arXiv subjects

Benedict von Harling

Publications and source records attributed to Benedict von Harling.

At least 19 recordsLinked to original sources

arXiv2023

Status of Electroweak Baryogenesis in Minimal Composite Higgs

We present an update on the status of electroweak baryogenesis in minimal composite Higgs models. The particularity of this framework is that the electroweak phase transition can proceed simultaneously with the confinement phase transition of the new strong dynamics that produces the composite Higgs. The latter transition is controlled by the dilaton - the pseudo-Goldstone boson of an approximate scale invariance of the composite sector. Since it naturally is first-order, the electroweak phase transition becomes first-order too. Another appealing aspect is that the necessary additional source of CP violation can arise from the variation of the quark Yukawa couplings during the phase transition, which is built-in naturally in this scenario. These two features address the shortcomings of electroweak baryogenesis in the Standard Model. We confront this scenario with the latest experimental bounds derived from collider searches for new resonances and measurements of the Higgs couplings and electric dipole moments. All these constraints provide (or will be able to provide in the near future) important bounds on the considered scenario, with the most stringent ones coming from LHC searches for new resonances which constrain the dilaton mass and couplings. We identify the viable region of parameter space which satisfies all the constraints, and is characterized by a dilaton mass in the $300-500$ GeV range and a Higgs decay constant $f \lesssim 1.1$ TeV. We discuss its future tests.

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arXiv2023

High-Temperature Electroweak Baryogenesis with Composite Higgs

Electroweak Baryogenesis (EWBG) paired with the Composite Higgs (CH) scenario provides a well-motivated and testable framework for addressing the questions of the origin of the matter-antimatter asymmetry and the naturalness of the electroweak scale. The appeal of both concepts however experiences increasing pressure from the experimental side, as no conclusive signs of the corresponding new physics have been observed. In this note we present a modification of the minimal CH EWBG model, where electroweak symmetry breaking persists to temperatures far above the usually obtained upper bound of ~ 100 GeV. This allows for an increase of the mass of the main actor of EWBG in this scenario - the dilaton. Such a modification results in relaxing the tension with experimental data, generally modifying the phenomenology, and pointing at collider searches for the heavy dilaton as the main direction for its future tests.

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arXiv2023

Dilaton at the LHC: Complementary Probe of Composite Higgs

The dilaton is predicted in various extensions of the standard model containing sectors with an approximate spontaneously-broken conformal invariance. As a Goldstone boson of a spontaneously broken symmetry, the dilaton can naturally be one of the lightest new physics particles, and therefore may be the first new physics imprint observed in collider experiments. In particular, it can arise in composite Higgs models which are often assumed to have approximate conformal invariance in the UV. The dilaton is then a composite state, generated by the same sector that produces the Higgs. We continue the exploration of composite dilaton signatures at the LHC, using the latest experimental data and analysing the future detection prospects. We elaborate on the connection of the dilaton properties with the properties of the Higgs potential, clarifying in particular the relation between the scale relevant for electroweak fine tuning and the scale controlling the dilaton couplings. This relation is then used to derive the experimental sensitivity to the dilaton in natural composite Higgs scenarios, which reaches ~3 TeV in dilaton mass for generic parameter choices. At the same time, we show that dilaton searches are a complementary direction to probe Higgs boson compositeness, with the sensitivity comparable or exceeding that of Higgs coupling measurements.

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arXiv2022

Wilson Coefficients and Natural Zeros from the On-Shell Viewpoint

We show how to simplify the calculation of the finite contributions from heavy particles to EFT Wilson coefficients by using on-shell methods. We apply the technique to the one-loop calculation of $g-2$ and $Hγγ$, showing how finite contributions can be obtained from the product of tree-level amplitudes. In certain cases, due to a parity symmetry of these amplitudes, the total contribution adds up to zero, as previously found in the literature. Our method allows to search for new natural zeros, as well as to obtain non-zero contributions in a straightforward way.

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arXiv2021

Baryogenesis via gauge field production from a relaxing Higgs

We show that the baryon asymmetry of the universe can be explained in models where the Higgs couples to the Chern-Simons term of the hypercharge group and is away from the late-time minimum of its potential during inflation. The Higgs then relaxes toward this minimum once inflation ends which leads to the production of (hyper)magnetic helicity. We discuss the conditions under which this helicity can be approximately conserved during its joint evolution with the thermal plasma. At the electroweak phase transition the helicity is then converted into a baryon asymmetry by virtue of the chiral anomaly in the standard model. We propose a simple model which realizes this mechanism and show that the observed baryon asymmetry of the universe can be reproduced.

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arXiv2021

Anomalous Dimensions of Effective Theories from Partial Waves

On-shell amplitude methods have proven to be extremely efficient for calculating anomalous dimensions. We further elaborate on these methods to show that, by the use of an angular momentum decomposition, the one-loop anomalous dimensions can be reduced to essentially a sum of products of partial waves. We apply this to the SM EFT, and show how certain classes of anomalous dimensions have their origin in the same partial-wave coefficients. We also use our result to obtain a generic formula for the one-loop anomalous dimensions of nonlinear sigma models at any order in the energy expansion, and apply our method to gravity, where it proves to be very advantageous even in the presence of IR divergencies.

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arXiv2020

Peccei-Quinn Phase Transition at LIGO

The LIGO observatories can potentially detect stochastic gravitational waves arising from phase transitions which happened in the early universe at temperatures around $T\sim 10^{8}$ GeV. This provides an extraordinary opportunity for discovering the phase transition associated with the breaking of the Peccei-Quinn symmetry, required in QCD axion models. Here we consider the simplest Peccei-Quinn models and study under which conditions a strong first-order phase transition can occur, analyzing its associated gravitational wave signal. To be detectable at LIGO, we show that some supercooling is needed, which can arise either in Coleman-Weinberg-type symmetry breaking or in strongly-coupled models. We also investigate phase transitions that interestingly proceed by first breaking the electroweak symmetry at large scales before tunneling to the Peccei-Quinn breaking vacuum. In this case, the associated gravitational wave signal is more likely to be probed at the proposed Einstein Telescope.

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arXiv2019

Baryogenesis from axion inflation

The coupling of an axion-like particle driving inflation to the Standard Model particle content through a Chern-Simons term generically sources a dual production of massless helical gauge fields and chiral fermions. We demonstrate that the interplay of these two components results in a highly predictive baryogenesis model, which requires no further ingredients beyond the Standard Model. If the helicity stored in the hyper magnetic field and the effective chemical potential induced by the chiral fermion production are large enough to avoid magnetic diffusion from the thermal plasma but small enough to sufficiently delay the chiral plasma instability, then the non-vanishing helicity survives until the electroweak phase transition and sources a net baryon asymmetry which is in excellent agreement with the observed value. If any of these two conditions is violated, the final baryon asymmetry vanishes. The observed baryon asymmetry can be reproduced if the energy scale of inflation is around $H_\text{inf} \sim 10^{10}$ - $10^{12}$ GeV with a moderate dependence on inflation model parameters.

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arXiv2019

The Super-Planckian Axion Strikes Back

We present a novel framework for obtaining large hierarchies in axion decay constants as well as trans-Planckian field excursions, with no need for tuning or a large number of fields. We consider a model with two or more CFTs with a common cutoff, that are linked by a gauged diagonal symmetry. This construction is dual to the geometry of a warped space with two or more throats glued at a common brane, allowing for calculability. Many applications of our setup are possible,such as ultra-light axions, natural inflation, and relaxion models.

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arXiv2018

Electroweak Phase Transition and Baryogenesis in Composite Higgs Models

We present a comprehensive study of the electroweak phase transition in composite Higgs models, where the Higgs arises from a new, strongly-coupled sector which confines near the TeV scale. This work extends our study in Ref. [1]. We describe the confinement phase transition in terms of the dilaton, the pseudo-Nambu-Goldstone boson of broken conformal invariance of the composite Higgs sector. From the analysis of the joint Higgs-dilaton potential we conclude that in this scenario the electroweak phase transition can naturally be first-order, allowing for electroweak baryogenesis. We then extensively discuss possible options to generate a sufficient amount of CP violation - another key ingredient of baryogenesis - from quark Yukawa couplings which vary during the phase transition. For one such an option, with a varying charm quark Yukawa coupling, we perform a full numerical analysis of tunnelling in the Higgs-dilaton potential and determine regions of parameter space which allow for successful baryogenesis. This scenario singles out the light dilaton region while satisfying all experimental bounds. We discuss future tests. Our results bring new opportunities and strong motivations for electroweak baryogenesis.

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arXiv2018

A warped relaxion

We construct a UV completion of the relaxion in a warped extra dimension. We identify the relaxion with the zero mode of the fifth component of a bulk gauge field and show how hierarchically different decay constants for this field can be achieved by different localizations of anomalous terms in the warped space. This framework may also find applications for other axion-like fields. The cutoff of the relaxion model is identified as the scale of the IR brane where the Higgs lives, which can be as high as $10^6$ GeV, while above this scale warping takes over in protecting the Higgs mass.

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arXiv2018

The Baryon Asymmetry from a Composite Higgs

We study the nature of the electroweak phase transition (EWPT) in models where the Higgs emerges as a pseudo-Nambu-Goldstone boson of an approximate global symmetry of a new strongly-interacting sector confining around the TeV scale. Our analysis focusses for the first time on the case where the EWPT is accompanied by the confinement phase transition of the strong sector. We describe the confinement in terms of the dilaton, the pseudo-Nambu-Goldstone boson of spontaneously broken conformal invariance of the strong sector. The dilaton can either be a meson-like or a glueball-like state and we demonstrate a significant qualitative difference in their dynamics. We show that the EWPT can naturally be strongly first-order, due to the nearly-conformal nature of the dilaton potential. Furthermore, we examine the sizeable scale variation of the Higgs potential parameters during the EWPT. In particular, we consider in detail the case of a varying top quark Yukawa coupling, and show that the resulting CP violation is sufficient for successful electroweak baryogenesis. We demonstrate that this source of CP violation is compatible with existing flavour and CP constraints. Our scenario can be tested in complementary ways: by measuring the CP-odd top Yukawa coupling in electron EDM experiments, by searching for dilaton production and deviations in Higgs couplings at colliders, and through gravitational waves at LISA.

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arXiv2017

QCD-induced Electroweak Phase Transition

Phase transitions associated with nearly conformal dynamics are known to lead to significant supercooling. A notorious example is the phase transition in Randall-Sundrum models or their CFT duals. In fact, it was found that the phase transition in this case is first-order and the tunneling probability for the radion/dilaton is so small that the system typically remains trapped in the false vacuum and the phase transition never completes. The universe then keeps expanding and cooling. Eventually the temperature drops below the QCD scale. We show that the QCD condensates which subsequently form give an additional contribution to the radion/dilaton potential, an effect which had been ignored so far. This significantly reduces the barrier in the potential and allows the phase transition to complete in a substantially larger region of parameter space. Due to the supercooling, electroweak symmetry is then broken simultaneously. This class of models therefore naturally leads to an electroweak phase transition taking place at or below QCD temperatures, with interesting cosmological implications and signatures.

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arXiv2016

Cosmological evolution of Yukawa couplings: the 5D perspective

The cosmological evolution of standard model Yukawa couplings may have major implications for baryogenesis. In particular, as highlighted recently, the CKM matrix alone could be the source of CP-violation during electroweak baryogenesis provided that the Yukawa couplings were large and varied during the electroweak phase transition. We provide a natural realisation of this idea in the context of Randall-Sundrum models and show that the geometrical warped approach to the fermion mass hierarchy may naturally display the desired cosmological dynamics. The key ingredient is the coupling of the Goldberger-Wise scalar, responsible for the IR brane stabilisation, to the bulk fermions, which modifies the fermionic profiles. This also helps alleviating the usually tight constraints from CP-violation in Randall-Sundrum scenarios. We study how the Yukawa couplings vary during the stabilisation of the Randall-Sundrum geometry and can thus induce large CP-violation during the electroweak phase transition. Using holography, we discuss the 4D interpretation of this dynamical interplay between flavour and electroweak symmetry breaking.

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arXiv2016

The Effective Bootstrap

We study the numerical bounds obtained using a conformal-bootstrap method - advocated in ref. [1] but never implemented so far - where different points in the plane of conformal cross ratios $z$ and $\bar z$ are sampled. In contrast to the most used method based on derivatives evaluated at the symmetric point $z=\bar z =1/2$, we can consistently "integrate out" higher-dimensional operators and get a reduced simpler, and faster to solve, set of bootstrap equations. We test this "effective" bootstrap by studying the 3D Ising and $O(n)$ vector models and bounds on generic 4D CFTs, for which extensive results are already available in the literature. We also determine the scaling dimensions of certain scalar operators in the $O(n)$ vector models, with $n=2,3,4$, which have not yet been computed using bootstrap techniques.

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arXiv2015

SUSY Implications from WIMP Annihilation into Scalars at the Galactic Centre

An excess in gamma-rays emanating from the galactic centre has recently been observed in the Fermi-LAT data. This signal can be interpreted as resulting from WIMP annihilation, with the spectrum well-fit by dark matter annihilating dominantly into either bottom-quark or Higgs pairs. Supersymmetric models provide a well-motivated framework to study the implications of this signal in these channels. With a neutralino dark matter candidate, the gamma-ray excess cannot be easily accommodated in the minimal supersymmetric model, which in any case requires tuning below the percent level to explain the observed Higgs mass. Instead we are naturally led to consider the next-to-minimal model with a singlet superfield. This not only allows for the annihilation channel into bottom-quark pairs to be implemented, but also provides new possibilities for annihilation into Higgs-pseudoscalar pairs. We show that the fit to the gamma-ray excess for the Higgs-pseudoscalar channel can be just as good as for annihilation into bottom-quark pairs. Moreover, in the parameter range of interest, the next-to-minimal supersymmetric model solves the mu-problem and can explain the 125 GeV Higgs mass with improved naturalness. We also consider an extension by adding a right-handed neutrino superfield with the right-handed sneutrino acting as a dark matter candidate. Interestingly, this allows for the annihilation into pseudoscalar pairs which also provide a good fit to the gamma-ray excess. Furthermore, in the case of a neutralino LSP, the late decay of a sneutrino NLSP can non-thermally produce the observed relic abundance. Finally, the WIMP annihilation into scalar pairs allows for the possibility of detecting the Higgs or pseudoscalar decay into two photons, providing a smoking-gun signal of the model.

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arXiv2015

Bound-state formation for thermal relic dark matter and unitarity

We show that the relic abundance of thermal dark matter annihilating via a long-range interaction, is significantly affected by the formation and decay of dark matter bound states in the early universe, if the dark matter mass is above a few TeV. We determine the coupling required to obtain the observed dark matter density, taking into account both the direct 2-to-2 annihilations and the formation of bound states, and provide an analytical fit. We argue that the unitarity limit on the inelastic cross-section is realized only if dark matter annihilates via a long-range interaction, and we determine the upper bound on the mass of thermal-relic dark matter to be about 197 (139) TeV for (non)-self-conjugate dark matter.

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arXiv2014

Bounds on OPE Coefficients in 4D Conformal Field Theories

We numerically study the crossing symmetry constraints in 4D CFTs, using previously introduced algorithms based on semidefinite programming. We study bounds on OPE coefficients of tensor operators as a function of their scaling dimension and extend previous studies of bounds on OPE coefficients of conserved vector currents to the product groups SO(N)xSO(M). We also analyze the bounds on the OPE coefficients of the conserved vector currents associated with the groups SO(N), SU(N) and SO(N)xSO(M) under the assumption that in the singlet channel no scalar operator has dimension less than four, namely that the CFT has no relevant deformations. This is motivated by applications in the context of composite Higgs models, where the strongly coupled sector is assumed to be a spontaneously broken CFT with a global symmetry.

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