arXiv subjects

Jean-Marc Gerard

Publications and source records attributed to Jean-Marc Gerard.

At least 19 recordsLinked to original sources

arXiv2022

Baryon number violation at the LHC: the top option

Subject to strong experimental constraints at low energies, baryon number violation is nonetheless well motivated from a theoretical point of view. We examine the possibility of observing baryon-number-violating top-quark production or decay at hadron colliders. We adopt a model independent effective approach and focus on operators with minimal mass-dimension. Corresponding effective coefficients could be directly probed at the Large Hadron Collider (LHC) already with an integrated luminosity of 1 inverse femtobarn at 7 TeV, and further constrained with 30 (100) inverse femtobarns at 7 (14) TeV.

hep-ph↗

arXiv2020

Isospin-breaking in $\varepsilon'/\varepsilon$: Impact of $η_0$ at the Dawn of the 2020s

For direct CP-violation in $K\toππ$ decays, the usual isospin-breaking effects at the percent level are amplified by the dynamics behind the $ΔI=1/2$ rule and conventionally encoded in $Ω_{\rm IB}$ parameters. The updated prediction $Ω_{\rm IB}^{(8)}=(15.9\pm 8.2)\times 10^{-2}$ of the Chiral Perturbation Theory for the strong isospin-breaking due to $π_3-η_8$ mixing confirms such a tendency but is quite sensitive to the theoretical input value of the low-energy constant corresponding to the flavour-singlet $η_0$ exchange contribution in this truncated octet scheme. We rather exploit the phenomenological $η_8-η_0$ mixing as a probe for the non-negligible flavour-singlet component of the physical $η$ pole to find $Ω_{\rm IB}^{(9)}=(35\pm7)\times 10^{-2}$ in a complete nonet scheme. A large central value in the nonet scheme is thus substituted for a large uncertainty in the octet one. Including the experimental $π^+-π^0$ mass difference as the dominant electromagnetic isospin-breaking, we obtain for the effective parameter entering the ratio $ε'/ε$ an improved result $\hatΩ_{\rm eff}^{(9)}=(29\pm7)\times 10^{-2}$ to be compared with $\hatΩ_{\rm eff}^{(8)}=(17\pm9)\times 10^{-2}$ used in recent analyses of $ε'/ε$. Accordingly, we get a reduction from $(ε'/ε)_{\rm SM}^{(8)}=(17.4\pm 6.1)\times 10^{-4}$ to $(ε'/ε)_{\rm SM}^{(9)}=(13.9\pm 5.2)\times 10^{-4}$ and thereby an effective suppression of $(ε'/ε)_{\rm SM}$ by isospin-breaking corrections as large as $40\%$ relative to the recent RBC-UKQCD value.

hep-ph↗

arXiv2019

BSM Hadronic Matrix Elements for $ε'/ε$ and $K\toππ$ Decays in the Dual QCD Approach

We calculate for the first time all four-quark hadronic matrix elements of local operators possibly contributing to $K\toππ$ decays and in particular to the ratio $ε'/ε$ beyond the Standard Model (BSM). To this end we use the Dual QCD (DQCD) approach. In addition to 7 new mirror operators obtained from the SM ones by flipping the chirality, we count 13 BSM four-quark operators of a given chirality linearly independent of each other and of the aforesaid 14 operators for which hadronic matrix elements are already known. We present results in two bases for all these operators, one termed DQCD basis useful for the calculation of the hadronic matrix elements in the DQCD approach and the other called SD basis suited to the short distance renormalization group evolution above the 1~GeV scale. We demonstrate that the pattern of long distance evolution (meson evolution) matches the one of short distance evolution (quark-gluon evolution), a property which to our knowledge cannot be presently achieved in any other analytical framework. The highlights of our paper are chirally enhanced matrix elements of tensor-tensor and scalar-scalar BSM operators. They could thereby explain the emerging $ε'/ε$ anomaly which is strongly indicated within DQCD with some support from lattice QCD. On the other hand we do not expect the BSM operators to be relevant for the $ΔI=1/2$ rule.

hep-ph↗

arXiv2018

$K\toππ$ and $K-π$ Matrix Elements of the Chromomagnetic Operators from Dual QCD

We perform for the first time a direct calculation of on-shell $K\toππ$ hadronic matrix elements of chromomagnetic operators (CMO) in the Standard Model and beyond. To his end, we use the successful Dual QCD (DQCD) approach in which we also consider off-shell $K-π$ matrix elements that allows the comparison with lattice QCD calculations of these matrix elements presented recently by the ETM collaboration. Working in the SU(3) chiral limit, we find for the single $B$ parameter $B_{\rm CMO}=0.33$. Using the numerical results provided by the ETM collaboration we argue that only small corrections beyond that limit are to be expected. Our results are relevant for new physics scenarios in the context of the emerging $ε^\prime/ε$ anomaly strongly indicated within DQCD and supported by RBC-UKQCD lattice collaboration.

hep-ph↗

arXiv2018

Dual QCD Insight into BSM Hadronic Matrix Elements for $K^0-\bar K^0$ Mixing from Lattice QCD

We calculate BSM hadronic matrix elements for $K^0-\bar K^0$ mixing in the Dual QCD approach (DQCD). The ETM, SWME and RBC-UKQCD lattice collaborations find the matrix elements of the BSM density-density operators $\mathcal{O}_i$ with $i=2-5$ to be rather different from their vacuum insertion values (VIA) with $B_2\approx 0.5$, $B_3\approx B_5\approx 0.7$ and $B_4\approx 0.9$ at $μ=3~GeV$ to be compared with $B_i=1$ in the VIA. We demonstrate that this pattern can be reconstructed within the DQCD through the non-perturbative meson evolution from very low scales, where factorization of matrix elements is valid, to scales of order $(1~GeV)$ with subsequent perturbative quark-gluon evolution to $μ=3~GeV$. This turns out to be possible in spite of a very different pattern displayed at low scales with $B_2=1.2$, $B_3=3.0$, $B_4=1.0$ and $B_5\approx 0.2$ in the large $N$ limit, $N$ being the number of colours. Our results imply that the inclusion of meson evolution in the phenomenology of any non-leptonic transition like $K^0-\bar K^0$ mixing and $K\toππ$ decays is mandatory. While meson evolution, as demonstrated in our paper, is hidden in LQCD results, to our knowledge DQCD is the only analytic approach for non-leptonic transitions and decays which takes this important QCD dynamics into account.

hep-ph↗

arXiv2016

Final State Interactions in $K\toππ$ Decays: $ΔI=1/2$ Rule vs. $\varepsilon^\prime/\varepsilon$

Dispersive effects from strong $ππ$ rescattering in the final state (FSI) of weak $K\toππ$ decays are revisited with the goal to have a global view on their {\it relative} importance for the $ΔI=1/2$ rule and the ratio $\varepsilon^\prime/\varepsilon$ in the Standard Model (SM). We point out that this goal cannot be reached within a pure effective (meson) field approach like chiral perturbation theory in which the dominant current-current operators governing the $ΔI=1/2$ rule and the dominant density-density (four-quark) operators governing $\varepsilon^\prime/\varepsilon$ cannot be disentangled from each other. But in the context of a dual QCD approach, which includes both long distance dynamics and the UV completion, that is QCD at short distance scales, such a distinction is possible. We find then that beyond the strict large $N$ limit, $N$ being the number of colours, FSI are likely to be important for the $ΔI=1/2$ rule but much less relevant for $\varepsilon^\prime/\varepsilon$. The latter finding diminishes significantly hopes that improved calculations of $\varepsilon^\prime/\varepsilon$ would bring its SM prediction to agree with the experimental data, opening thereby an arena for important new physics contributions to this ratio.

hep-ph↗

arXiv2015

Upper Bounds on epsilon'/epsilon Parameters B_6^{(1/2)} and B_8^{(3/2)} from Large N QCD and other News

We demonstrate that in the large N approach developed by the authors in collaboration with Bardeen, the parameters B_6^{(1/2)} and B_8^{(3/2)} parametrizing the K\toππmatrix elements _0 and _2 of the dominant QCD and electroweak operators receive both negative O(1/N) corrections such that B_6^{(1/2)} < B_8^{(3/2)}<1 in agreement with the recent lattice results of the RBC-UKQCD collaboration. We also point out that the pattern of the size of the hadronic matrix elements of all QCD and electroweak penguin operators Q_i contributing to the K\to ππamplitudes A_0 and A_2, obtained by this lattice collaboration, provides further support to our large N approach. In particular, a very precise lattice result for the matrix element _0 implies for the corresponding parameter B_8^{(1/2)}=1.0\pm 0.2 to be compared with large N value B_8^{(1/2)}=1.1\pm 0.1. We discuss briefly the implications of these findings for the ratio epsilon'/epsilon. In fact, with the precise value for B_8^{(3/2)} from RBC-UKQCD collaboration, our upper bound on B_6^{(1/2)} implies epsilon'/epsilon in the SM roughly by a factor of two below its experimental value (16.6\pm 2.3)\times 10^{-4}. We also briefly comment on the parameter \hat B_K and the ΔI=1/2$ rule.

hep-ph↗

arXiv2014

Large N Approach to Kaon Decays and Mixing 28 Years Later: Delta I = 1/2 Rule, \hat B_K and Delta M_K

We review and update our results for K-> pipi decays and K^0-\bar K^0 mixing obtained by us in the 1980s within an approach based on the dual representation of QCD as a theory of weakly interacting mesons for large N colours. In our analytic approach the dynamics behind the enhancement of ReA_0 and suppression of ReA_2, the so-called Delta I = 1/2 rule for K-> pi pi decays, has a simple structure: the usual octet enhancement through quark-gluon renormalization group evolution down to the scales O(1 GeV) is continued as a meson evolution down to zero momentum scales at which the factorization of hadronic matrix elements is at work. The inclusion of lowest-lying vector meson contributions in addition to the pseudoscalar ones and of Wilson coefficients in a momentum scheme improves significantly the matching between quark-gluon and meson evolutions. In particular, the anomalous dimension matrix governing the meson evolution exhibits the structure of the known anomalous dimension matrix in the quark-gluon evolution. The recent results on ReA_2 and ReA_0 from the RBC-UKQC collaboration give support for our approach. In particular, the signs of the two main contractions found numerically by these authors follow uniquely from our analytic approach. At NLO in 1/N we obtain ReA_0/ReA_2= 16.0\pm 1.5 which amounts to an order of magnitude enhancement over the strict large N limit value sqrt{2}. QCD penguins contribute at 15% level to this result. We also find \hat B_K=0.73\pm 0.02, with the smallness of 1/N corrections to the large N value \hat B_K=3/4 resulting within our approach from an approximate cancellation between pseudoscalar and vector meson one-loop contributions. We summarize the status of Delta M_K in this approach.

hep-ph↗

arXiv2012

Flavor Mixing Democracy and Minimal CP Violation

We point out that there is a unique parametrization of quark flavor mixing in which every angle is close to the Cabibbo angle θ_C \simeq 13^\circ with the CP-violating phase ϕ_q around 1^\circ, implying that they might all be related to the strong hierarchy among quark masses. Applying the same parametrization to lepton flavor mixing, we find that all three mixing angles are comparably large (around π/4) and the Dirac CP-violating phase ϕ_l is also minimal as compared with its values in the other eight possible parametrizations. In this spirit, we propose a simple neutrino mixing ansatz which is equivalent to the tri-bimaximal flavor mixing pattern in the ϕ_l \to 0 limit and predicts \sinθ_13 = 1/\sqrt{2} \sin (ϕ_l/2) for reactor antineutrino oscillations. Hence the Jarlskog invariant of leptonic CP violation J_l = (\sinϕ_l)/12 can reach a few percent if θ_13 lies in the range 7^\circ \leq θ_13 \leq 10^\circ.

hep-ph↗

arXiv2011

An effective approach to same sign top pair production at the LHC and the forward-backward asymmetry at the Tevatron

We study the phenomenology of same sign top pair production at the LHC in a model-independent way. The complete set of dimension six operators involving two top (or anti-top) quarks is introduced and the connection with all possible t- or s-channel particle exchanges is established. Only in the former case, same and opposite sign top pair production can be related. We find that while current Tevatron data disfavor t-channel models, other production mechanisms are viable and can be tested at the LHC.

hep-ph↗

arXiv2011

An upper bound on the Kaon B-parameter and Re(epsilon_K)

New precise data in B physics and theoretical developments in K physics lead us to reconsider the weak K^0-\bar{K}^0 transition from a large-N_c viewpoint, N_c being the number of colors. In this framework, we infer an upper limit on \hat{B}_K and the Kaon indirect CP violation.

hep-ph↗

arXiv2010

Cosmological perturbation in f(R,G) theories with a perfect fluid

In order to classify modified gravity models according to their physical properties, we analyze the cosmological linear perturbations for f(R,G) theories (R being the Ricci scalar and G, the Gauss-Bonnet term) with a minimally coupled perfect fluid. For the scalar type perturbations, we identify in general six degrees of freedom. We find that two of these physical modes obey the same dispersion relation as the one for a non-relativistic de Broglie wave. This means that spacetime is either highly unstable or its fluctuations undergo a scale-dependent super-luminal propagation. Two other modes correspond to the degrees of freedom of the perfect fluid, and propagate with the sound speed of such a fluid. The remaining two modes correspond to the entropy and temperature perturbations of the perfect fluid, and completely decouple from the other modes for a barotropic equation of state. We then provide a concise condition on f(R,G) theories, that both f(R) and R+f(G) do fulfill, to avoid the de Broglie type dispersion relation. For the vector type perturbation, we find that the perturbations decay in time. For the tensor type perturbation, the perturbations can be either super-luminal or sub-luminal, depending on the model. No-ghost conditions are also obtained for each type of perturbation.

astro-ph.CO↗

arXiv2010

Cosmological perturbations of a perfect fluid and noncommutative variables

We describe the linear cosmological perturbations of a perfect fluid at the level of an action, providing thus an alternative to the standard approach based only on the equations of motion. This action is suited not only to perfect fluids with a barotropic equation of state, but also to those for which the pressure depends on two thermodynamical variables. By quantizing the system we find that (1) some perturbation fields exhibit a noncommutativity quite analogous to the one observed for a charged particle moving in a strong magnetic field, (2) local curvature and pressure perturbations cannot be measured simultaneously, (3) ghosts appear if the null energy condition is violated.

gr-qc↗

arXiv2009

Unconventional phenomenology of a minimal two-Higgs-doublet model

Two-Higgs-doublet models (2HDM) are simple extensions of the Standard Model (SM) where the scalar sector is enlarged by adding a weak doublet. As a result, the Higgs potential depends in general on several free parameters which have to be carefully chosen to give predictions consistent with the current precision data. We consider a 2HDM invariant under a twisted custodial symmetry and depending only on three extra parameters beyond the SM ones. This model naturally features an inverted mass spectrum with a light pseudoscalar state and a heavy SM-like Higgs boson. We thoroughly analyze direct and indirect constraints and present a few unconventional though promising signatures at the LHC.

hep-ph↗

arXiv2008

Mass Issues in Fundamental Interactions

Driven by the mass problem, we raise some issues of the fundamental interactions in terms of non trivial commutation relations implemented within toy theories.

hep-ph↗

arXiv2007

The strong equivalence principle from gravitational gauge structure

Gravitational self-interactions are assumed to be determined by the covariant derivative acting on the Riemann-Christoffel field strength. Once imposed on a metric theory, this Yang-Mills gauge constraint extends the equality of gravitational mass and inertial mass to compact bodies with non-negligible gravitational binding energy. Applied to generalized Brans-Dicke theories, it singles out one tensor theory and one scalar theory for gravity but also suggests a way to implement a minimal violation of the strong equivalence principle.

gr-qc↗

arXiv2006

Tracking down penguins at the poles

QCD penguins are responsible for about 2/3 of the Delta I=1/2 rule in K to pi pi decays, as inferred from a combined analysis of K to pi pi and KL to gamma gamma. Further tests based on the decays KS to pi0 gamma gamma and K+ to pi+ gamma gamma are proposed. New insights into the treatment of pi0, eta, eta' pole amplitudes are also reported.

hep-ph↗

arXiv2005

Radiative Kaon Decays and the Penguin Contribution to the Delta I = 1/2 Rule

A consistent census of penguins in the Delta I = 1/2 rule is taken from the eta0 pole contribution to the radiative KL to gamma gamma, KS to pi0 gamma gamma and K+ to pi+ gamma gamma decay modes. We briefly comment on its impact for KL to pi0 pi0 gamma gamma, KL to pi+ pi- gamma and check its compatibility with the KL - KS mass difference and the CP violating epsilon-prime / epsilon parameter.

hep-ph↗