LEP measurements on production, mass, lifetime of beauty particles


Coma, Italy, June 21 -25, 1993

Presented at "Hadron 93 "

Guy Wormscr

LIFETIME OF BEAUTY PARTICLES

LEP MEASUREMENTS ON PRODUCTION, MASS,

October 1993 OCR Output

LAL 93-60





be 5373 ;l: 4 MeV/cz OCR Output
for Eb production has been reported. Finally, the B2 meson mass has been measured to
duction rate of each of these particles is measured at the 20% level. Preliminary evidence
Ab . The Ab lifetime is found to be smaller than 7];+ with a 2.7 0 significance. The pro
B3 and B+ are found to be equal within 10% whilst a 15% precision is reached for B2 and
ferent beauty particles using the results of the LEP experiments. Individual lifetimes for

This report details the present knowledge about the individual properties of the dif

Abstract

Université de Paris—Sud, 91405 Orsay Cedex, France
Laboratoire de 1’Accélérateur Linéaire, IN2P3—CNRS et

G. Wormser

MASS, LIFETIME OF BEAUTY PARTICLES

LEP MEASUREMENTS ON PRODUCTION,

October 1993

OCR OutputLAL 93-60



D0 —lepton sample contains typically 75% B' decays and 25% B3 decays. OCR Output
the Dj —lepton samples correspond to Bf decays in more than 90% of the cases, while a
and due to the inefficiency to identify D0 coming from a D* decay as such. As a result,
of D** —lepton events where the initial D·lepton correlation can be lost in the D** decay
and D0 —lepton correlation but some mixing is to be expected because of the production
q$—lepton and A-lepton (or proton-lepton) respectively. BQ and B‘ lead to D"'(D*)-lepton
decay products, namely ¢,A and protons, it will also be possible to tag B2 and Ab by
beauty baryon decay by a Ab —lepton correlation. Since Dj' and Ab also lead to specific
in semileptonic decays. A BQ decay will then be tagged by a Dj —lepton correlation, a
final state. This correlation is made possible by the smallness of non spectator effects
distinction of each B particle through the reconstruction of the charm particle in the
in semileptonic decays which offer much larger branching fractions and still allow a good
experiment. To overcome this problem, specific B states can be partially reconstructed
of completely reconstruted B particles is too small, typically a few tens of B2, B+ per
applied at LEP (see below) but is not yet the method of choice because the sample
to perform a complete exclusive reconstruction of that particle. This method has been

The usual method to perform the lifetime measurement of a given particle is of course

1. Specific lifetime measurements

Section 3.

Section 2 deals with production rates and the exclusive reconstruction is detailed in
Section 1 contains the description of the lifetime measurements for each beauty species,
beauty case to the one in the charm case. This report is thus organized as follows
heavy quark dynamics, for instance by comparing the SU3 symmetry breaking in the
Ab is opening up. A precise mass measurement of these states will give useful insight on

With the present statistics, the field of the complete exclusive reconstruction of BQ and
average mixing results to extract B3 and Bf oscillation frequencies.
for a b quark to hadronize in each species will be especially useful when interpreting

The measurement of the production rate of the different B mesons, i.e the probability

shorter Ab lifetime.
suppressed. These diagrams should also occur in the beauty case, leading to a somewhat
Ab short lifetime is due to the presence ofi annihilation diagrams which are not helicity
lead to a discrepancy between TB+ and 7*Bo smaller than 10%. For the baryon case, the
occur but with a reduced strength because of the larger available phase space and should
final state which leads to a destructive interference. In B+ decays, the same effect will
first effect is commonly believed to be due to the presence of two identical d quark in the
larger than the D0 one and the Ab lifetime is two times smaller than the D0 one. The
This approximation is not valid for charmed particles where the D+ lifetime is 2.5 times
In such a framework, all different species of B particles should have the same lifetime.
mation will measure the validity of the spectator model approximation in B weak decays.
ferent beauty particles using the results of the LEP experiments. Precise lifetime infor

This report details the present knowledge about the individual properties of the dif



Table 1 : B3 and B+ Lifetime (in ps) OCR Output

Total 1.43 ;t 0.14 1.51 :1; 0.13 I LEP

Charge countingll.56 zh 0.25 :1; 0.17 :1: 0.07| 1.56 :1: 0.19 ;l; 0.13 IDELPHI

1.51i3:§;t8;%i
’1.51i8;§:£5¥¥. OPAL

D6 1.17i3;§§ 1 0.15 1 0.5 |1.3i3;§§ 1 0.15 :1;.05 DELPHI
1.52% ii3;?§ 1.47i3;¥§ i3;1i I ALEPH

Exclusive 1.19i3;;g 1 0.14 | 1.77;*3;;.33 1 0.14 | ALEPH

Method

to extract the B2 lifetime from the neutral sample.
Ab mesons. Recent measurements of B2 and Ay, lifetimes and production rates were used
sample is almost pure in B+ whereas the neutral sample contains a mixture of B2, B2 and
for the bias towards long lived events introduced by the selection procedure. The charged
to the results of Monte—Carlo simulation program. This program is also used to correct
monitored through the amount of unphysical doubly charged vertices which is compared
vertex are selected. The number of wrong charge measurements at the secondary vertex is
case, events where tracks can be unambiguously attributed to a primary and a secondary
chamber or emulsion experiments and has been applied to DELPHI recently[4]. In that
distinguish between neutral and charged B decays. This method has been used in bubble

Counting the number of charged tracks at the B secondary vertex also allows to
measure from the D0 —lepton in a event—by-event basis.
surement of the jet visible energy allows a precise measurement (8%) of the B energy to
B+ ——> D0 —lepton X decay, the neutrino is the only unmeasured B fragment, a mea
lepton energy with a precision of 15%. DELPHI has taken advantage that since in
while the B energy is usually obtained through a parametrization of the observed D
event in a straigthforward manner : the D—lepton vertex provides the B decay length
collaboration[2] reported similar results. The proper time is extracted from the D—lepton
correlation in the Kvrvr decay mode (Fig. 1-d) as measured by DELPHl[3]. The OPAL
(Fig. 1-b), direct D0 —lepton in the Kvr decay mode (Fig. 1-c) and for the D+ —lepton
ilar signals are obtained for the D"‘—lepton correlation using K1r1r1r as D0 decay mode
in ALEPH[1] where a clear D0 signal can be seen for D° coming from D"‘ decays. Sim

Fig. 1-a shows the Kr mass spectrum obtained in correlation with an high pt lepton



A, its trajectory cannot be reconstructed with the microvertex detectors and therefore OCR Output
by the OPAL[10] results shown in Fig. 3. Because of the long decay distance of the
vation of A lepton correlations reported by the three LEP experiments and illustrated
to exact nature of the initial beauty baryon. The first set of results concern the obser
either a Ac or a A in their semileptonic decay since the experiments do not have access

In the following, Ab will be used as the generic name for all B baryons which produce

1.3 Ab lifetime

Table 2 : B2 Lifetime (in ps)

Total I 1.25 zi; 0.20 l LEP

Inclusive D, |0.801gZgg:l: 0.24 I DELPHI

aiepion | 1.24i3;§;4 0.2 |nEL1¤H1

n,-1€pum |2.26’;8;23¤& 0.12| ALEPH

1.1:si3;§g4 .00 | oPAL

above.
sion of 15% and is not significantly different from the B3 and B+ measurements reported
extent. The results are summarized on Table 2. The LEP average has a relative preci
tions and inclusive Dj production which is also sensitive to B2 production to a lesser
OPAL[8]. DELPHI[9] has reported two other measurements based on gb-lepton correla
found over a negligible background. In addition to similar results from ALEPH[7] and
preliminary DELPHI result[6] including 1992 data where 20 Dj events can be clearly
reconstructed through the ¢7r and K" K decay modes. Fig. 2 shows as an example the

The method of choice is as explained above to use Dj —lepton correlations. DQ" are

1.2 B2 lifetime

D" production that will follow from the direct observation of D=•<>•= -lepton correlation.
rapidly improve with the addition of data collected in 1992 and a better handle on
better than 10%, an impressive achievement. The quality of this measurement should
B+ lifetimes are found to be consistent with each other with a relative precision slightly
using the exclusively reconstructed BQ and B+ samples (see Fig. 8). The average B2 and
three LEP experiments including the preliminary measurements performed by ALEPH[5]

Table 1 summarizes the different measurements presented at the conference by the



DOW. OCR Output

probable than the precise hierarchy of the B lifetime will be measured in a year from
significantly smaller, with a 2.7 cr effect compared to the B+ meson. It is now highly
contrast with the charm case. On the other hand, the Ab lifetime is beginning to be
achievement. B3 and B+ lifetimes are found to be equal at the 10% level, in striking
precision with which they are measured is comprised between 10% and 15%, a remarkable

Table 5 summarizes the results described above on the four individual lifetimes. The

1.4 Lifetime summary

Table 3 : Ab lifetime (in ps)

Total | 1.00 :1: 0.14 ps | LEP

DELPHI1.1t3;§

A. —z |1.10i3;g§4 0.07| ALEPH

0.0s~;3;§§1 0.12 | DELPHI (Aire)

A - 2 |1.01jBb?,84 0.0s| OPAL( Alv)

1.121334 0.16 |ALEPH (Lepton)

conference.

of course required and encouraging results from DELPHI have been submitted to the
brought by the observation of proton-lepton correlations. Good particle identification is
awaiting for more data to confirm this expected effect, some more information could be
(TBM = 1.51 j: 0.13 ps), the statistical significance is 2.7 standard deviation. While
than the beauty mesons’ one. Compared to the largest one, the B+ lifetime,
The average Ab lifetime TA, = 1.00 i 0.14 ps is beginning to be significantly lower
any kinematic reflexion from D+ or Dj decays. Table 3 summarizes all these results.
for ALEPH and OPAL[14] or on the RICH in the DELPHI[12] case is used to avoid
Ab baryon is reconstructed in the pK1r mode and proton identification based on dE/ dx
correlation with high pt leptons, illustrated by the ALEPH[13] result on Fig. 4. The

These results are complemented by the clean observation of fully reconstructed Ab in
and the third has a lower efiiciency.
The first two methods suffer some dilution from the poor determination of the Ab vertex
DELPHl[12] looks for an extra pion coming from the Ab decay to get a precise Ab vertex.
eter distribution, OPAL uses nevertheless the intersection of the lepton and the A while
ments have chosen three different methods : ALEPH[11] users the lepton impact param
the Ab vertex cannot be reconstructed as precisely as in the B2 case. The three experi



Dj decays into qbir for the ALEPH collaboration, whereas DELPHI considers in addition OCR Output
Cerenkov counter[20]. The selected modes are .]/1/; q5, (ii) 1/1'¢ and (iii) Dj rr where the
experiment takes also advantage of its good Kaon identification provided by its RICH
lies in the requirement of a q$ meson in the BQ or daugther Dj decay. The DELPHI
the more abundantly produced B3 meson. A strong protection against such a problem
a negligible combinatoric background and, above all, a small reflection probability from
experiments have focused on the few hadronic decay modes which offer a good visibility,
this conference precise mass determinations, as well as the CDF experiment[23]. These
an unfortunately very large mass error. ALEPH[21] and DELPHI[22] have reported at
J /1/J 45 event attributed to a B2 decay with a invariant mass of 5360 zh 70 MeV/c2 , with
121 zh 10 MeV/c2 . The OPAL[8] collaboration had also published the observation of a
T(5S) which gave two solutions for the Bf -B3 mass difference of 82 i 2.5 MeV/cz or
was an indirect CUSB rneasurement[19] based on the inclusive photon spectrum at the
therefore no mass determination was possible. The only information available so far

Up to now, evidence from BQ production at LEP came from semileptonic decays and

3. ITIEISS I'I`1€3S`l1I`€I'I'I€Ht»

agreement with the value 0.75 expected from spin counting.
is visible at 50 MeV corresponding to a ratio NB./NB = 0.82 zh 0.08 zh 0.12 in good
photon energy distribution in the B center of mass displayed on Fig. 6. A clear excess
containing a high pt lepton and boosted back in a approximate B rest frame to obtain the
dence on the production of B* mesons. Low momenta photons were searched for in events

To conclude this section, let us mention that the L3 collaboration[18] reported evi
most one event is found in wrong sign combination (Fig. 5-b)
sign correlation with a lepton of pt above 1.2 GeV/ c. Nine candidates are found while at
would indicate the production of a Eb baryon. Fig. 5-a shows the Avr invariant for right

The DELPHI collaboration[17] reported the evidence of a E-lepton correlation which
thus fbarygn = 0.19 zh 0.06, a somewhat large result but with a large error too.
4% zh 1% branching ratio for Ab into pK1r, fb,,,yb,, is 0.24 zh 0.10. The combined result is
AC —lepton correlation, fbb,yb,, .BR(Ab —> Ab lz/).BR(Ac —+ pK1r )= 7.7 :h 1.6 10‘4 with
lower than the average b lifetime), fbbbybn is measured to be 0.16 zh 0.07. Using now the
ing ratio has to be smaller than the average semileptonic BR since the Ab lifetime is
of A in Ab decays and 8% zh 2% for the semileptonic Ab branching ratio (this branch
BR(Ab —> A X) is (0.34 zh 0.06) %. Using 27% zh 9% as the inclusive branching fraction
give fb = 0.15 zh 0.03 ;h 0.03[16]. For the Ab, the average fbmyw, .BR(Ab —> Ab lu).
for the semileptonic branching fraction, the average over the three LEP experiments
tion. Assuming a 2.8% branching ratio for the Dj —+ q$1r branching ratio and 11%

The BQ meson production rate f, is measured using the Dj —lepton correla
leads to fb +fb,,,yb,, = 0.24 zh 0.12.
a D0 meson does not change from 10 to 91 GeV. The result is fd = 0.38 zh 0.06 which
T(4S) . The underlying assumption is that the probability for a c quark to hadronize as
coming from a b quark to the D mesons rate coming from BQ or B+ mesons at the
B2 meson, fb . The DELPHI collaboration[15] has compared the amount of D mesons
know the probability with which a b quark will fragment into a B3 meson, fd , and a

lt is highly desirable in order to interpret meaningfully the average mixing results to



effects are only important in Ab decays. The B2 mass has been precisely measured OCR Output
now reasonably well established and in good agreement with expectations I non spectator
allow to pin down the Ab mass to 10 MeV in a near future. The beauty lifetime hierarchy is
although no results were reported at this conference, it is expected that a few events will
will probably reach the 10% precision level and concerning Ab exclusive reconstruction,
and some areas will soon be improved : the production rates of the different particles
given by the recent results from LEP experiments. The achievement is already impressive

The table 5 summarizes the present knowledge concerning the B particles mostly

4. Conclusion

Table 4 I B, mass in MeV/cz

5373.1 zh 4.2Overall average

CDF14.0 i 4.7 J/2/¤¢’sI5383.3 zh 4.5 zh 5.0
OPAL5360 i 70 I 35J/¢¢

average I5368.4 zh 5.6 zh 1.5
5368.4 A; 5.6 I 41.7 45 IALEPHz/¤’¢

1.65401 ;h 77 I 40.9Dj(¢1r)1r

average I 5357 zh 12 zh 6

2.45387 :1: 19 I 36.7J/1/¤qS
12 IDELPHI5341 d: 23 I 26.8Dj(¢vr)a]
0.1Dj(K°K)¢r I 5340 1 30 I 27.8
4.55333 j; 27 I 28.0Dj(¢¢r)rr

Decay mode ( BQ mass (MeV) |E(GeV)|Flight (mm)| Expt.

tribution is shown on Fig. 9, where 17 :1; 7 B2 events are observed.
DELPHI[24] looking for KKvr7r and KK4vr final states. The corresponding mass dis—

A more inclusive analysis based to strict Kaon identification was performed by
close to the Dj —D+ one (99.5 i 0.6 MeV/c2 ), as expected in the heavy quark limit.
5373 :l: 4 MeV/c2. The B3 -B2 mass difference (94.5 :1: 4.6) turns therefore to be quite
group[5]. The 1993 average for the B2 mass including the CDF measurement is thus
procedures as those described above. Fig. 8 shows the reference peak used by ALEPH
from the PDG value of reconstructed B3 and B+ signals using similar channels and fitting
the uncertainty in the absolute mass scale. This is controlled by the observed deviation
resonances (except for the OPAL event). The main systematic error source comes from
constrained fit imposing the correct vertex topology and the PDG value for intermediate
reconstructed at LEP so far. The individual mass errors are the result of the kinematic
be found on Fig. 7. Table 4 summarizes the properties of the 7 B2 events unambiguously
modes (i), (iii), (iv) and The invariant mass spectrum of the DELPHI candidates can
Dj into ¢vr. ALEPH has found one candidate in modes (ii) and (iii) and DELPHI one in
(iv) the K* K decay of the Dj in the Dj 7r mode and (v) the Dj a1(1260) mode with the

v11



[21] D. Buskulic et al., ALEPH collab., CERN—PPE/93-97. OCR Output
[20] E. G. Anassontzis et al., Nucl. lnstrum. Methods A323 (1992) 351.
[19] J. Lee—Franzini et al., CUSB collab., Phys. Rev. Lett. 65 (1990) 2947.

France.

[18} S. Goldfarb et al., L3 collab., proceeedings of the 1993 EPS conference, Marseille,
[17] P. Abreu et al., DELPHI collab., DELPHI note 93-95 PHYS 322.

ference, Cornell, Ithaca NY (USA).
[16] W. Venus, Rapporteur talk on B—physics, 1993 Lepton-Photon. International Con
[15] P. Abreu et al., DELPHI collab., CERN—PPE/93-70.

P. D. Acton et al., OPAL collab., OPAL note PN—082.[14]
[13] D. Buskulic et al., ALEPH collab., CERN—PPE/92-73.

93-88 PHYS 315.

[12] P. Abreu et al., DELPHI collab., Phys. Lett B311 (1993) 3799 and DELPHI note
[11] D. Buskulic et al., ALEPH collab., Phys. Lett. B297 (1992) 449.

P. D. Acton et al., OPAL collab., OPAL note PN—092.[10]
[9] P. Abreu et al., DELPHI collab., DELPHI note 93-86 PHYS 313.
[8] P. D. Acton et al., OPAL collab., Phys. Lett. B295 (1992) 357.
[7] D. Buskulic et al., ALEPH collab., Phys. Lett. B294 (1992) 145.

93-87 PHYS-314.

[6] P. Abreu et al., DELPHI collab., Phys. Lett. B289 (1992) 199 and DELPHI note
B. Jacobsen and H. G. Moser, ALEPH collab., private communication.[5}

[4] P. Abreu et al., DELPHI collab., Phys. Lett. B312(1993) 253.
P. Abreu et al., DELPHI collab., Z. Phys. C 57(1993),181.[3]
P. D. Acton et al., OPAL collab., OPAL note PN106.[2}
D. Buskulic et al., ALEPH collab., CERN-PPE/93-42.lil

References

Table 5 : Summary of B particle properties

baryons

EKU!Others I Eb seen

Ab I 5641 zh 50 I1.00 :1; 0.14 fb Z, 0.10 I ?

zh 0.09 I D,7r,D,a1
B2 | 5366 -|; 6 |1.25 i 0.20 f, = 0.25 ;l: 0.08I ¤bq$,gb'<15

fd = 0.38 :1: 0.06I 0.21B+ I 5278.6 ;l: 2 l1.51 d; 0.13
fd = 0.38 dz 0.06| 0.3- 133 [5278.7 i 2.1|1.43 i 0.14

Prod. Rate IE BR seenLEP93l Mass l Lifetime

T(4S) experiments.
will become accessible. LEP experiments are in this respect quite complementary to
using the "g01d plated" decay modes. In the future, the general pattern of B2 decays

vm



(from DELPHI) OCR Output
Figure 9 : KK1r and KK47r mass spectrum with two RICH identified kaons

Figure 8 : Knvr mass spectrum for exclusive BQ and B+ reconstruction (from ALEPH)

(d) J/z/z dz mass spectrum
D:a1(l260), normalized to the number of observed B2 candidates.
histograms shows the expected contribution from B2 ——> D; 1r and

(c) KK4vr in the Dj a1(1260) channel. The dotted line in these three
(b) KK1rrr mass spectrum in the Dj vr channel and Dj —> K* K

Figure 7 : (a) KK1r1r mass spectrum in the Dj 7T channel and Dj ——+ gbvr

without B* production.
a high pt lepton (from L3). The shaded histogram corresponds to a simulation
Photon energy spectrum in the approximate B rest frame for events containingFigure 6 :

(b) A1r` mass spectrum in correlation with a wrong sign (l+) high pt lepton.
(from DELPHI)

Figure 5 (a) A7r` mass spectrum in correlation with a right sign (I`) high pt lepton

(b) pK'vr+ mass spectrum in correlation with a wrong sign (l+) high pt lepton.
(from ALEPH).

Figure 4 (a) pK`1r+ mass spectrum in correlation with a right sign (I') high pt lepton

The shaded histogram corresponds to wrong sign correlations.
Proton vr mass spectrum in correlation with a high pt lepton (from OPAL).Figure 3

KK7r mass spectrum in correlation with high pt lepton (from DELPHI)Figure 2

(d) Kvrvr mass spectrum in correlation with a high pt lepton (from DELPHI)
(c) Kvr mass spectrum in correlation with a high pt lepton (from ALEPH)

correlation with a high pt lepton (from ALEPH)
(b) Kmrrr mass spectrum for D candidates coming from D" decays in

with a high pt lepton (from ALEPH)
(a) Kvr mass spectrum for D candidates coming from D* decays in correlationFigure 1

Figure Captions

[24] P. Abreu ct al., DELPHI collab., DELPHI note 93-50 PHYS 280.
[23] K. Abc ct al., CDF collab., FERMILAB-PUB-93/141-E.
[22] P. Abrcu ct al., DELPHI collab., DELPHI note 93-107 PHYS 334.

1x