The LHC and Discovery of the Higgs Boson - 50 years of...
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The LHC and Discovery of the Higgs Boson -
50 years of Higgs’Mechanism
Christophe Royon IRFU-SPP, CEA Saclay, France
Colloquium at UFRJ, Rio de Janeiro, February 6 2014
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LHC: Exploration of a New Energy Frontier
Physics exploitation for the next 20 years: The LHC will possibly answersome of the most fundamental questions in modern physics:
• The origin of mass: The Higgs boson
• Unification of fundamental forces
• Matter-Antimatter asymmetry
• New forms of matter (quark-gluon plasma...)
• Extra-dimensions of space time
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Outline
• Introduction to particle physics
• A typical detector: the example of ATLAS
• What is particle physics: a few Physics results
• The Higgs boson discovery at the LHC
• Looking for extra-dimensions in the universe
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CERN-Brazil and French-Brazil collaboration
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• Many people performed their PhD or post-doc in France, in Saclay forinstance
• Long term collaboration in D0: as an example Result of the week ofFermilab in April 2010
• Presently, we have a COFECUB/CAPES collaboration between Saclay,Sao Paulo, Rio, Pelotas and Porto Alegre
• Brazil will be officially associated to CERN like USA, China... in a verynear future
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Scales and tools
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Energy units
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Energy and time
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A long history: searching for fundamental particles
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Starting point: Rutherford
• The Rutherford team discovered the existence of the atomic nucleus
• α particles originating from radium directed onto a very thin gold foil,the α particles are scattered by the nuclei
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Standard Model of particle physics
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Three families of Quarks
Three families of quarks in nature!
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What do we use? The Large Hadron Collider
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LHC components
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LHC: one of the hottest spot in the universe!
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LHC: one of the coldest spot on earth!
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LHC: one of the largest instrument on earth!
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ATLAS (“A Toroidal LHC Apparatus”) detector
An example of a typical generic detector in high energy experiment (CMS ismore compact)
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Detector size
Two general experiments: ATLAS and CMS
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Detector mass
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ATLAS (“A Toroidal LHC Apparatus”) detector
An example of a typical generic detector in high energy experiment (CMS ismore compact)
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The ATLAS Collaboration
ATLAS: ∼ 3000 scientists from 174 institutes and 38 countries
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The elementary particle zoo
SM particles: What about Higgs boson?
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Particle signatures
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ATLAS by pictures
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Magnetic spectrometer
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ATLAS inner detector
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ATLAS performance: tracking and rediscovering resonances
Excellent data/MC agreement for basic tracking quantities
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ATLAS Calorimeter
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ATLAS trigger
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ATLAS trigger
• Trigger: Keep all interesting events (40 MHz nominal collision rate atthe LHC)
• ATLAS different level trigger: ∼75 kHz at first level, >200 Hz outputwritten on tape
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ATLAS computing
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ATLAS computing
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ATLAS computing
• GRID is essential: up to 150k jobs running in parallel
• Extra-resources up to 7.5k CPU cores
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What is a proton?
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Proton proton collisions
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Basic processes at the LHC
• Hard scattering processes represent only a tiny fraction of the totalinelastic and elastic pp cross sections
• Selection of interesting processes using triggers
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Different physics topics
Before looking for the Higgs boson: check the standard model
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Understanding the proton structure
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What happens when hadron collide?
• Difficult to calculate: non-perturbative physics...: example of a“minimum bias” event at 7 TeV
• Fundamental aspects: confinement, hadronic mass generation...
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Jet production at 7 TeV: a new territory
• Extension of measurement up to ∼2 TeV allows to probe standardmodel in a new territory and measure the proton structure in terms ofquark and gluon densities
• Good agreement with Quantum Chromodynamics calculation
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High pT jet and dijet mass events
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The elementary particle zoo: top quark
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Rediscovering the Top quark
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Selection of tt̄ events
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Measurement of tt̄ cross section
tt̄ cross section in agreement with SM
[TeV]s
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ATLAS+CMS Preliminary
TOPLHCWG
-1Tevatron combination L = 8.8 fb-1ATLAS dilepton L = 0.7 fb
-1CMS dilepton L = 2.3 fb-1ATLAS lepton+jets L = 0.7 fb
-1CMS lepton+jets L = 2.3 fb-1TOPLHCWG combination L = 1.1 fb
-1ATLAS dilepton L = 20.3 fb-1CMS dilepton L = 2.4 fb
-1ATLAS lepton+jets L = 5.8 fb-1CMS lepton+jets L = 2.8 fb
NNLO+NNLL (pp)
)pNNLO+NNLL (pCzakon, Fiedler, Mitov, PRL 110 (2013) 252004
uncertainties according to PDF4LHCSα ⊕ = 172.5 GeV, PDF topm
[TeV]s
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7 8100
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The elementary particle zoo: Higgs boson
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Higgs mechanism - part 1
Imagine a room full of journalists quietly chattering; it is like space filledonly with the Higgs field
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Higgs mechanism - part 2
A well-known person (President, Queen of England) enters in the room,This creates a disturbance as he moves through the room and attracts a
group of journalists with each step
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Higgs mechanism - part 3
This increases the resistance to movement of that famous person. In otherwords, he acquires mass just like a particle moving through the Higgs field
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Higgs mechanism - part 4
Now something different: a rumour crosses the room, “the Higgs boson isdiscovered!” for instance
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Higgs mechanism - part 5
It creates the same kind of clustering but this time between the journaliststhemselves. In this analogy, these clusters are the Higgs particles.
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Higgs mechanism
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How the Higgs boson can be produced at the LHC?
• The Higgs boson can be produced via 2 gluons originating from theprotons
• The Higgs boson can decay into two photons (one of the best signals tosee the Higgs boson)
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Two examples of decays of the Higgs boson
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Difficulty of the analysis: pile up!
Many interactions occur in the same bunch crossing
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Looking for the Higgs boson decaying into two photons
• Higgs boson decays to two high energetic photons: Higgs massdetermined from energies and angles of photons
• Background processes look similar but creates no peak!
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Higgs decaying into two photons
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Higgs decaying into two photons
• Diphoton mass distributions
• Both CMS and ATLAS see a peak at about 126 GeV!
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Higgs decaying into two Z bosons
• Look in another channel: Higgs boson decaying into two Z bosons
• Many additional channels studied that show the same presence of theHiggs boson
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Higgs discovery paper
Leads to the Higgs discovery paper: already 2193 citations for atlas (2179for CMS) in ∼1.5 year
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50 years of Higgs mechanism: Nobel Prize 2013
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Higgs boson discovery
Meeting at CERN to celebrate the Nobel Prize
Special tribute to R. Brout who passed away in 2011
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The future? Looking for extradimensions in the universe
• We live in a 4 dimension space: time and space
• Gravity might live in extra-dimensions: exploration at the LHC forinstance in looking for new couplings
• If discovered at the LHC, this might lead to major changes in the waywe see the world
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The future: tagging intact protons at high luminosity
• Very special events: the protons remain intact after interactions! (NB:analogy: accident between two big trucks, and not only the trucksremain intact but some cars are produced in addition in the final state!)
• Tagging intact protons allows to get one of the best methods to lookfor extra-dimensions in the universe
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Looking for extradimensions
γγ
γγ
p
p p
p γγ/mmissppm
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Eve
nts
-110
1
10
210
310 SignalExcl. background
+ pileupγγ
= 14 TeVs-1L = 300 fb
= 50µ-4 GeV-12 = 10
1ζ
-4 GeV-13 = 101
ζ
• Looking for instance for diphoton production with intact protons
• 4-photon coupling enhanced if extra-dimensions exist
• Background can be controlled by comparing the information fromtagged protons and photons
• Might lead to important discoveries: stay tuned
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Measuring the position and time-of-flight of protons
• Very precise measurement of the proton positions: 5-10 µm using Sitechnology
• Measurement of proton time-of-flight: ∼ 10 ps precision Manyapplications for instance in medicine: gain of about a factor 10 inresolution for PET imaging
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Conclusion
• “Rediscovering” the standard model of particle physics: detector wellunderstood
• Nobel Prize 2013 in physics: Higgs boson discovery
• Exactly 40 years after the Higgs mechanism was invented by Brout,Englert and Higgs!
• Tagging protons in the final state: motivated for instance to probeextra-dimension in the universe; Position and timing detectors to beused
• Many devolopments performed/in progress for the project and extremlyuseful for the future in particle physics or medical applications: 3D Si,timing detectors
• Recall also that CERN was at the origin of WEB which everybody usestoday
• Last but not least: Happy birthday for the physics institute, and bestwishes for the ongoing collaboration between France and Brazil!
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