Measurements of jet quenching using semi-inclusive hadron+jet distributions in pp and central Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV

The ALICE Collaboration reports measurements of the semi-inclusive distribution of charged-particle jets recoiling from a high transverse momentum (high $p_{\rm T}$) charged hadron, in pp and central Pb$-$Pb collisions at center of mass energy per nucleon$-$nucleon collision $\sqrt{s_{\rm NN}}=5.02$ TeV. The large uncorrelated background in central Pb$-$Pb collisions is corrected using a data-driven statistical approach, which enables precise measurement of recoil jet distributions over a broad range in $p_{\rm T,ch\,jet}$ and jet resolution parameter $R$. Recoil jet yields are reported for $R=0.2$, 0.4, and 0.5 in the range $7 <~ p_{\rm T,ch\, jet} <~ 140$ GeV$/c$ and $\pi/2<~\Delta\varphi<~\pi$, where $\Delta\varphi$ is the azimuthal angular separation between hadron trigger and recoil jet. The low $p_{\rm T,ch\,jet}$ reach of the measurement explores unique phase space for studying jet quenching, the interaction of jets with the quark-gluonnplasma generated in high-energy nuclear collisions. Comparison of $p_{\rm T,ch\,jet}$ distributions from pp and central Pb$-$Pb collisions probes medium-induced jet energy loss and intra-jet broadening, while comparison of their acoplanarity distributions explores in-medium jet scattering and medium response. The measurements are compared to theoretical calculations incorporating jet quenching.

 

Accepted by: PRC
HEP Data
e-Print: arXiv:2308.16128 | PDF | inSPIRE
CERN-EP-2023-188
Figure group

Figure 1

Trigger-normalized recoil jet distributions ($\rr=0.4$) as a function of \dphi and \pTreco, in \pp collisions (left) and in \PbPb collisions (right) at $\snn=5.02$ \TeV, for \TT{20}{50} \gev. The azimuthal region of the analysis is indicated by the vertical dashed lines.

Figure 2

$\rho$ distributions for central \PbPb\ collisions at $\sqrtsNN=5.02$ TeV. Upper panel: \TTSig\ and \TTRef\ distributions, and the \TTRef\ distribution shifted by $\drho=1.7$ \gev\ (Sec. \ref{sect:rhoaligngment}). Lower panel: ratio of \TTRef\ and shifted \TTRef\ to \TTSig\ distribution. The mean ($\mu$) and RMS of the distributions are given in the legend. The vertical lines on the data points are the statistical uncertainties, and the shaded band on the ratio of the \TTSig\ over the shifted \TTRef\ distributions represents the systematic uncertainty of the procedure.

Figure 3

Trigger-normalized semi-inclusive recoil jet distributions for \TTSig\ and \TTRef-selected populations in central \PbPb\ collisions at $\sqrtsNN=5.02$ \TeV, for $\rr=0.2$ (upper panels), 0.4 (middle panels), and 0.5 (lower panels). The \drho\ correction (Sec. \ref{sect:rhoaligngment}) has been applied to the \TTRef\ distributions. The left column shows distributions in the \dphi\ acceptance of the \DrecoilpT\ analysis. The remaining columns show distributions in selected \dphi\ bins for the \Drecoilphi\ analysis, with the second column having the largest deviation from $\dphi=\pi$ and the rightmost column at $\dphi\sim\pi$.

Figure 4

Ratio of \TTSig\ and \TTRef-selected distributions from Fig. \ref{fig:cref-PbPb-pt}, with the same panel layout. The horizontal dashed line and band show the value of \cRef\ and its uncertainty. The vertical solid grey line is at $\pTreco=0$, and horizontal solid grey line is at unity. See text for details.

Figure 5

Scaling factor \cRef\ of \Drecoilphi\ for $\rr=0.2$, 0.4, and 0.5. The vertical bars indicate the statistical uncertainties. The value of \cRef\ for the \DrecoilpT\ analysis, which integrates over the region $|\dphi - \pi| < 0.6$, is displayed as a horizontal bar for each value of \rr, with shaded bands indicating statistical uncertainty.

Figure 6

Trigger-normalized semi-inclusive recoil jet distributions for \TTSig\ and \TTRef-selected populations in central \PbPb\ collisions at $\sqrtsNN=5.02$ \TeV, for $\rr=0.2$ (top), 0.4 (middle), and 0.5 (bottom). The \TTRef\ distribution has the \drho\ calibration applied and is scaled by \cRef. The resulting \Drecoil\ distribution is also shown. Left panels: Distributions as a function of \pTreco\ in the \dphi\ acceptance of the \DrecoilpT\ analysis. Right panels: Distributions as a function of \dphi, for $\pTreco\in[20,30$] \gev. Data points with a negative value for \Drecoil\ are not shown, but all such points are consistent with zero within statistical error.

Figure 7

Left: trigger-normalized semi-inclusive recoil jet distributions with $\rr=0.4$ for \TTSig\ and \TTRef-selected populations in \pp\ collisions at $\s=5.02$ \TeV; Right: ratio of the two distributions. The horizontal blue line indicates the fit to the ratio close to $\pTreco=0$ for the determination of \cRef, the value of which is also given in the figures.

Figure 8

Trigger-normalized semi-inclusive recoil jet distributions for \TTSig\ and \TTRef-selected populations in \pp collisions at $\sqrts=5.02$ \TeV, for $\rr=0.2$ (top), 0.4 (middle), and 0.5 (bottom). The \TTRef\ distribution is scaled by \cRef. The resulting \Drecoil\ distribution is also shown. Left panels: Distributions as a function of \pTreco\ in the \dphi\ acceptance of the \DrecoilpT\ analysis. Right panels: Distributions as a function of \dphi, for $\pTreco\in[20,30$] \gev. \TTSig\ and \TTRef\ distributions in left middle panel are the same as in Fig. \ref{fig:ppcref}, left panel.

Figure 9

Closure test of \Drecoilphi\ analysis for central \PbPb\ collisions, for $\rr=0.2$, 0.4, and 0.5 in selected \pTjetch\ bins.

Figure 11

Upper panels: corrected \DrecoilpTch\ distributions measured for $\rr=0.2$ (left), 0.4 (middle), and 0.5 (right) in \pp\ collisions at $\sqrts=5.02$ TeV, compared to calculations from JETSCAPE  , JEWEL  , PYTHIA8  , and POWHEG  . Lower panels: ratio of the data and calculations to a functional fit of the measured \DrecoilpTch\ distributions.

Figure 12

Corrected \Drecoilphi\ distributions for \pp\ collisions at $\sqrts=5.02$ TeV for $\rr=0.2$ (top), 0.4 (middle), and 0.5 (bottom) in \pTjetch\ bins (left to right): [10,20], [20,30], [30,50], and [50,100] \gev. JETSCAPE, JEWEL, PYTHIA8, and POWHEG calculations are also shown Upper sub-panels show the individual distributions, while lower sub-panels show their ratio to a functional fit of the measured data.

Figure 13

Upper panels: corrected \DrecoilpTch\ distributions measured for $\rr=0.2$ (left), 0.4 (middle), and 0.5 (right) in \PbPb\ collisions at $\sqrtsNN=5.02$ TeV, compared to theoretical calculations from JETSCAPE and JEWEL. Lower panels: ratio of the data and calculations to a functional fit of the measured \DrecoilpTch\ distributions.

Figure 14

Corrected \Drecoilphi\ distributions for \PbPb\ collisions at $\sqrtsNN=5.02$ TeV, for $\rr=0.2$ (top), 0.4 (middle), and 0.5 (bottom) in \pTjetch\ bins (left to right): [10,20], [20,30], [30,50], and [50,100] \gev. JETSCAPE and JEWEL calculations are also shown. Upper sub-panels show the individual distributions, while lower sub-panels show their ratio to a functional fit of the data.

Figure 15

\IAApT\ from the \DrecoilpT\ distributions measured for $\rr=0.2$ (top), 0.4 (middle), and 0.5 (bottom) in central \PbPb\ (Fig. \ref{fig:PbPb_Drecoil_pT}) and \pp\ collisions (Fig. \ref{fig:pp_Drecoil_pT}) JETSCAPE, JEWEL, and the Hybrid Model calculations are also shown.

Figure 16

\IAApT\ for $\rr=0.2$ and 0.4, for various \TTSig\ selections. The data points have been displaced for clarity, and the vertical dashed grey lines indicate the bin edges. JETSCAPE predictions for the corresponding \TTSig\ selections are also shown.

Figure 17

Ratio of \DrecoilpTch\ distributions in \pp\ collisions at $\sqrts=5.02$ TeV using the data from Fig. \ref{fig:pp_Drecoil_pT}, for $\rr=0.2/\rr=0.4$ (left) and $\rr=0.2/\rr=0.5$ (right), compared to calculations from PYTHIA8 and JETSCAPE. The corresponding ratios of cross sections for inclusive jets are also shown for \pp\ collisions at $\sqrts = 5.02$ \TeV  and $\sqrts = 13$ \TeV . The uncertainties in the ratio take into account the correlation of uncertainties between numerator and denominator.

Figure 18

Ratio of \DrecoilpTch\ distributions with different \rr\ for \PbPb\ collisions at $\sqrtsNN=5.02$ TeV from the data in Fig. \ref{fig:PbPb_Drecoil_pT}, together with the ratios for \pp\ collisions from Fig. \ref{fig:RRatios-pp}, for $\rr=0.2/\rr=0.4$ (left) and $\rr=0.2/\rr=0.5$ (right). The uncertainty in the ratio takes into account the correlation of uncertainties between numerator and denominator. JETSCAPE calculations for \pp\ and \PbPb\ collisions are also shown.

Figure 19

Distributions as a function of \dphi\ for $\rr=0.2$. Upper panels: \Drecoilphi\ in intervals of \pTjetch\ measured in \pp\ and \PbPb\ collisions. Lower panels: \IAApT, the ratio of the \pp\ and \PbPb\ distributions in the corresponding upper panel. Predictions from JETSCAPE, JEWEL, Hybrid model, and a pQCD calculation are also shown.