We present the capabilities of the CMS experiment to explore the heavy-ion physics program offered by the CERN Large Hadron Collider (LHC). The prime goal of this research is to test the fundamental theory of the strong interaction (QCD) in extreme conditions of temperature, density and parton momentum fraction by colliding nuclei at energies of sqrt(s_NN) = 5.5 TeV. This presentation will give the overview of the potential of the CMS to carry out a full set of representative Pb-Pb measurements both in "soft" and "hard" regimes. Measurements include "bulk" observables -- charged hadron multiplicity, low pT inclusive hadron identified spectra and elliptic flow -- which provide information on the collective properties of the system; as well as perturbative processes - such as quarkonia, heavy-quarks, jets, gamma-jet, and high pT hadrons -- which yield "tomographic" information of the hottest and densest phases of the reaction.
Jet studies provide an experimental method to explore the features of energy loss in the strongly interacting quark-gluon plasma. Recent jet results from 2.76 TeV PbPb and pp collisions measured with the CMS detector are presented. Jets in the most head-on (central) PbPb collisions are quenched in comparison to pp, and the jets fragment in different ways in the two systems. Measurements from pPb collisions at 5.02 TeV show that the jet and charged particle suppression seen in central PbPb measurements are not due to initial state nuclear effects.
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…
The heavy ion program of the CMS experiment will examine the QCD matter under extreme conditions, through the study of global observables and specific probes.
This article presents a brief overview of the CMS experiment capabilities to study the hot and dense matter created in relativistic heavy-ion collisions. The CERN Large Hadron Collider will provide collisions of Pb nuclei at 5.5 TeV per nucleon. The CMS heavy ion group has developed a plethora of physics analyses addressing many important aspects of heavy-ion physics in preparation for a competitive and successful program.
This paper presents the CMS measurements of quarkonia and open heavy flavor production in \pp, \pPb, and \PbPb collisions at \sqrtsnn = 2.76 and 5.02 TeV. A brief outlook of the near-future CMS heavy flavor physics analyses is provided at the end.
The production of a strongly interacting medium in heavy-ion collisions is identified through suppression of high transverse momentum jets leading to an effect known as jet quenching. Detailed measurements of nuclear modification factors and energy flow of the aforementioned quenched jets are performed with the CMS detector. These new results extend previous measurements to large angles with respect to the quenched jets and increase reach in the transverse momentum range. High statistics pp, pPb and PbPb data taken in 2011-13 are used in the following analysis. A new data-driven method to esti…
The CMS detector is well equipped to provide unique measurements in heavy ion collisions at LHC. It will provide measurements of the J/Psi and Upsilon families with good separation of the different resonances. Jet quenching will be studied by analyzing monojet-to-dijet ratios and Z0(gamma)+jet events.
The capabilities of the CMS detector at the LHC will be described for measuring high-pT hadrons, photons and jets in heavy ion collisions. Detailed simulations of various studies planned with the CMS apparatus, including charged particle tracking, jet reconstruction using calorimetry, dimuon and isolated photon detection and the measurement of in-medium fragmentation functions using high-pT photon-jet correlations will be discussed.
Heavy ion collisions at the Large Hadron Collider (LHC) will produce strongly interacting matter at unprecedented energy densities. At LHC collision energies, new hard probes of the dense initial collision system will become readily available. We present an overview of the capabilities of the Compact Muon Solenoid (CMS) detector to use these probes for a detailed study of QCD phenomenology at the highest energy densities.
We present a study of the CMS trigger system in heavy-ion collisions. Concentrating on two physics channels, dimuons from decays of quarkonia and single jets, we evaluate a possible trigger strategy for Pb+Pb running that relies on event selection solely in the High-Level Trigger (HLT). The study is based on measurements of the timing performance of the offline algorithms and event-size distributions using full simulations. Using a trigger simulation chain, we compare the physics reach for the jet and dimuon channels using online selection in the HLT to minimum bias running. The results demons…
At the Relativistic Heavy Ion Collider, jets have been a useful tool to probe the properties of the hot, dense matter created. At the Large Hadron Collider, collisions of Pb+Pb at $\sqrt{s_{NN}}$ = 5.5 TeV will provide a large cross section of jets at high $E_T$ above the minimum bias heavy ion background. Simulations of the Compact Muon Solenoid (CMS) experiment's capability to measure jets in heavy ion collisions are presented. In particular, $γ$-jet measurements can estimate the amount of energy lost by a jet interacting strongly with the medium, since the tagged photon passes through unaff…
The capabilities of the CMS experiment to study properties of hot and dense QCD-matter created in heavy ion collisions at the CERN Large Hadron Collider with the perturbative processes (so-called "hard probes") are presented. Detailed studies from complete simulations of the CMS detectors in Pb+Pb collisions at $\sqrt{s}=5.5$ TeV per nucleon pair are presented in view of two hard probes: quarkonium and $γ$-jet production.
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…
Studies of heavy-ion collisions at the LHC will benefit from an array of qualitatively new probes not readily available at lower collision energies. These include fully formed jets at ET > 50 GeV, Z0's and abundantly produced heavy flavors. For Pb+Pb running at LHC design luminosity, the collision rate in the CMS interaction region will exceed the available bandwidth to store data by several orders of magnitude. Therefore an efficient trigger strategy is needed to select the few percent of the incoming events containing the most interesting signatures. In this report, we will present the heavy…
Quarkonia have been studied in different collision system and energy in order to understand the effects of the hot and dense medium created in heavy-ion collisions. CMS is well suited to measure quarkonia decays to muons given the muon identification and charged particle tracking capability. We report here prompt, non-prompt J/ψ, and Υ production measured by the CMS experiment in pp collisions at \sqrt{s}=7 TeV. In addition, the J/ψ and Υ production in PbPb at \sqrt{s_{NN}}=2.76 TeV and pp collisions at the same per nucleon energy are measured and compared. Prompt and non-prompt J/ψ contributi…
How can we gain a detailed insight into the hydrodynamic response of the system created in heavy ion collisions to the fluctuating initial geometry and viscous effects? Do we create a strongly interacting medium in proton-nucleus and proton-proton collisions, or rather a system of partons undergoing few scatterings? To what extent can we discriminate between initial momentum correlations and flow generated as a response to the initial geometry via interactions in the final state? Do measurements of identified particle flow confirm the observations from inclusive charged hadrons? An experimenta…
There is a rich program of forward physics measurements within the CMS Collaboration covering a wide range of topics. In many cases there is a connection to quantities and effects relevant for very high energy cosmic ray interaction. Some of the recent measurements in the fields of exclusive final states, low-pT inclusive and diffractive cross sections, underlying event, multi parton interactions, double parton scattering, final state particle correlations and minimum bias results are briefly summarized here.
In a relativistic heavy ion collision, heavy flavor (charm and bottom) quarks are mostly created via hard processes at the early stage of the collisions. We present the latest results of the azimuthal anisotropy coefficients $v_{\text{n}}$ for prompt and nonprompt $\text{D}^{0}$ mesons in PbPb, pPb, and pp collisions from the CMS experiment. The studies are about collectivity phenomena in smaller systems (pp and pPb collisions), searches for the effects of very strong electromagnetic fields created in the initial stages of ultrarelativistic PbPb collisions, and charm quark energy loss in the q…
In the particle-flow approach information from all available sub-detector systems is combined to reconstruct all stable particles. The global event reconstruction has been shown to improve, in particular, the resolution of jet energy and missing transverse energy in pp collisions compared to purely calorimetric measurements. This improvement is achieved primarily by combining the precise momentum determination of charged hadrons in the silicon tracker with the associated energy depositions in the calorimeters. By resolving individual particles inside jets, particle flow reduces the sensitivity…
We review the results on the bottomonium system from the CMS experiment at the Large Hadron Collider. Measurements have been carried out at different center-of-mass energies in proton collisions and in collisions involving heavy ions. These include precision measurements of cross sections and polarizations, shedding light on hadroproduction mechanisms, and the observation of quarkonium sequential suppression, a notable indication of quark-gluon plasma formation. The observation of the production of bottomonium pairs is also reported along with searches for new states. We close with a brief out…
The CMS heavy-ion program will probe QCD matter under extreme conditions. Its capabilities for the study of global observables and soft probes are described.
Ultrarelativistic heavy ion collisions at the laboratory provide a unique chance to study quantum chromodynamics (QCD) under extreme temperature (${\approx}150\,\mathrm{MeV}$) and density (${\approx}1\,\mathrm{GeV}/\mathrm{fm}^3$) conditions. Over the past decade, experimental results from LHC have shown further evidence for the formation of the quark-gluon plasma (QGP), a phase that is thought to permeate the early Universe and is formed in the high-density neutron-star cores. Various QCD predictions that model the behavior of the low-$x$ gluon nuclear density, a poorly explored region, are a…
The CMS experiment at the LHC is a general-purpose apparatus with a set of large acceptance and high granularity detectors for hadrons, electrons, photons and muons, providing unique capabilities for both proton-proton and ion-ion collisions. The data collected during the November 2010 PbPb run at sqrt{s_NN} = 2.76 TeV was analyzed and multiple measurements of the properties of the hot and dense matter were obtained. Global event properties, detailed study of jet production and jet properties, isolated photons, quarkonia and weak bosons were measured and compared to pp data and Monte Carlo sim…
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…
The CMS experiment will provide good quality measurements of yields and spectra of identified charged and neutral particles, both in p-p and heavy-ion collisions, thus contributing to the study of soft hadronic physics at the LHC energies.
The CMS heavy ion program can study quark matter over an unprecedented range of Bjorken x and mass. CMS is equipped with excellent detectors to exploit the new physics probes available at_/Snn = 5.5 TeV. The high rate capability and wide rapidity coverage will allow us to study the correlations between low x and high mass phenomena.
This work presents the latest results on open heavy-flavour production in proton-proton, proton-lead, lead-lead and xenon-xenon collisions from the ALICE, ATLAS, CMS and LHCb Collaborations at the LHC.
The unprecedented center-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. Electroweak boson production is an important benchmark process at hadron colliders. Precise measurements of 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 mu…
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…
This writeup summarizes recent CMS results on quarkonia measurements in pp, pPb and PbPb collisions at LHC. The excellent muon detection capability of CMS allows measurement of charmonia states at high transverse momentum ($p_T$) while the $Υ$ states can be reconstructed starting at zero $p_T$. The absolute and relative yields of different charmonia and bottomonia states modified in PbPb collisions (over pp collisions) are described. The vertexing capability of CMS enables measurement of B meson energy loss via its decay to J/$ψ$. An overview of these measurements is given. How these measureme…
The measurement of the charmonium ($J/ψ$, $ψ'$) and bottomonium ($Υ$, $Υ'$, ${Υ''}$) resonances and $Z^0$ boson in nucleus-nucleus collisions provides crucial information on high density QCD matter. The observation of anomalous suppression of $J/ψ$ at the CERN-SPS and RHIC is well established but the clarification of some important questions requires equivalent studies of the $Υ$ family, only possible at LHC energies. The $Z^0$ boson will be produced for the first time in heavy-ion collisions at the LHC and, since its dominant production channel is through $q{\bar q}$ fusion, it is an excellen…
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…
The heavy ion (HI) program at the LHC has proven to be a successful and indispensable part of the LHC physics program. Its chief aim had been the detailed characterization of the quark-gluon plasma (QGP) in lead-lead collisions. Using additional data sets of proton-lead, proton-proton, and xenon-xenon collisions, the program has also included many advances, for example, in the understanding of the partonic nuclear structure, collectivity in smaller collision systems, and electromagnetic interactions. This Letter of Interest outlines the CMS Heavy Ion Group point of view regarding the scientifi…
The Zero Degree Calorimeter (ZDC) is integral part of the CMS experiment, especially, for heavy ion studies. The design of the ZDC includes two independent calorimeter sections: an electromagnetic section and a hadronic section. Sampling calorimeters using tungsten and quartz fibers have been chosen for the energy measurements. An overview of the ZDC is presented along with a current status of calorimeter's preparation for Day 1 of LHC.
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.
The comparison of heavy-flavour hadron production in proton-proton, proton-Pb and Pb-Pb collisions at the LHC offers the opportunity to investigate the properties of the high-density colour-deconfined state of strongly-interacting matter (Quark Gluon Plasma, QGP) that is expected to be formed in high-energy collisions of heavy nuclei. A review of the main quarkonium and open heavy-flavour results obtained by the ALICE, ATLAS and CMS experiments is presented.
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…
Jets are an important tool to probe the hot, dense medium produced in ultra-relativistic heavy-ion collisions. At the collision energies available at the Large Hadron Collider (LHC), there is copious production of hard processes, such that high p_T jets may be differentiated from the heavy-ion underlying event. The multipurpose Compact Muon Solenoid (CMS) detector is well designed to measure hard scattering processes with its high quality calorimeters and high precision silicon tracker. Jet quenching has been studied in CMS in PbPb collisions at sqrt(s_NN)= 2.76 TeV. As a function of centralit…
These proceedings report recent studies performed by the ALICE, ATLAS and CMS experiments on jet substructure in pp and Pb+Pb collisions at 5.02 TeV. The dependence of the modification of the jet shape on the dijet transverse moment um asymmetry is presented. Measurements of particles recoiling against the Z boson without being constrained to jet cone delimitations are shown. Fully-corrected measurements of the N-subjettiness within anti-kt R = 0.4 charged-particle jets are also described.
A brief summary of results on Pb+Pb collisions from ALICE, ATLAS, and CMS is presented covering global event properties, anisotropic flow, jet quenching, and quarkonia.
At the end of 2010, the CERN Large Hadron Collider started operation with heavy ion beams, colliding lead nuclei at a centre-of-mass energy of 2.76 TeV/nucleon and opening a new era in ultra-relativistic heavy ion physics at energies exceeding previous accelerators by more than an order of magnitude. This review summarizes the results from the first year of heavy ion physics at LHC obtained by the three experiments participating in the heavy ion program, ALICE, ATLAS, and CMS.
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.
The ALICE experiment at LHC is mainly dedicated to heavy-ion physics. An overview of its performances, some predictions related to its first measurements and QGP observable measurements will be given.
Beauty quarks are considered as one of the best probes of the strongly interacting medium created in relativistic heavy-ion collisions because they are predominantly produced via initial hard scatterings. Our measurements of B mesons productions provide information on the diffusion of beauty quarks and the flavor dependence of in-medium energy loss. In these studies, clarifying the hadronization mechanism is crucial for understanding the transport properties of beauty quarks. We propose to show our experimental results on three sets of particles: nonprompt $J/ψ$ and $D^0$, $B^0_s$ and $B^+$, a…
In this short paper we provide an overview of new results from the ATLAS physics program at the LHC as of spring 2015. We separately summarize the results from p+Pb collisions and Pb+Pb collisions along with some of their interpretations.
Beauty quarks are considered as one of the best probes of the strongly interacting medium created in relativistic heavy-ion collisions because they are predominantly produced via initial hard scatterings. Measurements of B meson production provide information about the diffusion of beauty quarks and the flavor dependence of in-medium energy loss. In these studies, clarifying the hadronization mechanism is crucial for understanding the transport properties of beauty quarks. Measurements of $B^{0}_{s}$ production can shed light on the mechanisms of beauty recombination in the medium and provide …
We review the physics performance of the ATLAS detector for hard probes in heavy ion collisions, focusing on three topics: jet reconstruction, direct photon measurement, and heavy flavor jet tagging.
A new experimental program to study hadron production in collisions of nuclei at the CERN SPS is presented. The program will focus on measurements of fluctuations and long correlations with the aim to identify properties of the onset of deconfinement and search for the critical point of strongly interacting matter.
In this contribution, we present some predictions for the production of D and B mesons in ultrarelativistic heavy ion collisions at RHIC and LHC energies and confront them with experimental results obtained so far by the STAR, PHENIX, ALICE and CMS collaborations. We next discuss some preliminary results obtained with an improved description of the medium based on EPOS initial conditions, and its possible implications on the nuclear modification factor and on the elliptic flow of heavy quarks.
For the first time in the CERN history two experimental programs devoted to study nucleus-nucleus collisions at high energies are performed in parallel. In the SPS ion program, carried out by NA61/SHINE, interactions of light and medium size ions in the energy range 5-20 GeV are investigated. The program aims to discover the critical point of strongly interacting matter as well as establish properties of the onset of deconfinement. In 2010 ALICE, ATLAS and CMS at LHC recorded first data on Pb+Pb collisions at the highest energy reached up to now, 2760 GeV. This opens a new exciting area in the…
Apparently universal trends have been observed in relativistic nucleus-nucleus collisions up to RHIC energies. Here, we review these trends and we discuss their agnostic extrapolation to heavy ion collisions at the LHC.
