ALICE in Vienna: from antinuclei to quark–gluon plasma

25 September 2015

The 2015 edition of the European Physical Society Conference on High Energy Physics (EPS-HEP 2015), which took place in Vienna in July (“Vienna hosts a high-energy particle waltz”), provided an opportunity for the ALICE collaboration to present the latest results from analysis of data from Run 1 of the LHC. While many of the presentations centred on the properties of the quark–gluon plasma (QGP) as produced in the collisions of heavy ions, there was also an interesting glimpse of other kinds of physics that ALICE can investigate.


Once in a while in the heavy-ion collisions, a few protons and neutrons are created close enough in phase space such that they coalesce into a nucleus. The heavier the nucleus (the larger the number of nucleons), the lower the probability that it is created, but about once in 10 thousand events, for example, a 3He nucleus can be created and detected within ALICE’s tracking and particle-identification set-up. Moreover, the lead–ion collisions at the LHC also provide a copious source of antiparticles, such that nuclei and the corresponding antinuclei are produced at nearly equal rates.

This allows ALICE to make a detailed comparison of the properties of the nuclei and antinuclei that are most abundantly produced. At EPS-HEP 2015, the collaboration presented a new limit on the conservation in nucleon–nucleon interactions of CPT symmetry – the fundamental symmetry that implies that all of the laws of physics are the same under the simultaneous reversal of charges (charge conjugation, C), reflection of spatial co-ordinates (parity transformation, P) and time inversion (T). The new test of CPT invariance was extracted from measurements of the mass-to-electric-charge ratios of the deuteron/antideuteron and the 3He/3He nuclei. The combined results of the difference of the mass-over-charge ratio for each pair of the nucleus/antinucleus species allowed the extraction of differences in their relative binding energies. The measurements, published in Nature Physics, confirm CPT invariance to an unprecedented precision in the sector of light nuclei (ALICE Collaboration 2015).

The strongly interacting hot and dense matter, the QGP, produced in heavy-ion collisions is characterized by the smallest ratio of sheer viscosity to entropy density of all known materials – a substance that flows almost as a perfect liquid. This QGP is a system of quarks and gluons where the mean free path is very short – a so-called strongly coupled system. A parton traversing such a medium, even a highly energetic one, is exposed to the medium and loses part of its energy. The new measurements by ALICE presented at EPS-HEP 2015 indicate that the heavier charm and beauty quarks also lose a significant part of their energy in the dense QGP. For relatively low quark momenta, the interaction with the bulk of the partons in the medium may follow exclusively through elastic scatterings. For high-energy quarks, a number of soft gluons can be radiated, carrying a fraction of quark energy into the medium. These processes are a QCD analogue of phenomena known from QED: the physics of a parton traversing a droplet of QGP resembles the scenario of an electrically charged particle traversing ordinary matter.

In other measurements, the ALICE collaboration has compared data on the production of D mesons (containing a charm quark) with data from CMS on non-prompt J/ψ mesons (the decay products of heavier mesons containing a beauty quark). The comparison shows that the heavier the quark, the less energy it loses inside the medium. Indeed, this was one of the most striking predictions of theoretical models describing strongly coupled QCD matter – the plasma is less opaque to heavy quarks as compared to light quarks and gluons. So, these new measurements at last provide the first confirmation of these predictions.

The nature of the interactions between the heavy quarks and the medium can also be deduced from the azimuthal asymmetry of the production of heavy-flavour hadrons: the magnitude of the asymmetry is proportional to the collective flow of the medium. Measurements of the asymmetry presented by ALICE confirm that the heavy quarks participate in the collective flow of QGP. These results are critical to establishing the focus of future theoretical work on the transport properties of the plasma, while from the experimental point of view, the ALICE collaboration is looking forward to the improved precision from the measurements in LHC Run 2.

The droplet of QGP produced in heavy-ion collisions constantly expands, and lasts at most about 10 fm/c (30 × 10−24 s). After that time, the temperature drops below the critical temperature (about 155 MeV) and the energy density falls below a critical density of about 0.5 GeV/fm3. At that point, the distances between the quarks become large and, owing to the nature of the strong force, the partons are re-confined/combined into colour-neutral hadrons. Following this hadronization process, the system becomes a gas of hadrons and, while the gas is still hot, the hadrons may still interact. The most useful messengers from this phase of the collision are the short-lived hadronic resonances. At the conference, ALICE presented extensive studies of the short-lived mesons and baryons. Their production rates provide sensitive information on the strength of the hadron–hadron interactions, and thus are a vital source for understanding the properties of the hadron gas. Knowing the equation-of-state of the hadron gas allows the genuine QGP signals to be unravelled in greater detail.

Finally, ALICE presented signatures of collective particle production in an extended pseudorapidity range in proton–lead collisions (“ALICE goes forward with the ridge in pPb collisions”). Such collective behaviour, known from heavy-ion collisions, was not initially expected for the smaller proton–lead system. The new measurement provides qualitatively new constraints to theoretical models attempting to explain the novel phenomena.

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