Introduction to particle physics Lecture 7 - ippp.dur.ac.ukkrauss/Lectures/IntroToParticlePhysics/Lecture... · LHC: Accelerator and Detectors Physics at the LHC The quest for the

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


Outline

1 LHC: Accelerator and Detectors

2 Physics at the LHC

3 The quest for the Higgs boson

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CERN and the LHC

Distribution of CERN users

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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).

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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

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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

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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.

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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).

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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.

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Scheme of particle detection

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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.

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Different field configurations of ATLAS and CMS:

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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.

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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)

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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.

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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

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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

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Higgs production cross sections at hadron colliders

(from M.Spira, hep-ph/9810289)

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Higgs decays

(from V.Buescher & K.Jakobs, Int. J. Mod. Phys. A 20 (2005) 2523)

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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)

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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”.

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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)

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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.

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Introduction to particle physics Lecture 7 - ippp.dur.ac.ukkrauss/Lectures/IntroToParticlePhysics/Lecture... · LHC: Accelerator and Detectors Physics at the LHC The quest for the