IL NUOV0 CIMENT0 VOL. 80 A, N. 3 1 Aprile 1984

Study of 7:-p ~ ~'~n in a Search for Glueballs.

I H E P (a) - I ISN (b). LAPP (~) COLLABORATIO~

F. BINON (b)~ C. BRIC]KAN (0)~ S . V . DO~SKOV (a), lb. DUTEIL (d)

M. GOUA~]~RE (c), A . V . I:NYAKIN (a)~ V . A . KACItANOV (a)

D . B . KAKAURIDZE (a), G . V . I{HAUSTOV (a), .A.V. KULIK (a) J . 1 D. LAGNAUX (b), A . A . LEDNEV (a), YU. V. :3[IKKA:IXLOV (a) TH. }/[OUTttUY (b)~ 5 . 1 ). 1DEIGNEUX (c), A. POSSOZ (a), "~U. D. PROKOSHKTN (a)

YU. V. ]~OD~OV (a), S. -4. SADOVSKY (a), p . M . SHkGIN (a), D. SILLOU (c)

A . V . SI~GOVSKY (a), j . p . STI~OOT (b) ~nd V . P . SUGO~YAEV (~)

Join t Experiment o] I H E P - Serpukhov, U S S R CERJV - Geneva, Switzerland

(ricevuto 1'1 Febbraio 1984)

S u m m a r y . - ~'~ pair production in the charge exchange reaction ~ - p - ~ ' ~ n has been observed and studied in a search for glueballs with the 38 GeV/e pion beam of the 70 GeV I H E P accelerator. The T-rays from the decay ~'~ --~ 4,f were detected with the hodoscope spectrometer GAMS-2000. The ~'~ events in the mass range from threshold up to 1.8 GeV arc decays of the scalar G(1590)-meson which has been already observed in its ~ decay mode. The measured ratio of the partial widths of the G --~ ~'~ and the G --~ "q~ decay channels is 2.7 4- 0.8 in agreement with the value expected for gluebal]s.

PACS. 13.85. - IIadron-induced high- and super-high-energy interac- tions, energy > 10 GeV. PACS. 14.40. - Mesons and meson resonances. PACS. 13.25. - Hadronic decays of mesons.

(a) Institute for High Energy Physics, Serpukhov, USSR. (b) Inst i tut Interunivcrsitaire des Sciences Nucl6aires, Brussels, Belgium. (c) Laboratoire d'Annecy de Physique des Particules, :Prance. (d) CERN, Geneva, Switzerland.

363

3{~ F. BINON, C. BRICMAN, S. V. DONSKOV, P. DUTEIL ETC.

1 . - I n t r o d u c t i o n .

During the last two years, experiments on the search for bound gluon states (glueballs) have been intensively pursued (*). The occurrence of such states wi thout quarks as a pure gluonio state of mat te r is one of the most important predictions of quantum chromodynamics.

A very at tract ive way of searching for glueballs lies in the s tudy of ~ and ~'~ systems which m a y be favoured final states of gluobaU decays (2). Already a tensor meson, 0(1700)[I~(J ~) C ~ 0+(2 +) ~- ] decaying into an ~} pair has been discovered. I t is interpreted as a glueball candidate (3). The scalar G(1590)-meson [0+(0 +) -~] also decaying in ~ is another possible glueban (%

The s tudy of other G(1590) decay channels is of great interest, part icularly G -+ B'~. Would the G(1590) meson be of ghonie nature, the ratio of decay probabilities in both ~B and ~'B channels could be theoretically predicted rather precisely (5) using the octet-singlet mixing angle 0 ~ -- 18 ~ r

~ ~)~<3.6. (1) 23 B I ~ ( G -> '

The present experiment has been made with the aim of measuring neutral meson states decaying into ~'~ pairs in the mass region up to 1.8 GeV.

(1) S. ~r Glueballs, V I I International Con]erenve on Experimental Mesou Spectroscopy, BNL (1983); PRE-26748, UCLA (1983) (see further references). (2) J .F . DONOGHUE: UMHEP-157, University of Massachussetts (1981). (3) C. EDWARDS, R. PARTRIDGE, C. PECK, F C. PORTER, D. ANTREASYAN, Y . F . Gu, W. KOLLMANN, M. RICHARDSON, K. STRAUCH, K. WACKER, A. WE1NSTEIN, A. ASCHMAN, T. BIYRNETT, M. CAVALLI-SFORZA, D. COYNE, C. NEWMAN, H. ~. W. SADROZINSKI, D. GELPItYiAN, R. HOFSTADTER, •. HORISBERGER, I. KIRKBRIDE, J. KOLANOSKI, K. K('~NIGSMANN, R. LEE, A. LIBERMAN, J. 0'RE]LLY, A. 0STERItELD, B. POLLOCK, J. TOMPKINS, E. BLOOM, F. BULOS, R. CHESTNUT, J. GAISER, G. GODFREY, C. KIESLING, W. LOCKI~AN, M. 0REGLIA and D.L. SCHARRE: Phys. Rev. Lett., 48, 458 {1982); K. GOT~FRIED: Proceedings of the International Con]erence on High Energy Physics (Brigh- ton, 1983), p. 743. (4) F. BINON, S.V. DONSKOV, P. DUTEIL, ~r GOUAN]~-RE, A.V. INYAKIN, V.A. KACHANOV, D.B. KAKAURIDZE, G.V. KHAUSTOV, A. V. KULIK, J .P . LAGNAUX, A.A. LEDNEV, YU. V. MIKI=[AILOV, J . P . PEIGNEUX, YU. D. PROKOSHKIN, YU. V. RODNOV, S.~k. SADOVSKY, A.V. SINGOVSKY, J .P . STROOT and V.P. SUGONYAEV: .NUOVO Ci- mento A, 78, 313 (1983); Yad. _Fiz., 38, 934 (1983). (5) S.S. GERSHTEIN, A.K. LIKHODED ~nd Yu. D. PROKOSItKIN: preprint IHEP 83-148, Serpukhov (1983); Yad. Fiz., 39, 251 (1984). (e) W.D. ArEL, K. H. AVGENSTEIN, E. BE~OLVCCI, S. V. DONSKOV, A. V. INVAXlN, R. JOHNSON, V. A. KACHANOV, V. KITTENBElZGER, R. N. KRASNOKUTSKY, M. KROGER, G. LEDER, A A. LEDNEV, I. MANELLI, YU. V. MIKFIAiLOV, H. Mi31J~ER, YIy. D. PRo-

STUDY OF ::-p ~ ~'~n IN A SEARCg FOR GLUEBALLS 365

2. - E x p e r i m e n t a l p r o c e d u r e .

The set-up has been described previously (4,7). The measurements have been made in a 38 GoV/c negative-pion beam. Charge exchange processes leading to neutra l final states which u l t imate ly decay into photons have been detec ted

~-p --> M~ (2)

t--,ku

The photons were measure4 in the hodoscope spectrometer GAMS-2000. Even ts with all kinds of multiplicities are recorded including those of

mult ipl ici ty k = 4 which would be produced b y decays of the type

Mo --> ~'~ (3) ~ d y

and which have the same topology as have events in the react ion ~:-p - . ~ n through which G(1590) was discovered in the same set-up (4).

The react ion 7:--p-+~'~n has not been observed before. The difficulty lies in the low decay probabi l i ty ~ '?~-.dy (7.10 -3) and the large eombinatoriM background f rom the processes of 7~ ~ and ?] pair product ion which are three orders of magni tude more abundant .

3. - R e a c t i o n 7~-p--> ~ ' ~ n .

The following criteria have been used to suppress the background con- t r ibut io~ to events of react ion (2) with k = 4 multiplicity, selected b y the re-

KOSttKIN, G. ~'[. PIERAZZINI, M. QUAGLIA, D. SCmNZE[, A. SCRIBANO, H. SCIt:NEID~R, F. SERGIAMFIETRI, R. S. SHD'VALOV, G. SIGURDSSON, M. STEUER and M. L. VINCELLI: -Phys. Lett. B, 83, 131 (1979); A. WEINSTEIN, D. ANTREASYAN, Y.F. GU, W. KOLL~A~r ~V[. RICHARDSON, K. STRAUCtt, K. WACKER, C. EDWARDS, R. PARTRIDGE, C. PECK, F. C. PORTER, D. ASCHMAN, •. CAVALLI-SFORZA, D. COYNE, C. NEWMAN-HOLMES, I~. F. W. SADROZINSKI, D. GELPHMAN, R. HOFSTADTER, R. HORISBERGER, I. KIRK- BRIDE, H. KOLANOSKI, K. KONIGSMANN, ]~. LEE, A. LIBERMAN, J. O'REILLY, A. OSTER- HELD, B. POLLOCK, J. TOMPKINS, E. BLOOM, F. BULOS, t~. CHESTNUT, Z. GAISER, G. GODFREY, C. KIESLING, W. LOCKMAN, M. 0REGLIA and D. L. SCHARR]~: Phys. l~ev. D, 28, 2896 (1983). (7) F. BINON, C. BRICMAN, V.A. DAVYDOV, S.V. DONSKOV, J. DUFOURNAUD, P. DUTEIL, M. GOUAN~RE, A.V. INYAKIN, V.A. KACHANOV, D.B. KAKAURIDZE, G.V. KHAUSTOV, A.V. KULIK, J .P . LAGNAUX, A.A. LEDNEV, YU. V. MIKHAILOV, J. I ). PEIGNEVX, YU. D. PROKOSttKIN, YU. V. :RODNOV, S.A. SADOVSKY, A.V. SINGOVSKY, A.V. STARZEV, J. P. STReeT and V. P. SUGONYAEV" NUOVO Cimento A, 71, 497 (1982); Yad. _Fiz., 36, 670 (1980).

$6~ F. BINON, C. BRICMAN, 8. V. DONSKOV, P. DUTEIL ETC.

const ruct ion p rogram (7) :

a) the effective mass of each pair of g a m m a s M~m should be larger

t h a n 180 ~ e V in order to el iminate neut ra l pions ( ') ;

b) events wi th bo th pairs of g a m m a s in the ~-mesoa range f rom 450 MeV

to 650 ~ e V are rejected in order to exclude the react ion 7 r p -~ ~ n .

These cri teria dramat ica l ly decrease the background and allow to securely ident i fy events (3) in reaction (2). F igure lb) shows the mass spec t rum of one

g a m m a pa i r T3T4 under the condit ion t h a t the mass of the other pair Y1y~ lies inside

200

N

100

80

N

40

/ I

400

~)

600 800 1000 1200 N/u

60

N

40

20

0

,L, 4-00 600 800

Mu

b)

~r L - L j

800 1000 1200 M~, 3 .~4 (Mev)

c)

I 10100 1200

Fig. 1. - a) Effective-mass spectrum of y pairs in reaction (2) for events with a mul- tiplicity of k = 4 (6 entries for one event). Arrows show the tabulated masses of ~l and ~'. b) Idem for one pair (Y3Y4) when the mass of the other pair (Y1Y2) is in the mass range shown by arrows, c) Idem for My,v, when My,v, is in the ~' mass range shown by arrows.

STUDY OF ~-p---,','-~t~]n IN A B~ARCII FOR GLU~BALL8 367

the ~ mass range (470 ~[eV ~ My.y, < 650 MeV). Figure lc) shows the analogous distribution for the mass My.y, when the second pair y3y~ has the mass of ~' (870 ~ e V < My.y, < 1080 Meu In both spectra, ~ and ~)' meson peaks are distinctly present, in contrast with the mass spectrum of all arbi t rary pairs My,xj (fig. la)) . The background under the ~' peak is about 20 %. I t is negligibly small under the ~ peak.

4. - Mass spectrum and angular distributions.

The kinematical characteristics of the ~'~ systems produced in the reaction ~ - p - > ~'~n have been determined by a 3C fit of the selected events fixing MT.y, = m~, MT,y, = m~, and the recoil neutron mass. The detection efficiency e has been evaluated with the method in ref. (4,7) using a data base of real showers. I t does practically depend neither on the mass of the ~)'~ system, nor on the decay angle in the Gottff icd-Jackson frame.

100

N

50

0

/ /

/ /

1500 1600 1700 1800 ~r~l(MeV)

Fig. 2 . - ~}'~ mass spectrum produced in reaction ~-p- -~ '~n from the threshold to 1.83 GcV. In this figure and in the next one, Z r has been corrected for efficiency (e~ 0.25). The full line is a Breit-Wigner fit (4) with parameters (5). The dashed line is the phase space normalized to the number of events in the measured mass interval.

The mass spectrum of ~'~ is given in fig. 2. I t shows a fast increase very near the kinematical threshold, as expected for the decay G(1590)-->~'~.

In the explored mass region near the threshold, M~,~ < 1.8 Geu S-waves

36~ F. BINON, C. BRICIvIAN, S. V. DONSKOV, P. DIYTEIL ~TO.

dominate, the contribution of higher waves being decreased by the barrier factor (4). This is confirmed by the isotropic angular distribution of fig. 3.

I t also fixes the quantum numbers of the scalar ~1'~1 system: 0+(0 +) -1-.

200

N

150

100

-1 i I I

--0.5 0 0.5 C O S 063

Fig. 3 . - Polar angular distribution of ~1'~1 decay in the Gottfried-Jackson frame. 1.51 GeV < M~,~ < 1.83 GeV. 0Gz = 0 corresponds to the trajectory of ~ along the beam direction.

~-p-->~'~n events are concentrated in the small-emission-angle region. The shape of their t-distribution is characteristic of one-pion exchange. I t is similar to that obtained in the reaction producing the G(1590)-meson decaying into ~Pl ('). I t is described by da/dt ~ t(t - - m~) -2 exp [bt] with b ___ 5 (GeV/c) -~.

5. - G(1590)~* ~1'~1 and G(1590)- -* ~1~1 decays.

The ~t'~ muss spectrum has been fitted with a Breit-Wigner distribution of the form

do A q(M) (4)

d M ( M - ~17) 2 ~- (/"/2) ~ q(2~) '

where q(M) -~ [M ~ - (m~, -I- m~)~]t[M 2 - ( m ~ , - m~)~]t/2M is the phase space (M ~ M~,~) and M, F and A are free parameters (fig. 2). I t gives the following best values of the parameters:

(5) M -~ (1568• lWeV, /'----- (260:~60) IV~eV.

The width F is evaluated by taking into account the instrmnental resolution

STUDY OF g-1O-+ ~'~n IN A SEARCH FOR GLUEBALLS 369

aM--~25 MeV (a~,~/M.-~ 1.5%). These values are in good agreement wi thin the exper imenta l error l imits with those of the mass and the width of the

G(1590)-meson (M o = (1592-P25)MeV, _F o = (210:L40)MeV) (4). So all measured characteris t ics (quan tum mtmbers , resonance mass and

width, t-dependence of the cross-section) of the observed B'~ events are con-

sistent with those of G(1590). This allows us to conclude t h a t the decay mode

(6) G(1590) -+ ~'~

has been observed. The rat io of G decay ra tes th rough ~'~ and th rough ~ evalua ted f rom the

da ta of this exper iment and of the previous one (a)

(7) BI~(G -+ ~'~) 2.7-V0.8 BI~(G -+ ~))

is in agreement wi th the value predic ted b y the a l ready ment ioned model in

which the G(1590)-meson is described as a glueball with a small quark ad- mix tu re (5).

I f one takes into accotmt the relat ive phase spaces, the ~'B decay channel

of G(1590) is enhanced b y a fac tor 7 compared to the BB decay channel which is itself several t imes enhanced compared to the 7:= and K K channels. This is a characteris t ic fea ture of glueball.

We would like to t h a n k S .S . GElCSHTEIN: A . K . LIKHODED and V . F . OB~AZTSOV for discussing the results. We also t h a n k the I t t E P and CERI~

directorates for their support .

O R I A S S U N T O (*)

stata osservata e studiata la produzione di coppie ~'~ nella reazione di scambio di carica ~-p --~ ~'~n nella ricerca di sfere di gluoni con il fascio di pioni a 38 GeV/c del- l'acceleratore IHEP a 70 GeV. I raggi gamma derivanti dal decadimento ~'~--~4y sono stati rivelati con lo spettrometro odoscopieo GAMS-2000. Gli eventi ~'~ nell'in- tervallo di massa dalla soglia fine a 1.8 GeV sono decadimenti del mesone scalare G(1590) che ~ state gis osservato nel sue mode di decadimento ~ . I1 rapporto mi- surato delle ampiezze parziali dei canali di deeadimento G --~ ~'~ e G --~ ~) ~ 2.7 -V 0.8 in accordo con il valore atteso per le sfere di gluoni.

(*) Traduzione a cura della ~edazione.

24 - II 2Vuovo C i m e n t o A .

370 F. BINON, C. BRICMAlq, S. V. DONSKOV, P. D U T E I L ETC.

I/][cc~e~oBaHHe p e a c h ~ - ~ ) ~ ~ '~n B HOHC~ax ~( FJIIO6OHOB )')'.

Pe3mMe (*). - - Ha6mogaeTcg ri rlccae~yeTcg po~r~erme ~'~ n a p B pear t t r ta nepe3apz~ca~ 7r-p-+ ~ '~n c uem, io no~cKa ~< FTIIO6OTIOB )), Hcno~b3yg ILvIOIaHl, t~ IlytlOK C KMnyJ~bCOM 38 FaB/c yc ropa re s i~ 70 FaB I H E P . 7-~yqa ri3 p a c n a ~ a ~ '~ -+ 4y ~eTer rnpymTc~ c noMomj,~o CHeKTpOMeTpa G A M S - 2 0 0 0 . ~'~ co6J, ITna B o6nacxrI Macc OT n o p o r a Bnaows ~o 1.8 FaB npe~craBrmior p a c n a ~ i c ran~pHoro G(1590)-Meaona, rowopl, n~ y~re Ha6am- ~a.rtcg B MO~e p a c n a ~ a ~ . I43MepeHHOe OTHOIHenne napurmm, rmLx mrlpnH Kana~oB pacrla~toB G - + ~'~ n G ~ ~ , cocxaBzI~er 2 .84-0.8 , ~xo c o r z a c y e r c a c o~ria~aeMo~ Bem~- ~HHOfi ~n~ << r a m 6 o n a >>.

(*) 1-lepeae3eno peOamlue(t.