kb/data/en.wikipedia.org/wiki/ALICE_experiment-5.md

title	chunk	source	category	tags	date_saved	instance
ALICE experiment	6/7	https://en.wikipedia.org/wiki/ALICE_experiment	reference	science, encyclopedia	2026-05-05T13:02:56.303258+00:00	kb-cron

the study of the thermalization of partons in the quark-gluon plasma (QGP), with focus on the massive charm and beauty quarks and understanding the behavior of these heavy quarks in relation to the strongly coupled medium of QGP the study of the mechanisms of energy loss that occur in the QGP medium and the dependencies of energy loss on the parton species the dissociation of quarkonium states, which can be a probe of deconfinement and of the temperature of the medium the production of thermal photons and low-mass dileptons emitted by the QGP, which allows for assessing the initial temperature and degrees of freedom of the systems as well as the chiral nature of the phase transition. The ALICE collaboration presented its first results from LHC proton collisions at a centre-of-mass energy of 7 TeV in March 2010. The results confirmed that the charged-particle multiplicity is rising with energy faster than expected while the shape of the multiplicity distribution is not reproduced well by standard simulations. The results were based on the analysis of a sample of 300,000 proton–proton collisions the ALICE experiment collected during the first runs of the LHC with stable beams at a centre-of-mass energy of 7 TeV. In 2011, the ALICE Collaboration measured the size of the system created in Pb-Pb collisions at a centre-of-mass energy of 2.76 TeV per nucleon pair. ALICE confirmed that the QCD matter created in Pb-Pb collisions behaves like a fluid, as expected by hydrodynamic equations.

=== A perfect liquid at the LHC === Off-center nuclear collisions, with a finite impact parameter, create a very asymmetric "almond-shaped" fireball. However, experiments cannot measure the spatial dimensions of the interaction (except in special cases, for example in the production of pions). Instead, they measure the momentum distributions of the emitted particles. A correlation between the measured azimuthal momentum distribution of particles emitted from the decaying fireball and the initial spatial asymmetry can arise only from multiple interactions between the constituents of the created matter; that is, the distribution provides information about how the matter flows, which is related to its equation of state and its thermodynamic transport properties. The measured azimuthal distribution of particles in momentum space can be decomposed into Fourier coefficients. The second Fourier coefficient, called elliptic flow, is particularly sensitive to the internal friction or viscosity of the fluid, or more precisely, η/s, the ratio of the shear viscosity (η) to entropy (s) of the system. For a "thin" liquid such as water, the η/s ratio is small. A "thick" liquid, such as honey, has large values of η/s. In heavy-ion collisions at the LHC, the ALICE collaboration found that the hot matter created in the collision behaves like a fluid with little friction, with η/s close to its lower limit, that is, almost zero viscosity.

=== Measuring the highest temperature on Earth === In August 2012, ALICE scientists announced that their experiments produced quark–gluon plasma with temperatures of around 5.5 trillion kelvin (470 MeV), the highest temperature matter achieved in any physical experiments thus far. This temperature is about 38% higher than the previous record of about 4 trillion kelvin (340 MeV), achieved in the 2010 experiments at the Brookhaven National Laboratory. The ALICE results were announced at the August 13 Quark Matter 2012 conference in Washington, D.C. The quark–gluon plasma produced by these experiments approximates the conditions in the universe that existed microseconds after the Big Bang, before matter coalesced into atoms.

=== Energy loss === A basic process in QCD is the energy loss of a fast parton in a medium composed of color charges. This phenomenon, "jet quenching", is especially useful in the study of QGP, using the naturally occurring jets produced from the hard scattering of quarks and gluons from the incoming nuclei. A highly energetic parton (a color charge) probes the colored medium similarly to how an X-ray probes ordinary matter. The production of these partonic probes in hadronic collisions is well understood within perturbative QCD. The theory also shows that a parton traversing the medium will lose a fraction of its energy by emitting many low energy gluons. The amount of radiated energy is proportional to the density of the medium and to the square of the path length travelled by the parton in the medium. Theory also predicts that the energy loss depends on the flavor of the parton. Jet quenching was first observed at RHIC by measuring the yield rates of hadrons with high transverse momentum. These particles are produced via fragmentation of energetic partons. The yields of these high transverse momentum particles in central nucleus–nucleus collisions were found from the measurements in proton–proton reactions to be a factor of five lower than expected. ALICE has recently published measurements of charged particles in central heavy-ion collisions at the LHC. As at RHIC, the production of high transverse momentum hadrons at the LHC is significantly lower than expected. However, the observations at the LHC show qualitatively new features. The observation from ALICE is consistent with reports from the ATLAS and CMS collaborations on direct evidence for parton energy loss within heavy-ion collisions. These reports are based on fully reconstructed back-to-back jets of particles associated with hard parton scatterings. The latter two experiments have shown a strong energy imbalance between the jet and its recoiling partner. This imbalance is thought to arise because one of the jets traversed the hot and dense matter, transferring a substantial fraction of its energy to the medium in a way that is not recovered by the reconstruction of the jets.

=== Studying quarkonium hadroproduction ===