| 1. |
- Chauvin, G., et al.
(author)
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Discovery of a warm, dusty giant planet around HIP 65426
- 2017
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 605
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Journal article (peer-reviewed)abstract
- Aims. The SHINE program is a high-contrast near-infrared survey of 600 young, nearby stars aimed at searching for and characterizing new planetary systems using VLT/SPHERE's unprecedented high-contrast and high-angular-resolution imaging capabilities. It is also intended to place statistical constraints on the rate, mass and orbital distributions of the giant planet population at large orbits as a function of the stellar host mass and age to test planet-formation theories.Methods. We used the IRDIS dual-band imager and the IFS integral field spectrograph of SPHERE to acquire high-contrast coronagraphic differential near-infrared images and spectra of the young A2 star HIP 65426. It is a member of the similar to 17 Myr old Lower Centaurus-Crux association. Results. At a separation of 830 mas (92 au projected) from the star, we detect a faint red companion. Multi-epoch observations confirm that it shares common proper motion with HIP 65426. Spectro-photometric measurements extracted with IFS and IRDIS between 0.95 and 2.2 mu m indicate a warm, dusty atmosphere characteristic of young low-surface-gravity L5-L7 dwarfs. Hot-start evolutionary models predict a luminosity consistent with a 6-12 M-Jup, T-eff = 1300-1600K and R = 1.5 +/- 0.1 R-Jup giant planet. Finally, the comparison with Exo-REM and PHOENIX BT-Settl synthetic atmosphere models gives consistent effective temperatures but with slightly higher surface gravity solutions of log(g) = 4.0-5.0 with smaller radii (1.0-1.3 R-Jup).Conclusions. Given its physical and spectral properties, HIP 65426 b occupies a rather unique placement in terms of age, mass, and spectral-type among the currently known imaged planets. It represents a particularly interesting case to study the presence of clouds as a function of particle size, composition, and location in the atmosphere, to search for signatures of non-equilibrium chemistry, and finally to test the theory of planet formation and evolution.
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| 2. |
- Grandjean, A., et al.
(author)
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Constraining the properties of HD 206893 B A combination of radial velocity, direct imaging, and astrometry data
- 2019
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 627
-
Journal article (peer-reviewed)abstract
- Context. High contrast imaging enables the determination of orbital parameters for substellar companions (planets, brown dwarfs) from the observed relative astrometry and the estimation of model and age-dependent masses from their observed magnitudes or spectra. Combining astrometric positions with radial velocity gives direct constraints on the orbit and on the dynamical masses of companions. A brown dwarf was discovered with the VLT/SPHERE instrument at the Very Large Telescope (VLT) in 2017, which orbits at similar to 11 au around HD 206893. Its mass was estimated between 12 and 50 M-J from evolutionary models and its photometry. However, given the significant uncertainty on the age of the system and the peculiar spectrophotometric properties of the companion, this mass is not well constrained. Aims. We aim at constraining the orbit and dynamical mass of HD 206893 B. Methods. We combined radial velocity data obtained with HARPS spectra and astrometric data obtained with the high contrast imaging VLT/SPHERE and VLT/NaCo instruments, with a time baseline less than three years. We then combined those data with astrometry data obtained by HIPPARCOS and Gaia with a time baseline of 24 yr. We used a Markov chain Monte Carlo approach to estimate the orbital parameters and dynamical mass of the brown dwarf from those data. Results. We infer a period between 21 and 33 yr and an inclination in the range 20-41 degrees from pole-on from HD 206893 B relative astrometry. The RV data show a significant RV drift over 1.6 yr. We show that HD 206893 B cannot be the source of this observed RV drift as it would lead to a dynamical mass inconsistent with its photometry and spectra and with HIPPARCOS and Gaia data. An additional inner (semimajor axis in the range 1.4-2.6 au) and massive (similar to 15 M-J) companion is needed to explain the RV drift, which is compatible with the available astrometric data of the star, as well as with the VLT/SPHERE and VLT/NaCo nondetection.
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| 3. |
- Keppler, M., et al.
(author)
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Discovery of a planetary-mass companion within the gap of the transition disk around PDS 70
- 2018
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 617
-
Journal article (peer-reviewed)abstract
- Context. Young circumstellar disks are the birthplaces of planets. Their study is of prime interest to understand the physical and chemical conditions under which planet formation takes place. Only very few detections of planet candidates within these disks exist, and most of them are currently suspected to be disk features.Aims. In this context, the transition disk around the young star PDS 70 is of particular interest, due to its large gap identified in previous observations, indicative of ongoing planet formation. We aim to search for the presence of an embedded young planet and search for disk structures that may be the result of disk-planet interactions and other evolutionary processes.Methods. We analyse new and archival near-infrared images of the transition disk PDS 70 obtained with the VLT/SPHERE, VLT/NaCo, and Gemini/NICI instruments in polarimetric differential imaging and angular differential imaging modes.Results. We detect a point source within the gap of the disk at about 195 mas (similar to 22 au) projected separation. The detection is confirmed at five different epochs, in three filter bands and using different instruments. The astrometry results in an object of bound nature, with high significance. The comparison of the measured magnitudes and colours to evolutionary tracks suggests that the detection is a companion of planetary mass. The luminosity of the detected object is consistent with that of an L-type dwarf, but its IR colours are redder, possibly indicating the presence of warm surrounding material. Further, we confirm the detection of a large gap of similar to 54 au in size within the disk in our scattered light images, and detect a signal from an inner disk component. We find that its spatial extent is very likely smaller than similar to 17 au in radius, and its position angle is consistent with that of the outer disk. The images of the outer disk show evidence of a complex azimuthal brightness distribution which is different at different wavelengths and may in part be explained by Rayleigh scattering from very small grains.Conclusions. The detection of a young protoplanet within the gap of the transition disk around PDS 70 opens the door to a so far observationally unexplored parameter space of planetary formation and evolution. Future observations of this system at different wavelengths and continuing astrometry will allow us to test theoretical predictions regarding planet-disk interactions, planetary atmospheres, and evolutionary models.
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| 4. |
- Langlois, M., et al.
(author)
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The SPHERE infrared survey for exoplanets (SHINE) : II. Observations, data reduction and analysis, detection performances, and initial results
- 2021
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 651
-
Journal article (peer-reviewed)abstract
- Context. In recent decades, direct imaging has confirmed the existence of substellar companions (exoplanets or brown dwarfs) on wide orbits (>10 au) around their host stars. In striving to understand their formation and evolution mechanisms, in 2015 we initiated the SPHERE infrared survey for exoplanets (SHINE), a systematic direct imaging survey of young, nearby stars that is targeted at exploring their demographics.Aims. We aim to detect and characterize the population of giant planets and brown dwarfs beyond the snow line around young, nearby stars. Combined with the survey completeness, our observations offer the opportunity to constrain the statistical properties (occurrence, mass and orbital distributions, dependency on the stellar mass) of these young giant planets.Methods. In this study, we present the observing and data analysis strategy, the ranking process of the detected candidates, and the survey performances for a subsample of 150 stars that are representative of the full SHINE sample. Observations were conducted in a homogeneous way between February 2015 and February 2017 with the dedicated ground-based VLT/SPHERE instrument equipped with the IFS integral field spectrograph and the IRDIS dual-band imager, covering a spectral range between 0.9 and 2.3 μm. We used coronographic, angular, and spectral differential imaging techniques to achieve the best detection performances for this study, down to the planetary mass regime.Results. We processed, in a uniform manner, more than 300 SHINE observations and datasets to assess the survey typical sensitivity as a function of the host star and of the observing conditions. The median detection performance reached 5σ-contrasts of 13 mag at 200 mas and 14.2 mag at 800 mas with the IFS (YJ and YJH bands), and of 11.8 mag at 200 mas, 13.1 mag at 800 mas, and 15.8 mag at 3 as with IRDIS in H band, delivering one of the deepest sensitivity surveys thus far for young, nearby stars. A total of sixteen substellar companions were imaged in this first part of SHINE: seven brown dwarf companions and ten planetary-mass companions.These include two new discoveries, HIP 65426 b and HIP 64892 B, but not the planets around PDS70 that had not been originally selected for the SHINE core sample. A total of 1483 candidates were detected, mainly in the large field of view that characterizes IRDIS. The color-magnitude diagrams, low-resolution spectrum (when available with IFS), and follow-up observations enabled us to identify the nature (background contaminant or comoving companion) of about 86% of our subsample. The remaining cases are often connected to crowded-field follow-up observations that were missing. Finally, even though SHINE was not initially designed for disk searches, we imaged twelve circumstellar disks, including three new detections around the HIP 73145, HIP 86598, and HD 106906 systems.Conclusions. Nowadays, direct imaging provides a unique opportunity to probe the outer part of exoplanetary systems beyond 10 au to explore planetary architectures, as highlighted by the discoveries of: one new exoplanet, one new brown dwarf companion, and three new debris disks during this early phase of SHINE. It also offers the opportunity to explore and revisit the physical and orbital properties of these young, giant planets and brown dwarf companions (relative position, photometry, and low-resolution spectrum in near-infrared, predicted masses, and contrast in order to search for additional companions). Finally, these results highlight the importance of finalizing the SHINE systematic observation of about 500 young, nearby stars for a full exploration of their outer part to explore the demographics of young giant planets beyond 10 au and to identify the most interesting systems for the next generation of high-contrast imagers on very large and extremely large telescopes.
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| 5. |
- Mesa, D., et al.
(author)
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Limits on the presence of planets in systems with debris discs : HD92945 and HD107146
- 2021
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 503:1, s. 1276-1289
-
Journal article (peer-reviewed)abstract
- Recent observations of resolved cold debris discs at tens of au have revealed that gaps could be a common feature in these Kuiper-belt analogues. Such gaps could be evidence for the presence of planets within the gaps or closer in near the edges of the disc. We present SPHERE observations of HD 92945 and HD 107146, two systems with detected gaps. We constrained the mass of possible companions responsible for the gap to 1–2 MJup for planets located inside the gap and to less than 5 MJup for separations down to 20 au from the host star. These limits allow us to exclude some of the possible configurations of the planetary systems proposed to explain the shape of the discs around these two stars. In order to put tighter limits on the mass at very short separations from the star, where direct-imaging data are less effective, we also combined our data with astrometric measurements from Hipparcos and Gaia and radial-velocity measurements. We were able to limit the separation and the mass of the companion potentially responsible for the proper-motion anomaly of HD 107146 to values of 2–7 au and 2–5 MJup, respectively.
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| 6. |
- Vigan, A., et al.
(author)
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The SPHERE infrared survey for exoplanets (SHINE) : III. The demographics of young giant exoplanets below 300 au with SPHERE
- 2021
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 651
-
Journal article (peer-reviewed)abstract
- The SpHere INfrared Exoplanet (SHINE) project is a 500-star survey performed with SPHERE on the Very Large Telescope for the purpose of directly detecting new substellar companions and understanding their formation and early evolution. Here we present an initial statistical analysis for a subsample of 150 stars spanning spectral types from B to M that are representative of the full SHINE sample. Our goal is to constrain the frequency of substellar companions with masses between 1 and 75 MJup and semimajor axes between 5 and 300 au. For this purpose, we adopt detection limits as a function of angular separation from the survey data for all stars converted into mass and projected orbital separation using the BEX-COND-hot evolutionary tracks and known distance to each system. Based on the results obtained for each star and on the 13 detections in the sample, we use a Markov chain Monte Carlo tool to compare our observations to two different types of models. The first is a parametric model based on observational constraints, and the second type are numerical models that combine advanced core accretion and gravitational instability planet population synthesis. Using the parametric model, we show that the frequencies of systems with at least one substellar companion are 23.0−9.7+13.5, 5.8−2.8+4.7, and 12.6−7.1+12.9% for BA, FGK, and M stars, respectively. We also demonstrate that a planet-like formation pathway probably dominates the mass range from 1–75 MJup for companions around BA stars, while for M dwarfs, brown dwarf binaries dominate detections. In contrast, a combination of binary star-like and planet-like formation is required to best fit the observations for FGK stars. Using our population model and restricting our sample to FGK stars, we derive a frequency of 5.7−2.8+3.8%, consistent with predictions from the parametric model. More generally, the frequency values that we derive are in excellent agreement with values obtained in previous studies.
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| 7. |
- Acharya, S., et al.
(author)
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Accessing the strong interaction between Λ baryons and charged kaons with the femtoscopy technique at the LHC
- 2023
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In: Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics. - 0370-2693. ; 845
-
Journal article (peer-reviewed)abstract
- The interaction between Λ baryons and kaons/antikaons is a crucial ingredient for the strangeness S=0 and S=−2 sector of the meson–baryon interaction at low energies. In particular, the ΛK‾ might help in understanding the origin of states such as the Ξ(1620), whose nature and properties are still under debate. Experimental data on Λ–K and Λ–K‾ systems are scarce, leading to large uncertainties and tension between the available theoretical predictions constrained by such data. In this Letter we present the measurements of Λ–K⊕+Λ‾–K− and Λ–K⊕−Λ‾–K+ correlations obtained in the high-multiplicity triggered data sample in pp collisions at s=13 TeV recorded by ALICE at the LHC. The correlation function for both pairs is modeled using the Lednický–Lyuboshits analytical formula and the corresponding scattering parameters are extracted. The Λ–K⊕−Λ‾–K+ correlations show the presence of several structures at relative momenta k⁎ above 200 MeV/c, compatible with the Ω baryon, the Ξ(1690), and Ξ(1820) resonances decaying into Λ–K− pairs. The low k⁎ region in the Λ–K⊕−Λ‾–K+ also exhibits the presence of the Ξ(1620) state, expected to strongly couple to the measured pair. The presented data allow to access the ΛK+ and ΛK− strong interaction with an unprecedented precision and deliver the first experimental observation of the Ξ(1620) decaying into ΛK−. © 2023 The Author(s)
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| 8. |
- Acharya, S., et al.
(author)
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ALICE luminosity determination for Pb-Pb collisions at √ s NN= 5.02 TeV
- 2024
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In: Journal of Instrumentation. - 1748-0221. ; 19:2
-
Journal article (peer-reviewed)abstract
- Luminosity determination within the ALICE experiment is based on the measurement, in van der Meer scans, of the cross sections for visible processes involving one or more detectors (visible cross sections). In 2015 and 2018, the Large Hadron Collider provided Pb-Pb collisions at a centre-of-mass energy per nucleon pair of √s NN= 5.02 TeV. Two visible cross sections, associated with particle detection in the Zero Degree Calorimeter (ZDC) and in the V0 detector, were measured in a van der Meer scan. This article describes the experimental set-up and the analysis procedure, and presents the measurement results. The analysis involves a comprehensive study of beam-related effects and an improved fitting procedure, compared to previous ALICE studies, for the extraction of the visible cross section. The resulting uncertainty of both the ZDC-based and the V0-based luminosity measurement for the full sample is 2.5%. The inelastic cross section for hadronic interactions in Pb-Pb collisions at √s NN= 5.02 TeV, obtained by efficiency correction of the V0-based visible cross section, was measured to be 7.67 ± 0.25 b, in agreement with predictions using the Glauber model. © 2024 Institute of Physics. All rights reserved.
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| 9. |
- Acharya, S., et al.
(author)
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Anisotropic flow and flow fluctuations of identified hadrons in Pb–Pb collisions at √sNN = 5.02 TeV
- 2023
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In: Journal of High Energy Physics. - 1029-8479. ; 2023:5
-
Journal article (peer-reviewed)abstract
- The first measurements of elliptic flow of π ±, K ±, p + p ¯ , KS0 , Λ + Λ ¯ , ϕ, Ξ −+ Ξ ¯ + , and Ω −+ Ω ¯ + using multiparticle cumulants in Pb–Pb collisions at sNN = 5.02 TeV are resented. Results obtained with two- (v 2{2}) and four-particle cumulants (v 2{4}) are shown as a function of transverse momentum, p T, for various collision centrality intervals. Combining the data for both v 2{2} and v 2{4} also allows us to report the first measurements of the mean elliptic flow, elliptic flow fluctuations, and relative elliptic flow fluctuations for various hadron species. These observables probe the event-by-event eccentricity fluctuations in the initial state and the contributions from the dynamic evolution of the expanding quark–gluon plasma. The characteristic features observed in previous p T-differential anisotropic flow measurements for identified hadrons with two-particle correlations, namely the mass ordering at low p T and the approximate scaling with the number of constituent quarks at intermediate p T, are similarly present in the four-particle correlations and the combinations of v 2{2} and v 2{4}. In addition, a particle species dependence of flow fluctuations is observed that could indicate a significant contribution from final state hadronic interactions. The comparison between experimental measurements and CoLBT model calculations, which combine the various physics processes of hydrodynamics, quark coalescence, and jet fragmentation, illustrates their importance over a wide p T range. [Figure not available: see fulltext.]. © 2023, The Author(s).
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| 10. |
- Acharya, S, et al.
(author)
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Anisotropic flow of identified hadrons in Xe-Xe collisions at √sNN = 5.44 TeV
- 2021
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In: Journal of High Energy Physics. - 1029-8479. ; 2021:10
-
Journal article (peer-reviewed)abstract
- Measurements of elliptic (v2) and triangular (v3) flow coefficients of π±, K±, p+p ¯ , KS0, and Λ+Λ ¯ obtained with the scalar product method in Xe-Xe collisions at sNN = 5.44 TeV are presented. The results are obtained in the rapidity range |y| < 0.5 and reported as a function of transverse momentum, pT, for several collision centrality classes. The flow coefficients exhibit a particle mass dependence for pT< 3 GeV/c, while a grouping according to particle type (i.e., meson and baryon) is found at intermediate transverse momenta (3 < pT< 8 GeV/c). The magnitude of the baryon v2 is larger than that of mesons up to pT = 6 GeV/c. The centrality dependence of the shape evolution of the pT-differential v2 is studied for the various hadron species. The v2 coefficients of π±, K±, and p+p ¯ are reproduced by MUSIC hydrodynamic calculations coupled to a hadronic cascade model (UrQMD) for pT< 1 GeV/c. A comparison with vn measurements in the corresponding centrality intervals in Pb-Pb collisions at sNN = 5.02 TeV yields an enhanced v2 in central collisions and diminished value in semicentral collisions. [Figure not available: see fulltext.] © 2021, The Author(s).
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| 11. |
- Acharya, S, et al.
(author)
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(Anti-)deuteron production in pp collisions at √s=13TeV
- 2020
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In: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 80:9
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Journal article (peer-reviewed)abstract
- The study of (anti-)deuteron production in pp collisions has proven to be a powerful tool to investigate the formation mechanism of loosely bound states in high-energy hadronic collisions. In this paper the production of (anti-)deuterons is studied as a function of the charged particle multiplicity in inelastic pp collisions at s=13 TeV using the ALICE experiment. Thanks to the large number of accumulated minimum bias events, it has been possible to measure (anti-)deuteron production in pp collisions up to the same charged particle multiplicity (d Nch/ d η∼ 26) as measured in p–Pb collisions at similar centre-of-mass energies. Within the uncertainties, the deuteron yield in pp collisions resembles the one in p–Pb interactions, suggesting a common formation mechanism behind the production of light nuclei in hadronic interactions. In this context the measurements are compared with the expectations of coalescence and statistical hadronisation models (SHM). © 2020, The Author(s).
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| 12. |
- Acharya, S., et al.
(author)
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Antihelium-3 fluxes near Earth using data-driven estimates for annihilation cross section
- 2022
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In: 37th International Cosmic Ray Conference (ICRC2021) - GAI - Gamma Ray Indirect. - Trieste, Italy : Sissa Medialab. - 1824-8039. ; 395
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Conference paper (peer-reviewed)abstract
- Antinuclei found in cosmic rays could provide a smoking gun signal for dark matter as this signal is virtually background free. The study of 3He cosmic rays requires the knowledge of their production, propagation in the galaxy and annihilation cross-section. While the former two have been already estimated with data-driven methods, there were no experimental data available for the 3He inelastic cross section. We measured for the first time the inelastic cross section of 3He using the ALICE detector itself as a target. To study the effect of 3He annihilation in the galaxy and estimate the transparency of the galaxy, the 3He source functions and annihilation cross sections were implemented in GALPROP. © Copyright owned by the author(s) under the terms of the Creative Commons.
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| 13. |
- Acharya, S., et al.
(author)
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Azimuthal correlations of heavy-flavor hadron decay electrons with charged particles in pp and p–Pb collisions at √sNN = 5.02 TeV
- 2023
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In: European Physical Journal C. - 1434-6044. ; 83:8
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Journal article (peer-reviewed)abstract
- The azimuthal (Δ φ) correlation distributions between heavy-flavor decay electrons and associated charged particles are measured in pp and p–Pb collisions at sNN=5.02 TeV. Results are reported for electrons with transverse momentum 4 < pT< 16 GeV / c and pseudorapidity | η| < 0.6 . The associated charged particles are selected with transverse momentum 1 < pT< 7 GeV / c , and relative pseudorapidity separation with the leading electron | Δ η| < 1 . The correlation measurements are performed to study and characterize the fragmentation and hadronization of heavy quarks. The correlation structures are fitted with a constant and two von Mises functions to obtain the baseline and the near- and away-side peaks, respectively. The results from p–Pb collisions are compared with those from pp collisions to study the effects of cold nuclear matter. In the measured trigger electron and associated particle kinematic regions, the two collision systems give consistent results. The Δ φ distribution and the peak observables in pp and p–Pb collisions are compared with calculations from various Monte Carlo event generators. © 2023, The Author(s).
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| 14. |
- Acharya, S, et al.
(author)
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Azimuthal correlations of prompt D mesons with charged particles in pp and p–Pb collisions at √sNN=5.02TeV
- 2020
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In: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 80:10
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Journal article (peer-reviewed)abstract
- The measurement of the azimuthal-correlation function of prompt D mesons with charged particles in pp collisions at s=5.02TeV and p–Pb collisions at sNN=5.02TeV with the ALICE detector at the LHC is reported. The D , D +, and D ∗ + mesons, together with their charge conjugates, were reconstructed at midrapidity in the transverse momentum interval 30.3GeV/c and pseudorapidity | η| < 0.8. The properties of the correlation peaks appearing in the near- and away-side regions (for Δ φ≈ 0 and Δ φ≈ π, respectively) were extracted via a fit to the azimuthal correlation functions. The shape of the correlation functions and the near- and away-side peak features are found to be consistent in pp and p–Pb collisions, showing no modifications due to nuclear effects within uncertainties. The results are compared with predictions from Monte Carlo simulations performed with the PYTHIA, POWHEG+PYTHIA, HERWIG, and EPOS 3 event generators. © 2020, The Author(s).
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| 15. |
- Acharya, S, et al.
(author)
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Calibration of the photon spectrometer PHOS of the ALICE experiment
- 2019
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In: Journal of Instrumentation. - 1748-0221. ; 14:May 2019
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Journal article (peer-reviewed)abstract
- The procedure for the energy calibration of the high granularity electromagnetic calorimeter PHOS of the ALICE experiment is presented. The methods used to perform the relative gain calibration, to evaluate the geometrical alignment and the corresponding correction of the absolute energy scale, to obtain the nonlinearity correction coefficients and finally, to calculate the time-dependent calibration corrections, are discussed and illustrated by the PHOS performance in proton-proton (pp) collisions at s=13 TeV. After applying all corrections, the achieved mass resolutions for π0 and η mesons for pT > 1.7 GeV/c are σm π0 = 4.56 ± 0.03 MeV/c2 and σm η = 15.3 ± 1.0 MeV/c2, respectively.
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| 16. |
- Acharya, S, et al.
(author)
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Centrality dependence of J/ψ and ψ(2S) production and nuclear modification in p-Pb collisions at √sNN = 8.16 TeV
- 2021
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In: Journal of High Energy Physics. - 1029-8479. ; 2021
-
Journal article (peer-reviewed)abstract
- The inclusive production of the J/ψ and ψ(2S) charmonium states is studied as a function of centrality in p-Pb collisions at a centre-of-mass energy per nucleon pair sNN = 8.16 TeV at the LHC. The measurement is performed in the dimuon decay channel with the ALICE apparatus in the centre-of-mass rapidity intervals −4.46 < ycms< −2.96 (Pb-going direction) and 2.03 < ycms< 3.53 (p-going direction), down to zero transverse momentum (pT). The J/ψ and ψ(2S) production cross sections are evaluated as a function of the collision centrality, estimated through the energy deposited in the zero degree calorimeter located in the Pb-going direction. The pT-differential J/ψ production cross section is measured at backward and forward rapidity for several centrality classes, together with the corresponding average 〈pT〉 and 〈pT2〉 values. The nuclear effects affecting the production of both charmonium states are studied using the nuclear modification factor. In the p-going direction, a suppression of the production of both charmonium states is observed, which seems to increase from peripheral to central collisions. In the Pb-going direction, however, the centrality dependence is different for the two states: the nuclear modification factor of the J/ψ increases from below unity in peripheral collisions to above unity in central collisions, while for the ψ(2S) it stays below or consistent with unity for all centralities with no significant centrality dependence. The results are compared with measurements in p-Pb collisions at sNN = 5.02 TeV and no significant dependence on the energy of the collision is observed. Finally, the results are compared with theoretical models implementing various nuclear matter effects. [Figure not available: see fulltext.] © 2021, The Author(s).
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| 17. |
- Acharya, S., et al.
(author)
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Characterizing the initial conditions of heavy-ion collisions at the LHC with mean transverse momentum and anisotropic flow correlations
- 2022
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In: Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 834
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Journal article (peer-reviewed)abstract
- Correlations between mean transverse momentum [pT] and anisotropic flow coefficients v2 or v3 are measured as a function of centrality in Pb–Pb and Xe–Xe collisions at sNN=5.02 TeV and 5.44 TeV, respectively, with ALICE. In addition, the recently proposed higher-order correlation between [pT], v2, and v3 is measured for the first time, which shows an anticorrelation for the presented centrality ranges. These measurements are compared with hydrodynamic calculations using IP-Glasma and TRENTo initial-state shapes, the former based on the Color Glass Condensate effective theory with gluon saturation, and the latter a parameterized model with nucleons as the relevant degrees of freedom. The data are better described by the IP-Glasma rather than the TRENTo based calculations. In particular, Trajectum and JETSCAPE predictions, both based on the TRENTo initial state model but with different parameter settings, fail to describe the measurements. As the correlations between [pT] and vn are mainly driven by the correlations of the size and the shape of the system in the initial state, these new studies pave a novel way to characterize the initial state and help pin down the uncertainty of the extracted properties of the quark–gluon plasma recreated in relativistic heavy-ion collisions. © 2022 The Author(s)
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| 18. |
- Acharya, S, et al.
(author)
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Charged-particle multiplicity fluctuations in Pb–Pb collisions at √sNN = 2.76 TeV
- 2021
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In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6052. ; 81:11
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Journal article (peer-reviewed)abstract
- Measurements of event-by-event fluctuations of charged-particle multiplicities in Pb–Pb collisions at sNN = 2.76 TeV using the ALICE detector at the CERN Large Hadron Collider (LHC) are presented in the pseudorapidity range | η| < 0.8 and transverse momentum 0.2 < pT< 2.0 GeV/c. The amplitude of the fluctuations is expressed in terms of the variance normalized by the mean of the multiplicity distribution. The η and pT dependences of the fluctuations and their evolution with respect to collision centrality are investigated. The multiplicity fluctuations tend to decrease from peripheral to central collisions. The results are compared to those obtained from HIJING and AMPT Monte Carlo event generators as well as to experimental data at lower collision energies. Additionally, the measured multiplicity fluctuations are discussed in the context of the isothermal compressibility of the high-density strongly-interacting system formed in central Pb–Pb collisions. © 2021, The Author(s).
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| 19. |
- Acharya, S, et al.
(author)
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Charged-particle production as a function of multiplicity and transverse spherocity in pp collisions at √s=5.02 and 13 TeV
- 2019
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In: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 79:10
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Journal article (peer-reviewed)abstract
- We present a study of the inclusive charged-particle transverse momentum (pT) spectra as a function of charged-particle multiplicity density at mid-pseudorapidity, d Nch/ d η, in pp collisions at s=5.02 and 13 TeV covering the kinematic range | η| < 0.8 and 0.15 < pT< 20 GeV/c. The results are presented for events with at least one charged particle in | η| < 1 (INEL>0). The pT spectra are reported for two multiplicity estimators covering different pseudorapidity regions. The pT spectra normalized to that for INEL>0 show little energy dependence. Moreover, the high-pT yields of charged particles increase faster than the charged-particle multiplicity density. The average pT as a function of multiplicity and transverse spherocity is reported for pp collisions at s=13 TeV. For low- (high-) spherocity events, corresponding to jet-like (isotropic) events, the average pT is higher (smaller) than that measured in INEL>0 pp collisions. Within uncertainties, the functional form of ⟨ pT⟩ (Nch) is not affected by the spherocity selection. While EPOS LHC gives a good description of many features of data, PYTHIA overestimates the average pT in jet-like events. © 2019, CERN for the benefit of the ALICE collaboration.
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| 20. |
- Acharya, S., et al.
(author)
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Charged-particle production as a function of the relative transverse activity classifier in pp, p–Pb, and Pb–Pb collisions at the LHC
- 2024
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In: Journal of High Energy Physics. - 1029-8479. ; 2024:1
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Journal article (peer-reviewed)abstract
- Measurements of charged-particle production in pp, p–Pb, and Pb–Pb collisions in the toward, away, and transverse regions with the ALICE detector are discussed. These regions are defined event-by-event relative to the azimuthal direction of the charged trigger particle, which is the reconstructed particle with the largest transverse momentum in the range 8 < < 15 GeV/c. The toward and away regions contain the primary and recoil jets, respectively; both regions are accompanied by the underlying event (UE). In contrast, the transverse region perpendicular to the direction of the trigger particle is dominated by the so-called UE dynamics, and includes also contributions from initial- and final-state radiation. The relative transverse activity classifier, , is used to group events according to their UE activity, where is the charged-particle multiplicity per event in the transverse region and is the mean value over the whole analysed sample. The energy dependence of the RT distributions in pp collisions at = 2.76, 5.02, 7, and 13 TeV is reported, exploring the Koba-Nielsen-Olesen (KNO) scaling properties of the multiplicity distributions. The first measurements of charged-particle pT spectra as a function of RT in the three azimuthal regions in pp, p–Pb, and Pb–Pb collisions at = 5.02 TeV are also reported. Data are compared with predictions obtained from the event generators PYTHIA 8 and EPOS LHC. This set of measurements is expected to contribute to the understanding of the origin of collective-like effects in small collision systems (pp and p–Pb).
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| 21. |
- Acharya, S, et al.
(author)
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Charged-particle pseudorapidity density at mid-rapidity in p–Pb collisions at √sNN = 8.16 TeV
- 2019
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In: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 79:4
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Journal article (peer-reviewed)abstract
- The pseudorapidity density of charged particles, d N ch / d η, in p–Pb collisions has been measured at a centre-of-mass energy per nucleon–nucleon pair of sNN = 8.16 TeV at mid-pseudorapidity for non-single-diffractive events. The results cover 3.6 units of pseudorapidity, | η| < 1.8. The d N ch / d η value is 19.1 ± 0.7 at | η| < 0.5. This quantity divided by ⟨ N part ⟩ / 2 is 4.73 ± 0.20 , where ⟨ N part ⟩ is the average number of participating nucleons, is 9.5% higher than the corresponding value for p–Pb collisions at sNN = 5.02 TeV. Measurements are compared with models based on different mechanisms for particle production. All models agree within uncertainties with data in the Pb-going side, while HIJING overestimates, showing a symmetric behaviour, and EPOS underestimates the p-going side of the d N ch / d η distribution. Saturation-based models reproduce the distributions well for η> - 1.3. The d N ch / d η is also measured for different centrality estimators, based both on the charged-particle multiplicity and on the energy deposited in the Zero-Degree Calorimeters. A study of the implications of the large multiplicity fluctuations due to the small number of participants for systems like p–Pb in the centrality calculation for multiplicity-based estimators is discussed, demonstrating the advantages of determining the centrality with energy deposited near beam rapidity. © 2019, CERN for the benefit of the ALICE collaboration.
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| 22. |
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| 23. |
- Acharya, S., et al.
(author)
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Closing in on critical net-baryon fluctuations at LHC energies: Cumulants up to third order in Pb–Pb collisions
- 2023
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In: Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 844
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Journal article (peer-reviewed)abstract
- Fluctuation measurements are important sources of information on the mechanism of particle production at LHC energies. This article reports the first experimental results on third-order cumulants of the net-proton distributions in Pb–Pb collisions at a center-of-mass energy sNN=5.02 TeV recorded by the ALICE detector. The results on the second-order cumulants of net-proton distributions at sNN=2.76 and 5.02 TeV are also discussed in view of effects due to the global and local baryon number conservation. The results demonstrate the presence of long-range rapidity correlations between protons and antiprotons. Such correlations originate from the early phase of the collision. The experimental results are compared with HIJING and EPOS model calculations, and the dependence of the fluctuation measurements on the phase-space coverage is examined in the context of lattice quantum chromodynamics (LQCD) and hadron resonance gas (HRG) model estimations. The measured third-order cumulants are consistent with zero within experimental uncertainties of about 4% and are described well by LQCD and HRG predictions. © 2022 The Author
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| 24. |
- Acharya, S, et al.
(author)
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Coherent J / ψ and ψ ′ photoproduction at midrapidity in ultra-peripheral Pb–Pb collisions at √sNN=5.02 TeV
- 2021
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In: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 81:8
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Journal article (peer-reviewed)abstract
- The coherent photoproduction of J/ψ and ψ′ mesons was measured in ultra-peripheral Pb–Pb collisions at a center-of-mass energy sNN−−−√ = 5.02 TeV with the ALICE detector. Charmonia are detected in the central rapidity region for events where the hadronic interactions are strongly suppressed. The J/ψ is reconstructed using the dilepton (l+l−) and proton–antiproton decay channels, while for the ψ′ the dilepton and the l+l−π+π− decay channels are studied. The analysis is based on an event sample corresponding to an integrated luminosity of about 233 μb−1. The results are compared with theoretical models for coherent J/ψ and ψ′ photoproduction. The coherent cross section is found to be in a good agreement with models incorporating moderate nuclear gluon shadowing of about 0.64 at a Bjorken-x of around 6×10−4, such as the EPS09 parametrization, however none of the models is able to fully describe the rapidity dependence of the coherent J/ψ cross section including ALICE measurements at forward rapidity. The ratio of ψ′ to J/ψ coherent photoproduction cross sections was also measured and found to be consistent with the one for photoproduction off protons.
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| 25. |
- Acharya, S, et al.
(author)
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Coherent J/ψ photoproduction at forward rapidity in ultra-peripheral Pb–Pb collisions at sNN=5.02 TeV
- 2019
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In: Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 798
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Journal article (peer-reviewed)abstract
- The ALICE collaboration performed the first rapidity-differential measurement of coherent J/ψ photoproduction in ultra-peripheral Pb–Pb collisions at a center-of-mass energy sNN=5.02 TeV. The J/ψ is detected via its dimuon decay in the forward rapidity region (−4.0
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