Transcript The Higgs Boson What it is and how to find it
The Higgs Boson
What it is and how to find it Roger Barlow Manchester University
Particle Physics: the Goal
To deduce the laws of physics using the minimum number of arbitrary assumptions
"What really interests me is whether God had any choice in the creation of the world." --Albert Einstein
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Elementary Particles: (1) The electron
Known for 100 years e • Very common • Very light: mass of 9.109 10 -31 kg • Very small (pointlike?) • Described by Quantum Mechanics. Wave function (r,t), a solution of the Schr ödinger Equation –(ħ 2 /2m) 2 =E Slide 3/26
(2) The photon
Argument:Wave function has an arbitrary phase Constant change of phase: e i does not change physics It would be ‘nice’ if variable change of phase: physics…but e i (r) did not change terms mess up Schr ödinger Equation Modify S.E. new term And if e i then A –(ħ 2 /2m) ( -ieA) 2 A+(1/e) =E (Gauge Transformation) A(r) describes another particle: Gauge Boson. Spin 1, interacts with electron, has zero mass (no A 2 term)… the photon Hence electromagnetism,Maxwell’s Equations,Etc Everything predicted except the actual value of e Slide 4/26
(3) The positron
Relativity: Schr ödinger Equation replaced by Dirac Equation -i ħ a .( -ieA) + m =E is not just one complex function but 4.
Extra components describe spin (up/down) and particle/antiparticle Antiparticle has opposite charge Many more processes possible e QuantumElectroDynamics QED e + Slide 5/26
(4) The quark
quark - like an electron (has charge, spin ½, has antiparticle) But also has an extra (triple) quantum number. Called ‘colour’ – red (1,0,0), green (0,1,0), blue (0,0,1) Needed because of the Pauli Exclusion Principle in particles such as the D ++ , made of 3 otherwise identical quarks.
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(5) The gluon
Argue: the choice of red-green-blue axes arbitrary. Physics should not change if we switch around Or even if we rotate the axes in r-g-b space. Rotation matrix
R…
Even if
R
varies with position+time… extra
R
terms in equations.
Need extra function in equation with appropriate gauge transformation New massless particle – Another Gauge Boson the gluon Similar to QED but more complicated due to matrix structure:QuantumChromoDynamics - QCD.
Arbitrary constant is much larger than e.
Strong
force.
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Pause for breath
Understand Electromagnetism and the Strong (nuclear) force, apart from a few arbitrary(?) constants. And technical details of calculations That’s everything except gravity and beta decay. Not a ‘Theory of Everything’ but a ‘Theory of quite a lot’ Can’t do gravity…. But should manage beta decay Slide 8/26
Beta decay as it ought to be…
n p e d u e Quarks in protons/neutrons/nuclei are in two ‘flavours’: u and d. (Different charges and masses) u and d are two states of the same fundamental entity - the quark e and are two states of the same fundamental entity – the lepton (Weak) isospin up or down.
Run gauge theory argument again for up down… predicts Gauge Bosons W + , W 0 , W e W d Slide 9/26 u
Slight(?) problem
Gauge Bosons have got to be massless.* Or the Gauge Invariance of the equations breaks down.
• Photons • Gluons • The W bosons They exist alright – but have masses ~80 GeV.
Theory stuck here for some time * Mass: The minimum energy needed to create a particle Slide 10/26
The Higgs Field
Suppose there is a field called H(r,t) that interacts with the electron, quark, W etc OK, why not Suppose that the lowest-energy stats is not H(r,t)=0 but H(r,t)=V Seriously weird Slide 11/26
Masses that are not masses
1. As a W propagates through space and time, it interacts with this nonzero Higgs field… 2.
Which gives it an energy….
3.
Even if it has no kinetic or potential energy… 4. Which means it has, to all intents and purposes, a mass. Without breaking gauge invariance Happens to quarks and leptons too Slide 12/26
The Standard Model
• Quarks and Leptons (x3 ‘generations’) • Gauge Symmetries for the Weak, Strong and EM force • Higgs mechanism giving masses to the W bosons • Also mixing/unifying Weak and EM forces • Also explains weak decays between generations (with a few more parameters) Slide 13/26
Is the Standard Model true?
Yes!
Predicts W/Z mass ratio Predicts cross sections and branching ratios in many many particle decays Accounts for parity violation Accounts for CP violation in K and B sectors No experimental results in disagreement No!
Does not predict quark and lepton masses Or coupling constants… 28 free parameters altogether Or why there are 3 generations Or why there is parity violation Higgs is an ad-hoc addition Slide 14/26
Testing Higgs: from field to particle
H?
Higgsness • Quantum excitations of the H field are H particles (Same as any particle, though usually about 0) • The Higgs coupling of any particle is proportional to its mass. (actually the other way round…) H is best made by massive particles H will decay to the heaviest allowed particles Slide 15/26
Is the Higgs true?
• Probably not – it’s a very arbitrary kludge • Many alternative theories have been proposed that are more elegant/beautiful/natural • All have very similar effects until you get to high (TeV) energies Slide 16/26
e+ e Z*
First Attempt: LEP
q Collide electrons and positrons at energies of 200 GeV Z q b H b Slide 17/26
Saw some events, but..
Consistent with background M H >114 GeV Slide 18/26
Second Attempt: the LHC
Proton proton collisions at 14 TeV Start operation next year Slide 19/26
Experiments: ATLAS and CMS
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Common features
Tracking • Magnetic Field • Measure charged particle tracks with drift chambers or Silicon • Curvature gives momentum Calorimetry • Material so Neutral particles interact • Measure total energy by scintillator etc Muon detection • Muons get through the calorimeter Slide 21/26
Looking for signals
Decay depends on M H Plots shows signal if M H fairly large Smaller values more difficult Slide 22/26
Handling the data
• Collision rate 40 MHz • Several events/collision • Each event gives massive amount of data • Massive data stream. >10 TB/y • Tiny number of interesting events Handled by Grid of computers all over Europe - and the world Slide 23/26 10,000+ CPUs
Third Attempt: the ILC
Electron positron collisions at 1 TeV Still at the design stage Straight (not circular) Chicago? Japan??
38 km? $6Bn?
Start 2015+? Slide 24/26
Why?
• LHC is a proton-proton collider • Protons are made of quarks • LHC is actually a quark quark collider • Quarks share proton energy in a random way 500 GeV 1 TeV 500 GeV Precision measurements junk ?
junk 7 TeV Exploration • A 14 TeV proton proton collision gives a whole spectrum of energies for quark quark collisions • And the unused energy appears as background particles
The Future
• LHC will start next year • First serious data 2008+ • Interesting results 2-3 years? after that • Should find Higgs - probably not quite as expected • Other new particles/new effects predicted by speculative models (SUSY? GUTs?) • Exploration will be followed by precision measurements at the ILC • Build Beyond the Standard Model theory with fewer arbitrary parameters • Understand the universe we live in a little bit better Slide 26/26