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Register a new userOptimizing the parton shower model in PYTHIA with pp collision data at $sqrt{s}$ = 13 TeV
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Production of quarks and gluons in hadron collisions tests Quantum Chromodynamics (QCD) over a wide range of energy. Models of QCD are implemented in event generators to simulate hadron collisions and evolution of quarks and gluons into jets of hadrons. PYTHIA8 uses the parton shower model for simulating particle collisions and is optimized using experimental observations. Recent measurements of event shape variables and jet cross-sections in pp collisions at $sqrt{s}$ = 13 TeV at the Large Hadron Collider have been used to optimize the parton shower model as used in PYTHIA8.
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Understanding the production of quarks and gluons in high energy collisions and their evolution is a very active area of investigation. Monte carlo event generator PYTHIA8 uses the parton shower model to simulate such collisions and is optimized using experimental observations. Recent measurements of event shape variables and differential jet cross-sections in pp collisions at $surd{s} = 13, TeV$ at the Large Hadron Collider have been used to investigate further the parton shower model as used in PYTHIA8.
In a previous paper, hereafter referred to as I, we have analyzed the 7 TeV LHC data on W + jets events from the standpoint of IR-improved DGLAP parton shower effects, using the IR-improved Herwiri1.031 parton shower MC in comparison with the Herwig6.5 parton shower MC in the context of the exact $O(alpha_s)$ matrix element matched parton shower framework provided by MG5_aMC@NLO. In the current paper, we extend this analysis to the LHC 8 and 13 TeV data to investigate the energy dependence of the results obtained in I. Specifically, W~+ jet events are generated in the MADGRAPH5_aMC@NLO framework and showered by HERWIG6.521 and HERWIRI1.031 with $mathtt{PTRMS}=2.2$ and 0 GeV, respectively. The differential cross sections are reported as functions of jet multiplicity, transverse linear momenta ($P_{T}$), the jet pesudo-rapidity ($eta$) and the scalar sum of jet transverse momenta ($H_{T}$) for different jet multiplicities 1--3. The dijet cross sections as functions of transverse linear momenta, invariant mass of the dijet and the jet separation are shown as well. Distributions of angular correlations between the jets and the muon are examined as well and the corresponding cross sections are presented. The respective measured cross sections are compared with the exact next-to-leading-order (NLO) matrix element matched parton shower theoretical predictions provided by MADGRAPH5_aMC@NLO/HERWIRI1.031~($mathtt{PTRMS}=0)$ and MADGRAPH5_aMC@NLO/HERWIG6.521~($mathtt{PTRMS}=2.2~mathrm{GeV})$ and the phenomenological consequences are discussed with an eye toward their energy dependence.
The production cross-sections of $Upsilon(1S)$, $Upsilon(2S)$ and $Upsilon(3S)$ mesons in proton-proton collisions at $sqrt{s}$= 13 TeV are measured with a data sample corresponding to an integrated luminosity of $277 pm 11$ $rm pb^{-1}$ recorded by the LHCb experiment in 2015. The $Upsilon$ mesons are reconstructed in the decay mode $Upsilontomu^{+}mu^{-}$. The differential production cross-sections times the dimuon branching fractions are measured as a function of the $Upsilon$ transverse momentum, $p_{rm T}$, and rapidity, $y$, over the range $0 < p_{rm T}< 30$ GeV/c and $2.0 < y < 4.5$. The ratios of the cross-sections with respect to the LHCb measurement at $sqrt{s}$= 8 TeV are also determined. The measurements are compared with theoretical predictions based on NRQCD.
The production of $mathit{Xi}_{cc}^{++}$ baryons in proton-proton collisions at a centre-of-mass energy of $sqrt{s}=13$ TeV is measured in the transverse-momentum range $4<p_mathrm{T}<15~mathrm{GeV}$/$c$ and the rapidity range $2.0<y<4.5$. The data used in this measurement correspond to an integrated luminosity of 1.7 $mathrm{fb}^{-1}$, recorded by the LHCb experiment during 2016. The ratio of the $mathit{Xi}_{cc}^{++}$ production cross-section times the branching fraction of the $mathit{Xi_{cc}^{++} to Lambda_{c}^{+} K^- pi^+ pi^+}$ decay relative to the prompt $mathit{Lambda_c^{+}}$ production cross-section is found to be $(2.22pm 0.27 pm 0.29)times 10^{-4}$, assuming the central value of the measured $mathit{Xi_{cc}^{++}}$ lifetime, where the first uncertainty is statistical and the second systematic.
In the era of precision physics measurements at the LHC, efficient and exhaustive estimations of theoretical uncertainties play an increasingly crucial role. In the context of Monte Carlo (MC) event generators, the estimation of such uncertainties traditionally requires independent MC runs for each variation, for a linear increase in total run time. In this work, we report on an automated evaluation of the dominant (renormalization-scale and non-singular) perturbative uncertainties in the PYTHIA 8 event generator, with only a modest computational overhead. Each generated event is accompanied by a vector of alternative weights (one for each uncertainty variation), with each set separately preserving the total cross section. Explicit scale-compensating terms can be included, reflecting known coefficients of higher-order splitting terms and reducing the effect of the variations. The formalism also allows for the enhancement of rare partonic splittings, such as $g to b bar{b}$ and $qto q gamma$, to obtain weighted samples enriched in these splittings while preserving the correct physical Sudakov factors.
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