arXiv subjects

Kristjan Kannike

Publications and source records attributed to Kristjan Kannike.

At least 55 records · Page 3Linked to original sources

arXiv2017

A frame independent classification of single field inflationary models

Seemingly unrelated models of inflation that originate from different physical setups yield, in some cases, identical predictions for the currently constrained inflationary observables. In order to classify the available models, we propose to express the slow-roll parameters and the relevant observables in terms of frame and reparametrisation invariant quantities. The adopted invariant formalism makes manifest the redundancy that afflicts the current description of inflation dynamics and offers a straightforward way to identify classes of models which yield identical phenomenology. In this Letter we offer a step-to-step recipe to recast every single field inflationary model in the proposed formalism, detailing also the procedure to compute inflationary observables in terms of frame and reparametrisation invariant quantities. We hope that our results become the cornerstone of a new categorisation of viable inflationary models and open the way to a deeper understanding of the inflation mechanism.

hep-ph↗

arXiv2017

Large loop-coupling enhancement of a heavy pseudoscalar from a light dark sector

The small background and the sensitivity to charged particles via a leading order loop coupling make the diphoton channel a privileged experimental test for new physics models. We propose a simple archetypal scenario to generate a sharp di-photon resonance as a result of threshold enhancements in the effective coupling between a heavy pseudoscalar particle and new vector-like leptons. We therefore study three different scenarios consistent with the current experimental limits and deviating from the Standard Model at the 2~$σ$ level. The model also introduces a natural dark matter candidate able to match the observed dark matter abundance and comfortably respect the current direct detection constraints.

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arXiv2016

Relaxion Cosmology and the Price of Fine-Tuning

The relaxion scenario presents an intriguing extension of the standard model in which the particle introduced to solve to the strong CP problem, the axion, also achieves the dynamical relaxation of the Higgs boson mass term. In this work we complete this framework by proposing a scenario of inflationary cosmology that is consistent with all the observational constraints: the relaxion hybrid inflation with an asymmetric waterfall. In our scheme, the vacuum energy of the inflaton drives inflation in a natural way while the relaxion slow-rolls. The constraints on the present inflationary observables are then matched through a subsequent inflationary epoch driven by the inflaton. We quantify the amount of fine-tuning of the proposed inflation scenario, concluding that the inflaton sector severely decreases the naturalness of the theory.

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arXiv2016

The Higgs of the Higgs and the diphoton channel

LHC results do not confirm conventional natural solutions to the Higgs mass hierarchy problem, motivating alternative interpretations where a hierarchically small weak scale is generated from a dimension-less quantum dynamics. We propose weakly and strongly-coupled models where the field that breaks classical scale invariance giving mass to itself and to the Higgs is identified with a possible new resonance within the LHC reach. As an example, we identify such resonance with the 750 GeV diphoton excess recently reported by ATLAS and CMS. Such models can be extrapolated up to the Planck scale, provide Dark Matter candidates and eliminate the SM vacuum instability.

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arXiv2016

Linear inflation from quartic potential

We show that if the inflaton has a non-minimal coupling to gravity and the Planck scale is dynamically generated, the results of Coleman-Weinberg inflation are confined in between two attractor solutions: quadratic inflation, which is ruled out by the recent measurements, and linear inflation which, instead, is in the experimental allowed region. The minimal scenario has only one free parameter -- the inflaton's non-minimal coupling to gravity -- that determines all physical parameters such as the tensor-to-scalar ratio and the reheating temperature of the Universe. Should the more precise future measurements of inflationary parameters point towards linear inflation, further interest in scale-invariant scenarios would be motivated.

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arXiv2015

Muon g-2 and galactic centre $γ$-ray excess in a scalar extension of the 2HDM type-X

We consider an extension of the lepto-specific 2HDM with an extra singlet $S$ as a dark matter candidate. Taking into account theoretical and experimental constraints, we investigate the possibility to address both the $γ$-ray excess detected at the Galactic centre and the discrepancy between the Standard Model prediction and experimental results of the anomalous magnetic moment of the muon. Our analyses reveal that the $SS \to τ^+ τ^-$ and $SS \to b \bar b$ channels reproduce the Galactic centre excess, with an emerging dark matter candidate which complies with the bounds from direct detection experiments, measurements of the Higgs boson invisible decay width and observations of the dark matter relic abundance. Addressing the anomalous magnetic moment of the muon imposes further strong constraints on the model. Remarkably, under these conditions, the $SS \to b \bar b$ channel still allows for the fitting of the Galactic centre. We also comment on a scenario allowed by the model where the $SS \to τ^+ τ^-$ and $SS \to b \bar b$ channels have comparable branching ratios, which possibly yield an improved fitting of the Galactic centre excess.

hep-ph↗

arXiv2015

Dynamically Induced Planck Scale and Inflation

Theories where the Planck scale is dynamically generated from dimensionless interactions provide predictive inflationary potentials and super-Planckian field variations. We first study the minimal single-field realisation in the low-energy effective field theory limit, finding the predictions $n_s \approx 0.96$ for the spectral index and $r \approx 0.13$ for the tensor-to-scalar ratio, which can be reduced down to $\approx 0.04$ in presence of large couplings. Next we consider agravity as a dimensionless quantum gravity theory finding a multi-field inflation that converges towards an attractor trajectory that predicts $n_s\approx 0.96$ and $0.003<r<0.13$, interpolating between the quadratic and Starobinsky inflation. These theories relate the smallness of the weak scale to the smallness of inflationary perturbations: both arise naturally because of small couplings, implying a reheating temperature of $10^{7-9}$ GeV. A measurement of $r$ by Keck/Bicep3 would give us information on quantum gravity in the dimensionless scenario.

astro-ph.CO↗

arXiv2014

Embedding inflation into the Standard Model - more evidence for classical scale invariance

If cosmological inflation is due to a slowly rolling single inflation field taking trans-Planckian values as suggested by the BICEP2 measurement of primordial tensor modes in CMB, embedding inflation into the Standard Model challenges standard paradigm of effective field theories. Together with an apparent absence of Planck scale contributions to the Higgs mass and to the cosmological constant, BICEP2 provides further experimental evidence for the absence of large $M_{\rm P}$ induced operators. We show that classical scale invariance, the paradigm that all fundamental scales in Nature are induced by quantum effects, solves the problem and allows for a remarkably simple scale-free Standard Model extension with inflaton without extending the gauge group. Due to trans-Planckian inflaton values and vevs, a dynamically induced Coleman-Weinberg-type inflaton potential of the model can predict tensor-to-scalar ratio $r$ in a large range, converging around the prediction of chaotic $m^2ϕ^2$ inflation for a large trans-Planckian value of the inflaton vev. Precise determination of $r$ in future experiments will single out a unique scale-free inflation potential, allowing to test the proposed field-theoretic framework.

hep-ph↗

arXiv2014

The universal Higgs fit

We perform a state-of-the-art global fit to all Higgs data. We synthesise them into a 'universal' form, which allows to easily test any desired model. We apply the proposed methodology to extract from data the Higgs branching ratios, production cross sections, couplings and to analyse composite Higgs models, models with extra Higgs doublets, supersymmetry, extra particles in the loops, anomalous top couplings, invisible Higgs decay into Dark Matter. Best fit regions lie around the Standard Model predictions and are well approximated by our 'universal' fit. Latest data exclude the dilaton as an alternative to the Higgs, and disfavour fits with negative Yukawa couplings. We derive for the first time the SM Higgs boson mass from the measured rates, rather than from the peak positions, obtaining $M_h = 125.0 \pm 1.8$ GeV.

hep-ph↗

arXiv2014

Minimal semi-annihilating $\mathbb{Z}_N$ scalar dark matter

We study the dark matter from an inert doublet and a complex scalar singlet stabilized by $\mathbb{Z}_N$ symmetries. This field content is the minimal one that allows dimensionless semi-annihilation couplings for $N > 2$. We consider explicitly the $\mathbb{Z}_3$ and $\mathbb{Z}_4$ cases and take into account constraints from perturbativity, unitarity, vacuum stability, necessity for the electroweak $\mathbb{Z}_N$ preserving vacuum to be the global minimum, electroweak precision tests, upper limits from direct detection and properties of the Higgs boson. Co-annihilation and semi-annihilation of dark sector particles as well as dark matter conversion significantly modify the cosmic abundance and direct detection phenomenology.

hep-ph↗

arXiv2014

Towards Completing the Standard Model: Vacuum Stability, EWSB and Dark Matter

We study the standard model (SM) in its full perturbative validity range between $Λ_QCD$ and the $U(1)_Y$ Landau pole, assuming that a yet unknown gravitational theory in the UV does not introduce additional particle thresholds, as suggested by the tiny cosmological constant and the absence of new stabilising physics at the EW scale. We find that, due to dimensional transmutation, the SM Higgs potential has a global minimum at 10^26 GeV, invalidating the SM as a phenomenologically acceptable model in this energy range. We show that extending the classically scale invariant SM with one complex singlet scalar S allows us to: (i) stabilise the SM Higgs potential; (ii) induce a scale in the singlet sector via dimensional transmutation that generates the negative SM Higgs mass term via the Higgs portal; (iii) provide a stable CP-odd singlet as the thermal relic dark matter due to CP-conservation of the scalar potential; (iv) provide a degree of freedom that can act as an inflaton in the form of the CP-even singlet. The logarithmic behaviour of dimensional transmutation allows one to accommodate the large hierarchy between the electroweak scale and the Landau pole, while understanding the latter requires a new non-perturbative view on the SM.

hep-ph↗

arXiv2013

Vacuum Stability Conditions From Copositivity Criteria

A scalar potential of the form $λ_{ab} ϕ_a^2 ϕ_b^2$ is bounded from below if its matrix of quartic couplings $λ_{ab}$ is copositive -- positive on non-negative vectors. Scalar potentials of this form occur naturally for scalar dark matter stabilised by a $\mathbb{Z}_2$ symmetry. Copositivity criteria allow to derive analytic necessary and sufficient vacuum stability conditions for the matrix $λ_{ab}$. We review the basic properties of copositive matrices and analytic criteria for copositivity. To illustrate these, we re-derive the vacuum stability conditions for the inert doublet model in a simple way, and derive the vacuum stability conditions for the $\mathbb{Z}_2$ complex singlet dark matter, and for the model with both a complex singlet and an inert doublet invariant under a global U(1) symmetry.

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arXiv2013

One or more Higgs bosons?

Now that one has been found, the search for signs of more scalars is a primary task of current and future experiments. In the motivated hypothesis that the extra Higgs bosons of the next-to-minimal supersymmetric Standard Model (NMSSM) be the lightest new particles around, we outline a possible overall strategy to search for signs of the CP-even states. This work complements Ref. arXiv:1304.3670.

hep-ph↗

arXiv2013

Exploring the Higgs sector of a most natural NMSSM

The Next to Minimal Supersymmetric Standard Model (NMSSM) with a Higgs-singlet coupling λ close to unity, moderate tan β and stop masses below 1 TeV minimizes the fine tuning of the electroweak scale, making possible that the lightest new particles, except perhaps for the LSP, be part of the extended Higgs system. We show how the measurements of the couplings of the 126 GeV Higgs boson constrain the region of the physical parameters of a generic NMSSM most relevant to this context. In the same region of parameter space we determine the cross section for the production of a heavier CP-even scalar together with its total width and its most relevant branching ratios. For comparison we show the same analysis for the MSSM. We also comment on how a coupling λ ~ 1 can be compatible with gauge coupling unification.

hep-ph↗

arXiv2013

Fermiophobic Higgs boson and supersymmetry

If a light Higgs boson with mass 125 GeV is fermiophobic, or partially fermiophobic, then the MSSM is excluded. The minimal supersymmetric fermiophobic Higgs scenario can naturally be formulated in the context of the NMSSM that admits Z_3 discrete symmetries. In the fermiophobic NMSSM, the SUSY naturalness criteria are relaxed by a factor N_c y_t^4/g^4 \sim 25, removing the little hierarchy problem and allowing sparticle masses to be naturally of order 2--3 TeV. This scale motivates wino or higgsino dark matter. The SUSY flavour and CP problems as well as the constraints on sparticle and Higgs boson masses from b \to sγ, B_s \to μμ and direct LHC searches are relaxed in fermiophobic NMSSM. The price to pay is that a new, yet unknown, mechanism must be introduced to generate fermion masses. We show that in the fermiophobic NMSSM the radiative Higgs boson branchings to γγ, γZ can be modified compared to the fermiophobic and ordinary standard model predictions, and fit present collider data better. Suppression of dark matter scattering off nuclei explains the absence of signal in XENON100.

hep-ph↗

arXiv2013

$Z_3$ Scalar Singlet Dark Matter

We consider the minimal scalar singlet dark matter stabilised by a $Z_3$ symmetry. Due to the cubic term in the scalar potential, semi-annihilations, besides annihilations, contribute to the dark matter relic density. Unlike in the $Z_2$ case, the dark matter spin independent direct detection cross section is no more linked to the annihilation cross section. We study the extrema of the potential and show that a too large cubic term would break the $Z_3$ symmetry spontaneously, implying a lower bound on the direct detection cross section, and allowing the whole parameter space to be tested by XENON1T. In a small region of the parameter space the model can avoid the instability of the standard model vacuum up to the unification scale. If the semi-annihilations are large, however, new physics will be needed at TeV scale because the model becomes non-perturbative. The singlet dark matter mass cannot be lower than 53.8 GeV due to the constraint from Higgs boson decay into dark matter.

hep-ph↗

arXiv2012

Anthropic solution to the magnetic muon anomaly: the charged see-saw

We present models of new physics that can explain the muon g-2 anomaly in accord with with the assumption that the only scalar existing at the weak scale is the Higgs, as suggested by anthropic selection. Such models are dubbed "charged see-saw" because the muon mass term is mediated by heavy leptons. The electroweak contribution to the g-2 gets modified by order one factors, giving an anomaly of the same order as the observed hint, which is strongly correlated with a modification of the Higgs coupling to the muon.

hep-ph↗

arXiv2012

Is the resonance at 125 GeV the Higgs boson?

The recently discovered resonance at 125 GeV has properties remarkably close to those of the Standard Model Higgs boson. We perform model-independent fits of all presently available data. The non- standard best-fits found in our previous analyses remain favored with respect to the SM fit, mainly but not only because the γγ rate remains above the SM prediction.

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