Rm CH Research Articles

Laser-induced persistent orientation of chiral molecules

We show, both classically and quantum mechanically, enantioselective orientation of gas phase chiral molecules excited by laser fields with twisted polarization. Counterintuitively, the induced orientation, whose direction is laser controllable, does not disappear after the excitation, but stays approximately constant long after the end of the laser pulses, behavior unique to chiral systems. We computationally demonstrate this long-lasting orientation using propylene oxide molecules (${\rm CH_{3}CHCH_{2}O}$, or PPO) as an example, and consider two kinds of fields with twisted polarization: a pair of delayed cross-polarized pulses, and an optical centrifuge. This novel chiral effect opens new avenues for detecting molecular chirality, measuring enantiomeric excess and separating enantiomers with the help of inhomogeneous external fields.

Beam Energy and Centrality Dependence of Direct-Photon Emission from Ultrarelativistic Heavy-Ion Collisions.

The PHENIX collaboration presents first measurements of low-momentum (0.4<p_{T}<3 GeV/c) direct-photon yields from Au+Au collisions at sqrt[s_{NN}]=39 and 62.4GeV. For both beam energies the direct-photon yields are substantially enhanced with respect to expectations from prompt processes, similar to the yields observed in Au+Au collisions at sqrt[s_{NN}]=200. Analyzing the photon yield as a function of the experimental observable dN_{ch}/dη reveals that the low-momentum (>1 GeV/c) direct-photon yield dN_{γ}^{dir}/dη is a smooth function of dN_{ch}/dη and can be well described as proportional to (dN_{ch}/dη)^{α} with α≈1.25. This scaling behavior holds for a wide range of beam energies at the Relativistic Heavy Ion Collider and the Large Hadron Collider, for centrality selected samples, as well as for different A+A collision systems. At a given beam energy, the scaling also holds for high p_{T} (>5 GeV/c), but when results from different collision energies are compared, an additional sqrt[s_{NN}]-dependent multiplicative factor is needed to describe the integrated-direct-photon yield.

Charged jet cross section and fragmentation in proton-proton collisions at s=7 TeV

We report the differential charged jet cross section and jet fragmentation distributions measured with the ALICE detector in proton-proton collisions at a centre-of-mass energy $\sqrt{s}=$ 7 TeV. Jets with pseudo-rapidity $\left| \eta \right| < {\rm 0.5}$ are reconstructed from charged particles using the anti-$k_{\rm T}$ jet finding algorithm with a resolution parameter $R$ = 0.4. The jet cross section is measured in the transverse momentum interval 5 $\leq p_{\rm T}^{\rm ch \; jet} <$ 100 GeV/$c$. Jet fragmentation is studied measuring the scaled transverse momentum spectra of the charged constituents of jets in four intervals of jet transverse momentum between 5 GeV/$c$ and 30 GeV/$c$. The measurements are compared to calculations from the PYTHIA model as well as next-to-leading order perturbative QCD calculations with POWHEG + PYTHIA8. The charged jet cross section is described by POWHEG for the entire measured range of $p_{\rm T}^{\rm ch \; jet}$. For $p_{\rm T}^{\rm ch \; jet}$ $>$ 40 GeV/$c$, the PYTHIA calculations also agree with the measured charged jet cross section. PYTHIA6 simulations describe the fragmentation distributions to 15%. Larger discrepancies are observed for PYTHIA8.

Hydrogen transfer reactions of interstellar complex organic molecules

Radical recombination has been proposed to lead to the formation of complex organic molecules (COMs) in ices rich in carbon monoxide (CO) in the early stages of star formation. These COMs can then undergo hydrogen addition and abstraction reactions leading to a higher or lower degree of saturation. Here, we have studied 14 hydrogen transfer reactions for the molecules glyoxal, glycoaldehyde, ethylene glycol, and methylformate, and an additional three reactions where |${\rm CH}_n{\mathrm{ O}}$| fragments are involved. Over-the-barrier reactions are possible only if tunneling is invoked in the description at low temperature. Therefore, the rate constants for the studied reactions are calculated using instanton theory that takes quantum effects into account inherently. The reactions were characterized in the gas phase, but this is expected to yield meaningful results for CO-rich ices due to the minimal alteration of reaction landscapes by the CO molecules. We found that rate constants should not be extrapolated based on the height of the barrier alone, since the shape of the barrier plays an increasingly larger role at decreasing temperature. It is neither possible to predict rate constants based only on considering the type of reaction, the specific reactants and functional groups play a crucial role. Within a single molecule, though, hydrogen abstraction from an aldehyde group seems to be always faster than hydrogen addition to the same carbon atom. Reactions that involve heavy-atom tunneling, e.g. breaking or forming a C–C or C–O bond, have rate constants that are much lower than those where H transfer is involved.

Predictions for multiplicities and flow harmonics in 5.44 TeV Xe+Xe collisions at the CERN Large Hadron Collider

We present the next-to-leading-order event-by-event EKRT model predictions for the centrality dependence of the charged hadron multiplicity in the pseudorapidity interval $|\eta|\le 0.5$, and for the centrality dependence of the charged hadron flow harmonics $v_n\{2\}$ obtained from 2-particle cumulants, in $\sqrt{s_{NN}}=5.44$ TeV Xe+Xe collisions at the CERN Large Hadron Collider. Our prediction for the 0-5 \% central charged multiplicity is $dN_{\rm ch}/d\eta =1218\pm 46$. We also predict $v_n\{2\}$ in Xe+Xe collisions to increase more slowly from central towards peripheral collisions than those in a Pb+Pb system. We find that at $10 \dots 50$\% centralities $v_2\{2\}$ is smaller and $v_3\{2\}$ is larger than in the Pb+Pb system while $v_4\{2\}$ is of the same magnitude in both systems. We also find that the ratio of flow harmonics in Xe+Xe collisions and in Pb+Pb collisions shows a slight sensitivity to the temperature dependence of the shear-viscosity-to-entropy ratio. As we discuss here, the new nuclear mass-number systematics especially in the flow harmonics serves as a welcome further constraint for describing the space-time evolution of a heavy-ion system and for determining the shear viscosity and other transport properties of strongly interacting matter.

Measurements of azimuthal anisotropy and charged-particle multiplicity in d + Au collisions at sNN=200 , 62.4, 39, and 19.6 GeV

We present measurements of the elliptic flow, $v_2$, as a function of transverse momentum ($p_T$), pseudorapidity ($\eta$), and centrality in $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=$200, 62.4, 39, and 19.6 GeV. The beam-energy scan of $d$$+$Au collisions provides a testing ground for the onset of flow signatures in small collision systems. We measure a nonzero $v_2$ signal at all four collision energies, which, at midrapidity and low $p_T$, is consistent with predictions from viscous hydrodynamic models. Comparisons with calculations from parton transport models (based on the {\sc ampt} Monte Carlo generator) show good agreement with the data at midrapidity to forward ($d$-going) rapidities and low $p_T$. At backward (Au-going) rapidities and $p_T>1.5$ GeV/$c$, the data diverges from {\sc ampt} calculations of $v_2$ relative to the initial geometry, indicating the possible dominance of nongeometry related correlations, referred to as nonflow. We also present measurements of the charged-particle multiplicity ($dN_{\rm ch}/d\eta$) as a function of $\eta$ in central $d$$+$Au collisions at the same energies. We find that in $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=$200 GeV the $v_2$ scales with $dN_{\rm ch}/d\eta$ over all $\eta$ in the PHENIX acceptance. At $\sqrt{s_{_{NN}}}=$62.4, and 39 GeV, $v_2$ scales with $dN_{\rm ch}/d\eta$ at midrapidity and forward rapidity, but falls off at backward rapidity. This departure from the $dN_{\rm ch}/d\eta$ scaling may be a further indication of nonflow effects dominating at backward rapidity.

The detonation of a sub-Chandrasekhar-mass white dwarf at the origin of the low-luminosity Type Ia supernova 1999by

While Chandrasekhar-mass ($M_{\rm Ch}$) models with a low $^{56}\rm{Ni}$ yield can match the peak luminosities of fast-declining, 91bg-like Type Ia supernovae (SNe Ia), they systematically fail to reproduce their faster light-curve evolution. Here we illustrate the impact of a low ejecta mass on the radiative display of low-luminosity SNe Ia, by comparing a sub-$M_{\rm Ch}$ model resulting from the pure central detonation of a C-O White Dwarf (WD) to a $M_{\rm Ch}$ delayed-detonation model with the same $^{56}\rm{Ni}$ yield of 0.12 M$_{\odot}$. Our sub-$M_{\rm Ch}$ model from a 0.90 M$_{\odot}$ WD progenitor has a $\sim$5 day shorter rise time in the integrated UV-optical-IR (uvoir) luminosity, as well as in the $B$-band, and a $\sim$20 per cent higher peak uvoir luminosity ($\sim$1 mag brighter peak $M_B$). This sub-$M_{\rm Ch}$ model also displays bluer maximum-light colours due to the larger specific heating rate, and larger post-maximum uvoir and $B$-band decline rates. The luminosity decline at nebular times is also more pronounced, reflecting the enhanced escape of gamma rays resulting from the lower density of the progenitor WD. The deficit of stable nickel in the innermost ejecta leads to a notable absence of forbidden lines of [Ni II] in the nebular spectra. In contrast, the $M_{\rm Ch}$ model displays a strong line due to [Ni II] 1.939 $\mu\rm{m}$, which could in principle serve to distinguish between different progenitor scenarios. Our sub-$M_{\rm Ch}$ model offers an unprecedented agreement with optical and near-infrared observations of the 91bg-like SN 1999by, making a strong case for a WD progenitor significantly below the Chandrasekhar-mass limit for this event and other low-luminosity SNe Ia.

Full-dimensional treatment of short-time vibronic dynamics in a molecular high-order-harmonic-generation process in methane

We present derivation and implementation of the Multi-Configurational Strong-Field Approximation with Gaussian nuclear Wave Packets (MC-SFA-GWP) -- a version of the molecular strong-field approximation which treats all electronic and nuclear degrees of freedom, including their correlations, quantum-mechanically. The technique allows, for the first time, realistic simulation of high-harmonic emission in polyatomic molecules without invoking reduced-dimensionality models for the nuclear motion or the electronic structure. We use MC-SFA-GWP to model isotope effects in high-harmonics generation (HHG) spectroscopy of methane. The HHG emission in this molecule transiently involves strongly vibronically-coupled $^2F_2$ electronic state of the $\rm CH_4^+$ cation. We show that the isotopic HHG ratio in methane contains signatures of: a) field-free vibronic dynamics at the conical intersection (CI); b) resonant features in the recombination cross-sections; c) laser-driven bound-state dynamics; as well as d) the well-known short-time Gaussian decay of the emission. We assign the intrinsic vibronic feature (a) to a relatively long-lived ($\ge4$ fs) vibronic wave packet of the singly-excited $\nu_4$ ($t_2$) and $\nu_2$ ($e$) vibrational modes, strongly coupled to the components of the $^2F_2$ electronic state. We demonstrate that these physical effects differ in their dependence on the wavelength, intensity, and duration of the driving pulse, allowing them to be disentangled. We thus show that HHG spectroscopy provides a versatile tool for exploring both conical intersections and resonant features in photorecombination matrix elements in the regime not easily accessible with other techniques.

Ejecta mass diagnostics of Type Ia supernovae

We present one-dimensional non-local thermodynamic equilibrium time-dependent radiative transfer simulations (using CMFGEN) of two sub-Chandrasekhar (sub-$M_{\rm Ch}$), one $M_{\rm Ch}$ and one super-$M_{\rm Ch}$ Type Ia SN ejecta models. Three originate from $M_{\rm Ch}$ delayed detonation models, and the fourth is a sub-$M_{\rm Ch}$ detonation model. Ejecta masses are 1.02, 1.04, 1.40, and 1.70 M$_\odot$, and all models have 0.62 M$_\odot$ of $^{56}{\rm Ni}$. Sub-$M_{\rm Ch}$ model light curves evolve faster, reaching bolometric maximum 2--3 days earlier and having 3--4 days shorter bolometric half light widths. The models vary by $\sim$12 per cent at maximum bolometric luminosity and by 0.17 mag in $B_{\rm max}$. While $\Delta M_{15}(B)$ increases with ejecta mass it only varies by $\sim$5 per cent around 1 mag. Sub-$M_{\rm Ch}$ models are 0.25 mag bluer in $B-R$ at $B_{\rm max}$. Optical spectra share many similarities, but lower mass models exhibit less UV line blanketing during the photospheric phase. At nebular times, significant NIR spectroscopic differences are seen. In particular, emission lines of the Ca II NIR triplet; [S III] $\lambda\lambda$9068,9530; [Ca II] $\lambda\lambda$7291,7324; [Ar III] $\lambda\lambda$7135,7751; and [Ni II] 1.939 $\mu$m are stronger in higher mass models. The [Ni II] 1.939 $\mu$m line is absent in the sub-$M_{\rm Ch}$ detonation model, and provides a valuable potential tool to distinguish sub-$M_{\rm Ch}$ explosions from $M_{\rm Ch}$ explosions. In general, the nebular phase models are too highly ionized. We attribute this to the neglect of clumping and/or the distribution of intermediate mass and iron group elements. The two sub-$M_{\rm Ch}$ models, while exploded by different mechanisms, can be distinguished in the $J$ and $H$ bands at late times (e.g., $+200$ days).

Molecular simulation of caloric properties of fluids modelled by force fields with intramolecular contributions: Application to heat capacities.

The calculation of caloric properties such as heat capacity, Joule-Thomson coefficients, and the speed of sound by classical force-field-based molecular simulation methodology has received scant attention in the literature, particularly for systems composed of complex molecules whose force fields (FFs) are characterized by a combination of intramolecular and intermolecular terms. The calculation of a thermodynamic property for a system whose molecules are described by such a FF involves the calculation of the residual property prior to its addition to the corresponding ideal-gas property, the latter of which is separately calculated, either using thermochemical compilations or nowadays accurate quantum mechanical calculations. Although the simulation of a volumetric residual property proceeds by simply replacing the intermolecular FF in the rigid molecule case by the total (intramolecular plus intermolecular) FF, this is not the case for a caloric property. We describe the correct methodology required to perform such calculations and illustrate it in this paper for the case of the internal energy and the enthalpy and their corresponding molar heat capacities. We provide numerical results for cP, one of the most important caloric properties. We also consider approximations to the correct calculation procedure previously used in the literature and illustrate their consequences for the examples of the relatively simple molecule 2-propanol, CH3CH(OH)CH3, and for the more complex molecule monoethanolamine, HO(CH2)2NH2, an important fluid used in carbon capture.

Transverse momentum fluctuations in ultrarelativistic Pb + Pb and p + Pb collisions with “wounded” quarks

We analyze the phenomenon of size-flow transmutation in ultrarelativistic nuclear collisions in a model where the initial size fluctuations are driven by the wounded quarks and the collectivity is provided by viscous hydrodynamics. It is found that the model properly reproduces the data for the transverse momentum fluctuations measured for Pb+Pb collisions at 2.76TeV by the ALICE Collaboration. The agreement holds for a remarkably wide range of centralities, from 0-5% up to 70-80%, and displays a departure from a simple scaling with $(dN_{\rm ch}/d\eta)^{1/2}$ in the form seen in the data. The overall agreement in the model with wounded quarks is significantly better than with nucleon participants. This feature joins the previously-found wounded quark multiplicity scaling in the argumentation in favor of subnucleonic degrees of freedom in the early dynamics. We also examine in detail the correlations between measures of the initial size and final average transverse momentum of hadrons. Predictions are made for the transverse momentum fluctuations in p+Pb collisions at 5.02TeV.

Evidence for sub-Chandrasekhar-mass progenitors of Type Ia supernovae at the faint end of the width–luminosity relation

The faster light-curve evolution of low-luminosity Type Ia supernovae (SNe Ia) suggests that they could result from the explosion of white dwarf (WD) progenitors below the Chandrasekhar mass ($M_{\rm Ch}$). Here we present 1D non-LTE time-dependent radiative transfer simulations of pure central detonations of carbon-oxygen WDs with a mass ($M_\rm{tot}$) between 0.88 M$_{\odot}$ and 1.15 M$_{\odot}$, and a $^{56}\rm{Ni}$ yield between 0.08 M$_{\odot}$ and 0.84 M$_{\odot}$. Their lower ejecta density compared to $M_{\rm Ch}$ models results in a more rapid increase of the luminosity at early times and an enhanced $\gamma$-ray escape fraction past maximum light. Consequently, their bolometric light curves display shorter rise times and larger post-maximum decline rates. Moreover, the higher $M(^{56}\rm{Ni})/M_\rm{tot}$ ratio at a given $^{56}\rm{Ni}$ mass enhances the temperature and ionization level in the spectrum-formation region for the less luminous models, giving rise to bluer colours at maximum light and a faster post-maximum evolution of the $B-V$ colour. For sub-$M_{\rm Ch}$ models fainter than $M_B\approx -18.5$ mag at peak, the greater bolometric decline and faster colour evolution lead to a larger $B$-band post-maximum decline rate, $\Delta M_{15}(B)$. In particular, all of our previously-published $M_{\rm Ch}$ models (standard and pulsational delayed detonations) are confined to $\Delta M_{15}(B) < 1.4$ mag, while the sub-$M_{\rm Ch}$ models with $M_\rm{tot}\lesssim 1$ M$_{\odot}$ extend beyond this limit to $\Delta M_{15}(B)\approx 1.65$ mag for a peak $M_B\approx -17$ mag, in better agreement with the observed width-luminosity relation (WLR). Regardless of the precise ignition mechanism, these simulations suggest that fast-declining SNe Ia at the faint end of the WLR could result from the explosion of WDs whose mass is significantly below the Chandrasekhar limit.

BPS/CFT correspondence II: Instantons at crossroads, moduli and compactness theorem

Gieseker-Nakajima moduli spaces $M_{k}(n)$ parametrize the charge $k$ noncommutative $U(n)$ instantons on ${\bf R}^{4}$ and framed rank $n$ torsion free sheaves $\mathcal{E}$ on ${\bf C\bf P}^{2}$ with ${\rm ch}_{2}({\mathcal{E}}) = k$. They also serve as local models of the moduli spaces of instantons on general four-manifolds. We study the generalization of gauge theory in which the four dimensional spacetime is a stratified space $X$ immersed into a Calabi-Yau fourfold $Z$. The local model ${\bf M}_{k}({\vec n})$ of the corresponding instanton moduli space is the moduli space of charge $k$ (noncommutative) instantons on origami spacetimes. There, $X$ is modelled on a union of (up to six) coordinate complex planes ${\bf C}^{2}$ intersecting in $Z$ modelled on ${\bf C}^{4}$. The instantons are shared by the collection of four dimensional gauge theories sewn along two dimensional defect surfaces and defect points. We also define several quiver versions ${\bf M}_{\bf k}^{\gamma}({\vec{\bf n}})$ of ${\bf M}_{k}({\vec n})$, motivated by the considerations of sewn gauge theories on orbifolds ${\bf C}^{4}/{\Gamma}$. The geometry of the spaces ${\bf M}_{\bf k}^{\gamma}({\vec{\bf n}})$, more specifically the compactness of the set of torus-fixed points, for various tori, underlies the non-perturbative Dyson-Schwinger identities recently found to be satisfied by the correlation functions of $qq$-characters viewed as local gauge invariant operators in the ${\mathcal{N}}=2$ quiver gauge theories. The cohomological and K-theoretic operations defined using ${\bf M}_{k}({\vec n})$ and their quiver versions as correspondences provide the geometric counterpart of the $qq$-characters, line and surface defects.

Inferring the evolutionary stages of the internal structures of NGC 7538 S and IRS1 from chemistry

To unambiguously diagnose the detailed physical mechanisms and the evolutionary status of high-mass star-forming regions (HMSFRs), we have performed 0.4" ($\sim1000$ AU) resolution observations towards NGC7538S and IRS1, using the Plateau de Bure Interferometer (PdBI). This paper presents a joint analysis of the 1.37 mm continuum emission and the line intensity of 15 molecular species. Assuming local thermal equilibrium, we derived molecular column densities and molecular abundances for each internal gas substructure which is spatially resolved. These derived quantities are compared with a suite of 1-D gas-grain models. NGC7538S is resolved into at least three dense gas condensations. Despite the comparable continuum intensity of these condensations, their differing molecular line emission is suggestive of an overall chemical evolutionary trend from the northeast to the southeast. Since these condensations are embedded within the same parent gas core, their differing chemical properties are the most likely due to the different warm-up histories, rather than the different dynamic timescales. Despite remaining spatially unresolved, in IRS1 we detect abundant complex organic molecules, indicating that IRS1 is the most chemically evolved hot molecular core presented here. We observe a continuum that is dominated by absorption features with at least three strong emission lines, potentially from $\rm CH_3OH$. The $\rm CH_3OH$ lines which are purely in emission have higher excitation than the ones showing purely absorption. Potential reasons for that difference are discussed. This is the first comprehensive comparison of observations of the high-mass cores NGC7538S and IRS1 and a chemical model. We have found that different chemical evolutionary stages can coexist in the same natal gas core. Our achievement illustrates the strength of chemical analysis for understanding HMSFRs.

Global properties of proton-proton collisions at s = 100 $$ \sqrt{\mathrm{s}}=100 $$ TeV

The global properties of the final states produced in hadronic interactions of protons at centre-of-mass energies of future hadron colliders (such as FCC-hh at CERN, and SppC in China), are studied. The predictions of various Monte Carlo (MC) event generators used in collider physics (PYTHIA 6, PYTHIA 8, and PHOJET) and in ultrahigh-energy cosmic-rays studies (EPOS, and QGSJET) are compared. Despite their different underlying modeling of hadronic interactions, their predictions for proton-proton (p-p) collisions at $\sqrt{s}$ = 100 TeV are quite similar. The average of all MC predictions (except PHOJET) for the different observables are: (i) p-p inelastic cross sections $\sigma_{\rm inel}$ = 105 $\pm$ 2 mb; (ii) total charged multiplicity $\rm N_{_{\rm ch}}$ = 150 $\pm$ 20; (iii) charged particle pseudorapidity density at midrapidity $\rm dN_{ch}/d\eta|_{\eta=0} = 9.6 \pm 0.2$; (iv) energy density at midrapidity $\rm dE/d\eta|_{\eta=0} = 13.6 \pm 1.5$ GeV, and $\rm dE/d\eta|_{\eta=5} = 670 \pm 70$ GeV at the edge of the central region; and (v) average transverse momenta at midrapidities $\rm < p_{T}> = 0.76 \pm 0.07$ GeV/c. At midrapidity, EPOS and QGSJET-II predict larger per-event multiplicity probabilities at very low ($\rm N_{\rm ch}<3$) and very high ($\rm N_{\rm ch}>100$) particle multiplicities, whereas PYTHIA 6 and 8 feature higher yields in the intermediate region $\rm N_{\rm ch}\approx$ 30--80. These results provide useful information for the estimation of the detector occupancies and energy deposits from pileup collisions at the expected large FCC-hh/SppC luminosities.

Couplings between QCD axion and photon from string compactification

The QCD axion couplings of various invisible axion models are presented. In particular, the exact global symmetry U(1)$_{\rm PQ}$ in the superpotential is possible for the anomalous U(1) from string compactification, broken only by the gauge anomalies at one loop level, and is shown to have the resultant invisible axion coupling to photon, $c_{a\gamma\gamma}\ge \frac83-c_{a\gamma\gamma}^{\rm ch\,br}$ where $c_{a\gamma\gamma}^{\rm ch\,br}\simeq 2$. However, this bound is not applicable in approximate U(1)$_{\rm PQ}$ models with sufficiently suppressed U(1)$_{\rm PQ}$-breaking superpotential terms. We also present a simple method to obtain $c_{a\gamma\gamma}^0$ which is the value obtained above the electroweak scale.

Two scales in Bose–Einstein correlations

We argue that the secondaries produced in high energy hadron collisions are emitted by small size sources distributed over a much larger area in impact parameter space occupied by the interaction amplitude. That is, Bose-Einstein correlation of two emitted identical particles should be described by a `two-radii' parametrization ansatz. We discuss the expected energy, charged multiplicity and transverse momentum of the pair (that is, $\sqrt{s},~N_{\rm ch}, k_t$) behaviour of both the small and large size components.

Scaling properties of fractional momentum loss of high-pThadrons in nucleus-nucleus collisions atsNNfrom 62.4 GeV to 2.76 TeV

Measurements of the fractional momentum loss ($S_{\rm loss}\equiv{\delta}p_T/p_T$) of high-transverse-momentum-identified hadrons in heavy ion collisions are presented. Using $\pi^0$ in Au$+$Au and Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}=62.4$ and 200 GeV measured by the PHENIX experiment at the Relativistic Heavy Ion Collider and and charged hadrons in Pb$+$Pb collisions measured by the ALICE experiment at the Large Hadron Collider, we studied the scaling properties of $S_{\rm loss}$ as a function of a number of variables: the number of participants, $N_{\rm part}$, the number of quark participants, $N_{\rm qp}$, the charged-particle density, $dN_{\rm ch}/d\eta$, and the Bjorken energy density times the equilibration time, $\varepsilon_{\rm Bj}\tau_{0}$. We find that the $p_T$ where $S_{\rm loss}$ has its maximum, varies both with centrality and collision energy. Above the maximum, $S_{\rm loss}$ tends to follow a power-law function with all four scaling variables. The data at $\sqrt{s_{_{NN}}}$=200 GeV and 2.76 TeV, for sufficiently high particle densities, have a common scaling of $S_{\rm loss}$ with $dN_{\rm ch}/d\eta$ and $\varepsilon_{\rm Bj}\tau_{0}$, lending insight on the physics of parton energy loss.

Centrality dependence of pion freeze-out radii in Pb-Pb collisions atsNN=2.76 TeV

We report on the measurement of freeze-out radii for pairs of identical-charge pions measured in Pb--Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV as a function of collision centrality and the average transverse momentum of the pair $k_{\rm T}$. Three-dimensional sizes of the system (femtoscopic radii), as well as direction-averaged one-dimensional radii are extracted. The radii decrease with $k_{\rm T}$, following a power-law behavior. This is qualitatively consistent with expectations from a collectively expanding system, produced in hydrodynamic calculations. The radii also scale linearly with $\left< \mathrm{d}N_{\rm ch}/\mathrm{d}\eta \right>^{1/3}$. This behaviour is compared to world data on femtoscopic radii in heavy-ion collisions. While the dependence is qualitatively similar to results at smaller $\sqrt{s_{\rm NN}}$, a decrease in the $R_{\rm out}/R_{\rm side}$ ratio is seen, which is in qualitative agreement with specific predictions from hydrodynamic models. The results provide further evidence for the production of a collective, strongly coupled system in heavy-ion collisions at the LHC.

Transverse energy production and charged-particle multiplicity at midrapidity in various systems fromsNN=7.7to 200 GeV

Measurements of midrapidity charged particle multiplicity distributions, $dN_{\rm ch}/d\eta$, and midrapidity transverse-energy distributions, $dE_T/d\eta$, are presented for a variety of collision systems and energies. Included are distributions for Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$, 130, 62.4, 39, 27, 19.6, 14.5, and 7.7 GeV, Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}=200$ and 62.4 GeV, Cu$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV, U$+$U collisions at $\sqrt{s_{_{NN}}}=193$ GeV, $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV, $^{3}$He$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV, and $p$$+$$p$ collisions at $\sqrt{s_{_{NN}}}=200$ GeV. Centrality-dependent distributions at midrapidity are presented in terms of the number of nucleon participants, $N_{\rm part}$, and the number of constituent quark participants, $N_{q{\rm p}}$. For all $A$$+$$A$ collisions down to $\sqrt{s_{_{NN}}}=7.7$ GeV, it is observed that the midrapidity data are better described by scaling with $N_{q{\rm p}}$ than scaling with $N_{\rm part}$. Also presented are estimates of the Bjorken energy density, $\varepsilon_{\rm BJ}$, and the ratio of $dE_T/d\eta$ to $dN_{\rm ch}/d\eta$, the latter of which is seen to be constant as a function of centrality for all systems.