Transcript g - Experimental High Energy Physics

Higgs rush at CERN
Frank Filthaut
Radboud Universiteit Nijmegen / Nikhef
Contents:
Particle physics: general picture
The Higgs particle
The recent results
Particle Physics: the General Picture
Going beyond the naked eye
Antoni van Leeuwenhoek, 16321723:
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
The atom “cracked”
Idea: use particles to “see” smaller structures
Rutherford: scattering of α-particles
(4He nuclei, Eα≈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
State of the art
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
QuickTime™ and a
Sorenson Video 3 decompressor
are needed to see this picture.
experiments analyze outcome
of collisions and select
“interesting” events
stochastic process, no control
over outcome of individual
collision
➠ can only select after the fact
6
Quantum Electrodynamics
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
Techniques
•
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)
mΛ
(not the Higgs particle...)
8
The Weak Interaction
•
e∓
Exchange / production of heavy particles!
W-boson
Z-boson
production
(and
decay)
and decay
e±
W and Z particles are heavy!
e±
Most collisions between
MW = 80.398(25) GeV (~ Sr, Kr)
protons involve the strong
interaction ➟
MZ = 92.188(2) GeV (~ Ru)
look for leptons (only EM
Discovered in p-p̅ collisions, ECM = 630 GeV
and weak interactions)
9
The Higgs Particle
The Particle Family
“Leptons”??
particles not involved in the strong
interaction (only weak, EM)
also heavier counterparts of the
electron and its neutral partner, νe
5⋅10-3
1.5
5⋅10-3
0.3
173
91
0
80
0
Quarks:
particles also susceptible to the
strong interaction
5⋅10-4
0.1
1.8
< 10-6
again, 3 “generations” involving
heavier partners than the u, d
that are constituents of the proton
Force carriers:
photon (EM), W/Z (weak interaction), gluon(s) (strong interaction)
The only missing family member: the Higgs particle
11
Rotational Symmetry
The Universe at large is isotropic and homogeneous!
CMWB:
temperature
fluctuations ~ 10-5 K
WMAP 7-year results, full
sky
12
Extended Symmetry
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
The Higgs Mechanism
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
Magnetic Analogues
Spontaneous symmetry breaking
equivalent
ground states
excitation
Massive photons
Meißner effect:
superconductor repels
magnetic field lines
massive photons
but needs a medium (e- pair
condensate)!
In the particle physics case,
the “medium” is the vacuum!
QuickTime™ and a
decompressor
are needed to see this picture.
15
Constraints & Previous Searches
MH unknown, but for given MH all Higgs boson properties fixed
➟ know “exactly” what to look for
direct searches at previous
colliders
indirect evidence from
precision measurements
16
The Recent Results
The LHC: a Success Story!
Expectations for 2011
exceeded by a factor 5
18
Experimental conditions
We need this performance!
interactions at hadron colliders
dominated by strong interaction
when searching for Higgs boson
production, need to suppress
backgrounds by ~ 1010
Look for striking signatures
setting the Higgs boson apart
from more ubiquitous
“background” processes
1 pb = 10-36 cm2
√s ≡ ECM
19
now
+
WW
H→
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
H → γγ
Requires excellent discrimination
between single high-energy photons
from hadrons
but offers good energy resolution
Looking for small excess on top
of large (but smooth) background
21
H → ZZ
Very rare process, especially
with both Z particles decaying
to leptons
but very clean, and with good
mass resolution
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)
22
Combining It All
23
The Competition: CMS
H→W+W-
H→γγ
24
The Competition: CMS (2)
H→ZZ
Found 2 candidate events near 126
GeV
1 in e+e-e+e1 in e+e-μ+μ-
Combination
25
Summary
CMS:
ATLAS:
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 (123 GeV)
excess in γγ final state (126
Caveat emptor!
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
But
onewe
way
theHiggs
other,particle
we expect
to rule
make
Either
findorthe
or we
outa
much morethe
definite
statement
within a year
Standard
Model!
26
Thank you!
27
Further Information
CERN press release including pointers to further information:
http://press.web.cern.ch/press/PressReleases/Releases2011/PR25.11E.html
28
Finally...
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
Outlook
30
Symmetries and Conserved Quantities
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
31
Emmy
Noether
The Electron’s Magnetic Dipole Moment
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!
th order
Applying
the gauge
principle
to
the
G. Gabrielse
et al.,
2008
subset
contributions
of
contributions
at
lowest
at
orders
5
Observe
a
single
electron
for
months
The comparison:
Dirac
equationPenning
(relativistic
Cylindrical
trapequation
of motion for spin-1/2 particles):
g=2
A triumph for QED!
Computing quantum corrections:
expansion in powers (up to fifth
±) looks much more like a
A
similar
measurement
for
the
muon
(μ
power) of fine structure constant
regular HEP setting...
32
A Colourful Interaction
Three quarks forming baryons (and quark-antiquark pairs forming
mesons): a new symmetry (and interaction), colour
“gauge principle” interaction with gluons:
quarks change identity (colour) under exchange of a gluon!
Quark confinement at low energy
33
The Weak Interaction
Responsible for all nucleonic transmutations
and particle decays
fusionDecays of heavy particles
radioactivity (β decay)
Truly a weak interaction:
solar ν flux on Earth: ~ 6⋅1014 m-2 s-1
•
during your lifetime, at most a few will interact with your body at
all!
34
Standard Model Summary
Three fermion “generations”
doublet structure
ordered by mass
W-boson couples charged leptons
to ν (and up- to down-type quarks)
35
QCD at High Energies
At high energies, quarks and gluons do manifest themselves as “free”
particles → hadron jets
e-
electron-proton
scattering: 27.5 GeV +
920 GeV
jet
36
A Weighty Issue...
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
p̂
λ= +½
Ŝ
left- and right-handed fermions should be different particles
this requires them to be strictly massless
37
The Higgs Mechanism to the Rescue
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
The Higgs Hunters
ATLAS...and CMS
39
Particle Detection
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 b-quarks:
lifetime of B-hadrons, τB ≈1.5 ps)
QuickTime™ and a
Sorenson Video 3 decompressor
are needed to see this picture.
40
“long”
Higgs Boson Production and Decay
Total inelastic
scattering cross section (strong interaction) ~ 60
Strategy:
use leptons!
mb:
low MH (≲ 135 GeV): VH associated production, leptonic V decay
(V=W,Z)
background suppression by 10-11 orders of magnitude required
• high
H→ W+W-, both
Wstrong
bosons
decaying leptonically
H (≳ 135 GeV):
use M
signatures
not overwhelmed
by the
interaction
A straightforward strategy, but leading to a large number of final
states
41
LEP Higgs Boson Search
Searches at LEP dominated by ZH associated production (“Higgsstrahlung”)
Example distribution (also other variables used)
42
The Tevatron Collider
pp̅ collisions,
1.96 TeV
√s =
CDF
mature collider and
experiments
DØ
Tevatron
running since 2001
Main Injector
8.0 fb-1
7.1 fb-1
•
•
43
1 fb = 10-43 cm2
if σ =1 fb: need L=1 fb-1
to produce one event
many interesting
processes have σ ~
100-1000 fb
How Credible is All This?
DØ’s discovery track record...
kin. cuts leptonic FS
jets
evidenc
e only...
Especially interesting:
single top production:
final state as WH→lνbb̅
same
similarly for WW: irreducible bg
to high-MH search channel
44
Combinations
Note the consistent (but
not yet significant)
signal-like behaviour for
low MH:
a first
hint?!
45
DØ only
Tevatron
Limits
No significant signal-like excess observed... ➟ set limits
Procedure:
Compare data compatibility with s+b / b-only hypotheses (each MH)
Calibrate outcome with toy experiments
Compare resulting
distributions with
observed Q
observed
CLb/s+b ≡ fraction of
backgroundonly/signal+bg
experiments less signallike
than
data
Reject
s+b
hypothesis if CLs+b <
0.05
CLs+b
1-CLb
46