We present data on dielectron emission in proton induced reactions on a Nb target at 3.5 GeV kinetic beam energy measured with HADES installed at GSI. The data represent the first high statistics ...measurement of proton-induced dielectron radiation from cold nuclear matter in a kinematic regime, where strong medium effects are expected. Combined with the good mass resolution of 2%, it is the first measurement sensitive to changes of the spectral functions of vector mesons, as predicted by models for hadrons at rest or small relative momenta. Comparing the e+e− invariant mass spectra to elementary p+p data, we observe for e+e− momenta Pee<0.8 GeV/c a strong modification of the shape of the spectrum, which we attribute to an additional ρ-like contribution and a decrease of ω yield. These opposite trends are tentatively interpreted as a strong coupling of the ρ meson to baryonic resonances and an absorption of the ω meson, which are two aspects of in-medium modification of vector mesons.
.
The combination of a production target for secondary beams, an optimized ion optical beam line setting, in-beam detectors for minimum ionizing particles with high rate capability, and an efficient ...large acceptance spectrometer around the reaction target constitutes an experimental opportunity to study in detail hadronic interactions utilizing pion beams impinging on nucleons and nuclei. For the 0.4-2.0GeV/c pion momentum regime such a facility is located at the heavy ion synchrotron accelerator SIS18 in Darmstadt (Germany). The layout of the apparatus, performance of its components and encouraging results from a first commissioning run are presented.
The study of baryon excitation spectra provides insight into the inner structure of baryons. So far, most of the world-wide efforts have been directed towards N * and Delta spectroscopy. ...Nevertheless, the study of the double and triple strange baryon spectrum provides independent information to the N * and Delta spectra. The future antiproton experiment (P) over bar ANDA will provide direct access to final states containing a (Xi) over bar Xi pair, for which production cross sections up to mu b are expected in (p) over barp reactions. With a luminosity of L = 10(31) cm(-2) s(-1) in the first phase of the experiment, the expected cross sections correspond to a production rate of similar to 10(6) events/day. With a nearly 4 pi detector acceptance, (P) over bar ANDA will thus be a hyperon factory. In this study, reactions of the type (p) over barp -> (Xi) over bar (+)Xi*(-) as well as (p) over barp -> (Xi) over bar*(+)Xi(-) with various decay modes are investigated. For the exclusive reconstruction of the signal events a full decay tree fit is used, resulting in reconstruction efficiencies between 3 and 5%. This allows high statistics data to be collected within a few weeks of data taking.
New results on the differential cross section in deuteron-proton elastic scattering are obtained at the deuteron kinetic energy of 2.5 GeV with the HADES spectrometer. The angular range of 69° – 125° ...in the center of mass system is covered. The obtained results are compared with the relativistic multiple scattering model calculation using the CD-Bonn deuteron wave function. The data at fixed scattering angles in the c.m. are in qualitative agreement with the constituent counting rules prediction.
We present first data on sub-threshold production of Ks0 mesons and Λ hyperons in Au+Au collisions at sNN=2.4 GeV. We observe an universal 〈Apart〉 scaling of hadrons containing strangeness, ...independent of their corresponding production thresholds. Comparing the yields, their 〈Apart〉 scaling, and the shapes of the rapidity and the pt spectra to state-of-the-art transport model (UrQMD, HSD, IQMD) predictions, we find that none of them can simultaneously describe these observables with reasonable χ2 values.
Abstract
The antiproton experiment PANDA at FAIR is designed to bring hadron physics to a new level in terms of scope, precision and accuracy. In this work, its unique capability for studies of ...hyperons is outlined. We discuss ground-state hyperons as diagnostic tools to study non-perturbative aspects of the strong interaction, and fundamental symmetries. New simulation studies have been carried out for two benchmark hyperon-antihyperon production channels:
$${\bar{p}}p \rightarrow {\bar{\varLambda }}\varLambda $$
p
¯
p
→
Λ
¯
Λ
and
$${\bar{p}}p \rightarrow {\bar{\varXi }}^+\varXi ^-$$
p
¯
p
→
Ξ
¯
+
Ξ
-
. The results, presented in detail in this paper, show that hyperon-antihyperon pairs from these reactions can be exclusively reconstructed with high efficiency and very low background contamination. In addition, the polarisation and spin correlations have been studied, exploiting the weak, self-analysing decay of hyperons and antihyperons. Two independent approaches to the finite efficiency have been applied and evaluated: one standard multidimensional efficiency correction approach, and one efficiency independent approach. The applicability of the latter was thoroughly evaluated for all channels, beam momenta and observables. The standard method yields good results in all cases, and shows that spin observables can be studied with high precision and accuracy already in the first phase of data taking with PANDA.
Abstract
The study of baryon excitation spectra provides insight into the inner structure of baryons. So far, most of the world-wide efforts have been directed towards
$$N^*$$
N
∗
and
$$\varDelta $$
...Δ
spectroscopy. Nevertheless, the study of the double and triple strange baryon spectrum provides independent information to the
$$N^*$$
N
∗
and
$$\varDelta $$
Δ
spectra. The future antiproton experiment
$$\overline{\text{ P }}$$
P
¯
ANDA will provide direct access to final states containing a
$${\overline{\varXi }}\varXi $$
Ξ
¯
Ξ
pair, for which production cross sections up to
$$\mu \text{ b }$$
μ
b
are expected in
$$\bar{\text{ p }}$$
p
¯
p reactions. With a luminosity of
$$L=10^{31}$$
L
=
10
31
cm
$$^{-2}$$
-
2
s
$$^{-1}$$
-
1
in the first phase of the experiment, the expected cross sections correspond to a production rate of
$$\sim 10^6\, \text{ events }/\text{day }$$
∼
10
6
events
/
day
. With a nearly
$$4\pi $$
4
π
detector acceptance,
$$\overline{\text{ P }}$$
P
¯
ANDA will thus be a hyperon factory. In this study, reactions of the type
$$\bar{\text{ p }}$$
p
¯
p
$$\rightarrow $$
→
$${\overline{\varXi }}^{+}$$
Ξ
¯
+
$$\varXi ^{*-}$$
Ξ
∗
-
as well as
$$\bar{\text{ p }}$$
p
¯
p
$$\rightarrow $$
→
$${\overline{\varXi }}^{*+}$$
Ξ
¯
∗
+
$$\varXi ^{-}$$
Ξ
-
with various decay modes are investigated. For the exclusive reconstruction of the signal events a full decay tree fit is used, resulting in reconstruction efficiencies between 3 and 5%. This allows high statistics data to be collected within a few weeks of data taking.
Abstract
This paper reports on Monte Carlo simulation results for future measurements of the moduli of time-like proton electromagnetic form factors,
$$|G_{E}|$$
|
G
E
|
and
$$|G_{M}|$$
|
G
M
|
, ...using the
$$\bar{p} p \rightarrow \mu ^{+} \mu ^{-}$$
p
¯
p
→
μ
+
μ
-
reaction at
$$\overline{\text {P}}\text {ANDA}$$
P
¯
ANDA
(FAIR). The electromagnetic form factors are fundamental quantities parameterizing the electric and magnetic structure of hadrons. This work estimates the statistical and total accuracy with which the form factors can be measured at
$$\overline{\text {P}}\text {ANDA}$$
P
¯
ANDA
, using an analysis of simulated data within the PandaRoot software framework. The most crucial background channel is
$$\bar{p} p \rightarrow \pi ^{+} \pi ^{-}$$
p
¯
p
→
π
+
π
-
, due to the very similar behavior of muons and pions in the detector. The suppression factors are evaluated for this and all other relevant background channels at different values of antiproton beam momentum. The signal/background separation is based on a multivariate analysis, using the Boosted Decision Trees method. An expected background subtraction is included in this study, based on realistic angular distributions of the background contribution. Systematic uncertainties are considered and the relative total uncertainties of the form factor measurements are presented.
This paper reports on Monte Carlo simulation results for future measurements of the moduli of time-like proton electromagnetic form factors,
|
G
E
|
and
|
G
M
|
, using the
p
¯
p
→
μ
+
μ
-
reaction ...at
P
¯
ANDA
(FAIR). The electromagnetic form factors are fundamental quantities parameterizing the electric and magnetic structure of hadrons. This work estimates the statistical and total accuracy with which the form factors can be measured at
P
¯
ANDA
, using an analysis of simulated data within the PandaRoot software framework. The most crucial background channel is
p
¯
p
→
π
+
π
-
, due to the very similar behavior of muons and pions in the detector. The suppression factors are evaluated for this and all other relevant background channels at different values of antiproton beam momentum. The signal/background separation is based on a multivariate analysis, using the Boosted Decision Trees method. An expected background subtraction is included in this study, based on realistic angular distributions of the background contribution. Systematic uncertainties are considered and the relative total uncertainties of the form factor measurements are presented.
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