Higgs rush at CERN Frank Filthaut Radboud Universiteit Nijmegen / Nikhef.
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Transcript of Higgs rush at CERN Frank Filthaut Radboud Universiteit Nijmegen / Nikhef.
Higgs rush at CERNHiggs rush at CERN
Frank FilthautFrank FilthautRadboud Universiteit Nijmegen / NikhefRadboud Universiteit Nijmegen / Nikhef
Contents:Contents:
Particle physics: general Particle physics: general picturepictureThe Higgs particleThe Higgs particleThe recent resultsThe recent results
Particle Physics: the General Particle Physics: the General PicturePicture
4
Going beyond the naked eyeGoing beyond the naked eye
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 ~ λlimit when using visible light:
0.5 μmimprovement to ~ 1Å possible
using STM, AFM
Minimum discernible dimensions ~ λlimit when using visible light:
0.5 μmimprovement to ~ 1Å possible
using STM, AFM
5
The atom “cracked”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
Repeated in the 60’s with scattering of 180 MeV electrons on protons➠ the proton (r ~ 1 fm) contains further sub-structure (quarks)!
charge distribution with nucleus(Rutherford)
diffusecharge distribution(Thomson)
Planck’s constantprojectile momentum
6
State of the artState of the artPresent scheme in CERN’s Large Hadron Collider:
accelerate proton beams toenergies of 3.5 TeV per proton
v/c ≈ 0.99999996 (energyto be doubled in 2014: 6→8)
in both directions!
make them collide in thecentres of the detectors
experiments analyze outcomeof collisions and select“interesting” events
stochastic process, no controlover outcome of individualcollision➠ can only select after the fact
QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.
7
Quantum ElectrodynamicsQuantum ElectrodynamicsEinstein (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)
8
TechniquesTechniquesHigh 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 thedecaying particle (if all decayproducts are measured)
mΛ
(not the Higgs particle...)
9
The Weak InteractionThe Weak InteractionExchange / production of heavy particles!
W-boson production
(and decay)
e±
Z-boson production
and decay
e±
e∓
W and Z particles are heavy!•MW = 80.398(25) GeV (~ Sr, Kr)•MZ = 92.188(2) GeV (~ Ru)
Discovered in p-pY collisions, ECM = 630 GeV
Most collisions between protons involve the strong interaction ➟look for leptons (only EM and weak interactions)
The Higgs ParticleThe Higgs Particle
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The Particle FamilyThe Particle Family“Leptons”??
particles not involved in the stronginteraction (only weak, EM)
also heavier counterparts of theelectron and its neutral partner, νe
Quarks:• particles also susceptible to the
strong interaction
again, 3 “generations” involvingheavier partners than the u, dthat 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 The only missing family member: the Higgs particle
5⋅10-3
5⋅10-3
1.5
0.3
173
4.8
5⋅10-4 0.1 1.8
91
80
0
0
< 10-6
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Rotational SymmetryRotational SymmetryThe Universe at large is isotropic and homogeneous!
CMWB: temperature
fluctuations ~ 10-
5 K
WMAP 7-year results, full sky
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Extended SymmetryExtended SymmetryRotational 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:This is what the Higgs field does:
interactions obey symmetry• ground state does not
The Standard Model is invalid w/o Higgs!The Standard Model is invalid w/o Higgs!
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The Higgs MechanismThe Higgs MechanismParticles 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
Photons in the medium are effectively massive
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Magnetic AnaloguesMagnetic Analogues
Spontaneous symmetry breaking
Massive photons
equivalentground states
excitation
QuickTime™ and a decompressor
are needed to see this picture.
Meißner effect: superconductor repels magnetic field linesmassive photonsbut needs a medium (e-
pair condensate)!In the particle physics case, the “medium” is the vacuum!
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Constraints & Previous SearchesConstraints & Previous Searches
MH unknown, but for given MH all Higgs boson properties fixed➟ know “exactly” what to look for
direct searches at previous colliders
direct searches at previous colliders
indirect evidence fromprecision measurements
indirect evidence fromprecision measurements
The Recent ResultsThe Recent Results
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The LHC: a Success Story!The LHC: a Success Story!
Expectations for 2011Expectations for 2011exceeded by a factor 5exceeded by a factor 5
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Experimental conditionsExperimental conditions
We need this performance!
interactions at hadron collidersdominated by strong interaction
when searching for Higgs boson production, need to suppress backgrounds by ~ 1010
√s ≡ ECMnow
1 pb = 10-36 cm2
Look for striking signatures setting the Higgs boson apart from more ubiquitous “background” processes
Look for striking signatures setting the Higgs boson apart from more ubiquitous “background” processes
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H → WH → W++WW--
Relatively large event rate, but leptonicW boson decays lead to unobservedneutrinos
cannot reconstruct mass of a systemdecaying to WW
Large mass range excluded already in Summer
Large mass range excluded already in Summer
21
H → γγH → γγRequires excellent discriminationbetween single high-energy photonsfrom hadrons
but offers good energy resolution
Looking for small excess on top of large (but smooth) backgroundLooking for small excess on top of large (but smooth) background
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H → ZZ H → ZZ Very rare process, especiallywith both Z particles decayingto leptons
but very clean, and with goodmass 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)
23
Combining It AllCombining It All
24
The Competition: CMSThe Competition: CMS
H→W+W-H→W+W-
H→γγH→γγ
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The Competition: CMS (2)The Competition: CMS (2)
H→ZZH→ZZ
Found 2 candidate events near 126 GeV•1 in e+e-e+e-
•1 in e+e-μ+μ-
CombinationCombination
26
SummarySummaryATLAS:
excess in W+W- final states: broad but compatible with low-mass Higgs boson
excess in ZZ final state (124 GeV)
excess in γγ final state (126 GeV)
CMS:
excess in W+W- final states: broad but compatible with low-mass Higgs boson
excess in ZZ final state (126 GeV)
excess in γγ final state (123 GeV)Caveat emptor!Caveat emptor!
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 tellBut one way or the other, we expect to make a much more definite statement
within a year
But one way or the other, we expect to make a much more definite statement
within a year
Either we find the Higgs particle or we rule out the Standard Model!
Either we find the Higgs particle or we rule out the Standard Model!
27
Thank you!Thank you!
28
Further InformationFurther InformationCERN press release including pointers to further information:
http://press.web.cern.ch/press/PressReleases/Releases2011/PR25.11E.html
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Finally...Finally...
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
Finding the Higgs boson does not mean particle physics is finished!
30
OutlookOutlook
31
Symmetries and Conserved Symmetries and Conserved QuantitiesQuantities
Noether Theorem:
also indicates how to construct these conserved quantities, given the symmetry
Examples:
translational symmetry ⇔ conservation of momentum
rotational symmetry ⇔ conservation of angular momentum
Every symmetry of a physical system comes with an associated conserved quantity (and vice versa)
Every symmetry of a physical system comes with an associated conserved quantity (and vice versa)
Emmy Noether
32
The Electron’s Magnetic Dipole The Electron’s Magnetic Dipole MomentMoment
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. Gabrielse et al., 2008Cylindrical Penning trap
Observe a single electron for monthsApplying the gauge principle to the Dirac equation (relativistic equation of motion for spin-1/2 particles): g=2
Computing quantum corrections: expansion in powers (up to fifth power) of fine structure constant
subset of contributions at 5th orderApplying the gauge principle to the Dirac equation (relativistic equation of motion for spin-1/2 particles): g=2
Computing quantum corrections: expansion in powers (up to fifth power) of fine structure constant
contributions at lowest ordersThe comparison:
A triumph for QED!
A similar measurement for the muon (μ±) looks much more like a regular HEP setting...
33
A Colourful InteractionA Colourful InteractionThree 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
34
The Weak InteractionThe Weak InteractionResponsible for all nucleonic transmutations
fusion radioactivity (β decay)Decays of heavy particles
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!
and particle decays
35
Standard Model SummaryStandard Model Summary
Three fermion “generations”
doublet structure
ordered by mass
W-boson couples charged leptons to ν (and up- to down-type quarks)
36
QCD at High EnergiesQCD at High EnergiesAt high energies, quarks and gluons do manifest themselves as “free” particles → hadron jets e-
jet
electron-proton scattering: 27.5 GeV + 920 GeV
37
A Weighty Issue...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
left- and right-handed fermions should be different particles
this requires them to be strictly massless
λ= -½ λ= +½pi
S i
38
The Higgs Mechanism to the RescueThe 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:
39
The Higgs HuntersThe Higgs HuntersATLAS... and CMS
40
Particle DetectionParticle DetectionIn 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:
“long” lifetime of B-hadrons, τB ≈1.5 ps)
41
Higgs Boson Production and DecayHiggs Boson Production and Decay
Total inelastic scattering cross section (strong interaction) ~ 60 mb: background suppression by 10-11 orders of magnitude
required use signatures not overwhelmed by the strong interaction
Total inelastic scattering cross section (strong interaction) ~ 60 mb: background suppression by 10-11 orders of magnitude
required use signatures not overwhelmed by the strong interaction
Strategy: use leptons!• low MH (≲ 135 GeV): VH associated production, leptonic V decay (V=W,Z) high MH (≳ 135 GeV): H→ W+W-, both W bosons decaying
leptonically
Strategy: use leptons!• low MH (≲ 135 GeV): VH associated production, leptonic V decay (V=W,Z) high MH (≳ 135 GeV): H→ W+W-, both W bosons decaying
leptonicallyA straightforward strategy, but leading to a large number of final states
42
LEP Higgs Boson SearchLEP Higgs Boson SearchSearches at LEP dominated by ZH associated production (“Higgs-strahlung”)
Example distribution (also other variables used)
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1 fb = 10-43 cm2
if σ =1 fb: need L=1 fb-1 to produce one eventmany interesting processes have σ ~ 100-1000 fb
The Tevatron ColliderThe Tevatron Collider
ppY collisions, √s = 1.96 TeV
mature collider and experiments
running since 2001
Tevatron
Main Injector
CDF DØ
7.1 fb-
1
8.0 fb-
1
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kin. cuts
leptonic FS jets
evidence
only...
How Credible is All This?How Credible is All This?
DØ’s discovery track record...
Especially interesting:
single top production: same final state as WH→lνbbY
• similarly for WW: irreducible bg to high-MH search channel
45
CombinationsCombinations DØ only
Tevatron
Note the consistent (but not yet significant) signal-like behaviour for low MH: a first hint?!
46
observed
1-CLb
CLs+b
LimitsLimits
No significant signal-like excess observed... ➟ set limits
Procedure:
Compare data compatibility with s+b / b-only hypotheses (each MH)
Calibrate outcome with toy experimentsCompare resulting distributions with observed Q
CLb/s+b ≡ fraction of background-only/signal+bg experiments less signal-like than dataReject s+b hypothesis if CLs+b < 0.05