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\begin{center}
  \begin{Large}
    \shadowbox{\bf{\Blue{Evidence for a Narrow Anti-Charmed Baryon State}}} \\
    \shadowbox{\bf{\Blue{The H1 Collaboration}}}
  \end{Large}
\end{center}

In this paper, the H1 collaboration reports the first evidence for a new
class of particle, a {\em charmed pentaquark}, which is a
bound state of two light up ($u$) flavoured quarks, 
two light down ($d$) flavoured quarks and a heavy anti-charm ($\bar{c}$) 
quark. The
evidence comes in the form of a narrow resonance (peak)
observed in the distribution of invariant
mass combinations of $D^*$ mesons and protons at a mass close to
$3100 \ {\rm MeV}$.

Quarks and antiquarks are bound together by the strong nuclear force,
which is described by the theory of 
{\em quantum chromodynamics} (QCD). 
The known bound states of quarks and antiquarks (hadrons) are classified 
by the {\em constituent quark model} into multiplets 
built from their quark flavour composition. The only allowed 
combinations are those in which the colour charges of the quarks and 
antiquarks combine to produce a hadron with no net colour. 
The simplest allowed states are 
baryons, consisting of three quarks, and mesons, consisting of a 
quark and an antiquark. Well over a hundred such states 
have been observed, of which
the most well known is the proton, a baryon composed of two up quarks and
a down quark ($uud$). For decades,
little evidence has been put forward for different, more exotic, 
quark and antiquark combinations, though such states are allowed by the quark 
model. Interest in searching for such exotic hadrons was revived
following a recent prediction using another QCD-based model, the 
{\em chiral soliton model}, that there should be a pentaquark that is 
meta-stable (i.e. that survives for a longer time before it decays than 
most ordinary hadrons) with a mass near $1530 \ {\rm MeV}$.
The long lifetime implies that a resonance produced by the pentaquark 
should be rather narrow.  
This prediction has been confirmed in the past year, with
several experiments reporting the observation of a narrow state
near to the predicted mass,
in $K^+ n$ or $K^0_s p$
invariant mass combinations.
Since this state decays strongly to a neutron (a $udd$ baryon) 
and a $K^+$ (an anti-strange $u \bar{s}$ meson) 
its minimal quark composition
is $uudd \bar{s}$. It has thus been interpreted
as a five quark state, named the $\theta^+$. 
The discovery of pentaquarks in the strange sector immediately raises
the question of whether similar states exist with the anti-strange 
quark replaced by an anti-charm quark, corresponding to the charmed 
analogue $\theta^0_c$ of the $\theta^+$. 

This paper reports the first
evidence for such a state, obtained by the H1 Collaboration looking at
the particles emerging from high energy inelastic collisions between 
electrons and protons at the HERA accelerator at DESY, Hamburg. 
HERA is the world's only colliding beam facility for electron-proton
scattering, providing a centre-of-mass energy of over 300 GeV. The H1 
Collaboration consists of around 350 physicists from 39 institutes in
13 countries, who built and now operate the H1 detector in order to
study many aspects of fundamental physics, including the 
{\em deep inelastic scattering} (DIS)
of electrons from the quarks within the proton, in order to measure
precisely the proton structure.

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The evidence for the new particle is a resonance observed
when studying the
invariant mass combinations of $D^{* \, -}$ 
anti-charmed mesons ($d \bar{c}$) with 
protons ($uud$) and the anti-matter equivalent, $D^{* \, +}$ mesons
($\bar{d} c$)
with anti-protons ($\bar{u} \bar{u} \bar{d}$). 
These hadrons are produced along with other particles 
in DIS events. 
The resulting invariant mass distribution of the $D^* p$ 
combinations is shown in the figure. A strong signal is observed at a
mass of about $3100 \ {\rm MeV}$, sitting on a moderate background. 
The resonance is remarkably narrow, in fact so narrow
that the true width could not be measured because it is smaller
than the experimental resolution. This is also the case for all experiments
which have observed the $\theta^+$ pentaquark.
The red curve in the figure shows the result of a fit to 
determine the mass of the resonance, which is measured to
be $3099 \pm 6 \ {\rm MeV}$. It is very unlikely indeed that the resonance is
produced by a statistical fluctuation in the background distribution. 
The fluctuation probability based on a cautious
estimate of the background (the blue curve in the figure) 
is $4 \cdot 10^{-8}$. The peak contains roughly equal contributions
from $D^{* \, -} p$ and $D^{* \, +} \bar{p}$ combinations. It survives
all reasonable variations in the selection criteria and many other
careful tests. A resonance 
with compatible mass and width is also observed in an independent 
H1 `photoproduction' data sample. 

There is thus strong evidence for the production of a new particle
in the H1 data. The decay to a $D^*$ and a proton implies that
the minimal quark composition is $uudd \bar{c}$. 
Not much more is yet known. Other experiments 
now have to look in their data to see whether they can confirm the 
observation. The exact interpretation 
of the state will depend on more
detailed future measurements and theoretical work. 
Is this the $\theta^0_c$, or an excited state
with spin 3/2 instead of spin 1/2, or something completely
different? If the state is confirmed, it is likely to be the first
step towards a whole new 
spectroscopy of charmed pentaquarks, 
which could lead to an improved understanding
of the forces that bind quarks together. 
It will be very exciting to watch how things develop and see what explanations
are put forward in the coming months.   

\section*{\Blue{Further Details}}

\underline{Paper submitted to Phys. Lett. {\bf B}} \\
\Red{
http://www-h1.desy.de/h1/www/publications/htmlsplit/DESY-04-038.long.html}

\noindent
\underline{H1 homepage} \\
\Red{http://www-h1.desy.de}

\noindent
\underline{DESY homepage} \\
\Red{http://www.desy.de/html/home/index.html}

\section*{\Blue{Contact Details}}
Please address questions to the H1 spokesman
Max Klein (\Red{klein@ifh.de}),
deputy spokesman Tim Greenshaw (\Red{green@mail.desy.de}) 
or physics coordinators Paul Newman (\Red{newmanpr@mail.desy.de}) 
and Vladimir Chekelian (\Red{shekeln@mail.desy.de}).

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