The 26th International Nuclear Physics Conference, 11-16 September, 2016, Adelaide, Australia

Ahn, J.K.

Akazawa, Y.

Aoki, K.

Botta, E.

Ekawa, H.

Evtoukhovitch, P.

Feliciello, A.

Fujita, M.

Gogami, T.

Hasegawa, S.

Hasegawa, T.

Hayakawa, S.H.

Hayakawa, T.

Honda, R.

Hosomi, K.

Ichikawa, Y.

Imai, K.

Jung, W.

Kanatsuki, S.

Kim, S.H.

Kimbara, S.

Kobayashi, K.

Lee, J.Y.

Marcello, Simonetta

Miwa, K.

Moon, T.

Nagae, Tomofumi

Nakada, Y.

Nakagawa, M.

Nanamura, T.

Naruki, M.

Sakaguchi, A.

Sako, H.

Sasaki, Y.

Sato, S.

Shirotori, K.

Sugimura, H.

Takahashi, T.

Tamura, H.

Tanida, Kiyoshi

Tsamalaidze, Z.

Ukai, M.

Yamamoto, T.O.

English

Tomofumi Nagae

S. Kanatsuki

T. Gogami

H. Ekawa

T. Nanamura

M. Naruki

S. Hasegawa

K. Hosomi

Y. Ichikawa

K. Imai

H. Sako

S. Sato

H. Sugimura

Kiyoshi Tanida

K. Kobayashi

S.H. Hayakawa

T. Hayakawa

R. Honda

Y. Nakada

M. Nakagawa

A. Sakaguchi

Y. Akazawa

M. Fujita

K. Miwa

Y. Sasaki

H. Tamura

T.O. Yamamoto

K. Aoki

T. Takahashi

M. Ukai

J.K. Ahn

W. Jung

S.H. Kim

E. Botta

A. Feliciello

Simonetta Marcello

P. Evtoukhovitch

Z. Tsamalaidze

J.Y. Lee

T. Moon

S. Kimbara

T. Hasegawa

K. Shirotori

Crossref

Search For A Ξ Bound State In The 12C(K-,K+)X Reaction At 1.8 Gev/c

Kyoto University Research Information Repository

RIGHT:URL:CITATION:AUTHOR(S):ISSUE DATE:TITLE:Search For A Ξ Bound State In The12C(K-,K+)X Reaction At 1.8 Gev/cNagae, Tomofumi; Kanatsuki, S.; Gogami, T.;Ekawa, H.; Nanamura, T.; Naruki, M.; Hasegawa, S.;... Kimbara, S.; Hasegawa, T.; Shirotori, K.Nagae, Tomofumi ...[et al]. Search For A Ξ Bound State In The 12C(K-,K+)X Reaction At 1.8Gev/c. Proceedings of Science 2017, 281: 038.2017-05-09http://hdl.handle.net/2433/259423Copyright owned by the author(s) under the term of the CreativeCommons Attribution-NonCommercial-NoDerivatives 4.0 InternationalLicense.PoS(INPC2016)038Observation of Ξ bound states in the 12C(K−,K+)Xreaction at 1.8 GeV/c in J-PARCT. Nagae∗, †a S. Kanatsuki,a T. Gogami,ad H. Ekawa,a T. Nanamura,a M. Naruki,aS. Hasegawa,b K. Hosomi,b Y. Ichikawa,b K. Imai,b H. Sako,b S. Sato,b H. Sugimura,beK. Tanida,b K. Kobayashi,c S.H. Hayakawa,c T. Hayakawa,c R. Honda,ce Y. Nakada,cM. Nakagawa,c A. Sakaguchi,c Y. Akazawa,d M. Fujita,d K. Miwa,d Y. Sasaki,dH. Tamura,d T.O. Yamamoto,de K. Aoki,e T. Takahashi,e M. Ukai,e J.K. Ahn, fW. Jung, f S.H. Kim, f E. Botta,gm A. Feliciello,g S. Marcello,gm P. Evtoukhovitch,hZ. Tsamalaidze,h J.Y. Lee,i T. Moon,i S. Kimbara, j T. Hasegawa,k K. Shirotorila Department of Physics, Kyoto University, Kitashirakawa, Japanb JAEA, Tokai, Japanc Department of Physics, Osaka University, Toyonaka, Japand Department of Physics, Tohoku University, Sendai, Japane IPNS, KEK, Tsukuba, Japanf Korea University, Seoul, Koreag INFN Sezione di Torino, Torino, Italyh JINR, Dubna, Russiai Seoul National University, Seoul, Koreaj Gifu University, Gifu, Japank Kitasato University, Sagamihara, Japanl RCNP, Osaka University, Ibaraki, Japanm Università di Torino, Torino, ItalyE-mail (speaker): nagae@scphys.kyoto-u.ac.jpWe have carried out a pilot data taking of the J-PARC E05 experiment to search for the boundstate peaks of 12Ξ Be in the12C(K−,K+)X reaction at 1.8 GeV/c. The measurement was performedat the K1.8 beam line of the J-PARC hadron experimental hall with a typical K− beam intensity of6×105 every six seconds. So far the best energy resolution of about 6 MeVFWHM was achievedwith the existing SKS spectrometer. With a reasonable statistics, we have succeeded to observepeak structures in the bound region, which seems to suggest that the potential depth of Ξ wouldbe deeper than 14 MeV estimated in the previous measurements.The 26th International Nuclear Physics Conference11-16 September, 2016Adelaide, Australia∗Speaker.†This work was supported by JSPS KAKENHI Grant Numbers 23000003 and JP16H02186.c⃝ Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/A Self-archived copy inKyoto University Research Information Repositoryhttps://repository.kulib.kyoto-u.ac.jpPoS(INPC2016)038Observation of Ξ Bound States in J-PARC T. Nagae1. IntroductionBy extending the nucleon-nucleon force in flavor SU(3), we would like to establish a modernpicture of generalized nuclear force models, in relation to quantum chromo-dynamics (QCD). Theforces between two octet baryons, such as N-N, Y -N, Y -Y , are our subjects. Here the Y standsfor a baryon with strangeness, that is a hyperon (Λ,Σ,Ξ). Because of the short life time (10−10sec.) of the hyperons, it is difficult to carry out Y -N, Y -Y scattering measurements. Therefore,we have been investigating the spectroscopy of hypernuclei to extract the information on the Y -Nand Y -Y interactions. For example, the fine structure of p-shell Λ hypernuclei has revealed spin-dependent interactions of Λ-N force. A precious information on the Λ-Λ force was obtained in the"Nagara" event in emulsion [1, 2]. Knowing that the Σ-N force is repulsive on average, the lastkey information is the Ξ-N interaction. In a recent study of Lattice QCD calculations [3], the Ξ-Ninteraction potentials are available for various S =−2 channels, although the pion mass is still largecompared to its physical mass. Such a comparison between theory and experiment in near futurewould be very much interesting.Whether the Ξ hypernuclei exist as bound states or not has been a long-standing issue instrangeness nuclear physics. A recent observation of the "Kiso" event [4] in emulsion stronglysuggests that a bound state of 14N-Ξ− system exists with a binding energy larger than the 3Datomic-state binding energy of 0.17 MeV. They observed the process Ξ−+14N→10Λ Be+5ΛHe withthe binding energy of Ξ− to be 4.38±0.25 MeV1. If the 10Λ Be is in an excited level, the bindingenergy is 1.11±0.25 MeV.There were, so far, two measurements on the 12C(K−,K+)X reaction by KEK E224 [5] andBNL E885 [6]. The latter measurement has better statistics. In fact, the BNL E885 claimed ev-idence for the 12Ξ Be bound state from a shape analysis near the binding threshold. The potentialdepth of Ξ was estimated to be about 14 MeV, for which the binding energy was about 4.5 MeV.However, they were not able to observe any bound-state peaks because of their poor energy reso-lution of 14 MeVFWHM.The J-PARC E05 experiment is looking for the 12Ξ Be bound states as clear peaks to determinethe binding energies. A new spectrometer called "Strangeness -2 Spectrometer (S-2S)" is now un-der construction2 dedicated to the (K−,K+) reaction spectroscopy with an energy resolution of ≲2MeVFWHM. The solid angle acceptance is about 55 msr. With this energy resolution, the infor-mation on the conversion width of the Ξ bound state would be obtained. If the conversion widththrough the Ξ−p → ΛΛ is large, there is a possibility to excite double-Λ hypernuclei, especiallytheir excited levels, directly in the (K−,K+) reaction. However, the present K− beam intensityat J-PARC hadron experimental hall is too low to achieve both good energy resolution and highstatistics. Thus, we planned to perform a pilot run having reasonable statistics and moderate en-ergy resolution. With a thick target, we are not able to have a good energy resolution due to energyloss straggling in the target, while we can get more statistics.1Updated values were reported in this conference by K. Nakazawa.2A talk on "S-2S" was presented by S. Kanatsuki in this conference.1A Self-archived copy inKyoto University Research Information Repositoryhttps://repository.kulib.kyoto-u.ac.jpPoS(INPC2016)038Observation of Ξ Bound States in J-PARC T. Nagae2. E05 pilot runWe have carried out a pilot run of the J-PARC E05 experiment at the K1.8 beam line of J-PARC hadron experimental hall, from October 26 to November 19 in 2015. The typical K− beamintensity at 1.8 GeV/c was 6× 105K− per spill (2.4-sec. beam duration every 6 seconds) with aprimary proton beam power of 39 kW. The K−/π− ratio was about 0.8. We took data using a9.36-g/cm2 12C target for about 10 days and a 9.54-g/cm2 CH2 target for about 2 days. The CH2target was used for the p(K−,K+)Ξ− reactions at five different incident momenta between 1.5 and1.9 GeV/c. The total number of K−’s irradiated on targets amounted to 100 G.0 3 mQ13Q12D4BH2BACBC4BC3Q11Q10MS2Q9BFTBH1SKSSDC1SDC2SDC3SDC4TOFACLCTargetFigure 1: A schematic view of the experimental setup during the E05 pilot run.The experimental setup of the E05 pilot run is shown in Fig. 1. In the upstream part of the K1.8beam line, we have a double-stage electrostatic separator system, and the K− beam was finally se-lected through a mass slit (MS2). The incident K− beam was defined with two sets of plasticscintillation counters (BH1 and BH2) and an aerogel Čerenkov counter (BAC) to suppress π−’s atthe on-line trigger level. The beam momentum was analyzed track by track with the tracking de-tectors (BFT, BC3 and BC4). The beam line spectrometer was composed of 4 quadrupole magnets2A Self-archived copy inKyoto University Research Information Repositoryhttps://repository.kulib.kyoto-u.ac.jpPoS(INPC2016)038Observation of Ξ Bound States in J-PARC T. Nagae(Q10-Q13) and one dipole magnet (D4). The design momentum resolution of the spectrometer was∆p/p ∼ 5×10−4FWHM .The outgoing K+ was momentum analyzed with the SKS spectrometer with four sets of driftchambers (SDC1-4). In the pilot run, the SKS central momentum was set at about 1.3 GeV/ccovering about 110 msr. The momentum resolution of the SKS was ∆p/p ∼ 3×10−3. These aregreat advantages of the SKS compared with the spectrometers used for the (K−,K+) reaction inthe past.The particles detected in the SKS were identified with a time of flight measured with a plasticsscintillation counter wall of "TOF" correcting with the flight path and momentum obtained fromthe tracking chambers. The aerogel Čerenkov counter wall "AC" was installed to suppress pions inthe trigger level, and the "Lucite Counter (LC)" for suppression of protons.3. Analysis for the p(K−,K+)Ξ− reactionThe overall energy resolution was evaluated with the peak for the p(K−,K+)Ξ− reaction at1.8 GeV/c as shown in Fig. 2, and was confirmed to be 5.4 MeVFWHM . It is a factor of two betterwith respect to the 10 MeV of the BNL E885 for proton target. In the Carbon kinematics, theBNL E885 resolution was estimated to be 14 MeV, while our estimate is about 6 MeV which wasdominated by the target energy loss straggling. Absolute energy scale of the (K−,K+) missingmass was adjusted with this peak position. About 6,000 Ξ−’s were identified per day.Missing Mass (GeV)1.3 1.31 1.32 1.33 1.34 1.35Counts /0.5 MeV0100200300400500) at 1.8 GeV/c+, K-Missing mass p(KFigure 2: Missing-mass spectrum for the p(K−,K+)X reaction at 1.8 GeV/c obtained with a CH2 target. Thepeak corresponds to the reaction on proton and the continuous background below the peak is the contributionof quasi-free Ξ production from 12C.The incident momentum dependence of the forward cross sections of the K−+ p → K++Ξ−reaction was obtained at 1.5, 1.6, 1.7, 1.8 and 1.9 GeV/c. The statistics at each momentum wasmore than about 20 times of the statistics of the old bubble chamber data. At 1.8 GeV/c, thestatistics was about 100 times to obtain the angular distribution in detail. In a preliminary analysis,we have confirmed with improved statistics that the relative strengths of the cross sections peaks atabout 1.8 GeV/c as suggested by Dover and Gal [7].3A Self-archived copy inKyoto University Research Information Repositoryhttps://repository.kulib.kyoto-u.ac.jpPoS(INPC2016)038Observation of Ξ Bound States in J-PARC T. Nagae-B.E. [MeV]180− 160− 140− 120− 100− 80− 60− 40− 20− 0 20Counts /2 MeV0246810121416182022) (Carbon + CH2)+, K-C(K12Binding energy Unphysical BackgroundPreliminary-B.E. (MeV)50− 0 50 100 150 200 250 300 350 400(Counts /2 MeV)0100200300400500600700) (Carbon)+, K-C(K12Binding energy a) b)Figure 3: Missing-mass spectra for the 12C(K−,K+)X reaction at 1.8 GeV/c. a) in a wide energy range, andb) a close-up view near the binding energy threshold indicated with a dashed oval in a).4. Analysis for the 12C(K−,K+) reactionThe missing mass spectrum of the 12C(K−,K+) reaction is shown in Fig. 3-a) as a function ofnegative values of binding energy (B.E.) of Ξ− in 11B. Owing to the large momentum acceptanceof the SKS, we were able to obtain the spectrum in a wide energy range. The largest part ofthe spectrum comes from the quasi-free production of Ξ’s peaking at about 110 MeV, and thecontribution of Ξ∗ can be seen in the higher energy region. Due to the large momentum transferredto Ξ (∼550 MeV/c) in this reaction, the sticking probability is small, so that there are very fewevents in the bound region (−B.E. < 0) in this vertical scale. A large shift of the quasi-free peak isalso due to the large momentum transfer.Next we magnify the view in the bound region as indicated in Fig. 3-a) as a dashed oval, andplot it in Fig. 3-b). Here, there should be no physical processes with a binding energy value largerthan about 40 MeV (−B.E.≤ −40 MeV), where the ground state of 12ΛΛBe exists. Therefore, wethink these events in this region should be the background, mostly coming from Kaon decay-in-flight and having almost flat distribution (1.08 counts/ 2 MeV). Then, we observed a significantevent excess of about 55 events in the binding energy region between 0 and 20 MeV (−20 MeV≥−B.E.≥0 MeV) shown in red in Fig. 3-b). This is the same level of statistics obtained by the BNLE885 (42-67 events). Above the binding threshold (−B.E.≥ 0 MeV), we clearly see a rise for thequasi-free production.The momentum acceptance of the SKS corresponding to this energy region is almost flat. We,therefore, tried a fit to the spectrum assuming there exist two gaussian peaks in the bound regionand a quasi-free component as a straight line convoluted with the detector energy resolution of 5.4MeV. We also assumed a flat background component as suggested from the Fig. 3-b). The fittingresult is shown in Fig. 4. The red solid line is the fit where the dotted lines show each component.The peak positions were obtained to be about 9 MeV and 2 MeV. If this is the case, the potentialdepth of the Ξ in A = 12 must be deeper than 14 MeV. Alternative idea could be that there existsone broad structure in the bound region; assuming such a fit model the width was about 16 MeVand the peak position was still deep at around 7 MeV. Such a large width is not expected in Ξ4A Self-archived copy inKyoto University Research Information Repositoryhttps://repository.kulib.kyoto-u.ac.jpPoS(INPC2016)038Observation of Ξ Bound States in J-PARC T. Nagae-B.E. [MeV]80− 70− 60− 50− 40− 30− 20− 10− 0 10 20Counts /2 MeV0246810121416182022) (Carbon + CH2)+, K-C(K12Binding energy PreliminaryFigure 4: Missing-mass spectrum for the 12C(K−,K+)X reaction at 1.8 GeV/c near the binding energythreshold fitted with two gaussian peaks. The width was fixed at 6 MeVFWHM .hypernuclei, and it is statistically significant of the two peak structure.5. SummaryIn the pilot run of the J-PARC E05 experiment, we were able to observe peak structures inthe bound region of the 12C(K−,K+)X reaction. The energy resolution was estimated to be about6 MeVFWHM which was a great improvement from the previous resolution of 14 MeV. With thisimprovement of the energy resolution, we were able to observe the peaks in the bound region; atleast, two peaks. The analysis is still preliminary, but a fit to the spectrum with two gaussianssuggest the binding energy would be about 9 MeV. It implies that the potential depth of the Ξ inA = 12 would be deeper than 14 MeV.References[1] H. Takahashi et al., Observation of a 6ΛΛHe double hypernucleus, Phys. Rev. Lett. 87, 212502 (2001).doi:10.1103/PhysRevLett.87.212502[2] J. K. Ahn et al. [E373 (KEK-PS) Collaboration], Double-Λ hypernuclei observed in a hybridemulsion experiment, Phys. Rev. C 88, no. 1, 014003 (2013). doi:10.1103/PhysRevC.88.014003[3] K. Sasaki et al. [HAL QCD Collaboration], Coupled-channel approach to strangeness S = −2baryonfbayron interactions in lattice QCD, PTEP 2015, no. 11, 113B01 (2015)doi:10.1093/ptep/ptv144 [arXiv:1504.01717 [hep-lat]].[4] K. Nakazawa et al., The first evidence of a deeply bound state of Ξ14N system, PTEP 2015, no. 3,033D02 (2015). doi:10.1093/ptep/ptv0085A Self-archived copy inKyoto University Research Information Repositoryhttps://repository.kulib.kyoto-u.ac.jpPoS(INPC2016)038Observation of Ξ Bound States in J-PARC T. Nagae[5] T. Fukuda et al. [E224 Collaboration], Cascade hypernuclei in the (K−,K+) reaction on 12C, Phys.Rev. C 58, 1306 (1998). doi:10.1103/PhysRevC.58.1306[6] P. Khaustov et al. [AGS E885 Collaboration], Evidence of Ξ hypernuclear production in the12C(K−,K+)12Ξ Be reaction, Phys. Rev. C 61, 054603 (2000) doi:10.1103/PhysRevC.61.054603[nucl-ex/9912007].[7] C. B. Dover and A. Gal, Ξ Hypernuclei, Annals Phys. 146, 309 (1983).doi:10.1016/0003-4916(83)90036-26A Self-archived copy inKyoto University Research Information Repositoryhttps://repository.kulib.kyoto-u.ac.jp

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Search For A Ξ Bound State In The 12C(K-,K+)X Reaction At 1.8 Gev/c

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