Inclusive Charged Hadron Production in Two-Photon Collisions at LEP
a r X i v :h e p -e x /0301025v 1 15 J a n 2003
EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH
CERN-EP/2002-081November 5,2002
Inclusive Charged Hadron Production in Two-Photon Collisions at LEP
The L3Collaboration
Abstract
Inclusive charged hadron production,e +e ?→e +e ?h ±X,is studied using 414pb ?1of data collected at LEP with the L3detector at centre-of-mass energies be-tween 189and 202GeV.Single particle inclusive di?erential cross sections are mea-sured as a function of the particle transverse momentum,p t ,and pseudo-rapidity,η.For p t ≤1.5GeV,the data are well described by an exponential,typical of soft hadronic processes.For higher p t ,the onset of perturbative QCD processes is observed.The π±production cross section for p t >5GeV is much higher than the NLO QCD predictions.
Submitted to Phys.Lett.B
1Introduction
Two-photon collisions are the main source of hadron production in the high-energy regime of LEP via the process e+e?→e+e?γ?γ?→e+e?hadrons.In the Vector Dominance Model (VDM),each photon can transform into a vector meson with the same quantum numbers,thus
initiating a strong interaction process with characteristics similar to hadron-hadron interactions. This process dominates in the“soft”interaction region,where hadrons are produced with a
low transverse momentum,p t,with respect to the beam direction.Hadrons with high p t are produced by the direct QED processγ?γ?→qˉq or by QCD processes originating from the partonic content of the photon.QCD calculations are available for single particle inclusive
production in two-photon interactions at next-to-leading order(NLO)precision[1,2].
The L3collaboration recently published results on inclusiveπ0and K0S production[3].The π0di?erential cross section measured as a function of p t exhibits a clear excess over QCD
calculations.A comparison of these results with other single particle inclusive production at high p t is therefore important.In this Letter,the inclusive charged hadron production is
studied for a centre-of-mass energy of the two interacting photons,Wγγ,greater than5GeV. The hadrons are measured in the transverse momentum range0.4GeV≤p t≤20GeV and in the pseudo-rapidity1)interval|η|≤1.The contributions fromπ±and K±are also derived.
The data used for this analysis were collected by the L3detector[4]at centre-of-mass √s=194GeV, energies
for an integrated luminosity of414pb?1.Results on inclusive charged hadron production for a
√
smaller data sample at lower
s, in order to exclude e+e?annihilation events.The total energy in the electromagnetic calorimeter is required to be greater than500MeV,to suppress beam-gas and beam-wall backgrounds.
?An anti-tag condition.Events with a cluster in the luminosity monitor with an energy greater than30GeV and an electromagnetic shower shape are excluded.
?A mass cut.The visible mass of the event must be greater than5GeV.
About2million hadronic events are selected by these criteria.The overall background level is less than1%and is mainly due to the e+e?→qˉq(γ)and e+e?→e+e?τ+τ?processes.
Charged hadrons are measured with high quality tracks in the inner tracking detector. These tracks have a transverse momentum greater than400MeV and a distance of closest approach to the primary vertex in the transverse plane less than4mm.The number of hits
must be greater than80%of that expected from the track length.Tracks are analysed in the|η|<1and p t<20GeV range where the detector resolution is optimal.A resolution /p t?0.015(GeV?1)×p t is achieved.
σp
t
3Di?erential cross section
The di?erential cross sections of inclusive charged hadron production as a function of p t are measured for an e?ective mass of theγγsystem Wγγ≥5GeV,with a mean value of Wγγ ?30GeV,a photon virtuality Q2≤8GeV2and an average photon virtuality Q2 ?0.2GeV2. This phase space is de?ned by cuts at the Monte Carlo generator level.Results are presented in12p t bins between0.4and20GeV.
The distribution of the detected charged hadrons in these p t bins is presented in Figure1a. The background remains very low over the whole p t range.Events from the e+e?→e+e?τ+τ?process dominate the background at low p t while annihilation events dominate it at high p t.To measure the cross section,the background is subtracted bin-by-bin and the data are corrected for the selection e?ciency,including acceptance,calculated bin-by-bin with PYTHIA.This selection e?ciency varies from62%to84%.At low p t,the e?ciency decreases due to the e?ect of the mass and energy cuts.At high p t,it decreases because of the multiplicity cut,since high p t particles are mainly produced in low multiplicity events.
The level1trigger e?ciency is obtained by comparing the number of events accepted by the independent track and calorimetric energy[16]triggers.It varies from95%to98%.The e?ciency of higher level triggers is about95%and is measured using prescaled events.The overall e?ciency,taking into account selection and trigger e?ciencies is given in Table1.
Sources of systematic uncertainties on the cross section measurements are the trigger e?-ciency estimation,the background subtraction,the selection procedure and the Monte Carlo modeling.Their contributions are shown in Table2.The uncertainty on the trigger e?ciency and on the background subtraction are of a statistical nature.The uncertainty due to the
selection procedure is evaluated by repeating the analysis with di?erent selection criteria:the
√multiplicity cut is moved from5to7objects,the energy cut is moved to0.35
results.This was veri?ed by performing a one-step Bayesian unfolding[17]of the track p t distribution which give results compatible,within errors,with those obtained using the bin-by-bin correction.
The steep decrease of dσ/dp t in the range0.4
4Charged pions and charged kaons
Assuming the fragmentation function implemented in JETSET are correct,theπ±and the K±inclusive cross sections are extracted from the charged hadron cross section.Their ratios relative to charged hadrons are estimated bin-by-bin from Monte Carlo.Above5GeV,they are almost constant.Their uncertainty is calculated in the same way as the uncertainty on the Monte Carlo modeling of the selection e?ciency,by using di?erent subprocesses in PYTHIA. This gives an additional systematic uncertainty of from2%to12%for pions and from14%to 24%for kaons.
The di?erential cross sections forπ±and K±production as a function of p t are presented in Figure2and in Table3.Theπ±data are compared to the previousπ0data[3]scaled up by a factor4:a factor2to correct for the|η|<0.5interval used for theπ0measurement and a factor2to take into account the isospin symmetry.A good agreement is found between these two measurements as shown in Figure2.The K±data are compared to the previous results of K0S data[3]scaled up by a factor4/3:a factor2/3to correct for the|η|<1.5interval of the K0S measurement and a factor2to take into account unobserved K0L decays.Good agreement is found between these two measurements as shown in Figure2.These agreements show a good consistency with the data of the fragmentation functions as implemented in JETSET.
The di?erential cross section ofπ±production as a function of|η|for p t>1GeV is shown in Figure3and in Table5.The cross section is almost constant in thisηrange.It agrees well with theπ0measurement[3].For di?erent p t cuts,Monte Carlo and QCD predictions describe well the uniformηdistribution,while the agreement in the absolute rate depends on the p t range considered.
In Figure4a the data are compared to analytical NLO QCD predictions[19,2].For this calculation,the?ux of quasi-real photons is obtained using the Equivalent Photon Approxi-mation[20],taking into account both transverse and longitudinal virtual photons.The inter-acting particles can be point-like photons or partons from theγ→qˉq process,which evolve into quarks and gluons.The NLO parton density functions of Reference21are used and all elementary2→2and2→3processes are considered.New NLO fragmentation functions[22] are used.The renormalization,factorisation and fragmentation scales are taken to be equal:μ=M=M F=ξp t[2],withξ=1for the central value.The scale uncertainty in the NLO calculation is estimated by varying the value ofξfrom0.5to2.0.The agreement with the data is poor in the high-p t range for any choice of scale.
To test NLO QCD calculations in regions where non-perturbative subprocesses are better
suppressed,we have also measured di?erential cross sections ofπ±production for Wγγ>10, 30and50GeV.The results are shown in Table4and Figure4b.The discrepancy between the calculations and data at high p t is not signi?cantly reduced by these stringent more Wγγcuts.
Similar calculations were previously compared toγp reactions at HERA up to a p t of 12GeV and toˉp p collisions up to a p t of20GeV.Good agreement was found[23].In theγγchannel,an excess of data with respect to NLO QCD was observed in tagged events at PETRA experiments[2].No discrepancy is observed with the OPAL data which explore a p t range up to10GeV.In this range,our data and the OPAL ones are well in agreement within the quoted uncertainties.A discrepancy with NLO QCD is revealed by our data which extend the measurement to higher p t values.
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Author List
The L3Collaboration:
P.Achard,20O.Adriani,17M.Aguilar-Benitez,24J.Alcaraz,24,18G.Alemanni,22J.Allaby,18A.Aloisio,28M.G.Alviggi,28 H.Anderhub,46V.P.Andreev,6,33F.Anselmo,8A.Are?ev,27T.Azemoon,3T.Aziz,9,18P.Bagnaia,38A.Bajo,24
G.Baksay,25L.Baksay,25S.V.Baldew,2S.Banerjee,9Sw.Banerjee,4A.Barczyk,46,44R.Barill`e re,18P.Bartalini,22
M.Basile,8N.Batalova,43R.Battiston,32A.Bay,22F.Becattini,17U.Becker,13F.Behner,46L.Bellucci,17R.Berbeco,3
J.Berdugo,24P.Berges,13B.Bertucci,32B.L.Betev,46M.Biasini,32M.Biglietti,28A.Biland,46J.J.Blaising,4S.C.Blyth,34 G.J.Bobbink,2A.B¨o hm,1L.Boldizsar,12B.Borgia,38S.Bottai,17D.Bourilkov,46M.Bourquin,20S.Braccini,20
J.G.Branson,40F.Brochu,4J.D.Burger,13W.J.Burger,32X.D.Cai,13M.Capell,13G.Cara Romeo,8G.Carlino,28
A.Cartacci,17J.Casaus,24F.Cavallari,38N.Cavallo,35C.Cecchi,32M.Cerrada,24M.Chamizo,20Y.H.Chang,48
M.Chemarin,23A.Chen,48G.Chen,7G.M.Chen,7H.F.Chen,21H.S.Chen,7G.Chiefari,28L.Cifarelli,39F.Cindolo,8
I.Clare,13R.Clare,37G.Coignet,4N.Colino,24S.Costantini,38B.de la Cruz,24S.Cucciarelli,32J.A.van Dalen,30
R.de Asmundis,28P.D′e glon,20J.Debreczeni,12A.Degr′e,4K.Dehmelt,25K.Deiters,44D.della Volpe,28E.Delmeire,20 P.Denes,36F.DeNotaristefani,38A.De Salvo,46M.Diemoz,38M.Dierckxsens,2C.Dionisi,38M.Dittmar,46,18A.Doria,28 M.T.Dova,10,?D.Duchesneau,4M.Duda,1B.Echenard,20A.Eline,18A.El Hage,1H.El Mamouni,23A.Engler,34
F.J.Eppling,13P.Extermann,20M.A.Falagan,24S.Falciano,38A.Favara,31J.Fay,23O.Fedin,33M.Felcini,46T.Ferguson,34 H.Fesefeldt,1E.Fiandrini,32J.H.Field,20F.Filthaut,30P.H.Fisher,13W.Fisher,36I.Fisk,40
G.Forconi,13
K.Freudenreich,46C.Furetta,26Yu.Galaktionov,27,13S.N.Ganguli,9P.Garcia-Abia,5,18M.Gataullin,31S.Gentile,38
S.Giagu,38Z.F.Gong,21G.Grenier,23O.Grimm,46M.W.Gruenewald,16M.Guida,39R.van Gulik,2V.K.Gupta,36
A.Gurtu,9L.J.Gutay,43D.Haas,5R.Sh.Hakobyan,30D.Hatzifotiadou,8T.Hebbeker,1A.Herv′e,18J.Hirschfelder,34
H.Hofer,46M.Hohlmann,25G.Holzner,46S.R.Hou,48Y.Hu,30B.N.Jin,7L.W.Jones,3P.de Jong,2I.Josa-Mutuberr′?a,24 D.K¨a fer,1M.Kaur,14M.N.Kienzle-Focacci,20J.K.Kim,42J.Kirkby,18W.Kittel,30A.Klimentov,13,27A.C.K¨o nig,30
M.Kopal,43V.Koutsenko,13,27M.Kr¨a ber,46R.W.Kraemer,34A.Kr¨u ger,45A.Kunin,https://www.wendangku.net/doc/4e7578022.html,dron de Guevara,24
https://www.wendangku.net/doc/4e7578022.html,ktineh,https://www.wendangku.net/doc/4e7578022.html,ndi,17M.Lebeau,18A.Lebedev,13P.Lebrun,23P.Lecomte,46P.Lecoq,18P.Le Coultre,46
J.M.Le Go?,18R.Leiste,45M.Levtchenko,26P.Levtchenko,33C.Li,21S.Likhoded,45C.H.Lin,48W.T.Lin,48F.L.Linde,2 L.Lista,28Z.A.Liu,7W.Lohmann,45E.Longo,38Y.S.Lu,7C.Luci,38L.Luminari,38W.Lustermann,46W.G.Ma,21
L.Malgeri,20A.Malinin,27C.Ma?n a,24D.Mangeol,30J.Mans,36J.P.Martin,23F.Marzano,38K.Mazumdar,9R.R.McNeil,6 S.Mele,18,28L.Merola,28M.Meschini,17W.J.Metzger,30A.Mihul,https://www.wendangku.net/doc/4e7578022.html,cent,18G.Mirabelli,38J.Mnich,1
G.B.Mohanty,9G.S.Muanza,23A.J.M.Muijs,2B.Musicar,40M.Musy,38S.Nagy,15S.Natale,20M.Napolitano,28
F.Nessi-Tedaldi,46H.Newman,31A.Nisati,38H.Nowak,45R.O?erzynski,46
https://www.wendangku.net/doc/4e7578022.html,antini,38C.Palomares,18P.Paolucci,28 R.Paramatti,38G.Passaleva,17S.Patricelli,28T.Paul,10M.Pauluzzi,32C.Paus,13F.Pauss,46M.Pedace,38S.Pensotti,26 D.Perret-Gallix,4B.Petersen,30D.Piccolo,28F.Pierella,8M.Pioppi,32P.A.Pirou′e,36E.Pistolesi,26V.Plyaskin,27
M.Pohl,20V.Pojidaev,17J.Pothier,18D.O.Proko?ev,43D.Proko?ev,33J.Quartieri,39G.Rahal-Callot,46M.A.Rahaman,9 P.Raics,15N.Raja,9R.Ramelli,46P.G.Rancoita,26R.Ranieri,17A.Raspereza,45P.Razis,29D.Ren,46M.Rescigno,38
S.Reucroft,10S.Riemann,45K.Riles,3B.P.Roe,3L.Romero,24A.Rosca,45S.Rosier-Lees,4S.Roth,1C.Rosenbleck,1
B.Roux,30J.A.Rubio,18G.Ruggiero,17H.Rykaczewski,46A.Sakharov,46S.Saremi,6S.Sarkar,38J.Salicio,18E.Sanchez,24 M.P.Sanders,30
C.Sch¨a fer,18V.Schegelsky,33H.Schopper,47
D.J.Schotanus,30C.Sciacca,28L.Servoli,32S.Shevchenko,31 N.Shivarov,41V.Shoutko,13
E.Shumilov,27A.Shvorob,31D.Son,42C.Souga,23P.Spillantini,17M.Steuer,13
D.P.Stickland,36B.Stoyanov,41A.Straessner,18K.Sudhakar,9G.Sultanov,41L.Z.Sun,21S.Sushkov,1H.Suter,46
J.D.Swain,10Z.Szillasi,25,?X.W.Tang,7P.Tarjan,15L.Tauscher,5L.Taylor,10B.Tellili,23D.Teyssier,23
C.Timmermans,30Samuel C.C.Ting,13S.M.Ting,13S.C.Tonwar,9,18J.T′o th,12C.Tully,36K.L.Tung,7J.Ulbricht,46
E.Valente,38R.T.Van de Walle,30R.Vasquez,43V.Veszpremi,25G.Vesztergombi,12I.Vetlitsky,27D.Vicinanza,39
G.Viertel,46S.Villa,37M.Vivargent,4S.Vlachos,5I.Vodopianov,25H.Vogel,34H.Vogt,45I.Vorobiev,34,27
A.A.Vorobyov,33M.Wadhwa,5X.L.Wang,21Z.M.Wang,21M.Weber,1P.Wienemann,1H.Wilkens,30S.Wynho?,36
L.Xia,31Z.Z.Xu,21J.Yamamoto,3B.Z.Yang,21C.G.Yang,7H.J.Yang,3M.Yang,7S.C.Yeh,49An.Zalite,33Yu.Zalite,33 Z.P.Zhang,21J.Zhao,21G.Y.Zhu,7R.Y.Zhu,31H.L.Zhuang,7A.Zichichi,8,18,19B.Zimmermann,46M.Z¨o ller.1
1III.Physikalisches Institut,RWTH,D-52056Aachen,Germany§
2National Institute for High Energy Physics,NIKHEF,and University of Amsterdam,NL-1009DB Amsterdam, The Netherlands
3University of Michigan,Ann Arbor,MI48109,USA
4Laboratoire d’Annecy-le-Vieux de Physique des Particules,LAPP,IN2P3-CNRS,BP110,F-74941 Annecy-le-Vieux CEDEX,France
5Institute of Physics,University of Basel,CH-4056Basel,Switzerland
6Louisiana State University,Baton Rouge,LA70803,USA
7Institute of High Energy Physics,IHEP,100039Beijing,China△
8University of Bologna and INFN-Sezione di Bologna,I-40126Bologna,Italy
9Tata Institute of Fundamental Research,Mumbai(Bombay)400005,India
10Northeastern University,Boston,MA02115,USA
11Institute of Atomic Physics and University of Bucharest,R-76900Bucharest,Romania
12Central Research Institute for Physics of the Hungarian Academy of Sciences,H-1525Budapest114,Hungary?13Massachusetts Institute of Technology,Cambridge,MA02139,USA
14Panjab University,Chandigarh160014,India.
15KLTE-ATOMKI,H-4010Debrecen,Hungary?
16Department of Experimental Physics,University College Dublin,Bel?eld,Dublin4,Ireland
17INFN Sezione di Firenze and University of Florence,I-50125Florence,Italy
18European Laboratory for Particle Physics,CERN,CH-1211Geneva23,Switzerland
19World Laboratory,FBLJA Project,CH-1211Geneva23,Switzerland
20University of Geneva,CH-1211Geneva4,Switzerland
21Chinese University of Science and Technology,USTC,Hefei,Anhui230029,China△
22University of Lausanne,CH-1015Lausanne,Switzerland
23Institut de Physique Nucl′e aire de Lyon,IN2P3-CNRS,Universit′e Claude Bernard,F-69622Villeurbanne,France 24Centro de Investigaciones Energ′e ticas,Medioambientales y Tecnol′o gicas,CIEMAT,E-28040Madrid,Spain?
25Florida Institute of Technology,Melbourne,FL32901,USA
26INFN-Sezione di Milano,I-20133Milan,Italy
27Institute of Theoretical and Experimental Physics,ITEP,Moscow,Russia
28INFN-Sezione di Napoli and University of Naples,I-80125Naples,Italy
29Department of Physics,University of Cyprus,Nicosia,Cyprus
30University of Nijmegen and NIKHEF,NL-6525ED Nijmegen,The Netherlands
31California Institute of Technology,Pasadena,CA91125,USA
32INFN-Sezione di Perugia and Universit`a Degli Studi di Perugia,I-06100Perugia,Italy
33Nuclear Physics Institute,St.Petersburg,Russia
34Carnegie Mellon University,Pittsburgh,PA15213,USA
35INFN-Sezione di Napoli and University of Potenza,I-85100Potenza,Italy
36Princeton University,Princeton,NJ08544,USA
37University of Californa,Riverside,CA92521,USA
38INFN-Sezione di Roma and University of Rome,“La Sapienza”,I-00185Rome,Italy
39University and INFN,Salerno,I-84100Salerno,Italy
40University of California,San Diego,CA92093,USA
41Bulgarian Academy of Sciences,Central Lab.of Mechatronics and Instrumentation,BU-1113So?a,Bulgaria
42The Center for High Energy Physics,Kyungpook National University,702-701Taegu,Republic of Korea
43Purdue University,West Lafayette,IN47907,USA
44Paul Scherrer Institut,PSI,CH-5232Villigen,Switzerland
45DESY,D-15738Zeuthen,Germany
46Eidgen¨o ssische Technische Hochschule,ETH Z¨u rich,CH-8093Z¨u rich,Switzerland
47University of Hamburg,D-22761Hamburg,Germany
48National Central University,Chung-Li,Taiwan,China
49Department of Physics,National Tsing Hua University,Taiwan,China
§Supported by the German Bundesministerium f¨u r Bildung,Wissenschaft,Forschung und Technologie
?Supported by the Hungarian OTKA fund under contract numbers T019181,F023259and T037350.
?Also supported by the Hungarian OTKA fund under contract number T026178.
?Supported also by the Comisi′o n Interministerial de Ciencia y Tecnolog′?a.
?Also supported by CONICET and Universidad Nacional de La Plata,CC67,1900La Plata,Argentina.
△Supported by the National Natural Science Foundation of China.
p t
[GeV][%][pb/GeV]
0.48(23.4±0.1± 3.7)×103
0.6?0.864.5± 6.9
0.88(48.0±0.1± 5.9)×102
1.0?1.57
2.4± 4.8
1.68(28.5±0.1±
2.2)×10
2.0?
3.077.2±
4.3
3.36(13.1±0.2± 1.0)
4.0?
5.069.5± 5.8
5.79(15.3±0.4± 1.6)×10?1
7.5?10.065.2±8.7
11.98(21.0±1.2± 3.8)×10?2
15.0?20.059.8±14.7
background Monte Carlo [GeV]e?ciency[%]procedure[%]
0.4?0.60.110.1
<0.110.6
0.8?1.00.28.4
<0.1 6.6
1.5?
2.00.4 5.9
0.1 5.6
3.0?
4.00.9 3.1
0.98.2
5.0?7.5 1.2 1.6
2.51
3.3
10.0?15.0 1.2 1.1
4.224.6
Table2:Systematic uncertainty on the charged hadron cross section due to trigger e?ciency, background subtraction,selection procedure and Monte Carlo modeling.
p t
[pb/GeV][pb/GeV]
(23.7±0.1± 5.1)×102
0.68(88.1±0.1±12.5)×102
(80.7±0.2±15.3)×10
1.14(10.2±0.1± 1.0)×102
(54.3±0.4±9.2)
2.31(44.7±0.3±
3.5)
(21.3±0.4± 3.8)×10?1
4.39(37.8±1.0± 3.6)×10?1
(20.6±0.8± 4.2)×10?2
8.46(41.0±2.1± 6.2)×10?2
(27.7±2.1±7.7)×10?3
17.36(81.3±7.1±22.5)×10?3
dσ/dp t[pb/GeV]dσ/dp t[pb/GeV]dσ/dp t[pb/GeV] [GeV]
0.48(13.7±0.1± 3.2)×103(30.1±0.2±17.0)×102
0.68(60.6±0.1±12.5)×102(13.8±0.1±7.5)×102
0.88(25.7±0.1± 4.5)×102(59.3±0.6±31.2)×10
1.14(74.3±0.2±10.3)×10(18.9±0.2±9.5)×10
1.68(15.6±0.1± 1.0)×10(39.4±0.7±9.4)
2.31(36.4±0.3± 2.2)(96.1±2.3±2
3.4)×10?1
3.36(88.8±1.5± 5.8)×10?1(25.5±1.1± 6.5)×10?1
4.39(3
5.7±1.0± 2.9)×10?1(9
6.5±6.0±26.0)×10?2
5.79(11.7±0.4± 1.2)×10?1(3
6.5±2.1±10.5)×10?2
8.46(37.8±2.0± 5.6)×10?2(13.1±1.2± 4.4)×10?2 11.98(16.4±1.0± 3.4)×10?2(84.1±7.4±31.5)×10?3 17.36(78.9±7.0±23.9)×10?3(61.3±7.5±27.2)×10?3 Table4:Di?erential cross section as a function of p t for inclusiveπ±production for|η|<1 and di?erent Wγγcuts.The?rst uncertainty on the cross section is statistical and the second systematic.
|η|
0.0?0.2638±3±80
0.2?0.4677±3±84
0.4?0.6693±4±86
0.6?0.8719±4±90
0.8?1.0687±4±86
Table5:Di?erential cross section as a function of|η|for inclusiveπ±production for Wγγ>5 GeV and p t>1GeV.The?rst uncertainty on the cross section is statistical and the second systematic.
500
1000
|η|
d σ / d |η| [p b ]
Figure 3:Inclusive π±di?erential cross section dσ/d |η|for p t >1GeV compared to the inclusive π0measurement [3]scaled by a factor 2and two Monte Carlo predictions.Statistical and systematic uncertainties are shown.