Extraction and validation of a new set of CMS pythia8 tunes from underlying-event measurements

Abstract

New sets of CMS underlying-event parameters ("tunes") are presented for the pythia8 event generator. These tunes use the NNPDF3.1 parton distribution functions (PDFs) at leading (LO), next-to-leading (NLO), or next-to-next-to-leading (NNLO) orders in perturbative quantum chromodynamics, and the strong coupling evolution at LO or NLO. Measurements of charged-particle multiplicity and transverse momentum densities at various hadron collision energies are fit simultaneously to determine the parameters of the tunes. Comparisons of the predictions of the new tunes are provided for observables sensitive to the event shapes at LEP, global underlying event, soft multiparton interactions, and double-parton scattering contributions. In addition, comparisons are made for observables measured in various specific processes, such as multijet, Drell-Yan, and top quark-antiquark pair production including jet substructure observables. The simulation of the underlying event provided by the new tunes is interfaced to a higher-order matrix-element calculation. For the first time, predictions from pythia8 obtained with tunes based on NLO or NNLO PDFs are shown to reliably describe minimum-bias and underlying-event data with a similar level of agreement to predictions from tunes using LO PDF sets.

Fig. 1

Illustration of several $\varphi$ regions relative to the leading object that are sensitive to the underlying event. See the text for the details on the definitions of the regions

Fig. 2

The (left column) transMIN and (right column) transMAX  charged-particle $p_{\mathrm{T}}^{\text{sum}}$ (upper row), and multiplicity (lower row) densities for particles with $p_{\mathrm{T}}>0.5\text{GeV}$ in $|\eta |<2.0$, as a function of the transverse momentum of the leading charged particle ($p_{\mathrm{T}}^{\text{max}}$), from the CMS $\sqrt{s}=13\text{TeV}$ analysis [16]. The data are compared with the pythia8 tune Monash, the CMS pythia8 tunes CUETP8S1-CTEQ6L1 and CUETP8M1, and the herwig7 (labelled as “H7”) tune UE-MMHT. The ratios of the simulations to the data (MC/Data) are also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 3

Distribution of number of MPI simulated by the tunes Monash, CP2, CP3, CP4, and CP5 (left). Gluon distribution function at a reference scale of $\mu =3\text{GeV}$ (right) for the NNPDF2.3LO PDF set and the different versions of the NNPDF3.1 PDF set: LO, NLO, and NNLO. The ratio of NNPDF3.1 gluon distribution functions to the NNPDF2.3LO gluon distribution function are also shown

Fig. 4

The transMIN (upper left) charged-particle and charged $p_{\mathrm{T}}^{\text{sum}}$ (upper right) densities and the transMAX (lower left) charged-particle and charged $p_{\mathrm{T}}^{\text{sum}}$ (lower right) densities, as a function of the transverse momentum of the leading charged particle, $p_{\mathrm{T}}^{\text{max}}$, from the CMS $\sqrt{s}=13\text{TeV}$ analysis [16]. Charged hadrons are measured with $p_{\mathrm{T}}>0.5\text{GeV}$ in $|\eta |<2.0$. The transMIN densities are more sensitive to the MPI, whereas the transMAX densities are more sensitive to ISR and FSR. The data are compared with the CMS pythia8 LO-PDF tunes CP1 and CP2. The ratios of the simulations to the data (MC/Data) are also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 5

The transMIN (upper left) charged-particle and charged $p_{\mathrm{T}}^{\text{sum}}$ (upper right) densities and the transMAX (lower left) charged-particle and charged $p_{\mathrm{T}}^{\text{sum}}$ (lower right) densities, as a function of the transverse momentum of the leading charged particle, $p_{\mathrm{T}}^{\text{max}}$, from the CMS $\sqrt{s}=13\text{TeV}$ analysis [16]. Charged hadrons are measured with $p_{\mathrm{T}}>0.5\text{GeV}$ in $|\eta |<2.0$. The data are compared with the CMS pythia8 (N)NLO-PDF tunes CP3, CP4, and CP5. The ratios of simulations to the data (MC/Data) are also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 6

The transMIN (upper left) charged-particle and charged $p_{\mathrm{T}}^{\text{sum}}$ (upper right) densities and the transMAX (lower left) charged-particle and charged $p_{\mathrm{T}}^{\text{sum}}$ (lower right) densities, as a function of the transverse momentum of the leading charged particle, $p_{\mathrm{T}}^{\text{max}}$, from the CMS $\sqrt{s}=7\text{TeV}$ analysis [10]. Charged hadrons are measured with $p_{\mathrm{T}}>0.5\text{GeV}$ in $|\eta |<0.8$. The data are compared with the CMS pythia8 LO-PDF tunes CP1 and CP2. The ratios of simulations to the data (MC/Data) are also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 7

The transMIN (upper left) charged-particle and charged $p_{\mathrm{T}}^{\text{sum}}$ (upper right) densities and the transMAX (lower left) charged-particle and charged $p_{\mathrm{T}}^{\text{sum}}$ (lower right) densities, as a function of the transverse momentum of the leading charged particle, $p_{\mathrm{T}}^{\text{max}}$, from the CMS $\sqrt{s}=7\text{TeV}$ analysis [10]. Charged hadrons are measured with $p_{\mathrm{T}}>0.5\text{GeV}$ in $|\eta |<0.8$. The data are compared with the CMS pythia8 (N)NLO-PDF tunes CP3, CP4, and CP5. The ratios of simulations to the data (MC/Data) are also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 8

The transMIN (upper left) charged-particle and charged $p_{\mathrm{T}}^{\text{sum}}$ (upper right) densities and the transMAX (lower left) charged-particle and charged $p_{\mathrm{T}}^{\text{sum}}$ (lower right) densities, as a function of the transverse momentum of the leading charged particle, $p_{\mathrm{T}}^{\text{max}}$, from the CDF $\sqrt{s}=1.96\text{TeV}$ analysis [9]. Charged hadrons are measured with $p_{\mathrm{T}}>0.5\text{GeV}$ in $|\eta |<0.8$. The data are compared with the CMS pythia8 LO-PDF tunes CP1 and CP2. The ratios of simulations to the data (MC/Data) are also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 9

The transMIN (upper left) charged-particle and charged $p_{\mathrm{T}}^{\text{sum}}$ (upper right) densities and the transMAX (lower left) charged-particle and charged $p_{\mathrm{T}}^{\text{sum}}$ (lower right) densities, as a function of the transverse momentum of the leading charged particle, $p_{\mathrm{T}}^{\text{max}}$, from the CDF $\sqrt{s}=1.96\text{TeV}$ analysis [9]. Charged hadrons are measured with $p_{\mathrm{T}}>0.5\text{GeV}$ in $|\eta |<0.8$. The data are compared with the CMS pythia8 (N)NLO-PDF tunes CP3, CP4, and CP5. The ratios of simulations to the data (MC/Data) are also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 10

The pseudorapidity distribution of charged hadrons measured in $|\eta |<2$ for an inclusive selection in inelastic proton-proton collisions, with zero magnetic field strength (B = 0 T), from the CMS $\sqrt{s}=13\text{TeV}$ analysis [18]. The data are compared with the CMS pythia8 LO-PDF tunes CP1 and CP2 (left), and with the CMS pythia8 NLO-PDF tune CP3 and the CMS pythia8 NNLO-PDF tunes CP4 and CP5 (right). The ratios of simulations to the data (MC/Data) are also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 11

The normalized cross sections as a function of event-shape variables, oblateness (upper left), sphericity (upper right), thrust (lower left), and thrust major (lower right) from the ALEPH $\sqrt{s}=91.2\text{GeV}$ analysis [45], compared with the predictions by mg5_amc $+$ pythia8 with $k_{\mathrm{T}}$–MLM merging, for tunes CP2, CP3, and CP5. The ratio of the simulations to the data (MC/Data) is also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 12

The normalized cross sections as a function of event-shape variables, oblateness (upper left), sphericity (upper right), thrust (lower left), and thrust major (lower right) from the ALEPH $\sqrt{s}=91.2\text{GeV}$ analysis [45], compared with the predictions by mg5_amc $+$ pythia8 with $k_{\mathrm{T}}$–MLM merging, for tune CP5, CP5 with CMW rescaling, CP5 FSR up, and CP5 FSR down. The ratio of the simulations to the data (MC/Data) is also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 13

The pseudorapidity distribution ($p_{\mathrm{T}}>0.5\text{GeV}$, $|\eta |<2.4$) for the NSD-enhanced (left) and the SD-enhanced (right) event selection of charged particles in inelastic proton-proton collisions, from the CMS $\sqrt{s}=13\text{TeV}$ analysis [24]. The data are compared with the CMS pythia8 LO-PDF tunes CP1 and CP2, the CMS pythia8 NLO-PDF tune CP3, and the CMS pythia8 NNLO-PDF tunes CP4 and CP5. The ratio of the simulations to the data (MC/Data) is also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 14

The transMIN charged-particle multiplicity (left column) and $p_{\mathrm{T}}$ sum densities (right column) for particles with $p_{\mathrm{T}}>0.5\text{GeV}$ in $|\eta |<2.0$ as a function of the transverse momentum of the leading charged-particle jet, $p_{\mathrm{T}}^{\text{jet}}$, from the CMS $\sqrt{s}=13\text{TeV}$ analysis [16]. The upper-row plots show the LO tunes, while the lower-row plots show the higher-order tunes. The ratio of the simulations to the data (MC/Data) is also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 15

The energy flow measured in an inclusive selection as a function of pseudorapidity, from the CMS $\sqrt{s}=13\text{TeV}$ analysis [47]. The data are compared with the CMS pythia8 LO-PDF tunes CP1 and CP2 (left), and with the CMS pythia8 NLO-PDF tune CP3 and the CMS pythia8 NNLO-PDF tunes CP4 and CP5 (right). The ratio of the simulations to the data (MC/Data) is also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 16

The total energy spectrum measured in the pseudorapidity interval $-6.6<\eta <-5.2$, from the CMS $\sqrt{s}=13\text{TeV}$ analysis [49]. The data are compared with the CMS pythia8 LO-PDF tunes CP1 and CP2 (left), and with the CMS pythia8 NLO-PDF tune CP3 and the CMS pythia8 NNLO-PDF tunes CP4 and CP5 (right). The ratio of the simulations to the data (MC/Data) is also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 17

The azimuthal difference $\Delta {\varphi }_{1,2}$ between the leading two jets with $|\eta |<2.4$ in dijet events with leading-jet transverse momentum in the range (left) $200<p_{\mathrm{T}}^{\text{lead}}<300\text{GeV}$ and (right) $300<p_{\mathrm{T}}^{\text{lead}}<400\text{GeV}$, from the CMS $\sqrt{s}=13\text{TeV}$ analysis [57]. The jets are reconstructed using the anti-$k_{\mathrm{T}}$ jet finding algorithm [58, 59] with a distance parameter of 0.4. The data are compared with predictions of the NLO dijet ME calculation from powheg, interfaced to the pythia8 tunes CUETP8M1, CP2, CP4, and CP5. Tunes CP1 and CP3 are not shown in the plot but present a similar behavior as tunes CP2 and CP4. The ratios of simulations to the data (MC/Data) are also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 18

The correlation observable $\Delta$S measured in 4j (left) and 2b2j (right) production, compared to predictions of pythia8 tunes CUETP8M1, CP2, CP4, and CP5, from the CMS $\sqrt{s}=7\text{TeV}$ analyses [61, 63]. Tunes CP1 and CP3 are not shown in the plot but show a similar behaviour as, respectively, tunes CP2 and CP4. The ratios of simulations to the data (MC/Data) are also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 19

The normalized $\mathrm{t}\overline{\mathrm{t}}$ cross section in the lepton$+$jets channel, as a function of the transverse momentum of the top quark for leptonically decaying top quarks ($t_{\ell }$) (upper), the invariant mass of the $\mathrm{t}\overline{\mathrm{t}}$ system, m($\mathrm{t}\overline{\mathrm{t}}$) (middle), and in bins of number of additional jets (lower) from CMS $\sqrt{s}=13\text{TeV}$ analysis [69]. The data are compared with the predictions of powheg (left) and mg5_amc [FxFx] (right). In both cases, the PS simulation is done with the pythia8 tunes CUETP8M1, CP2, CP4, or CP5. Tunes CP1 and CP3 are not shown in the plot but present a similar behaviour as, respectively, tunes CP2 and CP4. The ratios of simulations to the data (MC/Data) are also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 20

Comparison with the measurement [70] of the angle between two groomed subjets, $\Delta R_g$ in $\mathrm{t}\overline{\mathrm{t}}$ events predicted by powheg  $+$ pythia8 for the different tunes. The data are compared to tunes CUETP8M1, CP2, CP4, and CP5 (left). Tunes CP1 and CP3 are not displayed but they present a similar behavior as tunes CP2 and CP4, respectively. The data are also compared to CP5, CP5 FSR up, CP5 FSR down, and CP5 with CMW rescaling (right). The ratios of simulations to the data (MC/Data) are also shown, where the shaded band indicates the total experimental uncertainty in data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Horizontal bars indicate the associated bin width

Fig. 21

The charged-particle multiplicity (left) and $p_{\mathrm{T}}^{\text{sum}}$ (right) in the toward (upper), transverse (middle), and away (lower) regions measured as a function of the Z boson $p_{\mathrm{T}}$ in Drell–Yan events at $\sqrt{s}=13\text{TeV}$ [21], and compared with the predictions obtained by an inclusive NLO ME calculated by mg5_amc, interfaced to the UE simulation of pythia8 with the CUETP8M1, CP2, CP4, and CP5 tunes. Tunes CP1 and CP3 are not shown in the plot but present a similar behaviour as, respectively, tunes CP2 and CP4. The ratios of simulations to the data (MC/Data) are also shown, where the shaded band indicates the total experimental uncertainty of the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 22

Comparison with the measurement [71, 72] of the inclusive jet multiplicity in Z $+$jets (upper) and W $+$jets (lower) events predicted by mg5_amc $+$ pythia8 with $k_{\mathrm{T}}$–MLM merging (left) and FxFx merging (right) for the different tunes. Tunes CP1 and CP3 are not shown in the plot but present a similar behaviour as, respectively, tunes CP2 and CP4. The ratios of simulations to the data (MC/Data) are also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 23

Comparison with the measurement [71] of the $p_{\mathrm{T}}$ balance predicted by mg5_amc $+$ pythia8 with $k_{\mathrm{T}}$–MLM merging (left) and FxFx merging (right) for the different tunes for events with at least one jet. Tunes CP1 and CP3 are not shown in the plot but they present a similar behaviour as tunes CP2 and CP4, respectively. The ratios of simulations to the data (MC/Data) are also shown, where the shaded band indicates the total experimental uncertainty in the data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 24

Comparison with the measurement [71] of the $p_{\mathrm{T}}$ (Z) predicted by mg5_amc $+$ pythia8 with $k_{\mathrm{T}}$–MLM merging (left) and FxFx merging (right) for the different tunes. Tunes CP1 and CP3 are not shown in the plot but they present a similar behaviour as tunes CP2 and CP4, respectively. The ratios of simulations to the data (MC/Data) are also shown, where the shaded band indicates the total experimental uncertainty in data. Vertical lines drawn on the data points refer to the total uncertainty in the data. Vertical lines drawn on the MC points refer to the statistical uncertainty in the predictions. Horizontal bars indicate the associated bin width

Fig. 25

The variations allowed by the CP5 tune when ${\alpha }_S^{\mathrm{ISR}}(m_{\mathrm{Z}})$ (blue band) and ${\alpha }_S^{\mathrm{FSR}}(m_{\mathrm{Z}})$ (red band) are left free in the fit for charged-particle (left) and charged $p_{\mathrm{T}}^{\text{sum}}$ (right) density in the transMIN region at $\sqrt{s}=13\text{TeV}$. Vertical lines drawn on the data points refer to the total uncertainty in the data. The grey band represents the total UE uncertainty for the tune CP5. Horizontal bars indicate the associated bin width