observation of a narrow structure in the pp elastic scattering observable aoonn at tkin = 2.11 gev

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

Observation of a narrow structure in the p p elastic scattering observable Aoonn at 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/IN2P 3 et CEA/DSM, CE-Saclay, 91191 Gtfi sur- 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 Phys- ical Engineering, Czech Technical University, Brehov~ 7, 11519 Prague 1, Czech Republic.

206

The spin correlation parameter Aoonn in pp elastic

scattering was measured at SA T U RN E II using a po-

larized proton beam and a polarized proton target.

One aim of the experiment was the determination of

the energy and angular dependence o f Aoonn around a beam kinetic energy of 2.1 GeV, corresponding to

a mass of 2.73 GeV, in order to search for a possi-

ble structure. At 2.0 GeV proton beam energy the au-

Elsevier Science B.V. SSDI 0370-2693 (93)E1480-L

Volume 320, number 1,2 PHYSICS LETTERS B 6 January 1994

thors offers. [ 1-5 ] predicted the existence o f a dibary- onic resonance in the 1S 0 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 au- thors 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 ]. Indica- tions 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 SATURNE II complete sets of observables [10]. Additional evidence for structure is suggested by the measurement of the analyz- ing 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 ° CM) . This result was confirmed by new measurements of the an- alyzing power and the differential cross section energy dependence in the same reaction at SATURNE 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 nota- tion o f the observables.

At 90 ° C M only three independent amplitudes sur- vive: 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 90°CM:

[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 spin- singlet resonance on the behaviour o f the right-hand-

side of eq. (1) may be diluted by contributions o f the spin-triplet amplitudes.

The differential cross section of the elastic pp scat- terlng 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 momentum 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 pos- sibly differed from one ZGS experiment to another. In the present paper we use two different fits to sep- arate both groups of d a / d O data.

The present measurements were carried out at SAT- URNE 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, Aooon and Aoonn. As a by- product we determined the rescattering observables Do,o~ and Kon,o with lower statistics. In this paper we report the Aoonn data. Numerical tables o f the data will be given in a forthcoming article. The data were obtained in the angular region from 58 ° to 97°CM 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 momentum 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 momentum 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 de- polarizing resonance 7G = 6 at 2.2016 GeV which cannot be removed by tuning up the accelerator. For

207


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. How- ever, the particle energy spectrum after the degrader is very large. The energy spread due to electromag- netic 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 de- sired 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 sim- ply by using the measured asymmetry and analyz- ing power data interpolated from the results around 2.1 GeV when a strong variation of Aoono in 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 prin- ciple. 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 di- rection, 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 differ- ences in the beam polarization. From the difference Aoono and Aooon we deduce that the systematic errors are of the same magnitude as the statistical ones. For- tunately 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. Aspe-

O6

O4

02

0 0¢

O4

02

t I i I

+ Aoonn p-p •e%+

I I

_ o • •

1 96 GeV

I I I I I

• e e o c e

~+ 4L,o • ' • ~. 2 04 GeV

i I I i i 60 80 100

I i I I I I I I I I I i I I i 1

,,•*~°e% ~,+ ~,+* ~÷ ***%** _

198 GeV 200 GeV 202 ~eV_

I q I I I I I I I I I I I I I

IIII IIII D~I I~I Iiii1o~ -

l I i ~ 0 e e • e

206 GeV 208 GeV. 212 GeV ; I I I I I I I I I I I I q I

6 0 8 0 1 0 0 6 0 8 0 1 0 0 6 0 8 0 1 0 0

0CM (deg)

06

O4

02

0 O.6

O4

O2

0

i i I [ I

- Aoonn

P-Poo,,ee o o

- e l •

214 GeM

I I I I I

l o l l l e o •

- 2 21 GeV • [ I I I I

6O 80 100

a i I L I i i i I I

o o l ~ o o o i ~ o o 0

• .-~ P 216 GeV ÷~ 18 Ge~/

I I I I I I I I I I

+...."-'" ~,...'"'% 222 GeV 223 GeV-

q I q i I I i E I I 6 0 8 0 1 0 0 6 0 8 0 1 0 0

OcM(deg)

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 Aoonn with values av- eraged 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 gener- ally 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

208


0 5

0 4

0 3

[ I I J I J I a I i I i I J I * I l I ~ I I I l

~ A°°nn

,

I I I I [ i I I I r ! ~ [ I I I I I I ~ I I [ 16 18 20 22 24 26

Tkm(GeV)

Fig. 2. Energy dependence of Aooa~ at 90°CM. Black dots: this experiment, open circles: data from ref. [22], solid line: prediction from ref. [3 ].

009

008

0 0 7

OOE

00~

004~

O 0 2

0 O 2

0 0 1

i ' [ ' I F 1 : I ~ l ' i ' I ' [ ' I i I

Ibl 2 p-p

90 ° CM

I ~ I i I f I ~ [ , I i I ~ [ I I I I ~ [ , ] 16 18 20 22 2.4 26

Tkj n (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: Aoorm from 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 da/dO 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 Aooan results were used ( + for the present experiment, x for the older SATURNE II results). A typi- cal relative statistical error in Jb] 2 is about 4-3%. The

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 90°CM between 1.96 and 2.23 GeV suggests a possible resonance in a spin- singlet 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 con- stituent 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 energy- dependent phase shift analysis using all the SAT- U 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. I f 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


T. Kirk, J.M. Laget, N.A. Russakovich, J. Saudinos, J. Tolar, Ts. Vylov, and A. Yokosawa for support of 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. Winterni tz have solved several problems. The tuning and control of the accelerator source and of the beam extraction were successfully accomplished by the SATURNE operator crew. The operat ion of the polarized target owes a lot to Ph. Marlet and Ph. Chesny. We thank T. Lambert , E. Perrin, J. Poupard, and J.P. Richeux for their efficient help in preparat ion o f the experiment. This experiment was supported in part by the US Depar tment of Energy, Contract No W-31-109-ENG- 38, and by the Swiss Nat ional Science Foundat ion.

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observation of a narrow structure in the pp elastic scattering observable aoonn at tkin = 2.11 gev

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