department of physics and astron

Massive black holes in high-redshift quasar hosts: formation and evolution

Date: 
Wednesday, October 20, 2021 - 3:00pm
Location: 
Blazer Dining 339 (on zoom)
Type of Event (for grouping events):
The observations of high redshift quasars up to z~7 tell us that massive black holes (MBHs) were already in place, with masses well above 10^9 solar masses, when the Universe was less than 1 Gyr old. According to Soltan’s argument MBHs gain most of their mass via radiatively efficient accretion, hence we expect they formed early in the Universe as smaller seeds. To date, the common formation mechanism advocated to explain the most massive MBHs at high redshift is the direct collapse scenario, which leads to the formation of seed MBHs of about 10^4-5 Msun. However, because of the peculiar conditions required by this formation mechanism, its plausibility is still debated. After highlighting the main conditions required by this scenario, I will discuss whether the peculiar environment in which high-redshift massive galaxies evolve provides ideal conditions for the formation of such massive seeds, and the processes that may potentially inhibit the process. I will also discuss the subsequent evolution of these protogalaxies and their central MBHs up to the observed masses, a result that strongly depends on the interaction with its galaxy host, and how the MBH obesity found by observations is not necessarily real.

First Results from the SIDIS Program at CLAS12

Date: 
Thursday, April 15, 2021 - 2:00pm to 3:00pm
Location: 
online
Type of Event (for grouping events):

Semi-inclusive deep-inelastic scattering (SIDIS) is an essential tool to probe the quark-gluon structure of the proton and thus for our understanding of non-perturbative QCD dynamics. The CLAS12 experiment has been taking physics data at the upgraded Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Laboratory since 2018. It takes advantage of the world record luminosities provided by CEBAF to perform an ambitious program of 3D imaging of the proton in momentum and position space. This talk will present the first results from the SIDIS program. I will focus on the first observation of beam-spin asymmetries in di-pion production in SIDIS. From the measured di-pion correlations a first extraction of the collinear twist-3 PDF e(x), which is sensitive to quark-gluon correlations in the proton, can be performed. Furthermore spin-orbit correlations in the hadronization of longitudinal polarized quarks into pions can be studied for the first time.

 
Meeting Recording:
 

Heterodyne Detection of Axion Dark Matter

Date: 
Thursday, April 29, 2021 - 2:00pm to 3:00pm
Type of Event (for grouping events):
Detecting ultralight axion dark matter has recently become one of the benchmark goals of future direct detection experiments. I will discuss a new idea to detect such particles whose mass is well below the micro-eV scale, corresponding to Compton wavelengths much greater than the typical size of tabletop experiments. The approach involves detecting axion-induced transitions between two quasi-degenerate resonant modes of a superconducting accelerator cavity. Compared to more traditional setups, the sensitivity is parametrically enhanced for ultralight axions, allowing for the exploration of vast new areas of parameter space relevant to the QCD axion and astrophysically long-ranged fuzzy dark matter.
 

 

Smoothing things over: Building better magnetic fields

Date: 
Thursday, May 6, 2021 - 2:00pm to 3:00pm
Type of Event (for grouping events):

Next generation searches for beyond standard model physics such as nEDM and neutron spin rotation rely on exquisitely uniform magnetic fields. Recent advances in theory and computation have yielded a streamlined pipeline to design coils satisfying increasingly stringent design requirements. nEDM@LANL is a natural progression of nEDM measurements, and we expect it will ultimately be limited by the well-known systematic of magnetic gradients, which ultimately induce a false EDM. In spite of our best efforts to design and build a perfect magnetically shielded room (MSR) and B0 coils, we will need a coil package capable of shimming away specific unwanted gradients. We are designing our gradient correction coils in two generations. The focus of this talk will be on the second-generation coils. We are pursuing coils designed with modern CAD tooling: I'll discuss stream-function generated coils as well as a family of continuously wound coils designed with a nod to the weakly perturbed scalar potential method before introducing the fabrication and validation of these coils and their implementation as a full gradient correction package.

 
Meeting Recording:
 

Untangling the neutron’s electroweak radiative corrections

Date: 
Thursday, April 22, 2021 - 2:00pm to 3:00pm
Type of Event (for grouping events):
The success of the Standard Model of particle physics derives from its impressive capacity for calculating quantum corrections to extremely high precision. This is particularly the case for the neutron - a system which interacts strongly, weakly and electromagnetically - which is rich in physics without being prohibitively complex, making it a prime candidate for searching for Beyond Standard Model (BSM) physics. Besides top-row unitarity tests of the Cabibbo-Kobayashi-Maskawa matrix, the rise of precise lattice QCD calculations in recent years have enabled direct searches for exotic right-handed weak currents which arise naturally in many BSM scenarios. Both of these require exquisite control of electroweak radiative corrections, the latter of which has seen a surge of activity in the past three years. Recently, we have calculated for the first the complete electroweak radiative corrections to the nucleon axial charge, gA, up to next-to-leading order, indicating that experimental results for gA must be corrected for when comparing to lattice results. Additionally, differences between two main approaches for calculating corrections to the nucleon vector charge were shown to be reconcilable, confirming the tremendous physics reach of the neutron.
 
 

 

An accurate determination of the neutron skin thickness of Pb208 through parity-violation in electron scattering

Date: 
Thursday, April 1, 2021 - 2:00pm to 3:00pm
Type of Event (for grouping events):

While the proton radius of a nucleus is precisely measured by performing electron elastic scattering on the nucleus, the neutron radius previously measured by hadronic scattering experiments and other methods suffers from uncontrolled uncertainties due to hadronic dynamics. Opposite to the electromagnetic charge, the neutron has a much larger weak charge than the proton. Utilizing this, in PREX-2, we extracted the neutron radius of Pb208 by measuring the parity violating asymmetry obtained through performing longitudinal polarized electron scattering on the lead target. The heavy nuclei Pb208 shares the same equation of state with the neutron stars, even though it is 18 orders of magnitude smaller than neutron stars. The neutron skin thickness of Pb208 extracted from PREX-2 will accurately constrain the density dependence of the symmetry energy of nuclear matter near saturation density, which is important in the determination of the neutron star size. An overview of the PREX-2 experiment and its results will be presented.

 

Meeting Recording:
 

Novel Nucleon Structure with Transverse Polarization

Date: 
Thursday, March 4, 2021 - 2:00pm to 3:00pm
Type of Event (for grouping events):
Abstract: In this talk, we will present our recent research on transverse spin physics, focusing on two topics: (1) transverse spin sum rule; (2) single transverse spin asymmetries in dijet production at RHIC. 
 
Meeting Recording:
 

Nucleon and nuclear structure from measurements in muonic and normal atoms

Date: 
Thursday, March 11, 2021 - 2:00pm to 3:00pm
Location: 
online
Type of Event (for grouping events):

Laser spectroscopy of simple atoms is sensitive to properties of the atomic
nucleus, such as its charge and magnetization distribution, or its
polarizability. This allows determining the nuclear parameters from atomic
spectroscopy, but also limits the attainable precision for the determination of
fundamental constants or the test of QED and the Standard Model.

In light muonic atoms and ions, one negative muon replaces all atomic electrons,
resulting in a calculable hydrogen-like system. Due to the muon's large mass
(200 times the electron mass), the muon orbits the nucleus on a 200 times
smaller Bohr radius, increasing the sensitivity of muonic atoms to nuclear
properties by 200^3 = 10 million.

This has resulted in a 10fold increase in the precision of the charge radius of
the proton, deuteron, and the stable helium nuclei. The consequences for atomic
and nuclear physics, the determination of fundamental constants, and the test of
QED and the Standard Model are discussed.

 

https://www.dropbox.com/sh/69sdbudfg8245pj/AACTb2WyBF_R2ujBHJkIx6zja?dl=0

Meeting Recording:
 
 

First-principles calculation of the nucleon axial charge

Date: 
Thursday, February 11, 2021 - 2:00pm to 3:00pm
Type of Event (for grouping events):
Understanding theoretically the neutrino-nucleus cross section is critical to the success of future neutrino oscillation experiments such as DUNE and Hyper-K. For neutrino energy at around 1 GeV, the nucleon axial form factor limits the precision of the cross section model as it is difficulty to measure experimentally. In this talk, I will discuss how lattice QCD can provide first-principles calculations of nucleon matrix elements. In particular, I will focus on the first calculation of the nucleon axial charge, the nucleon axial form factor at zero momentum transfer, with an efficient lattice fermion discretization, and discuss prospects in extending our work to the full momentum dependence.
 
 
 
 
 
 
 
 
 

 

Radiative corrections in neutrino scattering

Date: 
Thursday, March 25, 2021 - 2:00pm to 3:00pm
Location: 
online
Type of Event (for grouping events):
Neutrino physics is entering a precision era that requires a careful treatment of percent-level effects. In this talk, I am going to discuss the role of radiative corrections in modern and future experiments with artificial neutrino sources.
 
One-loop radiative corrections introduce the flavor dependence in the coherent elastic neutrino-nucleus scattering at the percent level. To consistently account for radiative corrections, we start from the effective field theory of neutrino-lepton and neutrino-quark interactions, embed quarks into nucleons and nucleons into nuclei. We present cross sections at energies below 100 MeV and provide a complete error budget accounting for all uncertainties at nuclear, nucleon, hadronic, and quark levels.
 
Precise knowledge of neutrino-nucleon charged-current quasielastic scattering is crucial for successful measurements of neutrino oscillation parameters at accelerator-based facilities. Exploiting effective field theory, we factorize neutrino-nucleon quasielastic cross sections into soft, collinear, and hard contributions. We evaluate soft and collinear functions from QED and provide a model for the hard contribution. Performing resummation, we account for logarithmically-enhanced higher-order corrections of percent level and evaluate precise cross sections quantifying the resulting error. We discuss the relevance of radiative corrections depending on conditions of accelerator-based neutrino experiments.
 
 

 

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