by Andrea Dainese & Gianluca Usai. Published: 05 December 2012

The main physics motivation for the upgrade of the Inner Tracking System of the ALICE experiment is to perform new measurements on heavy flavour (charm and beauty) and thermal dilepton production in heavy-ion collisions, which address important questions about the QGP properties that cannot be answered with the present experimental setup.

Heavy quarks play a special role in heavy-ion physics because they constitute a tagged (identified) probe (from production to observation), which enables a unique access to their interactions in the QGP. This allows us to gain microscopic insights into the transport properties of the medium. Heavy-flavour particles may be thought of as “Brownian motion” markers, the kinematical distributions of which (especially in momentum and azimuthal angle) reflect their reinteraction history. The large mass makes complete thermalization very difficult, thus most likely preserving a “memory” of the interaction history.

The two main open questions concerning heavy-flavour interactions with the QGP medium —and the corresponding experimental handles— are:
–Thermalization and hadronization of heavy quarks in the medium, which can be studied by measuring in the charm sector the baryon/meson ratio (Λc/D) and the strangeness enhancement (Ds/D), the azimuthal anisotropy v2 for charm mesons and baryons, and the possible in-medium thermal production of charm quarks.

–Heavy-quark in-medium energy loss and its mass dependence, which can be addressed by measuring the nuclear modification factors RAA of the pT distributions of D and B mesons separately in a wide momentum range.

The measurement of low-mass dilepton production provides access to the bulk properties and the space-time evolution of the hot and dense QCD matter formed in ultra-relativistic heavy-ion collisions, and reveals microscopic properties such as the relevant degrees of freedom and the hadronic excitation spectrum in medium. Electromagnetic radiation is produced at all stages of the collision, and since leptons couple only weakly to the surrounding medium, their spectrum retains information of the entire system evolution.

The fundamental questions to be addressed by a comprehensive measurement of low-mass dileptons in heavy-ion collisions at the LHC are the following:
– The generation of hadron masses is driven by the spontaneous breaking of chiral symmetry of QCD in the vacuum. Lattice QCD predicts that this fundamental symmetry is restored in the QGP, leading to substantial modifications of the vector meson spectral functions. Such modifications, in particular of the ρ–meson, can be inferred from low-mass dilepton spectra.

–Dilepton production is intimately related to the temperature of the system at all stages of the collision and encoded in the transverse momentum and invariant mass dependencies.

The replacement of the current ITS with a new detector with better resolution, smaller material thickness and with high-rate readout capabilities is a fundamental cornerstone to address these questions.

Within our Working Group, in addition to elaborating some of the main directions of the future physics programme of the experiment, we have carried out a first round of simulation studies to assess the expected performance of the ALICE detector, with the new ITS, and a general upgrade of the readout and data-handling capabilities.

The studies are based on a detector consisting of seven concentric cylindrical layers covering a radial extension from 22 mm to 430 mm with respect to the beamline. In particular, the innermost layer is significantly closer to the interaction point than in the present detector. All layers are segmented in pixels with dimensions of 20x20 μm increasing the pixel density by a factor of 50. In addition, the use of state-of-the-art pixel technology will allow the silicon material budget per layer to be reduced by a factor of 7 in comparison to the present ITS (50 μm instead of 350 μm). Distribution of electrical power and signals, mechanics, cooling and other detector elements can also be improved with respect to the present ITS design. Combining all these new elements together, it should be possible to build a detector with a thickness of 0.3% of the radiation length (X0) per layer or better.

The new detector will provide an improvement of the impact parameter resolution by a factor of 3 in the transverse direction (see also the work of the 2nd Working Group ), decreasing from 60 μm to 20 μm at 1 GeV/c transverse momentum. This is very important to measure the production of particles like the Λc, which has a mean proper decay length (cτ) of 60 μm, that is 2-5 times smaller than that of D mesons.

Another important and challenging aspect is the readout capability. The new detector will be able to record Pb-Pb collision events at a rate of 50 kHz. The new physics programme is based on a minimum-bias data sample corresponding to an integrated luminosity of 10 nb-1, a factor 100 larger than in the programme planned for the current detector.

The main conclusions obtained from the present studies for the heavy-flavour sector are:

–The quark mass dependence of elliptic flow (see upper panel of the figure) and of in-medium energy loss will be accessible down to very low pT via the separation of the prompt (from charm) and delayed (from B decays) components of the D meson yields.
–The baryon/meson ratio for charm (Λc/D) will also be accessible for the first time (see lower panel of the figure).
–The strangeness enhancement in the charm sector (Ds/D) will be measured with high precision.
–There is a positive outlook for improved beauty production measurement in Pb-Pb via displaced J/ψ, via displaced single electrons and via full reconstruction of exclusive B decay channels.
- Additionally, there are on-going studies to assess the possibility of measuring even more exotic particles, like the Λb baryon.

For what concerns low-mass dielectrons, an essential aspect is the high-rate capability of the upgraded ALICE. Moreover, the measurement will benefit from the reduced material budget of the new ITS, that limits the combinatorial electron background from photon conversions. The excellent impact parameter resolution of the new ITS will provide a handle to reject charm-decay electrons, another important background source. The reduced background will translate into significantly smaller systematic uncertainties.

The main conclusions for low-mass dileptons are:

–Quantitative aspects as the temperature of the thermal radiation from the QGP, inferred from the exponential shape of the invariant mass in the region between the φ and the J/ψ mesons, or from the corresponding transverse momentum spectra, will become accessible for the first time.

–The excess dielectron spectrum from the ρ meson can be isolated with a good statistical significance allowing to study the behaviour of the spectral function in the nuclear medium at the highest energy ever reached. This is particularly interesting because the in-medium modifications of hadronic spectral functions can be related to chiral symmetry restoration.

A new vertex detector with the capabilities described above will thus place ALICE in a unique position in comparison to the other LHC experiments. Indeed, for a number of heavy-flavour and dilepton measurements, the other experiments can only explore momenta above a certain threshold of a few GeV/c.

Figure: expected physics performance for the measurement of the charm and beauty elliptic flow with prompt and delayed D0 mesons (upper panel) and of the baryon/meson ratio in the charm sector with Λc/D0 (lower panel). For both measurements the upgraded ALICE detector with a new ITS and a data sample corresponding to an integrated luminosity of 10 nb-1 are considered.

The results achieved so far by the Working Group, as reported in the second chapter of the ITS Upgrade CDR and in the ALICE Upgrade Letter of Intent , owe mostly to the enthusiasm and valuable work of several students, postdocs and young researchers (C. Bianchin, C. Di Giglio, G.M. Innocenti, M. Kweon, M. Mager, A. Mastroserio, L. Molnar, S. Moretto, P. Reichelt, R. Romita, A. Rossi, J. Stiller, C. Terrevoli). The activity of the Working Group benefits from the close collaboration with the Working Group on Detector Requirements (WG2). This fruitful synergy will continue towards the preparation of a Technical Design Report (TDR), where we will present the outcome of detailed studies based on the full simulation of the new detector in its close-to-final design.

Read also: Heavy Flavours through Quark Matter 2012