Article
Reference
Inclusive Lambda production in two-photon collisions at LEP
L3 Collaboration
ACHARD, Pablo (Collab.), et al.
Abstract
The reactions e+e−→e+e−ΛX and e+e−→e+e−ΛX are studied using data collected at LEP with the L3 detector at centre-of-mass energies between 189 and 209 GeV. Inclusive differential cross sections are measured as a function of the lambda transverse momentum, pt, and pseudo-rapidity, η, in the ranges 0.4 GeV
L3 Collaboration, ACHARD, Pablo (Collab.), et al . Inclusive Lambda production in two-photon collisions at LEP. Physics Letters. B , 2004, vol. 586, no. 3-4, p. 140-150
DOI : 10.1016/j.physletb.2004.02.030
Available at:
http://archive-ouverte.unige.ch/unige:42591
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Inclusive Lambda production in two-photon collisions at LEP
L3 Collaboration
P. Achard
t, O. Adriani
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0370-2693/$ – see front matter 2004 Published by Elsevier B.V.
doi:10.1016/j.physletb.2004.02.030
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aaIII. Physikalisches Institut, RWTH, D-52056 Aachen, Germany1
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
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, China6 hUniversity of Bologna and INFN, Sezione di Bologna, I-40126 Bologna, Italy
iTata Institute of Fundamental Research, Mumbai (Bombay) 400 005, India jNortheastern University, Boston, MA 02115, USA
kInstitute of Atomic Physics and University of Bucharest, R-76900 Bucharest, Romania
lCentral Research Institute for Physics of the Hungarian Academy of Sciences, H-1525 Budapest 114, Hungary2 mMassachusetts Institute of Technology, Cambridge, MA 02139, USA
nPanjab University, Chandigarh 160 014, India oKLTE-ATOMKI, H-4010 Debrecen, Hungary3
pDepartment of Experimental Physics, University College Dublin, Belfield, Dublin 4, Ireland 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, 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 Californa, 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 Laboratory 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, Germany
atEidgenössische Technische Hochschule, ETH Zürich, CH-8093 Zürich, Switzerland auUniversity of Hamburg, D-22761 Hamburg, Germany
avNational Central University, Chung-Li, Taiwan, ROC awDepartment of Physics, National Tsing Hua University, Taiwan, ROC
Received 9 December 2003; received in revised form 11 February 2004; accepted 13 February 2004 Editor: L. Rolandi
Abstract
The reactions e+e−→e+e−X and e+e−→e+e−X are studied using data collected at LEP with the L3 detector at¯ centre-of-mass energies between 189 and 209 GeV. Inclusive differential cross sections are measured as a function of the lambda transverse momentum,pt, and pseudo-rapidity,η, in the ranges 0.4 GeV< pt<2.5 GeV and|η|<1.2. The data are compared to Monte Carlo predictions. The differential cross section as a function ofptis well described by an exponential of the formAexp(−pt/pt).
2004 Published by Elsevier B.V.
1. Introduction
Two-photon collisions are the main source of had- ron production in high-energy e+e− interactions at LEP via the process e+e− →e+e−γ∗γ∗→ e+e− hadrons. The outgoing electron and positron carry al- most the full beam energy and their transverse mo- menta are usually so small that they escape, unde- tected, along the beam pipe. In this process, the neg- ative four-momentum squared of the photons, Q2, has an average valueQ2 0.2 GeV2 and they can therefore be considered as quasi-real. In the Vector Dominance Model (VDM), each photon can trans- form into a vector meson with the same quantum num- bers, thus initiating a strong interaction process with characteristics similar to hadron–hadron interactions.
This process dominates in the “soft” interaction re- gion, where hadrons are produced with a low trans- verse momentum,pt. Hadrons with highpt may also be produced by the QED process γ γ →qq (direct¯ process) or by QCD processes originating from the partonic content of the photon (resolved processes).
The L3 Collaboration has previously studied inclu- siveπ0, K0S[1] and charged hadron [2] production in quasi-real two-photon collisions. In this Letter, the in- clusiveand¯ production7from quasi-real photons is studied for a centre-of-mass energy of the two inter- acting photons,Wγ γ, greater than 5 GeV. The results are expressed in bins of transverse momentum, pt, and pseudo-rapidity,η, in the ranges 0.4 GeV< pt<
2.5 GeV and|η|<1.2. Theηrange is further divided in two subsamples with 0.4 GeV< pt1 GeV and 1 GeV< pt <2.5 GeV. The data sample corresponds to a total integrated luminosity of 610 pb−1collected
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 T037350.
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.
7 If not mentioned otherwise, the symbolrefers to bothand , as both charge-conjugate final states are analysed.¯
with the L3 detector [3], at e+e−centre-of-mass ener- gies√
s=189–209 GeV, with a luminosity weighted average value of√
s=198 GeV. Results on inclusive production for a smaller data sample at lower√
s were previously reported by the TOPAZ Collaboration in the range 0.75 GeV< pt <2.75 GeV [4]. The H1 Collaboration investigated the photoproduction at HERA [5].
2. Monte Carlo simulation
The process e+e−→e+e− hadrons is modelled with the PYTHIA [6] and PHOJET [7] event gener- ators with two times more luminosity than the data.
Both generators include the VDM, direct and resolved processes. In PYTHIA, including these processes for each photon leads to six classes of interactions.
A smooth transition between these classes is then ob- tained by introducing a transverse momentum scale.
The two-photon luminosity function is based on the EPA approximation [8] with a cut at the mass of the rho meson. The SaS-1D parametrisation is used for the photon structure [9]. PHOJET relies on the Dual Parton Model [10], with a soft and a hard component.
The two-photon luminosity functions is calculated in the formalism of Ref. [8]. The leading-order GRV parametrisation is used for the photon structure [11].
For both programs, matrix elements are calculated at the leading order and higher-order terms are simu- lated by parton shower in the leading-log approxima- tion. The fragmentation is performed using the Lund string fragmentation scheme as implemented in JET- SET [6], which is also used to simulate the hadroni- sation process. The strangeness suppression factor in JETSET is chosen as 0.3, while a valueαS(mZ)=0.12 is used for the strong coupling constant.
From a study of Monte Carlo events, the hard com- ponent is found to be larger than the soft component.
Their ratio goes from around two at low values ofpt to around three at high values ofpt.
The following Monte Carlo generators are used to simulate the background processes: KK2f [12] for the annihilation process e+e−→qq(γ ); KORALZ [13]¯ for e+e−→τ+τ−(γ ); KORALW [14] for e+e−→ W+W− and DIAG36 [15] for e+e−→e+e−τ+τ−. The response of the L3 detector is simulated using the GEANT [16] and GHEISHA [17] programs. Time-
dependent detector inefficiencies, as monitored dur- ing each data taking period, are included in the sim- ulations. All simulated events are passed through the same reconstruction program as the data.
3. Event selection
Two-photon interaction events are mainly collected by the track triggers [18], with a low transverse mo- mentum threshold of about 150 MeV, and the calori- metric energy trigger [19]. The selection of e+e−→ e+e− hadrons events [20] is based on information from the central tracking detectors and the electromag- netic and hadronic calorimeters. It consists of:
• A multiplicity cut. To select hadronic final states, at least six objects must be detected, where an object can be a track satisfying minimal quality requirements or an isolated calorimetric cluster of energy greater than 100 MeV.
• Energy cuts. The total energy deposited in the calorimeters must be less than 40% of √
s to suppress events from the e+e− → qq(γ )¯ and e+e− → τ+τ−(γ ) processes. In addition, the total energy in the electromagnetic calorimeter is required to be greater than 500 MeV in order to suppress beam-gas and beam-wall interactions and less than 50 GeV to remove events from the annihilation process e+e−→qq(γ ).¯
• An anti-tag condition. Events with a cluster in the luminosity monitor, which covers the angular re- gion 31 mrad< θ <62 mrad, with an electro- magnetic shower shape and energy greater than 30 GeV are excluded from the analysis. In ad- dition, events with an electron scattered above 62 mrad are rejected by the energy cuts.
• A mass cut. The invariant mass of all visible particles,Wvis, must be greater than 5 GeV. In this calculation, the pion mass is attributed to tracks while isolated electromagnetic clusters are treated as massless. The distribution ofWvisfor data and Monte Carlo after all other cuts are applied is shown in Fig. 1. Values of Wvis up to 200 GeV are accessible.
About 3 million hadronic events are selected by these criteria with an overall efficiency of 45%. The
Fig. 1. The distribution of the visible mass Wvis. The Monte Carlo distributions are normalized to the data luminosity. Various contributions to the background (bkg) are shown as cumulative histograms.
background level of this sample is less than 1% and is mainly due to the e+e−→ qq(γ )¯ and e+e− → e+e−τ+τ− processes. The background from beam- gas and beam-wall interactions is found to be negli- gible.
The baryons are reconstructed using the decay →pπ. Secondary decay vertices are selected which are formed by two oppositely charged tracks. The secondary vertices must satisfy the following criteria:
• The distance, in the plane transverse to the beam direction, between the secondary vertex and the primary e+e− interaction point must be greater than 3 mm.
• The angle between the total transverse momentum vector of the two outgoing tracks and the direction in the transverse plane between the primary inter- action point and the secondary vertex must be less than 100 mrad.
Fig. 2. Distribution of the variables used for the selection of secondary vertices: (a) the distance, in the plane transverse to the beam direction, between the secondary vertex and the primary e+e−interaction point, d⊥, and (b) the angle between the total transverse momentum vector of the two outgoing tracks and the direction in the transverse plane between the primary interaction point and the secondary vertex,α. In each plot, all other selection criteria are applied. The predictions of the PHOJET Monte Carlo are shown as the full line and the contribution due to baryons as the dashed line. The Monte Carlo distributions are normalized to the data luminosity.
Table 1
The number ofbaryons estimated by the fit, together with themass, the overall efficiency and the corresponding differential cross section as a function ofptfor|η|<1.2. The first uncertainty on the cross section is statistical and the second systematic
pt (GeV)
pt (GeV)
Number of Mass of
(MeV)
Efficiency (%)
dσ/dpt (pb/GeV)
0.4–0.6 0.50 3412±71 1116.0±0.1 10.2±0.1 273±6±36
0.6–0.8 0.69 4408±85 1115.9±0.1 13.7±0.1 264±5±29
0.8–1.0 0.89 3420±81 1116.1±0.1 15.3±0.2 183±4±15
1.0–1.3 1.12 3201±87 1115.7±0.1 16.8±0.2 104±3±7
1.3–1.6 1.43 1222±55 1115.5±0.1 17.7±0.4 38±2±3
1.6–2.0 1.77 578±41 1115.0±0.2 15.9±0.6 15±1±1
2.0–2.5 2.21 292±25 1115.9±0.3 17.2±1.3 6±1±1
The distributions of these variables are presented in Fig. 2. The proton is identified as the track with the largest momentum. Monte Carlo studies show that this association is correct for more than 99% of the vertices. In addition, thedE/dx measurement of both proton and pion candidates must be consistent with this hypothesis with a confidence level greater than 0.01.
After these cuts, about 70 000 events are selected.
The distribution of the invariant mass of the pπ
system, m(pπ ), shows a clear peak, as shown in Figs. 3 and 4 for the differentpt bins listed in Table 1.
The resolution ofm(pπ )is found to be around 3 MeV and is well reproduced by Monte Carlo simulations.
4. Differential cross sections
The differential cross sections forproduction as a function ofpt and|η|are measured forWγ γ >5 GeV,
Fig. 3. The invariant mass of the pπsystem for (a) 0.4 GeV< pt0.6 GeV, (b) 0.6 GeV< pt0.8 GeV, (c) 0.8 GeV< pt1 GeV and (d) 1 GeV< pt1.3 GeV. The signal is modelled with a Gaussian and the background by a fourth-degree Chebyshev polynomial.
with a mean value of 30 GeV, and a photon virtuality Q2 <8 GeV2 with Q2 0.2 GeV2. This phase space is defined by cuts at the generator level of the Monte Carlo.
The number of baryons in eachpt and|η|bin is evaluated by means of a fit to them(pπ )spectrum in the interval 1.085 GeV< m(pπ ) <1.145 GeV. The signal is modelled with a Gaussian and the background
Fig. 4. The invariant mass of the pπsystem for (a) 1.3 GeV< pt1.6 GeV, (b) 1.6 GeV< pt2 GeV and (c) 2 GeV< pt<2.5 GeV. The signal is modelled with a Gaussian and the background by a fourth order Chebyshev polynomial.
by a fourth-degree Chebyshev polynomial. All para- meters, including the mass and width of the peak, are left free. The results are given in Tables 1, 2 and 3.
The overall efficiencies, also listed in Tables 1, 2 and 3, include reconstruction and trigger efficien-
cies and take into account the 64% branching fraction of the decay→pπ. The reconstruction efficiency, which includes effects of the acceptance and the selec- tion cuts, is calculated with the PHOJET and PYTHIA Monte Carlo generators. As both generators reproduce
Table 2
The number ofbaryons estimated by the fit, together with the overall efficiency and the corresponding differential cross section as a function of pseudorapidity for 0.4 GeV< pt1 GeV. The first uncertainty on the cross section is statistical and the second systematic
|η| Number of Efficiency (%)
dσ/d|η| (pb) 0.0–0.3 2953±72 13.8±0.2 59±1±3 0.3–0.6 2742±63 13.4±0.2 56±1±3 0.6–0.9 2904±70 13.0±0.2 61±1±4 0.9–1.2 2774±89 11.1±0.1 68±2±8
Table 3
The number ofbaryons estimated by the fit, together with the overall efficiency and the corresponding differential cross section as a function of pseudorapidity for 1 GeV< pt<2.5 GeV. The first uncertainty on the cross section is statistical and the second systematic
|η| Number of Efficiency (%)
dσ/d|η| (pb) 0.0–0.3 1458±60 18.8±0.5 21±1±1 0.3–0.6 1411±62 18.2±0.4 21±1±1 0.6–0.9 1480±62 19.3±0.5 21±1±1 0.9–1.2 1007±63 14.3±0.4 19±1±3
well the shapes of the experimental distributions of hadronic two-photon production [20], their average is used to calculate the selection efficiency. The effi- ciency does not depend on theQ2cut-off. The track trigger efficiency is calculated for each data taking pe- riod by comparing the number of events accepted by the track and the calorimetric energy triggers. The ef- ficiency of the higher level triggers is measured using prescaled events. The total trigger efficiency increases from 82% forpt<0.4 GeV to 85% in the highpt re- gion.
The differential cross section as a function of pt
is first measured for the three different data samples collected in 1998, 1999 and 2000 at different values of
√sand for different trigger and machine background conditions. Good agreement is obtained between the three measurements. The different data samples are therefore combined into a single data sample.
Sources of systematic uncertainties on the cross section measurements are: background subtraction, scale and resolution uncertainties, Monte Carlo mod- elling and the accuracy of the trigger efficiency mea- surement. Their contributions are listed in Table 4. The dominant source of systematic uncertainty is due to background subtraction. It is estimated using differ- ent background parameterisations and fit intervals in the fitting procedure. The scale and resolution uncer- tainties are assessed by varying the selection cuts. The main contributions arises from the secondary vertex selection (3.2%) and the proton and pion identification criteria (2.5%). The uncertainty due to the selection of e+e−→e+e−hadrons events is 1%. The Monte Carlo modelling uncertainty, taken as half the relative differ- ence between PHOJET and PYTHIA, increases with pt from 1% to 5%. A systematic uncertainty of 2% is assigned to the determination of the trigger efficiency, which takes into account the determination procedure and time stability.
5. Results
The average multiplicity ofbaryons in the range Wγ γ >5 GeV, 0.4 GeV< pt <2.5 GeV and |η|<
1.2 is(1.57±0.11)×10−2per e+e−→e+e−hadrons
Table 4
Systematic uncertainty on the cross section of the e+e−→e+e−X and e+e−→e+e−X processes due to background subtraction, scales¯ and resolution uncertainties, Monte Carlo modelling and trigger efficiency. The total systematic uncertainty is the quadratic sum of the different contributions
pt (GeV)
Background subtraction (%)
Scales and resolutions (%)
Monte Carlo modelling (%)
Trigger efficiency (%)
Total (%)
0.4–0.6 12.1 4.2 1.4 2.0 13.0
0.6–0.8 9.8 4.2 1.6 2.0 11.0
0.8–1.0 6.6 4.2 1.9 2.0 8.3
1.0–1.3 5.3 4.2 2.1 2.0 7.3
1.3–1.6 5.8 4.2 2.4 2.0 7.8
1.6–2.0 6.4 4.2 2.8 2.0 8.4
2.0–2.5 19.4 4.2 4.8 2.0 20.5
Fig. 5. The differential cross section as a function ofptfor the e+e−→e+e−X and e+e−→e+e−X processes for¯ |η|<1.2: (a) with the exponential fit described in the text and (b) compared to the predictions of the PHOJET and PYTHIA Monte Carlo programs.
Fig. 6. The differential cross section as a function of |η| for the e+e− →e+e−X and e+e− → e+e−X processes for:¯ (a) 0.4 GeV< pt1 GeV and (b) 1 GeV< pt<2.5 GeV. The data are compared to the predictions of the PHOJET and PYTHIA Monte Carlo programs.
event. The uncertainty includes both statistical and systematic uncertainties. This result is below the value (1.80±0.01)×10−2predicted by PHOJET and above the value(1.43±0.01)×10−2predicted by PYTHIA.
The ratio ofto¯ baryons, as determined from the charge of the most energetic track of the vertex, is found to be N()/N()¯ =0.99±0.04.
The sum of the differential cross sections for the e+e−→e+e−X and e+e−→e+e−X processes¯ as a function ofptfor|η|<1.2 is presented in Table 1 and Fig. 5. Mass effects explain the lower value ob- tained in the first bin. The behaviour of the cross sec- tion for 0.75 GeV< pt <2.5 GeV is well described by an exponential of the form Aexp(−pt/pt), as shown in Fig. 5(a), with a mean valuept =368± 17 MeV. For comparison, the valuespt 230 MeV andpt 290 MeV are obtained for inclusiveπ0and K0Sproduction, respectively [1].
The data are compared to PHOJET and PYTHIA Monte Carlo predictions in Fig. 5(b). The region pt<0.6 GeV is well described by PYTHIA, whereas PHOJET overestimates the cross section. On the contrary, the region 0.6 GeVpt 1 GeV is better reproduced by PHOJET while PYTHIA lies below the data. For pt >1 GeV both PYTHIA and PHOJET underestimate the data. This level of agreement is not unusual in two-photon physics.
The differential cross sections as a function of
|η| for 0.4 GeV< pt 1 GeV and 1 GeV< pt <
2.5 GeV are given in Tables 2 and 3 and shown in Fig. 6. Both Monte Carlo programs describe well the almost uniform η shape, while the size of the discrepancy on the absolute normalisation depends on thept range.
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