The Z^0-tagged jet event asymmetry in heavy-ion collisions at the CERN Large Hadron Collider

Tagged jet measurements provide a promising experimental channel to quantify the similarities and differences in the mechanisms of jet production in proton-proton and nucleus-nucleus collisions. We present the first calculation of the transverse momentum asymmetry of Z^0/gamma^*-tagged jet events in sqrt{s}=2.76$ TeV reactions at the LHC. Our results combine the O(G_F\alpha_s^2) perturbative cross sections with the radiative and collisional processes that modify parton showers in the presence of dense QCD matter. We find that a strong asymmetry is generated in central lead-lead reactions that has little sensitivity to the fluctuations of the underlying soft hadronic background. We present theoretical model predictions for its shape and magnitude.Comment: 4 pages, 4 figures, as published in PR

Thermal field theory derivation of the source term induced by a fast parton from the quark energy-momentum tensor

I derive the distribution of energy and momentum transmitted from a fast parton to a medium of thermalized quarks, or the source term, in perturbative thermal field theory directly from the quark energy-momentum tensor. The fast parton is coupled to the medium by adding an interaction term to the Lagrangian. The thermal expectation value of the energy-momentum tensor source term is then evaluated using standard Feynman rules at finite temperature. It is found that local excitations, which are important for exciting an observable Mach cone structure, fall sharply as a function of the energy of the fast parton. This may have implications for the trigger $p_T$ dependence of measurements of azimuthal dihadron particle correlations in heavy-ion collisions. In particular, a conical emission pattern would be less likely to be observed for increasing trigger $p_T$. I show that the results presented in this paper can be generalized to more realistic modeling of fast parton propagation, such as through a time dependent interaction term, in future studies.Comment: Version as accepted by Physical Review D. New version has several clarifications and added references. 5 pages, 3 figure

A derivation of the source term induced by a fast parton from the quark energy-momentum tensor

The distribution of energy and momentum deposited by a fast parton in a medium of thermalized quarks, or the source term, is evaluated in perturbative thermal field theory. The calculation is performed by directly evaluating the thermal expectation value of the quark energy-momentum tensor. The fast parton is coupled to the medium by adding an interaction term to the Lagrangian. I show that this approach is very general and can be modified to consider more realistic modeling of fast parton propagation, such as a fast parton created in an initial hard interaction or the evolution of a parton shower due to medium induced radiation. For the scenario considered here, it is found that local excitations fall sharply as a function of the energy of the fast parton. These local excitations couple directly to the sound mode in hydrodynamics and are important for generating an observable shockwave structure. This may have implications for the trigger pT dependence of measurements of azimuthal dihadron particle correlations in heavy-ion collisions. In particular, one would be less likely to observe a conical emission pattern for increasing trigger pT.Comment: 7 pages and 3 figures. Contribution to the proceedings of the 27th Winter Workshop on Nuclear Dynamics held in Winter Park, Colorad

Sonic Mach Cones Induced by Fast Partons in a Perturbative Quark-Gluon Plasma

We derive the space-time distribution of energy and momentum deposited by a fast parton traversing a weakly coupled quark-gluon plasma by treating the fast part on as the source of an external color field perturbing the medium. We then use our result as a source term for the linearized hydrodynamical equations of the medium. We show that the solution contains a sonic Mach cone and a dissipative wake if the parton moves at a supersonic speed.Comment: Final version accepted for publicatio

Explicit generating functional for pions and virtual photons

We construct the explicit one-loop functional of chiral perturbation theory for two light flavours, including virtual photons. We stick to contributions where 1 or 2 mesons and at most one photon are running in the loops. With the explicit functional at hand, the evaluation of the relevant Green functions boils down to performing traces over the flavour matrices. For illustration, we work out the pi+ pi- -> pi0 pi0 scattering amplitude at threshold at order p^4, e^2p^2.Comment: 20 pages, 2 figures; version accepted for publication, minor typographical changes, acknowledgments adde

The Mach cone signal and energy deposition scenarios in linearized hydrodynamics

Particle correlation measurements associated with a hard or semi-hard trigger in heavy-ion collisions may reflect Mach cone shockwaves excited in the bulk medium by partonic energy loss. This is of great interest because, when compared with theory, such measurements can provide information on the transport properties of the medium. Specifically, the formation of Mach cone shockwaves is sensitive to the viscosity and speed of sound, as well as the detailed nature of the jet medium interaction. However, modeling the physics of shockwave excitation to obtain a meaningful comparison with the measured correlations is very challenging since the correlations arise from an interplay of perturbative as well as non-perturbative phenomena at different momentum scales. In this work we take a step in that direction by presenting a systematic study of the dependence of azimuthal particle correlations on the spatio-temporal structure of energy deposition into the medium. Our results indicate that detailed modeling of the evolution of an initially produced hard parton and the interaction of this evolving state with the medium is crucial, as both magnitude and shape of the shockwave signal show a strong dependence on the assumptions being made.Comment: Expanded version, as accepted for publication in the Physical Review C. Now contains better discussion and more figures, including the effect of modifying the speed of sound on our result

Implementing High Availability with Cots Components and Open-Source Software

High Availability of IT services is essential for the successful operation of large experimental facilities such as the LHC experiments. In the past, high availability was often taken for granted and/or ensured by using very expensive high-end hardware based on proprietary, single-vendor solutions. Today's IT infrastructure in HEP is usually a heterogeneous environment of cheap, off the shelf components which usually have no intrinsic failure tolerance and can thus not be considered reliable at all. Many services, in particular networked services like the Domain Name Service, shared storage and databases need to run on this unreliable hardware, while they are indispensable for the operation of today's control systems. We present our approach to this problem which is based on a combination of open-source tools, such as the Linux High Availability Projet and home-made tools to ensure high-availability for the LHCb Experiment Control system, which consists of over 200 servers, several hundred switches and is controlling thousands of devices ranging from custom made devices, connected to the LAN, to the servers of the event-filter farm

Jets associated with Z^0 boson production in heavy-ion collisions at the LHC

The heavy ion program at the LHC will present unprecedented opportunities to probe hot QCD matter, that is, the quark gluon plasma (QGP). Among these exciting new probes are high energy partons associated with the production of a Z^0 boson, or Z^0 tagged jets. Once produced, Z^0 bosons are essentially unaffected by the strongly interacting medium produced in heavy-ion collisions, and therefore provide a powerful signal of the initial partonic energy and subsequent medium induced partonic energy loss. When compared with theory, experimental measurements of Z^0 tagged jets will help quantify the jet quenching properties of the QGP and discriminate between different partonic energy loss formalisms. In what follows, I discuss the advantages of tagged jets over leading particles, and present preliminary results of the production and suppression of Z^0 tagged jets in relativistic heavy-ion collisions at LHC energies using the Guylassy-Levai-Vitev (GLV) partonic energy loss formalism.Comment: To appear in the proceedings of the 2010 Winter Workshop on Nuclear Dynamics, which was held in Ocho Rios, Jamaica, mon