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The LHCb experiment @ CERN
R. Graciani (U. de Barcelona, Spain)for the LHCb Collaboration
International ICFA Workshop on Digital DivideMexico City, October 2007
ICFA Workshop Mexico 2007
The LHCb experiment @ CERN
2
Overview
• Introduction
• The LHCb experiment
• The LHCb Computing Model
• Making Physics @ LHCb
• Summary
Introduction
ICFA Workshop Mexico 2007
The LHCb experiment @ CERN
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The Large Hadron Collider
LHC = 27 km
CERN is a laboratory where scientists unite to study the building blocks of matter and the forces that hold them together.
CERN
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The LHCb experiment @ CERN
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The structure of Matter
PARTICLES:
LEPTONS&QUARKS
FORCES:
The Standard Model=
+
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The LHCb experiment @ CERN
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The Large Hadron Collider
Geneva
protons
protons
14 TeV = 14·1012 eV
The LHCb experiment
MATTER:
ANTI-MATTER:
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The LHCb experiment @ CERN
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Where is anti-matter?
• In principle:– particles and anti-particles are produced in pairs
and,– they annihilate in pairs.
E = mc2
• But, our world is made out of matter.• So, where is the anti-matter?
– There must be some difference to explain its absence.
• One of the ingredients is CP asymmetry.
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B physics @ LHCb
• The aim of the experiment, is to study with very good precision:– CP asymmetry in B systems, and– rare B decays.
• The Standard Model predicts differences between matter and anti-matter for B-mesons.
• BaBar (@ PEP-II) and Belle (@ KEKB) experiments have recorded 500·106 bb pairs each.
• But @ LHC:– there is a much larger bb cross section:
• 500 μb 1012 bb pairs / year
– All B-hadrons are produced:• B+(40%), B0(40%), Bs(10%), b(10%)
ICFA Workshop Mexico 2007
The LHCb experiment @ CERN
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679 Physicists
49 Institutions
15 Countries
The LHCb Apparatus (I)
B
B
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The LHCb experiment @ CERN
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The LHCb Apparatus (II)Average RAW event size:Trigger 1.0 kBVelo 5.5 kBRICH 4.3 kBTrackers 14.8 kBMuon 1.3 kBCalo 9.7 kBTotal 36.5 kB
ICFA Workshop Mexico 2007
The LHCb experiment @ CERN
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The LHCb Apparatus (III)
ICFA Workshop Mexico 2007
The LHCb experiment @ CERN
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Taking Data
40 MHz
0.2 kHz35 GB/s
1.8 kHz700 TB/y
2·1010 Evt./y
pp collisions occur every 25 ns
LHCb selects interesting signals
Online reconstruction and
selection
The result is 20 billion events with ~2 billion are full bb pairs.
1 MHz
ICFA Workshop Mexico 2007
The LHCb experiment @ CERN
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Studying B decays
• Final states must be fully reconstructed:– “Simple” 2 body decays:
– “Easy” to find leptonic decays:
– Or “complicated” multi-hadron decays:
• Different physics studies require different B decays to be reconstructed.
• From the first level of the trigger events are classified in different streams (up ~30).
0s
0 )(/B KeeJ
0B KK0sB
)(B0s KKDs
0s
0 )(/B KJ
The LHCb Computing Model
ICFA Workshop Mexico 2007
The LHCb experiment @ CERN
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Processing Data
DST75kB/evt
RAW35kB/evt
Strip
pin
grDST
20kB/evt
Analysis
Reconstruction
nTuple10kB/evt
@ CERN + 6 Tier1’s
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The LHCb experiment @ CERN
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Simulating Data
DST250kB/evt
Produced at Tier2’s, stored @ CERN + 6 Tier1’s
Simulating 1 event ~ Reconstructing 100 events
ICFA Workshop Mexico 2007
The LHCb experiment @ CERN
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Computing Resources
• Reconstruction & Stripping: – @ CERN + 6 Tier1’s (CNAF, GRIDKA, IN2P3,
NIKHEF-SARA, PIC, RAL).– Several iterations per year, require:
– ~ 5.3 M SpecInt2000
• Monte Carlo Simulation:– @ Tier2’s– 4 billion evt./year, require:
– 11.4 M SpecInt2000
• Storage:– ~ 4.0 PB of Tape– ~ 3.2 PB of Disk
Making Physics @ LHCb
ICFA Workshop Mexico 2007
The LHCb experiment @ CERN
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Analysis @ LHCb
• Require full reconstruction of B hadrons from their decay products.
• Many tens of channels are interesting.– Expect > 106 events for some channels– Expect < 100 events for other channels
• There are many other channels.– Backgrounds, contaminations, …
• Use different data samples for different parts of their analysis.
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Typical analysis
• Use a signal channel: O(105 evt).– Produce several selections or ntuples to study their
properties.• Use few control channels: O(105 evt).
– Produce several selections or ntuples to study backgrounds, systematics, contaminations,…
• Use some inclusive sample: O(106 evt).– Produce several selections or ntuples to study efficiency,
resolutions,…• Use Simulated data: O(105 evt).
– To cross check models, hypothesis,…• Estimate: 107 evt. x 10 kB / evt. = 100 GB
– Produced at Tier1’s and transfer to Local resources for further reduction/processing/analysis.
• For larger statistics up to 1 TB are expected.
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The LHCb experiment @ CERN
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Typical analysis
DSTAnalysisnTuple
DSTAnalysisnTuple
DSTAnalysisnTuple
DSTAnalysisnTuple
Signal
Monte Carlo
Control
Inclusive
Now the physicist iterates over these data to prepare his publications.
@ Tier1’s
@ Tier3
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The LHCb experiment @ CERN
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Response for Analysis
• Assume all data can be re-analyzed @Tier1’s in O(1 day).– Reasonable since all data will be on disk.
• How long does the physicist need to get back the result?– 100 Mbs link @ 50 %
• 100 GB / 50 Mbs = 4.5 hours• 1 TB / 50 Mbs = 45 hours
Summary
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Summary
• Processing LHCb data will need large amounts of CPU, Storage and Networking.
• LHCb has decided for a Computing Model where most of the network traffic is between Tier1’s (including CERN).
• Tier2 contributions to simulation are not limited by bandwith (in total few MB/s).
• Our Brazilian colleges are actively participating in the preparations for the analysis of LHCb data and are expected to contribute as successfully in the future.
• Response time for analysis may be slow if “good” connectivity is not guaranteed.
• LHC will start in 2008 and we hope to be at full speed by 2009.• LHCb will measure CP violation and improve our understanding
of the difference between matter and anti-matter.
