Heavy Ion Physics at the LHC with the Compact Muon Solenoid Detector Matthew Searle UC Davis Nuclear Physics Group 25 Aug 2004
CMS Geometry Transverse view of the CMS Detector.
CMS Geometry Longitudinal view of a quadrant of the CMS detector.
CMS Geometry • CMS is designed to identify and precisely measure muons, electrons, photons, and jets over a broad energy and rapidity range. • Main CMS detecting systems • Tracker • Electromagnetic Calorimeter (ECAL) • Hadronic Calorimeter (HCAL) • Muon Chambers • CASTOR and the Forward detectors • Zero-Degree Calorimeters (ZDC’s)
Tracker • The Tracker is made up of two types of detectors: the pixel layers and the silicon strip counters. • Pixel detector: 3 pixel barrel layers at 4.5, 7.5, and 10 cm from beam axis and 2 endcap disks in forward and backward directions. Barrel layers cover up to |η| < 2.1. • The barrel layers contain ~ 9.6, 16, and 22.4 million pixels, respectively. Pixel dimensions: 100x150 μm2.
Tracker • continued: • The inner Si strip counter consists of 4 barrel layers an 3 disks in each endcap. • The outer MSGC’s has 6 barrel layers & 9 disks in each endcap.
Tracker Longitudinal view of the Tracker.
Calorimeters • ECAL: • Made up of ~ 76,000 scintillating crystals of PbWO4. Light is detected with avalanche photodiodes (barrel) and vacuum phototriodes (endcap). • EB covers up to |η| < 1.48. • Crystals are 23 cm long, corresponding to 25.8 X0.
Calorimeters Longitudinal view of the ECAL.
Calorimeters • HCAL: • Split into two parts: HB and HE. • HB |η| < 3 • HE 3 < |η| < 5 • The HCAL is a sampling calorimeter made up of scintillator/copper • plates. The copper absorber plates are 5(8) cm think in the • barrel(endcaps). The scintillator is 4mm thick. • The barrel calorimeter is 79cm thick corresponding to 5.15 nuclear • interaction lengths.
Calorimeters Longitudinal view of the HCAL.
Muon Chambers • The muon system uses three different detecting elements: drift tubes (DT’s), cathode strip chambers (CSC’s), and resistive plate chambers (RPC’s) dedicated to triggering. The muon system covers up to |η| < 2.4.
Muon Chambers Longitudinal view of the Muon Chambers
CASTOR and the Forward Region • CMS will have a suite of detectors beyond η = 5 that are unique at the LHC: CASTOR and T2 at 5 < η < 7, the TOTEM Roman pots 7 < η < 10, and the ZDC’s. • The ZDC’s will be able to measure neutrons and photons at 0 degrees. • The ZDC’s are vital for beam tuning and make possible or enhance measurements of: • Cross section measurements • Centrality, through number of spectators • UPC’s through detection of mutual Coulomb dissociation • Energy flow • pA collisions and calibration of cosmic ray experiments
Intro to CMS • LHC provides a unique opportunity to study the strong interaction. • Studies at RHIC at √SNN=200 GeV strongly suggest that an equilibriated, strongly-coupled partonic system exists. • Extrapolation to LHC energies hint at new discoveries at the TeV scale.
Intro to CMS • The Compact Muon Solenoid is an ideal Heavy Ion detector: large acceptance for tracking & calorimetry, high granularity & resolution, fast detector technologies, & sophisticated triggering. • The Heavy Ion community can gain access to CMS for a small fraction of developing a new collider.
Intro to CMS • The LHC & CMS will be able to explore the dense matter formed in a heavy ion collision at higher density, higher temperatures, and for longer lifetimes of the fireball. • LHC energies will provide qualitatively new probes: high pT jets, y, Z0, Y family, D and B mesons, & high-mass dileptons. • CMS has a dedicated amount of beam time for Heavy Ion physics (1 month per year).
Intro to CMS • Why CMS is currently the ideal Heavy Ion detector: • High rate: pp collisions at L=1034 cm-2 s-1 , or pp collision rate of 40 MHz, and 25 pp collisions per bunch at full pp luminosity. • High resolution and granularity: • 4T field and pixel layers give ΔpT/pT < 1.5% up to pT ≈ 100 GeV/c • b resolution < 50 μm (< 20 μm at pT > 10 GeV/c) • Calorimetry: 16% jet energy resolution for 100 GeV jets w/ dN/dy = 5000 • ECAL spatial resolution in η and φ of .028 and .032, respectively.
Intro to CMS • Continued: • Large acceptance: • CASTOR, 5 < η < 7 • T2 silicon detector, 5 < η < 7 • TOTEM Roman Pots, 7 < η < 10 • CMS performance in most categories far exceeds capabilities • of existing or planned heavy-ion detectors. • CMS will have significant but not excellent performance in low pT • spectra and 2-particle correlations.
Intro to CMS • Whereas RHIC can approach x ~ 0.02, LHC ≈ RHIC x 30 (energy), and will approach x ~10-6-10-7. • Nonlinear evolution of gluon density with Q2 and gluon saturation is expected to be seen. • The Final State will be hotter and longer lived: LHC ≈ RHIC x 20 (energy density), with the initial temperature T0 doubling, and the fireball living roughly 3 times longer.
Physics Studies • Primary focus of heavy ion physics at CMS will center around such processes as quarkonia, jets, and gauge bosons. • Experience at RHIC has shown that global variables are essential for event categorization in various analysesand for placing important constraints on fundamental properties of particle production. • Breaking of energy scaling is expected at LHC energies (14 TeV pp, 5.5 TeV Pb-Pb).
Global Observables Global Observables: • Charged particle multiplicity, dN/dy • Transverse energy, ET & d ET/dy • Azimuthal anisotropy, v2 • Zero-degree energy of neutral spectators, E0 • The dependence of these observables on collision geometry allows for characterization of the events. • They will play an important role in testing models of particle and energy production and may allow selection of rare & exotic events with high dN/dy or ET.
Global Observables • On the left is an example of the charged particle multiplicity for a single event reconstructed from pixel layer hits for a central Pb-Pb event. • The largest contribution to the systematic error comes from the uncertainty in the yield of secondaries from the surrounding material and weakly decaying particles. This is expected to be reduced as experimental data becomes available at the LHC.
Flow & Azimuthal Anisotropy • Measurement of azimuthal flow provides info about the initial spatial geometry of the collision region, and is sensitive to the early conditions and thermalization of the system. • In hydrodynamic models, v2 arises from anisotropic pressure gradients in the transverse plane due to the almond shape of the overlap regionof the nuclei. • Deviation from linear dependence on centrality may indicate a phase transition or thermal disequilibrium.
Measurement of Flow • CMS proposes a new calorimetry-based analysis of flow using the azimuthal distribution of reconstructed jets. The proposed method is based on correlations between the azimuthal position of a jet axis & the angles of particles not incorporated in the jet. Reconstuction of the reaction plane is avoided & estimation of the jet energy is not necessary.
Impact Parameter • Many phenomena in a heavy ion collision depend crucially on the event centrality. Thus, the measurement of the impact parameter, b, is essential to characterize the event. CMS can do this by measuring the transverse energy, ET, with its hadronic and electromagnetic calorimeters. • The strong correlation between ET and b makes a measurement of the impact parameter to less than 1 fm possible.
Impact Parameter • Simulations of the calorimeter system during Pb+Pb collisions indicate that over 80% of ET will be detected. • Shown on the right, the upper plot shows the relationship between the summed transverse energy in the calorimeter and the impact parameter. The lower plot shows the accuracy of the impact parameter measured with ET flow in the HF calorimeter ( 3 < eta < 5 ) for Ar+Ar collisions.
Quarkonia ( c/c-bar, b/b-bar) • CMS will focus on detecting quarkonia through their decay into muon pairs. • Excellent momentum resolution for muons leads to a Y mass resolution of 50 MeV/c2. This provides a clean separation between the members of the Y family. • On the right, Y detection in the CMS detector for different species with hadronic background calculated with the highest multiplicity estimates for each system.
Quarkonia • Signal in Pb+Pb collisions after background subtraction in both the J/psi (left) and Y mass regions (right).
Jet Physics at CMS • CMS is ideally suited to study high pT jets. This makes it possible to study a wide range of observables key for understanding the modification of jet properties due to parton energy loss in a dense nuclear medium. • Dijet quenching & monojets may be signals of dense matter formation in a relativistic nuclear collision. • At leading order, hard jets are produced with p1=-p2. A monojet occurs if one of the members of a dijet loses so much evergy traversing the medium that only a single jet cone is observable.
Jet Physics • The dijet rate in AA relative to pp collisions can be studied by using a reference process, unaffected by energy loss and with a rate proportional to the number of nucleon-nucleon collisions , such as Drell-Yan production, or Z0 production. • This normalization is necessary to remove systematic errors in the luminosity. A measurement relative to a reference process requires pp and Pb+Pb runs at the same energy. However, pp runs at 5.5 TeV will count against the heavy ion beam time. Therefore, this data will not be available when heavy ion data taking starts at the LHC.
Jet Physics • In the figure on the left, the monojet/dijet ratio as a function of the threshold jet energy ET in central Pb-Pb collisions for different quenching scenarios: • The scaled PYTHIA result for the dijet spectrum is shown as the solid curve at a ratio of 2. Solid line: no quenching Dashed line : ideal plasma Dotted line: maximally viscous plasma
Tagged Heavy quark jets • Since there is an expected difference in the quenching mechanism for heavy quarks, b-quark jets will yield important information regarding energy loss in the medium. • Dead cone effect: for heavy quarks, gluon bremsstralung at small angles is suppressed, leading to significantly smaller energy loss. • Heavy quark jets are tagged by reconstructing secondary vertices leading to D and B mesons. • CMS will provide good tagging efficiency with low contamination of light quark/gluon jets.
Three jet events • Three jet events offer an interesting opportunity to study energy loss by comparing gluon and quark jets.
Z0-jet and y-jet channels • Processes where a hard parton jet is tagged by an “unquenched” ( not strongly interacting ) particle such as a Z0 or y are ideal to measure jet energy loss of the corresponding away-side jet. • Given the high granularity hadronic and electromagnetic calorimeters of CMS, a good rejection factor can be achieved against misidentified π0’s ( vs. y-jet events).
Z0-bosons • The Z0 provides a unique opportunity to study quark distributions in the nucleus at high Q2=m2z0, and CMS should be able to measure nuclear modifications as a function of Z0 rapidity.
pA collisions • pA collisions provide the cleanest measure of the initial state for AA collisions. • It is postualted that when viewed a fast probe, a nucleus may appear to resemble a sheet (or pancake) of highly correlated gluons known as a Color Glass Condensate (CGC). • In a CGC, two soft gluons can merge to form a harder gluon. This can lead to a suppression of low pT hadrons in dAu collisions compared to pp collisions. These effects should be stronger at forward rapidities where x is smaller but gluon density is higher.
pA collisions • At CMS: • Central rapidities: at |η|< 2.4, silicon tracking, photon, jet, & muon measurements can give very detailed descriptions of the collision. • Forward rapidities: CMS is able to reconstruct jets up to |η| < 5. • The CASTOR and T2 detectors may push jet reconstruction up to η = 7.
pA collisions • Almost hermetic calorimeter coverage of CMS will allow very precise measurements of energy stopping in pA collisions. This data will provide strong constraints on AA collisions. • pA measurements will also serve to “calibrate” the energy scale of ultra-high energy cosmic ray experiments which currently rely on extrapolations of GeV measurements to the PeV range to simulate their detector response.
Forward physics • The forward detectors, CASTOR & the ZDC’s will play an important role at CMS. Forward coverage is essential for measuring parton (especially gluon) distribution functions in protons and nuclei. • CMS will be able to study x as low as ~10-6-10-7 . Nonlinear evolution of parton densities & saturation effects will be able to be mapped out in x and Q2.
Forward Physics • Near hermetic coverage is also important for the study of diffractive processes. Diffractive events are characterized by large rapidity gaps in collision products. Hard diffractive production of heavy quarks & jets may lead to a further understanding of the Pomeron.
Forward Physics • PHOBOS and BRAHMS have shown the value of studying Au-Au & dAu collisions over a large pseudorapidity range. Based on extrapolation from BRAHMS, it is expected that CASTOR will cover the region of maximum baryon density at CMS. Thus, CMS will be able to study partonic matter over a very large range of baryo-chemical potential.
Ultra-peripheral collisions • UPC’s can shed light on a number of physics topics including nuclear parton distributions and meson spectroscopy. • Gluon distribution functions can be measured by studying photoproduction (yg q/q-bar) of heavy quarks (usually c or b, t being available at LHC energies). • It is important to determine if/how the nuclei break up after a UPC. The ZDC system is therefore essential to measure neutral particle flux very near the beam.
Summary • LHC will push energies up to 5.5 TeV for Pb-Pb collisions, and 14 TeV for pp collisions. Many new phenomena are expected at these energies. • CMS is the ideal Heavy Ion detector for studying these new phenomena at these energies.
Acknowledgements • It should be noted that this presentation was meant to inform the UC Davis Nuclear Physics Group and in no way represents original research of my own. • I would like to thank Dr. Daniel Cebra, Dr. Juan Romero, Roppon Picha, and David Cherney for useful discussions related to this presentation. • This presentation is based of “Heavy Ion Physics at the LHC with the Compact Muon Solenoid detector” by • R. Arcidiacono et al. • Pictures were obtained from http://cmsinfo.cern.ch/Welcome.html/CMSdetectorInfo/CMSdetectorInfo.html and its subpages.