by Michele Floris. Published: 11 August 2017

The 2017 edition of the European Physical Society Conference on High Energy Physics (EPS-HEP) took place in Venice, Italy, between July 5th and 12th.

EPS-HEP is one of the major conferences in the field, and an opportunity for the experiments to showcase their most recent results.  With over 950 participants, it has been one of the main appointments of the year. Many new measurements were released by ALICE and the other collaborations.

ALICE Results

The new ALICE results from pp, p-Pb and Pb-Pb collisions marked an important advancement in the understanding of identified particle production, collective effects and hard processes.

The extensive campaign of measurements in pp and p-Pb collisions suggests the possibility that a small droplet of quark-gluon plasma can be created in these smaller systems. The similarity of the different colliding systems, with respect to effects that were considered to be hallmarks of heavy ion collisions, was confirmed. In particular, measurements of collective effects and strangeness enhancement were presented. These measurements provide a pathway to understand the microscopic emergence of the quark-gluon plasma. They also challenge some of the established models of soft QCD (for what concerns the universality of string fragmentation) and of the quark-gluon plasma (how close does the system gets to thermal equilibrium?).

The recent Run-II Pb-Pb measurements in collisions at ÖsNN = 5.02 TeV confirm the behaviours and trends observed at ÖsNN = 2.76 TeV with increased statistics and data quality, paving the road for a precise characterization of the quark-gluon plasma.

  • Identified Particles

The results on strangeness enhancement in pp collisions, recently published in Nature Physics, were complemented with new measurements on strange particle production in Pb-Pb collisions at ÖsNN =5.02 TeV [talk by P. Kalinak]. In central Pb-Pb collisions, particle abundances are known to reflect (with some tensions) a thermal distribution [J. Otwinowski], whose origin is not yet understood. The smooth evolution of ratios across colliding systems and size represents an opportunity to clarify the dynamical origin of this apparent equilibrium.

Lambda/K0 ratio vs transverse momentum (pT) in several centrality classes (V0M) in Pb-Pb collisions at 5.02 TeV from LHC run 2. Error bars depict statistical uncertainties while boxes show systematic uncertainties. [Credits: ALICE/CERN]

 

New results for identified light flavour particles in minimum bias pp collisions at ÖsNN = 5.02 TeV and 13 TeV were also shown [G. Bencedi]. These provide the baseline for the upcoming measurements as a function of multiplicity, exploiting the high-statistics samples collected in Run II.

An important element for the interpretation in terms of a thermalised system is the emission of thermal radiation. The search for such contribution in the di-electron channel in pp collisions, as well as the status of di-electron measurements in all colliding systems, were discussed [R. Bailhache].

Complementary information can be obtained from the study of heavy flavour hadrons in pp and p-Pb collisions. The results presented at EPS-HEP include a recent measurement of J/psi production as a function of multiplicity [J. Crkovska], which provides key information on the evolution of multiple-parton interactions, and the first measurement of the heavy flavour baryons XC in pp collisions and ΛC in p-Pb collisions [A. De Caro] [C. Terrevoli], which gives crucial information on charm hadronisation.

Finally, it should not be forgotten that heavy-ion collisions are also an excellent factory for the production of nuclei and hyper-nuclei. Two highlights from the presentation of [S. Trogolo] are the new measurements of light nuclei in Pb-Pb collisions at ÖsNN = 5.02 TeV and one of the most precise measurement of the lifetime of the hyper-triton (a bound state of a Lambda hyperon, a proton and a neutron). The latter, in particular, is compatible with the free Lambda lifetime (as it would be expected for the loosely-bound hyper-triton), and could resolve a small tension between this expectation and the world average, prior to the ALICE measurement.

This figure shows the lifetime values availabel in literature and the results obtained from the analysis of the (anti-)hypertriton 2-body decay channel on the 2015 data sample of Pb-Pb collisions at 5.02 TeV. The result shown as Preliminary, which is the official value, is the one obtained from the "ct-spectra" analysis (exponential fit to the corrected ct-spectra). [Credits: ALICE/CERN]

 

  • Collective Effects

The status of collective effects of light-flavour hadrons measured in large and small systems was discussed in various talks [J. Otwinowski] [S. Noor Alam].  This provides the baseline for the study of heavy-flavour particles: Run II results on the v2 of D mesons and J/ Ψ, with a much-reduced uncertainty as compared to Run I, were presented [X. Zhang] [I. Das]. The v2 is now found to be significant at more than 5σ for both mesons. These important results indicate that charm quarks thermalise with the quark-gluon plasma. They pave the road for the detailed heavy-flavour collectivity studies that ALICE will complete following the upgrade (in Run 3 and 4), which will ultimately elucidate the process of thermalisation in the deconfined QCD medium.

  • Hard Probes and Heavy Flavours

Hard probes and heavy flavours play an important role in the study of the quark-gluon plasma. They can be considered as well-calibrated, tagged, probes of the microscopic structure of the medium and of the thermalisation process.

Hard QCD processes can be computed accurately using perturbative QCD. Studying in details the modifications of high momentum partons as they interact with the quark-gluon plasma is the analogous of the Rutherford experiment, and provides information on the microscopic degrees of freedom of hot QCD matter.

The status of jet measurements in pp and heavy-ion collisions was summarised in two separate contributions [A. Shabetai][C. Nattrass]. Among the many results discussed, it is worth to emphasise recent measurements on jet shapes, which allow the study of possible modification to the fragmentation (induced by energy loss or by the response of the medium), and correlation studies that give access to lower energy jets. (On this latter point, see also the contribution by X. Peng on p0-hadron correlations.)

A precise measurement of the parton-type and mass dependence of the energy loss is a crucial prerequisite for a detailed study of the energy loss process, in particular to disentangle between radiative and collisional energy loss.

Many new results for heavy-flavour hadrons were presented at the conference, including a new measurement of various D mesons RAA in semi-central collisions [X. Zhang]. Most notably, these results show a hint of a smaller suppression of Ds compared to other D mesons at intermediate pt, which support a contribution from coalescence (due to the enhancement of strange quarks in Pb-Pb collisions).

Progress was also made in the study of heavy flavour and high-momentum processes in p-Pb collisions. New results for the J/ Ψ i suppression as a function of pt and rapidity were presented [B. Paul]. These measurements are traditionally motivated by the need to estimate the so-called "Cold Nuclear Matter" (CNM) effects (for instance, modifications of the parton-distribution functions or saturation effects).

The recent hints for the creation of a hot, interacting, medium in small systems, however, also call for the search of energy loss and heavy-flavour collectivity in small systems.   In particular, new results were presented for the RpPb of D mesons and of the LC baryon [C. Terrevoli, 6/7 15:45]. The data show a hint of a bump in the D RpPb at low pt, which could be induced by collective effects, but the precision of the measurement does not yet allow to discriminate between different scenarios.

An alternative strategy to study initial state CNM effects are "Ultra-Peripheral Collisions" (UPC), in which a photon produced by the strong electromagnetic field of the "projectile" nucleus interacts with the "target" one. This allows using the LHC as a photon-nucleus collider, providing a very clean probe of the initial state [V. Pozdniakov].

Inclusive J/ψ QpPb-mult shown as a function of ⟨Ncoll-mult⟩ in p-Pb. The QpPb-mult at √sNN = 8.16 TeV (red circles) are compared to those obtained at √sNN = 5.02 TeV (blue squares). Systematic uncertainties are shown as boxes around the symbols, while statistical ones are shown as bars. [Credits: ALICE/CERN]

 

  • Upgrade, Methodology and Diversity

The EPS-HEP conference also provides a forum for discussing instrumental and collaborative aspects of the large high-energy physics experiments.

The ALICE ambitious upgrade programme, which aims at collecting Pb-Pb collisions at high luminosity for the study of rare processes, was discussed in three talks focusing on the status of the most important detector upgrades [C. Lippmann][P. Camerini][M. Slupecki].

Progress on the application of machine learning methods to physics analysis, which bear the promise of a significant improvement in the measurement of low mass di-electrons and tagged jets were summarised by R. Haake. With these new methods, ALICE joins the other LHC collaborations in the usage of state-of-the-art techniques for the full exploitation of the collected data.

Finally, the dynamic ALICE junior community and its major contributions to all aspects of the experiment were presented in the "Outreach, Education, and Diversity" session [H. Beck].

Other Heavy Ion Results

Overall, far too many results were presented at the conference to be summarised in this short article.

The heavy ions session was very lively, with contributions from theorists and experimental collaborations, spanning the entire energy range of the field, from Hades to the LHC.

Noteworthy updates from the heavy-ion experiments include:

  • New LHCb results on open heavy flavour [P. Robbe] and quarkonia [F. Bossu] in p-Pb collisions and from the fixed-target programme [L. Anderlini];
  • An update on the ATLAS measurement of photon-photon interactions in ultra-peripheral collisions [B. Cole] and final results on jet production and fragmentation [M. Spousta];
  • Final measurements of charmonium production in p-Pb and Pb-Pb collisions [A. G. Ståhl] and new results on identified π, K, p production as a function of multiplicity in pp collisions at ÖsNN = 13 TeV from CMS [F. Sikler];
  • The STAR measurement of U production at RHIC [O. Matonoha].

In the plenary heavy-ion session, besides the summary of the ALICE contributions [M. Floris], two talks addressed state-of-the-art results on flavour [R. Arnaldi], jets and correlations [M. Nguyen]. The 30-years-long history of heavy-ion physics was summarised by J. Stachel.

Michele Floris presenting the highlights of the ALICE experiment. [Credits: Andrea Dainese]

 

HEP Highlights

For what concerns other high-energy physics results, it is worth mentioning a few highlights.

LHCb presented an update on tensions with lepton universality from various semileptonic decays, which are at the moment at the 2-3 s level and have the potential of uncovering new physics [U. Egede]. The collaboration also reported the measurement of the first doubly-charmed baryon (Xcc).

Atlas and CMS presented the status of the searches for many beyond-the-standard-model particles (plenary talk by D. Tovey for ATLAS and J. Alcaraz Maestre for CMS), and new measurements on the coupling of the Higgs, in particular to fermions, essential to ascertain the nature of the H scalar measured at the LHC [P. Meridiani].

Very interesting results from non-accelerator programs were also discussed, in particular with the progress on the measurement of gravitational waves [M. Punturo].

 

For more details, see the EPS-HEP agenda and the conference summary [P. Sphicas].