Antoni van Leeuwenhoek, 1632-
1723: invention of the microscope discovery first bacteria (“kleine beestjes”), 0.5 - 500 μm
E. coli (size ~ 1 μm)
Minimum discernible dimensions ~ λ limit when using visible light: 0.5
μm improvement to ~ 1Å possible using STM, AFM
4

Idea: use particles to “see” smaller structures
Rutherford : scattering of α-particles
( 4 He nuclei, E α
≈3 MeV) off a gold foil
Quantum mechanical translation: de Broglie wavelength λ ~ h/p
Planck’s constant projectile momentum diffuse charge distribution
(Thomson) charge distribution with nucleus
(Rutherford)
Repeated in the 60’s with scattering of 180 MeV electrons on protons
➠ 5 the proton (r ~ 1 fm) contains further sub-structure (quarks)!

Present scheme in CERN’s Large Hadron Collider: accelerate proton beams to energies of 3.5 TeV per proton v/c ≈ 0.99999996 (energy to be doubled in 2014: 6→8) in both directions!
make them collide in the centres of the detectors experiments analyze outcome of collisions and select
“interesting” events stochastic process, no control over outcome of individual collision
➠ can only select after the fact
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6

Einstein (1905): photo-electric effect
➠ particle nature of light
Paradigm change!
EM interaction described as photon exchange
Graphical representation:
Feynman diagrams
(intuitive way to compute outcome of scattering processes in QM)
7

• High energy allows for the creation of other, usually short-lived particles
τ < 10 -22 s for
“interesting” particles in collisions : convert kinetic energy into mass in decay processes: reconstruct mass of the decaying particle (if all decay products are measured) m Λ
(not the Higgs particle...)
8

•
Exchange / production of heavy particles!
e ∓ e
± e
±
W and Z particles are heavy!
Most collisions between
M
W
= 80.398(25) GeV (~ Sr, Kr)
M
Z
= 92.188(2) GeV (~ Ru)
Discovered in pp̅ collisions, E
CM protons involve the strong interaction
➟ look for leptons (only EM
= 630 GeV
9

“Leptons”??
particles not involved in the strong interaction (only weak, EM)
5 ⋅ 10 -3 1.5
173 also heavier counterparts of the electron and its neutral partner, ν e
Quarks: particles also susceptible to the strong interaction again, 3 “generations” involving heavier partners than the u , d that are constituents of the proton
Force carriers:
5 ⋅ 10 -3 0.3
5 ⋅ 10 -4 0.1
1.8
< 10 -6
91
80 photon (EM), W/Z (weak interaction), gluon(s) (strong interaction)
The only missing family member: the Higgs particle
0
0
11

The Universe at large is isotropic and homogeneous !
CMWB: temperature fluctuations ~ 10 -5 K
WMAP 7-year results, full sky
12

Rotational symmetry: laws of physics do not depend on any direction
Symmetries are important in many areas of physics e.g. conserved quantities like angular momentum in the case of rotational symmetry
In particle physics, this idea is extended to internal symmetries that can turn particles into one another the origin of our description of all (EM, weak, strong) interactions but this symmetry must be broken!
This is what the Higgs field does: interactions obey symmetry ground state does not
The Standard Model is invalid w/o Higgs!
13

Particles become “effectively” massive by means of their interaction with the Higgs field!
More physical analogy: refractive index caused by different speed of light in medium caused by forward scattering of light by the medium
Photons in the medium are effectively massive
14

Spontaneous symmetry breaking equivalent ground states
Massive photons
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excitation
Meißner effect: superconductor repels magnetic field lines massive photons but needs a medium (e condensate)!
pair
15
In the particle physics case, the “medium” is the vacuum!

M
H unknown, but for given M
H all Higgs boson properties fixed
➟ know “exactly” what to look for direct searches at previous colliders indirect evidence from precision measurements
16

Expectations for 2011 exceeded by a factor 5
18

We need this performance!
interactions at hadron colliders dominated by strong interaction when searching for Higgs boson production, need to suppress backgrounds by ~ 10 10
Look for striking signatures setting the Higgs boson apart from more ubiquitous
“background” processes
1 pb = 10 -36 cm 2
√s ≡ E
CM now
19

+
-
Relatively large event rate, but leptonic
W boson decays lead to unobserved neutrinos cannot reconstruct mass of a system decaying to WW
Large mass range excluded already in Summer
20

Requires excellent discrimination between single high-energy photons from hadrons but offers good energy resolution
21
Looking for small excess on top of large (but smooth) background

Very rare process, especially with both Z particles decaying to leptons but very clean, and with good mass resolution
22
Found 3 candidate events at low mass:
2 in e + e μ + μ final state (124.3 GeV,
123.6 GeV)
1 in μ + μ μ + μ final state (124.6 GeV)

23

H→W + W -
H→γγ
24

H→ZZ
Found 2 candidate events near 126
GeV
1 in e + e e + e -
1 in e + e μ + μ -
Combination
25

ATLAS: CMS: excess in W + W final states: broad but compatible with lowmass Higgs boson excess in W + W final states: broad but compatible with lowmass Higgs boson excess in ZZ final state (124
GeV) excess in ZZ final state (126
GeV) excess in γγ final state (126
Caveat emptor!
GeV) excess in γγ final state (123 GeV) each individual excess not statistically significant masses in γγ, ZZ are close but do not match
➠ questions: are the energy calibrations as well understood as we think?
is this just a statistical fluctuation after all?
Time (and additional investigation) will tell
26

27

CERN press release including pointers to further information: http://press.web.cern.ch/press/PressReleases/Releases2011/PR25.11E.html
28

Finding the Higgs boson does not mean particle physics is finished!
The Standard Model cannot incorporate gravity in a consistent way
The Higgs boson’s mass is not stable against radiative corrections
The Standard Model does not explain Dark Matter / Dark Energy
29

30

Noether Theorem:
Every symmetry of a physical system comes with an associated conserved quantity (and vice versa) also indicates how to construct these conserved quantities, given the symmetry
Examples: translational symmetry ⇔ conservation of momentum rotational symmetry ⇔ conservation of angular momentum
Emmy
Noether
31

Well-known system: interaction of magnetic dipole moments with external magnetic field
Zeeman splitting of (atomic) energy levels
Spin precession around B-field axis, Larmor frequency ω=ϒB
Unlike regular QM, QED provides a prediction for g !
g =2
A triumph for QED!
expansion in powers (up to fifth
A similar measurement for the muon (μ ±
) looks much more like a regular HEP setting...
32

Three quarks forming baryons (and quark-antiquark pairs forming mesons): a new symmetry (and interaction), co lo ur
“gauge principle” interaction with gluons : quarks change identity (colour) under exchange of a gluon!
33
Quark confinement at low energy

Responsible for all nucleonic transmutations and particle decays fusion radioactivity (β decay)
•
Truly a weak interaction: solar ν flux on Earth: ~ 6 ⋅ 10 14 m -2 s -1 during your lifetime, at most a few will interact with your body at all!
34

Three fermion “generations” doublet structure ordered by mass
W-boson couples charged leptons to ν (and up - to down -type quarks)
35

At high energies, quarks and gluons do manifest themselves as
“free” particles → hadron jets e electron-proton scattering: 27.5 GeV +
920 GeV
36 jet

QED, QCD: photon & gluons are strictly massless
Weak interaction: massive W and Z bosons fermion masses: (and similarly for quarks)
And worse!
W-boson deals with left-handed fermions (right-handed anti-fermions) only
λ= -½ λ= +½ p̂
Ŝ left- and right-handed fermions should be different particles this requires them to be strictly massless
37

Required: a mechanism to break the EW symmetry spontaneously
Lagrangian maintains full EW symmetry but the ground state does not !
Achieved through the introduction of the (complex scalar) Higgs field
With μ < 0 : minimum at ϕ≠0
Generation of fermion masses through “Yukawa” couplings:
38

39

In addition to individually observable particles: neutrinos (from apparent lack of momentum conservation) hadron jets (from calorimeter energy deposits/tracks)
τ leptons (very narrow “hadronic jet”) b-jets (from hadronisation of bquarks: “long” lifetime of Bhadrons, τ
B
≈1.5 ps)
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40

mb:
H
( ≲ 135 GeV): VH associated production, leptonic V decay background suppression by 10-11 orders of magnitude required
•
A straightforward strategy, but leading to a large number of final states
41

Searches at LEP dominated by ZH associated production (“Higgsstrahlung”)
Example distribution (also other variables used)
42

pp̅ collisions, √s =
1.96 TeV mature collider and experiments running since 2001
CDF
Tevatron
8.0 fb -1
7.1 fb -1
Main Injector
43
DØ
•
•
1 fb = 10 -43 cm 2 if σ =1 fb: need L=1 fb -1 to produce one event many interesting processes have σ ~
100-1000 fb

DØ’s discovery track record... kin. cuts leptonic FS jets evidenc e only...
Especially interesting: single top production: same final state as WH →lνbb̅ similarly for WW: irreducible bg to high-M
H search channel
44

Note the consistent (but not yet significant) signal-like behaviour for low M
H
: a first hint?!
45

No significant signal-like excess observed...
➟ set limits
Procedure:
Compare data compatibility with s+b / b-only hypotheses (each M
H
)
Calibrate outcome with toy experiments
Compare resulting distributions with observed Q
CL b/s+b
≡ fraction of backgroundonly/signal+bg experiments less signal-
1-CL b hypothesis if CL s+b
<
0.05
46 observed
CL s+b