\subsection{Exotic hadrons}

Improving our understanding of quark confinement
in hadrons requires deepening our insight into the 
gluonic fields within those particles.  
Exploration of the spectrum and properties of exotic hybrid mesons 
has emerged as a key path forward for exploring quark
confinement. Indeed, that topic 
was a major motivation for the upgrade of JLab,
and is specifically mentioned multiple times 
in the 2015 NSAC Long-Range Plan \cite{Geesaman:2015fha}.

To deepen understanding of how confinement arises, ASU is working 
with the GlueX Collaboration in Hall D to search for 
exotic hybrids  with masses up to $2.5$~GeV.
Exotic hadrons (``exotics") are classified as one of three types: glueballs,
 tetraquarks, and hybrids. 
Glueballs are states that completely lack valence
quarks. The lightest glueballs should have $J^{PC} = 0^{++}$ or $2^{++}$,
and are expected to mix with constituent quark states with the same quantum
numbers. 
Tetraquarks are comprised of two $q \overline{q}$ states, 
and are the product of two $SU$(3) octets that 
can be represented as
\begin{equation*}
\bf{8} \otimes \bf{8} = \bf{27} \oplus \bf{10} \oplus \bf{\overline{10}} \oplus \bf{8_1} \oplus \bf{8_2} \oplus \bf{1}.
\end{equation*}
Hybrid states in a constituent $q \overline{q} g$ model 
are built from the usual $q \overline{q}$ constituent-quark representation 
with the promotion of one or more gluons from virtual to real.
For a hybrid state, a  $q \overline{q}$ colored state interacts with real
constituent colored gluon(s) to form an overall colorless state. 
In contrast to glueballs and tetraquarks,  
hybrids are only expected to exist in nonets similar to those
for the constituent quark model. 
Exotics, however, can have quantum numbers $J^{PC}$ that differ from those of hadrons 
constructed with the simple constituent quark model. 
Finding particles with exotic quantum numbers $J^{PC}$ 
serves as the ``smoking gun'' for determining if a state is exotic. 

Some guidance in the search for hybrid states has come from lattice QCD predictions. For example,
a lattice calculation performed by the Hadron Spectrum Collaboration (HSC) in 2013~\cite{Dudek:2013yja} 
found hybrid states  in supermultiplets with  
$J^{PC}$ = $0^{+-}$, $1^{-+}$, $2^{+-}$, and $1^{--}$. 
This HSC model description suggests that
those hybrids have the structure 
($\bf{[q\overline{q}]_{8_c} \otimes G_{8_c}}$).  
States of particular importance for the search of hybrids have the exotic quantum 
numbers of
$J^{PC} = 0^{+-}, 1^{-+}, 2^{+-}$.

Table ~\ref{tbl:exotic} shows several possible decays of exotic mesons of interest for the GlueX experiment.
Currently, there are two exotic $J^{PC} = 1^{-+}$ states ($\pi_1(1400)$ and $\pi_1(1600)$) 
listed by the Particle Data Group \cite{Olive:2016xmw}. 
For the $\pi_1(1400)$, the decay modes that have been seen
are $\eta \pi^0$ and $\eta \pi^-$, while for $\pi_1(1600)$ the decays seen
are $b_1 \pi$, $\eta^{\prime} \pi^-$ and $f_1 \pi$.
The interpretation of the $\pi_1(1400)$ as a hybrid is problematic \cite{Chung:2002fz,Iddir:2000yb,Close:1987aw}. 
A hybrid meson ($q \overline{q} g$) state
can either have an angular excitation between the quark+anti-quark system and the gluon 
(said to be ``gluon excited'') or
the excitation can be exclusively between the quark and anti-quark (``quark excited''). 
For the gluonic angular-excitation, within
the $q \overline{q} g$ constituent model,
a decay of a hybrid into two fundamental mesons is suppressed. 
This would make the 
$\pi(1400) \rightarrow \eta \pi$ decay unlikely in the gluon-excited case. 
However, a quark excitation can proceed into two fundamental mesons, 
though the branch into the $\eta$-singlet
is favored over the $\eta$-octet. 
Since no search for $\pi(1400) \rightarrow \eta^{\prime} \pi$ 
has been successful, there is some doubt that the $\pi_1(1400)$ is a hybrid. 
Instead, some have
suggested that the $\pi_1(1400)$ is a tetraquark state \cite{Chung:2002fz,Iddir:2000yb,Close:1987aw}.


\begin{table}[h]
\vspace{5mm}
\caption{
Possible decay modes of exotic hybrids that will be explored by GlueX.
} \label{tbl:exotic}
\centering
\begin{tabular}{|c|c|c|c|c|}
\hline
Hybrid &                 & Reachable with           & Reachable with  		& Challenging even  \\
 meson         & $J^{PC}$  & limited running &  longer running            & with longer running  \\
\hline
$b_0$ & $0^{+-}$ &   --- & ---   & $\pi(1300) \pi$, $h_1 \pi$,  \\
          &                &  &  &  $f_1 \pi$, $b_1 \pi$ \\
$h_0$ & $0^{+-}$  & --- & $b_1 \pi$ & $h_1 \eta$ \\
$h^{\prime}_0$ & $0^{+-}$  & ---  & $K_1(1270) K$  & $K(1460) K$, $h_1 \eta$ \\
\hline
$\pi_1$ & $1^{-+}$  & $\pi \rho$ & $\pi b_1$, $\pi f_1$, $\pi \eta^{\prime}$ & $\eta a_1$ \\
$\eta_1$ & $1^{-+}$  & $\eta f_2$, $a_2 \pi$ & $\eta f_1$, $\eta \eta^{\prime}$ & $\pi(1300) \pi$, $a_1 \pi$  \\
$\eta^{\prime}_1$ & $1^{-+}$  & $K^{*}K$ & $K_1(1270) K$, $K_1(1410) K$, & --- \\
                            &                 &                   & $K^{*}_2 K$, $\eta \eta^{\prime}$ &  \\
\hline
$b_2$ & $2^{+-}$  & $\omega \pi$, $a_2 \pi$, $\rho \eta$ & ---  & $f_1 \rho$, $a_1 \pi$, $h_1 \pi$, $b_1 \eta$ \\
$h_2$ & $2^{+-}$  & $\rho \pi$ & $b_1 \pi$, $\omega \eta$ & $f_1 \omega$ \\
$h^{\prime}_2$ & $2^{+-}$  & --- & $K_1(1270) K$, $K_1(1410) K$  & $f_1 \phi$ \\
                         &                  &     &  $K^{*}_2 K$, $\phi \eta$         &                  \\
\hline
\end{tabular}
\vspace{-5mm}
\end{table}


In addition to the $\pi_1$ exotic states, there should be corresponding isospin $I$ = 0
nonet members if the $\pi_1$ is a hybrid. 
Establishing these isoscalar members for the nonets is important 
in order to verify these exotics as being hybrids. 
The tetraquark state with $J^{PC} = 1^{-+}$ excludes the presence of an isosinglet, and is
fully contained within the $\bf{20}$-plet ($\bf{10} \oplus \bf{\overline{10}}$).
Thus, identifying an isosinglet state with $J^{PC} = 1^{-+}$ 
excludes that state as being a tetraquark. 

%In contrast to the approach taken by the CLAS Collaboration, 
%the GlueX Collaboration does not consider any particular physics reaction or observable 
%as being reserved for analysis by any particular university group or subset of collaboration members.
%Instead, the GlueX Collaboration considers every physics reaction available for
%analysis by anyone in the collaboration. 
%Thus, in Hall D, the ASU group will continue to contribute to the
%analyses of GlueX data in a way that maximizes our utility to the collaboration while
%paying special attention to the reactions of primary interest for the group.
%In addition to our keen interest in the search for exotics, 
%we also have the parallel interest in ``very strange'' particles as noted in the previous section.
%Final states detected in GlueX containing multiple final-state
%kaons thus are of particular interest to ASU, since 
%analyses of those states provide a possible path to extend our
%CLAS12 $\Omega$ and cascade studies to the dataset obtained in Hall D.
%Thus, by concentrating our Hall D analysis efforts on multi-kaon final states,
%we not only participate in the effort to harvest important physics on exotics there,  
%but also maintain the possibility of a synergy
%between our Hall B and Hall D efforts looking for very strange hadrons.

%%%%%%%%%%%%%%%%%%%%%%%


%%%%%%%%%%%%%%%%%%%%%%%
As detailed further below, we participated in GlueX analyses of single meson final states
with the initial data obtained in the Spring 2016 running. 
The first GlueX physics paper has been published
for the photon beam asymmetry $\Sigma$ 
from $\gamma p \rightarrow p \pi^0$ and $\gamma p \rightarrow p \eta$ reactions \cite{AlGhoul:2017nbp}.
This paper was made possible by the photon beam polarization measurements 
determined by the ASU group using
TPOL, described further below.
No other measurement of the photon beam polarization was available
since the coherent bremsstrahlung spectral analysis (CBSA) approach
originally envisioned proved unreliable. 
PI Dugger, who spearheaded the triplet polarization analysis, 
served on the three-member GlueX analysis/paper review committee. 

The collection of physics results we intend to focus on in Hall D
and those from other analyses by our collaborators in the GlueX Collaboration,
will clarify the existence of exotic states,
determine the properties of any such particles found,
and, in turn, lead to progress in understanding how quarks and gluons assemble into hadrons. 

\subsubsection{The GlueX detector in Hall D}

The Hall D GlueX detector is being used to search for
gluonic excitations within mesons
with masses up to $2.5$~GeV \cite{gluexPac30,gx2year}.
As noted above, distinguishing states with gluonic excitations from other
meson states will require the determination of exotic
quantum numbers, $J^{PC}$ that are unlike the assignments possible with traditional constituent
quark models of meson states.
A full partial wave analysis will be
needed to obtain $J^{PC}$ for the observed states.
To minimize possible
ambiguities in obtaining the required  $J^{PC}$ information
from the partial wave analysis, all decay products from the candidate
hybrid meson must be identified with good resolution. The identification
of all decay products is greatly enhanced when the detector has
a full acceptance in decay angles. For these reasons the GlueX detector
(Fig.~\ref{fig:hd})
was designed to have $4\pi$ solid angle coverage, with nearly
full hermeticity, and good momentum and energy resolution
of photons and charged particles.

\begin{wrapfigure}[]{r}{0.6\textwidth}
\vspace{-10pt}
\begin{center}
\includegraphics[width=\linewidth]{gx_detect.eps}
\end{center}
\vspace{-13pt}
\caption{Diagram of Hall D
and the GlueX detector. 
\newline
\vspace{-5pt}
}
{\label{fig:hd}}
\end{wrapfigure}
Electrons with energies of 12~GeV
incident on a thin (50 or 20~$\mu$m) diamond radiator produce
a beam of linearly polarized photons
($\approx$40\% polarization) with $E_\gamma=$ 8.5-9~GeV.
The tagging of the post-bremsstrahlung electron
determine the photon energy to within 0.1\%.
The photons strike a $30$-cm-long liquid hydrogen target
surrounded by a start counter.
Past the start counter, the next detector elements are
cylindrical drift-chambers, which 
%in turn 
are surrounded by a
cylindrical electromagnetic calorimeter made of lead and scintillating fibers.
The target, start counter, drift chambers, and electromagnetic calorimeters
are all contained within a refurbished 2.25~T superconducting solenoid that
provides the momentum-analyzing magnetic field. On the downstream side of the solenoid
is a time-of-flight system and a
3000-element lead-glass electromagnetic calorimeter.
For 9 GeV incident photons, the GlueX detector has $4 \pi$ acceptance
for photons and charged particles to facilitate the
partial-wave analyses of many-particle final states.
The acceptance of final-state particles is typically better
than 95\%, and is very uniform over the detector.
Moreover, the near hermetic design of the detector is well suited
for determining the masses and $J^{PC}$ for mesons with
masses from $1.5$ to $2.5$ GeV/c$^{2}$.

For increased precision in kaon particle identification, the newly acquired DIRC
(Detection of Internally Reflected Cherenkov)  
is placed just in front of the FCAL. The DIRC has as the main component four
boxes filled with fused silica bars from the BaBar experiment.
When installed in the BaBar experiment, these boxes provided good kaon separation 
up to 4 GeV~\cite{Adam:2004fq}.
Similar performance is expected when utilized for the GlueX experiment,
greatly improving kaon identification, which
will facilitate many analyses including those ASU is particularly targeting.


\subsubsection{TPOL: the ASU triplet polarimeter}

The partial-wave analyses to be undertaken
in the GlueX search for hybrid mesons require an 
absolute uncertainty of $\pm$0.04 in the photon beam
polarization. When ASU joined the GlueX Collaboration,
we proposed a direct 
measurement of the photon polarization through pair production from an
atomic electron (``triplet photoproduction'')
as a method to determine this critical parameter. 
In triplet production, the photon interacts
with the field of an atomic electron to produce an electron-positron pair; 
the recoil atomic electron has a relatively low momentum, 
and can attain large polar angles
that correlates with the
production plane. 
The process is a straightforward QED process for which the 
primary contributing diagrams are given in Fig.~\ref{fig:fdia}.
The large polar angle for the recoil atomic electron  
simplifies its detection.

\begin{wrapfigure}[]{r}{0.5\textwidth}
\vspace{-15pt}
\begin{center}
\includegraphics[width=\linewidth]{fdia.eps}
\end{center}
\vspace{-18pt}
\caption{
Half the Feynman QED diagrams for the triplet photoproduction process. 
Remaining diagrams
are obtained by swapping electron 2 with electron 4. 
\newline
\vspace{-5pt}
}
{\label{fig:fdia}}
\end{wrapfigure}
The azimuthal angular distribution of the recoil electron yields information 
on the beam polarization.
For polarized photons, the cross section for the triplet photoproduction process can be written as
$\sigma=\sigma_0\left[1+P\Sigma\cos(2 \phi)\right]$,
where $\sigma_0$ is the unpolarized cross section, $P$ is the photon beam polarization,
$\Sigma$ is the photon beam asymmetry, and $\phi$ is the recoil electron azimuthal angle. 
For GlueX, the photoproduced pair is detected in the existing
pair spectrometer located downstream of the triplet polarimeter. 
A QED calculation of the beam asymmetry arising from scattering from
an orbital electron yields the analyzing power $\Sigma$ for the reaction,
which with the Hall D beamline and geometry is approximately 0.2. 
To determine the photon beam polarization, the azimuthal distribution
of the recoil electrons is fit to the function
$A\left[1 + B \cos(2 \phi)\right]$, where the variables $A$ and $B$ are
fit parameters. Once $B$ has been extracted from the data, the polarization is $P = B/\Sigma$.

Our group designed and constructed
a detector system (called TPOL) that measures photon beam asymmetries with this triplet production process,
and installed the system in the Hall D beamline prior to the Spring 2015 test run.
TPOL uses an S3, 
double-sided silicon strip detector (SSD) manufactured by Micron Semiconductor
to measure timing, energy deposition, 
and the azimuthal distribution of recoil electrons coming
from the triplet production process. 
Fig.~\ref{fig:Mpol} shows the interior of the TPOL vacuum chamber;
the beam transits that figure from right to left. 
The SSD has 32 azimuthal sectors on the ohmic side and 
24 concentric rings on the junction side, with an outer active diameter of 
70 mm and an inner active diameter of 22 mm. 

The TPOL installation in the Hall D beamline
is shown in Fig.~\ref{fig:tripBGR2}, seen from beam right. 
The housings consist of three boxes attached to one another.
The largest rearmost box in the figure is the vacuum chamber, the middle box is
the preamp enclosure, and the smallest box is the distribution box that
routes voltages and
signals to and from the preamps. 
Cables attached to the
distribution box include the signal lines (white), high voltage bias (red) and
low voltage (gray) lines. 

\begin{figure}
\begin{minipage}[t][2.85in][t]{.5\linewidth}
 { \centering
  \captionsetup{margin=0.5cm}
  \includegraphics[width=0.8\linewidth]{polInsideKei.eps}
  \caption{Interior of TPOL vacuum chamber with SSD
  and production target ladder assembly (from beam left).
  %\vspace{-5pt}
  }
  \label{fig:Mpol}}
\end{minipage}%
\begin{minipage}[t][2.85in][t]{.5\linewidth}
 { \centering
  \captionsetup{margin=0.5cm}
  \includegraphics[width=0.95\linewidth]{tripBGR2.eps}
  \caption{TPOL vacuum chamber and electronics housings installed in Hall D beamline
  (from beam right).
  %\vspace{-5pt}
  }
  \label{fig:tripBGR2}}
\end{minipage}
\end{figure}


To determine the degree of photon beam polarization $P$ in practice,
the value of $B$ obtained from fitting the azimuthal distribution to
$A\left[1 + B \cos(2 \phi)\right]$ is divided by the analyzing power $\Sigma$. 
A plot showing the polarization for PARA and PERP orientations 
(i.e., photon beam polarization parallel and perpendicular to the floor of the experimental area)
as a function of incident
photon energy
is given  in Fig.~\ref{fig:polPlots}, where the red points are for PARA and the blue points
are for the PERP orientation.
For the Spring 2016 runs, the degree of polarization $P$,
in the energy range used for the published $\pi^0$ and $\eta$ beam asymmetries \cite{AlGhoul:2017nbp}
($8.4 > E_{\gamma} < 9.0$ GeV) in the next subsection was found to be
$P^{\rm{PARA}}_{\gamma} = 0.440 \pm 0.009(\rm{stat.}) \pm 0.006(\rm{syst.})$ and
$P^{\rm{PERP}}_{\gamma} = 0.382 \pm 0.008(\rm{stat.}) \pm 0.006(\rm{syst.})$.
Additional information on the TPOL detector can be found in our recently published paper \cite{Dugger:2017zoq}.

\begin{wrapfigure}[]{r}{0.6\textwidth}
\vspace{-15pt}
\begin{center}
\includegraphics[width=\linewidth,trim={0 0.0cm 0 1.0cm},clip]{polPPv2.eps}
\end{center}
\vspace{-20pt}
\caption{
Polarization versus $E_{\gamma}$. Red squares represent
results from the PARA setting, while blue circles represents PERP results.
\newline
\vspace{-5pt}
}
{\label{fig:polPlots}}
\end{wrapfigure}
Thus far, we have only instrumented the 32 sectors on the ohmic side of the SSD. 
During the proposed award period, we
plan on upgrading the TPOL device to include readout for the ring side of the SSD. 
The readout of the ring side will give us access to additional polar angle information
for the recoil electron that will allow us
to further understand and reduce systematic uncertainties.   
The preamplifiers required for the upgrade
have already been obtained, and JLab will produce the ASU-designed filter electronics.
In order to house the additional electronics, the distribution box (smallest box shown in Fig.~\ref{fig:tripBGR2})
has to be expanded, and part of the preamp enclosure (middle box in Fig.~\ref{fig:tripBGR2}) has to
be modified. JLab will fabricate the new distribution box, while the ASU group will 
be responsible for the modifications to the preamp enclosure.

To our knowledge, no other photon beam polarimeter operates at such energies.
Motivated by our success with TPOL, the ASU group has begun conversations
with the University of Glasgow on the possibility of designing a polarimeter utilizing pixel detectors
to measure photon polarization through
asymmetries in pair production. The advantage of using the pair production process, as
opposed to the triplet process, is that the statistical precision and analyzing power
for a pair production asymmetry measurement could be much better than that of a TPOL 
type device owing to the larger production cross section. 
The difficulty, however, in using the pair process, is that the opening
angle between the photoproduced leptons is very small and requires high precision in 
the spatial resolution of the pair-detector elements.
 
The ASU group has provided the Glasgow group with
our event generator and GEANT simulation software, and they have modified 
that package for the purpose
of exploring a pair production polarimeter (PPOL). 
From their studies, the Glasgow group
found that the PPOL concept should work and  
has obtained funding through a grant from
the Science and Technology Facilities Council 
(United Kingdom). The 
Glasgow development of a PPOL will begin in earnest starting this year.  
In the meantime, the ASU group has performed additional feasibility studies
of the PPOL concept using GlueX data and simulations. The results of the ASU analysis look
promising, and Ken Livingston of the Glasgow group is working with us on the 
possibility of installing several pixel detectors in the GlueX pair
spectrometer as a way to have a hybrid TPOL/PPOL polarization measurement.
 
ASU is thus positioned to continue providing the important polarization information for
all Hall D experiments with TPOL during the proposed award period, 
as well as primed to further exploit new approaches in photon beam polarimetry
that might be extended to other photon beams around the world. 

\subsubsection{The first physics publication from GlueX}

\begin{wrapfigure}[]{r}{0.59\textwidth}
\vspace{-10pt}
\begin{center}
\includegraphics[width=\linewidth,trim={0 0.2cm 0 0.2cm},clip]{gluexresult1.png}
\end{center}
\vspace{-13pt}
\caption{
Beam asymmetry $\Sigma$ for $\pi^0$ photoproduction from the proton
with 8.4 to 9 GeV linearly-polarized photons
from the first GlueX physics publication~\cite{AlGhoul:2017nbp}.
\newline
\vspace{-5pt}
}
{\label{fig:gluexresult}}
\end{wrapfigure}
As an example of the performance of the GlueX detector, TPOL, and an example of
ASU's participation in the collaboration, we briefly
discuss here the first physics publication arising from GlueX analyses. 

The Spring 2016 running period in Hall D coincided with the 
JLab accelerator providing its first 12 GeV electron beam.
A 50-micron-thick diamond was placed in the beamline using a
goniometer, and polarized photons with a coherent edge energy of 9 GeV were produced.
Most of the running period was consumed
by commissioning activities, but 45 data runs with polarized photons had conditions sufficient for physics production.
From those 45 polarized photon-beam runs, 21 were taken with the polarization
parallel to the lab floor and 24 with the polarization perpendicular.


Using this small initial sample of production data,
the GlueX collaboration was able to obtain photon beam asymmetries $\Sigma$
for $\pi^0$ and $\eta$ photoproduction from the proton 
($\pi^0$ results are shown in Fig.~\ref{fig:gluexresult}),
which included the first $\Sigma$ measurements for $\eta$ photoproduction at 9 GeV.
The $\pi^0$ results possessed much higher precision than any previous measurement.
These $\Sigma$ measurements  became the first physics publication for the GlueX Collaboration \cite{AlGhoul:2017nbp}.
The results would not have been possible without the TPOL work of the ASU group
that provided the polarization measurements illustrated above in Fig.~\ref{fig:polPlots}
since no other approach to determine the beam polarization 
attempted by the GlueX Collaboration was successful.


The dip in the SLAC $\pi^0$ results shown in Fig.~\ref{fig:gluexresult}
occurs where large corrections were required due to Compton backgrounds.
In the SLAC article~\cite{Anderson:1971xh}, the authors stated
``The extracted $\pi^0$ cross section in the dip depends very critically on this
correction. For example, at 15 GeV and t = -0.5 (GeV/c), two-thirds of the
observed yield is due to Compton scattering.'' In terms of physics, 
a large positive value of the beam asymmetry corresponds to the expected case, where
the $\pi^0$ is created through a $t$-channel process with a vector meson acting as
a messenger particle. This particular type of exchange results in  a natural parity
exchange. The dip shown in Fig.~\ref{fig:gluexresult}, however, would correspond to
a large contribution from a pseudovector meson exchange, like a $b$ or $h$ meson. 
Such a pseudovector exchange would lead to an unnatural parity exchange. 
This sensitivity to the reaction mechanism is illustrative
of how polarization information can be useful in determining, in this case, the type
of meson being exchanged. Such information is critical for extracting 
the $J^{PC}$ information required for the classification of a $t$-channel produced 
meson as being exotic.

These first results illustrate that progress toward understanding the 
GlueX detector has been swift, that the triplet polarization approach to 
measuring the photon beam polarization is sound, 
and that the partial wave analyses needed in the search for exotics will
not be hampered by ambiguities in measurements of the photon beam
polarization. As additional data is obtained in future running periods during
the proposed award, the searches undertaken in Hall D should proceed
apace, with ASU both participating in those analyses and providing the beam
polarization information essential for achieving the physics goals for Hall D. 

\subsubsection{Example of ASU analysis of GlueX data}

To illustrate the types of analysis we can currently perform with GlueX data,
we discuss here portions of the analysis of multi-kaon states undertaken by
ASU graduate student Sebastian Cole on the existing GlueX dataset. 
In ASU's Hall B studies, the group extensively utilized the $\eta$ and
$\eta^{\prime}$ to filter out $I$ = 3/2 resonances from the nucleon
resonance spectrum. For Hall D, we will similarly use
the $\eta_1^{\prime}$ to filter out tetraquark states from the
spectrum of exotic meson states. The $K^{*} K$ branch is a prime
candidate for early detection of the $\eta_1^{\prime}$ hybrid.

%Since the $K^{*} \rightarrow K \pi$ 100\% of the time, the 
%detected particles necessary for the reconstruction of the 
%$\eta_1^{\prime} \rightarrow K^{*} K$ are $K K \pi$. The particular 
%charge states that Sebastian is pursuing in his search for the 
%$\eta_1^{\prime}$, is $K^+ K^- \pi^0$. To begin his search, Sebastian 
%has taken the strategy of making sure that he can construct the well 
%known $\phi \rightarrow K^+ K^-$ meson state reaction, and then 
%include an additional $\pi^0$. In Fig.~\ref{fig:seb2d}, Sebastian has 
%constructed a plot showing the invariant mass of $p K^-$ versus the 
%invariant mass of $K^+ K^-$. That plot shows strong enhancement bands in
%the vertical and horizontal directions. Along with the data shown on the 
%plot, there is a vertical white line at 1.4 GeV and a horizontal one at 
%2 GeV. These white lines represent analysis cuts performed on the data 
%in order to extract the $\phi$ and also, as a bonus, the $\Lambda(1520)$. 

\begin{wrapfigure}[]{r}{0.59\textwidth}
\vspace{-10pt}
\begin{center}
\includegraphics[width=\linewidth,trim={0.5cm 0.4cm 1.0cm 1.6cm},clip]{seb2dV3_1.png}
\end{center}
\vspace{-13pt}
\caption{
Invariant mass of $p K^+$ versus the invariant mass of $K^+ K^-$. The white
lines denote different cut selections (described in text).
\newline
\vspace{-5pt}
}
{\label{fig:seb2d}}
\end{wrapfigure}
Since the $K^{*} \rightarrow K \pi$ 100\% of the time, the
detected particles necessary for the reconstruction of the
$\eta_1^{\prime} \rightarrow K^{*} K$ are $K K \pi$. 
The particular
branches that Cole is isolating in his search for the
$\eta_1^{\prime}$ are $K^+ K^- \pi^0$ and $K^0_s K^{\pm} \pi^{\mp}$.
To begin his search, he has reconstructed
the well-known $\phi \rightarrow K^+ K^-$ meson decay, as well
as the $\Lambda(1520) \rightarrow p K^-$ baryon decay. This work
will assist in improving pre-DIRC kaon identification in order to search
for $\eta_1^{\prime}$. Fig.~\ref{fig:seb2d} 
shows the invariant mass of $p K^-$ versus the invariant mass of
$K^+ K^-$ 
%using improvements to the standard GlueX kaon identification and
%requiring the kinematic fit $\chi^{2}/NDF > 3.5$ on $gamma p \rightarrow p K^+ K^-$
%events in 
from the August 2018 reconstruction of Spring 2016 GlueX data. Fig.~\ref{fig:seb2d}
shows strong enhancement bands $\sim1.02$ GeV in the vertical direction, and $\sim1.52$ GeV and
$\sim1.8$ GeV in the horizontal direction. (The vertical enhancement at $\sim1.2$ GeV due to
the misidentification of pions with low opening angles and high momentum as kaons.) 
Also shown in Fig.~\ref{fig:seb2d} is a vertical white line at 1.4 GeV and a horizontal
white line at 2 GeV, which indicate the location of cuts on the data  to
further isolate $\phi(1020)$ and $\Lambda(1520)$.

%By taking events above the horizontal white line shown in 
%Fig.~\ref{fig:seb2d}, a plot of the $K^+ K^-$ invariant mass 
%showing a strong, and fairly clean, $\phi$ signal is created 
%(Fig.~\ref{fig:sebKpKm}). Likewise, by using events to the 
%right of the vertical white line, the $\Lambda(1520)$ is 
%clearly visible in Fig.~\ref{fig:sebPKm}. Now that Sebastian 
%has finished his study on isolating the 
%$\phi \rightarrow K^+ K^-$ reaction, he is simply waiting
%for a $p K^+ K^- \pi^0$ skim of the GlueX data. Once that 
%$p K^+ K^- \pi^0$ data skim has been obtained, a subset of 
%$K^{*} \rightarrow K \pi$ candidate events will be identified 
%and the $\eta_1^{\prime}$ hybrid search will begin. 

\begin{figure}[h!]
\vspace{2pt}
\begin{minipage}[b]{3.4in}
 { \centering
  \captionsetup{margin=0.5cm}
  \includegraphics[width=\linewidth,trim={0 0.2cm 0 0.2cm},clip]{sebKpKmV2_1.png}
  \caption{Invariant mass of $p K^-$. The vertical red line shows
    the PDG value for the mass of the $\phi$ meson.
    \newline
    \vspace{-5pt}
  }
  \label{fig:sebKpKm}}
\end{minipage}%
\begin{minipage}[b]{3.4in}
 { \centering
  \captionsetup{margin=0.5cm}
  \includegraphics[width=\linewidth,trim={0 0.2cm 0 0.2cm},clip]{sebPKmV2_1.png}
  \caption{Invariant mass of $p K^-$. The vertical red line shows
    the PDG value for the mass of the $\Lambda(1520)$ baryon.
    \newline
    \vspace{-5pt}
  }
  \label{fig:sebPKm}}
\end{minipage}
\end{figure}
By taking events above the horizontal white line shown in
Fig.~\ref{fig:seb2d}, a plot of the $K^+ K^-$ invariant mass
shows a strong $\phi$ signal, as seen in Fig.~\ref{fig:sebKpKm}. 
Likewise, by selecting events to the
right of the vertical white line, the $\Lambda(1520)$ is
clearly visible in Fig.~\ref{fig:sebPKm}. 

\begin{figure}[t!]
\vspace{2pt}
\begin{subfigure}{0.63\textwidth}
 { \centering
   \includegraphics[width=\linewidth,trim={0 0.2cm 0 0.2cm},clip]{KpPimInvMass}
   \vspace{-15pt}
 }
 \caption{} \label{fig:sebKpPim}
\end{subfigure} 
\begin{subfigure}{0.37\textwidth}
  \vspace{10pt}
 { \centering
   \includegraphics[width=\linewidth,trim={0 0.0cm 0 0.0cm},clip]{E852KStarV3.png}
  \caption{}  \label{fig:E852KStar}} 
\end{subfigure}
  \caption{(a) Invariant mass of $K^+ \pi^-$. The vertical red line indicates
     the PDG value for the mass of the $K^{*}(892)$. (b) Missing mass spectrum
     from Ref.~\cite{Adams:2001sk}. The shaded area indicates results where $|t| > 0.1$~GeV$^2$.
  } 
 \end{figure} 
 
At the time of this writing,
we await $p K^+ K^- \pi^0$ and $p K^{\pm} 2\pi^{\mp} \pi^{\pm}$
skims of the GlueX data from the August reconstruction launches of 2016 and 2017
data, as well as the first reconstruction launch of 2018 data to occur in Fall 2018, 
to dig deeper into this search.
These reconstruction launches are critical for analyses in GlueX due to major
improvements to GlueX tracking, and energy deposition determination in the CDC
and FDC, leading to a recovery of approximately 30\% more events than current analysis
efforts. A subset of the $p K^+ K^- \pi^0$ and $p K^{\pm} 2\pi^{\mp} \pi^{\pm}$ skims
containing $K^{*} \rightarrow K \pi$ candidate events will be identified
and the $\eta_1^{\prime}$ hybrid search will begin in earnest.
In preparation for this newly reconstructed data, Cole has
begun analyzing $p K_s^{0} K^{+} \pi^{-}$, where $K_s^{0} \rightarrow \pi^{+} \pi^{-}$,
skims from the initial reconstruction of low and high intensity 2017 data.
Fig.~\ref{fig:sebKpPim} shows evidence of $K^{*}(892)$ in the high intensity 2017 running of GlueX. 
The reconstruction of a $K^{*}(892)$ is a necessary step in the search for 
$\eta_1^{\prime}$ in $K^{*} K_{s}^{0}$.
While still at a relative early stage of analysis, this spectrum is remarkably 
similar to prior published data on this channel \cite{Adams:2001sk}, as seen in Fig.~\ref{fig:E852KStar}.

%The next 12 months is a particularly exciting time for the 
%exploration of reaction signatures that include kaons. 
%Recently, the GlueX Collaboration installed a DIRC 
%(Detection of Internally Reflected Cherenkov) for in-beam 
%commissioning slated for Fall 2018, with DIRC-included 
%production runs starting in Fall 2019. The DIRC will greatly 
%improve the particle identification for kaons needed for the
%search of the $\eta^{\prime}_1$ hybrid.



The next 12 months is a particularly exciting time for the
exploration of reaction signatures that include kaons.
Recently, the GlueX Collaboration installed a DIRC
%(Detection of Internally Reflected Cherenkov) 
for in-beam
commissioning slated for Fall 2018, with DIRC-included
production runs starting in Fall 2019. The DIRC will greatly
improve the particle identification for kaons needed for the
search of the $\eta^{\prime}_1$ hybrid.





