Shape, transverse size, and charged-hadron multiplicity of jets in pp collisions at sqrt(s) = 7 TeV

TL;DR

This CMS study analyzes jet substructure in pp collisions at sqrt(s) = 7 TeV using 36 pb^-1 to measure jet shapes ($\\rho(r)$, $\\Psi(r)$), mean charged-hadron multiplicity ($\\langle N_{\\text{ch}}\\rangle$), and jet width ($\\langle \\delta R^2\\rangle$). Jets are reconstructed with anti-($k_T$) algorithms using PF and JPT inputs, and observables are unfolded to particle level with MC-based corrections, enabling comparison to multiple QCD MC generators (PYTHIA6 tunes D6T, Z2, Perugia2010; PYTHIA8; HERWIG++). The results show jets become progressively narrower with higher $p_T$ and display mild rapidity dependence; PYTHIA6 Z2 describes low-$p_T$ shapes well, while D6T and Perugia2010 perform better at higher $p_T$, while the models generally overpredict charged multiplicity. These measurements provide essential constraints for tuning parton showers and MPIs, and for understanding quark vs gluon jet composition in collider analyses.

Abstract

Measurements of jet characteristics from inclusive jet production in proton-proton collisions at a centre-of-mass energy of 7 TeV are presented. The data sample was collected with the CMS detector at the LHC during 2010 and corresponds to an integrated luminosity of 36 inverse picobarns. The mean charged hadron multiplicity, the differential and integral jet shape distributions, and two independent moments of the shape distributions are measured as functions of the jet transverse momentum for jets reconstructed with the anti-kT algorithm. The measured observables are corrected to the particle level and compared with predictions from various QCD Monte Carlo generators.

Figures (10)

• Figure 1: Pictorial definition of the differential (top) and integrated (bottom) jet shape quantities. Analytical definitions of these quantities are given in the text.
• Figure 2: Differential jet shape as a function of the distance from the jet axis for central jets ($|y|<1$) with jet transverse momentum ranging from 20 to 125${\,\text{Ge\spaceV\space/\space}c}$ for representative jet $p_{\mathrm{T}}\xspace$ bins. The data are compared to particle-level herwig++, pythia8, and pythia6 predictions with various tunes. Statistical uncertainties are shown as error bars on the data points and the shaded region represents the total systematic uncertainty of the measurement. Data points are placed at the bin centre; the horizontal bars show the size of the bin. The ratio of each MC prediction to the data is also shown in the lower part of each plot.
• Figure 3: Differential jet shape as a function of the distance from the jet axis for central jets ($|y|<1$) with jet transverse momentum ranging from 140 to 1000${\,\text{Ge\spaceV\space/\space}c}$ for representative jet $p_{\mathrm{T}}\xspace$ bins. The data are compared to particle-level herwig++, pythia8, and pythia6 predictions with various tunes. Statistical uncertainties are shown as error bars on the data points and the shaded region represents the total systematic uncertainty of the measurement. Data points are placed at the bin centre; the horizontal bars show the size of the bin. The ratio of each MC prediction to the data is also shown in the lower part of each plot.
• Figure 4: Measured integrated jet shape, $1 - \Psi(r=0.3)$, as a function of jet $p_{\mathrm{T}}\xspace$ in the central rapidity region $|y|<1$, compared to herwig++, pythia8, and pythia6 predictions with various tunes. Statistical uncertainties are shown as uncertainties on the data points and the shaded region represents the total systematic uncertainty of the measurement. Data points are placed at the bin centre; the horizontal bars show the size of the bin. The ratio of each MC prediction to the data is also shown in the lower part of each plot.
• Figure 5: Measured integrated jet shape, $1 - \Psi(r=0.3)$, as a function of jet $p_{\mathrm{T}}\xspace$ in different jet rapidity regions, compared to herwig++, pythia8, and pythia6 predictions with various tunes. Statistical uncertainties are shown as error bars on the data points and the shaded region represents the total systematic uncertainty of the measurement. Data points are placed at the bin centre; the horizontal bars show the size of the bin. The ratio of each MC prediction to the data is also shown in the lower part of each plot.