Article
Reference
Measurement of inclusive ω and η′ production in hadronic Z decays
L3 Collaboration
AMBROSI, Giovanni (Collab.), et al.
Abstract
We present a study of the inclusive ω and η′ production based on 3.1 million hadronic Z decays recorded with the L3 detector at LEP during 1991–1994. The production rates per hadronic Z decay have been measured to be 1.17±0.17 ω mesons and 0.25±0.04 η′ mesons.
The production rates and the differential cross sections have been compared with predictions of the JETSET and the HERWIG Monte Carlo models. We have observed that the differential cross sections can be described by an analytical quantum chromodynamics calculation.
L3 Collaboration, AMBROSI, Giovanni (Collab.), et al . Measurement of inclusive ω and η′
production in hadronic Z decays. Physics Letters. B , 1997, vol. 393, p. 465-476
DOI : 10.1016/S0370-2693(96)01683-8
Available at:
http://archive-ouverte.unige.ch/unige:113054
Disclaimer: layout of this document may differ from the published version.
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ELSEVIER
13 February 1997
PHYSICS LElTERS B
Physics Letters B 393 (1997) 465-476
Measurement of inclusive w and 7’ production in hadronic 2 decays
M. Acciarriab, 0. Adrianiq, M. Aguilar-Benitezm, S. Ahlen k, B. Alpat ‘, J. Alcaraz=, G. Alemanni w, J. Allaby *, A. Aloisio ad, G. Alverson ‘, M.G. Alviggi ad, G. Ambrosi t,
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L3 Collaboration
0370-2693/97/$17.00 Copyright 0 1997 Published by Elsevier Science B.V. All rights reserved.
PII SO370-2693(96)01683-S
466 L3 Collaboration/Physics Letters B 393 (1997) 465-476
A. Gurtuj, L.J.
Gutay“-‘, B. Hartmann”, A. Hasan ae, D. Hatzifotiadou i, T. Hebbeker h, A. Her+‘, W.C. van Hoekaf, H. Hofer ay, H. Hoorani t, S.R. Hou ba, G. Hu e, V. Innocente r,
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PG. Rancoitaab, M. Rattaggi ab, G. Raven ‘O, P. Razis ae, K. Read as, D. Renay, M. Rescigno ae, S. Reucroft ‘, T. van Rhee at, S. Riemann aw, B.C. Riemers au, K. Riles ‘, 0. Rind c, S. Ro ar, A. Robohm ay, J. Rodin P, F.J. Rodriguez aa, BP Roe ‘, L. Romero aa,
S. Rosier-Leesd, Ph. Rosselet w, W. van Rossumat, S. Rotha, J.A. Rubio’,
H. Rykaczewski ay, J. Salicio’, E. Sanchez aa, A. Santocchia”‘, M.E. Sarakinos “, S. Sarkarj, M. Sassowsky a, G. Sauvaged, C. Schafer”, V. Schegelsky am, S. Schmidt-Kaerst ‘, D. Schmitz a, P Schmitz a, M. Schneegans d, N. Scholz ay, H. Schopper az, D.J. Schotanus af,
J. Schwenke”, G. Schwering”, C. Sciaccaad, D. Sciarrino t, J.C. Sens ba, L. Servoli”‘, S. Shevchenko&, N. Shivarovaq, V. ShoutkoaC, J. Shuklay, E. ShumilovaC, A. Shvorobah,
T. Siedenburg a, D. Son =, A. Sopczak”“, V Soulimovad, B. Smithr, P Spillantiniq,
.M. Steuer P, D.P. Stickland ak, H. Stoneak, B. Stoyanovaq, A. Straessner”, K. Strauch’, K. Sudhakarj, G. Sultanov ‘, L.Z. Sun “, G.F. Susinno t, H. Suter”Y, J.D. Swains, X.W. Tang g, L. Tauscher f, L. Taylor ‘, Samuel C.C. Ting P, S.M. Ting P, M. Tonutti ‘,
S.C.
Tonwarj, J. T&h “, C. Tully *, H. Tuchscherer “, K.L. Tung s, Y. Uchida
P,J. Ulbricht ay, U. Uwer r, E. Valente &, R.T. Van de Walle af, G. Vesztergombi “,
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w Collaboration/ Physics Letters B 393 (1997) 465-476 461
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Z.Z. Xu”, B.Z. Yang”, C.G. Yangg, X.Y. Yaog, J.B. Ye”, S.C. Yehba, J.M. Youi, An. Zalite am, Yu. Zalite am, P. Zemp ay, Y. Zeng a, Z. Zhang g, Z.P. Zhang “, B. Zhou k,
Y. Zhou ‘, G.Y. Zhu g, R.Y. Zhu ah, A. Zichichi i,r*s, F. Ziegler aw
a I. Physikalisches Institut, RWTH, D-52056 Aachen, FRG ’ III. Physikalisches lnstitut, RWTH, D-52056 Aachen, FRG ’
h National Institute for High Energy Physics, NlKHEF and University of Amsterdam, NL-1009 DB Amsterdam, The Netherlands c University of Michigan, Ann Arbor. Ml 48109, USA
* Laboratoire d’Annecy-le-Vieux de Physique des Particules. LAPPlN2P3-CNRS, BP 110, F-74941 Annecy-le-Vieux CEDEX, France e Johns Hopkins University. Baltimore, MD 21218, USA
’ Institute of Physics, University of Basel, CH-4056 Basel, Switzerland 8 Institute of High Energy Physics, IHEP 100039 Beijing, China6
h Humboldt University, D-10099 Berlin, FRG ’ i INFN-Sezione di Bologna, I-40126 Bologna, Italy j Tata Institute of Fundamental Research, Bombay 400 005, India
k Boston University, Boston, MA 02215, USA r Northeastern University, Boston, MA 02115, USA
m Institute of Atomic Physics and University of Bucharest, R-76900 Bucharest, Romania
” Central Research Institute for Physics of the Hungarian Academy of Sciences, H-1525 Budapest 114, Hungary2
’ Harvard University, Cambridge, MA 02139, USA v Massachusetts Institute of Technology, Cambridge, MA 02139, USA 4 INFN Sezione di Firenze and University of Florence, I-50125 Florence, Italy
’ European Laboratory for Particle Physics, CERN, CH-1211 Geneva 23, Switzerland s World Laboratory, FBLJA Project, CH-1211 Geneva 23, Switzerland
’ University of Geneva, CH-1211 Geneva 4, Switzerland
u Chinese University of Science and Technology, VSTC, Hefei, Anhui 230 029, China 6 v SEFT, Research Institute for High Energy Physics, PO. Box 9, SF-00014 Helsinki, Finland
w University of Lausanne, CH-1015 Lausanne, Switzerland
x INFN-Sezione di Lecce and Universitd Degli Studi di Lecce, l-73100 Lecce. Italy Y Los Alamos National Laboratory, Los Alamos, NM 87544, USA
’ lnstitut de Physique Nucleaire de Lyon, lN2P3-CNRS,Vniversite Claude Bernard, F-69622 Villeurbanne. France Ba Centro de lnvestigaciones Energeticas, Medioambientales y Tecnologicas, CIEMAT, E-28040 Madrid, Spain3
a’ INFN-Sezione di Milano, I-20133 Milan, Italy
a’ Institute of Theoretical and Experimental Physics, lTEP Moscow, Russia ad INFN-Sezione di Napoli and University of Naples, i-80125 Naples. Italy Z Department of Natural Sciences. University of Cyprus, Nicosia, Cyprus a[ University of Nymegen and NlKHEF, NL-6525 ED Nymegen, The Netherlands
‘g Oak Ridge National Laboratory Oak Ridge, TN 37831, USA ah California Institute of Technology, Pasadena, CA 91125, USA
ai INFN-Sezione di Perugia and Vniversitd Degli Studi di Perugia, I-06100 Perugia, Italy aj Carnegie Mellon University, Pittsburgh, PA 15213, USA
ak Princeton University, Princeton, NJ 08544, USA
ae INFN-Sezione di Roma and University of Rome, “La Sapienza”, l-00185 Rome, Italy am Nuclear Physics Institute, St. Petersburg, Russia
illl University and INFN, Salerno, l-84100 Salerno, Italy i1o University of California, San Diego, CA 92093, USA
Q Dept. de Fisica de Particulas Elementales, Univ. de Santiago, E-15706 Santiago de Compostela, Spain aq Bulgarian Academy of Sciences, Central Laboratory of Mechatronics and Instrumentation, BV-1113 Sojiaia, Bulgaria
8r Center for High Energy Physics, Korea Advanced Inst. of Sciences and Technology, 305-701 Taejon, South Korea as University of Alabama, Tuscaloosa, AL 35486. USA
at Utrecht University and NIKHEE NL-3584 CB Vtrecht, The Netherlands Bu Purdue University West Lafayette, IN 47907, USA
Bv Paul Scherrer lnstitut, PSI, CH-5232 Villigen, Switzerland
468 L3 Collaboration/Physics Letters B 393 (1997) 465-476
w DESY-Institut fir Hochenergiephysik. D-15738 Zeuthen, FRG
ay Eidgeniissische Technische Hochschule, ETH Ziirich, CH-8093 Ziirich. Switzerland a’ University of Hamburg, D-22761 Hamburg, FRG
‘a High Energy Physics Group, Taiwan, ROC Received 27 November 1996
Editor: K. Winter
Abstract
We present a study of the inclusive o and 7’ production based on 3.1 million hadronic Z decays recorded with the L3 detector at LEP during 1991-1994. The production rates per hadronic Z decay have been measured to be 1.17 XIC 0.17 w mesons and 0.25 f 0.04 7’ mesons. The production rates and the differential cross sections have been compared with predictions of the JETSET and the HERWIG Monte Carlo models. We have observed that the differential cross sections can be described by an analytical quantum chromodynamics calculation.
1. Introduction
Experimental studies of spectra and composition of particles in e+e- annihilation are important for un- derstanding the fragmentation of quarks and gluons into hadrons. Fragmentation is not perturbatively cal- culable and at present it can only be described by phenomenological models. Many particle species have been extensively studied in e+e- annihilation at lower energies [ I], and recently, a large amount of informa- tion on identified hadron species has become available at LEP [ 21. However, few e+e- experiments [ 3-61 have measured the production of w and r]’ mesons.
The q’ production rate is of interest since it provides information on the transformation of the SU( 3) states (78, ~1) into the observed states (7, v’). Also, un- certainties on the 7’ production rate can have an ef- fect on other measurements, for example, the Bose- Einstein correlation of charged pions [ 71 and particle multiplicities. Furthermore, the measured 7’ produc- tion rate [ 41 differs significantly from the prediction of JETSET 7.3 [ 83.
’ Supported by the German Bundesministerium fiir Bildung, Wis- senschaft, Forschung und Technologie.
* Supported by the Hungarian OTKA fund under contract number T14459.
3 Supported also by the Comisi6n Interministerial de Ciencia y Technologia.
J Also supported by CONICET and Universidad National de La Plats, CC 67, 1900 La Plata, Argentina.
5 Also supported by Panjab University, Chandigarh-160014, India.
’ Supported by the National Natural Science Foundation of China.
In this paper, we report on a measurement of inclu- sive production of w and q’ mesons in hadronic Z de- cays using the L3 detector at LEI? The w is identified via its decay channel w + n-+r-r’, while the T’ is re- constructed via its decay channels 7’ --+ T+~Y-V and rl’ + p’y. The no and 77 mesons are detected through their two-photon decay channels as narrow peaks in the yy invariant mass distribution, while the p” meson is reconstructed via its decay channel p” --+ %-;TT+m-.
The production rates and differential cross sections as a function of the scaled momentum xp = pIpbeam are measured and compared with the predictions of two Monte Carlo generators. A parton cascade based on perturbative QCD calculations is used in both gener- ators, while the non-perturbative hadronization phase is described by either string (JETSET 7.3) or cluster fragmentation (HERWIG 5.4 [ 91) models.
We also measure the dependence of the differen- tial cross section on the variable 5 = In( 1 /xp) . Under the local parton-hadron duality assumption [ 10,111, the measured cross section can be compared with pre- dictions of the modified leading log approximation (MLLA) of QCD [ lo].
2. The L3 detector
The L3 detector is described in detail in Ref. [ 121.
It consists of a central tracking chamber, a high res- olution electromagnetic calorimeter composed of bis- muth germanium oxide (BGO) crystals, a ring of plas-
L3 Collaboration/Physics Letters B 393 (1997) 465-476 469 tic scintillation counters, an uranium and brass hadron
calorimeter with proportional wire chamber readout, and an accurate muon chamber system. These detec- tors are installed in a 12 m diameter magnet which provides uniform field of 0.5 T along the beam direc- tion.
The central tracking chamber (TEC) is a time ex- pansion chamber with high spatial resolution in the plane normal to the beam. A Z-chamber, mounted just outside the TEC, supplements the r/q5 measurements with z-coordinates. Starting from 1994, tracks are also measured by a silicon micro-vertex detector consist- ing of two layers of silicon with double-sided read- out [ 131.
The material preceding the barrel part of the elec- tromagnetic detector amounts to less than 10% of a radiation length. In this region the energy resolution is 5% for photons and electrons of energy around 100 MeV, and is less than 2% for energies above 1 GeV.
The angular resolution of electromagnetic clusters is better than 0.5” for energies above 1 GeV.
For this analysis, we use the data collected in the following polar angle ranges: for the central track- ing chamber 40” < 0 < 140”; for the electromag- netic calorimeter 11” < 0 < 169”; and for the hadron calorimeter 5” < 0 < 175”.
3. Data selection
For this analysis, we use events collected at center of mass energies near the Z mass (88.4 I fi I 93.7 GeV) during the 199 l-1994 LEP running periods.
The data correspond to an integrated luminosity of 112 pb-
’ .
The selection of events of the type e+e- -+ hadrons is based on tracking information as well as on the en- ergy measured in the electromagnetic detector and in the hadron calorimeter. Events are accepted if they have high multiplicity and high and well balanced vis- ible energy [ 141. In total, 3.1 million events pass the selection cuts.
About 2.3 million hadronic events generated by the Monte Carlo program JETSET 7.3 [ 81 are used in the analysis. In addition, 430,000 events generated using HERWIG 5.4 [ 91 are used to check for systematic ef- fects due to different fragmentation models. The val- ues for the QCD scale and the fragmentation param-
eters in both Monte Carlo programs were determined from fits to our previous measurements [ 151. The gen- erated events are passed through the L3 detector sim- ulation [ 161, which implements a detailed description of the detector and takes into account the effects of en- ergy loss, multiple scattering, interactions and decays in the detector materials and the beam pipe. These events are processed with the same reconstruction and analysis programs as used for the experimental data.
Applying the same selection of hadronic Z decays to the simulated events as for the data, we find that more than 99% of the hadronic decays from the Z are ac- cepted. The contamination from e’e- and r+r- final states and from hadronic production via two-photon processes is estimated to be less than 0.2% and is ig- nored in the following analysis.
3.1. Photon selection
Photon candidates are recognized as isolated clus- ters in the electromagnetic calorimeter. The photon energy is calculated from the energy of the cluster by applying a position-dependent correction. Assum- ing that the photon originates at the e+e- interaction point, its direction is determined from the center of gravity of the shower.
The photons used in this analysis are required to be in the barrel region, ) cos 8,( < 0.74. Their energy has to be greater than 100 MeV. The lateral shower shape of the photon candidate has to be consistent with that of an electromagnetic shower, as determined by an artificial neural network [ 171 trained on a sample of Monte Carlo data. To further reduce the background from hadrons, the photon candidate must be separated by at least 50 mrad from any charged particle. The selection efficiency for inclusive photons in hadronic Z events is about 30% with a purity of about 70%.
3.2. Track selection
To have good resolution in the invariant mass, we apply relatively tight selection cuts to charged tracks.
Charged tracks have to be well measured in the central tracking chamber as well as the Z-chamber, with a momentum transverse to the beam axis of more than 100 MeV. Their distance of closest approach to the primary e+e- vertex in the R#J plane must be smaller than 2 mm. In addition, we exclude tracks passing
410 L3 Collaboration/Physics Letters B 393 (1997) 465-476
i3
h W-m+~-TcO
16000 t 12000 :
8000 :
4000 :
Ok’.‘.
11,’ l’,‘,l,,‘I’4
0.5 0.75 1 1.25 1.5 mn+-,p( GeV)
Fig. I. The measured r+r-r” invariant mass distribution. The solid line represents the result of the fit to the data using a sum of a Gaussian distribution and a background shape as described in the text. The dashed line indicates the fitted background.
through low resolution regions adjacent to the anode and cathode planes.
3.3. Extraction of the w signal
The w meson is measured in the decay channel w -+ rf7r-?ro. In a first step we reconstruct the n-O through its two-photon decay channel. In order to re- duce the combinatorial background, the yy invariant mass spectrum is determined from pairs of photons produced in the same hemisphere defined with respect to the event thrust axis. For the analysis, only & can- didates with a reconstructed mass within &24 MeV (f3g) of the nominal mass of 135.0 MeV are used.
The selected 9 candidates are then combined with two charged tracks assumed to be pions. To reduce the large combinatorial background, we subtract the spec- tra for like-sign combinations (rr*&?rO) from those for unlike-sign combinations (r+rr-d). As will be discussed below in the study of systematic errors, the final results are relatively insensitive to the procedures used to subtract the backgrounds.
The resulting ‘background-subtracted’ spectrum is shown in Fig. 1. An w signal is clearly visible. There is also a small but clear Q signal close to the kine- matic threshold in the mass spectrum. We fit the spec- trum with two Gaussian distributions for the w and r]
signal respectively, plus a background function which is described by either a fifth-order polynomial or the
following threshold function BG( M) = (M - M*)P’
. exp(p2M + pM2 + p4M3),
where Mth is the threshold invariant mass of the de- cay products. The threshold function is found to yield better x2 and is therefore taken, while the polynomial is used in evaluating systematic uncertainties. The fit gives 25260 & 1901 as the observed number of w mesons. The mass is compatible with the PDG [ 181 value and the width is 28 MeV, in agreement with the expected detector resolution.
3.4. Extraction of the 7’ signal
The 8’ is measured in its two dominant decay chan- nels, Q’ ---f 1~~7r-v and r]’ + p’y. For the decay channel 77’ --+ &7.r-v, the same procedure is applied as used for the extraction of the w signal in the w -+
rr+r-rr’ channel, except that the 77 is used instead of the r”. The 77 is reconstructed through its two-photon decay channel. In order to reduce the combinatorial background related to 1~’ decays, we exclude all pho- tons entering into a two-photon combination with in- variant mass inside the 1~’ window. The invariant mass window for 7 candidates is set to be 550 MeV (f3a) around the nominal mass of 547.5 MeV. Because of limited phase space, rrrr pairs from the q’ decay tend to have low invariant mass. To further reduce the com- binatorial background in combinations of ~nv, we require the invariant mass of rrr pairs to be smaller than 0.4 GeV. The resulting ‘background-subtracted’
spectrum is shown in Fig. 2a. From the fit, in which a fourth-order polynomial is used for the background, we obtain 2989 f 221 as the observed number of 7’
mesons. The mass is compatible with the PDG value and the width is 14 MeV, smaller than the measured width of the w due to the lower energy available in the decay 77’ -+ ~‘~-11.
For the decay channel 7’ + p’y, we first recon- struct the p” in the channel p” + &rTT-. All combina- tions of two unlike-sign tracks with an invariant mass in a window of 150 MeV around the p” nominal mass are used. To reduce the combinatorial background in combinations of rrry, a number of additional cuts are applied. The reconstructed direction of the p” and of the y must both be inside a cone around the direction
L3 Collaboration /Physics Letters B 393 (1997) 465-476 471 Table I
Measured production rate of o and 7’ mesons per hadronic Z decay. The measured values, obtained with JETSET and HERWIG Monte Carlo programs for the efficiency calculation, are reported separately. The average of the two is listed in the last column as the final result.
The first error is statistical and the second systematic.
Decay channel xP Range JETSET
meas. pred.
HERWIG
meas. pred.
Production rate measured 0 - rr+$r-flo 0.02 < x,, < 0.30 0.92 1.07 0.95 0.66 0.94 f 0.07 f 0.12
O<x,<I 1.15 1.33 I .20 0.83 1.17f0.09f0.1S
7’ -+ rrf?i-rJ 0.02 < XP < 0.30 0.20 0.54 0.23 0.20 0.21 f 0.02 * 0.04
O<X,<l 0.25 0.67 0.28 0.25 0.27 f 0.03 f 0.05
rl’ - P’? 0.01 < xP < 0.24 0.20 0.54 0.19 0.20 0.20 f 0.02 f 0.04
O<X,<l 0.25 0.67 0.24 0.25 0.24 & 0.02 f 0.05
600
OYL .l,,..l.,..‘.,..lA
1 1.2 1.4 1.6
‘““y , j
0.0 1.2
mx+i-Y (GeV)
1.4
Fig. 2. The measured (a) &P-T and (b) ~‘7 invariant mass distributions. The solid lines represent the results of the fits to the data using a sum of a Gaussian distribution and a background shape as described in the text. The dashed lines indicate the fitted background.
of the 7’ candidate, with a half opening angle vary- ing linearly with the v’ momentum p from 90” at p = 0 GeV to 30” at p = 8 GeV. The cosine of the an- gle 0* between the direction of the y with respect to the 7’ direction in the 7’ rest frame is required to be cos(8*) > -0.8. As the v’ is a spin 0 meson and p”
and y have spin 1, a helicity cut is applied. The cosine of the helicity angle 0** between the y and a pion in the in p” rest frame is required to be 1 cos( fil**) 1 <
0.8. In addition, it is required that the most energetic pion carries more than 20% of the energy of the pay system. Finally, we subtract the spectra for like-sign combinations (n*r*y) from those for unlike-sign combinations (&n--y). The resulting ‘background- subtracted’ spectra are shown in Fig. 2b. The residual background is fitted with a threshold function and the signal with a Gaussian function. From the fits, a total of 10812 & 910 71’ are observed in 0.01 < xy < 0.24.
The mass is compatible with the PDG value and the width is 19 MeV, consistent with the detector resolu- tion.
4. Inclusive production rates
The selection efficiency for the w and 7’ signal is determined from the Monte Carlo as the ratio of the
‘observed’ number of w (or v’ ) mesons to the num- ber of generated w (or v’ ) mesons. In this way, our results are independent of the Monte Carlo simulation of the background and of the width of the signal. The resulting production rates determined from the w and two 7’ decay channels are listed in Table 1 and com- pared with the Monte Carlo predictions. The rates are
472 L3 Collaboration/Physics Letters B 393 (1997) 465-476 Table 2
Contributions to the systematic errors of the measured w and 7’ production rates, added to the total in quadrature.
Source of systematic error
selection
detector efficiencies fitting
background subtraction Monte Carlo models Monte Carlo statistics total
On 0 rate w - ??-+??-7P 0.09
0.07 0.0s 0.05 0.03 0.07 0.1.5
On r)’ rate 9’ --+ ?rfrr--~
0.03 0.02 0.02 0.02 0.02 0.02 0.05
On 7j’ rate 7’ -+ POY 0.04 0.01 0.02 0.02 0.01 0.02 0.05
extrapolated to the full momentum range by means of the corresponding Monte Carlo predictions. Due to the different fragmentation schemes in the two mod- els, there is a difference between IETSET and HER- WIG in the isolation of photons. Therefore, the two Monte Carlo models give different selection efficien- cies. As both models otherwise provide a reasonable description of the data [ 151, we take the average of the production rates determined by the two models as our result, assigning half of the differences to the sys- tematic errors.
The first error in the last column of Table 1 is sta- tistical, the second one is systematic. Besides the sys- tematic error due to the choice of fragmentation mod- els, other errors are obtained by varying the photon se- lection cuts, switching off the requirement on shower isolation from charged particles and changing the ?r”
background suppression procedure. Errors due to the uncertainty in the detector efficiency are determined from a comparison of the # and 7 rate in differ- ent geometrical regions of the detector. An additional small error due to the uncertainty of the TEC and the Z-chamber efficiency is taken into account. As men- tioned above, two different functions have been used to describe the background shape in the fit. The func- tion leading to the better x2 has been taken while the other was used in evaluating systematic uncertainties.
For the decay channels w -+ &r-?r” and 77’ -+
&r-v, we also use the 7~’ and r] sideband subtrac- tion to estimate systematic effects in the background subtraction. In this case, yy combinations from a n-O (or 71) sideband, combined with a &rTT- pair, are used for the background subtraction. Finally, the sta- tistical error on the Monte Carlo events is included in
rn,, (GeV)
Fig. 3. The measured &‘y invariant mass distribution. The solid line represents the result of the fit to the data using a sum of a Gaussian distribution and a background shape as described in the text. The dashed line indicates the fitted background.
the systematic errors. The size of these contributions is summarized in Table 2.
From the procedure described above, the w rate per hadronic Z event is determined to be
N, = 1.17 f 0.17.
In an independent analysis, the w signal is studied via the decay channel w ---f n-‘y, whose branching ratio is a factor of 10 smaller than that of the decay channel w + n-+r-?ro. The invariant mass spectrum of r”r from 930,000 hadronic Z events collected in 199 1 and
1992 is shown in Fig. 3. We obtain 1.12f0.22( stat) w mesons per hadronic Z decay, which is in a good agree- ment with the result obtained with the decay channel w + r+rr-n-‘. The channel w -+ ?r”r is not used for
L3 Collaboration/Physics Letters B 393 (1997) 465-476 413 Table 3
Differential cross section for inclusive OJ production from o --t r+lr-?~~.
XP X/IV
0.02-0.04 0.030 0.04-0.06 0.049 0.06-0.08 0.069 0.08-O. 12 0.098 0.12-0.18 0.146 0.18-0.24 0.206 0.24-0.30 0.268
l/u,, daldx, I I .7 f 2.5
8.8 f 1.8 5.8 f 1.3 3.53 f 0.82 2.10f0.78 I .60 f 0.52 0.84 f 0.42
5P C%V
3.91-3.22 3.51 3.22-2.8 1 3.02
2X-2.53 2.65
2.53-2.12 2.28 2.12-1.72 I .90 1.72- 1.43 I.56 1.43- 1.20 I .30
t/flh..d/dep 0.34 f 0.07 0.43 f 0.09 0.40 f 0.09 0.35 f 0.08 0.31 i 0.12 0.33f0.11 0.23 f 0. I1
the final result. Within errors, the prediction of JET- SET for the w rate is consistent with our measured rate, while the rate predicted by HERWIG is too low.
In hadronic Z decays, the vector mesons w and p”
are expected to be predominantly produced by frag- mentation. Since these two mesons have the same quark content, the same spin and nearly equal mass, and thus differ only by their isospin, their production rate is expected to be equal. Using our measured pro- duction rate of w mesons and the ALEPH and DEL- PHI results for p” [ 19,3], N(p’) = 1.29 % 0.12, the ratio N(o)/N(pO) is determined to be 0.91 f 0.16.
This result is in good agreement with the ARGUS measurement [ 51 of N(o) /N( p”) = 0.91 f 0.20 and data from pp interactions [ 201 of N(w) /N(p’) =
1.02 f 0.08.
Averaging the results obtained from the two 7’ de- cay channels and taking into account the common sys- tematic errors (uncertainties in detector efficiencies and Monte Carlo models), the production rate of the 7)’ is determined to be
N,,, = 0.25 f 0.04,
which is consistent with the prediction of HERWIG, but much lower than 0.67, the value predicted by JET- SET 7.3.
Using our previously measured production rate of the7 [21] of0.91f0.02fO.ll,the$to~ratioisde-
termined to be 0.2760.06, which is significantly lower than the value of 0.55 from JETSET 7.3. In JETSET, the v and 7’ are described with the same strangeness content, which is the case if the value of the 7-v mix- ing angle 0~ is close to -10’. According to this de- scription, in the string fragmentation model, the v and 17’ would be produced essentially with the same rate from fragmentation. Several measurements [ 18,221,
however, indicate a value of BP around -20°, which assigns more strangeness to 7’ and less to v. Applying the strangeness suppression factor used in JETSET to the 7 and 7’ production according to their strangeness content, the ratio v’/v from JETSET would be re- duced by a factor of about 1.5, to 0.37, which is closer to our measured value. The ratio v’/~ could be also reduced by the production of mesons with orbital ex- citation, which favor strongly the q over the 7’ decay modes [ 181. Such mesons are not included in JET- SET 7.3, but might be produced with non-negligible production rates.
5. Differential cross sections
To determine the X~ distribution, the above mea- surement of the production rate is repeated for differ- ent xp intervals. Since the two Monte Carlo generators give similar efficiencies for both w and 7’ mesons, we only use the high statistics JETSET Monte Carlo sample to measure the differential cross sections.
The normalized w rate obtained in each xp interval is listed in Table 3, where the scaled momentum value, xl,,,, is the appropriate position [ 231 within the xy bin used to plot the differential cross section. JETSET is used to calculate the value of xlw (the result from HERWIG is essentially the same). In the same way we measure the momentum spectrum of 7’ mesons and the results are listed in Table 4 and Table 5. The xp spectrum for w and 7’ mesons is shown in Fig. 4a and Fig. 4b, respectively, together with the predictions of both models. Within errors, the shape of the measured w and 7’ spectra is consistent with the prediction of both models.
For comparison with analytical calculations per-
474 W Collaboration/Physics Letters B 393 (1997) 465-476 Table 4
Differential cross section for inclusive 7’ production from r]’ + rr+‘ir-q.
XP X/V I /Ch d CT/d xp
0.02-0.04 0.03 1 2.69 zt 0.80
0.04-0.06 0.050 1.82 ??0.54
0.06-0.08 0.070 1.31 f0.43
0.08-0.12 0.097 1.07 f 0.34
0.12-0.18 0.145 0.60 It 0.19
0.18-0.24 0.208 0.39 f 0.14
5P &w
3.91-3.22 3.54
3.22-2.8 1 2.99
2.81-2.53 2.67
2.53-2.12 2.36
2.12-1.72 1.88
1.72- 1.43 I .55
l/gh. daldtp
0.078 f 0.023 0.090 f 0.027 0.092 f 0.030 0.105 f 0.033 0.089 f 0.028 0.080 zk 0.029
Table 5
Differential cross section for inclusive t7’ production from ?I’ + p”y.
XP X/W
0.01-0.04 0.028
0.04-0.08 0.057
0.08-O. 12 0.097
0.12-0.16 0.138
0.16-0.24 0.198
~/~,,‘dd/dxp 2.32 * 0.67 1.56 z!c 0.48 0.89 f 0.29 0.72 f 0.30 0.40 zt 0.12
61) &jl” 1 /(T,, d U/d c.fp
4.61-3.22 3.59 0.050 f 0.014
3.22-2.53 2.86 0.090 f 0.028
2.53-2.12 2.36 0.087 f 0.028
2.12-1.83 1.96 0.099 f 0.041
1.83-1.43 1.60 0.080 f 0.024
Table 6
Differential cross section for inclusive 7’ production, obtained by combining the results from 7’ -+ rr+rr-v and 7’ * pay.
*in 1 /uh d u/d X,, 0.030 2.47 f 0.57 0.059 1.53 * 0.30 0.097 0.96 k 0.23 0.142 0.63 f 0.18 0.203 0.40 * 0.10
h,, 1 /CT,, d u/d tp 3.57 0.058 zt 0.013 2.84 0.091 & 0.018 2.36 0.095 zt 0.023 1.92 0.092 f 0.025 1.58 0.080 f 0.020
Table 7
Measured parameters of QCD MLLA corresponding to EJ.q. ( 1).
6,’ denotes the position of the maximum corresponding to the given value of &a.
Meson N Aelt (GeW
w 0.127 rt 0.019 0.88 f 0.24 2.86 f 0.20
?I’ 0.033 f 0.005 1.14ztO.16 2.69 f 0.10
formed in the framework of the modified leading log approximation (MLLA) of QCD, we use the variable 8)) = ln(l/+). The tP values and the corresponding measured normalized rates are listed in Table 3, Ta- ble 4 and Table 5. The value hw referred to in the tables is calculated in the same way as the value ~1,.
The combined results from the two v’ decay channels
are listed in Table 6. The measured distributions for w and v’ are shown in Fig. 5a and Fig. 5b, respec- tively, where for the v’, the combined results from the two 7’ decay channels are shown. Following the same method used in our previous publications [ 2 1,241, we fit the measured tP spectra with the MLLA expression for the limiting spectrum:
where @h&j is the total hadronic cross section, and the function f is specified in Ref. [ lo]. Eq. ( 1) contains only two free parameters: an overall normalization fac- tor N (which describes the hadronization and depends on the center of mass energy fi and on the particle type) and an effective scale parameter h,rr. Eq. ( 1) is valid in the range 1 < (/, < In (0.5 fi/h,~) . The fit results are given in Table 7, where the errors of the parameters include also the uncertainty due to a vari- ation of the range of data points included in the fit.
6. Summary and conclusions
We have studied the production of w and r]’ from hadronic Z decays. The production rate of the w is determined to be 1.17 & 0.17, in agreement with the
W Collaboration/Physics Letters B 393 (1997) 465-476 415
- JETSET 7.3 --. HERWIG 5.4
xP
0
I_
L3 . DATA ~,‘-m+n~,, 0 DATA q’+p’y - JETSET 7.3 -.. HERWIG 5.4
0.1 0.2 0.:
xP
I
Fig. 4. The x,, distribution of the (a) w and (b) 7’ at the Z resonance normalized to the total hadronic cross section in comparison with the predictions of JETSET and HERWIG. The hin ranges are given in Tables 3-5.
JETSET prediction, but higher than the HERWIG pre- diction. The measured production rate of the 77’ and the ratio of production rates v//r] are measured to be 0.25 f 0.04 and 0.27 f 0.06 respectively, significantly lower than the predictions of JETSET 7.3. The low value of the ratio $/T could be explained partially by the description of the v and 71’ quark content used in JETSET. The prediction of HERWIG for the produc- tion rate of the 9’ is consistent with the measurement.
The measured w and 7’ production rates are consis- tent with the ALEPH results [ 3,4].
The shape of the inclusive momentum spectrum of w and v’ is in agreement with the predictions of both Monte Carlo models. The differential cross sections as a function of cp can be described by analytical QCD calculations.
u.F
0.5
9
:
:
- OCD(M 0 0.4 :
-0 f 0.3 : 2 - 0.2 :
0.1 :
Oo 1 2 F3 4 5
LI :
%f 0.12 r
- CCD(MLLA)
B 0.1 B d 0.08 - 0.08
~:~~, ,
‘Y,, 1
1
2jP3 4 5
Fig. 5. The t,, distribution of the (a) o and (b) 7’ at the 2 resonance normalized to the total hadronic cross section, compared with analytical QCD calculations in the range of validity of F.q.
( 1). In Fig. 5b the results from the two 7’ decay channels are combined. The bin ranges are given in Tables 3 and 6.
Acknowledgments
We wish to express our gratitude to the CERN ac- celerator divisions for the excellent performance of the LEP machine. We acknowledge the contributions of all the engineers and technicians who have partici- pated in the construction and maintenance of this ex- periment. Those of us who are not from member states thank CERN for its hospitality and help.
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