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department of physics and astron

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

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.

Date:
Location:
Blazer Dining 339 (on zoom)
Type of Event (for grouping events):

First Results from the SIDIS Program at CLAS12

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:
 
Date:
-
Location:
online
Type of Event (for grouping events):

Nucleon and nuclear structure from measurements in muonic and normal atoms

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:
 
 
Date:
-
Location:
online
Type of Event (for grouping events):

Radiative corrections in neutrino scattering

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.
 
 

 

Date:
-
Location:
online
Type of Event (for grouping events):

Experimental Opportunities for Measuring Neutrino Nucleon Scattering with H$_2$/D$_2$ Detectors

 

Many models of neutrino-nucleus scattering are guided by data taken during the 1970's and 1980's by the Argonne, Brookhaven, and Fermilab bubble-chamber experiments, which have limited data sample sizes and large systematic uncertainties.  The long-baseline neutrino facility (LBNF) will provide a neutrino beam, primarily composed of $\nu_\mu$ when it runs with forward horn current, and $\bar\nu_\mu$ when it runs with reverse horn current.  The beam intensity is driven by the power of the proton beam on the target, which is slated to start at 1.2 MW and will be upgraded to 2.4 MW.  The near detectors currently being designed are optimized for DUNE's determination of the neutrino mass ordering and the measurement of $\delta_{\rm{CP}}$.  This powerful beam provides unprecedented opportunities to measure the cross sections of neutrinos on protons and neutrons via hydrogen/deuterium targets with integrated particle detection capability.  Options range from using the hydrogen in the scintillating plastic in the SAND near detector component, to adding hydrogen-rich gas to the high-pressure gas TPC near detector component, to building a H$_2$/D$_2$ bubble chamber in a separate hall upstream of the DUNE near detector hall. Polarized targets are also under consideration, though these will necessarily involve other elements along with the hydrogen and deuterium, but they will allow the first measurements of neutrino scattering on polarized nuclei.

https://www.dropbox.com/sh/z67mq9zfdwm6wj8/AAC2v83jGvFJOujFrVi2Nmr2a?dl=0

 
Meeting Recording:
 
Date:
-
Location:
online
Type of Event (for grouping events):

Direct detection for heavy wimp dark matter

In this talk, I'll introduce wimps as a large class of viable dark matter candidates and experiments aiming to search for them. I'll focus on heavy wimps (mass greater than electroweak scale) direct detection experiment rate calculations, particularly for higgsino-like and wino-like particles. There's a cancellation between leading order amplitudes and the overall cross sections are much suppressed, leaving them at least an order below current direct detection experimental limits. It is important to consider all the subleading effects to see whether they will change the leading order result and to predict reliable benchmark results for next generation experiments.
 

 

Date:
-
Location:
online
Type of Event (for grouping events):

Beyond leading-twist PDFs from lattice QCD: GPDs and twist-3 PDFs

Lattice QCD (LQCD) is a theoretical non-perturbative approach for studying QCD dynamics numerically from first principles. LQCD is widely used for hadron structure calculations and is becoming a reliable tool, striving to control various sources of systematic uncertainties. Parton distribution functions (PDFs) have a central role in understanding the hadron structure and have been calculated in lattice QCD mainly via their Mellin moments.

In this talk, I will present selected results using an alternative new method to access PDFs proposed by X. Ji in 2013. This is the so-called quasi-distribution method, which relies on matrix elements of fast-moving hadrons and non-local operators. These are matched to the light-cone PDFs using Large Momentum Effective Theory (LaMET). The main focus of the talk is to demonstrate a novel calculation of twist-3 gT PDF, explored in lattice QCD for the first time. I will also show results from the first calculation of the chiral-even unpolarized and helicity quark generalized parton distributions (GPDs), extracted from numerical simulations of lattice QCD. The calculation is performed on one ensemble of two degenerate light, a strange and a charm quark (Nf=2+1+1) of maximally twisted mass fermions with a clover term, reproducing a pion mass of 260 MeV.

Date:
-
Location:
online
Type of Event (for grouping events):

New Limit on the Permanent Electric Dipole Moment of 129Xe using 3He Comagnetometry and SQUID Detection

We report results of a new technique to measure the electric dipole moment of 129Xe with 3He comagnetometry. Both species are polarized using spin-exchange optical pumping, transferred to a measurement cell, and transported into a magnetically shielded room, where SQUID magnetometers detect free precession in applied electric and magnetic fields. The result from a one-week measurement campaign in 2017 and a 2.5 week campaign in 2018, combined with detailed study of systematic effects, is dA(129Xe)=(1.4±6.6stat±2.0syst)×10−28 ecm. This corresponds to an upper limit of |dA(129Xe)|<1.4×10−27ecm (95% CL), a factor of five more sensitive than the limit set in 2001.

 

Meeting Recording:
 
Date:
-
Location:
online
Type of Event (for grouping events):

A Theoretically Grand Tour of Compton Scattering and Nucleon Polarisabilities

I review the theory side of the synergetic international effort of experimentalists and theorists in Compton scattering on one- and few-nucleon systems. It is an excellent opportunity to probe the symmetries and strengths of nucleonic and nuclear interactions and relate to lattice-QCD computations of fundamental hadronic properties. Rich information is encoded in the polarisabilities, which parametrise the stiffness of charge distributions against deformations. The spin polarisabilities are particularly interesting since they parametrise the stiffness of the spin in external electro-magnetic fields (nucleonic Faraday effect) and probe the spin-dependent component of the pion-nucleon interaction. I then discuss tests and extractions of the polarisabilities of the proton and neutron from data. Comprehensive studies show how sensitive observables for nuclei up to ${}^4$He are on the individual nucleonic scalar and spin polarisabilities, and to their combinations. We are also developing statistical methods to identify experiments with the likely biggest impact on extracting values from future data. This facilitates planning and analysis of the new generation of Compton experiments.

Date:
-
Location:
online
Type of Event (for grouping events):

Gravitational Wave Detection, Dark Matter Searches, and Fundamental Tests of Gravity with Atom Interferometry

Atom interferometers exploit the quantum mechanical, wavelike nature of massive particles to make a broad range of highly precise measurements.  Recent technological advances have opened a path for atom interferometers to contribute to two areas at the forefront of modern physics: gravitational wave astronomy and the search for dark matter.  In this talk, I will describe a new experiment, MAGIS-100, that will use a 100-meter-tall atom interferometer to pursue these directions.  MAGIS-100 will serve as a prototype gravitational wave detector in the mid-band frequency range 0.1 Hz to 10 Hz, which is complementary to the frequency bands addressed by laser interferometers such as LIGO and the planned LISA experiment.  I will discuss the scientific motivation for gravitational wave detection in the mid-band.  In addition, I will explain how MAGIS-100 can look for ultralight dark matter, a well-motivated class of dark matter candidates that behave as coherently oscillating fields.  Finally, I will briefly introduce a related atom interferometry experiment we are building at Northwestern to perform fundamental tests of gravity.

 
Meeting Recording:
 
Date:
-
Location:
online
Type of Event (for grouping events):
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