Leading lead through the LHC

Three of the LHC experiments - ALICE, ATLAS and CMS - will be studying the upcoming heavy-ion collisions. Given the excellent results from the short heavy-ion run last year, expectations have grown even higher in experiment control centres. Here they discuss their plans:

ALICE

For the upcoming heavy-ion run, the ALICE physics programme will take advantage of a substantial increase of the LHC luminosity with respect to last year’s heavy-ion run.  The emphasis will be on the acquisition of rarely produced signals by implementing selective triggers. This is a different operation mode to that used during the first low luminosity heavy-ion run in 2010, when only minimum-bias triggered events were collected. In addition, ALICE will benefit from increased acceptance coverage by the electromagnetic calorimeter and the transition radiation detector. In order to double the amount of recorded events, ALICE will exploit the maximum available bandwidth for mass storage at 4 GB/s and the new event compression algorithm running on the High Level Trigger computer farm. The 2011 data will allow ALICE to extend the physics reach of identified particles, quarkonia, heavy flavours, jets and photons thanks to measurements with largely increased statistics and extended transverse momentum reach.

In anticipation of a future proton-ion run, LHC will perform feasibility studies just before the heavy-ion run starts. Measurements of proton-nucleus collisions are an essential component of the ALICE scientific programme. On the one hand, they will provide an insight into the structure of the entrance channel that leads in nucleus-nucleus collisions to the formation of hot and dense nuclear matter. On the other hand, they will enable us to disentangle cold nuclear matter effects from hot nuclear matter effects on the relevant observables providing information on the properties of the Quark Gluon Plasma.

Yves Schutz and Despina Hatzifotiadou

ATLAS

Less than a year ago, no one knew what would be discovered from the lead collisions at LHC energies. Yet, from the 2010 data, ATLAS found the first direct evidence of jet quenching and measured J/psi suppression. We also reported the apparent absence of similar effects for Z and W production, and performed advanced studies of particle flow and two-particle correlations.

With the new 2011 lead ion data, we can thoroughly characterize the di-jet asymmetry, which was the first sign of jet quenching, and events with a single jet balanced by an energetic photon can be used to gain further insights. As well, we can find answers to many other questions, for instance concerning the behaviour of Zs and Ws in the hot, dense soup of quarks and gluons.

This year, the LHC again offers something completely new: the possibility of proton-ion collisions. While this year's run is an accelerator feasibility test, ATLAS is optimistically hoping also to obtain the first sample of proton-ion collisions. The basic properties of such collisions at these energies are yet unknown, and they can be determined from even a few LHC fills.

We are all excited about these fascinating prospects!

Will Brooks and Sasha Milov
ATLAS Heavy Ion conveners

CMS

We are eagerly awaiting the start-up of the new heavy-ion run. All preparatory work to cope with the challenges of the new data-taking phase has been successfully completed. The short period of lead-lead collisions in 2010 generated such a fantastic amount of results that there are great expectations for the coming run. The machine teams are promising us an increase of data by a factor of ten or more. That would be simply fantastic. By being able to record new clean signals, such as photons recoiling against jets, we’ll be able to study - in great detail - the striking phenomena connected with the quenching of jets in the new state of matter produced in lead-lead collisions. We’ll produce plenty of electroweak vector bosons (Ws and Zs) to perform precision measurements that could challenge the predictions of the current models. It will be possible to study in more detail the suppression of the excited states of the Ypsilon resonance whose appearance in the 2010 data was one of the major results of the year. With large statistics it will also be possible to characterise up to the smallest detail the properties of the fluid produced in high centrality heavy-ion collisions.

Lastly, the pilot fill of proton-ion collisions that is currently planned will hopefully be a major step towards the very interesting 2012 physics programme.

Guido Tonelli