ELSEVIER
UCLEAF PHYSICS
Nuclear Physics B (Proc. Suppl.) 115 (2003) 145-149
PROCEEDINGS SUPPLEMENTS
www.elsevier.com/locate/npe
K L O E Results on Kaon Decay,' s arid Phi Radiative Decays
P. B r a n c h i n i , S e z i o n e I N F N di R.oma 3 U n i v e r s i t h " l b m l a 3", Via Della Vasca N a v a l e 84, Rorna, I t a l y on b e h a l f o f T h e K L O E C o l l a b o r a t i o n 1
T h e K L O E experiment has been collecting d a t a since April 1999 at the DAONE e * - e - collider at a center of mass energy equal to the C-meson mass. A total amount of 5 x 10 ~ ¢ decays have been analyzed. Re- suits c o n c e r n i n g t h e measurement of the semileptonie branching ratio of the K s , the measurement of the ratio ! B R ( K s --+ r r + l r - ) / B R ( K s --+ 7r°rr °) and the s t u d y of the radiat, ive decays of the ¢ involving r/, 7? , f0(980) and a 0 ( 9 8 0 ) are presented.
1. I n t r o d u c t i o n
T h e K L O E [1] d e t e c t o r a t I)A@NI". [2], the P r a s e a t i ~ f a c t o r y , s t a r t e d d a t a t a k i n g in april 1999. T h e DAq~NE e + - e - eollider operau.'s at I.he c e n t e r o f m a s s o f t h e ¢ ( 1 0 2 0 ) m e s o n p r o d u c i n g a l m o s t m o n o c h r o m a t i c K ° a n d K °, K ~ a n d K p a i r s a n d all o t h e r ¢ d e c a y p r o d u c t s . M o r e o v e r a t DArI, N E k a o n s a r e p r o d u c e d with a ~-, 110 - 125 M e V / c m o m e n t u m a n d t h e i r d e c a y h m g t h s are As ~ 0.6 em, A L ~ "~40 cm a n d A:!:: ~ 90 cm. T h e u n i q u e f e a t u r e o f a ~ t ; a c t o r y is th<, tagging: l.he d e t e c t i o n o f a l o n g liw.,d n e u t r a l kaon g u a r a n t e e s t h e p r e s e n c e o f a K!~ of given m o m e n t u m a n d di- r e c t i o n a n d v i e e v e r s a , t h e s a m e h o l d s for c h a r g e d kaons.
T h e a n a l y s e s are b a s e d on 17 p b -1 of i n t e g r a t e d l u m i n o s i t y , c o r r e s p o n d i n g t o "~ 5 x 10 r cb. At.
p r e s e n t D A e p N E d e l i v e r s _~ 2.5 p b - 1 / d a y a n d its p e r f o r m a n c e s a r e c o n s t a n t l y i r n p r o v i n g .
1A. Aloisio, F. Ambrosino, A. Antonelli, M. Ant,onelli, C. Bacci, G. Beneivenni, S. Bertolucci, C. Bini, C. Bloisc, V. Bocci, F. Bossi, P. Branchini, S. A. Bulychjov, I%. Caloi, P. Carnpana, G. Capon, G. Carboni, M. Casarsa, V. Casavola, G. Gataldi, F. Ceradini, F. Cervelli, F. Cevenini, G. Chiefari, P. Giambrone, S. Coneu,i, E. De Lucia, G. De Robertis, P. De Simone, (3. I)e Zorzi, S. Dell'Agnello, A. Denig, A. Di Domenico, C. 1)i l)onato, S. Di Falco, A. Doria, M. Dreucei, O. Erriqucv., A. Far- illa, G. Felici, A. Ferrari, M. L Ferret, G. F'inoechiaro, C. Forti, A. FYanceasehi, P. l~anzini, C. Gatti, P. Cauzzi, S. Giovannella, E. Gorini, F. Grancagnolo, E. G'raziz~ni, S. W. Han, M. Incagli, L. lngr(~so, W. Kluge, C. Kuo, V. Kulikov, F. Lac, awa, G. l,anfranchi, .l. I,ee-l~anzini, D. Leone, F. Ira, M. Martemianov, M. Mal;syuk, W. Mei, L. Merola, R. Messi, S. Miscetti, M. Moulson, S. Miiller, F. Murtas, M. Napolil.ano, A. Nedosckin, F. Nguycn.
M. Palul,an, [,. Paoluzi, l';. Pasqualucci, L. I}assalacqua.
2. T h e K L O E d e t e c t o r
T h e I ( I , ( ) E d e t e c t o r [1,3] c o n s i s t s of a l a r g e c y l i n d r i c a l d r i f t c h a r n b e r s u r r o u n d e d b y a her- t a c t i c ch;cl:rornttgnel,ic c a l o r i m e t e r . A s u p e r c o n - d u c t i n g coil a n d an iron y o k e s u r r o u n d i n g t h e whole d e t e c t o r p r o v i d e a 0.52 T m a g n e t i c field.
T h e drift c h a m b e r [4] ( I ) C ) w i t h 4 m d i a m e t e r a n d 3.3 m lengt.h, h a s a t o t a l n u m b e r o f 52140 wires, a r r a n g e d in 12582 cells d i s t r i b u t e d over 58 c o n c e n t r i c l a y e r s arm with an a l l - s t e r e o g e o m e - try. In o r d e r to r n a x i m i z e t r a n s p a r e n c y to p h o - tons a n d r e d u c e 1(~ r e g e n e r a t i o n the o p e r a t i n g gas m i x t u r e is 90 ~ h e l i u m - 10 % i s o b u t a n e . T h e achieved posit;ion r e s o l u t i o n is a r 6 ~ 150/zm a n d crz ~ 2 a m while v e r t i c e s a r e r e c o n s t r u c t e d with a r e s o l u t i o n a,, ~ 3rnm. T h e m o m e n t u m r e s o l u t i o n is c,(p L ) / p t "~ 0 . 4 % .
T h e e l e c t r o m a g n e t i c c a l o r i m e t e r [3,5] ( E m C ) is a l e a d - s c i n t i l l a t i n g fibers s a m p l i n g c a l o r i m e t e r m a d e of 88 m o d u l e s , d i v i d e d into a b a r r e l see- Lion a n d two C - s h a p e d e n d - c a p s , e n s u r i n g 98%
c o v e r a g e of the solid angle. T h e r e a d - o u t o f I.he rn()dules is pe.rforrned on b o t h e n d s w i t h
4 .,1 x 4.4crrt 2 g r a n u l a r i t y tbr at t o t a l of 4880 p h o - I.cmmltipliers. T h e c a l o r i m e t e r h a s to d e t e c t w i t h very high elficicncy p h o t o n s with e n e r g i e s down I,o 20 MeV a n d t,o a c c u r a t e l y m e a s u r e t h e i r en- ergy a n d t i m e of flight. T h e a c h i e v e d e n e r g y res-
A. P;~,;scri, V. Patera, E. Petrolo, I,. Pontecorvo, M. Pri- mavera, F'. Ruggieri, P. Santangelo, E. Santovetti, G. Sara- cmo, It. D. Scharnberger, B. Sciascia, A. Sciubba, F. Seuri, 1. Sfiligoi, T. Sl'mdaro, E. Spiriti, G. L. Tong, L. Tor- tora. F:. \;akmt.( h P. Valente, B. Valeriani, G. Venanzoni, S. \'¢:nczian(), A. Ventura, G. Xu, G. W. Yu.
0920-5632/03/$ - see front matter © 2003 Elsevier Science B.V. All rights reserved.
doi: 10.1016/S0920-5632(02)01975-8
146 P. Branchini/Nuclear Physics B (Proc. Suppl.) 115 (2003) 145-149
olution is 5 . 7 % / ~ ( G e V ) , The time resolution is at=(54/vZ-E~(OeV) ® 50) ps. At the present luminosity the K I , O E trigger system [6] works at d a t a acquisition rate of ~ 2.5 kllz, resulting in a t h r o u g h p u t to the 1)AQ system [7] of ,-~ 3 M b y t e s / s .
3. K ° d e c a y s
Neutra] kaons from ¢ meson decays are pro- duced in a p u r e q u a n t u m s t a t e (,1 I'c = l ) therefore the final kaon pair is always k'o /,to and ' ~ S - ' ' L the observation of a K~ tags t,he presence of the K°s in the opposite hemisphere. \Vc use the Lime of flight identification of K °, interacting in the E m C ( " E M C K°signal ''), characterized hy a neu- tral cluster with rather slow velocity (fl ~ (L22) in the event, as K ° tagging strategy. About one half of the K ° 's reach the calorimeter belbre de- caying, thus the " E m C K¢{-signal '' tag provides a particularly clean and a b u n d a n t K ° mesons sam- ple.
3.1. K° s ~ ~r+rr-(7) a n d K~ --+ rr°rr ° d e c a y s T h e ratio of l:he partial decay widl.hs inl.o two charged and into two neutral pions represenl.s the first step towards the rneasurement of the. C P vio- lation p a r a m e t e r in the neutral kaon sysu'.rn. The K ° decaying into two neutral pions events are se- lected by requiring the presence of at least three neutral p r o m p t clusters in the E m C with e.nergy larger t h a n 20 MeV. A p r o m p t photon must sat- isfy ] t C L - le'c .l/cl _< 5 x at, with tct, being the cluster time, IFCL] its position and o~. the en- ergy dependent time resolution. The final selec- tion efficiency for the K ° -~ ~%r ° decay channel is eo0=(90.1-4-0.2)% and is dominat.ed by accep- tance. T h e selection of K ° ~ rr~r, s ( T ) events [8] requires two tracks with opposite charge, cwig- inating in a cylinder of 4 cm radius and 10 crn length around the interaction point, with po- lar angle in the interval 30 ° < 0 < 150 ° and reaching the E m C . A further cut is applied on the measured m o m e n t a to remove the residu'd background due to charged kaon tracks: 12() <
p ( M e V / c ) < 300. No requirements have been done on photons therefore this is a lully inclusive me,~surement,. T h e track reconstructJon e.giciency
as a flmction of the m o m e n t u m and polar angle is rneasured from d a t a samples. T h e final selec- tion efficiency is e. +__=(57.6-1-0.2)%, again domi- nat,ed by acceptance. T h e trigger efficiency and the probability for having at least one good clus- ter Lo determine the to of the event are evalu- ated lbr both decay types using data. We obtain (!)9.69 4- 0.04)% for the neutral decay and (97.9 4- 0.03)% for the charged one. Background levels are kept well below 1% for both decay types.
The final result is [9]:
• 0
l (K., - , ('r))
I.,(K~ ~ rr%O ) = 2.236 4- 0.003,~tat 4- 0.015~y~
This value, has t,o be, c o m p a r e d with the present I)I)G (particle d a t a group) vahm of2.197 + 0.026.
It is worl.hwhilc t,o noLe that the experiments con- tribul,ing to the PI)G vahm do not, have a clear indical,ion of the cut on the photon energy and l:he. corresponding efficiency, while we present a flflly inclusive rneasurement of K ° --* rr+Tr - (7).
3.2. K ° -~ ~:~:e:~u(u) d e c a y s
If C P T invariance and /kS = A Q hold, the S t a n d a r d Model (:an predict BR(K ° rr!e~p(u)) using the K° s and K~]. lifetimes and
-,
°,
Therefore a precise measu,rement of this decay is a U.'st of l, he A S = A Q rule. T h e K{)~ + rrJ:e~:~,(u) decay candidates are selected asking for events with a "EmC K°signal '', and two tracks of op- posite charge with associated g m C clusters and forming a vertex in the previously defined cylin- der around the interaction point. In order to re- jeer t:he background from K ° ~ 7r+rr - decays, fi~rther cuts arc'. applied on the two tracks invari- ant mass, evaluated in the pion mass hypothe- sis, and on I,he resulting K ° m o m e n t u m in the ¢ resl, fi'ame. Electrons and pkms discrimination is performed with time of flight measurement.
The time of flight identification efficiency is esti- m a t e d using the high-purity d a t a sample of KLS°' de.caying into rreu before the DC internal wall.
The vertex reconstruction efficiency and fiducial cuts are estimated by Monte Carlo. The track reconstructiorl elfieiency is evaluated using both Monte Carlo arm d a t a from K ° ~ rr+rr--sample.
The elficiency for the signal selection is (20.8 +
P Branchini/Nuclear Physics B (Proc. SuppL) 115 (2003) 145-149 147
300
200
100
Eve~ta/1 MeV
• Data 1 7 p b -~
- D M C fit
6~0 events
i t
-30
-20 -100 10
v
/ %
MeV
I I
20 30 40
- O-+(fo+o)%,
"'.. ..,..,-"
~_~ . . . '.':':-~-:-:'"': ...
3 O 0 4 0 O 5 O 0 6(~) 7 0 0 8 O O 9 0 0 I0(1~
M = (MeV)
F i g u r e 1. Distribution of the difference bet.ween missing e n e r g y a n d missing m o m e n t u m for K ° --+
rr±eTO(u) candidate, s. T h e peak at zero is the signal. T h e fit to t h e d i s t r i b u t i o n uses the Monte Carlo for signal a n d b a c k g r o u n d in the r a n g e -40 MeV + 40 MeV.
l:'i~ure 2. d H R / d M as a timer.ion of M ~ from (,~ -~ JiF,e and f0 ~ rr%r°. Points are data. T h e solid line is the fit result, a,~suming c o n t r i b u t i o n s frorn fo, rr a.nd the interference between them.
Individual cont,ributions are also shown.
0.4) %. K ° --+ rr:Leq:O(u) events h a w ~. t,o sal.isfy E,~,i.,, (missing energy)- p,,,i.,,, (missing m o m e n - t u m ) = 0. Figure 1 shows the distribution of E m i s s -- Pmiss. A clean peak of the signal is ev- ident a r o u n d zero. T h e fit to the d i s t r i b m i , m is done using M o n t e Carlo s p e c t r a for b o t h signal a n d b a c k g r o u n d .
T h e m e a s u r e d yield is N ( K ~ ~ rr :! e ~ P(p)) = (;27 4- 30 events. T h e total n u m b e r of ewmt,s is t,hen divided by t h e r m m b e r of observed K ° --* 7r' rr events, to give [10]: BI?.(K~! --* rr:~'e'P(,,)) =
(6.92
4- 0.34 ,t=t 4- 0.15.,v,L ) x 10 - 4. Th is result is in a g r e e m e n t with the S t a n d a r d Model e x p e c t a t i o n of B R ( K ° --~ rr:~e:F#(u)) = (6.70 4- 0 . 0 7 ) x 1 0 4, o b t a i n e d using t h e K ° and K° L lifetimes and F ( K ° ~ ~r:%::P(u))=r(K°~ -* r r : i : e r ' 4 " ) ) which follows from C P T invarianee and A S = A Q .4. R a d i a t i v e ¢ d e c a y : ¢ -~ fo~' -+ u o ~ % a n d ¢ --+ ao7 -~ ,qwoy.
Various models exist for the description of the n a t u r e of t h e f0 a n d a0 mesons ( o r d i n a r y qq me-
son, qqqq state, KK molecule). T h e s e models make different predict:ions [11,12] for B R ( ¢ --*
f0"7), I~;1{(¢ -~ ao3') and their mass spectra, there- t~m; a precise m e a s u r e m e n t of these quantities would clarify the interl)retation on the n a t u r e of these mesons. T h e fo meson has been stud- led in the following decay chain: ¢ ~ f 0 7 with ./}~ -~ 7r% °. Ccmcerrfing t.he ao we have inves- t.igaled the channel: cb --* a07 with a0 --* rpr ° and lhc following decays of the rl: r I ~ 7 7 and r/--, ~ ' ~ no. These final s t a t e s are c h a r a c t e r i z e d by 5 p r o m p t 7's and this is the f r s t observation of events with ao ~ 'q~r ° and 'q --* rr+~ -rr °. T h e
!ffl~lill b a c k g r o u n d processes a r e : e ~ e ~ cart 0 with cv --~ rr°'y or a; --* WT, and the r e s o n a n t pro- tess ~,') -~ p°rr° with po ___+ rro.r or pO __~ r]7. On l,he ~thcr h a n d the a0~ final state with ~r ~ r r - 5 7 has a unique s i g n a t u r e with negligible physical back ground.
T h e events are identified by requiring five p r o m p t p h o t o n s within loose a c c e p t a n c e cuts. T h e n a first kinematic fit, is p e r f o r m e d requiring total en- ergy and m o m e n t u m conservation with the ad- ditional c o n s t r a i n t fl = I for all photons. T h e
148 P. Branchini/Nuclear Physics B (Proc. Suppl.) 115 (2003) 145-149
best p h o t o n pairing is found by m a t c h i n g the
"7"7 invariant masses to the i n t e r m e d i a t e p a r t M c s masses, either 27r ° or rlrc °, and then a second kine- m a t i c fit is p e r f o r m e d c o n s t r a i n i n g the masses.
B a c k g r o u n d events are rejected by m e a n s of cuts on k i n e m a t i c variables and on tile X 2 probability values o b t a i n e d in the k i n e m a t i c fits.
T h e overall detection efficiency is o b t a i n e d by t u n i n g t h e MC to r e p r o d u c e the obse, rved M~o,,, a n d Mn~o invariant mass distributions in case of
¢ ---+ rr°Tr°"7 and q5 ~ rite°"7 events, respectively.
Final result on the b r a n c h i n g ratio of
¢ --* rr°rr°'7 is: BI~. (oh --* 7r%°v) = (1.08 -{- O.03stat :k O.05syst) X l O 4 For I,he b r a n c h i n g ratio of (9 ~ rpr°7 we measure from the 'q ~ y'7 sample: I317. (0 ~ 'qvr'°7) = (8.5 "4- 0.5star q- 0.6syst) × 10 a and from the r I --* rr+w-rr ° sample: [3]~ (4--* 'q~°-e) = (8.0 :t: 0.6star 47 0.4sy,,t) X 10 - s .
In order e s t i m a t e the c o n t r i b u t i o n Lo 13I/.(6 -~
w°rr°'7) and BR.(¢ --~ rite°7) frorn ¢ ~ j}~w a.nd
¢ -4 ao~' respectively, we have p e r f o r m e d a fit t.o t h e invariant mass s p e c t r a M~o~o and A4,1~,,.
In t h e ¢ ~ rr°rr°y m o d e the process 6 --~ £'%
with a scalar meson S (fo or a) decayirlg i n l - rr°rr °, and t h e process q5 --~ p°rr°, with pO ~ ~o,y, c o n t r i b u t e to the M~%o spectrum[11]. To ill, lhe, dipion s p e c t r u m , t o g e t h e r with the fo we haw', lo include also the c o n t r i b u t i o n of the a[13] and a s t r o n g destructive interference between these two t e r m s (fig. 2). T h e c o n t r i b u t i o n from the d) --~
p°rr° channel comes o u t to be negligible.
I n t e g r a t i n g t h e f0 component, of the spec- trum[1 1] we obtain: t~I-~ ( ¢ ~ f0'7 ~ w°rr°7) = ( 1 . 4 9 : t : 0 . 0 7 ) x 1 0 4.
C o n c e r n i n g the ¢ ---+ r/rr°'7 case, we p e r f o r m e d a s i m u l t a n e o u s ill; of the two Mw, o spectra (fig. 3), from the r/--+ "77 a n d r/--* x v r r - 7r, ° samples. T h e ao mass p a r a m e t e r has been set to the P I)G value of 984.8 MeV.
/,From t h e fit we o b t a i n [12]:
B R (¢ ao'7, w °) = (7.4 + 0.7) x io 5. R a d i a t i v e ¢ d e c a y : ¢ -* r! t "7 a n d ~6 -* rlT.
Precise m e a s u r e m e n t s of the b r a n c h i n g ratios o f ¢ --4 r/'7 and ¢ --4 rl" 7 are of parl:icular inter- est in p r o b i n g the [s,~> and g h u m i u m content of
M ~ (MeV)
I
(b) ~
..Mn~ (MeV)
:vl ~ (Me'C)
Figure 3. T h e b a c k g r o u n d s u b t r a c t e d Mmr spec- tra resulting from 4) ~ ao"7 a n d ao --~ r/rr. Points in ploL (a) and (b) are data, (a) for the case where r! --* 2 % (b) lbr case where r I --+ rr + rr-rr °. His- l o g r a m s arc results of the fit, and (c) is the theo- retical c l l r v c for l,]'l(,' (I 0 .
the r/. Moreover t.he ratio of these two branch- ing ratios is rela.Led to Lhe "q - r/ mixing p a r a m e - i ters predictable in the chiral p e r t u r b a t i o n t h e o r y frame. T h e decay chain for the r/ selection is:
¢ - - * ' q "y w i t h r I ~ rr ~w r l a n d t h e n ' q ~ 7"y [14].
T h e r I is selected t h r o u g h the chain ¢ -+ r/"7 with rl -~ Tr+rc ;o. T h e final state (TroT r-"7"7"7) is the same for the two selections theretbre several sys- tematic errors cancel in the m e a s u r e m e n t of the ralio BR.(d + r / " y ) / B R ( ¢ i ~ rl"7). T h e selection R~r both channels requires: the presence of three p r o m p t p h o t o n s with e n e r g y larger than 7 MeV and two trac.ks of opposite charge with a vertex near l.he interaction region. T h e n a preliminary kinematic fit is p e r f o r m e d i m p o s i n g total energy arm r n o m e n t u m conservation, the c o n s t r a i n t fl = 1 for all photons, without any invariant mass con- sl.raint on i n t e r m e d i a t e particles. Simple kine- ma.tic cuts eliminate, the b a c k g r o u n d m a i n l y due lo 6b -~ ~-i~ ,,¢o events with an e x t r a p h o t o n and to ~,5 -~ K r . l ( s ~ rt ~ rr-Tr°rc ° events with
o n ( !
t)hl)ton
]~)sl.. T h e rmrnher o f signal events isP Branchini/Nuclear Physics B (Proc. Suppl.) 115 (2003) 145-149
149
Nev = 120 -F 12~t~t -t- 5s~.t. Normalizing the sig- nal events to the number of observe, d (b ~ 'rF~
events and correcting for detection el[iciency esti- mated from MC, we measure the ratio of the t, wo branching ratios
[14]:
/
B R
[¢ "-~ 'r]'7) = (4.70 + 0.47.,,t=t :t: 0 31.,,>s,) x 10 :'
\
B R (¢ ---,
~13')
LFYom this rat:io we obtain the following value tbr the pseudoscalar mixing angle ~p in the flavour
basis: ~p (41.8 +t9)o
1 . 6 'Moreover making use of the BR,(c~ ~ ,try ) value, from PDG, we obtain [14]: BP.(6 ~ "r! = (6.8:1: 0.6star 4" 0.bsw~t) X 10 5 . This is the. mosl precise determination to date.
13. l';.M.Aitala el, al., Ph, ys. Rev. Lett 86 770 (2001)
14. The KI,()E collaboration, hep-ex/0206010, submil.te.d t,o Phys. [,el.t,. B
R E F E R E N C E S
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12. The K L O E collaboration, I)h.ys. I;cl.l. B 5 3 6 20.q,(2002)
