A model for the spacetime evolution of heavy-ion collisions at RHIC

We investigate the space-time evolution of ultrarelativistic Au-Au collisions at full RHIC energy using a schematic model of the expansion. Assuming a thermally equilibrated system, we can adjust the essential scale parameters of this model such that the measured transverse momentum spectra and Hanbury-Brown Twiss (HBT) correlation parameters are well described. We find that the experimental data strongly constrain the dynamics of the evolution of the emission source although hadronic observables for the most part reflect the final breakup of the system.Comment: 9 pages, 3 figures, submitted to Phys. Rev.

Yang-Mills Equations of Motion for the Higgs Sector of SU(3)-Equivariant Quiver Gauge Theories

We consider SU(3)-equivariant dimensional reduction of Yang-Mills theory on spaces of the form R x SU(3)/H, with H equals either SU(2) x U(1) or U(1) x U(1). For the corresponding quiver gauge theory we derive the equations of motion and construct some specific solutions for the Higgs fields using different gauge groups. Specifically we choose the gauge groups U(6) and U(8) for the space R x CP^2 as well as the gauge group U(3) for the space R x SU(3)/U(1)xU(1), and derive Yang-Mills equations for the latter one using a spin connection endowed with a non-vanishing torsion. We find that a specific value for the torsion is necessary in order to obtain non-trivial solutions of Yang-Mills equations. Finally, we take the space R x CP^1 x CP^2 and derive the equations of motion for the Higgs sector for a U(3m+3) gauge theory.Comment: 21 pages, 4 figures; v2: figures added, references updated, published version (JMP

Hard dihadron correlations in heavy-ion collisions at RHIC and LHC

High transverse momentum (P_T) processes are considered to be an important tool to probe and understand the medium produced in ultrarelativistic heavy-ion collisions via the interaction of hard, perturbatively produced partons with the medium. In this context, triggered hard dihadron correlations constitute a class of observables set between hard single inclusive hadrons (dominated by the leading jet fragments) and fully reconstructed jets - while they probe some features of the perturbative QCD evolution of a parton shower in the medium, they do not suffer from the problem of finding a suitable separation between soft perturbative (jet-like) and soft non-perturbative (medium-like) physics as the identification of full jets does. On the other hand, the trigger requirement introduces non-trivial complications to the process, which makes the medium-modification of the correlation pattern difficult and non-intuitive to understand. In this work, we review the basic physics underlying triggered dihadron correlations and make a systematic comparison of several combinations of medium evolution and parton-medium interaction models with the available data from 200 AGeV Au-Au collisions at RHIC. We also discuss the expected results for 2.76 ATeV Pb-Pb collisions at the LHC.Comment: 13 pages, 7 figures, submitted to PR

Why a long-lived fireball can be compatible with HBT measurements

The common interpretation of HBT data measured at top SPS energies leads to apparent source lifetimes of 6-8 fm/c and emission duration of approximately 2-3 fm/c. We investigate a scenario with continuous pion emission from a long-lived (~17 fm/c) thermalized source in order to show that it is not excluded by the data. Starting from a description of the source's spacetime expansion based on gross thermodynamical properties of hot matter (which is able to describe a number of experimental observables), we introduce the pion emission function with a contribution from continuous emission during the source's lifetime and another contribution from the final breakup and proceed by calculating the HBT parameters R_out and R_side. The results are compared with experimental data measured at SPS for 158 AGeV central Pb-Pb collisions. We achieve agreement with data, provided that some minor modifications of the fireball evolution scenario are made and find that the parameter R_out is not sensitive to the fireball lifetime, but only to the duration of the final breakup, in spite of the fact that emission takes place throughout the whole lifetime. We explicitly demonstrate that those findings do not alter previous results obtained within this model.Comment: 13 pages, 5 figures, submitted to Phys. Rev. C. (revised description of fireball expansion

Efficient representation of fully many-body localized systems using tensor networks

We propose a tensor network encoding the set of all eigenstates of a fully many-body localized system in one dimension. Our construction, conceptually based on the ansatz introduced in Phys. Rev. B 94, 041116(R) (2016), is built from two layers of unitary matrices which act on blocks of $\ell$ contiguous sites. We argue this yields an exponential reduction in computational time and memory requirement as compared to all previous approaches for finding a representation of the complete eigenspectrum of large many-body localized systems with a given accuracy. Concretely, we optimize the unitaries by minimizing the magnitude of the commutator of the approximate integrals of motion and the Hamiltonian, which can be done in a local fashion. This further reduces the computational complexity of the tensor networks arising in the minimization process compared to previous work. We test the accuracy of our method by comparing the approximate energy spectrum to exact diagonalization results for the random field Heisenberg model on 16 sites. We find that the technique is highly accurate deep in the localized regime and maintains a surprising degree of accuracy in predicting certain local quantities even in the vicinity of the predicted dynamical phase transition. To demonstrate the power of our technique, we study a system of 72 sites and we are able to see clear signatures of the phase transition. Our work opens a new avenue to study properties of the many-body localization transition in large systems.Comment: Version 2, 16 pages, 16 figures. Larger systems and greater efficienc

Neutrino-Mass Hierarchies and Non-linear Representation of Lepton-Flavour Symmetry

Lepton-flavour symmetry in the Standard Model is broken by small masses for charged leptons and neutrinos. Introducing neutrino masses via dimension-5 operators associated to lepton-number violation at a very high scale, the corresponding coupling matrix may still have entries of order 1, resembling the situation in the quark sector with large top Yukawa coupling. As we have shown recently, in such a situation one may introduce the coupling matrices between lepton and Higgs fields as non-linear representations of lepton-flavour symmetry within an effective-theory framework. This allows us to separate the effects related to the large mass difference observed in atmospheric neutrino oscillations from those related to the solar mass difference. We discuss the cases of normal or inverted hierarchical and almost degenerate neutrino spectrum, give some examples to illustrate minimal lepton-flavour violation in radiative and leptonic decays, and also provide a systematic definition of next-to-minimal lepton-flavour violation within the non-linear framework.Comment: 17 pages, 1 figur