Ξ0 beta decays at KTeV

ELSEVIER Nuclear Physics B (Proc. Suppl.) 75B (1999) 36-40

m m u

P R O C E E D I N G S SUPPLEMENTS

_~0 beta decays at KTeV

Emmanuel Monnier (on behalf of the KTeV collaboration) a

aUniversity of Chicago, Enrico Fermi Institute, 5640 S. Ellis Avenue, Chicago, Illinois, 60637, USA

The new KTeV beam line and detector at the Fermilab Tevatron have been operating in 1996 and 1997. The experiment 799 at Fermilab was the rare decay run of the KTeV apparatus. Besides collecting a large number of neutral kaon decays, the apparatus was used to trigger for rare decays of neutral hyperons. We present the status of our measurements of the hyperon beta decays =0 _~ E+ e ue, as well as evidence for the first observation of ~6 _4 ~ - e + u~ and E ° -4 E + p - ~

1. I n t r o d u c t i o n

KTeV is a fixed target experiment at Fermilab. It is designed to study mainly CP violation and rare decays in the neutral kaon system. I t looks at a double neutral beam created from the interac- tion on a beryllium oxide target of a high intensity 800 GeV/c proton beam (up to 5 × 1012 protons per 19 s Tevatron spill every minute). At 90 m downstream from the target, this intense beam

contains not only K ° and ~-o (as well as neutrons and photons), but also a good fraction of high mo-

mentum A °, A °, S °, and ~ that decay in a 70 m vacuum region. Powerful particle identification, and outstanding resolution for both charged particles and photons make the KTeV detector an excellent facility for the s tudy of hyperon decays and particularly rare E ° decays. The KTeV/E799 phase of the experiment has taken six weeks of data in winter 1997 and four weeks in summer 1997. Analyzing these data, the hyperon group at KTeV has observed m ~ l y new semileptonic S ° decays. We report here oil the status of this analysis.

2. T h e K T e V d e t e c t o r

The KTeV detector[I,2] is based on a precise charged particle spectrometer and a high resolution electromagnetic calorimeter. The spec- t rometer which provides the momentum of the charged particles and identifies the charged vertex, consists of a high precision dipole magnet

surrounded by four large 4-plane drift chambers. The magnetic field imparts a 205 MeV/c horizon- tal momentum component to charged particles. The electromagnetic calorimeter is made of 3100 pure CsI crystals (27 radiation lengths, 1.4 inter- action lengths).. Its energy resolution is bet ter than 1% for electromagnetic particles between 2 and 60 GeV. It is used to identify electrons and photons and to measure their energies. To improve the pion/electron identification during the KTeV/E799 run, a nine chamber transit ion radiation detector was added in front of the calorimeter. In addition to these various elements, a set of sub-detectors, mainly scintillators, are used to define the fiducial volume.

3. H y p e r o n T r i g g e r s

All the informations provided by the various sub-detectors of the KTeV experiment are used by the different levels of the trigger system to select the best events to keep on tape. For the hyperon studies, a specific trigger component has been added to the trigger logic. I t is aimed to identify high momentum charged particles trav- eling down the beam holes since in their decays, hyperons usually transfer most of their momentum to the final s tate baryon. The hyperon trigger was optimized for studying the decay mode ~o __+ E+ e - Yc with the E + decaying into a proton and a 7: °. Therefore, events were selected online as hyperon events if they had a hit in either of the beam hole scintillators combined

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E. Monnier/Nuclear Physics B (Proc. Suppl.) 75B (1999) 36-40 37

with hits in the drift chambers in the beam hole regions corresponding to the hit scintillator, at least two clusters in the calorimeter and a total energy of 18 GeV or greater, and a loose veto on hadronic showers after the calorimeter. We also took prescaled triggers that contained no or very loose requirements on the calorimeter to select unbiased so--+ A ° ~0 decays for normalization purposes and to allow trigger efficiency studies. During winter 1997 and summer 1997, more than a couple of hundred million hyperon candi- date have been recorded on tapes.

4. S ° s e m i l e p t o n i c d e c a y s

The study of the S ° - + Z + e - p e a s well as

=0 _+ E e + ~edecay modes is interesting because under d and s quark interchange, this pro- cess is the direct analogue of the well-measured neutron beta decay, n -+ p e - ~ . Thus, in the flavor symmetr ic quark model (or likewise, the Cabibbo model), differences between these two decays arise solely from the differing particle masses and from the relevant CKM matr ix elements (Vus rather than V~,d). Flavor symmetry violation[4,5] can therefore be directly inves- tigated. In the symmetry limit, the predicted branching ratio is (2.61 + 0.11) × 10 -4. Stud- ies of pari ty violating interference effects in this decay should also prove possible because the final s tate E + polarization is directly measurable. Finally, in contrast to other hyperon be ta decays, the absence of a competing two-body decay containing a E + eliminates a major background. The _~o _+ E+ p - ~ decay mode is of special in- terest because the mass of the muon may induce non negligible third order form factors[6] contri- butions. Hints tha t these factors are non-zero and large might first appear if the branching ratio of this decay differs from the simple Cabibbo theory prediction which can be calculated to be 2.2 × 10 -6.

4.1. = o _ _ ~ E + e - p e In the =0 _+ E+ e - ~e decay, the final s tate par-

ticles are an electron and a high momentum proton and two photons tha t come from the decay of the E +. To filtered our da ta and identify

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Figure 1. Neutrino momentum spectrum in the =o rest frame. Points are da ta with statistical errors, and the his togram is our Monte Carlo simulation normalized to the same number of events. The acceptance has little effect on this distribution. Note the typical be ta decay spectrum shape.

this decay mode, we have applied several selection criteria. Only events with two well separated neutral clusters above 3 GeV and one charged cluster with 90% of its energy deposited in the calorimeter were accepted. The last criteria se- lects electrons. Since this decay is fairly asym- metric, the momen tum ratio between the proton and the electron is usually greater than one, in contrast to kaon decays. Thus only events with a ratio greater than 3.5 were accepted in order to remove the kaon background. In addition only events with a proton above 110 GeV/c and an electron above 2.5 GeV/c were kept. The E + vertex is obtained by finding the point along the proton flight pa th where the invariant mass of the two photons matches the 7r ° mass. The p~0 invariant mass can then be reconstructed. The closest approach between the electron track and the reconstructed E + track defines the _-0 vertex point. This vertex is required to be ups t ream of the E + vertex and within the decay volume of the KTeV apparatus , and the total transverse

38 E. Monnier/Nuclear Physics B (Proc. Suppl.) 75B (1999) 36-40

momentum of the decay products is required to be small. Several other fiducial cuts are applied to fully constrained the events in the detector region. Finally, we also require a suitable E + e - invariant mass (1.2-1.32 GeV/c 2) and a neutrino transverse momentum which is (within errors) less than the reconstructed neutrino momentum in the ~o rest frame. A similar technique with a A ° instead of a E + is used to reconstruct events with the hypothesis of a E ° ~ A ° 7r ° decay with a A ° --+ p lr- decay. The events previously selected that have a reconstructed E ° mass with this last hypothesis close to the expected value are re- jected. In addition, for normalization purposes, this reconstruction hypothesis is combined with similar fiducial and kinematic requirements than those used to select E ° - + E + e u~ events, and is applied to isolate E ° --+ A ° 7r ° events from the data selected online to be E ° --+ A ° 7r ° candidates. To eliminate electrons, accepted events are also required to have less than 80% of the nega- rive track (Tr-) energy deposited in the calorimeter and to have a A ° 7r ° invariant mass within +15 MeV/c z (+5 standard deviations) of the accepted ~_0 mass.

From the data taken during the winter period, 251 events have been isolated with a p~r ° invariant mass distribution presenting a clear peak of 179 events within +15 MeV/c 2 (+3 standard deviations) of the accepted E + mass. Estimating the background by counting events in two 15 MeV/c 2 mass regions on either side of the peak, we find a signal of 169 ± 13 ~o _+ E+ e-PC events on top of a 10 event background. The reconstructed E + and E ° decay distance distributions match the known lifetime values. Furthermore, distributions like the laboratory proton and electron momentum spectra and the neutrino spectrum in the E ° rest frame (Fig. 1) for simulated E0__+ E + e - ~ decays all agree with the data. These events represent the first observation of the E ° -+ E + e - ~ decay. From the same data taking period, 43 272 E ° -+ A ° 7r ° have been selected as well. Again, kinematic distributions for simulated events agree well with the data. Acceptance calculations and background investigations were carried out with a detailed Monte Carlo simulation of the beam and detector. One of the main

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Figure 2. The pTr ° invariant mass distribution for E ° -+ E+e-P~-~ event candidates from the 1997 KTeV runs.

remaining background is from E ° --+ A ° 7r ° events and more specifically from events in which the A ° decays semileptonicaly. Nevertheless it is strongly suppressed and ostimated to contribute only for five events. Other backgrounds such as radiative K~3 decay are also strongly suppressed. System- atic effects were studied very carefully with the data. With all this information we measure a branching ratio of (2.49 -I- 0.19 -t- 0.25) x 10 -4.

The first error is statistical and the second is systematic. The calculated branching ratio of 2.61 x 10 -4 based on exact, flavor symmetry is consistent with this result. The analysis of the two data taking periods is now under way and we have extracted a sample of the order of 1000 E ° --+ E + e-P~ decays as can be seen on the figure 2 that show the invariant v r ° mass for all the selected .~o __+ E + e - ~, events.

Due to trigger improvement, the accumulated statistics during the summer period is three times higher than accumulated during the winter period. Extra data were taken to improve the trig-

E. Monnier/Nuclear Physics B (Proc. Suppl.) 75B (1999) 36-40 39

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Figure 3. The cos(O~p) in the E + rest frame dis- tr ibution for =0 ~ E+e-P~ event candidates from the data (plain circle) and various Monte Carlo simulations (open symbols).

ger control as well. For these reasons, the asymmetry analysis has s tar ted on the summer run. Many steps are needed to complete such a difficult analysis and extract form factor values. Never- theless we are progressing and as a preliminary result, we present in figure 3, in the rest frame of the E + and without acceptance correction, the distribution of the cos(Oep) which is directly linked to the electron proton asymmetry. This quanti ty is interesting since the E + is almost 100% self ana- lyzed because of an C~o = -0 .98. Simulations per- formed with various first order axial vector over vector form factor values (ga/fl)[3] are compared with the data on the same figure. As expected, the known g l / f l value from the neutron be ta decay tend to be favored by the data. Combining this correlation with the neutrino proton and neutrino electron correlations will help to get a bet- ter precision on these form factor values. More results are expected in a few months.

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Figure 4. The p - l r ° invariant mass distribution

for =-~6 __+ ~+e+ue event candidates from the 1997 KTeV summer run.

4.2. ~ o _ ~ E e +re Our trigger conditions during the winter run

highly suppressed anti particle decay modes but this was modified for the summer run. : 0 were produced and selected online with a 1 to 12 relative production rate with respect to 20 . Then by performing the same analysis as the Eo ~ E + e - ~ a n a l y s i s but only inverting the charge of the high momentum particle and of the low momentum one, we extracted 247 events. Their pTr ° invaxiant mass distribution is presented in figure 4. It exhibits a clear peak of 70 events on top of a background of 45 events. This peak has been checked to be broadly distributed in time. Associated distributions such as the reconstructed neutrino momentum look very similar to those obtained with the : o __~ E+ e - ~e decay. This is the first observation of this =0 decay mode. Initial branching ratio calculations compare favorably with the be ta decay mode branching ratio. More work is underway to pursue these comparisons between the two modes.

40 E. Monnier/Nuclear Physics B (P~oc. Suppl.) 75B (1999) 36-40

5. E ° --+ E + ~ - v ,

Again, by performing similar analysis as the ~_o_+ ~+ e-V~ decay mode analysis, but select- ing a muon instead of an electron we looked for evidence of So _~ E+ # - p~ decays. Most of the selection criteria are identical or very close to those used for the beta decay analysis. Some had to be adjusted because the muon is heavier than the electron and thus the decay released less energy. To select muons, hits in the muon chambers are required in combination with almost no energy deposited by the muon in the calorimeter and no hadronic showering in the back of the calorimeter. In addition to usual kinematic criteria, the ~ + # - ~ 0 reconstructed mass is required to be greater than 0.49 GeV/c 2 to remove K ° -+ ~°~+~r- with ~r- -+ # -P~ background. The remaining effects of this background has been studied with wrong sign events since anti-hyperons are highly suppressed at production. Mass cuts were also used to remove most of the background of E0 _~ A 0 u0 events with A ° -+ p u - and the ~ - either decay in flight or fake a muon. Finally, of all the data taken, six events remain with an es- t imated background of 0 events as can be seen on figure 5. The number of events outside of the box is also consistent with expectations for the background level and for the data. This represents the first observation of this decay mode. More- over, initial branching ratio calculations normalized to the measured ~_0 __+ ~+ e - ~ branching ratio compare favorably with the expectations. Improvement in the knowledge of this mode is expected with the new run of KTeV in 1999.

6. C o n c l u s i o n

During this first year of analysis, the unique combination in the KTeV detector of an outstanding electromagnetic calorimeter and a very accurate spectrometer gave us the opportunity to observe for the first time several hyperon semileptonic decays. We hope that in a near future precise measurement of the S0 form factors would be possible. A new run of the KTeV detector and beam-line is planned to occur in 1999 with a fore- seen increase by 4 of the statistics for most of the decay modes cited here.

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Mp~o (GeV/cal Figure 5. E ° reconstructed transverse momentum squared versus the p~0 invariant mass for _~0 __+ E+ # - ~ candidates from the data (plain circle), from a Monte Carlo simulation of the expected signal (dots) , from opposite sign data (kaon background) (open triangle) and from simulated ~o __+ ~o A --+ p ~ - ~ # - ~ background events (open square). Superimposed is a box that contains 90% of the simulated signal events.

R E F E R E N C E S

1. K. Arisaka et al., K T e V (Kaons at the Teva- tron) Design Report, Fermilab Report No. FN-580, 1992.

2. J. Adams et al., Phys. Rev. Lett. 80, 4123 (1998).

3. Particle Data Group, R.M. Barnett et al., Phys. Rev. D 54, 568 (1996).

4. See, for example, J. Dai et al., Phys. Rev. D 53, 273 (1996); A. Garcia, R. Huerta, and P. Kielanowski, Phys. Rev. D 45, 879 (1992); C. Avenarius, Phys. Lett. B 272, 71 (1991); M. Roos, Phys. Lett. B 246, 179 (1990); P. G. Ratcliffe, this proceeding; L. J. Carson, R. J. Oakes, and C. R. Willcox, Phys. Rev.D 37, 3197 (1988) wherein flavor symmetry violation is considered in a variety of contexts.

5. A. Garcia and P. Kielanowski, The Beta De- cay of Hyperons (Lecture Notes in Physics, 222, Springer-Verlag, 1985).

6. V. Linke, Nucl. Phys. Lett. B 12,669 (1969).

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Ξ0 beta decays at KTeV