Study of the centrally produced ππ and KK systems

Nuclear Physics B (Proc. Suppl .) 21 (1991) 49-54 North-Holland

Study of the centrally produced

49

7r7r

and

KK

systems

presented by Antimo Palano, Dipartimento di Fisica and INFN, Bari WA76 Collaboration T .A . Armstrong, M . Benayoun5 , W. Beusch4 , I.J. Bloodworth 3 , J .N. Carney3 , R. Childs3 , C. Evangelista 2 , B.R. French4 , B. Ghidini2 , M. Girone 2 , A. Jacholkowski4 , J . Kahane5 , J .B. Kinson3 , A. Kirk4 , K. Knudson4 , V. Lenti2 , Ph. Leruste5, A. Malamant5 , J.L. Narjoux5 , F. Navach2 , A. Paaan0 2, E. Quercigh4 , N. Redaelli4 L. Rossi4 , M . SenO, R. SenO, M. Stassinaki l , M .T. Trainor 3 , G. Vassiliadis', O. Villslobos Baillie3 , M.F. Votruba3 1) 2) 3) 4) 5)

Athens University, Nuclear Physics Department, Athens, Greece Dipartimento di Fisica dell'Universitâ and Sezione INFN, Bari, Italy University of Birmingham, Physics Department, Birmingham, U.K. CERN, CH-1211 Geneva 23, Switzerland Collège de France, F-75231 Paris, France

We have studied the 7r7r and KK systems centrally produced in proton proton collisions at 300 GeV/c. Clear evidence for S*/fo(975) production is observed. An analysis performed on the 7r+7r - mass spectrum in the 1.0 GeV region, using a coupled channel formalism, shows that it is possible to describe the S*/fo(975) effect with one single resonance, and requires interference of the S*/fo (975) with the S-wave background. The resulting S* /fo (975) parameters are mo = 979 i 4 MeV, g r = 0.28 f 0.04, 9K = 0.56 ± 0 .18. Evidence is found for a structure having a mass of 1472 ± 12 MeV and a width of 195 ± 33 MeV. 1. Introduction The understanding of meson spectroscopy is of fundamental importance in the search for gluonium or hybrid states which are ex pected, from QCD, to populate the low mass region of the hadrons spectrum . Two gluonium candidates already exist : the J PC = 0 - + c/q(1440) observed in J/~b decay' and the J PC = 2++ 0/f2(1720) observed in J/0 decay2 and central production 3 . Several theoretical models predict the J PC = 0++ glueball to be the lightest and to lie in the mass region between 1 .0 and 1 .5 GeV4 . In this paper we describe the results from

a search for gluonium states in the centrally produced -r7r and KK final states in the reaction pp -4 pf (Xo )ps at 300 GeV/c incident beam momentum . The data come from the WA76 experiment performed at the CERN 9 spectrometer . Details of the layout of the apparatus, trigger conditions and data processing have been given in a previous publication s . 2 . Study of the The reaction

09205632/91/$03 .50 © 1991 - Elsevier Science Publishers BX . (North-Holland)

7r+7r -

system .

r + 7r PP -* Pf ( 7

)p8

(1 )

50

WA 76 Collaboration/Centrally produced

has been selected from the sample of 4-prong events by requiring momentum balance. Events having one of the central particles identified by the Cherenkov system as a K, p or ambiguous K/p, have been, removed . The energy balance was ensured by requiring the Ehrlich mass squared' to be in the range from -0.14 to 0.15 GeV2 . The final sample consists of 303000 events whose 7r+7r- effective mass spectrum is -shown in fig. 1 . We observe a small p(770) signal, some f2(1270) and a sharp drop around 1 GeV. In order to extract the S* /fo (975) parameters, a fit to the 7r+7r- mass spectrum has been performed using relativistic P and D wave Breit-Wigner formulae to describe the p(770), f2(1270), 0/f2(1720) and a coupled channel Breit-Wigner 7 to describe the S*/fo(975) region. Taking v l i to be the coupling of the exchanged particles to the X o system, VF, and *10 12

3

10

ca>

7r7r

and KK systems

to describe the couplings of the X o system to the 7r7r and KK final states respectively and neglecting final state interaction effects, we have written the coupled channel Breit-Wigner which describes the S*/ fo(975) resonance as: V/l7K

F,r(m)

in the

7r7r

FK(m)

=

Mo VI iyla - imo (I',r

m2o - m2

-I- rK )

channel and =

2

MO -

K m2 - imo (r.r -}- I'K ) MO V l

iy 1

1

1 .25

1 .75

m (7T* 7T-) GeV Figure 1 effective mass distribution . The line is the result from the fit described in the text . 7r+7r-

(3)

in the KK channel. The value of ri is unknown and has been absorbed into the intensity of the resonance . IF, and rK describe the S*/fo(975) partial widths to 7r7r and KK : r7r = g,r(m2/4 - m2)1/2

= 9K/2[(m2/4_m2x+)1/2+(m2/4_MKo)1/2] where g,r and 9K are the squares of the couplings of S*/fo (975) to 7r7r and KK systems. The background has been parametrized as G(m) _ (m - m t h)ae -Qm -ym2 where mth is the 7r7r threshold mass. Inserting incoherently the F, (m) in the fit gives a poor description of the data. In order to have a satisfactory description of the S* / fo (975) region it was found necessary to introduce an interference between the coupled channel Breit Wigner F, (m) and the background . The S-wave contribution was parametrized as rK

Bs(m) = Il + AsF,(m)e 'bi 12 0 u11 1 0.25 0.75

(2)

(4)

where 61 is a mass independent phase. In order to have a satisfactory fit in the low mass region around 0.5 GeV, it was also necessary to introduce an w(783) reflection in the low mass region of the 7r+7r- mass spectrum . The coupled channel Breit-Wigner used to describe the S*/fo(975) resonance has been

WA76 Collaboration /Centrally produced a7r and Kdf systems

convoluted with a Gaussian representing the experimental resolution which is ±10 MeV in the 1 .0 GeV region. In this hypothesis the fit gives a good description of the data in the first part of the mass spectrum but fails to describe it in the 1 .5 GeV region. Several attempts have been made to describe this region. We tried first an interference between the f2(1270) and a possible 7rir decay mode of the f2(1525) but this hypothesis did not improve the fit by a large amount . The inclusion of a new incoherent Breit-Wigner did not describe the data satisfactorily either . It was found that by introducing an interference between the f2(1270), f2(1525) and the background the fit improved but the fit probability remained low at 0.2%. In this hypothesis we measure a branching ratio r(f2 -> 7r7r)/r(f2 -- KK) = 0.21 ± 0.08 . If, however, we include a Breit-Wigner shape having free parameters and interfering with the background i.e. by replacing (4) with the expression: Bs (m) = I l + As F,r (m)eib, + Ax Fx (m)eib2 1 2 the fit probability increases to 51% and gives: mx = 1472 ± 12Mev, rx = 195 f 33MeV, 62 = 159 ± 80 , as parameters for this new Breit-Wigner . The resulting S* / fo(975) parameters are the following: mo = 979±4MeV, g,r = 0.28 ± 0 .04, 9K = 0.56 ± 0.18, 61 = 23 4 0 , with a pole position on the II sheet at (1001 ± 2) - i(36 f 4) MeV . The result of the fit is shown, as a curve superimposed on the data, in fig. 1 . 3. Comparison with the KK data Using the results from the fit of the 7r+7r spectrum it is possible, by using expression (3) multiplied by the intensity coming from the fit, to have an absolute prediction of the expected number of events for the S* / f0 (975) K+K..- spectrum . in the

51

The black band drawn in the threshold region of fig. 2 represents the predicted S*/ fo (975) contribution after having taken into account the different geometrical acceptance between the two channels and ClebschGordon coefficients . The band shows the prediction of ila where a is the statistical error . Such a contribution is easily accommodated in the threshold region of the K+K- mass spectrum . It is interesting to compare the above result with the KSKs mass spectrum . Although this channel suffers from low statistics, it has the advantage of not being complicated by the presence of the 0(1020) in the threshold region . The curve drawn in the threshold region of fig. 3 represents the S*/fo(975) contribution to the KSKs final state. The 320 280 240 0 cv 0

200

ô 160 c 120 a> 80 40

m(K* K-) GeV

Figure 2 K+K_ effective mass spectrum . The band drawn in the threshold region represents the estimated contribution frum the S*/fo(975) measured in the 7r+7r - spectrum with one standard deviation .

WA76 Collaboration /Centrally produced rr and KK systems

52

K+K - and KsKs channels have been normalized by firstly correcting the channels for the geometrical acceptance of the apparatus and for the losses due to the detection of two Ko's and then by using the presence of the f2'(1525) seen in both mass spectra. It can be seen that the projected curve is able to describe correctly the shape and the magnitude of the structure observed in the threshold region of the KOKO mass spectrum .

120 100 80 ca 60

11-1

C

40

4. Study of the t dependence

20 1 .5

2.

We have studied the t dependence (where t = 14 + t2l where ti and t2 are the fourmomentum transfer from the upper and lower protons vertices respectively) of the resonances observed in the 7r+7r - mass spectrum by dividing the data sample into different t regions. The ,r+7r - mass spectra have been fitted by fixing the S*/fo(975) parameters to the values obtained from the fit to the total mass spectrum but allowing the interference

2.5

m(K°s K°5) GeV Figure 3 Ii sh effective mass distribution. The curve represent the S*/fo (9 75) contribution from the fit to the ,r+ 7r - data.

s

104

a) S®

f

b) fa(1270)

c) p(770)

C

-0--

\ 102

10

0.

0.4 t GeV'

0.8

0.

0.4 t GeV'

0 .8

0.

0.4

0.8

t GeV'

Figure 4 1/t dN/dt distributions for : a) S*/ fo(975), b) f2(1270) and c) p( 7 170) . Open points: uncorrected data, black points: data corrected for geometrical acceptance .

WA 76 Collaboration /Centrally produced ra and KK systems

phases 61 and 62 to vary. The description of the data is good in all cases, with fit probabilities which range between 23% and 89% . The corrected and uncorrected t distributions for S*/fo(975), f2(1275) and p(770) are shown in fig. 4. Since an interference term is involved in the description of the S* /fo (975), the Breit-Wigner intensity has been plotted . It can be noticed that, while the S*/fo(975) behaves like a simple exponential having a slope of 8.7 ± 0.2 GeV -2 , the p(770) and f2 (1270) are consistent with zero in the first t interval suggesting a turn over at low t. I± ;c intareçting to compare the t behaviour of the f2 (1275) with that of the two other I=0 tensors observed in the same experiment, the f2(1525) and the 9/f2 (1720), observed in the K+K - channel. By dividing the K+K - data into four regions of t and correcting for geometrical acceptance, we obtain the t distributions shown in fig. 5 which have been fitted by simple exponentials having slopes 6.1 ± 0.4 GeV-2 and 9.6 ± 0.4 GeV-2 , respectively for the f2(1525) and 9/f2 (1720) . We observe enhanced production of 0/f2 (1720) at low i, in contrast with the behaviour of f2(1275) and f2(1525) . Thus, the data indicate a different production mechanism for the gluonium candidate 0/f2(1720) with respect to the other two I=0 tensors f2(1275) and f2(1525), in the central Feynman -I~F region covered by this experiment. 5 . Conclusions The study of 7r7r and KK systems centrally produced in proton proton collisions at 300 GeV/c shows that it is possible to de scribe the S*/fo(975) effect with one single resonance interfering with the S-wave background. The resulting S*_/ fo (975) parameters were determined to be mo = 979 ± 4 MeV, g,r = 0.28 ± 0.04, 9K = 0.56 ± 0.18 giv-

â3

104 a) fi (1525)

103

10

0 1

1

0.

1

1

0.4 t Ge1P

1

0.8

104 -v

b) 0/f2(1720)

103

I

1

I

0.4 t GeV2

I

0.8

Figure 5 1/t dN/dt distributions for: a) f2(1525), b) 0/f2 (1720) . Open points: uncorrected data, black points: corrected data. ing a pole position on the II sheet at (1001 f 2) - i(36 f 4) MeV and rs-/fo(975) = 72 f 8 MeV. The value 9K/97r = 2 .0 f 0.9 suggest a substantial ss contribution in the S*/fo(975) wavefunction . In order to test if the formalism and S* / fo (975) parameters used in the present analysis are also able to describe the data coming from other experiments, we have fitted the 7r+ r- mass spectrum from the ISR experiment s and from hadronic JN decay to

WAî6 Collaboration /Centrally produced ir7r and KK systems

54

m(7r+

7T - )

GeV

Figure 6 7-, + 7.-- effective mass distribution from ISRSFM [8] . The line is the result from the fit described in the text. Q? + ,r -9 . The fits have been performed by using the same formalism described in sect . 2 but fixing the 5*/fo(975) and X(1470) parameters while the interference phases as well as the resonances amplitudes have been allowed to vary. The results from the fits are shown in figs. 6 and 7 for ISR and MarkIII data respectively, showing good agreement with the data. eferences 1 D .L . Scharre et al., Phys . Lett. 97B (1980) 329 C . Edwards et al., Phys . Rev . Lett. 49 (1982) 259 2 C. Edwards et al., Phys . Rev . Lett. 48 (1982) 458 R.M . Baltrusaitis et al., Phys. Rev . D35 (1987) 2077

m(n+ n-) GeV

Figure 7 effective mass from J/0 decay to 07r+7r The line is the result from the fit described in the text. +7r -

3 4

5 6 7 8

9

J.E. Augustin et al., Phys . Rev . Lett . 60 (1988) 2238 T .A. Armstrong et al., Phys. Lett . 227B (1989) 186 S. Sharpe, Proc . of BNL Workshop on Glueballs, Hybrids and Exotic Hadrons, 1988, BNL T. Armstrong et al ., Nucl. Instr. and Meth. A274 (1989) 165 R. Ehrlich et al., Phys. Rev . Lett. 20 (1968) 686 S .M. Flattè et al., Phys. Lett . 38B (1972) 232 K .L. Au et al., Phys . Rev . D35 (1987) 1633 T. Akesson et al., Nucl. Phys. B264 (1986) 154 B . Lockman, Proc . of the 3rd Int . Conf. on Hadron Spectroscopy, Ed. Frontieres 1989