Pion femtoscopy measurements in ALICE at the LHC

We present the results of two-pion Bose-Einstein correlations measured in Pb--Pb collisions at a center-of-mass energy sqrt(s_NN) = 2.76 TeV recorded by ALICE at the Large Hadron Collider. These types of correlations allow to extract, using the technique of femtoscopy (also known as Hanburry-Brown Twiss interferometry, or shortly HBT), the space-time characteristics of the source from the correlation calculated as a function of the pair momentum difference. The femtoscopic analysis was performed using both the Spherical Harmonics decomposition and the standard 3D Cartesian representation of the correlation function. The source sizes in three dimensions, the HBT radii, were extracted by fitting the experimental correlation functions. The resulting dependencies of the radii as a function of centrality and pair transverse momentum are shown. The results indicate the existence of a flowing medium and provide constraints on existing dynamical models. The ALICE Pb-Pb HBT radii are also compared to the pp analysis and other heavy-ion experiments in order to test the multiplicity scaling between different systems.Comment: 7 pages, 5 figures, submitted to the European Physical Journal: Web of Conferences (proceedings of ICNFP 2013 conference

Studies of final state interactions via femtoscopy in ALICE

Femtoscopy is a technique enabling measurements of the space-time characteristics of particle-emitting sources. However, the femtoscopic analysis is also sensitive to the interaction cross-section. In this paper we show the first preliminary measurements of $\rm K^0_SK^{\pm}$ correlation functions in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV. These correlations originate from the final-state interactions which proceed through the $a_0(980)$ resonance only and can be employed to constrain its parameters. A similar approach can be applied to baryon pairs to extract the unknown interaction cross-sections for some (anti-)baryon-(anti-)baryon pairs. We show baryon--baryon and baryon--anti-baryon correlation functions of protons and lambdas, as well as discuss shortly the fitting method.Comment: 4 pages, 3 figures, proceedings from Strangeness in Quark Matter 2016 conferenc

Soft QGP probes with ALICE

In heavy-ion collisions at the LHC a hot and dense medium of deconfided partons, the Quark-Gluon Plasma (QGP), is created. Its global properties can be characterized by the measurements of particles in the low transverse momentum (or "soft") regime, which represent the majority of created particles. In this report we outline a selection of measurements of the soft probes by the ALICE experiment in pp, p--Pb, and Pb--Pb collisions. The paper focuses on recent flow measurements via angular correlations and femtoscopic studies. The first ever preliminary analysis of $\mathrm{K}^0_{\rm S}\mathrm{K}^{\pm}$ femtoscopy is also presented.Comment: 16 pages, 13 figures, proceedings of Cracow Epiphany Conference on the Physics in LHC Run

New developments for ALICE MasterClasses and the new Particle Therapy MasterClass

International MasterClasses (IMC), an outreach activity of the International Particle Physics Outreach Group (IPPOG), has been bringing cutting-edge particle physics research to schoolchildren for over 15 years now. All four LHC experiments participate in the event, including ALICE, the experiment optimised for the study of heavy-ion collisions. Heavy-ion physics is actively contributing to IMC with new developments including experimental measurements but also applications for society, such as treatment of cancer with ions. In particular, ALICE provides three MC measurements related to the main observables used to characterize the properties of the produced Quark-Gluon Plasma. Historically, those MC measurements were developed independently, inheriting from the first one, by several ALICE groups. Since all of them are based on the ROOT EVE package, a project to integrate them into a common framework was undertaken. ALICE delivers now a single and easy-to-use application, compiled under Linux, MacOS, and, for the first time, Windows. Then, in line with current IPPOG goals to increase the global reach and scope of the IMC programme a newly developed measurement on medical applications of particle physics, the Particle Therapy MasterClass (PTMC) was introduced in the IMC2020 programme. It is a simplified version of matRad, a MATLAB-based toolkit for calculation of dose deposition in the body and allows for planning of radiotherapy using different modalities and highlighting the benefits of treatment with ions.Comment: 7 pages, 3 figures, proceedings of the 24th International Conference on Computing in High Energy and Nuclear Physics (CHEP 2019

GPU propagation and visualisation of particle collisions with ALICE magnetic field model

The ALICE Collaboration at CERN developed a 3D visualisation tool capable of displaying a representation of collected collision data (particle trajectories, clusters and calorimeter towers) called the Event Display. The Event Display is constantly running in the ALICE Run Control Center as part of the Quality Assurance system, providing the monitoring personnel with visual cues about possible problems of both hardware and software components during periods of data gathering. In the software, particle trajectories (which are curved due to presence of magnetic field inside the detector) are generated from physical parameters of detected particles, such as electrical charge and momentum. Previously this process in the Event Display used a uniform, constant magnetic field for these calculations, which differs from the spatial variations of the real magnetic field and does not model one of the two magnets used in the detector. Recently, a detailed model of ALICE magnetic field was made available as a shader program for execution on the GPU. In this work we attempt to implement the reconstruction algorithm in a shader form as well, allowing us to combine it with the detailed model to create a full solution for rendering trajectories from collision event data directly on the GPU. This approach has several possible advantages, such as better performance and the ability to alter the magnetic field properties in real-time. This was not previously done for ALICE and as such could be used in the future to upgrade the Event Display

Two-Dimensional Phononic Crystals: Disorder Matters

The design and fabrication of phononic crystals (PnCs) hold the key to control the propagation of heat and sound at the nanoscale. However, there is a lack of experimental studies addressing the impact of order/disorder on the phononic properties of PnCs. Here, we present a comparative investigation of the influence of disorder on the hypersonic and thermal properties of two-dimensional PnCs. PnCs of ordered and disordered lattices are fabricated of circular holes with equal filling fractions in free-standing Si membranes. Ultrafast pump and probe spectroscopy (asynchronous optical sampling) and Raman thermometry based on a novel two-laser approach are used to study the phononic properties in the gigahertz (GHz) and terahertz (THz) regime, respectively. Finite element method simulations of the phonon dispersion relation and three-dimensional displacement fields furthermore enable the unique identification of the different hypersonic vibrations. The increase of surface roughness and the introduction of short-range disorder are shown to modify the phonon dispersion and phonon coherence in the hypersonic (GHz) range without affecting the room-temperature thermal conductivity. On the basis of these findings, we suggest a criteria for predicting phonon coherence as a function of roughness and disorder.Comment: 19 pages, 4 figures, final published version, Nano Letters, 201

A novel high resolution contactless technique for thermal field mapping and thermal conductivity determination: Two-Laser Raman Thermometry

We present a novel high resolution contactless technique for thermal conductivity determination and thermal field mapping based on creating a thermal distribution of phonons using a heating laser, while a second laser probes the local temperature through the spectral position of a Raman active mode. The spatial resolution can be as small as $300$ nm, whereas its temperature accuracy is $\pm 2$ K. We validate this technique investigating the thermal properties of three free-standing single crystalline Si membranes with thickness of 250, 1000, and 2000 nm. We show that for 2-dimensional materials such as free-standing membranes or thin films, and for small temperature gradients, the thermal field decays as $T(r) \propto ln(r)$ in the diffusive limit. The case of large temperature gradients within the membranes leads to an exponential decay of the thermal field, $T \propto exp[-A \cdot ln(r)]$. The results demonstrate the full potential of this new contactless method for quantitative determination of thermal properties. The range of materials to which this method is applicable reaches far beyond the here demonstrated case of Si, as the only requirement is the presence of a Raman active mode

Rayleigh surface waves propagating in (111) Si substrate decorated with Ni phononic nanostructure

The paper reports results of the Surface Brillouin Light Scattering at the silicon (111) surface loaded with a periodic 2D nickel nanostructure. Measurements were made for samples loaded with nanostructures of different period (different size) but of the same height. The relation between the nanostructure size and the velocity of surface Rayleigh waves was proved to be nonlinear. Anisotropy of the surface Rayleigh wave velocity was compared with the results of theoretical modelling based on the Finite Element Method