discrepancies between the measured and predicted multiplicity are observed forpT >0.7GeV. The discrepancies increase withpT. The best agreement with the data is obtained by AMBT1, Perugia0 andPhojet, which describe thepT spectrum to within 20%. The ATLAS MC09 tune predicts too many particles at highpT, while DW predicts too few.
Figure 10.1(c) shows the charged particle multiplicity distribution. ThePythia-based models predict more events withnch = 1 than in data, but fewer events fornch&10. This results in an average number of charged particle lower than in data as noted in Figs. 10.1(a) and 10.1(b). Phojet and AMBT1 describe thenchspectrum to within 10%. Most models do not predict sufficient high multiplicity events. The discrepancy for events withnch&40is almost a factor of two, but the data are limited by the systematic uncertainties because the uncertainty on the efficiency to reconstruct each track is additive.
The averagepT as a function ofnchis shown in Fig. 10.1(d). The average transverse momentum increases with the number of charged particles. The slope of the distribution changes around nch= 10as previously noted by CDF [8]. The Perugia0 and AMBT1 models describe the data well, but the other models have discrepancies at the 10% level. The otherPythia-based models predict too large an average transverse momentum in high multiplicity events, whilePhojetpredicts too small an averagepT. The average pT as a function ofnch is particularly sensitive to the values of the colour reconnection parameters in the models. Therefore the ATLAS MC09c tune, which was based on the ATLAS MC09 tune, but with colour reconnection parameters retuned using the CDF tune ofhpTivsnchprovides a significantly better description of the data.
η / dchN d⋅evN1/
0.6 0.8 1 1.2 1.4 1.6 1.8
Data 2009
PYTHIA ATLAS AMBT1 PYTHIA ATLAS MC09 PYTHIA DW PYTHIA Perugia0 PHOJET
| < 2.5 > 500 MeV, | η pT
1,
ch≥ n
= 0.9 TeV s η / dchN d⋅evN1/
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η -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5
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]-2 [ GeVTpdη/dchN2) dTpπ 1/(2evN1/
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Data 2009
PYTHIA ATLAS AMBT1 PYTHIA ATLAS MC09 PYTHIA DW PYTHIA Perugia0 PHOJET
| < 2.5 > 500 MeV, | η pT
1,
ch≥ n
= 0.9 TeV s ]-2 [ GeVTpdη/dchN2) dTpπ 1/(2evN1/
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(b)
chn/dev N d⋅evN1/
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Data 2009
PYTHIA ATLAS AMBT1 PYTHIA ATLAS MC09 PYTHIA DW PYTHIA Perugia0 PHOJET
| < 2.5 > 500 MeV, | η pT
≥ 1, nch
= 0.9 TeV s
chn/dev N d⋅evN1/
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5 10 15 20 25 30 35 40 45
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nch
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(c)
[ GeV ]〉Tp〈
0.4 0.6 0.8 1 1.2 1.4
Data 2009
PYTHIA ATLAS AMBT1 PYTHIA ATLAS MC09 PYTHIA DW PYTHIA Perugia0 PHOJET
| < 2.5 η > 500 MeV, | pT
1,
ch≥ n
= 0.9 TeV [ GeV ]〉p〈T s
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nch
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(d)
Figure 10.1: Charged particle multiplicities for events with nch ≥ 1 within the kinematic range pT >500MeV and|η|<2.5at√
s= 900GeV. The panels show the charged particle multiplicity as a function of the pseudorapidity (a) the charged particle multiplicity as a function of the transverse momentum (b), the charged particle multiplicity (c), and the average transverse momentum as a function of the number of charged particles in the event (d) [4]. The markers represent the data and the curves predictions from different Monte Carlo models. The vertical bars represent the statistical uncertainties, while the green shaded bands show the statistical and systematic uncertainties added in quadrature. The values of the ratio histograms used the bin centroids.
7TeV the larger range means that the multiplicity varies by ten orders of magnitude. None of the models describe the shape of the pT spectrum. They predict a lower multiplicity at lowpT and a higher multiplicity at highpT than measured in data. The best agreement at low pT is for the ATLAS MC09 and AMBT1 tunes, while at mid-pT Phojet provides a slightly better description of the data.
None of the models correctly describes the multiplicity distribution in Fig. 10.2(c). They predict more events at lownch and fewer events at highnch. The slope of the averagepT as a function of nchin Fig. 10.2(d) changes around nch= 10 as at√
s= 900GeV. All the models favour a higher averagepT, with the most accurate prediction being provided by the Perugia0 and AMBT1 tunes.
At√
s= 7TeV the charged particle multiplicity is higher than at√
s= 900GeV as expected.
However, most models did not predict a sufficient increase in the multiplicity when the centre of mass energy increased from√
s= 900GeV to√
s= 7TeV. Therefore larger discrepancies between data and simulation are observed at√
s= 7TeV in all distributions. In particular, this means that the parameters controlling the extrapolation in the multiplicity with the centre of mass energy, needed retuning.
Because of these large discrepancies, the ATLAS collaboration has produced the AMBT1 tune using the LHC data at √
s = 900 GeV and √
s = 7 TeV. To limit the contribution from the large uncertainties on the modelling of diffractive processes in Pythia, the tune was based on the distributions with nch > 6, which eliminates the contribution from diffractive events [126].
Results from ATLAS underlying event measurements at √
s = 900 GeV and √
s = 7 TeV [129]
were also used. AMBT1 was based on the ATLAS MC09c tune, but five parameters describing the colour reconnection and multiple parton interactions were varied. The parameters describing the distribution of hadronic matter and colour reconnection were adjusted to improve the description of the shape of the nch and pT distributions. The parameters describing the cut-off for multiple parton interactions and its extrapolation with energy, on the other hand, did not change. The resulting tune describes the minimum bias data at √
s= 900GeV and √
s= 7 TeV well with the highpT region described to an accuracy of 10%.
η / dchN d⋅evN1/
1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3
Data 2010
PYTHIA ATLAS AMBT1 PYTHIA ATLAS MC09 PYTHIA DW PYTHIA Perugia0 PHOJET
| < 2.5 > 500 MeV, | η pT
1,
ch≥ n
= 7 TeV s η / dchN d⋅evN1/
1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3
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1.2 Data Uncertainties MC / Data
η -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5
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]-2 [ GeV Tpdη/dchN2 ) d Tpπ 1/(2evN1/
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Data 2010
PYTHIA ATLAS AMBT1 PYTHIA ATLAS MC09 PYTHIA DW PYTHIA Perugia0 PHOJET
| < 2.5 > 500 MeV, | η pT
1,
ch≥ n
= 7 TeV s ]-2 [ GeV Tpdη/dchN2 ) d Tpπ 1/(2evN1/
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Data 2010
PYTHIA ATLAS AMBT1 PYTHIA ATLAS MC09 PYTHIA DW PYTHIA Perugia0 PHOJET
| < 2.5 > 500 MeV, | η pT
≥ 1, nch
= 7 TeV s
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(c)
[ GeV ]〉 Tp〈
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Data 2010
PYTHIA ATLAS AMBT1 PYTHIA ATLAS MC09 PYTHIA DW PYTHIA Perugia0 PHOJET
| < 2.5 η > 500 MeV, | pT
≥ 1, nch
= 7 TeV [ GeV ]〉p〈 T s
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Data Uncertainties MC / Data
nch
10 20 30 40 50 60 70 80 90
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0.8 1 1.2
(d)
Figure 10.2: Charged particle multiplicities for events with nch ≥ 1 within the kinematic range pT >500 MeV and |η|<2.5 at √
s= 7 TeV. The panels shows the charged particle multiplicity as a function of the pseudorapidity (a) and of the transverse momentum (b), the charged particle multiplicity (c), and the average transverse momentum as a function of the number of charged particles in the event (d). The markers represent the data and the curves predictions from different Monte Carlo models. The vertical bars represent the statistical uncertainties, while the green shaded ares show the statistical and systematic uncertainties added in quadrature. The values of the ratio histograms used the bin centroids [127].