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I. Strakovsky

Publications and source records attributed to I. Strakovsky.

31 records · Page 2Linked to original sources

A precision measurement of the $p$($e,e^\prime p\,$)$π^0$ reaction at threshold

New results are reported from a measurement of $π^0$ electroproduction near threshold using the $p(e,e^{\prime} p)π^0$ reaction. The experiment was designed to determine precisely the energy dependence of $s-$ and $p-$wave electromagnetic multipoles as a stringent test of the predictions of Chiral Perturbation Theory (ChPT). The data were taken with an electron beam energy of 1192 MeV using a two-spectrometer setup in Hall A at Jefferson Lab. For the first time, complete coverage of the $ϕ^*_π$ and $θ^*_π$ angles in the $p π^0$ center-of-mass was obtained for invariant energies above threshold from 0.5 MeV up to 15 MeV. The 4-momentum transfer $Q^2$ coverage ranges from 0.05 to 0.155 (GeV/c)$^2$ in fine steps. A simple phenomenological analysis of our data shows strong disagreement with $p-$wave predictions from ChPT for $Q^2>0.07$ (GeV/c)$^2$, while the $s-$wave predictions are in reasonable agreement.

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Analysis Tools for Next-Generation Hadron Spectroscopy Experiments

The series of workshops on New Partial-Wave Analysis Tools for Next-Generation Hadron Spectroscopy Experiments was initiated with the ATHOS 2012 meeting, which took place in Camogli, Italy, June 20-22, 2012. It was followed by ATHOS 2013 in Kloster Seeon near Munich, Germany, May 21-24, 2013. The third, ATHOS3, meeting is planned for April 13-17, 2015 at The George Washington University Virginia Science and Technology Campus, USA. The workshops focus on the development of amplitude analysis tools for meson and baryon spectroscopy, and complement other programs in hadron spectroscopy organized in the recent past including the INT-JLab Workshop on Hadron Spectroscopy in Seattle in 2009, the International Workshop on Amplitude Analysis in Hadron Spectroscopy at the ECT*-Trento in 2011, the School on Amplitude Analysis in Modern Physics in Bad Honnef in 2011, the Jefferson Lab Advanced Study Institute Summer School in 2012, and the School on Concepts of Modern Amplitude Analysis Techniques in Flecken-Zechlin near Berlin in September 2013. The aim of this document is to summarize the discussions that took place at the ATHOS 2012 and ATHOS 2013 meetings. We do not attempt a comprehensive review of the field of amplitude analysis, but offer a collection of thoughts that we hope may lay the ground for such a document.

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First Measurement of the Polarization Observable E in the $\vec p(\vec γ,π^+)n$ Reaction up to 2.25 GeV

First results from the longitudinally polarized frozen-spin target (FROST) program are reported. The double-polarization observable E, for the reaction $\vec γ\vec p \to π^+n$, has been measured using a circularly polarized tagged-photon beam, with energies from 0.35 to 2.37 GeV. The final-state pions were detected with the CEBAF Large Acceptance Spectrometer in Hall B at the Thomas Jefferson National Accelerator Facility. These polarization data agree fairly well with previous partial-wave analyses at low photon energies. Over much of the covered energy range, however, significant deviations are observed, particularly in the high-energy region where high-L multipoles contribute. The data have been included in new multipole analyses resulting in updated nucleon resonance parameters. We report updated fits from the Bonn-Gatchina, Jülich, and SAID groups.

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$K^+Λ$ and $K^+Σ^0$ photoproduction with fine center-of-mass energy resolution

Measurements of $γp \rightarrow K^{+} Λ$ and $γp \rightarrow K^{+} Σ^0$ cross-sections have been obtained with the photon tagging facility and the Crystal Ball calorimeter at MAMI-C. The measurement uses a novel $K^+$ meson identification technique in which the weak decay products are characterized using the energy and timing characteristics of the energy deposit in the calorimeter, a method that has the potential to be applied at many other facilities. The fine center-of-mass energy ($W$) resolution and statistical accuracy of the new data results in a significant impact on partial wave analyses aiming to better establish the excitation spectrum of the nucleon. The new analyses disfavor a strong role for quark-diquark dynamics in the nucleon.

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Observation of a narrow structure in p(gamma,K_s)X via interference with phi-meson production

We report observation of a narrow peak structure at ~1.54 GeV with a Gaussian width sigma=6 MeV in the missing of K_s in the reaction gamma+p = pK_sK_L. The observed structure may be due to the interference between a strange (or anti-strange) baryon resonance in the pK_L system and the phi(K_sK_L) photoproduction leading to the same final state. The statistical significance of the observed excess of events estimated as the log likelihood ratio of the resonant signal+background hypothesis and the phi-production based background only hypothesis corresponds to 5.3 sigma.

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Differential cross sections of the charge-exchange reaction pi- p --> pi0 n in the momentum range from 103 to 178 MeV/c

Measured values of the differential cross sections for pion-nucleon charge exchange, pi- p --> pi0 n, are presented for pi- momenta of 103, 112, 120, 130, 139, 152, and 178 MeV/c. Complete angular distributions were obtained by using the Crystal Ball detector at the Alternating Gradient Synchrotron at Brookhaven National Laboratory. Statistical uncertainties of the differential cross sections vary from 3% to 6% in the backward angle region, and from 6% to about 20% in the forward region with the exception of the two most forward angles. The systematic uncertainties are estimated to be about 3% for all momenta.

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How is Exotics Produced ? Where to Search for it ?

On the basis of existing data, we suggest such a mechanism of production for exotic hadrons that can explain, at least qualitatively, why the Theta+-baryon is seen in some experiments and not in others. With our hypothesis, production of exotic hadrons is a new kind of hard processes. We also can propose new experiments to check (and confirm?) existence of exotics and to provide new important information about both exotic and conventional hadrons [The talk is based on the papers by Ya. Azimov, K. Goeke, and I. Strakovsky, Phys Rev D 76, 074013 (2007)]

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A possible explanation why the Theta+ is seen in some experiments and not in others

To understand the whole set of positive and null data on the Theta+(1530)-production, we suggest the hypothesis that multiquark hadrons are mainly generated from many-quark states, which emerge either as short-term hadron fluctuations, or as hadron remnants in hard processes. This approach allows us to describe both non-observation of the Theta+ in current null experiments and peculiar features of its production in positive experiments. Further, we are able to propose new experiments that might be decisive for the problem of the Theta+ existence. Distributions of the Theta+ in such experiments can give important information both on higher Fock components of conventional hadrons and about structure and hadronization properties of hadron remnants produced in hard processes. We also explain that description of multiquark hadrons may require a modified form of the constituent quark model, with quark masses and couplings being intermediate between their values for the familiar constituent quarks and the current ones.

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K*-couplings for the antidecuplet excitation

We estimate the coupling of the K* vector meson to the N-->Theta+ transition employing unitary symmetry, vector meson dominance, and results from the GRAAL Collaboration for eta photoproduction off the neutron. Our small numerical value for the coupling constant is consistent with the non-observation of the Theta+ in recent CLAS searches for its photoproduction. We also estimate the K*-coupling for the N-->Sigma* excitation, with Sigma* being the Sigma-like antidecuplet partner of the Theta+-baryon.

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Extraction of radiative decay width for the non-strange partner of Theta^+

Using the results of the GRAAL collaboration on the ηphotoproduction from the neutron target, we attempt to extract the partial radiative width of the possible new nucleon resonance N^*(1675). The obtained estimates support this resonance to be a very attractive candidate for the non-strange member of the exotic antidecuplet of baryons -- a partner of the Θ^+ pentaquark. Our phenomenological value for the transition magnetic moment μ(n^* n), appears to be in good agreement with predictions of the Chiral Quark Soliton Model.

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Multipole Analysis of Recent Pion Electroproduction Data

We have performed a series of global and single-Q2 fits to the growing database for pion electroproduction. We compare to some other recent fits which have used different formalisms, more selective datasets, and truncated multipole expansions. We present results for the E2/M1 and S2/M1 ratios, as a function of Q2.

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Precise Measurement of Sigma Beam Asymmetry for Positive Pion Photoproduction on the Proton from 800 to 1500 Mev

The Sigma beam asymmetry for positive pion photoproduction on the proton has been measured over an angular range of 40-170 deg at photon energies from 0.8 to 1.5 GeV. The resulting data set includes 237 accurate points, 136 of these belonging to an almost unexplored domain above 1.05 GeV. Data of such high precision provide severe constraints for partial wave analyses. The influence of this experiment on the GW multipole analysis is demonstrated. Significant changes are found in multipoles connected to the S31(1620) and P13(1720) resonances. Comparisons using the MAID analysis are also presented.

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Multipole Analysis of a Benchmark Data Set for Pion Photoproduction

We have fitted low- and medium-energy benchmark datasets employing methods used in the MAID/SAID and dynamical model analyses. Independent fits from the Mainz, RPI, Yerevan, and Kharkov groups have also been performed over the low-energy region. Results for the multipole amplitudes are compared in order to gauge the model-dependence of such fits, given identical data and a single method for error handling.

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