Pion exchange at high energies

We estimate the reaction mechanisms for the photoproduction of the 1540 resonance on the nucleon, through K and K Regge exchanges. We compare the size of the cross sections for the n ! K ÿ and p ! K 0 reactions and investigate their sensitivity to the spin-parity assignments J P 1 2; 3 2 for the resonance. The model allows one to estimate the cross sections corresponding with a given upper bound on the width of the . Within this model, the cross sections on the neutron are found to be around a factor of 5 larger than the ones on the proton, due to the presence of charged K exchange for the reaction on a neutron target. Furthermore, the photon asymmetry is found to display a pronounced sensitivity to the parity of the , making it a very promising observable to help determine the quantum numbers of the resonance.

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“…Refs. [55,61]. In particular, one notices that at the K pole, the prefactors t ÿ m 2 K in the second terms of Eqs.…”

Section: Spin-1=2 Amplitudementioning

confidence: 97%

Photoproduction of theΘ+resonance on the nucleon in a Regge model

et al. 2005

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“…The -pole amplitude by itself is not gauge invariant and charge conservation requires an addition of the electric part of the s-channel nucleon Born term (third diagram in Fig. 1) [7,11]. When considering realistic vertex functions, which include form factors, current conservation is violated and one has to restore the gauge invariance of the model [12].…”

mentioning

confidence: 97%

“…To account for these states we replace in J the -Feynman propagator by the Regge propagator [7]. Furthermore, since the s-channel Born term can generate the pion pole itself [11], we factor out the pion propagator in the sum J þ J s following Ref. [7] and Reggeize it according to the above prescription.…”

mentioning

confidence: 99%

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Deeply inelastic pions in the exclusive reactionp(e,e′π+)nabove the resonance region

2008

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A model for the pðe; e 0 þ Þn reaction which combines an improved treatment of gauge invariant mesonexchange currents and hard deep-inelastic scattering (DIS) of virtual photons off nucleons is proposed. It is shown that DIS dominates and explains the transverse response at moderate and high photon virtualities Q 2 , whereas the longitudinal response is dominated by hadronic degrees of freedom and the pion electromagnetic form factor. This leads to a combined description of the longitudinal and transverse components of the cross section in a wide range of photon virtuality Q 2 and momentum transfer to the target t and solves the longstanding problem of the observed large transverse cross sections. The latter are shown to be sensitive to the intrinsic transverse momentum distribution of partons.At Jefferson Laboratory (JLAB) the exclusive reaction pðe; e 0 þ Þn has been investigated for a range of photon virtualities up to Q 2 ' 5 GeV 2 at an invariant mass of the þ n system around the onset of deep-inelastic regime, W ' 2 GeV [1-3]. A separation of the cross section into the transverse T and longitudinal L components has been performed. The longitudinal cross section L is well understood in terms of the pion quasielastic knockout mechanism [4] because of the pion pole at low Àt. This makes it possible to study the charge form factor of the pion at momentum transfer much bigger than in the scattering of pions from atomic electrons [5]. On the other hand, the T is predicted to be suppressed by $1=Q 2 with respect to L for sufficiently high Q 2 ) Ã 2 QCD [6]. However, the data from the À CT experiment [3] show that T is large at JLAB energies. At Q 2 ¼ 3:91 GeV 2 T is by about a factor of 2 larger than L and at Q 2 ¼ 2:15 GeV 2 it has the same size as L in agreement with previous JLAB measurements [1]. Theoretically, the model of Ref. [7], which is generally considered to be a guideline for the experimental analysis and extraction of the pion form factor, underestimates T at Q 2 ¼ 2:15 GeV 2 and at Q 2 ¼ 3:91 GeV 2 by about 1 order of magnitude [3]. Previous measurements at values of Q 2 ¼ 1:6ð2:45Þ GeV 2 [1] show a similar problem in the understanding of T . Even at smaller JLAB [2] and much higher Cornell [8] values of Q 2 , there is a disagreement between model calculations based on the hadron-exchange scenario and experimental data; see Ref.[9] for a possible interpretation and references therein.In this paper we first generalize the treatment of Ref.[7] for the longitudinal contribution. We then propose a resolution of the T problem. The idea followed here is to complement the soft hadronlike interaction types shown in Fig. 1 which dominate in photoproduction and low Q 2 electroproduction by direct hard interaction of virtual photons with partons followed by the hadronization process into þ n channel, to form the þ -electroproduction framework. As we shall show, then the large T in the reaction pðe; e 0 þ Þn can be readily explained and both L and T can be described from low up to high values of Q 2 .The exclus...

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“…A detailed review of the above models and references to the original works are given by Jones. 6 Theoretical understanding of the CEX region has been hampered by a lack of information about the spin dependence of the interaction over the range in u from 0 to about -0.05 (GeV/c) 2 in which the peak is observed. Except for the analyzing power the only available spin-dependent data 7 are very imprecise values for K NN andif ss near 600 MeV.…”

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confidence: 98%

Spin Dependence in Neutron-Proton Charge Exchange at 790 MeV

Ransome

Hollas²,

Riley³

et al. 1982

Phys. Rev. Lett.

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The analyzing power A and spin-transfer parameters K NN> K ss , K SL , and K LL have been measured in the np charge-exchange (np~~pn) region at 790 MeV. These data provide new and unique information on the spin dependence of the np interaction in the charge-exchange region. Models which explain the charge-exchange peak in the np elastic differential cross section as being due to interference between one-pion exchange and a slowly varying background are in basic agreement with the data.PACS numbers: 13.75.Cs, 21.30.+y During the past 25 years, a number of measurements have been made of the np differential cross section in the charge-exchange (CEX) region over a large energy range. 1 The differential cross section shows a pronounced peak in this region, which when plotted as a function of four-mo menturn transfer squared, u, has a shape nearly independent of energy from about 200 MeV to beyond 60 GeV. This peak has a width of about 0.02 (GeV/c) 2 , roughly equal to that of the mass of the pion (m-^) squared. The width of the peak strongly suggests that one-pion exchange (OPE) is the dominant mechanism; however, the cross section calculated from OPE is proportional to [u/ (u -m*)T in the first Born approximation. This clearly gives a dip in the CEX region, rather than the observed peak, and also gives cross sections too large outside this region. The predictions of Reggeized OPE 2 are closer in magnitude to the observed cross section than simple OPE, 3 but still give a dip in the cross section at u =0. In 1963 Phillips 4 noted that the peak is consistent with a nearly constant background which interferes destructively with the OPE contribution. The many subsequent attempts to explain the origin of such a background fall into two categories, namely absorption and cut models, both based in Regge theory. Absorption models assume that the pion pole is modified by inelastic channels. Regge-cut models evaluate double exchanges. Chia, 5 for example, models the pion-Pomeron exchange and calculates its contribution to np CEX and other reactions, with some success. A detailed review of the above models and references to the original works are given by Jones. 6 Theoretical understanding of the CEX region has been hampered by a lack of information about the spin dependence of the interaction over the range in u from 0 to about -0.05 (GeV/c) 2 in which the peak is observed. Except for the analyzing power the only available spin-dependent data 7 are very imprecise values for K NN andif ss near 600 MeV. In that experiment an average value for the entire CEX region was obtained, and hence little can be deduced about the mechanism responsible for the peak. We report here new measurements of np spin-dependent observables in the CEX region.The experimental setup is shown in Fig. 1. A 790-MeV polarized neutron beam (P"^0.40) is produced via polarization transfer in the reaction p po id-~n pol pp (]K LL = -0.65 andP^0.7) 8 at 0°, and is collimated to a diameter of about 10 cm at the liquid hydrogen target. Two dipole magnets, M x (vert...

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Pion exchange at high energies

Cited by 15 publications

References 154 publications

Photoproduction of theΘ+resonance on the nucleon in a Regge model

Photoproduction of theΘ+resonance on the nucleon in a Regge model

Deeply inelastic pions in the exclusive reactionp(e,e′π+)nabove the resonance region

Spin Dependence in Neutron-Proton Charge Exchange at 790 MeV

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