arXiv subjects

Gilberto Tetlalmatzi-Xolocotzi

Publications and source records attributed to Gilberto Tetlalmatzi-Xolocotzi.

33 records · Page 2Linked to original sources

arXiv2021

Mapping out the Space for New Physics with Leptonic and Semileptonic $B_{(c)}$ Decays

Decays of $B$ mesons with leptons in the final state offer an interesting laboratory to search for possible effects of physics from beyond the Standard Model. In view of puzzling patterns in experimental data, the violation of lepton flavour universality is an interesting option. We present a strategy, utilising ratios of leptonic and semileptonic $B$ decays, where the elements $|V_{ub}|$ and $|V_{cb}|$ of the Cabibbo--Kobayashi--Maskawa (CKM) matrix cancel, to constrain the short-distance coefficients of (pseudo)-scalar, vector and tensor operator contributions. The individual branching ratios allow us then to extract also the CKM matrix elements, even in the presence of new-physics contributions. Bounds on unmeasured leptonic and semileptonic decays offer important additional constraints. In our comprehensive analysis, we give also predictions for decays which have not yet been measured in a variety of scenarios.

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arXiv2021

Triple Higgs Boson Production at the Large Hadron Collider with Two Real Singlet Scalars

We investigate the production of three Higgs bosons in the Two Real Singlet extension of the Standard Model, where the scalar sector is augmented by two additional real scalar fields which are singlets under the Standard Model gauge group. The model contains three neutral CP-even scalars, allowing for resonant production and asymmetric decay chains. We focus on the signature $p p\,\rightarrow\,h_3\,\rightarrow\,h_1\,h_2\,\rightarrow\,h_1\,h_1\,h_1$, where we identify $h_3$ as the heaviest scalar state, $h_2$ as the second heaviest and the lightest, $h_1$, as the Standard Model-like Higgs boson discovered by the Large Hadron Collider experiments. The dominant final state occurs when all three Higgs bosons decay to bottom-anti-bottom quark pairs, $h_1\,\rightarrow\,b\,\bar{b}$, leading to 6 $b$-jets. Taking into account all current theoretical and experimental constraints, we determine the discovery prospects for this channel in future runs of the Large Hadron Collider, as well as in the high-luminosity phase.

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arXiv2020

Model-independent bounds on new physics effects in non-leptonic tree-level decays of B-mesons

We present a considerably improved analysis of model-independent bounds on new physics effects in non-leptonic tree-level decays of B-mesons. Our main finding is that contributions of about $\pm 0.1 $ to the Wilson coefficient of the colour-singlet operator $Q_2$ of the effective weak Hamiltonian and contributions in the range of $\pm 0.5$ (both for real and imaginary part) to $Q_1$ can currently not be excluded at the $90\%$ C.L.. Effects of such a size can modify the direct experimental extraction of the CKM angle $γ$ by up to $10^{\circ}$ and they could lead to an enhancement of the decay rate difference $ΔΓ_d$ of up to a factor of 5 over its SM value - a size that could explain the D0 dimuon asymmetry. Future more precise measurements of the semi-leptonic asymmetries $a_{sl}^q$ and the lifetime ratio $τ(B_s) / τ(B_d)$ will allow to shrink the bounds on tree-level new physics effects considerably. Due to significant improvements in the precision of the non-perturbative input we update all SM predictions for the mixing obervables in the course of this analysis, obtaining: $ΔM_s = (18.77 \pm 0.86 ) \, \mbox{ps}^{-1}$, $ΔM_d = (0.543 \pm 0.029) \, \mbox{ps}^{-1}$, $ΔΓ_s = (9.1 \pm 1.3 ) \cdot 10^{-2} \, \mbox{ps}^{-1}$, $ΔΓ_d = (2.6 \pm 0.4 ) \cdot 10^{-3} \, \mbox{ps}^{-1}$, $a_{sl}^s = (2.06 \pm 0.18) \cdot 10^{-5}$ and $a_{sl}^d = (-4.73 \pm 0.42) \cdot 10^{-4}$.

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arXiv2019

Triple Higgs boson production to six $b$-jets at a 100 TeV proton collider

We investigate the production of three Higgs bosons at a proton-proton collider running at a centre-of-mass energy of 100 TeV, all of which decay into $b$-jets. This final state encapsulates by far the largest fraction of the total cross section of triple Higgs boson production, approximately $20\%$. We examine, by constructing detailed phenomenological analyses, two scenarios: (i) one in which the triple and quartic Higgs boson self-couplings are modified independently by new phenomena with respect to their Standard Model (SM) values and (ii) an extension of the SM by a gauge-singlet scalar that could drive first-order electroweak phase transition, within the context of the so-called xSM. In the former, we find that competitive constraints of $\mathcal{O}(1)$ can be placed on the quartic coupling and in the latter we demonstrate that it will be possible to obtain important information on the structure of the extended scalar sector.

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arXiv2018

Probing Lepton Universality with (Semi)-Leptonic B decays

The most recent measurements of the observables $R_{D^{(*)}}$ are in tension with the Standard Model offering hints of New Physics in $b\rightarrow c \ell \barν_{\ell}$ transitions. Motivated by these results, in this work we present an analysis on their $b\rightarrow u \ell \barν_{\ell}$ counterparts (for $\ell=e, ~μ, ~τ$). Our study has three main objectives. Firstly, using ratios of branching fractions, we assess the effects of beyond the Standard Model scalar and pseudoscalar particles in leptonic and semileptonic $B$ decays ($B^-\rightarrow \ell^- \barν_{\ell}$, $\bar{B}\rightarrow π\ell \barν_{\ell}$ and $\bar{B}\rightarrow ρ\ell \barν_{\ell}$). Here a key role is played by the leptonic $B$ processes, which are highly sensitive to new pseudoscalar interactions. In particular, we take advantage of the most recent measurement of the branching fraction of the channel $B^-\rightarrow μ^-\barν_μ$ by the Belle collaboration. Secondly, we extract the CKM matrix element $|V_{ub}|$ while accounting simultaneously for New Physics contributions. Finally, we provide predictions for the branching fractions of yet unmeasured leptonic and semileptonic $B$ decays.

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arXiv2018

Decoding (Pseudo)-Scalar Operators in Leptonic and Semileptonic $B$ Decays

We consider leptonic $B^-\to \ell^- \barν_\ell$ and semileptonic $\bar B \to π\ell^- \barν_\ell$, $\bar B \to ρ\ell^- \barν_\ell$ decays and present a strategy to determine short-distance coefficients of New-Physics operators and the CKM element $|V_{ub}|$. As the leptonic channels play a central role, we illustrate this method for (pseudo)-scalar operators which may lift the helicity suppression of the corresponding transition amplitudes arising in the Standard Model. Utilising a new result by the Belle collaboration for the branching ratio of $B^-\to μ^- \barν_μ$, we explore theoretically clean constraints and correlations between New Physics coefficients for leptonic final states with $μ$ and $τ$ leptons. In order to obtain stronger bounds and to extract $|V_{ub}|$, we employ semileptonic $\bar B \to π\ell^- \barν_\ell$ and $\bar B \to ρ\ell^- \barν_\ell$ decays as an additional ingredient, involving hadronic form factors which are determined through QCD sum rule and lattice calculations. In addition to a detailed analysis of the constraints on the New Physics contributions following from current data, we make predictions for yet unmeasured decay observables, compare them with experimental constraints and discuss the impact of CP-violating phases of the New-Physics coefficients.

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arXiv2018

Rare Leptonic B decays

The transitions $B_{s,d}\rightarrow \ell^+\ell^-$ (for $\ell=e, μ, τ$), although extremely suppressed within the SM, are exceptionally clean channels particularly sensitive to New Physics contributions. So far only the branching ratio for the process $B_{s}\rightarrow μ^+μ^-$ has been measured experimentally and, although the results are in agreement with the SM prediction, there is still room for NP effects. Here we provide an overview of the theoretical status of rare decays. We present strategies that can allow us to discriminate between different NP scenarios. Finally, we include a discussion of the effects of nontrivial new CP violating phases on the observables associated with $B_{s}\rightarrow μ^+μ^-$.

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arXiv2018

On the ultimate precision of meson mixing observables

Meson mixing is considered to be an ideal candidate for new physics searches. Current experimental precision has greatly increased over the recent years, excelling in several cases the theoretical precision. A possible limit in the theoretical accuracy could be a hypothetical breakdown of quark-hadron duality. We propose a simple model for duality violations and give stringent bounds on such effects for mixing observables, indicating regions, where future measurements of $ΔΓ_d$, $a_{sl}^d$ and $a_{sl}^s$ would be clear signals of new physics. Finally, we turn our attention to the charm sector, and reveal that already a modest duality violation of about $20 \%$ could explain the huge difference between HQE predictions for D-mixing and experimental data.

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arXiv2018

Rare top quark decays at a 100 TeV proton-proton collider: $t \rightarrow bWZ$ and $t\rightarrow hc$

We investigate extremely rare top quark decays at a future proton-proton collider with centre-of-mass energy of 100 TeV. We focus on two decay modes: radiative decay with a $Z$ boson, $t \rightarrow b WZ$, and flavour-changing neutral decay with a Higgs boson, $t \rightarrow h c$, the former being kinematically suppressed with a branching ratio of $\mathcal{O}(10^{-6})$, and the latter highly loop-suppresed, with a branching ratio of $\mathcal{O}(10^{-15})$. We find that $t \rightarrow b WZ$ will be very challenging to observe in top quark pair production, even within well-motivated beyond-the-Standard Model scenarios. For the mode $t\rightarrow h c$ we find a stronger sensitivity than that obtained by any future LHC measurement by at least one order of magnitude.

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arXiv2017

CP Violation in Leptonic Rare $B^0_s$ Decays as a Probe of New Physics

The decay $B^0_s\toμ^+μ^-$ is a key probe for the search of physics beyond the Standard Model. While the current measurements of the corresponding branching ratio agree with the Standard Model within the uncertainties, significant New-Physics effects may still be hiding in $B^0_s\toμ^+μ^-$. In order to reveal them, the observable $\mathcal{A}^{μμ}_{ΔΓ_s}$, which is provided by the decay width difference $ΔΓ_s$ of the $B^0_s$-meson system, plays a central role. We point out that a measurement of a CP-violating observable ${\cal S}_{\rm μμ}$, which is induced through interference between $B^0_s$-$\bar B^0_s$ mixing and $B_s\toμ^+μ^-$ decay processes, is essential to obtain the full picture, in particular to establish new scalar contributions and CP-violating phases. We illustrate these findings with future scenarios for the upgrade(s) of the LHC, exploiting also relations which emerge within an effective field theory description of the Standard Model, complemented with New Physics entering significantly beyond the electroweak scale.

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arXiv2017

Double-charming Higgs identification using machine-learning assisted jet shapes

We study the possibility of identifying a boosted resonance that decays into a charm pair against different sources of background using QCD event shapes, which are promoted to jet shapes. Using a set of jet shapes as input to a boosted decision tree, we find that observables utilizing the simultaneous presence of two charm quarks can access complementary information compared to approaches relying on two independent charm tags. Focusing on Higgs associated production with subsequent $H\to c \bar{c}$ decay and on a CP-odd scalar $A$ with $m_A \leq 10$ GeV we obtain the limits $\mathcal{B}r(H\rightarrow c\bar{c})\leq 6.09\%$ and $\mathcal{B}r(H\rightarrow A(\rightarrow c\bar{c}) Z)\leq 0.01\%$ at $95\%$ C. L..

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arXiv2017

In Pursuit of New Physics with $B^0_{s,d}\to\ell^+\ell^-$

Leptonic rare decays of $B^0_{s,d}$ mesons offer a powerful tool to search for physics beyond the Standard Model. The $B^0_{s}\toμ^+μ^-$ decay has been observed at the Large Hadron Collider and the first measurement of the effective lifetime of this channel was presented, in accordance with the Standard Model. On the other hand, $B^0_{s}\toτ^+τ^-$ and $B^0_{s}\to e^+e^-$ have received considerably less attention: while LHCb has recently reported a first upper limit of $6.8\times10^{-3}$ (95% C.L.) for the $B^0_s\toτ^+τ^-$ branching ratio, the upper bound $2.8\times 10^{-7}$ (90% C.L.) for the branching ratio of $B^0_s\to e^+e^-$ was reported by CDF back in 2009. We discuss the current status of the interpretation of the measurement of $B^0_{s}\toμ^+μ^-$, and explore the space for New-Physics effects in the other $B^0_{s,d}\to\ell^+\ell^-$ decays in a scenario assuming flavour-universal Wilson coefficients of the relevant four-fermion operators. While the New-Physics effects are then strongly suppressed by the ratio $m_μ/m_τ$ of the lepton masses in $B^0_s\toτ^+τ^-$, they are hugely enhanced by $m_μ/m_e$ in $B^0_s\to e^+e^-$ and may result in a $B^0_s\to e^+e^-$ branching ratio as large as about 5 times the one of $B^0_{s}\toμ^+μ^-$, which is about a factor of 20 below the CDF bound; a similar feature arises in $B^0_{d}\to e^+e^-$. Consequently, it would be most interesting to search for the $B^0_{s,d}\to e^+e^-$ channels at the LHC and Belle II, which may result in an unambiguous signal for physics beyond the Standard Model.

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arXiv2015

On new physics in $ΔΓ_d$

Motivated by the recent measurement of the dimuon asymmetry by the DØ collaboration, which could be interpreted as an enhanced decay rate difference in the neutral $B_d$-meson system, we investigate the possible size of new-physics contributions to $ΔΓ_d$. In particular, we perform model-independent studies of non-standard effects associated to the dimension-six current-current operators $(\bar{d} p)(\bar p^{\hspace{0.25mm}\prime} b)$ with $p,p^\prime= u,c$ as well as $(\bar{d}b) (\barττ)$. In both cases we find that for certain flavour or Lorentz structures of the operators sizable deviations of $ΔΓ_d$ away from the Standard Model expectation cannot be excluded in a model-independent fashion.

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arXiv2015

New physics in $ΔΓ_d$

We analyze the possibility of having new physics effects in the decay rate difference, $ΔΓ_d$, of neutral $B_d$ mesons. Three different sources of enhancement are considered, CKM unitarity violations, beyond standard model effects in the tree-level dimension six operators $(\bar{d}p)(\bar{p}'b)$ with $p,p'=u,c$; and large enhancements of the almost unconstrained operators $(\bar{d}b)(\barττ)$. We find that deviations of several hundred per cent from the standard model prediction of $ΔΓ_d$ are not excluded by current experimental data.

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arXiv2014

New physics effects in tree-level decays

We critically review the assumption that no new physics is acting in tree-level $B$-meson decays and study the consequences for the ultimate precision in the direct determination of the CKM angle $γ$. In our exploratory study we find that sizable universal new physics contributions, $ΔC_{1,2}$, to the tree-level Wilson coefficients $C_{1,2}$ of the effective Hamiltonian describing weak decays of the $b$ quark are currently not excluded by experimental data. In particular we find that Im $ΔC_1 $ and Im $ΔC_2 $ can easily be of order $\pm 10%$ without violating any constraints from data. Such a size of new physics effects in $C_1$ and $C_2$ corresponds to an intrinsic uncertainty in the CKM angle $γ$ of the order of $|δγ| \approx 4^\circ$, which is slightly below the current experimental precision. The accuracy in the determination of $γ$ can be improved by putting stronger constraints on the tree-level Wilson coefficients, in particular $C_1$. To this end we suggest a more refined theoretical study as well as a more precise measurements of the observables that currently provide the strongest bounds on hypothetical new weak phases in $C_1$ and $C_2$. We note that the semi-leptonic CP asymmetries seem to have the best prospect for improving the bound on the weak phase in $C_1$.

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