Separating fact from fantasy in the proton's spin*: the chiral anomaly and the proton spin puzzle
The prospects of extracting new physics signals in a coherent elastic neutrino-nucleus scattering (CEvNS) process are limited by the precision with which the underlying nuclear structure physics, embedded in the weak nuclear form factor, is known. In this talk, I will present a microscopic nuclear structure physics calculations of charge and weak nuclear form factors and CEvNS cross sections on $^{12}$C, $^{16}$O, $^{40}$Ar, $^{56}$Fe and $^{208}$Pb nuclei. We obtain the proton and neutron densities, and charge and weak form factors by solving Hartree-Fock equations with a Skyrme (SkE2) nuclear potential. We validate our approach by comparing $^{208}$Pb and $^{40}$Ar charge form factor predictions with elastic electron scattering data. In view of the worldwide interest in liquid-argon based neutrino and dark matter experiments, we pay special attention to the $^{40}$Ar nucleus and make predictions for the $^{40}$Ar weak form factor and the CEvNS cross sections. Furthermore, we attempt to gauge the level of theoretical uncertainty pertaining to the description of the $^{40}$Ar form factor and CEvNS cross sections by comparing relative differences between recent microscopic nuclear theory and widely-used phenomenological form factor predictions. Future precision measurements of CEvNS will potentially help in constraining these nuclear structure details that will in turn improve prospects of extracting new physics.
Meeting Recording:
Lattice Quantum Chromodynamics QCD provides a way to have a precise calculation and a new way of understanding the hadrons from first principles. From this perspective, I will first present a precise calculation of the pion form factor using overlap fermions on six ensembles of 2+1-flavor domain-wall configurations generated by the RBC/UKQCD collaboration with pion masses varying from 137 to 339 MeV. With a z-expansion fitting of our data, we find the pion mean square charge radius to be $\braket{r^2}_\pi = 0.437(7)(7) {\rm{fm^2}}$, including the systematic uncertainties from pion mass, lattice spacing and finite volume dependence. It agrees with the experimental value $\braket{r^2}_\pi = 0.434(5) {\rm{fm^2}}$ at a percent level. The second topic is lattice calculation of proton momentum and angular momentum fractions. As confirmed from experiment and lattice QCD calculation, the total helicity contribution from quark is just about $\sim 30\%$ of the proton spin. Determination of the rest contributions from quarks and gluons to the nucleon spin is a challenging and important problem. On the lattice side, one way to approach this problem is using the nucleon matrix element of the traceless, symmetric energy-momentum tensor (EMT) to determine the momentum and angular momentum distributions of up, down, strange and glue constituents. Since the EMT of each parton species are not separately conserved, we summarized their final angular momentum fractions by considering mixing and non-perturbative renormalization at $\overline{\rm{MS}}(\mu = 2 \ {\rm{GeV}})$ and use the momentum and angular momentum sum rules to normalize them.
Seminar slides: https://www.dropbox.com/sh/8k11s7xapdzwd0a/AACUkAq5Wd-GglwdVcX8USMVa?dl=0
The CP-violating effects observed thus far appear in flavor-changing processes and in a manner more or less consistent with the predictions of the Standard Model (SM). However, it has long been thought that the observed size of the cosmic baryon asymmetry suggests that mechanisms of CP violation beyond the CKM paradigm should exist. Permanent electric dipole moment searches are exquisite probes of new sources of P and CP violation, whereas processes that would break C and CP are not well studied. The decay $\eta\to\pi^+\pi^-\pi^0$ is an ideal process in which to search for flavor-diagonal C and CP violation. The patterns of C and CP violation that could emerge from an observed violation of mirror symmetry in the Dalitz plot distribution of $\eta\to\pi^+\pi^-\pi^0$ decay would speak to patterns of new physics as well. In particular, the isospin of the underlying C- and CP-violating structures can be reconstructed from their kinematic representation in the Dalitz plot. Our analysis of recent KLOE-2 data reveals that the C- and CP-violating amplitude with total isospin $I = 2$ is much more severely suppressed than that with total isospin $I = 0$. We conclude with a discussion of the constraints on possible new C- and CP-odd operators as derived from SM effective field theory.
Recorded talk: https://www.dropbox.com/sh/iwaagzhif5cfrm5/AAB0LSByK5kBTwlXw__ViIBaa?dl=0
It is known from the Sakharov conditions that baryon number violation is required to explain the current observable universe's baryon abundance (BA) and associated matter-antimatter asymmetry; further, it seems that Standard Model (SM) B-L conserving processes (such as the sphaleron) are unable to naturally generate an adequate asymmetry without high-scale, effectively untestable physics being invoked to create a primordial lepton asymmetry. An alternative, testable theoretical framework (post-sphaleron baryogenesis) predicts low-scale (observable) B-L violating neutron-antineutron transformations which can produce a viable BA. Such transformations can be sought in future free neutron beam (European Spallation Source, ESS) and intranuclear bound neutron (DUNE, Hyper-Kamiokande) experiments; I will review past incarnations of these experiments, their future sensitivities, and advocate for their complimentary necessity. Current experimental work at ORNL in neutron disappearance phenomena will also be discussed as a stepping stone to these ultimate goals. These topics will be a focus of an upcoming Amherst Center for Fundamental Interactions workshop, a review (pre-CDR) paper from the ESS NNBar Collaboration, and the 2021 Snowmass Process.
With the availability of cost-effective digitizers and powerful pipeline processors, modern spectroscopy can be performed completely in the digital domain with minimal analog front-end signal processing. These systems offer the flexibility and extensibility of digital signal processing algorithms to simultaneously extract multiple waveform parameters such as pulse height and start time from faint signals buried in background noise. However, pulse fitting was traditionally a computationally intensive task performed offline on computing clusters in the final analysis. I present new algorithms we have developed to perform full nonlinear covariant least-squares fits for optimal resolution of 30 keV proton pulses in the Nab experiment. These algorithms can be implemented in digitizer firmware to increase the resolution and efficiency of our hardware trigger as well as on graphics processing units (GPUs) for high precision online analysis of triggered waveforms.
The framework of chiral perturbation theory (ChPT) sets an excellent basis for the description of low-energy electroweak processes involving hadrons in general. After a brief introduction to this theory, I will discuss some applications we have worked on, namely the processes of photon- and neutrino-induced pion production close to threshold, and the predictive power therein. Furthermore, I will show that ChPT can be of use also in other physics fields, with the example of setting constraints on CP-violating decay rates by connection to the neutron EDM calculation.
Recorded talk: https://www.dropbox.com/sh/4mkuvuxkahkjrgs/AAChvZitTcnirQeQqpu9YvvRa?dl…
In the past decade, renewed interest in the nucleon electromagnetic form factors was sparked by new measurements of electron-proton scattering at low Q^2 by the A1 Collaboration and of the proton charge radius in muonic hydrogen by the CREMA Collaboration. Subsequent theoretical developments re-examined longstanding assumptions on the parameterizations of these form factors. In this talk, I will review some of these developments, then present new parameterizations of the form factors that are the result of work in the past few years. Finally, I will outline applications of these form factors in both atomic physics and for the US program of precision neutrino measurements.
Recorded talk and slides: https://www.dropbox.com/sh/sek9spcry22xorw/AABHxLmJwsrae-E0zEftz7HWa?dl=0
In 2017, the COHERENT collaboration made the first observation of coherent elastic neutrino-nucleus scattering (CEvNS) using a 14.6 kg CsI scintillating crystal detector located at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. In addition to neutrons, the 1.4 MW pulsed (60 Hz) proton beam at the SNS produces charged pions which subsequently decay to yield a large neutrino flux with a well known energy spectrum and time structure. COHERENT employs a suite of detectors at the SNS to search for CEvNS in different target nuclei and to measure potential backgrounds. This multi-target program allows for testing of Standard Model predictions for CEvNS as well as for verifying the $N^2$-dependence of the cross section of this interaction. CENNS-10, a 24 kg liquid argon scintillation detector, has been actively taking data at the SNS since the spring of 2017. This talk will detail the methods and results of a search for and detection of CEvNS in CENNS-10 data.
Host: Ryan MacLellan