More details on the ALICE DCAL

The DCal will extend significantly the jet quenching measurements enabled by the EMCal in ALICE, by providing large acceptance for back-to-back correlation measurements of jets and hadrons. The EMCal has superb capabilities for inclusive jet measurements (ALICE EMCal Physics Performance Report, 20091 ).

However, experience with jet quenching measurements at RHIC has shown that the combination of inclusive and correlation measurements is a much more powerful probe of jet quenching than inclusive measurements alone. Addition of the DCal in ALICE makes hadron+jet and jet+jet correlation measurements possible at high statistics over a very broad kinematic range, thereby expanding ALICE physics capabilities significantly. The most significant measurements enabled by the DCal, and those which are the focus of this discussion, are as follows:

  • the correlation of a jet recoiling back-to-back from a triggered high pT π0 exploits the phenomenology of jet quenching to provide a unique geometric bias, maximizing the path length of the recoiling jet in the hot medium. In one heavy ion running year (0.5 nb-1  for Pb+Pb) the DCal and EMCal will measure several thousand fully reconstructed jets with energy greater than 150 GeV recoiling from a π0 trigger with pT > 20 GeV/c.
  • the DCal and EMCal will measure the correlation of fully reconstructed jet pairs with large cone radius (R ~ 0.4), with 1000 counts above ET ~ 140 GeV. This likewise enables unique jet quenching measurements, for instance the study of energy balance between recoiling jet pairs.

This is not an exhaustive list of all measurements the DCal will carry out, but rather a representative set that illustrates its physics reach. Jet quenching effects are estimated utilizing a PYTHIA Monte Carlo model, which is the only model currently available that incorporates a theoretically well-motivated quenching mechanism.

The DCal uses the same type of detectors as the EMCal and will be operated in conjunction with the PHOS detector, another existing lead-tungstate calorimeter with smaller acceptance but very high precision. 

The full acceptance of the DCal electromagnetic calorimeter consists of a barrel section providing coverage for a 60o arc in azimuth and 1.4 units of pseudo rapidity along the beam direction. Figure VI.1 shows the beam view of EMCal and DCal. Three 20 degree φ- segments of EMCal are back-to-back with three 20 degree φ-segments of DCal. This is the maximum back-to-back coverage possible in ALICE given the installed location of the EMCal and the ALICE space frame support system. Each 20 degree φ-segment of EMCal comprises two EMCal super modules each spanning Δη=0.7 units of pseudo rapidity. The resulting full EMCal coverage for a total of 10 super modules is Δη=1.4 x Δφ=100o . The corresponding full coverage for DCal is Δη=1.4 x Δφ=60o but is built up from effectively 6 separate DCal “super modules” and 3 PHOS modules spanning in combination the full acceptance. This is illustrated in figure VI.2 which shows the 6 DCal super modules and a total of 4 PHOS Modules – only three of which are contiguous with DCal super modules. At the moment, only these 3 PHOS super modules are installed in ALICE and the other one is pending funding. In any event, the single missing PHOS module is not relevant to the proposed DCal physics programme. 

DCal super modules are built up exactly as they are in EMCal, out of strip modules with each strip containing 12 modules of 4 towers each. All of these building blocks including all readout and trigger electronics are identical and interchangeable between EMCal and DCal. The details of the module and strip module construction, optical read out, electronics and cosmic ray calibration are explained in detail in the EMCal Technical Design Report. 

Perspective view of the DCal and PHOS integrated on a common support. The PHOS charged particle veto detectors are in place in front of the PHOS modules. As discussed in the text, the support structure is a component of the full international project scope. Four PHOS modules are shown although only three, those contiguous with the DCal, are installed in ALICE at the moment and considered part of DCal. Also shown on the support structure, to the left of DCal, are five super modules of a hypothetical Very High Momentum Particle Detector (VHMPID) which is currently under discussion in ALICE.

The Dcal has been built by the same international team from institutes in France, Italy and the US that built the EMCal, with additional new contributions from institutes in Japan and China.