arXiv subjects

Jaume Guasch

Publications and source records attributed to Jaume Guasch.

At least 19 recordsLinked to original sources

arXiv2022

Using Machine Learning techniques in phenomenological studies in flavour physics

An updated analysis of New Physics violating Lepton Flavour Universality, by using the Standard Model Effective Field Lagrangian with semileptonic dimension six operators at $Λ= 1\,\mathrm{TeV}$ is presented. We perform a global fit, by discussing the relevance of the mixing in the first generation. We use for the first time in this context a Montecarlo analysis to extract the confidence intervals and correlations between observables. Our results show that machine learning, made jointly with the SHAP values, constitute a suitable strategy to use in this kind of analysis.

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arXiv2022

Anomalies in B mesons decays: A phenomenological approach

The experimental measurements on flavour physics, in tension with Standard Model predictions, exhibit large sources of Lepton Flavour Universality violation. We perform an analysis of the effects of the global fits on the Wilson coefficients assuming a model independent effective Hamiltonian approach, by including a set of different scenarios in which the New Physics contributions to the Wilson coefficients are present in one, two or three of the Wilson coefficients at a time. We compare the results of the global fit with respect to two cases: the Standard Model and the more general case in which New Physics modifies three independent Wilson coefficients. The last mentioned scenario is the favoured one for explaining the tension between Standard Model predictions and B-physics anomalies, but a specific more restricted scenario can provide similar goodness with a smaller set of free parameters. A discussion of the implications of our analysis in leptoquark models is included.

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arXiv2021

Exploring B-physics anomalies at colliders

Several experimental measurements of $B$ meson decays, in tension with Standard Model predictions, exhibit large sources of Lepton Flavour Universality violation. We perform an analysis of the effects of the global fits to the Wilson coefficients assuming a model independent effective Hamiltonian approach, by including a proposal of different scenarios to include the New Physics contributions. Both the current fits at the LHC and the ILC projections are considered. We found that for a simultaneous analysis of predictions for the $R_{D^{(*)}}$ and $R_{K^{(*)}}$ observables, the scenarios with three non-universal Wilson coefficients are favoured.

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arXiv2021

Anomalies in B mesons decays: Present status and future collider prospects

The experimental measurements on flavour physics, in tension with Standard Model predictions, exhibit large sources of Lepton Flavour Universality violation. This note summarises an analysis of the effects of the global fits to the Wilson coefficients assuming a model independent effective Hamiltonian approach, by including a proposal of different scenarios to include the New Physics contributions. Additionally, we include an overview of the impact of the future generation of colliders in the field of B-meson anomalies.

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arXiv2019

Some results on Lepton Flavour Universality Violation

Motivated by recent experimental measurements on flavour physics, in tension with Standard Model predictions, we perform an updated analysis of New Physics violating Lepton Flavour Universality, by using the effective Lagrangian approach and in the Z' and S_3 leptoquark models. We explicitly analyze the impact of considering complex Wilson coefficients in the analysis of B-anomalies, by performing a global fit of R_K and R_K*0 observables, together with ΔMs and A_CP^mix. The inclusion of complex couplings provides a slightly improved global fit, and a marginally improved ΔMs prediction.

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arXiv2013

Effective squark/chargino/neutralino couplings: MadGraph implementation

We have included the effective description of squark interactions with charginos/neutralinos in the MadGraph MSSM model. This effective description includes the effective Yukawa couplings, and another logarithmic term which encodes the supersymmetry-breaking. We have performed an extensive test of our implementation analyzing the results of the partial decay widths of squarks into charginos and neutralinos obtained by using FeynArts/FormCalc programs and the new model file in MadGraph. We present results for the cross-section of top-squark production decaying into charginos and neutralinos.

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arXiv2009

Effective description of squark interactions

We propose an effective description of squark interactions with charginos/neutralinos. We recompute the strong corrections to squark partial decay widths, and compare the full one-loop computation with the effective description. The effective description includes the effective Yukawa couplings, and another logarithmic term which encodes the supersymmetry-breaking. The proposed effective couplings reproduce correctly the radiative-corrected partial decay widths of the squark decays into charginos and neutralinos in all relevant regions of the parameter space.

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arXiv2008

FCNC-induced heavy-quark events at the LHC from Supersymmetry

We analyze the production and subsequent decay of the neutral Higgs bosons of the MSSM into electrically neutral quark pairs qq'=bs,tc of different flavors at the LHC and compare with the direct FCNC production mechanisms. The cross-sections are computed in the unconstrained MSSM with minimal flavor-mixing sources and taking into account the stringent bounds from radiative B-meson decays. We extend the results previously found for these FCNC processes, which are singularly uncommon in the SM. Specifically, we report here on the SUSY-EW contribution of the Higgs-mediated FCNC cross-section into bs and tc final states and the SUSY-QCD and SUSY-EW contributions to bs-production. In this way, the complete map of MSSM predictions for the qq'-pairs produced at the LHC becomes available. The upshot is that the most favorable channels are: 1) the Higgs boson FCNC decays into bs, and 2) the direct production of tc pairs, both of them at the 1 pb level and mediated by SUSY-QCD effects. If, however, the latter are suppressed, we find a small SUSY-EW yield for the tc-production through Higgs decays but, at the same time, a cross-section of 0.1-1 pb for bs-production, which implies a significant number (10^4-10^5) of bs-pairs per 100 inverse femtobarn of integrated luminosity.

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arXiv2008

Triple Higgs boson production at the ILC within a generic Two-Higgs-Doublet Model

We present a study of triple Higgs boson (3H) production at the International Linear Collider (ILC) within the general Two-Higgs-Doublet Model (2HDM). We compute the production cross-sections at the leading-order for the 3H final states and find values up to sigma ~ 0.1 pb. This result represents a large enhancement with respect to the corresponding MSSM cross-sections, which stay typically at the level of sigma ~ 10^(-6) pb or less. Furthermore, since the 3H cross-sections in the general 2HDM can be of the order of the double Higgs production cross-sections, such 3H processes could be a competitive (if not the dominant) mechanism for Higgs boson production at the ILC. In practice, these 3H events could be identified through the tagging of 6 heavy-quark jet final states and, in this case, they would provide strong evidence of an extended Higgs boson sector -- likely of non-supersymmetric nature.

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arXiv2008

Production of single top-quark final states at the LHC from supersymmetric FCNC interactions

We discuss the production of single top-quark final states by direct supersymmetric flavor-changing interactions at the LHC. The total cross section pp(gg)->t\bar{c}+\bar{t}c is computed at the 1-loop order within the unconstrained MSSM. We prove that SUSY-QCD effects may furnish sizeable production rates amounting up to barely 10^5 t\bar{c}(c\bar{t}) events per 100 fb^{-1} of integrated luminosity, in full compliance with the stringent low-energy constraints from b->s gamma. Furthermore, we show that the cooperative SUSY-EW effects can be sizeable on their own, regardless of the SUSY-QCD contribution, with maximum production rates of the order of 10^3 events per 100 fb^{-1}. Owing to the fact that FCNC production of electrically neutral heavy-quark pairs is virtually absent within the SM, we conclude that the observation of such pp(gg)->t\bar{c}+\bar{t}c processes at the LHC could lead to evidence of new physics - of likely supersymmetric nature.

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arXiv2007

Single top-quark production by strong and electroweak supersymmetric flavor-changing interactions at the LHC

(Abridged) We report on a complete study of the single top-quark production by direct supersymmetric flavor-changing neutral-current (FCNC) processes at the LHC. The total cross section for pp(gg)->t\bar{c}+\bar{t}c is computed at the 1-loop order within the unconstrained Minimal Supersymmetric Standard Model (MSSM). The present study extends the results of the supersymmetric strong effects (SUSY-QCD), which were advanced by some of us in a previous work, and includes the computation of the full supersymmetric electroweak corrections (SUSY-EW). Our analysis of pp(gg)->t\bar{c}+\bar{t}c in the MSSM has been performed in correspondence with the stringent low-energy constraints from b->s gamma. In the most favorable scenarios, the SUSY-QCD contribution can give rise to production rates of around 10^5 events per 100 fb^{-1} of integrated luminosity. Furthermore, we show that there exist regions of the MSSM parameter space where the SUSY-EW correction becomes sizeable. In the SUSY-EW favored regions, one obtains lower, but still appreciable, event production rates that can reach the 10^3 level for the same range of integrated luminosity. We study also the possible reduction in the maximum event rate obtained from the full MSSM contribution if we additionally include the constraints from B^0_s-\bar{B}^0_s. In view of the fact that the FCNC production of heavy quark pairs of different flavors is extremely suppressed in the SM, the detection of a significant number of these events could lead to evidence of new physics -- of likely supersymmetric origin.

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arXiv2007

Triple Higgs boson production in the Linear Collider

Triple Higgs boson production (3H) may provide essential information to reconstruct the Higgs potential. We consider 3H-production in the International Linear Collider (ILC) both in the Minimal Supersymmetric Standard Model (MSSM) and in the general Two-Higgs-doublet Model (2HDM). We compute the total cross-section for the various 3H final states, such as H^+ H^- h^0, H^0 A^0 h^0, etc. and compare with the more traditional double Higgs (2H) boson production processes. While the cross-sections for the 2H final states lie within the same order of magnitude in both the MSSM and 2HDM, we find that for the 3H states the maximum 2HDM cross-sections, being of order 0.1 pb, are much larger than the MSSM ones which in most cases are of order 10^{-6} pb or less. Actually, the 3H processes could be the dominant mechanism for Higgs boson production in the 2HDM. Ultimately the origin of the remarkable enhancement of the 3H channels in the 2HDM case (for both type I and type II models) originates in the structure of the trilinear Higgs boson couplings. The extremely clean environment of the ILC should allow a relatively comfortable tagging of the three Higgs boson events. In view of the fact that the MSSM contribution is negligible, these events should manifest themselves mainly in the form of 6 heavy-quark jet final states. Some of these signatures could be spectacular, and in case of being detected would constitute strong evidence of an extended Higgs sector of non-supersymmetric origin.

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arXiv2006

Single top-quark production by direct supersymmetric flavor-changing neutral-current interactions at the LHC

Production of (electrically neutral) heavy-quark pairs, such as t{\bar c} and {\bar t}c, is extremely suppressed in the SM. In supersymmetric (SUSY) theories, such as the MSSM, the number of these events can be significantly enhanced thanks (mainly) to the FCNC couplings of gluinos. We compute the efficiency of this mechanism for FCNC production of heavy quarks at the LHC. We find that σ(pp\to t\bar{c}+\bar{t}c) can reach 1 pb, and therefore one can expect up to 10^{5} events per 100 fb^{-1} of integrated luminosity (with no counterpart in the SM). Their detection would be instant evidence of new physics, and could be a strong indication of underlying SUSY dynamics.

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arXiv2006

SUSY Higgs boson flavor-changing neutral currents at the LHC

We compute and analyze the Flavor-Changing Neutral Current (FCNC) interactions of Minimal Supersymmetric Standard Model Higgs bosons (h = h0, H0, A0) with heavy quarks (top and bottom), focusing on the strongly-interacting sector. We correlate the Higgs bosons production cross-section at the LHC with the FCNC decay branching ratios and find the maximum allowed values of the production rates, sigma(pp -> h -> qq') = sigma(pp -> h) x B(h -> qq') (qq'= tc or bs) after taking into account limits from low energy data on flavor-changing interactions. We single out the top channel, with a maximum production rate of sigma^max(pp -> h -> tc) =~ 10^-3 - 10^-2 pb, as the most promising FCNC channel to be detected at the LHC.

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arXiv2005

MW and sin^2θ_eff in Split SUSY: present and future expectations

We analyse the precision electroweak observables MW and sin^2θ_eff and their correlations in the recently proposed Split SUSY model. We compare the results with the Standard Model and Minimal Supersymmetric Standard Model predictions, and with present and future experimental accuracies. Present experimental accuracies in (MW, sin^2θ_eff) do not allow constraints to be placed on the Split SUSY parameter space. We find that the shifts in (MW, sin^2θ_eff) induced by Split SUSY can be larger than the anticipated accuracy of the GigaZ option of the International Linear Collider, and that the most sensitive observable is sin^2θ_eff. These large shifts are possible also for large chargino masses in scenarios with small tan(β) =~ 1.

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arXiv2005

Production and FCNC decay of supersymmetric Higgs bosons into heavy quarks in the LHC

We analyze the production and subsequent decay of the neutral MSSM Higgs bosons (h = h^0, H^0, A^0) mediated by flavor changing neutral currents (FCNC) in the LHC collider. We have computed the h-production cross-section times the FCNC branching ratio, σ(pp -> h -> qq') = σ(pp -> h) B(h -> qq'), in the LHC focusing on the strongly-interacting FCNC sector. Here qq' is an electrically neutral pair of quarks of different flavors, the dominant modes being those containing a heavy quark: tc or bs. We determine the maximum production rates for each of these modes and identify the relevant regions of the MSSM parameter space, after taking into account the severe restrictions imposed by low energy FCNC processes. The analysis of σ(pp -> h -> qq') singles out regions of the MSSM parameter space different from those obtained by maximizing only the branching ratio, due to non-trivial correlations between the parameters that maximize/minimize each isolated factor. The production rates for the bs channel can be huge for a FCNC process (0.1-1 pb), but its detection can be problematic. The production rates for the tc channel are more modest (10^{-3}-10^{-2} pb), but its detection should be easier due to the clear-cut top quark signature. A few thousand tc events could be collected in the highest luminosity phase of the LHC, with no counterpart in the SM.

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arXiv2004

Higgs Boson Flavor-Changing Neutral Decays into Bottom Quarks in Supersymmetry

We analyze the maximum branching ratios for the Flavor Changing Neutral Current (FCNC) decays of the neutral Higgs bosons of the Minimal Supersymmetric Standard Model (MSSM) into bottom quarks, h -> b\bar{s} (h=h^0,H^0,A^0). We consistently correlate these decays with the radiative B-meson decays (b-> sγ). A full-fledged combined numerical analysis is performed of these high-energy and low-energy FCNC decay modes in the MSSM parameter space. Our calculation shows that the available data on B(b->s γ) severely restricts the allowed values of B(h->b\bar{s}). While the latter could reach a few percent level in fine-tuned scenarios, the requirement of naturalness reduces these FCNC rates into the modest range B(h->b\bar{s}) ~ 10^{-4}-10^{-3}. We find that the bulk of the MSSM contribution to B(h->b\bar{s}) could originate from the strong supersymmetric sector. The maximum value of the FCNC rates obtained in this paper disagree significantly with recent (over-)estimates existing in the literature. Our results are still encouraging because they show that the FCNC modes h->b\bar{s} can be competitive with other Higgs boson signatures and could play a helpful complementary role to identify the supersymmetric Higgs bosons, particularly the lightest CP-even state in the critical LHC mass region m_{h^0} ~= 90-130 GeV.

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arXiv2003

Higgs Boson Flavor-Changing Neutral Decays into Top Quark in a General Two-Higgs-Doublet Model

Higgs boson decays mediated by flavor changing neutral currents (FCNC) are very much suppressed in the Standard Model, at the level of 10^{-15} for Higgs boson masses of a few hundred GeV. Therefore, any experimental vestige of them would immediately call for new physics. In this paper we consider the FCNC decays of Higgs bosons into a top quark in a general two-Higgs-doublet model (2HDM). The isolated top quark signature, unbalanced by any other heavy particle, should help to identify the potential FCNC events much more than any other final state. We compute the maximum branching ratios and the number of FCNC Higgs boson decay events at the LHC collider at CERN. The most favorable mode for production and subsequent FCNC decay is the lightest CP-even state in the Type II 2HDM, followed by the other CP-even state, if it is not very heavy, whereas the CP-odd mode can never be sufficiently enhanced. Our calculation shows that the branching ratios of the CP-even states may reach 10^{-5}, and that several hundred events could be collected in the highest luminosity runs of the LHC. We also point out some strategies to use these FCNC decays as a handle to discriminate between 2HDM and supersymmetric Higgs bosons.

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