Measurement of the underlying event activity in pp collisions at the LHC using leading tracks at √s = 7 TeV and comparison with √s = 0.9 TeV

Abstract

A measurement of the underlying event activity is performed on proton-proton collisions for $\sqrt{s}$ = 0.9 TeV and 7 TeV at CMS. The charged particles in the azimuthal region transverse to the leading track with $p_{T}$ > 0.5 GeV/c and within $|\eta|\!<\!0.8$ are studied. The aim of the study is to compare the measurements with various MC predictions to get extra constraints for the tuning parameters, as well as getting a better understanding for the underlying event phenomena. A significant increase in the average multiplicity and the scalar $\sum p_T$ in the transverse region is observed up to a leading track $p_T$ of few GeV/c followed by a slower rate of increase. Both quantities are observed to increase with $\sqrt{s}$.

Results at √s=7 TeV

Nch profile plot $\sum p_T$ profile plot description
Profile_Nch_pT_7TeV.png Profile_SumpT_pT_7TeV.png The corrected measurements of charged particles with pT > 0.5 GeV/c and \η\ < 0.8 in the transverse region, $60^o$ < $\Delta\phi$ < $120^o$, as a function of the $p_T$ of the leading track: The average multiplicity per unit of pseudorapidity and per radian in $\Delta\phi$ (upper left), and the average scalar $\sum p_T$ per unit of pseudorapidity per radian (upper right). The inner error bars represent the statistical uncertainty, and the outer error bars show the systematic and statistical uncertainties added in quadrature. Predictions of {\sc PYTHIA6} (tunes Z1 and D6T), {\sc PYTHIA8} tune 1 and the default tune of Herwig++ are compared to the data and the ratio of the MC prediction over the data are presented in the lower row.
Profile_Nch_pT_MCoD_7TeV.png Profile_Sum_pT_MCoD_7TeV.png  

Results for √s=0.9 TeV

Nch profile plot $\sum p_T$ profile plot description
Profile_Nch_pT_09TeV.png Profile_SumpT_pT_09TeV.png Fully corrected measurements of charged particles with pT > 0.5 GeV/c and \η\ < 0.8 in the transverse region, $60^o$ < $\Delta\phi$ < $120^o$: (upper left) The average multiplicity, and (upper right) the average scalar $\sum p_T$ , per unit of pseudorapidity per radian, as functions of the leading track $p_T$, for data at $\sqrt{s} = 0.9$ TeV. The inner error bars represent the statistical uncertainty, and the outer error bars represent systematic and statistical uncertainties added in quadrature. The lower plots show the ratio of the MC predictions over the data, as functions of the leading track $p_T$, with pT > 0.5 GeV/c and \η\ < 0.8 in the transverse region for the average multiplicity (lower left) and the average scalar $\sum p_T$ (lower right). The inner bands correspond to the average statistical uncertainty and the outer error bands correspon to the total experimental uncertainty (statistical and systematic uncertainties added in quadrature). The outer error bands are hardly visible since the regions of high leading track $p_{T}$ where the statistical uncertainty dominates in the results shown.
Profile_Nch_pT_MCoD_09TeV.png Profile_Sum_pT_MCoD_09TeV.png  

Center-of-mass energy dependence

Nch profile plot $\sum p_T$ profile plot description
Profile_Nch_pT_7TeV_09TeV.png Profile_Sum_pT_7TeV_09TeV.png Fully corrected measurements of charged particles with pT > 0.5 GeV/c and \η\ < 0.8 in the transverse region : The average multiplicity (upper left plot), and the average scalar $\sum p_T$ (upper right plot), per unit of pseudorapidity and per radian, as functions of the leading track $p_T$, for data at $\sqrt{s} = 0.9$ TeV and $\sqrt{s}$ =7 TeV. The inner error bars represent statistical uncertainty, and the outer error bars represent systematic and statistical uncertainties added in quadrature except for correlated systematic uncertainty sources. The lower row shows the ratio of the hadronic activity in the transverse region for the two center of mass energies: The average multiplicity (lower left plot) and the average scalar $\sum p_T$ (lower right plot).
Profile_Nch_pT_7over09TeV.png Profile_Sum_pT_7over09TeV.png  

Comparison with ALICE

Nch profile plot $\sum p_T$ profile plot description
CommonPlots_Profile_Nch_pT_7TeV.png CommonPlots_Profile_SumpT_pT_7TeV.png Fully corrected measurements of charged particles with pT > 0.5 GeV/c and \η\ < 0.8 in the transverse region compared to ALICE data: (left plots) The average multiplicity, and (right plots) the average scalar $\sum p_T$, per unit of pseudorapidity and per radian, as functions of the leading track $p_T$, for data at $\sqrt{s}$ = 0.9 TeV (lower row) and $\sqrt{s}$ =7 TeV (upper row).
CommonPlots_Profile_Nch_pT_09TeV.png CommonPlots_Profile_SumpT_pT_09TeV.png  

a link to the document: http://cdsweb.cern.ch/record/1478982?ln=en

-- MohammedZakaria - 26-Dec-2011

-- MohammedZakaria - 25-Sep-2012

Topic attachments
I Attachment History Action Size Date Who Comment
PNGpng CommonPlots_Profile_Nch_pT_09TeV.png r1 manage 18.2 K 2012-09-25 - 05:30 MohammedZakaria  
PNGpng CommonPlots_Profile_Nch_pT_7TeV.png r1 manage 17.5 K 2012-09-25 - 05:30 MohammedZakaria  
PNGpng CommonPlots_Profile_SumpT_pT_09TeV.png r1 manage 19.3 K 2012-09-25 - 05:30 MohammedZakaria  
PNGpng CommonPlots_Profile_SumpT_pT_7TeV.png r1 manage 18.8 K 2012-09-25 - 05:30 MohammedZakaria  
PNGpng Profile_Nch_pT_09TeV.png r1 manage 22.4 K 2012-09-25 - 05:31 MohammedZakaria  
PNGpng Profile_Nch_pT_7TeV.png r1 manage 22.4 K 2012-09-25 - 05:31 MohammedZakaria  
PNGpng Profile_Nch_pT_7TeV_09TeV.png r1 manage 19.5 K 2012-09-25 - 05:30 MohammedZakaria  
PNGpng Profile_Nch_pT_7over09TeV.png r1 manage 16.2 K 2012-09-25 - 05:30 MohammedZakaria  
PNGpng Profile_Nch_pT_MCoD_09TeV.png r1 manage 23.3 K 2012-09-25 - 05:30 MohammedZakaria  
PNGpng Profile_Nch_pT_MCoD_7TeV.png r1 manage 23.7 K 2012-09-25 - 05:32 MohammedZakaria  
PNGpng Profile_Sum_pT_7TeV_09TeV.png r1 manage 21.3 K 2012-09-25 - 05:30 MohammedZakaria  
PNGpng Profile_Sum_pT_7over09TeV.png r1 manage 16.6 K 2012-09-25 - 05:30 MohammedZakaria  
PNGpng Profile_Sum_pT_MCoD_09TeV.png r1 manage 23.2 K 2012-09-25 - 05:30 MohammedZakaria  
PNGpng Profile_Sum_pT_MCoD_7TeV.png r1 manage 23.6 K 2012-09-25 - 05:32 MohammedZakaria  
PNGpng Profile_SumpT_pT_09TeV.png r1 manage 23.1 K 2012-09-25 - 05:31 MohammedZakaria  
PNGpng Profile_SumpT_pT_7TeV.png r1 manage 24.1 K 2012-09-25 - 05:31 MohammedZakaria  
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Topic revision: r2 - 2014-10-30 - XavierJanssen
 
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