arXiv subjects

Goran Senjanović

Publications and source records attributed to Goran Senjanović.

At least 19 recordsLinked to original sources

arXiv2025

Particle physics: a personal view

The reader surely knows what particles physics is about: finding building blocks of nature that appear elementary at a given time and study their interactions - so why in the world this essay? The problem is how to arrive at a fundamental theory, and maybe even more important, what is the theory supposed to do? I follow here a simple but profound prescription of Feynman for a true, self-contained theory, and then show how the Standard Model beautifully illustrates this prescription. That the issue is not trivial, is clear from the fact that we had theories dominate our field for decades, in spite of completely failing Feynman's requirements and offering no clear predictions whatsoever. I next critically review two unique candidates for a Beyond Standard Model theory: the Left-Right Symmetric theory of electro-weak interactions and the minimal grand unified theory based on SU(5) gauge symmetry. I also comment on some generic aspects of grand unification, and what hope we may have in testing it.

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arXiv2025

The importance of being discrete

Although continuous symmetries may be more appealing, especially the local gauge ones, I argue that discrete symmetries may still play a fundamental role in shaping our understanding of the physics beyond the Standard Model. I exemplify this on the case of left-right symmetry and recall how its spontaneous breaking leads naturally to new weak interactions, and even more interestingly, to neutrino mass and the seesaw mechanism behind its smallness. The minimal realization of this program is shown to be a self-contained theory of the origin and nature of neutrino mass, and of quark mixing angles. I also discuss some new results on the extension of the theory based on Pati-Salam quark-lepton symmetry, and its embedding in $SO(10)$ grand unified theory, where LR symmetry becomes gauged discrete transformation in the form of charge conjugation.

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arXiv2025

A minimal Pati-Salam theory: from cosmic defects to gravitational waves and colliders

We discuss a minimal renormalizable Pati-Salam theory based on the $SU(4)_{\rm C}\,\times\,SU(2)_{\rm L}\,\times\,SU(2)_{\rm R}$ gauge group, with unification scale Higgs multiplets taken as $SU(2)_{\rm L}$ and $SU(2)_{\rm R}$ doublets, which lead to neutrino Dirac picture. Although a number of scalar particles could be light, even lying at the LHC energies, the unification scale is hopelessly out of reach in any foreseeable future. Moreover, phase transition in the early Universe leads to the production of magnetic monopoles and domain walls, both incompatible with the standard cosmological model. A small explicit breaking of the discrete left-right symmetry allows the domain walls to decay, and in the process possibly sweep away the monopoles, analogously to the previously discussed case of $SU(5)$ grand unified theory. This leaves an important imprint of gravitational waves, within the reach of next generation searches, correlated with monopole detection and new light particles at collider energies. The theory has a dark matter candidate in the form of an inert scalar doublet, with a mass below TeV, which can further trigger electroweak baryogenesis.

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arXiv2025

$SO(10)$ theory on the plateau: the importance of being renormalizable

We revisit a minimal renormalisable $SO(10)$ grand unified theory, with the Higgs representation $45_{\rm H}$, $126_{\rm H}$ and complex $10_{\rm H}$, responsible for the unification, intermediate and the weak scale symmetry breaking, respectively. We perform the study of unification constraints and find that it allows for the Left-Right symmetric scale to be accessible even at the LHC, and the Quark-Lepton unification scale as low as its phenomenological limit around $10^5\,$GeV. Moreover, one can have neutron - anti neutron oscillations at the level of the present day sensibility in both of the above cases, while in the former case one can have simultaneously neutrinoless double beta decay induced by new light scalar states, reachable today - with both electrons emerging as left-handed, as in the neutrino exchange through its possible Majorana mass. We also discuss a recently raised issue of the fine-tuning of the light Higgs mass and its potential conflict with low intermediate mass scales.

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arXiv2024

Minimal SO(10) ante portas: the importance of being effective

The minimal $SO(10)$ grand unified theory, augmented by higher-dimensional operators, is based on the following Higgs representations: adjoint $45_{\rm H}$, spinor $16_{\rm H}$ and complex vector $10_{\rm H}$. It was recently realized that, as opposed to the conventional wisdom, any intermediate mass scale has to lie close to the grand unification one. This bears profound consequences for the scalar particles mass spectrum and the proton decay lifetime. A complete analysis of the parameter space shows the following interesting correlation under the requirement of perturbativity of the theory: A color octet below approximately $5\,\rm TeV$ would imply a lower bound on neutrino mass $m_ν\gtrsim 0.2\,\rm eV$, soon to be probed, a scalar weak triplet below $500\,\rm GeV$ and a scalar weak doublet, color triplet below $3\,\rm TeV$. If one were to stick to no flavor cancellations in proton decay amplitudes, one would recover a recent result of all these states being accessible at collider energies. Moreover, the proton decay lifetime would then be tantalizingly close to the present experimental bound.

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arXiv2024

Minimal SU(5) theory on the edge: the importance of being effective

It is well known that the minimal renormalizable $SU(5)$ grand unified theory is ruled out: it predicts same masses of down quarks and charged leptons, the gauge couplings do not unify and neutrinos are massless. We show here that all this can be cured simultaneously by the addition of higher-dimensional effective operators. However, the theory lives on the edge since the unification scale turns out as low as roughly $10^{14}\,$GeV, threatening proton longevity. If the lower bound on the proton lifetime was to be increased by an order of magnitude, the usual desert in energies between the weak and unification scales would be populated. We also revisit two minimal extensions of this theory that offer a dynamical seesaw origin of neutrino mass, and discuss the resulting consequences.

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arXiv2023

Grand unification, small vs large representations, hadron colliders and all that

We review minimal realistic grand unified models based on $SU(5)$ and $SO(10)$ gauge groups. The models with small Higgs representations and higher dimensional operators - under the assumption of no cancellations in proton decay amplitudes - predict nearby oases with new light particles. Two of them stand out: real fermion and scalar weak triplets. The former generates dynamically neutrino Majorana mass through the so-called type III seesaw mechanism, while the latter naturally gets a small vacuum expectation value and thus generically modifies $W$-boson mass. On the contrary, the renormalisable versions of these theories fail to determine the particle spectra. In particular, in the renormalisable version of the $SO(10)$ theory the mass of the right-handed gauge boson - normally assumed (and allowed) to lie far in the desert - could be even accessible at the LHC. Last but not least, we show that in the minimal supersymmetric $SU(5)$ theory all the superpartners can lie orders of magnitude above the weak scale, unless one requires strict naturalness. Thus, the so-called split supersymmetry does not imply any light states.

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arXiv2023

Spontaneous Parity Violation

We review here the central features of the idea of spontaneous breakdown of parity, in the context of its minimal realization, the Left-Right symmetric gauge theory.

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arXiv2023

$SU(5)$ grand unification and $W$-boson mass

A realistic extension of the minimal $SU(5)$ theory consisting of the addition of an adjoint fermion is known to predict light real fermion and scalar weak triplets, potentially accessible at the LHC. These particles, in addition to playing a key role in gauge coupling unification, have profound phenomenological implications. The fermion triplet, that through the seesaw mechanism offers a testable origin of neutrino mass, has been already extensively discussed. The scalar triplet develops a vacuum expectation value that modifies the $W$-boson mass. We show that its low-energy effective theory is remarkably predictive: in the leading approximation, all the relevant physical processes involving the scalar triplet depend only on its mass and the deviation from the Standard Model $W$-mass value.

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arXiv2017

Higgs Sector of the Left-Right Symmetric Theory

We perform an in-depth analysis of the Higgs sector in the Minimal Left-Right Symmetric Model and compute the scalar mass spectrum and associated mixings, offering simple physical and symmetry arguments in support of our findings. We identify the tree-level quartic and cubic potential couplings in terms of the physical states and compute the quantum corrections for the latter ones. The deviations from the Standard Model prediction of the cubic Higgs doublet coupling are considered. Moreover we discuss the possible implications concerning the stability of the potential under the renormalization-group-equations evolution. In particular we examine three possible energy scales of parity restoration: LHC reach, next hadronic collider and very high energy relevant for grand unification.

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arXiv2016

Probing Seesaw with Parity Restoration

We present a novel way of testing the seesaw origin of neutrino mass in the context of the minimal Left-Right Symmetric Model. It is based on the connection between the leptonic interactions of the doubly charged scalars, whose presence is at the core of the seesaw mechanism, and the neutrino Dirac Yukawa couplings which govern, among other processes, the right-handed neutrino decays into left-handed charged leptons. We prove that any physical quantity depending on these couplings is a function of the hermitian part only which can significantly simplify their future experimental determination.

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arXiv2015

Restoration of Parity and the Right-Handed Analog of the CKM Matrix

In a recent Letter we determined analytically the right-handed quark mixing matrix in the minimal Left-Right symmetric theory with generalized Parity. We derived its explicit form as a series expansion in a small parameter that measures the departure from hermiticity of quark mass matrices. Here we analyze carefully the convergence of the series by including higher order terms and by comparing with numerical results. We apply our findings to some phenomenological applications such as the production and decays of the right-handed gauge boson $W_R$, the neutrinoless double beta decay, the decays of the heavy scalar doublet, the strong CP parameter and the theoretical limits on the new mass scale from the $K$ and $B$-meson physics. In particular, we demonstrate that the relevant coupling for the production of the $W_R$ gauge boson at hadronic colliders and for the neutrinoless double beta decay equals its left-handed counterpart, within a percent. We also demonstrate that the stability of the theoretical lower limit on the $W_R$ mass from the $K$-meson physics is due to a partial cancellation of the external phases of the right-handed mixing matrix.

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arXiv2015

Right Handed Quark Mixing in Left-Right Symmetric Theory

We give exact formulas for the right-handed analog of the CKM matrix in the minimal Left-Right symmetric theory, for the case when the Left-Right symmetry is generalized Parity as in the original version of the theory. We derive its explicit form and give a physical reason for the known and surprising fact that the right-handed mixing angles are close in value to the CKM ones, in spite of the Left-Right symmetry being badly broken in nature. We exemplify our results on the production of the right-handed charged gauge boson and the computation of the neutral K meson mass difference.

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arXiv2011

Three Extra Mirror or Sequential Families: a Case for Heavy Higgs and Inert Doublet

We study the possibility of the existence of extra fermion families and an extra Higgs doublet. We find that requiring the extra Higgs doublet to be inert leaves space for three extra families, allowing for mirror fermion families and a dark matter candidate at the same time. The emerging scenario is very predictive: it consists of a Standard Model Higgs boson, with mass above 400\,\GeV, heavy new quarks between 340 and 500\,\GeV, light extra neutral leptons, and an inert scalar with a mass below $M_Z$.

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arXiv2011

Left-Right Symmetry: from LHC to Neutrinoless Double Beta Decay

The Large Hadron Collider has a potential to probe the scale of left-right symmetry restoration and the associated lepton number violation. Moreover, it offers hope of measuring the right-handed leptonic mixing matrix. We show how this, together with constraints from lepton flavor violating processes, can be used to make predictions for neutrinoless double beta decay. We illustrate this deep connection in the case of the type-II seesaw.

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arXiv2011

First Limits on Left-Right Symmetry Scale from LHC Data

We use the early Large Hadron Collider data to set the lower limit on the scale of Left-Right symmetry, by searching for the right-handed charged gauge boson $W_R$ via the final state with two leptons and two jets, for 33/pb integrated luminosity and 7 TeV center-of-mass energy. In the absence of a signal beyond the Standard Model background, we set the bound M_WR > 1.4 TeV at 95% C.L.. This result is obtained for a range of right-handed neutrino masses of the order of few 100 GeV, assuming no accidental cancelation in right-handed lepton mixings.

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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.

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arXiv1997

Supersymmetry and Broken Symmetries at High Temperature

It is generally believed that internal symmetries are necessarily restored at high temperature in supersymmetric theories. We provide simple and natural counterexamples to this no-go theorem for systems having a net background charge. We exemplify our findings on abelian models, for both cases of global and local symmetries and discuss their possible implications.

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