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Ruben Sandapen

Publications and source records attributed to Ruben Sandapen.

At least 19 recordsLinked to original sources
arXiv2026

The 2027-2034 Vision for Nuclear Physics in Canada, with an outlook to 2041

The Canadian subatomic physics community establishes its scientific, and thus funding, priorities through periodic Long-Range Plans (LRP). The community is now putting together a new LRP, which will be in effect from 2027 through 2034, with its scope extending through 2041. As part of this process, the Canadian Institute of Nuclear Physics (CINP) has put together a strategic report, following an extensive consultation process. The report describes the broad and ambitious research program undertaken by the Canadian nuclear physics research community, both onshore and abroad, touching on key questions regarding the origin, evolution, and structure of visible matter in the universe. This document provides a grid of different Canadian nuclear physics projects undertaken now and in the future, and their associated timelines. It concludes with specific recommendations for maximizing Canadian scientific output in nuclear physics.

nucl-ex↗
arXiv2025

A holographic analysis of the pion

Being simultaneously a bound state and a pseudo-Goldstone boson of chiral symmetry breaking, the pion is an ideal probe of the intertwined phenomena of confinement and chiral symmetry breaking in QCD. Here, we compute the low energy pion observables with light-front holography, taking into account longitudinal dynamics using an ansatz that explores the strong degeneracy of the `t Hooft and Li-Vary models as well as their underlying connection in $\mathrm{AdS}_3$ as recently noted by Vegh.

hep-ph↗
arXiv2024

A holographic analysis of the pion

Inspired by light-front holography, we compute the pion mass, charge radius, decay constant, electromagnetic form factor and electromagnetic transition form factor. To do so, we model the longitudinal quark dynamics using potentials due to 't Hooft and to Li & Vary. We find a longitudinal wavefunction that is rather more peaked about $x \sim 1/2$ than in previous studies. We also explore the strong degeneracy between these two potentials and conclude by noting that one scenario that accords well with the data also maps onto an equation previously noted by Vegh that describes the dynamics of a four-segmented string in $\mathrm{AdS}_3$.

hep-ph↗
arXiv2022

Pion spectroscopy and dynamics using the holographic light-front Schrödinger equation and the 't Hooft equation

We show that the holographic Schrödinger equation of light-front chiral QCD, together with the 't Hooft equation of (1+1)-dimensional QCD in the large $N_c$ limit, can simultaneously describe pion spectroscopy as well as the pion decay constant, charge radius, electromagnetic form factor, photon-to-pion transition form factor, Parton Distribution Function (PDF) and Distribution Amplitude (DA). Furthermore, the chiral-limit constraints, as encoded in the Gell-Mann-Oakes-Renner (GMOR) relation, are satisfied.

hep-ph↗
arXiv2021

Two Schrödinger-like Equations for hadrons

In this talk, based on [1,2], I argue that the holographic Schrödinger Equation of $(3+1)$-dim, conformal light-front QCD and the 't Hooft Equation of $(1+1)$-dim, large $N_c$ QCD, can be complementary to each other in providing a first approximation to hadron spectroscopy. Together, the two equations play a role in hadronic physics analogous that of the ordinary Schrödinger Equation in atomic physics.

hep-ph↗
arXiv2021

Extending light-front holographic QCD using the 't Hooft Equation

We show the 't Hooft Equation and the light-front holographic Schrödinger Equation are complementary to each other in governing the transverse and longitudinal dynamics of colour confinement in quark-antiquark mesons. Together, they predict remarkably well the light, heavy-light and heavy-heavy meson spectroscopic data. The universal emerging hadronic scale of light-front holography, $κ\approx 0.5$ GeV, controls the transverse dynamics of confinement in all mesons. In heavy-heavy mesons, it also coincides numerically with the 't Hooft coupling which governs longitudinal confinement, thus reflecting the restoration of manifest 3-dimensional rotational symmetry

hep-ph↗
arXiv2021

Hadron spectroscopy using the light-front holographic Schrödinger equation and the 't Hooft equation

Light-front holographic QCD provides a successful first approximation to hadron spectroscopy in the chiral limit of $(3+1)$-dim light-front QCD, where a holographic Schrödinger-like equation, with an emerging confining scale, $κ$, governs confinement in the transverse direction. In its supersymmetric formulation, light-front holography predicts that each baryon has two superpartners: a meson and a tetraquark, with their degenerate masses being generated by the same scale, $κ$. In nature, this mass degeneracy is lifted by chiral symmetry breaking and longitudinal confinement. In this paper, we show that the latter can be successfully captured by the 't Hooft equation of $(1+1)$-dim, large $N_c$, QCD. Together, the holographic Schrödinger equation and the 't Hooft equation, provide a good global description of the data across the full hadron spectrum with a universal $κ$.

hep-ph↗
arXiv2021

Nucleon electroweak form factors using spin-improved holographic light-front wavefunctions

We construct spin-improved holographic light front wavefunctions for the nucleons (viewed as quark-diquark systems) and use them to successfully predict their electromagnetic Sachs form factors, their electromagnetic charge radii, as well as the axial form factor, charge and radius of the proton. The confinement scale is the universal mass scale of light-front holography, previously extracted from spectroscopic data for light hadrons. With the Dirac and Pauli form factors normalized using the quark counting rules and the measured anomalous magnetic moments respectively, the masses of the quark and diquark are the only remaining adjustable parameters. We fix them using the data set for the proton's Dirac-to-Pauli form factor ratio, and then predict all other data without any further adjustments of parameters. Agreement with data at low momentum-transfer is excellent. Our findings support the idea that light (pseudoscalar and vector) mesons and the nucleons share a nonperturbative universal holographic light-front wavefunction which is modified differently by their spin structures.

hep-ph↗
arXiv2020

Light-front holographic radiative transition form factors for light mesons

We predict the $\mathcal{V} \to \mathcal{P} γ$ decay widths and the $\mathcal{V} \to \mathcal{P} γ^{*}$ transition form factors, where $\mathcal{V}=(ρ, ω, K^*, ϕ)$ and $\mathcal{P}= (π,K, η,η^\prime)$, using spin-improved holographic light-front wavefunctions for the mesons. We find excellent agreement with the available data for both the decay widths and the timelike transition form factors extracted from the leptonic conversion decays $\mathcal{V} \to \mathcal{P} l^+ l^-$.

hep-ph↗
arXiv2020

An overview of light-front holography

Light-front holography offers a successful first semiclassical approximation to hadronic spectroscopy and dynamics. We review its underlying assumptions, its remarkable predictions as well as attempts to go beyond the semiclassical approximation in order to the describe a wide range of data with a universal AdS/QCD mass scale.

hep-ph↗
arXiv2019

Re-examination of the rare decay $B_s \to ϕμ^+ μ^-$ using holographic light-front QCD

We calculate the Standard Model (SM) predictions for the differential branching ratio of the rare $B_s \to ϕμ^+ μ^-$ decays using $B_s \to ϕ$ transition form factors (TFFs) obtained using holographic light-front QCD (hQCD) instead of the traditional QCD sum rules (QCDSR) . Our predictions for the differential branching ratio is in better agreement with the LHCb data. Also, we find that the hQCD prediction for $R_{K^*ϕ}$, the ratio of the branching fraction of $B \to K^* μ^+ μ^-$ to that of $B_s \to ϕμ^+ μ^-$ , is in excellent agreement with both the LHCb and CDF results in low $q^2$ range.

hep-ph↗
arXiv2019

Predicting the light-front holographic TMDs of the pion

We predict the twist-2 Transverse Momentum Dependent parton distribution functions (TMDs) of the pion, namely the unpolarized quark TMD, $f_{1}(x, k_\perp)$, and the transversely polarized quark TMD, also known as the Boer-Mulders function, $h^\perp_{1}(x, k_\perp)$, using a holographic light-front pion wavefunction with dynamical spin effects. These spin effects, in conjunction with gluon rescattering, are crucial to predict a non-zero holographic Boer-Mulders function. We investigate the use of a non-perturbative SU(3) gluon rescattering kernel, thus going beyond the usual approximation of perturbative U(1) gluons. We find that the non-perturbative color dynamics offer a more promising way to describe the available lattice data on the generalized Boer-Mulders shifts.

hep-ph↗
arXiv2019

Holographic QCD predictions for rare B decays

Light-front wavefunctions obtained from holographic light-front QCD are used to obtain the distributions amplitudes for $K^*$ vector meson. Consequently, alternate predictions for rare B transitions to $K^*$ form factors are presented. In this talk, I compare our results for some rare B decay channels to those obtained from QCD sum rules and available experimental data.

hep-ph↗
arXiv2018

Dynamical spin effects within the pseudo scalar nonet within holographic QCD

We investigate the importance of dynamical spin effects in the holographic light-front wavefunctions of the pseudoscalar mesons. We find that these effects are crucial to describe the pion data while they are not necessary to describe the available kaon data. For $η-η^\prime$ system, we find that dynamical spin effects are required to describe their transition form factors data.

hep-ph↗
arXiv2018

Probing transition form factors in the rare $B\to K^*ν\barν$ decay

We compare the Standard Model (SM) predictions for the differential branching ratio of the rare $B\to K^*ν\barν$ decays using $B \to K^*$ form factors obtained from holographic light-front QCD (hLFQCD) and Sum Rules (SR) Distribution Amplitudes. For the total branching ratio, we predict $\mathcal{BR}(\process)_{\rm hLFQCD}=(6.36^{+0.59}_{-0.74})\times 10^{-6}$ and $\mathcal{BR}(\process)_{\rm SR}=(8.14^{+0.16}_{- 0.17})\times 10^{-6}$. More interestingly, we find that the two model predictions for the differential branching ratio are sufficiently different at low momentum transfer, so that future measurements at Belle II may be able to discriminate between them. We also confirm numerically that the $K^*$ longitudinal polarization fraction $F_L$ has little sensitivity to the non-perturbative form factors and is thus an excellent observable to probe New Physics signals. We predict $F_L=0.40^{+0.02}_{-0.01}(0.41\pm 0.01)$ using hLFQCD (SR).

hep-ph↗
arXiv2018

Dynamical spin effects in the holographic light-front wavefunctions of light pseudoscalar mesons

We quantify the importance of dynamical spin effects in the holographic light-front wavefunctions of the pion, kaon, $η$ and $η^\prime$. Using a universal AdS/QCD scale and constituent quark masses, we find that such effects are maximal in the pion where they lead to an excellent simultaneous description of a wide range of data: the decay constant, charge radius, spacelike EM and transition form factors, as well as, after QCD evolution, both the parton distribution function and the parton distribution amplitude data from Fermilab. These dynamical spin effects lead up to a $30\%$ chance of finding the valence quark and antiquark with aligned spins in the pion. The situation is very different for the kaon, where a simultaneous description of the available data (decay constant, radius and spacelike EM form factor) prefer no dynamical spin effects at all. The situation is less clear for the $η$ and $η^\prime$: while their radiative decay widths data are consistent with dynamical spin effects only in $η^\prime$, the data on their spacelike transition form factors clearly favor maximal dynamical spin effects in both mesons.

hep-ph↗
arXiv2018

Comparing AdS/QCD and Sum Rules predictions for $B\to K^*ν\barν$

Using the form factors obtained from holographic AdS/QCD and QCD sum rules, we predict the differential branching ratio and longitudinal polarization fraction for the rare $B\to K^* ν\barν$ decay. This is an interesting decay channel as it does not suffer from hadronic uncertainties beyond the form factors. We point out that the future measurement of the $B\to K^* ν\barν$ with around 30% accuracy can discriminate between the two models.

hep-ph↗