arXiv subjects

U. A. Wiedemann

Publications and source records attributed to U. A. Wiedemann.

At least 19 recordsLinked to original sources

arXiv2020

QCD Challenges from pp to A-A Collisions

This paper is a write-up of the ideas that were presented, developed and discussed at the third International Workshop on QCD Challenges from pp to A-A, which took place in August 2019 in Lund, Sweden. The goal of the workshop was to focus on some of the open questions in the field and try to come up with concrete suggestions for how to make progress on both the experimental and theoretical sides. The paper gives a brief introduction to each topic and then summarizes the primary results.

hep-ph↗

arXiv2019

Future physics opportunities for high-density QCD at the LHC with heavy-ion and proton beams

The future opportunities for high-density QCD studies with ion and proton beams at the LHC are presented. Four major scientific goals are identified: the characterisation of the macroscopic long wavelength Quark-Gluon Plasma (QGP) properties with unprecedented precision, the investigation of the microscopic parton dynamics underlying QGP properties, the development of a unified picture of particle production and QCD dynamics from small (pp) to large (nucleus--nucleus) systems, the exploration of parton densities in nuclei in a broad ($x$, $Q^2$) kinematic range and the search for the possible onset of parton saturation. In order to address these scientific goals, high-luminosity Pb-Pb and p-Pb programmes are considered as priorities for Runs 3 and 4, complemented by high-multiplicity studies in pp collisions and a short run with oxygen ions. High-luminosity runs with intermediate-mass nuclei, for example Ar or Kr, are considered as an appealing case for extending the heavy-ion programme at the LHC beyond Run 4. The potential of the High-Energy LHC to probe QCD matter with newly-available observables, at twice larger center-of-mass energies than the LHC, is investigated.

hep-ph↗

arXiv2019

Future heavy-ion facilities: FCC-AA

The operation of the Future Circular Collider (FCC) with heavy ions would provide Pb-Pb and p-Pb collisions at sqrt{s_NN}= 39 and 63 TeV, respectively, per nucleon-nucleon collision, with projected per-month integrated luminosities of up to 110/nb and 29/pb, respectively. This document outlines the unique and broad physics opportunities with heavy ions at the energy frontier opened by FCC.

hep-ph↗

arXiv2017

Physics with ions at the Future Circular Collider

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 $Υ(1S)$, 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 $γγ$ (ultraperipheral) collisions at c.m. energies up to 1 TeV.

hep-ex↗

arXiv2016

Heavy ions at the Future Circular Collider

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. This is a report by the working group on heavy-ion physics of the FCC Study. First ideas on the physics opportunities with heavy ions at the FCC are presented, covering the physics of the Quark-Gluon Plasma, of gluon saturation, of photon-induced collisions, as well as connections with other fields of high-energy physics.

hep-ph↗

arXiv2016

Effective description of dark matter as a viscous fluid

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.

astro-ph.CO↗

arXiv2016

Nuclear collisions at the Future Circular Collider

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.

hep-ph↗

arXiv2012

Morphology of High-Multiplicity Events in Heavy Ion Collisions

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. Our exploratory study indicates, however, that these redundancies may offer novel opportunities for a detailed characterization of those event-wise fluctuations that remain after subtraction of the dominant collective flow signatures. By construction, the proposed approach allows also for the characterization of more complex collective phenomena like higher-order flow and other sources of fluctuations, and it may be extended to the characterization of phenomena of non-collective origin such as jets.

hep-ph↗

arXiv2012

The contribution of medium-modified color flow to jet quenching

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. By interfacing simple branching histories of medium-modified color flow with the Lund hadronization model, we analyze how the medium modification of color correlations can affect the distribution of hadronic fragments in jets. Importantly, we observe that jet-medium interactions give rise to the medium-induced color decoherence of gluons from the parton shower. Since hadronization respects color flow and since each color singlet in a parton shower is hadronized separately, this medium-induced color decoherence leaves characteristic signatures in the jet fragmentation pattern. In particular, it can contribute to the quenching of leading hadron spectra. Moreover, it can increase strongly the yield of soft hadronic fragments from a jet, while the distribution of more energetic hadrons follows naturally the shape of a vacuum-like fragmentation pattern of lower total energy.

hep-ph↗

arXiv2011

Medium-induced color flow softens hadronization

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. This effect is seen in more than half of the most energetic color-singlet fragments of medium-modified parton branchings. Applying a standard cluster hadronization model, we find that it leads to a characteristic additional softening of hadronic spectra. A fair description of the nuclear modification factor measured at the LHC may then be obtained for relatively low momentum transfers from the medium.

hep-ph↗

arXiv2011

Suppression of high-pT particle production in AA collisions: the role of in-medium color-flow

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.

hep-ph↗

arXiv2011

Proton-Nucleus Collisions at the LHC: Scientific Opportunities and Requirements

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.

hep-ph↗

arXiv2007

The Physics of Ultraperipheral Collisions at the LHC

We discuss the physics of large impact parameter interactions at the LHC: ultraperipheral collisions (UPCs). The dominant processes in UPCs are photon-nucleon (nucleus) interactions. The current LHC detector configurations can explore small $x$ hard phenomena with nuclei and nucleons at photon-nucleon center-of-mass energies above 1 TeV, extending the $x$ range of HERA by a factor of ten. In particular, it will be possible to probe diffractive and inclusive parton densities in nuclei using several processes. The interaction of small dipoles with protons and nuclei can be investigated in elastic and quasi-elastic $J/ψ$ and $Υ$ production as well as in high $t$ $ρ^0$ production accompanied by a rapidity gap. Several of these phenomena provide clean signatures of the onset of the new high gluon density QCD regime. The LHC is in the kinematic range where nonlinear effects are several times larger than at HERA. Two-photon processes in UPCs are also studied. In addition, while UPCs play a role in limiting the maximum beam luminosity, they can also be used a luminosity monitor by measuring mutual electromagnetic dissociation of the beam nuclei. We also review similar studies at HERA and RHIC as well as describe the potential use of the LHC detectors for UPC measurements.

nucl-ex↗

arXiv2006

Heavy-quark energy loss at RHIC and LHC

The attenuation of heavy-flavored particles in nucleus-nucleus collisions tests the microscopic dynamics of medium-induced parton energy loss and, in particular, its expected dependence on the identity (color charge and mass) of the parent parton. We discuss the comparison of theoretical calculations with recent single-electron data from RHIC experiments. Then, we present predictions for the heavy-to-light ratios of D and B mesons at LHC energy.

hep-ph↗

arXiv2005

How sensitive are high-pt electron spectra at RHIC to heavy quark energy loss?

In nucleus-nucleus collisions, high-pt electron spectra depend on the medium modified fragmentation of their massive quark parents, thus giving novel access to the predicted mass hierarchy of parton energy loss. Here we calculate these spectra in a model, which supplements the perturbative QCD factorization formalism with parton energy loss. In general, we find - within large errors - rough agreement between theory and data on the single inclusive electron spectrum in pp, its nuclear modification factor, and its azimuthal anisotropy. However, the nuclear modification factor depends on the relative contribution of charm and bottom production, which we find to be affected by large perturbative uncertainties. In order for electron measurements to provide a significantly more stringent test of the expected mass hierarchy, one must then disentangle the b- and c-decay contributions, for instance by reconstructing the displaced decay vertices.

hep-ph↗

arXiv2005

Heavy-to-light ratios as a test of medium-induced energy loss at RHIC and the LHC

The ratio of nuclear modification factors of high-$p_T$ heavy-flavored mesons tolight-flavored hadrons (heavy-to-light ratio) is shown to be a sensitive tool to test medium-induced energy loss at RHIC and LHC energies. Heavy-to-light ratios of $D$ mesons at RHIC in the region $7<p_T<12$ GeV, and of $D$ and $B$ mesons at the LHC in the region $10<p_T<20$ GeV, are proposed for such a test. Finally, the different contributions to the nuclear modification factor for electrons at RHIC are analyzed. Preliminary PHENIX and STAR data are compatible with radiative energy loss provided the contribution of electrons from beauty decays is small compared to that from charm.

hep-ph↗

arXiv2005

Nuclear size and rapidity dependence of the saturation scale from QCD evolution and experimental data

The solutions of the Balitsky-Kovchegov evolution equations are studied numerically and compared with known analytical estimations. The rapidity and nuclear size dependences of the saturation scale are obtained for the cases of fixed and running coupling constant. These same dependences are studied in experimental data, on lepton-nucleus, deuteron-nucleus and nucleus-nucleus collisions, through geometric scaling and compared with the theoretical calculations.

hep-ph↗

arXiv2004

Numerical analysis of the Balitsky-Kovchegov equation with running coupling: dependence of the saturation scale on nuclear size and rapidity

We study the effects of including a running coupling constant in high-density QCD evolution. For fixed coupling constant, QCD evolution preserves the initial dependence of the saturation momentum $Q_s$ on the nuclear size $A$ and results in an exponential dependence on rapidity $Y$, $Q^2_s(Y) = Q^2_s(Y_0) \exp{[ \barα_s d (Y-Y_0) ]}$. For the running coupling case, we re-derive analytical estimates for the $A$- and $Y$-dependences of the saturation scale and test them numerically. The $A$-dependence of $Q_s$ vanishes $\propto 1/ \sqrt{Y}$ for large $A$ and $Y$. The $Y$-dependence is reduced to $Q_s^2(Y) \propto \exp{(Δ^\prime\sqrt{Y+X})}$ where we find numerically $Δ^\prime\simeq 3.2$. We study the behaviour of the gluon distribution at large transverse momentum, characterizing it by an anomalous dimension $1-γ$ which we define in a fixed region of small dipole sizes. In contrast to previous analytical work, we find a marked difference between the fixed coupling ($γ\simeq 0.65$) and running coupling ($γ\sim 0.85$) results. Our numerical findings show that both a scaling function depending only on the variable $r Q_s$ and the perturbative double-leading-logarithmic expression, provide equally good descriptions of the numerical solutions for very small $r$-values below the so-called scaling window.

hep-ph↗