# Summer 2014 Seminar

Talks are 1:00 pm on Wednesdays in Olin 451, unless otherwise noted.

June 4, 2014. TIME: 11:00 am. LOCATION: Olin 264

## Muon g-2: precision particle physics at Fermilab

**Abstract:** I work on the Muon g-2 experiment, a
precision particle physics experiment which is being built
at Fermilab. We plan to measure the anomalous magnetic
moment of the muon to 0.14 ppm precision, a factor of four
improvement over the current experimental value. Our
measurement is desirable from the standpoint of particle
physics because it will be a fine probe of the standard
model, as well as various beyond-the-standard-model
scenarios. I will give an overview of the experiment -- its
purpose, method, and progress so far. I will also talk about
my own work simulating the response of the electromagnetic
calorimeters. Finally, I will share some thoughts about what
it's like to be a graduate student in experimental particle
physics, and how I ended up doing this work.

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FRIDAY, June 13, 2014

## Quantum Mechanics meet Fluid Dynamics: Visualizing
Superfluid Helium

### Matt Paoletti '05, Johns Hopkins University Applied
Physics Laboratory

**Abstract:** Turbulence in classical fluids may readily
be found in the mixing of a cup of coffee or the intricate
patters in the atmosphere. While such examples are
commonplace, fluids cooled to temperatures near absolute
zero exhibit turbulent phenomena that differ from their
classical counterparts, owing to quantum effects. Quantum
fluids, such as superfluid helium-4, are typically
described as a mixture of two interpenetrating fluids with
distinct velocity fields: a viscous normal fluid akin to
water and an inviscid superfluid. In this “two-fluid
model,” there is no conventional viscous dissipation in the
superfluid component. While vortices in classical fluids
can be any size and strength, from whirlpools in a bathtub
to tornadoes, every vortex in superfluid helium-4 is
atomically thin and has quantized circulation. Our group
discovered that micron-sized particles of frozen hydrogen
may be used for flow visualization in superfluid
helium-4. The particles can trace the motions of the
normal fluid or be trapped by the quantized vortices, which
enables one to characterize the dynamics of both the normal
fluid and superfluid components for the first time. By
directly observing and tracking these particles, we have
confirmed the two-fluid nature of quantum fluids, observed
vortex rings and quantized vortex reconnection,
characterized thermal counterflows, and observed the very
peculiar nature of quantum turbulence.

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June 18, 2014

## Probabilistic foundations for quantum mechanics

### Alex
Wilce, Department of Mathematics, Susquehanna
University

**Abstract:** Here are two ways to look at quantum
mechanics: (i) as a dynamical theory with a fairly standard
mathematical structure, but a strange probabilistic
interpretation; (ii) as a probabilistic theory with a fairly
standard interpretation, but a strange mathematical
structure. The first point of view has to deal with the
measurement problem. The second wants a convincing account
of how the Hilbert space structure of quantum theory emerges
from deeper operational, probabilistic or
information-theoretic principles. In fact, there is a long
history of attempts to give such an account. After briefly
surveying some of these, I'll sketch a recent approach that
is particularly simple and economical.

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June 25, 2014

## Why Aren't There More Stars? The Battle Against Gravity in
our Galaxy's Nurseries

### Ned Ladd, Bucknell University

**Abstract:** Stars form via the gravitational collapse
of large diffuse molecular clouds that occupy much of the
disk of our galaxy. However, not all of this cloud material
participates in the collapse process, mostly because energy
injected on a range of size scales “stirs up”
the cloud material. The competition between gravitational
attraction and dynamical support ultimately determines how
many stars form from any cloud. I'll discuss some of the
basic ideas behind the process, and describe how astronomers
measure the amount and dynamical state of cloud material
using radio wavelength astronomical observations.

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July 2, 2014

## Phase Field Modelling and Numerical Instability

**Abstract:** Phase field models are widely used in
computational physics and materials science. These models
are capable of spontaneously generating interfaces with
desired thermodynamic properties, and then evolving them in
time without the need for interface tracking.
Unfortunately, phase field models come with a cost: they
become numerically unstable unless the time step size is
held below a rather small threshhold value. I will
introduce phase field modelling, demonstrate some important
examples, and illustrate the problem associated with
numerical instability. Then I will describe recent work in
developing stable integration methods that will allow us to
keep the benefits of phase field models without paying the
cost.

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July 9, 2014

## Particle vs. Wave: The Ongoing Saga of Light

**Abstract:** The answer to the question “What is
Light?” has always been fraught with difficulties in
attempts to explain its behavior by placing it into the
category of either particle-like or wave-like. Historically
this characterization has vacillated between these two
categories several times. In this talk I will show the
results of some experiments that demonstrate the rather
bizarre behavior of light and imply that an understanding of
light is even trickier than a simple hybridization of
particle-wave concepts.

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July 16, 2014

## The Sensitivity of Non-Uniform Sampling NMR

### David Rovnyak,
Department of Chemistry, Bucknell University

**Abstract:** The advent of multi-dimensional NMR,
grounded in the use of the Fast-Fourier Transform (FFT)
algorithm, ignited decades of innovation and discovery in
studying the structures and dynamics of molecules. A
modified method for acquiring multi-dimensional NMR data,
termed non-uniform sampling (NUS), can informally be
described as “skipping” certain data
points. Although offering several advantages, NUS data can
not take advantage of the FFT and the theorems associated
with its use. We have been developing exact analytic
expressions, with extensive experimental verification, that
place some of the advantages of NUS methods on stronger
theoretical foundations. We recently solved the exact
sensitivity of NUS-NMR of decaying signals, showing that NUS
leads to significant and predictable sensitivity
enhancements that enable new science. In particular: NUS can
simultaneously improve resolution and sensitivity, a
statement that cannot be applied to conventional data
acquisition. The theory also indicates road maps for further
optimizations of NUS resolution and sensitivity that are
currently being developed in our lab.

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July 23, 2014

## Doppelganger Defects

### Melinda Andrews '06, University of Pennsylvania

**Abstract:** Generally, different fundamental physics
theories lead to different predictions about the existence
and nature of topological defects such as domain walls and
cosmic strings. Thus, observing the properties of such
objects would open a window on higher-energy physics than we
can access with experiments. However, we find that some
interesting non-standard theories can have defect solutions
that are identical to those of a standard theory. In this
talk, I will introduce topological defects and their
importance in cosmology, explain the motivation for studying
non-standard fundamental theories, and explore the
implications for defect solutions.

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