Strangeness at the new energy frontier, from pp to PbPb

Abelev Betty

Dr. Abelev Betty - Lawrence Livermore National Laboratory

Physicists from the CMS, ATLAS, LHCb and LHCf collaborations presented their data and plans for the future. The results from the only dedicated heavy ion experiment at the LHC, ALICE, were prominently featured in the introductory and concluding plenary talks, and also during the dedicated heavy ion day. One of the presentations, focused on the aspects of production of particles containing the strange quark, or strange particles for short, as measured by ALICE.

PLHC was a conference hosted by the Canada's premiere high energy and nuclear physics facility, TRIUMF, located on the University of British Columbia campus.

Ordinary matter around us is made of protons and neutrons, which in turn are composed of up (u) and down (d) quarks. The next quark that can be liberated from the sea of quark-anti-quark pairs that populate the vacuum is the strange quark (s-quark). It's heavier than u and d, yet close enough in mass to undergo production and modification processes in similar manner. That, and the relative abundance of the strange quark in high-energy interactions, make the s-quark a very useful study tool for proton-proton and heavy nucleus collisions.

Standard Model of Elementary Particles

ALICE is the only experiment at the LHC equipped with a large-volume time projection chamber (TPC), a four-dimensional digital camera that captures the tracks and momenta of particles emerging from the collision region to a radius of up to 2.5 meters. Strange particles, K-mesons (Kaons, made up of a strange and a non-strange quark pair), Lambda (uds), Xi (dss), and Omega (sss) baryons have an appreciable lifetime before they decay into ordinary matter. These decays have a characteristic geometrical configuration, which allows an effective reconstruction of strange particles across a large momentum range using the information obtained by the TPC.

At the conference, we presented the measurements obtained in the pp collisions at two energies, 0.9 and 7 TeV, and the data obtained in 2.76 TeV Pb-Pb collisions.

The strangeness data obtained in pp collisions is particularly important to improve the overall modeling of those collisions. PYTHIA is a software package that is able to generate events from a model. The model parameterizes the low-momentum processes taking place in elementary collisions, and calculates the higher energy processes up to the next to leading order perturbative term in the perturbative QCD expansion. The latest PYTHIA versions describe the general properties of real collisions fairly closely, but significantly underestimate the yields of strange particles: the more the strangeness content of the particle, the worse is the discrepancy. One of the recent PYTHIA versions, PYTHIA Perugia-2011, made significant modifications to its s-quark cross-section, and as a result has gotten very close to reproducing the yields of baryons with multiple strange quarks, especially at higher transverse momentum. ALICE is the only experiment at the LHC to have measured Omega baryon yields in pp collisions.

In addition, the pp measurement serves as a baseline for the measurements in Pb-Pb collisions, where we expect to produce the Quark-Gluon-Plasma (QGP). An enhancement in the production of particles with strange quarks has long been thought to be a signature of extra degrees of freedom available in the QGP. Indeed, this enhancement has been seen at lower energies as well: the larger the volume of the collision, the more the number of Lambda, Xi and Omega baryons increases with respect to the baseline (a pp or a Be-Be collision). This is also observed at 2.76 TeV Pb-Pb collisions, however, with a caveat: the enhancement is smaller than that at lower energies! We think this is due to the complexity of our baseline: at these high energies, pp collisions to which we compare are more complex and produce much more strangeness than events at lower energies.

Enhancement in the production of particles with strange quarks

Further results characterized the effects of collective movement of quarks within the plasma (the measurement of parton flow), and the suppression of high momentum strange particles due to their passing through the dense matter formed in more violent (central) Pb-Pb collisions. We also compared to lower energies with surprising results: at an energy of more than x30 (previous data was taken at 0.2 TeV), strange particles follow very similar patterns of flow and suppression. By comparing strange to non-strange particle ratio to thermal model fits, we are able to access the temperature at which these particles were produced. Currently it seems that the formation of protons emerging from the fireball and heavier particles, such as the multi-strange baryons, happened at slightly different temperature surfaces.

The talk was well received, and raised an interesting question about how well we can model the collective behavior of particles with our current understanding of the underlining heavy ion collision. We are currently working on refining the strangeness results for the upcoming Quark Matter conference in US.

Alice Matters