The ratio of forward K+ production on copper, silver and gold targets to that
on carbon has been measured at proton beam energies between 1.5 and 2.3 GeV as
a function of the kaon momentum p_K using the ANKE spectrometer at
COSY-Juelich. The strong suppression in the ratios observed for p_K<200-250
MeV/c can be ascribed to a combination of Coulomb and nuclear repulsion in the
K+A system. This opens a new way to investigate the interaction of K+-mesons in
the nuclear medium. Our data are consistent with a K+A nuclear potential of
V_K~20 MeV at low kaon momenta and normal nuclear density. Given the
sensitivity of the data to the kaon potential, the current experimental
precision might allow one to determine V_K to better than 3 MeV.Comment: 9 pages, 3 figures; changed conten

Buescher, M.

Cassing, W.

Hartmann, M.

Hejny, V.

Kleber, V.

Koch, H. R.

Koptev, V.

Maeda, Y.

Maier, R.

Merzliakov, S.

Mikirtychiants, S.

Nekipelov, M.

Ohm, H.

Petrus, A.

Prasuhn, D.

Rathmann, F.

Rudy, Z.

Schleichert, R.

Schneider, H.

Sistemich, K.

Stein, H. J.

Stroeher, H.

Watzlawik, K. -H.

Wilkin, C.

Physics Letters B

English

arXiv

The ratio of forward K+ production on copper. silver and gold targets to that on carbon has been measured at proton beam energies between 1.5 and 2.3 GeV as a function of the kaon momentum p(K) using the ANKE spectrometer at COSY-Julich. The strong suppression in the ratios observed for p(K) < 200-250 MeV/c may be ascribed to a combination of Coulomb and nuclear repulsion in the K+ A system. This opens a new way to investigate the interaction of K+-mesons in the nuclear medium. Our data are consistent with a K+ A nuclear potential of V-K(0) approximate to 20 MeV at low kaon momenta and normal nuclear density, Given the sensitivity of the data to the kaon potential, the current experimental precision might allow one to determine V-K(0) to better than 3 MeV. (C) 2002 Elsevier Science B.V. All rights reserved

Büscher, M.

Ströher, H.

Watzlawik, K.-H.

Juelich Shared Electronic Resources

arXiv:nucl-ex/0202021v2  17 May 2002Evidence of kaon nuclear and Coulomb potential effectson soft K+ production from nuclei.M. Nekipelov a,b,1, M. Bu¨scher a, W. Cassing c, M. Hartmann a,V. Hejny a, V. Kleber a, H.R. Koch a, V. Koptev b, Y. Maeda a,R. Maier a, S. Merzliakov d, S. Mikirtychiants b, H. Ohm a,A. Petrus d, D. Prasuhn a, F. Rathmann a, Z. Rudy a,e,R. Schleichert a, H. Schneider a, K. Sistemich a, H.J. Stein a,H. Stro¨her a, K.-H. Watzlawik a, C. Wilkin faInstitut fu¨r Kernphysik, Forschungszentrum Ju¨lich, D-52425 Ju¨lich, GermanybHigh Energy Physics Department, Petersburg Nuclear Physics Institute, 188350Gatchina, RussiacInstitut fu¨r Theoretische Physik, Justus Liebig Universita¨t Giessen, D-35392Giessen, GermanydLaboratory of Nuclear Problems, Joint Institute for Nuclear Research, Dubna,141980 Dubna, Moscow Region, RussiaeM. Smoluchowski Institute of Physics, Jagellonian University, Reymonta 4,PL-30059 Cracow, PolandfPhysics Department, UCL, Gower Street, London WC1 6BT, EnglandAbstractThe ratio of forward K+ production on copper, silver and gold targets to that oncarbon has been measured at proton beam energies between 1.5 and 2.3 GeV asa function of the kaon momentum pK using the ANKE spectrometer at COSY-Ju¨lich. The strong suppression in the ratios observed for pK < 200–250 MeV/c maybe ascribed to a combination of Coulomb and nuclear repulsion in the K+A system.This opens a new way to investigate the interaction of K+-mesons in the nuclearmedium. Our data are consistent with a K+A nuclear potential of V 0K ≈ 20MeV atlow kaon momenta and normal nuclear density. Given the sensitivity of the data tothe kaon potential, the current experimental precision might allow one to determineV 0K to better than 3MeV.Key words: Kaon production, Coulomb suppressionPACS: 13.60.Le, 13.75.Jz, 14.40.Aq, 24.40.-h1 Corresponding author. E-mail address: m.nekipelov@fz-juelich.dePreprint submitted to Elsevier Preprint 8 February 2008Final state interactions of K+ mesons in nuclei are generally considered tobe rather small, due to their strangeness of S = +1. As a consequence, theproduction of K+-mesons in proton-nucleus collisions is of great importanceto learn about either cooperative nuclear phenomena or high momentum com-ponents in the nuclear many-body wave function. This is particularly the casesince the production of kaons, being relatively heavy as compared to pions,requires strong medium effects.Several groups have made experimental and theoretical studies of total anddoubly-differential K+-production cross sections over a wide range of protonbeam energies [1–12]. These studies show that a two-step reaction mechanism,involving the production of an intermediate ∆ or pi, dominates below thethreshold of the elementary pN → K+ΛN reaction (Tp = 1.58 GeV). A strongtarget mass dependence of the production rate may be a good indicator forthe dominance of such secondary mechanisms. At higher energies the role ofthe secondary effects decreases, especially in the high momentum part of thekaon spectra, where direct production dominates [13].It is, however, clear that the repulsive Coulomb potential in the target nucleuswill distort the soft part of the momentum spectrum. Furthermore, since theK+ nuclear potential, though small, is also repulsive [14], with a strengthrather similar to that of the Coulomb for a heavy nucleus, this distortionwill be reinforced. For this reason there have been several publications whichhave stressed the importance of including the effects of Coulomb and nuclearpotentials on the propagation of mesons in the nuclear medium [14–16]. Sucheffects can change the interpretation of the shape of the K+ spectrum as wellas of the mass dependence of the cross sections; therefore they have to betaken into account in the interpretation of the experimental results.The influence of final state rescattering effects in meson production can beinvestigated experimentally for high momentum mesons by measuring two-body reactions, e.g. pA → (A + 1)∗ pi+ for pions or pA → Λ(A + 1)∗K+ forkaons, using high precision spectrometers. However, just as for β±-decay, muchstronger Coulomb effects are expected in the very low momentum part of themeson spectrum, but there have as yet been no direct experimental tests, atleast for kaons. A reliable way of studying this phenomenon is by measuringdirectly ratios of cross sections for different nuclei, since many of the possiblesystematic errors cancel out. Measurements of cross section ratios for mesonsof different charges, e.g. pi+/pi− or K+/K− are, as a rule, clouded by the dif-ferences in reaction mechanisms. Since most of the measurements were carriedout for high pion and kaon momenta, the existing experimental data [17–20]are not very informative regarding both Coulomb and kaon potentials. Thisis changing with the commissioning of the ANKE spectrometer, which is cur-rently the only device that is able to measure K+ mesons with momenta downto ≈ 150 MeV/c.2Measurements of K+ momentum spectra resulting from proton-nucleus colli-sions have been performed with the ANKE spectrometer [21] at the COoolerSYnchrotron COSY-Ju¨lich. A detailed description of the kaon detection sys-tem is given in Ref. [22]. The criteria for the kaon identification and the pro-cedure of measurements are briefly described as follows: The COSY protonbeam, with an intensity of (2− 4)× 1010 protons and a cycle time of ∼ 60 s,was accelerated to the desired energy in the range Tp = 1.5− 2.3 GeV on anorbit below the target. The targets were thin strips of C, Cu, Ag or Au with athickness of (40–1500)µg/cm2. Over a period of ∼ 50 s, the beam was slowlybrought up to the target by steerers, keeping the trigger rate in the detectorsnearly constant at (1000–1500) s−1, a level that could be handled by the dataacquisition system with a dead time of less than 25%. Ejectiles with horizontalangles in the range ±12◦, vertical angles up to ±7◦, and momenta between 150and 600MeV/c, were deflected by the ANKE dipole magnet, passed one of 23plastic scintillation counters and 6 planes of 2.5mm wire-step MWPCs. Theywere then focussed onto one of the 15 kaon range telescopes, each consisting ofthree plastic scintillator counters (stop, ∆E and veto) and two degraders. The10 cm wide telescopes, placed at the focal surface of the ANKE spectrometer,defined ∼ 10% momentum bites in ejectile momenta. The momentum rangescovered by each telescope were kept constant for the different beam energiesby operating ANKE at constant magnetic field strength and maintaining therelative target-dipole-detector geometry. The thicknesses of the scintillatorsand degraders in the telescopes were chosen so as to stop kaons in the de-grader in front of the veto counters. The kaons subsequently decay with amean life time of ≈ 12.4 ns and the products of this decay (pions or muons)are detected by the veto counters with a delay of more than 1.3 ns with re-spect to the signals from the corresponding stop counters. The combination ofthe time-of-flight between the start and stop counters, energy losses in all thescintillators, delayed particle signals, and information from MWPCs resultedin clean kaon spectra, with a background of less than 10% for all beam ener-gies of 1.5GeV and above. The MWPC track reconstruction allowed us alsoto vary the angular and momentum acceptances of the individual telescopes.The ratio R(A/C)pK of the kaon production cross sections from heavy (A) tocarbon (C) targets for a given kaon momentum pK can be calculated from theobserved number of kaons n(K+) in the individual telescopes as:R(A/C)pK =[nA(K+)nC(K+)]pK×LCLA· (1)LC and LA denote the integrated luminosities during data taking with a par-ticular target. The luminosity ratio could be obtained from the number n(pi+)of 500MeV/c pions, measured during pion calibration runs for every energy3and for each target.LCLA=[AC]1/3×[nC(pi+)nA(pi+)]ppi=500MeV/c· (2)All numbers of detected pions and kaons in Eq. (1) and Eq. (2), nC andnA, were individually normalised to the relative luminosities during the cor-responding runs. This relative normalisation was obtained by monitoring theinteraction of the proton beam with the target to an accuracy of 2% usingstop counters 2–5 in four-fold coincidence directly looking at the target [22],thereby selecting ejectiles, produced in the target by hadronic interactions,which bypassed the spectrometer dipole. Pion production cross sections inproton-nucleus reactions have been measured by several groups in the forwarddirection in the 0.73–4.2GeV energy range [18,19,23]. The combined analysisof these data showed that, to within 10%, the ratios of the pion productioncross sections can be scaled with the target mass number as A1/3 [24], whichis used in Eq.(2).The absolute values of the doubly-differential cross sections can be obtainedfrom the numbers of kaons n(K+) identified by each telescope, after correc-tion for luminosities, detection efficiencies in the scintillators and MWPCs,kaon decay between the target and the telescopes, and angle-momentum ac-ceptances. For the cross section ratios, used in our present analysis, absolutevalues are not needed and many uncertainties of the efficiency corrections can-cel out. The presentation of normalised cross sections is deferred until a laterpublication.The gold/carbon ratio is shown in Fig. 1a for proton beam energies of 2.3, 1.75,and 1.5GeV. This ratio has a broadly similar shape at all three energies, withclear maxima for pmax ≈ 245MeV/c coinciding within 2MeV/c. For higherkaon momenta the ratios decrease monotonically with pK and in this regionthe K+ production in gold is relatively stronger at 2.3GeV than at lower ener-gies, reflecting changes in the production mechanism with bombarding energy.For low kaon momenta one sees a dramatic fall in the ratio R(Au/C). To en-sure that this phenomenon is not an artefact of the ANKE detection system,the 2.3GeV run was repeated with a reduced dipole magnetic field, resultingin a change in the values of the momenta that are focussed onto individualrange telescopes. The low pK suppression remained unchanged. Identical ra-tios were also obtained when the polar K+ emission angles in data analysiswere restricted to lie below ϑK = 3◦ with the help of the MWPC information(see Fig.1b).Ratios of kaon-production cross sections for copper, silver and gold targetsmeasured at 2.3GeV are presented in Fig. 2. All data exhibit similar shapes,rising steadily with decreasing kaon momenta, passing a maximum and falling40246810120 100 200 300 400 500 0 100 200 300 400 500 600a bR(Au/C)p (MeV/c)pK (MeV/c)- Tp=1.5GeV- Tp=1.75GeV- Tp=2.3GeV- pi+- K+, θK<12o- K+, θK<3oFig. 1. Ratios of K+(pi+) production cross sections for Au and C measured at differ-ent beam energies (left figure) and kinematic conditions (right figure) as functionsof the laboratory meson momentum. a) All the symbols correspond to kaons mea-sured in the full ANKE acceptance (θ < 12◦). b) The open circles are as in the leftfigure, whereas the closed circles correspond to kaons measured in the restrictedangular interval (θ < 3◦). The pi+-production cross-section ratios of Au to C atTp = 2.3GeV are designated by crosses.steeply at low momenta. The position of the maximum varies with the nucleus,a fit to the data results in pmax(A/C) = 245± 5, 232± 6, and 211± 6MeV/cfor Au, Ag, and Cu, respectively. The error bars include contributions from asystematic uncertainty of about 1% in the absolute value of momentum as wellas the using of different functional forms to fit the points near the maxima.R(A/C)246810100 400- Ag/C- Cu/C- Au/CpK (MeV/c)200 300 5000Fig. 2. Ratios of the K+ production cross sections on Cu, Ag, and Au measured atTp = 2.3GeV as a function of the laboratory kaon momentum.5It is clear from Fig. 1 that the suppression of R(Au/C) at low pK is largelyindependent of beam energy and of the angular acceptance of the spectrom-eter, suggesting that the phenomenon is principally due to the interaction ofthe K+ with the residual nucleus. On the other hand, Fig. 2 shows that theposition of the maximum in R(A/C) increases with A. The situation has aparallel in the well-known suppression of β+ emission in heavy nuclei at lowpositron momenta arising from the repulsive Coulomb field. Thus a K+ pro-duced at rest at some radius R in the nucleus would, in the absence of allother interactions, acquire a momentum of pmin =√2mKVC(R). Taking R tobe the nuclear edge, this purely classical argument leads to a minimum K+momentum for Au of pmin ≈ 130MeV/c.It is, moreover, known from K+ elastic scattering experiments at higher ener-gies [25] that the K+A potential is mildly repulsive, and this is in accord withone-body optical potentials based upon low-energy K+N scattering parame-ters [26]. At normal nuclear density, ρ0 ≈ 0.16 fm−3, the predicted repulsiveK+A potential of strength V 0K ≈ 20− 25MeV [14] would shift pmin to highervalues.In order to see whether the observed low momentum suppression is compati-ble with such a combination of Coulomb and nuclear repulsion, we performedcalculations in the framework of the coupled channel transport model [13,27].In this approach the different mechanisms for the kaon production and theinfluence of average Coulomb and nuclear potentials, as well as hadron rescat-tering effects, which can cause a sudden change of the kaon trajectory whenkaon comes close to a nucleon, can be taken into account using realistic den-sity distributions. Results of the calculations for the R(Au/C) ratio are shownin Fig. 3. Without including the Coulomb and kaon potentials (dashed linein Fig. 3a) the ratio exhibits a smooth momentum dependence with a steadyincrease towards low momenta resulting from the stronger K+ rescatteringprocesses for the Au target. A behaviour of this type was observed in pi+ pro-duction, which was also measured in a short test experiment (see Fig. 1b).In this case the influence of the Coulomb potential is expected to show upbelow ppi ≈ 80MeV/c, which was below our acceptance limit. For kaons thepure Coulomb interaction leads to a distortion of the momentum spectrumand provides a maximum at pK ≈ 200MeV/c (dashed-dotted line in Fig. 3a).It has been checked that a 10% change in the charge radius would move themaximum by less than ±2MeV/c.A much larger change is observed in the calculations when a repulsive kaonnuclear potential is also considered, giving a 40 and 80 MeV/c shift with apotential strength of 20 and 40 MeV, respectively. When a kaon potential ofV 0K = 20MeV is used in the calculations, a reasonable agreement with theexperiment is achieved, with a maximum close to the experimental value of245 ± 5MeV/c (solid line in Fig. 3b). To take into account the absorption6of the incident proton, a baryon potential has also been included. The latterdoes not change the position of the maximum, but makes the agreement withthe experimental data better. A precision of 5 MeV/c in the position of themaximum, taken together with the sensitivity of the data to kaon potentialshown above, might allow one to determine the strength of V 0K to better than3 MeV.024681012141618200 100 200 300 400 500 600pK (MeV/c)R(Au/C)ab024681012Fig. 3. Ratios of K+ production cross sections for Au/C at Tp = 2.3GeV as afunction of the kaon momentum. a) The dash-dotted line is obtained from transportcalculations including only the Coulomb potential, the dotted line corresponds tocalculations with the addition of a kaon potential of 20 MeV at ρ0, whereas thedashed-double-dotted line shows the result where a kaon potential of 40 MeV hasbeen used. The broken line corresponds to simulations without Coulomb and nuclearkaon potentials. In all cases considered here,K+ rescattering in the nucleus has beentaken into account. b) The open circles are the experimental data. The solid lineshows the result of transport calculations starting from the dotted line in the topfigure with a baryon potential added.In summary, we have observed a strong suppression of the ratio of K+ produc-tion by protons on heavy nuclei to that on carbon at low kaon momenta. The7independence of this effect from beam energy and the variation of the struc-ture with A provides clear evidence for the influence of the K+ Coulomb andnuclear interaction potentials. The sensitivity found within our model suggeststhat a careful study of this region will provide a new way to investigate theK+A optical potential at low momenta. For this to be successful, more exten-sive transport calculations or other phenomenological descriptions have to bedeveloped. Our preliminary analysis suggests that the K+ nuclear potentialat normal nuclear matter density is of the order of 20MeV, which is in linewith K+ elastic scattering experiments [25] and low-energy K+N scatteringparameters [26]. Data on K+ production from heavy-ion reactions at GSI alsopoint towards K+ nuclear potentials of about the same strength [28,29]. Wehope that the accuracy of our data will stimulate further development of modelcalculations to provide a better description of the momentum spectra. If thisis done we could expect the strength of the kaon potential to be extractedwith an accuracy better than 3MeV.We wish to acknowledge the assistance we received from the COSY staff whenperforming these measurements at ANKE. Financial support from the fol-lowing funding agencies was invaluable for our work: Georgia (Department ofScience and Technology), Germany (BMBF: grants WTZ-POL-001-99, WTZ-RUS-649-96, WTZ-RUS-666-97, WTZ-RUS-685-99, WTZ-POL-007-99; DFG:436 RUS 113/337, 436 RUS 113/444, 436 RUS 113/561, State of North-RhineWestfalia), Poland (Polish State Committee for Scientific Research: 2 P03B101 19), Russia (Russian Ministry of Science, Russian Academy of Science:99-02-04034, 99-02-18179a) and European Community (INTAS-98-500).References[1] V. Koptev et al., Sov. Phys. JETP 67 (1988) 2177.[2] S. Schnetzer et al., Phys. Rev. C 40 (1989) 640.[3] W. Cassing et al., Phys. Lett. 238 (1990) 25.[4] W. Cassing et al., Z. Phys. A 349 (1994) 77.[5] A.A. Sibirtsev and M. Bu¨scher, Z. Phys. A 347 (1994) 191.[6] M. Bu¨scher et al., Z. Phys. A 335 (1996) 93.[7] M. Debowski et al., Z. Phys. A 356 (1996) 313.[8] A. Badala` et al., Phys. Rev. Lett. 80 (1998) 4863.[9] Yu.T. Kiselev et al., J. Phys. G 25 (1999) 381.[10] E.Ya. Paryev, Eur. Phys. J. A 5 (1999) 307; Eur. Phys. J. A 9 (2000) 521.8[11] V. Koptev et al., Phys. Rev. Lett. 87 (2001) 022310.[12] M. Bu¨scher et al., Phys. Rev. C 65 (2002) 014603.[13] Z. Rudy et al., E-print Archive: nucl-th/0201069.[14] A. Sibirtsev and W. Cassing, Nucl. Phys. A 641 (1998) 476.[15] A. Ayala and J. Kapusta, Phys. Rev. C 56 (1997) 407.[16] S. Teis et al., Z. Phys. A 359 (1997) 297.[17] D.R.F. Cochran et al., Phys. Rev. D 6 (1972) 3085.[18] J.F. Crawford et al., Phys. Rev. C 22 (1980) 1184.[19] V. V. Abaev et al., J. Phys. G 14 (1988) 903.[20] I. Chemakin et al., Phys. Rev. C 65 (2002) 024904.[21] S. Barsov et al., Nucl. Instr. Meth. A 462/3 (2001) 364.[22] M. Bu¨scher et al., Nucl. Instr. Meth. A 481 (2002) 378.[23] J. Papp et al., Phys. Rev. Lett. 34 (1975) 601.[24] S. Barsov et al., Acta Phys. Pol. B 31 (2000) 2159.[25] D. Marlow et al., Phys. Rev. C 25 (1982) 2619.[26] B.R. Martin, Nucl. Phys. B 94 (1975) 413.[27] W. Cassing and E. Bratkovskaya, Phys. Rep. 308 (1999) 65.[28] F. Laue et al., Eur. Phys. J. A 9 (2000) 397; Phys. Rev. Lett. 82 (1999) 1640.[29] Y. Skin et al., Phys. Rev. Lett. 81 (1998) 1576.9

Evidence of kaon nuclear and Coulomb potential effects on soft K+ production from nuclei

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.285.8386

Evidence of kaon nuclear and Coulomb potential effects on soft K+
  production from nuclei

Evidence of kaon nuclear and Coulomb potential effects on soft K+ production from nuclei

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