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In many real-world decision problems there is partially observed, hidden or latent information that remains fixed throughout an interaction. Such decision problems can be modeled as Latent Markov Decision Processes (LMDPs), where a latent variable is selected at the beginning of an interaction and is not disclosed to the agent. In the last decade, there has been significant progress in solving LMDPs under different structural assumptions. However, for general LMDPs, there is no known learning algorithm that provably matches the existing lower bound (Kwon et al., 2021). We introduce the first sample-efficient algorithm for LMDPs without any additional structural assumptions. Our result builds off a new perspective on the role of off-policy evaluation guarantees and coverage coefficients in LMDPs, a perspective, that has been overlooked in the context of exploration in partially observed environments. Specifically, we establish a novel off-policy evaluation lemma and introduce a new coverage coefficient for LMDPs. Then, we show how these can be used to derive near-optimal guarantees of an optimistic exploration algorithm. These results, we believe, can be valuable for a wide range of interactive learning problems beyond LMDPs, and especially, for partially observed environments. 

A<sc>bstract</sc> The azimuthal anisotropy of particles associated with jets (jet particles) at midrapidity is measured for the first time in p-Pb and Pb-Pb collisions at$$ \sqrt{{\textrm{s}}_{\textrm{NN}}} $$ $\sqrt{s_{\mathrm{NN}}}$= 5.02 TeV down to transverse momentum (p_T) of 0.5 GeV/cand 2 GeV/c, respectively, with ALICE. The results obtained in p-Pb collisions are based on a novel three-particle correlation technique. The azimuthal anisotropy coefficientv₂in high-multiplicity p-Pb collisions is positive, with a significance reaching 6.8σat lowp_T, and its magnitude is smaller than in semicentral Pb-Pb collisions. In contrast to the measurements in Pb-Pb collisions, thev₂coefficient is also found independent ofp_Twithin uncertainties. Comparisons with the inclusive charged-particlev₂and with AMPT calculations are discussed. The predictions suggest that parton interactions play an important role in generating a non-zero jet-particlev₂in p-Pb collisions, even though they overestimate the reported measurement. These observations shed new insights on the understanding of the origin of the collective behaviour of jet particles in small systems such as p-Pb collisions, and provide significant stringent new constraints to models. 

Abstract The ALICE experiment was proposed in 1993, to study strongly-interacting matter at extreme energy densities and temperatures. This proposal entailed a comprehensive investigation of nuclear collisions at the LHC. Its physics programme initially focused on the determination of the properties of the quark–gluon plasma (QGP), a deconfined state of quarks and gluons, created in such collisions. The ALICE physics programme has been extended to cover a broader ensemble of observables related to Quantum Chromodynamics (QCD), the theory of strong interactions. The experiment has studied Pb–Pb, Xe–Xe, p–Pb and pp collisions in the multi-TeV centre of mass energy range, during the Run 1–2 data-taking periods at the LHC (2009–2018). The aim of this review is to summarise the key ALICE physics results in this endeavor, and to discuss their implications on the current understanding of the macroscopic and microscopic properties of strongly-interacting matter at the highest temperatures reached in the laboratory. It will review the latest findings on the properties of the QGP created by heavy-ion collisions at LHC energies, and describe the surprising QGP-like effects in pp and p–Pb collisions. Measurements of few-body QCD interactions, and their impact in unraveling the structure of hadrons and hadronic interactions, will be discussed. ALICE results relevant for physics topics outside the realm of QCD will also be touched upon. Finally, prospects for future measurements with the ALICE detector in the context of its planned upgrades will also be briefly described. 

Recent measurements of charm-baryon production in hadronic collisions have questioned the universality of charm-quark fragmentation across different collision systems. In this work the fragmentation of charm quarks into charm baryons is probed, by presenting the first measurement of the longitudinal jet momentum fraction carried by ${\mathrm{\Lambda }}_c^{+}$baryons, $z_{\parallel }^{\mathrm{ch}}$, in hadronic collisions. The results are obtained in proton-proton ( $pp$) collisions at $\sqrt{s}=13\mathrm{TeV}$at the LHC, with ${\mathrm{\Lambda }}_c^{+}$baryons and charged (track-based) jets reconstructed in the transverse momentum intervals of $3\le p_T^{{\mathrm{\Lambda }}_c^{+}}<15\mathrm{GeV}/c$and $7\le p_T^{\text{jet ch}}<15\mathrm{GeV}/c$, respectively. The $z_{\parallel }^{\mathrm{ch}}$distribution is compared to a measurement of ${\mathrm{D}}^0$-tagged charged jets in $pp$collisions as well as to 8 simulations. The data hints that the fragmentation of charm quarks into charm baryons is softer with respect to charm mesons, in the measured kinematic interval, as predicted by hadronization models which include color correlations beyond leading-color in the string formation. © 2024 CERN, for the ALICE Collaboration2024CERN 

The production of the $\psi \left(2S\right)$charmonium state was measured with ALICE in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$, in the dimuon decay channel. A significant signal was observed for the first time at LHC energies down to zero transverse momentum, at forward rapidity ( $2.5<y<4$). The measurement of the ratio of the inclusive production cross sections of the $\psi \left(2S\right)$and $\mathrm{J}/\psi$resonances is reported as a function of the centrality of the collisions and of transverse momentum, in the region $p_T<12\mathrm{GeV}/c$. The results are compared with the corresponding measurements in $pp$collisions, by forming the double ratio $[{\sigma }^{\psi \left(2S\right)}/{\sigma }^{J/\psi }{]}_{\mathrm{Pb}\text{−}\mathrm{Pb}}/[{\sigma }^{\psi \left(2S\right)}/{\sigma }^{J/\psi }{]}_{pp}$. It is found that in Pb-Pb collisions the $\psi \left(2S\right)$is suppressed by a factor of $\sim 2$with respect to the $J/\psi$. The $\psi \left(2S\right)$nuclear modification factor $R_{\mathrm{AA}}$was also obtained as a function of both centrality and $p_T$. The results show that the $\psi \left(2S\right)$resonance yield is strongly suppressed in Pb-Pb collisions, by a factor of up to $\sim 3$with respect to $pp$. Comparisons of cross section ratios with previous Super Proton Synchrotron findings by the NA50 experiment and of $R_{\mathrm{AA}}$with higher- $p_T$results at LHC energy are also reported. These results and the corresponding comparisons with calculations of transport and statistical models address questions on the presence and properties of charmonium states in the quark-gluon plasma formed in nuclear collisions at the LHC. © 2024 CERN, for the ALICE Collaboration2024CERN 

A<sc>bstract</sc> The fractions of non-prompt (i.e. originating from beauty-hadron decays) D⁰and D⁺mesons with respect to the inclusive yield are measured as a function of the charged-particle multiplicity in proton-proton collisions at a centre-of-mass energy of$$ \sqrt{s} $$ $\sqrt{s}$= 13 TeV with the ALICE detector at the LHC. The results are reported in intervals of transverse momentum (p_T) and integrated in the range 1< p_T<24 GeV/c. The fraction of non-prompt D⁰and D⁺mesons is found to increase slightly as a function ofp_Tin all the measured multiplicity intervals, while no significant dependence on the charged-particle multiplicity is observed. In order to investigate the production and hadronisation mechanisms of charm and beauty quarks, the results are compared to PYTHIA 8 as well as EPOS 3 and EPOS 4 Monte Carlo simulations, and to calculations based on the colour glass condensate including three-pomeron fusion. 

Context. Molecular filaments and hubs have received special attention recently thanks to new studies showing their key role in star formation. While the (column) density and velocity structures of both filaments and hubs have been carefully studied, their magnetic field (B-field) properties have yet to be characterized. Consequently, the role of B-fields in the formation and evolution of hub-filament systems is not well constrained. Aims. We aim to understand the role of the B-field and its interplay with turbulence and gravity in the dynamical evolution of the NGC 6334 filament network that harbours cluster-forming hubs and high-mass star formation. Methods. We present new observations of the dust polarized emission at 850 μ m toward the 2 pc × 10 pc map of NGC 6334 at a spatial resolution of 0.09 pc obtained with the James Clerk Maxwell Telescope (JCMT) as part of the B-field In STar-forming Region Observations (BISTRO) survey. We study the distribution and dispersion of the polarized intensity ( PI ), the polarization fraction ( PF ), and the plane-of-the-sky B-field angle ( χ B_POS ) toward the whole region, along the 10 pc-long ridge and along the sub-filaments connected to the ridge and the hubs. We derived the power spectra of the intensity and χ B POS along the ridge crest and compared them with the results obtained from simulated filaments. Results. The observations span ~3 orders of magnitude in Stokes I and PI and ~2 orders of magnitude in PF (from ~0.2 to ~ 20%). A large scatter in PI and PF is observed for a given value of I . Our analyses show a complex B-field structure when observed over the whole region (~ 10 pc); however, at smaller scales (~1 pc), χ B POS varies coherently along the crests of the filament network. The observed power spectrum of χ B POS can be well represented with a power law function with a slope of − 1.33 ± 0.23, which is ~20% shallower than that of I . We find that this result is compatible with the properties of simulated filaments and may indicate the physical processes at play in the formation and evolution of star-forming filaments. Along the sub-filaments, χ B POS rotates frombeing mostly perpendicular or randomly oriented with respect to the crests to mostly parallel as the sub-filaments merge with the ridge and hubs. This variation of the B-field structure along the sub-filaments may be tracing local velocity flows of infalling matter in the ridge and hubs. Our analysis also suggests a variation in the energy balance along the crests of these sub-filaments, from magnetically critical or supercritical at their far ends to magnetically subcritical near the ridge and hubs. We also detect an increase in PF toward the high-column density ( N H 2 ≳ 10 23  cm −2 ) star cluster-forming hubs. These latter large PF values may be explained by the increase in grain alignment efficiency due to stellar radiation from the newborn stars, combined with an ordered B-field structure. Conclusions. These observational results reveal for the first time the characteristics of the small-scale (down to ~ 0.1 pc) B-field structure of a 10 pc-long hub-filament system. Our analyses show variations in the polarization properties along the sub-filaments that may be tracing the evolution of their physical properties during their interaction with the ridge and hubs. We also detect an impact of feedback from young high-mass stars on the local B-field structure and the polarization properties, which could put constraints on possible models for dust grain alignment and provide important hints as to the interplay between the star formation activity and interstellar B-fields.