N ELSEVIER Nuclear Physics A675 (2000) 241c-244c

N U C L E : A I ~ P H y B I I : ] ~ I J

www.elsevier.nl/locate/npe

A study of the wr l system produced in the reaction ~-p --+ ~+z-4 7 n

at 18 G e V / c

Paul Eugenio for the Brookhaven E852 Collaboration* Carnegie Mellon Universi ty P i t t sburgh, PA 15213, USA

Results are presented on a s tudy of s tructure in the cot/ final s ta te produced in ~r-p interact ions at 18 G e V / c where co --+ ~r+~r-~r °, ~r ° --+ 2")' , and t/--+ 27. In the extended flux tube model, the cot/decay of an isoscalar y v o = 1 - - hybrid meson is expected. The observed cot/invariant mass dis tr ibut ion rises rapidly from threshold but in general shows no obvious indicat ion of resonant structure. However the result of a par t ia l wave analysis finds both a yPC = 1 - - signal consistent with the previously observed 0:(1600) and a dominant yPC = 1 +- par t ia l wave which exhibits phase motion relative to the co(1600). The decay co(1600) --+ cot/has not been previously reported.

1. I N T R O D U C T I O N

Exper iment E852 at Brookhaven National Labora tory is an experiment in meson spec- t roscopy with a focus on s tudying both neutral and charged final states of ~r-p collisions in a search for meson states incompat ible with the consti tuent quark model. (A descrip- t ion of Brookhaven E852 is given in these proceedings[i].) . An analysis effort of the E852 da t a is current ly underway which studies the wt/ system produced in the react ion ~r-p --+ ~r+~r-~r°t/n. A tota l of 750 million triggers were acquired during the 1995 da ta run of which 108 million were of a type designed to enrich the exclusive final s ta te events ~r-p --+ n~r+~r-47. A to ta l of 6 million events of this type were fully reconstructed. The da ta were kinematicMly fi t ted to select events consistent with an mr+~r-lr° t /hypothesis . To e l iminate incorrect ly hypothesized events a min imum acceptable confidence level was set to 10%.

One problem in interpreting the results of the kinematic fit is deciding between hypothe- ses for ambiguous events. There are six ways to combine the 4 7 into pairs. Twenty-nine percent of the fitted glr+Tr-~r°t/events were also found to fit to an nTr+~r-Tr%r ° hypothesis. Events with this ambiguity were removed from the data. A total of 113000 n~+~r-~r°t/ events remained for further analysis.

Two prominent resonances are seen in the Ir+~r-~r ° invariant mass dis tr ibut ion( see Figure la ) . These are the well known t/ and the w. In Figure lb , the cot/invariant mass dis t r ibut ion of 19530 o:7/final s ta te events is shown. The co selection is 750 M e V / c 2 _<

*The E852 Collaboration authors are listed in the contribution to this conference by P. Eugenio, titled Meson Spectroscopy in ~r-p Interactions at 18 GeV/c -Recent Results from Brookhaven E852.

0375-9474/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII $0375-9474(00)00259-1

242c P. Eugenio /Nuclear Physics A675 (2000) 241c-244c

Mass[3~r] <_ 830 M e V / c 2. The invariant mass dis tr ibut ion rises rapidly near threshold, then increases at a slower ra te to a max imum near 1600 M e V / c 2. The strong threshold enhancement might be explained by a state being produced near the wT/mass threshold. However, the mass dis tr ibut ion shows no clear evidence for structures indicat ing the presence of resonant states.

{B

~oo a )

~oo

o

00,2 ~4 0 ,60J~ t 12 14 16 1,8 $ 22

M a s s ( ~ + ~ - ~ ~) C ~ V / c 2

+÷+. '~ ++ '+ b )

~ ' ~ + +

~ +

~1~ +

~oo + + +÷

4-

÷ ÷

01.2 IA 1.6 LU .

Mass(~q) c~v/~

Figure 1. [a] The ~r+zr-~r ° effective mass distr ibution. [b] The wy effective mass distr ibu- tion.

2. P A R T I A L W A V E A N A L Y S I S

The da ta used in the par t ia l wave analysis (PWA) are shown in Figure lb . Typica l ly there are 1000 events per 40 M e V / c 2 mass bin. A rank 2 fit was performed in twenty overlapping 80 M e V / c 2 wide mass bins at 40 M e V / c 2 intervals from 1340 to 2200 M e V / c 2 and in the t region: 0 ~ It[ _< 1.2 (GeV/c2) 2. All par t ia l waves for L < 4 and M < 2 were considered in this analysis. The par t ia l waves JPCM~L are expressed in the reflectivi ty basis which takes into account par i ty conservation in the product ion process by a t ransformat ion of helicity states to eigenstates of the reflection operator in the product ion plane[2]. Ampl i tudes of different reflectivity, e = q-, do not interfere.

The most significant par t ia l waves found in the analysis are the I + - 0 + S and 1 - - I + P waves. The contributions of the 2 + - , 3 - - , 3 + - and 4 - - are considerably smaller and relat ively featureless. These waves tend to become more impor tan t at masses higher than 1700 M e V / c 2, and the par t ia l waves contr ibuting to the 2 - - intensi ty have large error- bars and do not have any obvious structure. It is impor tant to note tha t the exotic waves ( j p c = 0 - - , 2 + - ) do not show any clear resonant-like structures.

The only repor ted isoscalar j p c = 1 +- states are the h1(1160) and h~(1380)[3], and both of these states have masses much lower than the observed s t ructure in the I + - 0 + S wave. There are two isoscalar jPC = 1 - - states l isted between the 1000-2000 M e V / c 2 range in the PDG[3]. These are the the w(1420) and the w(1600). The mass and width of

P. Eugenio/Nuclear Physics A675 (2000) 241c-244c 243c

the w(1600) (1649 4- 24 MeV/c 2 and 222 4-35 MeV/c 2 respectively) are in good agreement with the observed structure in the 1 - - I + P wave.

The absolute sign of the relative phase between waves for the wrl system is un- known. This leads to two ambiguous so- lutions which result in the same partial wave intensities, but with a relative phase of opposite sign. Figure 2 shows the phase difference for both solutions between the 1 - - 1 - P and I+-0=S waves. The physical interpretation of the 1 - - 1 - P and I + - 0 - S waves is made more difficult by a cross- ing over at zero of the relative phases for the two solutions in the region of 1450 MeV/c2: These ambiguities lead to 4 pos- sible phase motions: [1] a relative phase ¢1+-0+s - - ¢l--l+P always positive, [2] a relative phase ¢1+-0+s - ¢l--X÷P always negative, [3] a relative phase ¢1+-0+s- ¢1--1+P positive at low mass changing nag-

P h a s e : 1 ~ @ + S

i[t t Jt,tt 1'.6 1 . s

Mass[ emil

1 - I÷P

I1 • i

I1

2.0 2.Z GeVIc 2

Figure 2: The phase difference for both am- ative at high mass, and [4] a relative phase biguous solutions between the 1 - - 1 - P and ¢1+-o+~- ¢1--1+P negative at low mass 1+-0-5 ' waves. changing positive at high mass.

3. M A S S D E P E N D E N T A N A L Y S I S

In order to better understand the nature of the I+-0+S and the 1 - - I + P observed structures, a mass dependent analysis of the PWA results was performed. The input quantities included the l+-0+S and 1 - - I + P wave intensities, and their relative phase for each mass bin. Relativistic Breit-Wigner forms were used to parameterize the amplitudes for the two waves.

Many mass dependent fits were performed for different hypotheses and for different relative phase motions. The result for the best fit finds both waves resonating with the phase described by Figure 3c. The mass and width of the I+-0+S state are m a s s =

1542 4- 13 MeV/c 2 and F = 285 -t- 30 MeV/c 2. The mass and width of the 1 - - I + P state are m a s s = 1664 4- 24 MeV/c 2 and F = 219 4- 63 MeV/c 2. Figure 3 displays the results of the mass dependent analysis: 3a, 3b, and 3c show results of this fit overlaid on the partial wave intensities and relative phase. Figure 3d shows the absolute Breit-Wigner phases for 1 - - and 1 +- and the relative constant production phase.

Assuming that the observed 1 - - I + P state is identified with the w(1600) and that the I+-0+S object is a single resonance, then the I+-0+S wave is also resonating with a mass of 1542 4- 13 MeV/c 2 and a width of 285 4- 30 MeV/c2; this state would be interpreted as a new ha(1540) state.

244c P. Eugenio /Nuclear Physics A675 (2000)241c-244c

Possible interpreta t ions for an h1(1540) include an hi radial exci tat ion, an hi hy- brid, or a radia l -hybrid mix- ture. In the Godfrey-Isgur potent ia l model[4], a mass of 1780 M e V / c ~ was predicted for the 23p0 h~ radial excita- t ion. However, their model also predicts a high mass for the h1(1160) and a high mass for the h~(1380) (1220 MeV/c 2 and 1470 MeV/c 2, respectively), therefore one might expect the radial s ta te to actual ly lie 100- 200 MeV/2 below the Godfrey- Isgur prediction. More recently, a calculation by T. Barnes[5] using a 3P0 model suggests tha t a 1700 M e V / c 2 hi radial ex- ci tat ion should decay almost equally to 1 + - S and I + - D par- t ia l waves 2 which are not what we observe. Alternat ively, since the f lux-tube model predicts a jPC = 1 + - hybr id near 1900-2000 MeV/c2[6] and ex- otic s tates have been repor ted at masses lower than the flux-

E v e n l s : 1 ~ O * S

l~(a~s [ <m~] G e v

P h ; ~ s e : 1 ~ O ÷ S - 1 ~ I ÷ P 3 -~i . . . . . .

e

, i i I . i i

ii :i

~ 3ooo

~soo

] E v e n t s : 1-- I * P

~iq, i

i ~ ~ . i !b . i i i i ~ i / 1

3'. .i:. . ~i' : 3 ~ ! o 1,4 1.5 !.6 1.7 1,.~ 1,9 2.0

P h ~ e : B W 1 , B W 2 , I ) r o d u c ~ o ~

2 I . . . . 1.4 L S 1 .~ 1 .7 l~'g 1 .9 2 .0

Figure 3: Mass dependent analyses of the I + - 0 + S and 1 - - I + P waves show tha t the 2 waves are well described by two interfering Brei t -Wigner resonances. The 1 - - s ta te of mass = 1664 =k 24 M e V / c 2 and F = 219 -4- 63 M e V / c 2 is in good agreement with the PDG values for the w(1600) of mass = 1649-t-24 M e V / c 2 and F = 220=t= 35 M e V / e 2. This is the first observation of ¢o(1600) --+ wT/. A new 1 +- s ta te with mass = 1542 -4- 13 M e V / c 2 and P = 285 =k 30 M e V / c 2 is observed.

tube model expectations[7,8], the hi (1540) can be a candidate for a hybr id or radia l -hybr id mixed state.

R E F E R E N C E S

1. P. Eugenio, Meson Spectroscopy in 7r-p Interactions at 18 GeV/c -Recent Results from Brookhaven E852, these proceedings.

2. S.U. Chung and T.L. Trueman, Phys. Rev. D l l 633, (1975). 3. Par t ic le Da ta Group, Phys. Rev. D 5 4 1, (1996). 4. S. Godfrey and N. Isgur, Phys. Rev. D32 189, (1985). 5. Barnes, Close, Page, and Swanson, Phys. Rev. D55 4157, (1997). 6. F. Close and P. Page, Nucl. Phys. B443 233, (1995). 7. D . R . Thompson et. al., Phys. Rev. Left. */9, 1630, (1997). 8. G . S . Adams et. al., Phys. Rev. Left. 81, 5760,(1998).

2The calculation is mass dependent, and one expects the S to D wave ratio to increase for a lower mass hi state due to angular momentum barrier effects.