Event-by-event mean $p_{\rm T}$ fluctuations in pp and Pb-Pb collisions at the LHC

Event-by-event fluctuations of the mean transverse momentum of charged particles produced in pp collisions at $\sqrt{s}$ = 0.9, 2.76 and 7 TeV, and Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV are studied as a function of the charged-particle multiplicity using the ALICE detector at the LHC. Dynamical fluctuations indicative of correlated particle emission are observed in all systems. The results in pp collisions show little dependence on collision energy. The Monte Carlo event generators PYTHIA and PHOJET are in qualitative agreement with the data. Peripheral Pb-Pb data exhibit a similar multiplicity dependence as that observed in pp. In central Pb-Pb, the results deviate from this trend, featuring a significant reduction of the fluctuation strength. The results in Pb--Pb are in qualitative agreement with previous measurements in Au-Au at lower collision energies and with expectations from models that incorporate collective phenomena.

Figures

Figure 1

Relative fluctuation $\sqrt{C_m}/M(\pt)_m$ as a function of $\langle \rm{d}N_{\rm ch}/\rm d\eta \rangle$ in pp collisions at $\sqrt{s}$ = 0.9, 2.76 and 7 TeV.
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Figure 2

Relative dynamical mean transverse momentum fluctuations in pp collisions as a function of $\sqrt{s}$. The ALICE results for $\sqrt{C}/M(\pt)_m$ are compared to the quantity $R$ measured at the ISR (see text and~\cite{isr}).
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Figure 3

Results for $\langle\pt\rangle_m$ as a function of $\langle \rm{d}N_{\rm ch}/\rm d\eta \rangle$ in pp collisions at $\sqrt{s}$ = 7 TeV from different event generators.
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Figure 4

Left: Relative dynamical fluctuation $\sqrt{C_m}/M(\pt)_m$ for data and different event generators in pp collisions at $\sqrt{s}$= 7 TeV as a function of $\langle \rm{d}N_{\rm ch}/\rm d\eta \rangle$. Right: Ratio models to data. The red error band indicates the statistical and systematic data uncertainties added in quadrature.
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Figure 5

Relative dynamical fluctuation $\sqrt{C_m}/M(\pt)_m$ as a function of $\langle \rm{d}N_{\rm ch}/\rm d\eta \rangle$ in pp and Pb-Pb collisions at $\snn$ = 2.76 TeV. Also shown are results from HIJING and power-law fits to pp (solid line) and HIJING (dashed line) (see text).
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Figure 6

Mean transverse momentum fluctuations in central heavy-ion collisions as a function of $\snn$. The ALICE data point is compared to data from the CERES~\cite{ceres-2} and STAR~\cite{star-2} experiments. For STAR only statistical uncertainties are available.
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Figure 7

Left: Relative dynamical fluctuation $\sqrt{C_m}/M(\pt)_m$ as a function of $\langle \rm{d}N_{\rm ch}/\rm d\eta \rangle$ in Pb-Pb collisions at $\snn$=2.76 TeV from ALICE compared to results from STAR in Au-Au collisions at $\snn$=200 GeV \cite{star-2}. Also shown as dashed lines are results from power-law fits to the data (see text). Right: same data as a function of $\langle N_{part}\rangle$.
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Figure 8

Relative dynamical fluctuation $\sqrt{C_m}/M(\pt)_m$ normalized to $\langle \rm{d}N_{\rm ch}/\rm d\eta \rangle^{-0.5}$ (see text) as a function of $\langle \rm{d}N_{\rm ch}/\rm d\eta \rangle$ in pp and Pb-Pb collisions at $\snn$=2.76 TeV. The ALICE data are compared to results from HIJING and AMPT.
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