1. CMS Electromagnetic Calorimeter and V-V fusion processes detection (general aspects of my research activity and status of my thesis)
I.N.F.N. Turin

2. Summary (I) Electronics of Electromagnetic Calorimeter
physics goals of CMS
the CMS detector
the CMS Electromagnetic Calorimeter (ECAL)
ECAL read-out chain components
motherboard testing
VFE testing
Weak bosons fusion for Higgs detection
Higgs production in LHC
Weak vectors fusion
PHASE MonteCarlo simulation
Signal extraction from PHASE generated sample
19/01/06

3. Higgs boson channels
The experiment CMS and ATLAS at LHC have as main goal to detect new particles:
Higgs boson(s), predicted by symmetry-breaking model
supersymmetric companions of known particles, predicted by supersymmetric models
LHC parameters:
cross section [mb] 100 (60 inelastic)
energy 2× 7.0 TeV
particles/bunch 1.15·1011
bunches 2808
crossing rate 40 MHz
beam size [cm] 5.3
luminosity [cm-2s-1] 2∙1033 ÷ 1034
19/01/06

4. Compact Muon Solenoid (CMS)
An uniform magnetic field of 4 tesla includes Trackers and Calorimeters (Compact!)
19/01/06

5. Electromagnetic Calorimeter (I)
Electromagnetic Calorimeter guide lines:
small (it must be placed within magnetic solenoid together with hadronic calorimeter)
homogeneous calorimeter of scintillating lead crystals to gather most of released energy
high granularity and low Moliere radius to cope with pile-up, allow better spatial resolution, even of photons from π0 decay
goal resolution (barrel; energy in GeV):
𝐸 𝐸 = 2.7% 𝐸 ⊕5.5⊕ 200𝑀𝑒𝑉 𝐸
19/01/06

6. Electromagnetic Calorimeter (II)
Coverage:
azimuth (φ) direction: full (2π)
pseudo-rapidity: |η| ≤ 3
Granularity:
∆η × ∆φ = (0.0175)2 (barrel)
Thickness: ~26 X0
Dynamic range: ~16 bit, 30 MeV ÷ 2 TeV
Endcap (25 X0)
(1.48 < |η| < 3)
Preshower (3X0)
(1.65 < |η| < 2.6)
Barrel (25.8 X0)
(|η| < 1.48)
Barrel:
36 supermodules with 1700 crystals each, 20 along φ and 85 along η, grouped 4 modules and in 68 5×5 trigger towers η M1 M2 M3 M4
19/01/06

7. ECAL electronics overview×5 ×5 ×5
LVR FE
VFE MB
19/01/06

8. Lead crystals and photomultipliers
active material PbWO4
average size 22×22×230 mm3
density 8.28 g/cm3
X cm
λI 22.4 cm
rM 2.19 cm
spectrum centred on 420 nm (blue)
decay times ~15 ns
light yield 50~100 photons/MeV
-2%/K
Photomultiplier requirements:
can operate under a 4T magnetic field
radiation hard
big gain
Avalanche PhotoDiodes (APD):
gain: 50
quantum efficiency: 75%
but:
small surface: 25 mm2 each
very sensitive gain: 3.3%/V at 400 V, -2.2%/K
19/01/06

9. Motherboards Motherboards Connections test
designed, managed and tested in Turin
connect 5 photodiode pairs, temperature sensors, a Low Voltage Regulator and 5 VFEs
distribute high and low voltage respectively from external Power Supply Units and Low Voltage Regulator boards
flexible connections (kapton) are employed
many capacitors provide power stability
they arrive to Turin from manufacturer with no test behind
Connections test
design: custom board, PC (GPIB) driven
checks all 240 connections in about 5 minutes
19/01/06

10. Motherboards: burn-in test
Motherboards are left under 500V high voltage for at least 72 hours to verify the components resistance to stress.
During the test motherboards are constantly monitored and disconnected on failure. Measured current (ideally null) decreases fast; part of this current is generated by impurities and it fades out in time; another residual part is made mainly of superficial currents, which can't be removed.
Current [μA] in 18 MB
in 5 days-run
19/01/06

11. Very Front-End boards: a close view
Very Front-End boards digitise signals coming from APDs and temperatures. Each VFE handles 5 channels, each one made of:
a capacitor
a Multi-Gain Pre-Amplifier (MGPA), an analogue amplifier with three different gains (1×, 6×, 12×) following a common pre-amplifier;
a custom AD41240 ADC: it has 4 channels, 12 bits each; it digitises all three amplified channels from MGPA, then returns the highest, non-saturated value (12 bit) and which that channel was (2 bits)
a buffer of 256 words
VFEs convert input signals continuously at 40 MHz frequency; each signal is 10 samples long.
ADC
MGPA
buffer
To FE
To motherboard
19/01/06

12. Very Front-End boards: calibration
Charge [pC] → ADC count
12× : σS/S = 1.24%
6× : σS/S = 1.14%
1× : σS/S = 1.25%
19/01/06

13. Very Front-End boards: database
We have defined a “protocol” for VFE life-cycle; each passage is recorded in a MySQL-based database; there are currently three physical databases in Lyon, Turin and at CERN.
19/01/06

14. Statistics Motherboard (full process in Turin; 68/SuperModule):
all 2650 MB will be ready in July 2006
1300 MB are ready now
production and testing rate: 3 SM/month = 210 MB/month
problems found:
bad soldering (can be repaired in Turin laboratory)
inverted kapton soldering
bad kapton
burn-in failure
VFE (CERN, Lyon, Turin; 340/SuperModule):
all barrel VFEs will be ready in Spring 2006
all VFEs are assembled, 7500 are calibrated, 7000 VFEs are ready now
no burn-in failure so far (9000 VFEs)
functionality test failed on 69 VFEs (0.5%)
physical defects (bad assembled housing, broken connectors: 0.8%)
barcodes repetition (35 VFEs)
19/01/06

15. Summary (II) Electronics of Electromagnetic Calorimeter
physics goals of CMS
the CMS detector
the CMS Electromagnetic Calorimeter (ECAL)
ECAL read-out chain components
motherboard testing
VFE testing
Weak bosons fusion for Higgs detection
Higgs production in LHC
Weak vectors fusion
PHASE MonteCarlo simulation
Signal extraction from PHASE generated sample
19/01/06

16. Higgs production processes
Due to the way Higgs boson couples with particles, the most probable channel is one-loop gluon-gluon fusion, with a top inner triangle, followed by direct production by W+W-/Z0Z0 fusion.
19/01/06

17. Weak vector bosons fusion
Note: Higgs boson compensate a divergence in VL-VL scattering in S.M.; missing H0, a new way must occur to avoid unitariness violation
Tags:
weak boson decaying into leptons
two jets in end-caps
Examples of core reactions in V-V fusion processes:
19/01/06

18. PHASE MonteCarlo generator
designed with V-V fusion in mind: qq → qq qq ℓνℓ
exact computation of matrix element following S.M., currently at order αem6
thousand diagrams reduced to 16 groups made of 4 sets: 4W, 2Z2W, one b-quark pair and two b-quark pairs processes
uses modularity to avoid computing again the same parts of diagrams
Higgs boson can be “switched off”
some approximations: massless fermions (except b and t quarks), no Higgs-light quark coupling (only H-b)
customizable cuts are used; e.g.:
electron: E > 20 GeV, pt > 10 GeV/c, || < 3
quarks (jets): E > 20 GeV, pt > 10 GeV/c, || < 6.5, mjj > 10 GeV/c2
forward/backward quarks required (|| >1)
19/01/06

19. PHASE: V-V fusion processes
processes with Higgs boson: V-V fusion with Higgs boson in s or t channel
processes without Higgs boson: electroweak V-V fusion (always present!)
19/01/06

20. PHASE: non-V-V fusion processes
PHASE MonteCarlo generates events with an exact computation, which sums up all possible diagrams; it's not possible to neglect or forget unwanted (non V-V fusion) processes during generation, and they must be cut later.
19/01/06

21. Other background processes
There are also some classes of processes that PHASE doesn't simulate at all, which can produce or mimic V-V fusion final state
same final states:
mimic V-V fusion final states:
 𝑠 2  𝑤 4 :𝑔𝑔 𝑞 𝑞 𝑞 𝑞𝑉𝑊
 𝑠 4  𝑤 2 :𝑔𝑔 𝑞 𝑞 𝑞 𝑞𝑗𝑗𝑊
 𝑠 4  𝑤 2 : 𝑞 𝑞 𝑞 𝑞𝑉𝑊
 𝑠 3  𝑤 2 : 𝑞 𝑞 𝑞 𝑞𝑗𝑊
19/01/06

22. PHASE data format
PHASE output is a tagged text file; each event has 8 particles and 12 lines.
Incoming quarks are extracted from 7 TeV/c protons, according to CTEQ5L PDF
Particles ID, following PDG standard,
(here: b b b b u d e νe)
Particle direction
(-1 = incoming, 1 = outgoing)
IDUP
ISTUP
E E E E+03
E E E E+02
E E E E+03
E E E E+02
E E E E+03
E E E E+03
E E E E+03
E E E E+03
ICOLUP
SCALUP E+03 AQEDUP E-02 AQCDUP E+01
Particles 4-momentum
(cpx;cpy;cpz;E) [GeV]
αem(Q2)
αs(Q2) (unused)
𝑄 2 ≡ 𝑚 𝑊 2  1 6 𝑖=1 6 𝑐𝑝 ⊥,𝑖 2
Strong charge streams, passed to hadronisation
19/01/06

23. Tag quarks cut
To cut away processes with incoming quarks annihilation in γ, W± or Z0 (or Higgs!), the first requirement is that event must be compatible with a event topology where each incoming quarks emits a W± or Z0, identifying in this way a pair of “tag quark lines”. In this process, each outgoing particle is assigned a role including “tag quark”, “central quark” (non-tag quarks, deriving from a weak boson) or lepton. Apart from the latter, all assignment suffer from potential ambiguities, which are resolved (arbitrary) choosing the pair closest to W/Z mass as central quarks and the minimum tag quarks scattering angle.
Event statistics for: no-Higgs sample
cross section (σ) 0.69 pb
total PHASE events
without tag quark lines
combinations with same tag candidates 6811
multiple central quarks combinations
fusion candidates
initial quark/antiquark state
19/01/06

24. PHASE: incoming quarks
Distributions of incoming light quarks (d, u, s, c) follow Parton Distribution Functions of proton; bottom quarks presence, instead, is enhanced by the big cross section for events simulated by PHASE, mainly developing into a bb → tt, s- and t-channel processes. Incoming top quarks are simply not included. d t b c s u
No-Higgs sample
(288k events)
Note: particles are indicated in purple, antiparticles in underlined orange.
19/01/06

25. PHASE: weak bosons “resolution”
Invariant mass of “central” quarks (left) and leptons (right) shows that emitted weak bosons are usually on shell, with a width of about 2 GeV for both W± and Z0. mW ΓW mZ ΓZ
19/01/06

26. Three bosons cut Window: 10 GeV/c2
Cut away events whose tag quarks pair has an invariant mass less than 10 GeV/c2 away from W/Z.
Cut results in / events last with ε = 96%; cutting only W has ε = 98% but purity < 99% (leaving ~2705 Z events)
19/01/06

27. Top quark cut (I) b b t t b b
A “top candidate” is a bottom tag quark coupled with the correct W boson; such a couple is labelled “top” if its invariant mass is closer than 20 GeV/c2 to nominal top mass (175 GeV/c2)
19/01/06

28. Top quark cut (II)
Within PHASE, no V-V fusion event can have a top quark: an event is discarded if it has even one top candidate; / events pass the cut, with ε* = 95%. These are my current signal (about 25% of starting sample).
Two points per event!
19/01/06

29. A closer look: weak bosons
Leptons (right) and “central” quarks (left) have η direction strongly correlated, meaning that their boson has a non-negligible boost.
On last plot the PHASE cut on electrons (compatible with ECAL acceptance) can be seen.
19/01/06

30. A closer look: pseudorapidity
qt,2
qt,1
Cuts have slightly “moved” tag quark closer to beam direction.
before cuts (288k)
after cuts (100k)
qt,2
qt,1 qV
19/01/06

31. The show must go on
the V-V fusion sample (100k events) must be hadronized
hadronized products must pass through CMS detectors simulation
a proper set of cuts must be defined to maximise the signal against the huge background
quantities are to be identified which are sensitive enough to reveal the presence of resonances
quantify the resolution on Higgs boson from this channel
19/01/06

32. (when you haven't enough)
Backup slides
(when you haven't enough)

33. Analogue signal path
19/01/06

34. (hey, you aren't supposed to see these!)
Old slides
(hey, you aren't supposed to see these!)

35. Large Hadron Collider (LHC)
located in the old LEP tunnels at CERN
four main experiments: ALICE, ATLAS, CMS, LHCb
two beam pipes for two protons or 208Pb82+ nuclei, 8 cross points
Beams data for protons nuclei
cross section [mb] 100 (60 inelastic)
energy 2× 7.0 TeV 2× 208× 2.76 TeV
particles/bunch 1.15·1011 7·107
bunches 2808 ~600
crossing rate 40 MHz 8 kHz
beam size [cm]
luminosity [cm-2s-1] 2∙1033 ÷ ·1027
19/01/06

36. Front-End boards
Front-End boards (FE) collect data from 5 VFE and compute partial energy sums to be transmitted for trigger generation; on board:
7 FENIX chips, able to support three working modes:
sums (5 chips): each chip provides sum of digitized and calibrated energy of 5 crystals (along φ direction)
total sum: sum of 5 partial sums plus “fine grain” bit which describes whether energy deposit is very narrow
data formatting for transmission
a Communication and Control Unit (CCU) for slow control via a double-linked token ring
two Gigabit OptoHybrid (GOH) data Mb/s for data and trigger transmission
To reduce data load, zero suppression is applied on each crystal first; a selective readout is directed by off-detector boards to read only interesting trigger towers.
Trigger
GOH
CCU
FENIX
Readout
19/01/06

37. Lead crystals (PbWO4) active material PbWO4 average size 22×22×230 mm3
density 8.28 g/cm3
X cm
λI 22.4 cm
rM 2.19 cm
n 2.3 (θγ > 25 are totally reflected)
spectrum centred on 420 nm (blue)
decay times 5 ns (39%), 15 ns (60%), 100 ns (1%)
light yield 50~100 photons/MeV
-2%/K
19/01/06

38. Avalanche PhotoDiodes
Main requirements for ECAL barrel photomultiplier are:
to be able to operate under a strong magnetic field
radiation hardness
big gain
Avalanche PhotoDiodes (APD) fulfil these requirements with some drawbacks:
based on 200 μm silicon
gain: ×150 and over (50 was chosen with ~400V feed)
quantum efficiency: 75%
but:
small surface: 25 mm2 each (two of them are used on each crystal)
4.5 photoelectrons/MeV
very sensitive gain: 3.3%/V at 400 V, -2.2%/K
As for crystals, temperature control and monitoring are needed.
19/01/06