ELSEVIER

IUCLEAR PHYSICS

Nuclear Physics B (Proc. Suppl.) 54A (1997) 281-285

P R O C E E D I N G S S U P P L E M E N T S

Charmed Baryons and Two-photon Interactions at CLEO M. Sivertz Physics Department, University of California at San Diego, La Jolla, California 92093 USA

We use the CLEO detector at the Cornell e+e - storage ring, CESR, to study QCD from a wide variety of perspectives. With over 5 fb- 1 of integrated luminosity accumulated, we investigate charmed baryon spectroscopy helping to complete the understanding of the isospin multiplets. We present results on color suppression in charmed baryon decay. We also report on a variety of two-photon processes; the first observation of AA, and the measurement of the form factors of the Ir °, 7/, and ~/' in tagged two-photon events.

1. C h a r m e d B a r y o n s

The CLEO Collaboration has collected over 5 fb -1 of data since the commissioning of the CLEO-II detector[l]. Making use of the excellent charged particle tracking and high resolution CsI electromagnetic calorimeter we have been able to identify two previously unobserved charmed baryons, the E *° and the E *+. These are observed in their decays to a Ec and a charged r . The -e=+ decay modes[2] we reconstruct are E - r + r +, = ° r + r ° , and E + K *°. The -c=° decay modes[3] reconstructed are E-w +, f~ -K +, E - r + r °, and E o r + r - .

An enhancement in the invariant mass distri- bution of ~-~+ combined with 7r- are identified as the J P = 3/2 + state, Ec °. Figure 1 shows the mass difference between the M ( E + r - ) and M(E +) with a clear peak at 178.2 + 0.5 + 1.0 MeV/c ~ where the first error is statistical and the second is systematic. We search for excited states using the mass difference plots because there is bet ter mass resolution in the mass difference than in the resonance mass. The E~ ° width was deter- mined to be less than 5.5 MeV (90% CL).

The Ec + is observed in its decay to _c r = ° +. Figure 2 shows the mass difference between the M ( E ° r +) and M(E°c) with a peak at 174.3+0.5+ 1.0 MeV/c 2 and a width less than 3.1 MeV (90% CL). The mass difference between the E~ + and the E~ ° is consistent with zero.

We have also observed the J P = 3/2 + states, Zc ++ and ~ o . Figure 3a shows the mass dif- ference between combinations of A+r + pairs and the A + mass, while Figure 3b shows the mass dif-

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ference M ( A + r - ) - M(A+). The large peak on the left is the unexcited J P : 1/2 + ~++ s t a t e while the shoulder below 0.2 GeV/c 2 is the par- tial reconstruction of A *+ -+ A + r + r - when one of the charged pions is missing. The broad en- hancements near 0.23 GeV/c 2 are evidence of the ~c candidates. We identify these enhancements as the J P = 3]2 + excited states of the ~c baryons

282 M. Sivertz/Nuclear Physics B (Proc. Suppl.) 54A (1997) 281 285

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because the mass splitting between the ground and excited states is in agreement with theoreti- cal predictions, for which the splitting scales like the inverse of the heavy quark mass, 1/mQ.

2. Colour-Suppress ion in C h a r m Decay

CLEO has also observed the Cabibbo- suppressed colour-suppressed decay model4], A+ --+ PC, and measured the magnitude of the colour-suppression factor by comparing the branching ratio of A + ~ PC to the Cabibbo- suppressed colour-favoured mode A + --~ p K + K - . This ratio of branching ratios is predicted to be 1/15 by simple colour-suppression. The study of colour-suppression in charmed baryons was spurred by the lack of evidence for it in charmed meson decay, such as D + -~ ¢~r + versus D + -+ K * ° K + where the expected ratio of branching ra- tios was 1/9 whereas the measured ratio was close to 1.0. Alternative approaches using factorization

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Figure 3. a) Mass difference, M(A+~r+)-M(A+), in GeV/c 2. b) Mass difference, M ( A % r - ) - M(A+), in GeV/c 2. The lines below the signal are produced by combining r+ with candidates from the h + mass sidebands.

and taking the large Nc limit[5] predict no colour suppression.

Figure 4 shows the reconstructed mass of p K + K - combinations when the K + K - system is consistent with the ¢ mass (top line), and from the ¢ sidebands (bottom line). From this sig- nal we have been able to measure the relative branching ratio of the A + to PC- When no con- straints are placed on the mass of the K + K - system, we measure the non-resonant branching ratio A + -~ p K + K - . The ratio of these two branching ratios is 0.62 ± 0.2 =k 0.12 rather than the expected value of 1/15. This demonstrates that colour-suppression is no more effective in charmed baryon decays than in charmed meson decays, and lends support to the approach which uses factorization and a 1/Nc expansion.

3. T w o - P h o t o n P r o d u c t i o n o f AA

After finding good agreement between the pre- dicted[7] and measured[6] cross-sections for the two-photon production of pp, CLEO has looked

M. Sivertz/Nuclear Physics B (Proc. Suppl.) 54A (1997) 281-285 283

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Figure 4. Invariant mass of A + candidates observed in the Cabibbo-suppressed colour- suppressed decay mode A + ~ PC. The top his- togram shows events where the K + K - are con- sistent with ¢ decay, while the shaded histogram has events from the ¢ sidebands.

for ways to test the quark-diquark model more fully. We have recently tested the predictions of AA production.

CLEO has made a first observation of the pro- cess 77 ~ AA. Two-photon events occur when both the electron and positron emit photons, typ- ically at very small values of [q2[ (the photon 4- momentum). These photons can then undergo in- teractions while the e + and e- are undetected and are usually lost down the beampipe. Two photon events are characterized by large missing energy because of the undetected leptons, but with well balanced transverse momentum, p2 ,,, 0.0

In looking for AA events, we begin by requir- ing 4 reconstructed charged tracks with zero net charge. The protons are identified by specific ion- ization measurements ( d E / d x ) made in the drift chamber. For the events of interest, the proton is well separated from any other particle species. Additionally we require the total observed energy in the event to be less than 6.0 GeV and the sum of the transverse momentum components to be

Figure 5. Mass of A --+ plr- candidates versus ~ p r + candidates. Both scales are in GeV/c 2.

The clear peak at 1.115 GeV/c 2 is the first obser- vation of two-photon production of AA.

less than 0.6 GeV/c. Finally, in order to com- pare with the theoretical predictions, we measure the cross-section for events with ]cos 0*i < 0.6, where 0* is the angle between the A and the two- photon collision axis in the two-photon centre of mass frame.

Figure 5 shows the mass of the prr- versus the /5~r +, with a clear enhancement at the intersection of the A mass. A possible background, although interesting in its own right, is the two-photon pro- duction of either ~o~o or a A~ pair with the sub- sequent decay of the ~0 via ~/A. Although we do not have sufficient statistics to make a measure- ment of this background process, we have fit to the inclusive A~, spectrum and made a small sub- traction to account for a possible contribution.

The derived cross-section for the process ~/~/ AA with cos0* < 0.6 is shown in Figure 6 along with the previously measured[6] pp cross-section and the quark-diquark model predictions for the two[7]. The cross-section is shown as a function of W~x, the two-photon invariant mass. The pro- duction cross-section of AA seems to be above expectations, especially at low W~.

284 M. Sivertz/Nuclear Physics B (Proc. Suppl.) 54A (1997) 281-285

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Figure 6. Comparison of the measured cross sec- tion for 77 -+ AA and the cross section predicted using the quark-diquark model for cos 0* < 0.6. Also shown axe the measurements and predictions for the cross section 77 -+ PP-

Figure 7. r ° electromagnetic transition form fac- tor, multiplied by Q2, to show the development of the form factor as a function of Q2. Solid points are CLEO results, open points are from CELLO.

4. P s e u d o s c a l a r F o r m F a c t o r s

Although there is a long history[8-10] of cal- culations of 7"Y -+ ~0, there has recently been renewed activity[ll-15] in the form factor of the ~r ° and in the onset of perturbative QCD.

In order to measure the electromagnetic tran- sition form factor of the ~r ° and other light pseu- doscalar mesons, CLEO examines he processes e+e - --~ e+e - (~r°/y/~'). To lowest order these processes are described as single-meson produc- tion by two space-like photons, one emitted by each electron. In our experiment we detect the decay products of the meson and one scattered electron which is referred to as the "tag". The other electron is scattered at a small angle and remains undetected (untagged). We measure the meson-production rates as a function of the mo- mentum transfer, Q2. The study of the deviation of the meson-production rate from that which is predicted for a point-like meson gives a form fac- tor measurement.

Previous experiments[16,17] have measured the

~? transition form factor in the Q2 < 3.4 (GeV/c) 2. Statistically significant measure- ments[16,17] of the y' electromagnetic form fac- tor in the space-like region exist for Q2 < 3.4 (GeV/c) 2. Our study furnishes information in the range from 2.0 < Q2 < 12.0(GeV/c)2 for the

and from 2.0 < Q2 < 20.0(GeV/c)2 for the ~'. The only measurement of the ro form fac- tor in the space-like region[17] is limited to Q2 < 2.7(GeV/c) 2. We have measured the lr ° transi- tion form factor in the region 3.0 < Q2 < 8.0 (GeV/c) 2.

In our analysis we observe the ~0, ~ and ~' form factors using the decay channels ~0 __+ 77, Y 77, ~ ~ ~+Tr-~r°, ~' -+ P°7(P ~ ~+Tr-), and ~' ~ ~+Tr-~ (~ --+ 77).

To obtain the event reconstruction efficiency we use simulation programs based on the formalism of Budnev[18] with a simple double-pole vector meson dominance (VMD) form factor model.[19]

We extract the Q2 dependence of the form factor by comparing the measured production rate with that which is predicted for point-like

M. Sivertz/Nuclear Physics B (Proc. Suppl.) 54A (1997) 281-285 285

Table 1 Summary of the results on the pole mass, Ap in MeV, describing the momentum transfer depen- dence of the form factors and the fp [MeV], pseu- doscalar meson coupling constant. Errors shown are statistical.

TPC/27 CELLO CLEO A~o 748+30 771+18 A n 700±80 839±63 787±23 A n, 850±70 794±44 812±15 f~o 84 ± 3 87 ± 2 f~ 91± 6 94+ 7 89 ± 3 fn, 78 4- 5 89 ± 5 92 ± 2

pseudoscalar mesons. We interpret our results in terms of Ap, the mass parameter which de- scribes Q2 development of the form factor for pseudoscalar meson "p". This parameter Ap is obtained by fitting our data with a function of the following form:

]~-(Q~, q2)[2 × M3/647r = FT~/(1 + Q2/A~)2

Here ~-(Q2, q2) is the electromagnetic transition form factor which is a function of Q2 and q2 (the momentum transfer due to the photon emitted by the undetected electron), M is the mass of the meson "p" and F77 is its two-photon partial width (7.35 eV, 0.463 keV and 4.3 keV for the ~r °, q and q', respectively).[20] We estimate the unmeasured momentum transfer q2 to be below 0.001 (GeV/c) 2 using simulations, and ignore this effect in our analyses, assuming q2 = 0. The val- ues obtained for A v are given in Table I along with results from previous experiments. The re- sults of our current measurement are in agree- ment with earfier results, but are more precise. Figure 7 shows the behavior of the lr ° form fac- tor as a function of Q2, where we have multiplied the form factor by Q2 to show deviations from point-like dependence.

It is predicted[10] that in the asymptotic limit (Q2 __4 ~ ) the pole mass can be expressed in terms of the pseudoscalar meson coupling con- stant fv as ]2p __ A2/(8~r~). Table I gives the results for a fit to fp.

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