Subject Grouping: Physics

One of the key ways of looking into what happens when high-energy hadrons collide is to measure the relationship between the number, or multiplicity, of particles produced and their momentum transverse to the direction of the colliding beams. The results cast light on processes ranging from the interactions of individual partons (quarks and gluons) to the collective motion of hot, dense matter containing hundreds of partons. The ALICE experiment is investigating effects across the range of possibilities, using data collected with proton–proton (pp), proton–lead (pPb) and lead–lead collisions (PbPb) in the LHC – and the results are showing some surprises.

A correlation between the average transverse momentum 〈p_T〉 and the charged particle multiplicity N_ch was first observed at CERN’s SppS collider and has since been measured in pp(p) collisions over a range of centre-of-mass energies, culminating recently at the LHC. The strong correlation observed led to a change in paradigm in the modelling of such collisions, with the proposal of mechanisms that go beyond independent parton–parton collisions.

In pp collisions, one way to understand the production of high multiplicities is through multiple parton interactions, but the incoherent superposition of such interactions would lead to the same 〈p_T〉 for different values of multiplicity. The observation of a strong correlation thus led to the introduction, within the models of the PYTHIA event simulator, of colour reconnections between hadronizing strings. In this mechanism, which can be interpreted as a collective final-state effect, strings from independent parton interactions do not independently produce hadrons, but fuse before hadronization. This leads to fewer, but more energetic, hadrons. Other models that employ similar mechanisms of collective behaviour also describe the data.

In PbPb collisions, high-multiplicity events are the result of a superposition of (single) parton interactions taking place in a large number of nucleon–nucleon collisions. In this case, substantial rescattering of constituents is thought to lead to a redistribution of the particle spectrum, with most particles being part of a locally thermalized medium that exhibits collective, hydrodynamic-type, behaviour. The moderate increase of 〈p_T〉 seen in PbPb collisions (shown in figure 1 for N_ch around 10 or larger) is thus usually attributed to collective flow.

Now, the first measurements by ALICE of two-particle correlations in the intermediary system of pPb collisions have sparked an intense debate about the role of initial- and final-state effects. The pPb data on 〈p_T〉 indeed exhibit features of both pp and PbPb collisions, at low and high multiplicities, respectively. However, the saturation trend of 〈p_T〉 versus N_ch is less pronounced in pPb collisions than in PbPb and at high multiplicities leads to a much higher value of 〈p_T〉 than in PbPb. Is this nevertheless a fingerprint of collective effects in pPb collisions? Predictions that incorporate collective effects within the hadron interaction model EPOS describe the data well, but alternative explanations, based on initial-state effects (gluon saturation), have also been put forward and are being tested by these data (ALICE collaboration 2013 a).

Other recent measurements of particle production in proton–nucleus collisions have shown unexpected behaviour that is reminiscent of quark–gluon plasma (QGP) signatures. But what could cause such behaviour and is a QGP the only possible explanation? To answer this in more detail, it is important to separate particle species, as collective phenomena should follow an ordering in mass. To this end, ALICE has measured the transverse-momentum spectra of identified particles in pPb collisions at √s_NN = 5.02 TeV and their dependence on multiplicity (ALICE collaboration 2013b).

The measurements show that the identified particle spectra become progressively harder with multiplicity, just as in PbPb collisions, where the hardening is more pronounced for particles of higher mass. In heavy-ion collisions, this mass ordering is interpreted as a sign of a collective radial expansion of the system. To check if such an idea describes the observations, a blast-wave parameteriz ation can be used. This assumes a locally thermalized medium that undergoes a collective expansion in a common velocity field, followed by an instantaneous common freeze-out.

As figure 2 shows, the blast-wave fit describes the spectra well at low p_T, where hydrodynamics-like behaviour should dominate. The description fails at higher momenta, however, where the non-thermal components should contribute significantly. But are QGP-like interpretations such as this one unique in describing these measurements? The colour-recombination mechanism present in PYTHIA, discussed above, leads qualitatively to similar features to those observed in the data.

The presence of flow and of a QGP in high multiplicity pPb collisions is thus not ruled out, but since other non-QGP effects could mimic collective phenomena, further investigation is needed. Nevertheless, these results are certainly a crucial step towards a better comprehension not only of pPb collisions but also of high-energy collisions involving nuclei in general.

The ALICE collaboration had a significant presence at two recent major conferences, the 2013 European Physical Society Conference on High-Energy Physics (EPSHEP 2013), in Stockholm (EPS-HEP2013: these are good times for physics), and the 14th Topical Conference on Strangeness in Heavy Flavour Production in Heavy-Ion Collisions – Strangeness in Quark Matter 2013 (SQM2013) – that took place on 22–27 July at Birmingham University in the UK.

The many contributed talks and plenary presentations, in particular at SQM2013, highlighted new results from the proton–lead (pPb) data recorded in early 2013. While this run was initially intended to provide control data sets, several unexpected, and currently unexplained, results have been observed.

Presentations also covered updates on both soft (low p_T) and hard (high p_T and heavy flavour) probes of PbPb collisions

Most intriguingly, both the spectra of identified particles and charged-hadron correlations in high-multiplicity pPb events reveal signals suggestive of collective flow, which are similar to those observed in heavy-ion collisions, as the figure shows. These mass-dependent phenomena do not arise trivially in either the colour glass condensate or in the gluon saturation framework that describes the initial state of the colliding nuclei at the relevant small values of Bjorken-x.

Also in pPb collisions, ALICE’s measurements of minimum-bias spectra for a variety of hadronic species and jets, reveal no strong deviations from the expectations of the scaled number of nucleon–nucleon (binary) collisions. This confirms that the striking suppressions observed so far for all final-state hadrons in lead–lead (PbPb) collisions are a specific feature of quark and/or gluon energy loss via interactions with the quark–gluon plasma (QGP).

Presentations also covered updates on both soft (low p_T) and hard (high p_T and heavy flavour) probes of PbPb collisions. Higher precision results on nuclear-modification factors and elliptic flow – including measurements on heavy quarks – as a function of the event centrality gave the most detailed picture to date of partonic interactions with the QGP. The measurements of D mesons also indicate that the initial density and temperature of the QGP are so high that the heavy, charm quarks thermalize with the QGP before hadronization. Interestingly, the J/ψ results reveal much less suppression than at Brookhaven’s Relativistic Heavy-Ion Collider, suggesting that significant late-stage regeneration of these quarkonia states occurs as a result of the initial copious production of charm quarks in the heavy-ion collisions at the LHC.

Last, the high-precision soft physics results from the PbPb data underscored the potential significance of a hadronic re-scattering phase at the end of the produced medium’s evolution at the LHC. This phase has not previously been considered important when predicting signatures of the QGP, but it must now be accounted for to model accurately the full dynamics of a heavy-ion collision at the LHC.

There was lively debate at both conferences about the possible interpretations of all of these interesting new results, continuing well after the talks were over. Future studies were proposed that should help to unravel the origin of these intriguing phenomena observed in both pPb and PbPb collisions.

Two teams working on experiments at CERN’s ISOLDE facility have published results that extend knowledge in different areas of nuclear and atomic physics. The ISOLTRAP collaboration has measured the masses of exotic calcium nuclei using the new multi-reflection time-of-flight (MR-TOF) instrument, while a team working at the resonant-ionization laser ion source (RILIS) has made the first determination of the ionization potential of the radioactive-element astatine. The results from the two experiments demonstrate well the versatility of the ISOLDE facility.

The ISOLTRAP team used the facility to make exotic isotopes of calcium, with the aim of finding out how their nuclear “shell structure” evolves with increasing numbers of neutrons. By integrating the MR-TOF system into the experiment, the team has made precise determinations of the masses of calcium isotopes up to ⁵⁴Ca. While the new device has already been applied successfully as a mass separator, this first use as a mass spectrometer has already led to a key finding and promises further important results in the future.

The results strengthen the prominence in calcium of a “magic number” that was not foreseen in the original nuclear shell model, for which Maria Goeppert-Mayer and Hans Jensen received the Nobel prize in 1963, exactly 50 years ago. In this model, the protons and neutrons in a nucleus form independent “shells” that are similar to those of electrons in atoms. The magic numbers correspond to full nuclear shells, in which the constituents are bound more tightly, leading to greater stability and lighter masses. With 20 protons and 20 neutrons, standard calcium, ⁴⁰Ca, is doubly magic, while the rare and naturally occurring, long-lived isotope ⁴⁸Ca has 28 neutrons – another magic number. The measurements by the ISOLTRAP team indicate a new closed-shell structure in ⁵²Ca and therefore a new magic number of 32 (Wienholtz et al. 2013). Its shell strength of about 4 MeV rivals that of the classic magic numbers.

These measurements cast light on how nuclei can be described in the context of the fundamental strong force, in particular in terms of predictions using state-of-the-art theory that includes three-body forces, from physicists at the Technical University of Darmstadt. Calcium is the heaviest isotopic chain for which three-nucleon forces – based on an effective field theory of QCD – have been applied. The ISOLTRAP results are in excellent agreement with the theoretical calculations and they show that a description of extremely neutron-rich nuclei can be closely connected to a deeper understanding of nuclear forces.

One of the strengths of the ISOLDE facility is the RILIS source, which produces many of the beams. At the source, bunches of protons at 1.4 GeV from CERN’s Proton Synchrotron Booster are fired at a thick target of uranium carbide or thorium dioxide. The collisions produce nuclei of many different elements, which diffuse inside a metal cavity held at around 2000°C. Shining overlapping laser beams of chosen wavelengths into this cavity results in the selective ionization of some of the neutral atoms inside. After electrostatic extraction and magnetic mass-separation, the result is a pure beam of one isotope that travels on to a detector.

The latest element to come under scrutiny at RILIS is astatine. With a half-life of just over eight hours for its longest-lived isotope, ²¹⁰As, astatine is the rarest naturally occurring element and one of the least known. Now, a team at ISOLDE has measured the element’s ionization potential for the first time, giving a result of 9.31751 eV (Rothe et al. 2013).

The measurement fills a long-standing gap in the Periodic Table because astatine is the last element present in nature for which this fundamental property remained unknown. It is of particular interest because isotopes of astatine are candidates for the creation of radiopharmaceuticals for cancer treatment by targeted alpha-particle therapy. The experimental value for astatine also serves as a benchmark for theories that predict the atomic and chemical properties of super-heavy elements, in particular the recently discovered element 117, which is an astatine homologue.

These two results demonstrate beautifully the wealth of ISOLDE’s tool-box for exploring nuclear physics. They complement well the recent results on the shape of radon nuclei that were observed in post-accelerated beams.