The η-carbon potential at low meson momenta Nanova, M.; Friedrich, S.; Metag, V. ...
The European physical journal. A, Hadrons and nuclei,
2018/10, Letnik:
54, Številka:
10
Journal Article
Recenzirano
Odprti dostop
.
The production of
η
mesons in coincidence with forward-going protons has been studied in photon-induced reactions on
12
C and on a liquid hydrogen (LH
2
) target for incoming photon energies of ...1.3-2.6 GeV at the electron accelerator ELSA. The
η
mesons have been identified via the
η
→
π
0
π
0
η
→
6
γ
decay registered with the CBELSA/TAPS detector system. Coincident protons have been identified in the MiniTAPS BaF
2
array at polar angles of
2
∘
≤
θ
p
≤
11
∘
. Under these kinematic constraints the
η
mesons are produced with relatively low kinetic energy (
≈
150
MeV) since the coincident protons take over most of the momentum of the incident-photon beam. For the C-target this allows the determination of the real part of the
η
-carbon potential at low meson momenta by comparing with collision model calculations of the
η
kinetic energy distribution and excitation function. Fitting the latter data for
η
mesons going backwards in the center-of-mass system yields a potential depth of
V
=
-
(
44
±
16
(
s
t
a
t
)
±
15
(
s
y
s
t
)
)
MeV, consistent with earlier determinations of the potential depth in inclusive measurements for average
η
momenta of
≈
1
.
1
GeV/
c
. Within the experimental uncertainties, there is no indication of a momentum dependence of the
η
-carbon potential. The LH
2
data, taken as a reference to check the data analysis and the model calculations, provide differential and integral cross sections in good agreement with previous results for
η
photoproduction off the free proton.
The Crystal Barrel is an electromagnetic calorimeter consisting of 1380 CsI(Tl) scintillators, and is currently installed at the CBELSA/TAPS experiment where it is used to detect decay products from ...photoproduction of mesons. The readout of the Crystal Barrel has been upgraded in order to integrate the detector into the first level of the trigger and to increase its sensitivity for neutral final states. The new readout uses avalanche photodiodes in the front-end and a dual back-end with branches optimized for energy and time measurement, respectively. An FPGA-based cluster finder processes the whole hit pattern within less than 100 ns. The important downside of APDs -- the temperature dependence of their gain -- is handled with a temperature stabilization and a compensating bias voltage supply. Additionally, a light pulser system allows the APDs' gains to be measured during beamtimes.
The reaction $\gamma p \rightarrow p \pi^0 \eta$ has been studied with the
CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for
incident photon energies from threshold up to ...3.1 GeV. This paper has been
motivated by the recently claimed observation of a narrow structure in the
M$_{N\eta}$ invariant mass distribution at a mass of 1678 MeV/$c^2$. The
existence of this structure cannot be confirmed in the present work. Instead,
for E$_{\gamma}$ = 1400 - 1500 MeV and the cut M$_{p\pi^0} \le 1190 $ MeV/$c^2$
a statistically significant structure in the M$_{p\eta}$ invariant mass
distribution near 1700 MeV/$c^2$ is observed with a width of $\Gamma\approx 35$
MeV/$c^2$ while the mass resolution is $\sigma_{res}$ = 5 MeV/$c^2$. Increasing
the incident photon energy from 1420 to 1540 MeV this structure shifts in mass
from $\approx$ 1700MeV/c$^2$ to $\approx$ 1725 MeV/$c^2$; the width increases
to about 50 MeV/$c^2$ and decreases thereafter. The cross section associated
with this structure reaches a maximum of $\approx$ 100 nb around E$_{\gamma}
\approx$ 1490 MeV (W $\approx $ 1920 MeV), which coincides with the $p a_0$
threshold. Three scenarios are discussed which might be the origin of this
structure in the M$_{p\eta}$ invariant mass distribution. The most likely
interpretation is that it is due to a triangular singularity in the $\gamma p
\rightarrow p a_0 \rightarrow p \pi^0 \eta$ reaction
The production of \(\eta^\prime\) mesons in coincidence with forward-going protons has been studied in photon-induced reactions on \(^{12}\)C and on a liquid hydrogen (LH\(_2\)) target for incoming ...photon energies of 1.3-2.6 GeV at the electron accelerator ELSA. The \(\eta^\prime\) mesons have been identified via the \(\eta^\prime\rightarrow \pi^0 \pi^0\eta \rightarrow 6 \gamma\) decay registered with the CBELSA/TAPS detector system. Coincident protons have been identified in the MiniTAPS BaF\(_2\) array at polar angles of \(2^{\circ} \le \theta _{p} \le 11^{\circ}\). Under these kinematic constraints the \(\eta^\prime\) mesons are produced with relatively low kinetic energy (\(\approx\) 150 MeV) since the coincident protons take over most of the momentum of the incident-photon beam. For the C-target this allows the determination of the real part of the \(\eta^\prime\)-carbon potential at low meson momenta by comparing with collision model calculations of the \(\eta^\prime\) kinetic energy distribution and excitation function. Fitting the latter data for \(\eta^\prime\) mesons going backwards in the center-of-mass system yields a potential depth of V = \(-\)(44 \(\pm\) 16(stat)\(\pm\)15(syst)) MeV, consistent with earlier determinations of the potential depth in inclusive measurements for average \(\eta^\prime\) momenta of \(\approx\) 1.1 GeV/\(c\). Within the experimental uncertainties, there is no indication of a momentum dependence of the \(\eta^\prime\)-carbon potential. The LH\(_2\) data, taken as a reference to check the data analysis and the model calculations, provide differential and integral cross sections in good agreement with previous results for \(\eta^\prime\) photoproduction off the free proton.
The reaction \(\gamma p \rightarrow p \pi^0 \eta\) has been studied with the CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for incident photon energies from threshold up to ...3.1 GeV. This paper has been motivated by the recently claimed observation of a narrow structure in the M\(_{N\eta}\) invariant mass distribution at a mass of 1678 MeV/\(c^2\). The existence of this structure cannot be confirmed in the present work. Instead, for E\(_{\gamma}\) = 1400 - 1500 MeV and the cut M\(_{p\pi^0} \le 1190 \) MeV/\(c^2\) a statistically significant structure in the M\(_{p\eta}\) invariant mass distribution near 1700 MeV/\(c^2\) is observed with a width of \(\Gamma\approx 35\) MeV/\(c^2\) while the mass resolution is \(\sigma_{res}\) = 5 MeV/\(c^2\). Increasing the incident photon energy from 1420 to 1540 MeV this structure shifts in mass from \(\approx\) 1700MeV/c\(^2\) to \(\approx\) 1725 MeV/\(c^2\); the width increases to about 50 MeV/\(c^2\) and decreases thereafter. The cross section associated with this structure reaches a maximum of \(\approx\) 100 nb around E\(_{\gamma} \approx\) 1490 MeV (W \(\approx \) 1920 MeV), which coincides with the \(p a_0\) threshold. Three scenarios are discussed which might be the origin of this structure in the M\(_{p\eta}\) invariant mass distribution. The most likely interpretation is that it is due to a triangular singularity in the \(\gamma p \rightarrow p a_0 \rightarrow p \pi^0 \eta\) reaction
The photoproduction of $\omega$ and $\eta^\prime$ mesons off carbon and
niobium nuclei has been measured as a function of the meson momentum for
incident photon energies of 1.2-2.9 GeV at the ...electron accelerator ELSA. The
mesons have been identified via the $\omega \rightarrow \pi^0 \gamma
\rightarrow 3 \gamma$ and $\eta^\prime\rightarrow \pi^0 \pi^0\eta \rightarrow 6
\gamma$ decays, respectively, registered with the CBELSA/TAPS detector system.
From the measured meson momentum distributions the momentum dependence of the
transparency ratio has been determined for both mesons. Within a Glauber
analysis the in-medium $\omega$ and $\eta^\prime$ widths and the corresponding
absorption cross sections have been deduced as a function of the meson
momentum. The results are compared to recent theoretical predictions for the
in-medium $\omega$ width and $\eta^\prime$-N absorption cross sections. The
energy dependence of the imaginary part of the $\omega$- and
$\eta^\prime$-nucleus optical potential has been extracted. The finer binning
of the present data compared to the existing data allows a more reliable
extrapolation towards the production threshold. The modulus of the imaginary
part of the $\eta^\prime$ nucleus potential is found to be about three times
smaller than recently determined values of the real part of the
$\eta^\prime$-nucleus potential, which makes the $\eta^\prime$ meson a suitable
candidate for the search for meson-nucleus bound states. For the $\omega$
meson, the modulus of the imaginary part near threshold is comparable to the
modulus of the real part of the potential. As a consequence, only broad
structures can be expected which makes the observation of $\omega$ mesic states
very difficult experimentally.
The excitation function and momentum distribution of {\eta}' mesons have been measured in photoproduction off 93^Nb in the energy range of 1.2-2.9 GeV. The experiment has been performed with the ...combined Crystal Barrel and MiniTAPS detector system, using tagged photon beams from the ELSA electron accelerator. Information on the sign and magnitude of the real part of the {\eta}'-Nb potential has been extracted from a comparison of the data with model calculations. An attractive potential of -(41 \pm 10(stat) \pm 15(syst)) MeV depth at normal nuclear matter density is deduced within model uncertainties. This value is consistent with the potential depth of -(37 \pm 10(stat) \pm 10(syst)) MeV obtained in an earlier measurement for a light nucleus (carbon). This relatively shallow {\eta}'-nucleus potential will make the search for {\eta}'-nucleus bound states more difficult.
The photoproduction of \(\omega\) and \(\eta^\prime\) mesons off carbon and niobium nuclei has been measured as a function of the meson momentum for incident photon energies of 1.2-2.9 GeV at the ...electron accelerator ELSA. The mesons have been identified via the \(\omega \rightarrow \pi^0 \gamma \rightarrow 3 \gamma\) and \(\eta^\prime\rightarrow \pi^0 \pi^0\eta \rightarrow 6 \gamma\) decays, respectively, registered with the CBELSA/TAPS detector system. From the measured meson momentum distributions the momentum dependence of the transparency ratio has been determined for both mesons. Within a Glauber analysis the in-medium \(\omega\) and \(\eta^\prime\) widths and the corresponding absorption cross sections have been deduced as a function of the meson momentum. The results are compared to recent theoretical predictions for the in-medium \(\omega\) width and \(\eta^\prime\)-N absorption cross sections. The energy dependence of the imaginary part of the \(\omega\)- and \(\eta^\prime\)-nucleus optical potential has been extracted. The finer binning of the present data compared to the existing data allows a more reliable extrapolation towards the production threshold. The modulus of the imaginary part of the \(\eta^\prime\) nucleus potential is found to be about three times smaller than recently determined values of the real part of the \(\eta^\prime\)-nucleus potential, which makes the \(\eta^\prime\) meson a suitable candidate for the search for meson-nucleus bound states. For the \(\omega\) meson, the modulus of the imaginary part near threshold is comparable to the modulus of the real part of the potential. As a consequence, only broad structures can be expected which makes the observation of \(\omega\) mesic states very difficult experimentally.
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