arXiv subjects

K. Hicks

Publications and source records attributed to K. Hicks.

At least 109 records · Page 6Linked to original sources

arXiv2009

Near-threshold photoproduction of $Λ(1520)$ from protons and deuterons

Photoproduction of $Λ$(1520) with liquid hydrogen and deuterium targets was examined at photon energies below 2.4 GeV in the SPring-8 LEPS experiment. For the first time, the differential cross sections were measured at low energies and with a deuterium target. A large asymmetry of the production cross sections from protons and neutrons was observed at backward K$^{+/0}$ angles. This suggests the importance of the contact term, which coexists with t-channel K exchange under gauge invariance. This interpretation was compatible with the differential cross sections, decay asymmetry, and photon beam asymmetry measured in the production from protons at forward K$^+$ angles.

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arXiv2008

Evidence of the Θ^+ in the γd \to K^+K^-pn reaction

The γd \to K^+K^-pn reaction has been studied to search for the evidence of the Θ^+ by detecting K^+K^- pairs at forward angles. The Fermi-motion corrected nK^+ invariant mass distribution shows a narrow peak at 1.524 +- 0.002 + 0.003 GeV/c^2. The statistical significance of the peak calculated from a shape analysis is 5.1 σ, and the differential cross-section for the γn \to K^- Θ^+ reaction is estimated to be 12 +- 2 nb/sr in the photon energy range from 2.0 GeV to 2.4 GeV in the LEPS angular range by assuming the isotropic production of the Θ^+ in the γn center-of-mass system. The obtained results support the existence of the Θ^+.

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arXiv2008

Cross Sections and Beam Asymmetries for K+ Sigma*- photoproduction from the deuteron at 1.5-2.4 GeV

The $Σ(1385)$ resonance, or $Σ^*$, is well-known as part of the standard baryon decuplet with spin $J=3/2$. Measurements of the reaction $γp \to K^+ Σ^{*0}$ are difficult to extract due to overlap with the nearby $Λ(1405)$ resonance. However, the reaction $γn \to K^+ Σ^{*-}$ has no overlap with the $Λ(1405)$ due to its charge. Here we report the first measurement of cross sections and beam asymmetries for photoproduction of the \sigs from a deuteron target. % where the $K^+$ and $π^-$ are detected in the LEPS spectrometer. The cross sections at forward angles range from 0.4 to 1.2 $μ$b, with a broad maximum near $E_γ\simeq 1.8$ GeV. The beam asymmetries are negative, in contrast to postive values for the $γn \to K^+Σ^-$ reaction.

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arXiv2008

Measurements of polarized photo-pion production on longitudinally polarized HD and Implications for Convergence of the GDH Integral

We report new measurements of inclusive pion production from frozen-spin HD for polarized photon beams covering the Delta(1232) resonance. These provide data simultaneously on both H and D with nearly complete angular distributions of the spin-difference cross sections entering the Gerasimov-Drell-Hearn (GDH) sum rule. Recent results from Mainz and Bonn exceed the GDH prediction for the proton by 22 microbarns, suggesting as yet unmeasured high-energy components. Our pi0 data reveal a different angular dependence than assumed in Mainz analyses and integrate to a value that is 18 microbarns lower, suggesting a more rapid convergence. Our results for deuterium are somewhat lower than published data, considerably more precise and generally lower than available calculations.

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arXiv2008

Photoproduction of Lambda(1405) and Sigma^{0}(1385) on the proton at E_γ= 1.5-2.4 GeV

Differential cross sections for $γp \to K^+Λ(1405)$ and $γp \to K^+Σ^0(1385)$ reactions have been measured in the photon energy range from 1.5 to 2.4 GeV and the angular range of $0.8<\cos(Θ)<1.0$ for the $K^+$ scattering angle in the center-of-mass system. This data is the first measurement of the $Λ(1405)$ photoproduction cross section. The lineshapes of \LamS measured in $Σ^+π^-$ and $Σ^-π^+$ decay modes were different with each other, indicating a strong interference of the isospin 0 and 1 terms of the $Σπ$ scattering amplitudes. The ratios of \LamS production to \SigS production were measured in two photon energy ranges: near the production threshold ($1.5<E_γ<2.0$ GeV) and far from it ($2.0 <E_γ<2.4$ GeV). The observed ratio decreased in the higher photon energy region, which may suggest different production mechanisms and internal structures for these hyperon resonances.

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arXiv2008

Parton fragmentation in the vacuum and in the medium

We present the mini-proceedings of the workshop on ``Parton fragmentation in the vacuum and in the medium'' held at the European Centre for Theoretical Studies in Nuclear Physics and Related Areas (ECT*, Trento) in February 2008. The workshop gathered both theorists and experimentalists to discuss the current status of investigations of quark and gluon fragmentation into hadrons at different accelerator facilities (LEP, B-factories, JLab, HERA, RHIC, and Tevatron) as well as preparations for extension of these studies at the LHC. The main physics topics covered were: (i) light-quark and gluon fragmentation in the vacuum including theoretical (global fits analyses and MLLA) and experimental (data from e+e-, p-p, e-p collisions) aspects, (ii) strange and heavy-quark fragmentation, (iii) parton fragmentation in cold QCD matter (nuclear DIS), and (iv) medium-modified fragmentation in hot and dense QCD matter (high-energy nucleus-nucleus collisions). These mini-proceedings consist of an introduction and short summaries of the talks presented at the meeting.

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arXiv2008

Forward coherent $ϕ$-meson photoproduction from deuterons near threshold

Differential cross sections and decay asymmetries for coherent $ϕ$-meson photoproduction from deuterons were measured for the first time at forward angles using linearly polarized photons at $E_γ$= 1.5-2.4 GeV. This reaction offers a unique way to directly access natural-parity Pomeron dynamics and gluon exchange at low energies. The cross sections at zero degrees increase with increasing photon energy. The decay asymmetries demonstrate a complete dominance of natural-parity exchange processes, showing that isovector unnatural-parity $π$-meson exchange is small. Nevertheless the deduced cross sections of $ϕ$-mesons from nucleons contributed by isoscalar t-channel exchange processes are not well described by the conventional Pomeron model.

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arXiv2007

Cross Sections for the $γp \to K^{*0}Σ^+$ Reaction at $E_γ= 1.7 - 3.0$ GeV

Differential cross sections for the reaction $γp \to K^{*0} Σ^+$ are presented at nine bins in photon energy in the range from 1.7 to 3.0 GeV. The \kstar was detected by its decay products, $K^+π^-$, in the CLAS detector at Jefferson Lab. These data are the first \kstar photoproduction cross sections ever published over a broad range of angles. Comparison with a theoretical model based on the vector and tensor $K^*$-quark couplings shows good agreement with the data in general, after adjusting the model's two parameters in a fit to our data. Disagreement between the data at forward angles and the global angle-energy fit to the model suggests that the role of scalar $κ$ meson exchange in $t$-channel diagrams should be investigated.

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arXiv2007

Backward-angle photoproduction of $π^0$ mesons on the proton at $E_γ$ = 1.5--2.4 GeV

Differential cross sections and photon beam asymmetries for $π^0$ photoproduction have been measured at $E_γ$ = 1.5--2.4 GeV and at the $π^0$ scattering angles, --1 $<$ cos$Θ_{c.m.} <$ --0.6. The energy-dependent slope of differential cross sections for $u$-channel $π^0$ production has been determined. An enhancement at backward angles is found above $E_γ$ = 2.0 GeV. This is inferred to be due to the $u$-channel contribution and/or resonances. Photon beam asymmetries have been obtained for the first time at backward angles. A strong angular dependence has been found at $E_γ>$ 2.0 GeV, which may be due to the unknown high-mass resonances.

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arXiv2007

Measurement of the $\vecγ p \to K^+ Λ$ Reaction at Backward Angles

Cross sections for the $γp \to K^+ Λ$ have been measured at backward angles using linearly polarized photons in the range 1.50 to 2.37 GeV. In addition, the beam asymmetry for this reaction has been measured for the first time at backward angles. The $Λ$ was detected at forward angles in the LEPS spectrometer via its decay to $pπ^-$ and the K^+ was inferred using the technique of missing mass. These measurements, corresponding to kaons at far backward angles in the center-of-mass frame, complement similar CLAS data at other angles. Comparison with theoretical models shows that the reactions in these kinematics provide further opportunities to investigate the reaction mechanisms of hadron dynamics.

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arXiv2007

First measurement of coherent $ϕ$-meson photoproduction on deuteron at low energies

The cross section and decay angular distributions for the coherent ϕmeson photoproduction on the deuteron have been measured for the first time up to a squared four-momentum transfer t =(p_γ-p_ϕ)^2 =-2 GeV^2/c^2, using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cross sections are compared with predictions from a re-scattering model. In a framework of vector meson dominance, the data are consistent with the total ϕ-N cross section σ_{ϕN} at about 10 mb. If vector meson dominance is violated, a larger σ_{ϕN} is possible by introducing larger t-slope for the ϕN \to ϕN process than that for the γN \to ϕN process. The decay angular distributions of the ϕare consistent with helicity conservation.

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arXiv2006

Differential cross section and photon beam asymmetry for the gamma n -> K+ Sigma- reaction at Egamma=1.5-2.4 GeV

Differential cross sections and photon beam asymmetries have been measured for the gamma n -> K+ Sigma- and gamma p -> K+ Sigma0 reactions separately using liquid deuterium and hydrogen targets with incident linearly polarized photon beams of Egamma=1.5-2.4 GeV at 0.6<cosTheta<1. The cross section ratio of sigma(K+Sigma-)/sigma(K+Sigma0), expected to be 2 on the basis of the isospin 1/2 exchange, is found to be close to 1. For the K+Sigma- reaction, large positive asymmetries are observed indicating the dominance of the K*-exchange. A large difference between the asymmetries for the K+Sigma- and K+Sigma0 reactions can not be explained by simple theoretical considerations.

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arXiv2006

Search for the $Θ^+$ pentaquark in the reaction $γd \to p K^- K^+ n$

A search for the \thp in the reaction $γd \to pK^-K^+n$ was completed using the CLAS detector at Jefferson Lab. A study of the same reaction, published earlier, reported the observation of a narrow \thp resonance. The present experiment, with more than 30 times the integrated luminosity of our earlier measurement, does not show any evidence for a narrow pentaquark resonance. The angle-integrated upper limit on \thp production in the mass range of 1.52 to 1.56 GeV/c$^2$ for the $γd \to pK^-Θ^+$ reaction is 0.3 nb (95% CL). This upper limit depends on assumptions made for the mass and angular distribution of \thp production. Using \lamstar production as an empirical measure of rescattering in the deuteron, the cross section upper limit for the elementary $γn \to K^-Θ^+$ reaction is estimated to be a factor of 10 higher, {\it i.e.}, $\sim 3$ nb (95% CL).

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arXiv2006

K^*0 Photoproduction off the proton at CLAS

We report here differential cross sections for the reaction $γp \to K^{*0}Σ^+$ for the first time with high statistics. The measurements were done using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. Data were taken at 3.115 GeV electron beam energy using tagged photons incident upon a liquid hydrogen target. In addition, our analysis provides information about nucleon resonances and their couplings to $K^*$ decay. Our data will be used to test predictions of theoretical models of K$^*$ production, and to provide constraints on the $K^*ΣN$ coupling constant used in these models.

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arXiv2005

Diffractive $ϕ$-meson photoproduction on proton near threshold

Photoproduction of $ϕ$-meson on protons was studied by means of linearly polarized photons at forward angles in the low-energy region from threshold to $E_γ$= 2.37 GeV. The differential cross sections at $t = -|t|_{min}$ do not increase smoothly as $E_γ$ increases, but show a local maximum at around 2.0 GeV. The angular distributions demonstrate that $ϕ$-mesons are photo-produced predominantly by helicity-conserving processes, and the local maximum is not likely due to unnatural-parity processes.

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arXiv2005

Experimental Search for Pentaquarks

The experimental evidence for pentaquarks, both old an new, is discussed. Constraints due to $K^+N$ scattering data from previous decades is first reviewed, followed by experiments with positive evidence and those with null results. Finally, the problem of the narrow width of the Theta+ pentaquark is discussed, along with theoretical implications.

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arXiv2004

An Experimental Review of the $Θ^+$ Pentaquark

Evidence for the $Θ^+$ pentaquark is still sketchy at the present time. This state, if it exists, has a small width and consequently a small production cross section. No single experiment has overwhelming evidence for the $Θ^+$ and some non-observations of the $Θ^+$ are difficult to understand unless this exotic baryon also has an exotic production mechanism. However, new data from the LEPS and CLAS collaborations with higher statistics are on the way, which will likely resolve the question of whether the $Θ^+$ exists.

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arXiv2004

Comment on The Evidence for a Pentaquark and Kinematic Reflections

The Regge exchange model used by Dzierba et al. is shown to be questionable, since the pion pole term is not allowed. Hence the Regge amplitudes in their calculation are exaggerated. The amount of kinematic reflection in the mass spectrum of the (nK+) system, which is one decay channel of a possible pentaquark, is not well justified in the fitting procedure used by Dzierba et al., as shown by comparison with the (K+K-) invariant mass spectrum, which is one decay channel of the a_2 and f_2 tensor mesons. While kinematic reflections are still a concern in some papers that have presented evidence for the pentaquark, better quantitative calculations are needed to demonstrate the significance of this effect.

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