What's up with the Higgs-like boson?

Marc Merlin, Director

Atlanta Science Tavern

marc@atlantasciencetavern.com

Licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

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Goals for this talk

• introduce you to the Standard Model of particle physics
• explain why its success relies on the existence of the Higgs boson
• describe how the Higgs "confers" mass on elementary particles
• discuss efforts underway at CERN to produce and detect it
• review the announcement of its "discovery"

The hype

• The Higgs boson has been called the "God Particle" because it gives particles mass.
• Leon Lederman originally called it the "goddamn particle" because it was so elusive.

"The funniest book about physics ever written." - Dallas Morning News

Known deficiencies in the Standard Model

• It does not include gravity.
• It does not account for the disparity in the amounts of matter and antimatter in the universe.
• It does not include dark matter.
• It does not include dark energy.

Through a Universe Darkly

Rejected Standard Model taglines

• "a theory of everything"
• "a theory of almost everything"
• "a theory of almost nothing"

WE ARE THE

4%

Everyday particles

• proton
• neutron
• electron
• neutrino

The nuclear family (Cory Doctorow)

Dinesh D'Souza, in the Dark about Dark Matter and about Science

We ain't scared of no antimatter

• Every particle has its antiparticle, sometimes itself.
• There is nothing mysterious about antimatter.
• An antiparticle "mirrors" the properties of its particle.

A positron mirrors an electron's charge and parity. (credit)

Everyday forces

• electromagnetism
• strong nuclear force
• weak nuclear force

Electromagnetism

• describes electrical and magnetic phenomena
• accounts for light, chemistry, biology and the mechanical properties of ordinary objects
• moderate strength / infinite range

Strong nuclear force

• binds protons and neutrons in the atomic nucleus
• accounts for nuclear fission and fusion that fuels the sun
• very strong / short range

Weak nuclear force

• is responsible for radioactive decay of elements
• accounts for much of the heat produced in the Earth's interior
• very weak / very short range

Scientists using a neutrino detector have measured how much heat is generated in the interior of the Earth way by capturing geoneutrinos released during radioactive decay. (credit LBL)

Who ordered that?

• A new particle called the muon was discovered in 1936 in cosmic ray studies.
• I. I. Rabi remarked in response to the news, "who ordered that?"

Simulated cosmic ray shower over the lakefront in Chicago. (credit)

The Physics of Unwelcome Surprises

Disturbing the universe with "atom- smashers"

• Charged particles can be accelerated to high speeds using electromagnetic fields.
• Because of the equivalence of mass and energy, new particles can be created when collisions occur.

First cyclotron in 1930 was 4.5 inches in diameter (AIP).

The particle zoo

• By the late 1960s dozens of new particles and "resonances" had been observed.
• All these particles are short-lived and decay ultimately into protons, neutrons, electrons and neutrinos.

Particle Properties (wallet sheet), 1968. (credit)

Quarks to the rescue

• Fractionally charged particles called quarks were proposed to tame the particle zoo.
• They combine in twos and threes to make hadrons, particles that participate in the strong interaction.

Baryons and mesons are made up of combinations of quarks. (credit)

Quarks in ordinary matter

• Protons and neutrons are composed of up and down quarks.
• These quarks are lightweight and relatively stable inside the nucleus.
• You are made up mostly of up and down quarks.

A proton is constructed from 2 up quarks and 1 down quark. (credit)

Quarks with silly names

• Quarks with the names strange, charm, bottom and top were proposed to account for particles with unusual properties.
• These quarks are heavier than up and down and very short-lived.

A positive kaon is composed of an up quark and an anti-strange quark. (credit)

The Physics of Silly Names

Quark confinement

• Experiments at SLAC in 1969 indicated that nucleons are composed of point-like particles.
• Quarks are never observed in isolation.
• Paradoxically, the strong force becomes weaker at very short distances.

Aerial view of the 2-mile long Stanford Linear Accelerator Center. (credit)

Leptons complete the picture

• Electrons have short-lived, heavy cousins, the muon and the tau.
• Like the electron, the muon and the tau come with neutrino partners.
• These particles are collectively called leptons.

4 force carriers to bind them all

• photon - electromagnetism
• W and Z bosons - weak force
• gluon - strong force

Diagram of elementary particle interactions. (credit)

What's missing is a mathematical theory

• Electromagnetism is described to remarkable precision by a gauge field theory (QED).
• This kind of theory has been adapted to the strong interaction to solve the problem of quark confinement.

When enough energy is added, gluon field snaps, producing a new quark antiquark pair (credit).

The weak interaction is the gauge theory holdout

• Gauge theories only work with zero-mass force carriers.
• The short range of the weak interaction, though, implies that the W and Z bosons are quite heavy.

The Z boson, on sale now at the Particle Zoo.

The Higgs mechanism

• A mathematical trick called the Higgs mechanism solves the problem of heavy W's and Z's.
• It unifies the weak and electromagnetic interactions.
• The Higgs particle is predicted as a result.

The Standard Model of particle physics. (credit FNAL)

Higgs with benefits

• As a side-effect the Higgs interacts with some elementary particles.
• These particles acquire their masses in proportion to the strength of their interaction with the Higgs.

Range of lepton and quark masses (credit).

Physical model for the Higgs

• ~10^-43 sec after the Big Bang forces are unified and particles are massless.
• ~10^-12 sec this symmetry is broken, the forces separate and a pervasive Higgs field forms giving quarks and leptons mass.

The evolution of the Universe since the Big Bang. (Max Planck Institute for Physics)

Cocktail party metaphor for the Higgs field

The Higgs field is imagined to be a room at cocktail party full of guests interested in hobnobbing with celebrities. (David J. Miller)

A celebrity walks through the party

A celebrity walking through the room attracts a swarm of guests and he finds his progress slowed as a result.

A rumor circulates around the room

Even without a celebrity, a rumor can excite a swarm of guests which circulates around the room on its own.

Taking stock

• Forces carriers, quarks and leptons provide a compelling description of particle physics.
• But a rigorous mathematical framework appears to require gauge theories.
• An out-of-the-box gauge theory is incompatible with heavy Z and W bosons.
• The Higgs mechanism is a workaround.
• It rescues gauge theories, unifying the weak and electromagnetic interactions and, in the process, gives some particles mass.

How to create and detect a Higgs boson

• Accelerate counter rotating proton beams to very high energies.
• Have the protons collide as many times as possible.
• Take a "snapshot" of the debris from these collisions, paying attention to signatures that are characteristic of the decay of a Higgs boson.
• Determine statistically that the number of events you record can't all be attributed to other processes.

Large Hadron Collider (LHC)

• 16.6 miles in circumference
• 2 proton beamlines / 4 intersections
• 7 TeV energy per collision
• ~800 million collisions per second

CERN, home of the LHC, is situated in the northwest suburbs of Geneva on the Franco–Swiss border. (CERN)

Possible Higgs decay modes

Decay modes of the Higgs depend upon its mass (Higgs Interactive FNAL).

• The Higgs lifetime is ~10^-24 seconds.
• Direct observation is impossible.
• Its decay results in a cascade of particles which can contain telltale signatures.

CMS (Compact Muon Solenoid)

The CMS along with the ATLAS detector are involved in the search for the Higgs at the LHC (credit).

4-muon candidate event

Candidate event in which 4 high energy muons (red lines) are observed. The yellow lines are the measured tracks of other particles produced in the collision. (CERN)

2-photon candidate event

Candidate event including two high-energy photons whose energy (depicted by red towers) is measured in the CMS electromagnetic calorimeter. The yellow lines are the measured tracks of other particles produced in the collision. The pale blue volume shows the CMS crystal calorimeter barrel (CERN).

It's a numbers game

• Standard Model background processes produce candidate events with identical final state signatures.
• So no individual event can be attributed conclusively to the Higgs.
• But the number of background events can be calculated.
• An excess of events above the predicted background would constitute evidence for the creation of the Higgs boson.

Playing the odds

• The quantity σ is used to indicate the confidence in a statistical conclusion.
• By convention, a value of 5σ is required to announce a "discovery."

ATLAS Detector

• Statistical uncertainty is not the only source of error.
• Errors can also result from systematic mistakes.
• Confidence requires independent experimental verification.

"Black Hole" event superimposed over a classic image of the ATLAS detector. (CERN)

Two-photon mass distribution (July 4, 2012 CERN seminar)

• Excess of two-photon events with a significance of 4.1 sigma at a mass near 125 GeV.
• Two-photon final state implies that the new particle is a boson.

Observation of a New Particle with a Mass of 125 GeV

Four-lepton mass distribution (July 4, 2012 CERN seminar)

• Mass distribution of four-lepton events yields an excess of 3.2 sigma above background at a mass near 125 GeV.

Bottom line (July 4, 2012 seminar)

• CMS observes an excess of events at a mass of approximately 125 GeV with a significance of 5 sigma above background.
• The probability of the background alone fluctuating up by this amount or more is about one in three million.
• "We interpret this to be due to the production of a previously unobserved particle with a mass of around 125 GeV."

A Higgs too far

• Up and down quarks account for 1% of the proton mass.
• Their energy of motion and the energy density of gluons account for the rest.
• The majority of mass of ordinary matter is not due to the Higgs.

WE ARE

THE

99%

Checking the Higgs Arithmetic

The God particle delusion

• The Higgs is not the origin of mass in any fundamental way.
• It does give mass to some known elementary particles.
• The mass of ordinary matter is due mostly to gluons inside protons and neutrons.

The Higgs strikes back

• With the discovery of the Higgs, the Standard Model will be complete.
• A mathematical trick, born of theoretical necessity, will have led to a deeper understanding of the nature of the microscopic world today and an instant after the Big Bang.
• The Standard Model, deficient and fraying as it may be, will be established as the cornerstone on which theories of "new" physics will be built.

The last piece of the puzzle

MinutePhysics

Why Higgs-like instead of Higgs?

• The new particle is a boson and its observed decay modes are consistent with the Standard Model Higgs.
• The number of events in hand is inadequate to say whether the relative frequency of the observed decay modes is as it should be.
• More data and a more exact characterization of decay modes is required to reach a definitive conclusion.

Checks on decay rates

The first checks indicate that the new boson is compatible with being the Higgs boson. But the precision is still too low to tell (Quantum Diaries, 2012.09.20)

First piece of the next puzzle?

MinutePhysics