in the $500-10^6 $TeV range. The squarks can be either light or heavy, depending on their contribution to radiative neutrino masses. On the other hand, the Fermi-LAT data imply heavy superpartners, all out of LHC reach, simply on the grounds of the energy scale involved, for the gravitino must weigh more than 2 TeV. The perturbativity of the theory also implies an upper bound on its mass, approximately $6-7 $TeV.

hep-ph↗

arXiv2009

Probing leptonic CP phases in LFV processes

We study a CP and T violating triple (spin) correlation in the muon to electron conversion in nuclei in the context of the seesaw mechanism. After concluding that the results are negative for all three seesaw types, we turn to the left-right symmetric theories as the original source of seesaw. We find that in general this correlation is of order one which offers a hope of observing CP violation in lepton flavor violating processes for a L-R scale below around 10-30 TeV. We discuss the conditions that could render to (unlikely) conspiracies as to suppress the CP violating effects.

hep-ph↗

arXiv2009

Collider Signatures for Heavy Lepton Triplet in Type I+III Seesaw

The minimal SU(5) theory augmented by the fermionic adjoint representation restores the coupling constant unification and gives realistic neutrino masses and mixing through the hybrid Type I and Type III seesaw. The crucial prediction of the theory is an SU(2) lepton triplet with the mass below TeV. We study the signature of these heavy leptons and propose the strategy to test this mechanism at the hadron and lepton colliders. The smoking gun evidence of the theory is Delta L=2 lepton number violation through events of a pair of like-sign leptons plus four jets without significant missing energy at hadron colliders. We find that via this unique channel, the heavy lepton can be searched for up to a mass of 200 GeV at the Tevatron with 8 fb^-1, and up to 450 (700) GeV at the LHC of 14 TeV C.M.energy with 10 (100) fb^-1. The signal rate at the 10 TeV LHC is reduced to 60-35% for a mass of 200-700 GeV. We also comment on how to distinguish this theory from other models with similar heavy leptons. Finally, we compare the production rates and angular distributions of heavy leptons in e+e- collisions for various models.

hep-ph↗

arXiv2008

Metastable gauged O'Raifeartaigh

We study the possibility of obtaining metastable supersymmetry breaking vacua in a perturbative gauge theory without singlet fields, thus allowing for scenarios where a grand unified symmetry and supersymmetry are broken by the same sector. We show some explicit SU(5) examples. The minimal renormalizable example requires the use of two adjoints, but it is shown to inevitably lead to unwanted light states. We suggest various alternatives, and show that the viable possibilities consist of allowing for non-renormalizable operators, of employing four adjoints or of adding at least one field in a different representation.

hep-ph↗

arXiv2008

Minimal SO(10) splits supersymmetry

A good fit of the fermion masses and mixings has been found in the minimal renormalizable supersymmetric SO(10). This solution needs a strongly split supersymmetry breaking scenario with gauginos and higgsinos around 100 TeV, sfermions close to 10^14 GeV and a low GUT scale of around 6 10^15 GeV. We predict fast proton decays through SO(10) type of d=6 operators and the leptonic mixing angle theta_13 approximately 0.1.

hep-ph↗

arXiv2007

Probing seesaw at LHC

We have recently proposed a simple SU(5) theory with an adjoint fermionic multiplet on top of the usual minimal spectrum. This leads to the hybrid scenario of both type I and type III seesaw and it predicts the existence of the fermionic SU(2) triplet between 100 GeV and 1 TeV for a conventional GUT scale of about 10^{16} GeV, with main decays into W (Z) and leptons, correlated through Dirac Yukawa couplings, and lifetimes shorter than about 10^{-12} sec. These decays are lepton number violating and they offer an exciting signature of Delta L=2 dilepton events together with 4 jets at future pp (p\bar p) colliders. Increasing the triplet mass endangers the proton stability and so the seesaw mechanism could be directly testable at LHC.

hep-ph↗