Download Computing at the Large Hadron Collider in the CMS

Computing at the
Large Hadron Collider
in the CMS experiment
www.iphc.cnrs.fr/-cms-.html
Daniel Bloch
at the heart of matter
électron : < ~10-20 m
>10–9 m
10–10 m
< ~10–20 m
10–14 m
10–15 m
molecule
atom
nucleus
proton/neutron
quark
chemistry
particle physics
electromagnetic interaction
strong interaction
weak interaction
2
the standard model
§ 
§ 
§ 
§ 
§ 
Elaborated in years 1960-70
Describes both elementary particles (grouped in 3 families) and
electroweak (unifies em and weak int.) and strong interactions
Relies on symetry properties
(conservation laws)
Experimentally tested with great precision
(~10-3 for em and weak unification)
Higgs mecanism: to explain the origin
of particle masses
=> a new
particle:
Higgs boson:
discovered
at LHC
in 2012
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beyond standard model: supersymetry ?
§ 
Supersymetry (Susy) is an
elegant theory which
considers matter and
interaction particles on an
equal footing:
§  many new Susy particles
are predicted,
but not their mass !
(+ many other parameters)
§  accessible at LHC ?
§ 
dark matter candidate
~
χ01
~χ0
3
~χ0
2
χ~0
~±
χ
4
Susy would solve some of our fundamental questions:
§  the Higgs boson has been found, but why is it so light ?
§  the lightest Susy particle (neutral, weakly interacting) would be an
excellent candidate for dark matter
§  electroweak and strong interactions can be unified in Susy models
§ 
But there are also known complications with Susy:
§ 
§ 
not found yet ! It would be ``natural’’ to get it at TeV mass scale
the minimal Susy model is (amost) ruled out => the number of free
parameters may be large (between > 5 and up to 124 …)
proton collisions at LHC
• 
• 
• 
• 
2800 bunches of protons
energy of each proton : 6.5 TeV
100 billions protons / bunch
beam crossing rate: 40 MHz
bunches
•  in the experiments at each
crossing:
~ 20-50 proton-proton collisions
~ 1500 particles produced
protons
•  1 billion interactions / second
constituents
(quarks, gluons)
•  impossible to record everything !
•  a Higgs boson to find
within 5 billions of
collisions…
5
proton collisions at LHC
• 
• 
• 
• 
2800 bunches of protons
energy of each proton : 6.5 TeV
100 billions protons / bunch
beam crossing rate: 40 MHz
bunches
•  In the experiments at each
crossing:
~ 20-50 proton-proton collisions
~ 1500 particules prodiuced
protons
•  1 billion interactions / second
constituents
(quarks, gluons)
•  A Higgs boson to find
within 5 billions of
collisions…
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the CMS collaboration
•  38 countries
•  183 institutes
•  ~3000 scientists
(permanent,
post-doc, students)
•  ~100 French people
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the CMS detector
as a 3D camera
of 14000 t, 29 m length, 15 m height
with 75 millions of pixels and taking
40 millions of pictures per second
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the CMS detector
concentric layers, each with well defined
detection purpose
Tracker
magnetic field 3.8 T
reconstruct charged particles
pixels : 100×150 µm2
66M chanels, 1m2
Silicon strips :
9M chanels, 210 m2
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the CMS detector
Calorimeter(s)
Measure the energy of particles
(except muons and neutrinos)
em. (ECAL) :
76k cristals PbWO4
hadronic (HCAL) :
scintillators/Cu
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the CMS detector
Muon Detectors
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what happens during collisions ?
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trigger and data acquisition
1rst level trigger: 100 kHz
660 000 MB/s
from all sub-detectors
high level trigger: 1 kHz (100 ms / event)
600 MB/s
raw data
600
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first data reconstruction at CERN
7
800
30
600
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The World LHC Computing Grid
a worldwide net of many computing sites:
CERN CERN çè national centers çè academic centers
Tier 0
Tiear 1 : 12 sites
Tier 2 : 140 sites
15 the grid in France and at Strasbourg
CERN
Tier 0
çè
CC IN2P3
çè
Tier 1
CERN 8 sites
Tier 2
at IPHC Strasbourg:
•  Tier 2 for CMS and Alice
•  15% of ressources for other
local usage (subatomic
physics, protéomic, bioinformatics, …)
20M h of computing/year,
2000 slots,
1400 TB disk space
16 event size and reconstruction time
•  depends on the intensity of the beams:
•  superimposed interactions ~ 20 to 50
•  => affects the size of the events and their reconstruction time !
•  event size ~0.2 to 0.9 MB
•  reconstruction time per event ~15-80 s
•  need also to generate and simulate data: ~50 s / events
•  total number of events per year (real data + simulated): 5 Billions
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CPU and disk needs
TIER 0
CMS
CPU
DISK
6k cores
15 PB (+35 PB tape)
25k cores
30 PB (+70 PB tape)
60k cores
30 PB
2015
TIER 1
CMS
2015
TIER 2
CMS
2015
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event reconstruction
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§ 
§ 
What we look for:
example of the production of a
Susy-top pair
experimental signature:
top-quark pair +MET
§ 
What we observe in the detector…
§ 
§ 
§ 
§ 
Information in calorimeters: ECAL,HCAL.
Trajectories of charged particles.
Muons (red line).
Missing Transverse Energy (MET: arrow).
χ0
χ0
event reconstruction (cont)
point of decay
of a jet from b quark
(accuracy ~100 µm)
•  Jets of particles with
the trajectograph and
calorimeters
•  Identification of jet
from b decay
(« b-tagging ») with
pixel detector: distance
of decay flight
pp collision
point
•  Missing Tranverse
Energy (MET) :
hermetic detectors =>
allows one to measure
the energy and
direction of invisible
particles (neutrino,
dark mater if any) in
the plane transverse to
the beam axis
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event reconstruction (cont)
Reconstruction
of jets
Rejection of
events from
pile-up pp int.
Identification
of the event
Selection of jets
need to reject the large background
§ 
§ 
from different processes, but giving a similar signature
can be :
§  Physical background (irreductible), with same final state
§  Instrumental background: due to badly reconstructed particles
jet
jet
jet
jet
jet
§ 
need to define the sensitive variables to enhance the signal:
§  production rate (cross section).
§  invariant mass (or related quantities) of the initial particles
electron
+ MET
processing flow for a typical analysis
§  trigger: electron+jets or muon+jets or ee, eµ, µµ
rate ~200 Hz at 8 TeV (about 1/3 of all recorded events)
§  selection of the reconstructed events
and writing of reduced size information (Ttrees)
§  input, read on the Grid:
§ 
§ 
§ 
§ 
data: 500M events
simulation: Susy signal = 150M events, backgrounds = 150M events
overall size: 0.4 MB × 500M + 0.7 × 300M ~ 400 TB
processing time: 2 s / event
repeated 2-3 times after each new version of the data reconstruction
total: 1M hours of computing time
§  output, stored at IPHC on the Grid (can be used worldwide):
§  100M data events, 150M simulated events
§  overall size: 30 TB with full information, 5 TB with reduced information
§  processing time (fast) : 2.5 ms / event on local cluster
=> ~200 hours for all data and simulation
but repeated many times (~10)
search for the Susy-top
§  no Susy-top observed so far (at 8 TeV collision energy):
set limits on its mass and on dark matter particle
§  need to go higher in energy: 13 TeV collisions from this year