arXiv subjects

Benjamín Grinstein

Publications and source records attributed to Benjamín Grinstein.

At least 19 recordsLinked to original sources

arXiv2026

Counting axions with IAXO

The existence of multiple axion species is a generic prediction of a number of extensions of the Standard Model. If more than one axion couples to photons, their combined signal in helioscope experiments may mimic that of a single axion with different parameters. This raises a fundamental question: if a next-generation helioscope such as IAXO detected a signal, would we be able to disentangle whether it originated from one or multiple axions? To answer this question, we first recast current CAST bounds and derive IAXO/IAXO+ projections in the two-axion parameter space, identifying the regions where a signal could be observed. Then, we analyze the spectral signatures of axion flavor oscillations in both the quasi-degenerate and hierarchical mass regimes, and point out where IAXO can discriminate a two-axion signal from the single-axion hypothesis given the expected energy resolutions of the detector. Finally, we show that these results extend to a broad class of $N$-axion systems.

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arXiv2025

Taming forward scattering singularities in partial waves

Perturbative partial-wave amplitudes diverge in cases with a massless exchanged particle in the $t$-channel. We argue that the divergence is an artifact of perturbation theory and give a prescription for the all-orders correction factor that renders the partial waves finite. As an example, we apply this to longitudinal $W^+W^-$ elastic scattering, for which there is a photon exchange $t$-channel contribution, and derive improved quasi-perturbative unitarity bounds on the mass of the Higgs. The method is also useful for $t$-channel exchange of very light particles.

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arXiv2024

Most general EFTs from spurion analysis: Hilbert series and Minimal Lepton Flavor Violation

We derive a saturation theorem for general Effective Field Theories (EFTs) constructed using spurion analysis. Let $S$ be a set of spurion fields introduced to organize the breaking of a global symmetry $G_f$, and $H_S$ be the subgroup of $G_f$ that remains unbroken under a generic vacuum expectation value $\langle S\rangle$; we show that the EFT Lagrangian constructed from the spurion analysis $saturates$ the EFT Lagrangian without the spurions but restricted to $H_S$ invariance, provided that arbitrary powers of the spurion fields are allowed. As examples, we study several implementations of the Minimal Lepton Flavor Violation (MLFV) principle, corresponding to various origins of the neutrino masses. In each scenario, we compute the Hilbert series to obtain the numbers of independent lepton flavor covariants that appear in the corresponding EFT at mass dimension 6. These numbers agree with the number of $H_S$ invariants in the EFT without the spurions, demonstrating the saturation theorem. Motivated by phenomenological connections, we provide linearly independent spurion polynomials for selected lepton flavor covariants. An ancillary file is supplied at https://github.com/HilbertSeries/Group_Invariants_and_Covariants , which is a Mathematica notebook that provides functions for computing general Hilbert series of invariants and covariants of compact classical groups. It presents examples demonstrating the use of the code, including the Hilbert series for our MLFV scenarios.

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arXiv2024

Accidental Symmetries, Hilbert Series, and Friends

Accidental symmetries in effective field theories can be established by computing and comparing Hilbert series. This invites us to study them with the tools of invariant theory. Applying this technology, we spotlight three classes of accidental symmetries that hold to all orders for non-derivative interactions. They are broken by derivative interactions and become ordinary finite-order accidental symmetries. To systematically understand the origin and the patterns of accidental symmetries, we introduce a novel mathematical construct - a (non-transitive) binary relation between subgroups that we call $friendship$. Equipped with this, we derive new criteria for all-order accidental symmetries in terms of $friends$, and criteria for finite-order accidental symmetries in terms of $friends\ ma\ non\ troppo$. They allow us to verify and identify accidental symmetries more efficiently without computing the Hilbert series. We demonstrate the success of our new criteria by applying them to a variety of sample accidental symmetries, including the custodial symmetry in the Higgs sector of the Standard Model effective field theory.

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arXiv2023

Hilbert series for covariants and their applications to Minimal Flavor Violation

We elaborate how to apply the Hilbert series method to enumerating group covariants, which transform under any given representation, including but going beyond group invariants. Mathematically, group covariants form a module over the ring of the invariants. The number of independent covariants is given by the rank of the module, which can be computed by taking a ratio of two Hilbert series. In many cases, the rank equals the dimension of the group covariant representation. When this happens, we say that there is a \textit{rank saturation}. We apply this technology to revisit the hypothesis of Minimal Flavor Violation in constructing Effective Field Theories beyond the Standard Model. We find that rank saturation is guaranteed in this case, leading to the important consequence that the MFV symmetry principle does not impose any restriction on the EFT, \textit{i.e.} MFV SMEFT = SMEFT, in the absence of additional assumptions.

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arXiv2022

A novel determination of the $B_c$ lifetime

To reduce the current theory uncertainties a novel way to determine the $B_c$ lifetime, $τ_{B_c}$, is proposed. Taking the difference of the $B_c$, and $B$ and $D$ meson decay rates eliminates the leading contributions from the calculation, which exhibit large scale and scheme dependence. The uncertainties in the proposed determination of $τ_{B_c}$ are analyzed and improvements are proposed. The method predicts a value of $τ_{B_c}$ in tension with the experimental determination. Several explanations are considered, including underestimation of uncertainties, duality violation and new physics. We discuss quantitative evidence that our method is vitiated by duality violation in the large mass OPE used to calculate nonleptonic decay rates of $B$ and $D$ mesons.

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arXiv2022

The Strange Physics of Dark Baryons

Dark sector particles at the GeV scale carrying baryon number provide an attractive framework for understanding the origin of dark matter and the matter-antimatter asymmetry of the universe. We demonstrate that dark decays of hadronic states containing strange quarks -- hyperons -- offer excellent prospects for discovering such dark baryons. Building up on novel calculations of the matrix elements relevant for hyperon dark decays, and in view of various collider, flavor, and astrophysical constraints, we determine the expected rates at hyperon factories like BESIII and LHCb. We also highlight the interesting theoretical connections of hyperon dark decays to the neutron lifetime anomaly and Mesogenesis.

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arXiv2022

The $B_c$ decay rate in the Standard Model

The Standard Model decay rate of the $B_c$ meson is discussed together with a novel approach based on the usage of experimental data in combination with an operator product expansion. In the new method differences of $B,\,D$ and $B_c$ meson decay rates are considered, for which the free-quark contributions drop out, leading to a reduction of the theory prediction.

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arXiv2022

$τ_{B_c}$ in the Standard Model

The Standard Model prediction of the $B_c$ lifetime is discussed, together with the dominant uncertainties and strategies on how to improve them. Furthermore, a new method to compute the $B_c$ lifetime based on the operator product expansion is proposed. It relies on differences of $B,\,D$ and $B_c$ meson decay rates, in which the free-quark contributions cancel out, reducing the uncertainty of the theory prediction.

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arXiv2021

The $B_c$ lifetime in the Standard Model

Using an operator product expansion (OPE) approach an updated Standard Model prediction of the $B_c$ lifetime is presented. The computation in three different mass schemes for the heavy quarks leads to three different values consistent with each other and with experiment. Furthermore a novel way to compute the $B_c$ lifetime is presented, taking differences of $B,D$ and $B_c$ meson decay rates. In this approach the leading contributions from free-quark decays cancel out, leading to a reduction of scale and scheme dependence.

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arXiv2021

Implications of new evidence for lepton-universality violation in $b\to s\ell^+\ell^-$ decays

Motivated by renewed evidence for new physics in $b \to s\ell\ell$ transitions in the form of LHCb's new measurements of theoretically clean lepton-universality ratios and the purely leptonic $B_s\toμ^+μ^-$ decay, we quantify the combined level of discrepancy with the Standard Model and fit values of short-distance Wilson coefficients. A combination of the clean observables $R_K$, $R_{K^*}$, and $B_s\to μμ$ alone results in a discrepancy with the Standard Model at $4.0σ$, up from $3.5σ$ in 2017. One-parameter scenarios with purely left-handed or with purely axial coupling to muons fit the data well and result in a $\sim 5 σ$ pull from the Standard Model. In a two-parameter fit of %$C_9$ and $C_{10}$, new-physics contributions with both vector and axial-vector couplings to muons the allowed region is much more restricted than in 2017, principally due to the much more precise result on $B_s \to μ^+ μ^-$, which probes the axial coupling to muons.Including angular observables data restricts the allowed region further.A by-product of our analysis is an updated average of $\text{BR}(B_s \to μ^+ μ^-) = (2.8\pm 0.3) \times 10^{-9}$.

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arXiv2021

Standard Model prediction of the $B_c$ lifetime

Applying an operator product expansion approach we update the Standard Model prediction of the $B_c$ lifetime from over 20 years ago. The non-perturbative velocity expansion is carried out up to third order in the relative velocity of the heavy quarks. The scheme dependence is studied using three different mass schemes for the $\bar b$ and $c$ quarks, resulting in three different values consistent with each other and with experiment. Special focus has been laid on renormalon cancellation in the computation. Uncertainties resulting from scale dependence, neglecting the strange quark mass, non-perturbative matrix elements and parametric uncertainties are discussed in detail. The resulting uncertainties are still rather large compared to the experimental ones, and therefore do not allow for clear-cut conclusions concerning New Physics effects in the $B_c$ decay.

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arXiv2020

Towards the discovery of new physics with lepton-universality ratios of $b\to s\ell\ell$ decays

Tests of lepton-universality as rate ratios in $b\to s \ell\ell$ transitions can be predicted very accurately in the Standard Model. The deficits with respect to expectations reported by the LHCb experiment in muon-to-electron ratios of the $B\to K^{(*)}\ell\ell$ decay rates thus point to genuine manifestations of lepton non-universal new physics. In this paper, we analyse these measurements in the context of effective field theory. First, we discuss the interplay of the different operators in $R_K$ and $R_{K^*}$ and provide predictions for $R_{K^*}$ in the Standard Model and in new-physics scenarios that can explain $R_K$. We also provide approximate numerical formulas for these observables in bins of interest as functions of the relevant Wilson coefficients. Secondly, we perform frequentist fits to $R_{K}$ and $R_{K^*}$. The SM disagrees with these measurements at $3.7σ$ significance. We find excellent fits in scenarios with combinations of $\mathcal O_{9(10)}^\ell=\bar sγ^μb_L~\ellγ_μ(γ_5) \ell$ operators, with pulls relative to the Standard Model in the region of $4σ$. An important conclusion of our analysis is that a lepton-specific contribution to $\mathcal O_{10}$ is essential to understand the data. Under the hypothesis that new-physics couples selectively to the muons, we also present fits to other $b\to sμμ$ data with a conservative error assessment, and comment on more general scenarios. Finally, we discuss new lepton universality ratios that, if new physics is the origin of the observed discrepancy, should contribute to the statistically significant discovery of new physics in the near future.

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arXiv2019

Revisiting the new-physics interpretation of the $b\to cτν$ data

We revisit the status of the new-physics interpretations of the anomalies in semileptonic $B$ decays in light of the new data reported by Belle on the lepton-universality ratios $R_{D^{(*)}}$ using the semileptonic tag and on the longitudinal polarization of the $D^*$ in $B\to D^*τν$, $F_L^{D^*}$. The preferred solutions involve new left-handed currents or tensor contributions. Interpretations with pure right-handed currents are disfavored by the LHC data, while pure scalar models are disfavored by the upper limits derived either from the LHC or from the $B_c$ lifetime. The observable $F_L^{D^*}$ also gives an important constraint leading to the exclusion of large regions of parameter space. Finally, we investigate the sensitivity of different observables to the various scenarios and conclude that a measurement of the tau polarization in the decay mode $B\to Dτν$ would effectively discriminate among them.

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arXiv2017

On the Heat Kernel and Weyl Anomaly of Schrödinger invariant theory

We propose a method inspired from discrete light cone quantization (DLCQ) to determine the heat kernel for a Schrödinger field theory (Galilean boost invariant with $z=2$ anisotropic scaling symmetry) living in $d+1$ dimensions, coupled to a curved Newton-Cartan background starting from a heat kernel of a relativistic conformal field theory ($z=1$) living in $d+2$ dimensions. We use this method to show the Schrödinger field theory of a complex scalar field cannot have any Weyl anomalies. To be precise, we show that the Weyl anomaly $\mathcal{A}^{G}_{d+1}$ for Schrödinger theory is related to the Weyl anomaly of a free relativistic scalar CFT $\mathcal{A}^{R}_{d+2}$ via $\mathcal{A}^{G}_{d+1}= 2πδ(m) \mathcal{A}^{R}_{d+2}$ where $m$ is the charge of the scalar field under particle number symmetry. We provide further evidence of vanishing anomaly by evaluating Feynman diagrams in all orders of perturbation theory. We present an explicit calculation of the anomaly using a regulated Schrödinger operator, without using the null cone reduction technique. We generalise our method to show that a similar result holds for one time derivative theories with even $z>2$.

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arXiv2017

Lectures on Flavor Physics and CP Violation

Two short introductory lectures on Flavor Physics delivered at CLASHEP 2015. Among included topics: The KM matrix and the KM model of CP-violation, Determination of KM Elements, FCNC and GIM, New Physics and Flavor, Neutral Meson Mixing and CP Asymmetries. Many problems for the student, and solutions to selected problems, included.

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arXiv2016

Weyl Consistency Conditions in Non-Relativistic Quantum Field Theory

Weyl consistency conditions have been used in unitary relativistic quantum field theory to impose constraints on the renormalization group flow of certain quantities. We classify the Weyl anomalies and their renormalization scheme ambiguities for generic non-relativistic theories in 2+1 dimensions with anisotropic scaling exponent z=2; the extension to other values of z are discussed as well. We give the consistency conditions among these anomalies. As an application we find several candidates for a C-theorem. We comment on possible candidates for a $C$-theorem in higher dimensions.

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arXiv2016

The lifetime of the $B_c^-$ meson and the anomalies in $B\to D^{(*)}τν$

We investigate a new constraint on new-physics interpretations of the anomalies observed in $B\to D^{(*)}τν$ decays making use of the lifetime of the $B_c^-$ meson. A constraint is obtained by demanding that the rate for $B_c^-\toτ^-\barν$ does not exceed the fraction of the total width that is allowed by the calculation of the lifetime in the standard model. This leads to a very strong bound on new-physics scenarios involving scalar operators since they lift the slight, but not negligible, chiral suppression of the $B_c^-\toτ^-\barν$ amplitude in the standard model. The new constraint renders a scalar interpretation of the enhancement measured in $R_{D^*}$ implausible, including explanations implementing extra Higgs doublets or certain classes of leptoquarks. We also discuss the complementarity of $R_{D^{(*)}}$ and a measurement of the longitudinal polarization of the $τ$ in the $B\to D^*τν$ decay in light of our findings.

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