A Summary Trigger Unit
for the ALICE Electromagnetic Calorimeters
Olivier Bourrion1), Tatsuya Chujo2), Rachid Guernane1),2)
1,2)
Ritsuya Hosokawa , Hiroki Yokoyama for the ALICE Collaboration
1) Laboratoire de Physique Subatomique et de Cosmologie de Grenoble,
2) University of Tsukuba, JapanThe STU, a FPGA-based embedded electronic device implementing the ALICE Level-1 (L1) calorimeter triggers, has demonstrated during LHC Run 1, efficient realtime selection of events with high transverse momentum jets and photons. During the recent LHC Long Shutdown 1, the ALICE calorimetry system was upgradedwith DCal, a second arm situated back-to-back in azimuth to EMCal and surrounding PHOS. Taking advantage of this two-arm setup, and through the flexibility andscalability of STU hardware design, ALICE has decided to extend the use of STU to both DCal and PHOS calorimeters. A new L1 jet trigger algorithm was thenproposed for Pb-Pb data taking, aggregating DCal and PHOS regions to estimate on an event-by-event basis, an energy density used to correct EMCal jet patchesfrom soft background contamination, and vice versa. The ALICE STUs are now under commissioning since summer 2015 at CERN with the aim to have themroutinely included in data taking.

Physics Motivation

Jet nuclear modification factor RAA for the 10%
most central collisions from ALICE. RAA measuresdeviations in yields from a simple superpositionsof incoherent proton-proton collisions.[1]ALICE Electromagnetic Calorimeters

The ALICE experiment is dedicatedto the study of Quark Gluon Plasma(QGP). Jets are recognized as avaluable probe of the QGP since
their yield is expected to be modified
by the QGP formation. QGP indeed
induces partonic energy loss whichstrongly reflects its properties. As aresult of medium induced energy
loss, the observed jet energy isreduced (Jet Quenching), and the jet
shape is modified. We can assess
QGP effects quantitatively throughcomparison between nucleusnucleus collisions and proton-protoncollisions.

EMCal

PHOS
Calorimeters structure in
ALICE detector system

Summary Trigger Unit (STU)

The electromagnetic calorimeters (EMCal,DCal, and PHOS) significantly enhance
ALICE capabilities to measure high-pt
processes at LHC Run 2. These detectors areable to provide fast triggering, precise energy
measurement, and efficient identification ofphotons and electrons.
‣ EMCal and DCal
‣ |η|<0.7, Δφ=107° (EMCal)
DCal ‣ 0.22<|η|<0.7 ,Δφ=67° (DCal)‣ Pb-Scintillator sandwich type calorimeter‣ 12288 (EMCal), 5376 (DCal) towers‣ tower size: 6.0x6.0 cm2‣ PHOS‣ |η|<0.12, Δφ=70°‣ lead-tungstate crystal (PWO) based‣ 12544 crystals‣ crystal size: 2.2x2.2 cm2L1 data transmitting
L0_confirm

PHOS
PHOS
TRU TRUPHOSTRU
PHOS TRU

L0_localdata(12bit x 112)
40MHz CLK

PHOS
STU

PHOS_L0
PHOS_L1_gamma_low
PHOS_L1_gamma_mid
PHOS_L1_gamma_high

CTP…

*28TRUsjet primitive data
19bit *36subregion

DCAL
DCAL
TRUTRU
DCAL
TRU
DCAL TRU

L0_localdata(12bit x 96)
40MHz CLK

DCAL
STU

DCAL_L0
DCAL_L1_gamma_low
DCAL_L1_gamma_high
DCAL+PHOS_L1_Jet_low
DCAL+PHOS_L1_Jet_high…

*14TRUs

BKG density
18bit

EMCAL
DCAL
TRUTRU
DCAL
TRU
DCAL TRU

The Summary Trigger Unit (STU) computes Level-1 Jet and Photontriggers. The trigger algorithm is implemented on an FPGA (XilinxVertex5) and it can be configured remotely. STU and Trigger Region Unit(responsible for L0 triggers) are connected with 120MHz serial links
while STUs communicate through 80MHz serial links. This lastconnection is needed to compute the Jet trigger.

L0_localdata(12bit x 96)
40MHz CLKEMCALSTU

EMCAL_L0
EMCAL_L1_gamma_low
EMCAL_L1_gamma_high
EMCAL_L1_Jet_low
EMCAL_L1_Jet_high…

*32TRUs

Each STU receives module (2x2 towers) amplitudes from the TriggerRegion Unit (TRU) and applies L1 algorithms to generate L1-gamma
and L1-Jet signals. For L1-Jet, PHOS data are shared with DCal at STUlevel due to the large Jet patch size. Also DCal+PHOS (EMCal)
estimated background density data will be transmitted to EMCal (DCal)
for Jet background subtraction. The Central Trigger Processor (CTP)
collects all trigger output signals from STUs and generates the global
trigger decision for all the ALICE detectors.

L1-algorithm
L1-gamma algorithm
jet primitive(DCAL)
This algorithm is based on module (2x2 towers) amplitudes. The sum
8x8 towersof module amplitudes in a sliding gamma-patch (2x2 modules) arecalculated
andcomparedwiththresholddefinedbytheV0-detector.
jet primitive(PHOS)
6x22 crystals
The on-the-fly calculation is done during serial transmission between
jet primitive(PHOS)TRU and STU.
22x22 crystals‣ L1-Jet algorithm
jet primitive(PHOS)
6x20 crystals
This algorithm is based on jet-primitive (8x8 towers) amplitudes.The
jet primitive(PHOS)
sliding Jet-patch (2x2 or 4x4 jet-primitives) amplitudes and22x20 crystalsbackground patch (2x2 jet-primitives) amplitudes are calculated. The
computed
medianvalue
frombackground
patchesissubtractedby
L1 jet patch
2x2 jet primitives
jet patches of the opposite side detector and compared to a constantthreshold set by DCS slow control. This background subtraction
methodallowstoreduce
theimpactofbackgroundfluctuationsin
L1 jet patch
4x4 jet primitives (option) heavy-ion collisions. The background subtraction will be applied onlyat heavy-ion collisions. The constant threshold will be used in othersystem.
‣121690

23

78

323

L1-Jet trigger patch for DCAL and PHOS side

TRU#X!

8x12[module]

0

12345

6

70

8

91011

12

13

14

15

816

17

18

19

20

21

22

23

16

24

25

26

27

28

29

30

31

24

32

33

34

35

36

37

38

39

32

40

41

42

43

44

45

46

47

40

48

49

50

51

52

53

54

55

48

56

57

58

59

60

61

62

63

56

64

65

66

67

68

69

70

71

64

72

73

74

75

76

77

78

79

72

80

81

82

83

84

85

86

87

80

88

89

90

91

92

93

94

95

88

0

1

23

45670

gamma-patch = 4x4 towers
XXmodule = 2x2 towers

L1-gamma trigger patch[1] The ALICE collaboration, J.Adam et al., Measurement of jet suppression in central Pb-Pb collisions at √sNN = 2.76 TeV, Physics Letters B 746 (2015) 1–14