The LHCb experiment @ CERN

R. Graciani (U. de Barcelona, Spain)for the LHCb Collaboration

International ICFA Workshop on Digital DivideMexico City, October 2007

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Overview

• Introduction

• The LHCb experiment

• The LHCb Computing Model

• Making Physics @ LHCb

• Summary

Introduction

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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 structure of Matter

PARTICLES:

LEPTONS&QUARKS

FORCES:

The Standard Model=

+

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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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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%)

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679 Physicists

49 Institutions

15 Countries

The LHCb Apparatus (I)

B

B

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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

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The LHCb Apparatus (III)

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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

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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

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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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Simulating Data

DST250kB/evt

Produced at Tier2’s, stored @ CERN + 6 Tier1’s

Simulating 1 event ~ Reconstructing 100 events

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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

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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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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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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.