E.Kistenev Large area Electromagnetic Calorimeter for ALICE What EMC can bring to ALICE Physics and engineering constrains One particular implementation How much it will cost Schedule

Large area calorimeter will: deliver the rate for high Pt photons; make possible the low level triggering on electrons and photons(*); allow precision jet measurements; allow triggering on jets (e/m component is good enough); allow for correlated photon-jets physics; allow for parton dE/dx measurement via leading particle spectra in tagged jets (direct access to measuring modifications to fragmentation function); (*) Neither TRD nor EMCal can do this job alone, pion decays in flight will become a main source for TRD triggers, large energy deposits from hadrons will dominate the EMCal trigger.

STAR Design

Problems&Solutions Too High Occupancy. Relevant parameters are: E low pt in the angular cone in which the shower is measured; overlap probability (two hits in the same calorimeter cell). Handles: calorimeter density and/or granularity; calorimeter depth and longitudinal segmentation: very high energy shower has much of its energy at depths where the low pt showers have died away. PS. Overlaps are irrelevant to the high Pt showers.

Problems&Solutions Energy measurements: Photons and electrons In the central AuAu event at LHC the average “foreign” energy per tower is ~ 25 MeV - use “essential contributors” only. Pile-up does limit the precision of the energy measurements for the lower end of the shower energy range, but not in the “natural range for High Density QCD at LHC ” around ~ few GeV;

Problems&Solutions Energy measurements: Jets In the most of LHC experiments it is the uncertainties of jet definition what limits the resolution not the shower- type dependence E jet = (E EMCal (depth > 1Labs) ~ 0.75 E impigent ) + corrections from tracking;

If functionality (energy and position) is not separated reaching few mm goal within the framework of traditional design requires matching cell size to radiation length (one needs a reasonable amount of energy to leak out of the hit cell to measure impact position) -> cost prohibitive for large area devices. Problems&Solutions Position measurements: Have no effect on Pt measuremnts; Only secondary to effective mass measurements; Constrains are set by track-to-shower matching: few mm resolution is certainly sufficient.

Problems&Solutions Angular measurements: very useful to reject non-vertex background; nearly a must if diamond is large and more then one event per crossing is possible; costly - but desirable

Particle Id: primarily e/h separation but can do better

Energy measurements (E - P matching)x 100 (*) Lateral shower shape x 50 (*) Longitudinal shower shape x 2 (*) Signal timing structure? (*) Unfortunately - calorimeter based criteria are correlated: practical limit to hadron rejection in a stand-alone calorimeter is ~200 for a few GeV/c hadrons.

ANTIBARYON SHOWERS Late arrivals in EMCal (  -flash corrected > 2.5 ns) Shoulder consistent with antibaryon contribution EMCal ToFeffective at low energies, works nicely for antyneutrons

Something about time segm.

ALICE EM calorimeter (1) full coverage (rate&jets) but hermeticity is not a must; (2) energy resolution of (15-20)% at 1 GeV-> comparable tracking and calorimeter resolution at a lower limit of the “natural range for High Density QCD at LHC ” (3) deps of ~ 25 Lrad / 1 Labs (em resolution + jets); (4) high density to limit shower size (it also helps to limit the cost); (5) relatively coarse granularity - two high Pt showers are unlikely to overlap, limit is set by  0 background to prompt photons; (6) some degree of a pointing capability; (7) high light yield to retain ToF capability; (8) upgradability -> to offset initial cost.

May EMC be designed and built along these lines and still be reasonably costed: The answer would be YES if design allows to resolve internal contradictions between density, granularity and ability to point. B.Aubert et al, NIM, A309, 438 (1991)

Sampling fraction = 10.5% Energy resolution = 15% (3mm plates)

Why Accordion… very uniform; no dead areas; very linear - autocompensation for light attenuation in the fibers; best possible position resolution for a given cell size; shower shape is very sensitive to impact angle - built-in pointing; multiple options for longitudinal segmentation, relatively easy industrialization.

Energy resolution ~ 15%

Basics of costing: PHENIX EMCal Design-> $US PHENIX EMCal Mechanics -> $US (*) Fibers-> $US Assembly&testing-> $US PHENIX EMCal Readout PMT’s-> $US HV-> $US LV-> $US FEM-> $US (4k/FEM - production cost only) Total-> ~ $US + FEM development costs (~ $US) (*) Cost per kg of active media $15

ALICE large area EMCal (mechanics) Cost/kg (active media) 20 $US Contingency 50% Cost (active media - mechanics)~ $US Industrial comp. (fibers etc)~ $US ______________________________________________________ Development costs (incl. R&D)~ $US Support structures (10%)~ $US ______________________________________________________ Total~ $US

ALICE large area EMCal (readout) Cost per channel: APD’s(  =5 mm)$ 50 (*) readout$ 20 power$ 5 Total per channel $75 Channel count: 5x5 cm260k-> $US 7x7 cm230 k-> $US 10x10 cm2 (staged)15k-> $US (*) Smaller size APD’s are the option - we may use smaller diameter fibers and loose some light but regain the timing - all this is the subject for optimization

Fine tuning the specifications Baseline simulation of the EMCal performance & optimization Decision on longitudinal segmentation Prototype design: multiple options Readout evaluation Prototype construction Envelope studies Infrastructure design Test beam Prototype readout Detector Design Construction 2 Years 1.5 Years 6 months Time scale for the project to complete