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Journal ArticleDOI

A study of kinematical correlations between charmed particles produced in π-Cu interactions at √s = 26 GeV

26 Sep 1996-Physics Letters B (North-Holland)-Vol. 385, Iss: 1, pp 487-492

AbstractA sample of 475 events, in which two charmed-particle decays are observed, is analyzed to determine distributions of two-particle kinematic variables. One charmed particle with x F > 0 is fully reconstructed and the other is at least partially recontructed. The distributions of Δo and p T 2 are compared with a next-to-leading order QCD calculation.

Topics: Quantum chromodynamics (50%)

Summary (2 min read)

1 Introduction

  • Hadroproduction of heavy quarks is an important testing ground for Quantum Chro- modynamics (QCD).
  • The observed experimental correlations between charmed particles provide a test of the NLO QCD calculations.
  • These variables are useful in subsequent comparisons with the predictions of charm production models.
  • The DkD has analogue readout so that secondary interactions in the detector material may be identi ed by their large energy deposits.
  • More details of the description and performance of the experimental apparatus can be found in Refs. [3, 4].

2 Event selection

  • Events are selected in which one charmed particle is fully reconstructed, while recon- struction of the decay vertex is all that is required for the second.
  • At this stage no cut is applied on the charmed candidate's invariant mass, except that 2-prong vertices compatible with K0 or 0 decays are rejected.
  • From the events which satisfy the previous requirements, 690 (2251) in the 1992 (1993) data, the authors extract two samples: the signal sample if the mass of the fully reconstructed vertex in the hypothesis D !.
  • The range and width of the side-bands has been chosen to allow the subtraction of a linear background distribution and at the same time to minimize the statistical error of this subtraction.
  • They were found to be compatible, so only the combined results are presented.

3 Momentum estimator

  • As previously noted, the momenta of the charmed particles is not needed for the measurement of ; however, it is required for the determination of the other correlation variables.
  • Thus it is necessary to have an estimator for the momentum of the partially reconstructed vertex which takes into account the unseen decay products.
  • The charm momentum can be deduced imposing the charm mass: pD = D DMD: A comparison between these two methods and the simulation indicates that both methods give useful and independent information; however the rst method systematically underestimates the momentum while the second method systematically overestimates it.
  • The optimal weights are acceptance dependent; for their experimental setup equal weights give a good result.
  • The resulting fractional errors on the correlation variables are well below 10% over most of the range of their measurements.

4 Background subtraction and acceptances

  • The invariant mass distribution for the fully reconstructed vertex is shown in Fig. 2a.
  • Fitting the data mass distributions with a Gaussian peak above a linear background, the authors nd that the background in the signal region is 15% for both the 1992 and 1993 data sets.
  • Comparison of the data and Monte Carlo distributions shows that the charm events with reconstruction errors account for all the background in this mass interval.
  • 3 and 4, therefore acceptance corrections were evaluated by dividing each distribution of reconstructed events by the corresponding distribution of generated events.

5 Results

  • 3 and 4 show the distributions of the charm correlation variables after back- ground subtraction and acceptance correction.
  • Table 1 reports the mean values found by the present analysis for all measured correlation variables.
  • The asymmetry in these distributions is thought to result from asymmetry in quark contents of the beam and target particle.
  • The and p2T distributions are plotted in Fig. 3 along with the results of a model based on a NLO QCD calculation [22] which includes non-perturbative e ects such as hadronization and initial transverse momentum of the incident partons [23].
  • Within the context of this model, no meaningful prediction is available for comparison with the data.

6 Conclusions

  • This paper presents correlations observed in the WA92 experiment between two charmed particles produced in {Cu interactions at p s = 26GeV, where one of the charmed particles, which is fully reconstructed, has positive xF .
  • The distributions observed are similar in shape and statistics to the previous highest statistics experiment.
  • A comparison has been made between the and p2T distributions observed and a model based on NLO QCD.

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH
CERN{PPE/96{108
26 July 1996
A study of kinematical correlations between charmed particles
pro duced in
{Cu interactions at
p
s
= 26 GeV
The BEATRICE Col laboration
M. Adamovich
5)
, M. Adinol
3)
, Y. Alexandrov
5)
, C. Angelini
6)
, D. Barberis
3)
,
D. Barney
4)
, J. Batten
4)
, C. Bruschini
3)
, A. Cardini
6)
, F. Ceradini
8)
, C. Cianfarani
1)
,
G. Ciapetti
7)
, M. Dameri
3)
, G. Darb o
3)
, A. Duane
4)
, V. Flaminio
6)
,A.Forino
1)
,
B. R. French
2)
,A.Frenkel
7)
, C. Gemme
3)
, K. Harrison
7)
, N. Hummadi
4)
, R. Hurst
3)
,
A. Kirk
2)
, C. Lazzeroni
6)
, L. Malferrari
1)
, G. Martellotti
7)
,P. Martinengo
2)
,
P. Mazzanti
1)
, J. G. McEwen
9)
,P. Nechaeva
5)
, A. Nisati
7)
, D. Orestano
8)
, B. Osculati
3)
,
M. Palutan
7)
,M. Passaseo
2)
,G.Penso
7)
,L.Pontecorvo
7)
, A. Quareni
1)
, H. Rotscheidt
2)
,
C. Roda
6)
, L. Rossi
3)
,S.Veneziano
7)
,M. Verzocchi
7)
,D. Websdale
4)
, L. Zanello
7)
and
M. Zavertyaev
5)
.
Abstract
A sample of 475 events, in whichtwocharmed-particle decays are observed, is an-
alyzed to determine distributions of two-particle kinematic variables. One charmed
particle with
x
F
>
0 is fully reconstructed and the other is at least partially recon-
tructed. The distributions of
and
p
2
T
are compared with a next-to-leading order
QCD calculation.
(Tobe submitted to Physics Letters B)
1)
Universita di Bologna and INFN, Bologna, Italy.
2)
CERN, Geneva, Switzerland.
3)
Universita di Genova and INFN, Genoa, Italy.
4)
Blackett Lab oratory, Imp erial College, London, United Kingdom.
5)
Lebedev Physical Institute, Moscow, Russian Federation.
6)
Universita di Pisa and INFN, Pisa, Italy.
7)
Universita di Roma \La Sapienza" and INFN, Rome, Italy.
8)
Universita di Roma \Roma Tre" and INFN, Rome, Italy.
9)
University of Southampton, Southampton, United Kingdom.

1 Intro duction
Hadroproduction of heavy quarks is an important testing ground for Quantum Chro-
modynamics (QCD). Recent perturbative calculations to next-to-leading order (NLO),
i.e.
O
(
3
S
), are believed to be of sucient accuracy that useful comparisons can b e made be-
tween experimental data and theoretical predictions [1]. For charm production, however,
the mass is not suciently large for higher order p erturbative eects to be negligible,
thus signicant discrepancies with exp erimental results might still exist. The observed
experimental correlations between charmed particles provide a test of the NLO QCD
calculations.
In this letter we rep ort measurements of correlation variables which relate kinematic
quantities for twocharmed particles produced in a single eventby exp erimentWA92. We
have previously rep orted results on the azimuthal angle
between charmed particles
[2]. Nowwe improve our previous analysis in three ways. First, we include measurements
of kinematic correlation variables, in addition to the topological variable
. Second, we
use the full statistics of the exp eriment, increasing the sample size by approximately a
factor of 5; third, we include the corrections due to an improved study of the acceptance
of the detector.
The measurementof
does not require knowledge of the magnitudes of the two
charmed-particle momenta; however, this information is required for other kinematic cor-
relation variables and must b e either measured or estimated. The variables wehave mea-
sured, in addition to
, include the invariant mass
M
(
DD
), the rapidity dierence
y
=
y
(
D
)
y
(
D
), the Feynman
x
dierence
x
F
=
x
F
(
D
)
x
F
(
D
), the Feynman
x
of the
DD
system, and the square of the transverse momentum of the
DD
pair
p
2
T
(
DD
),
where
p
T
is relative to the incident b eam particle direction. These variables are useful in
subsequent comparisons with the predictions of charm pro duction models.
Data were collected using a 350 GeV/
c
beam incident on a 2 mm copper target
in the CERN Spectrometer. WA92 was conceived as a xed-target beauty hadropro-
duction exp eriment, so the apparatus was designed to study short-lived particles. The
main features of the experimental hardware are a fast online secondary vertex trigger
and a high precision silicon microstrip detector array which provides excellent track vi-
sualization. The trigger, although designed to select beauty decays, also has signicant
eciency for charm decays, which p ermits the present measurement. The silicon tracking
array is divided into a Decay Detector (DkD) consisting of 17 planes of 10
m pitch sili-
con microstrips covering the rst 2
:
5cm downstream of the target, followed byaVertex
Detector (VxD) consisting of 12 planes of 25
m pitch and 5 planes of 50
m pitch. The
DkD has analogue readout so that secondary interactions in the detector material may
be identied by their large energy deposits. Cuts on the recorded pulse heights in the
vicinity of secondary vertices reduce by
91% the number of vertices due to secondary
interactions in the material of the DkD. More details of the description and p erformance
of the exp erimental apparatus can b e found in Refs. [3, 4]. The exp eriment collected data
in 1992 and 1993 with a total luminosityof8
:
1nb
1
.
2 Event selection
Events are selected in which one charmed particle is fully reconstructed, while recon-
struction of the decayvertex is all that is required for the second. Requiring b oth charmed
particles to b e fully reconstructed would reduce the sample size by a factor
45 (
i.e.
to
10 events). This factor is consistent with charm branching fractions and with our ac-
1

ceptance and reconstruction eciency. Due to detector acceptance, all fully reconstructed
charmed particle decays have a p ositivevalue of Feynman
x
; the partially reconstructed
decays mayhave either p ositive or negative
x
F
. Only events with two secondary vertices
within 6 cm of the target are considered. The reconstructed positions of the secondary
vertices are required to be outside of the target and to be separated from the primary
vertex by at least 6
(1
:
2mmonaverage), where
is the r.m.s. precision in the vertex
separation. In addition, they are required to b e separated from each other by at least
100
m in the plane transverse to the beam direction. Since our vertex reconstruction has
a transverse accuracy of
3{10
m, this last requirement excludes with high probability
events in which a single vertex is split into twovertices by reconstruction errors.
As the fully reconstructed charm vertex must b e physically compatible with a
Cabibbo favoured decay
D
!
Kn
(
n
=1
;
2
;
3), its total charge is required to b e 0
or
1 and its total momentum vector is required to p oint to the primary vertex within
30
m. At this stage no cut is applied on the charmed candidate's invariant mass, except
that 2-prong vertices compatible with
K
0
or
0
decays are rejected.
Softer cuts are applied to the second vertex. Any pairs of tracks belonging to it which
have opp osite charge and whose momentum sum points to the primary vertex within
60
mmust be incompatible with
K
0
or
0
decays.
e
+
e
pairs from photon conversions
are automatically excluded as they would b e detected as a single track in the silicon
microstrip detectors which are lo cated in a zone of weak magnetic eld. The invariant
mass of the vertex, assuming pion mass for all particles, must be less than 1
:
9 GeV/
c
2
.
There is no requirement on the vertex charge, on the number of tracks, nor on p ointing
to the primary vertex.
From the events which satisfy the previous requirements, 690 (2251) in the 1992
(1993) data, we extract two samples: the signal sample if the mass of the fully recon-
structed vertex in the hypothesis
D
!
Kn
equals the
D
mass
15 MeV/
c
2
; and a
side-band sample if the invariant mass
M
is in the region 1
:
69
<M <
1
:
78 GeV/
c
2
or
1
:
96
<M <
2
:
05 GeV/
c
2
. The range and width of the side-bands has been chosen to allow
the subtraction of a linear background distribution and at the same time to minimize the
statistical error of this subtraction. There are 123 (441) events in the signal sample and
129 (407) events in the side-band sample for the 1992 (1993) data. Since our apparatus
cannot distinguish b etween
and
K
, mass assignments for 2- and 4-prong vertices are
ambiguous. All combinations are considered equally.Thus it is p ossible for a 2-prong
vertex, for example, to be used twice | once in the signal sample and once, with the
opposite mass assumptions, in the side-band sample. As the width of the side-bands is 6
times the width of the signal region, all combinations in the side-bands are given weight
1/6. The 1992 and 1993 experimental setups were slightly dierent so the two data sets
were analyzed indep endently. They were found to be compatible, so only the combined
results are presented.
3 Momentum estimator
As previously noted, the momenta of the charmed particles is not needed for the
measurementof
;however, it is required for the determination of the other correlation
variables. Thus it is necessary to have an estimator for the momentum of the partially
reconstructed vertex which takes into account the unseen decay products. We use a com-
bination of two metho ds that are based on dierentphysical considerations. The rst
method consists in closing the charm decayby adding a missing particle, assumed to have
2

Figure 1:
Error of the momentum estimator describedinSection 3. The dierencebe-
tween the reconstructed and the simulated charm momentum is displayed as a function
of the simulated momentum. The vertical error bars represent the r.m.s. width of the
reconstructed momentum distribution.
a pion mass. Requiring the vertex to have the charm mass and requiring that its total
momentum vector p oint to the primary vertex, a second order equation is obtained with
two possible solutions for the charm momentum. Wecho ose the lower of the twocharm
momentum solutions since the apparatus has no acceptance for low-momentum charged
particles (below
1 GeV
=c
) and, therefore, there is a greater probability that a missing
particle makes a small contribution to the total momentum. The second metho d has b een
used by other experiments [10, 11] and is based on the assumption that in the rest frame
of the charmed particle the unseen momentum is emitted on average in a direction per-
pendicular to the charm laboratory momentum. The Lorentz b oost
D
from the charm
rest frame to the lab oratory frame is then given by
D
=
E
vis
q
M
2
vis
+
p
2
T
vis
where
E
vis
is the visible energy,
p
T
vis
is the visible transverse momentum, and
M
vis
is the
mass of the visible system, calculated assigning the pion mass to all the detected particles.
The charm momentum can be deduced imp osing the charm mass:
p
D
=
D
D
M
D
:
A comparison b etween these two metho ds and the simulation indicates that both metho ds
give useful and independent information; however the rst metho d systematically under-
estimates the momentum while the second method systematically overestimates it. The
3

best estimator for the charm momentum is a weighted average of the results of the two
methods. The optimal weights are acceptance dependent; for our experimental setup equal
weights give a goo d result. Finally, a 10% correction comp ensates for the underestimation
of the visible mass due to the assignments of pion masses to kaons, yielding an accuracy
of
p
=p
=0
:
012 + 3
:
2
10
4
p
where
p
is in GeV
=c
(see Fig. 1). The resulting fractional errors on the correlation vari-
ables are well b elow 10% over most of the range of their measurements. Obviously,if
the secondary vertex momentum sum p oints to the primary vertex and the visible mass
is compatible with the
D
mass, neither of the two metho ds is applied, and the visible
momentum is used.
Figure 2:
Invariant mass distributions for: a) data; b) simulated
c
c
events.
4 Background subtraction and acceptances
The invariant mass distribution for the fully reconstructed vertex is shown in Fig. 2a.
Fitting the data mass distributions with a Gaussian peak above a linear background, we
nd that the background in the signal region is
15% for both the 1992 and 1993 data
sets.
The main contribution to the background is from poorly reconstructed vertices in
charm events and has b een estimated using a Monte Carlo simulation. The
c
c
events
were produced by a combination of Pythia 5.4 [5] for leading order QCD generation of
the initial
c
c
quark pair, Jetset 7.3 [6] for their hadronization, and Fluka [7] for nuclear
eects in the target; detector resp onses were simulated with Geant 3.21 [8] interfaced with
Fluka and Jetset 7.4 [9] as described in ref. [4]. Monte Carlo events were selected by the
4

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