arXiv subjects

Mikael Chala

Publications and source records attributed to Mikael Chala.

At least 37 records · Page 2Linked to original sources

arXiv2022

Theory Techniques for Precision Physics -- Snowmass 2021 TF06 Topical Group Report

The wealth of experimental data collected at laboratory experiments suggests that there is some scale separation between the Standard Model (SM) and phenomena beyond the SM (BSM). New phenomena can manifest itself as small corrections to SM predictions, or as signals in processes where the SM predictions vanish or are exceedingly small. This makes precise calculations of the SM expectations essential, in order to maximize the sensitivity of current and forthcoming experiments to BSM physics. This topical group report highlights some past and forthcoming theory developments critical for maximizing the sensitivity of the experimental program to understanding Nature at the shortest distances.

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arXiv2022

Towards the renormalisation of the Standard Model effective field theory to dimension eight: Bosonic interactions I

We compute the one-loop renormalisation group running of the bosonic Standard Model effective operators to order $v^4/Λ^4$, with $v\sim 246$ GeV being the electroweak scale and $Λ$ the unknown new physics threshold. We concentrate on the effects triggered by pairs of the leading dimension-six interactions, namely those that can arise at tree level in weakly-coupled ultraviolet completions of the Standard Model. We highlight some interesting consequences, including the interplay between positivity bounds and the form of the anomalous dimensions; the non renormalisation of the $S$ and $U$ parameters; or the importance of radiative corrections to the Higgs potential for the electroweak phase transition. As a byproduct of this work, we provide a complete Green basis of operators involving only the Higgs and derivatives at dimension-eight, comprising 13 redundant interactions.

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arXiv2022

Theoretical developments in the SMEFT at dimension-8 and beyond

In this contribution to the Snowmass 2021 process we review theoretical developments in the Standard Model Effective Field Theory (SMEFT) with a focus on effects at the dimension-8 level and beyond. We review the theoretical advances that led to the complete construction of the operator bases for the dimension-8 and dimension-9 SMEFT Lagrangians. We discuss the possibility of obtaining all-orders results in the $1/Λ$ expansion for certain SMEFT observables as well as the current status of renormalization group running and implications for positivity, and briefly present the on-shell approach to constructing SMEFT amplitudes. Finally we present several new phenomenological effects that first arise at dimension-8 and discuss the impact of these terms on experimental analyses.

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arXiv2022

New Higgs decays to axion-like particles

We investigate the interactions of a light scalar with the Higgs boson and second-generation fermions, which trigger new rare decays of the Higgs boson into $4μ$, $2μ2γ$, $6μ$ and $4μ2j$. We recast current LHC searches to constrain these decays and develop new collider analyses for those channels which are only poorly tested by existing studies. With the currently collected data we can probe branching ratios as small as $1.5 \times10^{-5}$, $8.7 \times10^{-5}$, $5.7 \times10^{-8}$ and $1.6 \times10^{-7}$, respectively. For the High-Luminosity LHC run, considered here to involve 3 ab$^{-1}$ of integrated luminosity, these numbers go down to $1.3 \times10^{-5}$, $2.0 \times10^{-6}$, $3.0 \times10^{-9}$ and $5.4 \times10^{-9}$, respectively. We also comment on other channels that remain still unexplored.

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arXiv2021

A Green's basis for the bosonic SMEFT to dimension 8

We present a basis of dimension-eight Green's functions involving Standard Model (SM) bosonic fields, consisting of 86 new operators. Rather than using algebraic identities and integration by parts, we prove the independence of these interactions in momentum space, including a discussion on evanescent bosonic operators. Our results pave the way for renormalising the SM effective field theory (SMEFT), as well as for performing matching of ultraviolet models onto the SMEFT, to higher order. To demonstrate the potential of our construction, we have implemented our basis in matchmakereft and used it to integrate out a heavy singlet scalar and a heavy quadruplet scalar up to one loop. We provide the corresponding dimension-eight Wilson coefficients. Likewise, we show how our results can be easily used to simplify cumbersome redundant Lagrangians arising, for example, from integrating out heavy fields using the path-integral approach to matching.

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arXiv2021

Positivity bounds in the Standard Model effective field theory beyond tree level

Focusing on four-Higgs interactions, we analyse the robustness of tree-level-derived positivity bounds on Standard Model effective field theory (SMEFT) operators under quantum corrections. Among other results, we demonstrate that: (i) Even in the simplest extensions of the Standard Model, e.g. with one new scalar singlet or with a neutral triplet, some positivity bounds are strictly violated; (ii) the mixing of the dimension-eight operators under renormalisation, which we compute here for the first time, can drive them out of their positivity region; (iii) the running of the dimension-eight interactions triggered by solely dimension-six terms respects the positivity bounds. Our results suggest, on one hand, that departures from positivity within the SMEFT, if ever found in the data, do not necessarily imply the breaking of unitarity or causality, nor the presence of new light degrees of freedom. On the other hand, they lead to strong constraints on the form of certain anomalous dimensions.

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arXiv2021

Neutrino masses in the Standard Model effective field theory

We compute the leading-logarithmic correction to the neutrino mass matrix in the Standard Model effective field theory (SMEFT) to dimension seven. In the limit of negligible lepton and down-type quark Yukawa couplings, it receives contributions from the Weinberg dimension-five operator as well as from 11 dimension-six and five dimension-seven independent interactions. Two of the main implications we derive from this result are the following. First, we find dimension-seven operators which, despite violating lepton number, do not renormalise neutrino masses at one loop. And second, we demonstrate that the presence of dimension-six operators around the TeV scale can modify the Standard Model prediction by up to $\mathcal{O}(50\,\%)$. Our result comprises also one step forward towards the renormalisation of the SMEFT to order $v^3/Λ^3$.

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arXiv2021

Detecting gravitational waves from cosmological phase transitions with LISA: an update

We investigate the potential for observing gravitational waves from cosmological phase transitions with LISA in light of recent theoretical and experimental developments. Our analysis is based on current state-of-the-art simulations of sound waves in the cosmic fluid after the phase transition completes. We discuss the various sources of gravitational radiation, the underlying parameters describing the phase transition and a variety of viable particle physics models in this context, clarifying common misconceptions that appear in the literature and identifying open questions requiring future study. We also present a web-based tool, PTPlot, that allows users to obtain up-to-date detection prospects for a given set of phase transition parameters at LISA.

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arXiv2020

Novel flavour-changing neutral currents in the top quark sector

We demonstrate that flavour-changing neutral currents in the top sector, mediated by leptophilic scalars at the electroweak scale, can easily arise in scenarios of new physics, and in particular in composite Higgs models. We moreover show that such interactions are poorly constrained by current experiments, while they can be searched for at the LHC in rare top decays and, more generally, in the channels $pp\to tS(S)+j$, with $S\to\ell^+\ell^-$. We provide dedicated analyses in this respect, obtaining that cut-off scales as large as $Λ\sim$ 90 TeV can be probed with an integrated luminosity of $\mathcal{L} = 150$ fb$^{-1}$.

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arXiv2020

One-loop running of dimension-six Higgs-neutrino operators and implications of a large neutrino dipole moment

We compute the one-loop running of the dimension-six CP-even Higgs operators in the Standard Model effective field theory involving the right-handed component of the would-be Dirac neutrinos. We discuss the implications of a large Dirac neutrino magnetic dipole moment. In particular, we demonstrate that a neutrino magnetic moment explaining the recent XENON1T excess induces Higgs and $Z$ invisible decays with branching ratios in the range $[10^{-18}, 10^{-12}]$. These numbers are unfortunately beyond the reach of current and near future facilities.

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arXiv2020

Effective field theory for vector-like leptons and its collider signals

We argue that in models with several high scales; e.g. in composite Higgs models or in gauge extensions of the Standard Model (SM), vector-like leptons can be likely produced in a relatively large $\sqrt{s}$ region of the phase space. Likewise, they can easily decay into final states not containing SM gauge bosons. This contrasts with the topology in which these new particles are being searched for at the LHC. Adopting an effective field theory approach, we show that searches for excited leptons must be used instead to test this scenario. We derive bounds on all the relevant interactions of dimension six; the most constrained ones being of about $0.05$ TeV$^{-2}$. We build new observables to improve current analyses and study the impact on all single-field UV completions of the SM extended with a vector-like lepton that can be captured by the effective field theory at tree level, in the current and in the high-luminosity phase of the LHC.

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arXiv2020

The effective field theory of low scale see-saw at colliders

We study the Standard Model effective field theory ($ν$SMEFT) extended with operators involving right-handed neutrinos, focussing on the regime where the right-handed neutrinos decay promptly on collider scales to a photon and a Standard Model neutrino. This scenario arises naturally for right-handed neutrinos with masses of the order $m_N \sim 0.1 \dots 10\, \text{GeV}$. We limit the relevant dimension-six operator coefficients using LEP and LHC searches with photons and missing energy in the final state as well as pion and tau decays. While bounds on new physics contributions are generally in the TeV scale for order one operator coefficients, some coefficients, however, remain very poorly constrained or even entirely evade bounds from current data. Consequently, we identify such weakly constrained scenarios and propose new searches for rare top and tau decays involving photons to probe potential new physics in the $ν$SMEFT parameter space. Our analysis highlights the importance of performing dedicated searches for new rare tau and top decays.

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arXiv2020

One-loop matching in the SMEFT extended with a sterile neutrino

We study the phenomenology of the simplest renormalisable model that, at low energy, leads to the effective field theory of the Standard Model extended with right-handed neutrinos ($ν$SMEFT). Our aim is twofold. First, to contextualise new collider signatures in models with sterile neutrinos so far studied only using the bottom-up approach. And second and more important, to provide a thorough example of one-loop matching in the diagrammatic approach, of which other matching techniques and automatic tools can benefit for cross-checks. As byproducts of this work, we provide for the first time: (i) a complete off-shell basis for the $ν$SMEFT and explicit relations between operators linked by equations of motion; (ii) a complete basis for the low-energy effective field theory ($ν$LEFT) and the tree-level matching onto the $ν$SMEFT; (iii) partial one-loop anomalous dimensions in the $ν$LEFT. This way, our work comprises a new step forward towards the systematisation of one-loop computations in effective field theories, especially if the SM neutrinos are Dirac.

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arXiv2019

Mapping the shape of the scalar potential with gravitational waves

We study the dependence of the observable stochastic gravitational wave background induced by a first-order phase transition on the global properties of the scalar effective potential in particle physics. The scalar potential can be that of the Standard Model Higgs field, or more generally of any scalar field responsible for a spontaneous symmetry breaking in beyond-the-Standard-Model settings thatprovide for a first-order phase transition in the early universe.Characteristics of the effective potential include the relative depth of the true minimum ($E_α^4$), the height of the barrier that separates it from the false one ($E_m^4$) and the separation between the two minima in field space ($v$), all at the bubble nucleation temperature. We focus on a simple yet quite general class of single-field polynomial potentials, with parameters being varied over several orders of magnitude. It is then shown that gravitational wave observatories such as aLIGO O5, BBO, DECIGO and LISA are mostly sensitive to values of these parameters in the region $E_α\sim (0.1-10) \times E_m$. Finally, relying on well-defined models and using our framework, we demonstrate how to obtain the gravitational wave spectra for potentials of various shapes without necessarily relying on dedicated software packages.

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arXiv2019

Higgs phenomenology as a probe of sterile neutrinos

Physics beyond the Standard Model can manifest itself as both new light states and heavy degrees of freedom. In this paper, we assume that the former comprise only a sterile neutrino, $N$. Therefore, the most agnostic description of the new physics is given by an effective field theory built upon the Standard Model fields as well as $N$. We show that Higgs phenomenology provides a sensitive and potentially crucial tool to constrain effective gauge interactions of sterile neutrinos, not yet probed by current experiments. In parallel, this motivates a range of new Higgs decay channels with clean signatures as candidates for the next LHC runs, including $h\to γ+p_T^\text{miss}$ and $h\to γγ+p_T^\text{miss}$.

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arXiv2019

$ΔA_{CP}$ within the Standard Model and beyond

In light of the recent LHCb observation of CP violation in the charm sector, we review standard model (SM) predictions in the charm sector and in particular for $ΔA_{CP}$. We get as an upper bound in the SM $| ΔA_{CP} ^{\rm SM}| \leq 3.6 \times 10^{-4}$, which can be compared to the measurement of $ΔA_{CP} ^{\rm LHCb2019} = (-15.4 \pm 2.9) \times 10^{-4}$. We discuss resolving this tension within an extension of the SM that includes a flavour violating $Z'$ that couples only to $\bar{s}s$ and $\bar{c}u$. We show that for masses below 80 GeV and flavour violating coupling of the order of $10^{-4}$, this model can successfully resolve the tension and avoid constraints from dijet searches, $D^0-\overline{D}^0$ mixing and measurements of the $Z$ width.

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arXiv2019

Probes of the Standard Model effective field theory extended with a right-handed neutrino

If neutrinos are Dirac particles and, as suggested by the so far null LHC results, any new physics lies at energies well above the electroweak scale, the Standard Model effective field theory has to be extended with operators involving the right-handed neutrinos. In this paper, we study this effective field theory and set constraints on the different dimension-six interactions. To that aim, we use LHC searches for associated production of light (and tau) leptons with missing energy, monojet searches, as well as pion and tau decays. Our bounds are generally above the TeV for order one couplings. One particular exception is given by operators involving top quarks. These provide new signals in top decays not yet studied at colliders. Thus, we also design an LHC analysis to explore these signatures in the $t\overline{t}$ production. Our results are also valid if the right-handed neutrinos are Majorana and long-lived.

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arXiv2019

Novel $B$-decay signatures of light scalars at high energy facilities

We study the phenomenology of light scalars of masses $m_1$ and $m_2$ coupling to heavy flavour-violating vector bosons of mass $m_V$. For $m_{1,2}\lesssim $ few GeV, this scenario triggers the rare $B$ meson decays $B_s^0\to 3μ^+ 3μ^-$, $B^0\to 3μ^+ 3μ^-$, $B^+\to K^+ 3μ^+ 3μ^-$ and $B_s^0\to K^{0*} 3μ^+ 3μ^-$; the last two being the most important ones for $m_1\sim m_2$. None of these signals has been studied experimentally; therefore we propose analyses to test these channels at the LHCb. We demonstrate that the reach of this facility extends to branching ratios as small as $6.0\times 10^{-9}$, $1.6\times 10^{-9}$, $5.9\times 10^{-9}$ and $1.8\times 10^{-8}$ for the aforementioned channels, respectively. For $m_{1,2}\gg \mathcal{O}(1)$ GeV, we show that slightly modified versions of current multilepton and multitau searches at the LHC can probe wide regions of the parameter space of this scenario. Altogether, the potential of the searches we propose outperform other constraints such as those from meson mixing.

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