RADIATIVE DECAYS OF \lr(3095)

Transcription

1 SLAC-PUB ~~~-5382 September 1976 (T/E) RADIATIVE DECAYS OF \r(3095) AND q(3684)+ J. S. Whitaker, W. Tanenbaum, G. S. Abrams, M. S. Aam, A. M. Boyarski, M. Breidenbach, W. Chinowsky, R. G. DeVoe, G. J. Fedman, C. E. Friedberg, D. Fryberger, G. Godhaber, G. Hanson, J. A. Jaros, A. D. Johnson, J. A. Kadyk, ++ v. Ltith, H. L. Lynch, R. J. Madaras, E: E: k?~~~~k~~*"'~~',:. Nguyen,* J. M. Paterson, M. L. Per, I. Peruzzi, ** M. P&010, E. M. Pierre,*** T. P. Pun, P. Rapidis, B. Richter, B. Sadouet, R. H. Schinder, R. F. Schwitters, J. Siegrist, G. H. Triing, F. Vannucci,** J. E. Wiss Lawrence Berkeey Laboratory and Department of Physics University of Caifornia, Berkeey, Caifornia and Stanford Linear Acceerator Center Stanford University, Stanford, Caifornia ABSTRACT We present the incusive photon spectra observed in $(3095) and q(3684) decays. The decay $(3684) + y-x(3415) is observed with a branching fraction of ko.026. Evidence is presented for three intermediate states in the decay sequence q(3684) -a Y-Y-d 3095) with masses of 3504 MeV, 3543 MeV, and 3455 MeV or 3340 MeV. (Submitted to Phys. Rev. Letters) i- Work supported by the Energy Research and Deveopment Administration. ++Feow of Deutsche Forschungsgemeinschaft. +++Permanent address: Varian Associates, Pao Ato, Caifornia. *Permanent address: LPNHE, Universit& Paris VI, Paris, France. **Permanent address: Laboratori Nazionai, Frascati (Rome), Itay. ***Permanent address: DPhPE, Centre d*etudes Nucdaires de SaCay, France. *Permanent address: EP Division, CERN, Geneva, Switzerand. **Permanent address: Institut de Physique Nucdaire, Orsay, France.

2 -2- States of even charge conjugation intermediate in mass to $(3095) (Z $) and q(3684) (E $'> h ave been observed in radiative decays of $(36&) by their - hadronic decays 192 and by their decays to f, 394 There is aso evidence from DESY for a state beow the $. 5 We present here the incusive photon spectra from 142,000 $ and 309,000 q' events observed in the SLAC-LBL magnetic detector6 at SPEAR. We aso present resuts on intermediate states in $' + W$I decays based on a data sampe three times arger than for our previous Letter. 3 Photons are detected by their conversion to eectron-positron pairs in materia near the interaction region. The effective converter consists of the vacuum pipe, two scintiation counters, and two proportiona chambers, a tota of radiation engths of materia ocated 8 to 22 cm from the beam axis. The conversion.probabiity for normay incident photons is at 0.15 GeV and rises to at 2 GeV. The conversion products are tracked through the four-kiogauss magnetic fied by the two proportiona chambers and by four - doube-gap spark chambers which cover the fu azimuth and from 50 to 130' in poar ange 0. Partices must traverse at east the two inner spark chambers to be detected, impying a minimum transverse momentum per partice of GeV/c. Pairs of oppositey-charged partices with invariant mass (assuming eectron masses) smaer than GeV are seected as photon conversions; this cut admits m 20% accidenta hadronic background (estimated from ike-charged pairs). The photon energy is cacuated as the sum of the energies of the two partices. Charged partice momentum resoution and energy oss by ionization or radiation after conversion ead to an rms photon energy resoution of 2-4% from 0.2 to 2 GeV, The insert in Fig. a shows the cacuated photon detection efficiency Y- versus energy; conversion probabiity, detector anguar acceptance, and the transverse momentum cutoff are incuded. An isotropic photon distribution is assumed; E r drops by 16% for a + cos2ig distribution.

3 -3- The incusive photon spectra for $ and I#' are presented in Figs. a,b in 4% energy bins. The ine in Fig. a is the incusive photon spectrum obtained from a simpe phase space Monte-Caro mode of IJI events, normaized to the data in the GeV interva. The spectra shapes of the Monte-Caro resuts and the data agree we above m 0.2 GeV; the Monte-Caro photon conversion yied per event agrees with the data within the N 10% systematic uncertainties of the mode. The 9' spectrum shows a > 5 u peak at N 260 MeV, having a width consistent with the resoution. The peak position and width are not affected by an increase in the charged partice transverse momentum cutoff, indicating the peak is not shaped by the acceptance. After a correction for mean energy oss by ioniza- tion the peak is at 261~ 10 MeV, corresponding to a state of mass 3413t1 MeV; we identify this as the X( 3415) observed in hadronic channes.172 The branching fraction for the transition v' + rx(341-5) is cacuated as BF = (No. of signa evts.)/(ey * RC efr) (NO. of tota evts.)/efs1. Here RC, cacuated to be a N 0.8, is a correction for radiative osses, etr is the average trigger efficiency- for a events, and E S is the trigger effitr S a ciency for signa events; etr is taken to be equa to etr enhanced by the contribution from the converted photon -- this assumes the X(3415) has mutia s body decays simiar to I# or IJI'. The ratio etr /I&, determined by counting the fraction of events in which the photon conversion was essentia to trigger, is There is a further 7% correction to the branching ratio to account for interna conversions. 7 The branching fraction is for an isotropic photon distribution and k0.026 for a + cos26 distribution (which is favored by the data2). The 35% reative error is the sum in quadrature of the 17% statistica error and estimated systematic errors of 20%

4 -4- in the background subtraction, in the photon detection efficiency, 10% in a s the etr /etr cacuation, and 5% in the radiative correction. This resut is - in good agreement with the preiminary resuts of the SPEAR SP-27 coaboration and sighty arger than the upper imits found by the HEPL experiment at SPEAR.Y There are no other cear monochromatic signas in the $ or Jr, spectra above 250 MeV; beow 250 MeV the sma and rapidy varying photon detection efficiency makes monochromatic signas difficut to observe. Upper imits for monochromatic signas were cacuated by repacing the number of signa events in the branching fraction cacuation by the excess over the smooth background pus twice the error on the tota within the E'WHM photon energy resoution; aowing for sys- tematic errors, these shoud be interpreted as 90% confidence eve upper imits. Tabe I gives upper imits for monochromatic photon ines, separated from other ines by at east the FWHM energy resoution, for representative energies above 250 MeV at 9 and q'. The question of additiona signas in the +' spectrum is compicated by the Dopper broadening of the photons from intermediate states X, X + rq, due to the motion of the X state. The broadening is sufficient to more than doube the expected width and to merge together the contributions from the various X states, making the background beneath any possibe signa in the MeV region difficut to estimate. Photon conversions enabe us to identify q' + m events, where one of the photons converts and the J) is detected by its epton pair decay. The observed epton pair mass is required to ie between 2.97 and 3.22 GeV and is the constrained to the Q mass. Eectron pair events are rejected when the ange between the eectron and the positron is > 177*5O, eiminating > 95% of the radiative background from $' -+ e+e- but ony - 8% of rea +' + y-r+, f- $4 ee events. Radiative events were further reduced by rejecting events if the converted photon was coinear within 10 0 with one of the eptons; five events were rejected by this cut. Background from $' + n+n-jr where the ~'n- 8

5 -5- simuate a converted photon is negigibe, since the r['~[- mass peaks strongy at arge vaues. 10 Figure 2 shows the missing-mass-squared, $, recoiing against the qy- system for the 54 events surviving the seection cuts. There is a peak at zero < corresponding to true 9' * Y-r-q events and a broad background from Q' + 00 v'sr 2- where a photon from a n: 0 converted. Additiona photons may be detected in the shower counters, which cover N 65% of 4n and have resoution of - oo in azimutha ange. The shading in Fig. 2 fags the detection of additiona photons:~ darky shaded events have a shower counter signa consistent 11 with the missing photon direction assuming a $f decay, ighty shaded events have no unconverted photons detected, and unshaded events have one or more photons detected in counters inconsistent with the missing photon direction (as expected for qr' + n"nov events). The high correation of shading with sma I$ corroborates the identification of events with sma g as $' + m events. The q shaded events with (GeV/c2)2 < e < 0.03 (GeV/c2)' are kept. The unconverted photon is detected in 15 of these events, consistent with detector acceptance and efficiency. Six events with missing-mass-squared recoiing against the \Ir greater than 0.27 (GeV/c2)2 are removed as possibe $' + 97, TJ + yy events (four s&h events are expected). The remaining 21 events are constrained to fit $I' -+ m\r. There are two vaues of the w mass for each event; these are potted against one another in Fig. 3. The shaded events in Fig. 3 have the unconverted photon detected in the proper counter. The expected background is one event. There are three main custers of events in Fig. 3. The mass spread of the high mass projection of each custer is consistent with the expected rms resoution of N 8 MeV; the masses of the three possibe states are 3543fO MeV, O MeV, and MeV. The mass spread of the ow mass projections of each of the custers is consistent with the expected Dopper-broadened

6 -6- resoution of MeV; each custer has a confidence eve of,< for the ow mass projection assuming the 8 MeV unbroadened resoution. However, the events at 3454 MeV in the high mass projection coud be interpreted as a state at 3340 MeV with three events and one background event. The states X(3545) and X(3505) h ave been observed through hadronic decays 2 ; however, there is no evidence in hadronic channes for a state at 3455 MeV or 33cO MeV, and such a state awaits confirmation. Correcting for $ branching ratios and photon detection efficiency, we cacuate the branching ratio products for Qr' + TX, x + y-t~ to be 0.8?0.4%, %, and % for the X(3455), X(3505), and X( 3545) respectivey. The singe event at 3413 MeV is either from the x(345)172 or background. If it is taken as signa, the branching ratio is %. In concusion, the transition q' + yx( 3415) has been observed in the If' incusive photon spectrum with branching fraction o. 026, assuming a +cos20 anguar distribution. No other monochromatic signas are observed above 250 MeV in the \i or 9, incusive photon spectra, with 90% confidence eve upper imits of - 4% f or the branching fractions. Identifying X( 3415) with the 'PO charmonium state, the observed branching fraction is a factor of 2 smaer than the prediction of Eitihten et a. 12 Aso, the 90% confidence eve upper imit of N 1% for transitions from \Ir' to a state5 near 2.8 GeV is a factor of four smaer than their prediction if we assume the 2.8 GeV state is the pseudoscaar partner of the \ir. From studying the $r masses in q' + w$ events we find evidence for two states X at O MeV and MeV with branching fraction products for q' 3 TX, X + rv of % and % respectivey. There is aso an indication of a state at 3455 MeV or 3340 MeV with branching fraction product of w 0.8%; this state awaits corroboration. The estabished states at 3415, 3505, and 3545 MeV are candidates for the predicted tripet P wave states of

7 -7- charmonium. However, there is serious disagreement between theoretica prediction and experiment for the branching fraction product $' + rx(3&55), X(3455) -+ rq if the X( 3455 ) is identified with the pseudoscaar partner of the q'. 13 Whie such issues remain to be resoved, there is now an emerging picture of Q and X spectroscopy in quaitative agreement with the charmonium picture. Tabe I. 90% confidence eve branching fraction imits for monochromatic photon production above GeV at + and qt. 9 9 Er (GeV) Limit Limit ~ o *Limit for a narrow peak. See text for a discussion of compica- tions due to the cascade photons.

8 -a FOOTNOTES AND REFERENCES 1. G. J. Fedman et a., Phys..Rev. Letters 2, 821 (1975). 2. G. Godhaber, Proceedings of the Internationa Conference on Production of Partices with New Quantum Numbers (University of Wisconsin, Madison, Wisconsin, 1976). 3. W. Tanenbaum et a., Phys. Rev. Letters 35, 1323 (1975). 4. W. Braunschweig et a., Phys. Letters 57B, 407 (1975); D. Schmitz, Invited Tak presented at the Internationa Neutrino Conference, Aachen, Germany (1976); B. Wiik, Rapporteur's Tak at the XVIIIth Internationa Conference on High Energy Physics, Tbiisi, USSR, Juy 15-21, Invited taks by B. Wiik and J. Heintze in Proceedings of the 1975 Interna- tiona Symposium on Lepton and Photon Interactions at High-Energies, W. T. Kirk, editor (Stanford Linear Acceerator Center, Stanford, Caifornia, ~ 6)~ PP. 69 and 9% 6. J.-E. Augustin et a., Phys. Rev. Letters 2, 233 (1975). 7. N. M. Kro and W. Wada, Phys. Rev. 98, 1355 (1955). 8. D. H. Badtke et a., paper submitted to the XVIIIth Internationa Conference on High Energy Physics, Tbiisi, USSR, Juy 15-21, J. W. Simpson et a., Phys. Rev. Letters 2, 699 (1975). 10. W. Tanenbaum et a., Phys. Rev. Letters 36, 402 (1976). 11. The consistency was tested ony in the transverse pane, due to poor resoution aong the beam axis. 12. E. Eichten et a., Phys. Rev. Letters 3, 500 (1976). 13. M. Chanowitz and F. Giman, Phys. Letters B, to be pubished.

9 -9- FIGURE CAPTIONS Fig. 1. The incusive photon spectra in 4% bins of the photon energy for (a) JI and (b) 9'. The smooth curve in (a) is the incusive photon spectrum from a phase space Monte-Caro mode of $ decays, normaized to the data in the region GeV. The insert in (a) shows the photon detection efficiency versus energy. Fig. 2. Missing-mass-squared I$ against \Iry-; the shading is expained in the text. Fig. 3. Scatter pot of the two soutions for the mass of intermediate states in ICI' + Y-r* events.

10 Ey (GeV) - 9 \o 0 2 e CL t + i 9 I@# i t ++h h** PHOTON ENERGY (GeV) Fig. 1

11 EVENTS/O.01 ( G&/c~)~ I I I :.:.:.:.:.:.:.:.:.:.:.:.:.:.:.~:.~:..:.:.:.:.:.~:.:.: CD CQ 0. < h) o\. 0

12 L e+e- Z 0 I- 3 5: 3 ^r 3 CA 2 I g 3.F_._ ::. :::::...~.~....~..Z.~..~.~... - ::::...._ ::. _.: 2 ::..~...~_~...~.,.:::::. ;_.::::. =i 1 g t 0 e 3.20 MASS ($7) HIGH SOLUTION (GeWc2)2 IPI2e.2 Fig. 3