arXiv subjects

Emanuele Mereghetti

Publications and source records attributed to Emanuele Mereghetti.

At least 55 records · Page 3Linked to original sources

arXiv2022

The pion-nucleon sigma term from Lattice QCD

We summarize recent evidence, both from lattice QCD and chiral perturbation theory, that suggests that larger-than-expected excited-state contamination could be the reason for the tension between phenomenological determinations and previous direct lattice-QCD calculations of the pion--nucleon sigma term $σ_{πN}$. In addition, we extend the $χ$PT analysis by calculating the corrections due to including the $Δ(1232)$ resonance as an explicit degree of freedom. This correction is found to be small, thereby corroborating the excited-state effects found in the $Δ$-less calculation and the result for $σ_{πN}$.

hep-lat↗

arXiv2022

Neutrinoless Double-Beta Decay: A Roadmap for Matching Theory to Experiment

The observation of neutrino oscillations and hence non-zero neutrino masses provided a milestone in the search for physics beyond the Standard Model. But even though we now know that neutrinos are massive, the nature of neutrino masses, i.e., whether they are Dirac or Majorana, remains an open question. A smoking-gun signature of Majorana neutrinos is the observation of neutrinoless double-beta decay, a process that violates the lepton-number conservation of the Standard Model. This white paper focuses on the theoretical aspects of the neutrinoless double-beta decay program and lays out a roadmap for future developments. The roadmap is a multi-scale path starting from high-energy models of neutrinoless double-beta decay all the way to the low-energy nuclear many-body problem that needs to be solved to supplement measurements of the decay rate. The path goes through a systematic effective-field-theory description of the underlying processes at various scales and needs to be supplemented by lattice quantum chromodynamics input. The white paper also discusses the interplay between neutrinoless double-beta decay, experiments at the Large Hadron Collider and results from astrophysics and cosmology in probing simplified models of lepton-number violation at the TeV scale, and the generation of the matter-antimatter asymmetry via leptogenesis. This white paper is prepared for the topical groups TF11 (Theory of Neutrino Physics), TF05 (Lattice Gauge Theory), RF04 (Baryon and Lepton Number Violating Processes), NF03 (Beyond the Standard Model) and NF05 (Neutrino Properties) within the Theory Frontier, Rare Processes and Precision Frontier, and Neutrino Physics Frontier of the U.S. Community Study on the Future of Particle Physics (Snowmass 2021).

hep-ph↗

arXiv2022

Excited states and precision results for nucleon charges and form factors

The exponentially falling signal-to-noise ratio in all nucleon correlation functions, and the presence of towers of multihadron excited states with relatively small mass gaps makes extraction of matrix elements of various operators within the ground state nucleon challenging. Theoretically, the allowed positive parity states with the smallest mass gaps are the $N(\bm p)π(-\bm p)$, $N(\bm 0)π(\bm 0)π(\bm 0)$, $N(\bm p)π(\bm 0)$, $N(\bm 0)π(\bm p),\ \ldots$, states. A priori, the contribution of these states arises at one loop in chiral perturbation theory ($χ$PT), however, in many cases the contributions are enhanced. In this talk, I will review four such cases: the correlation functions from which the axial form factors, electric and magnetic form factors, the $Θ$-term contribution to neutron electric dipole moment (nEDM), and the pion-nucleon sigma term are extracted. Including appropriate multihadron states in the analysis can lead to significantly different results compared to standard analyses with the mass gaps taken from fits to 2-point functions. The $χ$PT case for $N π$ states is the most clear in the axial/pseudoscalar form factors which need to satisfy the PCAC relation between them. Our analyses, supported by $χ$PT, suggests similarly large effects in the calculations of the $Θ$-term and the pion-nucleon sigma term that have significant phenomenological implications.

hep-lat↗

arXiv2022

Calculation of neutron electric dipole moment due to the QCD topological term, Weinberg three-gluon operator and the quark chromoelectric moment

We present results for the neutron electric dipole moment due to the dimension 4 and dimension 6 gluonic CP violation, and the isovector quark chromoelectric dipole moment using clover valence quarks on HISQ dynamical ensembles generated by the MILC Collaboration. For the gluonic operators, we use the gradient flow scheme to obtain divergence-free continuum results. For the chromoelectric dipole moment operator, we use the unflowed local operator but discuss how the quadratically divergent mixing with the pseudoscalar operator can be controlled nonperturbatively.

hep-lat↗

arXiv2022

Pion-induced radiative corrections to neutron beta-decay

We compute the electromagnetic corrections to neutron beta decay using a low-energy hadronic effective field theory. We identify and compute new radiative corrections arising from virtual pions that were missed in previous studies. The largest correction is a percent-level shift in the axial charge of the nucleon proportional to the electromagnetic part of the pion-mass splitting. Smaller corrections, comparable to anticipated experimental precision, impact the $β$-$ν$ angular correlations and the $β$-asymmetry. We comment on implications of our results for the comparison of the experimentally measured axial charge with first-principle computations using lattice QCD and on the potential of $β$-decay experiments to constrain beyond-the-Standard-Model interactions.

nucl-th↗

arXiv2021

The pion-nucleon sigma term from lattice QCD

We present an analysis of the pion-nucleon $σ$-term, $σ_{πN}$, using six ensembles with 2+1+1-flavor highly improved staggered quark action generated by the MILC collaboration. The most serious systematic effect in lattice calculations of nucleon correlation functions is the contribution of excited states. We estimate these using chiral perturbation theory ($χ$PT), and show that the leading contribution to the isoscalar scalar charge comes from N$π$ and N$ππ$ states. Therefore, we carry out two analyses of lattice data to remove excited-state contamination, the standard one and a new one including N$π$ and N$ππ$ states. We find that the standard analysis gives $σ_{πN} = 41.9(4.9)$ MeV, consistent with previous lattice calculations, while our preferred $χ$PT-motivated analysis gives $σ_{πN} = 59.6(7.4)$ MeV, which is consistent with phenomenological values obtained using $π$N scattering data. Our data on one physical pion mass ensemble was crucial for exposing this difference, therefore, calculations on additional physical mass ensembles are needed to confirm our result and resolve the tension between lattice QCD and phenomenology.

hep-lat↗

arXiv2021

A low-energy perspective on the minimal left-right symmetric model

We perform a global analysis of the low-energy phenomenology of the minimal left-right symmetric model (mLRSM) with parity symmetry. We match the mLRSM to the Standard Model Effective Field Theory Lagrangian at the left-right-symmetry breaking scale and perform a comprehensive fit to low-energy data including mesonic, neutron, and nuclear $β$-decay processes, $ΔF=1$ and $ΔF=2$ CP-even and -odd processes in the bottom and strange sectors, and electric dipole moments (EDMs) of nucleons, nuclei, and atoms. We fit the Cabibbo-Kobayashi-Maskawa and mLRSM parameters simultaneously and determine a lower bound on the mass of the right-handed $W_R$ boson. In models where a Peccei-Quinn mechanism provides a solution to the strong CP problem, we obtain $M_{W_R} \gtrsim 5.5$ TeV at $95\%$ C.L. which can be significantly improved with next-generation EDM experiments. In the $P$-symmetric mLRSM without a Peccei-Quinn mechanism we obtain a more stringent constraint $M_{W_R} \gtrsim 17$ TeV at $95\%$ C.L., which is difficult to improve with low-energy measurements alone. In all cases, the additional scalar fields of the mLRSM are required to be a few times heavier than the right-handed gauge bosons. We consider a recent discrepancy in tests of first-row unitarity of the CKM matrix. We find that, while TeV-scale $W_R$ bosons can alleviate some of the tension found in the $V_{ud,us}$ determinations, a solution to the discrepancy is disfavored when taking into account other low-energy observables within the mLRSM.

hep-ph↗

arXiv2021

Leptonic anomalous magnetic moments in $ν$SMEFT

We investigate contributions to the anomalous magnetic moments of charged leptons in the neutrino-extended Standard Model Effective Field Theory ($ν$SMEFT). We discuss how $ν$SMEFT operators can contribute to a lepton's magnetic moment at one- and two-loop order. We show that only one operator can account for existing electronic and muonic discrepancies, assuming new physics appears above $1$ TeV. In particular, we find that a right-handed charged current in combination with minimal sterile-active mixing can explain the discrepancy for sterile neutrino masses of $\mathcal O(100)$ GeV while avoiding direct and indirect constraints. We discuss how searches for sterile neutrino production at the (HL-)LHC, measurements of $h\rightarrow μ^+ μ^-$ and searches for $h\rightarrow e^+ e^-$, neutrinoless double beta decay experiments, and improved unitarity tests of the CKM matrix can further probe the relevant parameter space.

hep-ph↗

arXiv2021

Towards grounding nuclear physics in QCD

Exascale computing could soon enable a predictive theory of nuclear structure and reactions rooted in the Standard Model, with quantifiable and systematically improvable uncertainties. Such a predictive theory will help exploit experiments that use nucleons and nuclei as laboratories for testing the Standard Model and its limitations. Examples include direct dark matter detection, neutrinoless double beta decay, and searches for permanent electric dipole moments of the neutron and atoms. It will also help connect QCD to the properties of cold neutron stars and hot supernova cores. We discuss how a quantitative bridge between QCD and the properties of nuclei and nuclear matter will require a synthesis of lattice QCD (especially as applied to two- and three-nucleon interactions), effective field theory, and ab initio methods for solving the nuclear many-body problem. While there are significant challenges that must be addressed in developing this triad of theoretical tools, the rapid advance of computing is accelerating progress. In particular, we focus this review on the anticipated advances from lattice QCD and how these advances will impact few-body effective theories of nuclear physics by providing critical input, such as constraints on unknown low-energy constants of the effective (field) theories. We also review particular challenges that must be overcome for the successful application of lattice QCD for low-energy nuclear physics. We describe progress in developing few-body effective (field) theories of nuclear physics, with an emphasis on HOBET, a non-relativistic effective theory of nuclear physics, which is less common in the literature. We use the examples of neutrinoless double beta decay and the nuclear-matter equation of state to illustrate how the coupling of lattice QCD to effective theory might impact our understanding of symmetries and exotic astrophysical environments.

nucl-th↗

arXiv2021

Charged Lepton Flavor Violation at the EIC

We present a comprehensive analysis of the potential sensitivity of the Electron-Ion Collider (EIC) to charged lepton flavor violation (CLFV) in the channel $ep\to τX$, within the model-independent framework of the Standard Model Effective Field Theory (SMEFT). We compute the relevant cross sections to leading order in QCD and electroweak corrections and perform simulations of signal and SM background events in various $τ$ decay channels, suggesting simple cuts to enhance the associated estimated efficiencies. To assess the discovery potential of the EIC in $τ$-$e$ transitions, we study the sensitivity of other probes of this physics across a broad range of energy scales, from $pp \to e τX$ at the Large Hadron Collider to decays of $B$ mesons and $τ$ leptons, such as $τ\to e γ$, $τ\to e \ell^+ \ell^-$, and crucially the hadronic modes $τ\to e Y$ with $Y \in \{ π, K, ππ, K π, ...\}$. We find that electroweak dipole and four-fermion semi-leptonic operators involving light quarks are already strongly constrained by $τ$ decays, while operators involving the $c$ and $b$ quarks present more promising discovery potential for the EIC. An analysis of three models of leptoquarks confirms the expectations based on the SMEFT results. We also identify future directions needed to maximize the reach of the EIC in CLFV searches: these include an optimization of the $τ$ tagger in hadronic channels, an exploration of background suppression through tagging $b$ and $c$ jets in the final state, and a global fit by turning on all SMEFT couplings, which will likely reveal new discovery windows for the EIC.

hep-ph↗

arXiv2021

Determining the leading-order contact term in neutrinoless double $\boldsymbolβ$ decay

We present a method to determine the leading-order (LO) contact term contributing to the $nn \to pp e^-e^-$ amplitude through the exchange of light Majorana neutrinos. Our approach is based on the representation of the amplitude as the momentum integral of a known kernel (proportional to the neutrino propagator) times the generalized forward Compton scattering amplitude $n(p_1) n(p_2) W^+ (k) \to p(p_1^\prime) p(p_2^\prime) W^- (k)$, in analogy to the Cottingham formula for the electromagnetic contribution to hadron masses. We construct model-independent representations of the integrand in the low- and high-momentum regions, through chiral EFT and the operator product expansion, respectively. We then construct a model for the full amplitude by interpolating between these two regions, using appropriate nucleon factors for the weak currents and information on nucleon-nucleon ($N\! N$) scattering in the $^1S_0$ channel away from threshold. By matching the amplitude obtained in this way to the LO chiral EFT amplitude we obtain the relevant LO contact term and discuss various sources of uncertainty. We validate the approach by computing the analog $I = 2$ $N\! N$ contact term and by reproducing, within uncertainties, the charge-independence-breaking contribution to the $^1S_0$ $N\! N$ scattering lengths. While our analysis is performed in the $\overline{\rm MS}$ scheme, we express our final result in terms of the scheme-independent renormalized amplitude ${\cal A}_ν(|{\bf p}|,|{\bf p}^\prime|)$ at a set of kinematic points near threshold. We illustrate for two cutoff schemes how, using our synthetic data for ${\cal A}_ν$, one can determine the contact-term contribution in any regularization scheme, in particular the ones employed in nuclear-structure calculations for isotopes of experimental interest.

nucl-th↗

arXiv2021

Towards complete leading-order predictions for neutrinoless double $β$ decay

The amplitude for the neutrinoless double $β$ ($0νββ$) decay of the two-neutron system, $nn\to ppe^-e^-$, constitutes a key building block for nuclear-structure calculations of heavy nuclei employed in large-scale $0νββ$ searches. Assuming that the $0νββ$ process is mediated by a light-Majorana-neutrino exchange, a systematic analysis in chiral effective field theory shows that already at leading order a contact operator is required to ensure renormalizability. In this work, we develop a method to estimate the numerical value of its coefficient in analogy to the Cottingham formula and validate the result by reproducing the charge-independence-breaking contribution to the nucleon-nucleon scattering lengths. Our central result, while derived in the $\overline{\text{MS}}$ scheme, is given in terms of the renormalized amplitude $\mathcal{A}_ν(|\mathbf{p}|,|\mathbf{p}^\prime|)$, matching to which will allow one to determine the contact-term contribution in regularization schemes employed in nuclear-structure calculations. Our results thus greatly reduce a crucial uncertainty in the interpretation of searches for $0νββ$ decay.

nucl-th↗

arXiv2021

Contribution of the QCD $Θ$-term to nucleon electric dipole moment

We present a calculation of the contribution of the $Θ$-term to the neutron and proton electric dipole moments using seven 2+1+1-flavor HISQ ensembles. We also estimate the topological susceptibility for the 2+1+1 theory to be $χ_Q = (66(9)(4) \rm MeV)^4$ in the continuum limit at $M_π= 135$ MeV. The calculation of the nucleon three-point function is done using Wilson-clover valence quarks. The CP-violating form factor $F_3$ is calculated by expanding in small $Θ$. We show that lattice artifacts introduce a term proportional to $a$ that does not vanish in the chiral limit, and we include this in our chiral-continuum fits. A chiral perturbation theory analysis shows that the $N(0) π(0)$ state should provide the leading excited state contribution, and we study the effect of such a state. Detailed analysis of the contributions to the neutron and proton electric dipole moment using two strategies for removing excited state contamination are presented. Using the excited state spectrum from fits to the two-point function, we find $d_n^Θ$ is small, $|d_n^Θ| \lesssim 0.01 \overline Θe$ fm, whereas for the proton we get $|d_p^Θ| \sim 0.02 \overline Θe$ fm. On the other hand, if the dominant excited-state contribution is from the $N π$ state, then $|d_n^Θ|$ could be as large as $0.05 \overline Θe$ fm and $|d_p^Θ| \sim 0.07 \overline Θe$ fm. Our overall conclusion is that present lattice QCD calculations do not provide a reliable estimate of the contribution of the $Θ$-term to the nucleon electric dipole moments, and a factor of ten higher statistics data are needed to get better control over the systematics and possibly a $3σ$ result.

hep-lat↗

arXiv2020

Electric dipole moments of three-nucleon systems in the pionless effective field theory

We calculate the electric dipole moments (EDMs) of three-nucleon systems at leading order in pionless effective field theory. The one-body contributions that arise from permanent proton and neutron EDMs and the two-body contributions that arise from CP-odd nucleon-nucleon interactions are taken into account. Neglecting the Coulomb interaction, we consider the triton and ${}^3$He, and also investigate them in the Wigner-SU(4) symmetric limit. We also calculate the electric dipole form factor and find numerically that the momentum dependence of the electric dipole form factor in the Wigner limit is, up to an overall constant (and numerical accuracy), the same as the momentum dependence of the charge form factor.

nucl-th↗

arXiv2020

Non-perturbative renormalization scheme for the CP-odd three-gluon operator

We define a regularization-independent momentum-subtraction scheme for the $CP$-odd three-gluon operator at dimension six. This operator appears in effective field theories for heavy physics beyond the Standard Model, describing the indirect effect of new sources of $CP$-violation at low energies. In a hadronic context, it induces permanent electric dipole moments. The hadronic matrix elements of the three-gluon operator are non-perturbative objects that should ideally be evaluated with lattice QCD. We define a non-perturbative renormalization scheme that can be implemented on the lattice and we compute the scheme transformation to $\overline{\text{MS}}$ at one loop. Our calculation can be used as an interface to future lattice-QCD calculations of the matrix elements of the three-gluon operator, in order to obtain theoretically robust constraints on physics beyond the Standard Model from measurements of the neutron electric dipole moment.

hep-ph↗

arXiv2020

Sterile neutrinos and neutrinoless double beta decay in effective field theory

We investigate neutrinoless double beta decay ($0νββ$) in the presence of sterile neutrinos with Majorana mass terms. These gauge-singlet fields are allowed to interact with Standard-Model (SM) fields via renormalizable Yukawa couplings as well as higher-dimensional gauge-invariant operators up to dimension seven in the Standard Model Effective Field Theory extended with sterile neutrinos. At the GeV scale, we use Chiral effective field theory involving sterile neutrinos to connect the operators at the level of quarks and gluons to hadronic interactions involving pions and nucleons. This allows us to derive an expression for $0νββ$ rates for various isotopes in terms of phase-space factors, hadronic low-energy constants, nuclear matrix elements, the neutrino masses, and the Wilson coefficients of higher-dimensional operators. The needed hadronic low-energy constants and nuclear matrix elements depend on the neutrino masses, for which we obtain interpolation formulae grounded in QCD and chiral perturbation theory that improve existing formulae that are only valid in a small regime of neutrino masses. The resulting framework can be used directly to assess the impact of $0νββ$ experiments on scenarios with light sterile neutrinos and should prove useful in global analyses of sterile-neutrino searches. We perform several phenomenological studies of $0νββ$ in the presence of sterile neutrinos with and without higher-dimensional operators. We find that non-standard interactions involving sterile neutrinos have a dramatic impact on $0νββ$ phenomenology, and next-generation experiments can probe such interactions up to scales of $\mathcal O(100)$ TeV.

hep-ph↗

arXiv2020

Novel angular dependence in Drell-Yan lepton production via dimension-8 operators

We study the effects of dimension-8 operators on Drell-Yan production of lepton pairs at the Large Hadron Collider (LHC). We identify a class of operators that leads to novel angular dependence not accounted for in current analyses. The observation of such effects would be a smoking-gun signature of new physics appearing at the dimension-8 level. We propose an extension of the currently used angular basis and show that these effects should be observable in future LHC analyses for realistic values of the associated dimension-8 Wilson coefficients.

hep-ph↗

arXiv2019

$\boldsymbol{C\!P}\!$ violation in Higgs-gauge interactions: from tabletop experiments to the LHC

We investigate the interplay between the high- and low-energy phenomenology of $C\!P$-violating interactions of the Higgs boson with gauge bosons. For this purpose we use an effective field theory approach and consider all dimension-6 operators arising in so-called universal theories. We compute their loop-induced contributions to electric dipole moments and the $C\!P$ asymmetry in $B\to X_sγ$, and compare the resulting current and prospective constraints to the projected sensitivity of the LHC. Low-energy measurements are shown to generally have a far stronger constraining power, which results in highly correlated allowed regions in coupling space, a distinctive pattern that could be probed at the high-luminosity LHC.

hep-ph↗