
In the previous analysis, hard cuts on {\em raw} missing transverse
energy and additional jet activity were used reduce the background to
an acceptable level.  For the analysis presented here, a different
strategy is used in which a looser selection gives both larger signal
and larger background but with approximately the same statistical
power as the previous analysis.

\vspace{0.2in}
{\bf Event samples} \\ \nopagebreak

The signature of $ZH$ decays is two $b$-jets with large invariant mass
and either large missing energy ($\met$), or two opposite sign leptons
with mass near that of the $Z$.  These requirements effectively
eliminate generic multijet events, as shown below, and leave the
following background processes: $Zb\bar{b}$, $ZZ$, $\ttbar$, and
single top production. The $\met$ selection also picks up events with
$W\rightarrow l \nu$ decay, particularly if the leptons escape
detection.  This introduces additional backgrounds from $WZ$ and
$Wb\bar{b}$ processes and adds $WH$ events to the effective signal.
$ZH$ and $WH$ events are generated using PYTHIA (for Higgs mass values
of 90, 100, 110, 120, and 130 GeV$/c^2$). PYTHIA is also used for the
$ZZ$, $WZ$, $\ttbar$, and single $t$ backgrounds, and CompHEP is
used for $Zb\bar{b}$ and $Wb\bar{b}$. The $Zb\bar{b}$ simulation is
checked against ISAJET, and the results agree to within 8\%.  Cross
sections and branching ratios for both the signal and background
processes are taken directly from the generators without any
correction factors applied, except for $\ttbar$ events which are
normalized to the measured cross section.  Detector
simulation for all samples is performed using the SHW package.

As in the previous analysis, no estimate for the level of background 
from $\bb$ dijet production is made here, as it will require actual
data. 
\pagebreak

\vspace{-0.2in}
{\bf Selection Criteria} \\ \nopagebreak

$ZH \rightarrow \nu \bar{\nu} b \bar{b}$ events are selected by the following
criteria:

\begin{itemize}
  \item 2 $b$-jets: 
  \begin{itemize}
    \item[-] one tight $b$-tagged jet with $E_T>20$ GeV and $|\eta|<2$
    \item[-] one loose $b$-tagged jet with $E_T>15$ GeV and $|\eta|<2$ 
  \end{itemize}
  \item missing transverse energy $\met > 35$ GeV
  \item the angle between the $\met$ and its closest jet $\delta\phi$ $>0.5$
  \item reject events with one or more isolated leptons with ($p_T>8$ GeV/c) 
  \item scalar sum of hadronic energy $H_T<175$ GeV
\end{itemize}

The $b$-tagging and lepton identification efficiencies are all taken
from SHW.  The effect of the cuts on both the 100~GeV$/c^2$~Higgs
signal and background samples is shown in Table~\ref{evflow}. As shown
in Figure~\ref{met_ht}, the $\met$ cut removes generic multijet
background, and the additional cut on $\delta\phi$ removes multijet
events with $\met$ due to mismeasured jets.  Combined with the
requirement of two $b$-tagged jets, few generic multijet events
remain; based on Run~1 data, we believe that the background from
multijet events will be no more than 50\% of the total background from
all sources.  The isolated lepton cuts reduce the backgrounds
involving $W$ decay.  However, significant contributions remain due to
muon detection inefficiencies (which also add to the apparent $\met$
of an event). This also means that $WH$ events contribute to the
effective signal.

It should be noted that the $WH$ search mode selection requires the
detection of a high $p_T$ lepton, so the two samples are independent.
The $H_T$ cut helps remove $\ttbar$ background (see
Figure~\ref{met_ht}), and cuts much less of the signal than the third
jet veto used to veto top in previous analyses (increasing the
significance by as much as 30\%). Energy corrections are applied to
the $b$-jets to correct for final state radiation and undetected
energy from semileptonic decays. Only the two highest $E_T$ $b$-jets
in an event are used as the candidate jets from the Higgs decay.

\begin{table}
\caption{$ZH\rightarrow \nn\bb$ channel, SHW analysis.
 Data reduction for 100 GeV$/c^2$~Higgs and backgrounds.}
\label{evflow}
\begin{center}
\begin{tabular}{lccccccccc} 
channel                & $ZH$ & $WH$ & $ZZ$ & $WZ$ & $Zb\bar{b}$ & $Wb\bar{b}$ & $\ttbar$ & $tb$ & $tq+tbq$  \\
\tableline
$\sigma\times BR$ (fb) & 27.0 & 71.5 & 137  & 135  &     668     &     2530    &    6800    & 1000 &    807 \\ \tableline
generated              & 7000 & 7000 & 7000 & 7000 &   20000     &    20000    &  270000    & 6223 &  55000 \\
2 $b$-jets             & 3022 & 2798 & 2437 & 2254 &    3694     &     3802    &  119713    & 2012 &   5036 \\
$\met > 35$ GeV        & 2177 & 1744 &  941 & 1297 &    2573     &     1924    &   55559    &  555 &   3686 \\
$\Delta\phi >0.5$      & 1983 & 1538 &  840 & 1171 &    2330     &     1704    &   48657    &  491 &   3324 \\
$H_T < 175$ GeV        & 1527 &  831 &  668 &  802 &    1592     &     1526    &    2004    &  200 &   1660 \\
no isolated leptons    & 1376 &  364 &  609 &  401 &    1458     &      680    &     767    &   99 &    902 \\ 
$80 < m_{bb} < 125$    & 1014 &  271 &  412 &  268 &     314     &       96    &     286    &   32 &    194 \\
%\hline 
\end{tabular}
%\caption{$ZH\rightarrow \nn\bb$ channel, SHW analysis.
% Data reduction for 100 GeV$/c^2$~Higgs and backgrounds.}
%\label{evflow}
\end{center}
\vspace{-1pc}
\end{table}


\begin{figure}
  \begin{center}
    \parbox{6.0in}{\epsfxsize=\hsize\epsfbox{zhvv/met_ht.eps}}
  \end{center}
  \caption{$ZH\to\nn\bb$ channel, SHW analysis.  Distributions of
           missing transverse energy and scalar sum of hadronic 
           energy ($H_T$).}
  \label{met_ht}
\end{figure}

\vspace{0.2in}
{\bf Results and Conclusions} \\ \nopagebreak

Signal mass windows for each Higgs mass are determined using the
nominal mass resolution obtained from SHW with $b$-jet corrections
($\approx 15\%$ for 100 GeV$/c^2$~Higgs).  The number of signal and
background events expected in 1~fb$^{-1}$ of data is shown in
Table~\ref{nunu_sig}. Numbers obtained assuming a 10\% 
mass resolution are also shown in the table. These results assume a
100\% trigger efficiency.  Efficiencies close to this should be
obtainable with a multijet-$\met$~trigger with the $\met$ threshold
set as low as the rates will allow. The addition of secondary vertex
tagging at the trigger level would help keep the rates down if
necessary, as would a veto on back--to--back jets.

\begin{table*}[tbp]
\vspace{1cm}
\caption{$ZH\rightarrow \nn\bb$ channel, SHW analysis.  Expected number 
         of $\met b\bar{b}$ signal and background events in 1 fb$^{-1}$ 
         of data (per experiment) for various Higgs masses. Numbers in 
         parentheses are for 10\% $\bb$ mass resolution.}
\label{nunu_sig}
\begin{center}
\begin{tabular}{lccccc}
$m_H$ (GeV$/c^2$) & 90           & 100          & 110          & 120          & 130          \\ \tableline
$\Delta m$        & 70--110      & 80--125      & 85--130      & 90--140      & 95--150      \\ 
                  &(80--105)     &(90--120)     &(95--125)     &(105--135)    &(115--145)    \\ \tableline
$ZH$              & 4.9          & 3.9          & 2.8          & 1.9          & 1.3          \\
$WH$              & 4.0          & 2.9          & 1.8          & 1.3          & 0.8          \\
signal            & 8.9          & 6.7          & 4.6          & 3.2          & 2.1          \\ \tableline
$ZZ$              & 8.5  (8.5)   & 8.0  (6.0)   & 7.1  (2.9)   & 5.8  (0.2)   & 4.4  (0.0)   \\
$WZ$              & 6.0  (5.9)   & 5.9  (5.1)   & 5.4  (2.7)   & 4.7  (0.2)   & 3.6  (0.0)   \\
$Zb\bar{b}$       & 12.3 (7.0)   & 10.3 (6.3)   & 9.2  (5.6)   & 8.7  (4.6)   & 8.1  (3.5)   \\
$Wb\bar{b}$       & 15.1 (8.5)   & 12.1 (7.1)   & 11.1 (6.7)   & 10.5 (5.4)   & 10.1 (4.2)   \\
single $t$        & 7.4  (4.6)   & 8.0  (5.4)   & 7.8  (5.3)   & 8.0  (4.7)   & 7.9  (3.8)   \\
$\ttbar$          & 6.3  (4.0)   & 7.2  (4.7)   & 7.0  (4.7)   & 7.3  (4.3)   & 7.2  (3.5)   \\ %\hline
background$^a$    & 55.6 (38.5)  & 51.5 (34.6)  & 47.6 (27.9)  & 45.0 (19.4)  & 41.3 (15.0)  \\
%\hline
%$S/\sqrt{B}$     & 1.19 (1.43)  & 0.93 (1.14)  & 0.67 (0.87)  & 0.47 (0.73)  & 0.33 (0.54)  \\
\end{tabular}
$^a$This does not include generic $\bb$ dijet production
processes, which must be estimated from data; see text.\\
\end{center}
%\vspace{0.2in}
%\caption{$ZH\rightarrow \nn\bb$ channel, SHW analysis.  Expected number 
%         of $\met b\bar{b}$ signal and background events in 1 fb$^{-1}$ 
%         of data (per experiment) for various Higgs masses. Numbers in 
%         parentheses are for 10\% $\bb$ mass resolution.}
%\label{nunu_sig}
\end{table*}


