arXiv subjects

Alain Le Yaouanc

Publications and source records attributed to Alain Le Yaouanc.

At least 19 recordsLinked to original sources

arXiv2026

Influence of the KK graviton decay into hh on the triple Higgs measurement at LHC

Evidence for two KK graviton candidates has been previously reported at 380 GeV and 700 GeV. Following a Randall Sundrum interpretation, two extra resonances should appear at 1000 GeV and 1300 GeV. Recently ATLAS, in its search for triple Higgs coupling, has reported an excess in that mass region in conformity with this prediction. Local cross sections are therefore clearly in excess of the standard predictions even for large values of kl. While still marginally significant, this effect appears in a mass region with low background which allows to expect good prospects of discovery with RUN3 data. Such a result could therefore allow to interpret an excess in the measurement of kl and confirm the existence of a series of KK graviton resonances observable at LHC. Other opportunities seem to appear in searches for heavy resonances decaying into ZZ/WW in semi-leptonic and fully hadronic modes. Indirect evidences for the RS model coming from precision measurements are also presented.

hep-ex↗

arXiv2026

Evidence for BSM spin 0 and spin 2 resonances at LHC Possible Interpretations

Nine statistically significant decay channels are observed in LHC data around a mass of 650 GeV. We interpret three of them as coming from a 20 GeV wide resonance observed in e+e-, 2 photons and ZZ into four leptons which could be a J=2 Kaluza Klein graviton resonance called T700 (T for tensor with J=2). This hypothesis is reinforced by noting that this signal is produced by VBF and not by ggF since it disappears in ZZ when treated as a scalar. Given that the six other excesses have poor mass resolution, one cannot exclude the presence of an additional wide scalar resonance called H650. Assuming a Randall Sundrum RS model, we conclude that LHC observes the predicted sequence T380, T700 and T1000, with prospects to discover heavier states by reconstructing hadronic states. At variance with the RS model, T700 decouples from gluon and photon pairs, suggesting a composite model interpretation in which preons are charge neutral and colourless. T700 does significantly couple to $e^+e^-$ which has implications for future $e^+e^-$ colliders. Perturbative unitarity requirements predict $T^{++} \to W^+W^+$ and $T^+ \to ZW$ resonances, again indicated by LHC data. With BR($e^+e^-$)~0.25%, measured by ATLAS and CMS, this scenario offers excellent prospects for abundantly (Gigafactory) producing a sequence of narrow resonances at future $e^+e^-$ colliders. For heavy scalars, the situation is less clear. Following ATLAS and CMS, we expect that the top loop contribution to the gluon-gluon fusion mechanism $ggF$ could produce a deficit rather than an excess in the mass distribution of top pairs, which prevents a standard estimate of the statistical significance for heavy resonances. It seems that the pseudo-scalar and scalar resonances A490 and H650, indicated by other channels, create observable deviations in the $t \bar t$ analyses presented by ATLAS and CMS.

hep-ph↗

arXiv2025

X650 $\to$ ZZ/WW/H125H95/A450Z/$γγ$ -- $J=1$, $J=2$, narrow or wide resonance?

At ICHEP 2024 CMS has published RUN2 results for ZZ->4 leptons and reached the conclusion that there is no evidence for a scalar resonance H650 decaying into ZZ, as was also concluded by ATLAS using an MVA analysis optimised for a scalar particle. Since this resonance is indicated into ZZ by ATLAS in a Cut Based Analysis and into three other modes: WW and h95h125 by CMS, ttZ by ATLAS , the present paper is an attempt to understand these apparent contradictions. Our conclusion is that strict selections for the reaction H650->ZZ cut as much into the signal than into the background, therefore reducing its statistical significance. A plausible interpretation of this behaviour is that these selections are inadequate for a spin 2 particle produced by the VBF process and decaying into ZZ with an angular distribution similar to the Drell Yan background. If true, our description of the various findings in term of the Georgi Machacek model needs to be deeply revised, which is attempted in the present note. A scenario with T690 as a spin 2 narrow resonance is presented which naturally accommodates WW/ZZ and h95h125 decays, while we suggest that A450Z could come from a H650 nearby resonance. We propose a scheme in which scalar resonances are accommodated in three scalar isodoublets, while T690, T450++, T375+ and T350 belong to two tensor isoquintuplets. In this scenario, T690 could be a narrow resonance which interferes with the SM background, as confirmed by its observation in the two photon mode, reinforcing a Kaluza Klein graviton interpretation. If, as indicated by ATLAS and CMS, T690 couples to e+e-, this scenario leads to a dramatic prediction for future $e^+e^-$ colliders.

hep-ph↗

arXiv2024

Triple Higgs couplings at LHC

A global interpretation of several significant indications of scalar resonances observed at LHC is achieved using the Georgi Machacek model. Among many other consequences, one predicts large cross sections for the processes ggFH(320)->h(125)h(125) and A(151)A(151) and ggF->A(420)->H(320)Z, where H(320) has been observed in A(420)->H(320)Z->bbbbl+l- and where A(151) is observed into two photons accompanied by a b jet, a lepton or missing transverse energy. We predict that ggF->H(320) has a cross section of about 2000 fb with a BR into h(125)(125) of 17% and into A(151)A(151) of 45%. Henceforth we predict that ggF->H(320)->hh will dominate over the SM process h*->hh, where h is the SM h(125) scalar. One expects that H(320)->A(151)A(151) can be observed into bbbb, becoming one of the most significant BSM phenomena identified so far. Arguments in favor of a SUSY version of GM are also provided.

hep-ph↗

arXiv2024

H(650) -> W+W-/ZZ predicts H++ -> W+W+ and H+ -> ZW+, as indicated by LHC data

Several indications for neutral scalars are observed at the LHC. One of them, a broad resonance peaked at about 650 GeV which we call H(650), was first observed by an outsider combining published histograms from ATLAS and CMS on ZZ -> 4 leptons searches, and this combination shows a local significance close to 4 s.d. Since then, CMS has reported two other indications at the same mass, with similar local significances: H->WW->2leptons+neutrinos and H(650)->bbh(125) where mbb~90 GeV and h(125)->gam gam. ATLAS has completed its analysis of ZZ->4 leptons from which we infer an indication for H(650) with 3.5 s.d. significance. Combining these three results, one gets a global statistical significance above 6 s.d. H(650) has a coupling to WW similar to h(125) and therefore we argue that a sum rule (SR) required by unitarity for W+W- scattering implies that there should be a compensating effect from a doubly charged scalar H++, with a large coupling to W+W+. We therefore predict that this mode should become visible through the vector boson fusion process W+W+->H++, naturally provided by LHC. A recent indication for H++(450)->W+W+ from ATLAS allows a model independent interpretation of this result through the SR constraint which gives BR(H++->W+W+)~10%, implying the occurrence of additional decay modes H+W+ and H+H+ from one or several light H+ with masses below mH++ - mW or MH++/2, that is mH+ < 370 GeV or 225 GeV. A similar analysis is performed for H+(375)->ZW, indicated by ATLAS and CMS. Both channels suggest a scalar field content similar to the Georgi Machacek model with triplets, at variance with the models usually considered. Implications on precision measurements are presented followed by a complete extraction of the GM parameters. An alternate interpretation of the 650 GeV resonance as a tensor is also briefly discussed. Implications for precision measurements are presented.

hep-ph↗

arXiv2022

Searches for scalars at LHC and interpretation of the findings

In view of the future Higgs factories, this work presents the status of scalar searches at the LHC with an emphasis on the H(650) resonance which has been observed in WW, ZZ and h(95)h(125) channels, with a cumulative evidence of about 7 s.d. global significance. Its interpretation in models, restricted to extension of the scalar sector by SU(2) singlets and doublets, is clearly excluded, while its interpretation in models with additional triplets requires an extension with respect to the conventional Georgi-Machacek model. A general picture of these searches is updated, showing that h(95) is also reaching a similar level of evidence while two other candidates, A(400) and h(151), although less prominent, are above the 4 s.d. global evidence.

hep-ph↗

arXiv2021

A model for NL and SL decays by $\overline{B}^0 \to D^{**}$ transitions with $BR(j=1/2) \ll BR(j=3/2)$ using the $LLSW$ scheme

We present a model for the vector and axial form factors of the transitions $\overline{B}^0 \to D^{**}$ in good agreement with the presently available data and based on the present theoretical knowledge, combining a) the safe lattice QCD predictions at $m_Q=\infty$ and $w=1$ ; b) the predictions at general $w$ of a relativistic, covariant quark model at $m_Q=\infty$, including the well tested Godfrey and Isgur spectroscopic model and which agrees with lattice QCD at $w=1$ ; c) the constraint of Bjorken and Neubert relating Semi-leptonic (SL) and Class I Non-leptonic (NL) decays, which shows that $\overline{B}^0 \to D_0(2300)^{+} π^-$ strongly constrains $τ_{1/2}(w)$ to be much smaller than $τ_{3/2}(w)$, in agreement with the theoretical expectation; d) the general HQET expansion which constrains the $1/m_Q$ corrections (cf \cite{LLSW}, denoted hereafter as $LLSW$). An important element in the understanding of data is the large contribution of virtual $D^{(*)}_V$ to the broad structures seen in SL decays at low $D^{(*)} π$ masses - which makes it difficult to isolate the broad resonances denoted as $D_{1/2}$ in the following.

hep-ph↗

arXiv2018

Determining the photon polarization of the b --> s gamma using the B --> K1(1270) gamma --> (K pi pi) gamma decay

Recently the radiative B decay to the strange axial-vector mesons, B --> K1(1270) gamma, has been observed with rather large branching ratio. This process is particularly interesting as the subsequent K1 decay into its three body final state allows us to determine the polarization of the photon, which is mostly left- (right-)handed for Bbar (B) in the SM while various new physics models predict additional right- (left-)handed components. A new method is proposed to determine the polarization, exploiting the full Dalitz plot distribution, which seems to reduce significantly the statistical errors. This polarization measurement requires however a detailed knowledge of the K1--> K pi pi strong interaction decays, namely, the various partial wave amplitudes into the several possible quasi two-body channels, as well as their relative phases. The pattern of partial waves is especially complex for the K1(1270). We attempt to obtain the information through the combination of an experimental input and a theoretical one, provided by the 3P0 quark-pair-creation model.

hep-ph↗

arXiv2017

Unexpected pattern of transitions to radial excitations by heavy quark current

Motivated by the claimed possibility of a large contribution of the first radial excitation of the $D^{(\ast )}$ to the $B$ semileptonic decay into charmed mesons, also invoked to solve the "$1/2$ vs. $3/2$ semileptonic puzzle", we discuss the transitions to heavy-light radial excitations by a heavy $b \to c$ quark current. We first consider a HQET sum rule, which provides a bound on the slopes of Isgur-Wise functions which we then calculate in the Bakamjian-Thomas framework which both guaranties covariance in the heavy quark limit and satisfies a set of HQET sum rules. We observe a remarkable property that for a large variety of wave functions the transition to the first radial excitation is very small while the transition matrix element to the second radial excitation is large and dominant in saturating the HQET sum rule. This is opposite to what is found in non-relativistic models, where the transition to the first radial excitation dominates the sum rule. The relative magnitude of the transition to the second excitation appears to be weakly dependent on the dynamical scale (radius of the bound states), and the same holds true for the slope of the elastic transition. These features could be tested in the heavy mass limit of lattice QCD. This pattern is shown to be related to the general structure of the Bakamjian-Thomas model, it is independent of the spin structure of the approach and derives mainly from the Lorentz transformation of the spatial wave function, a feature often disregarded in quark models.

hep-ph↗

arXiv2017

$B$ decays to radially excited $D$ mesons in Heavy Quark Effective Theory

Semileptonic transitions $\overline{B} \to D^{(n)} \ell \overlineν$, where $D^{(n)} (n \not = 0)$ is a radially excited meson, have recently attracted much attention as a way to understand some puzzles between theory and data. Following closely the formalism of Falk and Neubert for the elastic case, we study the $1/m_Q$ corrections to the heavy quark limit, in which the inelastic Isgur-Wise function vanishes at zero recoil, $ξ^{(n)}(1) = 0\ (n \not = 0)$. We find simple formulas that involve the derivative $ξ^{(n)'}(1)$, and we propose a number of ways of isolating this quantity in practice. We formulate also a generalization to the inelastic case of Luke's theorem. On the other hand, although some $1/m_Q$ HQET results are satisfied in the Bakamjian-Thomas relativistic quark model, we emphasize the problems concerning these corrections in this scheme.

hep-ph↗

arXiv2014

Insight into $D/B\to π\ell ν_\ell$ decay using the pole models

After improving the knowledge about residua of the semileptonic form factor at its first two poles we show that $f_+^{Dπ}(q^2)$ is not saturated when compared with the experimental data. To fill the difference we approximate the rest of discontinuity by an effective pole and show that the data can be described very well with the position of the effective pole larger than the next excitation in the spectrum of $D^\ast$ state. The results of fits with experimental data also suggest the validity of superconvergence which in the pole models translates to a vanishing of the sum of residua of the form factor at all poles. A similar discussion in the case of $B\to π\ellν_\ell$ leads to the possibility of extracting $\vert V_{ub}\vert$, the error of which appears to be dominated by $g_{B^\ast Bπ}$, which can be nowadays computed on the lattice. In evaluating the residua of the form factors at their nearest pole we needed the vector meson decay constants $f_{D^\ast}$ and $f_{B^\ast}$, which we computed by using the numerical simulations of QCD on the lattice with $N_{\rm f}=2$ dynamical quarks. We obtain, $f_{D^\ast}/f_D=1.208(27)$ and $f_{B^\ast}/f_B=1.051(17)$.

hep-ph↗

arXiv2014

In Memoriam Nikolai Uraltsev : Uraltsev's and other Sum Rules, Theory and Phenomenology of $D^{**}$'s

We first discuss Uraltsev's and other sum rules constraining the $B \to D^{**}(L=1)$ weak transitions in the infinite mass limit, and compare them with dynamical approaches in the same limit. After recalling these well established facts, we discuss how to apply infinite mass limit to the physical situation. We provide predictions concerning semi-leptonic decays and non-leptonic ones, based on quark models. We then present in more detail the dynamical approaches: the relativistic quark model à la Bakamjian-Thomas and lattice QCD. We summarise lattice QCD results in the infinite mass limit and compare them to the quark model predictions. We then present preliminary lattice QCD results with finite $b$ and $c$ quark masses. A systematic comparison between theory and experiment is performed. We show that some large discrepancies exist between different experiments. Altogether the predictions at infinite mass are in fair agreement with experiment for non-leptonic decays contrary to what happens for semileptonic decays. We conclude by considering the prospects to clarify both the experimental situation, the theoretical one and the comparison between both.

hep-ph↗

arXiv2013

On the significance of B-decays to radially excited D

We discuss the possibilities of testing the recent claims of a relatively large value of the B --> D' transition form factor through B0--> D' pi+ and B- --> D' pi- decay modes. To estimate the width of the latter we need the decay constant of the radially excited D-meson that we computed in lattice QCD with Nf=2 dynamical flavors and after taking the continuum limit we found f(D')=117(25) MeV. We also provide an update for the values of the D-meson decay constants obtained by using maximally twisted mass QCD.

hep-ph↗

arXiv2012

Proposal to study $B_s \to \bar D_{sJ}$ transitions

It is proposed to clear some of the puzzles of B decay to the broad $\bar{D}^{\ast\ast}$ states by studying the corresponding decay with strange states $B_s^0 \to D_{s0}^{\ast -} π^+$ at LHCb. Interpretation of the results should be easier due to the narrowness of the $D_{s0}^{\ast -}$ state.

hep-ph↗

arXiv2012

Future prospects for the determination of the Wilson coefficient $C_{7γ}^\prime$

We discuss the possibilities of assessing a non-zero $C_{7γ}^\prime$ from the direct and the indirect measurements of the photon polarization in the exclusive $b \to sγ^{(*)}$ decays. We focus on three methods and explore the following three decay modes: $B \to K^*(\to K_Sπ^0)γ$, $B \to K_1(\to Kππ)γ$, and $B \to K^*(\to Kπ)\ell^+\ell^-$. By studying different New Physics scenarios we show that the future measurement of conveniently defined observables in these decays could provide us with the full determination of $C_{7γ}$ and $C_{7γ}^\prime$.

hep-ph↗

arXiv2012

Pionic couplings to the lowest heavy-light mesons of positive and negative parity

We present the method and compute the strong couplings of the lowest and first orbitally excited heavy-light mesons to a soft pion in the static heavy quark limit on the lattice. Besides the usual g^ and g couplings, we were able to make the first computation of the coupling h using the relevant radial distributions. Our results are obtained from the simulations of QCD with Nf=2 light Wilson-Clover quarks, combined with the improved static quark actions. The hierarchy among couplings that emerges from our study is g < g^ < h.

hep-lat↗

arXiv2011

The strong decays of K1 resonances

We investigate the K1--> K pi pi strong interaction decays. Using the 3P0 quark-pair-creation model to derive the basic parametrization, we discuss in detail how to obtain the various partial wave amplitudes into the possible quasi-two-body decay channels as well as their relative phases from the currently available experimental data. We obtain the K1 mixing angle to be thetaK1= 60 deg, in agreement with previous works. Our study can be applied to extract the information needed for the photon polarization determination of the radiative B--> K1 gamma decay.

hep-ph↗

arXiv2009

Soft dynamics in heavy-light mesons

We compute the radial distributions of the scalar, vector and axial charge density in the heavy-light mesons. We present the results obtained for both the lowest lying static heavy-light mesons as well as for their nearest excitations, with $N_f=2$ dynamical quarks of Wilson (Clover) type. We used various improvements of the static heavy-quark actions. From these distributions we were able to compute the corresponding charges and their radii $< r^2>$, the results of which are also presented.

hep-lat↗