Energy-energy correlators are constructed by averaging the number of charged particle pairs within jets, weighted by the product of their transverse momenta, as a function of the angular separation of the particles within a pair. They are sensitive to a multitude of perturbative and nonperturbative quantum chromodynamics phenomena in high-energy particle collisions. Using lead-lead data recorded with the CMS detector, energy-energy correlators inside high transverse momentum jets are measured in heavy ion collisions for the first time. The data are obtained at a nucleon-nucleon center-of-mass energy of 5.02 TeV and correspond to an integrated luminosity of 1.70 nb$^{-1}$. A similar analysis is done for proton-proton collisions at the same center-of-mass energy to establish a reference. The ratio of lead-lead to proton-proton energy-energy correlators reveals significant jet substructure modifications in the quark-gluon plasma. The results are compared to different models that incorporate either color coherence or medium response effects, where the two effects predict similar substructure modifications.
The energy-energy correlator distributions constructed with charged particles with $p_{\mathrm{T}} > 1$ GeV for energy weight $n=1$ and jet $p_{\mathrm{T}}$ selection $120 < p_{\mathrm{T,jet}} < 140$ GeV. The results are shown for different centrality bins in PbPb collisions and for pp collisions.
The energy-energy correlator distributions constructed with charged particles with $p_{\mathrm{T}} > 1$ GeV for energy weight $n=1$ and jet $p_{\mathrm{T}}$ selection $120 < p_{\mathrm{T,jet}} < 140$ GeV. The results are shown for different centrality bins in PbPb collisions and for pp collisions.
The energy-energy correlator distributions constructed with charged particles with $p_{\mathrm{T}} > 1$ GeV for energy weight $n=1$ and jet $p_{\mathrm{T}}$ selection $140 < p_{\mathrm{T,jet}} < 160$ GeV. The results are shown for different centrality bins in PbPb collisions and for pp collisions.
The production of prompt $\Lambda^+_\mathrm{c}$ baryons is measured via the exclusive decay channel $\Lambda^+_\mathrm{c}$$\to$ pK$^-\pi^+$ at a center-of-mass energy per nucleon pair of 5.02 TeV, using proton-proton (pp) and lead-lead (PbPb) collision data collected by the CMS experiment at the CERN LHC. The pp and PbPb data were obtained in 2017 and 2018 with integrated luminosities of 252 and 0.607 nb$^{-1}$, respectively. The measurements are performed within the $\Lambda^+_\mathrm{c}$ rapidity interval $\vert y \vert$$\lt$ 1 with transverse momentum ($p_\mathrm{T}$) ranges of 3-30 and 6-40 GeV/$c$ for pp and PbPb collisions, respectively. Compared to the yields in pp collisions scaled by the expected number of nucleon-nucleon interactions, the observed yields of $\Lambda^+_\mathrm{c}$ with $p_\mathrm{T}$$\gt$ 10 GeV/$c$ are strongly suppressed in PbPb collisions. The level of suppression depends significantly on the collision centrality. The $\Lambda^+_\mathrm{c}$ / D$^0$ production ratio is similar in PbPb and pp collisions at $p_\mathrm{T}$$\gt$ 10 GeV/$c$, suggesting that the coalescence process does not play a dominant role in prompt $\Lambda^+_\mathrm{c}$ baryon production at higher $p_\mathrm{T}$.
The product of acceptance and efficiency ($A \epsilon$) as a function of $p_{\mathrm{T}}$ for prompt $\Lambda^+_c$ in pp collisions and within centrality regions of 0-90, 0-10, 10-30, 30-50 and 50-90% in PbPb collisions.
The $p_{\mathrm{T}}$ differential cross sections for prompt $\Lambda^+_c$ production in pp collisions. The global fit uncertainty is 8.6%.
The $\mathrm{T_{AA}}$-scaled $\Lambda^+_c$ yields as a function of $p_{\mathrm{T}}$ for PbPb collisions within centrality regions of 0-90, 0-10, 10-30, 30-50 and 50-90%.
The multiplicity dependence of jet production in pp collisions at the centre-of-mass energy of $\sqrt{s} = 13\ \mathrm{TeV}$ is studied for the first time. Jets are reconstructed from charged particles using the anti-$k_\mathrm{T}$ algorithm with resolution parameters $R$ varying from $0.2$ to $0.7$. The jets are measured in the pseudorapidity range $|\eta_{\rm jet}|< 0.9-R$ and in the transverse momentum range $5<p_\mathrm{T,jet}^{\rm ch}<140\ \mathrm{GeV}/c$. The multiplicity intervals are categorised by the ALICE forward detector V0. The $p_{\mathrm{T}}$ differential cross section of charged-particle jets are compared to leading order (LO) and next-to-leading order (NLO) perturbative quantum chromodynamics (pQCD) calculations. It is found that the data are better described by the NLO calculation, although the NLO prediction overestimates the jet cross section below $20\ \mathrm{GeV}/c$. The cross section ratios for different $R$ are also measured and compared to model calculations. These measurements provide insights into the angular dependence of jet fragmentation. The jet yield increases with increasing self-normalised charged-particle multiplicity. This increase shows only a weak dependence on jet transverse momentum and resolution parameter at the highest multiplicity. While such behaviour is qualitatively described by the present version of PYTHIA, quantitative description may require implementing new mechanisms for multi-particle production in hadronic collisions.
Inclusive charged-particle jet cross sections in pp collisions at $\sqrt{s}$ = 13 TeV using the anti-kT algorithm for different jet resolution parameters R from 0.2 to 0.7, with UE subtraction. Statistical uncertainties are displayed as vertical error bars. The total systematic uncertainties are shown as solid boxes around the data points.
Ratio of charged-particle jet cross section for resolution parameter R = 0.2 to other radii R = X, with X ranging from 0.3 to 0.7, after UE subtraction. Data are compared with LO (PYTHIA) and NLO (POWHEG+PYTHIA8) predictions as shown in the bottom panels. The systematic uncertainties of the cross section ratios from data are indicated by solid boxes around data points in the upper panel and shaded bands around unity in the mid and lower panels. No uncertainties are shown for theoretical predictions for better visibility.
Charged-particle jet yields in different V0M multiplicity percentile intervals for resolution parameters R varied from 0.2 to 0.7 in pp collisions at s = 13 TeV. Statistical and total systematic uncertainties are shown as vertical error bars and boxes around the data points, respectively.
Forum