ALICE mission

The ALICE Collaboration has built a detector optimized to study the collisions of nuclei at the ultra-relativistic energies provided by the LHC. The aim is to study the physics of strongly interacting matter at the highest energy densities reached so far in the laboratory. In such conditions, an extreme phase of matter - called the quark-gluon plasma - is formed. Our universe is thought to have been in such a primordial state for the first few millionths of a second after the Big Bang, before quarks and gluons were bound together to form protons and neutrons. Recreating this primordial state of matter in the laboratory and understanding how it evolves will allow us to shed light on questions about how matter is organized and the mechanisms that confine quarks and gluons. For this purpose, we are carrying out a comprehensive study of the hadrons, electrons, muons, and photons produced in the collisions of heavy nuclei (208Pb). ALICE is also studying proton-proton and proton-nucleus collisions both as a comparison with nucleus-nucleus collisions and in their own right. In 2021 ALICE is completing a significant upgrade of its detectors to further enhance its capabilities and continue its scientific journey at the LHC for many years to come.

Recent highlights

 

Recent highlights

The ALICE collaboration at the Large Hadron Collider (LHC) has made the first direct observation of the dead-cone effect – a fundamental feature of the theory of the strong force that binds quarks and gluons together into protons, neutrons, and, ultimately, all atomic nuclei. In addition to confirming this effect, the observation, reported in a paper published in Nature, provides direct experimental access to the mass of a single charm quark before it is confined inside hadrons. Read more...
The ALICE Collaboration reports a wide range of new physics results at the Large Hadron Collider Physics conference (LHCP2022) this week (16-21 May 2022). The status of the upgraded detector commissioning for the LHC Run 3, prospects for further upgrades for Run 4, and the completely new heavy-ion detector ALICE 3 for Run 5 and beyond are also presented. The ALICE Collaboration participates in LHCP2022 with 6 plenary, 17 parallel and 11 poster presenters. The plenary presentation on 16 May reports the highlights of new ALICE measurements and the status of the experiment. A selection of these topics is presented in the following sections. Read more...
The ALICE Collaboration reports a wide range of new physics results at the Quark Matter 2022 conference this week (4-10 April 2022). The status of the upgraded detector commissioning for the LHC Run 3, prospects for further upgrades for Run 4, and the completely new heavy-ion detector ALICE 3 for Run 5 and beyond are also presented. The ALICE Collaboration participates in Quark Matter 2022 with 1 plenary, 35 parallel, and 94 poster presenters. Read more...

Latest ALICE Submissions

Dielectron production at midrapidity at low transverse momentum in peripheral and semi-peripheral Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeVThe first measurement of the ${\rm e}^{+}{\rm e}^{-}$ pair production at low lepton pair transverse momentum ($p_{\rm T,ee}$) and low invariant mass ($m_{\rm ee}$) in non-central Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV at the LHC is presented. The dielectron production is studied with the ALICE detector at midrapidity ($|\eta_{\rm e}| < ~ 0.8$) as a function of invariant mass ($0.4 \leq m_{\rm ee} < ~ 2.7$ GeV/$c^2$) in the 50$-$70% and 70$-$90% centrality classes for $p_{\rm T,ee} < ~ 0.1$ GeV/$c$, and as a function of $p_{\rm T,ee}$ in three $m_{\rm ee}$ intervals in the most peripheral Pb$-$Pb collisions. Below a $p_{\rm T,ee}$ of 0.1 GeV/$c$, a clear excess of ${\rm e}^{+}{\rm e}^{-}$ pairs is found compared to the expectations from known hadronic sources and predictions of thermal radiation from the medium. The $m_{\rm ee}$ excess spectra are reproduced, within uncertainties, by different predictions of the photon$-$photon production of dielectrons, where the photons originate from the extremely strong electromagnetic fields generated by the highly Lorentz-contracted Pb nuclei. Lowest-order quantum electrodynamic (QED) calculations, as well as a model that takes into account the impact-parameter dependence of the average transverse momentum of the photons, also provide a good description of the $p_{\rm T,ee}$ spectra. The measured $\sqrt{\langle p_{\rm T,ee}^{2} \rangle}$ of the excess $p_{\rm T,ee}$ spectrum in peripheral Pb$-$Pb collisions is found to be comparable to the values observed previously at RHIC in a similar phase-space region.
2204.11732
W$^\pm$-boson production in p$-$Pb collisions at $\sqrt{s_{NN}} = 8.16$ TeV and PbPb collisions at $\sqrt{s_{NN}} = 5.02$ TeVThe production of the W$^\pm$ bosons measured in p$-$Pb collisions at a centre-of-mass energy per nucleon$-$nucleon collision $\sqrt{s_{NN}} = 8.16$ TeV and Pb$-$Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV with ALICE at the LHC is presented. The W$^\pm$ bosons are measured via their muonic decay channel, with the muon reconstructed in the pseudorapidity region $-4 < ~ \eta^\mu_{\rm lab} < ~ -2.5$ with transverse momentum $p_{\rm T}^\mu > 10$ GeV/$c$. While in Pb$-$Pb collisions the measurements are performed in the forward ($2.5 < ~ y^\mu_{\rm cms} < ~ 4$) rapidity region, in p$-$Pb collisions, where the centre-of-mass frame is boosted with respect to the laboratory frame, the measurements are performed in the backward ($-4.46 < ~ y^\mu_{\rm cms} < ~ -2.96$) and forward ($2.03 < ~ y^\mu_{\rm cms} < ~ 3.53$) rapidity regions. The W$^{-}$ and W$^{+}$ production cross sections, lepton-charge asymmetry, and nuclear modification factors are evaluated as a function of the muon rapidity. In order to study the production as a function of the p$-$Pb collision centrality, the production cross sections of the W$^{-}$ and W$^{+}$ bosons are combined and normalised to the average number of binary nucleon$-$nucleon collision $\langle N_\mathrm{coll} \rangle$. In Pb$-$Pb collisions, the same measurements are presented as a function of the collision centrality. Study of the binary scaling of the W$^\pm$-boson cross sections in p$-$Pb and Pb$-$Pb collisions is also reported. The results are compared with perturbative QCD (pQCD) calculations, with and without nuclear modifications of the Parton Distribution Functions (PDFs), as well as with available data at the LHC. Significant deviations from the theory expectations are found in the two collision systems, indicating that the measurements can provide additional constraints for the determination of nuclear PDF (nPDFs) and in particular of the light-quark distributions.
2204.10640
Photoproduction of low-$p_{\rm T}$ J/$ψ$ from peripheral to central Pb$-$Pb collisions at 5.02 TeVAn excess of J/$\psi$ yield at very low transverse momentum ($p_{\rm T} < ~ 0.3$ GeV/$c$), originating from coherent photoproduction, is observed in peripheral and semicentral hadronic Pb$-$Pb collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{\rm NN}} = 5.02$ TeV. The measurement is performed with the ALICE detector via the dimuon decay channel at forward rapidity ($2.5 < ~y < ~4$). The nuclear modification factor at very low $p_{\rm T}$ and the coherent photoproduction cross section are measured as a function of centrality down to the 10% most central collisions. These results extend the previous study at $\sqrt{s_{\rm NN}} = 2.76$ TeV, confirming the clear excess over hadronic production in the $p_{\rm T}$ range 0$-$0.3 GeV/$c$ and the centrality range 70$-$90%, and establishing an excess with a significance greater than 5$\sigma$ also in the 50$-$70% and 30$-$50% centrality ranges. The results are compared with earlier measurements at $\sqrt{s_{\rm NN}} = 2.76$ TeV and with different theoretical predictions aiming at describing how coherent photoproduction occurs in hadronic interactions with nuclear overlap.
2204.10684
System-size dependence of the charged-particle pseudorapidity density at $\sqrt{s_{\rm NN}} = 5.02$ TeV for pp, p-Pb, and Pb-Pb collisionsWe present and compare the charged-particle pseudorapidity densities for pp, p-Pb, and Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV measured over a wide pseudorapidity range (${-3.5 < ~\eta < ~5}$), using ALICE at the Large Hadron Collider. The distributions for p-Pb and Pb-Pb collisions are determined as a function of the centrality of the collisions, while results from pp collisions are reported for inelastic events with at least one charged particle at midrapidity. The charged-particle pseudorapidity densities are, under simple and robust assumptions, transformed to charged-particle rapidity densities. This allows for the calculation and the presentation of the evolution of the width of the rapidity distributions and of a lower bound on the Bjorken energy density, as a function of the number of participants in all three collision systems. We find a decreasing width of the particle production, and roughly a ten fold increase in the energy density, as the system size grows.
2204.10210
First measurement of antideuteron number fluctuations at energies available at the Large Hadron ColliderThe first measurement of event-by-event antideuteron number fluctuations in high energy heavy-ion collisions is presented. The measurements are carried out at midrapidity ($|\eta| < ~ 0.8$) as a function of collision centrality in Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV using the ALICE detector. A significant negative correlation between the produced antiprotons and antideuterons is observed in all collision centralities. The results are compared with coalescence calculations, which fail to describe the measurement, in particular if a correlated production of protons and neutrons is assumed. Thermal-statistical model calculations describe the data within uncertainties only for correlation volumes that are different with respect to those describing proton yields and a similar measurement of net-proton number fluctuations.
2204.10166
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Upcoming Conferences (Next Week)

Diversity and Inclusivity in ALICE

The ALICE Collaboration embraces and values the diversity of its team members and colleagues. We are committed to fostering an inclusive environment for all people regardless of their nationality/culture, profession, age/generation, family situation and gender, as well as individual differences such as but not limited to ethnic origin, sexual orientation, belief, disability, or opinions provided that they are consistent with the Organization’s values.

News of cards

ALICE DCS measured the effects of Tonga volcano eruption (and confirmed the speed of sound)

The Fast Interaction Trigger is the final piece of the puzzle of ALICE’s LS2 sub-detector installations.

The two barrels of the largest pixel detector ever built have been successfully lowered into the cavern and stand ready for commissioning.

The new ITS Outer Barrel was installed in March 2021.

The new Muon Forward Tracker, one of ALICE’s main sub-detectors, was installed in the cavern in December 2020.