Article
Reference
Λ and Σ
0pair production in two-photon collisions at LEP
L3 Collaboration
ACHARD, Pablo (Collab.), et al.
Abstract
Strange baryon pair production in two-photon collisions is studied with the L3 detector at LEP.
The analysis is based on data collected at e+e− centre-of-mass energies from 91 GeV to 208 GeV, corresponding to an integrated luminosity of 844 pb−1. The processes γγ→ΛΛ and γγ→Σ0Σ0are identified. Their cross sections as a function of the γγ centre-of-mass energy are measured and results are compared to predictions of the quark–diquark model.
L3 Collaboration, ACHARD, Pablo (Collab.), et al . Λ and Σ
0pair production in two-photon collisions at LEP. Physics Letters. B , 2002, vol. 536, no. 1-2, p. 24-33
DOI : 10.1016/S0370-2693(02)01781-1
Available at:
http://archive-ouverte.unige.ch/unige:43267
Disclaimer: layout of this document may differ from the published version.
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Physics Letters B 536 (2002) 24–33
www.elsevier.com/locate/npe
and 0 pair production in two-photon collisions at LEP
L3 Collaboration
P. Achard
t, O. Adriani
q, M. Aguilar-Benitez
x, J. Alcaraz
x,r, G. Alemanni
v, J. Allaby
r, A. Aloisio
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u, G. Grenier
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0370-2693/02/$ – see front matter 2002 Published by Elsevier Science B.V.
PII: S 0 3 7 0 - 2 6 9 3 ( 0 2 ) 0 1 7 8 1 - 1
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ae, H.L. Zhuang
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at, M. Zöller
aaI. Physikalisches Institut, RWTH, D-52056 Aachen, FRG III. Physikalisches Institut, RWTH, D-52056 Aachen, FRG1
bNational Institute for High Energy Physics, NIKHEF, and University of Amsterdam, NL-1009 DB Amsterdam, The Netherlands cUniversity of Michigan, Ann Arbor, MI 48109, USA
26 L3 Collaboration / Physics Letters B 536 (2002) 24–33
dLaboratoire d’Annecy-le-Vieux de Physique des Particules, LAPP, IN2P3-CNRS, BP 110, F-74941 Annecy-le-Vieux CEDEX, France eInstitute of Physics, University of Basel, CH-4056 Basel, Switzerland
fLouisiana State University, Baton Rouge, LA 70803, USA gInstitute of High Energy Physics, IHEP, 100039 Beijing, PR China6
hHumboldt University, D-10099 Berlin, FRG1
iUniversity of Bologna and INFN, Sezione di Bologna, I-40126 Bologna, Italy jTata Institute of Fundamental Research, Mumbai (Bombay) 400 005, India
kNortheastern University, Boston, MA 02115, USA
lInstitute of Atomic Physics and University of Bucharest, R-76900 Bucharest, Romania
mCentral Research Institute for Physics of the Hungarian Academy of Sciences, H-1525 Budapest 114, Hungary2 nMassachusetts Institute of Technology, Cambridge, MA 02139, USA
oPanjab University, Chandigarh 160 014, India pKLTE-ATOMKI, H-4010 Debrecen, Hungary3
qINFN, Sezione di Firenze and University of Florence, I-50125 Florence, Italy rEuropean Laboratory for Particle Physics, CERN, CH-1211 Geneva 23, Switzerland
sWorld Laboratory, FBLJA Project, CH-1211 Geneva 23, Switzerland tUniversity of Geneva, CH-1211 Geneva 4, Switzerland
uChinese University of Science and Technology, USTC, Hefei, Anhui 230 029, PR China6 vUniversity of Lausanne, CH-1015 Lausanne, Switzerland
wInstitut de Physique Nucléaire de Lyon, IN2P3-CNRS, Université Claude Bernard, F-69622 Villeurbanne, France xCentro de Investigaciones Energéticas, Medioambientales y Tecnológicas, CIEMAT, E-28040 Madrid, Spain4
yFlorida Institute of Technology, Melbourne, FL 32901, USA zINFN, Sezione di Milano, I-20133 Milan, Italy
aaInstitute of Theoretical and Experimental Physics, ITEP, Moscow, Russia abINFN, Sezione di Napoli and University of Naples, I-80125 Naples, Italy
acDepartment of Physics, University of Cyprus, Nicosia, Cyprus adUniversity of Nijmegen and NIKHEF, NL-6525 ED Nijmegen, The Netherlands
aeCalifornia Institute of Technology, Pasadena, CA 91125, USA
afINFN, Sezione di Perugia and Università Degli Studi di Perugia, I-06100 Perugia, Italy agNuclear Physics Institute, St. Petersburg, Russia
ahCarnegie Mellon University, Pittsburgh, PA 15213, USA aiINFN, Sezione di Napoli and University of Potenza, I-85100 Potenza, Italy
ajPrinceton University, Princeton, NJ 08544, USA akUniversity of California, Riverside, CA 92521, USA
alINFN, Sezione di Roma and University of Rome “La Sapienza”, I-00185 Rome, Italy amUniversity and INFN, Salerno, I-84100 Salerno, Italy
anUniversity of California, San Diego, CA 92093, USA
aoBulgarian Academy of Sciences, Central Lab. of Mechatronics and Instrumentation, BU-1113 Sofia, Bulgaria apThe Center for High Energy Physics, Kyungpook National University, 702-701 Taegu, Republic of Korea
aqPurdue University, West Lafayette, IN 47907, USA arPaul Scherrer Institut, PSI, CH-5232 Villigen, Switzerland
asDESY, D-15738 Zeuthen, FRG
atEidgenössische Technische Hochschule, ETH Zürich, CH-8093 Zürich, Switzerland auUniversity of Hamburg, D-22761 Hamburg, FRG
avNational Central University, Chung-Li, Taiwan, ROC awDepartment of Physics, National Tsing Hua University, Taiwan, ROC
Received 6 February 2002; received in revised form 12 April 2002; accepted 18 April 2002
Editor: L. Rolandi
Abstract
Strange baryon pair production in two-photon collisions is studied with the L3 detector at LEP. The analysis is based on data collected at e+e−centre-of-mass energies from 91 GeV to 208 GeV, corresponding to an integrated luminosity of 844 pb−1. The processesγ γ→¯ andγ γ→00are identified. Their cross sections as a function of theγ γcentre-of-mass energy are measured and results are compared to predictions of the quark–diquark model.2002 Published by Elsevier Science B.V.
1. Introduction
Electron–positron colliders are a suitable place for the study of two-photon interactions, via the process e+e−→e+e−γ∗γ∗→e+e−X, where γ∗ denotes a virtual photon. The outgoing electron and positron carry almost the full beam energy and are usually undetected, due to their small transverse momenta.
The final state X has, therefore, a low mass relative to the e+e− centre-of-mass energy, √
s. The small photon virtuality allows the extraction of the cross sectionσ (γ γ →X)in real photon collisions, once the photon flux is calculated by QED [1].
The processγ γ →baryon–antibaryon is sensitive to the quark structure of the baryon. Calculations of the cross section for this process were performed using the hard scattering approach [2]. Due to the failure of a three-quark calculation [3] to correctly predict the γ γ → pp cross section in the GeV region [4],¯ an alternative quark–diquark model was proposed [5].
This model includes non-perturbative effects through the use of diquarks, a qq bound state within the baryon [6].
In this Letter we present the first measurements at LEP of the cross sectionsσ (γ γ →)¯ andσ (γ γ→ 00). We analysed a total integrated luminosity of
1 Supported by the German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie.
2 Supported by the Hungarian OTKA fund under contract numbers T019181, F023259 and T024011.
3 Also supported by the Hungarian OTKA fund under contract number T026178.
4 Supported also by the Comisión Interministerial de Ciencia y Tecnología.
5 Also supported by CONICET and Universidad Nacional de La Plata, CC 67, 1900 La Plata, Argentina.
6 Supported by the National Natural Science Foundation of China.
844 pb−1 collected with the L3 detector [7]. Out of this sample, 157 pb−1 were collected around the Z peak and 687 pb−1 at centre-of-mass energies from 161 GeV to 208 GeV. The analysis is based on the central tracking system and the high resolution BGO electromagnetic calorimeter. The events are selected by the track triggers [8].
Monte Carlo events are generated [9] for each beam energy and for each process within the formalism of Ref. [1]. An uniform spectrum as a function of the two-photon mass, Wγ γ, from threshold to 5 GeV is used. The two-body final states ¯ and 00 are generated isotropically in the centre-of-mass of the γ γ system. The events are then passed through the full L3 detector simulation using the GEANT [10]
and GEISHA [11] programs and are reconstructed following the same procedure as for the data. Time dependent detector inefficiencies, as monitored during the data taking period, are taken into account.
The CLEO, TPC/2γ and VENUS Collaborations [12–14] searched for the reaction e+e−→e+e−¯ at √
s =10.6 GeV, 14.5 GeV and 58 GeV, respec- tively. Only CLEO and TPC/2γ observe a signal. No results for the e+e−→e+e−00cross section were reported so far. Our results are compared to these ex- periments and to theoretical predictions of the quark–
diquark model.
2. Event selection
In order to study the ¯ and 00 final states, the0→γ,0→ ¯γ,→pπ−and¯ → ¯pπ+ decays are considered. The preselection of events is based on charged tracks and proceeds as follows:
• There must be four charged tracks in the tracking chamber with a net charge of zero. These tracks
28 L3 Collaboration / Physics Letters B 536 (2002) 24–33
must be reconstructed from at least 12 hits out of a maximum of 62.
• There must be two secondary vertices at a distance from the primary interaction vertex greater than 3 mm in the transverse plane.
• The angle in the transverse plane between the flight direction of eachcandidate and the total momentum vector of the two outgoing tracks must be less than 0.15 rad.
• Events with a secondary vertex due to a photon conversion are rejected. A conversion is identified if by assigning the electron mass to the two tracks, their effective mass is below 0.05 GeV.
2.1. identification
The two secondary vertices are assigned to the and ¯ decays. At each vertex, the p or the p are¯ identified as the highest momentum tracks. Monte Carlo studies show that this is the correct configuration in more than 99% of the cases. To suppress the dominant e+e−→e+e−K0sK0s →e+e−π+π−π+π− background, the following criteria are applied:
• ThedE/dxmeasurement must be consistent with theor¯ hypotheses. A confidence level CL>
0.001 is required for both proton, antiproton and pions candidates. This cut rejects 85% of the K0sK0s background.
• The particle assignment is considered to be correct if either,
(a) there is at least one of the tracks associated to a proton or to an antiproton with more than 30 hits and a dE/dx confidence level ratio CL(p)/CL(π ) >10, or
(b) the ratio between the electromagnetic trans- verse energy,ET, and the transverse momen- tum,pT, of the antiproton candidate is greater than 0.7. This cut eliminates 70% of the pi- ons and keeps 77% of the antiproton signal, as shown in Fig. 1.
The dE/dx identification has a high discrimi- nating power for particles with momentum be- low 700 MeV, whereas theET/pT cut becomes more efficient for higher momentum antiprotons.
These criteria suppress 83% of the remaining K0sK0s background.
Fig. 1. Distribution of the ratio of the transverse energy deposited in the electromagnetic calorimeter,ET, and the transverse momentum, pT, for pions and antiprotons. Theπ−data distribution is obtained from a high purity K0sK0s sample [16]. Theπ−Monte Carlo dis- tribution corresponds to a simulated e+e−→e+e−K0sK0s sample normalized to the number of K0sK0s observed in data [16]. The an- tiproton distribution is obtained from simulated e+e−→e+e−¯ events, with arbitrary normalization.
• If two K0s candidates are reconstructed, the event is rejected. A K0s candidate is defined as a sys- tem with a reconstructed π+π− mass within a
±30 MeV interval around the nominal K0s mass.
Only 1% of the original K0sK0s background re- mains after this cut.
In addition to the previous requirements, a cut
|cosθ∗|<0.6 is applied, whereθ∗is the polar angle of the in the two-photon centre-of-mass system, to match the experimental acceptance with the range of the theoretical predictions. Cleanand¯ signals are observed in the distributions of the pπ−andpπ¯ + masses, presented in Figs. 2a and b. The and ¯ masses are found to bem=1.113±0.006 GeV and m¯ =1.115±0.006 GeV, respectively, in agreement with the nominal value of 1.116 GeV [15]. The final
Fig. 2. Effective mass distribution of the (a) pπ−system and (b)pπ¯ +system. The two-dimensional distribution is shown in (c) and (d). A radius of 40 MeV around the nominal mass value ofmdefines the inclusive¯ sample.
sample contains 66 inclusive ¯ candidates. They are selected within a radius of 40 MeV around the nominalmass in the plane of the effective masses m(pπ−)vs m(pπ¯ +), shown in Figs. 2c and d. The remaining K0sK0s contamination is estimated by Monte Carlo simulation to be less than 1%. The normalisation of the K0sK0s Monte Carlo background is determined from our previous measurement of this channel [16].
Within the available statistics, the hypothesis of an isotropic distribution of the¯ signal is verified.
2.2. 0identification
The reconstruction of 0 and 0 candidates is performed by combining the selected and¯ with photon candidates. A photon candidate is defined as a shower in the electromagnetic calorimeter with at least two adjacent crystals and an energy between 50 MeV and 200 MeV. Monte Carlo studies show that
91% of the photons emitted by a0 have an energy below 200 MeV, which is compatible with the nominal 0 mass of 1.193 GeV. Photon isolation criteria are also applied; there must be no charged tracks within 200 mrad around the photon direction and the cosine of the angle between the antiproton and the photon directions must be less than 0.8. To identify the 0, the mass differencem=m(pπ γ )−m(pπ )is used, as presented in Fig. 3. A0candidate corresponds to the mass interval 47 MeV< m <107 MeV. Out of the 66 selected¯ events, 31 have0candidates.
3. Exclusive¯ and00identification
In order to select the events from the exclusive reactionsγ γ →¯ andγ γ →00, the transverse momentum of the four charged particles, PT, is required to be less than 0.5 GeV. This cut rejects
30 L3 Collaboration / Physics Letters B 536 (2002) 24–33
Fig. 3. Distribution of the mass difference between theγ(pπ γ) and the(pπ) systems. All possible combinations of a photon and aor¯ candidate are shown.
Table 1
Integrated luminosity, overall efficiency and number of selected e+e−→e+e−¯ and e+e−→e+e−00events for each data taking period. The efficiency refers to the phase-space cuts: 2.23<
m¯ <3.5 GeV, |cosθ∗|<0.6 andPT <0.5 GeV. The quoted uncertainties are statistical
√s(GeV) Luminosity (pb−1) Efficiency (%) Events
91 157 2.38±0.11 8
161–172 20 1.72±0.07 3
183 52 1.97±0.09 1
189 172 1.78±0.07 3
192–202 230 1.94±0.07 10
205–208 213 1.75±0.07 8
events containing contributions from other final states such as00or0(1385)0(1385). Some of these states can still pass thePT cut, but their contribution to the final sample is negligible, given the magnitude of their cross sections [6]. ThePT requirement rejects less than 8% of events corresponding to the exclusive final states¯ and00. Since the photons emitted by the0 candidates have a low energy, they give a small contribution to the total transverse momentum
Fig. 4. A typicalγ γ→¯event, displayed in the transverse plane.
It illustrates the higher momentum of the proton and antiproton in theand¯ decays and the separation in the electromagnetic calorimeter between the large antiproton signal and the small energy deposit of pions and protons.
imbalance. The final sample contains 33 events. The numbers of selected events for the different e+e− centre-of-mass energies are listed in Table 1. A typical ¯ event is shown in Fig. 4.
The relative proportions of ¯ and 00 final states in the sample are determined as follows. The event is labelled as 00-like if a 0 or a 0 candidate is observed and as-like otherwise. With¯ these criteria, 19 -like and 14¯ 00-like events are found in the data. The true fractions rj (j = , ¯ 00)of the two components are determined by a maximum extended likelihood fit with the constraint r¯ +r00 = 1. The likelihood function to be maximized is:
L=nNtte−nt Nt!
i
nNiie−ni Ni! ,
whereNt andnt correspond, respectively, to the total number of observed and expected events, andNi and ni to the number of observed and expected i-like
Table 2
Numbers of observed eventsNiidentified as-like and¯ 00- like and relative probabilitiespij of identifying an event generated in the final statejasi-like
Identified as Ni Selection probabilitypij(%) Generated as¯ Generated as00 -like¯ 19 88.0±0.8 39.1±1.0
00-like 14 12.0±0.8 60.9±1.0
Table 3
Results of the fit for the fractionsrj and the numberntrj of¯ and00final states
Final state Fractionrj Eventsntrj
¯ 0.38±0.18 12.5±6.1
00 0.62±0.18 20.5±6.5
events. The latter is given by:
ni=
j
pijrj
nt,
where pij is the probability of identifying an event corresponding to the final statejasi-like. The relative probabilities pij are determined by Monte Carlo and shown in Table 2 together with their statistical uncertainties. The fractions rj and the number of eventsntrj obtained by the fit are given in Table 3.
The cross sections for theγ γ →0 andγ γ → 0¯ processes are predicted to be negligible com- pared to the other channels [6]. In order to test this assumption, also an analysis with the three compo- nents,¯ 0+0¯ and00is carried out. The 0+0¯ fraction is measured to be compatible with zero within a large uncertainty.
4. Results
The production cross sectionsσ (e+e−→e+e−)¯ andσ (e+e−→e+e−00)are measured as a func- tion of the centre-of-mass energy. They refer to the following phase-space cuts: the effective mass of the ¯ pair,m¯, less than 3.5 GeV,|cosθ∗|<0.6 and PT <0.5 GeV. In the cross section determination it is assumed that the fractionsri are independent of√
s.
The results are summarised in Table 4.
Table 4
The e+e−→e+e−¯ and e+e−→e+e−00 cross sections for 2.23< m¯ <3.5 GeV,|cosθ∗|<0.6 andPT <0.5 GeV. The first uncertainty is statistical, the second systematic
√s σ (e+e−→e+e−)¯ σ (e+e−→e+e−00)
(GeV) (pb) (pb)
91 0.81±0.48±0.07 1.33±0.61±0.12 161–208 0.75±0.39±0.07 1.23±0.43±0.11
Table 5
Average acceptance of the different selection cuts used in the analy- sis. The overall efficiency takes also into account the acceptance of the trigger system (80%)
Cut Acceptance (%)
Four tracks 10
Two secondary vertices 55
Photon conversion rejection 93
dE/dxcompatibility 98
dE/dxorET/pT 77
K0sK0srejection 92
|cosθ∗| 83
vs¯ mass 94
PT 92
Total 2.5
The detection efficiency is determined by Monte Carlo for each data taking period. It takes into account the →pπ branching ratio and track geometrical acceptance (6%), the baryon identification criteria (26%) and the track trigger efficiency (10%). The efficiency of higher level triggers (90%) is estimated from the data themselves, using prescaled events.
The contribution of the different selection cuts to the detection efficiency is detailed in Table 5. The total efficiencies for each data set are listed in Table 1.
The dominant source of systematic uncertainty is the selection procedure (7%); other sources are the fi- nite Monte Carlo statistics (5%) and the determination of the trigger efficiency (3%). The Monte Carlo contri- bution includes the uncertainty on thepij probabilities used in the determination of the fractionsri.
The cross sections σ (γ γ →)¯ and σ (γ γ → 00) in real photon collisions are extracted as a function ofWγ γ by deconvoluting the two-photon lu- minosity function and the form factor [17]. They are presented in Table 6. For the γ γ →00 case, the number of selected events as a function of Wγ γ is obtained from the corresponding m¯ distribution,
32 L3 Collaboration / Physics Letters B 536 (2002) 24–33
Table 6
Theγ γ→¯ andγ γ→00cross sections as a function ofWγ γfor|cosθ∗|<0.6 andPT <0.5 GeV. The central valueWγ γof each bin corresponds to an average according to aW−8distribution. The first uncertainty is statistical, the second systematic
Wγ γ(GeV) Wγ γ(GeV) σ (γ γ→)¯ (pb) Wγ γ(GeV) Wγ γ(GeV) σ (γ γ→00)(pb)
2.2–2.5 2.34 226±111±23 2.4–2.7 2.51 369±116±41
2.5–2.7 2.59 92±45±10 2.7–2.9 2.77 151±48±17
2.7–3.1 2.86 44±22±5 2.9–3.3 3.06 72±23±8
3.1–3.5 3.27 18±9±2 3.3–3.8 3.50 30±9±3
Fig. 5. Measurements of theγ γ→¯andγ γ→00cross sections as a function ofWγ γ. In (a) theγ γ→¯ cross section is compared to the one obtained by CLEO[4]. The dashed line shows the power law fit described in the text. In (b) and (c) theσ (γ γ →)¯ and σ (γ γ→00)measurements are compared to the calculations of Ref. [6]. Statistical and systematic uncertainties are added in quadrature.
within a 4.0% uncertainty. The efficiencies and lumi- nosity functions are evaluated for eachWγ γ interval and centre-of-mass energy. The efficiencies increase withWγ γ reflecting the expected rise in the detector acceptance. The trigger and track identification effi- ciencies do not depend onWγ γ. An additional system- atic uncertainty of 5%, due to the choice of the photon form factor, is included.
Fig. 5a compares the presentσ (γ γ →)¯ mea- surement with that of CLEO. The mass dependence of CLEO is steeper than the one we observe. Our data, fitted with a function of the formσ ∝W−n, gives a value n=7.6±3.9. The quark–diquark model pre- dicts n=6, and a three quark model n=10 [18].
In Figs. 5b and c, theγ γ →¯ andγ γ →00 cross section measurements are compared to the pre- dictions of recent quark–diquark model calculations [6]. This model considers three different distribution amplitudes (DA) for the diquarks. The absolute predic-
tions using the standard distribution amplitude (Stan- dard DA) reproduce well our data. The asymptotic DA [19] and DZ-DA [20] models are excluded.
Acknowledgements
We thank C.F. Berger and W. Schweiger for very useful discussions and for providing us their theoreti- cal predictions.
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