arXiv subjects

P. Roig

Publications and source records attributed to P. Roig.

At least 37 records · Page 2Linked to original sources

arXiv2017

A broad 750 GeV diphoton resonance? Not alone

The hint for a possible resonance in the diphoton channel with mass of 750~GeV disappeared in the data presented at ICHEP'16 by ATLAS and CMS. However, the diphoton final state remains as one of the golden channels for new physics discoveries at the TeV scale in the LHC experiments. This motivates us to analyze model-independently the implications of an O(TeV) bump in the $γγ$ final state. By means of forward sum-rules for $γγ$ scattering, we show that a spin-0 resonance with mass of the order of the TeV and a sizable $γγ$ partial width -of the order of a few GeV- must be accompanied by higher spin resonances with $J_R\geq 2$ with similar properties, as expected in strongly coupled extensions of the Standard Model or, alternatively, in higher dimensional deconstructed duals. Furthermore, independently of whether the putative O(TeV) candidate is a scalar or a tensor, the large contribution to the forward sum-rules in the referred scenario implies the presence of states in the spectrum with $J_R\geq 2$, these high spin particles being a manifestation of new extra-dimensions or composite states of a new strong sector.

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arXiv2017

Sum-rule constraints on possible diphoton resonances at LHC

The study of the forward scattering amplitude $V(k,λ) V(k'λ')\to V(k,λ) V(k',λ')$ of real massless gauge bosons $V$, e.g. photons or gluons, leads to a sum-rule that can be used to investigate beyond the Standard Model signals at LHC in the $γγ$ channel. The sum-rule only relies on general properties such as analyticity, unitarity and crossing. We use the now buried 750 GeV diphoton resonance as a case of study to exemplify the constraints that the forward sum-rule requires to any new physics candidate. In the case of a large $γγ$ or $gg$ partial width, of the order of 10 GeV in our 750 GeV analysis, one finds that an infinite tower of states with spin $J_R=2$ and higher must be ultimately incorporated to the beyond Standard Model theory in order to fulfill the sum-rule. We expect these techniques may be useful in next diphoton searches at LHC and future colliders.

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arXiv2016

Lepton flavor violation in the Simplest Little Higgs model

Little Higgs Models are a possible elegant solution to the hierarchy problem on the Higgs mass. As they predict naturally small deviations with respect to SM results, they are in agreement with all current experimental data. In this contribution, we review lepton flavor violation in the Simplest Little Higgs model focusing on semileptonic lepton flavor violating tau decays (where some new results are presented) and $H\to\ell\ell'$. Within this model, the most promising decay channels for discovering lepton flavor violation are $μ\to e$ conversion in nuclei, $μ\to eγ^{(*)}$, $τ\to (e/μ)γ$ and $τ\to (e/μ) (π^+π^-/ρ)$.

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arXiv2016

Five-body leptonic decays of muon and tau leptons

We study the five-body decays $μ^- \to e^- e^+ e^- ν_μ \barν_{e}$ and $τ^- \to \ell^- \ell'^+\ell'^-ν_τ\barν_{\ell}$ for $\ell, \ell'=e, μ$ within the Standard Model (SM) and in a general effective field theory description of the weak interactions at low energies. We compute the branching ratios and compare our results with two previous, mutually discrepant, SM calculations. By assuming a general structure for the weak currents we derive the expressions for the energy and angular distributions of the three charged leptons when the decaying lepton is polarized, which will be useful in precise tests of the weak charged current at Belle II. In these decays, leptonic $\mathbf{T}$-odd correlations in triple products of spin and momenta --which may signal time reversal violation in the leptonic sector-- are suppressed by the tiny neutrino masses. Therefore, a measurement of such $\mathbf{T}$-violating observables would be associated to neutrinoless lepton flavor violating (LFV) decays, where this effect is not extremely suppressed. We also study the backgrounds that the SM five-lepton lepton decays constitute to searches of LFV $L^- \to \ell^- \ell'^+\ell'^-$ decays. Searches at high values of the invariant mass of the $\ell'^+\ell'^-$ pair look the most convenient way to overcome the background.

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arXiv2016

Lepton Flavour Violation in Hadron Decays of the Tau Lepton in the Simplest Little Higgs Model

We study Lepton Flavour Violating hadron decays of the tau lepton within the Simplest Little Higgs model. Namely we consider $τ\rightarrow μ(P, V, PP)$ where $P$ and $V$ are short for a pseudoscalar and a vector meson. We find that, in the most positive scenarios, branching ratios for these processes are predicted to be, at least, four orders of magnitude smaller than present experimental bounds.

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arXiv2015

$L\,\to\, l\, l'\, l'\, ν_l\, ν_L$ in the SM and beyond

We study the $L\to l l' l' ν_l ν_L$ decays ($L=τ,μ$; $l,l'=μ,e$) in the Standard Model (SM) and in the effective field theory (EFT) description of the weak charged current at low energy, both for polarized and unpolarized $L$, keeping the $l,l'$ mass dependence. We clarify the discrepancy between two previous SM calculations of the decay rate improving their precision. The recent $3.5\,σ$ anomaly found in $τ\to e γ\barν_eν_τ$ could be checked using our precise prediction for the $τ\to e e e \barν_eν_τ$ decays, which shall be measured analyzing already existing data from the first generation B-factories. It is shown how measurements of the di-$l'$ mass distribution (with appropriate cuts) and $T$-asymmetries are able to reveal the corresponding lepton flavor violating (LFV) processes without neutrinos in the final state.

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arXiv2015

Long-distance weak annihilation contribution to the $B^{\pm}\to (π^{\pm},K^{\pm}) \ell^+\ell^-$ decays

We propose an alternative evaluation of the long-distance weak annihilation (WA, also called one-photon exchange in this paper) contribution to the rare semileptonic $B^{\pm}\to (π^{\pm},K^{\pm})\ell^+\ell^-$ ($\ell=e,\,μ$) decays. This hadronic description at low energies is matched at intermediate energies to its short-distance counterpart in terms of quark and gluon degrees of freedom. Although the WA contribution does not contribute to solve the possible breaking of lepton-universality observed by LHCb in the $B^{\pm}\to K^{\pm}(μ^+μ^-/e^+e^-)$ ratio, nor provides an important hadronic contamination to their decay rates, its contribution to the branching ratios (and direct CP asymmetry) of the $B^{\pm}\to π^{\pm}\ell^+\ell^-$ transitions turns out to be significant. This hadronic pollution should be taken into account when looking for new physics effects in decays into pions, which suggests to restrict these searches to squared lepton-pair invariant mass in the $(1,8)$ GeV$^2$ range. The interference of the one-photon exchange contribution with the dominant short-distance one-loop amplitude induces a sizable CP asymmetry in these rare decays, which calls for dedicated measurements.

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arXiv2015

A new long distance contribution to $B^\pm\to K^\pm/π^\pm\ell^+\ell-$ decays

We have identified a missing long-distance (one-photon exchange) contribution to the $B^\pm\to K^\pm/π^\pm\ell^+\ell^-$ decays. Although it does not help to explain the anomaly measured by the LHCb Coll. in the ratio $R_K = \mathcal{B}(B^\pm\to K^\pmμ^+μ^-)/\mathcal{B}(B^\pm\to K^\pm e^+e^-)\sim0.75$ (2.6 $σ$ deviation with respect to $R_K=1+\mathcal{O}(10^{-4})$ in the Standard Model), it provides a sizable contribution to the branching ratio of the $B^\pm\to π^\pm\ell^+\ell^-$ decays. The new decay mechanism gives rise to a measurable CP asymmetry, which is of order 1\% in both channels. These predictions can be tested in forthcoming LHCb measurements.

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arXiv2015

RChL currents in Tauola: implementation and fit parameters

We present the results of a partial upgrade to the Monte Carlo event generator TAUOLA using Resonance Chiral Theory for the two and three meson final states. These modes account for 88% of total hadronic width of the tau meson. The first results of the model parameters have been obtained using Preliminary BaBar data for the three pion mode.

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arXiv2014

Combined analysis of the decays $τ^-\to K_Sπ^-ν_τ$ and $τ^-\to K^-ην_τ$

In a combined study of the decay spectra of $τ^-\to K_Sπ^-ν_τ$ and $τ^-\to K^-ην_τ$ decays within a dispersive representation of the required form factors, we illustrate how the $K^*(1410)$ resonance parameters, defined through the pole position in the complex plane, can be extracted with improved precision as compared to previous studies. While we obtain a substantial improvement in the mass, the uncertainty in the width is only slightly reduced, with the findings $M_{K^{*\prime}}=1304 \pm 17\,$MeV and $Γ_{K^{*\prime}} = 171 \pm 62\,$MeV. Further constraints on the width could result from updated analyses of the $Kπ$ and/or $Kη$ spectra using the full Belle-I data sample. Prospects for Belle-II are also discussed. As the $K^-π^0$ vector form factor enters the description of the decay $τ^-\to K^-ην_τ$, we are in a position to investigate isospin violations in its parameters like the form factor slopes. In this respect also making available the spectrum of the transition $τ^-\to K^-π^0ν_τ$ would be extremely useful, as it would allow to study those isospin violations with much higher precision.

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arXiv2014

Assessing the accuracy of the Brodsky-Lepage prediction

We show that a consistent set of short-distance conditions on the odd-intrinsic parity resonance chiral Lagrangian couplings can be derived in the large-$N_C$ limit and the single resonance approximation. These constraints are satisfied both by the VVP Green function and all its associated form-factors. Moreover, they are in agreement with earlier results found in the normal parity sector, which further supports the overall consistency of the approach. Here we take a new perspective and examine the consequences of these high-energy relations on the precise coefficient of the $1/Q^2$-term of meson form-factors, which rules their asymptotic behavior. In particular, we show that the coefficient derived using our consistent set of short-distance constraints is in remarkable accordance with the Brodsky-Lepage prediction, being alternative results for these coefficients disfavored.

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arXiv2014

The VV'P form factors in resonance chiral theory and the pi-eta-eta' light-by-light contribution to the muon g-2

The description of the VV'P form factors (V,V' stands for vector particles and P for a pseudoscalar meson) for different particles virtualities remains a challenge for the theory of strong interactions. While their chiral limit is well understood, recent measurements of the gamma^* omega pi^0 and gamma^* gamma pi^0 form factors at high photon virtualities seem to depart from the simplest scaling behavior suggested by QCD. Here we attempt to describe them in their whole measured energy regimes within the Resonance Chiral Theory, a framework which naturally incorporates the chiral limit constraints and extends to higher energies by including the resonances as active fields. Specifically, we obtained an accurate description of the data up to 2 GeV on the former form factor by including three multiplets of vector resonances. Good agreement with measurements of the latter was possible even in the single resonance approximation, although we propose to measure the e^+e^- to mu^+mu^-pi^0 cross-section and di-muon invariant mass distribution to better characterize this form factor. We have then evaluated the pion exchange contribution to the muon g-2 obtaining (6.66\pm0.21)x10^{-10} with an accurate determination of the errors. We have also recalled that approximating the whole pion exchange by the pion pole contribution underestimates the corresponding result for the anomaly (by (15,20)%). Based on these results, we have predicted the eta^(\prime) transition form factors obtaining good agreement with data and obtained their respective contributions to the muon anomaly. In this way, the contribution of the three lightest pseudoscalars to it yields (10.47\pm0.54)x10^{-10}, in agreement with previous evaluations but with smaller error.

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arXiv2013

Mini-Proceedings of the 14th meeting of the Working Group on Rad. Corrections and MC Generators for Low Energies

The mini-proceedings of the 14th Meeting of the "Working Group on Rad. Corrections and MC Generators for Low Energies" held in Frascati on September 13, 2013, as a satellite meeting of the PHIPSI13 conference in Rome, are presented. These meetings, started in 2006, have as aim to bring together experimentalists and theorists working in the fields of meson transition form factors, hadronic contributions to (g-2)_μand the effective fine structure constant, and development of MonteCarlo generators and Radiative Corrections for precision e+e- and tau physics.

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arXiv2013

Resonance Chiral Lagrangian Currents and Experimental Data for $τ^-\toπ^{-}π^{-}π^{+}ν_τ$

In this paper we document the modifications introduced to the previous version of the Resonance Chiral Lagrangian current ({\it Phys.Rev.} {\bf D86} (2012) 113008) of the $τ^\pm \to π^\pm π^\pm π^\mp ν_τ$ decay which enable the one dimensional distributions measured by the BaBar collaboration to be well modeled. The main change required to model the data is the addition of the $σ$ resonance. Systematic errors, theoretical and experimental ones, limitations due to fits of one dimensional distributions only, and resulting difficulties and statistical/systematic errors for fitted parameters are addressed. The current and fitting environment is ready for comparisons with the fully exclusive experimental data. The present result for $τ^\pm \to π^\pm π^\pm π^\mp ν_τ$ is encouraging for work on other $τ$ decay modes and Resonance Chiral Lagrangian based currents.

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arXiv2013

$τ^-\to K^-η^{(\prime)}ν_τ$ decays in Chiral Perturbation Theory with Resonances

We have studied the $τ^-\to K^-η^{(\prime)}ν_τ$ decays within Chiral Perturbation Theory including resonances as explicit degrees of freedom. We have considered three different form factors according to treatment of final-state interactions. In increasing degree of soundness: Breit-Wigner, exponential resummation and dispersive representation. We find that although the first one fails in accounting for the data on the $Kη$ mode, the other two approaches provide good fits to them which are sensitive to the $K^\star(1410)$ pole parameters, that are determined to be $M_{K^{\star\prime}}\,=\,(1330^{+27}_{-41})$ MeV and $Γ_{K^{\star\prime}}\,=\,(217^{+68}_{-122})$ MeV. These values are competitive with the standard determination from $τ^-\to(Kπ)^-ν_τ$ decays. The corresponding predictions for the $τ^-\to K^-η^{\prime}ν_τ$ channel respect the current upper bound and hint to the discovery of this decay mode in the near future.

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arXiv2013

Dispersive representation of the pion vector form factor in $τ\toππν_τ$ decays

We propose a dispersive representation of the charged pion vector form factor that is consistent with chiral symmetry and fulfills the constraints imposed by analyticity and unitarity. Unknown parameters are fitted to the very precise data on $τ^-\toπ^-π^0ν_τ$ decays obtained by Belle, leading to a good description of the corresponding spectral function up to a $ππ$ squared invariant mass $s\simeq 1.5$ GeV$^2$. We discuss the effect of isospin corrections, and obtain the values of low energy observables. For larger values of $s$, this representation is complemented with a phenomenological description to allow its implementation in the new TAUOLA hadronic currents.

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arXiv2013

$τ^-\toπ^-η^{(\prime)}ν_τ$ decays

The vector and scalar form factors relevant to the second-class current $τ^-\toπ^-η^{(\prime)}ν_τ$ processes are evaluated supplementing Chiral Perturbation Theory with the inclusion of resonances as explicit degrees of freedom. While the vector contribution is essentially fixed from knowledge of the two-pion vector form factor, the scalar form factor demands a coupled channel dispersive analysis to obtain robust predictions for the detection of these decay modes.

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arXiv2013

The weak radiative pion vertex in τ^- ->π^-ν_τl^+ l^- decays

We carry out a detailed study of the branching fractions and lepton pair invariant-mass spectrum of τ^- ->π^- ν_τl^+l^- decays (l=e,μ). In addition to the model-independent (QED) contributions, we include the structure-dependent (SD) terms, which encode information on the hadronization of QCD currents. The form factors describing the SD contributions are evaluated by supplementing Chiral Perturbation Theory with the inclusion of the lightest multiplet of spin-one resonances as active degrees of freedom. The Lagrangian couplings have been determined demanding the known QCD short-distance behaviour to the relevant Green functions and associated form factors in the limit where the number of colours goes to infinity. As a result, we predict BR(τ^- ->π^-ν_τe^+e^-)= (1.7^{+1.1}_{-0.3})x10^{-5} and BR(τ^- -> π^-ν_τμ^+μ^-) = [0.03,1.0]x10^{-5}. According to this, the first decay could be measured in the near future, which is not granted for the second one.

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