Publications

Order by: publication date number of citations

• Collective dynamics of heavy ion collisions in ATLAS

  Helena Santos

  SciPost Phys. Proc. 10, 034 (2022) · published 11 August 2022 | Toggle abstract · pdf

  The latest measurements of collective behaviour in a variety of collision systems with the ATLAS detector at the LHC, including pp collisions at 13 TeV, Xe+Xe collisions at 5.44 TeV, and Pb+Pb collisions at 5.02 TeV, are presented. They include vn-[pT] correlations, which carry important information about the initial-state geometry of the quark-gluon plasma and can shed light on any quadrupole deformation in the Xe nucleus, and measurements of flow decorrelations differential in rapidity, which probe the longitudinal structure of the colliding system. These measurements furthermore provide stringent tests of the theoretical understanding of the initial state in heavy ion collisions.

• On irregularities in the cosmic ray spectrum of \(10^{16}-10^{18}\) eV range

  Stanislav P. Knurenko, Igor S. Petrov

  SciPost Phys. Proc. 13, 020 (2023) · published 28 September 2023 | Toggle abstract · pdf

  Existing small, medium and large arrays for the study of cosmic rays of ultra-high energies are aimed for obtaining information about our galaxy and extragalactic space, namely to search and study astronomical objects that produce the flux of relativistic particles. The drift and interaction of such particles with magnetic fields and shock waves taking place in interstellar space causes the same interest. The shape of the energy spectrum of cosmic rays in the energy range $10^{15}-10^{18}$ eV, where the "knee" and the "second knee" are observed, can be formed as a superposition of the partial spectra of various chemical elements. Verification of galactic models, using recent experimental spectral data, makes it possible to study the nature of the galactic and extragalactic components of cosmic rays. The paper presents the result of the energy spectrum of cosmic rays in the range $10^{16}-10^{18}$ eV of measurements obtained with the Small Cherenkov array - a part of the Yakutsk array.

• Measurements of charm production in pp collisions with ALICE

  Tiantian Cheng

  SciPost Phys. Proc. 10, 030 (2022) · published 11 August 2022 | Toggle abstract · pdf

  The production of charm-hadrons at midrapidity in pp collisions at $\sqrt{s}$ = 5.02 and 13 TeV was recently measured by the ALICE Collaboration. The results show that the baryon-to-meson yield ratios differ from predictions where the hadronisation has been tuned to measurements obtained in $\rm e^{+}e^{-}$ and ep collisions for different charm-baryon species. These observations suggest that the charm fragmentation fractions are not universal and that the baryon-to-meson ratio depends on the collision systems. The results are compared to the predictions from Monte Carlo event generators and theoretical calculations.

• Time evolution of effective central charge and signatures of RG irreversibility after a quantum quench

  Axel Cortes Cubero

  SciPost Phys. 4, 016 (2018) · published 27 March 2018 | Toggle abstract · pdf

  At thermal equilibrium, the concept of effective central charge for massive deformations of two-dimensional conformal field theories (CFT) is well understood, and can be defined by comparing the partition function of the massive model to that of a CFT. This temperature-dependent effective charge interpolates monotonically between the central charge values corresponding to the IR and UV fixed points at low and high temperatures, respectively. We propose a non-equilibrium, time-dependent generalization of the effective central charge for integrable models after a quantum quench, $c_{\rm eff}(t)$, obtained by comparing the return amplitude to that of a CFT quench. We study this proposal for a large mass quench of a free boson, where the charge is seen to interpolate between $c_{\rm eff}=0$ at $t=0$, and $c_{\rm eff}\sim 1$ at $t\to\infty$, as is expected. We use our effective charge to define an "Ising to Tricritical Ising" quench protocol, where the charge evolves from $c_{\rm eff}=1/2$ at $t=0$, to $c_{\rm eff}=7/10$ at $t\to\infty$, the corresponding values of the first two unitary minimal CFT models. We then argue that the inverse "Tricritical Ising to Ising" quench is impossible with our methods. These conclusions can be generalized for quenches between any two adjacent unitary minimal CFT models. We finally study a large mass quench into the "staircase model" (sinh-Gordon with a particular complex coupling). At short times after the quench, the effective central charge increases in a discrete "staircase" structure, where the values of the charge at the steps can be computed in terms of the central charges of unitary minimal CFT models. When the initial state is a pure state, one always finds that $c_{\rm eff}(t\to\infty)\geq c_{\rm eff}(t=0)$, though $c_{\rm eff}(t)$, generally oscillates at finite times. We explore how this constraint may be related to RG flow irreversibility.

• The full coverage approach to the detection of Extensive Air Showers

  Giuseppe Di Sciascio

  SciPost Phys. Proc. 13, 019 (2023) · published 28 September 2023 | Toggle abstract · pdf

  A shower array exploiting the full coverage approach with a high segmentation of the readout allow to image the front of atmospheric showers with unprecedented resolution and detail. The grid distance determines the energy threshold (small energy showers are lost in the gap between detectors) and the quality of the shower sampling. Therefore, this experimental solution is needed to detect showers with a threshold in the 100 GeV range. The full coverage approach has been exploited in the ARGO-YBJ experiment. In this contribution we will summarise the advantages of this technique and discuss possible applications in new wide field of view detectors.

• Dynamically restoring conformal invariance in (integrable) $\sigma$-models

  Rigers Aliaj, Konstantinos Sfetsos, Konstantinos Siampos

  SciPost Phys. 14, 043 (2023) · published 21 March 2023 | Toggle abstract · pdf

  Integrable $\lambda$-deformed $\sigma$-models are characterized by an underlying current algebra/coset model CFT deformed, at the infinitesimal level, by current/parafermion bilinears. We promote the deformation parameters to dynamical functions of time introduced as an extra coordinate. It is conceivable that by appropriately constraining them, the beta-functions vanish and consequently the $\sigma$-model stays conformal. Remarkably, we explicitly materialize this scenario in several cases having a single and even multiple deformation parameters. These generically obey a system of non-linear second-order ordinary differential equations. They are solved by the fixed points of the RG flow of the original $\sigma$-model. Moreover, by appropriately choosing initial conditions we may even interpolate between the RG fixed points as the time varies from the far past to the far future. Finally, we present an extension of our analysis to the Yang--Baxter deformed PCMs.

• Multi-differential studies to explore strangeness enhancement in pp with ALICE at the LHC

  Francesca Ercolessi

  SciPost Phys. Proc. 10, 028 (2022) · published 11 August 2022 | Toggle abstract · pdf

  The study of energy and multiplicity dependence of strange hadron production in proton-proton collisions provides a powerful tool to understand similarities and differences between small and large collision systems. In order to better understand the origin of strangeness enhancement in pp new multi-differential analyses have been performed. The first separates the contribution of soft and hard processes, such as jets, to strange hadron production through two-particle correlation techniques. The second exploits the concept of the effective energy available for particle production in the event, which is estimated by an anti-correlation with the energy deposited in ALICE's Zero Degree Calorimeters. The results indicate that strangeness production emerges from the growth of the underlying event and confirm it is related to the final state multiplicity.

• Mechanically-driven stem cell separation in tissues caused by proliferating daughter cells

  Johannes C. Krämer, Edouard Hannezo, Gerhard Gompper, Jens Elgeti

  SciPost Phys. 16, 097 (2024) · published 8 April 2024 | Toggle abstract · pdf

  The homeostasis of epithelial tissue relies on a balance between the self-renewal of stem cell populations, cellular differentiation, and loss. Although this balance needs to be tightly regulated to avoid pathologies, such as tumor growth, the regulatory mechanisms, both cell-intrinsic and collective, which ensure tissue steady-state are still poorly understood. Here, we develop a computational model that incorporates basic assumptions of stem cell renewal into distinct populations and mechanical interactions between cells. We find that the model generates unexpected dynamic features: stem cells repel each other in the bulk tissue and are thus found rather isolated, as in a number of in vivo contexts. By mapping the system onto a gas of passive Brownian particles with effective repulsive interactions, that arise from the generated flows of differentiated cells, we show that we can quantitatively describe such stem cell distribution in tissues. The interaction potential between a pair of stem cells decays exponentially with a characteristic length that spans several cell sizes, corresponding to the volume of cells generated per stem cell division. Our findings may help understanding the dynamics of normal and cancerous epithelial tissues.

• Measuring the attenuation length of muon number in the air shower with muon detectors of 3/4 LHAASO array

  Xiaoting Feng, Hengying Zhang, Cunfeng Feng, Lingling Ma on behalf of the LHAASO Collaboration

  SciPost Phys. Proc. 13, 017 (2023) · published 28 September 2023 | Toggle abstract · pdf

  LHAASO KM2A consists of 5915 scintillation detectors and 1188 muon detectors, and the muon detectors cover 4% area of the whole array with 30 m interdetector spacing. The muon number in air shower events, with very high energy, is investigated with the data recorded by muon detector of the 3/4 LHAASO array in 2021. The attenuation length of muon number in the air shower is measured by fitting the muon number with constant flux in various zenith angles, based on the constant intensity cut method. The variation of the attenuation length as shower energy from hundreds TeV to tens PeV is presented. The results of simulation also is presented for comparing.

• Classifying irreducible fixed points of five scalar fields in perturbation theory

  Junchen Rong, Slava Rychkov

  SciPost Phys. 16, 040 (2024) · published 1 February 2024 | Toggle abstract · pdf

  Classifying perturbative fixed points near upper critical dimensions plays an important role in understanding the space of conformal field theories and critical phases of matter. In this work, we consider perturbative fixed points of N=5 scalar bosons coupled with quartic interactions preserving an arbitrary subgroup $G\subset O(5)$. We perform an exhaustive algorithmic search over the symmetry groups G which are irreducible and satisfy the Landau condition, so that the fixed point can be reached by fine-tuning a single mass term and there is no need to tune the cubic couplings. We also impose stability of the RG flow in the space of quartic couplings, and reality. We thus prove that there exist no new stable fixed points in d=4-$\epsilon$ dimensions beyond the two known ones: namely the O(5) nvariant fixed point and the Cubic(5) fixed point. This work is a continuation of the classification of such fixed points with N=4 scalars by Toledano, Michel, Toledano and Brézin in 1985 [Phys. Rev. B 31, 7171 (1985)].