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% declarations for front matter
\title{Baryonic $B$ Meson Decays}
 
\author{M.Z.~Wang\address{Department of Physics,
        National Taiwan University, \\
        Taipei, Taiwan, R.O.C.}%
  \thanks{on behalf of the Belle Collaboration}
}
 
\runtitle{2-column format camera-ready paper in \LaTeX}
\runauthor{M.Z. Wang}

\begin{document}

\begin{abstract}

Recent results of the baryonic $B$ decays from the two b-factories, 
BABAR and Belle, are presented. These include studies of
charmonium decays from $\bp \to \ppk$ and $\bp \to \LLK$; observations of 
$\bp \to J/\psi\bar{\Lambda}p$, $B \to \Lambda_c^+ \bar\Lambda_c^- K$, and
$\bp \to \bar{\Xi}^0_c  \Lambda_c^+$ and study of of inclusive $B$ decays
to $\Lambda_c$.

%
\noindent{\it PACS:} 13.25.Hw, 13.60.Rj, 13.40.Hq, 14.20Lq, 14.20.Dh, 14.40.Nd,
12.15Hh

\end{abstract}

\maketitle

\section{Introduction}

The weak decays of $B$ mesons offer not only the important
study of CP violation but also a profound searching ground
for the final states containing a baryon-antibaryon pair
because of the large mass of $B$ mesons. Following the  pioneering work
done by CLEO~\cite{CLEOfu}, many exclusive baryonic $B$ meson decays
have been found recently by the two b-factories, BABAR~\cite{BABAR} and  
Belle~\cite{Belle}.

In b-factory, it is an over-constrained system to determine decays
from B meson since not only the mass but also 
the energy of the $B$ meson are known in the center-of-mass (CM)
frame.
One can use two kinematic variables in the CM frame to identify the
reconstructed $B$ meson candidates: the beam energy
constrained mass $\mb = \sqrt{E^2_{\rm beam}-p^2_B}$, and the
energy difference $\de = E_B - E_{\rm beam}$, where $E_{\rm
beam}$ is the beam energy, and $p_B$ and $E_B$ are the momentum and
energy, respectively, of the reconstructed $B$ meson. Alternatively, 
one can use  $\mb$ and $\Delta M_B$, where $\Delta M_B$ is the mass 
difference between the reconstructed $B$ mass and the 
nominal $B$ mass~\cite{PDG}. 
To determine the $B$ yields, normally an unbinned extended 
likelihood fit is applied by using the above two variables as
inputs for all candidate events. The signal probability density function
(PDF) of the two variables is typically obtained by Monte Carlo samples and 
the background PDF is determined from sideband (i.e. non-signal region) data.


\section{Charmless modes}

After the first observation of the charmless baryonic $B$ meson decay,
$\bp \to \ppk$~\cite{belleppk,conjugate}, 
many charmless three-body baryonic decays are
found~\cite{plpi}. The dominant contributions for these decays
are presumably via the $b \to s$ penguin diagram except that for $\bp \to
\pppi$ which is believed to be the $b \to u$ tree diagram. There is a common 
feature of these decays that the dibaryon mass 
spectra show an enhancement near threshold. This feature was conjectured 
in Refs.~\cite{HS} and aroused more theoretical interests
%~\cite{glueball} 
lately. Detailed information from
the polar angle  distributions~\cite{polar} and Dalitz plot~\cite{BABARppk}
offer better understanding of the underlying dynamics. 
 

\begin{figure}[htb]
\includegraphics[width=0.48\textwidth]{ppk.eps}
\includegraphics[width=0.48\textwidth]{LLK.eps}
\caption{ 
(a) $B$ signal yield versus
$M_{\rm p\bar p }$ and (b) $B$ signal yield versus
$M_{\rm\Lambda\bar\Lambda}$. The inset shows the $\eta_{c}$-$J/\psi$ mass
region. The curves represent the unbinned likelihood fits to the data.
}
\label{fgppk}
\end{figure}


Fig.~\ref{fgppk} shows the recent results~\cite{alpha} from $\bp \to \ppk$
and $\bp \to \LLK$ obtained by
Belle with $386 \times 10^6 B\bar{B}$ pairs.  
There are clear threshold enhancement, $\eta_c$ and $J/\psi$ peaks 
%along with threshold enhancement events
in the baryon-antibaryon mass spectrum.
%A fit to the data is shown in the inset.
A maximum likelihood fit to the data is shown in the inset,
with a relativistic Breit-Wigner function for the $\eta_c$ peak,
a Gaussian for the $J/\psi$ peak, and
a linear function for the non-resonant background. 
The Breit-Wigner function
is convolved with the detector response function, which is taken from the
$J/\psi$ peak.
One then obtains an $\eta_c$ mass of
M$_{\eta_c}=2971\pm 3^{+2}_{-1}$ MeV$/c^2$ ($2974\pm 7^{+2}_{-1}$ MeV$/c^2$) 
and a width of
$\Gamma (\eta_c)=48^{+8}_{-7}\pm 5 $ MeV$/c^2$ ($40 \pm 19\pm 5 $ MeV$/c^2$)
for the $\eta_c \to \pp$ ($\eta_c \to \LL$) mode.
%The systematic errors are determined from the differences of $J/\psi$ peaks
%between data and the PDG~\cite{PDG} value, and by varying different fit
%shapes for $\eta_c$ signal and background assuming
%no interference effect between them.
The width is larger than the PDG average but is consistent with recent
BABAR~\cite{BABARppk} and previous Belle~\cite{Fang:2002gi} measurements.
This is also the first observation of $\eta_c \to \LL$ decays with
${\mathcal B}(\eta_c \to \LL) = ( 0.87^{+0.24}_{-0.21} (stat) 
{+0.09\atop -0.14} (syst) \pm 0.27 (PDG))
\times 10^{-3}$.

From this data set, one can also determine the branching fraction ratios: ${\mathcal B}$($\eta_{\rm c} \to \LL$)/${\mathcal B}$($\eta_{\rm c} \to \pp$) = $0.67^{\rm+0.19}_{\rm-0.16} \pm 0.12$
and ${\mathcal B}$($J/\psi \to \LL$)/${\mathcal B}$($J/\psi \to \pp$) =
$0.90^{\rm+0.15}_{\rm-0.14} \pm 0.10$,
where common systematic errors in the numerator and denominator cancel.
The observed ratio 
is consistent with theoretical expectation~\cite{etactoBB} based on
a quark-diquark model for baryons. 


\section{Charmed Modes}

The $b \to c$ process is the dominant process for $B$ decays. Many decay modes
with  $\Lambda_c^+$ in the final states have been found, including the
first observation of a two-body decay:$\bz \to p \bar\Lambda_c^-$~\cite{Lcp}.
It is interesting to see that
the two-body baryonic decay is suppressed in exclusive $B$ decays.
Comparing with mesonic $B$ decays, two-body and three-body decays are 
comparable. This 
indicates that for the formation of a baryon-antibaryon pair, giving off extra 
energy is much favored.

\begin{figure}[htb]
\includegraphics[width=0.41\textwidth]{jpsi1.eps}
\includegraphics[width=0.44\textwidth]{jpsi2.eps}
\caption{ 
(a) The ($\mb, \Delta M_B$) scatter plot of  
$\bp \to J/\psi\bar{\Lambda}p$ candidates and its projection onto 
(b) $\Delta M_B$ with 5.265 GeV/$c^2 < \mb < 5.29$ GeV/$c^2$ and (c) 
$\mb$ with $|\Delta M_B| < 0.03$ GeV/$c^2$. The dashed lines and solid boxes
indicate the signal regions. The curves are the results of the fit.
}
\label{JLP}
\end{figure}


The excess in the low momentum region of the inclusive $J/\psi$ 
spectrum%~\cite{CLEOJX} 
for
$B$ decays is not consistent with predictions from non-relativistic 
QCD calculations.
%~\cite{theoJX}
With the hint
of threshold enhancement, this triggers the search for decay modes like 
$\bp \to J/\psi\bar{\Lambda}p$. Evidence is reported by BABAR~\cite{BABARJLp}
with $8.9 \times
10^6$ $B\bar{B}$ pairs. Using larger data set,  $275 \times
10^6$ $B\bar{B}$ pairs, Belle has made the observation~\cite{JpsiLp} 
and obtained a branching
fraction of  
${\mathcal B}(\bp \to J/\psi\bar{\Lambda}p) = (11.6\pm 2.8^{+1.8}_{-2.3}
)\times 10^{-6}$. This is not large enough to explain the observed excess.
However, it stimulates more efforts to search for similar modes.
Fig.~\ref{JLP} shows the $\mb - \Delta M_B$ scatter plots and their projections
for candidate events.



Similar decay process via the $b \to c\bar{c}s$ transition and
 limited phase space has been found for 
$\bp \to \Lambda_c^+ \bar\Lambda_c^- K^+$ and $\bz \to \Lambda_c^+ 
\bar\Lambda_c^- K^0$~\cite{LcLcK}
 with a  $386 \times 10^6 B\bar{B}$ data sample.
The measured branching fractions are unexpectedly large: 
${\mathcal B}(\bp \to \Lambda_c^+ \bar\Lambda_c^- K^+) = 
(6.5^{+1.0}_{-0.9} \pm 1.1 \pm 3.4)\times 10^{-4}$ and 
${\mathcal B}(\bz \to \Lambda_c^+ \bar\Lambda_c^- K^0) = 
(7.9^{+2.9}_{-2.3} \pm 1.2 \pm 4.1)\times 10^{-4}$. 
Again this large rate can be understood by the threshold enhancement
phenomenon.
Observation of this kind
of decay is important for the determination of the charm particle yield
per $B$ decay. Decays like $B \to \Lambda_c^+ \bar\Lambda_c^- K$ would 
give a wrong-sign $\Lambda_c^+$, where for most cases only  
$\bar\Lambda_c^-$'s are present in the final state from $B$ decays. 


Along the doubly charmed baryon track, another two-body decay
has been found in the same data set. It's decay diagram is shown
in Fig.~\ref{fgScLc1} which is quite similar to that of $\bz \to 
\bar\Lambda_c^-p$. One only needs to replace the $c\bar{s}$ with $u\bar{d}$
and $d\bar{d}$ with $u\bar{u}$ to make the change. Therefore, one would
expect these two decay modes have similar branching fractions. However,
the measured  branching fraction product~\cite{XiLc} 
${\mathcal B}(\bp \to \bar{\Xi}^0_c  \Lambda_c^+) \times 
{\mathcal B}(\bar{\Xi}^0_c \to  \bar{\Xi}^+\pi^-) =
(4.8^{+1.0}_{-0.9} \pm 1.1 \pm 1.2)\times 10^{-4}$ is  too big. Assuming
${\mathcal B}(\bar{\Xi}^0_c \to \bar{\Xi}^+\pi^-)$ is at 1\% level, then
${\mathcal B}(\bp \to \bar{\Xi}^0_c  \Lambda_c^+) \sim 10^{-3}$. This is 
about 100
times bigger than that of $\bz \to \bar\Lambda_c^-p$. This is another example
of enhancement for smaller available energy in the baryon-antibaryon system.
Fig.~\ref{fgScLc2} shows the observed signals and hints for background with 
similar final state particles.

The charm counting problem of $B$ decays is a fundamental issue
to be solved. Using fully reconstructed $B$ sample (flavor tagging), one
can study the correlated ($b \to c$) and 
anti-correlated ($b \to \bar{c}$) charm production
 from other $B$ decays.
Presumably the correlated charm production is dominant and the anti-correlated
is suppressed with a phase space factor. Experimentally, one can determine
the correlated/anti-correlated charm production by studying the inclusive
$B$ decay rates to a limited charm hadrons, e.g. $\Lambda_c^+$, because
all other heavier charm particles decay into one of these special cases. 
Recently, BABAR used a $231  \times
10^6 B\bar{B}$ sample and made the measurement~\cite{taggedLc}.  The 
charm yield per $B$ decay is around 1.2.

\section{Summary}
The exclusive baryonic $B$ decays are well established 
after a few year's running
of the two b-factories. It seems that to understand the threshold enhancement 
is a key  for constructing the proper decay mechanism. The study 
of baryonic $B$ decays
is booming with fast accumulated data samples. Many unexpected things
like rare decays from both charm and beauty mesons, and even exotic states 
(e.g. pentaquark) could be the by-products.


\begin{figure}[htb]
\includegraphics[width=0.38\textwidth]{ScLc1.eps}
\caption{ The quark diagram for the $\bp \to \bar{\Xi}^0_c  \Lambda_c^+$ decay.
}
\label{fgScLc1}
\end{figure}




\begin{figure}[htb]
\includegraphics[width=0.48\textwidth]{ScLc2.eps}
\caption{ The (a) $\de$ and (b) $\mb$ distributions for 
the $\bp \to \bar{\Xi}^0_c  \Lambda_c^+$ candidates. The hatched histograms
show the combined $\bar{\Xi}^0_c$ and $\Lambda_c^+$ mass sidebands normalized to the signal region. The excess around low $\de$ region maybe be due to 
decays with extra final state particle, e.g. $\bar{\Xi}^0_c  
\Lambda_c^+ \pi^0$. The (c) $\bar{\Xi}^0_c$ and (d) $\Lambda_c^+$ mass 
distributions for candidates taken from the $B$-signal region. The overlaid 
curves are the fit results.
}
\label{fgScLc2}
\end{figure}






%\begin{figure}[htb]
%\includegraphics[width=0.48\textwidth]{incLc1.eps}
%\caption{ Charm (left) and anti-charm (right) mass spectra
%in the recoil of $\bp$ candidates, for the subsample of events
%with $\mb > 5.27$ GeV/$c^2$. The solid curve show the fit result. The dark
%shaded areas show the contribution of reconstructed $\Lambda_c^+$ and  
%$\Lambda_c^-$ signal in the recoil of combinatorial $\bp_{\rm rec'd}$ 
%background. The light shaded area corresponds to the fitted combinatorial 
%(anti-) charm background.   
%}
%\label{fgincLc1}
%\end{figure}


\begin{thebibliography}{9}


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 X.~Fu {\it et al.} (CLEO Collaboration),
 Phys. Rev. Lett. {\bf 79}, 3125 (1997).



\bibitem{BABAR}
B.~Aubert {\it et al.} (BABAR Collaboration),
Nucl. Instr. and Meth. A {\bf 479}, 1 (2002).


\bibitem{Belle}
A.~Abashian {\it et al.} (Belle Collaboration),
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\bibitem{PDG}
S.~Eidelman {\it et al.} (Particle Data Group), Phys. Lett. B {\bf
592}, 594 (2004).


\bibitem{belleppk}
 K.~Abe {\it et al.} (Belle Collaboration),
 Phys. Rev. Lett. {\bf 88}, 181803 (2002).

\bibitem{conjugate} {Throughout this report,
inclusion of charge conjugate mode is always implied unless
otherwise stated.}




\bibitem{plpi}
%M.Z.~Wang {\it et al.} (Belle Collaboration), 
%Phys. Rev. Lett. {\bf 90}, 201802 (2003);
M.Z.~Wang {\it et al.} (Belle Collaboration), 
Phys. Rev. Lett. {\bf 92}, 131801 (2004).
%Y.J.~Lee {\it et al.} (Belle Collaboration), 
%Phys. Rev. Lett. {\bf 93}, 211801 (2004);
%Y.J.~Lee {\it et al.} (Belle Collaboration),
%Phys. Rev. Lett. {\bf 95}, 061802 (2005).


\bibitem{HS} W.S. Hou and A. Soni,
 Phys. Rev. Lett. {\bf 86}, 4247 (2001).

%\bibitem{glueball}
%C.K.~Chua, W.S.~Hou and S.Y.~Tsai,
%Phys. Lett. B {\bf 544}, 139 (2002);
%A.~Datta and P.J.~O'Donnell, Phys. Lett. B {\bf 567}, 273
%(2003); J.L. Rosner, 
%Phys. Rev. D {\bf 68}, 014004 (2003);
%B.~Kerbikov, A.~Stavinsky, and V.~Fedotov, 
%Phys. Rev. C {\bf 69}, 055205 (2004).

\bibitem{polar}
M.Z.~Wang {\it et al.} (Belle Collaboration),
Phys. Lett. B {\bf 617}, 141 (2005).



\bibitem{BABARppk}
B.~Aubert {\it et al.} (BABAR Collaboration),
Phys. Rev. D {\bf 72}, 051101 (2005).



\bibitem{alpha}
C.H.~Wu {\it et al.} (Belle Collaboration), 
hep-ex/0606022.


\bibitem{Fang:2002gi}
  F.~Fang {\it et al.} (Belle Collaboration),
  Phys.\ Rev.\ Lett.\  {\bf 90}, 071801 (2003).


\bibitem{etactoBB}{
M.~Anselmino, F.~Caruso, S.~Forte and B.~Pire,
Phys.\ Rev.\ D {\bf 38}, 3516 (1988).
}

\bibitem{Lcp} N. Gabyshev {\it et al.} (Belle Collaboration),
 Phys. Rev. Lett. {\bf 90}, 141802 (2003).


%\bibitem{CLEOJX}
%R.~Balest {\it et al.} (CLEO Collaboration), 
%Phys. Rev. D {\bf 52}, 2661 (1995).


%\bibitem{theoJX}
%M. Beneke, G.A. Schuler, and S. Wolf, 
%Phys. Rev. D {\bf 62}, 034004 (2000).


\bibitem{BABARJLp}
B. Aubert {\it et al.} (BABAR Collaboration), 
Phys. Rev. Lett. {\bf 90}, 231 (2003).

\bibitem{JpsiLp}
Q.L.~Xie {\it et al.} (Belle Collaboration), 
Phys. Rev. D {\bf 72}, 051105 (2005).



\bibitem{LcLcK}
N.~Gabyshev {\it et al.} (Belle Collaboration), 
hep-ex/0508015.


\bibitem{XiLc}
R.~Chistov {\it et al.} (Belle Collaboration), 
hep-ex/0510074.



\bibitem{taggedLc}
B. Aubert {\it et al.} (BABAR Collaboration), 
hep-ex/0606026.










\end{thebibliography}
 

\end{document}
