Photoproduction of pi0 omega off protons for E(gamma) < 3 GeV
EPJ manuscript No.
(will be inserted by the editor)
Photoproduction of π0ω off protons for Eγ ≤ 3GeV
The CB-ELSA Collaboration
J. Junkersfeld 1, A.V. Anisovich 1,2, G. Anton 3, R. Bantes 4, O. Bartholomy 1, R. Beck 1, Yu. Beloglazov 2,
R. Bogendörfer 3, R. Castelijns 5,a, V. Crede 1,6, A. Ehmanns 1, J. Ernst 1, I. Fabry 1, H. Flemming 8,b, A. Fösel 3,
M. Fuchs 1, Ch. Funke 1, R. Gothe 4,c, A. Gridnev 2, E. Gutz 1, St. Höffgen 4, I. Horn 1, J. Hößl 3, H. Kalinowsky 1,
F. Klein 4, E. Klempt 1, H. Koch 8, M. Konrad 4, B. Kopf 8, B. Krusche 9, J. Langheinrich 4,c, H. Löhner 5, I. Lopatin 2,
J. Lotz 1, H. Matthäy 8, D. Menze 4, J. Messchendorp 7,d, V.A. Nikonov 1,2, D. Novinski 2, M. Ostrick 4,e, A. Radkov 2,
A.V. Sarantsev 1,2, S. Schadmand 7,a, C. Schmidt 1, H. Schmieden 4, B. Schoch 4, G. Suft 3, V. Sumachev 2,
T. Szczepanek 1, H. van Pee 1,7, U. Thoma 1,7, D. Walther 4, and Ch. Weinheimer 1,f
1 Helmholtz-Institut für Strahlen- und Kernphysik, Universität Bonn, Germany
2 Petersburg Nuclear Physics Institute, Gatchina, Russia
3 Physikalisches Institut, Universität Erlangen, Germany
4 Physikalisches Institut, Universität Bonn, Germany
5 Kernfysisch Versneller Instituut, Groningen, The Netherlands
6 Department of Physics, Florida State University, Tallahassee, FL, USA
7 II. Physikalisches Institut, Universität Gießen, Germany
8 Institut für Experimentalphysik I, Universität Bochum, Germany
9 Institut für Physik, Universität Basel, Switzerland
a Present address: Institut für Kernphysik, Forschungszentrum Jülich, Germany
b Present address: GSI, Darmstadt, Germany
c Present address: University of South Carolina, Columbia, SC, USA
d Present address: Kernfysisch Versneller Instituut, Groningen, The Netherlands
e Present address: Institut für Kernphysik, Universität Mainz, Germany
f Present address: Institut für Kernphysik, Universität Münster, Germany
Received: August 15, 2019/ Revised version:
Abstract. Differential and total cross-sections for photoproduction of γp → pπ0ω and γp → ∆+ω were
determined from measurements of the CB-ELSA experiment, performed at the electron accelerator ELSA
in Bonn. The measurements covered the photon energy range from the production threshold up to 3GeV.
PACS. 13.30.-a Decays of baryons – 13.60.Le Meson production – 14.20.Gk Baryon resonances with S=0
1 Introduction
A large number of baryon resonances has been estab-
lished experimentally [1]. Below a mass of 1.8GeV/c2,
most of these states are well reproduced by constituent
quark models [2,3,4]. The models differ in details of the
predicted mass spectrum but have a common feature: above
1.8GeV/c2, they predict many more states than have been
seen experimentally. A natural explanation for these miss-
ing resonances is that they have escaped detection. The
majority of known non-strange baryon resonances stems
from πN scattering experiments. Model calculations show
that for some of these missing resonances only a small
coupling to πN is expected [5]. In elastic scattering, the
coupling to πN enters in the entrance and exit channel so
Correspondence to: klempt@hiskp.uni-bonn.de
these resonances contribute only very weakly. By contrast,
these resonances are predicted to have normal photo cou-
plings [6] and some of them should be observed in channels
like Nη, KΛ, KΣ [7], ∆η or ∆ω [8]. In comparison to the
Nπ final state, most of the above provide a distinctive
advantage: they act as isospin filters; only N∗ resonances
contribute to the Nη and KΛ final states while resonances
in ∆η, and ∆ω belong to the ∆∗ series.
A partial-wave analysis of various photoproduction data
suggested the existence of several new resonances [9]. The
analysis included data from CB-ELSA on π0 and η photo-
production [10,11], Mainz-TAPS data on η photoproduc-
tion [12], beam-asymmetry measurements of π0 and η [13,
14,15], data on γp → nπ+ [16] and from the compilation
of the SAID database [17], and data on photoproduction
http://arxiv.org/abs/0704.0710v1
2 J. Junkersfeld et al.: Photoproduction of π0ω off protons
Fig. 1. Contributions to ∆ω photoproduction: left, produc-
tion of ∆∗ intermediate states; right: production of ω mesons
via t-channel pion exchange.
of γp → K+Λ and γp → K+Σ0 from SAPHIR [18,19],
CLAS [20,21], and LEPS [22].
The reaction γp → pω is known to receive large contri-
butions from t-channel exchange processes [23,24]. A sim-
ilar mechanism may contribute also to ∆ω photoproduc-
tion: the incoming photon may couple to ωπ0, the virtual
π0 excites the nucleon to a ∆ and the ω escapes, pref-
erentially in forward direction. Fig. 1 shows a Feynman
diagram for this reaction mechanism and for the produc-
tion of a ∆ resonance decaying into ∆ω.
This paper reports on a measurement of differential
and total cross sections for the reaction
γp → pπ0ω , (1)
with ω → π0γ and the π0 detected in its two photon decay.
From this data the total cross-section for
γp → ∆+ω (2)
with the subsequent decays
∆+ → pπ0 and ω → π0γ
was extracted and compared to an earlier measurement
at higher energies [25]. The low statistics for reactions (1)
and (2) does not yet provide a sufficiently large data sam-
ple for a partial-wave analysis, but may serve as a guide
for what to expect from future experiments and is thus of
exploratory character.
2 Experimental setup
The experiment was performed at the Electron Stretcher
Accelerator ELSA [26] at the University of Bonn. Elec-
trons were extracted at an energy of 3.2GeV and brems-
strahlung was produced in a radiator foil with a thickness
of 3/1000 of a radiation length (Fig. 2). Electrons deflected
in a magnet were detected with a tagging system cover-
ing the photon energy range from 750 to 2970MeV. The
tagging system consisted of 14 thick scintillation counters
and two proportional wire chambers with a total of 352
wires. The scintillation counters were used to derive a fast
timing signal and the wire chambers to determine the pho-
ton energy. The γ energy resolution varied from 30MeV
dipole magnet beam
radiator
e  beam
g beam
Crystal Barrel
scifi detector
target
quadrupole
H  liquifier2
6,6 m
Fig. 2. Setup of the CB-ELSA experiment
at the lower end to 0.5MeV at the upper end of the spec-
trum not taking into account the energy distribution of
the electron beam of 3−5MeV [26]. This is well matched
with the overall resolution of the detector for this reac-
tion of 30MeV (FWHM) (see below). Typical rates were
1 − 3 × 106 photons/s. The photon beam hit a liquid H2
target of 5.3 cm length and 3 cm diameter. The absolute
normalisation was derived from a comparison of our dif-
ferential angular distributions for the reaction γp → pπ0
with the SAID model SM02. The normalisation uncer-
tainty was estimated to be 15% [10,27].
Charged reaction products were detected by a three-
layer scintillating fibre detector covering polar angles from
15◦ to 165◦ [28]. The outer layer was parallel to the beam
axis, the fibres of the other two layers were bent ±25◦
with respect to the first layer to allow for a spatial re-
construction of hits. Photons and charged particles were
detected in the Crystal Barrel detector [29], a calorimeter
consisting of 1380 CsI(Tl) crystals with photodiode read-
out, covering 98% of 4π solid angle. The detector with its
high granularity and energy resolution is excellently suited
for the detection of multi-photon final states.
Electromagnetic showers typically extended over up to
30 crystals in the calorimeter. Photons were reconstructed
with an energy resolution of σE/E = 2.5%/
E[GeV] and
an angular resolution of σθ,φ ≈ 1.1
◦. Hits due to charged
particles induce smaller clusters with typical 3 – 6 crystals.
A fast first-level trigger signal was derived from a coin-
cidence between a hit in the tagging system and a signal in
at least two out of three layers of the inner fibre detector.
The second-level trigger required a minimum number of
hits in the calorimeter. For part of the data the minimal
number of hits in the calorimeter was 2, otherwise at least
3 hits were requested, in order to reduce the dead-time.
Dead-time losses were below 70%, and below 20% for the
more restrictive trigger.
A more detailed description of the experimental setup
and the event reconstruction can be found in [27].
3 The reaction γp → pπ0ω
3.1 Event selection
The reaction γp → pπ0ω, ω → π0γ, leads to a final state
with five photons and a proton. The π0ω photoproduction
threshold is at Eγ = 1365MeV; a cut on a tagged photon
energyEγ > 1315MeV was applied right at the beginning.
J. Junkersfeld et al.: Photoproduction of π0ω off protons 3
 [MeV]
γ  0π
500 600 700 800 900 1000 1100 1200
 0.9 MeV± = 783.8  ωm
  83± = 2017  ωN
 [MeV]
γ  0π
500 600 700 800 900 1000 1100 1200
Fig. 3. ω signal in π0γ invariant mass
The first step in the analysis is the identification of the
five photons and the reconstruction of their energies and
directions. Protons with Ekin below ∼ 95MeV only pro-
duce a signal in the inner detector but not in the calorime-
ter. Hence in the analysis, events were selected with 5 or 6
hits in the Crystal Barrel calorimeter and 1 – 3 hits in the
inner detector. (A three–hit pattern can arise from three
single hits in each layer not crossing in a single point.) At
least two layers of the inner detector had to have a sig-
nal. For each pair of fibre and barrel hits it was tested if
the two vectors pointing from the target centre to a fibre–
detector-hit and to a Crystal-Barrel-hit form an angle of
20◦ or less; in this case the Crystal Barrel hit was identi-
fied as a proton, otherwise as a photon. The 20◦ matching
angle was chosen to allow for the extension of the target
and the uncertainties in the measurement. Events with
five photons were kept for further analysis. In case of 6
hits in the barrel, one of them had to match the proton
identification.
Surviving events were kinematically fitted to the hy-
pothesis γp → pmiss π
0π0γ with a missing proton, neglect-
ing identified charged hits and using all remaining photon
candidates. The kinematic fit assumed that the reaction
took place in the target centre. Since the momentum of
the proton is unknown and needs to be reconstructed, en-
ergy and momentum conservation give one constraint, the
π0 masses two constraints. A cut on a confidence level of
2% was applied, optimised to lose only few good events.
From the fit, the flight direction of the proton was deter-
mined and compared to hits in the inner detector. Again,
the direction of the missing proton and the direction to
a hit in the inner fibre detector had to form an angle of
±20◦ or less for the hit to be identified as proton.
The pπ0π0γ events were used to identify pπ0ω events
with ω decaying into π0γ. Fig. 3 shows the π0γ mass dis-
tribution with two entries per event. The fit using a Voigt
function (a Breit-Wigner convoluted with a Gaussian) im-
posing the ω width of Γ = 8.49MeV/c2 assigns about
2000 events to reaction (1). The ω mass was determined
to (783.8± 0.9stat± 1.0syst)MeV/c
2. The systematic error
was estimated from the comparison of η, η′, and ω masses
in different reactions with the PDG values. The mass res-
olution is determined to σ = 16MeV/c2.
 [MeV]γE
1500 2000 2500 3000
ω 0 π
 MCω 0πp 
 [MeV]γE
1500 2000 2500 3000
ω 0 π
0.005
0.015
 MC0π 0π 0πp 
Fig. 4. Acceptance of pπ0ω events (left) and the misidentifi-
cation probability of p 3π0 events (right).
) [MeV]γ0πm(
600 800 1000 12000
  26± = 253  ωN
600 800 1000 12000
 < 2382 MeVγ2256 < E
) [MeV]γ0πm(
600 800 1000 12000
  30± = 364  ωN
600 800 1000 12000
 < 2677 MeVγ2495 < E
Fig. 5. The ω signal with background. The background is
predicted in height and shape by simulations of p 3π0 (dark-
grey) and pπ0ω combinatorial background (light-grey).
The background in the π0γ distribution of pπ0π0γ
events has two main sources. A large fraction stems from
p 3π0 events. Fig. 4 shows that, in the energy region 2000 <
Eγ < 3000MeV, p 3π
0 events have a high probability to be
misidentified as pπ0ω. The misidentification probability is
only one order of magnitude smaller than the acceptance
for pπ0ω. However, the branching ratio of p 3π0 → p 6γ is
(96.44±0.09)% compared with (8.71±0.25)% for pπ0ω →
p 5γ. The cross-section for 3π0 photoproduction was esti-
mated using the cross-section for γp → pη [11], which
was determined from events with the η decaying into γγ
and 3 π0. The fractions of η and non-η events in the 3π0
event samples were determined and used to estimate the
cross-section of γp → p 3π0. Monte Carlo simulations were
performed using the p 3π0 cross-section estimate to deter-
mine the expected number of p 3π0 events surviving the
pπ0π0γ reconstruction. Fig. 5 shows for two photon en-
ergy ranges the predicted contribution of pπ0π0π0 events
to the background and the observed π0γ distribution.
The expected combinatorial background was determined
from the number of reconstructed pπ0ω events. It is shown
together with the p 3π0 part of the background in Fig. 5.
In the ω mass region, there is good agreement between the
simulated background distribution and the observed back-
ground. The study of simulated pπ0π0 and pπ0η events
shows misidentification probabilities of the order of 0.1%.
Their contributions were neglected.
The number of events due to reaction (1) in a given
energy range was determined by fitting the π0γ distribu-
tion using a Voigt function for the ω signal and a sec-
ond order polynomial for the background. The fit also re-
turned the number of background events below the peak.
For background subtraction, data histograms were filled
with events within the ω mass region (mω ± 40MeV/c
and background histograms with events falling into the
upper or lower sidebands (687 − 727MeV/c2 and 839 −
4 J. Junkersfeld et al.: Photoproduction of π0ω off protons
879MeV/c2, also shown in Fig. 3). The latter histograms
were scaled to contain the same number of events as found
in the background below the peak. For each energy and
angular region, the sideband histograms were subtracted
from the data histograms to extract the pπ0ω distribu-
tions. The same procedure was used to determine the pπ0ω
distributions for each energy region as function of the mo-
mentum transfer and the invariant mass respectively.
The acceptance was studied with a GEANT-based Mon-
te Carlo simulation using phase space distributed pπ0ω
and ∆+ω events. In the first iteration only pπ0ω Monte-
Carlo events were taken into account. cross-sections for
γp → pπ0ω and γp → ∆+ω were thus obtained, as will
be described in sections 3.3 and 3.4, and used to produce
Monte Carlo events with a realistic mixture of pπ0ω and
∆+ω events. This provides a more realistic acceptance
simulation. Stable results were achieved in the second iter-
ation. The simulated acceptance was different when only
events due to phase-space distributed pπ0ω events or ∆+ω
events were used for the simulation. The difference in the
acceptance was taken as a contribution to the systematic
error.
3.2 Differential cross-sections
The differential cross-sections were obtained from the side-
band subtracted histograms. We give in the centre of mass
system cross-sections differential in cos θω, cos θπ0 and |t−
tmin|,
dσ/dΩ(cos θω), dσ/dΩ(cos θπ0), dσ/dt (|t− tmin|),
respectively. Here t is the squared four-momentum trans-
fer from the photon beam to the pπ0 system given by
t = q2 = (pγ − pω)
and tmin is the minimal momentum transfer imposed by
kinematics.
Fig. 6 presents the differential cross-sections as a func-
tion of cos θω, in table they are given in numerical form.
The distributions are compatible with a description of the
(x) = a0 + a1 · e
a2x with x = cos θ. (4)
In the backward direction, the acceptance is small and the
errors large. The fit using Eq. (4) took into account only
data for which the acceptance ǫ was above 5% (thus re-
stricting the fit range to cos θω > −0.6 forEγ < 1800MeV,
and to cos θω > −0.8 for Eγ > 1800MeV), and was then
extrapolated to cover the full cos θω range. In the forward
direction, there is a strong increase in intensity, in partic-
ular at energies above 2GeV. Production of ω mesons via
t-channel exchange with simultaneous p → ∆(1232) ex-
citation seems to play an important role in the dynamics
of reaction (1).
From the cos θω distributions a total cross-section was
determined by summing over the measured values for which
the acceptance was above 5% and using extrapolated val-
ues in the remaining range.
0.8  < 1817 MeVγ1383 < E
0.8  < 2256 MeVγ2020 < E
 < 2495 MeVγ2382 < E
-1 -0.5 0 0.5 10
1.5  < 2845 MeVγ2677 < E
 < 2020 MeVγ1817 < E
 < 2382 MeVγ2256 < E
 < 2677 MeVγ2495 < E
ωθcos  
-0.5 0 0.5 1
 < 2970 MeVγ2845 < E
Fig. 6. Differential cross-sections dσ/dΩ(cos θω).
The differential cross-sections dσ/dΩ(cos θπ0) are shown
in Fig. 7 and listed numerically in 2. There are no ob-
vious structures beside some fluctuations in the forward
and backward regions. The data were fitted using a con-
stant. The fit was restricted to data points measured with
an acceptance of at least 5%, thus excluding for Eγ >
2380MeV the points with cos θπ0 > 0.8. From this dis-
tribution the total cross-section is derived from the data
points and the extrapolation was used for the points with
small acceptance.
Fig. 8 shows the differential cross-sections dσ/dt in
dependence of |t − tmin|, which are compatible with an
exponential behaviour in the low t region. This is charac-
teristic for production via t-channel exchange. The data
were fitted in the region below 0.8 · |tmax − tmin| (approx-
imately corresponding to ǫ > 5%) using
(|t− tmin|) = e
a+b|t−tmin| + c(E) (5)
where tmin is the minimum squared momentum transfer
imposed by kinematics. The non-t-dependent contribution
was described with a function c(E) = c0 + c1 · Eγ . The
parameters c0 and c1 were determined in a combined fit
of the differential cross sections.
The slope parameter b is shown in Fig. 9. The slope
is approximately constant over the covered energy range.
This indicates a strong contribution from ω production
via t-channel exchange processes.
J. Junkersfeld et al.: Photoproduction of π0ω off protons 5
Table 1. Differential cross-sections dσ/dΩ(cos θω). There is a common systematic error of 16%.
cos θω dσ/dΩ(cos θω) dσ/dΩ(cos θω) dσ/dΩ(cos θω) dσ/dΩ(cos θω)
[µb/sr] [µb/sr] [µb/sr] [µb/sr]
Eγ [MeV] 1383 - 1817 1817 - 2020 2020 - 2256 2256 - 2382
−1.00 −−0.80 0.16 ± 0.08 0.00 ± 0.13 0.29 ± 0.14 0.35 ± 0.23
−0.80 −−0.60 0.08 ± 0.06 0.05 ± 0.07 0.18 ± 0.09 0.23 ± 0.13
−0.60 −−0.40 0.04 ± 0.04 0.12 ± 0.06 0.15 ± 0.06 0.33 ± 0.12
−0.40 −−0.20 0.02 ± 0.03 0.03 ± 0.04 0.05 ± 0.04 0.22 ± 0.08
−0.20− 0.00 0.03 ± 0.02 0.17 ± 0.04 0.16 ± 0.05 0.10 ± 0.08
0.00 − 0.20 −0.02± 0.02 0.09 ± 0.05 0.17 ± 0.05 0.31 ± 0.09
0.20 − 0.40 0.06 ± 0.03 0.13 ± 0.04 0.22 ± 0.06 0.30 ± 0.08
0.40 − 0.60 0.03 ± 0.03 0.19 ± 0.06 0.33 ± 0.06 0.39 ± 0.11
0.60 − 0.80 0.04 ± 0.03 0.32 ± 0.06 0.46 ± 0.08 0.71 ± 0.13
0.80 − 1.00 0.11 ± 0.04 0.32 ± 0.07 0.75 ± 0.10 0.65 ± 0.17
Eγ [MeV] 2382 - 2495 2495 - 2677 2677 - 2845 2845 - 2970
−1.00 −−0.80 −0.10± 0.25 0.27 ± 0.18 0.70 ± 0.28 0.47 ± 0.23
−0.80 −−0.60 0.27 ± 0.14 0.44 ± 0.13 0.37 ± 0.16 0.42 ± 0.16
−0.60 −−0.40 0.44 ± 0.13 0.27 ± 0.09 0.15 ± 0.10 0.13 ± 0.12
−0.40 −−0.20 0.21 ± 0.10 0.17 ± 0.08 0.30 ± 0.09 0.25 ± 0.11
−0.20− 0.00 0.17 ± 0.09 0.09 ± 0.07 0.14 ± 0.09 0.25 ± 0.10
0.00 − 0.20 0.25 ± 0.09 0.28 ± 0.08 0.22 ± 0.08 0.17 ± 0.11
0.20 − 0.40 0.48 ± 0.11 0.26 ± 0.08 0.42 ± 0.10 0.32 ± 0.12
0.40 − 0.60 0.45 ± 0.13 0.42 ± 0.10 0.42 ± 0.11 0.51 ± 0.15
0.60 − 0.80 1.02 ± 0.18 0.57 ± 0.13 0.77 ± 0.18 0.99 ± 0.24
0.80 − 1.00 1.76 ± 0.27 1.23 ± 0.20 1.86 ± 0.28 1.79 ± 0.33
Table 2. Differential cross-sections dσ/dΩ(cos θ
). There is a common systematic error of 16%.
cos θ
dσ/dΩ(cos θ
) dσ/dΩ(cos θ
) dσ/dΩ(cos θ
) dσ/dΩ(cos θ
[µb/sr] [µb/sr] [µb/sr] [µb/sr]
Eγ [MeV] 1383 - 1817 1817 - 2020 2020 - 2256 2256 - 2382
−1.00−−0.80 0.05± 0.03 0.23 ± 0.07 0.30 ± 0.07 0.40± 0.11
−0.80−−0.60 0.05± 0.03 0.21 ± 0.05 0.32 ± 0.06 0.27± 0.09
−0.60−−0.40 0.07± 0.03 0.09 ± 0.05 0.29 ± 0.06 0.40± 0.11
−0.40−−0.20 0.04± 0.03 0.08 ± 0.05 0.27 ± 0.07 0.30± 0.11
−0.20− 0.00 −0.04± 0.03 0.17 ± 0.05 0.23 ± 0.06 0.42± 0.11
0.00 − 0.20 0.06± 0.03 0.17 ± 0.05 0.18 ± 0.06 0.17± 0.10
0.20 − 0.40 0.03± 0.03 0.11 ± 0.05 0.30 ± 0.06 0.40± 0.12
0.40 − 0.60 0.03± 0.03 0.16 ± 0.06 0.27 ± 0.07 0.32± 0.10
0.60 − 0.80 0.07± 0.04 0.18 ± 0.06 0.24 ± 0.07 0.33± 0.11
0.80 − 1.00 0.04± 0.04 0.18 ± 0.08 0.18 ± 0.10 0.64± 0.17
Eγ [MeV] 2382 - 2495 2495 - 2677 2677 - 2845 2845 - 2970
−1.00−−0.80 0.59± 0.14 0.67 ± 0.11 0.67 ± 0.14 0.61± 0.17
−0.80−−0.60 0.54± 0.12 0.29 ± 0.10 0.37 ± 0.10 0.50± 0.14
−0.60−−0.40 0.57± 0.13 0.37 ± 0.10 0.26 ± 0.11 0.32± 0.12
−0.40−−0.20 0.47± 0.13 0.31 ± 0.10 0.34 ± 0.12 0.23± 0.14
−0.20− 0.00 0.58± 0.13 0.46 ± 0.10 0.64 ± 0.12 0.43± 0.13
0.00 − 0.20 0.22± 0.12 0.12 ± 0.09 0.42 ± 0.12 0.49± 0.13
0.20 − 0.40 0.65± 0.14 0.39 ± 0.10 0.61 ± 0.12 0.60± 0.14
0.40 − 0.60 0.28± 0.12 0.29 ± 0.10 0.37 ± 0.13 0.58± 0.15
0.60 − 0.80 0.39± 0.15 0.39 ± 0.11 0.24 ± 0.12 0.09± 0.16
0.80 − 1.00 0.56± 0.27 0.15 ± 0.21 0.26 ± 0.16 0.35± 0.26
6 J. Junkersfeld et al.: Photoproduction of π0ω off protons
From these differential distributions the total cross-
section was obtained by integrating over function (5) from
tmin to tmax.
3.3 Total cross-section
The total cross-section was determined in three different
ways, by extrapolation and summation of the three types
of differential cross sections, dσ/dΩ(cos θω), dσ/dΩ(cos θπ0),
and dσ/dt (|t−tmin|) as described above. Statistical errors
of the total cross-sections were determined by error prop-
agation. As final result, the mean value of the total cross-
section and the mean statistical error are shown in Fig. 10
(left) as a function of the photon energy. The cross-section
rises with increasing photon energy, i. e. with the available
phase space.
A systematic uncertainty was derived from the spread
of the three different determinations of the total cross-
section, using data of Fig. 6, 7 and 8. A further error
of 5.7% was assigned to the Monte Carlo reconstruction
efficiency [30]. These contributions and the 15% normal-
isation error [27] were added in quadrature to yield the
total systematic error shown in Fig. 10.
3.4 The ∆+ω contribution to pπ0ω
Fig. 11 shows the differential cross-sections dσ/dm (pπ0),
which were used to disentangle the ∆+ω contribution to
 < 1817 MeVγ1383 < E
 < 2256 MeVγ2020 < E
1  < 2495 MeVγ2382 < E
-1 -0.5 0 0.5 10
1  < 2845 MeVγ2677 < E
 < 2020 MeVγ1817 < E
 < 2382 MeVγ2256 < E
 < 2677 MeVγ2495 < E
0πθcos  
-0.5 0 0.5 1
 < 2970 MeVγ2845 < E
Fig. 7. Differential cross-sections dσ/dΩ(cos θ
d -110
10  < 1817 MeVγ1383 < E
10  < 2256 MeVγ2020 < E
10  < 2495 MeVγ2382 < E
0 1 2
10  < 2845 MeVγ2677 < E
 < 2020 MeVγ1817 < E
 < 2382 MeVγ2256 < E
 < 2677 MeVγ2495 < E
]2| [(GeV/c)
|t - t
 < 2970 MeVγ2845 < E
Fig. 8. Differential cross-sections dσ/dt (|t − tmin|) of the
squared four-momentum transfer t to the pπ0 system.
 [MeV]γE
1500 2000 2500 3000
Fig. 9. Slope parameter of dσ/dt (|t− tmin|)
the total cross-section. The distributions show prominent
∆ signals. The ∆ peak was fitted by a phase space cor-
rected Breit-Wigner function (see e. g. [31] for details).
The non-resonant pπ0ω part was described by phase-space
distributed pπ0ω Monte Carlo events. Only the ampli-
tudes of the two contributions were left free in the fit.
The Breit-Wigner width of the ∆ was fixed to 120MeV/c2
the mass was fixed to 1232MeV/c2 for energies below
2500MeV and set to values between 1240 and 1250MeV/c2
for higher energies to improve the fit. With these two com-
ponents a good description of the ∆ peak and of the pπ0ω
J. Junkersfeld et al.: Photoproduction of π0ω off protons 7
phase space contribution to the differential cross-section
was achieved.
The Breit-Wigner distributions and the pπ0ω phase-
space contributions were integrated and their fractions de-
termined. The systematic uncertainty due to the disentan-
glement was estimated to 3− 10% and added in quadra-
ture to the systematic error. The cross-section for γp →
pπ0ω without ∆+ω contributions is shown in Fig. 10 (right).
The total cross-section of γp → ∆+ω was determined
from the observed fraction of ∆+ω events and the γp →
pπ0ω cross-section, taking into account the unseen ∆+ →
nπ+ decay mode. The resulting cross-section is shown in
Fig. 12 together with the results of the LAMP2 exper-
iment [25]. It is worthwhile to discuss how the LAMP2
cross-section was determined.
The LAMP2 experiment measured the reaction γp →
∆+ω by identifying ω mesons in their π+π−π0 decays. The
∆+ decay products were not observed. Instead, the ∆+
was identified in the missing mass spectrum of the γp →
ωX reaction. The missing mass distribution (Fig. 13) con-
tains signals for pω and ∆+ω production. The authors
give a 15% systematic uncertainty due to the difficulty in
disentangling the pω, pπ0ω and ∆+ω contributions.
In our analysis the fraction of pπ0 below the ∆ is sig-
nificant (see fig. 11) and larger than estimated by LAMP2.
Hence it seems possible that the LAMP2 cross-section is
overestimated.
The total cross-section for ∆+ω photoproduction in
Fig. 12 (see table 3) is consistent with a simple fit as-
suming a background amplitude in the form A · (E −
Ethreshold)
α/2 · (E − Eh)
β/2 (A, α, β, Ethreshold, Eh fit
parameters). The χ2 = 12.1 for NDoF = 6 corresponds to
an acceptable 6% probability.
4 Summary
We have studied the reaction γp → pπ0ω with ω → π0γ
from the ωπ0 production threshold up to 3GeV photon
energy using an unpolarised tagged photon beam and a
liquid hydrogen target. Differential cross-sections were de-
termined as functions of cos θω, cos θπ0 and |t− tmin|. The
distributions reveal strong contributions from isovector
 [MeV]γE
1500 2000 2500 3000
ω 0 π
 [MeV]γE
1500 2000 2500 3000
Fig. 10. Total cross-sections σ(γp → p π0ω) before (left) and
after (right) subtraction of the ∆+ω contribution. The grey
band represents the systematic uncertainty.
0.005
0.015
0.025
 < 1817 MeVγ1383 < E
0.04  < 2256 MeVγ2020 < E
0.04  < 2495 MeVγ2382 < E
1200 1400 1600 18000
0.04  < 2845 MeVγ2677 < E
 < 2020 MeVγ1817 < E
 < 2382 MeVγ2256 < E
 < 2677 MeVγ2495 < E
 [MeV]0πpm
1200 1400 1600 1800
 < 2970 MeVγ2845 < E
Fig. 11. Differential cross-sections dσ/dm (pπ0). They are fit-
ted with a combination of a Breit-Wigner (blue) and phase
space pπ0ω Monte Carlo events (red).
 [MeV]γE
2000 3000 4000 5000
 [MeV]γE
2000 3000 4000 5000
CB-ELSA
LAMP2
Fig. 12. Total cross-section σ(γp → ∆+ω), shown are the
data from this analysis and from the LAMP2 experiment [25].
The systematic errors are shown as an error band in light (CB-
ELSA) and dark grey (LAMP2). A fit to the data points is
shown, which is described in the text.
8 J. Junkersfeld et al.: Photoproduction of π0ω off protons
Fig. 13. Missing mass of the ω in the LAMP2 experiment
(from [25]). Note that the spectrum is dominated by pω events.
Table 3. Total cross-sections of γp → p π0ω (with and with-
out ∆+ω contribution) and γp → ∆+ω. The systematic error
is shown in Fig. 10 and 12.
Eγ σ(γp → pπ
0ω) σ(γp → pπ0ω) σ(γp → ∆+ω)
[MeV] [µb] (no ∆+ω) [µb] [µb]
1383− 1817 0.49 ± 0.13 0.35 ± 0.13 0.21 ± 0.17
1817− 2020 1.95 ± 0.24 0.93 ± 0.21 1.54 ± 0.35
2020− 2256 3.28 ± 0.28 1.46 ± 0.28 2.74 ± 0.44
2256− 2382 4.47 ± 0.47 2.44 ± 0.50 3.04 ± 0.63
2382− 2495 6.31 ± 0.56 3.58 ± 0.61 4.10 ± 0.76
2495− 2677 4.80 ± 0.43 3.49 ± 0.50 1.97 ± 0.50
2677− 2845 5.87 ± 0.52 3.92 ± 0.62 2.92 ± 0.63
2845− 2970 5.93 ± 0.63 4.18 ± 0.77 2.62 ± 0.74
exchange currents from the photon – converting to an
ω meson – to the proton which undergoes a p-∆ excita-
tion. The cross section for ∆ω production and for non-∆ω
events rises with increasing phase space; LAMP2 data in-
dicate a decrease of the cross-sections when going to larger
photon energies (3 - 5GeV).
We thank the technical staff at ELSA and at all the partici-
pating institutions for their invaluable contributions to the suc-
cess of the experiment. We acknowledge financial support from
the Deutsche Forschungsgemeinschaft (DFG) within the Son-
derforschungsbereich SFB/TR16. The collaboration with St.
Petersburg received funds from DFG and the Russian Founda-
tion for Basic Research. B. Krusche acknowledges support from
Schweizerischer Nationalfond. U. Thoma thanks for an Emmy
Noether grant from the DFG. A.V. Anisovich and A.V. Sarant-
sev acknowledge support from the Alexander von Humboldt
Foundation. This work comprises part of the PhD thesis of J.
Junkersfeld.
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	Introduction
	Experimental setup
	The reaction pp0
	Summary
