ELSEVIER

14 July 1994

Physics Letters B 332 (1994) 219-227

PHYSICS LETTERS B

K ° production in one-prong decays

ALEPH Collaboration

D. Buskulic a, D. Casper a, I. De Bonis a, D. Decamp a, p. Ghez a, C. Goy a, J.-E Lees a, M.-N. Minard a, p. Odier a, B. Pietrzyk a, F. Ariztizabal b, M. Chmeissani b, J.M. Crespo b, I. Efthymiopoulos b, E. Fernandez b, M. Fernandez-Bosman b, V. Gaitan b, L1. Garrido b,28,

M. Martinez b, T. Mattison b,29, S. Orteu b, A. Pacheco b, C. Padilla b, A. Pascual b, E Teubert b, D. Creanza c, M. de Palma c, A. Farilla c, G. Iaselli c, G. Maggi c, N. Marinelli c, S. Natali c, S. Nuzzo c, A. Ranieri c, G. Raso c, E Romano c, F. Ruggieri c, G. Selvaggi c, L. Silvestris c,

P. Tempesta °, G. Zito c, y. Chai d, D. Huang d, X. Huang d, j. Lin d, T. Wang d, y. Xie d, D. Xu d, R. Xu d, j. Zhang d, L. Zhang d, W. Zhao d, E. Blucher e.30, G. Bonvicini e,

e2 5 e e e e e e 3 J. Boudreau , , P. Comas , P. Coyle , H. Drevermann , R.W. Forty , G. Ganis , C. Gay ' , M. Girone e, R. Hagelberg e, j. Harvey e, R. Jacobsen e, B. Jost e, j. Knobloch e, I. Lehraus e,

M. Maggi e, C. Markou e, p. Mato e, H. Meinhard e, A. Minten e, R. Miquel e, p. Palazzi e, J.R. Pater e, J.A. Perlas e, p. Perrodo e, J.-F. Pusztaszeri e, F. Ranjard e, L. Rolandi e, J. Rothberg e'2, T. Ruan e, M. Saich e, D. Schlatter e, M. Schmelling e, F. Sefkow e'6,

W. Tejessy e, I.R. Tomalin e, R. Veenhof e, H. Wachsmuth e, S. Wasserbaech e,2, W. Wiedenmann e, T. Wildish e, W. Witzeling e, j. Wotschack e, Z. Ajaltouni f,

M. Bardadin-Otwinowska f, A. Barres f, C. Boyer f, A. Falvard f, P. Gay f, C. Guicheney f, P. Henrard f, J. Jousset f, B. Michel f, J-C. Montret f, D. Pallin f, P. Perret f, E Podlyski f,

J. Proriol f, F. Saadi f, T. Fearnley g, J.B. Hansen g, J.D. Hansen g, J.R. Hansen g, P.H. Hansen g, S.D. Johnson g, R. M011erud g, B.S. Nilsson g, A. Kyriakis h, E. Simopoulou h, I. Siotis h,

A. Vayaki h, K. Zachariadou h, j. Badier 1, A. Blondel 1, G. Bonneaud i, J.C. Brient 1, P. Bourdon i, G. Fouque l, L. Passalacqua i, A. Roug61, M. Rumpf 1, R. Tanaka i, M. Verderi i, H. Videau 1, D.J. CandlinJ, M.I. Parsons J, E. VeitchJ, E. Focardi k, L. Moneta k, G. Parrini k,

M. Corden e, M. Delfino e'12, C. Georgiopoulos e, D.E. Jaffe e, D. Levinthal e'15, A. Antonelli m, G. Bencivenni m, G. Bologna m,4, f . Bossi m, p. Campana m, G. Capon m, F. Cerutti m,

V. Chiarella m, G. Felici m, p. Laurelli in, G. Mannocchi m,5, F. Murtas m, G.P. Murtas m, M. Pepe-Altarelli m, S. Salomone m, p. Colrain n, I. ten Have n, I.G. Knowles n, J.G. Lynch n,

W. Maitland n, W.T. Morton n, C. Raine n, P. Reeves n, J.M. Scarr n, K. Smith n, M.G. Smith n, A.S. Thompson n, S. Thom n, R.M. Turnbull n, U. Becker o, O. Braun o, C. Geweniger o,

P. Hanke °, V. Hepp °, E.E. Kluge °, A. Putzer °'1 , B. Rensch °, M. Schmidt °, H. Stenzel °, K. Tittel o, M. Wunsch o, R. Beuselinck P, D.M. Binnie P, W. Cameron P, M. Cattaneo P,

0370-2693/94/$07 00 t~) 1994 Elsevier Science B.V All rights reserved SSDI 0 3 7 0 - 2 6 9 3 ( 94 ) 0 0 6 3 4 - J

220 ALEPH Collaboratwn / Physws Letters B 332 (1994) 219-227

D.J. Coiling P, RJ. Doman P, J.E Hassard P, N. Konstantinidis P, A. Moutoussi P, J. Nash P, D.G. Payne p, G. San Martin p, J.K. Sedgbeer p, A.G. Wright p, E Girtler q, D. Kuhn q,

G. Rudolph q, R. Vogl q, C.K. Bowdery r, T.J. Brodbeck r, A.J. Finch r, E Foster r, G. Hughes r, D. Jackson r, N.R. Keemer r, M. Nuttall r, A. Patel r, T. Sloan r, S.W. Snow r, E.E Whelan r,

A. Galla s, A.M. Greene s, K. Kleinknecht s, J. Raab s, B. Renk s, H.-G. Sander s, H. Schmidt s, S.M. Walther s, R. Wanke s, B. Wolf s, A.M. Bencheikh t, C. Benchouk t, A. Bonissent t,

D. Calvet t, j. Carr t, C. Diaconu t, E Etienne t, D. Nicod t, E Payre t, L. Roos t, D. Rousseau t, E Schwemling t, M. Talby t, S. Adlung u, R. Assmann u, C. Bauer a, W. Blum u, D. Brown u,

R Cattaneo u,23 B. Dehning u, H. Dietl u, E Dydak u.21, M. Frank u, A.W. Halley u, K. Jakobs u, H. Kroha u, j. Lauber u, G. Ltitjens u, G. Lutz u, W. M~inner u, H.-G. Moser u, R. Richter u,

J. Schr6der u, A.S. Schwarz u, R. Settles u, H. Seywerd u, U. Stierlin u, U. Stiegler u, R. St. Denis u, G. Wolf u, R. Alemany v, j. Boucrot v, O. Callot v, A. Cordier v, M. Davier v,

L. Duflot v, J.-E Grivaz v, Ph. Heusse v, E Janot v, D.W. Kim v,19, F. Le Diberder v, J. Lefranqois v, A.-M. Lutz v, G. Musolino v, H.-J. Park v, M.-H. Schune v, j._j. Veillet v,

I. Videau v, D. Abbaneo w, G. Bagliesi w, G. Batignani w, U. Bottigli w, C. Bozzi w, G. Calderini w, M. Carpinelli w, M.A. Ciocci w, V. Ciulli w, R. Dell 'Orso w, I. Ferrante w,

F. Fidecaro w, L. Fob w'l , E Forti w, A. Giassi w, M.A. Giorgi w, A. Gregorio w, E Ligabue w, A. Lusiani w, ES. Marrocchesi w, E.B. Martin w, A. Messineo w, E Palla w, G. Rizzo w,

G. Sanguinetti w, E Spagnolo w, j. Steinberger w, R. Tenchini w,1, G. Tonelli w.27, G. Triggiani w, A. Valassi w, C. Vannini w, A. Venturi w, EG. Verdini w, j. Walsh w, A.R Betteridge x, y. Gao x,

M.G. Green x, D.L. Johnson x, EV. March ×, T. Medcalf x, Ll.M. Mir*, I.S. Quazi x, J.A. Strong x, V. Bertin Y, D.R. Botterill Y, R.W. Clifft Y, T.R. Edgecock Y, S. Haywood Y,

M. Edwards Y, RR. Norton Y, J.C. Thompson Y, B. Bloch-Devaux z, E Colas z, H. Duarte z, S. Emery z, W. Kozanecki z, E. Lanqon z, M.C. Lemaire z, E. Locci z, B. Marx z, E Perez z,

J. Rander z, J.-E Renardy z, A. Rosowsky z, A. Roussarie z, J.-E Schuller z, j. Schwindling z, D. Si Mohand z, B. Vallage z, R.E Johnson aa, A.M. Litke aa, G. Taylor aa, j. Wear aa,

A. Beddall ab, C.N. Booth ab, S. Cartwright ab, E Combley ab, I. Dawson ab, A. Koksal at,, C. Rankin ab, L.E Thompson ab, A. B6hrer ac, S. Brandt ac, G. Cowan at.l, E. Feigl ac,

C. Grupen ac, G. Lutters ac, j. Minguet-Rodriguez ac, E Rivera ac,26, p. SaraJvaaC, U. Sch~ifer ac, L. Smolik ac, L. Bosisio ad, R. Della Marina ad, G. Giannini ad, B. Gobbo ad, L. Pitis ad,

E Ragusa ad,20 L. Bellantoni ae, W. Chen ae, J.S. Conway ae,24, Z. Feng ae, D.RS. Ferguson ae, Y.S. Gao ae, j. Grahl ae, J.L. Harton a~, O.J. Hayes ae, H. Hu ae, J.M. Nachtman ae Y.B. Pan ae, Y. Saadi a~, M. Schmitt ae, I. Scott ae, V. Sharma a~, J.D. Turk ae, A.M. Walsh ae, EV. Weber a~,

San Lan Wu ae, X. Wu ae, J.M. Yamartino ae, M. Zheng ae, G. Zobernig ae a Laboratmre de PhysLque des Parttcules (LAPP), IN2P3-CNRS, 74019 Annecy-le-Vleux Cedex, France

b Institut de Fiswa d'Altes Energws, Untversttat Autonoma de Barcelona, 08193 Bellaterra (Barcelona), Spare 7 c Dipartlmento di Fiswa, INFN Seztone dt Bari, 70126 Bari, Italy

d Inst, tute o f High-Energy Physws, Academia Smwa, Beljmg, People's Republic o f Chma 8 e European Laboratory for Parttcle Physics (CERN), 1211 Geneva 23, Switzerland

t Laboratoire de Physique Corpusculaire, Universitg Blaise Pascal, IN2P3-CNRS, Clermont-Ferrand, 63177 Aubidre, France g Nwls Bohr lnstttute, 2100 Copenhagen, Denmark 9

h Nuclear Research Center Demokrttos (NRCD), Athens, Greece i Laboratmre de Physique Nuclgatre et des Hautes Energws, Ecole Polytechnique, IN2P3-CNRS, 91128 Palatseau Cedex, France

ALEPH Collaboratton / Physws Letters B 332 (1994) 219-227 221

J Department of Physics, Umverslty of Edinburgh, Edmburgh EH9 3JZ, United Kingdom 10 k Dtpartimento dl Ftswa, Umversttgt dt Ftrenze, INFN Seztone di Ftrenze, 50125 Ftrenze, Italy

e Supercomputer Computations Research lnstttute, Flortda State Umversity, Tallahassee, FL 32306-4052, USA 13,14 m Laboratort Nazwnah delI'INFN (LNF-INFN), 00044 Frascatl, Italy

n Department of Physics and Astronomy, Umversay of Glasgow, Glasgow G12 8QQ, Umted Kmgdom 10 o Instttut fur Hochenergwphystk, Umversltg~t HeMelberg, 69120 HeMelberg, Germany 16

P Department of Physws, lmpertal College, London SW7 2BZ, Untted Kingdom 10 q Institutfiir Expertmentalphystk, Umversadt Innsbruck, 6020 lnnsbruck, Austria 18

r Department of Physws, Umverslty of Lancaster, Lancaster LA1 4YB, Untted Kmgdom 1o s Instttut fiir Phystk, Umversaat Mamz, 55099 Mainz, of Germany 16

t Centre de Phystque des Parttcules, Facultg des Sctences de Lummy, IN2P3-CNRS, 13288 Marsedle, France u Max-Planck-lnstttutftir Physlk, Werner-Hetsenberg-lnstitut, 80805 Miinchen, Germany 16

v Laboratotre de l'Acc~lgrateur Lmgatre, Umversttg de Parls-Sud, IN2P3-CNRS, 91405 Orsay Cedex, France w Dtpartlmento dt Flstca dell'Umverstt~, INFN Seztone dt Plsa, e Scuola Normale Supertore, 56010 Ptsa, Italy

× Department of Phystcs, Royal HoUoway & Bedford New College, Umverstty of London, Surrey TW20 OEX, Umted Kmgdom lo Y Parttcle Phystcs Dept, Rutherford Appleton Laboratory, Chdton, Dtdcot, Oxon 0 X l l OQX, Umted Kingdom 10

z Servtce de Physique des Parttcules, DAPNIA, CE-Saclay, 91191 Gtf-sur-Yvette Cedex, France 17 aa lnstttute for Parttcle Physws, Umverslty of Cahforma at Santa Cruz, Santa Cruz, CA 95064, USA 22

ab Department of Phystcs, Untverstty of Sheffield, Sheffield $3 7RH, Umted Kingdom lO ac Fachberewh Phystk, Umversttat Stegen, 57068 Swgen, Germany 16

ad Dtparttmento dt Fiswa, Untversltgz dt Trwste e INFN Seztone dt Trwste, 34127 Trwste, Italy ae Department of Phystcs, Umversay of Wzsconsm, Madtson, WI 53706, USA n

Recewed 26 April 1994 Editor' K Winter

Abstract

From a sample of about 75000 ~- decays identified with the ALEPH detector, K ° production in 1-prong hadronic decays is investigated by tagging the K~L component in a hadronic calorimeter. Results are given for the final states v r h - K ° and v~h-~r°K ° where the h - is separated into zr and K contributions by means of the d E / d x measurement in the central detector. The resulting branching ratios are: (Br --* v ~ - - K °) = (0 .88+ 0 .14+ 0.09)%, (B~"--+ v ~ K - K °) = ( 0 . 2 9 ± 0 1 2 ± 0 . 0 3 ) % , (B~- ---+ v~¢r-Tr°K °) = (0.33 ± 0.14 ± 0.07)% and (B~" ~ vrK-Tr°K °) = (0.05 ± 0.05 ± 0.01)%. The K* decay rate

m the K°cr channel agrees with that in the KTr ° mode: the combined value for the branching ratio is (B'f ~ v~K*- ) =

( 1 . 4 5 ± 0 . 1 3 ± 0 . 1 1 ) % .

1 Now at CERN, PPE Dwlslon, 1211 Geneva 23, Swttzedand 2 Permanent address University of Washington, Seattle, WA 98195, USA 3 Now at Harvard University, Cambridge, MA 02138, USA 4 Also Istituto di Flslca Generale, Unwerslt~t & Tonno, Tonno, Italy 5 Also Istttuto di Cosmo-Geofisica del C N R, Tonno, Italy 6 Now at DESY, Hamburg, Germany 7 Supported by CICYT, Spain. 8 Supported by the National Scmnce Foundation of China. 9 Supported by the Danish Natural Science Research Council 10 Supported by the UK Science and Engmeenng Research Council ii Supported by the US Department of Energy, contract DE-AC02- 76ER00881 12 On leave from Umvers~tat Autonoma de Barcelona, Barcelona, Spain. 13 Supported by the US Department of Energy, contract DE-FG05-

92ER40742 14 Supported by the US Department of Energy, contract DE-FC05- 85ER250000 is Present address' Lion Valley Vineyards, Cornelius, Oregon, USA 16 Supported by the Bundesmimstenum ftir Forschung und Tech- nologie, Germany 17 Supported by the DlrecUon des Sciences de la Mata~re, C E.A 18 Supported by Fonds zur FOrderung der wlssenschafthchen Forschung, Austria 19 Permanent address: Kangnung Nataonal University, Kangnung, Korea. 20 Now at Dipartlmento dl FlSlCa, Unlversith dl Mllano, Mdano, Italy 21 Also at CERN, PPE Dlvlslon, 1211 Geneva 23, Switzerland 22 Supported by the US Department of Energy, grant DE-FG03- 92ER40689. 23 Now at Umverstt~ th Pavia, Pavia, Italy

222 ALEPH Collaboration/Physics Letters B 332 (1994) 219-227

1. Introduct ion

To date measurements of K ° production in ~" decays have been essentially limited to the study of K*(892) in the ~" --~ u~K~sTr- channel, using the cr+Tr - decay of the K~s. More complete studies are worthwhile, not only to improve the general knowledge of~- decays, but also because K ° production can affect measurements of the main r-decay modes. In particular K~L'S have gone undectected in the past, and decays K~s --* 7r%r ° can be confused with other hadronic I" channels.

In this letter, the K ° content of one-prong hadronic r decays is extracted independently of the detection and identification o f the charged particle. Specifically, K ° 's are detected in the ALEPH detector by tagging their K°L component in a fine-grain hadronic calorimeter.

The complementarity between measurements of the charged particle, of the photons from 7r°'s and of the K°L'S allows the first measurement of several decay modes of the ~- lepton. This approach is also useful to determine the background in channels like ~- ~ v r K - and ~- ~ v~K-Tr ° [ 1] experimentally, and to improve our presently poor knowledge of the r decays involving strange particles.

The present results deal with K ° production in one- prong ~- decays. Branching ratios for the decay modes

r --* v r h - K ° (1)

r--, v ~ h - ~ ° ~ (2)

are measured by using d E / d x to identify statistically the charged hadron h - [ 1 ].

2. The A L E P H detector

A detailed description of the ALEPH detector can be found elsewhere [2]. Relevant information on the detection of charged particles in the time projection chamber (TPC), the reconstruction of photons in the

24 Now at Rutgers Umversity, Piscataway, NJ 08854, USA. 25 Now at Unwerslty of l~ttsburgh, Pittsburgh, PA 15260, USA 26 Partially supported by Colciencias, Colombia 27 Also at Isatuto di Matematlca e Fisica, Universlth dl Sassan, Sassari, Italy 28 Permanent address" Dept d'Estructura i Constituens de la Ma- teria, Umversltat de Barcelona, 08208 Barcelona, Spain 29 Now at SLAC, Stanford, CA 94309, USA 30 Now at Umversity of Chicago, Chicago, IL 60637, USA

electromagnetic calorimeter (ECAL) and trigger re- quirements, is given in the preceding letter [ 1 ].

The 1.2 m thick iron return yoke of the magnet is interleaved with 23 layers o f streamer tubes and acts as a hadronic calorimeter (HCAL) . Pad read-out is arranged in projective towers which provide an en- ergy measurement in 3.7 ° x 3.7 ° angular sectors, while a 2-dimensional digital pattern is obtained using the signals from strips running along the individual tubes which are parallel to the beam axis. The measurement of the hadronic energy has a Gaussian resolution o ' e / E

= 0.85/v/-E, with E in GeV. Including non-Gaussian tails, the effective resolution can be approximated by 0.90/x,/-E.

3. Event selection and calor imetr ic cuts

The present analysis is based on a data sample cor- responding to about one million hadronic Z decays collected by ALEPH in 1991 and 1992. The event selection is described in Ref. [ 1 ]: r+~ - - events are selected with high efficiency (78.1%) and low back- ground (1.6%). Decays with only one charged particle with a momentum larger than 2 GeV/c and identified as a hadron are selected. Next, 7r ° candidates are con- structed from photon pairs with a mass cut 31 between 90 and 190 MeV or from single photons with energy larger than 4 GeV. For this analysis, decays with at most one 7r ° are retained and classified in two samples, labelled 'h ' and 'h 7r ° ' . Because K°'s carry a large fraction of the ~- energy, the remaining total hadronic energy (as measured in the TPC and ECAL) must be less than 20 GeV (30 GeV) in the 'h ' ( ' h cr ° ' ) sam- pies, respectively. These selections yield a sample of 7948 one-prong r decays.

The K°L selection proceeds using only hadronic calorimetry. The characteristic K~L behaviour is a large energy deposition in HCAL, exceeding the expected amount from the charged hadron alone. Furthermore, there should be a displacement of the energy barycen- tre from the extrapolation point of the track in the HCAL. These two features are well-illustrated in Fig. 1, showing a typical event selected by the cuts de- scribed below. Note that only the tower read-out of

3L Compared to Ref. [ 1 ] the 7r ° mass cut is tighter because of more severe background conthtlons

ALEPH Collaboration/Physics Letters B 332 (1994) 219-227 223

ALEPH

Fig. 1. A typical r pair in the ALEPH detector (view transverse to the beams), where one of the ~"s decays into ~'K~L (upper part of the picture). The produced K~L is identified by an energy excess (27 GeV), measured on the tower readout and displayed as a histogram in the hadronic calorimeter (HCAL), which is offset in azimuth with respect to the impact of the 7r track (3 5 GeV/c) . The dlgatal pattern of the streamer tubes is a further check of the /~L shower

HCAL and its well-calibrated energy response are used. The detailed pattern of the hit tubes (projected into a plane normal to the beams) is not used in the se- lection because the shower topologies of the charged hadron and the K~L vary considerably depending on whether the respective showers begin in the ECAL, in the coil or in the H C A L However, the digital pat- terns can be very helpful in checking the selection procedure, and they have been used for verification in the visual scan of the selected events.

The displacement of the HCAL energy barycen- tre should occur mostly in the plane transverse to the beams, since the effect of track bending in the mag- netic field dominates over the typical opening angles

(a few degrees) in 7" decays at the Z peak. Therefore, two calorimetric variables sensitive to the presence of a K~L are defined as follows:

(/) The relative energy excess

EHCAL -- Ph 8E -- (3)

o" h

where EHCAL is the sum of the energy in HCAL clus- ters within a 30 ° cone around the charged hadron of momentum Ph and trh is the expected fluctuation of the charged hadron energy measured in HCAL (taken as 0.9 v /Ph(GeV/c ) ), and

(ii) The transverse angular offset

t~t~ = ¢ It~barycentre -- t~traclompaetl (4)

224 ALEPH Collaboratton / Physics Letters B 332 (1994) 219-227

ALEPH

"~ ~ * (a) signal

0 L ' - - : , i i , ~ e e t A . ' , ' , " , ,

-Io~1 " '~ '~H~ ' :4 , ~ - , z , , , p , , , , , , ", ,

0 5 10 15 20 25 30 5E

~_ (b) backgrounds

_ 2 0 ~ - I ~ ~ , I , ~ , , I . . . . I , , ~ j I . . . . I , ~ ~ ,

0 5 10 15 20 25 30

"~¢10~- A (c)Data "o •

. . . . . . : " . . .

-20~'1 , , , ~ , , ~ , .1 . . . . . ~ . . . . I ,* . . . . . , 0 5 10 15 20 25 30

5~ Fzg 2 Calonmetr ic cuts for the h K ° sample a) Monte Carlo s~mulatzon for mgnal (Tr K °, KK °) b) Monte Carlo simulation for backgrounds (~r ,p , az ,zr K ° /~o) c) data The Monte Carlo

simulatmn zs shown here wi th a statistzcs about 6 nines larger than

the data

where ~: = + 1 if the shift in ~b occurs in the direction of the track bending and ~: = - 1 in the opposite case.

Thus, the presence of a/~L should be signalled by a (generally large) positive value for &e and a (large) negative value for &~b.

Using a Monte Carlo simulation of ~- production and decay [3] ,the final cuts in the (&e, 8~b) plane are chosen to minimize the non-K°L backgrounds while keeping a reasonable efficiency. Three conditions must be simultaneously satisfied (see Fig. 2):

~ e > 1

~b < 1 ° (5)

-7&e + 88~b + 64 < 0

where 8~b is expressed in degrees. In addition, to avoid small fluctuations of the charged hadron shower en- ergy, an absolute cut is placed on the residual HCAL energy

EHeAL -- Ph > 4 GeV (6)

maintaining high efficiency, since the expected K°L energy spectrum vanishes below an energy of about 4 GeV.

Additional cuts are applied to remove further back- grounds:

(i) Final states with a charged hadron and ~-°'s in which part of the ~.0 energy leaks into HCAL (through the cracks between ECAL modules) can simulate a fake K°L signal. A cut on the q~ angle of the 'neu- tral' HCAL cluster (when different from the charged- particle associated cluster) is applied around the mod- ule edges of ECAL.

(ii) A small contribution from 3-prong events with only one (good) reconstructed track is also present. To reduce it to a negligible level, events with badly reconstructed tracks missing the interaction region are rejected.

4. R e s u l t s

The effect of the ( (~e, 8~b) calorimetric cuts is shown in Fig. 2 (3) for the hK~L (hTr ° K~L) samples. A clear tail, originating from K°L'S, is observed both in the data and the Monte Carlo. The shapes of the Be, 8~b and residual HCAL energy distributions have been checked and agree with the Monte Carlo predictions within statistics.

Whereas the analysis depends on the Monte Carlo to estimate the background from 7r ° energy leakage into HCAL, this is not the case for channels with only a single charged hadron. In the latter case, the 8~b dis- tribution should be essentially symmetric since shower fluctuations can occur on either side of the track im- pact. Thus, it is possible to check the Monte Carlo estimate of this background by counting events in a ( ~e, 8~b) region symmetric to the chosen cuts (5) with respect to 8~b. The number of background events is 2 (2) in the hK°L (hTr ° K(~L) samples, in fair agreement with the Monte Carlo expectation of 3.9 + 1.0 (4.3 -4- 1.0), respectively.

The charged hadron in each ~- decay is statistically identified as a pion or a kaon using the dE/dx mea- surement in the TPC, following the method described in Ref. [ 1 ]. The distribution of the kaon estimator PK [ 1 ] should show an excess above the distribution for pure pions near PK = 1. Fig. 4 shows clear evi- dence for the K - K ° channel in the hK°L sample, while

ALEPH Collaboration/Phystcs Letters B 332 (1994) 219-227 225

ALEPH

~ 1 0 1 * (a) signal

• , -

-~o E-I • ?" ~ . " • " • F I , , ; Y , ' , , " , . . . . , . . . . , , , , , ,

0 5 10 15 20 25 30 6c

~'~10I tt,~, * . • (b)backgrounds

0 5 10 15 20 25 6E

30

~ = (c) Data lo •

o ,,=t

-10 ** * ,

-20 , , I , , , , I , , , , I . . . . I . . . .

0 5 I 0 15 20 25 30 6E

Fzg 3 Calorimetric cuts h~'°K ° sample a) Monte Carlo simulation for szgnal (~.~o K o, KTr 0 K o) b) Monte Carlo simulatzon for backgrounds (p, al ) c) data The Monte Carlo simulation is shown here wzth a statzstzcs about 6 tzmes larger than the data

the hTr ° ~ sample contains only the hint of a signal. In both cases, since the number of events is quite low, the K fractions are obtained through an unbinned like- lihood fit, using the experimentally determined proba- bilities for pions and kaons for each particle [ 1 ]. From a hK~L sample of 74 events, the d E / d x fit yields a kaon content of 8.4 -4- 3.3, while the kaon rate is 1.4 4- 1.3 among the 17 hTr °/~L events.

The efficiencies and purities relevant for all ~- decay channels are obtained from the Monte Carlo [3], sup- plemented by a separate sample of the ~" ~ v ~ K - K °

mode which is not included in the present version of KORALZ 32. Almost all backgrounds correspond to decay modes with relatively well-known branching fractions, except for the ~ - K ° K ° mode. This chan- nel accounts for 50% of the background in the 7r- K ° channel using an estimate based on measurements of the related channel 7 r -K+ K - [4,5] which have large

~1o ,,>,

c Q) >

I J J

~ (o) hK*

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

P.

10 ~ (b) hTxOK *

. . . . . . : . . . . . . i .....

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 P,

Fig 4. Kaon estamator from dE/dx measurement m the TPC data thstnbutlon and Monte Carlo histograms (a) hK ° sample (b) h~'°K ° sample The shaded histogram is the Monte Carlo expectaUon for the plons, while the full histogram corresponds to the sum of the 7r and K contributions

uncertainties. An uncertainty of 4- 50% is assigned to this contribution in the systematic error.

The efficiencies are expressed ( Table 1) in terms of produced K °'s even though the method is sensitive mostly to the K°L component.The detected sample ac- tually corresponds to approximately 87% K~L and 13% /~s, and therefore the total K ° efficiency is quoted for clarity. The systematic uncertainties are dominated by the calorimetric cuts. Since the measured distributions do not show any significant deviations from the Monte Carlo expectations, it is possible to place limits on sys- tematic effects in the description of showers in HCAL (energy calibration, shower fluctuations). Varying the values for the cuts in the simulation within limits com- patible with the agreement between data and Monte Carlo would change the efficiencies by as much as -4- 7%, a value limited by the statistics of the data sample.

The final branching fractions, scaled to the full K ° rate, are determined:

32 the simulation of the r ~ vrK- K ° channel assumes a spin-one state for the hadronic system, as expected from the dominance of first-class currents

B ( r ~ v~-cr- K °) = (0.88 4- 0.14 4- 0.09)%

B(7- --~ ~,~-K- K °) = (0.29 4- 0.12 4- 0.03)%

226 ALEPH Collaboration/Physics Letters B 332 (1994) 219-227

Table 1 Results obtained for the branching ratios of one-prong ~- decays with K ° producUon, with statistmal and systematm uncertmntms, respectively The efficiencies are expressed relative to the produced/t°'s, even though essentially the /~L component is detected.

channel ~'K ° KK 0 ~'¢r ° K ° KTr°K °

decays 65 6 4- 8.3 8 4 4- 3 4 15 6 4- 4.0 1 4 4- 1.3 efficiency (%) 6 3 3.0 2.9 2 9 background 12.8 0.1 6.3 0 0 B(%) 0.884-0.144-0.09 0 294-0 124-0 03 0 334-0 144-0.07 0 054-0.054-0 01

B(~- --* v .~r-¢r°K °) = (0.33 4- 0.14 4- 0 .07)% > 18 z .

B(a----+v~.K-~°K °) = ( 0 . 0 5 4 - 0 . 0 5 4 - 0 . 0 1 ) % (7) o C ' )

Branching fractions for the decays ~- ~ v , .K-K °, ~ 1 6 v~qr-*r° K ° and vrK-¢r° K ° are measured here for the first time. The measurement of the K - K ° mode can ~> 14

ID

be compared to an experimental upper limit of 0.26% 12 [8] .

5. ~" decay into z, .K*(892)-

The 7rK ° final state is dominated by K* production as expected. The 7rK ° invariant mass distribution, plot- ted in Fig. 5, agrees with the Monte Carlo simulation and the fitted mass of 898 4- 23 MeV agrees with the standard value [6] . Scaled to the full K* decay modes using isospin invariance, the rate o f the ~- decay into K* is

B ( r ---+ v r K * ( 8 9 2 ) - ) = (1 .32- t -0 .21 4 -0 .13)%

(K°zr mode) (8)

This result, obtained with the K°~r mode, can be com- bined with the independent measurement using the KTr ° mode from in the preceding letter [ 1 ]

B ( r --. v~K* ( 8 9 2 ) - ) = ( 1.58 + 0.16 i 0 .20)%

(Kqr ° mode) (9)

to give

B(o- --o v ~ K * ( 8 9 2 ) - ) = (1.45 4- 0.13 4- 0 .11)%

(combined) ( 10 )

where the uncorrelated systematic uncertainties have been added in quadrature. The combined measurement is consistent with the previous world average [6] , (1.42 4- 0 .18)%.

ALEPH

K"

-

10

8

6

4

2

0 0 " ~ ' " ~ ' " __ 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 K°'~ m a s s (GeV/c ' )

Fig 5. Invanant mass dlstnbuUon for the K°~r system, data and Monte Carlo expectauons from K* and non-K°rr backgrounds.

6. Discussion

It is interesting to check the general consistency of the results by comparing the Cabibbo-suppressed channels to the allowed ones. Using ALEPH re- suits [ 1,7], the ratio

B ( 7 --. vrK* ) = 0 . 0 5 8 4 - 0 . 0 0 7 (11) B ( r ~ v~.p-)

is consistent with the similar ratio

B ( 7 ---, vTK- ) = 0.056 + 0.006 (12) B ( r ~ v r ~ ' - )

which only depends on the Cabibbo angle 0c, the ratio of the f.,K decay constants and known kinematical

ALEPH Collaboratwn / Physics Letters B 332 (1994) 219-227 227

factors [9] . Inserting the value for 0c [6] , the results (10) and ( 11 ) measure the ratio of the ~r and K decay constants and a similar ratio for the p and K* channels. These ratios are remarkably consistent:

f ~ = t a n O c , / B ( r - ~ v r K _ ) f x V B ( ~ - \ m y m ~ J

= 0.89 + 0.05

_ _ B (r ~ ~rP- ) /e m~. 2 m~:. f P = tan Oc 2 _ 2

f r* B ( r ~ v r K * - ) ~ m r m v

/ m y + 2m 2.

= 0.90 4- 0.06 (13)

The branching fraction for the decay ~- ~ v r K - K ° can be compared to theoretical estimates. In princi- ple, this decay rate could be extracted from e+e - K + K - , K~LK~s data by isolating the isovector compo- nent, but this is not possible with presently available data. Using the isovector channel e+e - ~ 7r+~ - - scaled by a kinematic factor to take into account the • r - K mass difference, estimates of (0.11 4- 0 .03)% [ 10] (0.16 4- 0.02) % [ 11 ] are obtained. These values are smaller than the present measurement, but they do not take into account SU(3) -break ing , which could result into a different resonance behaviour in the ~rTr and K~" channels.

7. Summary

Using a calorimetric analysis, K ° production in one- prong ~- decays is investigated using the K0L component. Branching ratios are obtained for four decay modes:

B ( r ---, v r ~ - - K 0) = (0.88 -4- 0.14 + 0 .09)%

B ( T --, v r K - K °) = (0.29 -4- 0.12 + 0 .03)%

B(~- --* vrTr-~r°K °) = ( 0 . 3 3 ± 0 . 1 4 + 0 . 0 7 ) %

B(7" --, v rK-qr°K °) = (0 .054 -0 .054 -0 .01 )% (14)

B ( r ~ vrK* ( 8 9 2 ) - ) = (1.45 + 0.13 4- 0 .11)%

(15)

The results presented in this letter and the preceding one offer a consistent and complete set o f measure- ments of kaon production in one-prong hadronic ~- de- cays.

Acknowledgments

We wish to thank our colleagues from the acceler- ator divisions for the successful operation o f LEP. We are indebted to the engineers and technicians in all our institutions for their contribution to the excellent per- formance of ALEPH. Those of us from non-member countries thank CERN for its hospitality.

References

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[4] GB. Mills et al, DELCO Coll., Phys Rev. Lett 54 (1985) 624

[5] DA. Bauer et al, TPC Coil, LEL preprint LBL-33037, (July 1993)

[6] Review of Partacle Propemes, Phys. Rev. D 45 (1992). [7] S. Snow, M Davier, Proceedings of the 2nd International

Workshop on ~" Lepton Physics (Columbus, 1992), K.K. Gan ed., World Scientific (1993)

[8] H Atharaet al, TPC Coll., Phys. Rev Lett 59 (1987) 751 [9] Y. Tsm, Phys. Rev. D 4 (1971) 2821

[10] S I. Eidelman, V.N Ivanchenko, Proceeding of the 1st International Workshop on ~" Lepton Physics (Orsay, 1990), M. Davier and B Jean-Mane, eds, Ediuons Fronti~res (1991)

[11] S Narison and A Plch, CERN prepnnt, CERN-TH 6769/9 (1993)

the latter three for the first time. The K* branching ratio is derived and combined with the new ALEPH measurement in the K~ -° mode [ 1 ] to give