Pentaquark searches at FOCUS
\footnotesize Kevin Stenson\footnote{on behalf of the FOCUS Collaboration (http://www-focus.fnal.gov/)}
We find no evidence for high-energy photoproduction of pentaquarks at 1540\,MeV$\!/c^2$, 1862\,MeV$\!/c^2$, or 3099\,MeV$\!/c^2$ using decay modes $pK_S^0$, $\Xi^-\pi^-$, and $D^{(*)-}p$, respectively. %No evidence is found in any case so upper limits are placed.
focus_pentaquark.tex
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\begin{document}

\markboth{Kevin Stenson}
{Pentaquark searches at FOCUS}

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\title{Pentaquark searches at FOCUS}
\author{\footnotesize Kevin Stenson\footnote{on behalf of the FOCUS Collaboration (http://www-focus.fnal.gov/)}}
\address{Department of Physics, University of Colorado, Campus Box 390\\
Boulder, CO 80309, USA}

\maketitle

\begin{abstract}
We find no evidence for high-energy photoproduction of pentaquarks at 1540\,MeV$\!/c^2$, 1862\,MeV$\!/c^2$, 
or 3099\,MeV$\!/c^2$ using decay modes $pK_S^0$, $\Xi^-\pi^-$, and $D^{(*)-}p$, respectively.
%No evidence is found in any case so upper limits are placed.
\end{abstract}

\section{Introduction}

%Pentaquarks have been reported in many decay modes by many experiments.  
A 4--7\,$\sigma$ significant pentaquark with a mass of
$\sim$1540\,MeV$\!/c^2$ decaying to $pK_S^0$ or $nK^+$ 
%with a significance of 4--7\,$\sigma$ 
has been reported by ten
experiments\,$^{\textrm{1--10}}$.%~\cite{leps,diana,clas1,saphir,asratyan,clas2,hermes,svd,cosytof,zeus}.  
Combining the
mass measurements of these experiments we find $M = 1533.6 \pm 1.2$\,MeV$\!/c^2$. 
The $\chi^2/dof$ for this averaging is $38.2/9$ giving a confidence level of $1.6 \times 10^{-5}$, a 
5.2\,$\sigma$ problem.  
%The existence of the $\Theta(1540)^+$ is very much under active investigation.
Pentaquarks with two strange quarks\,\cite{na49} and with a charm quark\,\cite{h1} have also been reported.

FOCUS ran during the 1996--7 fixed-target run at Fermilab.  
A photon beam from brehmsstrahlung of a 300 GeV electron and positron beam
impacts BeO targets.  16 silicon strip planes provide vertexing and tracking.
Charged particles are tracked and momentum analyzed as they pass
through up to two dipole magnets and up to five sets of multiwire proportional chambers.  
Three \v{C}erenkov counters, two EM calorimeters, and two muon detectors identify particles.
A hadronic trigger requiring $\sim$25\,GeV of energy passed 7 billion events for reconstruction.
Thus, these events are well above threshold for pentaquark production.  Charge
conjugates are assumed for these analyses and all pentaquarks are assumed to decay strongly.

\section{Search for $\Theta(1540)^+\!\rightarrow\! p K_S^0$}
We search for $\Theta(1540)^+\!\rightarrow\!p K_S^0$ and measure the production 
relative to two similar decays, $K^*(892)^+\!\rightarrow\!K_S^0\pi^+$ and 
$\Sigma(1385)^\pm\!\rightarrow\!\Lambda^0\pi^\pm$.  The data is from 
events with a reconstructed $K_S^0\!\rightarrow\!\pi^+\pi^-$ or $\Lambda^0\!\rightarrow\!p\pi^-$\,\cite{focus_vee}. 
%the reconstruction technique is described in Ref.\,\cite{focus_vee}.  
%We require \v{C}erenkov identification of the fast track to separate $K_S^0$ and $\Lambda^0$.  
%We apply a variety of clean-up cuts and 
Selecting vee candidates within $2.5\,\sigma$ of the nominal mass we obtain
$63\times 10^6$ $K_S^0$ ($8 \times 10^6$ $\Lambda^0$)
candidates with 92\% (96\%) purity.  The remaining good 
quality tracks must form a good vertex (CL $>$ 1\%).%, suppressing charm decays and reinteractions.  
The proton candidate must pass stringent \v{C}erenkov ID cuts, reducing the misidentification rate to $\sim$0.
%While $K^*(892)^-$ and $\Sigma(1385)^\pm$ should be fit with a P-wave Breit Wigner, a 
The $K^*(892)^-$ and $\Sigma(1385)^\pm$ are fit with a simple Breit-Wigner plus background of
$aq^b \exp{\left(cq + dq^2 + eq^3 +fq^4\right)}$ where $q$ is the energy release.  
We find $(8.29 \pm 0.01)\times 10^6$ $K^*(892)^-$, 
$(92 \pm 2)\times 10^3$ $\Sigma(1385)^+$, and $(146 \pm 3)\times 10^3$ $\Sigma(1385)^-$ signal events.
No pentaquark evidence is seen in the the $pK_S^0$ mass plot (Fig.\,\ref{fig:mpks}).  We obtain 95\% CL 
limits on the yield by determing how much the fitted yield must be increased to change the 
log-likelihood by 1.92 (continually minimizing the background parameters).  
%We do this in 1\,MeV$\!/c^2$ steps from 1.51--1.56\,GeV$\!/c^2$.  
This procedure is performed with a Breit-Wigner width of $0$ and
15\,MeV$\!/c^2$, both with Gaussian resolution varying from 2.36--3.07\,MeV$\!/c^2$.  
The maximum limit on the yield over the mass range 1.51--1.56\,GeV$\!/c^2$  is 
754 (2252) for $\Gamma$ of 0 (15) MeV$\!/c^2$.  To set cross section limits we generate pentaquarks the same as 
$\Sigma(1385)^+$.  Using \textsc{Pythia} production and FOCUS MC simulation of $K^*(892)^-$, $\Sigma(1385)^\pm$, 
and $\Theta(1540)^+$ events we convert the yield limits into cross section ratio limits.
For $1.51$$<$$M$$<$1.56\,GeV$\!/c^2$, the 95\% CL limit on the production of $\Theta(1540)^+$ relative to combined
$\Sigma(1385)^+$ and $\Sigma(1385)^-$ is 0.7\% (2.1\%) for 
%,$\frac{\sigma(\Theta(1540)^+)}{\sigma(\Sigma(1385)^+) + \sigma(\Sigma(1385)^-)}$, 
$\Gamma$ of 0 (15) MeV$\!/c^2$.  Relative to $K^*(892)^-$, the limit is 0.06\% (0.17\%) for 
$\Gamma$ of 0 (15) MeV$\!/c^2$.  We account for all branching ratios and assume 
$B(\Theta(1540)^+\!\rightarrow\!pK_S^0) = 0.25$.

\begin{figure}
\centerline{\includegraphics[width=5.0in,height=1.61in]{pentafit_exp4.eps}}
\caption{Fit to $M(pK_S^0)$ in search for $\Theta(1540)^+\!\rightarrow\!pK_S^0$.}
\label{fig:mpks}
\end{figure}

%\begin{figure}
%\centerline{\includegraphics[width=5.1in,height=2.5in]{pentaxlim_paper.eps}}
%\caption{Fit to $M(pK_S^0)$.}
%\label{fig:pks_limit}
%\end{figure}

\section{Search for $\phi(1860)^{--}\!\rightarrow\! \Xi^- \pi^-$}

FOCUS reconstruction of $\Xi^- \!\rightarrow\! \Lambda^0\pi^-$ is described in 
Ref.\,\cite{focus_vee}.  We select 800,000 $\Xi^-$ candidates of which 75\% are signal.
We search for $\Xi(1530)^0\!\rightarrow\!\Xi^-\pi^+$ and the $S=-2$ pentaquark candidate
$\phi(1860)^{--}\!\rightarrow\!\Xi^-\pi^-$.  We require the production and $\Xi^-\pi^\pm$ vertices
have CL $\!>\!$ 1\% and separated by less than $2\sigma$.
The pion candidate must have a \v{C}erenkov signature consistent with a pion.
We find $59391 \pm 536$ $\Xi(1530)^0$ events
and no evidence for $\phi(1860)^{--}$ as shown in Fig.\,\ref{fig:xiplots}.  The yield 
upper limit calculated at a mass of $1.862$\,GeV$\!/c^2$ is 114 (170) for $\Gamma$ of 0 (15) MeV$\!/c^2$ with
resolution $\sigma = 6.05$\,MeV$\!/c^2$.  Assuming production like $\Xi(1530)^0$ we
find $\frac{\sigma(\phi(1860)^{--}) \times B(\phi(1860)^{--}\!\rightarrow\!\Xi^-\pi^-)}{\sigma(\Xi(1530)^0)} < 0.25\%\: (0.37\%)$ at
95\% CL for $\Gamma$ of 0 (15) MeV$\!/c^2$.

\begin{figure}
\centerline{\includegraphics[width=2.48in,height=1.8in]{xi1530fit_exp4_edepp.eps}
\includegraphics[width=2.48in,height=1.8in]{xi1862fit_exp4_einds.eps}}
\caption{Fits to $M(\Xi^-\pi^+)$ (left) for $\Xi(1530)^0$ and $M(\Xi^-\pi^-)$ for $\phi(1860)^{--}$.}
\label{fig:xiplots}
\end{figure}

\section{Search for $\Theta_c(3099)^0\!\rightarrow\! D^{(*)-} p$}

Using standard FOCUS charm reconstruction techniques we obtain a clean
sample of $D^{*-}\!\rightarrow\!\overline{D}{}^0\pi^-$ 
$(\overline{D}{}^0\!\rightarrow\!K^+\pi^-)$ events and $D^-\!\rightarrow\!K^+\pi^-\pi^-$ events (Fig.~\ref{fig:thetacplots}).
Combining $35821 \pm 202$ $D^{*-}$ and $83940 \pm 303$ $D^-$ candidates with a
positively identified proton we find no evidence for a charm pentaquark as shown in Fig.~\ref{fig:thetacplots}.

\begin{figure}
\centerline{\includegraphics[width=2.48in,height=1.32in]{apr19_dstar.eps}
\includegraphics[width=2.48in,height=1.32in]{apr19_dstar_penta.eps}}
\centerline{\includegraphics[width=2.48in,height=1.32in]{apr19_dplus.eps}
\includegraphics[width=2.48in,height=1.32in]{apr19_dplus_penta.eps}}
\caption{We add a proton to the $D^{*-}$ events and $D^-$ events (left) to search for $\Theta_c(3099)$ (right).}
\label{fig:thetacplots}
\end{figure}


\begin{thebibliography}{0}
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%[arXiv:hep-ex/0301020]
\bibitem{diana} V.~V.~Barmin {\it et al.} [DIANA Collaboration], \PAN(66,1715,2003).
%[arXiv:hep-ex/0304040]
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%[arXiv:hep-ex/0307018]
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%\bibitem{asratyan} A.~E.~Asratyan, A.~G.~Dolgolenko and M.~A.~Kubantsev, \PAN(67,682,2004).
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\bibitem{clas2} V.~Kubarovsky {\it et al.}  [CLAS Collaboration], \PRL(92,032001,2004).
%[Erratum-ibid.\  {\bf 92}, 049902 (2004)].
%[arXiv:hep-ex/0311046]
\bibitem{hermes} A.~Airapetian {\it et al.}  [HERMES Collaboration], \PL(B585,213,2004).
%[arXiv:hep-ex/0312044]
\bibitem{svd} A.~Aleev {\it et al.}  [SVD Collaboration], arXiv:hep-ex/0401024
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%[arXiv:hep-ex/0403011]
%\bibitem{aslanyan} P.~Z.~Aslanyan, V.~N.~Emelyanenko and G.~G.~Rikhkvitzkaya, arXiv:hep-ex/0403044.
\bibitem{zeus} S.~Chekanov {\it et al.}  [ZEUS Collaboration], \PL(B591,7,2004)
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%[arXiv:hep-ex/0310014]
\bibitem{h1} A.~Aktas {\it et al.}  [H1 Collaboration], \PL(B588,17,2004).
%[arXiv:hep-ex/0403017]
\bibitem{focus_vee} J.~M.~Link {\it et al.}  [FOCUS Collaboration], \NIM(A484,174,2002).
%[arXiv:hep-ex/0109028]



\end{thebibliography}


\end{document}

hep-ex0412021.extracted_bib
\begin{thebibliography}{0}
\bibitem{leps} T.~Nakano {\it et al.} [LEPS Collaboration], \PRL(91,012002,2003).
%[arXiv:hep-ex/0301020]
\bibitem{diana} V.~V.~Barmin {\it et al.} [DIANA Collaboration], \PAN(66,1715,2003).
%[arXiv:hep-ex/0304040]
\bibitem{clas1} S.~Stepanyan {\it et al.}  [CLAS Collaboration], \PRL(91,252001,2003).
%[arXiv:hep-ex/0307018]
\bibitem{saphir} J.~Barth {\it et al.}  [SAPHIR Collaboration], \PL(B572,127,2003).
%[arXiv:hep-ex/0307083]
%\bibitem{asratyan} A.~E.~Asratyan, A.~G.~Dolgolenko and M.~A.~Kubantsev, \PAN(67,682,2004).
\bibitem{asratyan} A.~E.~Asratyan, {\it et al.}, \PAN(67,682,2004).
%[arXiv:hep-ex/0309042]
\bibitem{clas2} V.~Kubarovsky {\it et al.}  [CLAS Collaboration], \PRL(92,032001,2004).
%[Erratum-ibid.\  {\bf 92}, 049902 (2004)].
%[arXiv:hep-ex/0311046]
\bibitem{hermes} A.~Airapetian {\it et al.}  [HERMES Collaboration], \PL(B585,213,2004).
%[arXiv:hep-ex/0312044]
\bibitem{svd} A.~Aleev {\it et al.}  [SVD Collaboration], arXiv:hep-ex/0401024
\bibitem{cosytof} M.~Abdel-Bary {\it et al.}  [COSY-TOF Collaboration], \PL(B595,127,2004).
%[arXiv:hep-ex/0403011]
%\bibitem{aslanyan} P.~Z.~Aslanyan, V.~N.~Emelyanenko and G.~G.~Rikhkvitzkaya, arXiv:hep-ex/0403044.
\bibitem{zeus} S.~Chekanov {\it et al.}  [ZEUS Collaboration], \PL(B591,7,2004)
%[arXiv:hep-ex/0403051]
\bibitem{na49} C.~Alt {\it et al.}  [NA49 Collaboration],\PRL(92,042003,2004).
%[arXiv:hep-ex/0310014]
\bibitem{h1} A.~Aktas {\it et al.}  [H1 Collaboration], \PL(B588,17,2004).
%[arXiv:hep-ex/0403017]
\bibitem{focus_vee} J.~M.~Link {\it et al.}  [FOCUS Collaboration], \NIM(A484,174,2002).
%[arXiv:hep-ex/0109028]



\end{thebibliography}