Final-state Effects Research Articles

Recent results from the ALICE Experiment at LHC

ALICE (A Large Ion Collider Experiment) at the LHC performed high statistics measurements in Pb-Pb collisions at the top LHC energy, complemented with large recent reference datasets in elementary proton-proton collisions at the same energy. Elementary pp collisions are serving as baseline for testing QCD properties and allow the study of the changes induced by the hot and dense medium produced in heavy ion collisions. Key observables like nuclear modification factors, jet production, flow phenomena and spectra for identified particles, related to the different stages of collision evolution, are presented and compared with the most recent results from p-Pb and Xe-Xe collisions, thus allowing to probe both initial cold nuclear matter and final state effects, combined with the system size dependence of the measurements.

Probing the hadronic phase with resonances of different lifetimes in Pb-Pb collisions with ALICE

The ALICE experiment has measured the production of a rich set of hadronic resonances, such as ρ(770)0, K*(892)0, ϕ(1020), ∑±(1385), Λ(1520) and Ξ*0 in pp, p-Pb and Pb-Pb collisions at various energies at the LHC. A comprehensive overview and the latest results are presented in this paper. Special focus is given to the role of hadronic resonances for the study of final-state effects in high-energy collisions. In particular, the measurement of resonance production in heavy-ion collisions has the capability to provide insight into the existence of a prolonged hadronic phase after hadronisation. The observation of the suppression of the production of Λ(1520) resonance in central Pb-Pb collisions at [see formula in PDF] =2.76 TeV adds further support to the existence of such a dense hadronic phase, as already evidenced by the ratios K*(892)0/K and ρ(770)0/π.

Importance of initial and final state effects for azimuthal correlations in p+Pb collisions

We investigate the relative importance of initial and final state effects on azimuthal correlations of gluons in low and high multiplicity p+Pb collisions. To achieve this, we couple Yang-Mills dynamics of pre-equilibrium gluon fields (IP-GLASMA) to a perturbative QCD based parton cascade for the final state evolution (BAMPS) on an event-by-event basis. We find that signatures of both the initial state correlations and final state interactions are seen in azimuthal correlation observables, such as $v_2\left\lbrace2PC\right\rbrace(p_T)$, their strength depending on the event multiplicity and transverse momentum. Initial state correlations dominate $v_2\left\lbrace2PC\right\rbrace(p_T)$ in low multiplicity events for transverse momenta $p_T>2~{\rm GeV}$. While final state interactions are dominant in high multiplicity events, initial state correlations affect $v_2\left\lbrace2PC\right\rbrace(p_T)$ for $p_T>2~{\rm GeV}$ as well as the pT integrated $v_2\left\lbrace2PC\right\rbrace$.

Surface, final state, and spin effects in the valence-band photoemission spectra ofLaCoO3(001)

The electronic structure of a ${\mathrm{LaCoO}}_{3}$(001) perovskite is analyzed using simulated and measured angle-resolved valence-band photoemission spectroscopy data with emphasis on the effects of the surface truncation, the final state effects, and the spin degeneracy removal. Supercell simulations reveal instability towards the formation of nonuniform spin polarization in the Co-containing layers under tensile stress. Lattice expansion reduces antiferrodistortive octahedron tilting and tips the ratio between the crystal field parameter and the exchange energy in favor of local magnetization. While the magnitude of the local moment scales with lateral strain, the Curie temperature saturates for straight Co-O-Co chains below 75 K. The comparison of simulated photoemission spectra with experiments suggests Co-${\mathrm{O}}_{2}$ to be the most stable surface stoichiometry, as proposed from transport and catalytic studies.

X-ray photoelectron spectra for single-crystalTi2O3: Experiment and theory

We have measured high-resolution core-level and valence-band x-ray photoemission spectra for single-crystal $\mathrm{T}{\mathrm{i}}_{2}{\mathrm{O}}_{3}$ cleaved anoxically. The Ti(III) spectra for this lattice are considerably more complex than those measured for Ti(IV)-based oxides due to the presence of a single unpaired electron in the conduction band. This open-shell electron configuration leads to ligand-field split and frequently unresolved multiplets. The Ti $2p$ and $3p$ spectra have been calculated using relativistic Dirac-Hartree-Fock (DHF) theory with the sudden approximation for the intensities. Agreement between theory and experiment is excellent for the $3p$ spectrum, and very good for the $2p$ spectrum, the primary deficiency being a pair of features not captured by theory for the latter. The spectral line shapes are driven by final-state effects associated with angular momentum coupling of the unpaired valence electron with the core hole, one- and two-electron ligand-to-metal charge-transfer (shake) processes accompanying core photoionization, and core-hole screening by conduction-band electrons. The first two of these are accurately predicted by DHF theory with a small embedded cluster containing a single Ti cation and six oxygen ligands. The third effect is not predicted using this cluster in which screening of the core hole from electrons associated with more distant atoms is not possible.

Nuclear modification of forward Drell-Yan production at the LHC

Forward Drell-Yan production at high energy can provide important constraints on gluon densities at small $x$, in the saturation regime. In this work we focus on the nuclear modification of this process, which could be measured at the LHC in the near future. For this we employ the color dipole approach, using the optical Glauber model to relate the dipole cross section of a nucleus to the one of a proton. Combining these results with our earlier results for forward $J/\psi$ production, we compute the ratio of the nuclear modification factors of these two processes. This observable was recently suggested as a way to distinguish between initial and final state effects in forward particle production.

Double parton scattering in the CGC: Double quark production and effects of quantum statistics

We consider forward inclusive production of two quarks in the high energy p-A collisions in the CGC formalism. We demonstrate that the production cross-section is determined by the convolution of the proton generalized double transverse momentum-dependent distribution (2GTMD) functions with two independent eikonal scattering amplitudes: the product of two dipoles and a quadrupole. We explicitly demonstrate that the quadrupole amplitude term accounts for all the (initial and final state) effects of quantum statistics for identical fermions, and the correlations due to these effects. We also demonstrate that the effects due to quantum statistics (entirely encoded in the quadrupole) are parametrically leading contributions to the correlated particle production at large $N_c$. For non-identical quarks the quadrupole term also leads to correlated production which has characteristics similar to the HBT effect.

Next-to-leading-order electroweak corrections to the production of three charged leptons plus missing energy at the LHC

The production of a neutral and a charged vector boson with subsequent decays into three charged leptons and a neutrino is a very important process for precision tests of the Standard Model of elementary particles and in searches for anomalous triple-gauge-boson couplings. In this article, the first computation of next-to-leading-order electroweak corrections to the production of the four-lepton final states μ+μ−e+νe, {mu}^{+}{mu}^{-}{mathrm{e}}^{-}{overline{nu}}_{mathrm{e}} , μ+μ−μ+νμ, and {mu}^{+}{mu}^{-}{mu}^{-}{overline{nu}}_{mu } at the Large Hadron Collider is presented. We use the complete matrix elements at leading and next-to-leading order, including all off-shell effects of intermediate massive vector bosons and virtual photons. The relative electroweak corrections to the fiducial cross sections from quark-induced partonic processes vary between −3% and −6%, depending significantly on the event selection. At the level of differential distributions, we observe large negative corrections of up to −30% in the high-energy tails of distributions originating from electroweak Sudakov logarithms. Photon-induced contributions at next-to-leading order raise the leading-order fiducial cross section by +2%. Interference effects in final states with equal-flavour leptons are at the permille level for the fiducial cross section, but can lead to sizeable effects in off-shell sensitive phase-space regions.

Methanol on Anatase TiO2 (101): Mechanistic Insights into Photocatalysis.

The photoactivity of methanol adsorbed on the anatase TiO2 (101) surface was studied by a combination of scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) calculations. Isolated methanol molecules adsorbed at the anatase (101) surface show a negligible photoactivity. Two ways of methanol activation were found. First, methoxy groups formed by reaction of methanol with coadsorbed O2 molecules or terminal OH groups are photoactive, and they turn into formaldehyde upon UV illumination. The methoxy species show an unusual C 1s core-level shift of 1.4 eV compared to methanol; their chemical assignment was verified by DFT calculations with inclusion of final-state effects. The second way of methanol activation opens at methanol coverages above 0.5 monolayer (ML), and methyl formate is produced in this reaction pathway. The adsorption of methanol in the coverage regime from 0 to 2 ML is described in detail; it is key for understanding the photocatalytic behavior at high coverages. There, a hydrogen-bonding network is established in the adsorbed methanol layer, and consequently, methanol dissociation becomes energetically more favorable. DFT calculations show that dissociation of the methanol molecule is always the key requirement for hole transfer from the substrate to the adsorbed methanol. We show that the hydrogen-bonding network established in the methanol layer dramatically changes the kinetics of proton transfer during the photoreaction.

Fermi surface and effective masses in photoemission response of the (Ba1\u2212xKx)Fe2As2 superconductor

The angle-resolved photoemission spectra of the superconductor (Ba1−xKx)Fe2As2 have been investigated accounting coherently for spin-orbit coupling, disorder and electron correlation effects in the valence bands combined with final state, matrix element and surface effects. Our results explain the previously obscured origins of all salient features of the ARPES response of this paradigm pnictide compound and reveal the origin of the Lifshitz transition. Comparison of calculated ARPES spectra with the underlying DMFT band structure shows an important impact of final state effects, which result for three-dimensional states in a deviation of the ARPES spectra from the true spectral function. In particular, the apparent effective mass enhancement seen in the ARPES response is not an entirely intrinsic property of the quasiparticle valence bands but may have a significant extrinsic contribution from the photoemission process and thus differ from its true value. Because this effect is more pronounced for low photoexcitation energies, soft-X-ray ARPES delivers more accurate values of the mass enhancement due to a sharp definition of the 3D electron momentum. To demonstrate this effect in addition to the theoretical study, we show here new state of the art soft-X-ray and polarisation dependent ARPES measurments.

Neutral meson production in pp, p–Pb and Pb–Pb collisions with ALICE at the LHC

The ALICE experiment is mainly devoted to the study of QGP expected to be created in heavy-ion collisions at the LHC. In the early stage of such collisions, partons can be scattered and photons can be produced in energetic hard processes. High energy partons will interact with the QGP, and from the measurement of the particles originated from the parton fragmentation in vacuum, one can infer transport properties of the QGP. Therefore, study of high-pT hadrons production is important to constrain the properties of the hot and dense medium. The strong suppression of the high-pT hadrons in heavy-ion collisions has been explained by the energy loss of the partons in medium and it can be investigated experimentally via the nuclear modification factor of hadrons.ALICE measured neutral mesons (π0 and η) at mid-rapidity in a wide pT range in pp, p-Pb and Pb-Pb collisions. Neutral mesons are reconstructed via complementary methods, using the electromagnetic calorimeters, PHOS and EMCal, and by the central tracking system, identifying photons converted to e+e− pairs in the material of the inner barrel detectors (ITS and TPC). Measurement of neutral meson spectra in pp collisions at s=0.9,2.76,7 and 8 TeV provides valuable comparison data for pQCD calculations and allows to study scaling properties of hadron production at the LHC energies. The study of neutral mesons production in p-Pb collisions helps to disentangle initial and final state effects on the suppression itself. The latest results of neutral meson measurement in pp, p-Pb and Pb-Pb collisions will be shown in this paper.

Overview of ATLAS results

The heavy-ion programme in the ATLAS experiment at the Large Hadron Collider aims to probe and characterise hot and dense matter created in relativistic lead-lead collisions. Moreover, smaller collision systems involving nuclei and hadrons are of interest to disentangle initial- from final-state effects. This report presents new results based on lead-lead and proton-proton data collected at sNN=5.02 TeV in 2015, including measurements of bulk collectivity, charged-particle production, electroweak bosons, photon-jet correlations, and quarkonium suppression. First attempts to measure electromagnetic processes in ultra-peripheral collisions are also discussed.

Surface electronic structure of epi germanium (001)-2 × 1

Photoemission from an epi Ge(001)-2 × 1 surface is presented using synchrotron radiation as a probe. The topmost surface atoms are buckled with the up-dimer and down-dimer atoms exhibiting surface core-level shifts (SCLSs) of −0.492 and −0.178 eV, respectively. The subsurface layer shows a +0.083 eV SCLS. The final-state effect suffices to explain the sign of the shift. The electron affinity and ionization potential for the epi Ge surface are 4.36 and 5.09 eV, respectively. An argument contrasting the current results with those of existing reports with non-epi surfaces is also given. Non-epi surfaces possess Ge surfaces with isolated single atoms or small droplets that affect Ge’s contact with dielectric layers and the electric performances of the Ge metal–oxide–semiconductor structure.

Three-dimensional nature of the band structure of ZrTe5 measured by high-momentum-resolution photoemission spectroscopy

We have performed a systematic high-momentum-resolution photoemission study on ZrTe$_5$ using $6$ eV photon energy. We have measured the band structure near the $\Gamma$ point, and quantified the gap between the conduction and valence band as $18 \leq \Delta \leq 29$ meV. We have also observed photon-energy-dependent behavior attributed to final-state effects and the 3D nature of the material's band structure. Our interpretation indicates the gap is intrinsic and reconciles discrepancies on the existence of a topological surface state reported by different studies. The existence of a gap suggests that ZrTe$_5$ is not a 3D strong topological insulator nor a 3D Dirac semimetal. Therefore, our experiment is consistent with ZrTe$_5$ being a 3D weak topological insulator.

Epitaxial corundum-VTiO3 thin films grown on c-cut sapphire

Corundum structured VTiO3 has been grown as epitaxial films on c-cut sapphire by laser molecular beam epitaxy. The properties of the film were characterized by reflection high energy electron diffraction, X-ray diffraction, transmission electron microscopy (TEM), and photoemission spectroscopy. All the structural probes clearly indicate the corundum structure of the film. X-ray photoemission spectroscopy (XPS) indicates that V is in a 3+ charge state implying that Ti also should adopt a 3+ charge state in order for the corundum structure to form. However, the Ti-2p XPS, while clearly broadened to the lower binding energy side compared to TiO2, also exhibits a pronounced Ti4+ component. Similarly TEM-electron energy loss spectroscopy indicates a mixture of Ti3+ and Ti4+. This is tentatively assigned to a combination of final state effects in XPS measurements and existence of excess (interstitial) oxygen. The valence band spectra show occupation of 3d metal states that resemble more closely those of Ti2O3 than for V2O3, suggesting that mostly the a1g molecular states are occupied.

Microscopic self dynamics in liquids: Connections between the Gaussian approximation and the asymptotic impulsive regime

In this paper we have studied the Gaussian approximation and its violation when applied to quantum fluids. Deviations from the Gaussian approximation, the so-called non-Gaussian effects, have been presented in a very general framework and subsequently compared to the final state effects, i.e. the differences between the exact single-particle response function and the well-known impulsive approximation. We have shown that, similarly to final state effects, non-Gaussian effects too can be expressed as a power series of the inverse momentum transfer, where the first two terms can be approximately calculated making use of equilibrium properties of the system. Finally, we have accomplished a practical test on experimentally measured non-Gaussian effects in liquid parahydrogen (at T=15.7K and n=22.53nm−3) by comparing them with the mentioned power series calculations. However, the agreement between the two data sets turned out to be rather poor, except for a general intensity level. The origin of this discrepancy is not evident at the moment and further tests on these physical quantities seem highly needed.

Electronic structure of tetraphenylporphyrin layers on Ag(100)

The electronic structure of Mg and free-base tetraphenylporphyrin films on Ag(100) is investigated by one- and two-photon photoemission in combination with electronic structure calculations using density functional theory and the self-consistent $G{W}_{0}$ method. We determine the two highest occupied and the nearly degenerate lowest unoccupied molecular orbitals. Higher unoccupied states are seen in an enhanced emission as a final-state effect. For photon energies close to the prominent absorption of the Soret band we observe a strong electron emission attributed to the break up of the bound electron-hole pairs in the ${S}_{2}$ excited state. The experimental results on the occupied and unoccupied energy levels for the molecular films on Ag(100) nicely agree with calculated quasiparticle energies and experiments of the molecules in the gas phase.

Separation of the Charm and Beauty Production in p–Pb and Pb–Pb Collisions with ALICE

Measurements of heavy (charm and beauty) quarks provide useful insights into the properties of the quark–gluon plasma. The good particle identification capabilities of ALICE make a measurement based on the electrons from semi-leptonic decays of heavy-flavour hadrons feasible. This approach makes use of the large branching ratios (≈ 10 − 20%) of heavy–flavour hadrons into electrons. Separation of the contribution from beauty-hadron decay electrons was achieved by utilizing the large decay length of the associated hadrons, making the investigation of beauty quarks in the medium possible. By comparing measurements in p–Pb and Pb–Pb collisions, it is possible to disentangle effects of cold nuclear matter from those of the hot and dense medium. The results show a strong change in the transverse momentum distribution of electrons from beauty-hadron decays in central Pb–Pb collisions with respect to the proton–proton case. No significant change from proton–proton collisions can be seen in the p–Pb case, suggesting that the modification is a final state effect.

Efficient and accurate modeling of electron photoemission in nanostructures with TDDFT

We review different computational methods for the calculation of photoelectron spectra and angular distributions of atoms and molecules when excited by laser pulses using time-dependent density-functional theory (TDDFT) that are suitable for the description of electron emission in compact spatial regions. We derive and extend the time-dependent surface-flux method introduced in Reference [Tao L and Scrinzi A 2012 New Journal of Physics 14 013021] within a TDDFT formalism and compare its performance to other existing methods. We illustrate the performance of the new method by simulating strong-field ionization of C$_{60}$ fullerene and discuss final state effects in the orbital reconstruction of planar organic molecules.

Neutral meson production in p-Pb collisions with ALICE at the LHC

Strong suppression of high p T π 0 has been observed in heavy-ion collisions at the Large Hadron Collider (LHC) energies, which can be interpreted by involving various transport properties of the quantum chromodynamics (QCD) medium and initial state effects. Comparing particle production in pp, p-A, A-A reactions has frequently been used to separate initial state effects of colliding nuclei from final state effects in quark matter created by the collisions.We have measured π 0 and η meson emitted in p-Pb collisions at √ s NN = 5.02 TeV via complementary methods, using the ALICE electromagnetic calorimeters, PHOS and EMCal, and Photon Conversion Method (PCM), which identifies photons converted to e + e − pairs in the material of the ALICE inner detectors, TPC and ITS. In this paper, π 0 and η meson spectra in p-Pb collisions as well as the nuclear modification factor for π 0 ( R π 0 p-Pb ) will be shown.