# U. A. Wiedemann - CERN

## Contact Details

NameU. A. Wiedemann |
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AffiliationCERN |
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Location |
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## Pubs By Year |
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## External Links |
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## Pub CategoriesHigh Energy Physics - Phenomenology (43) Nuclear Theory (27) High Energy Physics - Experiment (12) Nuclear Experiment (11) Cosmology and Nongalactic Astrophysics (5) General Relativity and Quantum Cosmology (2) Physics - Accelerator Physics (2) High Energy Physics - Theory (1) Physics - Strongly Correlated Electrons (1) Quantum Physics (1) High Energy Physics - Lattice (1) Physics - Instrumentation and Detectors (1) Physics - Optics (1) |

## Publications Authored By U. A. Wiedemann

The unique physics opportunities accessible with nuclear collisions at the CERN Future Circular Collider (FCC) are summarized. Lead-lead (PbPb) and proton-lead (pPb) collisions at $\sqrt{s_{NN}}$ = 39 and 63 TeV respectively with $\mathcal{L}_{int}$ = 33 nb$^{-1}$ and 8 pb$^{-1}$ monthly integrated luminosities, will provide unprecedented experimental conditions to study quark-gluon matter at temperatures ${\cal O}$(1 GeV). The following topics are succinctly discussed: (i) charm-quark densities thrice larger than at the LHC, leading to direct heavy-quark impact in the bulk QGP properties, (ii) quarkonia, including $\Upsilon(1S)$, melting at temperatures up to five times above the QCD critical temperature, (iii) access to initial-state nuclear parton distributions (nPDF) at fractional momenta as low as $x\approx 10^{-7}$, (iv) availability of $5\cdot 10^5$ top-quark pairs per run to study the high-$x$ gluon nPDF and the energy loss properties of boosted colour-antennas, (v) study of possible Higgs boson suppression in the QGP, and (vi) high-luminosity $\gamma\gamma$ (ultraperipheral) collisions at c. Read More

Treating dark matter at large scales as an effectively viscous fluid provides an improved framework for the calculation of the density and velocity power spectra compared to the standard assumption of an ideal pressureless fluid. We discuss how this framework can be made concrete through an appropriate coarse-graining procedure. We also review results that demonstrate that it improves the convergence of cosmological perturbation theory. Read More

Following an approach of Matarrese and Pietroni, we derive the functional renormalization group (RG) flow of the effective action of cosmological large-scale structures. Perturbative solutions of this RG flow equation are shown to be consistent with standard cosmological perturbation theory. Non-perturbative approximate solutions can be obtained by truncating the a priori infinite set of possible effective actions to a finite subspace. Read More

**Authors:**A. Dainese

^{1}, U. A. Wiedemann

^{2}, N. Armesto

^{3}, D. d'Enterria

^{4}, J. M. Jowett

^{5}, J. -P. Lansberg

^{6}, J. G. Milhano

^{7}, C. A. Salgado

^{8}, M. Schaumann

^{9}, M. van Leeuwen

^{10}, J. L. Albacete, A. Andronic, P. Antonioli, L. Apolinario, S. Bass, A. Beraudo, A. Bilandzic, S. Borsanyi, P. Braun-Munzinger, Z. Chen, L. Cunqueiro Mendez, G. S. Denicol, K. J. Eskola, S. Floerchinger, H. Fujii, P. Giubellino, C. Greiner, J. F. Grosse-Oetringhaus, C. -M. Ko, P. Kotko, K. Krajczar, K. Kutak, M. Laine, Y. Liu, M. P. Lombardo, M. Luzum, C. Marquet, S. Masciocchi, V. Okorokov, J. -F. Paquet, H. Paukkunen, E. Petreska, T. Pierog, M. Ploskon, C. Ratti, A. H. Rezaeian, W. Riegler, J. Rojo, C. Roland, A. Rossi, G. P. Salam, S. Sapeta, R. Schicker, C. Schmidt, J. Stachel, J. Uphoff, A. van Hameren, K. Watanabe, B. -W. Xiao, F. Yuan, D. Zaslavsky, K. Zhou, P. Zhuang

**Affiliations:**

^{1}editors,

^{2}editors,

^{3}section editors,

^{4}section editors,

^{5}section editors,

^{6}section editors,

^{7}section editors,

^{8}section editors,

^{9}section editors,

^{10}section editors

The Future Circular Collider (FCC) Study is aimed at assessing the physics potential and the technical feasibility of a new collider with centre-of-mass energies, in the hadron-hadron collision mode, seven times larger than the nominal LHC energies. Operating such machine with heavy ions is an option that is being considered in the accelerator design studies. It would provide, for example, Pb-Pb and p-Pb collisions at sqrt{s_NN} = 39 and 63 TeV, respectively, per nucleon-nucleon collision, with integrated luminosities above 30 nb^-1 per month for Pb-Pb. Read More

**Authors:**Federico Antinori, Francesco Becattini, Peter Braun-Munzinger, Tatsuya Chujo, Hideki Hamagaki, John Harris, Ulrich Heinz, Boris Hippolyte, Tetsufumi Hirano, Barbara Jacak, Dmitri Kharzeev, Constantin Loizides, Silvia Masciocchi, Alexander Milov, Andreas Morsch, Berndt Müller, Jamie Nagle, Jean-Yves Ollitrault, Guy Paic, Krishna Rajagopal, Gunther Roland, Jürgen Schukraft, Yves Schutz, Raimond Snellings, Johanna Stachel, Derek Teaney, Julia Velkovska, Sergei Voloshin, Urs Achim Wiedemann, Zhangbu Xu, William Zajc

This document summarizes thoughts on opportunities in the soft-QCD sector from high-energy nuclear collisions at high luminosities. Read More

The Future Circular Collider is a new proposed collider at CERN with centre-of-mass energies around 100 TeV in the pp mode. Ongoing studies aim at assessing its physics potential and technical feasibility. Here we focus on updates in physics opportunities accessible in pA and AA collisions not covered in previous Quark Matter contributions, including Quark-Gluon Plasma and gluon saturation studies, novel hard probes of QCD matter, and photon-induced collisions. Read More

In hadronization models with color recombination, partons are allowed to regroup into color singlet structures that are different from those determined by the perturbative parton shower. This aims at modeling the possibility that soft interactions of partons with the underlying event can change color connections. If such an effect is at play in proton-proton collisions, it may be expected to be enhanced in proton-nucleus collisions due to the higher color charge density in the underlying event. Read More

Cosmological perturbations of sufficiently long wavelength admit a fluid dynamic description. We consider modes with wavevectors below a scale $k_m$ for which the dynamics is only mildly non-linear. The leading effect of modes above that scale can be accounted for by effective non-equilibrium viscosity and pressure terms. Read More

Recently, we have derived a novel and compact expression for how perturbations in the matter fields of the cosmological fluid can lead to deviations from the standard Friedmann equations. Remarkably, the dissipative damping of velocity perturbations by bulk and shear viscosity in the dark sector can modify the expansion history of the universe on arbitrarily large scales. In universes in which this effect is sufficiently sizeable, it could account for the acceleration of the cosmological expansion. Read More

**Authors:**Yasuyuki Akiba

^{1}, Aaron Angerami

^{2}, Helen Caines

^{3}, Anthony Frawley

^{4}, Ulrich Heinz

^{5}, Barbara Jacak

^{6}, Jiangyong Jia

^{7}, Tuomas Lappi

^{8}, Wei Li

^{9}, Abhijit Majumder

^{10}, David Morrison

^{11}, Mateusz Ploskon

^{12}, Joern Putschke

^{13}, Krishna Rajagopal

^{14}, Ralf Rapp

^{15}, Gunther Roland

^{16}, Paul Sorensen

^{17}, Urs Wiedemann

^{18}, Nu Xu

^{19}, W. A. Zajc

^{20}

**Affiliations:**

^{1}RIKEN,

^{2}Columbia University,

^{3}Yale University,

^{4}Florida State University,

^{5}Ohio State University,

^{6}University of California, Berkeley,

^{7}Stony Brook University,

^{8}Jyväskylä University,

^{9}Rice University,

^{10}Wayne State University,

^{11}Brookhaven National Laboratory,

^{12}Lawrence Berkeley National Laboratory,

^{13}Wayne State University,

^{14}Massachusetts Institute of Technology,

^{15}Texas A&M University,

^{16}Massachusetts Institute of Technology,

^{17}Brookhaven National Laboratory,

^{18}CERN,

^{19}Lawrence Berkeley National Laboratory,

^{20}Columbia University

The past decade has seen huge advances in experimental measurements made in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) and more recently at the Large Hadron Collider (LHC). These new data, in combination with theoretical advances from calculations made in a variety of frameworks, have led to a broad and deep knowledge of the properties of thermal QCD matter. Increasingly quantitative descriptions of the quark-gluon plasma (QGP) created in these collisions have established that the QGP is a strongly coupled liquid with the lowest value of specific viscosity ever measured. Read More

Density perturbations and their dynamic evolution from early to late times can be used for an improved understanding of interesting physical phenomena both in cosmology and in the context of heavy-ion collisions. We discuss the spectrum and bispectrum of these perturbations around a longitudinally expanding fireball after a heavy-ion collision. The time-evolution equations couple the spectrum and bispectrum to each other, as well as to higher-order correlation functions through nonlinear terms. Read More

The dissipation of energy from local velocity perturbations in the cosmological fluid affects the time evolution of spatially averaged fluid dynamic fields and the cosmological solution of Einstein's field equations. We show how this backreaction effect depends on shear and bulk viscosity and other material properties of the dark sector, as well as the spectrum of perturbations. If sufficiently large, this effect could account for the acceleration of the cosmological expansion. Read More

**Authors:**Federico Antinori, Nestor Armesto, Paolo Bartalini, Rene Bellwied, Peter Braun-Munzinger, Brian Cole, Andrea Dainese, Marek Gazdzicki, Paolo Giubellino, John Harris, Ulrich Heinz, Barbara Jacak, Peter Jacobs, Dmitri Kharzeev, Constantin Loizides, Silvia Masciocchi, Andreas Morsch, Berndt Mueller, Jamie Nagle, Guy Paic, Krishna Rajagopal, Gunther Roland, Karel Safarik, Jurgen Schukraft, Yves Schutz, Johanna Stachel, Peter Steinberg, Thomas Ullrich, Xin-Nian Wang, Johannes Wessels, Urs Achim Wiedemann

This document summarizes thoughts on opportunities from high-energy nuclear collisions. Read More

Initial fluctuations in hydrodynamic fields such as energy density or flow velocity give access to understanding initial state and equilibration physics as well as thermodynamic and transport properties. We provide evidence that the fluid dynamic propagation of fluctuations of realistic size can be based on a background-fluctuation splitting and a systematic perturbative expansion in the fluctuating fields. Initial conditions are characterized by a Bessel-Fourier expansion for single events, event-by-event correlations and probability distributions. Read More

The Future Circular Collider (FCC) design study is aimed at assessing the physics potential and the technical feasibility of a new collider with centre-of-mass energies, in the hadron-hadron collision mode including proton and nucleus beams, more than seven-times larger than the nominal LHC energies. An electron-positron collider in the same tunnel is also considered as an intermediate step, which would provide the electron-hadron option in the long term. First ideas on the physics opportunities with heavy ions at the FCC are presented, covering the physics of Quark-Gluon Plasma, gluon saturation, photon-induced collisions, as well as connections with ultra-high-energy cosmic rays. Read More

Based on parametric reasoning, we provide a simple dynamical picture of how a perturbative parton cascade, in interaction with a QCD medium, fills phase space as a function of time. Read More

An interesting opportunity to determine thermodynamic and transport properties in more detail is to identify generic statistical properties of initial density perturbations. Here we study event-by-event fluctuations in terms of correlation functions for two models that can be solved analytically. The first assumes Gaussian fluctuations around a distribution that is fixed by the collision geometry but leads to non-Gaussian features after averaging over the reaction plane orientation at non-zero impact parameter. Read More

We provide evidence from full numerical solutions that the hydrodynamical evolution of initial density fluctuations in heavy ion collisions can be understood order-by-order in a perturbative series in deviations from a smooth and azimuthally symmetric background solution. To leading linear order, modes with different azimuthal wave numbers do not mix. Quadratic and higher order corrections are small and can be understood as overtones with corresponding wave numbers. Read More

The kinetic freeze-out for the hydrodynamical description of relativistic heavy ion collisions is discussed using a background-fluctuation splitting of the hydrodynamical fields. For a single event, the particle spectrum, or its logarithm, can be written as the sum of background part that is symmetric with respect to azimuthal rotations and longitudinal boosts and a part containing the contribution of fluctuations or deviations from the background. Using a complete orthonormal basis to characterize the initial state allows one to write the double differential harmonic flow coefficients determined by the two-particle correlation method as matrix expressions involving the initial fluid correlations. Read More

As a consequence of the chiral anomaly, the hydrodynamics of hot QCD matter coupled to QED allows for a long-wavelength mode of chiral charge density, the chiral magnetic wave (CMW), that provides for a mechanism of electric charge separation along the direction of an external magnetic field. Here, we investigate the efficiency of this mechanism for values of the time-dependent magnetic field and of the energy density attained in the hot QCD matter of ultra-relativistic heavy ion collisions. To this end, we derive the CMW equations of motion for expanding systems by treating the CMW as a charge perturbation on top of an expanding Bjorken-type background field in the limit of small chemical potential. Read More

Event-by-event fluctuations in the initial conditions for a hydrodynamical description of heavy-ion collisions are characterized. We propose a Bessel-Fourier decomposition with respect to the azimuthal angle, the radius in the transverse plane and rapidity. This allows for a complete characterization of fluctuations in all hydrodynamical fields including energy density, pressure, fluid velocity, shear stress and bulk viscous pressure. Read More

We propose to study the fluid dynamic propagation of fluctuations in relativistic heavy ion collisions differentially with respect to their azimuthal, radial and longitudinal wavelength. To this end, we introduce a background-fluctuation splitting and a Bessel-Fourier decomposition of the fluctuating modes. We demonstrate how the fluid dynamic evolution of realistic events can be build up from the propagation of individual modes. Read More

The two arguably most generic phenomena seen in ultra-relativistic heavy ion collisions are the flow of essentially all soft hadronic observables and the quenching of essentially all hard hadronic observables. Limiting the discussion to these two classes of phenomena, I review what can be said so far about the properties of hot and dense QCD matter from the heavy ion programs at RHIC and at the LHC, and I discuss the opportunities for further progress in the coming years. Read More

We present a conceptually new framework for describing jet evolution in the dense medium produced in ultra-relativistic nucleus-nucleus collisions using perturbative QCD and its implementation into the Monte Carlo event generator JEWEL. The rescattering of hard partons in the medium is modelled by infrared continued pQCD matrix elements supplemented with parton showers. The latter approximate higher order real-emission matrix elements and thus generate medium-induced gluon emissions. Read More

**Authors:**J. L. Abelleira Fernandez, C. Adolphsen, P. Adzic, A. N. Akay, H. Aksakal, J. L. Albacete, B. Allanach, S. Alekhin, P. Allport, V. Andreev, R. B. Appleby, E. Arikan, N. Armesto, G. Azuelos, M. Bai, D. Barber, J. Bartels, O. Behnke, J. Behr, A. S. Belyaev, I. Ben-Zvi, N. Bernard, S. Bertolucci, S. Bettoni, S. Biswal, J. Blümlein, H. Böttcher, A. Bogacz, C. Bracco, J. Bracinik, G. Brandt, H. Braun, S. Brodsky, O. Brüning, E. Bulyak, A. Buniatyan, H. Burkhardt, I. T. Cakir, O. Cakir, R. Calaga, A. Caldwell, V. Cetinkaya, V. Chekelian, E. Ciapala, R. Ciftci, A. K. Ciftci, B. A. Cole, J. C. Collins, O. Dadoun, J. Dainton, A. De. Roeck, D. d'Enterria, P. DiNezza, M. D'Onofrio, A. Dudarev, A. Eide, R. Enberg, E. Eroglu, K. J. Eskola, L. Favart, M. Fitterer, S. Forte, A. Gaddi, P. Gambino, H. García Morales, T. Gehrmann, P. Gladkikh, C. Glasman, A. Glazov, R. Godbole, B. Goddard, T. Greenshaw, A. Guffanti, V. Guzey, C. Gwenlan, T. Han, Y. Hao, F. Haug, W. Herr, A. Hervé, B. J. Holzer, M. Ishitsuka, M. Jacquet, B. Jeanneret, E. Jensen, J. M. Jimenez, J. M. Jowett, H. Jung, H. Karadeniz, D. Kayran, A. Kilic, K. Kimura, R. Klees, M. Klein, U. Klein, T. Kluge, F. Kocak, M. Korostelev, A. Kosmicki, P. Kostka, H. Kowalski, M. Kraemer, G. Kramer, D. Kuchler, M. Kuze, T. Lappi, P. Laycock, E. Levichev, S. Levonian, V. N. Litvinenko, A. Lombardi, J. Maeda, C. Marquet, B. Mellado, K. H. Mess, A. Milanese, J. G. Milhano, S. Moch, I. I. Morozov, Y. Muttoni, S. Myers, S. Nandi, Z. Nergiz, P. R. Newman, T. Omori, J. Osborne, E. Paoloni, Y. Papaphilippou, C. Pascaud, H. Paukkunen, E. Perez, T. Pieloni, E. Pilicer, B. Pire, R. Placakyte, A. Polini, V. Ptitsyn, Y. Pupkov, V. Radescu, S. Raychaudhuri, L. Rinolfi, E. Rizvi, R. Rohini, J. Rojo, S. Russenschuck, M. Sahin, C. A. Salgado, K. Sampei, R. Sassot, E. Sauvan, M. Schaefer, U. Schneekloth, T. Schörner-Sadenius, D. Schulte, A. Senol, A. Seryi, P. Sievers, A. N. Skrinsky, W. Smith, D. South, H. Spiesberger, A. M. Stasto, M. Strikman, M. Sullivan, S. Sultansoy, Y. P. Sun, B. Surrow, L. Szymanowski, P. Taels, I. Tapan, T. Tasci, E. Tassi, H. Ten. Kate, J. Terron, H. Thiesen, L. Thompson, P. Thompson, K. Tokushuku, R. Tomás García, D. Tommasini, D. Trbojevic, N. Tsoupas, J. Tuckmantel, S. Turkoz, T. N. Trinh, K. Tywoniuk, G. Unel, T. Ullrich, J. Urakawa, P. VanMechelen, A. Variola, R. Veness, A. Vivoli, P. Vobly, J. Wagner, R. Wallny, S. Wallon, G. Watt, C. Weiss, U. A. Wiedemann, U. Wienands, F. Willeke, B. -W. Xiao, V. Yakimenko, A. F. Zarnecki, Z. Zhang, F. Zimmermann, R. Zlebcik, F. Zomer

The present note relies on the recently published conceptual design report of the LHeC and extends the first contribution to the European strategy debate in emphasising the role of the LHeC to complement and complete the high luminosity LHC programme. The brief discussion therefore focuses on the importance of high precision PDF and $\alpha_s$ determinations for the physics beyond the Standard Model (GUTs, SUSY, Higgs). Emphasis is also given to the importance of high parton density phenomena in nuclei and their relevance to the heavy ion physics programme at the LHC. Read More

**Authors:**J. L. Abelleira Fernandez, C. Adolphsen, P. Adzic, A. N. Akay, H. Aksakal, J. L. Albacete, B. Allanach, S. Alekhin, P. Allport, V. Andreev, R. B. Appleby, E. Arikan, N. Armesto, G. Azuelos, M. Bai, D. Barber, J. Bartels, O. Behnke, J. Behr, A. S. Belyaev, I. Ben-Zvi, N. Bernard, S. Bertolucci, S. Bettoni, S. Biswal, J. Blümlein, H. Böttcher, A. Bogacz, C. Bracco, J. Bracinik, G. Brandt, H. Braun, S. Brodsky, O. Brüning, E. Bulyak, A. Buniatyan, H. Burkhardt, I. T. Cakir, O. Cakir, R. Calaga, A. Caldwell, V. Cetinkaya, V. Chekelian, E. Ciapala, R. Ciftci, A. K. Ciftci, B. A. Cole, J. C. Collins, O. Dadoun, J. Dainton, A. De. Roeck, D. d'Enterria, P. DiNezza, M. D'Onofrio, A. Dudarev, A. Eide, R. Enberg, E. Eroglu, K. J. Eskola, L. Favart, M. Fitterer, S. Forte, A. Gaddi, P. Gambino, H. García Morales, T. Gehrmann, P. Gladkikh, C. Glasman, A. Glazov, R. Godbole, B. Goddard, T. Greenshaw, A. Guffanti, V. Guzey, C. Gwenlan, T. Han, Y. Hao, F. Haug, W. Herr, A. Hervé, B. J. Holzer, M. Ishitsuka, M. Jacquet, B. Jeanneret, E. Jensen, J. M. Jimenez, J. M. Jowett, H. Jung, H. Karadeniz, D. Kayran, A. Kilic, K. Kimura, R. Klees, M. Klein, U. Klein, T. Kluge, F. Kocak, M. Korostelev, A. Kosmicki, P. Kostka, H. Kowalski, M. Kraemer, G. Kramer, D. Kuchler, M. Kuze, T. Lappi, P. Laycock, E. Levichev, S. Levonian, V. N. Litvinenko, A. Lombardi, J. Maeda, C. Marquet, B. Mellado, K. H. Mess, A. Milanese, J. G. Milhano, S. Moch, I. I. Morozov, Y. Muttoni, S. Myers, S. Nandi, Z. Nergiz, P. R. Newman, T. Omori, J. Osborne, E. Paoloni, Y. Papaphilippou, C. Pascaud, H. Paukkunen, E. Perez, T. Pieloni, E. Pilicer, B. Pire, R. Placakyte, A. Polini, V. Ptitsyn, Y. Pupkov, V. Radescu, S. Raychaudhuri, L. Rinolfi, E. Rizvi, R. Rohini, J. Rojo, S. Russenschuck, M. Sahin, C. A. Salgado, K. Sampei, R. Sassot, E. Sauvan, M. Schaefer, U. Schneekloth, T. Schörner-Sadenius, D. Schulte, A. Senol, A. Seryi, P. Sievers, A. N. Skrinsky, W. Smith, D. South, H. Spiesberger, A. M. Stasto, M. Strikman, M. Sullivan, S. Sultansoy, Y. P. Sun, B. Surrow, L. Szymanowski, P. Taels, I. Tapan, T. Tasci, E. Tassi, H. Ten. Kate, J. Terron, H. Thiesen, L. Thompson, P. Thompson, K. Tokushuku, R. Tomás García, D. Tommasini, D. Trbojevic, N. Tsoupas, J. Tuckmantel, S. Turkoz, T. N. Trinh, K. Tywoniuk, G. Unel, T. Ullrich, J. Urakawa, P. VanMechelen, A. Variola, R. Veness, A. Vivoli, P. Vobly, J. Wagner, R. Wallny, S. Wallon, G. Watt, C. Weiss, U. A. Wiedemann, U. Wienands, F. Willeke, B. -W. Xiao, V. Yakimenko, A. F. Zarnecki, Z. Zhang, F. Zimmermann, R. Zlebcik, F. Zomer

This document provides a brief overview of the recently published report on the design of the Large Hadron Electron Collider (LHeC), which comprises its physics programme, accelerator physics, technology and main detector concepts. The LHeC exploits and develops challenging, though principally existing, accelerator and detector technologies. This summary is complemented by brief illustrations of some of the highlights of the physics programme, which relies on a vastly extended kinematic range, luminosity and unprecedented precision in deep inelastic scattering. Read More

In a suitably chosen back-to-back kinematics, four-jet production in hadronic collisions is known to be dominated by contributions from two independent partonic scattering processes, thus giving experimental access to the structure of generalized two-parton distributions 2GPDs. Here, we show that a combined measurement of the double hard four-jet cross section in proton-proton and proton-nucleus collisions will allow one to disentangle different sources of two-parton correlations in the proton, that cannot be disentangled with 4-jet measurements in proton-proton collisions alone. To this end, we analyze in detail the structure of 2GPDs in the nucleus (A), we calculate in the independent nucleon approximation all contributions to the double hard four-jet cross section in pA, and we determine corrections arising from the nuclear dependence of single parton distribution functions. Read More

**Authors:**J. L. Abelleira Fernandez, C. Adolphsen, A. N. Akay, H. Aksakal, J. L. Albacete, S. Alekhin, P. Allport, V. Andreev, R. B. Appleby, E. Arikan, N. Armesto, G. Azuelos, M. Bai, D. Barber, J. Bartels, O. Behnke, J. Behr, A. S. Belyaev, I. Ben-Zvi, N. Bernard, S. Bertolucci, S. Bettoni, S. Biswal, J. Blümlein, H. Böttcher, A. Bogacz, C. Bracco, G. Brandt, H. Braun, S. Brodsky, O. Brüning, E. Bulyak, A. Buniatyan, H. Burkhardt, I. T. Cakir, O. Cakir, R. Calaga, V. Cetinkaya, E. Ciapala, R. Ciftci, A. K. Ciftci, B. A. Cole, J. C. Collins, O. Dadoun, J. Dainton, A. De. Roeck, D. d'Enterria, A. Dudarev, A. Eide, R. Enberg, E. Eroglu, K. J. Eskola, L. Favart, M. Fitterer, S. Forte, A. Gaddi, P. Gambino, H. García Morales, T. Gehrmann, P. Gladkikh, C. Glasman, R. Godbole, B. Goddard, T. Greenshaw, A. Guffanti, V. Guzey, C. Gwenlan, T. Han, Y. Hao, F. Haug, W. Herr, A. Hervé, B. J. Holzer, M. Ishitsuka, M. Jacquet, B. Jeanneret, J. M. Jimenez, J. M. Jowett, H. Jung, H. Karadeniz, D. Kayran, A. Kilic, K. Kimura, M. Klein, U. Klein, T. Kluge, F. Kocak, M. Korostelev, A. Kosmicki, P. Kostka, H. Kowalski, G. Kramer, D. Kuchler, M. Kuze, T. Lappi, P. Laycock, E. Levichev, S. Levonian, V. N. Litvinenko, A. Lombardi, J. Maeda, C. Marquet, S. J. Maxfield, B. Mellado, K. H. Mess, A. Milanese, S. Moch, I. I. Morozov, Y. Muttoni, S. Myers, S. Nandi, Z. Nergiz, P. R. Newman, T. Omori, J. Osborne, E. Paoloni, Y. Papaphilippou, C. Pascaud, H. Paukkunen, E. Perez, T. Pieloni, E. Pilicer, B. Pire, R. Placakyte, A. Polini, V. Ptitsyn, Y. Pupkov, V. Radescu, S. Raychaudhuri, L. Rinolfi, R. Rohini, J. Rojo, S. Russenschuck, M. Sahin, C. A. Salgado, K. Sampei, R. Sassot, E. Sauvan, U. Schneekloth, T. Schörner-Sadenius, D. Schulte, A. Senol, A. Seryi, P. Sievers, A. N. Skrinsky, W. Smith, H. Spiesberger, A. M. Stasto, M. Strikman, M. Sullivan, S. Sultansoy, Y. P. Sun, B. Surrow, L. Szymanowski, P. Taels, I. Tapan, A. T. Tasci, E. Tassi, H. Ten. Kate, J. Terron, H. Thiesen, L. Thompson, K. Tokushuku, R. Tomás García, D. Tommasini, D. Trbojevic, N. Tsoupas, J. Tuckmantel, S. Turkoz, T. N. Trinh, K. Tywoniuk, G. Unel, J. Urakawa, P. VanMechelen, A. Variola, R. Veness, A. Vivoli, P. Vobly, J. Wagner, R. Wallny, S. Wallon, G. Watt, C. Weiss, U. A. Wiedemann, U. Wienands, F. Willeke, B. -W. Xiao, V. Yakimenko, A. F. Zarnecki, Z. Zhang, F. Zimmermann, R. Zlebcik, F. Zomer

The physics programme and the design are described of a new collider for particle and nuclear physics, the Large Hadron Electron Collider (LHeC), in which a newly built electron beam of 60 GeV, up to possibly 140 GeV, energy collides with the intense hadron beams of the LHC. Compared to HERA, the kinematic range covered is extended by a factor of twenty in the negative four-momentum squared, $Q^2$, and in the inverse Bjorken $x$, while with the design luminosity of $10^{33}$ cm$^{-2}$s$^{-1}$ the LHeC is projected to exceed the integrated HERA luminosity by two orders of magnitude. The physics programme is devoted to an exploration of the energy frontier, complementing the LHC and its discovery potential for physics beyond the Standard Model with high precision deep inelastic scattering measurements. Read More

Multiple interactions between parton showers and the surrounding QCD matter are expected to underlie the strong medium-modifications of jet observables in ultra-relativistic heavy ion collisions at RHIC and at the LHC. Here, we note that such jet-medium interactions alter generically and characteristically the color correlations in the parton shower. We characterize these effects in a color-differential calculation of the medium-induced gluon radiation spectrum to first and second order in opacity. Read More

We discuss opportunities that may arise from subjecting high-multiplicity events in relativistic heavy ion collisions to an analysis similar to the one used in cosmology for the study of fluctuations of the Cosmic Microwave Background (CMB). To this end, we discuss examples of how pertinent features of heavy ion collisions including global characteristics, signatures of collective flow and event-wise fluctuations are visually represented in a Mollweide projection commonly used in CMB analysis, and how they are statistically analyzed in an expansion over spherical harmonic functions. If applied to the characterization of purely azimuthal dependent phenomena such as collective flow, the expansion coefficients of spherical harmonics are seen to contain redundancies compared to the set of harmonic flow coefficients commonly used in heavy ion collisions. Read More

It is widely accepted that a phenomenologically viable theory of jet quenching for heavy ion collisions requires the understanding of medium-induced parton energy loss beyond the limit of eikonal kinematics formulated by Baier-Dokshitzer-Mueller-Peigne-Schiff and Zakharov (BDMPS-Z). Here, we supplement a recently developed exact Monte Carlo implementation of the BDMPS-Z formalism with elementary physical requirements including exact energy-momentum conservation, a refined formulation of jet-medium interactions and a treatment of all parton branchings on the same footing. We document the changes induced by these physical requirements and we describe their kinematic origin. Read More

We present a new formulation of jet quenching in perturbative QCD beyond the eikonal approximation. Multiple scattering in the medium is modelled through infra-red-continued (2 -> 2) scattering matrix elements in QCD and the parton shower describing further emissions. The interplay between these processes is arranged in terms of a formation time constraint such that coherent emissions can be treated consistently. Read More

Medium-induced parton energy loss, resulting from gluon exchanges between the QCD matter and partonic projectiles, is expected to underly the strong suppression of jets and high-$p_T$ hadron spectra observed in ultra-relativistic heavy ion collisions. Here, we present the first color-differential calculation of parton energy loss. We find that color exchange between medium and projectile enhances the invariant mass of energetic color singlet clusters in the parton shower by a parametrically large factor proportional to the square root of the projectile energy. Read More

We study how fluctuations in fluid dynamic fields can be dissipated or amplified within the characteristic spatio-temporal structure of a heavy ion collision. The initial conditions for a fluid dynamic evolution of heavy ion collisions may contain significant fluctuations in all fluid dynamical fields, including the velocity field and its vorticity components. We formulate and analyze the theory of local fluctuations around average fluid fields described by Bjorken's model. Read More

The strong modifications of dijet properties in heavy ion collisions measured by ATLAS and CMS provide important constraints on the dynamical mechanisms underlying jet quenching. In this work, we show that the transport of soft gluons away from the jet cone - jet collimation - can account for the observed dijet asymmetry with values of $\hat{q}\, L$ that lie in the expected order of magnitude. Further, we show that the energy loss attained through this mechanism results in a very mild distortion of the azimuthal angle dijet distribution. Read More

The suppression of high-pT single-hadron spectra in heavy-ion collisions is usually interpreted as due to parton energy-loss of high-momentum quarks and gluons propagating in the plasma. Here, we discuss to what extent this partonic picture must be complemented by a picture of medium-modified hadronization. In particular, we show how color-exchange with the medium modifies the properties of color singlet-clusters arising from the parton branchings, producing a softening of the hadron spectra. Read More

We study the evolution of local event-by-event deviations from smooth average fluid dynamic fields, as they can arise in heavy ion collisions from the propagation of fluctuating initial conditions. Local fluctuations around Bjorken flow are found to be governed by non-linear equations whose solutions can be characterized qualitatively in terms of Reynolds numbers. Perturbations at different rapidities decouple quickly, and satisfy (after suitable coordinate transformations) an effectively two-dimensional Navier-Stokes equation of non-relativistic form. Read More

We review the currently available formalisms for radiative energy loss of a high-momentum parton in a dense strongly interacting medium. The underlying theoretical framework of the four commonly used formalisms is discussed and the differences and commonalities between the formalisms are highlighted. A quantitative comparison of the single gluon emission spectra as well as the energy loss distributions is given for a model system consisting of a uniform medium with a fixed length of L=2 fm and L=5 fm (the `Brick'). Read More

**Authors:**C. A. Salgado

^{1}, J. Alvarez-Muniz, F. Arleo, N. Armesto, M. Botje, M. Cacciari, J. Campbell, C. Carli, B. Cole, D. D'Enterria, F. Gelis, V. Guzey, K. Hencken, P. Jacobs, J. M. Jowett, S. R. Klein, F. Maltoni, A. Morsch, K. Piotrzkowski, J. W. Qiu, T. Satogata, F. Sikler, M. Strikman, H. Takai, R. Vogt, J. P. Wessels, S. N. White, U. A. Wiedemann, B. Wyslouch, M. Zhalov

**Affiliations:**

^{1}Editor

Proton-nucleus (p+A) collisions have long been recognized as a crucial component of the physics programme with nuclear beams at high energies, in particular for their reference role to interpret and understand nucleus-nucleus data as well as for their potential to elucidate the partonic structure of matter at low parton fractional momenta (small-x). Here, we summarize the main motivations that make a proton-nucleus run a decisive ingredient for a successful heavy-ion programme at the Large Hadron Collider (LHC) and we present unique scientific opportunities arising from these collisions. We also review the status of ongoing discussions about operation plans for the p+A mode at the LHC. Read More

We review our recent progress in the production and characterization of tapered optical fibers with a sub-wavelength diameter waist. Such fibers exhibit a pronounced evanescent field and are therefore a useful tool for highly sensitive evanescent wave spectroscopy of adsorbates on the fiber waist or of the medium surrounding. We use a carefully designed flame pulling process that allows us to realize preset fiber diameter profiles. Read More

Monte Carlo (MC) simulations are the standard tool for describing jet-like multi-particle final states. To apply them to the simulation of medium-modified jets in heavy ion collisions, a probabilistic implementation of medium-induced quantum interference effects is needed. Here, we analyze in detail how the quantum interference effects included in the BDMPS-Z formalism of medium-induced gluon radiation can be implemented in a quantitatively controlled, local probabilistic parton cascade. Read More

Over the last decade, both experimental and theoretical advances have brought the need for strong coupling techniques in the analysis of deconfined QCD matter and heavy ion collisions to the forefront. As a consequence, a fruitful interplay has developed between analyses of strongly-coupled non-abelian plasmas via the gauge/string duality (also referred to as the AdS/CFT correspondence) and the phenomenology of heavy ion collisions. We review some of the main insights gained from this interplay to date. Read More

The ATLAS Collaboration recently reported strong modifications of dijet properties in heavy ion collisions. In this work, we discuss to what extent these first data constrain already the microscopic mechanism underlying jet quenching. Simple kinematic arguments lead us to identify a frequency collimation mechanism via which the medium efficiently trims away the soft components of the jet parton shower. Read More

Global perturbative QCD analyses, based on large data sets from e-p and hadron collider experiments, provide tight constraints on the parton distribution function (PDF) in the proton. The extension of these analyses to nuclear parton distributions (nPDF) has attracted much interest in recent years. nPDFs are needed as benchmarks for the characterization of hot QCD matter in nucleus-nucleus collisions, and attract further interest since they may show novel signatures of non-linear density-dependent QCD evolution. Read More

Global perturbative QCD analyses, based on large data sets from electron-proton and hadron collider experiments, provide tight constraints on the parton distribution function (PDF) in the proton. The extension of these analyses to nuclear parton distributions (nPDF) has attracted much interest in recent years. nPDFs are needed as benchmarks for the characterization of hot QCD matter in nucleus-nucleus collisions, and attract further interest since they may show novel signatures of non- linear density-dependent QCD evolution. Read More

Global perturbative QCD analyses, based on large data sets from electron-proton and hadron collider experiments, provide tight constraints on the parton distribution function (PDF) in the proton. The extension of these analyses to nuclear parton distributions (nPDF) has attracted much interest in recent years. nPDFs are needed as benchmarks for the characterization of hot QCD matter in nucleus-nucleus collisions, and attract further interest since they may show novel signatures of non-linear density-dependent QCD evolution. Read More

Elliptic flow is a hallmark of collectivity in hadronic collisions. Its measurement relies on analysis techniques which require high event multiplicity and could be applied so far to heavy ion collisions only. Here, we delineate the conditions under which elliptic flow becomes measurable in the samples of high-multiplicity ($dN_{\rm ch}/dy \geq 50$) p-p collisions, which will soon be collected at the LHC. Read More

"As you are well aware, many in the RHIC community are interested in the LHC heavy-ion program, but have several questions: What can we learn at the LHC that is qualitatively new? Are collisions at LHC similar to RHIC ones, just with a somewhat hotter/denser initial state? If not, why not? These questions are asked in good faith, and this talk is an opportunity to answer them directly to much of the RHIC community." With these words, the organizers of Quark Matter 2009 in Knoxville invited me to discuss the physics opportunities for heavy ion collisions at the LHC without recalling the standard arguments, which are mainly based on the extended kinematic reach of the machine. In response, I emphasize here that lattice QCD indicates characteristic qualitative differences between thermal physics in the neighborhood of the critical temperature (T_c < T < 400 - 500 MeV) and thermal physics at higher temperatures (T > 400-500 MeV), for which the relevant energy densities will be solely attainable at the LHC. Read More

This review article was prepared for the Landolt-Boernstein volume on Relativisitc Heavy Ion Physics. Read More

Extending the use of Monte Carlo (MC) event generators to jets in nuclear collisions requires a probabilistic implementation of the non-abelian LPM effect. We demonstrate that a local, probabilistic MC implementation based on the concept of formation times can account fully for the LPM-effect. The main features of the analytically known eikonal and collinear approximation can be reproduced, but we show how going beyond this approximation can lead to qualitatively different results. Read More