Observation of a narrow structure in the pp elastic scattering observable Aoonn at Tkin = 2.11 GeV

Physics Letters B 320 (1994) 206-210 North-Holland

PHYSICS LETTERS B

Observation of a narrow structure in the p p elastic scattering observable Aoonnat Tkin = 2.1 1 G e V J. Ball, P.A. Chamouard, M. Combet, J.M. Fontaine, R. Kunne, J.M. Lagniel, J.L. Lemaire, G. Milleret, J.L. Sans Laboratotre Natlonal SA TURNE, CNRS/IN2P3 et CEA/DSM, CE-Saclay, 91191 Gtfisur- Yvette Cedex, France

J. Bystricky, F. Lehar, A. de Lesquen, M. de Mall DAPNIA, CE Saclay, 91191 Gtf-sur-Yvette Cedex, France

Ph. Demierre, R. Hess, Z.F. Janout z, E.L. Lomon 2, D. Rapin, B. Vuaridel DPNC, Umverstty of Geneva, 24, quat Ernest-Anserrnet, 1211 Geneva 4, Swltzerland

L.S. Barabash, Z. Janout 3, V.A. Kalinnikov, Yu.M. Kazarinov, B.A. Khachaturov, V.N. Matafonov, I.L. Pisarev, A.A. Popov, Yu.A. Usov L N P - JINR, Dubna, P.O. Box 79, 101000 Moscow, Russtan Federation

M. Beddo, D. Grosnick, T. Kasprzyk, D. Lopiano, H. Spinka ANL-HEP, 9700 South Cass Ave., Argonne, 1L 60439, USA

A. Boutefnouchet, V. Ghazikhanian and C.A. Whitten UCLA, 405 Htlgard Ave., Los Angeles, CA 90024, USA Received 6 October 1993 Editor: L. Montanet

The angular dependence of the pp elastic scattering spin correlation parameter was measured in the angular range from 600 to 97°CM at 14 energms between 1.96 and 2.23 GeV. A rapid decrease of the energy dependence of this observable at 90 ° CM is observed around 2.11 GeV kinetic energy. This agrees with the behavior of Aoonn predicted on the basis of an exotic six-quark structure. The Aooma(90 ° CM), together with the known differential cross section, allows the determination of the absolute value of the pure spin-singlet amplitude. The energy dependence of this amplitude shows a shoulder centered at 2.11 GeV, corresponding to a total mass of 2.735 GeV and an estimated width of 17 MeV.

On leave of absence from the Computing Center of the Czech Technical Umversity, Zikova 4, 16635 Prague 6, Czech Republic. On leave of absence from the Center of TheoreUcal Physics, MIT, Cambridge, MA 02139, USA. Present address: Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University, Brehov~ 7, 11519 Prague 1, Czech Republic. 206

The spin correlation parameter Aoonn in pp elastic scattering was measured at S A T U R N E II using a polarized proton b e a m and a polarized proton target. One aim o f the experiment was the determination of the energy and angular dependence o f Aoonn around a b e a m kinetic energy of 2.1 GeV, corresponding to a mass o f 2.73 GeV, in order to search for a possible structure. At 2.0 GeV proton b e a m energy the auElsevier Science B.V. SSDI 0370-2693 (93)E1480-L

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thors offers. [ 1-5 ] predicted the existence o f a dibaryonic resonance in the 1S0 partial wave, implying an abrupt change in the angular dependence o f the Aoo,n and Aoo~ spin correlations above 55°CM. Previous data were measured at energies which were too widely separated to determine a narrow structure. Those authors pointed out that the predicted structure, as well as another one near T~n = 2.54 GeV, is suggested by the total cross section difference AaL(np) data [6,7]. The energy dependence of the pp unpolarized total cross section exhibits no pronounced structure, but there is an indication for a small anomaly at T~, around 2.1 GeV in the N I M R O D data [8 ]. Indications o f anomalies in this energy region were observed in the ANL-ZGS measurements [9]. Structure was also suggested by a direct reconstruction of the pp scattering matrix from S A T U R N E II complete sets of observables [10]. Additional evidence for structure is suggested by the measurement of the analyzing power Aoonn in the inelastic channel pp - d~ + [ 11,12]. The structure, centered around a mass of 2.7 GeV, was observed in the energy dependence of Aoom,(t = 0), Aoo~ (u = 0 ) and Aoonn(90 ° C M ) . This result was confirmed by new measurements of the analyzing power and the differential cross section energy dependence in the same reaction at S A T U R N E II [13]. Throughout this article we use the nucleon-nucleon scattering matrix formalism as given in ref. [ 14 ], with the amplitudes a, b, c, d, e, and the four-index notation o f the observables. At 90 ° C M only three independent amplitudes survive: a = 0, b = - c , d, and e. From the measured Aoom~observable and assuming that the pp elastic differential cross section d a / d O is a known and smooth function of energy, we can determine the absolute values of the spin-singlet amplitude at 9 0 ° C M : [b]2 = 1c]2 = ]Mss[2 -

doda 1 - Aoon,2

(1)

A resonance in the spin singlet state appears as a shoulder or a maximum in the [b(90°)l 2 amplitude energy dependence. From eq. (1) it follows that a fast decrease o f the single scattering observable Aoo., (90 ° ) in a small energy range would represent evidence for this structure. Away from 90 ° C M the effect of a spinsinglet resonance on the behaviour o f the right-hand-

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side of eq. (1) may be diluted by contributions o f the spin-triplet amplitudes. The differential cross section of the elastic pp scatterlng at 90 ° C M has not been measured in sufficiently small energy steps to determine the right-hand-side of eq. (1) without interpolation. The results reported in refs. [ 15-19 ] show higher values of the differential cross sections than obtained in an ANL-ZGS experiment [20]. The two groups of results cannot be made consistent by a simple renormalization, since their energy dependence is different. An energy shift could bring the results of the two sets of measurements into agreement. It should be noted that the ANL-ZGS was a weak-focussing accelerator with a m o m e n t u m spread of ±3.5% (ATk~n ~: I00 MeV around 2.2 GeV) and that the kinetic energy at the target was determined by the currents of the beam line magnets. Therefore, the beam energy may not have been well known, and possibly differed from one ZGS experiment to another. In the present paper we use two different fits to separate both groups of d a / d O data. The present measurements were carried out at SATU R N E II using the N N experimental set-up, the polarized proton beam and the polarized proton target [21 ]. The experiment measures principally the single scattering observables Aoono, Aooonand Aoonn. As a byproduct we determined the rescattering observables Do,o~ and Kon,o with lower statistics. In this paper we report the Aoonndata. Numerical tables o f the data will be given in a forthcoming article. The data were obtained in the angular region from 58 ° to 9 7 ° C M at 14 energies of the extracted proton beam: 1.96, 1.98, 2.00, 2.02, 2.04, 2.06, 2.08, 2.12, 2.14, 2.16, 2.18, 2.21, 2.22, and 2.23 GeV. The energy at the target center is about 5 MeV lower, taking into account the energy losses in the beam monitors and in the target. The m o m e n t u m spread of the SATURNE II internal beam is Ap/p = 1.435 x I0 -3 at 2.2 GeV and depends mainly on the orbit radius. The beam is extracted at the same radius and the particle momentum is effectively constant during the spill time interval. Consequently the extracted beam m o m e n t u m spread decreases an order of magnitude with respect to the internal beam. The measurement with a polarized beam around 2.2 GeV is difficult, since there exists a strong depolarizing resonance 7G = 6 at 2.2016 GeV which cannot be removed by tuning up the accelerator. For 207

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this reason the energies 2.23, 2.22, and 2.21 GeV were obtained by inserting different copper degraders in the 2.24 GeV beam. These degraders are not expected to have affected the beam polarization. However, the particle energy spectrum after the degrader is very large. The energy spread due to electromagnetic interactions does not exceed :t:6 MeV, but inelastic processes contribute considerably to the energy spread. Therefore the degraders were inserted close to the beam extraction point and a high resolution momentum analysis by four magnets was used in order to obtain a monoenergetic beam. The currents of the beam magnets are used to select the beam at the desired energy. The absolute value of the beam energy after the absorber is determined with an error estimated to be less than a few MeV. The absolute value of Ps cannot be determined simply by using the measured asymmetry and analyzing power data interpolated from the results around 2.1 GeV when a strong variation of Aoonoin this energy region is expected. Therefore P~ at each energy was determined by comparing the Aoono and Aooon angular distributions that must be equal by the Pauli principle. The absolute value of the target polarization is independent of the beam energy and is known to an absolute precision better than +0.03 by measurement with NMR. Relative errors during the experiment are smaller than 5:0.01. The error of Ps is then about the same as the error of PT. A scan over 14 energies required the reduction of statistics to the minimum necessary for determination of the single scattering observables. Because of limited beam time it was also necessary to measure several energies with the same target polarization direction, followed by the measurement of all these energies with the opposite PT. As a consequence, it is possible that the beam polarization PB may have been different for the two target polarizations at the same energy. The parameters of the accelerator and extraction, such as the extraction radius, may also have differed slightly, introducing uncontrolled small differences in the beam polarization. From the difference Aoono and Aooonwe deduce that the systematic errors are of the same magnitude as the statistical ones. Fortunately the Aoo~ measurement can be shown to be only negligibly affected by this possible systematic error. At 2.00 GeV, half of the data were taken without the magnetic field of the spectrometer magnet. Aspe208

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Fig. 1. Angular dependence of the spin correlation parameter Aoonn at 14 energies. cial treatment of these data was needed and the errors at this energy are large. The angular dependence of the spin correlation parameter Aoo~ at all energies is shown in fig. 1. Fig. 2 shows the energy dependence of Aoonnwith values averaged over the angular region from 85 ° to 95°CM. The previous SATURNE II results from ref. [22] are shown as well. The two sets of data are in excellent agreement. The existing ANL-ZGS data for Aoo~ are not plotted because of the uncertainty of the ZGS energy. The energy dependence of Aoonn(90 ° ) is a generally decreasing function between 1.8 and 3.5 GeV. This was estabhshed by the the previous ANL-ZGS and Saclay measurements. The present data show an additional sharp decrease in a small energy interval around 2.1 GeV and a minimum around 2.21 GeV. The solid line in fig. 2 show a good agreement of the predictions [3 ] and the present data. The Aoona observable measured in the present experiment and the known da/dO results allows us to

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22 2.4 26 Tkjn (GeV) Fig. 3. Amplitude Ib (90 ° CM)[ 2 a s a function of the beam energy. Black dots: this experiment, using the cross section data from refs. [15-19], open circles: Aoormfrom ref. [22] using the cross sections as above, crosses (+): this experiment, using the cross section data from ref. [20], crosses (x): Aoonn from ref. [22] using the cross sections from ref. [20]. determine the absolute values of the amplitude [b [ 2 a t 90°CM, using eq. ( 1 ). The amplitude Ib]2, calculated using the d a / d O values from the refs. [15-19] and the present Aoonn data are shown in fig. 3 as black dots. The open circles use the Aoo~n data from ref. [22]. Fig. 3 also shows [bl 2 using da/d£2 results from [20], which were fitted and extrapolated to 90°CM. The same Aooanresults were used ( + for the present experiment, x for the older S A T U R N E II results). A typical relative statistical error in Jb]2 is about 4-3%. The

6 January 1994

errors of the da/d£2 results are not quoted. I f they are added in quadrature the final error may increase by 2%. We observe that ]bl 2 is a decreasing function of energy that has a shoulder between 2.10 and 2.20 GeV. The structure has practically the same shape whether the differential cross sections of refs. [15-19] or of ref. [20] are used. The behaviour o f Ib[z at 9 0 ° C M between 1.96 and 2.23 GeV suggests a possible resonance in a spinsinglet state. We found the central energy value equal to 2.11 GeV, which corresponds to an invariant mass of 2.735 GeV. The width of the resonance can be roughly estimated. It is not larger than + 100 MeV in the beam kinetic energy scale, i.e. a resonance mass full width at half maximum about 17 MeV. The position of the resonance is consistent with the lowest lying exotic quark configurations in the isospin state I = 1 as predicted by Lomon, LaFrance and Gonzalez [2,3,5 ] using the Cloudy Bag Model and an R-matrix connection to long range meson exchange forces. The position is also in qualitative agreement with Resonating Group Method calculations for constituent quark models (CQM), as predicted by Wong [23] for the relativistic CQM, and by Kalashnikova, Narodetskii and Simonov [24] for the non-relativistic CQM. Such dibaryons, when first proposed, were predicted to be at substantially lower energies [25] using the MIT Bag Model, with an equilibrium radius that would be relevant if the multi-hadron system were confined and if there were no long range forces. For similar reasons, this lower range of predicted exotic masses was obtained by other early model calculations of exotic dibaryons (see also ref. [26] and references therein). An amplitude analysis cannot determine in which partial wave the resonance occurs. An energydependent phase shift analysis using all the SATU R N E II data measured at well-known energies in a large energy range would be required for this purpose. We conclude that our results represent a consistent experimental indication for a possible narrow resonance in the pp interaction. If the resonance suggested by our results is confirmed, its mass would be 2.735 GeV and its width was estimated to be about 17 MeV. We acknowledge J. Arvieux, M. Havlicek, E. Heer, 209

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T. Kirk, J.M. Laget, N.A. Russakovich, J. Saudinos, J. Tolar, Ts. Vylov, and A. Yokosawa for support o f this work. Discussions with R. Abegg, R. Beurtey, A. Boudard, J. Derrgel, J.M. Durand, F. Hinterberger, M. Huber, P. LaFrance, C. Lechanoine-Leluc, F. Perrot-Kunne, L.E. Price, A.N. Prokofiev, Th. Siemiarczuk, Y. Terrien, and P. Winternitz have solved several problems. The tuning and control o f the accelerator source and of the b e a m extraction were successfully accomplished by the S A T U R N E operator crew. The operation of the polarized target owes a lot to Ph. Marlet and Ph. Chesny. W e thank T. Lambert, E. Perrin, J. Poupard, and J.P. Richeux for their efficient help in preparation o f the experiment. This experiment was supported in part by the US D e p a r t m e n t o f Energy, Contract No W-31-109-ENG38, and by the Swiss National Science Foundation. References [1] E.L. Lomon, Colloq. Phys. (France) 46 (1985) C2329. [2] P. LaFrance and E.L. Lomon, Phys. Rev. D 34 (1986) 1341. [3] P. Gonzales, P. LaFrance and E.L. Lomon, Phys. Rev. D 35 (1987) 2142. [4] E.L. Lomon, 8th Intern. Symp. on High energy spin physics (Minneapolis, Minnesota, USA, September 1988), AIP Conf. Proc. No. 187 (AIP, New York, 1989), Particles and Fields Series 37 Vol. I, p. 655. [5] E.L. Lomon, Colloq. Phys. (France) 51 (1990) C6363. [6] I.P. Auer, E. Colton, H. Halpern, D. Hill, R.C. Miller, H. Splnka, N. Tamura, G. Theodosiou, K. Toshioka, D. Underwood, R. Wagner and A. Yokosawa, Phys. Rev. D 34 (1986) 2581. [7] I.P. Auer, E. Colton, W.R. Dltzler, H. Halpern, D. Hill, R.C. Miller, H. Spinka, N. Tamura, J.-J. Tavermer, G. Theodosiou, K. Toshioka, D. Underwood, R. Wagner and A. Yokosawa, Phys. Rev. Lett. 62 (1989) 2649. [8] D.V. Bugg, D.C. Salter, G.H. Stafford, R.F. George, K.F. Riley and RJ. Tapper, Phys. Rev. 146 (1966) 980. [9] H. Spinka, E. Colton, W.R. Ditzler, H. Halpern, K. Imai, R. Stanek, N. Tamura, G. Theodosiou, K. Toshmka, D. Underwood, R Wagner, Y. Watanabe, A. Yokosawa, G.R. Burleson, W.B. Cottingame, S.J. Greene, S Stuart and l.J. Jarmer, Nucl. Instrum. Methods 211 (1983) 239. [10] C.D. Lac, J. Ball, J. Bystricky, J. Der6gel, F. Lehar, A. de Lesquen, L. van Rossum, J.M. Fontaine, F. Perrot and P. Wmternitz, J. Phys. (Paris) 51 (1990) 2689. [11 ] R. Bertim, J. Arvleux, M. Bolvln, J.M. Durand, F. Soga, E. Descroix, J.Y. Grossiord, A. Guichard, J.R. P~zl, Th. Hennmo and L. Antonuk, Phys. Lett. B 162 (1985) 77. 210

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[12] R. Bertlnl, G. Roy, J.M. Durand, J. Arvieux, M. Bolvm, A. Boudard, C. Kerboul, J. Yonnet, M. Bedjidlan, J.Y. Grosslord, A. Guichard, J.R. Plzzi, Th. Hennino and L. Antonuk, Phys. Lett. B 203 (1988) 18. [13] J. Yonnet, R. Abegg, M. Boivin, A. Boudard, G. Bruge, P. Couvert, G. Galliard, M. Garcon, L.G. Greenlaus, D.A. Hutcheon, C. Kerboul and B. Mayer, in: Proc. seventh Intern. Conf. on Polarization phenomena in nuclear physics (Paris), eds. A. Boudard and Y. Terrlen, Colloq. Phys. (France) 51 (1990) C6-379. [14] J. Bystricky, F. Lehar and P. Winternitz, J. Phys. (Paras) 39 (1978) 1. [15] A.R. Clyde, Thesis UCRL - 16275, Berkeley (May 1966). [ 16] G.M. Ankenbrandt, A.R. Clark, B. Cork, T. Elioff, R.P. Kerth and W.A. Wenzel, Phys. Rev. 170 (1968) 1223. [ 17] M.G. Albrow, S. Andersson/Almehed, B. Bosniakovic, C. Daum, F.C. Erne, J.P. Lagnanx, J.C. Sens and F. Udo, Nucl. Phys. B 23 (1970) 445. [ 18 ] R.C. Kammerud, B.B. Brabson, R.R. Cnttenden, R.M. Heinz, A.H. Neal, H.W. Palk and R. Sidwell, Phys. Rev. D 4 (1971) 1309. [19 ] D.T. Williams, I.J. Bloodworth, E. Elsenhandler, W.R. Gibson, P.I.P. Kalmus, L.C.Y. Lee, Chi Kwong, G.TA. Arnlson, A. Astbury, S Gjesdale, E. Lillethun, B. Stave, O. Ullaland and I.L. Watkins, Nuovo Cimento 8A (1972) 447. [20] K.A. Jenkins, L.E. Price, R. Klem, R.J. Miller, P. Schreiner, M.L. Marshak, E.A. Peterson and K. Ruddick, Phys. Rev D 21 (1980) 2445. [21] J. Ball, Ph. Chesny, M. Combet, J.M. Fontaine, R. Kunne, J.L. Sans, J. Bystricky, F. Lehar, A. de Lesquen, M. de Mali, F Perrot-Kunne, L. van Rossum, P. Bach, Ph. Demierre, G. Gaillard, R. Hess, Z.F. Janout, D. Rapm, Ph. Sormam, B. Vuarldel, J.P. Goudour, R. Bmz, A Klett, E. R6ssle, H. Schmltt, L.S. Barabash, Z. Janout, V.A. Kalinnikov, Yu.M. Kazarlnov, B.A. Khachaturov, V.N. Matafonov, I.L. Ptsarev, A.A. Popov, Yu.A. Usov, M. Beddo, D. Grosnxck, T. Kasprzyk, D. Lopiano and H. Spinka, Nucl. Instrum. Methods A 327 (1993) 308. [22] F. Lehar, A. de Lesquen, J.P. Meyer, L. van Rossum, P. Chaumette, J. Der6gel, J. Fabre, J.M. Fontaine, F. Perrot, J. Ball, C.D. Lac, A. Michalowicz, Y. Onel, D. Adams, J. Bystricky, V. Ghazlkhanian, C.A. Whitten and A. Penzo, Nucl. Phys. B 294 (1987) 1013. [23] C.W. Wong, Prog. Part Nucl. Phys. 8 (1982) 223. [24]YUS. Kalashnikova, I.M. Narodeckil and Yu.A. Simonov, Yad. Fiz. 46 (1987) 1181 [Soy. J. Nucl. Phys. 46 (1987) 689]. [25] A.Th.M. Aerts, P.J.G. Mulders and J.J. de Swart, Phys. Rev. D 17 (1978) 260. [26] L.A. Kondratyuk and V.A. Vasflets, Nuovo Cimento 102A (1989) 25