Backward-angle photoproduction of ω and η ′ mesons from protons at E γ =1.5−3.0 GeV
Y.Morino, Y.Nakatsugawa, M.Yosoi, M.Niiyama, D.S.Ahn, J.K.Ahn, S.Ajimura, W.C.Chang, J.Y.Chen, S.Date, H.Fujimura, S.Fukui, K.Hicks, T.Hiraiwa, T.Hotta, S.H.Hwang, K.Imai, T.Ishikawa, Y.Kato, H.Kawai, M.J.Kim, H.Kohri, Y.Kon, P.J.Lin, K.Mase, Y.Maeda, M.Miyabe, N.Muramatsu, T.Nakano, H.Noumi, Y.Ohashi, T.Ohta, M.Oka, J.D.Parker, C.Rangacharyulus, S.Y.Ryu, T.Saito, T.Sawada, H.Shimizu, E.A.Strokovsky, Y.Sugaya, M.Sumihama, K.Suzuki, K.Tanida, A.Tokiyasu, T.Tomioka, T.Tsunemi, M.Uchida, R.Yamamura, T.Yorita
aa r X i v : . [ nu c l - e x ] J un Backward-angle photoproduction of ω and η ′ mesons from protons at E γ = 1 . − . Y. Morino a,b, ∗ , Y. Nakatsugawa c , M. Yosoi a , M. Niiyama d , D. S. Ahn b , J. K. Ahn e , S. Ajimura a ,W. C. Chang f , J. Y. Chen a , S. Dat´e g , H. Fujimura h , S. Fukui i , K. Hicks j , T. Hiraiwa a , T. Hotta a ,S. .H. Hwang k , K. Imai k , T. Ishikawa l , Y. Kato i , H. Kawai m , M. J. Kim n , H. Kohri a , Y. Kon a ,P. J. Lin f , K. Mase m , Y. Maeda o , M. Miyabe l , N. Muramatsu l , T. Nakano a , H. Noumi a , Y. Ohashi k ,T. Ohta a , M. Oka a , J. D. Parker d , C. Rangacharyulus p , S. Y. Ryu a , T. Saito m , T. Sawada a ,H. Shimizu l , E. A. Strokovsky q , Y. Sugaya r , M. Sumihama s , K. Suzuki r , K. Tanida n , A. Tokiyasu a ,T. Tomioka m , T. Tsunemi a , M. Uchida t , R. Yamamura a , T. Yorita a a Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan b RIKEN Nishina Center for Accelerator-Based Science, Wako, Saitama 351-0198, Japan c KEK, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan d Department of Physics, Kyoto University, Kyoto 606-8502, Japan e Department of Physics, Pusan National University, Busan 609-735, Korea f Institute of Physics, Academia Sinica, Taipei 11529, Taiwan g Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 679-5143, Japan h Wakayama Medical University , Wakayama 641-8509, Japan i Department of Physics and Astrophysics, Nagoya University, Nagoya, Aichi 464-8602, Japan j Department of Physics and Astronomy, Ohio University, Athens, OH 45701, USA k Japan Atomic Energy Agency, Tokai-mura, Ibaraki 319-1195, Japan l Research Center for Electron Photon Science, Tohoku University, Sendai, Miyagi 982-0826, Japan m Department of Physics, Chiba University, Chiba 263-8522, Japan n Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea o Proton Therapy Center, Fukui Prefectural Hospital, Fukui 910-8526, Japan p Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon SK S7N 5E2, Canada q Joint Institute for Nuclear Research, Laboratory of High Energy Physics, 141980, Dubna,Russia r Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan s Gifu University, Gifu 501-1193,Japan t Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan
Abstract
We report the measurement of differential cross sections for ω and η ′ photoproduction from protons atbackward angles ( − . < cos Θ XC.M < − .
8) using linearly polarized photons at E γ =1 . − . ω mesons are larger than the predicted u -channel contribution in the energy range2 . ≤ √ s ≤ . ω and η ′ mesons become closer to the predicted u -channel contribution at √ s > . √ s dependence of the differential crosssections for η ′ mesons was observed at √ s ∼ Keywords:PACS: π N scattering and π photopro-duction. It is well known that a large number of resonances predicted by the constituent quark model remainto be discovered (missing resonance problem)[1, 2]. Some of the missing resonances may not be observeddue to the weak coupling to the pion, but could be observed in the photo-production of other mesons. ∗ Corresponding author
Email address: [email protected] ( Y. Morino)
Preprint submitted to Elsevier September 15, 2018 mong various channels, η and η ′ photoproduction are of special interest since these mesons possess a largecomponent of s ¯ s . The interpretation of the large branching ratio of S (1535) → pη decay has been a topicof a much discussion; it could be a dynamically-generated state or a conventional three quark state[3, 4, 5].A systematic study of nucleon resonances with large couplings to η and η ′ mesons will give important infor-mation to solve this controversy. Recently, differential cross section and polarization variable of η , η ′ and ω mesons have been measured in experiments like CB-ELSA, GRAAL and CLAS with large acceptancespectrometers[6, 7, 8, 9, 10, 11]. Evidence and indication of new resonances have been obtained from par-tial wave analysis (PWA) of their results, although the list of resonances depends on models. There is asignificant contribution from nucleon resonances in the differential cross section of meson photoproductionat large scattering angle (Θ XC.M ∼ π/
2) at √ s ∼ u -channelexchange of Regge trajectories becomes significant. The differential cross section from the u -channel baryonexchange is expected to behave following a power law of s . In general, the differential cross section from the u -channel is much smaller than the one from the t -channel meson exchange. On the other hand, the angulardistribution of mesons from nucleon resonances could have a rapid change at forward and backward angleswhen the nucleon resonances have high angular momenta. The contribution of nucleon resonances withhigh angular momenta tends to be stronger at forward and backward angles than at intermediate angles.Therefore, the differential cross section at backward angles is sensitive and a good tool to identify and searchfor nucleon resonances with high angular momenta. A bump structure in the s dependence of differentialcross section at very backward angles has been observed at SPring-8/LEPS[14].A new measurement was carried out at SPring-8/LEPS with a time projection chamber (TPC) surround-ing the target in order to detect decay products of hadrons. Production of ω and η ′ mesons was clearlyidentified by detecting protons at the LEPS forward spectrometer and pions at the TPC. In comparisonwith the previous LEPS experiment, background events of ω and η ′ signals were reduced substantially byusing TPC [14]. In addition, E γ was extended to 3.0 GeV. In this article, we report the differential crosssections of ω and η ′ photoproduction at backward angles ( − . < cos Θ XC.M < − .
8) from protons in theenergy range E γ = 1.5-3.0 GeV.The experiment was carried out at the SPring-8/LEPS facility[15]. A linearly polarized photon beamin the energy range from 1.5 to 3.0 GeV was produced by backward-Compton scattering (BCS) from thehead-on collision between laser photons and 8-GeV electrons in the storage ring. Both 355-nm and deep-UV257-nm lasers were used to produce Compton-scattered photons in the range of 1.5 to 2.4 GeV and 1.5 to3.0 GeV, respectively. The energy of a scattered photon was determined by measuring the recoil electronfrom Compton scattering by a tagging counter. The energy resolution for the photon beam was about15 MeV. We used a liquid hydrogen (LH ) target with a length of 15 cm and a diameter of 40 mm. The datawas accumulated with 0 . × photons from 1.5 to 2.4 GeV at the target with the 355-nm laser, and with0 . × photons from 1.5 to 3.0 GeV with the 257-nm laser, respectively[17]. Half of the data with the355-nm laser (1.5-2.4 GeV) was taken with vertically polarized photons and the other half with horizontallypolarized photons. The data with the 257-nm laser (1.5-3.0 GeV) was taken only with vertically polarizedphotons.The LEPS forward spectrometer consisted of a dipole magnet, four multiwire drift chambers, a startcounter (SC) just downstream of the target, a silica-aerogel ˇCerenkov counter (AC), and a time-of-flight(TOF) hodoscope placed downstream of the tracking detectors. In this measurement, one multiwire driftchamber was used instead of a silicon-strip vertex detector, which was a different setup from previous LEPSexperiments. The angular coverage relative to the photon beam of the forward spectrometer was about ± ± target[16]. The strengthof the magnetic field was 2 T. The TPC had an active volume of hexagonal cylinder shape with a side lengthof 225 mm and a height of 750 mm. The TPC volume was filled with P gas (Ar:CH π and 0.35-2.25 rad in the laboratory system, respectively. Thesignals from the TPC were read through rectangular cathode pads with a length of 56 mm and 150 mm.2he typical spatial resolutions were 200-400 µ m in the pad plane and 400-4000 µ m in the beam directiondepending on the direction of charged tracks. Six scintillation counters surrounded the target inside theTPC and twelve scintillation counters were placed outside of the TPC. The trigger in this experiment wasgenerated from the coincidence among the tagging counter, any 1 of 6 inner counters and any 1 of 12 outercounters facing a hit inner counter. This trigger required at least one charged particle with p T ≥ .
09 GeV/cin the TPC acceptance. The trigger efficiency saturated at about 94% in the geometrical acceptance of thescintillation counters.
Figure 1: (color online) (a) Correlation plot of mass squared and momentum of positive charged particles measured by the spec-trometer. Dashed line shows boundary for proton identification. (b) Correlation plot of the energy deposition and momentumfor positive charged particles measured by the TPC. Dashed line shows boundary for pion identification.
The ω and η ′ mesons were measured via the following reactions. γp → pω → pπ + π − π (1) γp → pη ′ → pπ + π − η (2)Protons were measured by the LEPS forward spectrometer. Charged pions were detected in the TPC and π or η mesons were identified by missing mass information to select reaction (1) and (2). It was required thatthe number of reconstructed charged tracks in the TPC was one or two. The number of reconstructed electrontracks in the tagging counter was required to be one. Protons were selected by the mass reconstructed frommomentum and TOF information within 4 σ of the nominal value. The momentum of a proton was requiredto be more than 0.7 GeV/c because ω and η ′ mesons were produced only in this range. Figure 1(a) showsthe reconstructed mass square of positive charged particles as a function of momentum. Admixture of tracksfrom particles misidentified as protons was estimated to be negligible ( < . dE/dx ) inthe TPC. Figure 1(b) shows measured dE/dx of positive charged particles as a function of momentum. Thedashed line in Fig. 1(b) shows the boundary for pion identification with >
98% effeciency. The tracks with dE/dX below the boundary line were identified as pions. Although the dE/dx cut for pion identification wasnot tight, tracks from particles misidentified as pions were suppressed since a proton was already detectedin the forward spectrometer. The reconstructed tracks of pions had p T larger than ∼ .
08 GeV/c because3f the center hole in the TPC. The momentum resolution for tracks of the pions was 4%-25%, stronglydepending on momentum and polar angle of pions. A reaction vertex point was reconstructed as the closestpoint between a track reconstructed in the spectrometer and a track in the TPC. The spatial resolution ofthe reaction vertex along the z direction was 2.6 mm. Events produced from the target were selected bya cut on the z coordinate of the reaction vertex. The effect of acceptance, efficiency and resolution of thespectrometer and the TPC were evaluated using a Monte-Carlo simulation with the GEANT3 code[18].The systematic uncertainty for the target thickness including the target shape, fluctuations of the tem-perature, and pressure of the liquid hydrogen was estimated to be 2.0%. The systematic error of the photonnumber normalization was estimated to be 3% for data with the 355-nm laser and to be 4% for data with the257-nm laser, respectively. It includes fluctuation of proton yield per photon and transmission of the photonbeam. The systematic error of contamination from the events from the SC and the target cell was 1%. Thesystematic uncertainty for the efficiency of the spectrometer was 4%, including geometrical acceptance (3%),wire effeciency (1%), and proton identification effeciency (2%). The systematic uncertainty for the TPCefficiency was 4%.Figure 2(a) shows a missing mass spectrum for the γp → pX reaction ( M M ( p )), where the photonenergy is from 2.125 to 2.375 GeV and the scattering angle of protons is 0 . < cos Θ PC.M < .
00. Thespectra of missing mass squared for the γp → pπ ± X and γp → pπ + π − X reactions ( M M ( p, π ± ) and M M ( p, π + , π − )) are also shown in Fig. 2(b) and 2(c), respectively. The peaks of ρ / ω and η ′ mesonsare observed in Fig. 2(a) and the peaks of η and π mesons are not observed clearly due to the triggercondition. Although the peak of ω mesons overlaps with the one of ρ mesons, ω mesons can be separatedfrom ρ meson by identification of the decay products. In Fig. 2(b) and 2(c), the peaks due to 2 π and 3 π production are also seen. To reduce background events for ω and η ′ production, selection cuts were applied for M M ( p, π + / − ) and M M ( p, π + , π − ). Figure 2(d) and 2(e) show the M M ( p ) distribution with the ω and the η ′ selection cuts, respectively. The ω selection cut was − . < M M ( p, π + , π − ) < .
19 GeV /c and 0 . 44 GeV /c . 0 . < M M ( p, π + , π − ) < . 36 GeV /c and 0 . < M M ( p, π + / − ) < . 72 GeV /c were applied for the η ′ selection cut. The background events for ω and η ′ signals were reduceddrastically by applying the selection cuts. Figure 2(f) shows the M M ( p ) distribution with an inverse η ′ selection cut. The condition for M M ( p, π + , π − ) was reversed for the inverse cut (( M M ( p, π + , π − ) < . 24 GeV /c or M M ( p, π + , π − ) > . 36 GeV /c ) and 0 . < M M ( p, π + / − ) < . 72 GeV /c ). This plotwas prepared for the demonstration of understanding of the background shape.Yields of ω and η ′ mesons were extracted via the M M ( p ) distribution with the ω and η ′ selectioncut (Fig. 2(d)and 2(e)). Background events for ω and η ′ signals consisted of several reactions. To eval-uate these contributions, missing mass distributions of all reactions were prepared by using Monte-Carlosimulation. The relative yield of the each reaction was determined by minimizing of the χ between thesuperposition of the prepared distributions and the experimental data (a template fit). Events includingnon-resonant pions (from 2 to 5) and one proton were generated in the free N-body space as backgroundcomponents. Photoproductions of η , η ′ , ρ , ω , and φ mesons were also generated as known resonances. The M M ( p ), M M ( p, π ± ), and M M ( p, π + , π − ) shapes of these components were obtained by using Monte-Carlo simulation to take into account the detector response. The shape of M M ( p ) with a loose ω selectioncut (0 . < M M ( p, π ± ) < . /c ) was also prepared to determine to the ρ/ω ratio. The distributionsof M M ( p ), M M ( p, π ± ), M M ( p, π + , π − ), and M M ( p ) with the loose ω selection cut were used as the con-strain of the template fit simultaneously. Events were put in photon energy bins with an interval of 62.5 MeVand proton scattering angle bins with a 0.05 cos Θ PC.M interval. The template fit was performed for each binon the photon energy and on the scattering angle of proton. The reduced χ was 0.9 at minimum and 2.7at maximum, depending on the angular and energy bins. The contribution of each reaction in the fitting isshown in Fig. 2(a), 2(b), and 2(c), where the red solid lines represent the sum of all contributions and shouldbe compared with the experimental results. The M M ( p ) shapes with the ω and η ′ selection cuts in eachreaction were also obtained. To determine the background shape, these were summed up according to therelative yields in the fitting result, except the resonances corresponding to the selection cuts ( ω or η ′ ). Thenormalization of the background was determined by template fits with the background and signal shapes forthe M M ( p ) distributions with the selection cuts. The obtained background and each component are shownin Fig. 2(d), 2(e), and 2(f). In Fig. 2(d), 2(e), and 2(f) the light blue solid lines represent the determined4ackground and the red solid lines represent the sum of all contributions. The sum of all contributionsreproduces the experimental results successfully, including the result with the inverse cut (Fig. 2(f)). Theyields of ω and η ′ signals were extracted by the subtraction of the background. The yields were corrected bythe efficiency evaluated by the Monte-Carlo simulation to determine differential cross sections. The typicalefficiency of ω was 23% at − . < cos Θ ωC.M < − . 95 and 9% at − . < cos Θ ωC.M < − . 80, respectively.The typical efficiency of η ′ was 6% at − . < cos Θ ωC.M < − . ω and η ′ selection cut was estimated by applying looseselection cut and by varying the cut boundary. It was determined to be 3%. The systematic uncertainty forthe background shapes was estimated by varying the fit conditions (without the 5 π reaction, reducing the fitconstrain, strategy of minimizing χ , and the combination of these). It was determined to be 3% and 5% for ω and η ′ , respectively. Since there was only the vertical polarization data in E γ = 2 . − . E γ = 2 . − . E γ = 1 . − . ω production as a function of √ s . Each panel shows theresult at the ω scattering angle bin ((a) ∼ (d) ) and the u interval bin ((e) and (f)). In − . < cos Θ ωC.M < − . 8, the results of CLAS are consistent with the present result, although results of SAPHIR and CLASare not consistent with each other[6, 19]. The differential cross section decreases as √ s increases above √ s ∼ E γ = 2 . − . s − dependence at − . < u < − . . Since the differential cross section in the kinematic range of the Daresbury willbe determined by the u -channel process, the theoretical extrapolation to the LEPS energy range could beinterpreted as the u -channel contribution.The LEPS result shows that the present √ s dependence of dσ/d Ω is different from the theoreticalextrapolation and the power law behavior for √ s ≤ . √ s dependence isdifficult to explain by only the u -channel process, and that there is a significant contribution from nucleonresonances via s -channel process in this energy range. The theoretical curve becomes consistent with theLEPS and Daresbury result in √ s ≥ √ s dependence can be explained by only the u -channelprocess in √ s ≥ dσ/d Ω at2 ≤ √ s ≤ − . < cos Θ ωC.M < − . 80. The result ofthe Breit-Wigner fit for the excess depends on the ω scattering angle: its peak and width are 2.25 ± ∼ (2190), H (2220), and G (2250), allof which have all 4-star states [1]. Since all of these candidates have high angular momentum, the angulardistributions of these resonance decays could have rapid changes at backward angles. It could account for theexcess, which becomes smaller at the most backward scattering angle bin. The coupling of the G (2190) to pω decay is supported by the PWA of γp → pω at CLAS[1, 20]. There are no reports for other candidates inthe γp → pω reaction. The difference between the present data and the theoretical curve can be interpretedas an influence of the G (2190). However, this interpretation still does not explain the dip structure at2 ≤ √ s ≤ − . < cos Θ ωC.M < − . 95. The present results may require more than just theG (2190). It is useful for the identification of nucleon resonances with high angler momentum to includevery backward angles in the PWA. It is worth mentioning that the very steep s dependence of ω dσ/du at u ∼ − . 15 GeV , seen in the Daresbury data, are not observed in the LEPS energy range[12].Figure 4 shows the differential cross sections for η ′ production as a function of √ s . Each panel showsthe result at the certain η ′ scattering angle bin. Theoretical calculations are also shown in Fig. 4. These5alculations are tuned to reproduce the experimental results in a wide kinematic range except for the veryforward and the backward angles[24, 25]. The dotted lines in Fig. 4 show the u -channel contribution in thetheoretical calculation by F. Huang[25]. It should be mentioned that the theoretical u -channel contributionat √ s > . 35 GeV is just an extrapolation. The N N η ′ coupling constant g NNη ′ in this calculation wasassumed to be 1.0. While the best fit value for g NNη ′ depends on assumed resonances and parameters ofthe resonances, g NNη ′ was evaluated to be less than 2 by considering the extreme case[24]. If we comparethe present result with the u -channel contribution at √ s > . g NNη ′ = 1 seems to be a reasonableassumption. The dominant contribution at backward angles is from nucleon resonances at √ s < . 10 GeVand the √ s ∼ . 35 GeV region according to the comparison between the LEPS result and the u -channelcontribution. The LEPS result of η ′ shows an excess in dσ/d Ω at √ s ∼ u -channel contribution. This structure appears clearer in the most backward scattering angle bin, − . < cos Θ η ′ C.M < − . 90. If the bump structure is also due to nucleon resonance, the correspondingresonance might have high angular momentum and a large branching ratio to η ′ .Three nucleon resonances, the S (1535), P (1720), and D (2070), are suggested to have large branchingratio to η and/or η ′ , but this depends on the model[22, 23]. It is proposed that these resonances have thesame quantum numbers except for the orbital angular momentum[22]. According to this scenario, the fourthresonance with the strong coupling to η and/or η ′ may be a F with a mass at around 2.3 GeV by theestimation from Regge trajectory. There is a possibility that the observed bump structure is due to the F resonances. However, measurements including a wide kinematic range are necessary to explain the presenceof the bump structure at backward angles.In summary, the ω and η ′ photoproduction from protons at backward angle has been measured at E γ = 1 . − . ω and η ′ mesons were identified by detecting forwardscattered protons in the spectrometer and detecting pions from meson decay in the TPC surrounding thetarget. Background events for the ω and η ′ signals were reduced substantially in comparison with theprevious LEPS experiments by using a TPC. The differential cross sections for ω mesons are larger thanthe expected u -channel contribution in the range 2 . ≤ √ s ≤ . (2190) resonance is a possible explanation for this excess in the ω differential cross sections, a PWAincluding the present data is important to identify the possible resonances. A bump structure in the √ s dependence of the differential cross sections for η ′ mesons was observed at √ s ∼ η ′ differential cross sections.We thank the staff at SPring-8 for providing excellent experimental conditions. We thank A. Sibirtsevand H. Kamano for fruitful discussions. This work was supported in part by the Ministry of Education,Science, Sports and Culture of Japan; the National Science Council of the Republic of China (Taiwan); theNational Science Foundation (USA); References [1] Particle Data Group. Phys. Rev. D86 , 010001 (2012).[2] S. Capstic and N. Isgur, Phys. Rev. D34 , 2809 (1986).[3] N. Isgur and G. Karl, Phys. Rev. D 18 (1978) 4187.[4] N. Kaiser, P. B. Siegel, and W. Weise, Phys. Lett. B 362 (1995) 23.[5] L. Y. Glozman and D. O. Riska, Phys. Lett. B 366 (1996) 305.[6] M. 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Nakayama and H. Haberzettl Phys. Rev. C73 , 045211(2006).[25] F. Huang, H. Haberzettl and K. Nakayama arXiv:1208.2279. MM(p) (GeV/c c oun t / M e V / c <2.375 GeV γ (a) ) /c ) (GeV - π , + π (p, MM -0.4 -0.2 0 0.2 0.4 0.6 0.8 / c c oun t / M e V (c) ) MM(p) (GeV/c c oun t / M e V / c (d) ) MM(p) (GeV/c c oun t / M e V / c (e) ) MM(p) (GeV/c c oun t / M e V / c (f) RealMC sumMC background π Non Resonant 2 π Non Resonant 3 π Non Resonant 4 π Non Resonant 5 ωρ ’ ηφ ) /c )(GeV ± π (p, MM -0.5 0 0.5 1 1.5 / c c oun t / M e V <1.0 PC.M Θ (b) Figure 2: (color online) Missing mass spectra for events with 2 . < E γ < . 375 GeV and 0 . < cos Θ PC.M < . 0. Circlepoints represent the experimental result. Red solid lines represent sum of all contributions. Light blue lines represent sum ofcontributions without ω and η ′ contribution for panel (d) and (e), respectively. Blue dashed, dashed spaced, dotted and chainlines represent non-resonant 2, 3, 4, and 5 π production, respectively. Magenta, green, black and yellow lines represent ω , ρ , η ′ and φ , respectively. (a) MM ( p ) (b) MM ( p, π ± X ) (c) MM ( p, π + , π − ) (d) MM ( p ) with the ω selection cut (e) MM ( p )with the η ′ selection cut. (f) MM ( p ) with the inverse η ′ selection cut. (GeV)s b / s r ) µ ( Ω / d σ d (a) <-0.95 CM ) ω ( Θ -1.0 Figure 3: (color online) Differential cross sections for ω photoproduction as a function of √ s . The black circles are the presentresults. Shaded bars represent systematic uncertainty. The red triangles, blue squares and blue stars are the experimentalresults from SAPHIR, CLAS and Daresbury, respectively[6, 19, 12]. Smooth lines represent theoretical calculation and dashedlines represent the result of a power law fit for the Daresbury results[21]. (GeV)s b / s r ) µ ( Ω / d σ d <-0.8 CM ’) η ( Θ -0.9