Limits for the central production of Theta+ and Xi(--)pentaquarks in 920-GeV pA collisions.
ABSTRACT We have searched for Theta+(1540) and Xi(--)(1862) pentaquark candidates in proton-induced reactions on C, Ti, and W targets at midrapidity and square root of s = 41.6 GeV. In 2 x 10(8) inelastic events we find no evidence for narrow (sigma approximately 5 MeV) signals in the Theta+ --> pK0(S) and Xi(--) --> Xi- pi- channels; our 95% C.L. upper limits (UL) for the inclusive production cross section times branching fraction B dsigma/dy/(y approximately 0) are (4-16) mub/N for a Theta+ mass between 1521 and 1555 MeV, and 2.5 mub/N for the Xi(--). The UL of the yield ratio of Theta+/Lambda(1520) < (3-12)% is significantly lower than model predictions. Our UL of B Xi(--)/Xi(1530)0 < 4% is at variance with the results that have provided the first evidence for the Xi(--).
arXiv:hep-ex/0408048v1 13 Aug 2004
Limits for the central production of Θ+and Ξ−−pentaquarks in 920 GeV pA collisions
I. Abt,23M. Adams,10M. Agari,13H. Albrecht,12A. Aleksandrov,29V. Amaral,8A. Amorim,8S. J. Aplin,12
V. Aushev,16Y. Bagaturia,12,36V. Balagura,22M. Bargiotti,6O. Barsukova,11J. Bastos,8J. Batista,8C. Bauer,13
Th. S. Bauer,1A. Belkov,11Ar. Belkov,11I. Belotelov,11A. Bertin,6B. Bobchenko,22M. B¨ ocker,26A. Bogatyrev,22
G. Bohm,29M. Br¨ auer,13M. Bruinsma,28,1M. Bruschi,6P. Buchholz,26T. Buran,24J. Carvalho,8
P. Conde,2,12C. Cruse,10M. Dam,9K. M. Danielsen,24M. Danilov,22S. De Castro,6H. Deppe,14
X. Dong,3H. B. Dreis,14V. Egorytchev,12K. Ehret,10F. Eisele,14D. Emeliyanov,12S. Essenov,22
L. Fabbri,6P. Faccioli,6M. Feuerstack-Raible,14J. Flammer,12B. Fominykh,22M. Funcke,10Ll. Garrido,2
B. Giacobbe,6J. Gl¨ aß,20D. Goloubkov,12,33Y. Golubkov,12,34A. Golutvin,22I. Golutvin,11I. Gorbounov,12,26
A. Goriˇ sek,17O. Gouchtchine,22D. C. Goulart,7S. Gradl,14W. Gradl,14F. Grimaldi,6Yu. Guilitsky,22,35
J. D. Hansen,9J. M. Hern´ andez,29W. Hofmann,13T. Hott,14W. Hulsbergen,1U. Husemann,26O. Igonkina,22
M. Ispiryan,15T. Jagla,13C. Jiang,3H. Kapitza,12S. Karabekyan,25N. Karpenko,11S. Keller,26J. Kessler,14
F. Khasanov,22Yu. Kiryushin,11K. T. Kn¨ opfle,13H. Kolanoski,5S. Korpar,21,17C. Krauss,14P. Kreuzer,12,19
P. Kriˇ zan,18,17D. Kr¨ ucker,5S. Kupper,17T. Kvaratskheliia,22A. Lanyov,11K. Lau,15B. Lewendel,12T. Lohse,5
B. Lomonosov,12,32R. M¨ anner,20S. Masciocchi,12I. Massa,6I. Matchikhilian,22G. Medin,5M. Medinnis,12
M. Mevius,12A. Michetti,12Yu. Mikhailov,22,35R. Mizuk,22R. Muresan,9M. zur Nedden,5M. Negodaev,12,32
M. N¨ orenberg,12S. Nowak,29M. T. N´ u˜ nez Pardo de Vera,12M. Ouchrif,28,1F. Ould-Saada,24C. Padilla,12
D. Peralta,2R. Pernack,25R. Pestotnik,17M. Piccinini,6M. A. Pleier,13M. Poli,31V. Popov,22A. Pose,29
D. Pose,11,14S. Prystupa,16V. Pugatch,16Y. Pylypchenko,24J. Pyrlik,15K. Reeves,13D. Reßing,12H. Rick,14
I. Riu,12P. Robmann,30I. Rostovtseva,22V. Rybnikov,12F. S´ anchez,13A. Sbrizzi,1M. Schmelling,13B. Schmidt,12
A. Schreiner,29H. Schr¨ oder,25A. J. Schwartz,7A. S. Schwarz,12B. Schwenninger,10B. Schwingenheuer,13
F. Sciacca,13N. Semprini-Cesari,6S. Shuvalov,22,5L. Silva,8K. Smirnov,29L. S¨ oz¨ uer,12S. Solunin,11A. Somov,12
S. Somov,12,33J. Spengler,13R. Spighi,6A. Spiridonov,29,22A. Stanovnik,18,17M. Stariˇ c,17C. Stegmann,5
H. S. Subramania,15M. Symalla,12,10I. Tikhomirov,22M. Titov,22I. Tsakov,27U. Uwer,14C. van Eldik,12,10
Yu. Vassiliev,16M. Villa,6A. Vitale,6I. Vukotic,5,29H. Wahlberg,28A. H. Walenta,26M. Walter,29J. J. Wang,4
D. Wegener,10U. Werthenbach,26H. Wolters,8R. Wurth,12A. Wurz,20Yu. Zaitsev,22M. Zavertyaev,13,32
G. Zech,26T. Zeuner,12,26A. Zhelezov,22Z. Zheng,3R. Zimmermann,25T.ˇZivko,17and A. Zoccoli6
1NIKHEF, 1009 DB Amsterdam, The Netherlands
2Department ECM, Faculty of Physics, University of Barcelona, E-08028 Barcelona, Spain
3Institute for High Energy Physics, Beijing 100039, P.R. China
4Institute of Engineering Physics, Tsinghua University, Beijing 100084, P.R. China
5Institut f¨ ur Physik, Humboldt-Universit¨ at zu Berlin, D-12489 Berlin, Germany
6Dipartimento di Fisica dell’ Universit` a di Bologna and INFN Sezione di Bologna, I-40126 Bologna, Italy
7Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221, USA
8LIP Coimbra, P-3004-516 Coimbra, Portugal
9Niels Bohr Institutet, DK 2100 Copenhagen, Denmark
10Institut f¨ ur Physik, Universit¨ at Dortmund, D-44221 Dortmund, Germany
11Joint Institute for Nuclear Research Dubna, 141980 Dubna, Moscow region, Russia
12DESY, D-22603 Hamburg, Germany
13Max-Planck-Institut f¨ ur Kernphysik, D-69117 Heidelberg, Germany
14Physikalisches Institut, Universit¨ at Heidelberg, D-69120 Heidelberg, Germany
15Department of Physics, University of Houston, Houston, TX 77204, USA
16Institute for Nuclear Research, Ukrainian Academy of Science, 03680 Kiev, Ukraine
17J. Stefan Institute, 1001 Ljubljana, Slovenia
18University of Ljubljana, 1001 Ljubljana, Slovenia
19University of California, Los Angeles, CA 90024, USA
20Lehrstuhl f¨ ur Informatik V, Universit¨ at Mannheim, D-68131 Mannheim, Germany
21University of Maribor, 2000 Maribor, Slovenia
22Institute of Theoretical and Experimental Physics, 117259 Moscow, Russia
23Max-Planck-Institut f¨ ur Physik, Werner-Heisenberg-Institut, D-80805 M¨ unchen, Germany
24Dept. of Physics, University of Oslo, N-0316 Oslo, Norway
25Fachbereich Physik, Universit¨ at Rostock, D-18051 Rostock, Germany
26Fachbereich Physik, Universit¨ at Siegen, D-57068 Siegen, Germany
27Institute for Nuclear Research, INRNE-BAS, Sofia, Bulgaria
28Universiteit Utrecht/NIKHEF, 3584 CB Utrecht, The Netherlands
29DESY, D-15738 Zeuthen, Germany
30Physik-Institut, Universit¨ at Z¨ urich, CH-8057 Z¨ urich, Switzerland
31visitor from Dipartimento di Energetica dell’ Universit` a di Firenze and INFN Sezione di Bologna, Italy
32visitor from P.N. Lebedev Physical Institute, 117924 Moscow B-333, Russia
33visitor from Moscow Physical Engineering Institute, 115409 Moscow, Russia
34visitor from Moscow State University, 119899 Moscow, Russia
35visitor from Institute for High Energy Physics, Protvino, Russia
36visitor from High Energy Physics Institute, 380086 Tbilisi, Georgia
(Dated: February 4, 2008)
We have searched for Θ+(1540) and Ξ−−(1862) pentaquark candidates in proton-induced reactions
on C, Ti and W targets at mid-rapidity and√s = 41.6 GeV. In 2 · 108inelastic events we find no
evidence for narrow (σ ≈ 5MeV/c2) signals in the Θ+→pK0
upper limits (UL) for the inclusive production cross section times branching fraction B·dσ/dy|y≈0are
3.7 and 2.5 µb/N. The UL of the yield ratio of Θ+/Λ(1520)<2.7% is significantly lower than model
predictions. Our UL of B·Ξ−−/Ξ(1530)0<4% is at variance with the results that have provided
first evidence for the Ξ−−signal.
Sand Ξ−−→Ξ−π−channels; our 95% CL
PACS numbers: 14.20.Jn, 13.85.Rm, 12.39-x, 12.40-y
Recent experimental evidence suggests not only that
pentaquarks (PQs), i.e. baryons with at least five con-
stituent quarks, exist but that their production in high
energy collisions is common.
covery of the Θ+PQ (uudd¯ s) at 1540 MeV/c2in the
γn→K−K+n process on carbon , more than 10 ex-
periments using incident beams of photons, electrons,
kaons, protons or (anti)-neutrinos have observed reso-
nances within ±20 MeV/c2of this mass in either the
nK+ or the pK0
S[3, 4, 5] decay channels; the measured
widths have all been consistent with the experimental res-
olutions ranging from 20 MeV/c2to 2 MeV/c2. The
Θ+interpretation is based on a prediction  of the chi-
ral soliton model (CSM) according to which the Θ+is
expected to have a mass of 1530 MeV/c2, a width of less
than 15 MeV/c2, and to decay into the KN channel. In
both the CSM and the correlated quark model , the Θ+
is a member of an antidecuplet with two further exotic
isospin 3/2 states of S = −2, the Ξ−−(ddss¯ u) and the
candidate resonances for both the Ξ−−and its neutral
isospin partner have been found in the Ξ−π−and Ξ−π+
final states at the mass of 1862MeV/c2. Theoretically,
PQs are not restricted to the strange sector, and exper-
imental evidence for an anti-charmed PQ, Θ0
with a mass of 3.1 GeV/c2has recently been reported
. In this context also earlier already ‘forgotten’ c¯ c PQ
candidates  have been recalled .
On the other hand, criticism addressed to some of the
reported PQ signals includes the problem of kinematic
reflections , of spurious states , and of low statis-
tics . Other puzzles include the surprisingly narrow
width of the Θ+, the large and systematic  spread
of measured Θ+masses, and the non-observation of the
cin an equivalent experiment . Hence, for estab-
lishing the existence and character of the new resonances,
high statistics mass spectra are needed as well as mea-
surements of spin, parity, width and cross sections. In
After the possible dis-
3/2(uuss¯d). In pp collisions at√s ≈ 18 GeV, narrow
TABLE I: Statistics and experimental resolutions σ of the
relevant signals (charge-conjugate modes indicated by c.c.).
addition, considering the results of high statistics studies
which have found neither the Θ+signal in ψ(2S) and J/ψ
hadronic decays  nor the Ξ−−signal in Σ−-induced
reactions on nuclear targets , the need for a thor-
ough understanding of the PQ production mechanism
has been emphasized . Benchmarks for PQ produc-
tion exist based on statistical hadronization models; they
typically predict particle ratios such as Θ+/Λ(1520) in
heavy ion [21, 22, 23] and pp [23, 24, 25] collisions. Tak-
ing advantage of a large data sample with good mass
resolution (see Table I) HERA-B can contribute signifi-
cantly to many of these topics. The simultaneous study
cays in proton-nucleus collisions at√s=41.6 GeV allows
a test of these theoretical predictions and a comparison
with earlier experimental results including the possible
first confirmation of the Ξ−−signal.
HERA-B is a fixed target experiment at the 920 GeV
proton storage ring of DESY. It is a forward magnetic
spectrometer with a large acceptance centered at mid-
rapidity (ycm ≈ 0), featuring a high-resolution vertex-
ing and tracking system and excellent particle identi-
fication . The present study is based on a sample
of 2 · 108minimum bias events which were recorded at
√s = 41.6 GeV using carbon (C), titanium (Ti) and
tungsten (W) wire targets during the 2002/03 run pe-
counts / 0.5 MeV/c2
Λπ- + Λ
counts / 4 MeV/c2
pK- + p
of a) Ξ−→Λπ−and¯Ξ+→¯Λπ+, and b) Λ(1520) →pK−and
¯Λ(1520)→ ¯ pK+.
Signals obtained with the C target from decays
con vertex detector, the main tracking system, the ring-
imaging Cherenkov counter (RICH), and the electromag-
netic calorimeter (ECAL) was used.
With standard techniques described in , signals
are identified above a small background without parti-
cle identification (PID) requirements. Similar clean sig-
nals from Ξ−→ Λπ−and c.c. decays (Fig. 1a) are ob-
tained by requesting the Λπ−vertex to be at least 2.5 cm
downstream of the target and the event to exhibit a cas-
cade topology: a further downstream Λ vertex and the
Ξ−pointing back to the target wire (impact parameter
b < 1 mm). Table I summarizes the statistics of these
signals, together with their measured mass resolutions
σ. These resolutions are about 20% larger than those of
the Monte Carlo (MC) simulation, while all mass values
agree within <1MeV/c2with the nominal masses. For
all particle selections, invariant masses are required to be
within ±3σ of the respective nominal mass.
For the search for Θ+→pK0
least one reconstructed primary vertex were selected.
The proton PID was provided by the RICH. The cut
in proton likelihood of > 0.95 implies a misidentification
probability of less than 1% in the selected momentum
range from 22 to 55 GeV/c . The Λ and¯Λ contam-
inations  were removed  in the K0
invariant mass spectrum of the pK0
Fig. 2a) for the p+C data. The solid line represents the
background determined from event mixing after normal-
ization to the data. The spectrum exhibits a smooth
shape in the mass region from 1.45 to 1.7 GeV/c2. Using
the prescription of ref. , we have calculated from these
data upper limits at 95% confidence level, UL(95%), for
the inclusive production cross section of a narrow reso-
nance at mid-rapidity, B·dσ/dy|y≈0, (Fig. 2b); the ycm-
For this analysis the information from the sili-
S→ π+π−, Λ → pπ−and¯Λ → ¯ pπ+decays
Sdecays, events with at
Spairs is shown in
counts / 3 MeV/c2
p + C
counts / 5 MeV/c2
c) mult < 10
p + C
S mass, GeV/c2
d) mult < 10 + K-
FIG. 2: The pK0
the p+C collisions and the background estimate (continuous
line); b) deduced UL(95%) for the p+C inclusive cross section
at mid-rapidity; the dashed line shows our 95% CL sensitivity;
c,d) same as a) but requiring c) a track multiplicity of <10,
and d) in addition a K−particle in the event. The arrows
mark the masses of 1521, 1530 and 1555 MeV/c2.
Sinvariant mass distributions: a) data from
interval is ±0.3. The data have been fitted with a Gaus-
sian plus a background of fixed shape. The mean of the
Gaussian was varied in steps of 1 MeV/c2but fixed in
the fit; its width was fixed to the MC prediction multi-
plied by 1.2 and increased from 2.6 to 6.1 MeV/c2over
the considered range. At the Θ+mass, the width was
3.9 MeV/c2. The reconstruction efficiencies have been
determined by MC simulations assuming a flat rapid-
ity distribution and a p2
tdistribution proportional to
exp(−B · p2
an atomic mass dependence of A0.7for the production
cross section, the UL(95%) of B·dσ/dy varies from 3
to 22 µb/nucleon (N) for a Θ+mass between 1521 and
1555 MeV/c2. A systematic error of 14% was taken into
account. For the Θ+mass of 1530 MeV/c2(about the av-
erage of the mass values observed in the pK0
), our limit is B·dσ/dy< 3.7 µb/N. The ULs from all
target data are within ±30% of these values.
Further search strategies were tried including i) a cut
t) with B = 2.1(GeV/c)−2. Assuming
on the track multiplicity of the event (Fig. 2c) which
would otherwise peak at ≈ 13, ii) the request of a tag-
ging particle such as a Λ, Σ or K−in the event, or iii)
both conditions (Fig. 2d). None yielded a statistically
significant structure in Θ+mass region. Also, the effect
of lowering the cut on the RICH proton likelihood and the
corresponding increase of the proton momentum accep-
tance has been systematically studied without yielding
a Θ+signal. On the other hand, as shown in Fig. 1b,
when the same proton PID requirement used to produce
Fig. 2 is applied to pK−candidates, a strong Λ(1520)
signal results, further demonstrating the capabilities of
the RICH. The cut in the K−likelihood of > 0.95 im-
plies a selection of kaon momenta from 12 to 55 GeV/c.
With the same cut on the K0
branching ratio of B(Θ+→pK0
the particle ratio Θ+(1530)/Λ(1520) at ycm≈ 0 is 2.7%.
Both doubly-charged and neutral Ξ3/2PQ candidates
as well as their anti-particles have been searched for in
the Ξπ channels.The pion candidates were required
to originate from the primary vertex. The background
was further reduced by weak cuts on the PIDs from
the ECAL and RICH which eliminated all the tracks
with a positive electron, proton, or kaon PID. The his-
tograms of Fig. 3a) show the resulting Ξπ invariant
mass spectra obtained from the C target data.
smooth lines are the background estimates from event-
mixing normalized to the data.
nels the Ξ(1530)0resonance shows up as a prominent
signal of ≈ 103events (see Table I).
width (≈9.5 MeV/c2) of the Ξ(1530)0agrees with MC
simulations which imply an experimental resolution of
2.9 MeV/c2. None of these mass spectra shows evidence
for the narrow, less than 18 MeV/c2(FWHM) wide PQ
candidates at 1862 MeV/c2reported by the NA49 col-
laboration  nor for any other narrow state at masses
between 1.6 and 2.3 GeV/c2. Fig. 3b) shows the sum
of the four spectra of Fig. 3a) after background subtrac-
tion and can be compared directly to Fig. 3 of ref. .
The corresponding ULs(95%) of the production cross sec-
tions B·dσ/dy|y≈0 per carbon nucleus at mid-rapidity
(Fig. 3c) have been obtained in the same way as de-
scribed above for the pK0
Schannel; here the ycm-interval
is ±0.7, the experimental resolution increases from 2.9
to 10.6 MeV/c2in the considered mass range, and is
6.6 MeV/c2at 1862 MeV/c2. At this Ξ−π−mass, the
UL(95%) of B·dσ/dy is 2.5 µb/N; the corresponding lim-
its in the Ξ−π+,¯Ξ+π+, and¯Ξ+π−channels are 2.3, 0.85,
and 3.1 µb/N. With an A0.7dependence, the ULs from
all targets are 2.7, 3.2, 0.94, and 3.1 µb/N, respectively.
Table II lists our ULs(95%) of various relative yields
for the Θ+and Ξ−−. Reference states for the Θ+are
the Λ and the Λ(1520), and for the Ξ−−, the Ξ−and
the Ξ(1530)0.The Θ+and Ξ−−widths are assumed
to be equal to our experimental mass resolution and
their momentum distributions are assumed to be the
Smomenta, and assuming a
S)=0.25, the UL(95%) of
In the neutral chan-
counts / 3 MeV/c2
– + π-
– + π+
1.41.6 1.82 2.2
mass, GeV/ c2
the p+C collisions in indicated neutral and doubly-charged
channels and the background estimates (continuous lines); b)
sum of all four Ξπ spectra with the background subtracted,
and c) deduced UL(95%) for the p+C inclusive cross section
at mid-rapidity. The dashed line shows our 95% CL sensitiv-
ity. The arrows mark the mass of 1862 MeV/c2.
The Ξπ invariant mass distributions: a) data from
same as those of the reference states.
also predictions of various statistical hadronization mod-
els for the respective ratios.
tios show no significant variation between 17 <√s <
40 GeV, nor is there a significant difference between pre-
dictions for pp and AA collisions. We find our UL for
Θ+/Λ(1520)<2.7% to be more than one order of mag-
nitude lower than the model predictions. Also, the UL
of Θ+/Λ< 0.92% is lower than all predictions including
the model which uses the Gribov-Regge approach for de-
scribing the Θ+production and its√s dependence in
pp collisions . Our UL of the Ξ−−/Ξ−yield ratio is
Table II lists
We note that these ra-
TABLE II: Our 95% CL upper limits on the relative yields
of Θ+(1530) and Ξ−−(1862) PQs at ycm ≈ 0 and predictions
for pp and AA collisions. For a Θ+mass of 1540 MeV/c2, the
quoted values have to be multiplied by ≈4.
pA, y≈ 0
compatible with the model predictions. No theoretical
value is yet available for the Ξ−−/Ξ(1530)0ratio, but
our UL of <4%/B should be compared with the value
from the NA49 experiment which, however, is not explic-
itly quoted in the original paper  which reports only
the number of 38 Ξ−−events. According to ref. , the
number of Ξ(1530)0events is about 150 leading to a yield
ratio  in contradiction to our UL unless the relative
efficiencies for Ξ(1530)0and Ξ−−of NA49 (unpublished)
differ markedly from those of HERA-B.
In conclusion, having found no evidence for narrow
Θ+and Ξ−−signals, we have set UL(95%) for the cen-
tral production cross sections of resonances in the pK0
and Ξ−π−final states with widths less than our experi-
mental resolution of ≈5MeV/c2. For the Θ+(1530) and
the Ξ−−(1862) the respective ULs of B·dσ/dy|y≈0 are
3.7 and 2.5 µb/N. For the Θ+candidate observed in pA
collisions at√s = 11.5 GeV, the total cross section for
xF ≥ 0 was estimated to be 30 to 120 µb/N . A de-
crease of the central Θ+production with increasing√s
could be understood if the Θ+is produced by disinte-
gration of forward/backward peaked remnants . On
the other hand, our UL(95%) for Θ+/Λ(1520)< 2.7% is
significantly lower than statistical hadronization predic-
tions which yield a ratio of ≥0.5 in agreement with exper-
iments in which the Θ+candidate and Λ(1520) showed
similar yields . Our UL(95%) of B·Ξ−−/Ξ−<3% is
not low enough to contradict the theoretical predictions.
It is, however, inconsistent with the previously-published
 observation of the Ξ−−(1862) at mid-rapidity which is
based on a data sample of lower statistics (1.6k v. 12kΞ−)
and comparable mass resolution (7.6 v. 6.6MeV/c2).
The collaborating institutions wish to thank DESY for
its support and kind hospitality. This work is supported
by NSRC (Denmark); BMBF, DFG, and MPRA (Ger-
many); INFN (Italy); FOM (The Netherlands); RC (Nor-
way); POCTI (Portugal); MIST (No.SS1722.2003, Rus-
sia); MESS (Slovenia); CICYT (Spain); SNF (Switzer-
land); NAS and MES (Ukraine); DOE and NSF (U.S.A.);
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11.5 GeV studying four different final states of the Θ+¯K0
system. No evidence for a narrow PQ peak is found in any
of the studied channels .
et al. (SAPHIR Coll.),