ELSEVIER Nuclear Physics A675 (2000) 245c-248e

A

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A study of the rlrpr- and wrpr- systems produced in the reaction 7r-p -+ pTr+lr-~r-4~/at 18 GeV/c

Paul Eugenio and Patrick Randerson for the Brookhaven E852 Collaboration* Carnegie Mellon University Pittsburgh, PA 15213, USA

Preliminary results are reported on the production of rpl~r- and wrpr- in the reaction 7r-p --+ p~r+Tr-~r-~r°y at 18 GeV/c where both ~r ° and t/ decay to 27. The data were obtained using the MultiParticle Spectrometer at Brookhaven National Laboratory. In the yy~r- channel, a strong a0(980) signal decaying to rpr- is present. The accepted rppr- mass spectrum exhibits a structure consistent with the mass and width to the 7r(1800). In the wTpr- channel, the co7c- mass spectrum peaks at the mass of the b1(1235).

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

An analysis effort of the Brookhaven E852 data(A description of Brookhaven E852 is given in these proceedings[i].) is currently underway which focuses on the 7/yr¢- and ~o~/0r- systems produced in the reaction ~r-p -+ p0r+0r-Tr-47. The flux-tube model predicts characteristic decays of hybrid mesons to L = 0, L = 1 meson pairs[2]. Several states are predicted to decay into flyer and wTpr[3]. The 0r(1800) recently reported by the VES Collaboration[4] to have a large decay to qTpr has been argued to be a hybrid meson based on its decay to flux-tube favored modes[5]. The w~pr final state is also of considerable interest since this channel has not been previously studied.

The data were collected during the 1995 Brookhaven E852 run where 265 million triggers of the type designed to enrich this exclusive final state sample were acquired. Out of these data, 45.6 thousand events were fully reconstructed and kinematically fitted to the hypothesis of ~r-p --~ p0r+~--~r-0r°r/with a confidence level greater than 10%. To eliminate incorrectly hypothesized events arising from ambiguous fits of the 4 gammas into ~/~/pairs, events that also fitted a ~r-p --~ por%r-~r-~r°sr ° hypothesis were removed.

2. F E A T U R E S O F T H E ~r+~r-~r-~r°~ S Y S T E M

The accepted invariant mass distribution of the q~r+0r-~r-0r ° system produced in the reaction ~--p ~ por+~r-~--~r°r/is shown in Figure la. There are no obvious resonant-like structures in the distribution. The accepted invariant mass distribution of the ~r+~r-~r-0r ° subsystem is shown in Figure lb. This distribution exhibits a prominent enhancement at 1.3 G e V / c u with a width of 0.200 GeV/c 2. Assuming only isoscalar and isovector

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

0375-9474/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII S0375-9474(00)00260-8

246c P. Eugenio, P. Randerson/NucIear Physics A675 (2000)245c-248c

contributions, an overall charged 4~r system must have an I a = 1 +. Therefore the only known candida te for this s tructure is the b1(1235).

Figure 2a displays the accepted ~r%r-~r ° invariant mass distr ibution. Two prominent narrow resonances are observed: at 0.550 GeV/c 2, an r/signal; and at 0.785 GeV/c 2, an w signal.

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Mass( "q~z+n:-~:-'z 0) GeVld-

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Mass( ~*~-:x-~ °) GeV/ca

Figure 1. [a] The rpr+~'-~r-~r ° effective mass distr ibution. [b] The ~%r-Tr-~ -° effective mass dis tr ibut ion.

3. 7}~r- S Y S T E M

For an 7} selection of 0.500 G e V / c 2 < Mass[~r%r-~r °] < 0.580 G e V / c 2, 5000 events were used to s tudy the 7/rpr- system. The accepted invariant mass dis t r ibut ion of the rFr subsys tem is shown in Figure 2b. The main feature is a clear a0(980) signal. In addit ion, the dis t r ibut ion exhibits a shoulder-like s tructure in the region of 1.3 G e V / c 2 suggesting the presence of some a2(1320) signal. The accepted r/y invariant mass dis tr ibut ion, shown in Figure 3a, exhibits a broad s t ructure in the 1.4-1.5 GeV/c 2 region which appears correspond to events in the z/zpr Dalitz plot(not shown) where the a0(980) bands cross, suggesting tha t the ~/*/structure might be due to constructive interference of the a0 bands. Figure 3b shows the accepted 7ppr- invariant mass distr ibution. The mass spec t rum exhibits a resonant-l ike s tructure with a mass of 1.8 GeV/c 2 and a width of about 0.200 G e V / c 2. This mass and width is consistent with the ~r(1800)[6]. As shown by the dashed his togram there appears to be a strong a0(980)~/ component to this structure. A par t ia l wave analysis will be carried out to see if this is indeed correct.

P. Eugenio, P. Randerson/Nuclear Physics .4675 (2000) 245c-248c 247c

~OOo

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o.s e.~ I Tn. 1.4 1 ~ ~.a

I %(980)

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Figure 2. [a] The r+Tr-Tr ° effective mass distribution(2 entries/event). [b] The rpr- effective mass distribution of the r/qTr- system(2 entries/event).

160

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120

loo

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oi 1.1 1 ~ I .~ 1.4 I .~ 1.6 %7 1.8 1.9 Mass(~ln) ~v/e~

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Mass( ~1~ ") GeV/e2

Figure 3. [a] The fly effective mass distribution of the zppr- system. [b] The z/rpr- effective mass distribution; the dashed histogram corresponds to an a0(980)z/selection.

248c P. Eugenio, P. Randerson/Nuclear Physics A675 (2000) 245c-248c

4. coTpr- S Y S T E M

To study the ~orpr- system, 16,400 events were selected for an omega signal of 0.720 GeV/c 2 < Mass[Tr+Tr-Tr °] < 0.870 GeV/c 2. The accepted invariant mass distributions(not shown) of the wr/and wr- subsystems are featureless. The accepted wTr- mass spectrum, shown in Figure 4a, exhibits a peak with a mass and width consistent with the b1(1235) which suggests a b1(1235)7/ component in the data. Figure 4b shows the accepted wT/Tr- invariant mass distribution. The dashed histogram in the figure represents events con- sistent with a b1(1235)7/ selection. A partial wave analysis is in progress to determine if there are resonance structures in the wr/Tr- system.

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i ,_rL Is 2 2.2 2.4 ¢,ev/e2

, , , , i , 1.2 1.4 1.6 i

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Figure 4. [a] The w~r- effective mass distribution of the wN~r- system. [b] The wTpr- effective mass distribution.

R E F E R E N C E S

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

2. N. Isgur and J. Paton, Phys. Rev., D31, 2910, (1985); F. Close and P. Page, Nucl. Phys.,B443, 233, (1995).

3. Close, F., and Page, P., Nucl. Phys. B443 33, (1995). 4. D. Amelin et al., Phys. Lett., B356, 595, (1995). 5. F. Close and P. Page , Phys. Rev., D56, 1584, (1997). 6. Particle Data Group, Phys. Rev. D54 1, (1996).