Elementary Degrees Of Freedom Research Articles

Particles of a de Sitter Universe

The de Sitter spacetime is a maximally symmetric spacetime. It is one of the vacuum solutions to Einstein equations with a cosmological constant. It is the solution with a positive cosmological constant and describes a universe undergoing accelerated expansion. Among the possible signs for a cosmological constant, this solution is relevant for primordial and late-time cosmology. In the case of a zero cosmological constant, studies on the representations of its isometry group have led to a broader understanding of particle physics. The isometry group of d+1-dimensional de Sitter is the group SO(d+1,1), whose representations are well known. Given this insight, what can we learn about the elementary degrees of freedom in a four dimensional de Sitter universe by exploring how the unitary irreducible representations of SO(4,1) present themselves in cosmological setups? This article aims to summarize recent advances along this line that benefit towards a broader understanding of quantum field theory and holography at different signs of the cosmological constant. Particular focus is given to the manifestation of SO(4,1) representations at the late-time boundary of de Sitter. The discussion is concluded by pointing towards future questions at the late-time boundary and the static patch with a focus on the representations.

Skyrmion Spin Ice in Liquid Crystals.

We propose the first skyrmion spin ice, realized via confined, interacting liquid crystal skyrmions. Skyrmions in a chiral nematic liquid crystal behave as quasiparticles that can be dynamically confined, bound, and created or annihilated individually with ease and precision. We show that these quasiparticles can be employed to realize binary variables that interact to form ice-rule states. Because of their unique versatility, liquid crystal skyrmions can open entirely novel avenues in the field of frustrated systems. More broadly, our findings also demonstrate the viability of liquid crystal skyrmions as elementary degrees of freedom in the design of collective complex behaviors.

Symmetries of spatial meson correlators in high temperature QCD

Based on a complete set of $J = 0$ and $J=1$ spatial isovector correlation functions calculated with $N_F = 2$ domain wall fermions we identify an intermediate temperature regime of $T \sim 220 - 500$ MeV ($1.2T_c$--$2.8T_c$), where chiral symmetry is restored but the correlators are not yet compatible with a simple free quark behavior. More specifically, in the temperature range $T \sim 220 - 500$ MeV we identify a multiplet structure of spatial correlators that suggests emergent $SU(2)_{CS}$ and $SU(4)$ symmetries, which are not symmetries of the free Dirac action. The symmetry breaking effects in this temperature range are less than 5%. Our results indicate that at these temperatures the chromo-magnetic interaction is suppressed and the elementary degrees of freedom are chirally symmetric quarks bound into color-singlet objects by the chromo-electric component of the gluon field. At temperatures between 500 and 660 MeV the emergent $SU(2)_{CS}$ and $SU(4)$ symmetries disappear and one observes a smooth transition to the regime above $T \sim 1$ GeV where only chiral symmetries survive, which are finally compatible with quasi-free quarks.

Long-lived photoinduced response observed under extreme photoexcitation densities in a one-dimensional Peierls insulator

One-dimensional metal-halide compounds provide model systems to investigate the manner in which coupling between elementary degrees of freedom---here, electronic and vibrational---result in instabilities that give rise to both chemical and structural rearrangements. Here, we employ ``single-shot'' pump-probe spectroscopy to examine a one-dimensional platinum iodide compound (PtI(en)) under far-from-equilibrium conditions where repeated photoexcitation results in sample damage. It presents evidence for a distinct collective excited state lasting more than 100 ps upon self-trapped exciton generation at high densities, as measured by electronic signal amplitudes and phonon properties.

Super-Potts glass: A disordered model for glass-forming liquids

We introduce a new disordered system, the Super-Potts model, which is a more frustrated version of the Potts glass. Its elementary degrees of freedom are variables that can take M values and are coupled via pair-wise interactions. Its exact solution on a completely connected lattice demonstrates that for large enough M it belongs to the class of mean-field systems solved by a one step replica symmetry breaking Ansatz. Numerical simulations by the parallel tempering technique show that in three dimensions it displays a phenomenological behaviour similar to the one of glass-forming liquids. The Super-Potts glass is therefore the first long-sought disordered model allowing one to perform extensive and detailed studies of the Random First Order Transition in finite dimensions. We also discuss its behaviour for small values of M, which is similar to the one of spin-glasses in a field.

Finite size scaling in crossover among different random matrix ensembles in microscopic lattice models

Using numerical diagonalization we study the crossover among different random matrix ensembles (Poissonian, Gaussian orthogonal ensemble (GOE), Gaussian unitary ensemble (GUE) and Gaussian symplectic ensemble (GSE)) realized in two different microscopic models. The specific diagnostic tool used to study the crossovers is the level spacing distribution. The first model is a one-dimensional lattice model of interacting hard-core bosons (or equivalently spin 1/2 objects) and the other a higher dimensional model of non-interacting particles with disorder and spin–orbit coupling. We find that the perturbation causing the crossover among the different ensembles scales to zero with system size as a power law with an exponent that depends on the ensembles between which the crossover takes place. This exponent is independent of microscopic details of the perturbation. We also find that the crossover from the Poissonian ensemble to the other three is dominated by the Poissonian to GOE crossover which introduces level repulsion while the crossover from GOE to GUE or GOE to GSE associated with symmetry breaking introduces a subdominant contribution. We also conjecture that the exponent is dependent on whether the system contains interactions among the elementary degrees of freedom or not and is independent of the dimensionality of the system.

Black Hole Bose Condensation

General consensus on the nature of the degrees of freedom responsible for the black hole entropy remains elusive despite decades of effort dedicated to the problem. Different approaches to quantum gravity disagree in their description of the microstates and, more significantly, in the statistics used to count them. In some approaches (string theory, AdS/CFT) the elementary degrees of freedom are indistinguishable, whereas they must be treated as distinguishable in other approaches to quantum gravity (eg., LQG) in order to recover the Bekenstein-Hawking area-entropy law. However, different statistics will imply different behaviors of the black hole outside the thermodynamic limit. We illustrate this point by quantizing the Bañados-Teitelboim-Zanelli (BTZ) black hole, for which we argue that Bose condensation will occur leading to a "cold", stable remnant.

Charm diffusion in a pion gas implementing unitarity, chiral and heavy quark symmetries

We compute the charm drag and diffusion coefficients in a hot pion gas, such as is formed in a heavy ion collision after the system cools sufficiently to transit into the hadron phase. We fully exploit heavy quark effective theory (with both D and D ∗ mesons as elementary degrees of freedom during the collision) and chiral perturbation theory, and employ standard unitarization to reach higher temperatures. We find that a certain friction and shear diffusion coefficients are almost p 2 -independent at a fixed temperature which simplifies phenomenological analysis. At the higher end of reliability of our calculation, T ≃ 150 MeV , we report a charm relaxation length λ c ≃ 40 fm , in agreement with the model estimate of He, Fries and Rapp. The momentum of a 1 GeV charm quark decreases about 50 MeV per fermi when crossing the hadron phase.

Partially supersymmetric composite Higgs models

We study the idea of the Higgs as a pseudo-Goldstone boson within the framework of partial supersymmetry in Randall-Sundrum scenarios and their CFT duals. The Higgs and third generation of the MSSM are composites arising from a strongly coupled supersymmetric CFT with global symmetry SO(5) spontaneously broken to SO(4), whilst the light generations and gauge fields are elementary degrees of freedom whose couplings to the strong sector explicitly break the global symmetry as well as supersymmetry. The presence of supersymmetry in the strong sector may allow the compositeness scale to be raised to ∼ 10 TeV without fine tuning, consistent with the bounds from precision electro-weak measurements and flavour physics. The supersymmetric flavour problem is also solved. At low energies, this scenario reduces to the “More Minimal Supersymmetric Standard Model” where only stops, Higgsinos and gauginos are light and within reach of the LHC.

Exploring the light-quark interaction

Two basic motivations for an upgraded JLab facility are the needs: to determine the essential nature of light-quark confinement and dynamical chiral symmetry breaking (DCSB); and to understand nucleon structure and spectroscopy in terms of QCD's elementary degrees of freedom. During the next ten years a programme of experiment and theory will be conducted that can address these questions. We present a Dyson–Schwinger equation perspective on this effort with numerous illustrations, amongst them: an interpretation of string-breaking; a symmetry-preserving truncation for mesons; the nucleon's strangeness σ-term; and the neutron's charge distribution.

Working group report: Heavy ion physics

The 8th workshop on high energy physics phenomenology (WHEPP-8) was held at the Indian Institute of Technology, Mumbai, India during January 5–16, 2004. One of the four working groups, group III was dedicated to QCD and heavy ion physics (HIC). The present manuscript gives a summary of the activities of group III during the workshop (see also [1] for completeness). The activities of group III were focused to understand the collective behaviours of the system formed after the collisions of two nuclei at ultra-relativistic energies from the interactions of the elementary degrees of freedom, i.e. quarks and gluons, governed by non-abelian gauge theory, i.e. QCD. This was initiated by two plenary talks on experimental overview of heavy ion collisions and lattice QCD and several working group talks and discussions.

Expansion and hadronization of a chirally symmetric quark-meson plasma

Using a chirally symmetric Lagrangian, which contains quarks as elementary degrees of freedom and mesons as bound states, we investigate the expansion and hadronization of a fireball, which initially contains only quarks and produces mesons by collisions. For this model, we study the time scales of expansion and thermal and chemical equilibration. We find that the expansion progresses relatively fast, leaving not necessarily enough time to establish thermal and chemical equilibrium. Mesons are produced in the bulk of the fireball rather than at a surface, at a temperature below the Mott temperature. Initial density fluctuations become amplified during the expansion. These observations challenge the applicability of hydrodynamical approaches to the expansion of a quark-gluon plasma.

Expansion and hadronization of quark-meson plasma

Using a chirally symmetric Lagrangian, which contains quarks as elementary degrees of freedom and mesons as bound states, we investigate the expansion and hadronization of a fireball, which initially contains only quarks and produces mesons by collisions. For this model, we study the time scales of expansion and thermal and chemical equilibration. We find that the expansion progresses relatively fast, leaving not necessarily enough time to establish thermal and chemical equilibrium. Mesons are produced in the bulk of the fireball rather than at a surface, at a temperature below the Mott temperature. Initial density fluctuations become amplified during the expansion. These observations challenge the applicability of hydrodynamical approaches to the expansion of a quark-gluon plasma.

Relativistic transport theory for systems containing bound states

Using a Lagrangian which contains quarks as elementary degrees of freedom and mesons as bound states, a transport formalism is developed, which allows for a dynamical transition from a quark plasma to a state, where quarks are bound into hadrons. Simultaneous transport equations for both particle species are derived in a systematic and consistent fashion. For the mesons a formalism is used which introduces off-shell corrections to the off-diagonal Green functions. It is shown that these off-shell corrections lead to the appearance of elastic quark scattering processes in the collision integral. The interference of the processes $q\bar q\to\pi$ and $q\bar q\to\pi\to q\bar q$ leads to a modification of the $s$-channel amplitude of quark-antiquark scattering.

SPONTANEOUS SCALING EMERGENCE IN GENERIC STOCHASTIC SYSTEMS

We extend a generic class of systems which have previously been shown to spontaneously develop scaling (power law) distributions of their elementary degrees of freedom. While the previous systems were linear and exploded exponentially for certain parameter ranges, the new systems fulfill nonlinear time evolution equations similar to the ones encountered in Spontaneous Symmetry Breaking (SSB) dynamics and evolve spontaneously towards "fixed trajectories" indexed by the average value of their degrees of freedom (which corresponds to the SSB order parameter). The "fixed trajectories" dynamics evolves on the edge between explosion and collapse/extinction. The systems present power laws with exponents which in a wide range (α < –2.) are universally determined by the ratio between the minimal and the average values of the degrees of freedom. The time fluctuations are governed by Levy distributions of corresponding power. For exponents α > −2 there is no "thermodynamic limit" and the fluctuations are dominated by a few, largest degrees of freedom which leads to macroscopic fluctuations, chaos, and bursts/intermittency.