LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Introduction to particle physicsLecture 7
Frank Krauss
IPPP Durham
U Durham, Epiphany term 2009
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Outline
1 LHC: Accelerator and Detectors
2 Physics at the LHC
3 The quest for the Higgs boson
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
CERN and the LHC
Distribution of CERN users
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
The LHC
GeneralFirst beams successfully circulated on 10th September 2008.
Unfortunately on 19th September a serious fault developeddamaging a number of super-conducting magnets. Repair requires along technical intervention which overlaps with the planned wintershutdown.Therefore, no beam in LHC before September 2009.
LHC design: collide two counter-rotating beams of protons (or heavyions). pp collisions are foreseen at an energy of 7 TeV per proton.
Beams move around the ring inside a continuous vacuum.They are guided by super-conducting magnets, which are cooled by ahuge cryogenics system.The beams will be stored at high energy for hours. During this timecollisions take place inside the four main LHC experiments (i.e.detectors).
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Technical data27km circumference
High vacuum (10−10 Torr ≈ 3 millionmolecules/cm3, pressure at 1000 kmaltitude), 1.9 Kelvin, 36800 tons cold mass
1600 super-conducting magnets at 8T,each with mass around 22t.
pp collisions at 14 TeV c.m. energy( Ep = 7 TeV = 1.12 · 10−6 J ≈ 1 mosquito, vp = 0.999999991c)
Protons in 2808 bunches per beam, eachbunch with 200 billion (200 G) protons.( 1.29 · 105 J / bunch × 2808 bunches ≈ 360 MJ energy stored in one beam)
Interaction frequency: 40 MHz( collisions every 25ns)
Aerial and scheme
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Example: The LHC dipoles
Protons will be accelerated to v ≈ c . Tokeep them in the ring, they willpermanently be under a centripetalacceleration produced by Lorentz Force.
Centripetal acceleration:a ≈ c2/r =⇒ ac2.3 · 1013 m/s2
(About 2 · 1012 times the acceleration of gravity.)
This is achieved with the dipole magnets:B ≈ 8.3 T, L ≈ 14.3 m and m ≈ 35 tons
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Detectors
Some general thoughts
Basic principle: Interaction of particles with matter.(matter provided by material of the detector)
Obviously: The more matter, the more interaction.
Design principles:
Hermetic (don’t want to loose particles) – beams must still enter.Need to distinguish particles, therefore different subsystems.Detector needs “provisioning” (electronics, cooling, etc.), will induce“inactive” matter.
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Detecting charged particles
Typically, the following particles are “stable”, i.e. reach the detectorto be detected: e±, µ±, π±, K±, p, p̄.
They loose energy and are deflected through:
Inelastic collisions with atomic electrons(Leads to ionisation and therefore, electrical currents.)
Bremsstrahlung(In the field of a nucleus, light particles, such as positrons or electrons loose energy by emission of
“Bremsstrahlung” photons, scaling like Z2α3 in the field of nuclei with charge Z.)
Emission of Cherenkov photons(If the velocity of the particle is larger than the speed of light inside the material, photons are emitted,
forming a cone with angle cos θc = 1/(βn), where n is the refraction index of the material.)
Elastic scattering from nucleiNuclear reactions
In addition detector’s magnetic fields bend charged particles(allows for charge-to-mass ratio).
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Detecting neutral particles
Typically, the following particles are “stable”, i.e. reach the detectorto be detected: γ, K 0
L , n
For photons there’s a number of reactions:
Photoelectric effectCompton effectPair production
leading to full electromagnetic showers in the detectors whencombined with similar effects for the electrons.
Other neutrals experience nuclear reactions, depositing their energyin the detector. This is parametrised by the free hadronic pathlength X0.
Neutrinos typically do not interact with the detectors, since theyneed huge masses to compensate the small weak cross sections.
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Scheme of particle detection
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Multi-purpose detectors at the LHC: ATLAS and CMS
Main principle: Try to detect everything . . .. . . but at least the muons
Build detector in layers around interaction zone, add magneticfield(s) to bend charged particles’ trajectories.ATLAS and CMS have two different magnetic field configurations:toroid vs. solenoid.
Muons are interesting: at LHC high-energy muons come from thedecay of heavy objects.
Typically, they are simple to track/detect: They do not interact toomuch with the matter of the detector.So, naively, whatever charged particle leaves the detector, it must bea muon =⇒ build muon chambers at the outside.
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Multi-purpose detectors at the LHC: ATLAS and CMS
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Multi-purpose detectors at the LHC: ATLAS and CMS
Different field configurations of ATLAS and CMS:
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
For example: CMS
CMS technical data: Solenoid
Solenoid is a cylinder of super-conducting wires made fromNiobium-Titanium, cooled down to 4 KelvinCMS’ solenoid dimensions: length=13m, diameter = 5.9m, 20kA=⇒4T
CMS technical data: Tracker
220 m2 of silicon sensors. 6m long, 2.2m diameter, 60M electronicchannels for read-out.
CMS technical data: Electromagnetic Calorimeter
Scintillation light in lead-tungstate crystals (PbWO4) 64000 crystalsin barrel, 16000 in end-caps.
CMS technical data: Hadronic calorimeter
Barrel: 36 brass wedges, each with 35 tons; end-caps: recuperatedRussian brass ship artillery shells.
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Physics at the LHC
A long wish-list
There is a long list of questions to be addressed at LHC.
Find the Higgs boson:
verify/falsify our ideas about electroweak symmetry breaking.
Is there new physics beyond the Standard Model:
favourites: supersymmetry, extra dimensions, . . .
many of them have hot candidates for cold dark matter!
Constrain the standard model further:
Precision measurements of masses/widthsmore on CP-violation and the mixing in the quark sector
With heavy ions:
look for the quark-gluon plasma (a new state of matter)
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
General thoughts
Historical trend:Hadron colliders for discovery physicsLepton colliders for precision physics.
Historical trend: Shape your searches - know what youre looking for.This has never been truer.
In last decades: Theory triggers, experiment executes.Also true for the LHC?
There are no LHC events without QCD!!!For nearly all analyses the background is more or less a nightmare,need excellent control over Standard Model physics.
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
The quest for the Higgs boson
Basic findings
Higgs boson not yet found,therefore mH ≥ 114 GeV.
Precision data favour light mH
(see left fit)
Higgs boson couples to heavyobjects - all couplingsproportional to mass.
Problem: No heavy objectsinside beams (p).
0
1
2
3
4
5
6
10030 300
mH [GeV]∆χ
2
Excluded Preliminary
∆αhad =∆α(5)
0.02758±0.00035
0.02749±0.00012
incl. low Q2 data
Theory uncertaintyJuly 2008 mLimit = 154 GeV
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Tayloring search channels
Higgs production processes at hadron colliders
Common feature: Couple to heavy objects (top, W , Z )
Gluon fusion:
f
Higgs-Strahlung:
V = W,Z
Quark-associated: Weak boson fusion (WBF/VBF):
V = W,Z
V = W,Z
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Tayloring search channels
Higgs production cross sections at hadron colliders
(from M.Spira, hep-ph/9810289)
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Tayloring search channels
Higgs decays
(from V.Buescher & K.Jakobs, Int. J. Mod. Phys. A 20 (2005) 2523)
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Tayloring search channels
Some typical channels (mostly @ LHC)
gg → H → ZZ → 4µ, 2e2µ: “Golden plated” for mH > 140 GeV.Key ingredients: Mass peak from excellent mass resolution (leptons).
gg → H → W +W− → ℓℓ′ + E/⊥: nearly as good as ZZ
but no mass peak. Background killed with ∠ℓℓ′ etc..Very similar to current Tevatron analysis with huge stats.
gg → H → γγ: Good for small mH<∼ 120 GeV.
Key ingredient: mass resolution for γ’s & veto on π0’s.
WBF → H → ττ : Popular modeKey ingredient: QCD-backgrounds killed with rapidity gap
WBF → H → WW : ditto.
WBF → H → bb̄: in principle dittobut: Hard to trigger, pure QCD-like objects (jets)
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Summary
Discussed general physics of accelerators and detectors.
Highlighted some technical aspects.
Some physics at LHC, in particular the quest for the Higgs boson.
To read: Coughlan, Dodd & Gripaios, “The ideas of particlephysics”.
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Summary of the course
You should now know/understand the following:
Simple properties, symmetries and calculations:
special relativity & relativistic kinematics in two-body decaysaddition of spins/isospins/weak isospinsthe importance of resonancesparity and its violation as example for a discrete symmetryconserved quantities
Feynman diagrams:
how to draw Feynman diagrams for a given model, in particular QEDand the Standard Modelwhat they are (terms in pertubative expansion)
particles & anti-particles,
Dirac equation & interpretationhadrons (bound states): mesons and baryons, quantum numbers andquark content of the most important ones (p, n, π’s, K ’s, ∆’s)the proton as a bound state (form factor, parton picture)
F. Krauss IPPP
Introduction to particle physics Lecture 7
LHC: Accelerator and Detectors Physics at the LHC The quest for the Higgs boson
Summary of the course (cont’d)
You should now know/understand the following:
the Standard Model:
4 fundamental forces and which are included in the Standard Model,its construction principle (global & local gauge invariance),its particle content (elementary particles),their properties (quantum numbers, masses) and their interactions,strong, weak and electromagneticthe Feynman rules, i.e. vertices of the Standard Model.
principles of particle detection and basic facts about the LHC:
building principle of a detectordetection through interaction with matter - which specific effects arethere?some basics about the search for the Higgs boson.
F. Krauss IPPP
Introduction to particle physics Lecture 7