Lifetimes of charmed mesons

Volume 210, number 1,2 PHYSICS LETTERS B 18 August 1988

LIFETIMES OF CHARMED MESONS

A R G U S Col laborat ion

H. ALBRECHT, P. B O C K M A N N , R. GLASER, G. H A R D E R , A. K R U G E R , A. NIPPE, M. SCHAFER, W. S C H M I D T - P A R Z E F A L L , H. SCHRODER, H.D. SCHULZ, F. SEFKOW, J. SPENGLER, R. W U R T H , A. YAGIL DESK D-2000 Hamburg 52, Fed. Rep. Germany

R.D. A P P U H N , A. DRESCHER, D. KAMP, H. K O L A N O SK I , U. MATTHIESEN, H. SCHECK, G. SCHWEDA, B. SPAAN, A. WALTHER, D. W E G E N E R lnstitut fOr Physik ~, Universitiit Dortmund, D-4600 Dortmund 50, Fed. Rep. Germany

J.C. GABRIEL, T. RUF, K.R. SCHUBERT, J. STIEWE, K. STRAHL, R. WALDI, S. W E R N E R lnstitut fiir Hoehenergiephysik 2, Universitgit Heidelberg, D-6900 Heidelberg 1, Fed. Rep. Germany

K.W. E D W A R D S 3, W.R. F R I S K E N 4 D.J. G I L K I N S O N 5, D.M. G I N G R I C H 5, H. K A P I T Z A 3, R. K U T S C H K E s, D.B. M A C F A R L A N E 6, W. McLEAN 6, A.W. NILSSON 6, R.S. O R R 5, J.A. PARSONS 5, P.M. PATEL 6, J.D. P R E N T I C E 5, S.C. SEIDEL 5, H.C.J. SEYWERD 5, J.D. SWAIN 5, G. TSIPOLITIS 6, T.-S. Y O O N 5 Institute qf Particle Physics 7, Canada

R. AMMAR, S. BALL, D. COPPAGE, R. DAVIS, S. KANEKAL, N. K W A K University c~f Kansas 8, Lawrence, KS 66045, USA

B. BOSTJANCIC, G. KERNEL, P. K R I Z A N lnstitut J. Stefan and Oddelek za fiziko 9, Univerza v Ljubljani, 61111 Ljubljana, Yugoslavia

L. JONSSON Institute o f Physics Jo University of Lund, S-223 62 Lund, Sweden

A. BABAEV, M. DANILOV, B. F O M I N Y K H , A. G O L U T V I N , I. GORELOV, V. LUBIMOV, V. MATVEEV, A. SEMENOV, S. SEMENOV, V. SHEVCHENKO, V. SOLOSHENKO, V. TCHISTILIN, I. T I C H O M I R O V , Yu. ZAITSEV Institute of Theoretical and Experimental Physics, 117 259 Moscow, USSR

R. CHILDERS, C.W. D A R D E N and R.C. F E R N H O L Z University o f South Carolina ~ ~, Columbia. SC 29208, USA

Received 22 May 1988

Using the ARGUS detector at the e +e- storage ring DORIS II at DESY, we have measured the lifetimes of the D °, D + and (5.6_ i:>_ _+0.8) × 10-J3 s. Ds + mesons. We find ZDo = (4.8_+0.4_+0.3) × 10-13 s, rD. = (10.5 + 0.8 + 0.7) × 10-~3 s and rD+ = +~

For footnotes see next page.

267


In the spectator model of heavy flavour decays, light quarks play a minor role and it is expected that the lifetimes of all charmed hadrons are approximately equal. Initial experiments measuring the lifetimes [ 1 ] of charmed particles and the ratio of the D + to D o semileptonic branching fractions [ 2 ] indicated that this was not the case. Several mechanisms have been proposed to account for this difference ~L, but agreement with experiment is still unsatisfactory. Accu- rate measurements provide a check of previous results and constrain models which at tempt to explain lifetime differences.

We report here measurements of the D °, D +, and D~- lifetimes ~-~ using data collected by the ARGUS detector. The detector has been described in detail elsewhere [4]. This analysis relies primarily on the good spatial resolution of the vertex drift chamber [ 5 ] and the particle identification capabilities of the detector. The vertex drift chamber provides excellent resolution of less than 100 ~tm in the r -¢ plane at the interaction point for high m o m e n t u m tracks. The method of particle identification is described in ref. [6]. For charged tracks, dE/dx and time-of-flight measurements yield a Z 2 for the possible particle mass hypotheses e, ~t, n, K and p. A likelihood ratio, l,, for each of these hypotheses is then calculated:

w / . e x p ( - Z 2 / 2 ) l~ = y~jwi.exp ( _ z y / 2 ) , i,j=e, ~, ~, K, p,

where the w~ are relative particle production weights. These are set to five for the pion hypothesis and unity

t Supported by the German Bundesministerium f'tir Forschung und Technologie, under contract number 054DO51P.

2 Supported by the German Bundesministerium ftir Forschung und Technologie, under number 054HD24P.

3 Carleton University, Ottawa, Ontario, Canada KIS 5B6. 4 York University, Downsview, Ontario, Canada M3J IP3. s University of Toronto, Toronto, Ontario, Canada M5S 1A7. ~' McGill University, Montreal, Quebec, Canada H3A 2T8. 7 Supported by the Natural Sciences and Engineering Research

Council, Canada. Supported by the US National Science Foundation.

'~ Supported by Raziskovalna skupnost Slovenije and the Inter- nationales Btiro KfA, JiJlich.

~0 Supported by the Swedish Research Council. ~ Supported by the US Department of Energy, under contract

DE-AS09-80ER 10690. '~ For a discussion of these mechanisms, see ref. [3 ]. ~2 References in this paper to a specific charged state are to be

interpreted as implying the charge-conjugate state also.

for all others, in approximate agreement with obser- vation. A track is used as particle i if l, exceeds 0.05.

The data sample corresponds to an integrated lu- minosity of 166 pb -~ at centre-of-mass energies around 10 GeV. From our sample of mult i -hadron events, we have reconstructed D mesons decaying via the following channels:

(1) D * + ~ D ° ~ +, D°--*K-~ + ,

(2) D * + ~ D ° = +, D°~K-Tt+=+r t - ,

(3) D+-+K-r t+r t + ,

(4) D~--+~7c +, 9 - , K + K - .

For each measurement, events containing candidates consistent with the appropriate decay mode were chosen. Each of the tracks forming a candidate was required to have at least four hits in the vertex chamber and belong to the main event vertex within seven standard deviations. Those systems which passed our kinematic selection criteria (tables 1 and 2) were then fitted to a separate vertex using a three- dimensional parametric vertex fit [ 7 ]. Only systems which had a vertex Z 2 per degree-of-freedom less than five were accepted as charm decay candidates.

Since the events contain, in general, the decay products of charm quarks and other long-lived particles, it is not possible to measure the beam interac- t ion point on an event-by-event basis, for this reason, the beam position and beam widths were measured using tracks from the Bhabha events in each run (typically equivalent to about 25 n b - l of integrated luminosi ty) , which gave about 500 tracks in the bar- rel region of the detector. The beam center was deter- mined to an accuracy of approximately 30 ~tm in the

Table 1 Summary ofD ° data selection criteria. Momenta are in units of GeV/c, masses in GeV/c 2. AM refers to the D*+-D ° invariant mass difference.

D*+__,DOrt+

D°-~K ~t + OO~K-rc+~+rc

a~-G< 0.1 cm a~ 0.07 cm PK~> 2.5 PK3n> 2.5 1.818<MK~< 1 . 8 9 8 1.830<MK3~< 1,886 0.1435 <AM< 0.1475 0.1435 <&M< 0.1475

IM~+~ --MK~'J >0.03

268

Volume 2 t 0, number 1,2 PHYSICS LETTERS B 18 August 1988

Table 2 Summary ofD + and Ds ~ data selection criteria. 0~r~ is the angle between the ¢ and the D~ boost direction in the rest frame of the Ox system. 0~ is the helicity angle between the K- and the rt + in the rest frame of the ~ system.

D + ~K-rc+rt+ Ds ~ -.On +, , - ,K+K -

cr~< 0.08 cm PK~> 3.5 1.835 <MK~< 1.895 PK> 0.4, P~> 0.4

a/~<0.1 cm P~K.> 2.4 1,930 < MK~< 1.995 1.0115 <MKK< 1.0275 cos 0~D~ <0.9, ICOS 0~ [ >0.5

hor izonta l direct ion and 20 ~tm in the vert ical direction. The corresponding beam widths were found by studying impact pa ramete r d is t r ibut ions o f Bhabha tracks with respect to the beam center and were de- termined to be about 480 ~tm and 85 pm, respectively.

The decay length of each candida te was deter- mined by measuring the most probable path length in the r-O plane,

l,,. = x, t ,B~x +y, t,.B,.~ + Bx~ (Cy~. + t~ x, ) tZ .B , , -+ t~B~ , .+2 t , . t , ,B ..... '

where (x,., y,.) is the d isp lacement from the beam center o f the f i t ted secondary vertex, tx and t,, are the direct ion cosines of the decaying part ic le and the ma- trix B is the inverse o f the sum of the beam and vertex covariance matrices. The corresponding error is

aix,. = ( B~,vt~. + Bx.~t~ + 2 B , , , C t v ) - ' / 2

The proper decay length and error are then

It,. azr,, [ = cz = " ~ or/= - -

fly sin 0 ' f17 sin 0 '

where 0 is the polar angle of the part icle with respect to the beam axis.

To extract the proper decay length, lo, we used a m a x i m u m l ikel ihood fit where the single-event like- l ihood function is descr ibed by an exponent ia l con- voluted with a gaussian resolut ion function,

g ( l ) = ( 1/v2/2~a/ lo)

o o

X J e x p ( - ~/ /o)exp [ - ( l - ~ ) 2 / a [ ] d ~ . 0

In addi t ion, the effect o f background events in the

sample was accounted for by adding a gaussian, b ( l ) ,

o f width a~ and mean background decay length, lb. This gives the complete single-event l ikel ihood func- t ion as

L ( I) = ( 1 - f b ) g ( l ) + f b b ( l ) ,

wherefb is the fraction o f background con tamina t ion in the event sample.

The method was checked by a Monte Carlo simu- lat ion of each o f the decay channels, including a de- tai led s imulat ion of the A R G U S detector [8 ]. The decay length analysis reproduced the Monte Carlo input decay length within errors for all channels. We conclude that there is no bias in the method.

The background fractions were found for each of channels ( 1 ) - ( 4 ) by fit t ing the appropr ia te invariant mass dis t r ibut ions (figs. l a -4a , respect ively) with a gaussian for the signal and a po lynomia l pa- ramet r iza t ion for the background, the background proper decay lengths were measured by choosing events from the s idebands and passing them through the same cuts [9 ].

Table 3 summarizes the decay length analysis for all channels, including the results of the s ideband analysis. The m a x i m u m l ikel ihood fits to the proper decay lengths are shown in figs. l b - 4 b for the four channels, respectively. The systematic error on the decay length measurement for the D ° - , K - x + candidates was ob ta ined by adding in quadra ture the following contr ibut ions: + 2 ~tm from varying the background fraction within errors, _+ 1 ~tm by changing the background lifetime within errors, _+ 4 ~tm from allowing the beam sizes to vary by + 100/am in x and + 50 ~tm in y, _+0.6 Bm by allowing the beam posi- t ions to vary within errors, _+3 ~tm from changing the qual i ty cuts on the vertex Z e and a /wi th in reasonable l imits, and an addi t ional + 4.5 ~tm from uncerta in- ties in the error matr ix B. In the case of the D + mea-

Table 3 Summary of decay length analysis. The number of events quoted (N) includes the background fractionfb.

Decay N fb(%) /u( tam) /o(lam)

D°~K-rc + 431 8___2 19_+17 146+16 + 7 ~ D°-,K-rt+x+x- 452 16_+2 34_+10 142-+ I~ + D+ --,K-x+x + 825 56_+4 25_+ 5 315-+~+196 D~- -~QTt + 168 32_+4 2_+37 167 +39+24 --36--23

269


N

8 M e V / c z

80

60

40

20

100 , a m

60

(a)

1.4 1.6 1 .B 2.0 2.2

M(K- 'g+) [ G e V / c a]

40

20

0

0.3 -0 .2 -0,1 0.0 0.1 0 2 0 3

e ' r [ c m ]

Fig. 1. (a) K-n + invariant mass distribution. The region between 1.5 and 1.7 GeV/c-'. containing D ° decays into K - n + n °

where the n o is missed, is excluded from the fit. (b) Proper decay length distribution for D~-.K n + candidates. The hatched distribution shows the sideband decay lengths in relation to the D ° sample.

surement , a l though the same sources were considered, the backg round uncer ta in ty is the d o m i n a n t c o m p o - nent , con t r ibu t ing + ~ 4 - ~ 7 g m . F o r the D ~ , the e r ror ma- t r ix ( +__ 16 p.m) and b e a m wid th ( +_ ~jo g i n ) domina t e .

C o m b i n i n g the two avai lable channels , the l i f e t ime o f the D O meson is d e t e r m i n e d to be

"CD, = (4.8 +0.4+--0.3) X 10- 13 S .

The cor respond ing results for the D + and D ~ mesons are

5 M e V / c ~

80

60

40

20

(a)

0 ~ ~

1.G 1.7 1.9 2.1

M(K-n*n+rr -) [ G e V / c a]

100 ixm (b)

60

+ ++ + . + ¢ + ,

0 ~ , i , ~ ,

-0. : - 0 .2 -0.1 0.0 0.1 0,2 0 3

c ' r [ c m ]

Fig. 2. (a) K - n + n + n - invariant mass distribution. (b) De-~ K - n + n + n - proper decay length distribution.

rD+ = (10.5 +_0.8_+0.7) X 10-~3 S,

5.6_112 -t-0.8) X 10 -13 S . TDsl " -~ ( + I 3

F o r the ra t io o f charged to neut ra l D l i fet imes, we

f ind rD + / ZOO = 2.2 +_ 0.3 +_ 0.2 which should be equa l

to the rat io o f semi lep ton ic b ranch ing fract ions ,

B r ( D + - ~ e + X ) / B r ( D ° - - , e + X ) . Th is ra t io has been

m e a s u r e d by the M A R K III co l l abora t ion to be 2._,_o.4a+°s +0.1- [10] . F r o m the D~ and D O measure -

men t s we calcula te Z D ~ / r D o = l . 2 + 0 . 3 + 0 . 2 . These

results r epresen t the mos t precise c h a r m e d meson

l i fe t ime m e a s u r e m e n t s f r o m e + e - ann ih i l a t i on data,

270

Volume 210, number t.2 PHYSICS LETTERS B 18 August 1988

10 MeV/c a )

I50

1 O0

50

- - I

1.6 1.7 1,8 1.9 2.0 2.1

M(K-.+~ +) [ c e v / : ]

100 p m

120

80

40

! I

h +

-0.3 ~0.2

++ +

i . . . . . . . ~ 1 , J , ,

-0.1 0.0 0.1 0.2 0.3

o~ [¢m]

Fig, 3. (a ) K - n + n + invariant mass distribution. ( b ) D +--, K- n + n + proper decay length distribution.

10 MeV/c z

1.7 t ,8

100 ttrn (b) 2O

1.9 2,0 2.1 2.2

M(~x +) [GeV/c ~]

1

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3

c'r [cm]

Fig. 4. (a) 0r¢ + invariant mass distribution. The peak at a mass of 1.869 MeV/c 2 is from the decay D+-,¢rc +, (b) D~+-~¢r~ + proper decay length distribution.

a re in good a g r e e m e n t w i t h r e c e n t r e su l t s f r o m o t h e r

e x p e r i m e n t s ~3

d i r e c t o r a t e for t he s u p p o r t a n d k i n d h o s p i t a l i t y ex-

t e n d e d to t h e m .

I t is a p l e a s u r e to t h a n k U. D j u a n d a , E. K o n r a d , E.

M i c h e l a n d W. R e i n s c h for t h e i r c o m p e t e n t t e c h n i c a l

he lp in r u n n i n g t he e x p e r i m e n t a n d p r o c e s s i n g t h e

da ta . We t h a n k Dr . H. N e s e m a n n , B. Sa rau , a n d t h e

D O R I S g r o u p for the exce l l en t o p e r a t i o n o f t he s tor -

age r ing. T h e v i s i t i ng g r o u p s wish to t h a n k t he D E S Y

~3 For a review of recent results, including results only presented at conferences see ref. [ 11 ]. Results since then can be found in ref. [12].

R e f e r e n c e s

[ 1 ] E531 Collab, N. Ushida et al,, Phys. Rev. Lett. 45 (1980) 1049, 1053.

[ 2 ] DELCO Collab., W. Bacino et al., Phys. Rev. Left. 45 (1980) 329; MARK II Collab., R.H. Schindler et al., Phys. Rev. D24 (1981) 78.

[3] E.g.. R. Riickl, Habilitationsschrift, Universitiit Miinchen ( 1984); CERN preprint CERN-TH 4013/84 (1984).

271


[ 4 ] M. Danilov et al., Nucl. Instrum. Methods 217 ( 1983 ) 153; R. Heller et al., Nucl. Instrum. Methods A 235 ( 1985 ) 26; A. Drescher et al., Nuct. lnstrum. Methods 205 ( 1983 ) 125; 216 (1983) 35;A 237 (1985) 464; A. Arefiev et al., The ARGUS muon chambers, DESY report DESY 83-025 (1983).

[ 5 ] K.W. Edwards et al., Nucl. Instrnm. Methods A 252 (1986) 384.

[ 6 ] ARGUS Collab., H. Albrecht et al., Phys. Lett. B 150 ( 1985) 235.

[7 ] L.D. Gladney, Measurement of the lifetime of the neutral and charged D mesons, Ph.D. thesis, Stanford University (1985).

[8 ] H. Gennow, SIMARG: a program to simulate the ARGUS detector, DESY internal report DESY F 15-85-02 (1985).

[ 9 ] D.J. Gilkinson, Measurements of charmed meson lifetimes, Ph. D. thesis, University of Toronto ( 1987 ).

[ 10] MARK llI Collab., M. Baltrusaitis et al., Phys. Rev. Left. 54 (1985) 1976.

[ 11 ] E.g., K.R. Schubert, Proc. Intern. Europhysics Conf. on High energy physics (Uppsala, Sweden, 1987 ); TASSO Collab., W. Braunschweig et al., Z. Phys. C 33 (1987) 359; E691 Collab., J.R. Raab et al., Fermilab Pub-87/144-E.

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Lifetimes of charmed mesons