Prq. Parf. Nucl. Phyr.. Vol. 36, pp. 437-445. 1996

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SO146-641O(Y6)00051-8

Search for Exotic Mesons in IT-~ Interactions at 18 GeVlc

J. M. LOSECCO’, T. ADAMS’, J. M. BISHOP’,

N. M. CASON’, J. J. MANAK’, A. H. SANJARI’,

W. D. SHEPHARD’, D. L. STIENIKE’, S. A. TAEGAR’,

D. R. THOMPSON’, S. U. CHUNG*, R. W. HACKENBURG*,

C. OLCHANSKl*, D. P. WEYGAND*, H. J. WILLUTZKI*,

S. DENISOV3, A. DUSHKIN3, V. KOCHETKOV3, I. SHEIN3,

A. SOLDATOV”, B. B. BRABSON4, R. R. CRITTENDEN4,

A. R. DZIERBA4, J. GUNTER4, R. LINDENBUSCH4,

D. R. RUST4, E. SCOTT4, P. T. SMITH4, T. SULANKE4,

S. TEIGE4, Z. BAR-YAM5, J. P. DOWD5. P. EUGENIO’,

M. HAYEK’, W. KERN’, E. KING’, V. A. BODYAGIN’,

A. M. GRIBUSHIN6, 0. L. KODOLOVA6, M. A. KOSTIN6,

V. L. KOROTKIKH”, A. I. OSTROVIDOV”,

A. S. PROSKURYAKOV”, L. I. SARYCHEVA6,

N. B. SINEV6, I. N. VARDANYAN6, A. A. YERSHOV”,

D. S. BROWN73 T. K. PEDLAR7, K. K. SETH7, J. WISE7,

D. ZHA07, G. S. ADAMS8, J. NAPOLITAN08, M. NOZAR8,

J. A. SMITH” and M. WITKOWSKIX

’ Department of Physics, University of Notre Dome.

Notre Dame, IN 46556, US A. ?

Department oJPt!ysrc, Brookhoven R’nti,srinl Lohornrurj,

Upton, L.I.. NY 11973, KS A

3 lnstituteJor lligh Ener,qy Phy,$irJ,

Protvinu, Ru.sslnn Federatwn

4 Department of Physics. lndinnn Univenity,

Bloomington, IN 47405, U.S.A.

5 Department of Phys1c.r. Universiiy r,j‘Mos,~ochu,~ett.~ Dartmouth,

North Dartmouth, MA 02747, U.S.A.

’ Institute for Nuclear Physic.\. Moscow State Universrty,

Moscow, Russian Federatwn

7 Deportment of Physics, Northwestern Un~vrrsity,

Evanston. IL 60208. U.S.A.

8 Deportment ofPhysics. Rens.wloer Polytechnic Imtitute.

Troy, NY 12180, U.S.A.

437

438

Figure 1: Schematic \%w of the Detector

Introduction

Quantum Chromodynamics (QCD) is a theory of interacting quarks and gluons. The distinctive charge and color properties are responsible for t,hr quantum numbers of most of the known hadrons. Exotic mesons are by definition mesons that are not composed of a quark antiquark pair. Some of these exotic states can be identified by the fact that they have quantum numbers that can never be formed

from a quark antiquark pair. In particular the quantum numbers .Jpc= II--, O+-, l-+, 2+-, 3-+ etc. are explicitly chsotic. Observation of st,at,es wit.h t,hesr quantum numbers would be direct evidence for physics beyond t,he naivr quark model. Such stat,es can easily be formed in QCD as a quark antiquark glum bound st,at,p. McasurPmrnt of t,he mass. width and branching ratios of y@,~ exotics will give us a great dral of information about the role of the gluon in QCD.

Experiment

Bag model and lattice QCD estimates imply that exotics should exist in the same mass region as ordinary hadrons. This complicates the search for exotics by requiring that the experiment deal with

large numbers of conv&ional stat,es. On the other hand the presence of known states provides the opport,unity t,o observe t,he interference producrd by thca decay of difierent st,ates to the same final state.

Some guidance as to what final st,at,es may bfl populated by the tlpcay of exotic mesons is suggested by t,he “Flux Tube hlodel”[I]. It, is suggrsted that IIIPSOIIS wit,11 L = 1 arc’ preferred in such decays so that searches in the final states ~,,7r. fin, ~~7r. b17r are more likely to be fruitful than more conventional multi meson final st,atrs. 111 addition il number of groups 11al~ reported &dence for l-+ hybrids in the r/~[2] and ,j’,7r[3] final states.

Figure 2: 7~~~ mass distribution (left) and q.ir” mass distribution (right).

&arch for Exotic Mesons 439

Decay Asymmetry vs qlrc mass

d

M(~IT) GeV/c*

Figure 3: Forward-Backward Asymmetry for UT. The boxes are for v.rr” and the circles are for 1771

Studies of radiative J/I/J decay have also indicat,ed that the ‘r/ and 71’ mesons have a large “glue”

component and would be prevalent in the decay of states with an explicit gluon content. Brookhaven experiment 852 has been designed to search for exotics in R-I) reactions at 18 Gev/c.

The detector is shown in figure 1. Electromagnetic decays are detected in a 3049 element lead glass calorimeter located 5 met,ers downstream of a 30 cm liquid hydrogen target. Charged tracks are detected and measured with six drift, chambers c>ach of which contains 7 wire planes. The tracking region has a 1 Tesla magnetic field. TriggcJring is done with proportional wire chambers that measure charged multiplicity, and with a hardwired processor that is triggered on mass, enera or multiplicity in the

calorimeter.

Some evidence for an exotic l- + st,ate has been found in the 7p-r” final state[2]. We have looked at the q#, vv and V’X- final state. The first t,wo of these (figure 2) show strong evidence for the 2++ state. the a2(1320), which is an isovector.

Interference of the ~~(1320) with a 1. + amplitude will produce an asymmetry. Such an asymmetry has been observed. The asymmetry as a function of mass is plotted in figure 3. The asymmetry is

indicative of interference between odd and even waves. It is a necessary, but not sufficient condition for the presence of an exotic 1V’ resonance. The rxistcncr of a resonance must, be determined by a detailed partial wave analysis to study thr phase variat,ion of t,he l--+ amplitude. The phase is used to distinguish a resonance from a non resonant VET P wave. Work on the partial wave analysis is still in progress.

Figure 4: q’r- mass plot

440 J. M. Losccco et al

Figure 5: T/K asymmetry as a function of mass

The \‘I% esperiment has reported [I] significant, 1 ’ nmplitude in t,he r/r- final state. We observe t,his channel via the decay 7’ + r/7r’ in- (44%). Figure 1 shows t,he mass distribution for n’r-. There is some evidence for the 2 ’ + stat,r t,hr a( 1320).

Figure 5 shows a plot of the asymmetry as a function of mass of the r/‘~ syst,em. The asymmetry again indicat,es t,hr presence of l-‘~. Preliminary results from a part,ial wave analysis are in good agreement with VES [,l].

The .f,~ state has been identified with a l-+ rxot,ics candidat,e [3] in the 1.6-2.2 GeV/c’ mass range.

Figure 6 shows the q7ri 7r mass distribution in t,he ~7 ’ CC data sample. The q’(958) and the fr( 1285)

are clearly visible. Figure 7 shows the .f ,;r mass dist,ribut,ion. This stat,c is st,udied via the decay chain

,f, --f U,,K and u,~ + qir.

7r+71--7r”7r”

The i~‘x-i~“i~” state is pot,entially interesting since it, gives acwss to the preferred decay mode (LOT (figure 8) via the decay sequence (L% i /)rr (70’%) and 0 + i7R.

0.8 1 1.2 1.4 1.6 1.8

M (~7-r+C) Gev/c2

Figure 6: nr ‘IT- mass plot, from t,he ,rl37t sample. Note t,he n’(958) and fr(1285) peaks.

Starch for Exolic Mesons 441

M(f,n-) Gev/c2

Figure 7: f,~- mass plot

In addition t,he final states P+P~ (figure 9) and L&’ will populate this channel. UJ# is shown in

figure 10. The b, and p3(1690) are clearly visible.

The h17r states are preferred decay modes for hybrids in a number of models[l]. The bl and the 7~ are both isovector particles so t,he bin decay mode can be accessed by both isovector and isosinglet states. The bI (1235) decays almost entirely via WT. The w has a large branching ratio (89%) into 7rIT+~-@. So one would expect a strong signal for h17r in ~~+cR”KTT”. We note a strong bl signal in the T~K-K”K’ as shown in figure 10. In addition xt~-#x-7r” is coupled to WIJ.

Figure 11 shows the w mass region of the A+~T-T” mass plot in the 5 71 sample. A very clear w is visible. This w can be combined with either of t.he t,wo remaining K’S to look for a 6; or a by. Figure 12 shows both of these ti~n p1ot.s. Neither has much of an enhancement in the vicinity of the b1(1235). The absence of a clear bl signal makes it, less likely t,hat a strong signal will be found in the preferred bl r final state.

Figure 13 shows the wn7r mass plot. The ~~(1320) is clearly visible here.

n--p - -n--x-77-~7-Pn

Figure 8: QST mass plots.

442 I. M. Losccco et al

Figure 9: pi p mass plot,.

Thp q7?~ final stat,r gives BCCWS to both q17r and 02~ via the 7~ decay modes of the a0 and the u2 (15%:). The well known .f, (1285) has a substantial (44%) decay into no7r. The decay 7’ --t qn+~ (44%) is ii fairly good signature, in t.his channel. Figure 14 shows the st,rong 7)’ and .fr (1285) peaks in &.rr- and the ,f, (1285) and the E/L( 1420) in IL,~~.

The Q/I channel can be st,utlied via both the 71 + yp ard 17 + TT k~md’ modes. Figure 15 shows the 177 as measured in t,hr -I y mode. A similar response is found in the qq channel in which one 7 decays via 7r tiT- 7r”.

Figure 10: CJW mass plot. The br and ps(1690) are clearly visible

Search for Exotic Mesons 443

gure 12: ~371~ and d’ mass plots in the 5 71 sample. There is no clear 6,(1235) + wr peak

Figure 13: (L! -+ 7r ’ X”X~ in the 5 K sample. Note the presence of the ~(1320)

444 .I. M. Losccco et ul

Figuw 14: r/n 7i’ and u,,iP (strippfd) mass plots. Not,e t,he clear signal for q’(958) and the fl(l285)

70

60

50

40

30

20

10

0

Figure 15: T/T] --+ 4-i mass plot

&arch for Exotic Mesons 445

Conclusions

This experiment has access to a large number of final states that are favored in the decay of qqg exotics. Analysis of an initial sample of 2 x lo8 events is progressing with some initial results presented here. An additional 8 x 10” events has been acquired in the 1995 run and reconstruction of those events is in progress.

For additional information on the data discussed here see the proceedings of the Hadron ‘95 meeting [5]. Results on many of the favored states can be expected during the next year.

References

[l] N. Isgur and J. Paton, Phys. Lett. 124B, 247 (1983). N. Isgur and J. Paton, Phys. Rev. D31, 2910 (1985).

[2] D. Alde et al., Phys. Lett. 205B, 397 (1988).

[3] J.H. Lee et al., Phys. Lett. B323, 227 (1994).

[4] E. B. Berdnikov et al. II Nuovo Cimento 107A. 1941 (1994).

[5] N.M. Cason et al., in Proceedings of the Sixth Intern. Conf. on Hadron Spectroscopy, Manchester (1995). (to be published). G.S. Adams et al., in Proceedings of the Sixth Intern. Conf. on Hadron Spectroscopy, Manchester (1995). (to be published). B.B. Brabson et al., in Proceedings of the Sixth Intern. Conf. on Hadron Spectroscopy, Manchester (1995). (to be published).

J.P. Dowd et al., in Proceedings of the Sixth Intern. Conf. on Hadron Spectroscopy, Manchester (1995). (to be published). S.U. Chung et al., in Proceedings of the Sixth Intern. Conf. on Hadron Spectroscopy, Manchester (1995). (to be published). D.P. Weygand et al., in Proceedings of the Sixth Intern. Conf. on Hadron Spectroscopy, Manchester (1995). (to be published).