Great Experiments : Solar Neutrinos · 2008. 5. 1. · Great Experiments Easter Term 2008 Dr Mark...

Great ExperimentsEaster Term 2008Dr Mark Thomson

Dr M.A. Thomson Easter Term 2008 1

The W & Z Bosons

Y

XZ

Introduction: Forces in the Standard Model


Force Boson(s) JP m/GeV

Forces mediated by the exchange of spin-1 Gauge Bosons

Photon γ 1– 0EM (QED)W± / Z 1– 80 / 91Weak

Strong (QCD) 8 Gluons g 1– 0Graviton? 2+ 0Gravity (?)

g

• Fundamental interaction strength is given by charge g.• Related to the dimensionless coupling “constant”

Particle interactions described in terms of Feynman diagrams

e– µ–

e+ µ+

γ

“time”

INITIAL FINALFor today main points:•“time” runs from left – right, but

only in sense that:LHS of diagram is initial stateRHS of diagram is final stateMiddle is “how it happened”


Electroweak UnificationElectricity and Magnetism superficially look very differentMaxwell: described by different aspects of a unified theory of EM

Unification of Electricity and Magnetism:

Unification of Electromagnetism and the Weak Interaction:Electromagnetism and the weak interaction look very differentBut strong hints that they are related, …W-bosons are electrically charged ! “Strength of an interaction” has two components

• Intrinsic strength “coupling constant”• In QM “harder to exchange an energetic force carrying particle”

Intrinsic strength of EM and the WI are rather similar – coincidence ?

e–e–

e– e–

e–

e–

NO

The weak interaction is only weak because the W boson is massive

The Glashow-Weinberg-Salam ModelThe Glashow (1961), Weinberg (1967) and Salam (1968) model threats

the Weak and EM interactions in a unified manner• It is rather ad hoc• But, like all (useful) scientific theories it provides concrete predictions

which can be tested experimentally • Basic idea – start with 4 massless bosons . The

neutral bosons mix to give the physical particles that we see.

is the Weak Mixing angle• The W and Z bosons acquire mass through the Higgs Mechanism• Model gives exact predictions, e.g.

A nice idea, but correct ? When proposed the W and Z had not been discovered.


The need for high energyBy the mid-1970s, the discovery (or not) of the W and Z was the highest priority in particle physics At this time the expected W and Z masses were 65 GeV/c2 and 80 GeV/c2

To make these particles in an accelerator, need a high energies


Fixed target:

• e.g. a high energy beam of protons hitting a stationary target• conservation of momentum means after the collision still have a lot ofK.E., what matters is the total energy in the centre-of-mass (ZMF) frame

• At CERN, the 7km circumference Super-Proton-Synchrotron (SPS) startedoperation in 1976 with 300 GeV/c2 protons for fixed target experiments

• But insufficient to produce W and Z bosons

Colliding Beam:

• already in centre-of-mass frame, no energy “wasted”• BUT experimentally much more challenging !

The CERN Proton - Anti-proton ColliderIn 1976 a new CERN accelerator, the “Large Electron-Positron” LEPCollider, was being considered (see later). Long term project (started 1989)– and physicists wanted answers…

A revolutionary alternativeSimultaneously accelerate protons and anti-protons in the SPS ring

UA1• 3 bunches of protons• 3 bunches of anti-protons• Beams circulate for ~12 hours • Collisions at 6 points around ring• At two of these points build large

detectors (UA1 and UA2)UA2


Anti-protons made by smashing high energy protons into a targetAnti-protons have wide range of momenta and are not very localisedNeed to “cool” the anti-protons prior to injection to the SPSessentially reducing spread in space/transverse momenta

Stochastic Cooling

Looking along the beam line

Performed in the anti-proton accumulator


Betatron OscillationsAs particles circulate through an accelerator they oscillate aroundthe nominal beam orbit (betatron oscillations) Cooling ~ reducing the amplitude of these oscillations

Beam correctionsIf only one particle in the accelerator, in principle, could:

• detect its position using a beam pickup at one point in orbit• apply an electrostatic kick to move it closer to the nominal orbit

Stochastic Cooling


Stochastic coolingIn practice anti-proton accumulatorfilled with many particles

Beam pickup samples part of beamCorrection applied so as to move the average of those particles closer tonominal orbit

After many small corrections, beam iscooled and can be injected into theSPS and then accelerated

The Experiments


UA2 UA1 UA1

UA2

Two experiments : UA1, UA2 (Underground Area 1 and 2)Large “multipurpose detectors” (discussed later)Each a collaboration of ~150 physicistsCapable of tracking charged particles and identifying electrons and

muons

The Signal for W ProductionWhat to proton – anti-proton collisions usually look like ?

Most likely: a strong interaction between a quark in the proton and an anti-quark in the anti-proton which “knocks out” the struck quark/anti-quark leaving the remnants of the and travelling in roughly the original directionDue to an odd feature of the strong interaction, isolated quarks are never

observed, instead JETS of particles are observed


Can also produce a W boson by quark – anti-quark annihilationBut much rarer ~1:109

Spotting the WsThe experimental challenge

Find a handful of W bosons produced in approx. 1 billion interactionsLook for clear signatures of a W-boson10 % of time the W boson decays:Because the W bosons are massive (80 x proton mass), the electron and neutrino are energetic

Signal• isolated electron• missing “transverse

energy” from ν

Background• jets• “transverse energy”

balance


Discovery of the W and Z ! W bosons produced in the collisions leave very clear signatureJust run the machine…


Missing ET

e-

• Nov/Dec 1982: 109 interactions• 9 events (5 for UA1 and 4 for UA1)

consistent with fromwere observed

First Results

Discovery of the W-boson !

• April/May 1983• More W-bosons…

e.g. UA1 saw 54• Four events consistent

with and one

Second Run

Discovery of the Z-boson !

CommentsDiscovery of W and Z led to the 1984 Nobel Prize for physics being awarded Simon van der Meer and Carlo Rubbia for

"for their decisive contributions to the large project, which led to the discovery of the field particles W and Z, communicators of weak interaction"

Major technical achievement !From the point of view of the discovery of the W and Z bosons, the experimental signals are very clear and control of experimental systematic uncertainties not a major factor Also measured some of the properties of the produced particles to prove they were the W and Z



From discovery to precision physics…Proton – anti-proton colliders (or proton – proton colliders)Advantage(s):

• Relatively “easy” to accelerate protons to high energies• Good “discovery machines”

Disadvantage(s): • Not colliding fundamental particles• In addition to interesting interaction

also see remnants of proton • Not all energy goes into collision• Harder to make precise measurements

Electron – Positron collidersAdvantage(s):

• Fundamental particles• All energy goes into interaction• Clean environment for making precise

measurementsDisadvantage(s):

• Harder to accelerate (synchrotron radiation)• Larger accelerator for same energy

The Large Electron Positron ColliderThe Large Electron Positron (LEP) Collider at CERN (1989-2000) was designedto make precise measurements of the properties of the Z and W bosons.

1989-1995: Electron-Positron collisions at √s = 91.2 GeV17 Million Z bosons detected

1996-2000: Electron-Positron collisions at √s ~ 200 GeV30000 W+W- events detected

•26 km circumference acceleratorstraddling French/Swiss boarder

• Electrons and positrons collided at4 interaction points

•4 large detector collaborations (each with 300-400 physicists):

ALEPH, DELPHI,L3, OPAL

Opal

AlephL3

Delphie+

e-

Essentially a large Z and W factory:

SPS

LEP


e+e- Annihilation in Feynman Diagrams Electrons/positrons do not feel the strong force, interactions dominated byEM and weak force


At Z resonance: Zexchange dominant

Well below Z: photonexchange dominant

High energies:WW production

These are (amongst) the dominant processes – not much “background”i.e. most high energy interactions at LEP are “signal”

Practical Particle Physics

The particles produced interactions are observed and identified in large multi-purpose detectors

e- e+

All have same basic geometry: cylinder and two endcaps

Need to detect and identify particles as they cross the detector volume and measure their energies

(an overview of how measurements are made in a collider experiment) • What determines the design of a detector ?• How are measurements made in practice ?

• aim to detect all the particles coming from an interaction


e.g. The OPAL Experiment Detection and particle identification achieved with complex detectors consisting of

many different “sub-detectors”


θ ϕ

x

y

Hadron calorimeters

Electromagneticcalorimters

Muondetectors

Jetchamber

Vertexchamber

µVertexdetector

Solenoid andPressure vessel

SiW luminometerForwarddetector

4 main categories

Tracking Chambers• charged particles

ECAL (Electromagnetic calorimeter)• electrons/photonsHCAL (Hadronic calorimeter)• hadrons (i.e. protons, neutrons)MUON chambers• muons

Different technologies used by the four different experiments, e.g.

OPAL Tracking Chamber


AnodeWires

ChargedParticle

Ionization • positive ions and liberatedelectrons drift in electric field

• Charge collected on sense wires, producing an electrical pulse

• Particle bends in B-field,curvature particle momentum

• Charged particles ionize gas

• Reconstruct full path “track”of particle

Reconstruction (computer algs):

Detection

OPAL Electromagnetic Calorimeter (ECAL)Detect photons and measure energies of photons/electrons

e-

e-

γ e-e+

Ee

• When an e±/γ enters blockit produces a e±/γ cascade

• ECAL : e.g. 11705 Pb-Glassblocks (10x10x30 cm3)

• light detected using photo-multiplier tubes


Particle Identification

ELECTRON PHOTON MUON PION NEUTRINO

• Essential to be able to identify particles produced in interaction• e.g. if searching for decays, look for high energy electron

• Different particles leave characteristic signals in the different “sub-detectors” – making particle identification possible

HCAL MUONµ±

γ

e±ν

π±


Event Identification at LEP


At Z resonance mainly observe four types of event:

Each has a distinct topology in the detectors, e.g.

Annihilation rate at LEP few interactions per second Signals from the detector for each interaction are used to form reconstructed “events”From topology/particle ID; the events can almost always be identified clearly

In practice, events are identified by computer algorithms

Measurement of the Z Boson Mass

The Z-boson lives for ~10-25 sHow to measure its mass ?Remarkably simple !Production of the Z boson is a “resonant”process, the rate increases rapidly when the

energy is equivalent to the rest mass energy of the

In the first phase of LEP, the accelerator was run with a centre-of-massenergy close to the Z boson massZ bosons produced directly in the annihilation

To measure the mass, vary the energy ofthe accelerator in several steps close to peak of the resonanceDetermine the event rate, essentially countingFit to find the peak Millions of events fractional statistical errors (1/√N) are small.

Systematic Uncertainties ?Dr M.A. Thomson Easter Term 2008 23

including the unexpected Systematic Biases…To achieve 2 MeV precision – need to know energy of the colliding beams to better than 0.002 %. A this level of precision sensitive to unusual systematic effects…

As the moon orbits the Earth it distorts the rock in the Geneva area very slightly !The nominal radius of the accelerator of 4.3 km varies by ±0.15 mmChanges beam energy by ~10 MeV : need to correct for tidal effects !

Moon:

Final result


Leakage currents from the TGV railway line return to Earth followingthe path of least resistance.Travelling via the Versoix river and using the LEP ring as a conductor.

Each time a TGV train passed by, a smallcurrent circulated LEP slightly changingthe magnetic field in the accelerator

LEP beam energy changes by ~10 MeVOnce understood, not a problem

Trains:

0.002 % measurement of mZ !

W+W- ProductionFrom 1995-2000 LEP operated above the threshold for W-pair productionW bosons always produced in pairs (conserve charge)

W bosons decay either to leptons or hadrons with branching fractions:

Again gives rise to three distinct event topologies: easy to identify events !


W Boson MassUnlike , the process is not a resonant process

Different method to measure W-boson Mass•Measure energy and momenta of particles produced in the W boson decays, e.g.

Peak of reconstructed mass distributiongives

Neutrino energy and momentum inferredfrom missing energy and missingmomentum in event

Reconstruct masses of two W bosons

Here there are a number of importantsystematic errors: lack of time to discussin sufficient detail


The Power of PrecisionImpressive measurements – so what ?

Relations between standard model parameters

Hence, if you know any three of : predict the other two.

Within the SM of Electroweak unification:

Weak mixing angleStrength of EM Strength of Weak

From LEP and elsewhere have precise measurements – can test predictions and self-consistency of the Standard Model !

measure•e.g. predict:

•Therefore expect:butmeasure

Close, but not quite right – but have only considered lowest order predictionDr M.A. Thomson Easter Term 2008 27

Precision Tests of the Standard ModelIn QM, the W boson can “fluctuate” into pairs of virtual particlesMass of W boson also includes terms from virtual loops


Above “discrepancy” due to these virtual loops, i.e. by making very high precisionmeasurements become sensitive to the masses of particles inside the virtual loops !From virtual loop corrections and precise LEP data predicted the top quark mass:

In 1994 top quark observed at the Tevatron proton anti-proton collider at Fermilab– with the predicted mass !

+ Precise measurements also ruled out a host of alternative theories+ Major constraint on new theories – have to be consistent with these data

A Window to the HiggsThe (as yet) undiscovered Higgs boson is an essential part of the Standard Model

of particle physics. Without it, all particles have to be masslessThrough virtual loops, precise measurements sensitive to Higgs boson mass


The precision data from LEP and elsewhere, strongly suggest that the Higgs boson (if it exists) is relatively light

If correct, it will be discovered at the LHC !

Summary and the FutureThe Standard Model of Particle Physics is one of the great scientific triumphs of the late 20th century both in terms of the theory and the experimentalcollider physics programme

By the mid-1970s, the discovery (or not) of the Wand Z was the highest priority for particle physics

Their discovery in 1983 provided overwhelming evidence for the idea of electroweak unification


The subsequent precise measurements of properties of the W and Z validated the SM and places strong constraints on alternative theoretical models (many were killed off)

In 2008, the discovery (or not) of the Higgs is the highest priority for PP

the LHC proton-proton collider will push energy frontierfirst results in ~2009/2010?; perhaps the discovery of the Higgs? In the longer term, plans are developing for a new collider (the ILC) to study in detail the Higgs boson and/or whatever else the LHC uncovers…

Great Experiments : Solar Neutrinos · 2008. 5. 1. · Great Experiments Easter Term 2008 Dr Mark...

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