New physics results from the hyperon beam experiment WA89

NUCLEAR PHYSICS A Nuclear Physics A585 (1995) 183c-192c

ELSEVIER

New Physics Results from the H y p e r o n B e a m Experiment WA89 The WA89 collaboration* presented by Stephan Paul Max-Planck Institut ffir Kernphysik, Heidelberg, Germany 1. I n t r o d u c t i o n In the following talk we will present results from the CERN hyperon beam experiment WA89. This experiment aims at the study of strange final states with particular emphasis on exotic multiquark states and charmed strange baryons using E-of 330 GeV/c. Among the many topics studied in this experiment we will here concentrate on the production and decay properties of charmed baryons.

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Figure 1. SU(4) baryon multiplet

Charm production has long been subject of much speculation as cross sections measured showed large variations and the seemingly agreement of recent experiments with calculations has lead to a partial omission of early results difficult to be explained. Apart from e+e - collider experiments which focus their studies on the decay properties, however, all recent experiments have been using photon or pion beams. No exotic production phenomena are expected in photon beams and observed absolute and differential cross sections agree well with QCD predictions [1].

In meson beams so called 'leading' effects are observed which favour the forward production of final states which share at least one quark with the incoming projectile [2,3]. Such 'long distance' phenomena are partly reproduced within the Lund model [4] which has been shown to describe the hadronization process very well in e+e - experiments. The WA89 experiment offers a good alternative as the incoming E- hyperon carries strange and down quarks and leading effects, much more enhanced in baryon beams (see e.g. hyperon production by proton beams), could be singled out studying charmed baryons with and without strangeness or a large d-quark content. *Bristol-CERN-Genoa-Grenoble-HeidelbergUniv.-Heidelberg MPIfK-Mainz-Moscow-Rutgers 0375-9474/951509.50 © 1995 - Elsevier Science B.V. All rights reserved. SSDI 0375-9474(94)00563-X

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S. Paul /Nuclear Physics A585 (1995) 183c-192c

Besides production studies, which goes along with spectroscopy, our experiment also tries to contribute to the understanding of hadronic effects in weak interaction. For this purpose we want to measure the relative abundance of many different decay channels in order to study the relative importance of the various possible decay mechanisms. Although many different decay channels have been observed for charmed strange baryons so far, we lack information on their relative strength. Each single experiment only observes a few final states which can not be related to those of others since an absolute normalization is currently impossible. This situation is different for the case of non strange charmed baryons, however, they only constitute some pieces of the large hadronic jig saw puzzle. Such studies should go along with a precise determination of the lifetime of all ground state charmed baryons. Hadronic effects are the cause of the large lifetime differences observed among strange hadrons and are believed to be about 10 times smaller in the charm sector. After a brief description of the WA89 spectrometer and the analysis methods we will present spectral distributions of the production for A+, T ° and ~+ which show strong leading effects for charm production with a ~ - beam. Besides some studies of baryon decays we will show our first observations for ~0 and discuss the possible first evidence for the multiplet partner of ~+ (see fig. 1). Most of the results shown stem from our data taking in 1991. Where possible we will present data from our last beam time in 1993 with a much improved detector set up.

2. T h e E x p e r i m e n t

2.1. Experimental set-up Fig. 2 shows a view of the experimental set up used in 1993. The incoming beam (~r/E ~ 2) of 330 GeV/c is defined by silicon microstrip counters and a fast TRD for ~r rejection. Downstream of the longitudinally segmented target (Cu and C) 29 planes of silicon are used for precise track reconstruction. A 10 m long decay area filled with MWPC and DC serves for the detection of hyperons and K0. Charged track momenta are measured in the ~ magnetic spectrometer and particle identification is performed in a large Ring Imaging Cnerenkov counter (RICH). An electromagnetic (lead glass) and hadronic (spaghetti) calorimeter allow to measure photon and neutral particle energies.

2.2. Charm Reconstruction In order to identify the production of charmed hadrons we must strongly reduce background from other reactions as charm production amounts to about 1/1000 of the total cross section. On the one hand this is achieved by means of particle identification (where possible) either via signature in the RICH or by reconstruction of decay products forming a A, E or Ft. On the other hand we use the topological characteristics of such events searching for a decay vertex of the charm candidate being separated from the intial interaction point (primary vertex). Cuts on vertex reconstruction quality, primary-secondary vertex displacements, and/or vertex isolation criteria are commonly used.

S. Paul / Nuclear Physics A585 (1995) 183c-192c

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WA89 Hyperon beam 1 9 9 3 layout

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3. D i f f e r e n t i a l d i s t r i b u t i o n s o f various c h a r m e d b a r y o n s The invariant mass spectrum of p K - T r + final state is given in Fig 3a) showing a clear signal for A +. Using such A + candidates we connect these combinations with an additional r - to reconstruct E ° (fig. 3b). The phase space background is well reproduced using p K - ~ r + combinations off the nominal A+mass. Both spectra stem from the 1991 run period. Fig. 3c) depicts a signal for the E + decaying into AK-~r+lr + using only a small part of our 1993 data set. Underneath of each mass spectrum we show the background subtracted differential distributions d N / d z / (data points) corrected for trigger and reconstruction efficiency. A commonly used parametrization of the differential cross section is given by: da dx.edp~ - A . (1 - x l ) '~ . e -bp2*

(1)

The solid line represents the results of a fit. Values for n resulting from the fit are given in the plots. Within the still large errors for n which is a measure for the hardness of the spectra, no influence of the quark content seems visible. Such effect could manifest itself in a variation of the value of n, giving lower values for baryons with larger 'leading' effect. However, such fits can be misleading as most of the cross section (and usually also most of the number of observed events) is found at low values of x I and the fit is tightly constrained by these data points. In NLO-QCD calculations charm quark production is decsribed via the 'dominant' QCD processes q~ annihilation and gg-fusion which lead to a generanl value for n=6.9 [5]. If other production processes or exotic hadronization mechanisms play a role then they might show up at large x I. One therefore should look for a deviation of the spectral shape from a global (1 - x l ) T M behaviour at larger x l , with n N g being the parameter for non leading hadrons (the value n N g , however, can well be

S. Paul / Nuclear Physics A585 (1995) 183c-192c

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different for mesons and baryons). Although statistically not yet convincing, the spectrum for ~+ might show such a deviation at high x/. Earlier baryon beam experiments at lower energies with exclusive sensitivity into the very forward region (xf > .5) had indicated much harder spectra for charmed baryons than all recent meson beam experiments [6-8]. Within the present data set we cannot yet confirm this result at our energies. The Lund model describing the hadronization of charm quarks into mesons rather well (at least at higher values of x/) provides such a mechanism via the colour flow model. Assuming a 'hard' interaction of 2 partons only, the spectator quarks (diquarks) remain undisturbed keeping their initial colour. This way a colour correlation between the produced c-quarks and the spectator quarks can be kept which leads to an efficient recombination of the c-quark with the beam remnant. This process, although not important at low ] x/ ], leads to a large increase of the cross sections in the forward (or backward) region. The predictions of this model are also shown in fig. 3 (lower row) as the dotted histograms. The leading effect for ~0 baryons, however, is largely overestimated and can only be reduced assuming an unreasonable large value for the minimum Pt of the charm quarks produced. For baryons having only one quark in common with the projectile almost no such effect is predicted. This strongly indicates that the present scenario

S. Paul / Nudear Physics A585 (1995) 183c-192c

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involving a 'stable' diquark is oversimplified and should be modified. The interesting feature in our data, however, comes less from the spectral shape analysis but rather from large asymmetry in the observed yield of leading and non leading hadrons within our acceptance (x t > 0.2). No significant signal for the ~++could be observed leading to an upper limit for the production ratio ~'c++ /~c0 < 0.52 (90% c.1.). Another interesting observation is the large fraction of A+ steming from the strong decay of 0 + Ec---~A c ~r- . Taking into account acceptance effects we conclude that about .45+0.31 -4-.1 of all A+ are decay products of S °. If we further assume that as many Ec(+~A~Tr + 0) as A+ are genuinely produced and ignore the production of non leading E++ the ratio Agce n , ~ t n e / ~ 0 (leading/double leading) amounts to ~0.62. A similar effect can also be found for D-mesons, where we find the ratio of non leading to leading D + / D - = .47-4-0.14-4-.05. These features are in part well reproduced within the LUND model, in particular the A+/E ° ratio (see table 1). However, the large h + / D - ratio (11.3 4- 7.1) observed in this experiment, cannot be accounted for and has not been observed in meson beams. The spectral distributions in the transverse coordinate (pt2) exhibit a general exponential drop of the cross section with a slope b of ~ 1.2/(GeV/c) 2. This value which is found for all particles is in good agreement with naive expectations W ,'.~O(m~c). The large minimum pt required for the Lund model can not be observed. Finally we have measured the nuclear mass dependence of the production of --+using 1~C, 2ssi and 63Cu targets. The value of a ,,- A 's34-'24:l:'6s gives no conclusive answer on the question of surface effects. The best measurements on nuclear effects of charm production stem from the Drell Yang experiment E... which has observed a clear x/ dependence of this mass dependence. Much more statistics is needed to manifest such a behaviour in hadronic open charm production.

4. S o m e studies of c h a r m e d s t r a n g e b a r y o n

decays

Figure 4 (upper row) depicts signals for the --+ decaying into AK-~r%r + and -':-Tr%r+ from the 1991 data set. The corresponding decay channels for E0 are shown in fig 4(lower row). In order to extract branching ratios for those final states we have to first measure the lifetime of these states as the reconstruction efficiencies involving topological vertex cuts will be different for all channels. In the 1991 data set this has, however, only been possible so far with the AK-~r%r + final state. In order to reduce biases for the lifetime measurement difficult to control by any simulation we have replaced cuts on the significance of the vertex separations by an absolute minimum distance of the reconstructed charm decay point from the target edge. This has lead to only a small reduction of the events in the signal region. A binned likelihood method has then been applied to the lifetime distribution fitting simultaneously the number of events in the signal and the lifetime of the signal. No assumption on the shape of the background lifetime distribution has to be made as this is taken directly from the sidebands. We have obtained a value of r=+ = •32+'°s -.o6 3= .05 ps in good agreement with existing measurements (fig. 5) [6,9-11]. The present status of the analysis has not yet allowed a similar study for the neutral Ec partner. Using this lifetime we have estimated the branching ratio of the two decay channels

S. Paul / Nuclear Physics A585 (1995) 183c-192c

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with large systematical uncertainties. This value could be interpreted such that the splitting of the resulting two strange quarks into two different hadrons is preferred and could be understood from a large recoil transferred to the strange quark in the c-quark decay process. In D-meson decays two body final states are largely enhanced over many body decays. Observed multihadronic final states very often stem from the decay of intermediate resonances. Simple considerations predict that in the case of the -~+ baryon resonances should largely be suppressed and can only come from short range spin-spin interactions or from a mixing of the S+wavefunction (which is is mainly antisymmetric in the two lighter quarks) with the one from its multiplet partner -~+' (which is mainly symmetric) [12]. Our present analysis has shown no sign for a E.0 resonance in this decay _~+

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(4)

S. Paul /Nuclear Physics A585 (1995) 183c-192c

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Search for =' ~¢ Single-strange charmed baryons have so far only been observed as members of a SU(3) triplet with antisymmetric wavefunction in the two lighter quarks. The sextet states with symmetric wavefunction are expected to have a mass larger by about the size ~2' c of the A-E mass splitting. Most calculations predict a mass difference only allowing radiative decay into the antisymmetric triplet state. In order to search for those states we have selected E+ candidates in the two different decays modes described above and searched for associated photons. The mass differences of the two systems are shown in 4.1.

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S. Paul / Nudear Physics A585 (1995) 183c-192c

fig.6 for E + decaying into AK-Ir%r + and E-Tr+~r+. A small signal can be observed in both spectra at the same energy. The summed spectrum is also shown in fig. 6. Although statistically still weak we interpret this signal as first evidence for the observation of a ~--' c state. It should be noticed, however, that no such signal could yet be observed for the neutral charge state. 4.2. Search for ~0

The ssc (~0) is the least well known weakly decaying charmed strange baryon. Only tens of events had been seen until very recently in different experiments and different decay channels. The E687 collaboration has shown larger signals beginning of this year including ~, baryons in the final states. However, no particular decay mode has yet

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been seen by at least two experiments. Very recently we obtained our first f~0 signals in three different final states: ~-~r +, fl-lr-~r%r + and =-K-~r%r + (Fig. 7). No lifetime measurement for this baryon is yet existing. Theorists prediction ranges from z~c being comparable with the charged D-mesons lifetime to Tac being the shortest of all charmed hadrons [13,14]. With a larger sample at hand this question will be attacked by WA89 to measure the eventually fastest weak decay observed. 5. O u t l o o k Including data from a further beam time this year we expect to have up to thousand reconstructed charm events in the dominant decay modes. With such a large sample questions on production should be answered with higher reliability and more results on decay processes should allow to disantangle the variety of underlaying elementary processes. In particular, semileptonic decays should be an excellent tool to close the link between life time measurements and studies of branching fractions. However, only part of the new data will have some electron identification available to attack the latter problem. Observations of a possible polarization of charmed baryon could also be used to further understand production phenomena. Such effects have widely been observed in hyperon production but also here all existing data do not yet allow a simple theoretical explanation.

S. Paul / Nudear Physics A585 (1995) 183c-192c

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

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