arXiv subjects

A. Gillitzer

Publications and source records attributed to A. Gillitzer.

At least 19 recordsLinked to original sources

arXiv2020

Measurement of proton-proton elastic scattering into the Coulomb region at $\text{P}_\text{beam}$ = 2.5, 2.8 and 3.2 GeV/c

The proton--proton elastic differential cross section at very small four momentum transfer squared has been measured at three different incident proton momenta in the range of 2.5 to 3.2 GeV/c by detecting the recoil proton at polar angles close to $90^\circ$. The measurement was performed at COSY with the KOALA detector covering the Coulomb-nuclear interference region. The total cross section $σ_\text{tot}$, which has been determined precisely, is consistent with previous measurements. The values of the slope parameter $B$ and the relative real amplitude ratio $ρ$ determined in this experiment alleviate the lack of data in the relevant energy region. This precise data on $ρ$ might be an important check for a new dispersion analysis.

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arXiv2019

Rescattering effects in antiproton-induced exclusive $J/ψ$ and $ψ^\prime$ production on the deuteron

On the basis of the generalized eikonal approximation we study the exclusive reactions $\bar p d \to J/ψ\, n$ and $\bar p d \to ψ^\prime\, n$ in vicinity of the thresholds for charmonium production on a free proton target. It is shown that the rescattering of the incoming antiproton and outgoing charmonium on the spectator neutron leads to a depletion of the charmonium production at low- and to an enhancement at high transverse momenta. This is in qualitative agreement with previous studies of hard proton knockout in proton-deuteron collisions. We analyze different physical sources of uncertainty which may influence the extraction of the total charmonium-neutron cross section. The color transparency effect for the incoming $\bar p$ largely compensates the influence of charmonium rescattering both at low and high transverse momenta. Different choices of the deuteron wave function lead to significant uncertainties at high transverse momenta. As an outcome of the calculations of charmonium production, we also provide predictions on the production of open charm hadrons due to the dissociation of the charmonium on the neutron. It is shown that the open charm production cross section is proportional to the total charmonium-nucleon cross section and quite stable with respect to the variation of other parameters of the model. We thus suggest that open charm channels are most suited for future studies of charmonium-nucleon interactions at PANDA with a deuteron target.

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arXiv2018

Theoretical study of the $Δ^{++}-Δ^-$ configuration in the deuteron using antiproton beam

We study the manifestation of the $Δ^{++}-Δ^-$ component of the deuteron wave function in the exclusive reaction $\bar p d \to π^- π^- Δ^{++}$. Due to the large binding energy the internal motion in the $Δ-Δ$ system is relativistic. We take this into account within the light-cone (LC) wave function formalism and, indeed, found large differences between calculations based on the LC and non-relativistic (NR) wave functions. We demonstrate, that the consistent LC treatment of the $Δ-Δ$ system plays the key role in the separation of the signal and background. Within the LC approach, the characteristic shape of the momentum distribution of the $Δ-Δ$ bound system predicted by the meson-exchange model is well visible on the background of usual annihilations at beam momenta between 10 and 15 GeV/c.

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arXiv2018

Determination of N* amplitudes from associated strangeness production in p+p collisions

We present the first determination of the energy-dependent production amplitudes of N$^{*}$ resonances with masses between 1650 MeV/c$^{2}$ and 1900 MeV/c$^{2}$ for an excess energy between $0$ and $600$ MeV. A combined Partial Wave Analysis of seven exclusively reconstructed data samples for the reaction p+p $\rightarrow pKΛ$ measured by the COSY-TOF, DISTO, FOPI and HADES collaborations in fixed target experiments at kinetic energies between 2.14 and 3.5 GeV is used to determine the amplitude of the resonant and non-resonant contributions.

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arXiv2017

First Model-Independent Measurement of the Spin Triplet $pΛ$ Scattering Length from Final State Interaction in the $\vec{p}p \rightarrow pK^{+}Λ$ Reaction

The $\vec{p}p \rightarrow pK^{+}Λ$ reaction has been measured with the COSY-TOF detector at a beam momentum of $2.7\,\mathrm{GeV}/c$. The polarized proton beam enables the measurement of the beam analyzing power by the asymmetry of the produced kaon ($A_N^{K}$). This observable allows the $pΛ$ spin triplet scattering length to be extracted for the first time model independently from the final-state interaction in the reaction. The obtained value is $a_{t} = (-2.55 ^{+0.72}_{-1.39} {}_{\textrm{stat.}} \pm 0.6_{\textrm{syst.}} \pm 0.3_{\textrm{theo.}})\mathrm{fm}$. This value is compatible with theoretical predictions and results from model-dependent analyses.

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arXiv2016

Measurement of polarization observables of the associated strangeness production in proton proton interactions

The Lambda polarization, the analyzing power, and the Lambda spin transfer coefficient of the reaction pp -> p K+ Lambda were measured at beam momenta of 2.70 GeV/c and 2.95 GeV/c, corresponding to excess energies of 122 MeV and 204 MeV. While the analyzing power and the spin transfer coefficient do not change significantly with the excess energy, the Lambda polarization varies strongly and changes its sign. As this is the first measurement of polarization observables below an excess energy of 200 MeV, the change of the sign of the Lambda polarization was not observed before. The high statistics of the data (~200 k events for each momentum) enables detailed studies of the dependence of the Lambda polarization and the analyzing power on the center of mass momentum of the particles. The results of the spin transfer coefficient are in qualitative agreement with the DISTO experiment. The Lambda polarization data of 2.95 GeV/c are only conform with the DISTO experiment, while both the 2.70 GeV/c and 2.95 GeV/c data differ strongly from all previous measurements, whether exclusive or inclusive.

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arXiv2016

Extracting the depolarization coefficient D_NN from data measured with a full acceptance detector

The spin transfer from vertically polarized beam protons to Lambda or Sigma hyperons of the associated strangeness production pp -> pK Lambda (Sigma) is described with the depolarization coefficient D_NN. As the polarization of the hyperons is determined by their weak decays, detectors, which have a large acceptance for the decay particles, are needed. In this paper a formula is derived, which describes the depolarization coefficient D_NN by count rates of a 4 pi detector. It is shown, that formulas, which are given in publications for detectors with restricted acceptance, are specific cases of this formula for a 4 pi detector.

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arXiv2016

High precision measurement of the associated strangeness production in proton proton interactions

A new high precision measurement of the reaction pp -> pK+Lambda at a beam momentum of 2.95 GeV/c with more than 200,000 analyzed events allows a detailed analysis of differential observables and their inter-dependencies. Correlations of the angular distributions with momenta are examined. The invariant mass distributions are compared for different regions in the Dalitz plots. The cusp structure at the N Sigma threshold is described with the Flatté formalism and its variation in the Dalitz plot is analyzed.

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arXiv2014

A recoil detector for the measurement of antiproton-proton elastic scattering at angles close to 90$^{\circ}$

The design and construction of a recoil detector for the measurement of recoil protons of antiproton-proton elastic scattering at scattering angles close to 90$^{\circ}$ are described. The performance of the recoil detector has been tested in the laboratory with radioactive sources and at COSY with proton beams by measuring proton-proton elastic scattering. The results of laboratory tests and commissioning with beam are presented. Excellent energy resolution and proper working performance of the recoil detector validate the conceptual design of the KOALA experiment at HESR to provide the cross section data needed to achieve a precise luminosity determination at the PANDA experiment.

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arXiv2013

Final-State Interactions in the Process pp -> pKLambda

The possibility to determine the pLambda scattering length from the final-state interaction in the reaction pp->pKLambda is investigated experimentally. From a transversely polarized measurement, the K analyzing power (A_N) which, in principle, allows one to extract the spin triplet scattering length is studied. An unexpected energy dependence of the forward/backward symmetric part of A_N is found. The influence of N*-resonances on the pLambda invariant mass spectrum is investigated by exploiting the large acceptance for the process pp->pKLambda->pKppi and is found to be the main source of uncertainty for determining the pLambda scattering length.

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arXiv2013

Charmonium production in antiproton-nucleus reactions at low energies

The J/Ψ(1S) and Ψ'(2S) production near threshold in antiproton-nucleus reactions is calculated on the basis of the Glauber model. The model takes into account the antiproton (pre)absorption, proton Fermi motion, and charmonium formation length. We confirm earlier prediction that the charmonium production in $\bar pA$ collisions at $p_{lab}=3-10$ GeV/c is not influenced by formation length effects and is very well suited to determine the genuine charmonium-nucleon dissociation cross sections. The comparison is performed with the J/Ψphotoproduction at high energies, where formation length effects play a very important role. However, we demonstrate that the detailed structure of the proton and neutron density profiles has to be taken into account, if one wants to extract information on the J/ΨN dissociation cross section from J/Ψtransparency ratios. These studies are relevant for the upcoming PANDA experiment at FAIR.

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arXiv2013

On the SigmaN cusp in the pp -> pK+Lambda reaction

Measurements of the $pp \to pK^+Λ$ reaction at $T_p$ = 2.28 GeV have been carried out at COSY-TOF. In addition to the $Λp$ FSI and $N^*$ resonance excitation effects a pronounced narrow structure is observed in the Dalitz plot and in its projection on the $pΛ$-invariant mass. The structure appears at the $pp \to $N$K^+Σ$ threshold and is interpreted as $Σ$N cusp effect. The observed width of 20 MeV/$c^2$ is substantially broader than anticipated from previous inclusive measurements. Angular distributions of this cusp structure are shown to be dissimilar to those in the residual $pK^+Λ$ channel, but similar to those observed in the $pK^+Σ^0$ channel.

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arXiv2012

The pK0Σ+ final state in proton-proton collisions

This paper reports results from a study of the reaction pp->pK0Σ+ at beam momenta of p_{beam} = 2950, 3059, and 3200 MeV/c (excess energies of ε= 126, 161, and 206 MeV). Total cross sections were determined for all energies; a set of differential cross sections (Dalitz plots; invariant mass spectra of all two-body subsystems; angular distributions of all final state particles; distributions in helicity and Jackson frames) are presented for ε= 161 MeV. The total cross sections are proportional to the volume of available three-body phase-space indicating that the transition matrix element does not change significantly in this range of excess energies. It is concluded from the differential data that the reaction proceeds dominantly via the N(1710)P_{11} and/or N(1720)P_{13} resonance(s); N(1650)S_{11} and Δ(1600)P_{33} could also contribute.

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arXiv2010

Production of Lambda and Sigma^0 hyperons in proton-proton collisions

This paper reports results on simultaneous measurements of the reaction channels pp -> pK+Λand pp -> pK+Σ^0 at excess energies of 204, 239, and 284 MeV (Λ) and 127, 162, and 207 MeV (Σ^0). Total and differential cross sections are given for both reactions. It is concluded from the measured total cross sections that the high energy limit of the cross section ratio is almost reached at an excess energy of only about 200 MeV. From the differential distributions observed in the overall CMS as well as in the Jackson and helicity frames, a significant contribution of interfering nucleon resonances to the Λproduction mechanism is concluded while resonant Σ^0-production seems to be of lesser importance and takes place only through specific partial waves of the entrance channel. The data also indicate that kaon exchange plays a minor role in the case of Λ- but an important role for Σ^0-production. Thus the peculiar energy dependence of the Λ-to-Σ^0 cross section ratio appears in a new light as its explanation requires more than mere differences between the pΛand the pΣ^0 final state interaction. The data provide a benchmark for theoretical models already available or yet to come.

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arXiv2010

Influence of N*-resonances on hyperon production in the channel pp->K+ Lambda p at 2.95, 3.20 and 3.30 GeV/c beam momentum

Hyperon production in the threshold region was studied in the reaction pp -> K+Lp using the time-of-flight spectrometer COSY-TOF. Exclusive data, covering the full phase-space, were taken at the three different beam momenta of p_beam=2.95, 3.20 and 3.30 GeV/c, corresponding to excess energies of epsilon=204, 285 and 316 MeV, respectively. Total cross-sections were deduced for the three beam momenta to be 23.9+/-0.8 +/-2.0 ub, 28.4+/-1.3 +/-2.2 ub and 35.0+/-1.3 +/-3.0 ub. Differential observables including Dalitz plots were obtained. The analysis of the Dalitz plots reveals a strong influence of the N(1650)-resonance at p_beam=2.95 GeV/c, whereas for the higher momenta an increasing relative contribution of the N(1710)- and/or of the N(1720)-resonance was observed. In addition, the pL-final-state interaction turned out to have a significant influence on the Dalitz plot distribution.

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arXiv2010

Systematic study of the pp -> pp omega reaction

A systematic study of the production of omega-mesons in proton-proton-collisions was carried out in a kinematically complete experiment at three excess energies(epsilon= 92, 128, 173MeV). Both protons were detected using the large-acceptance COSY-TOF spectrometer at an external beam line at the Cooler Synchrotron COSY at Forschungszentrum Jülich. The total cross section, angular distributions of both omega-mesons and protons were measured and presented in various reference frames such as the overall CMS, helicity and Jackson frame. In addition, the orientation of the omega-spin and invariant mass spectra were determined. We observe omega-production to take place dominantly in Ss and Sp final states at epsilon = 92, 128 MeV and, additionally, in Sd at epsilon= 173 MeV. No obvious indication of resonant omega-production via N^*-resonances was found, as proton angular distributions are almost isotropic and invariant mass spectra are compatible with phase space distributions. A dominant role of ^3P_1 and ^1S_0 initial partial waves for omega-production was concluded from the orientation of the decay plane of the omega-meson. Although the Jackson angle distributions in the omega-p-Jackson frame are anisotropic we argue that this is not an indication of a resonance but rather a kinematical effect reflecting the anisotropy of the omega angular distribution. The helicity angle distribution in the omega-p-helicity frame shows an anisotropy which probably reflects effects of the omega angular momenta in the final state; this observable may be, in addition to the orientation of the omega decay plane, the most sensitive one to judge the validity of theoretical descriptions of the production process.

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arXiv2009

On the Production of $π^+π^+$ Pairs in pp Collisions at 0.8 GeV

Data accumulated recently for the exclusive measurement of the $pp\to ppπ^+π^-$ reaction at a beam energy of 0.793 GeV using the COSY-TOF spectrometer have been analyzed with respect to possible events from the $pp \to nnπ^+π^+$ reaction channel. The latter is expected to be the only $ππ$ production channel, which contains no major contributions from resonance excitation close to threshold and hence should be a good testing ground for chiral dynamics in the $ππ$ production process. No single event has been found, which meets all conditions for being a candidate for the $pp \to nn π^+π^+$ reaction. This gives an upper limit for the cross section of 0.16 $μ$b (90% C.L.), which is more than an order of magnitude smaller than the cross sections of the other two-pion production channels at the same incident energy.

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arXiv2009

Single-Pion Production in pp Collisions at 0.95 GeV/c (II)

The single-pion production reactions $pp\to dπ^+$, $pp\to npπ^+$ and $pp\to ppπ^0$ were measured at a beam momentum of 0.95 GeV/c ($T_p \approx$ 400 MeV) using the short version of the COSY-TOF spectrometer. The central calorimeter provided particle identification, energy determination and neutron detection in addition to time-of-flight and angle measurements from other detector parts. Thus all pion production channels were recorded with 1-4 overconstraints. Main emphasis is put on the presentation and discussion of the $npπ^+$ channel, since the results on the other channels have already been published previously. The total and differential cross sections obtained are compared to theoretical calculations. In contrast to the $ppπ^0$ channel we find in the $npπ^+$ channel a strong influence of the $Δ$ excitation already at this energy close to threshold. In particular we find a $(3 cos^2Θ+ 1)$ dependence in the pion angular distribution, typical for a pure s-channel $Δ$ excitation and identical to that observed in the $dπ^+$ channel. Since the latter is understood by a s-channel resonance in the $^1D_2$ $pn$ partial wave, we discuss an analogous scenario for the $pnπ^+$ channel.

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