Searches for New States

Figure: $M(D^*p)$ distribution from opposite-charge $D^*p$ combinations, compared with the results of a fit in which both signal and background components are included (solid line) and with the results of a fit in which only the background component is included (dashed line).

Since the year 2003 several experiments have reported the observation of a narrow resonance in the $K^+n$ channel at mass values around 1540 MeV [232]. The minimal constituent quark content of such a state is $uudd\bar{s}$. These measurements, supported by similar observations in the $K^0_S p$ channel [233], were interpreted as evidence for a pentaquark state. ZEUS performed searches for resonances decaying into $K^0_S p$ [53] and found a signal of around 1520 MeV which has a probability of being a statistical fluctuation of the background of $6 \cdot 10^{-6}$. No signal was found for the final state $\Xi\pi$ [54]. In analogy a charmed pentaquark state, with minimal quark contents $uudd\bar{c}$ as predicted in [234] might lead to an observable signal in the $D^*p$ invariant mass combination. H1 [51] and ZEUS [52] have investigated their data for signals and found different results.

H1 reports evidence for a resonance observed when studying the invariant mass combinations of $D^{*-}$ ($d\bar{c}$) with protons ($uud$) and the anti-matter equivalent, $D^{*+}$ mesons with anti-protons. A strong signal is observed at a mass of 3099 +/- 6 MeV, containing roughly equal contributions from $D^{*-}p$ and $D^{*+}\bar{p}$ combinations above a moderate background (fig.51). The resonance, corresponding to a minimal quark composition $uudd\bar{c}$ is produced along with other particles in DIS events. The fraction of events in the resonance peak compared to the number of $D^{*}$ events is about 1%. The probability that a background of $N_b=51.7$ events fluctuates to produce at least the number of events in the signal is $4 \cdot 10^{-8}$, assuming Poisson statistics. This number corresponds to 5.4 $\sigma$ when expressed as an equivalent number of Gaussian standard deviations.

ZEUS has performed a similar analysis and found no indication for a signal [52]. An upper limit on the fraction of $D^{*\pm}$ mesons originating from a resonance decay in the mass region around 3.1 GeV was calculated to be around $0.25\%$. There is hope that the apparent contradiction between the two experiments will be resolved with improved statistics and a more detailed analysis of the properties of the observed signal.