arXiv subjects

D. A. Clarke

Publications and source records attributed to D. A. Clarke.

13 recordsLinked to original sources

arXiv2026

Testing strong isospin breaking effects in QCD thermodynamics

Most modern lattice QCD calculations take the masses of the light quarks to be degenerate, setting $m_u=m_d$, a choice that, while not physical, increases computational efficiency and lowers the complexity of simulations and analysis. For many observables, the systematic error associated with this simplification is expected to be negligible, as $Δm_{ud}=m_d-m_u$ is significantly below the QCD scale. So far lattice studies have examined the role of strong isospin breaking (SIB) in hadron mass splittings, but SIB effects in thermodynamic observables have only been studied in terms of the crossover temperature with unphysically heavy quarks. Motivated by the recent observation of an unexpectedly large ratio between the abundances of charged versus neutral kaons in heavy-ion collision experiments, we investigate the effects of SIB in QCD thermodynamics by computing observables for three non-degenerate flavors ($N_f=1+1+1$) with physical quark masses using dynamical improved staggered fermions. In particular, we investigate the difference between the $u$- and $d$-quark condensates and extract the pseudocritical temperature, the equation of state, and conserved-charge fluctuations, comparing results to standard $N_f=2+1$ simulations.

hep-lat↗

arXiv2026

QCD Crossover at Low Temperatures from Lee-Yang Edge Singularity

We provide the first lattice-QCD estimate of the crossover line down to $T\simeq108$~MeV. We introduce a new method that combines the Lee-Yang edge in the complex plane of baryon chemical potential $μ_B$ with universal chiral scaling to determine the $μ_B$ dependence of the QCD chiral critical and pseudo-critical temperatures. By performing $(2\!+\!1)$-flavor lattice QCD simulations at $T\simeq108$~MeV and purely imaginary $μ_B$ with a single lattice spacing and two volumes, we compute $μ_B$-dependent baryon-number susceptibilities and extract the location of the Lee-Yang edge. Together with universal scaling near the QCD chiral transition, it constrains the mapping function between $\{T,μ_B\}$ and the scaling variable (\textit{i.e.}\ the argument of the universal scaling functions). This mapping function then yields the $μ_B$ dependence of the critical and pseudo-critical temperatures for $T\gtrsim108$~MeV. While our calculation is performed only at a single value of low temperature without explicit input from small-$μ_B$ expansion, the resulting $μ_B$ dependence of the pseudo-critical temperature is consistent with established lattice-QCD determinations at small $μ_B$ and compatible with chemical freeze-out parameters of heavy-ion collisions down to low temperatures, demonstrating the validity and robustness of the method. Application of this method can be systematically extended to additional temperatures and finer discretizations, opening a pathway to charting the QCD phase diagram in the low-$T$, high-$μ_B$ regime.

hep-lat↗

arXiv2025

The anomalous magnetic moment of the muon in the Standard Model: an update

We present the current Standard Model (SM) prediction for the muon anomalous magnetic moment, $a_μ$, updating the first White Paper (WP20) [1]. The pure QED and electroweak contributions have been further consolidated, while hadronic contributions continue to be responsible for the bulk of the uncertainty of the SM prediction. Significant progress has been achieved in the hadronic light-by-light scattering contribution using both the data-driven dispersive approach as well as lattice-QCD calculations, leading to a reduction of the uncertainty by almost a factor of two. The most important development since WP20 is the change in the estimate of the leading-order hadronic-vacuum-polarization (LO HVP) contribution. A new measurement of the $e^+e^-\toπ^+π^-$ cross section by CMD-3 has increased the tensions among data-driven dispersive evaluations of the LO HVP contribution to a level that makes it impossible to combine the results in a meaningful way. At the same time, the attainable precision of lattice-QCD calculations has increased substantially and allows for a consolidated lattice-QCD average of the LO HVP contribution with a precision of about 0.9%. Adopting the latter in this update has resulted in a major upward shift of the total SM prediction, which now reads $a_μ^\text{SM} = 116\,592\,033(62)\times 10^{-11}$ (530 ppb). When compared against the current experimental average based on the E821 experiment and runs 1-6 of E989 at Fermilab, one finds $a_μ^\text{exp} - a_μ^\text{SM} =38(63)\times 10^{-11}$, which implies that there is no tension between the SM and experiment at the current level of precision. The final precision of E989 (127 ppb) is the target of future efforts by the Theory Initiative. The resolution of the tensions among data-driven dispersive evaluations of the LO HVP contribution will be a key element in this endeavor.

hep-ph↗

arXiv2025

A generalized definition of the isothermal compressibility in (2+1)-flavor QCD

We introduce a generalized definition of the isothermal compressibility ($κ_{T,σ_Q^2}$) calculable by keeping net conserved charge fluctuations rather than total number densities constant. We present lattice QCD results for this isothermal compressibility, expressed in terms of fluctuations of conserved charges that are related to baryon ($B$), electric charge ($Q$) and strangeness ($S$) quantum numbers. This generalized isothermal compressibility is compared with hadron resonance gas model calculations as well as with heavy-ion collision data obtained at RHIC and the LHC. We find $κ_{T,σ_Q^2}=13.8(1.3)$~fm$^3$/GeV at $T_{pc,0}=156.5(1.5)$~MeV and $\hatμ_B=0$. This finding is consistent with the rescaled result of the ALICE Collaboration, where we replaced the number of charged hadrons ($N_{\rm ch}$) by the total number of hadrons ($N_{\rm tot}$) at freeze-out. Normalizing this result with the QCD pressure ($P$) we find that the isothermal compressibility on the pseudo-critical line stays close to that of an {\it ideal gas}, {\it i.e.} $P κ_{T,σ_Q^2}\simeq 1$.

hep-lat↗

arXiv2024

Searching for the QCD critical point using Lee-Yang edge singularities

Using $N_f=2+1$ QCD calculations at physical quark mass and purely imaginary baryon chemical potential, we locate Lee-Yang edge singularities in the complex chemical potential plane. These singularities have been obtained by the multi-point Padé approach applied to the net baryon number density. We recently showed that singularities extracted with this approach are consistent with universal scaling near the Roberge-Weiss transition. Here we study the universal scaling of these singularities in the vicinity of the QCD critical endpoint. Making use of an appropriate scaling ansatz, we extrapolate these singularities on $N_τ=6$ and $N_τ=8$ lattices towards the real axis to estimate the position of a possible QCD critical point. We find an approach toward the real axis with decreasing temperature. We compare this estimate with a HotQCD estimate obtained from poles of a [4,4]-Padé resummation of the eighth-order Taylor expansion of the QCD pressure.

hep-lat↗

arXiv2024

Detecting Lee-Yang/Fisher singularities by multi-point Padè

The Bielefeld Parma Collaboration has in recent years put forward a method to probe finite density QCD by the detection of Lee-Yang singularities. The location of the latter is obtained by multi-point Padè approximants, which are in turn calculated matching Taylor series results obtained from Monte Carlo computations at (a variety of values of) imaginary baryonic chemical potential. The method has been successfully applied to probe the Roberge Weiss phase transition and preliminary, interesting results are showing up in the vicinity of a possible QCD critical endpoint candidate. In this talk we will be concerned with a couple of significant aspects in view of a more powerful application of the method. First, we will discuss the possibility of detecting finite size scaling of Lee-Yang/Fisher singularities in finite density (lattice) QCD. Second, we will briefly mention our attempts at detecting both singularities in the complex chemical potential plane and singularities in the complex temperature plane. The former are obtained from rational approximations which are functions of the chemical potential at given values of the temperature; the latter are obtained from rational approximations which are functions of the temperature at given values of the chemical potential.

hep-lat↗

arXiv2024

Exploring the Critical Points in QCD with Multi-Point Padé and Machine Learning Techniques in (2+1)-flavor QCD

Using simulations at multiple imaginary chemical potentials for $(2+1)$-flavor QCD, we construct multi-point Padé approximants. We determine the singularties of the Padé approximants and demonstrate that they are consistent with the expected universal scaling behaviour of the Lee-Yang edge singularities. We also use a machine learning model, Masked Autoregressive Density Estimator (MADE), to estimate the density of the Lee-Yang edge singularities at each temperature. This ML model allows us to interpolate between the temperatures. Finally, we extrapolate to the QCD critical point using an appropriate scaling ansatz.

hep-lat↗

arXiv2023

QCD material parameters at zero and non-zero chemical potential from the lattice

Using an eighth-order Taylor expansion in baryon chemical potential, we recently obtained the (2+1)-flavor QCD equation of state (EoS) at non-zero conserved charge chemical potentials from the lattice. We focused on strangeness-neutral, isospin-symmetric QCD matter, which closely resembles the situation encountered in heavy-ion collision experiments. Using this EoS, we present here results on various QCD material parameters; in particular we compute the specific heat, speed of sound, and compressibility along appropriate lines of constant physics. We show that in the entire range relevant for the beam energy scan at RHIC, the specific heat, speed of sound, and compressibility show no indication for an approach to critical behavior that one would expect close to a possibly existing critical endpoint.

hep-lat↗

arXiv2023

Equation of state and speed of sound of (2+1)-flavor QCD in strangeness-neutral matter at non-vanishing net baryon-number density

We update results on the QCD equation of state in (2+1)-flavor QCD with non-zero conserved charge chemical potentials obtained from an eighth-order Taylor series. We present results for basic bulk thermodynamic observables of strangeness-neutral strong-interaction matter, i.e. pressure, number densities, energy and entropy density, and resum Taylor series results using Padé approximants. Furthermore, we calculate the speed of sound as well as the adiabatic compression factor of strangeness-neutral matter on lines of constant entropy per net baryon number. We show that the equation of state ($P(n_B), ε(n_B)$) is already well described by the $4^{\rm th}$-order Taylor series in almost the entire range of temperatures accessible with the beam energy scan in collider mode at the Relativistic Heavy Ion Collider.

hep-lat↗

arXiv2022

Isothermal and isentropic speed of sound in (2+1)-flavor QCD at non-zero baryon chemical potential

Recently interest in calculations of the speed of sound in QCD under conditions like constant temperature $c^2_T$ or constant entropy per net baryon number $c^2_s$ arose in the discussion of experimental results coming from heavy ion experiments. It has been stressed that the former in particular is closely related to higher order cumulants of conserved charge fluctuations that are calculated in lattice QCD. We present here results on $c^2_T$ and $c^2_s$ and compare results at vanishing strangeness chemical potential and vanishing net strangeness number with hadron resonance gas model calculations. We stress the difference of both observables at low temperature arising from the light meson sector, which does not contribute to $c^2_T$.

hep-lat↗

arXiv2021

Lattice QCD at Imaginary Chemical Potential in the Chiral Limit

We report on an ongoing study on the interplay between Roberge-Weiss (RW) and chiral transitions in simulations of (2+1)-flavor QCD with an imaginary chemical potential. We established that the RW endpoint belongs to the 3-$d$, $Z_2$ universality class when calculations are done with the Highly Improved Staggered Quark (HISQ) action in the RW plane with physical quark masses. We also have explored a range of quark masses corresponding to pion mass values, $m_π\geq40$~MeV and found that the transition is consistent with $Z_2$ universality class. We argue that observables that were usually used to determine the chiral phase transition temperature, e.g. the chiral condensate and chiral susceptibility, are sensitive to the RW transition and are energy-like observables for the $Z_2$ transition, contrary to the magnetic-like (order parameter) behavior at vanishing chemical potential. Moreover the calculations performed at $m_π\sim40$~MeV also put a stringent constraint for a critical pion mass at zero chemical potential for a possible first-order chiral phase transition.

hep-lat↗

arXiv2002

Three-Dimensional Simulations of Jets from Keplerian Disks: Self--Regulatory Stability

We present the extension of previous two-dimensional simulations of the time-dependent evolution of non-relativistic outflows from the surface of Keplerian accretion disks, to three dimensions. The accretion disk itself is taken to provide a set of fixed boundary conditions for the problem. The 3-D results are consistent with the theory of steady, axisymmetric, centrifugally driven disk winds up to the Alfvén surface of the outflow. Beyond the Alfvén surface however, the jet in 3-D becomes unstable to non-axisymmetric, Kelvin-Helmholtz instabilities. We show that jets maintain their long-term stability through a self-limiting process wherein the average Alfvénic Mach number within the jet is maintained to order unity. This is accomplished in at least two ways. First, poloidal magnetic field is concentrated along the central axis of the jet forming a ``backbone'' in which the Alfvén speed is sufficiently high to reduce the average jet Alfvénic Mach number to unity. Second, the onset of higher order Kelvin-Helmholtz ``flute'' modes (m \ge 2) reduce the efficiency with which the jet material is accelerated, and transfer kinetic energy of the outflow into the stretched, poloidal field lines of the distorted jet. This too has the effect of increasing the Alfvén speed, and thus reducing the Alfvénic Mach number. The jet is able to survive the onset of the more destructive m=1 mode in this way. Our simulations also show that jets can acquire corkscrew, or wobbling types of geometries in this relatively stable end-state, depending on the nature of the perturbations upon them. Finally, we suggest that jets go into alternating periods of low and high activity as the disappearance of unstable modes in the sub-Alfvénic regime enables another cycle of acceleration to super-Alfvénic speeds.

astro-ph↗

arXiv1997

Dynamics and Structure of Three-Dimensional Poloidally Magnetized Supermagnetosonic Jets

A set of 3D MHD simulations of magnetized jets has been performed. The jets contain an equipartition primarily poloidal magnetic field and the effect of jet density on jet dynamics and structure is evaluated. The jet is precessed at the origin to excite Kelvin-Helmholtz unstable helical modes. We extensively compare the structure in these simulations with linear stability theory. The jet that is dense with respect to the external medium develops a high speed core surrounded by a less dense sheath consisting of slower moving jet fluid. These simulations suggest that extended extragalactic jets propagate to such large distances because they are surrounded by a lobe or cocoon whose density is less than the jet density. (Abridged abstract.)

astro-ph↗