
\def\lsim{\mathrel{\raise.3ex\hbox{$<$\kern-.75em\lower1ex\hbox{$\sim$}}}}
\def\gsim{\mathrel{\raise.3ex\hbox{$>$\kern-.75em\lower1ex\hbox{$\sim$}}}}

\vspace{0.1in}
\small
\begin{center}
{\it Dhiman Chakraborty}
\end{center}
\normalsize\nopagebreak

In this section we consider the prospects for discovering or excluding
charged Higgs bosons produced in decays of top quarks at the 
Tevatron in Run~2.  If a charged Higgs boson exists, then it can be
directly produced in $p\bar{p}$ collisions.  However, as shown in
Section I.C.5.b, the cross section for inclusive charged Higgs
production is probably too small to be seen at the upgraded Tevatron
if $m_{H^+}\gsim m_t$.  However, if $t\to H^+b$ is kinematically
allowed then it can compete with $t\to W^+b$, which is by far the
dominant decay mode of top quark in the Standard Model (SM).

As previously noted, if $m_{H^+} < m_t - m_b$, and if $\tan\beta$ is
either very small or very large, then a significant fraction of those
events could contain charged Higgs bosons.  We focus on a search for
$p\bar{p}\to\ttbar\to H^+X$ based on the Type II two-Higgs doublet
model (2HDM) where one doublet couples exclusively to the down-type
fermions, and the second doublet couples exclusively to up-type
fermions~\cite{hhg}.\footnote{The MSSM Higgs sector is a Type II 2HDM.
However, the analysis of this section is more general, since the
results depend only on the charged Higgs boson mass and the parameter
$\tan\beta$ which parameterizes the coupling of $H^\pm$ to fermion
pairs [see eq.~\ref{hpmqq}].  No MSSM relations among Higgs sector
parameters are imposed.}

Recent results from searches for charged Higgs bosons can be found in
refs.~\cite{TEV,LEP,CLEO,ALEPH2}.  This study is an extension of a
search performed by D\O\ based on 109 pb$^{-1}$ from Run 1~\cite{TEV}.

\vskip 0.5cm
{\bf Parameter space}
\vskip 0.5cm

In the region $m_{H^+}< m_t - m_b$, the total inclusive cross-section 
for the production of either $H^+$ and/or $H^-$ is given by:
\begin{equation}
     \sigma(p\bar p\to \hpm+X)=\left(1-[{\rm BR}(t\to bW^+)]^2\right)
     \sigma(p\bar p\to t\bar t+X)\,,
\end{equation}
in the approximation that ${\rm BR}(t\to bH^+)+{\rm BR}(t\to bW^+) =
1$.  The parameters $m_{H^+}$ and $\tan\beta$ uniquely fix the top
quark branching ratios, given the top quark mass.  In this analysis,
we use the theoretical prediction that include NLO QCD
corrections~\cite{ttbarxsec}: $\sigma(p\bar{p}\to\ttbar) \simeq 7$ pb
at $\sqrt{s} = 2.0$ TeV.  However, only tree-level formulae for the
top-quark branching ratios have been employed.  Fig.~\ref{fg:params}
shows the region of the [$m_{H^+}$,$\tan\beta$] plane examined in this
study.  The minimum for $m_{H^+}$ is chosen at 50~GeV, somewhat below
the most recent lower limits from LEP.

This search is restricted to $m_{H^+} < 160$~GeV, somewhat less 
than $m_t - m_b$ (assuming $m_t = 175$~GeV); otherwise, the width of 
the charged Higgs $\Gamma(H^+)$ becomes too large ($> 7.5$~GeV) near 
the upper boundary on $\tan\beta$, and leading-order calculations become 
unreliable.  For the same reason, $\Gamma(t)$ is required to be $<15$~GeV.  
Since $\Gamma(t\to W^+b) \approx 1.5$~GeV, irrespective of 
[$m_{H^+}$,$\tan\beta$], this amounts to requiring $B(t\to H^+b) \le 0.9$, 
and thereby excludes from our analysis the dark-shaded regions at the two 
bottom corners of Fig.~\ref{fg:params}.  The cross-hatched regions correspond 
to $B(t\to H^+b) > 0.5$.  Also shown in Fig.~\ref{fg:params} are the decay
modes of $H^+$ that dominate in different parts of the parameter space.  
Analogous charge-conjugate results hold for $H^-$.

\begin{figure}
  \begin{center}
    \parbox{4in}{\epsfxsize=\hsize\epsffile{chiggs/gamma_lims.eps}}
  \end{center}
  \caption{The parameter space explored in this analysis.
           Regions where $B(t\to H^+b) > 0.5$ are shown cross-hatched, with 
           the labels for various decay modes of the charged Higgs 
           indicating their regions of dominance.  Regions where 
           $B(t\to H^+b) > 0.9$ (dark shaded areas) are not considered.}
  \label{fg:params}
\end{figure}

\vskip 0.5cm
{\bf Methodology of a disappearance search}
\vskip 0.5cm

A direct search (or ``appearance search'') for charged Higgs bosons
covering the entire parameter space is made difficult by the variety
of final state signatures.  The region dominated by $H^+\to c\bar{s}$
suffers from enormous QCD backgrounds.  While it is possible to look 
directly for $H^+\to\tau^+\nu$, and for $H^+\to W^+\bb$, both D\O\ and 
CDF have found that an indirect ``disappearance'' search was a 
competitive option in Run 1~\cite{TEV}.

With increasing statistics and enhanced detector performance, direct
searches will gain strength faster than a disappearance search, but
the latter, the only one considered in this study, is expected to
dominate until the integrated luminosity exceeds $\sim 2$ fb$^{-1}$.

In a disappearance search, one employs selection criteria optimized to
detect the SM decay of $\ttbar$.  Such criteria are expected to have
relatively low efficiency for decays involving $t\to H^+b$.  The top
quark pair-production cross section is not expected to be
significantly affected by the presence of a charged Higgs.
Consequently, if data agree well with SM-based predictions for
$\sigma_{\ttbar}$, regions of parameter space where $B(t\to H^+b)$ is
large can be excluded because in those regions one would expect fewer
events than seen in data.  We have considered final states containing
one or two charged leptons ($e$ or $\mu$), as was done in Run 1
analyses, although it is possible that all-hadronic final states also
will be utilized in future.

The published values of the $\ttbar$ production cross section
($\sigma_{\ttbar}$) and the mass of the top quark ($m_t$) from direct
observation at D\O\ \cite{d0cs,d0mt} and CDF~\cite{cdfcs,cdfmt} are
based on the assumption of BR($t\to W^+b$) = 1.  The topic of present
interest is based on a violation of this assumption, so those
measurements are not relevant in our analysis.  Hence, in the process
of setting limits in the [$m_{H^+}$,$\tan\beta$] plane, the values of
$\sigma_{\ttbar}$ and $m_t$ serve as input parameters.

\break
\vskip 0.5cm
{\bf Signal, background, and their modeling}
\vskip 0.5cm

Our search for $t \bar t \rightarrow H^\pm X$, are based on selection
criteria optimized for $\ttbar\to W^+ b W^- \bar{b}$ events
which are detailed in refs.~\cite{d0cs,cdfcs}.  $W+$jets and
$Z/\gamma^*+$jets are the main sources of physics background, while
measurement fluctuations resulting in false identification of
electrons, muons, and neutrinos ($\met$) in QCD multijet events
contribute to instrumental background.

All $\ttbar$ events are simulated using ISAJET~\cite{ISAJET} and a
GEANT-based simulation of the Run 1 D\O\ detector.  The efficiencies
are verified using PYTHIA~\cite{PYTHIA}, and improvements for Run 2
are estimated from studies conducted in the course of the technical
design report for D\O\ upgrade, as well as SHW.  $W+$jets and $Z+$jets
events are modeled using the VECBOS tree-level parton generator
interfaced with ISAJET for fragmentation and hadronization.  The
instrumental background is estimated using Run 1 data
\cite{d0cs,d0mt,cdfcs,cdfmt}, on which additional suppression factors
expected from detector improvements are applied.

\vskip 0.5cm
{\bf Event selection}
\vskip 0.5cm

The final states of $\ttbar$ decaying according to the SM are
characterized by the decays of the two $W$ bosons in the event.  We
focus on events where at least one $W$ decays into an $e$, or a $\mu$,
and the associated neutrino.  If the other $W$ also decays into $e
\nu$ or $\mu \nu$, then the event is classified as a ``dilepton''
event.  If it decays into $\qq$, we call it a ``single lepton''
event.  Dilepton events are characterized by two high-$\pt$ isolated
leptons, large $\met$, and two or more jets, both originated by $b$
quarks.  Single lepton events are characterized by one high-$\pt$
isolated lepton, large $\met$, and three or more (nominally, four) jets,
two of which are originated by $b$ quarks.  Tagging of the $b$-quark
jets, either by a decay vertex separated from the primary interaction
vertex, or by an associated non-isolated lepton from semileptonic
decay of $b$ offers strong suppression of backgrounds in single-lepton
channels.  Backgrounds in dilepton channels are so small that the
$b$-tag requirement is not necessary.

\begin{table}
\caption{The event selection criteria.}
\label{tb:cuts}
\begin{tabular}{lcc}
 &
single-lepton &
dilepton \\
\hline
$p_T(\ell)$ & $>$ 20~GeV & $>$ 20~GeV \\
$|\eta(\ell)|$ & $<$ 2.0  & $<$ 2.0  \\
$\met$ & $>$ 20~GeV & $>$ 20~GeV \\
leptons ($n_\ell$)& $\ge 1$ & $\ge 2$ \\
$E_T(j)$ & $>$ 20~GeV & $>$ 20~GeV \\
$|\eta(j)|$ & $<$ 2.0  & $<$ 2.0  \\
jets ($n_j$)& $\ge 3$ & $\ge 2$ \\
$\mu$-tags & 1 or 2 & $\le 2$  \\
Aplanarity & $>$ 0.04 & -- \\[2pt]
$H_T \equiv \sum_{i=1}^{n_j} E_T(j_i)$ & $>$ 110~GeV& $>$ 110~GeV\\[4pt]
\end{tabular}
%\caption{The event selection criteria.}
%\label{tb:cuts}
\end{table}

The selection criteria for dilepton and single-lepton channels are
summarized in Table~\ref{tb:cuts}.  These are similar to those used to
measure $\sigma_{\ttbar}$ in Run 1 \cite{d0cs,cdfcs}.  In addition to
these cuts, any event with an identified $\tau$ lepton (hadronic
decays only) with $E_T > 15$ GeV and $|\eta| < 2.0$ is rejected.
The dilepton and single-lepton selection criteria are combined in a
logical OR, so an event passing both sets of cuts is counted only
once, not twice.

\vskip 0.5cm
{\bf Efficiencies}
\vskip 0.5cm

For each top quark, there are four possible decay modes whose branching 
fractions depend on $m_{H^+}$ and $\tan\beta$: 
(1) $t\to W^+b$; (2) $t\to H^+b$, $H^+\to c\bar{s}$; (3) $t\to H^+b, H^+\to W^+\bb$; 
and (4) $t\to H^+b, H^+\to\tau^+\nu$.  If the decay mode of $t$ ($\bar t$) is
 denoted by $i$ ($j$), then the total acceptance for any set of selection criteria 
is given by
\begin{equation}
    A(m_H^+,\tan\beta) = {\displaystyle \sum_{i,j=1}^{4}} 
                     \epsilon_{i,j}(m_{H^+})  B_i(m_{H^+},\tan\beta) B_j(m_{H^+},\tan\beta),
\label{eq:acc}
\end{equation}
where $\epsilon_{i,j}$ is the efficiency for channel $\{i,j\}$, and
$B_i B_j$ is the branching fraction.  All $B_i$ depend strongly on
both $m_{H^+}$ and $\tan\beta$; $\epsilon_{1,1}$ depends on neither,
and all other $\epsilon_{i,j}$ depend on $m_{H^+}$, but not on
$\tan\beta$.


The efficiencies for various channels, for $m_{H^+}$ = 110 GeV, are given 
in  Table~\ref{tb:effs} in the form of a symmetric $4 \times 4$ matrix.
The 10 independent elements of the matrix are plotted as functions of
$m_{H^+}$ in Figs.~\ref{fg:eff_1} and \ref{fg:eff_2}.  Uncertainties in jet
energy scale, particle identification, and signal modeling are the
main components of systematic uncertainties.  The total systematic
uncertainty is estimated to be 10\% of the efficiency.  Statistical
uncertainties are negligible.

\begin{table}[ht!]
  \caption{The efficiencies $\epsilon_{i,j}$ of our selection criteria (in \%),
           for $m_t$ = 175~GeV and $m_{H^+}$ = 110~GeV, for various decay modes of 
           $\ttbar$.  Here $i$ is the row index and $j$ the column.
           Systematic uncertainties are assumed to be 10\%.
           Statistical uncertainties are negligible.}
  \label{tb:effs}
  \begin{center}
    \begin{tabular}{l|c| c| c| c} %\hline
  &
    $\overline{t} \to W^-\overline{b}$ &
      $\overline{t} \to H^-\overline{b}, H^- \to \bar{c}s$ &
        $\bar{t} \to H^-\bar{b}, H^- \to W^-\bb$ &
          $\bar{t} \to H^-\bar{b}, H^- \to \tau^-\bar{\nu}$ \\ \hline
$t \to W^+b$                   & $4.0$ & $2.7$ & $2.6$ & $1.1$ \\ 
$t \to H^+b, H^+ \to c\bar{s}$ & $2.7$ & $0.1$ & $2.0$ & $1.1$ \\ 
$t \to H^+b, H^+ \to W^+\bb$  & $2.6$ & $2.0$ & $2.0$ & $1.5$ \\ 
$t \to H^+b, H^+ \to \tau^+\nu$ & $1.1$ & $1.1$ & $1.5$ & $0.4$ \\ 
    \end{tabular}
  \end{center}
%  \caption{The efficiencies $\epsilon_{i,j}$ of our selection criteria (in \%),
%           for $m_t$ = 175~GeV and $m_{H^+}$ = 110~GeV, for various decay modes of 
%           $\ttbar$.  Here $i$ is the row index and $j$ the column.
%           Systematic uncertainties are assumed to be 10\%.
%           Statistical uncertainties are negligible.}
%  \label{tb:effs}
\end{table}

\begin{figure}[ht!]
  \begin{center}
    \parbox{4in}{\epsfxsize=\hsize\epsffile{chiggs/eff_run2_1.eps}}
  \end{center}
  \caption{The efficiencies of our selection criteria, as functions of 
           $m_{H^+}$, for channels in which no more than one top quark
           decays to charged Higgs.  The thick central curves and the 
           thin curves containing the uncertainty bands are parabolic fits, 
           respectively, to the points and the error bars.}
  \label{fg:eff_1}
\end{figure}


\begin{figure}
  \begin{center}
    \parbox{4in}{\epsfxsize=\hsize\epsffile{chiggs/eff_run2_2.eps}}
  \end{center}
  \caption{The efficiencies of our selection criteria, as functions of 
           $m_{H^+}$, for channels in which both top quarks decay to 
           charged Higgs.  The thick central curves and the thin curves 
           containing the uncertainty bands are parabolic fits, respectively, 
           to the points and the error bars.}
  \label{fg:eff_2}
\end{figure}



It is easy to see why the efficiencies are low in regions of parameter
space dominated by $H\to c\bar{s}$ (small $\tan\beta$, small
$m_{H^+}$).  When both top quarks decay to charged Higgs, and both
charged Higgs decay to $c\bar{s}$, there is no legitimate source for
an isolated $e$ or $\mu$, nor for large $\met$, in the final state.
Compared to the SM, efficiencies are lower in regions dominated by
$H^+\to\tau^+\nu$ (large $\tan\beta$) because $\tau$ leptons decay 
hadronically most of the time, and a veto is applied to events containing 
$\tau$s.  Even when a $\tau$ from an $H^\pm$ decays to an $e$ or a $\mu$, 
the lepton usually has a lower $p_T$ compared to one from a prompt decay of a
$W$.  Regions dominated by $H^+\to W^+\bb$ (small $\tan\beta$, large 
$m_{H^+}$) are more difficult to discriminate against because real $W$ 
bosons are produced.  However, such events are characterized by a large 
number of $b$ jets (up to 6), and can be suppressed somewhat by requiring 
that no more than two of the jets be tagged as $b$.


\vskip 0.5cm
\break


\begin{figure}[ht!]
  \begin{center}
    \parbox{4in}{\epsfxsize=\hsize\epsffile{chiggs/nobs_lklhd_run2.eps}}
  \end{center}
  \caption{Distribution of $n_{\rm obs}$ in simulated experiments with parameters
           given in Table~\ref{tb:exptpars}, assuming $B(t\to W^+b) = 1.$ 
           Lower efficiencies for $t \bar t \rightarrow H^\pm X$ events imply that
           the distribution would peak at smaller values of $n_{\rm obs}$ if 
           $B(t\to H^+b)$ is large.}
  \label{fg:lklhd_nobs}
\end{figure}

\begin{figure}[h!]
  \begin{center}
    \parbox{4in}{\epsfxsize=\hsize\epsffile{chiggs/tb_lklhd_run2.eps}}
  \end{center} 
   \caption{Posterior probability density for $\tan\beta$, for $m_{H^+}$ = 80 GeV, and 
         for four different values of the number of events
         experimentally observed in 2 fb$^{-1}$ integrated luminosity
         (based on simulated experiments with parameters given in
         Table~\ref{tb:exptpars}): (a) $n_{\rm obs}$ = 600, (b)
         $n_{\rm obs}$ = 500, (c) $n_{\rm obs}$ = 400, and (d) $n_{\rm
         obs}$ = 300.  As expected, for $n_{\rm obs}$ = 600, the most
         likely value of $\tan\beta$ is $\sqrt{{m_t}\over{m_b}} \approx 6.5$,
         or, $\log_{10}(\tan\beta) \approx 0.82$, where $B(t\to H^+b)$ is minimum
         (the SM-based prediction peaks at $n_{\rm obs}$ = 610, as
         seen in Fig.~\ref{fg:lklhd_nobs}).  Smaller values of $n_{\rm obs}$ 
        favor larger values of $B(t\to H^+b)$.}
  \label{fg:lklhd_tb}
\end{figure}

{\bf Likelihood analysis}
\vskip 0.5cm


For $n_{\rm obs}$ observed events, the joint posterior probability
density for $m_{H^+}$ and $\tan\beta$ is given by
\begin{equation}
    P(m_{H^+},\tan\beta | n_{\rm obs}) 
    \propto  {\displaystyle \int  G({\cal L}) \int  G(n_B)
    \int}  G(A) P(n_{\rm obs}|\mu) dA dn_B  d {\cal L},
\label{eq:n_obs}
\end{equation}
where $P(n_{\rm obs}| \mu)$, is the Poisson probability of observing
$n_{\rm obs}$ events, given a total (signal + background) expectation of
\begin{equation}
   \mu(m_{H^+},\tan\beta)  =  A(m_{H^+},\tan\beta) \sigma_\ttbar {\cal L}  + n_B,
\end{equation}
and $G$ represents a Gaussian distribution.
The means and widths of the Gaussians for the integrated luminosity $\cal L$,
and the number of background events $n_B$, are given in 
Table~\ref{tb:exptpars},
while those for the acceptance $A(m_{H^+},\tan\beta)$, are calculated using
Eq.~(\ref{eq:acc}), with parametrized functions for 
$\epsilon_{i,j}$, 
and leading order calculations of  $B_i$, $B_j$.


\begin{table}[ht!]
\caption{The integrated luminosity, and the
expectations from background and SM $\ttbar$ signal (assuming $m_t$ = 175~GeV,
$\sigma_{\ttbar}$ = 7.0~pb).}
\label{tb:exptpars}
\begin{tabular}{l|c}
Integrated luminosity, $\cal L $ & $2.0 \pm 0.1$ fb\\
Estimated background, $n_B$ & $50 \pm 5$ \\
Expected signal (SM), $n_S$ & $560 \pm $ 24\\
\end{tabular}
%\caption{The integrated luminosity, and the
%expectations from background and SM $\ttbar$ signal (assuming $m_t$ = 175~GeV,
%$\sigma_{\ttbar}$ = 7.0~pb).}
%\label{tb:exptpars}
\end{table}

Equation (\ref{eq:n_obs}), which we parametrize as a function of $m_{H^+}$
and $\tan\beta$, gives a Bayesian posterior probability density for those
parameters~\cite{Jaynes}.  The prior distribution is assumed to be
uniform in $m_{H^+}$ and in $\log\tan\beta$.  Assuming instead that the prior is
uniform in $m_{H^+}$ and in $B(H^+\to\tau^+\nu)$ does not significantly 
alter the posterior distribution.  To calculate probabilities, a Monte Carlo
integration is carried out by spanning the parameter space in steps of
5~GeV in $m_{H^+}$ from 50~GeV to 160~GeV, with 25 uniform steps in 
$\log\tan\beta$ covering the range $0.3 < \tan\beta < 150$ at each value 
of $m_{H^+}$, and performing 400,000 trials of Eq.~(\ref{eq:n_obs}) at each step.

\begin{figure}
  \begin{center}
    \parbox{4in}{\epsfxsize=\hsize\epsffile{chiggs/lklhd_surf_run2.eps}}
  \end{center}
  \caption{The likelihood of $n_{obs}$ = 600 (in arbitrary units), 
           as a function of $m_{H^+}$ and $\tan\beta$ (assuming $m_t$ = 175 GeV 
           and parameters given in Table~\ref{tb:exptpars}).}
  \label{fg:lklhd_surf}
\end{figure}

The likelihood predicted by the SM for observing $n_{\rm obs}$ events
is shown in Fig.~\ref{fg:lklhd_nobs}, while Fig.~\ref{fg:lklhd_tb}
shows the posterior probability density for $\tan\beta$ corresponding
to several values of $n_{\rm obs}$ for $m_{H^+}$ = 80~GeV.  For a
given value of $n_{\rm obs}$, the 95\% CL exclusion boundary in the
[$m_{H^+},\tan\beta$] plane is obtained by integrating the probability
density $P(m_{H^+},\tan\beta|n_{\rm obs})$, given by
Eq.~(\ref{eq:n_obs}), between contours of constant $P$.  The
likelihood surface corresponding to the parameters given in
Table~\ref{tb:exptpars}, and $n_{\rm obs}$ = 600, is shown in
Fig.~\ref{fg:excl_cs_shw} for three different values of $\int {\cal L}
dt$: 109 pb$^{-1}$ (Run 1, $\sqrt{s} = 1.8$ TeV ~\cite{TEV}), 2
fb$^{-1}$, and 10 fb$^{-1}$.

\begin{figure}
  \begin{center}
    \parbox{4in}{\epsfxsize=\hsize\epsffile{chiggs/excl_shw.eps}}
  \end{center}
\caption{The 95\% CL exclusion boundaries in the [$m_{H^+},\tan\beta$] plane 
         for  $m_t$ = 175~GeV and several values of the integrated luminosity:
         0.1 fb$^{-1}$ (at $\sqrt{s} = 1.8$ TeV, cross-hatched), 
         2.0 fb$^{-1}$ (at $\sqrt{s} = 2.0$ TeV, single-hatched), and 
         10 fb$^{-1}$ (at $\sqrt{s} = 2.0$ TeV, hollow), 
         if the $n_{\rm obs}$ continues to be where the SM-based prediction peaks.}
  \label{fg:excl_cs_shw}
\end{figure}


\vskip 0.5cm
{\bf Conclusions}
\vskip 0.5cm

Our study shows that, compared to Run 1~\cite{TEV}, the search for
charged Higgs bosons in decays of pair-produced top quarks will have a
much greater reach in Run 2.  A direct appearance search for charged
Higgs bosons in decays of top quark is also possible.  At the
Tevatron, it would primarily involve a search for $H\to\tau\nu$, which
limits its applicability to $\tan\beta > 4$.  CDF has published
results from such searches based on Run 1 data~\cite{CDFch,CDF_H+_2}
which are consistent with its disappearance search results.
Comparable results from an appearance search performed by D\O\ are
expected soon.  An appearance search in the region of $\tan\beta < 1$
will have to be based on $H^+\to c\bar{s}$.  This will be more
challenging since $c$ and $s$ quarks are more difficult to identify
than $\tau$s and $\nu$s.  However, if $m_{H^+}$ is substantially
larger than $M_W$, then it may be possible to distinguish ($t\to H^+b$,
$H\to c\bar{s}$) from ($t\to Wb$, $W\rightarrow q_1 \bar q_2$) based on
event kinematics.  The indirect disappearance search presented here is
expected to yield stronger null results than a direct appearance
search for integrated luminosities up to 2--4 fb$^{-1}$.  If there is a
charged Higgs lighter than the top quark, its presence can be detected
by a disappearance search if $\tan\beta$ is substantially different
from $\sqrt{{m_t}/{m_b}}$, prompting a direct search to confirm
discovery.  Beyond Run 2, appearance searches should lead to superior
results.


