Achievements

In the following sections, I will detail my primary research achievements, discuss my complementary expertise and research results, and provide a high-level overview of the broader implications of my work for nuclear and particle physics.

Neutrino Cross Sections for Oscillation Experiments

The primary goal of next-generation accelerator neutrino experiments is to determine whether neutrinos and antineutrinos oscillate in the same way, which would reveal potential violations of charge-conjugation and parity (CP) symmetry. As these experiments aim at percent-level accuracy, it is essential to account for radiative corrections, including both virtual and real photon emission and re-absorption.

Oscillation parameters are typically extracted from the survival probability of muon (anti)neutrinos and the appearance probability of electron (anti)neutrinos, which are expressed as the ratio of corresponding fluxes. To ensure accurate cross-section measurements and properly calibrate the normalization of the initial (anti)neutrino beam, I made key contributions to calculating radiative corrections for (anti)neutrino-electron scattering on atomic electrons¹. My approach starts with matching the Standard Model to the Low-Energy Effective Field Theory (LEFT) in the neutrino sector², incorporating electroweak and strong interactions. Afterward, I resum large electroweak logarithms to all orders and evolve the coupling constants from the electroweak scale down to the GeV and electron-mass energy scales. Using precisely determined coupling constants, I evaluated the quantum electrodynamics (QED) corrections in elastic (anti)neutrino-electron scattering, ensuring correct normalization of the dominant (anti)neutrino flux components. Additionally, I computed radiative corrections for the inverse muon decay reaction to accurately normalize the high-energy tails of the (anti)neutrino flux³. To improve the precision of solar neutrino physics, especially in experiments Super-Kamiokande, Borexino, and JUNO, I accounted for the electron mass in the calculations, focusing on the primary interaction channel (anti)neutrino-electron scattering. I also provided the effective field theory definition for the neutrino charge radius, which was subsequently adopted by the Particle Data Group⁴.

Given the need for precise control of (anti)neutrino fluxes, accurate cross-section inputs are indispensable. While muon (anti)neutrino cross sections are well-constrained experimentally at near detectors, the cross sections for the electron (anti)neutrinos, particularly those involved in the electron flavor appearance channel (critical for determining the CP-violating phase), must be derived from theoretical predictions. To address this problem, I systematically quantified radiative corrections to cross sections and cross-section flavor ratios⁵,⁶. As a first step, I developed and validated a novel factorization framework for radiative corrections in charged-current elastic (anti)neutrino-nucleon and (anti)neutrino-nucleus scattering. Within this framework, I provided precise predictions for cross sections and associated bremsstrahlung for modern and future accelerator neutrino experiments.

Coherent Elastic Neutrino-Nucleus Scattering (CEνNS)

CEνNS, one of the most recent experimental discoveries in neutrino interactions⁷, plays a critical role in various areas of physics. It is an unavoidable background in dark matter direct detection experiments, opens new avenues for measuring (anti)neutrino properties in small-scale experiments, and provides an essential tool for studying low-energy nuclear physics using (anti)neutrino beams. With next-generation detectors expected to achieve percent-level precision, accurate treatment of radiative corrections is vital for successful CEνNS applications.

To leverage the full potential of CEνNS, I developed an effective field theory framework that enables precise predictions for CEνNS cross sections and flavor-dependent cross-section ratios⁸. Building on previous work that matched the Standard Model with LEFT in the neutrino sector, I performed the first comprehensive uncertainty quantification, accounting for errors at the nuclear, nucleon, hadron, and quark levels, with a perturbative uncertainty added in quadrature. In the 20-100 MeV energy range, I found that the dominant uncertainty arises from the neutron distribution within the nucleus. While at lower energies, hadronic contributions and perturbative errors pose the main limitations. Furthermore, I provided precise predictions for flavor-dependent cross-section ratios in CEνNS, free from nuclear physics uncertainties. These contributions are critical for distinguishing (anti)neutrino flavors via neutral-current processes, marking a significant advancement for future neutrino physics experiments.

I also made significant advancements in reducing the theoretical uncertainties associated with (anti)neutrino flux predictions in CEνNS experiments. By accounting for radiative corrections, I achieved sub-permille-level precision in the neutrino and antineutrino energy spectra from kaon, pion, and muon decays⁹. These corrections are crucial as they introduce continuous and divergent spectral components near the energy spectrum endpoints. For the first time, I provided a detailed analysis of these spectral components and quantified the uncertainties arising from meson structure-dependent contributions. My work on muon decay further advanced the field by independently reproducing well-known quantum chromodynamics (QCD) spectra from B-meson decays. These results are essential for accurately calculating low-energy (anti)neutrino fluxes from pion-decay-at-rest sources and for refining high-energy flux predictions in precision experiments that aim to control (anti)neutrinos through charged-lepton tagging from meson and muon decays.

Neutron Decay and Cabibbo-Kobayashi-Maskawa (CKM) Matrix

Low-energy charged-current processes involving nucleons, such as neutron beta decay and (anti)neutrino-nucleon scattering, are measured with extraordinary precision down to sub-permille and permille levels, respectively. Achieving this level of accuracy requires careful treatment of leading and next-to-leading radiative corrections. To address this challenge, I developed a top-down effective field theory for low-energy charged-current processes. After matching the Standard Model to LEFT, I extended this framework by matching to heavy baryon chiral perturbation theory (HBChPT), expressing the relevant low-energy coupling constants (LECs) in terms of non-perturbative correlation functions of quark currents. I also resummed logarithmic terms between the hadronic and electron-mass scales, treating contributions from physics above the electron-mass scale as short-distance effects, which are captured by the vector and axial-vector coupling constants gV and gA¹⁰,¹¹,¹², respectively.

My work introduced several key innovations: (1) I incorporated next-to-leading logarithms in the electromagnetic coupling constant for evaluating the relevant LEFT Wilson coefficient. (2) I derived explicit expressions for the HBChPT LECs in terms of infrared-finite convolutions of simple kernels with single-nucleon matrix elements of time-ordered products of two- and three-quark bilinears (vector, axial-vector, and pseudoscalar), which are often overlooked in first-principles determinations of the axial-vector coupling constant. (3) I included two-loop anomalous dimensions in the evolution between the hadronic and electron-mass scales. (4) I utilized dimensional regularization with modified minimal subtraction and specified the regularization scheme at each step. It allowed me to include next-to-leading logarithms and their resummation consistently. (5) My framework offers a rigorous and systematic approach to estimating perturbative uncertainties. (6) I clarified the resummation of leading and subleading Coulomb-enhanced terms. (7) I mapped the deep inelastic scattering region of hadronic contributions onto the Wilson coefficient in LEFT, providing deeper insight into the hadronic structure. (8) Finally, I delivered the first determination of physically relevant combinations of electromagnetic and electroweak and all next-to-leading order HBChPT LECs.

Using this refined framework, I determined the low-energy vector coupling constant gV, which controls the neutron decay and low-energy (anti)neutrino-nucleon scattering. For the total corrections to the neutron decay rate, I found a result that is one standard deviation above previous calculations. It allowed me to update the extraction of the CKM matrix element Vud from neutron decay, consistently incorporating next-to-leading logarithms and Coulomb corrections, in contrast to earlier extractions. The first precise determination of the coupling constant gV sets the stage for accurately predicting the inverse beta decay reaction. This reaction is the signal process in the reactor antineutrino experiment at Jiangmen Underground Neutrino Observatory (JUNO).

For the axial-vector coupling constant gA, I performed the matching to HBChPT using two distinct methods: connecting to vector-vector-axial-vector three-point correlation functions and connecting to pseudoscalar-vector-vector three-point correlation functions. I verified the consistency of these calculations using Ward identities. The resulting expressions provide a basis for a more precise comparison between the experimental measurements and lattice-QCD extractions of the ratio gA/gV.

In addition to these central results, I clarified the amplitude decomposition for the vector-axial-vector two-point correlation functions and derived new sum rules for the corresponding nucleon amplitudes. I also provided model-independent low-energy behavior for all relevant hadronic objects in determining the axial-vector coupling constant. To validate this new framework, I derived and verified Ward identities between two- and three-point correlation functions of interest. These advances significantly enhance our understanding of neutron decay and (anti)neutrino-nucleon interactions, setting the stage for future precision measurements and theoretical investigations.

Nucleon Form Factors

Elastic scattering of leptons is a key tool for probing general properties of protons and light nuclei, such as their charge and magnetization distributions. Recently, a significant discrepancy known as the proton radius puzzle has emerged in the proton charge radius, with conflicting results from muonic hydrogen, hydrogen spectroscopy, and elastic electron-proton scattering. These discrepancies indicate the importance of radiative corrections beyond one-photon exchange. While corrections involving a single photon-nucleon coupling or additional soft or collinear photons can be analytically computed, more complex corrections involving multiple hard-photon exchanges remain the focus of ongoing theoretical and experimental studies. To improve the precision of proton charge radius extractions from electron- and muon-proton scattering data, I developed several innovative methods to calculate the two-photon exchange (TPE) contributions, an essential component responsible for the theoretical uncertainty in low-energy experiments.

The TPE corrections are expressed as a sum over intermediate proton states and inelastic contributions. For scattering at small angles, I included both proton and inelastic intermediate states in the elastic electron-proton and muon-proton scattering¹³,¹⁴,¹⁵. I derived several model-independent expressions for cross-sections and amplitudes in this regime, providing a more accurate theoretical framework for interpreting experimental data.

To address the most uncertain radiative corrections in elastic electron-proton scattering, I developed a novel dispersion relation framework for TPE corrections, incorporating proton and pion-nucleon intermediate states. A cornerstone of this work was the development of two innovative methods for the analytical continuation of one-particle¹⁶ and multiparticle¹⁷ intermediate states. These methods enabled the calculation of TPE contributions from the proton intermediate state across arbitrary kinematics and from pion-nucleon intermediate states at small momentum transfers (below 1 GeV)¹⁷,¹⁸,¹⁹,²⁰. This advancement allows for a more comprehensive understanding of the radiative corrections involved.

In studying the inelastic TPE contributions, I developed a new approach for evaluating the subtraction function in forward doubly-virtual Compton scattering¹⁵. I applied this method to experimental data, obtaining unexpected predictions, later confirmed by calculations within the Chiral Perturbation Theory. This new approach offers a powerful tool for refining the theoretical framework of nucleon structure.

My calculations for inelastic TPE corrections to elastic electron-proton scattering at small angles¹⁴ were included in the standard radiative correction package used in the analysis of the PRAD experiment at JLab²¹. These contributions played a crucial role in reducing systematic uncertainties of the proton charge radius.

Further, I contributed to the area of proton and neutron vector form factors by providing my TPE calculations. This analysis focused on the low-energy region and offered central values, uncertainties, and correlations for evaluating observables in atomic and neutrino physics²². With updated nucleon form factors, I evaluated the elastic TPE contributions to S energy levels for neutrons and protons, providing the first robust uncertainty estimate. This updated analysis revealed 3-5% shifts in the (anti)neutrino-nucleon scattering cross sections, driven by incorporating the most recent electron-proton scattering data from the A1 Collaboration²³,²⁴.

To compare electron-proton and muon-proton interactions in a complementary way, I calculated QED radiative corrections to lepton-pair photoproduction on a hydrogen target²⁵,²⁶. In doing so, I identified observables insensitive to two-photon exchange, thus eliminating corresponding hadronic uncertainties. These results are crucial for testing lepton universality in low-energy experiments, especially at the Mainz Energy-Recovering Superconducting Accelerator (MESA) facility²⁷.

In addition to vector form factors, I also contributed to the study of nucleon axial-vector and induced pseudoscalar form factors. Historically, the axial-vector form factor has been determined from (anti)neutrino-deuterium scattering measurements using bubble chambers. However, in 2023, the MINERvA Collaboration provided an alternative extraction of this form factor from antineutrino scattering on a hydrocarbon target²⁸. I analyzed this new data by comparing predictions based on phenomenological fits to the deuterium bubble-chamber data and lattice-QCD calculations²⁹. Both approaches were in agreement with the new MINERvA results. For the first time, I quantified the tension between lattice-QCD and bubble-chamber data at the 2.5 level. I also explored potential resolutions to this tension by investigating future measurements of antineutrino-hydrogen interactions with hydrocarbon targets³⁰. Using an experimental sensitivity study, I estimated the form-factor errors at the permille level that is below the current uncertainties in nucleon electromagnetic form factors. This work highlights the significance of my contributions to radiative corrections in (anti)neutrino-nucleon scattering. It also provides guidance for future extractions of both vector and axial-vector form factors in neutrino experiments.

Furthermore, I explored alternative methods to access axial-vector and induced pseudoscalar form factors by investigating all possible single-spin asymmetries in elastic (anti)neutrino-nucleon scattering³¹. I identified single-spin asymmetries sensitive to the axial-vector contributions at GeV energies. For the first time, I demonstrated that we can assess the pseudoscalar form factor through precise measurements with muon (anti)neutrinos of energies in the few hundred MeV range or with tau (anti)neutrinos. I also found that radiative corrections to these single-spin asymmetries largely cancel between the numerator and denominator, resulting in negligible effects³². These findings will guide future analyses of polarization effects in tau (anti)neutrino interactions and experimental efforts using polarized targets. Based on the amplitude decomposition of radiative corrections to (anti)neutrino-nucleon scattering⁵,⁶, I defined the general invariant amplitudes decomposition for elastic (anti)neutrino-nucleon scattering. With this new decomposition, I derived the most general expressions for unpolarized cross sections and single-spin asymmetries³². Furthermore, I explored how unpolarized and polarization measurements could constrain the normalization of invariant amplitudes, comparing them with existing constraints from β decay. Using high-quality MINERvA data, I was able to constrain the normalizations of invariant amplitudes (equivalently, muon-specific interactions at the nucleon and quark levels) and improve β-decay constraints on tensor and scalar neutrino-nucleon interactions in a broad class of new physics models³³.

Precision Physics of Simple Atoms

I contributed to the precision physics of simple atoms by providing the first fully data-driven evaluations of the TPE hadronic contributions to the shift of S energy levels in both regular and muonic hydrogen. I also established a relationship between the hyperfine splitting in electronic and muonic hydrogen through hadronic contributions and introduced a rigorous theoretical framework for TPE corrections in atomic systems.

TPE corrections are the dominant source of theoretical uncertainty in predicting the energy levels of light atoms³⁴,³⁵. Specifically, they exceed the experimental uncertainty of the 1S-2S transition measurement in regular hydrogen by a factor of 10. This transition is crucial for determining the fundamental Rydberg constant³⁶. I performed the first comprehensive data-driven calculation of the TPE correction to the S energy levels in regular hydrogen³⁷, accounting for significant recoil corrections for elastic intermediate states and all possible inelastic intermediate states over the full kinematic range. The uncertainty of this calculation is comparable to the experimental precision of the 1S-2S transition.

For heavier atoms, I completed the first data-driven calculation of the TPE correction to S energy levels on neutrons in both electronic and muonic atoms³⁸. This calculation included recoil corrections for elastic intermediate states and all inelastic intermediate states over the full kinematic range. I confirmed that the TPE correction on neutrons inside the nucleus is as sizable as on protons.

In muonic hydrogen, the theoretical uncertainty of TPE corrections and the experimental error of the Lamb shift measurement are of similar size. To reduce the theoretical error and provide more accurate TPE corrections, I developed a novel technique that replaces uncertain experimental data at long distances with well-known low-energy constants³⁹. I applied this technique to improve the TPE evaluation in muonic hydrogen.

Future measurements of the 1S hyperfine splitting in muonic hydrogen are expected to achieve accuracies 100 times smaller than the current theoretical uncertainty in the TPE correction. During my work, I uncovered a previously unknown relation between the hyperfine splitting in electronic and muonic hydrogen through the hadronic TPE correction⁴⁰. This discovery enabled me to reduce the uncertainty in the TPE correction to the hyperfine splitting in muonic hydrogen by a factor of 20 compared to previous data-based evaluations. After revisiting all known radiative corrections to the hyperfine splitting, I provided the most precise prediction for the hyperfine splitting in muonic hydrogen³⁷,³⁹. This prediction, which incorporates accurate measurements of the 21 cm line in regular hydrogen, will guide the choice of laser frequencies for upcoming 1S hyperfine splitting experiments.

In addition to these experimentally relevant results, I made a significant theoretical advancement in the precision physics of atomic systems. I developed and tested a new dispersion relation framework for TPE corrections to energy levels in both regular and muonic hydrogen⁴¹, that is based on lepton-proton rather than photon-proton scattering amplitudes. This framework offers a novel approach to calculating TPE corrections, advancing precision physics in atomic systems.

Nucleon and Nuclear Structure inside Large Nuclei

Building on my substantial contributions to radiative corrections at the single-nucleon level and the extraction of electromagnetic and axial-vector form factors from experimental data, I extended these concepts to interactions within the nuclear medium of large nuclei. This investigation led me to address an important, previously unexplored question: How do photon exchanges between charged leptons and protons inside the nucleus affect observables in (anti)neutrino-, electron-, and muon-nucleus scattering experiments? To answer this question, I formulated the QED nuclear medium effects for charged-current elastic (anti)neutrino-, electron-, and muon-nucleus scattering. I generalized soft-collinear effective field theory (SCET) with Glauber interactions by extending its application from QCD to QED⁴². Using this framework, I performed the first-ever estimate of these effects, considering both single rescattering inside the nucleus and multiple soft-photon interactions. For one rescattering, I computed these effects within the newly developed effective field theory. I validated the results by comparing them with full-QED calculations that involve no approximations.

When considering multiple rescattering, I found significant deviations of charged-lepton tracks, generating experimentally detectable perpendicular momenta at tens of MeV. This broadening sizably deflects electron tracks, leading to shifts in kinematic variables and impacting scattering cross sections accordingly⁴³.

In elastic electron-nucleus scattering, QED nuclear medium effects can lead to corrections at the percent level, especially for scattering in near-forward kinematics. These effects decrease as the incoming beam energy increases, making them more significant in experiments with lower beam energy, such as the anticipated China-based electron-ion collider EIcC, compared to the higher-energy US-based electron-ion collider EIC. For electron-ion colliders, I performed estimates for both elastic electron-nucleon scattering and neutral-current deep inelastic scattering as hard interaction processes inside a lead nucleus⁴⁴. These estimates confirmed the presence of percent-level effects, emphasizing that incorporating QED nuclear medium effects is crucial for precise, process-independent extraction of the multi-dimensional structure of nucleons within large nuclei and exploration of the nuclear modification of parton distribution functions.

Additionally, I evaluated the potential effects of the radiation of soft and collinear photons induced by the QED nuclear medium⁴⁵. While the modification to the bremsstrahlung spectra was negligible, the nuclear medium-induced radiation significantly impacted the broadening of charged leptons in the direction perpendicular to their propagation.

In (anti)neutrino-nucleus scattering, QED nuclear medium effects can only be relevant near kinematic endpoints, potentially influencing the interpretation of experimental searches for sterile neutrinos in accelerator-based neutrino experiments. My initial evaluations of these effects revealed a previously unaddressed systematic effect in neutrino oscillation analyses, confirming that these effects are negligible for modern oscillation physics.

Other research accomplishments

My research has significantly contributed to our understanding of the proton-neutron mass difference in the context of QED⁴⁶. I advanced the precision of data-driven predictions for the electromagnetic contributions to this mass difference through several key innovations: 1. Incorporating cutting-edge experimental data on neutron and proton form factors to refine theoretical prediction. 2. Applying a novel technique for evaluating subtraction functions via unsubtracted dispersion relations, effectively avoiding high-energy divergences and providing more reliable amplitude calculations. 3. Introducing experimental data on magnetic polarizabilities for protons and neutrons to normalize inelastic contributions, data that had previously been underutilized in this context. These improvements have resulted in the most accurate phenomenological estimate of the electromagnetic proton-neutron mass shift.

In astrophysics and cosmology, I have made substantial contributions to the field of cosmological magnetic fields, a subject with unresolved questions regarding their origin and evolution. I addressed this by investigating the evolution of cosmological magnetic instability⁴⁷, focusing on a model where lepton chiral asymmetry, generated before or during the electroweak phase transition in the early Universe, plays a central role in magnetic field generation. To test this mechanism, I analyzed the production of gravitational waves and their potential influence on cosmological evolution⁴⁸. I explored how these waves could leave observable imprints in cosmological data and examined the prospects for detecting such gravitational waves with Earth- and space-based interferometers. My work provided constraints on the parameters of magnetic field generation, informed by both cosmological data and gravitational wave measurements, advancing our understanding of the role these fields played in the early Universe.

Beyond theoretical work, I have participated in multinational experimental collaborations, including the ZEUS Collaboration at DESY and the DZero Collaboration at Fermilab. During my time with these teams, I was responsible for several technical tasks that significantly enhanced the precision of the experiments. For ZEUS, I evaluated the efficiency of muon reconstruction algorithms, ensuring accuracy in all analyses involving muons⁴⁹. At DZero, I assessed the purity of photon+jet events to calibrate the energy scale of the electromagnetic calorimeters. Additionally, I played an integral role in the data-taking process, monitoring critical systems such as the calorimeter, muon detector, and data acquisition system.

My involvement in these experimental efforts gave me a solid foundation in data analysis and machine learning techniques. Notably, I applied neural networks to precision studies of strange quark parton distribution functions in protons and antiprotons. This work included determining the ratio of cross sections for W boson and charm quark production relative to total W boson production with hadronic jets, as part of the QCD group’s physics program at DZero. The use of neural networks enhanced the accuracy and efficiency of the analysis, contributing to our understanding of quark and gluon interactions.

Concluding Remarks

My most valuable contributions to nuclear and particle physics have advanced the field by providing critical theoretical frameworks, radiative corrections, and phenomenological calculations for analyzing and interpreting cutting-edge experiments. These contributions have not only influenced the direction of future studies but also established a solid foundation for ongoing research. To date, I have provided key theoretical and phenomenological inputs to the precise extraction of quark and lepton mixing matrices within the Standard Model and for analyzing proton and nuclear radii, form factors, and other non-perturbative structure-dependent quantities. These quantities are crucial in analyses of accurate measurements of atomic energy levels and experiments involving (anti)neutrinos, electrons, and muons. I have also developed innovative methods to evaluate critical theoretical inputs and determine fundamental parameters in low-energy neutral-current interactions across all known elementary particles and nuclei, as well as in charged-current interactions involving nucleons and pions. Through these efforts, I have provided the most accurate values for key fundamental interaction parameters at the quark, nucleon, and nuclear levels. Additionally, I have conducted numerous sensitivity studies that guide the design of future neutrino physics experiments. These accomplishments represent significant progress in our understanding of nuclear and particle physics⁵⁰,⁵¹, setting the stage for continued experimental and theoretical research.

References