Gapless fracton quantum spin liquid and emergent photons found in 2D spin-1 model

Researchers have discovered a new type of quantum spin liquid, dubbed a gapless fracton quantum spin liquid, in a theoretical two-dimensional spin-1 model. This finding is significant because quantum spin liquids are exotic states of matter that do not order their spins in a conventional way, but instead exhibit long-range quantum entanglement. The particularity of this new state is its "gapless" nature, meaning there is no minimum energy to excite the system, and the emergence of "photons" as low-energy excitations, which distinguishes it from other known spin liquids. The concept of fractons, which are excitations with restricted mobility, has been an area of intense research in condensed matter physics. Until now, studied fracton spin liquids typically had an energy gap (gapped), meaning they required a minimum energy to generate excitations. The identification of a gapless fracton state in two dimensions, and the association of its excitations with particles that behave like photons, opens new avenues for understanding the interplay between topology and quantum dynamics in many-body systems. This theoretical model could serve as a basis for the design of new quantum materials with exotic properties. This advance is based on a spin-1 model, more complex than the commonly studied spin-1/2 models, allowing for a greater richness of quantum phenomena. The emergence of photons in this context does not refer to actual light particles, but to collective excitations of the system that exhibit photon-like properties, such as being massless and propagating at a constant speed. Understanding these exotic states is crucial for the development of future quantum technologies, including fault-tolerant quantum computing and high-precision quantum sensing, as the stability of quantum information often depends on the topological nature of the ground state.

Variational Gibbs State Preparation on Trapped-Ion Devices

Researchers have demonstrated a method for preparing variational Gibbs states on a trapped-ion device. This advancement is crucial for the quantum simulation of thermodynamic systems, as Gibbs states are fundamental for describing thermal equilibrium. The ability to efficiently generate these states on quantum hardware opens new avenues for exploring condensed matter phenomena and quantum chemistry at finite temperatures. The method employed uses a variational quantum algorithm, which combines classical optimization with execution on a quantum processor. In this case, it was applied to a system of Yb+ ions in a radiofrequency trap. Preparing Gibbs states is inherently complex due to the non-unitary nature of thermal evolution, making it difficult to implement directly in unitary quantum circuits. The variational approach circumvents this difficulty by searching for a state that minimizes a cost function related to the Helmholtz free energy. This work represents a significant step towards quantum simulation of open systems and quantum thermodynamics. The ability to prepare Gibbs states in a controlled manner on noisy intermediate-scale quantum (NISQ) platforms is a prerequisite for studying material properties at non-zero temperatures, such as phase transitions or transport properties. The obtained results validate the feasibility of these approaches on real hardware and suggest future applications in the design of new materials and the study of complex chemical reactions.

Leptogenesis Viable with Very Low Reheating Temperatures

A new study explores the possibility of generating the observed baryon asymmetry in the universe through leptogenesis, even with very low cosmic reheating temperatures, close to the Big Bang Nucleosynthesis (BBN) bound of approximately 4 MeV. Traditionally, leptogenesis requires significantly higher reheating temperatures. This work focuses on the canonical type-I seesaw framework, where the dominant production of right-handed neutrinos (RHN) is non-thermal, originating from inflaton decays (φ → NN). The research reveals that, while matter-like reheating (with an equation of state parameter w_φ=0) is incompatible with standard leptogenesis at very low temperatures, the situation changes drastically for generalized Starobinsky potentials, approximated by V(φ)∝φ^k with k≥4. In these scenarios, the observed baryon asymmetry can be readily obtained. A particular case studied in detail is radiation-like reheating (w_φ=1/3, k=4), where the evolving effective mass of the inflaton condensate leads to a kinematic shutoff of the φ → NN channel, qualitatively altering the leptogenesis dynamics. The authors include a detailed treatment of the effects of inflaton condensate fragmentation. Interestingly, the final baryon asymmetry primarily depends on only two parameters: the inflaton-RHN coupling (y_φNN) and the CP-violating parameter (|ε|). A key finding is that the final asymmetry is largely insensitive to the RHN mass, the reheating temperature, and the RHN decay rate. Although the study focuses on fermionic reheating, it is shown that the general features of these results also hold for bosonic reheating to scalars.

Analysis of Pion Nuclear Fragmentation Functions to Understand Hadronization

A new quantum chromodynamics (QCD) analysis has allowed the extraction of pion nuclear fragmentation functions (nFFs), which describe how hadronization is modified in high-energy nuclear collisions. This study, which simultaneously considers vacuum fragmentation functions and their nuclear modifications, is crucial for understanding the fundamental processes by which quarks and gluons transform into composite particles (hadrons) within a nuclear environment. Nuclear effects have been parameterized as functions of the nuclear mass number ($A$), the energy of the fragmenting parton in the target rest frame ($\nu$), and the hadron energy fraction ($z$). This parameterization has allowed the quantification of the dependence of these effects on these variables. The analysis incorporated semi-inclusive deep-inelastic scattering data on nuclear targets, applying specific kinematic cuts to ensure the applicability of perturbative QCD and collinear factorization. The resulting fit provides a good description of most datasets, with nFFs well constrained in the energy fraction range $z \in [0.2, 0.7]$. With these new nuclear fragmentation functions, next-to-leading order (NLO) predictions have been made for proton-proton ($pp$) and proton-nucleus ($pA$) collisions. These predictions show reasonable agreement with experimental data from the ALICE experiment, within current experimental uncertainties. This advance is significant for high-energy physics, as it provides a more precise tool for interpreting results from accelerator experiments like the LHC, where the properties of matter under extreme conditions are studied.

Heavy Quark Coalescence: Potential Influences Probability

A recent study has investigated the role of the interaction potential between heavy quarks in the coalescence process, a crucial phenomenon in hadron formation. Heavy quark coalescence, which is expected to have a probability close to unity at low momenta, is influenced by the nature of this potential. The research developed a phenomenological model for the heavy quark potential that successfully reproduces the vacuum masses of pseudoscalar and vector heavy mesons, providing a solid basis for the analysis. Using this potential, the researchers demonstrated that its inclusion enhances the coalescence probability. Furthermore, the study examined how medium-induced modifications of the heavy quark potential in the quark-gluon plasma (QGP) affect the coalescence process. The QGP is a state of matter that briefly existed in the early universe and is recreated in heavy-ion collision experiments, such as those at the LHC. The results indicate that the coalescence probability remains close to unity as long as the modification of the potential in the QGP is sufficiently moderate. This suggests that, even under extreme conditions like those in the QGP, hadron formation from heavy quarks remains an efficient process, provided that the interactions are not drastically altered. This finding is relevant for a deeper understanding of quantum chromodynamics and the formation of hadronic matter in high-energy environments.

Optimal Transport for Monte Carlo Event Weights at the LHC

Researchers have developed a new technique based on optimal transport to manage event weights generated by Monte Carlo simulations at the Large Hadron Collider (LHC). These weights, which can be negative or pathologically large, pose a significant computational challenge for experiments. The new approach uses cell resampling algorithms to locally redistribute event weights among nearby events in a metric space, improving the efficiency and accuracy of simulations. The study focuses on the performance of metrics defined in terms of optimal transport, specifically the Energy Mover's Distance and a spectral variant. These metrics are particularly useful because they are insensitive to the addition of soft and collinear radiation, allowing them to be applied directly to particles at any stage of event generation. This contrasts with previous methods that might require specific adjustments for different simulation phases. When this methodology was applied to samples simulated at next-to-leading-order in quantum chromodynamics, a significant reduction in bias was observed compared to other cell resampling techniques found in the literature. Furthermore, the researchers introduced the Cross-Section Mover's Distance as a general, unbinned figure of merit for quantifying the bias introduced by any full-phase-space reweighting. This advance is crucial for refining theoretical predictions and interpreting experimental data at the LHC, where precision is fundamental for discoveries in particle physics.