1. A Forward Calorimeter (FoCal) as upgrade for the ALICE experiment at CERN S. Muhuria , M. Reicherb and T. Tsujic a Variable Energy Cyclotron Centre, Kolkata, India b Department of Physics, Utrecht University, Utrecht, the Netherlands c CNS University of Tokyo, Japan
Motivation
As an upgrade of the ALICE experiment at the CERN-LHC, we would like to build and install a Forward Electromagnetic Calorimeter (FoCal) to be placed in the pseudorapidity region of 2.5 < η < 4.5, at the position of the existing Photon Multiplicity Detector (PMD). The basic motivation of including the calorimeter in the forward direction is to study outstanding fundamental QCD problems at low Bjorken-x values, such as parton distributions in the nuclei, test of pQCD predictions and to probe high temperature and high density matter in greater detail. A comprehensive measurement of p-p, p-Pb and Pb-Pb collisions at the highest LHC energies will be required. For these measurements, the detector needs to be capable of measuring photons for energies up to at least E ~200 GeV/c. It should allow discrimination of direct photons from neutral pions in a large momentum range and should also provide reasonable jet energy measurements. At present, two possible designs are being considered based on silicon-tungsten calorimetry. We will present physics motivation of this project, measurement items, conceptual detector candidates, and basic performance for the measurements
in this poster presentation.
120cm
Eta=2.5
Eta=3.0
Eta=4.0
132 tower
Conventional Pad+micro-pad readout
A tower of the FoCal is a single self-contained detector unit with full depth of the detectors. In order to fully absorb photons of 200 GeV a depth of 25 X0 is needed. The typical longitudinal segmentation will be 24 layers, made of a tungsten absorber and a detection plane of silicon sensors. The transverse size of a tower is driven by availability of silicon wafers in bulk which makes wafers of size either 6.3 cm×6.3 cm or 9.3 cm×9.3 cm. The transverse segmentation inside a tower should provide the required spatial resolution. This can be accomplished by layers with pads of typically 1 cm size, with some high resolution layers before shower maximum.
Detector Simulation
Detector simulation has been conducted using AliRoot or standalone GEANT4 to verify the detector performance. Basic characteristics such as sampling fraction, linearity, resolution, efficiency for single photon and pi0, sophisticated clustering logic have been checked.
Hardware Development
Since the channel density is so high, Application Specific Integrated Circuit (ASIC) has been developed. Specifications for the ASIC is summarized as follows. Prototype ASIC for pad readout has been fabricated and test will be done in the future.
Development of micro-pad has not been started yet. Candidates are MAPS (Monolithic Active Pixel Sensor) type or HAPS (Hybrid Active Pixel Sensor) with x mm2 pads and existing ASICS (HAL25, Beetle, etc). The issues for the latter case are to match the dynamic range (external CMOS attenuator or thinner sensor) and the way to get the signal out.
Physics Measurement
Accessible pT and parton x by the correlation measurement
Pi0 and Jets
High rates, large Q2 scale
Correlation to shrink x region
(due to g+g dominant process)
Prompt Photons
Low Q2 scale,
Small ratio of photon/pi0
Fragmentation contribution
Heavy-bosons
Large Q2 scale
Large pT dileptons
p0 (2<y<4) and jets (2<y<4)
p0 (2<y<4) and jets (-1<y<1)
x2~
x2~
First segment
Second segment
Third segment
Si micro-pad
Tungsten
Si pad
CPV
One of the possible specifications
W thickness: 3.5 mm (1X0)
wafer size: 9.3cmx9.3cmx0.525mm
Si pad size: 1.1x1.1cm2 (64 ch/wafer)
Si micro pad : MAPS or < x mm2 in preshower stage
W+Si pad : X0layers
3 or 4 longitudinal segments
Summing up raw signal longitudinally in one segment
Or readout from individual layers
Accessible pT and parton x by prompt photon measurement
x1 and x2
x1(left) and x2(middle)
vs. photon pT in prompt
photon production
(photons at eta 3-3.5).
Coverage of (x, Q2) with FOCAL
Si Pad sensor from Hamamatsu
Size: 9.3x9.3 cm2
Thickness: 535mm
Pad size: 1.1x1.1 cm2
Number of pads : 64
Cpad = 25pF, Idark =2- 10nA
Some examples of RHIC achievement in forward rapidity measurement
Pi0 suppression at forward rapidity
in d+Au at RHIC
Disappearance of away side peak
In pi0-pi0 correlation in d+Au at RHIC
linearity
<1% up to 200GeV
Resolution:
18.2%/sqrt(E) + 1.4%
Longitudinal shower
profile
linearity
<1% up to 200GeV
150 GeV
70 GeV
30 GeV
Resolution:
22%/sqrt(E) + 1.6%
Position resolution by micro-pad
Pi0 identification
Efficiency by pads
Pi0 identification
Efficiency by micro-pads
Efficiency > 50%
for E>60 GeV
Two gamma clusters
start to be merged
Efficiency >50%
for E<50 GeV 7
1.29 2
0.098
x cellid
y cellid 57
12.32 77
22.39 71
16.76 78
16.82 42
12.73 29
4.55
Legend
Data count
Total energy deposition in MeV
Embed 4 gammas with
5mm separation.
Cluster reconstructed
based on the dynamical
fuzzy c-mean Method (d-FCM).
200pF
Clow
560pF
Chigh
5.6nF
10pF
CSA PZ
ASIC
High side
Low side
3mm
4mm
4ch/chip
Prototype chip (TSMC 0.5um)
Pads readout :
Dynamic range: 10fc (1/5MIP) – 200pC (500GeV g)
Cross talk < 1%.
Trigger capability
Dual charge sensitive preamplifier with capacitive division
Single preamp+dual shaper
Dual transimpedance preamplifier
Micro-pad readout:
Dynamic range: 2fc – 500fc
Z = 1/wC + Zcf/A
First prototype
high
low
hspice simulation
2.5usec