Spectroscopy and pentaquarks at HERA
Leonid Gladilin
(on behalf of the H1 and ZEUS Collaborations)
Results of the H1 and ZEUS Collaborations on spectroscopy of light and charmed mesons and on pentaquark searches, obtained using the HERA I data, are summarised.
gladilin_proc.tex
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\title{Spectroscopy and pentaquarks at HERA}

%\classification{<Replace this text with PACS numbers; choose from this list:
%                \texttt{http://www.aip..org/pacs/index.html}>}
\classification{13.60.Le, 13.60.Rj, 14.20.Lq, 14.40.Lb}

\keywords      {spectroscopy, pentaquarks}

\author{Leonid Gladilin\\
(on behalf of the H1 and ZEUS Collaborations)
}{
address={
DESY, ZEUS experiment, Notkestr. 85, 22607 Hamburg, Germany \\
E-mail: gladilin@mail.desy.de
}
,address={On leave from Moscow State University,
supported by the U.S.-Israel BSF} % additional visiting address
}

\begin{abstract}

Results of the H1 and ZEUS Collaborations on
spectroscopy
of light and charmed mesons and on
pentaquark searches,
obtained using the HERA I data,
are summarised.

\end{abstract}

\maketitle

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\section{Introduction}

Light and charmed hadrons are produced copiously
in $ep$ collisions with a centre-of-mass energy of $318\,$GeV at HERA.
During first phase of HERA operation (1992-2000),
the H1 and ZEUS Collaborations accumulated data samples corresponding
to $\sim120\,$pb$^{-1}$ each.
The H1 and ZEUS
results on hadron spectroscopy and pentaquark searches are summarised
in this note.

\section{Spectroscopy of light and charmed mesons}

Inclusive photoproduction of $\eta$, $\rho^0$, $f_0(980)$ and $f_2(1270)$
mesons was measured at an average photon-proton centre-of-mass energy
$W=210\,$GeV~\cite{h1_f0f2}.
The differential cross sections for those mesons and for charged pions
as a function of $p_T+m$, where m is the meson's nominal mass, show
similar power-law behaviour. The results suggest a similar
mechanism of the mesons production in fragmentation processes.

Measurement of inclusive $K^0_s K^0_s$ production in deep inelastic
scattering (DIS) revealed a state at $1537\,$MeV,
consistent with $f^\prime_2(1525)$, and another
at $1726\,$MeV~\cite{zeus_f2f0}.
The state at $1726\,$MeV has a mass consistent with $f_0(1710)$,
and is found in a gluon-rich region of phase space.
This observation indicates that $f_0(1710)$ has
a sizeable gluonic component.

The production of excited charmed and charmed-strange mesons
was studied
using their decays to final states involving $D^{*\pm}$~\cite{zeus_dexc}.
The measured rates of $c$ quarks hadronising as $D^0_1$, $D^{*0}_2$ and
$D^\pm_{s1}$ mesons agree
with those obtained in $e^+e^-$ annihilations.
% with previous measurements. 
The measured value
of the helicity parameter for $D^\pm_{s1}$ mesons
is consistent with the observation of the
CLEO Collaboration that the spin-parity of the $D^\pm_{s1}$ is $1^+$.
A search for the radially excited $D^{*\prime\pm}$ meson revealed no signal.
The upper limit on
the product of the fraction of $c$ quarks hadronising as
a $D^{*\prime +}$ meson
and the branching ratio of the $D^{*\prime +}$ decay to
$D^{*+}\pi^+\pi^-$
was estimated to be $0.7\%$ ($95\%$ C.L.).

\section{Strange pentaquarks}

\begin{figure}
  \includegraphics[height=.32\textheight]{theta_sig_fit_2g.eps}
\hspace*{-0.3cm}
  \includegraphics[height=.32\textheight]{ksi_sig_q2_1.eps}
  \caption{
Invariant mass spectrum for the $K^0_s p ({\bar{p}})$ combinations
for $Q^2>20\,$GeV$^2$ (left) and for $\Xi \pi$ combinations for
$Q^2>1\,$GeV$^2$ (right)
}
\end{figure}

A peak in the $K^0_s p ({\bar{p}})$ invariant mass spectrum around $1520\,$MeV
was observed in DIS by the ZEUS Collaboration~\cite{zeus_theta}.
In Fig.~1(left), the spectrum is shown for
exchanged photon virtuality
$Q^2>20\,$GeV$^2$.
The statistical significance of the signal varies between 3.9$\sigma$
and 4.6$\sigma$ depending upon the treatment of the background.
The signal is seen in both $K^0_s p$ and $K^0_s {\bar{p}}$ samples.
If the signal corresponds to the pentaquark
$\Theta^+$, this provides the first evidence for an anti-pentaquark
with a quark content ${\bar u}{\bar u}{\bar d}{\bar d}s$.
A ratio of the $\Theta^+$ and $\Lambda^0$ production cross sections 
was measured to be
$4.2\pm0.9({\rm stat.})^{+1.2}_{-0.9}({\rm syst.})\%$~\cite{zeus_thexs}.

The ZEUS Collaboration performed also a search
for two other pentaquarks, reported by the NA49 Collaboration, and
observed no signal in the $\Xi \pi$ invariant mass spectrum~\cite{zeus_xi}.
In Fig.~1(right), the spectrum is shown for $Q^2>1\,$GeV$^2$.
A clear peak with more than 160 $\Xi^0(1530)$ baryons indicates
that the statistical sensitivity of the search is similar
to that of the NA49 Collaboration.

\section{Charmed pentaquarks}

\begin{figure}
  \includegraphics[height=.46\textheight]{fig_76.eps}
\hspace*{-0.3cm}
  \includegraphics[height=.494\textheight]{fig7.eps}
  \caption{
The distributions of $M(D^{*\pm}p^\mp)$
obtained by the H1 Collaboration (left)
and by the ZEUS Collaborations (right)
}
\end{figure}

An observation of a candidate
for the charmed pentaquark state, $\Theta^0_c = uudd{\bar c}$,
decaying to $D^{*\pm}p^\mp$
was reported by the H1 Collaboration~\cite{h1_ch5q}.
Fig.~2(left) shows the $D^{*\pm}p^\mp$
invariant-mass distributions
in DIS with $Q^2>1\,$GeV$^2$ and in photoproduction
with smaller $Q^2$ values.
A fit of the signal in DIS yielded $50.6\pm11.2$ signal events and
the mass of $3099\pm3({\rm stat.})\pm5({\rm syst.})\,$MeV.
%and the Gaussian width of $12\pm3({\rm stat.})\,$MeV.
%compatible with the experimental resolution.
The observed resonance was reported to contribute
roughly $1\%$ of the $D^{*\pm}$ production rate
in the kinematic range studied in DIS.

The observation of the H1 Collaboration was challenged
by the ZEUS collaboration~\cite{zeus_ch5q}.
Using a larger sample of $D^{*\pm}$ mesons,
ZEUS observed no signature of the narrow resonance in
the $M(D^{*\pm}p^\mp)$ spectra shown in Fig.~2(right).
The Monte Carlo $\Theta^0_c$ signals normalised to $1\%$
of the number of reconstructed $D^{*\pm}$ mesons are shown
on top of the fitted backgrounds.
The upper limit on the fraction of $D^{*\pm}$ mesons
originating from $\Theta^0_c$ decays was evaluated to be 
$0.23\%$ ($95\%$ C.L.). The upper limit for DIS with $Q^2>1\,$GeV$^2$
is $0.35\%$ ($95\%$ C.L.).

\bibliographystyle{aipproc}   % if natbib is available
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\begin{thebibliography}{9}

\bibitem{h1_f0f2}
H1 Collab.,
%{\it Measurement of the Inclusive Photoproduction of
%$\eta$, $\rho^0$, $f_0(980)$ and $f_2(1270)$ Mesons at HERA}.
Abstract 6-0184,
International Conference on High Energy Physics,
Beijing, China (ICHEP 2004), August 2004.
Available from http://www-h1.desy.de/.
%M.~P. Brown,  and K.~Austin, \emph{The New Physique}, Publisher Name,
%  Publisher City, 2000, pp. 212--213.

\bibitem{zeus_f2f0}
ZEUS Collab., S.~Chekanov {\it et al.},
\emph{Phys. Lett.} \textbf{B 578} (2004) 33.

\bibitem{zeus_dexc}
ZEUS Collab.,
%{\it Production of P-wave charm mesons at HERA}.
Abstract 854,
International Conference on High Energy Physics,
Osaka, Japan (ICHEP 2000), July-August 2000.\\
ZEUS Collab.,
%{\it Production of the excited charm-strange
%meson $D^\pm_{s1}(2536)$ at HERA}.
Abstract 497,
International Europhysics Conference on High Energy Physics,
Budapest, Hungary (EPS 2001), July 2001.
Available from http://www-zeus.desy.de/.

\bibitem{zeus_theta}
ZEUS Collab., S.~Chekanov {\it et al.},
\emph{Phys. Lett.} \textbf{B 591} (2004) 7.

\bibitem{zeus_thexs}
ZEUS Collab,
%{\it Cross section measurements of a narrow baryonic state
%decaying to $K^0_s p$ and $K^0_s {\bar p}$
%in deep inelastic scattering at HERA}.
Abstract 10-0273,
International Conference on High Energy Physics,
Beijing, China (ICHEP 2004), August 2004.
Available from http://www-zeus.desy.de/.

\bibitem{zeus_xi}
ZEUS Collab,
%{\it Search for pentaquark baryons in the
%$\Xi^- \pi^-$ and $\Xi^- \pi^+$ channels at HERA}.
Abstract 10-0293,
International Conference on High Energy Physics,
Beijing, China (ICHEP 2004), August 2004.
Available from http://www-zeus.desy.de/.

\bibitem{h1_ch5q}
H1 Collab., C.~Atkas  {\it et al.},
\emph{Phys. Lett.} \textbf{B 591} (2004) 7.

\bibitem{zeus_ch5q}
ZEUS Collab., S.~Chekanov  {\it et al.},
DESY 04-164 (2004),
accepted by \emph{Eur. Phys. J.} \textbf{C}.
% - Ms No. EPJC040916.

\end{thebibliography}

\end{document}

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hep-ex0412014.extracted_bib
\begin{thebibliography}{9}

\bibitem{h1_f0f2}
H1 Collab.,
%{\it Measurement of the Inclusive Photoproduction of
%$\eta$, $\rho^0$, $f_0(980)$ and $f_2(1270)$ Mesons at HERA}.
Abstract 6-0184,
International Conference on High Energy Physics,
Beijing, China (ICHEP 2004), August 2004.
Available from http://www-h1.desy.de/.
%M.~P. Brown,  and K.~Austin, \emph{The New Physique}, Publisher Name,
%  Publisher City, 2000, pp. 212--213.

\bibitem{zeus_f2f0}
ZEUS Collab., S.~Chekanov {\it et al.},
\emph{Phys. Lett.} \textbf{B 578} (2004) 33.

\bibitem{zeus_dexc}
ZEUS Collab.,
%{\it Production of P-wave charm mesons at HERA}.
Abstract 854,
International Conference on High Energy Physics,
Osaka, Japan (ICHEP 2000), July-August 2000.\\
ZEUS Collab.,
%{\it Production of the excited charm-strange
%meson $D^\pm_{s1}(2536)$ at HERA}.
Abstract 497,
International Europhysics Conference on High Energy Physics,
Budapest, Hungary (EPS 2001), July 2001.
Available from http://www-zeus.desy.de/.

\bibitem{zeus_theta}
ZEUS Collab., S.~Chekanov {\it et al.},
\emph{Phys. Lett.} \textbf{B 591} (2004) 7.

\bibitem{zeus_thexs}
ZEUS Collab,
%{\it Cross section measurements of a narrow baryonic state
%decaying to $K^0_s p$ and $K^0_s {\bar p}$
%in deep inelastic scattering at HERA}.
Abstract 10-0273,
International Conference on High Energy Physics,
Beijing, China (ICHEP 2004), August 2004.
Available from http://www-zeus.desy.de/.

\bibitem{zeus_xi}
ZEUS Collab,
%{\it Search for pentaquark baryons in the
%$\Xi^- \pi^-$ and $\Xi^- \pi^+$ channels at HERA}.
Abstract 10-0293,
International Conference on High Energy Physics,
Beijing, China (ICHEP 2004), August 2004.
Available from http://www-zeus.desy.de/.

\bibitem{h1_ch5q}
H1 Collab., C.~Atkas  {\it et al.},
\emph{Phys. Lett.} \textbf{B 591} (2004) 7.

\bibitem{zeus_ch5q}
ZEUS Collab., S.~Chekanov  {\it et al.},
DESY 04-164 (2004),
accepted by \emph{Eur. Phys. J.} \textbf{C}.
% - Ms No. EPJC040916.

\end{thebibliography}