Results on two-particle angular correlations are presented in proton-proton collisions at center of mass energies of 7 TeV, over a broad range of pseudorapidity and azimuthal angle. In very high multiplicity events at 7 TeV, a pronounced structure emerges in the two-dimensional correlation function for particle pairs with intermediate $p_{\mathrm{T}}$ of 1--3 GeV/c, in the kinematic region $2.0 < |Δη| < 4.8$ and small $Δϕ$. This structure, which has not been observed in pp collisions before, is similar to what is known as the "ridge" in heavy ion collisions. It is not predicted by commonly used proton-proton Monte Carlo models and is not seen in lower multiplicity pp collisions. Updated studies of this new effect as a function of particle transverse momentum, rapidity and event characteristics are shown.
Measurements of charged dihadron DeltaEta-DeltaPhi correlations from the CMS collaboration are presented for PbPb collisions at a center-of-mass energy of 2.76 TeV per nucleon pair over a broad range of pseudorapidity and the full range of azimuthal angle. A significant correlated yield is observed for pairs of particles with small DeltaPhi but large longitudinal separation DeltaEta, commonly known as the "ridge". The ridge persists up to at least |DeltaEta| = 4 and the dependence of the ridge region shape and yield on collision centrality and transverse momentum has been measured. A Fourier a…
Dihadron correlations are measured in d+Au collisions at 200 GeV by the STAR detector. The correlated yields with uniform background subtraction are studied in high- and low-multiplicity collisions. The effects of multiplicity selection bias on jet-like correlations are discussed. Finite correlated yields are observed on the near-side at large pseudo-rapidity separation in high-multiplicity collisions.
A very recent paper by the CMS collaboration \cite{cms_ridge} has created large discussion in the media, which call it important but did not explain why, in some places even calling it "unundestandable". While it is of course too soon to know what causes the correlations in question, a very similar observation in heavy ion collisions at RHIC has rather simple explanation related to explosion of high energy density matter. Perhaps this observation is the first hint for an explosive behavior in pp, which was anticipated and looked for for decades, yet never been seen.
This paper briefly reviews the striking experimental observation of a ridge-like dihadron correlation structure in high multiplicity proton-proton collisions at the Large Hadron Collider (LHC). Recent progress of both experimental and theoretical efforts on understanding the physical origin of the novel effect is reviewed. Outlook on future direction of possible new studies is discussed.
We discuss the physics of the ridge -- azimuthally collimated long range rapidity correlations -- in high multiplicity proton-proton and proton-nucleus collisions. We outline some of the theoretical discussions in the literature that address the systematics of these ridge correlations.
A phenomenological analysis is presented of recent two-particle angular correlation data on relative pseudorapidity ($η$) and azimuth reported by the Compact Muon Solenoid (CMS) Collaboration for $\sqrt{s}$ = 7 TeV proton-proton collisions. The data are described with an empirical jet-like model developed for similar angular correlation measurements obtained from heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC). The same-side (small relative azimuth), $η$-extended correlation structure, referred to as the {\em ridge}, is compared with three phenomenological correlation struct…
We demonstrate that the recent measurement of azimuthally collimated, long range rapidity ("ridge") correlations in $\sqrt{s}=13$ TeV proton-proton (p+p) collisions by the ATLAS collaboration at the LHC are in agreement with expectations from the color glass condensate effective theory of high energy QCD. The observation that the integrated near side yield as a function of multiplicity is independent of collision energy is a natural consequence of the fact that multiparticle production is driven by a single semi-hard saturation scale in the color glass condensate framework. We argue further th…
Measurements made by the ALICE Collaboration of single- and two-particle distributions in high-energy pp and p-Pb collisions are used to characterize the interactions in small collision systems, tune models of particle production in QCD, and serve as a baseline for heavy-ion observables. The measurements of charged-particle multiplicity density, $\langle dN_{ch}/dη\rangle$, and multiplicity distributions are shown in pp and p-Pb collisions, including data from the top center-of-mass energy achieved at the Large Hadron Collider (LHC), $\sqrt{s}$ = 13 TeV. Two-particle angular correlations in p-…
Correlations between particles separated by several units of pseudorapidity were discovered in high-multiplicity pp and p-Pb collisions at the LHC. These long-range structures observed in two-particle correlation functions are reminiscent of features seen in Pb-Pb collisions, where they are often viewed as a signature of collective behavior and the formation of a quark-gluon plasma (QGP). Therefore, the discovery of these `ridges' in small systems has implications for the study of collectivity in small systems as well as in heavy-ion collisions. The ridges in pp and p-Pb collisions have been s…
In a recent article (arXiv:1210.3890), we showed that high multiplicity di-hadron proton- proton (p+p) data from the CMS experiment are in excellent agreement with computations in the Color Glass Condensate (CGC) Effective Field Theory (EFT). This agreement of the theory with several hundred data points provides a non-trivial description of both nearside ("ridge") and away-side azimuthal collimations of long range rapidity correlations in p+p collisions. Our prediction in arXiv:1210.3890 for proton-lead (p+Pb) collisions is consistent with results from the recent CMS p+Pb run at $\sqrt{s}_{\rm…
Short range correlations of identified charged hadrons in pp (sqrt(s) = 0.9, 2.76, and 7 TeV), pPb (sqrt(sNN) = 5.02 TeV), and peripheral PbPb collisions (sqrt(sNN) = 2.76 TeV) are studied with the CMS detector at the LHC. Charged pions, kaons, and protons at low pT and in laboratory pseudorapidity |eta| < 1 are identified via their energy loss in the silicon tracker. The two-particle correlation functions show effects of quantum statistics, Coulomb interaction, and also indicate the role of multi-body resonance decays and mini-jets. The characteristics of the one-, two-, and three-dimensional…
The observation of long range ridge-like structure in the near-side region of the two particle $Δη-Δϕ$ correlations as measured by LHC experiments in high multiplicity p$-$p collisions indicated towards the presence of collective effects which are similar to that observed in p$-$A(nucleon-nucleus) and A$-$A (nucleus-nucleus) collisions. The two particle correlation between the charged particles in $Δη-Δϕ$ for p$-$p collisions at $\sqrt{s}$ = 7 TeV and 13 TeV is studied using Pythia 8 event generator within the framework of final-state partonic color reconnection effects as well as the microsco…
Short range correlations of identified charged hadrons in pp ($\sqrt{s} =$ 0.9, 2.76, and 7 TeV), pPb ($\sqrt{s_{_\mathrm{NN}}} =$ 5.02 TeV), and peripheral PbPb collisions ($\sqrt{s_{_\mathrm{NN}}} =$ 2.76 TeV) are studied with the CMS detector at the LHC. Charged pions, kaons, and protons at low momentum and in laboratory pseudorapidity $|η| < 1$ are identified via their energy loss in the silicon tracker. The two-particle correlation functions show effects of quantum statistics, Coulomb interaction, and also indicate the role of multi-body resonance decays and mini-jets. The characteristics…
The Compact Muon Solenoid (CMS) is fully equipped to measure hard probes in the di-muon decay channel in the high multiplicity environment of nucleus-nucleus collisions. Such probes are especially relevant for studying the quark-gluon plasma since they are produced at early times and propagate through the medium, mapping its evolution. CMS is able to distinguish non-prompt from prompt J/psi in pp and PbPb collisions. We report here the nuclear modification factor of prompt J/psi in PbPb as a function of transverse momentum, rapidity, and the number of nucleons participating in the collision.
Angular correlations between charged trigger and associated particles are measured by the ALICE detector in p-Pb collisions at a nucleon-nucleon centre-of-mass energy of 5.02 TeV for transverse momentum ranges within 0.5 < $p_{\rm T}^{\rm assoc}$ < $p_{\rm T}^{\rm trig}$ < 4 GeV/$c$. The correlations are measured over two units of pseudorapidity and full azimuthal angle in different intervals of event multiplicity, and expressed as associated yield per trigger particle. Two long-range ridge-like structures, one on the near side and one on the away side, are observed when the per-trigger yield …
The Compact Muon Solenoid (CMS) is fully equipped to measure leptonic decays of electroweak probes in the high multiplicity environment of nucleus-nucleus collisions. Electroweak boson production is an important benchmark process at hadron colliders. Precise measurements of W and Z production in heavy-ion collisions can help to constrain nuclear PDFs as well as serve as a standard candle of the initial state in PbPb collisions at the LHC energies. The inclusive and differential measurements of the Z boson yield in the muon and electron decay channel will be presented, establishing that no modi…
The centrality dependence of midrapidity charged-particle multiplicities at a nucleon-nucleon center-of-mass energy of 2.76 TeV from CMS are compared to PHOBOS data at 200 and 19.6 GeV. The results are first fitted with a two-component model which parameterizes the separate contributions of nucleon participants and nucleon-nucleon collisions. A more direct comparison involves ratios of multiplicity densities per participant pair between the different collision energies. The results support and extend earlier indications that the influences of centrality and collision energy on midrapidity char…
ATLAS has measured two-particle correlations as a function of relative azimuthal-angle, $Δϕ$, and pseudorapidity, $Δη$, in $\sqrt{s}$=13 and 2.76 TeV $pp$ collisions at the LHC using charged particles measured in the pseudorapidity interval $|η|$<2.5. The correlation functions evaluated in different intervals of measured charged-particle multiplicity show a multiplicity-dependent enhancement at $Δϕ\sim 0$ that extends over a wide range of $Δη$, which has been referred to as the "ridge". Per-trigger-particle yields, $Y(Δϕ)$, are measured over 2<$|Δη|$<5. For both collision energies, the $Y(Δϕ)$…
A measurement is presented of the charged hadron multiplicity in hadronic PbPb collisions, as a function of pseudorapidity and centrality, at a collision energy of 2.76 TeV per nucleon pair. The data sample is collected using the CMS detector and a minimum-bias trigger, with the CMS solenoid off. The number of charged hadrons is measured both by counting the number of reconstructed particle hits and by forming hit doublets of pairs of layers in the pixel detector. The two methods give consistent results. The charged hadron multiplicity density dN(ch)/d eta, evaluated at eta=0 for head-on colli…
The LHC offers unique opportunities for studying the properties of hot QCD matter created in Pb+Pb collisions at extreme temperatures and very low parton momentum fractions. With its high precision, large acceptance for tracking and calorimetry, and a trigger scheme that allows analysis of each minimum bias Pb+Pb event, CMS is fully equipped to measure dimuons in the high multiplicity environment of Pb+Pb collisions. Such probes are especially relevant since they are produced at early times and propagate through the medium, mapping its evolution. The capabilities of the CMS experiment to study…
An interpretation of the ridge phenomenon found in pp collisions at 7 TeV is given in terms of enhancement of soft partons due to energy loss of semihard jets. A description of ridge formation in nuclear collisions can directly be extended to pp collisions, since hydrodynamics is not used, and azimuthal anisotropy is generated by semihard scattering. Both the p_T and multiplicity dependencies are well reproduced. Some suggestions are made about other observables.
Final state variables and particle correlation will be discussed under a Multiple Parton Interaction (MPI) interpretation. The state of the art about the latest results on such variables will be provided. Furthermore the role played by event multiplicity in the deep understanding of particle correlation, in particular concerning the new results on the Long-Range Near-Side two particle correlations by the CMS Collaboration, will be discussed.
The study of charm production in heavy-ion collisions is considered an excellent probe for the properties of the hot and dense medium created in heavy-ion collisions. Measurements of D-meson nuclear modification can provide strong constraints into the mechanisms of in-medium energy loss and charm flow in the medium. The measurement of $D^0$ elliptic flow in pPb collisions helps us understand the strength of charm quarks coupling to significantly reduced systems which demonstrate hydrodynamic properties. In this paper, the measurements of the $D^0$ nuclear modification factor in PbPb collisions…
The unprecedented centre-of-mass energy available at the LHC offers unique opportunities for studying the properties of the strongly-interacting QCD matter created in PbPb collisions at extreme temperatures and very low parton momentum fractions. With its high precision, large acceptance for tracking and calorimetry, and a trigger scheme that allows analysis of each minimum bias PbPb event, CMS is fully equipped to measure dimuons in the high multiplicity environment of nucleus-nucleus collisions. Such probes are especially relevant for these studies since they are produced at early times and …
Measurements of two-particle angular correlations of inclusive charged particles as well as identified strange hadron in pp and pPb collisions are presented over a wide range in pseudorapidity and full azimuth. The data were collected using the CMS detector at the LHC, with nucleon-nucleon center-of-mass energy of 5.02 TeV for pPb collisions and 5, 7, 13 TeV for pp collisions. The results are compared to semi-peripheral PbPb collision data at center-of-mass energy of 2.76 TeV, covering similar charged-particle multiplicity ranges of the events. For high-multiplicity pp and pPb events, a clear …
Measurements of charged dihadron angular correlations are presented in proton-proton (pp) and Lead-Lead (PbPb) collisions, over a broad range of pseudorapidity and azimuthal angle, using the CMS detector at the LHC. In very high multiplicity pp events at center-of-mass energy of 7 TeV, a striking "ridge"-like structure emerges in the two-dimensional correlation function for particle pairs with intermediate pt of 1-3 GeVc, in the kinematic region 2.0<|Δη|<4.8 and small Δϕ, which is similar to observations in heavy-ion collisions. Studies of this new effect as a function of particle transverse m…
Inclusive J/$ψ$ yields and average transverse momenta in p-Pb collisions at a center-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}}$ = 8.16 TeV are measured as a function of the charged-particle pseudorapidity density with ALICE. The J/$ψ$ mesons are reconstructed at forward $(2.03 < y_{\rm cms} < 3.53)$ and backward ($-4.46 < y_{\rm cms} < -2.96$) center-of-mass rapidity in their dimuon decay channel while the charged-particle pseudorapidity density is measured around midrapidity. The J/$ψ$ yields at forward and backward rapidity normalized to their respective average values increase with…
Experiments at the Large Hadron Collider (LHC) have measured multiplicity distributions in proton-proton collisions at a new domain of center-of-mass energy ($\sqrt {s}$) in limited pseudorapidity intervals. We analyze multiplicity distribution data of proton-proton collisions at LHC energies as measured by the Compact Muon Solenoid (CMS) experiment in terms of characteristic parameters of the Negative Binomial Distribution (NBD) function that has played a significant role in describing multiplicity distribution data of particle production in high energy physics experiments, in the pre-LHC ene…
Hard probes are indispensable tools to study the hot and dense quark-gluon matter created in ultra-relativistic heavy ion collisions. These probes are created in the collision itself with a small cross section, and they serve as indicators of various properties of the medium, such as temperature, viscosity, energy density, transport coefficients. Hard probes measured by the CMS and ATLAS experiments at the LHC include highly energetic jets and charged particles, quarkonium states, and electroweak gauge bosons. An overview of those recent experimental results will be given that represent the pa…
We present the charged-particle multiplicity distributions over a wide pseudorapidity range ($-3.4<η<5.0$) for pp collisions at $\sqrt{s}=$ 0.9, 7, and 8 TeV at the LHC. Results are based on information from the Silicon Pixel Detector and the Forward Multiplicity Detector of ALICE, extending the pseudorapidity coverage of the earlier publications and the high-multiplicity reach. The measurements are compared to results from the CMS experiment and to PYTHIA, PHOJET and EPOS LHC event generators, as well as IP-Glasma calculations.
A measurement of the elliptic flow ($v_2$) of prompt J/$ψ$ mesons in high-multiplicity pPb collisions is reported using data collected by the CMS experiment at a nucleon-nucleon center-of-mass energy $\sqrt{s_\mathrm{NN}} =$ 8.16 TeV. Prompt J/$ψ$ mesons decaying into two muons are reconstructed in the rapidity region in the nucleon-nucleon center-of-mass frame ($y_\mathrm{cm}$), corresponding to either $-$2.86 $\lt y_{\mathrm{cm}} <$ $-$1.86 or 0.94 $\lt y_{\mathrm{cm}} <$ 1.94. The average $v_2$ result from the two rapidity ranges is reported over the transverse momentum (p$_\mathrm{T}$) ran…
The capabilities of the CMS experiment to explore the rich heavy-ion physics programme offered by the CERN Large Hadron Collider (LHC) are summarised. Various representative measurements in Pb-Pb collisions at sqrt(s) = 5.5 TeV are covered. These include "bulk" observables (charged hadron multiplicity, low-pT inclusive hadron spectra and elliptic flow) which provide information on the collective properties of the system; as well as perturbative processes (high-pT hadrons, jets, gamma-jet and quarkonium production) which yield "tomographic" information of the densest phases of the reaction.
First measurements of dihadron correlations for charged particles are presented for central PbPb collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV over a broad range in relative pseudorapidity, Delta(eta), and the full range of relative azimuthal angle, Delta(phi). The data were collected with the CMS detector, at the LHC. A broadening of the away-side (Delta(phi) approximately pi) azimuthal correlation is observed at all Delta(eta), as compared to the measurements in pp collisions. Furthermore, long-range dihadron correlations in Delta(eta) are observed for particles with simi…
Event-by-event correlations of azimuthal anisotropy Fourier coefficients ($v_n$) in 8.16 TeV pPb collision data, collected by the CMS experiment at the LHC, are extracted using a subevent four-particle cumulant technique. Each combination of four charged particles are selected from either two, three, or four distinct subevent regions of a pseudorapidity range from $-$2.4 to 2.4 of the CMS tracker, and with transverse momentum between 0.3 and 3.0 GeV. Using the subevent cumulant technique, correlations between $v_n$ of different orders are measured as functions of particle multiplicity and comp…
Experimental findings of CMS on properties of jets and underlying events at high multiplicities in proton-proton interactions at 7 TeV are interpreted as an indication of increasing role of central collisions with small impact parameters. We find an indication that the rates of different hard processes observed by CMS and ALICE universally depend on underlying event charged-particles multiplicity until it becomes four times more than average. It is shown that the increase of the overlap area of colliding protons is not sufficient for explanation of the rate of jet production in events with cha…
In nucleus-nucleus collisions, the linear dependence found for the elliptic flow harmonic of both positive or negative charged particles as a function of event charge asymmetry is predicted by the phenomenon known as the Chiral Magnetic Wave (CMW) due to its induced electric quadrupole moment. Here, the event charge asymmetry $A_{\rm ch}$ is defined as $\frac{N_{+}-N_{-}}{N_{+}+N_{-}}$, where $N_{+}$ and $N_{-}$ are the number of positive and negative charged particles, respectively. However, other scenarios are also possible and may provide alternative explanations for the experimental result…
In this Letter, comprehensive results on ${\rmπ}^\pm$, K$^\pm$, K$^0_S$, p, $\rm\bar{p}$, $\rm Λ$ and $\rm \barΛ$ production at mid-rapidity ($0 < y_{\rm cms} < 0.5$) in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV, measured by the ALICE detector at the LHC, are reported. The transverse momentum distributions exhibit a hardening as a function of event multiplicity, which is stronger for heavier particles. This behavior is similar to what has been observed in pp and Pb-Pb collisions at the LHC. The measured $p_{\rm T}$ distributions are compared to results at lower energy and with predicti…
Evidence for the production of top quarks in heavy ion collisions is reported in a data sample of lead-lead collisions recorded in 2018 by the CMS experiment at a nucleon-nucleon center-of-mass energy of $\sqrt{s_{_{\mathrm{NN}}}} =$ 5.02 TeV, corresponding to an integrated luminosity of $1.7\pm0.1\,\mathrm{nb}^{-1}$. Top quark pair ($\mathrm{t\bar{t}}$) production is measured in events with two opposite-sign high-$p_\mathrm{T}$ isolated leptons ($\ell^\pm\ell^\mp =\,\mathrm{e}^{+} \mathrm{e}^{-},\,μ^{+} μ^{-},\,\mathrm{and}\,\mathrm{e}^{\pm} μ^{\mp}$). We test the sensitivity to the $\mathrm{…
The Compact Muon Solenoid (CMS) is fully equipped to measure hard probes in the di-muon decay channel in the high multiplicity environment of nucleus-nucleus collisions. Such probes are especially relevant for studying the quark-gluon plasma since they are produced at early times and propagate through the medium, mapping its evolution. CMS has measured the nuclear modification factors of non-prompt J/psi (from b-hadron decays) and prompt J/psi in PbPb collisions at sqrt{s_{NN}} = 2.76 TeV. For prompt J/psi with relatively high p_T (p_T=6.5-30 GeV/c), a strong, centrality-dependent suppression …
Two-particle angular correlations between unidentified charged trigger and associated particles are measured by the ALICE detector in p-Pb collisions at a nucleon-nucleon centre-of-mass energy of 5.02 TeV. The transverse-momentum range 0.7 $ < p_{\rm{T}, assoc} < p_{\rm{T}, trig} <$ 5.0 GeV/$c$ is examined, to include correlations induced by jets originating from low momen\-tum-transfer scatterings (minijets). The correlations expressed as associated yield per trigger particle are obtained in the pseudorapidity range $|η|<0.9$. The near-side long-range pseudorapidity correlations observed in h…
Measurements of strange hadron (K0s, Lambda + anti-Lambda, and Xi+ + Xi-) transverse momentum spectra in pp, pPb, and PbPb collisions are presented over a wide range of rapidity and event charged-particle multiplicity. The data were collected with the CMS detector at the CERN LHC in pp collisions at sqrt(s) = 7 TeV, pPb collisions at sqrt(s[NN]) = 5.02 TeV, and PbPb collisions at sqrt(s[NN]) = 2.76 TeV. The average transverse kinetic energy is found to increase with multiplicity, at a faster rate for heavier strange particle species in all systems. At similar multiplicities, the difference in …
Results on two-particle angular correlations for charged particles emitted in pPb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV are presented. The analysis uses two million collisions collected with the CMS detector at the LHC. The correlations are studied over a broad range of pseudorapidity, eta, and full azimuth, phi, as a function of charged particle multiplicity and particle transverse momentum, pt. In high-multiplicity events, a long-range (2 < abs(Delta(eta)) < 4), near-side (Delta(phi) approximately 0) structure emerges in the two-particle Delta(eta)-Delta(phi) corr…
The elliptic azimuthal anisotropy coefficient ($v_2$) is measured for charm (D$^0$) and strange (K$_\mathrm{S}^0$, $Λ$, $Ξ^-$, and $Ω^-$) hadrons, using a data sample of pPb collisions collected by the CMS experiment, at a nucleon-nucleon center-of-mass energy $\sqrt{s_{_\mathrm{NN}}} =$ 8.16 TeV. A significant positive $v_2$ signal from long-range azimuthal correlations is observed for all particle species in high-multiplicity pPb collisions. The measurement represents the first observation of possible long-range collectivity for open heavy flavor hadrons in small systems. The results suggest…
The multiplicity dependence of electron production from heavy-flavour hadron decays as a function of transverse momentum was measured in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV using the ALICE detector at the LHC. The measurement was performed in the centre-of-mass rapidity interval $-1.07 < y_{\rm cms} < 0.14$ and transverse momentum interval 2 $< p_{\rm T} <$ 16 GeV/$c$. The multiplicity dependence of the production of electrons from heavy-flavour hadron decays was studied by comparing the $p_{\rm T}$ spectra measured for different multiplicity classes with those measured in pp col…
The measurement of prompt D-meson production as a function of multiplicity in p-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV with the ALICE detector at the LHC is reported. D$^0$, D$^+$ and D$^{*+}$ mesons are reconstructed via their hadronic decay channels in the centre-of-mass rapidity range $-0.96< y_{\mathrm{cms}}<0.04$ and transverse momentum interval $1<p_{\rm T}<24$ GeV/$c$. The multiplicity dependence of D-meson production is examined by either comparing yields in p-Pb collisions in different event classes, selected based on the multiplicity of produced particles or zero-degree energy…
Measurements of two- and four-particle angular correlations for charged particles emitted in pPb collisions are presented over a wide range in pseudorapidity and full azimuth. The data, corresponding to an integrated luminosity of approximately 31 inverse nanobarns, were collected during the 2013 LHC pPb run at a nucleon-nucleon center-of-mass energy of 5.02 TeV by the CMS experiment. The results are compared to 2.76 TeV semi-peripheral PbPb collision data, collected during the 2011 PbPb run, covering a similar range of particle multiplicities. The observed correlations are characterized by th…
Two-particle angular correlations are measured in high-multiplicity proton-proton collisions at $\sqrt{s} =13$ TeV by the ALICE Collaboration. The yields of particle pairs at short-($Δη$ $\sim$ 0) and long-range ($1.6 < |Δη| < 1.8$) in pseudorapidity are extracted on the near-side ($Δφ$ $\sim$ 0). They are reported as a function of transverse momentum ($p_{\mathrm T}$) in the range $1 <p_{\mathrm T}< 4$ GeV/$c$. Furthermore, the event-scale dependence is studied for the first time by requiring the presence of high-$p_{\rm T}$ leading particles and jets for varying $p_{\rm T}$ thresholds. The r…
The discovery of correlations between particles separated by several units of pseudorapidity in high-multiplicity pp and p-Pb collisions, reminiscent of structures observed in Pb-Pb collisions, was a challenge to traditional ideas about collectivity in heavy ion collisions. In order to further explore long-range correlations and provide information to theoretical models, correlations between forward trigger muons and mid-rapidity associated hadrons were measured in p-Pb collisions at $\sqrt{s_{\mbox{NN}}} = 5.02~\mbox{TeV}$. The results demonstrate that the nearside and awayside ridges extend …
Spectra of identified charged hadrons are measured in pPb collisions with the CMS detector at the LHC at sqrt(sNN) = 5.02 TeV. Charged pions, kaons, and protons in the transverse-momentum range pt approximately 0.1-1.7 GeV and laboratory rapidity abs(y) < 1 are identified via their energy loss in the silicon tracker. The average pt increases with particle mass and the charged multiplicity of the event. The increase of the average pt with charged multiplicity is greater for heavier hadrons. Comparisons to Monte Carlo event generators reveal that EPOS LHC, which incorporates additional hydrodyna…
For the first time a principle-component analysis is used to separate out different orthogonal modes of the two-particle correlation matrix from heavy ion collisions. The analysis uses data from sqrt(s[NN]) = 2.76 TeV PbPb and sqrt(s[NN]) = 5.02 TeV pPb collisions collected by the CMS experiment at the LHC. Two-particle azimuthal correlations have been extensively used to study hydrodynamic flow in heavy ion collisions. Recently it has been shown that the expected factorization of two-particle results into a product of the constituent single-particle anisotropies is broken. The new information…
Measurements of azimuthal anisotropies at forward and backward rapidity with muons in high-multiplicity p-Pb collisions at $\sqrt{s_{\rm NN}} = 8.16$ TeV
ALICE 在 8.16 TeV 高多重数 p-Pb 碰撞中用两粒子累积量和关联法测量前、后向μ子 v_2，发现显著正各向异性，为重味集体行为理论提供新约束。
Measurement of $ψ$(2S) production as a function of charged-particle pseudorapidity density in pp collisions at $\sqrt{s}$ = 13 TeV and p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 8.16 TeV with ALICE at the LHC
The angular correlations measured in proton-proton collisions at 7 TeV are decomposed into contributions from back to back emission and elliptic flow. Modeling the dominant term in the correlation functions as a momentum conservation effect or as an effect of the initial transverse velocity of the source, the remaining elliptic flow component can be estimated. The elliptic flow coefficient extracted from the CMS Collaboration data is 0.04-0.08. No additional small-angle, ridge-like correlations are needed to explain the experimental data.
