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Archil Kobakhidze

Publications and source records attributed to Archil Kobakhidze.

At least 55 records · Page 3Linked to original sources
arXiv2016

Unitarisation of EFT Amplitudes for Dark Matter Searches at the LHC

We propose a new approach to the LHC dark matter search analysis within the effective field theory (EFT) framework by utilising the K-matrix unitarisation formalism. This approach provides a reasonable estimate of the dark matter production cross section at high energies, and hence allows reliable bounds to be placed on the cut-off scale of relevant operators without running into the problem of perturbative unitarity violation. We exemplify this procedure for the effective operator D5 in monojet dark matter searches in the collinear approximation. We compare our bounds to those obtained using the truncation method and identify a parameter region where the unitarisation prescription leads to more stringent bounds.

hep-ph↗
arXiv2016

Heavy axion in asymptotically safe QCD

Assuming QCD exhibits an interacting fixed-point behaviour in the ultraviolet regime, I argue that the axion can be substantially heavier than in the conventional case of asymptotically free QCD due to the enhanced contribution of small size instantons to its mass.

hep-ph↗
arXiv2016

Gravitational Waves from the Phase Transition of a Non-linearly Realised Electroweak Gauge Symmetry

Within the Standard Model with non-linearly realised electroweak symmetry, the LHC Higgs boson may reside in a singlet representation of the gauge group. Several new interactions are then allowed, including anomalous Higgs self-couplings, which may drive the electroweak phase transition to be strongly first-order. In this paper we investigate the cosmological electroweak phase transition in a simplified model with an anomalous Higgs cubic self- coupling. We look at the feasibility of detecting gravitational waves produced during such a transition in the early universe by future space-based experiments. We find that for the range of relatively large cubic couplings, $111~{\rm GeV}~ \lesssim |κ| \lesssim 118~{\rm GeV}$, $\sim $mHz frequency gravitational waves can be observed by eLISA, while BBO will potentially be able to detect waves in a wider frequency range, $0.1-10~$mHz.

hep-ph↗
arXiv2016

Heavy Leptonium as the Origin of the 750 GeV Diphoton Excess

We propose a hypothetical heavy leptonium, the scalar bound state of an exotic lepton-antilepton pair, as a candidate for the recent 750 GeV resonance in the early LHC Run 2 data. Such a para-leptonium is dominantly produced via photon-photon fusion at the LHC and decays into a photon pair with a significant branching fraction. In addition, our model predicts a companion spin-1 ortho-leptonium bound state, which can decay to $W^+W^-$, $f\bar{f}$ and three photons. Under the LHC and the electroweak precision observables bounds, we find that the observed excess of 750 GeV diphoton events can be explained within $2σ$ accuracy for $Y_{L} \approx 4.8 - 7.2$ for the minimal case in our scenario. The observation of the ortho-leptonium in the dilepton channel will be the smoking gun for our scenario.

hep-ph↗
arXiv2016

750 GeV Composite Axion as the LHC Diphoton Resonance

We propose that the 750 GeV resonance, presumably observed in the early LHC Run 2 data, could be a heavy composite axion that results from condensation of a hypothetical quark in a high-colour representation of conventional QCD. The model, motivated by a recently proposed solution to the strong CP problem, is very economical and is essentially defined by the properties of the additional quark - its colour charge, hypercharge and mass. The axion mass and its coupling to two photons (via axial anomaly) can be computed in terms of these parameters. The axion is predominantly produced via photon fusion ($γγ\to {\cal A}$) which is followed by $ Z $ vector boson fusion and associated production at the LHC. We find that the total diphoton cross section of the axion can be fitted with the observed excess. Combining the requirement on the cross-section, such that it reproduces the diphoton excess events, with the bounds on the total width ($Γ_{tot} \leqslant 45$ GeV), we obtain the effective coupling in the range $1.6\times 10^{-4}$ GeV$^{-1}\gtrsim C_{\cal A} \gtrsim 6.5\times 10^{-5}$ GeV$^{-1}$. Within this window of allowed couplings the model favours a narrow width resonance and $ y_{Q}^2 \sim \mathcal{O}(10)$. In addition, we observe that the associated production $q\bar{q} \to {\cal A}γ\to γγγ$ can potentially produce a sizeable number of three photon events at future LHC and $ e^{+} e^{-} $ colliders. However, the rare decay $Z\to\mathcal{A}^*γ\to γγγ$ is found to be too small to be probed at the LHC.

hep-ph↗
arXiv2016

Electroweak Baryogenesis with Anomalous Higgs Couplings

We investigate feasibility of efficient baryogenesis at the electroweak scale within the effective field theory framework based on a non-linear realisation of the electroweak gauge symmetry. In this framework the LHC Higgs boson is described by a singlet scalar field, which, therefore, admits new interactions. Assuming that Higgs couplings with the eletroweak gauge bosons are as in the Standard Model, we demonstrate that the Higgs cubic coupling and the CP-violating Higgs-top quark anomalous couplings alone may drive the a strongly first-order phase transition. The distinguished feature of this transition is that the anomalous Higgs vacuum expectation value is generally non-zero in both phases. We identify a range of anomalous couplings, consistent with current experimental data, where sphaleron rates are sufficiently fast in the 'symmetric' phase and are suppressed in the 'broken' phase and demonstrate that the desired baryon asymmetry can indeed be generated in this framework. This range of the Higgs anomalous couplings can be further constrained from the LHC Run 2 data and be probed at high luminosity LHC and beyond.

hep-ph↗
arXiv2016

Observable Zitterbewegung in Curved Spacetimes

Zitterbewegung, as it was originally described by Schrödinger, is an unphysical, non-observable effect. We verify whether the effect can be observed in non-inertial reference frames/curved spacetimes, where the ambiguity in defining particle states results in a mixing of positive and negative frequency modes. We explicitly demonstrate that such a mixing is in fact necessary to obtain the correct classical value for a particle's velocity in a uniformly accelerated reference frame, whereas in cosmological spacetime a particle does indeed exhibit Zitterbewegung.

hep-th↗
arXiv2016

Natural Inflation with Hidden Scale Invariance

We propose a new class of natural inflation models based on a hidden scale invariance. In a very generic Wilsonian effective field theory with an arbitrary number of scalar fields, which exhibits scale invariance via the dilaton, the potential necessarily contains a flat direction in the classical limit. This flat direction is lifted by small quantum corrections and inflation is realised without need for an unnatural fine-tuning. In the conformal limit, the effective potential becomes linear in the inflaton field, yielding to specific predictions for the spectral index and the tensor-to-scalar ratio, being respectively: $n_s-1\approx -0.025\left(\frac{N_{\star}}{60}\right)^{-1}$ and $r\approx 0.0667\left(\frac{N_{\star}}{60}\right)^{-1}$, where $N_{\star}\approx 30-65$ is a number of efolds during observable inflation. This predictions are in reasonable agreement with cosmological measurements. Further improvement of the accuracy of these measurements may turn out to be critical in falsifying our scenario.

gr-qc↗
arXiv2016

750 GeV diphoton resonance in a top and bottom seesaw model

The top/bottom seesaw model, which extends the top seesaw in order to accomodate a 125 GeV Higgs boson, predicts vector-like top/bottom partners and these partners can be bounded to form several neutral and charged singlet composite scalars by some new strong dynamics. In this letter, we use such a singlet scalar to interpret the 750 GeV diphoton reseanance. This singlet scalar is dominantly produced through the gluon fusion process induced by the partners and its diphoton decay is induced by both the partners and the charged singlet scalars. We show that this scenario can readily account for the observed 750 GeV diphoton signal under the current LHC constraints. Further, this scenario predicts some other phenomenology, such as a strong correlation between the decays to $γγ$, $Z γ$ and $ZZ$, a three-photon singal from the associate production of a singlet scalar and a photon, as well as some signals from the partner cascade decays. These signals may jointly allow for a test of this framework in future LHC and ILC experiments.

hep-ph↗
arXiv2016

Solving the Strong CP Problem with High-Colour Quarks and Composite Axion

I propose a new axionic solution to the strong CP problem which involves a hypothetical vector-like quark(s) in a high-colour representation of the conventional QCD. There are two distinct scenarios. If the current mass of the exotic quark is zero, the strong CP phase can be trivially rotated away. The high-colour quark is `hidden' in various bounds states, the lightest being the composite axion field, with properties similar to the standard invisible axion. If the high-colour quark acquire a non-zero current mass due to the spontaneous chiral symmetry breaking, the composite axion can be heavy, while the strong CP phase is still cancelling out in the vacuum. The phenomenology and cosmological implications of the heavy composite axion differs drastically from the standard invisible axion.

hep-ph↗
arXiv2016

Closing up a light stop window in natural SUSY at LHC

Top squark (stop) plays a key role in the radiative stability of the Higgs boson mass in supersymmetry (SUSY). The LHC searches for stop have made a great progress and tightly constrained the stop mass during Run-1. In this work, we use the LHC Run-1 data to determine the lower mass limit of the right-handed stop in a natural SUSY scenario, where the higgsinos $\tildeχ^0_{1,2}$ and $\tildeχ^\pm_{1}$ are light and nearly degenerate. We find that the stop mass has been excluded up to 430 GeV for $m_{\tildeχ^0_1} \lesssim 250$ GeV and to 540 GeV for $m_{\tildeχ^0_1} \simeq 100$ GeV by the Run-1 SUSY searches for $2b+E^{miss}_T$ and $1\ell+jets+E^{miss}_T$, respectively. In a small strip of parameter space with $m_{\tildeχ^0_1} \gtrsim 190$ GeV, the stop mass can still be as light as 210 GeV and compatible with the Higgs mass measurement and the monojet bound. The 14 TeV LHC with a luminosity of 20 fb$^{-1}$ can further cover such a light stop window by monojet and $2b+E^{miss}_T$ searches and push the lower bound of the stop mass to 710 GeV. We also explore the potential to use the Higgs golden ratio, $D_{γγ}=σ(pp \to h \to γγ)/σ(pp \to h \to ZZ^* \to4\ell^\pm)$, as a complementary probe for the light and compressed stop. If this golden ratio can be measured at percent level at the high luminosity LHC (HL-LHC) or future $e^+e^-$ colliders, the light stop can be excluded for most of the currently allowed parameter region.

hep-ph↗
arXiv2015

ATLAS Z-peaked excess in MSSM with a light sbottom or stop

We attempt to explain the recent ATLAS 3$σ$ excess of dilepton events with an invariant mass near $Z$ peak through the gluino-mediated sbottom production in a simplified scenario inspired by the Minimal Supersymmetric Standard Model (MSSM). The additional $Z$ bosons can be produced through the cascade decay chain $\tilde{g} \to b \tilde{b}^{\dagger}_{1} \to b \bar{b} \tildeχ^{0}_{2,3} \to b \bar{b} \tildeχ^{0}_1 Z$, in which $\tilde b_1$ is the right-handed sbottom, $\tildeχ_1^0$ is the bino-like lightest supersymmetric particle (LSP) and $\tildeχ_{2,3}^0$ are two nearly degenerate higgsino-like next-to LSPs (NLSPs). Taking into account the constraint from the LHC search for the gluino-mediated sbottom production in the final states with the missing transverse energy and at least three $b$-jets, we find that the ATLAS on-$Z$ excess can only be marginally explained at $2σ$ level. We also note that within the scenario where the gluino predominantly decays to the right-handed stop instead of the sbottom, the excess can hardly be explained, since the $Z$-boson is not produced in the stop dominant decay channel, $\tilde{t}_1 \to b \tildeχ^+_1$.

hep-ph↗
arXiv2015

The Effective MSSM

We suggest an effective field theory framework to discuss deviations from the minimal supersymmetric Standard Model (MSSM) which is based on an alternative arrangement of the gauge-Higgs sector. In this effective MSSM (EffMSSM) nonlinearly realised $SU(2)\times U(1)$ gauge sector is described by an $SU(2)\times U(1)$-valued massive vector superfield, which contains a neutral CP-even and charged Higgs fields, while another neutral CP-even Higgs and the neutral CP-odd Higgs fields are residing in an $SU(2)\times U(1)$-singlet chiral superfield. Although the new theory contains the same particle content as the conventional MSSM, the unconventional representation of superfields allows for new type of interactions, which may lead to a significant modification of the phenomenology. As an illustrative example we consider EffMSSM with modified Higgs and electroweak gauge sector augmented by gaugino soft supersymmetry breaking masses, $M_i~ (i=1,2,3)$ and the Standard Higgs soft-breaking masses, $m_{H_u}=m_{H_d}$ and $B_μ$, and point out distinct features in the Higgs and gaugino sectors as compared to MSSM. In particular, we show that the lightest neutral CP-even Higgs boson with mass $\sim 125$ GeV can be easily accommodated within EffMSSM.

hep-ph↗
arXiv2015

Quantum relaxation of the Higgs mass

I put forward a qualitatively new dynamical mechanism for solving the electroweak hierarchy problem that does not require new physics at the electroweak. I argue that the infrared fluctuations of the gravitational field may provide a partial screening of the Higgs mass, similar to the infrared screening of the electric charge in quantum electrodynamics.

hep-ph↗
arXiv2015

Generating Luminous and Dark Matter During Inflation

We propose a new mechanism for generating both luminous and dark matter during cosmic inflation. According to this mechanism, ordinary and dark matter carry common charge which is associated with an anomalous $ U(1)_{X} $ group. Anomaly terms source $ \mathcal{CP} $ and $ U(1)_{X} $ charge violating processes during inflation, producing corresponding non-zero Chern-Simons numbers which are subsequently reprocessed into baryon and dark matter densities. The general framework developed is then applied to two possible extensions of the Standard Model with anomalous gauged $B$ and $B-L$, each with an additional dark matter candidate.

hep-ph↗
arXiv2014

Constraining Top partner and Naturalness at the LHC and TLEP

We investigate indirect constraints on the top partner within the minimal fermionic top partner model. By performing a global fit of the latest Higgs data, $B_s \to μ^+μ^-$ measurements and the electroweak precision observables we find that the top partner with the mass up to 830 GeV is excluded at $2σ$ level. Our bound on the top partner mass is much stronger than the bounds obtained from the direct searches at the LHC. Under the current constraints the fine-tuning measure is less than 9% and the branching ratio of $T \to tZ$ is bounded between 14% and 25%. We also find that precise measurements of Higgs couplings at 240 GeV TLEP will constrain the top partner mass in multi-TeV region.

hep-ph↗
arXiv2014

Anomalous Top-Higgs Couplings and Top Polarisation in Single Top and Higgs Associated Production at the LHC

In this paper, we put constraints on anomalous $\mathcal{CP}$-violating top-Higgs couplings using the currently available Higgs data and explore the prospect of measuring these couplings at 240 GeV TLEP. We find that the $\mathcal{CP}$-violating phase $ξ$ is currently limited in the range $|ξ|< 0.6π$ at 95\% C.L. and may be further constrained to $|ξ| <0.07π$ at TLEP. Under this consideration, we further investigate the observability of the scalar ($ξ=0$), pseudoscalar ($ξ=0.5π$) and mixed ($ξ=0.25π$) top-Higgs interactions through the channel $pp \to t(\to \ell^+ν_\ell b)h(\to b\overline{b})j$. We find that it is most promising to observe pure pseudoscalar interactions with $y_t=y_t^{SM}$, although this will be challenging due to a low signal to background ratio. We also find that the anomalous top-Higgs couplings can lead to sizeable differences in lepton forward-backward asymmetries and can be distinguished by measuring the lepton angular distributions from polarised top quarks at 14 TeV LHC.

hep-ph↗