\chapter{Introduction
\label{sec:intro}}

%GH: our discussion indicated that this intro needs to be more engaging, draw the reader in on why NP is interesting
Through the aegis of the Canadian Institute of Nuclear Physics (CINP), the
nuclear physics community has gathered together to outline its vision for the
next 5 years and beyond.  That vision is summarized in this report, prepared by
a representative committee consisting of the CINP Executive Director and the
Chairs of the five CINP Scientific Working Groups, ``The CINP White Paper Committee''.
Our duty was to gather community input and prepare a document placing the
Canadian nuclear physics contributions within a long-term and international
context, and make some overall recommendations.  We hope that it will be of
value to the Canadian Subatomic Physics Long Range Planning Committee as it works
to establish the vision and goals for the whole subatomic physics community in
Canada.

\section{Consultation process}

The consultation process we followed is outlined below.  
One of the fundamental aspects of the structure of the CINP are the Scientific 
Working Groups (SWG), which facilitate collaboration among researchers with common 
interests, and enhance the profile of specific research areas within Canada.
Each SWG is headed by a Chair, who is elected by its members and appointed by the CINP Board.
The SWG Chairs and the Executive Director form the CINP White Paper Committee, ensuring
representation from all of the sub-disciplines of nuclear physics.


In 2019, in preparation for the Long Range Plan, the CINP Board and Executive
Director undertook a review and renewal of its Scientific Working Group (SWG)
leadership, including their leadership and terms of reference.  
One SWG Chair decided to step down after two terms of service, and the terms of the other four chairs were renewed, leading to a very experienced White Paper Committee.

Due to the COVID-19 situation, a three-day virtual Town Hall meeting was held via Zoom on June 22-24, 2020.  Participants were requested to submit a draft written
document on their activities, plans and HQP training two weeks before the Town
Hall meeting (June 5), and all were given an opportunity to revise their
written briefs afterward, reflecting the discussions at the meeting.  
The response to the call for input was excellent, with 33 briefs received by 
the final deadline.

The Town Hall meeting consisted of plenary sessions, where the SWG Chairs provided 
overviews of the submitted briefs, and proponents of new projects invited to give 
presentations outlining their plans and opportunities for new research more thoroughly.
There were also breakout sessions, where each SWG Chair led discussions on the plans and 
priorities in each of their sub-fields of nuclear physics.  Finally, there was a plenary 
discussion to cover the main points to be emphasized in the CINP Report.

Following this extensive community input, the committee members meet several times by 
Zoom in July to discuss issues and develop a cohesive plan.  Four members of
committee met in person at TRIUMF August 10-11, 2020, with the remaining two connecting
remotely via Zoom, to prepare the main portions of the report.  After
further writing, online discussion, and edits, the draft report was released to the 
Canadian nuclear physics community for comment on September 6.  Written comments were requested by September 21, and a second draft incorporating these improvements was circulated on October 20.
A second virtual town hall meeting to gather further input and consensus
was held on October 26, with final comments due November 13.  The report was finalized and submitted on December 1.
Given this extensive process, the White Paper committee members are
confident this final document reflects the consensus of the CINP community.

\section{The key questions in nuclear physics}

\subsection{How do quarks and gluons give rise to the properties of strongly interacting particles?}

%GH: rephrased the question taken from 2017 LRP
We know that hadrons, strongly-interacting composite particles such as protons, neutrons and pions, are made up of quarks and gluons, but we only have partial answers on how quarks are distributed and moving inside.
Quantum Chromodynamics (QCD), the theory of the strong nuclear interaction between quarks and gluons, describes two regimes – the asymptotic freedom and confinement.  While the discovery of asymptotic freedom within the context of perturbative QCD was recognized by the Nobel Prize in 2004, we still do not have a complete solution in the confinement regime, where the quark coupling strength is too large to allow the use of perturbative methods. Thus, the explanation for the observed properties of the hadrons remains one of the central problems of modern physics, requiring advances in both theory and experiment. 
Recent advances in lattice QCD make it possible to extrapolate full lattice QCD simulations to physical quark masses, and thus allow direct comparison to experimental observables such as spectrum of hadrons. Further developments in computational methods making full use of new computing technologies and decisive breakthroughs are anticipated in the near future.  
In addition, the Chiral Perturbation Theory (ChPT), an effective field theory based on chiral symmetry of QCD, can provide stringent predictions of many fundamental properties of composite systems at low energies such as electromagnetic polarizabilities. 
Electromagnetic polarizabilities are fundamental properties of hadrons which are especially interesting because their values reﬂect dynamical response of hadrons to the electromagnetic probe at different energies, and their measurement provide an important test point for ChPT, dispersion relation approaches, and lattice QCD calculations.

Canadian theorists are major contributors to lattice calculations and to predictions based on ChPT and are closely working with experimentalists on planning the measurements and providing theory input, such as radiative corrections. 
Experiments designed to make detailed comparisons with QCD predictions are high-priority endeavours of research at facilities across the USA, Europe and Japan, with goals of obtaining: a tomographic
view of the quarks and their motion within the nucleon; the elucidation of the
role of gluons and gluon self-interactions in nucleons and nuclei; and a
detailed understanding of how QCD governs the transitions of quarks and gluons
into pions, protons and neutrons.
Hadron spectroscopy has become even more exciting since evidence for new types of groupings in heavy-quark systems have been found: tetraquark and pentaquark, candidates in the charmonium sector, and hybrids in the light-quark sector probed in pion production.

Canadians have leadership roles in a number of experiments at offshore
facilities, including detailed measurements of proton, kaon and pion structure, and
investigations of the spectrum of hybrid mesons containing explicit gluonic
degrees of freedom.  For example, a Canadian group led experimental efforts at JLab  to extract the photon-beam asymmetry for $\eta$ and $\eta^{\prime}$, resulting in a Ph.D. thesis 
%(T. Beattie, 2019, UofR President's Distinguished Medal) 
and a publication [The GlueX Collaboration, S.~Adhikari et al., Phys.\ Rev.\ C \textbf{100}, 052201(R) (2019)]. The extracted beam asymmetry afforded comparisons to theoretical models, %(see Figure~\ref{fig:physicsresults}a), 
and indicated the dominance of natural parity exchange in the reaction mechanism.  

\subsection{What are the phases of strongly interacting matter, and what roles do they play in the cosmos?}

% figure is duplicated in theory section 3.6
%\begin{figure}[!htb]
%\begin{center}
%\vskip 0.5cm
%\includegraphics[angle=0,width=0.5\textwidth]{Figs/rhic_graphics-fig1-hr.pdf}
%\caption{\footnotesize  The phase diagram of QCD, with the zones explored by %heavy-ion collisions performed at RHIC (in blue), and at the LHC  (in green). %Figure from Brookhaven National Laboratory.}
%\label{PhaseDiag}
%\end{center}
%\end{figure}

%GH: the text below is taken verbatim from the 2015 Brief.  The figure will need to be updated.

Nuclei make up 99.9\% of the visible matter in the universe. At the highest
densities, yet at still rather low temperatures, the quarks making up the
nucleons of nuclear matter may form a new state of matter, which is 
color-superconducting. Exotic nuclear matter can also be created by
colliding nuclei at 
relativistic energies. In this case, `nuclear temperatures' can reach values
that represent a state of matter (the quark-gluon plasma) as it existed
during the first moments after the Big Bang. This is an active field of study
at international facilities such as RHIC in the USA, and the LHC at
CERN.  There are a number of very active Canadian theorists who are
making significant contributions to our understanding of the phase diagram of
nuclear matter. Their work has significant bearing on the quest to characterize
the properties of the quark-gluon plasma, and for our understanding of
astrophysical phenomena
such as neutron star structure and the evolution of the early universe.

In recent years, the nuclear theory group at McGill University has developed and put forward a 3D, relativistic, viscous fluid-dynamical approach that has been successfully used to model the time-evolution of the high-energy nuclear collisions. 
A fundamental question in all studies of the many-body effects observed in high energy heavy ion collisions is whether the observed correlations develop dynamically or are already present in the initial states. A new approach has recently been constructed [Scott McDonald, Sangyong Jeon, Charles Gale.
Nucl. Phys. A982 (2019) 239]  which properly takes into account the
the evolution in full 3-dimensional space. Importantly, the physics of the IP-Glasma relies on that of {\em saturation}: the scale at which the non-linearities of the gluon field manifest themselves. The exploration of the {\em  saturation regime} is a central theme of the Electron-Ion Collider wich will push our study of QCD to new frontiers. 

\subsection{How does the structure of nuclei emerge from nuclear forces?
\label{sec:intro_ns}
}

%We know that there are four fundamental forces of nature; gravity, electromagnetic, the weak nuclear force and the strong nuclear force. These forces govern everything that happens in the universe and how we interact with the world around us. Gravity and the electromagnetic force are very well characterized. For example, accurate calculations of the gravitational force allowed the European Space Agency's Rosetta mission [https://sci.esa.int/web/rosetta] to land a capsule on the surface of a comet after a journey that took over ten years from earth. There are details of the weak and strong forces that are still not fully understood. 
Understanding the strong nuclear force binding the protons and neutrons to form the wide variety of complex nuclei in the universe has been a century-long challenge. Enormous progress has been made to this point, but it is a central pursuit of current nuclear structure research to reveal the fine details of the strong nuclear force that is responsible for the properties of nuclei and nuclear matter.

The rare isotopes are breaking the boundaries of our conventional knowledge and reforming our views on how nature organizes the building blocks, protons and neutrons, into a wide variety of complex atomic nuclei. Nuclear shells form the fundamental pillars that guide the characteristics of the atomic nucleus, influence the processes of element synthesis and are imprinted in the abundance of elements in nature. 

Over the past decades it has been determined that the well-established nuclear shells that were identified in stable isotopes in the 1950's, disappear in rare isotopes while new ones appear. This brings challenges to our understanding of nature's strong force that is at the heart of existence of all visible matter. Associated with the changes in nuclear shells the rare isotopes manifest unexpected forms with the creation of a thick surface, called a neutron skin, in isotopes with large neutron-to-proton ratio. This predominant neutron surface provides us with laboratory access to understand neutron-dominated objects in the universe and find new features, such as new excitation modes, that arise from it. While a few discoveries with light nuclei have opened a new paradigm, much of the rare isotopes remain unexplored. Canadians have a unique opportunity to make substantive advances in this field through utilizing the world-leading capabilities of the TRIUMF-ISAC and ARIEL facilities for rare-isotope production as well as leveraging complementary opportunities at rare-isotope laboratories world-wide.

\begin{figure}[ht!]
\centering
\includegraphics[width=0.6\columnwidth]{Figs/IRIS-10C-Highlight.pdf}
\caption{\label{fig:IRIS-10C-Highlight}
%
The differential cross section for $^{10}$C($p,p$) at $E_{cm}$ = 4.15\,MeV, adapted from [Phys. Rev. Lett. 118 262502 (2017)]. The symbols represent the experimental data. The curves represent no-core shell model with continuum calculation results with different chiral forces.}
\end{figure}

A recent highlight of the impact Canadian scientists are making is in revealing the imprints of the nuclear force in the elastic scattering of protons on $^{10}$C. The chiral effective field theory enables a link for its description connected with the theory of quantum chromodynamics but requires certain parameters that are not uniquely defined. From a measurement of proton elastic scattering on $^{10}$C using the IRIS facility at the TRIUMF-ISAC laboratory, and {\em ab-initio} nuclear reaction calculations performed by a collaboration led by the TRIUMF theory group, it was found that the shape and magnitude of the measured differential cross section is strongly sensitive to the nuclear force prescription (Fig. \ref{fig:IRIS-10C-Highlight}) \footnote{\href{https://link.aps.org/doi/10.1103/PhysRevLett.118.262502}{A. Kumar {\it et al.}, Phys. Rev. Lett. 118, 262502 (2017); Editors Suggestions and Viewpoint in Physics.}}.

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\subsection{What is the role of radioactive nuclei in shaping the visible matter in the universe?} \label{sec:intro:NA}

\textit{Per aspera ad astra} ("Through hardships to the stars") - Lucius Annaeus Seneca, (4~BC--65~AD)

Nuclear astrophysics -- connecting nuclear physics properties and the creation of nuclides in astrophysical events -- has grown to a mature research field that connects many aspects of experimental and theoretical nuclear physics. The experimental tools are multifaceted, ranging from small accelerators installed at universities or even in underground laboratories to larger setups installed at radioactive beam facilities. With this variety of tools measurements of astrophysically relevant parameters are carried out.

The by far most important event for the nuclear astrophysics community in the last 5 years was the detection of the gravitational wave signal GW170817 from a binary neutron star merger, followed about 1.7~s later by a short $\gamma$-ray burst (GRB170817A). This triggered a world-wide unique effort that coined the term "multi-messenger astronomy", and about 11~h later the astronomical transient (AT2017gfo), a "kilonova", was under close observation by dozens of telescopes in various wavelengths from radio to X-rays~\footnote{\href{https://doi.org/10.3847/2041-8213/aa91c9}{B.P. Abbott et al., Astrophys. J. Letters 848, L12 (2017)}} (see Fig.~\ref{Fig:NA-LIGO}). Not surprisingly, the Nobel Prize in Physics 2017 went to the three key players of the LIGO/Virgo collaboration, Rainer Weiss, Barry C. Barish, and Kip S. Thorne \textit{"for decisive contributions to the LIGO detector and the observation of gravitational waves"}.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{figure}[htb!]
\centering
\includegraphics[width=0.7\columnwidth]{Figs/NA_LIGO_GW170817.png}
\caption{\label{Fig:NA-LIGO} (Top) Binary neutron star merger event on August 17, 2017: Localization of the gravitational-wave, gamma-ray, and optical signals. The top left panel shows a projection of the 90\% credible regions from LIGO (light green), LIGO-Virgo (dark green), triangulation from the time delay between Fermi and INTEGRAL (light blue), and Fermi GBM (dark blue). The inset shows the location of the apparent host galaxy NGC 4993 in the Swope optical discovery image at 10.9 hours after the merger and the DLT40 pre-discovery image from 20.5 days prior to merger. (Bottom) The timeline for the discovery of GW170817, GRB170817A, AT2017gfo and the follow-up observations. Source: \href{https://www.ligo.org/science/Publication-GW170817MMA/}{LIGO Webpage}.
%
}
\end{figure}
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The event that triggered the gravitational wave emission on August 17th 2017 was identified as a merger of two neutron stars, a scenario that has never been observed before and is connected to the creation of about half of the elements heavier than iron in the so-called "rapid neutron capture ($r$) process". This astrophysical process is deeply connected to the nuclear physics of short-lived neutron-rich nuclei and the Equation of State (EoS) of neutron star matter, and was the prime motivation for the construction of the new generation of radioactive beam facilities worldwide, including the ARIEL facility at TRIUMF in Canada (see Sec.~\ref{sec:CanFac:ariel}), as well as FRIB in the USA and FAIR in Germany  \footnote{\href{https://iopscience.iop.org/article/10.1088/1361-6471/ab0849}{C.J. Horowitz et al., J. Phys. G: Nucl. Part. Physics 46, 083001 (2019)}} (see Sec.~\ref{sec:IntFac:others}).

A very successful "multi-messenger nuclear (astro)physics" program is already being carried out at many facilities and universities, with the aim to better understand various aspects of the creation of nuclei in stellar events. The detection of GW170817 and all follow-up observations have now laid a firm foundation for a new highway to the interior of stars on which the Canadian Nuclear Astrophysics community is already moving since many years.
The main focus of the experimental program will be the science enabled by the phased commissioning of the ARIEL facility, complemented by offshore experiments at radioactive beam facilities. 

The experimental tools for the measurement of important nuclear properties of the nuclei, like reaction cross sections, decay half-lives, nuclear masses, and particle emission probabilities, have been developed and are waiting to be leveraged at this new generation of facilities. With ARIEL, the increase of available radioactive ion beamtime 
%by a factor of 2 until 2023 and by a factor of 3 by 2026 
will not only allow the development of more beam species and more intense radioactive beams but also enable longer beamtimes to access more exotic nuclei and/or smaller cross sections which will greatly benefit the nuclear astrophysics program.

%%%%% Maybe shorten the following parts
Reaction cross section measurements have been limited so far to nuclei close to stability.
%, mainly due to the unavailability of intense beams that allow also measurements of non-resonant cross sections. 
This limitation will be overcome in the next decade and open a new avenue for nuclear astrophysics. Specifically the indirect determination of neutron-induced cross sections of radioactive nuclei via surrogate methods like $(d,p)$ reactions is an important extension to the ongoing efforts to measure proton- and $\alpha$-induced cross sections at the DRAGON and EMMA recoil separator facilities, as well as with the smaller TUDA particle detector at TRIUMF. 

The direct measurement of neutron-induced cross sections of short-lived nuclei might be still a bit down the road but first considerations for the world-wide unique coupling of a storage ring facility including an internal neutron generator target with radioactive beams from the ISAC facility are underway, and TRIUMF and the Canadian Nuclear Astrophysics community will play a pivotal role in this program in the next decade(s) (see Sec.~\ref{sec:isacupgrade}).

This experimental progress goes hand in hand with the tremendous progress on the nuclear theory side (see Sec.~\ref{sec:nucstruc:theory}). \emph{Ab-initio}, no-core shell model (NCSM), and valence space in-medium similarity renormalization group (VS-IMSRG) models for heavier nuclei have made a major step forward in the past years. Especially the VS-IMSRG model is a very promising tool since it extended the reach of \emph{ab-initio} nuclear structure calculations to medium-mass nuclei, and in future will be able to go to heavier masses up to even the superheavy nuclei.

This decade will be the start of a new era of radioactive beam facilities that will help us push our understanding of the creation of the lightest up to the heaviest elements further than anytime before.

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% I put my part of this section into its own TeX file, to make updates easier without having to copy and paste into a joint file.
\input{intro_1_funsym.tex}


~~~~\\[3ex]
In the sections following the Executive Summary, we will discuss the
contributions of Canadians to these questions in greater detail, placing this
work in the broader context of the field as a whole, and giving an indication
of where this work is expected to lead in the next 5 years and beyond.

%\section{How can the knowledge and technological progress provided by nuclear physics best be used to benefit society?}
\section{Benefits of nuclear physics research to society}

%GH: text below taken directly from 2015 brief.  Will need updating.
%also need to add EDI and outreach aspects, so this will be longer than last time

Canadian nuclear physicists are at the forefront of their fields - studies by the Council of Canadian Academies and Science Metrix both found that Canada is a ``world-leader'' in subatomic physics and astrophysics \footnote{\href{http://www.scienceadvice.ca/en/assessments/completed/science-tech.aspx}{``The State of Science and Technology in Canada, 2012.'' The Expert Panel on the State of Science and Technology in Canada, Council of Canadian Academies}}. Nuclear technology has an impact in a variety of scientific fields; only computers, microelectronics, and possibly laser technology surpass nuclear physics techniques as the most widely used set of tools in science.  For example, nuclear techniques are the gold standard for measuring the age of ancient objects, from the archaeological to the cosmic time scales. Nuclear-tracer techniques are used to unravel bio-chemical pathways, to determine the efficiency of chemical reaction vessels, and to measure the flow of ground water. Another example is that of the GlueX experiment, at JLab. That collaboration's requirement for stringent silicon photomultiplier (SiPM) specifications has produced tile arrays that are now used in medical imaging.  Most, if not all, of these technologies were not developed for the specific application for which they are used, but came about as a result of the pursuit of fundamental nuclear physics research.
   
An example of the importance of having a large base of knowledgeable and skilled researchers is evident in the recent success of an interdisciplinary team of researchers including Canadian nuclear physicists.  This team set out five years ago to develop a reliable, alternative means of producing a key medical isotope, $^{99}$Tc$^m$. The project resulted in over a dozen scientific publications, several provisional patents, and a training opportunity for more than 175 individuals. The team demonstrated true collaboration in solving this worldwide healthcare challenge and received the NSERC Brockhouse Canada Prize for Interdisciplinary Research in Science and Engineering for this groundbreaking, and life-saving, technique. \footnote{\href{http://www.triumf.ca/sites/default/files/NR-Feb-17-2015-Brockhouse-vF.pdf}{NSERC Brockhouse Canada Prize for Interdisciplinary Research in Science and Engineering 2015}} This achievement is a direct result of investment provided by the Government of Canada through programs at the Natural Sciences and Engineering Research Council, the Canadian Institutes of Health Research, and Natural Resources Canada.


%https://gereports.ca/canada-joins-the-big-leagues-in-superconducting/
%TRIUMF 2015-20 Five Year Plan; www.triumf.ca/five-year-plan-2015–2020