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

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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

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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

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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.

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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)

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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...)

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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

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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)

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Combining It AllCombining It All

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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

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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!

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Thank you!Thank you!

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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!

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OutlookOutlook

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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

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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...

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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

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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

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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)

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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

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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

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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:

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The Higgs HuntersThe Higgs HuntersATLAS... and CMS

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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)

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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

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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

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CombinationsCombinations DØ only

Tevatron

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

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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