ALICE Apparatus Research Articles

Measurement of the low-energy antitriton inelastic cross section

In this Letter, the first measurement of the inelastic cross section for antitriton–nucleus interactions is reported, covering the momentum range of 0.8≤p<2.4GeV/c. The measurement is carried out using data recorded with the ALICE detector in pp and Pb–Pb collisions at a centre-of-mass energy per nucleon of 13 TeV and 5.02 TeV, respectively. The detector material serves as an absorber for antitriton nuclei. The raw yield of (anti)triton nuclei measured with the ALICE apparatus is compared to the results from detailed ALICE simulations based on the ▪ toolkit for the propagation of (anti)particles through matter, allowing one to quantify the inelastic interaction probability in the detector material. This analysis complements the measurement of the inelastic cross section of antinuclei up to A=3 carried out by the ALICE Collaboration, and demonstrates the feasibility of the study of the isospin dependence of inelastic interaction cross section with the analysis techniques presented in this Letter.

ALICE at the LHC and astrophysics: From providing input for the search for antinuclei in space to constraining the equation of state of neutron stars

The ALICE experiment at the LHC was originally designed to study the creation of the quark-gluon-plasma in heavy-ion collisions, a new state of matter in which quarks and gluons are not confined into hadrons. More recently, the versatility of both the Large Hadron Collider and the ALICE apparatus have given additionally access to a unique set of nuclear and hadronic physics measurements. The main results from this new and dynamic research area will be presented. The application of these findings to astrophysical challenges, ranging from the equation of state of neutron stars to the search for dark matter in space, will be discussed.

Mathrm {K_S}^{0}- and (anti-)Lambda -hadron correlations in pp collisions at\xa0{sqrt{s}} = 13\xa0TeV

Two-particle Azimuthal correlations are measured with the ALICE apparatus in pp collisions at sqrt{s} = 13 TeV to explore strangeness- and multiplicity-related effects in the fragmentation of jets and the transition regime between bulk and hard production, probed with the condition that a strange meson (mathrm {K_S}^{0}) or baryon (Lambda ) with transverse momentum p_{mathrm T} >3 GeV/c is produced. Azimuthal correlations between kaons or Lambda hyperons with other hadrons are presented at midrapidity for a broad range of the trigger (3< p_mathrm {T}^mathrm {trigg} < 20 GeV/c) and associated particle p_{mathrm T} (1 GeV/c< p_mathrm {T}^mathrm {assoc} < p_mathrm {T}^mathrm {trigg} ), for minimum-bias events and as a function of the event multiplicity. The near- and away-side peak yields are compared for the case of either mathrm {K_S}^{0} or Lambda ({overline{Lambda }}) being the trigger particle with that of inclusive hadrons (a sample dominated by pions). In addition, the measurements are compared with predictions from PYTHIA 8 and EPOS LHC event generators.

Centrality dependence of J/\u03c8 and \u03c8(2S) production and nuclear modification in p-Pb collisions at sqrt{s_{mathrm{NN}}} = 8.16 TeV

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 sqrt{s_{mathrm{NN}}} = 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 leftlangle {p}_{mathrm{T}}^2rightrangle 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 sqrt{s_{mathrm{NN}}} = 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.

Light-flavour hadron production vs. multiplicity in pp and in p–Pb collisions with ALICE

The study of light-flavour hadron production plays an important role in understanding the dynamics of the strongly-interacting system created in high-multiplicity pp and p–Pb collisions at the LHC energies. Historically, it has been assumed that no QGP is formed in small collision systems, but this paradigm is challenged by the observation of collective-like behaviour and strangeness enhancement in high-multiplicity pp and p–Pb collisions. Exploiting its optimal particle identification capabilities, the ALICE experiment is able to measure pion, kaon, (anti-)proton, strange and multi-strange hadron production in a wide transverse momentum range. We report on the study of light-flavour hadron production as a function of multiplicity and center-of-mass energy in pp and p–Pb collisions using the ALICE apparatus. In addition, new multi-differential measurements are presented, comparing identified particle production in the underlying event to the one in jets.

Highlights of the production of (anti-)(hyper-)nuclei and exotica with ALICE at the LHC

Measurements of the production of light (anti-)nuclei and (anti-)hypernuclei have been performed in different colliding systems at the LHC, namely pp, p–Pb and Pb–Pb. The results of the production in Pb–Pb collisions of light (anti-)(hyper-)nuclei follows the expectation of the thermal model, whereas the production of nuclei in pp and p–Pb collisions shows a closer agreement with the expectations from coalescence models. Both models can give predictions for the production yields of hypothetical states such as bound states of two Λ hyperons or of a Λ and a neutron, which are expected to decay weakly. These states are searched for with the ALICE apparatus. Since no signal is found, upper limits, which are significantly below the model expectations, are set. Furthermore, the most recent measurement of the lifetime of the hypertriton determined in Pb–Pb collisions at at the LHC gives a value exactly between the world average and the free lifetime. In addition, the new result is in agreement with other recent measurements in heavy-ion collisions.

PID performance of the High Momentum Particle IDentification (HMPID) detector during LHC-Run 2

Abstract The ALICE apparatus is devoted collecting proton–proton (pp), proton–nucleus (p–A) and nucleus–nucleus (A–A) collision data provided by the LHC, to study the properties of strongly interacting matter under extremely high temperature and energy density conditions. In ALICE, the momentum of a track is evaluated by exploiting a solenoid magnetic field of 0.5 T. Among the ALICE PID detectors, the HMPID (High Momentum Particle Identification Detector) is devoted to the identification of charged hadrons . It consists of seven identical RICH (Ring Imaging Cherenkov) counters, with liquid C 6 F14 as Cherenkov radiator (n ≈ 1.298 at λ ph = 175 nm). Cherenkov photons and charged particles are detected by a MWPC (Multi Wire Proportional Chamber), equipped with a pad-segmented CsI coated photo-cathode. The HMPID provides 3 σ K– π and p–K separation up to p T = 3 and 5 GeV/ c , respectively. The detector performance depends on the experimental conditions, such as the event multiplicity and the intensity of the solenoid magnetic field. During the LHC Run 2 data-taking period (2015–2017), the HMPID collected data from different colliding systems with a magnetic field intensity B = 0.5 T and pp data with B = 0.2 T were also recorded. A review of the detector PID performance during the LHC Run 2 data-taking period will be shown. The contribution provided, so far, by the HMPID to the ALICE physics measurements, performed with the LHC Run 2 data, will also be presented.

Transverse momentum spectra and nuclear modification factors of charged particles in Xe–Xe collisions at [formula omitted

Transverse momentum (pT ) spectra of charged particles at mid-pseudorapidity in Xe–Xe collisions at sNN=5.44TeV measured with the ALICE apparatus at the Large Hadron Collider are reported. The kinematic range 0.15<pT<50GeV/c and |η|<0.8 is covered. Results are presented in nine classes of collision centrality in the 0–80% range. For comparison, a pp reference at the collision energy of s=5.44 TeV is obtained by interpolating between existing pp measurements at s=5.02 and 7 TeV. The nuclear modification factors in central Xe–Xe collisions and Pb–Pb collisions at a similar center-of-mass energy of sNN=5.02 TeV, and in addition at 2.76 TeV, at analogous ranges of charged particle multiplicity density 〈dNch/dη〉 show a remarkable similarity at pT>10GeV/c. The centrality dependence of the ratio of the average transverse momentum 〈pT〉 in Xe–Xe collisions over Pb–Pb collision at s=5.02 TeV is compared to hydrodynamical model calculations.

Extended analysis of the Trojan-horse attack in quantum key distribution

The discrete-variable QKD protocols based on BB84 are known to be secure against an eavesdropper, Eve, intercepting the flying qubits and performing any quantum operation on them. However, these protocols may still be vulnerable to side-channel attacks. We investigate the Trojan-Horse side-channel attack where Eve sends her own state into Alice's apparatus and measures the reflected state to estimate the key. We prove that the separable coherent state is optimal for Eve amongst the class of multi-mode Gaussian attack states. We describe how Alice may defend against this by adding thermal noise to the system and give an analytic expression of the resulting secret key rate. We also provide a bound on the secret key rate in the case where Eve may use any separable state, and describe an active defense system based on optical modulators.

Measurement of azimuthal correlations of D mesons with charged particles in pp collisions at sqrt{s}=7\xa0TeV and p\u2013Pb collisions at {sqrt{{{s}}_{scriptscriptstyle {mathrm{NN}}}}}=5.02\xa0TeV

The azimuthal correlations of D mesons with charged particles were measured with the ALICE apparatus in pp collisions at {sqrt{s}}=7~mathrm {TeV} and p–Pb collisions at {sqrt{{{s}}_{scriptscriptstyle {mathrm{NN}}}}}=5.02~mathrm {TeV} at the Large Hadron Collider. mathrm{D}^{0}, {mathrm{D}^{+}}, and {mathrm{D}^{*+}} mesons and their charge conjugates with transverse momentum 3<{p}_{mathrm{T}}<16~mathrm {GeV}/c and rapidity in the nucleon-nucleon centre-of-mass system |y_mathrm{cms}|<0.5 (pp collisions) and -0.96<y_mathrm{cms}<0.04 (p–Pb collisions) were correlated to charged particles with {p}_{mathrm{T}}>0.3~mathrm {GeV}/c. The yield of charged particles in the correlation peak induced by the jet containing the D meson and the peak width are compatible within uncertainties in the two collision systems. The data are described within uncertainties by Monte-Carlo simulations based on PYTHIA, POWHEG, and EPOS 3 event generators.

ϕ-Meson production at forward rapidity in p–Pb collisions at [formula omitted] and in pp collisions at [formula omitted

The first study of ϕ-meson production in p–Pb collisions at forward and backward rapidity, at a nucleon–nucleon centre-of-mass energy sNN=5.02 TeV, has been performed with the ALICE apparatus at the LHC. The ϕ-mesons have been identified in the dimuon decay channel in the transverse momentum (pT) range 1<pT<7 GeV/c, both in the p-going (2.03<y<3.53) and the Pb-going (−4.46<y<−2.96) directions — where y stands for the rapidity in the nucleon–nucleon centre-of-mass — the integrated luminosity amounting to 5.01±0.19 nb−1 and 5.81±0.20 nb−1, respectively, for the two data samples. Differential cross sections as a function of transverse momentum and rapidity are presented. The forward–backward ratio for ϕ-meson production is measured for 2.96<|y|<3.53, resulting in a ratio ∼0.5 with no significant pT dependence within the uncertainties. The pT dependence of the ϕ nuclear modification factor RpPb exhibits an enhancement up to a factor 1.6 at pT=3–4 GeV/c in the Pb-going direction. The pT dependence of the ϕ-meson cross section in pp collisions at s=2.76 TeV, which is used to determine a reference for the p–Pb results, is also presented here for 1<pT<5 GeV/c and 2.5<y<4, for a 78±3 nb−1 integrated luminosity sample.

The High Momentum Particle IDentification (HMPID) detector PID performance and its contribution to the ALICE physics program

The ALICE apparatus is dedicated to study the properties of strongly interacting matter under extremely high temperature and energy density conditions. For this, enhanced particle identification (PID) capabilities are required. Among the PID ALICE detectors, the ALICE-HMPID (High Momentum Particle IDentification) detector is devoted to the identification of charged hadrons, exploiting the Cherenkov effect. It consists of seven identical RICH modules, with liquid C6F14 as Cherenkov radiator (n ≈1.298 at λ=175nm). Photon and charged particle detection is performed by a MWPC, coupled with a pad segmented CsI coated photo-cathode. The total CsI active area is 10.3m2. The HMPID provides 3 sigma separation for pions and kaons up to pT=3GeV/c and for kaons and (anti-)protons up to pT=5GeV/c. A review of the HMPID PID performance, in particular in the challenging central Pb–Pb collisions, and its contribution to the ALICE physics program, using the LHC RUN1 (2010–2013) and RUN2 (2015) data, are presented.

Low-mass dimuon measurements in pp, p-Pb and Pb-Pb collisions with ALICE at the LHC

Low-mass dimuon production, including light vector mesons ρ, ω, ϕ, provides key information on the hot and dense state of strongly interacting matter produced in high-energy heavy-ion collisions. In particular, strangeness production can be studied via ϕ meson measurements, while the detailed description of the full dimuon mass spectrum down to the kinematic threshold can be used to reveal in-medium modifications of hadron properties and the thermal emission arising from the medium. Measurements in pp and p-A systems, in absence of hot nuclear matter effects, must be used as a reference to test our knowledge of the processes expected to contribute to dilepton production. Dimuon production is studied with the ALICE apparatus at the LHC at forward rapidity (2.5 < y < 4) with the Muon Spectrometer. In this contribution, results on low-mass dimuon production are shown, for various center-of-mass energies per nucleon pair, in pp, p-Pb, and Pb-Pb collisions.

Light (hyper-)nuclei production at the LHC measured with ALICE

The high center-of-mass energies delivered by the LHC during the last three years of operation led to accumulate a significant statistics of light (hyper-)nuclei in pp, p--Pb and Pb--Pb collisions. The ALICE apparatus allows for the detection of these rarely produced particles over a wide momentum range thanks to its excellent vertexing, tracking and particle identification capabilities. The last is based on the specific energy loss in the Time Projection Chamber and the velocity measurement with the Time-Of-Flight detector. The Cherenkov technique, exploited by a small acceptance detector (HMPID), has also been used for the most central Pb--Pb collisions to identify (anti-)deuterons at intermediate transverse momentum. Results on the production of stable nuclei and anti-nuclei in pp, p--Pb and Pb--Pb collisions are presented. Hypernuclei production rates in Pb--Pb are also described, together with a measurement of the hypertriton lifetime. The results are compared with the predictions from thermal and coalescence models. Moreover the results on the search for weakly-decaying light exotic states, such as the $\Lambda\Lambda$ (H-dibaryon) and the $\Lambda$-neutron bound states are discussed.

Light (hyper-)nuclei production at the LHC measured with ALICE

The high center-of-mass energies delivered by the LHC during the last three years of operation led to accumulate a significant statistics of light (hyper-)nuclei in pp, p–Pb and Pb–Pb collisions. The ALICE apparatus allows for the detection of these rarely produced particles over a wide momentum range thanks to its excellent vertexing, tracking and particle identification capabilities. The last is based on the specific energy loss in the Time Projection Chamber and the velocity measurement with the Time-Of-Flight detector. The Cherenkov technique, exploited by a small acceptance detector (HMPID), has also been used for the most central Pb–Pb collisions to identify (anti-)deuterons at intermediate transverse momentum.Results on the production of stable nuclei and anti-nuclei in pp, p–Pb and Pb–Pb collisions are presented. Hypernuclei production rates in Pb–Pb are also described, together with a measurement of the hypertriton lifetime. The results are compared with the predictions from thermal and coalescence models. Moreover the results on the search for weakly-decaying light exotic states, such as the ΛΛ (H-dibaryon) and the Λ-neutron bound states are discussed.

Exotica production with ALICE

The high collision energies reached at the LHC lead to significant production yields of light (anti-)nuclei and (hyper-)nuclei in proton–proton, proton–lead and, in particular, lead–lead collisions. The excellent particle identification capabilities of the ALICE apparatus, based on the specific energy loss in the Time Projection Chamber and the velocity information in the Time-Of-Flight detector, allow for the detection of these rarely produced particles. Further, the Inner Tracking System gives the possibility to separate primary nuclei from those coming from weak decay of heavier systems. One example of such a weak decay is the measurement of the (anti-)hypertriton decay to 3 He + π − (3 He + π + ). The aforementioned capabilities of the ALICE apparatus offer the unique opportunity to search for exotica, like the bound state of a Λ and a neutron which would decay into a deuteron and a pion, or the bound state of two Λ’s. Results on the production of stable nuclei in Pb–Pb collisions at √s NN = 2.76 TeV are presented, and compared with thermal model predictions. We further present the current status of the searches, by their upper limits on the production yields, and compare the results to thermal and coalescence model expectations.

Low-mass dilepton production with ALICE at the LHC

The production of low-mass dileptons, including those from ρ, ω and ϕ light vector meson decays, provides key information on the hot and dense state of strongly interacting matter produced in high-energy heavy-ion collisions. In particular, strangeness production can be studied through ϕ meson measurements, while the detailed description of the full dilepton mass spectra down to the kinematic threshold can be used to reveal in-medium modifications of hadron properties and the thermal emission arising from the medium. Measurements in pp and p–A, respectively, provide a reference for the observations in heavy-ion collisions and give insight on soft particle production in cold nuclear matter. Dilepton production is studied with the ALICE apparatus at the LHC both at central (|y|<0.9) and forward (2.5<y<4) rapidity, respectively in the dielectron and dimuon channel. In this contribution, we present recent results on low-mass dimuon production at forward rapidity in Pb–Pb and p–Pb collisions.

Pattern recognition and PID procedure with the ALICE-HMPID

The ALICE apparatus is dedicated to the study of pp, p–Pb and Pb–Pb collisions provided by LHC. ALICE has unique particle identification (PID) capabilities among the LHC experiments exploiting different PID techniques, i.e., energy loss, time-of-flight measurements, Cherenkov and transition radiation detection, calorimetry and topological ID. The ALICE-HMPID is devoted to the identification of charged hadrons. It consists of seven identical RICH counters, with liquid C6F14 as Cherenkov radiator (n≈1.299 at λph=175nm). Photons and charged particles detection is performed by a proportional chamber, coupled with a pad segmented CsI coated photo-cathode. In pp and p–Pb events HMPID provides 3 sigmas separation for pions and kaons up to pT=3GeV/c and for protons up to pT=5GeV/c. PID is performed by means of photon emission angle measurement, a challenging task in the high multiplicity environment of the most central Pb–Pb collisions. A dedicated algorithm has been implemented to evaluate the Cherenkov angle starting from the bi-dimensional ring pattern on the photons detector, it is based on the Hough Transform Method (HTM) to separate signal from background. In this way HMPID is able to contribute to inclusive hadrons spectra measurement as well as to measurements where high purity PID is required, by means of statistical or track-by-track PID. The pattern recognition, the results from angular resolution studies and the PID strategy with HMPID are presented.

Low mass vector meson production in pp and Pb-Pb collisions measured with ALICE at the LHC

Vector mesons are key probes of the hot and dense state of strongly interacting matter produced in heavy ion collisions. Their dileptonic decay channel is particularly suitable for these studies, since dileptons have negligible final state interactions in hadronic matter. In particular, strangeness production can be studied through the measurement of muon pairs coming from the decay of ϕ mesons. The ALICE apparatus at the LHC can access vector mesons produced at forward rapidity (2.5 < y < 4) through their decays in muon pairs. We present results on vector meson production in pp and Pb-Pb collisions at = 2.76 TeV. The ϕ meson production cross section is measured in pp collisions. In Pb-Pb collisions, a measurement of the ϕ/(ρ + ω) ratio and of the 0 nuclear modification factor as a function of the collision centrality is obtained.

Upsilon Production in Pb-Pb and p-Pb Collisions at Forward Rapidity with ALICE at the LHC

The ALICE apparatus at the LHC was designed and built to perform dedicated studies of the Quark-Gluon Plasma (QGP), a strongly interacting phase of QCD matter, expected to be created in heavy-ion collisions, where quarks and gluons are deconfined. In such collisions heavy flavors are produced at the very early stage of the interaction by the initial hard scattering processes and hence can be used to characterize the hot and dense medium. In particular the sequential suppression of quarkonia (charmonia and bottomonia) was proposed as a thermometer of the deconfined medium. The inclusive ϒ(1S) production has been measured down to zero transverse momentum in its dimuon decay channel at forward rapidity (2.5 < ylab < 4.0) using the muon spectrometer. Here results on the ϒ(1S) nuclear modification factor (RAA) in Pb-Pb collisions at = 2.76 TeV are discussed and compared to the measurement at mid-rapidity by the CMS Collaboration and to theoretical predictions. Also recent results on RpPb and forward-to-backward yield ratio (RFB) in p-Pb collisions at = 5.02 TeV are discussed.