arXiv subjects

S. Dymov

Publications and source records attributed to S. Dymov.

At least 55 records · Page 3Linked to original sources

arXiv2012

New determination of the mass of the eta meson at COSY-ANKE

A value for the mass of the eta meson has been determined at the COSY-ANKE facility through the measurement of a set of deuteron laboratory beam momenta and associated 3He center-of-mass momenta in the d+p -> 3He+X reaction. The eta was then identified by the missing-mass peak and the production threshold determined. The individual beam momenta were fixed with a relative precision of 3 x 10^-5 for values around 3 GeV/c by using a polarized deuteron beam and inducing an artificial depolarizing spin resonance, which occurs at a well-defined frequency. The final-state momenta in the two-body d+p -> 3He+eta reaction were investigated in detail by studying the size of the 3He momentum ellipse with the forward detection system of the ANKE spectrometer. Final alignment of the spectrometer for this high precision experiment was achieved through a comprehensive study of the 3He final-state momenta as a function of the center-of-mass angles, taking advantage of the full geometrical acceptance. The value obtained for the mass, m(eta)=(547.873 +- 0.005(stat) +- 0.027(syst)) MeV/c^2, is consistent and competitive with other recent measurements, in which the meson was detected through its decay products.

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arXiv2012

The production of K+K- pairs in proton-proton collisions at 2.83 GeV

Differential and total cross sections for the pp -> ppK+K- reaction have been measured at a proton beam energy of 2.83 GeV using the COSY-ANKE magnetic spectrometer. Detailed model descriptions fitted to a variety of one-dimensional distributions permit the separation of the pp -> pp phi cross section from that of non-phi production. The differential spectra show that higher partial waves represent the majority of the pp -> pp phi total cross section at an excess energy of 76 MeV, whose energy dependence would then seem to require some s-wave phi-p enhancement near threshold. The non-phi data can be described in terms of the combined effects of two-body final state interactions using the same effective scattering parameters determined from lower energy data.

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arXiv2012

Momentum dependence of the phi-meson nuclear transparency

The production of phi mesons in proton collisions with C, Cu, Ag, and Au targets has been studied via the phi -> K+K- decay at an incident beam energy of 2.83 GeV using the ANKE detector system at COSY. For the first time, the momentum dependence of the nuclear transparency ratio, the in-medium phi width, and the differential cross section for phi meson production at forward angles have been determined for these targets over the momentum range of 0.6 - 1.6 GeV/c. There are indications of a significant momentum dependence in the value of the extracted phi width, which corresponds to an effective phi-N absorption cross section in the range of 14 - 21 mb.

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arXiv2011

Differential cross section and analysing power of the quasi-free pn -> {pp}_s pi- reaction at 353 MeV

In order to establish links between p-wave pion production in nucleon-nucleon collisions and low energy three-nucleon scattering, an extensive programme of experiments on pion production is currently underway at COSY-ANKE. The final proton pair is measured at very low excitation energy, leading to an S-wave diproton, denoted here as {pp}_s. By using a deuterium target we have obtained data on the differential cross section and analysing power of the quasi-free pol{p}n -> {pp}_s pi^- reaction at 353 MeV. The spectator proton p_sp was either measured directly in silicon tracking telescopes or reconstructed using the momentum of a detected pi^-. Both observables can be described in terms of s-, p-, and d-wave pion production amplitudes. Taken together with the analogous data on the pol{p}p -> {pp}_s pi^0 reaction, full partial wave decompositions of both processes were carried out.

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arXiv2011

Differential cross section and analysing power of the pp -> {pp\}_s pi0 reaction at 353 MeV

In order to establish links between p-wave pion production in nucleon-nucleon collisions and low energy three-nucleon scattering, an extensive programme of experiments on pion production is currently underway at COSY-ANKE. The final proton pair is detected at very low excitation energy, leading to an S-wave diproton, denoted here as {pp}_s. We now report on measurements of the differential cross section and analysing power of the pol{p}p->{pp}_s pi^0$ reaction at 353 MeV. Both observables can be described in terms of s- and d-wave pion production and, by using the phase information from elastic pp scattering, unique solutions can be obtained for the corresponding amplitudes. This information is vital for the partial wave decomposition of the corresponding pn->{pp}_s pi^- reaction and hence for the extraction of the p-wave terms.

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arXiv2010

Measurement of the in-medium phi-meson width in proton-nucleus collisions

The production of phi mesons in the collisions of 2.83 GeV protons with C, Cu, Ag, and Au at forward angles has been measured via the phi -> K+K- decay using the COSY-ANKE magnetic spectrometer. The phi meson production cross section follows a target mass dependence of A^0.56+/-0.02 in the momentum region of 0.6-1.6 GeV/c. The comparison of the data with model calculations suggests that the in-medium phi width is about an order of magnitude larger than its free value.

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arXiv2010

Energy dependence of hard bremsstrahlung production in proton-proton collisions in the Delta(1232) region

Hard bremsstrahlung production in proton-proton collisions has been studied with the ANKE spectrometer at COSY-Juelich in the energy range of 353-800 MeV by detecting the final proton pair {pp}_s from the pp -> {pp}_s reaction with very low excitation energy. Differential cross sections were measured at small diproton c.m. angles from 0 to 20 degrees and the average over this angular interval reveals a broad peak at a beam energy around 650 MeV with a FWHM of about 220 MeV, suggesting the influence of Delta(1232)N intermediate states. Comparison with deuteron photodisintegration shows that the cross section for diproton production is up to two orders of magnitude smaller, due largely to differences in the selection rules.

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arXiv2010

The energy dependence of the pp->K+ n Sigma+ reaction close to threshold

The production of the Sigma+ hyperon through the pp->K+nSigma+ reaction has been investigated at four energies close to threshold, 1.826, 1.920, 1.958, and 2.020 GeV. At low energies, correlated K+pi+ pairs can only originate from Sigma+ production so that their measurement allows the total cross section for the reaction to be determined. The results obtained are completely consistent with the values extracted from the study of the K+-proton correlation spectra obtained in the same experiment. These spectra, as well as the inclusive K+ momentum distributions, also provide conservative upper limits on the Sigma+ production rates. The measurements show a Sigma+ production cross section that varies roughly like phase space and, in particular, none of the three experimental approaches used supports the anomalously high near-threshold pp->K+ nSigma+ total cross section previously reported [T. Rozek et al., Phys. Lett. B 643, 251 (2006)].

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arXiv2009

Precision study of the d+p->3He+eta reaction for excess energies between 20 and 60 MeV

The differential and total cross sections for the d+p->3He+eta reaction have been measured at COSY-ANKE at excess energies of 19.5, 39.4, and 59.4 MeV over the full angular range. The results are in line with trends apparent from the detailed near-threshold studies and also agree with those from CELSIUS, though the present data have higher precision. While at 19.5 MeV the results can be described in terms of s- and p-wave production, by 59.4 MeV higher partial waves are required. Including the 19.5 MeV point together with the near-threshold data in a global s- and p-wave fit gives a poorer overall description of the data though the position of the pole in the 3He scattering amplitude, corresponding to the quasi-bound or virtual state, is hardly changed.

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arXiv2009

Measurement of the isospin-filtering dd -> 4He K+ K- reaction at Q=39 MeV

The total cross section for the dd -> 4He K+ K- reaction has been measured at a beam momentum of 3.7 GeV/c, corresponding to an excess energy of 39 MeV, which is the maximum possible at the Cooler Synchrotron COSY-Jülich. A deuterium cluster-jet target and the ANKE forward magnetic spectrometer, placed inside the storage ring, have been employed in this investigation. We find a total cross section of sigma(tot) < 14 pb, which brings into question the viability of investigating the dd -> 4He a0(980) reaction as a means of studying isospin violation.

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arXiv2009

Measurement of the Spin-Dependence of the pbar-p Interaction at the AD-Ring

We propose to use an internal polarized hydrogen storage cell gas target in the AD ring to determine for the first time the two total spin-dependent pbar-p cross sections sigma_1 and sigma_2 at antiproton beam energies in the range from 50 to 450 MeV. The data obtained are of interest by themselves for the general theory of pbar-p interactions since they will provide a first experimental constraint of the spin-spin dependence of the nucleon-antinucleon potential in the energy range of interest. In addition, measurements of the polarization buildup of stored antiprotons are required to define the optimum parameters of a future, dedicated Antiproton Polarizer Ring (APR), intended to feed a double-polarized asymmetric pbar-p collider with polarized antiprotons. Such a machine has recently been proposed by the PAX collaboration for the new Facility for Antiproton and Ion Research (FAIR) at GSI in Darmstadt, Germany. The availability of an intense stored beam of polarized antiprotons will provide access to a wealth of single- and double-spin observables, thereby opening a new window on QCD spin physics.

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arXiv2009

Polarizing a stored proton beam by spin flip?

We discuss polarizing a proton beam in a storage ring, either by selective removal or by spin flip of the stored ions. Prompted by recent, conflicting calculations, we have carried out a measurement of the spin flip cross section in low-energy electron-proton scattering. The experiment uses the cooling electron beam at COSY as an electron target. The measured cross sections are too small for making spin flip a viable tool in polarizing a stored beam. This invalidates a recent proposal to use co-moving polarized positrons to polarize a stored antiproton beam.

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arXiv2009

Observation of an ABC effect in proton-proton collisions

The cross section for inclusive multipion production in the pp->ppX reaction was measured at COSY-ANKE at four beam energies, 0.8, 1.1, 1.4, and 2.0 GeV, for low excitation energy in the final pp system, such that the diproton quasi-particle is in the 1S0 state. At the three higher energies the missing mass Mx spectra show a strong enhancement at low Mx, corresponding to an ABC effect that moves steadily to larger values as the energy is increased. Despite the missing-mass structure looking very different at 0.8 GeV, the variation with Mx and beam energy are consistent with two-pion production being mediated through the excitation of two Delta(1232) isobars, coupled to S-- and D-- states of the initial pp system.

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arXiv2008

Measurement of the pn -> dK^+K^- total cross section close to threshold

Measurements of the pn -> p_spec dK^+K^- reaction, where p_spec is a spectator proton, have been undertaken at the Cooler Synchrotron COSY-Juelich by detecting a fast deuteron in coincidence with a K^+K^- pair in the ANKE facility. Although the proton beam energy was fixed, the moving target neutron allowed values of the non-resonant quasi-free pn -> dK^+K^- total cross section to be deduced up to an excess energy epsilon ~ 100 MeV. Evidence is found for the effects of K^-d and KKbar final state interactions. The comparison of these data with those of pp -> ppK^+K^- and pp -> dK^+Kbar shows that all the total cross sections are very similar in magnitude.

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arXiv2008

Neutron-proton charge-exchange amplitudes at 585 MeV

The differential cross section and deuteron analysing powers of the dp -> {pp}n charge-exchange reaction have been measured with the ANKE spectrometer at the COSY storage ring. Using a deuteron beam of energy 1170 MeV, data were obtained for small momentum transfers to a {pp} system with low excitation energy. A good quantitative understanding of all the measured observables is provided by the impulse approximation using known neutron-proton amplitudes. The proof of principle achieved here for the method suggests that measurements at higher energies will provide useful information in regions where the existing np database is far less reliable.

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arXiv2008

Observation of inverse diproton photodisintegration at intermediate energies

The reaction pp->{pp}_sγ, where {pp}_s is a proton pair with an excitation energy E_{pp}<3 MeV, has been observed with the ANKE spectrometer at COSY-Juelich for proton beam energies of T_p=0.353, 0.500, and 0.550 GeV. This is equivalent to photodisintegration of a free 1S_0 diproton for photon energies Eγ~ T_p/2. The differential cross sections measured for c.m. angles 0 deg.<θ_{pp}<20 deg. exhibit a steep increase with angle that is compatible with E1 and E2 multipole contributions. The ratio of the measured cross sections to those of np->dγis on the 10^{-3}-10^{-2} level. The increase of the pp->{pp}_sγcross section with T_p might reflect the influence of the Delta(1232) excitation.

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arXiv2008

Determination of target thickness and luminosity from beam energy losses

The repeated passage of a coasting ion beam of a storage ring through a thin target induces a shift in the revolution frequency due to the energy loss in the target. Since the frequency shift is proportional to the beam-target overlap, its measurement offers the possibility of determining the target thickness and hence the corresponding luminosity in an experiment. This effect has been investigated with an internal proton beam of energy 2.65 GeV at the COSY-Jülich accelerator using the ANKE spectrometer and a hydrogen cluster-jet target. Possible sources of error, especially those arising from the influence of residual gas in the ring, were carefully studied, resulting in a accuracy of better than 5%. The luminosity determined in this way was used, in conjunction with measurements in the ANKE forward detector, to determine the cross section for elastic proton-proton scattering. The result is compared to published data as well as to the predictions of a phase shift solution. The practicability and the limitations of the energy-loss method are discussed.

physics.acc-ph↗

arXiv2007

Energy dependence of forward 1S0 diproton production in the pp -> pp pi0 reaction

The pp -> {pp}_s pi0 differential cross section has been measured with the ANKE spectrometer at COSY-Juelich for seven proton beam energies T_p between 0.5 and 1.97 GeV. By selecting proton pairs with an excitation energy of less than 3 MeV it is ensured that the final {pp}_s system is in the 1S0 state. In the measured region of theta_{pp}^{cm} < 18 deg, the data reveal a forward dip for T_p =< 1.4 GeV whereas a forward peaking is seen at 1.97 GeV. The energy dependence of the forward cross section shows a broad peak in the 0.6-0.8 GeV region, probably associated with Delta(1232) excitation, and a minimum at 1.4 GeV. Some of these features are similar to those observed for the spin-isospin partner reaction, pp -> d pi+. However, the ratio of the forward differential cross sections of the two reactions shows a significant suppression of single pion production associated with a spin--singlet final nucleon pair.

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