Last week, two undergraduate researchers in our group, Evan Li and Julie Scherzer, completed their summer research experiences and presented their work at The Ohio State University. Their projects used ultra-peripheral heavy-ion collisions—encounters in which fast-moving nuclei pass close enough for their electromagnetic fields to interact without a direct nuclear collision—to investigate the structure of atomic nuclei.

Evan participated through the Ohio 5-OSU Summer Undergraduate Research Experience, while Julie conducted her project through the Physics Summer Research Program and presented at the Undergraduate Research Symposium.

Evan Li: probing oxygen-16 structure

Evan's project, “Probing the Structure of the O-16 Nucleus in High-Energy Ultra-Peripheral Collisions,” compared two pictures of the oxygen-16 nucleus: a smooth, spherical Woods–Saxon distribution and a model in which the nucleus is built from four alpha-particle clusters. The distinction matters because the spatial structure of a nucleus leaves an imprint on the particles produced in high-energy collisions.

Using simulated nuclear density distributions, thickness functions, and form factors, Evan fit the predicted momentum-transfer spectra to collider data. Both descriptions reproduced the measured spectrum well. In a combined fit, the Woods–Saxon contribution dominated, while an alpha-cluster component remained compatible with the data. Evan's work highlights both the promise and the challenge of these measurements: resolving geometry-sensitive differences will require more precise low-momentum-transfer data and reduced experimental backgrounds.

Download Evan's poster (PDF)

Julie Scherzer: photonuclear measurements with asymmetric ions

Julie's project, “First Photonuclear Measurements in Asymmetric Heavy Ion Collisions,” studied photonuclear processes in collisions between copper and gold nuclei. In an ultra-peripheral collision, the intense electromagnetic field of one ion can act as a beam of photons. These photons can produce vector mesons through interactions with the other nucleus, creating a powerful probe of nuclear size, shape, and quantum interference.

Julie analyzed copper–gold collisions at a center-of-mass energy of 200 GeV per nucleon pair. By reconstructing rho mesons and studying the momentum-transfer distribution, she extracted nuclear-radius information and examined interference patterns that are sensitive to the asymmetric collision geometry. Her results show clear effects associated with the copper nucleus in the diffractive component of the data and establish a foundation for the first ultra-peripheral-collision analysis with asymmetric ions.

Download Julie's poster (PDF)

Congratulations to Evan and Julie on an excellent summer of research and on sharing their results with the broader Ohio State community. I am grateful to the graduate-student mentors and collaborators who supported their work, including Xihe Han, Sam Corey, and Nicholas Jindal.

Location

• Partonic structure of protons and nuclei
• Physics at low-x and gluon saturation
• The initial stages and nuclear structure in heavy-ion collisions
• Collective dynamics from small to large systems
• New theoretical techniques at large and small coupling
• New facilities: DIS and hadronic experiments

OSU Contributions

Pictures

Scientific Agenda

• Beam Energy Scan
• Computing, Machine Learning, & AI
• Heavy Flavor & Quarkonia
• Jets
• Spin Physics, Cold QCD, & UPCs
• Flow & Vorticity
• Diversity, Equity, & Inclusion

OSU Contributions

OSU Physics highlights

Pictures

Location

• Bulk QCD
• Electromagnetic probes
• Heavy Flavor
• Quarkonia
• Jets
• Exotica
• Central Exclusive and Ultra Peripheral physics (CEP and UPC)
• Fixed Target physics at LHCb

OSU Contributions

Pictures

On May 20 - 24, we held a joint meeting of the Kent and OSU nuclear experimental groups. Since I and Zhangbu both started at our respective positions recently, we wanted to introduce our groups and cover a broad range of material beneficial for new students. The agenda allowed ample time for discussion and questions from everyone. We had the meeting at Kent State University on Monday - Wednesday. On Wednesday afternoon we took a group excursion to the Cuyahoga Falls National Park (see pictures below). On Thursday - Friday we hosted the meeting at OSU. We had about 20 people in attendance each day including undergraduates, pre- and post- candidacy students, post-docs and faculty from both universities. We had 21 talks on various topics in high-energy heavy ion physics.

On the final day we used this opportunity to also pilot another idea - to unify nearby institutions contributioning to the electron ion-collider and the ePIC experiment. On Friday we hosted the first ePIC Midwest meeting to discuss the ongoing efforts at Kent and OSU. Our agenda focuses on three topics according to the recent leadership positions taken by each institutions: 1) Zhangbu is the detector subsystem leader for the AC-LGAD TOF detector, 2) Leszek is the detector subsystem leader for the backward HCAL, and 3) I recently led the implementation of the initial electron-finder into the official EICRecon framework.

The meeting was a great success on several fronts. Scientifically, it provided a friendly and welcoming environment for new students to gain foundational knowledge about the field. Eight of the twenty participants were pre-candidacy or undergraduate students, and seven members gave their first presentation (outside of local group meetings). We also opened the ePIC Midwest session over ZOOM to welcomed University of Illinois Urbana-Champaign to the backward HCAL effort.

Meeting Agenda - Monday May 20

Meeting Agenda - Tuesday May 21

Meeting Agenda - Wednesday May 22

Meeting Agenda - Thursday May 23

Meeting Agenda - Friday May 24

Pictures

This year I helped organize the 39th Winter Workshop on Nuclear Dynamics from February 11 - 17 (Along with Ant, John, Caludia, Raghav). We held the workshop at the Snow King resort in Jackson, WY.

The conference information is available here:
https://indico.cern.ch/event/1345629/overview
and the timeline of talks is here:
https://indico.cern.ch/event/1345629/timetable/#all

We had 56 speakers from many different theoretical and experimental collaborations.

Below are some of my favorite pictures from the week.

Arriving into Jackson Hole!

Snow King had a beautiful fireplace burning in the lobby.

A full house!

Maria giving the HADES overview talk.

Yevhenia giving a talk about femtoscopy in STAR.

Ant Timmons and I on the ski lift, we were enjoing the green slopes together.

Kong was a great Ski instructor, by day 2 I was much better. We were about to go down a blue slope together.

About to go down a blue slope!

Waiting to go to the airport together.

We had three people from OSU at the meeting (Yevhenia, Maria, Daniel, and Yevhenia) - We made O-H-I-O at the airport

Getting on my flight at one of the most beautiful airports.

Until next time!

Pictures

Arrival

Fermilab Experimental Tours

BREAKING the Quantum Double Slit Experiment

to Peer Deep Within Atomic Nuclei

Pictures

A gallery of the most commonly used functional forms in HEP. Help to gain an intuition of the behavior of various functions that are commonly used to describe physical processes.

Functions

• Gaussian
• Landau
• Linear (pol0)
• Linear (pol1)
• Quadratic
• Exponential
• Logarithmic
• curves for (efficiency)
• Fourier
• Crystal Ball + ExpoGauss
• Student-t
• Pearson IV
• Breit-Wigner
• 1/r
• 1/r^2
• logistic

options:

• add random data so that the curves are a fit

Introduction

I got a macbook pro M1 Max 14 inch on January 1, 2023 as part of my new faculty position at the Ohio State University. I was upgrading from a 2018 macbook pro from my previous position. That one had the infamous butterfly keyboard which was unusable by the time I stopped using it. This is a writeup of how I’ve setup the new macbook and what I like / dislike about it.

Specs

Model Name:	MacBook Pro
Model Identifier:	MacBookPro18,4
Chip:	Apple M1 Max
Total Number of Cores:	10 (8 performance and 2 efficiency)
Memory:	64 GB
  Type:	LPDDR5
System Firmware Version:	7459.141.1
OS Loader Version:	7459.141.1

Free:	1.74 TB (1,743,555,956,736 bytes)
Capacity:	2 TB (1,995,218,165,760 bytes)

Physical Drive:
  Device Name:	APPLE SSD AP2048R
  Media Name:	AppleAPFSMedia
  Medium Type:	SSD
  Protocol:	Apple Fabric
  Internal:	Yes
  Partition Map Type:	Unknown
  S.M.A.R.T. Status:	Verified

Chipset Model:	Apple M1 Max
  Type:	GPU
  Bus:	Built-In
  Total Number of Cores:	24
  Vendor:	Apple (0x106b)
  Metal Family:	Supported, Metal GPUFamily Apple 7
  Displays:
Color LCD:
  Display Type:	Built-in Liquid Retina XDR Display
  Resolution:	3024 x 1964 Retina

Form Factor

I was comming from the larger 15 inch macbook pro size. I am much happier with the smaller size despite the fact that I dont have any external monitors yet. The screen still feels pretty roomy and the look

Keyboard & Trackpad

The keyboard is so much better

• Key travel and feel is much better than the old butterfly mechanism
• Having a physical escape key!
• No touch bar is better. Even though I paid at one point for BettterTouchTools I never liked or used the touchbar. On my previous laptop it often froze and would become unresponsive until after a reboot.
• Fingerprint reader is great and makes password entry a breeze.
• Trackad is as good as always.

Setup

Applications

After about 1 month of using the laptop daily this is what I have installed:

• Docker
• Dropbox
• OneDrive
• LaTeXiT
• Affinity Designer (personal choice)
• Adobe Photoshop & Illustrator (free through OSU)
• Adobe Acrobat Pro (free through OSU) - but it doesnt work and online formums seem to show no support from Adobe.
• Alfred - TODO buy powerpack
• Dato - a better menubar calendar
• Maccy - clipboard manager
• VS Code
• Sublime Text 4 - I purchased a while ago. It is still so much faster, especially for large files
• KeeWeb - Password manager
• Bitwarden - Password manager
• Good Notes - Along with my iPad
• Things - I bought this in an attempt to improve my organization
• Zotero
• Obsidian - for notes
• Microsoft products (email & teams used by department)
• Rectangle - window management
• iTerm2
• DiffusionBee
• Espanso
• Blender
• Sim Daltonism - check colorblind on plots etc.

Terminal

Shell

I use the zsh with oh-my-zsh and the powerlevel10k theme

• bat
• exa
• fzf
• espanso (homebrew cask)
• mactex (homebrew cask)
• root (research)
• python via conda

My .zshrc is not too complicated. A few additons:

# make file autocompletions!
zstyle ':completion:*:*:make:*' tag-order 'targets'
autoload -U compinit && compinit

alias fz="fzf --preview 'bat --color=always --style=numbers --line-range=:500 {}'"

vim setup

I am using Vundle for plugin management

git clone https://github.com/VundleVim/Vundle.vim.git ~/.vim/bundle/Vundle.vim

To install plugins listed in .vimrc

vim +PluginInstall +qall

OR to cleanup unused

vim +PluginClean +qall

set nocompatible              " be iMproved, required"
filetype off                  " required"

" set the runtime path to include Vundle and initialize"
set rtp+=~/.vim/bundle/Vundle.vim
call vundle#begin()

" Let Vundle manage Vundle"
Plugin 'VundleVim/Vundle.vim'
Plugin 'sheerun/vim-polyglot'
Plugin 'kristijanhusak/vim-hybrid-material'
Plugin 'preservim/nerdtree'
Plugin 'derekwyatt/vim-fswitch' " Switch between header and source "
Plugin 'tpope/vim-fugitive'
call vundle#end()            " required "
filetype plugin indent on    " required "

 " ROOT C macros "
autocmd BufNewFile,BufRead *.C set syntax=cpp

filetype plugin on
syntax on
" Color scheme "
set background=dark
let g:enable_bold_font = 1
let g:enable_italic_font = 0
" let g:hybrid_transparent_background = 1 "
colorscheme hybrid_material

set tabstop=4
set shiftwidth=4
set expandtab
set softtabstop=4   " number of spaces in tab when editing "
set number              " show line numbers"

set cursorline          " highlight current line"

set wildmenu            " visual autocomplete for command menu"
set showmatch           " highlight matching [{()}]"

set incsearch           " search as characters are entered"
set hlsearch            " highlight matches"

set splitbelow         " Always split below"
set mouse=a            " Enable mouse drag on window splits"


if (has("nvim"))
  "For Neovim 0.1.3 and 0.1.4 < https://github.com/neovim/neovim/pull/2198 >"
  let $NVIM_TUI_ENABLE_TRUE_COLOR=1
endif

"For Neovim > 0.1.5 and Vim > patch 7.4.1799 < https://github.com/vim/vim/commit/61be73bb0f965a895bfb064ea3e55476ac175162 >"
"Based on Vim patch 7.4.1770 (`guicolors` option) < https://github.com/vim/vim/commit/8a633e3427b47286869aa4b96f2bfc1fe65b25cd >"
" < https://github.com/neovim/neovim/wiki/Following-HEAD#20160511 >"
if (has("termguicolors"))
  set termguicolors
endif

" NERDTree options"
let NERDTreeShowBookmarks = 1   " Show the bookmarks table"
let NERDTreeShowHidden = 1      " Show hidden files"
let NERDTreeShowLineNumbers = 0 " Hide line numbers"
let NERDTreeMinimalMenu = 1     " Use the minimal menu (m)"
let NERDTreeWinPos = "left"     " Panel opens on the left side"
let NERDTreeWinSize = 31        " Set panel width to 31 columns"

nmap <F2> :NERDTreeToggle<CR>

" fswitch"
au! BufEnter *.cpp let b:fswitchdst = 'hpp,h'
au! BufEnter *.h let b:fswitchdst = 'cpp,c,cxx'

nmap <C-Z> :vsplit <bar> :wincmd l <bar> :FSRight<CR>

" gitgutter"
set updatetime=100

" folding"

set foldmethod=indent
set foldnestmax=10
set nofoldenable
set foldlevel=2

set terminal svg enhanced size 1000,700 set output 'out.svg' # set terminal svg size 600,400 dynamic enhanced fname 'arial' fsize 10 mousing name "heatmaps_3" butt solid # set output 'heatmaps.3.svg' set format cb "%4.1f" set view 49, 28, 1, 1.48 set samples 70, 70 set isosamples 60, 60 set ticslevel 0 set cbtics border in scale 0,0 mirror norotate offset character 0, 0, 0 autojustify set title "4D data (3D Heat Map)\nIndependent value color-mapped onto 3D surface" set title offset character 0, 1, 0 font "" norotate set xlabel "x" set xlabel offset character 3, 0, 0 font "" textcolor lt -1 norotate set xrange [ 5.00000 : 35.0000 ] noreverse nowriteback set ylabel "y" set ylabel offset character -5, 0, 0 font "" textcolor lt -1 rotate by -270 set yrange [ 5.00000 : 35.0000 ] noreverse nowriteback set zlabel "z" set zlabel offset character 2, 0, 0 font "" textcolor lt -1 norotate set pm3d implicit at s set colorbox user set colorbox vertical origin screen 0.9, 0.2, 0 size screen 0.03, 0.6, 0 front noborder Z(x,y) = 100. * (sinc(x,y) + 1.5) sinc(x,y) = sin(sqrt((x-20.)**2+(y-20.)**2))/sqrt((x-20.)**2+(y-20.)**2) color(x,y) = 10. * (1.1 + sin((x-20.)/5.)*cos((y-20.)/10.)) GPFUN_Z = "Z(x,y) = 100. * (sinc(x,y) + 1.5)" GPFUN_sinc = "sinc(x,y) = sin(sqrt((x-20.)**2+(y-20.)**2))/sqrt((x-20.)**2+(y-20.)**2)" GPFUN_color = "color(x,y) = 10. * (1.1 + sin((x-20.)/5.)*cos((y-20.)/10.))" splot '++' using 1:2:(Z($1,$2)):(color($1,$2)) with pm3d title "4 data columns x/y/z/color"

Output:

I purchased jdbburg.com for $12/year which is not bad.

I have been happy with vercel (previously now.sh) but I have used DigitalOcean for a long time and wanted to try out their new Apps platform. I also thought it might make things easier since I plan to host some other projects on sub-domains using digitalocean droplets.

I like the vercel platform because they provide serverless functions as part of your static site. Digitalocean provides “components” which seem more flexible (can be database or public/private workers) but not as simple/low cost.

The best feature of all these platforms is the continuous deployment features integrated with github/gitlab.

gnuplot is a powerful graphing utility that is capable of producing publication quality plots. However, it cannot interop with ROOT data natively. I have previously written a small tool to convert ROOT data (histograms, graphs, etc.) to gnuplot text-based data files. However, it still adds a substantial overhead when an extra step is needed to convert the data before plotting. It also makes it potentially susceptible to data being out-of-date. The goal of this tool is to provide a drop-in “replacement” for gnuplot that can work directly on root data. This is accomplished by converting the data in the background and processing the gnuplot script altogether.

Usage

1. Write gnuplot plotting scripts like normal
2. Add a comment line in your script like this

# root2gnuplot input.root:histogram_name output.dat "x y xlow xhigh ylow yhigh"

this line instructs rnuplot how to convert your ROOT data. It uses an internal version of the root2gnuplot tool. See details here. You can control the format of the conversion for custom plotting types in gnuplot. You can also plot ‘directly’ from ROOT TTrees.

3. Run your script with rnuplot instead of gnuplot and it will just work

Full usage details

Usage:
  rnuplot [OPTION...] [FILE]

  -c, --cache             Use cached data, set to true to use cached data
  -f, --file arg          File
  -d, --debug arg         Enable debugging
  -r, --root2gnuplot arg  Convert ROOT data using root2gnuplot (useful if
                          not specified in gnuplot script comment
  -h, --help              Print usage

By default the data is not cached - meaning that even if the data file exists

Example

set terminal pngcairo font "Helvetica, 18" size 1200,900 
set output 'demo.png'

set xlabel "𝛂" 
set ylabel "Counts"

set bars small  caps from error bars

# root2gnuplot input.root:histo_name h1.dat "x y xlow xhigh ylow yhigh"
plot 'h1.dat' u 1:2:3:4:($5):($6) with xyerr ls 2 pt 15 ps 0 lw 2 t 'ROOT histogram'

OR without modifying your gnuplot script at all you can:

rnuplot -r 'input.root:histo_name h1.dat "x y xlow xhigh ylow yhigh"' script.gnuplot

with a script like this:

set terminal pngcairo font "Helvetica, 18" size 1200,900 
set output 'h1.png'

set xlabel "x" 
set ylabel "y"

set bars small  caps from error bars
set style fill   solid 1.00 border

plot 'h1.dat' u ($1+($3-$4)/2.0):2  with fillsteps fc rgb "#aa00ff" t "h1", 'h1.dat' u 1:2:3:4:($5):($6) with xyerr ls 2 pt 15 ps 0 lw 2 lc rgb "#000000" notitle

Notice that these are valid gnuplot scripts. In fact, once the conversion step is done (first time) you can run future plots with gnuplot directly. Only comments with the root2gnuplot word are parsed for conversions

Build Steps

1. clone the repo
2. build with: make

Install

just put the rnuplot binary somewhere on the PATH.
Then you are ready to use rnuplot, just make sure gnuplot is on the PATH as well, since it is called internally

Requirements

• c++14
• ROOT (tested with v6-24-00@v6-24-00)
• gnuplot (tested with v5.4) on the PATH

Uses

• cxxopts as internal dependency

Ultra-relativistic heavy-ion collisions are expected to produce the strongest electromagnetic fields ($10^{13}-10^{16}$ Tesla) in the known Universe. These highly-Lorentz contracted fields can manifest themselves as linearly polarized quasi-real photons that interact via the Breit-Wheeler process. In this talk I will discuss recent experimental measurements that have spurred significant theoretical progress in our understanding of dilepton production in ultra-peripheral heavy ion collisions. Specifically, we now know that the energy and momentum distribution of the produced dileptons carry information about the strength and spatial distribution of the colliding fields. Moreover, the recent observation of quantum correlations between the interacting photon’s spin (polarization) and momentum provides a clear connection to the semi-classical electromagnetic field distribution, thus making it possible to measure the magnetic field produced in heavy ion collisions for the first time. I’ll end the talk with a look at how current and future measurements can be used to constraining the magnetic field and explore how they may provide novel input for the discussion of emergent magnetohydrodynamical phenomena driven by event-by-event fluctuations of the magnetic field.

Online here: https://hit.lbl.gov/#h.vyruxndze0en

Author: James Daniel Brandenburg for the STAR Collaboration

Only a handful of fundamental interactions between light and matter are allowed by QED, almost all of which have been observed in the 80 or so years since their prediction. Only the linear Breit-Wheeler process, the simplest mechanism for converting ’light quanta’ into matter and antimatter, has eluded observation for decades.

In this talk we will present measurements of $e^+e^-$ pair production in ultra-peripheral and peripheral Au$+$Au collisions at $\sqrt{s_{NN}} = 200$ GeV (Lorentz boost $\gamma=107$). A comprehensive study of the $e^+e^-$ pair kinematics is presented to distinguish the $\gamma\gamma \rightarrow e^+ e^-$ process from other possible production mechanisms and to definitively identify the Breit-Wheeler process. Furthermore, the measured distribution of electron-positron pairs reveals a striking fourth-order angular modulation which demonstrates the linear polarization of the colliding photons.

Title: Observation of the Breit-Wheeler Process in Heavy-Ion Collisions Quantum electrodynamics (QED), the relativistic quantum field theory of electromagnetism, is the best tested theory in physics. Only a handful of fundamental interactions between light and matter are allowed by QED, almost all of which have been observed in the 80 or so years since their prediction. Only the Breit-Wheeler process, the simplest mechanism for converting ’light quanta’ into matter and antimatter, has eluded observation for decades.

In this talk I will present STAR measurements of e+e- pair production in ultra-peripheral and peripheral Au+Au collisions at sqrt(sNN) = 200 GeV. A comprehensive study of the pair kinematics is presented to distinguish the gamma gamma to e+ e- process from other possible production mechanisms and to definitively identify the Breit-Wheeler process for the first time. Furthermore, the measured distribution of electron-positron pairs reveals a striking fourth-order angular modulation which demonstrates the linear polarization of the colliding photons. I’ll end the talk by briefly discussing applications of these observations and open questions that may be possible to address with future measurements in heavy-ion collisions.

Similar to my [UIUC Experimental Nuclear Physics Faculty Position] abstract

Ultra-relativistic heavy ion collisions are expected to produce some of the strongest magnetic fields ($10^{13}-10^{16}$ Tesla) in the Universe[1]. Recently, there has been increased interest in the magnetic fields produced by heavy ion collisions and their possible observational impacts through emergent magnetohydrodynamical phenomena in Quantum Chromodynamics, like the Chiral Magnetic Effect[2]. The initial strong electromagnetic fields produced in heavy ion collisions have been proposed as a source of linearly-polarized, quasi-real photons[3] that can interact via the Breit-Wheeler process to produce $e^+ e^-$ pairs[4].

In this talk I present STAR measurements of $e^+ e^-$ pair production in ultra-peripheral and peripheral Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. A comprehensive study of the pair kinematics is presented to distinguish the $\gamma\gamma \rightarrow e^+ e^-$ process from other possible production mechanisms. Furthermore, the measured distribution of $e^+e^-$ pairs reveals a striking fourth-order angular modulation which is related to vacuum birefringence[5], a phenomenon predicted in 1936 in which empty space can split light according to its polarization components when subjected to a strong magnetic field. These measurements provide the first direct experimental evidence that ultra-relativistic heavy ion collisions are capable of producing the strongest magnetic fields in the known Universe.

[1] V. Skokov, A. Illarionov, and V. Toneev. International Journal of Modern Physics A 24 (2009): 5925–32. [2] Kharzeev, D. E., et al. Prog. Part. Nucl. Phys., 88 (2016)1–28
[3] C. Weizsäcker, Zeitschrift für Physik 88 (1934): 612–25.
[4] G. Breit and J. A. Wheeler. Physical Review 46 (1934): 1087
[5] Heisenberg, W., and H. Euler. Zeitschrift für Physik, (1936) arXiv: physics/0605038

Slides here: https://indico.ihep.ac.cn/event/11115/contribution/44/material/slides/0.pdf

The intense electromagnetic fields produced by ultra-relativistic heavy nuclei have been proposed as a source of quasi-real photons i.e. in the Weizsacker-Williams equivalent photon method. A photon from one nucleus can fluctuate into a quark antiquark pair and interact directly with the other nucleus to produce a vector meson (e.g. $\rho^0$). It has been demonstrated that the interacting photons are linearly polarized, and therefore that the photon polarization should induce angular modulations in the final state particle distribution.

In this talk we present STAR measurements of diffractive photo-production of the $\rho^0$-meson (and direct $\pi^+\pi^-$ pairs) in ultra-peripheral Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. By employing a recently proposed technique, we measure the angular distribution of the final state $\pi^+\pi^-$ pairs and observe $\cos 2\Delta\phi$ and $\cos 4\Delta\phi$ modulations. Theoretical predictions suggest that such modulations may provide new insight into nuclear structure and may shed light on the transverse momentum dependent (TMD) distributions of gluons within large nuclei - a topic of great interest both at existing experiments and at a future Electron Ion Collider.

[1] C. Weizsäcker, Zeitschrift für Physik 88 (1934): 612–25. [2] H. Xing, C. Zhang, J. Zhou, and Y.-J. Zhou, ArXiv:2006.06206 [Hep-Ph] (2020). [3] A. Metz, and J. Zhou. Physical Review D 84 5 (2011).

Related: [Rho Polarization Analysis] [Ultra-Peripheral Heavy-ion Collisions] [equivalent photon method]

This photo of the sTGC prototype was uploaded to and is served by cloudinary.

Both are great as free services. I like the Netlify CMS since II can self-host it in the site itself. It also allows a little more customization.

I could not get it to work as they prescribed though. Specifically with the directory structure:

/static
   | - admin
        | - index.html
        | - config.yml

instead I had to do this:

/static
   | - admin
   |    | - index.html
   | - config.yml

That got it working. In fact this post was written in Netlify CMS.

Strip Detectors and Ghosts Hits

If we want to measure the 2D space point of a charged particle we can accomplish it with various types of detector technology. The small-strip thin-gap (sTGC) detectors used by ATLAS[1] provide reasonably good position resolution of $\sigma\approx100\mu$m and are significatly cheaper than competing technologies (like all silicon detectors).

However, the sTGC design is really two 1D detectors sandwiched to provide 2D space points. This design keeps the detectors simple and low cost, but has the disadvantage of introducing ambiguity when determinging the 2D space-point from the individual 1D measurements.

You can see this ambiguity in the demo below. Here the "real" hits are shown in green while the fake hits, e.g. "ghost" hits, are shown in red. The ghost hits appear at the crossing of all x-measurements with all y-measurements. For this reason, the number of ghost hits $(g)$ grows according to the combinations of real hits $(n)$ like: $$ g = n * (n - 1) $$

Adding UV strips

Additional strips can be added at an angle (often called U or V strips) to help reduce the ambiguity. Lets look at the same situation with an additional strip (U-strip) at 45° from the X and Y-strips. In this case, the U-strip can disambiquate some of the ghost hits. Now ghost hits only exist at the crossing of the XY-hit if they also fall on a U-hit also.

What angle is optimal for the U-strips?

Instead of 45° U-strips, lets look at some other angles to see how they compare for rejecting the ghost hits.

[1] Abusleme, A., et al. “Performance of a Full-Size Small-Strip Thin Gap Chamber Prototype for the ATLAS New Small Wheel Muon Upgrade.” Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, vol. 817, May 2016, pp. 85–92. ScienceDirect, doi:10.1016/j.nima.2016.01.087.

The particle azimuthal distribution measured with respect to the reaction plane is not isotropic; so it is customary to expand it in a Fourier series: $$ E \frac{d^3N}{d^3p} = \frac{1}{2\pi} \frac{d^2N}{p_T dp_T dy } \left( 1 + \sum 2 v_n \cos( n(\phi - \Psi) ) \right) $$
[1] Sergei A. Voloshin, Arthur M. Poskanzer, and Raimond Snellings "Collective phenomena in non-central nuclear collisions"

Explore the Fourier Expansion:

The parametric curves are given by: $$ x(\phi) = \bar{x} + R * \cos(\phi) + 2 R v_n \cos( n \phi ) $$ $$ y(\phi) = \bar{y} + R * \sin(\phi) + 2 R v_n \cos( n \phi ) $$

Probing Extreme Electromagnetic Fields with the Breit-Wheeler Process

Ultra-relativistic heavy-ion collisions are expected to produce the strongest magnetic fields ($10^{13}-10^{16}$ Tesla) in the Universe [1]. Recently, there has been increased interest in the magnetic fields produced by heavy-ion collisions and their possible observational impacts through emergent magnetohydrodynamic phenomena in Quantum Chromodynamics [2]. The initial strong electromagnetic fields produced in heavy ion collisions have been proposed as a source of linearly-polarized, quasi-real photons [3] that can interact via the Breit-Wheeler process to produce $e^+e^-$ pairs [4].

\noindent In this talk, I will present STAR measurements of $e^+e^-$ pair production in ultra-peripheral and peripheral Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. A comprehensive study of the pair kinematics is presented to distinguish the $\gamma\gamma \rightarrow e^+e^-$ process from other possible production mechanisms. Furthermore, I will present and discuss the first observation of a 4th-order azimuthal modulation of $e^+e^-$ pairs produced in heavy-ion collisions. The striking fourth-order angular modulation is a direct result of vacuum birefringence [5], a phenomenon predicted in 1936 in which empty space can split light according to its polarization components when subjected to a strong magnetic field. These measurements and their implications for the magnetic field produced in heavy-ion collisions will be discussed.

[1] V. Skokov, A. Illarionov, and V. Toneev. Int. J. Mod. Phys. A 24 (2009) 5925–32
[2] D. E. Kharzeev, et al. Prog. Part. Nucl. Phys., 88 (2016) 1–28
[3] C. Weizsäcker, Zeitschrift für Physik 88 (1934) 612–25
[4] G. Breit and J. A. Wheeler. Physical Review 46 (1934) 1087
[5] W. Heisenberg and H. Euler. Zeitschrift für Physik, (1936) arXiv: physics/0605038

$$ \vec{B} = \nabla \times A $$ Note several useful constants:

$$ \hbar c = 197.326 MeV fm $$ $$ e = \sqrt{4\pi \frac{1}{137}} $$ $$ \beta = \sqrt{1-1/\gamma^2} $$

To determine the field at point given by $\vec{x}$ $$ x = \vec{x}_0 / \hbar c $$ $$ y = \vec{x}_1 / \hbar c $$ $$ z = \vec{x}_2 / \hbar c $$

Then let: $$ R = \sqrt{ x^2 + y^2 + (z\gamma)^2} $$

Now the Magnetic field is : $$ \vec{B} = \beta \gamma ( x, y, z ) \times \vec{B_0} $$ $$ \vec{B} = (y \beta \gamma B_0, -x \beta \gamma B_0, 0) $$

where $B_0$ is: $$ B_0 = \frac{e^2}{4 \pi} \frac{Z_{Au} }{R^3} \frac{\int_0^{R/\hbar c} r^2 \rho(r) dr}{ \rho^\star } \hat{z} $$ $$ \rho^\star = \int_0^{\inf} r^2 \rho(r) dr $$

Compute $B(r)$ with $\vec{x} = (r, 0, 0)$ : $$ |B(r)| = r \beta \gamma \frac{e^2}{4 \pi} \frac{Z_{Au} }{R^3} \frac{\int_0^{R/\hbar c} r^2 \rho(r) dr}{ \rho^\star } $$

Using a spherically symmetric Woods-Saxson distribution: $$ \rho(r) = \frac{\rho_0 ( 1 + w^2 r^2 / R_{Au}^2 )}{ 1 + e^{ (r-R_{Au}) / a } } $$ with $$ \rho_0 = \frac{3 A} { 4 \pi R_{Au}^3 } $$

where: $a$ is the skin depth, $w$ is the ???, and $R_{Au}$ is the radius of the nucleus.

In this talk I will present STAR measurements of $e^+ e^-$ pair production in ultra-peripheral and peripheral Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. A comprehensive study of the pair kinematics is presented to distinguish the $\gamma\gamma \rightarrow e^+ e^-$ process from other possible production mechanisms. Furthermore, I will present and discuss the first observation of a 4th-order azimuthal modulation of $e^+ e^-$ pairs produced in heavy-ion collisions. The measured distribution of electron-positron pairs reveals a striking fourth-order angular modulation which is a direct result of vacuum birefringence[7], a phenomenon predicted in 1936 in which empty space can split light according to its polarization components when subjected to a strong magnetic field.

These measurements provide the first direct experimental evidence that ultra-relativistic heavy ion collisions are capable of produce ultra-strong magnetic fields approximately 10,000 times stronger than the magnetic fields found in the magnetosphere of magnetars (inferred to be $\approx 10^{10}-10^{12}$ Tesla), the strongest magnetic fields in the known Universe until now. These measurements provide constraints on existing models and provide an important experimental tool and baseline for the measurement of possible medium effects driven by strong final state magnetic fields or from Coulomb multiple scattering through the QGP[5,6]. If time permits I’ll further discuss the application of these discoveries to one of the “most intellectually pressing” questions that an electron-ion collider will address[8].

[1] V. Skokov, A. Illarionov, and V. Toneev. International Journal of Modern Physics A 24 (2009): 5925–32.
[2] Kharzeev, D. E., et al. Prog. Part. Nucl. Phys., 88 (2016)1–28
[3] C. Weizsäcker, Zeitschrift für Physik 88 (1934): 612–25.
[4] G. Breit and J. A. Wheeler. Physical Review 46 (1934): 1087
[5] STAR Collaboration, Phys. Rev. Lett. 121 (2018) 132301
[6] ATLAS Collaboration, Phys. Rev. Lett. 121 (2018) , 212301
[7] Heisenberg, W., and H. Euler. Zeitschrift für Physik, (1936) arXiv: physics/0605038
[8] EIC White Paper https://www.bnl.gov/npp/docs/EIC_White_Paper_Final.pdf

This is mostly a collection of notes for myself, to be sure I remember what I did and to give credit.

The site is built using hugo using the theme hello-friend-ng.

I added math support by creating a partial in layouts/partials/head.html to override the theme’s default. I copied from the theme and added:

{{ if or .Params.math .Site.Params.math }}
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/katex@0.11.1/dist/katex.min.css" integrity="sha384-zB1R0rpPzHqg7Kpt0Aljp8JPLqbXI3bhnPWROx27a9N0Ll6ZP/+DiW/UqRcLbRjq" crossorigin="anonymous">
<script defer src="https://cdn.jsdelivr.net/npm/katex@0.11.1/dist/katex.min.js" integrity="sha384-y23I5Q6l+B6vatafAwxRu/0oK/79VlbSz7Q9aiSZUvyWYIYsd+qj+o24G5ZU2zJz" crossorigin="anonymous"></script>

<script defer src="https://cdn.jsdelivr.net/npm/katex@0.11.1/dist/contrib/auto-render.min.js" integrity="sha384-kWPLUVMOks5AQFrykwIup5lo0m3iMkkHrD0uJ4H5cjeGihAutqP0yW0J6dpFiVkI" crossorigin="anonymous"
    onload='renderMathInElement(document.body, { delimiters:[
      {left: "$$", right: "$$", display: true},
      {left: "$", right: "$", display: false},
      {left: "\\(", right: "\\)", display: false},
      {left: "\\[", right: "\\]", display: true}
        ]} );'></script>
{{ end }}

so that any post with:

math: true

will automagically make the katex render in the body element. There could be some conflicts with using single ‘$’ but I left it for now to make it easier to paste in existing math content.

I can preview the site with:

hugo server -D

Note that the -D allows draft posts to be visible.

I created a new html page for my existing talk list with

hugo new talk-list.html

and a new markdown page for my curriculum vitae with:

hugo new curriculum-vitae.md

I was able to add these to the homepage top bar with these lines in the config (near similar):

[[menu.main]]
  identifier = "talks"
  name       = "Talks"
  url        = "/talk-list"
[[menu.main]]
  identifier = "cv"
  name       = "CV"
  url        = "/curriculum-vitae"

This site is hosted on a digital ocean droplet (smallest one) on Ubuntu 18.04. I use the caddy reverse proxy:

curl https://getcaddy.com | sudo bash -s personal

~# which caddy
/usr/local/bin/caddy
~# caddy -version
v1.0.4
~#

directions here: https://computingforgeeks.com/install-caddy-web-server-on-an-ubuntu-18-04-with-lets-encrypt-ssl/

sudo setcap 'cap_net_bind_service=+ep' /usr/local/bin/caddy
sudo groupadd -g 33 www-data
sudo useradd \
  -g www-data --no-user-group \
  --home-dir /var/www --no-create-home \
  --shell /usr/sbin/nologin \
  --system --uid 33 www-data

sudo mkdir /etc/caddy
sudo chown -R root:root /etc/caddy
sudo mkdir /etc/ssl/caddy
sudo chown -R root:www-data /etc/ssl/caddy
sudo chmod 0770 /etc/ssl/caddy

NOT using the part from above website, but different:

wget https://raw.githubusercontent.com/caddyserver/caddy/master/dist/init/linux-systemd/caddy.service
sudo cp caddy.service /etc/systemd/system/
sudo chown root:root /etc/systemd/system/caddy.service
sudo chmod 644 /etc/systemd/system/caddy.service
sudo systemctl daemon-reload
sudo systemctl start caddy.service

Unfortunately the caddy service file is no longer available. I copied from my previous working caddy systemd. Digital Ocean has a caddy image, maybe it is best to use that in the future.

Deploying the site

A few options exist, not sure what to use yet:

1. Build local, push site as tarball, extract on server
2. Install hugo (any and all dependencies) on server. Cron job to git pull and rebuild site. Performance on server?
3. Use a CI service to build the site (it runs for less than a second), push artifacts to server??

Both 2. and 3. require all assets in the git repo, maybe that is fine. But I wonder if I plan to ever include large assets that it could be a problem. Maybe that means assets should be stored elsewhere (CDN)?