Searcharxiv⌕ Search

arXiv subjects

Ll. Ametller

Publications and source records attributed to Ll. Ametller.

17 recordsLinked to original sources
arXiv2019

The Full Lepton Flavor of the Littlest Higgs Model with T-parity

We re-examine lepton flavor violation (LFV) in the Littlest Higgs model with T--parity (LHT) including the full T--odd (non-singlet) lepton and Goldstone sectors. The heavy leptons induce two independent sources of LFV associated with the couplings necessary to give masses to the T--odd mirror fermions and to their partners in right-handed $SO(5)$ multiplets, respectively. The latter, which have been neglected in the past, can be decoupled from gauge mediated processes but not from Higgs mediated ones and must therefore also be included in a general analysis of LFV in the LHT. We also further extend previous analyses by considering on-shell $Z$ and Higgs LFV decays together with the LFV processes at low momentum transfer. We show that current experimental limits can probe the LHT parameter space up to global symmetry breaking scales $f \sim 10$ TeV. For lower $f$ values $\gtrsim 1$ TeV, $μ-e$ transitions require the misalignment between the heavy and the Standard Model charged leptons to be $\lesssim 1 \, \%$. Future LFV experiments using intense muon beams should be sensitive to misalignments below the per mille level. For $τ$ LFV transitions, which could potentially be observed at Belle II and the LHC as well as future lepton colliders, we find that generically they can not discriminate between the LHT and supersymmetric models though in some regions of parameter space this may be possible.

hep-ph↗
arXiv2019

Lepton Flavor Changing Higgs decays in the Littlest Higgs Model with T-parity

We calculate loop induced lepton flavor violating Higgs decays in the Littlest Higgs model with T-parity. We find that a finite amplitude is obtained only when all contributions from the T-odd lepton sector are included. This is in contrast to lepton flavor violating processes mediated by gauge bosons where the partners of the right-handed mirror leptons can be decoupled from the spectrum. These partners are necessary to cancel the divergence in the Higgs mass introduced by the mirror leptons but are otherwise unnecessary and assumed to be decoupled in previous phenomenological studies. Furthermore, as we emphasize, including the partner leptons in the spectrum also introduces a new source of lepton flavor violation via their couplings to the physical pseudo-Goldstone electroweak triplet scalar. Although this extra source also affects lepton flavor changing gauge transitions, it decouples from these amplitudes in the limit of heavy mass for the partner leptons. We find that the corresponding Higgs branching ratio into taus and muons can be as large as $\sim 0.2 \times 10^{-6}$ for T-odd masses of the order a few TeV, a demanding challenge even for the high luminosity LHC.

hep-ph↗
arXiv2015

Insights into $Q^2 \bar{Q}^2$ states from an effective perspective

We discuss the two photon coupling of the lightest scalar meson on the basis of an extension of $χ$PT. Using low energy data on the pion form-factor and the $γγ\to π^+π^-(π^0π^0)$ cross-sections as inputs, we find $Γ(σ\toγγ) \cong 0.126~\rm{keV} $. The smallness of the result and the relative weight between its components, ${Γ_{γγ\to S_1}\over Γ{ γγ\toππ\to S_1}} < 1$, suggests that the scalar $0^{++}$ meson is mainly a $Q^2\bar{Q}^2$ state.

hep-ph↗
arXiv2005

QCD corrections to forward-backward charge asymmetries in l- l+ j production at hadron colliders

The large cross sections for gauge boson production at the Fermilab Tevatron and the CERN Large Hadron Collider (LHC) might give a chance to determine the electroweak parameters with high precision. We calculated two different forward-backward charge asymmetries (A^CS_FB and A^j_FB) of lepton pairs in events with a large transverse momentum jet p p (pbar) -> Z, gamma* + j -> e- e+ j at next-to-leading order (NLO), O(alpha_s) corrections, making use of the Monte Carlo programs MCFM and ALPGEN. These observables could provide a new determination of the weak mixing angle sin^2 theta^lept_eff (M_Z^2) with a statistical precision for each lepton flavour of 10^{-3} (7 x 10^{-3}) at LHC (Tevatron). If b jets are identified, a new asymmetry with respect to the b quark (A^b_FB) can also be measured with a statistical precision of 2 x 10^{-3} (4 x 10^{-2}) at LHC (Tevatron). Finally, we comment on the dependence of our results on various sources of uncertainties and compare, in the case of A^b_FB, the exact result with an approximation that might be more suitable when performing a realistic experimental analysis.

hep-ph↗
arXiv2005

NLO forward-backward charge asymmetries in p p (p bar p) -> l- l+ jet production at large hadron colliders

We consider the next-to-leading order corrections, O(alpha_s), to forward-backward charge asymmetries for lepton-pair production in association with a large transverse momentum jet at large hadron colliders. We find that the leading order results are essentially confirmed. Although experimentally challenging and in practice with large backgrounds, these observables could provide a new determination of the weak mixing angle sin2 theta^lept_eff (M_Z^2) with a statistical precision for each lepton flavour of ~10^(-3) (7 x 10^{-3}) at LHC (Tevatron), and if b jets are identified, of the b quark Z asymmetry A^b_{FB} with a statistical precision of ~ 2x10^{-3} (4x10{-2}) at LHC (Tevatron).

hep-ph↗
arXiv2002

Forward-backward asymmetries of lepton pairs in events with a large transverse momentum jet at hadron colliders

We discuss forward-backward charge asymmetries for lepton pair production in association with a large transverse momentum jet at hadron colliders. The measurement of the lepton charge asymmetry relative to the jet direction A_{FB}^j gives a new determination of the effective weak mixing angle sin^2{θ_{eff}^{lept}(M_Z^2)} with in principle a statistical precision after cuts of \sim 10^{-3} (8 x 10^{-3}) at LHC (Tevatron), due to the large cross section of the process with initial gluons. The identification of b jets also allows for the measurement of the bottom quark Z asymmetry A_{FB}^b at hadron colliders although with a lower precision than at LEP, the resulting statistical precision for sin^2{θ_{eff}^{lept}(M_Z^2)} being \sim 9 x 10^{-4} (2 x 10^{-2} at Tevatron).

hep-ph↗
arXiv2001

Electromagnetic corrections to gamma pi(+) -> pi(0) pi(+)

The amplitude for the anomalous transitions gamma pi(+) -> pi(0) pi(+) is analyzed within Chiral Perturbation Theory including electromagnetic interactions. The presence of a t-channel one-photon exchange contribution induces sizeable e^2 corrections which enhance the cross-section in the threshold region and bring the theoretical prediction into agreement with available data. In the case of the crossed reaction gamma pi(0) -> pi(+) pi(-), the same contribution appears in the s-channel and its effects are small.

hep-ph↗
arXiv1999

Zt and γt production via top flavour-changing neutral couplings at the Fermilab Tevatron

Associated single top production with a Z boson or a photon at large hadron colliders provides a precise determination of top flavour-changing neutral couplings. The best way to measure these couplings with the up quark at Tevatron is to search for events with three jets and missing energy or events with a photon, a charged lepton, a jet and missing energy. Other decay channels are also discussed.

hep-ph↗
arXiv1999

Low-energy photon-photon fusion into three pions in Generalized Chiral Perturbation Theory

The processes $γγ\toπ^0π^0π^0$ and $γγ\to π^+ π^- π^0$ are considered in Generalized Chiral Perturbation theory, in view of their potential sensitivity to the mechanism of spontaneous breaking of chiral symmetry and to various counterterms. The amplitudes are computed up to order ${\cal O}(p^6)$. The event production rates are estimated for the Daphne $ϕ$-Factory and for a future $τ$-Charm Factory.

hep-ph↗
arXiv1997

Chiral-loop and vector-meson contributions to eta -> pi pi gamma gamma decays

The process eta -> pi0 pi0 gamma gamma is discussed in Chiral Perturbation Theory (ChPT) extending two recent analyses. Special attention is devoted to one-loop corrections, eta-eta' mixing effects and vector-meson dominance of ChPT counter-terms. The less interesting eta -> pi^+ pi^- gamma gamma transition is briefly discussed too.

hep-ph↗
arXiv1996

Neural network analysis for gamma gamma -> 3pi at Daphne

We consider the possibility of using neural networks in experimental data analysis in Daphne. We analyze the process $γγ\to π^+ π^- π^0$ and its backgrounds using neural networks and we compare their performances with traditional methods of applying cuts on several kinematical variables. We find that the neural networks are more efficient and can be of great help for processes with small number of produced events.

hep-ph↗
arXiv1996

$γγ\to πππ$ to One Loop in Chiral Perturbation Theory

The $γγ\to π^0 π^0 π^0$ and $γγ\to π^+ π^- π^0$ amplitudes are discussed in the general context of Chiral Perturbation Theory (ChPT) to $O(p^6)$. Chiral loops are found to play a major role. This makes these processes a good test of ChPT, mainly in its anomalous sector. We correct earlier numerical results at tree level and determine the one-loop results as well.

hep-ph↗
arXiv1994

Enhancing the top signal at Tevatron using Neural Nets

We show that Neural Nets can be useful for top analysis at Tevatron. The main features of $t\bar t$ and background events on a mixed sample are projected in a single output, which controls the efficiency and purity of the $t\bar t$ signal.

hep-ph↗
arXiv1993

The $π^0\to e^+e^-$ and $η\to μ^+ μ^-$ Decays Revisited

The rare $π^0 \to e^+e^-$ and $η\to μ^+μ^-$ decays are calculated in different schemes, which are seen to be essentially equivalent to and produce the same results as conventional Vector-Meson Dominance. We obtain the theoretical predictions $B(π^0 \to e^+e^-) = (6.41 \pm 0.19)\times 10^{-8}$ and $B(η\to μ^+μ^-) = (1.14 +0.06 -0.03) \times 10^{-5}$ in agreement with recent experimental data.

hep-ph↗
arXiv1993

Semileptonic $π$ and $K$ Decays and the Chiral Anomaly at One-loop

We calculate the $O(p^6)$ corrections to the anomalous form factors appearing in $π^+$, $K^+ \to e^+ νγ,\ μ^+νγ$ and $K_{l4}$ decays in Chiral Perturbation Theory. The relevant dimension $6$ terms of the lagrangian are evaluated assuming their saturation by the vector meson contribution.

hep-ph↗
arXiv1992

Z' Decays into Four Fermions

If a new $Z'$ is discovered with a mass $\sim 1 \ TeV$ at LHC/SSC, its (rare) decays into two charged leptons plus missing transverse energy will probe the $Z'$ coupling to the lepton doublet $(ν,e)_L$ and to $W^+W^-$, allowing further discrimination among extended electroweak models.

hep-ph↗