Article
Reference
Inclusive D
*±production in two-photon collisions at LEP
L3 Collaboration
ACHARD, Pablo (Collab.), et al.
Abstract
Inclusive bb production in two-photon collisions is studied with the L3 detector at LEP, using 683 pb−1 of data collected at centre-of-mass energies from 183 to 209 GeV. Differential cross sections are determined as functions of the transverse momentum and pseudorapidity of the D*± mesons in the kinematic region 1 GeV
L3 Collaboration, ACHARD, Pablo (Collab.), et al . Inclusive D
*±production in two-photon collisions at LEP. Physics Letters. B , 2002, vol. 535, no. 1-4, p. 59-69
DOI : 10.1016/S0370-2693(02)01769-0
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http://archive-ouverte.unige.ch/unige:43049
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Inclusive D ∗± 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
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aaI. Physikalisches Institut, RWTH, D-52056 Aachen, FRG and 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
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 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 Lab. of Mechatronics and Instrumentation, BU-1113 Sofia, Bulgaria ap The 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, PR China awDepartment of Physics, National Tsing Hua University, Taiwan, PR China
Received 6 February 2002; received in revised form 8 March 2002; accepted 15 April 2002
Editor: L. Rolandi
Abstract
Inclusive D∗±production in two-photon collisions is studied with the L3 detector at LEP, using 683 pb−1of data collected at centre-of-mass energies from 183 to 209 GeV. Differential cross sections are determined as functions of the transverse momentum and pseudorapidity of the D∗±mesons in the kinematic region 1 GeV< PT <12 GeV and|η|<1.4. The cross sectionsσ (e+e−→e+e−D∗±X)in this kinematical region is measured and theσ (e+e−→e+e−ccX)¯ cross section is derived.
The measurements are compared with next-to-leading order perturbative QCD calculations. 2002 Published by Elsevier Science B.V.
1. Introduction
The measurement of open charm production in two-photon collisions provides a good test of perturba- tive QCD as the large physical scale set by the charm quark mass is expected to make the perturbative cal- culations more reliable. At LEP2 energies, the direct and single resolved processes, sketched in Fig. 1, are predicted [1] to give comparable contributions to the cross sectionσ (γ γ→cc). The contributions to open¯ charm production from soft processes described by the Vector Dominance Model (VDM) and from dou- ble resolved processes are expected to be small. The resolved photon cross section is dominated by the photon–gluon fusion processγg→cc. The cross sec-¯ tion for open charm production in two-photon colli- sions therefore depends on the charm quark mass and on the gluonic parton density function of the photon.
Measurements of open charm production in untagged two-photon collisions were performed at PEP [2], PE- TRA [3], TRISTAN [4], and LEP [5–8], where charm quarks were identified by the presence of D∗±mesons, leptons or K0Smesons.
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.
This Letter describes the results of a measurement of the e+e−→e+e−ccX cross section at centre-of-¯ mass energies ranging from 183 to 209 GeV. The data sample corresponds to a total integrated luminosity of 683 pb−1 collected with the L3 detector [9] at LEP.
Charm quarks are identified by reconstructing D∗+7 mesons in three decay channels:
(1) D∗+→D0πs+→
K−π+ πs+,
(2) D∗+→D0πs+→
K−π+π0 πs+,
(3) D∗+→D0πs+→
K−π+π−π+ πs+.
πs+ stands for a “slow pion”, so-called as the ki- netic energy release in D∗+ decays is only about 6 MeV. Differential cross sections for D∗± produc- tion as a function of its transverse momentumPT and pseudorapidityηare determined together with the vis- ible cross section. This is then extrapolated to derive σ (e+e−→e+e−ccX).¯
The PYTHIA [10] Monte Carlo program is used to model two-photon processes. It simulates γ γ events according to the current knowledge of hadronic inter- actions obtained from pp andγp studies. Two Monte Carlo samples are used: one with massless quark ma- trix elements, and another with matrix elements with a charm quark massmc=1.35 GeV. Both direct and re- solved diagrams are included in the generation. The re- solved processes are generated with the SaS1d photon parton density function [11]. A two-photon luminos- ity function corresponding to the Equivalent Photon Approximation [12] is used with a cut off Q2<m2ρ. The L3 detector is simulated with the GEANT pack- age [13] and Monte Carlo events are reconstructed in
7Charge conjugate states are assumed to be included.
Fig. 1. Diagrams contributing to charm production inγ γcollisions.
the same way as the data events. Time dependent de- tector inefficiencies, as monitored during the data tak- ing period, are also simulated.
2. Event selection and D∗±reconstruction
The event selection follows two steps: first, had- ronic final states from two-photon collisions are se- lected, then, D∗±mesons are reconstructed.
2.1. Selection of hadronic two-photon events
Hadronic events are required to contain at least four particles, with at least three charged tracks. A particle is defined as either a track in the central tracker or a calorimetric cluster not associated to a track. The background from e+e−→e+e−τ+τ− and e+e−→ τ+τ−events is highly suppressed by this requirement.
To reject annihilation events, the visible energy has to be less than 80 GeV. The analysis is restricted to events with visible massWvis>2 GeV, as calculated from tracks and calorimetric clusters, including those from the small angle luminosity monitor. All particles are considered to be pions, except for unmatched electromagnetic clusters considered as photons.
An antitagging condition is implemented on the scattered electron or positron, by requiring the energy of the most energetic cluster in the forward electro- magnetic calorimeter to be less than 0.2√
s. Only 6%
of events in the D∗±signal region are rejected by this
criterium. The antitagging requirement implies a small virtuality of the interacting photons:Q2 0.1 GeV2, as predicted by the PYTHIA Monte Carlo.
This preselection yields 4.6×106events. After ap- plying the above cuts, the contamination from annihi- lation processes is less than 1%.
2.2. D∗±reconstruction
The D∗± selection requires events with less than 15 tracks. Tracks are required to have at least 20 hits, a transverse momentum above 0.1 GeV and a distance of closest approach to the interaction point in the trans- verse plane below 1.5 mm. In addition, photon candi- dates are selected in the electromagnetic calorimeter as isolated showers with no close tracks. D∗±mesons are selected by first identifying D0mesons. Additional criteria reject the combinatorial background, and fi- nally, πs+ candidates are added. The D0 candidates are reconstructed from two-track combinations for the channel (1), two-track and two-photon combinations for the channel (2), and four-track combinations for the channel (3). Pion or kaon masses are assigned to the tracks. We define PK and Pπ as the probabili- ties for the kaon and pion mass hypotheses, calcu- lated from the energy loss,dE/dx, in the tracker. The joint probabilitiesPKPπ for channels (1) and (2) and PKPπ1Pπ2Pπ3 for channel (3), have to be greater than 0.5% and 0.1%, respectively. In addition, for chan- nel (3) the kaon track should satisfy the condition
PK/Pπ 2. The π0 candidates for channel (2) are constructed from two neutral clusters in the barrel electromagnetic calorimeter, with energy greater than 50 MeV and effective mass within ±20 MeV of the nominal π0 mass. The transverse momentum of the π0candidate with respect to the beam direction must be greater than 400 MeV.
The effective mass of the D0 candidate in the K−π+, K−π+π0, or K−π+π−π+ combinations is calculated for channels (1), (2) and (3), respectively.
This mass should be within ±75,±50 or±40 MeV around the nominal D0mass, respectively. These dif- ferent mass intervals are determined by the Monte Carlo D0mass resolution and reflect the better trans- verse momentum resolution for softer tracks and the high precisionπ0reconstruction in the BGO electro- magnetic calorimeter.
To reduce the combinatorial background in chan- nel (1), the opening angleα between the kaon and the D0 flight direction has to satisfy the condition cosα >−0.1.
A kinematical fit using as constraints the D0andπ0 masses is performed for channel (2). The fit prob- ability must be greater than 1%. In addition, the sum of the transverse momenta squared of these particles must exceed 0.9 GeV2. Since the decay D0→K−π+π0 proceeds dominantly through one of the quasi-two-body intermediate states K∗0π0, K∗−π+and K−ρ+[14], the K−π+and K−π0com- binations are required to lie within±60 MeV of the K∗(892)mass and theπ−π0ones within±150 MeV of theρmass. This last criterium also holds for chan- nel (3), that proceeds through the Kπρstate. In addi- tion, for channel (2) we require the helicity angleθ∗ of an intermediate decay state to satisfy the condi- tion |cosθ∗|>0.4, exploiting the P-wave properties of vector-particle decay into two scalar particles to re- ject background. The helicity angleθ∗ is defined as the angle between the direction of a decay product of the vector resonance (K∗0, K∗−orρ+) and the direc- tion of the pseudoscalar particle (π0,π+or K−) from the D0decay, calculated in the intermediate resonance rest frame.
D0decay tracks are required to originate near the interaction point. This is enforced by reconstructing a secondary vertex in the transverse plane for chan- nels (1) and (3). The D0impact parameterd with re-
Fig. 2. Distribution of the mass difference D∗+and D0candidates for D∗+ decays into (a) (K−π+)πs+, (b) (K−π+π0)πs+ and (c)(K−π+π−π+)πs+. The points are data, the error bars represent statistical uncertainties and the line is the result of the fit to the data points.
spect to the interaction point in the transverse plane must satisfy d <0.8 mm and d <0.6 mm, respec- tively, while the D0 signed flight distance s should satisfy−0.8< s <1.0 mm and−0.9< s <1.1 mm, respectively. No secondary vertex reconstruction is performed for channel (2), but the distance of clos- est approach of kaon and pion tracks to the interaction point in the transverse plane must be less than 1 mm.
Finally, a slow pion candidate track is added to the combination to form the D∗± candidate. The charge must be opposite to that of the kaon candidate, with a distance of closest approach to the interaction point in the transverse plane less than 4.5 mm.
The mass differenceM of the D∗±and D0can- didates is shown in Fig. 2. D∗± mesons are expected to form a peak around 145.4 MeV, while combinato- rial background yields a rising distribution, starting at 139.6 MeV, the mass of the charged pion. The recon- structed D∗±signal is clearly observed in all the three decay channels under study. The number of observed
Table 1
The number of observed D∗±mesons for the three different channels and the trigger (εtrig) and visible (εDvis∗±) efficiencies. Visible and total cross sections are also given. The uncertainties on the numbers of reconstructed D∗±mesons and on the efficiencies are statistical. For the cross section values, the first uncertainty is statistical, the second is systematic. The last uncertainty for the total charm cross section corresponds to the extrapolation to the total phase space
Channel (K−π+)πs+ (K−π+π0)πs+ (K−π+π−π+)πs+
NobsD∗± 245±27 139±18 99±16
εtrig, % 92.9±0.2 92.1±0.3 89.4±0.2
εvisD∗±, % 0.61±0.03 0.22±0.02 0.20±0.02
σvisD∗±, pb 63.3±6.7±8.9 100.9±13.0±18.2 81.1±12.5±17.8
σ (e+e−→e+e−c¯cX), nb 1.00±0.11±0.15+0.48−0.22 1.59±0.21±0.31+0.77−0.36 1.28±0.20±0.29+0.62−0.29
D∗± mesons is estimated by a fit to the M spec- trum. The background shape is modelled by Monte Carlo and checked using wrong-charge combinations in data, where the charge of the kaon candidate is the same as the charge of the slow pion candidate.
The background processes e+e−→γ /Z→qq and¯ e+e−→W+W−→qqq¯ q represent less than 0.2% of¯ the sample. The background shape is modelled by a function with two free parametersaandb:
(4) fbkg(M)=a(M−mπ)b,
where the background shape parameter b is deter- mined from the e+e−→e+e−qq Monte Carlo. A fit¯ to the M spectrum is performed, using a variable width Gaussian to describe the signal, and the fitted numbers of D∗±mesonsNobsD∗±found in each channel are presented in Table 1.
3. Results
3.1. Differential cross sections
The measured differential cross sections for inclu- sive D∗± production as a function of its transverse momentum PT and pseudorapidity η, dσ/dPT and dσ/d|η|, are shown in Tables 2 and 3 for the visible region defined by:
(5) 1 GeV< PT <12 GeV and |η|<1.4.
When evaluatingdσ/dPT, which has a steep depen- dence on PT, a correction factor, determined from Monte Carlo, is used to assign the differential cross sections to the centers of the corresponding bins. The selection efficiency is determined with PYTHIA, us- ing the D∗+ branching ratios of Reference [14] and
assuming a 1:1 mixture [1,7] of direct and single resolved processes. The sources of systematic uncer- tainty are discussed below and summarised in Table 4:
• Selection procedure. The uncertainty is estimated by a variation of the selection criteria. The largest contributions are the cut on the number of tracks (6.0%) and the D0mass window (4.6%) for chan- nel (1); the cut on the transverse momentum of the neutral pion (7.8%) and on the sum of transverse momenta (4.3%) for channel (2); the cut on the dE/dx probability (11.5%) and the D0 mass window (7.5%) for channel (3).
• Branching ratios. The uncertainties are taken from Ref. [14].
• Background. This uncertainty is obtained by changing the background fit function within the uncertainties on its parameters. The range of the fit is also varied.
• Ratio of the direct to resolved processes. The se- lection efficiency is re-evaluated changing the ra- tio from 1:1 to 3:1 or 1:3.
• Monte Carlo statistics.
• Trigger efficiency. The trigger efficiency and its uncertainty are determined from the data using a set of independent triggers.
Figs. 3 and 4 show the differential cross sections dσ/dPT anddσ/d|η|. The differential cross sections are compared to next-to-leading order (NLO) pertur- bative QCD calculations, based on massive matrix el- ements [15]. In this scheme, the charm quark is not considered to be one of the active flavours inside the photon. The Glück–Reya–Schienbein [16] photon par- ton density is used in the calculation. The renormaliza- tion scale,µR, and the factorization scale of the pho-
Table 2
Number of observed D∗±mesonsNobsD∗±and measured D∗±production cross sections in differentPT bins for|η|<1.4. Differentialdσ/dPT cross sections refer to the center of the bin andσis the integral over the bin
PT (GeV) NobsD∗± dσ/dPT (pb/GeV) σ(pb)
1–2 325±31 54.5±5.3±6.7 54.5±5.3±6.7
2–3 130±17 12.1±1.4±1.4 12.1±1.4±1.4
3–5 72±11 2.9±0.4±0.4 6.7±1.0±1.0
5–12 13±5 0.12±0.05±0.02 1.5±0.6±0.3
Table 3
Number of observed D∗±mesonsNobsD∗±and measured D∗±production cross sections for 1 GeV< PT <12 GeV. Differentialdσ/d|η|cross sections refer to the center of the bin andσis the integral over the bin
|η| NobsD∗± dσ/d|η|(pb) σ(pb)
0.0–0.4 194±23 49.4±6.0±6.0 19.8±2.4±2.4
0.4–0.8 232±24 60.3±6.0±7.2 24.1±2.4±2.9
0.8–1.4 92±17 39.4±7.2±5.6 23.6±4.3±3.4
Table 4
Sources of systematic uncertainty in % on the visible D∗±cross section and the total charm cross section. The total systematic uncertainty is calculated by adding all contributions in quadrature
Source (K−π+)πs+ (K−π+π0)πs+ (K−π+π−π+)πs+
Selection 7.8 9.9 16.8
Branching ratio [14] 3.1 7.2 4.9
Fit procedure 6.6 6.6 6.6
Direct/resolved ratio 8.6 8.6 8.6
Monte Carlo statistics 5.1 9.4 8.7
Trigger efficiency 0.2 0.3 0.2
Systematics visible 14.6 18.9 22.3
P (c→D∗+) 3.1 3.1 3.1
b¯b contribution 0.6 0.6 0.6
Systematics total 14.9 19.2 22.5
ton structure function,µF, are taken asµR=µF/2= mT =
p2T +m2c, wherepT is the transverse momen- tum of the charm quark whose mass ismc=1.5 GeV.
The calculations are repeated with different renor- malization scales separately for the direct and single- resolved contributions as well as with different charm quark masses, in order to estimate the maximal the- ory prediction uncertainty. The measurements are in agreement with the NLO QCD calculations within the theoretical uncertainties.
3.2. Visible cross section
The visible D∗± production cross section σ (e+e−→e+e−D∗±X)visis calculated in the kinemat-
ical region (5) as:
(6) σ
e+e−→e+e−D∗±X
vis= NobsD∗±
LεtrigεvisD∗±,
whereLis the total integrated luminosity. The trigger efficiency εtrig and the selection efficiency εvisD∗± are listed in Table 1 and the number of observed events in Tables 2 and 3. Results for eachPT andηbin are shown in Tables 2 and 3, while the integrated visible cross sections are given in Table 1 for each channel.
Averaging over the three channels gives:
σ
e+e−→e+e−D∗±X
vis=71.2±5.3±9.8 pb, where the first uncertainty is statistical and the second is systematic.
Fig. 3. The differential cross sectiondσ/dPT for inclusive D∗±pro- duction. The points are the data, the inner error bars represent the statistical uncertainty. The solid curve represents the NLO QCD cal- culations [15] with the Glück–Reya–Schienbein [16] parametriza- tion of the parton density of the photon. For the direct (dir) and resolved (res) processes, different values of the renormalisation and factorisation scalesµR andµF, as well as ofmc, are used. The PYTHIA prediction is shown as a histogram.
3.3. Total cross section
The total cross section of open charm production is calculated as:
σ
e+e−→e+e−ccX¯
= σ
e+e−→e+e−D∗±X
vis
−σ
e+e−→e+e−b¯bX→D∗±X
vis
× (7)
εDtot∗±2P
c→D∗+−1
.
The probability of a charm quark to hadronize into a D∗+ meson, P (c→D∗+), is equal to 0.241± 0.008 [17]. The efficiency εDtot∗± is a correction fac- tor to extrapolate from the visible D∗± kinematical region (5) to the full phase space. It follows from the
Fig. 4. The differential cross sectiondσ/d|η|for inclusive D∗±pro- duction. The points are the data, the inner error bars represent the statistical uncertainty. The solid curve represents the NLO QCD cal- culations [15] with the Glück–Reya–Schienbein [16] parametriza- tion of the parton density of the photon. For the direct (dir) and resolved (res) processes, different values of the renormalisation and factorisation scales µR andµF, as well as ofmc, are used. The PYTHIA prediction is shown as a histogram.
calculations of Ref. [15] as εDtot∗±=0.132+0.038−0.043. The two-photon b¯b production process also yields D∗±
mesons. The cross section of this contribution, σ
e+e−→e+e−bbX¯ →D∗±X
vis,
is estimated from a e+e−→e+e−bbX Monte Carlo¯ sample and the measuredσ (e+e−→e+e−bbX)¯ cross section [8] as 1.9±0.4 pb. It is subtracted and its uncertainty is included in the systematics.
The total open charm production cross section at the average centre-of-mass energy of 197 GeV is estimated to be:
σ
e+e−→e+e−ccX¯
=
1.12±0.09±0.16+−0.540.25
×103pb.
The first uncertainty is statistical and second is system- atic. The third uncertainty is that on the extrapolation
Fig. 5. The open charm production cross section in two-photon col- lisions as a function of the e+e−centre-of-mass energy. L3 mea- surements based on lepton tag [6,8] are plotted as the open circles.
The dashed lines correspond to the direct process contribution and the solid lines represent the NLO QCD prediction [1] for the sum of the direct and resolved processes. The effect of different values of mcis also shown.
from the visible phase space region to the full one. It corresponds to the integration [15] of the dotted and dashed lines in Figs. 3 and 4. A consistent estimate of this uncertainty is also obtained by comparing the extrapolation factors obtained using different Monte Carlo generators: the massive and massless PYTHIA samples yield extrapolation factors that differ by 24%.
The total inclusive charm production cross sec- tion as a function of the e+e− centre-of-mass energy is shown in Fig. 5 together with our previous mea- surements [6,8]. The data are compared to the NLO QCD predictions of Ref. [1] for the direct process and the sum of direct and resolved processes. In this calculation, the charm quark mass is taken to be 1.3 or 1.7 GeV, and the open charm threshold to be at 3.8 GeV. The resolved process is calculated using the Glück–Reya–Vogt [18] photon parton density func- tion. The renormalization and factorization scales are
taken equal to the charm quark mass. The use of the Drees–Grassie [19] parton density function results in a decrease of the cross section of 9% formc=1.3 GeV and of 3% formc=1.7 GeV. Changing the QCD scale frommcto 2mc decreases the predicted cross section by 30% formc=1.3 GeV and 15% formc=1.7 GeV.
The measured cross section is in good agreement with our previous measurements [6,8] based on lepton tag and with the QCD expectations. The contribution of the single-resolved process is needed to describe the data.
Acknowledgements
We thank S. Frixione, E. Laenen and M. Krämer for providing us with the results of QCD cross section calculations.
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