Evidence for a narrow resonance at 1530 MeV/c2 in the K0 p - system of the reaction pp -->Sigma+ K0 p from the COSY-TOF experiment

The hadronic reaction pp -->Sigma+ K0 p was measured exclusively at a beam momentum of 2.95 GeV/c using the TOF detector at the COSY storage ring. A narrow peak was observed in the invariant mass spectrum of the K0 p subsystem at 1530 +- 5 MeV/c2 with a significance of 4 - 6 standard deviations, depending on background assumptions. The upper limit of 18 +- 4 MeV/c2 (FWHM) for its width is given by the experimental resolution. The corresponding total cross section is estimated to be about 0.4 +- 0.1(stat) +- 0.1(syst) microbarn. Since a resonance in this subsystem must have strangeness S = + 1 we claim it to be the Theta+ state for which very recently evidence was found in various experiments.


Introduction
As presently understood, QCD does not forbid the existence of states other than quarkantiquark and three-quark systems as long as they form colour singlets. A priori, there should be no reason for a strong suppression of exotic states. In fact already in the early phase of QCD motivated models systems consisting of more than three quarks, in particular five quark systems have been discussed [1]. In more recent theoretical publications the possible existence of such pentaquark states has been worked out based on specific assumptions and production scenarios also including heavy quarks (e.g. [2,3] and others). One of the most cited publications by Diakonov, Petrov and Polyakov [4] is based on the soliton model assuming an antidecuplet as third rotational excitation in a three flavour system. The corners of this antidecuplet are occupied by exotic pentaquark states with the lightest state having a mass of ≈1530 MeV/c 2 , strangeness +1, spin 1/2 and isospin 0. This state, originally known as the Ζ + , has more recently been renamed Θ + . In this model the mass of the Θ + is fixed by the N * resonance at 1710 MeV/c 2 , which is assumed to be a member of the antidecuplet. The most striking property of the Θ + resonance is the predicted narrow width of Γ < 15 MeV/c 2 , which according to ref. [4] is connected with a narrow width of the 1710 MeV/c 2 N * resonance of 50 MeV or less. With the predicted quark content for the Θ + of s d d u u this pentaquark resonance is expected to decay into the channels n K + and p K 0 .
The first report on the discovery of a narrow resonance in the expected mass region came from the LEPS collaboration at SPring8 [5] where in the − K γ missing mass spectrum of the reaction n K K n − + → γ on C 12 a narrow resonance was observed at 1.54 ± 0.01 GeV/c 2 with a significance of 4.6 σ and an upper limit for the width of Γ = 25 MeV/c 2 . This was confirmed by the DIANA collaboration at ITEP [6] which observed a narrow resonance with a mass of 1539 ± 2 MeV/c 2 and a width of σ = 3 MeV/c 2 in the MeV/c 2 and a width of Γ < 21 MeV/c 2 and a narrow peak around 1555 MeV/c 2 from the reaction + − + → π γ K nK p [8]. A further γ-induced measurement of the reaction + → K K n p 0 γ has been reanalysed by the SAPHIR collaboration [9]. The observed mass of 1540 ± 4 ± 2 MeV/c 2 and the width of Γ < 25 MeV/c 2 is in agreement with the recent experiments.
Moreover evidence comes from neutrino-scattering on nuclei investigating the 0 S K p system at ITEP [10], where a peak was found at 1533 MeV/c 2 . Very recently positive results were also reported from the HERMES collaboration [11], where a narrow baryon state was found at 1528 MeV/c 2 in quasi-real photo production on a deuterium target in the decay channel − + → π π p K p S 0 , and from the SVD collaboration [12], where in the A p interaction a peak shows up at 1526 MeV/c 2 in the p K S 0 system.
In this paper we report on the search for the Θ + resonance using the COSY-TOF experiment.
Within the framework of the hyperon production program at COSY-TOF [13,14] the reaction  [15]. A deviation from a smooth invariant mass spectrum of the p K 0 system was already observed in a first measurement performed in 2000, but the extracted event sample was too small for a definitive statement [14]. To improve the statistical significance a second production run at the same beam momentum was performed in 2002, providing in addition the possibility of searching for systematic deviations between both independent measurements.
For the beam momenta of 3.2 GeV/c and 3.3 GeV/c which have been measured in addition the obtained event samples are too small to search for a resonance on a strong non resonant background. We want to mention that data were also taken of the reaction channel . This channel is in principle also suited for the θ + search. However, mainly due to the limited efficiency of the neutron detector used in this case the event samples are also too small for a detailed analysis of the spectra of interest.

Experimental setup and analysis
In the production runs in 2000 and 2002 reported in this paper the time-of-flight spectrometer COSY-TOF was used in its 3 m version [16]. The reaction is induced by focussing the extracted proton beam on a spot of about 1 mm ∅ on a liquid hydrogen target with a length of 4 mm [17]. The geometrical reconstruction of the related tracks and vertices is mainly realized by the start-detector system, a scheme of which is shown in Fig.1 together with an event of the type with a subsequent decay of the K 0 as a K S into a − + π π pair and the delayed decay of the Σ + into a + π n pair.    Both measurements show clear mass distributions peaking at the related corresponding masses of the Σ + ("primary mass") and K 0 ("secondary mass"), respectively, and they are identical within the statistical fluctuations as demonstrated in Fig. 2 and Fig. 3.
To get very clean samples for further investigations of the reaction of interest cuts on the resulting mass peaks have been applied. This is demonstrated in Fig. 4 where the spectra of the two runs are summed up. The upper and lower part show the sum of the spectra of Fig. 2 and Fig. 3. Applying the indicated cut (dashed lines) on the K 0 mass (actually between 480 MeV/c 2 and 510 MeV/c 2 ), a significant reduction of the width of the Σ + peak is obtained as can be seen in the middle part of Fig. 4 compared to the upper part. The same holds the other way round by cutting on the Σ + peak (dashed lines). Finally the cuts shown on the K 0 mass and the Σ + mass lead to two samples of 421 and 518 events for the two runs respectively, and accordingly 939 events for the total sample which is used for further analyses.

Results
To search for a possible resonance the data of the two runs at a beam momentum of p beam = 2.95 GeV/c have been investigated both separately and in sum. In Fig. 5   There is an obvious deviation from a smooth distribution in the spectra of both runs and in the spectrum of the summed samples (bottom) around 1.53 GeV/c 2 (indicated by the arrow in the summed spectrum).
Assuming a smooth background as obtained by a polynomial fit excluding the region between 1.51 GeV/c 2 and 1.54 GeV/c 2 (dashed curves in Fig. 5) the significance of the signal can be deduced. Following roughly the argumentation given in Ref. [11] three different expressions for the significance of the peak in the summed spectrum (Fig. 5 bottom)   To correct for the efficiency of the detector and the analysis, Monte Carlo simulations were used. The correction function is very smooth giving some enhancement at the edges of the phase space. In Fig. 7 the efficiency-corrected p K 0 invariant mass spectrum corresponding to the total sample is shown. In comparison to the uncorrected spectrum shown in Fig. 5 there is no major difference. Again there is a significant peak around 1.53 GeV/c 2 on top of a smooth background. For a more quantitative analysis a polynomial fit on the background and a Gaussian for the remaining signal are used (Fig.7 dotted lines). This yields a peak value of 1530 ± 5 MeV/c 2 . The deduced width of 18 ± 4 MeV/c 2 (FWHM) is in agreement with the value of the Monte Carlo analysis and accordingly only an upper limit for the physical width of the observed peak can be quoted.
The cross section of the observed peak around 1530 MeV/c 2 has been estimated by comparing with the measured total cross section of the reaction. The normalisation was deduced by comparison with the elastic p p scattering which was measured simultaneously and for which precise data are available from the COSY-EDDA experiment [19]. For the observed peak at 1530 MeV/c 2 we deduce a cross section of 0.4 ± 0.1 (stat.) ± 0.1 (sys.) b µ . This value is in rough agreement with theoretical estimations by Polyakov et al. [15] and Liu and Ko [20], where a total cross section in the order of 0.1 -1 b µ is predicted for the Θ + production in the threshold region in p p and n p induced reactions.

Summary and Outlook
The COSY-TOF experiment provides evidence for a narrow resonance in the p K 0 system at a mass of 1530 ±5 MeV/c 2 from the exclusively measured reaction The extracted width of about 18 MeV/c 2 reflects the experimental resolution. Since a resonance in this subsystem must have strangeness S = + 1 we claim it to be the + Θ state. The significance of the signal seen in the invariant mass spectrum of the p K 0 subsystem was deduced to be between 3.7 σ and 5.9 σ, depending on the background estimation. This is the first evidence on the Θ + resonance from an elementary hadron hadron induced reaction.
To get more insight into the nature of the Θ + resonance in an upcoming run the COSY-TOF experiment will use a polarized beam and in a further step also a deuterium target to investigate the reaction channel p K n p 0 Λ → for which our apparatus should be optimally suited. Moreover plans exist to use a polarized beam in combination with a polarized target to deduce the parity of this resonance [21].