Heavy-Ion Physics at the LHC with the Compact Muon Solenoid Detector

Hadron collisions at the LHC offer a unique opportunity to study strong interactions. The exciting data collected by the four RHIC experiments suggest that in heavy-ion collisions at sqrt(s_NN) = 200 GeV, an equilibrated, strongly-coupled partonic system is formed. An extrapolation of the existing data toward LHC energies suggests that the heavy-ion program at the LHC is in a situation comparable to that of the high-energy program, where new discoveries near the TeV-scale are expected. Similarly, heavy-ion studies at the LHC are bound to either confirm the theoretical picture emerging from RHIC or challenge and extend our present understanding of strongly interacting matter at extreme densities. The experience at RHIC shows that the ideal detector for future heavy-ion studies should provide large acceptance for tracking and calorimetry, high granularity, high resolution and use fast detector technologies as well as sophisticated triggering. The CMS detector at the LHC excels in each of these categories.Comment: 4 pages, 4 figures, prepared for the proceedings of the XXXIXth Rencontres de Moriond on QCD and Hadronic Interaction

Triggering on Hard Probes in Heavy-Ion Collisions with the CMS Experiment at the LHC

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-ion trigger strategy developed for the unique two-layer trigger system of the CMS experiment which consists of a ``Level-1'' trigger based on custom electronics and a High Level Trigger (HLT) implemented using a large cluster of commodity computers.Comment: 4 pages, 1 figure, To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse

Event-by-event fluctuations of particle ratios in central Pb + Pb collisions at 20 to 158 AGeV

In the vicinity of the QCD phase transition, critical fluctuations have been predicted to lead to non-statistical fluctuations of particle ratios, depending on the nature of the phase transition. Recent results of the NA49 energy scan program show a sharp maximum of the ratio of K+ to Pi+ yields in central Pb+Pb collisions at beam energies of 20-30 AGeV. This observation has been interpreted as an indication of a phase transition at low SPS energies. We present first results on event-by-event fluctuations of the kaon to pion and proton to pion ratios at beam energies close to this maximum

Simulation of Jet Quenching Observables in Heavy Ion Collisions at the LHC

Large transverse momentum jets provide unique tools to study dense QCD matter in high-energy heavy-ion collisions. Results from RHIC on suppression of high transverse momentum particles in Au+Au collisions indicate a significant energy loss of leading partons in the dense and strongly interacting matter formed in these collisions. The LHC will collide Pb ions at sqrt(s_NN)=5500 GeV, where the cross section of hard scattering will increase dramatically. Large production rates, the large acceptance of the CMS calorimeters and tracking system, combined with the capability of triggering on jets, will extend the transverse momentum reach of charged particle spectra and nuclear modification factors up to p_T>200 GeV/c.Comment: 6 pages, 5 figures. Presented at the 19th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions (QM2006), Shanghai, China, November 14-20, 2006. [Grant numbers added in the acknowledgment.

Heavy-Ion Physics with the CMS and ATLAS Experiments at the Large Hadron Collider

Heavy-ion collisions at the Large Hadron Collider (LHC) provide an opportunity for an unprecedented expansion of the study of Quantum Chromodynamics (QCD) in systems with extremely high energy density. In this report we will discuss and compare the capabilities of the ATLAS and CMS experiments for physics studies using heavy-ion collisions. The ATLAS and CMS detectors have been designed to perform high precision studies in pp collisions. Both feature precision tracking systems, hermetic calorimetry over a wide range of pseudorapidity and large acceptance muon spectrometers. We find that both experiments should perform very well for heavy-ion collisions, especially for studying hard probes of the dense partonic medium such as: heavy quarks and quarkonia with an emphasis on the b and Upsilon; high p_T jets; photons and Z^0 bosons

Particle Ratio Fluctuations in NA49

The results of the NA49 energy scan program show a sharp maximum of the ratio of K + to !+ yields in central Pb+Pb collisions at beam energies of 20-30 AGeV. This observation has been interpreted as an indication of a phase transition at low SPS energies. We present results on energy dependence of event-by-event fluctuations of the kaon to pion and proton to pion ratios at beam energies close to this maximum and complement this study with preliminary data from the STAR collaboration in the RHIC energy range. A significant increase of the fluctuation signal of the kaon to pion ratio at 20 and 30 AGeV is observed while it stays constant from the highest SPS energies out to the RHIC energy range

Efficient prediction of broadband trailing edge noise and application to porous edge treatment

Trailing edge noise generated by turbulent flow traveling past an edge of an airfoil is one of the most essential aeroacoustic sound generation mechanisms. It is of great interest for noise problems in various areas of industrial application. First principle based CAA with short response time are needed in the industrial design process for reliable prediction of spectral differences in turbulent-boundary-layer trailing-edge noise due to design modifications. In this paper, an aeroacoustic method is studied, resting on a hybrid CFD/CAA procedure. In a first step RANS simulation provides a time-averaged solution, including the mean-flow and turbulence statistics such as length-scale, time-scale and turbulence kinetic energy. Based on these, fluctuating sound sources are then stochastically generated by the Fast Random Particle-Mesh Method to simulate in a second CAA step broadband aeroacoustic sound. From experimental findings it is well known that porous trailing edges significantly lower trailing edge noise level over a large range of frequencies reaching up to 8dB reduction. Furthermore, sound reduction depends on the porous material parameters, e.g. geometry, porosity, permeability and pore size. The paper presents first results for an extended hybrid CFD/CAA method including porous materials with prescribed parameters. To incorporate the effect of porosity, an extended formulation of the Acoustic Perturbation Equations with source terms is derived based on a reformulation of the volume averaged Navier-Stokes equations into perturbation form. Proper implementation of the Darcy and Forchheimer terms is verified for sound propagation in homogeneous and anisotropic porous medium. Sound generation is studied for a generic symmetric NACA0012 airfoil without lift to separate secondary effects of lift and camber on sound from those of the basic edge noise treatments.Comment: 37 page

Jet measurements by the CMS experiment in pp and PbPb collisions

The energy loss of fast partons traversing the strongly interacting matter produced in high-energy nuclear collisions is one of the most interesting observables to probe the nature of the produced medium. The multipurpose Compact Muon Solenoid (CMS) detector is well designed to measure these hard scattering processes with its high resolution calorimeters and high precision silicon tracker. Analyzing data from pp and PbPb collisions at a center-of-mass energy of 2.76 TeV parton energy loss is observed as a significant imbalance of dijet transverse momentum. To gain further understanding of the parton energy loss mechanism the redistribution of the quenched jet energy was studied using the transverse momentum balance of charged tracks projected onto the direction of the leading jet. In contrast to pp collisions, a large fraction the momentum balance for asymmetric jets is found to be carried by low momentum particles at large angular distance to the jet axis. Further, the fragmentation functions for leading and subleading jets were reconstructed and were found to be unmodified compared to measurements in pp collisions. The results yield a detailed picture of parton propagation in the hot QCD medium.Comment: 7 pages, 5 figures, Quark Matter 2011 conference proceeding