lhc public lecture - ucla physics & astronomycousins/cousins_scaapt_lecture_3nov2012.pdfantoine...

© C

ERN

© C

ERN

On July 4, physicists at CERN outside Geneva described a discovery that appears to be “like the Higgs boson.” Two huge detectors, ATLAS and CMS, recorded and analyzed independent data. The press reaction was immediate…

ATLAS CMS

Physicists discover a candidate for the boson Higgs

“What in the World Is a Higgs Boson?”

To concentrate on this question, I will leave out most of our broad and deep physics research program, and focus on:

electromagnetic force field and the weak force field, which are connected in a profound way that leads us to the Higgs field and Higgs boson.

The story of the Higgs boson begins with this “photograph” taken in 1896 by Antoine Henri Becquerel in Paris: The film was in a drawer, wrapped in thick black paper, with uranium salts on top.

Becquerel was investigating whether or not sunlight caused uranium salts to emit X-rays.

He developed film after some cloudy days, and found the image of the uranium salts. It was caused by high-speed electrons dubbed beta-rays!

Beta decay is a spontaneous change due to the weak force: neutron → proton + electron + neutrino. (The neutrino was also previously unknown.)

The weak force is the only force that can change one type of elementary particle into another, as in beta decay… The sun could not “burn” without it…

The strong force powers the sun, but the weak force plays a unique crucial role.

The sun formed mostly from hydrogen:

The sun shines by forming heavier elements, all of which need neutrons in the nucleus. helium.

Only the weak force can change protons into neutrons, allowing this fusion furnace, so that the sun shines!

Why is the weak force so weak?

We need a theory of the weak force.

From 1896, it took 71 years of painstaking experiments and theoretical insights to discover the complete theory (equations) of the weak force in 1967.

It took 16 more years, until 1983, to confirm experimentally the major assumptions and predictions -- except one!

A crucial assumption (dating to 1962-67) was the existence of a new force field with four parts, which became known as the Higgs field.

It is extremely difficult experimentally to confirm or refute this assumption directly.

“It had to be there,” but the continuing lack of direct confirmation or falsification kept raising the stakes.

The search became one for a Santo Graal.

In fact, as I will discuss at the end, for us the excitement began with the discovery!

We knew something like a Higgs boson had to be there, but little was known about it!

There is now a whole “Higgs sector” of physics that we have begun to explore experimentally.

It was hard to make any measurements without the new particle! Now we have it – and maybe more to come.


What is the photon? What is mass? What is the electroweak theory? What is the Higgs field? What did the experiments “see”?

And then a look forward: What more must we try to learn about the Higgs field(s)?

In the 19th century, electric and magnetic force fields were understood to be parts of a single electromagnetic force field.

Ripples in the electromagnetic force field propagate out as waves. Einstein’s Nobel-prize-winning insight…

What is a photon?

“It seems to me that the observations are more readily understood if one assumes that the energy of light is discontinuously distributed in space… The energy of a light ray consists of energy quanta which are localized in space, which move without dividing, and which can only be produced and absorbed in complete units.”

Einstein, Annalen der Physik, 1905

Translation: Arons and Peppard, Amer. J. Phys, 1965

Concerning an Heuristic Point of View toward the Emission and Transformation of Light

What is a photon?

Satyendra Nath Bose (India) sent a paper to Einstein with an important consequence:

Photons tend to clump together due to (as we now know it) a deep Quantum-Mechanical effect since indistinguishable. Associated with integer spin.

Such particles are called bosons. All force fields have energy quanta These quanta all have integer spin and are bosons.

What is a photon?

E = mc2 is the energy of a particle at rest. If mass m is moving, with momentum p, then

The photon has m=0 (!) so

This applies to any particles moving at the speed of light. They have energy and momentum, but no mass!

What is a photon?

We need a unit to quantify amounts of energy E (and mc2). We use the electron-volt, eV, and units where c=1, dimensionless.

Energy of motion of each air atom at room temperature is about 0.04 eV.

Chemical reactions (moving atoms from one molecule to another) involve about 1 eV to a few eV per atom.

What is a photon?

Nuclear reactions (moving protons from one nucleus to another) involve about a million eV (MeV) or more per nucleus.

The energy mc2 in the mass of a proton at rest is about a billion eV (GeV), G for giga.

The LHC beams are each 4 trillion eV (TeV).

Photons have a huge range of energies, from radio waves (millionth of eV or less) to gamma rays (billions of eV or more). Regardless of energy, symbol for photon is γ

What is a photon?

Q: How do we make photons?

A: Shake some electric charge: photons will be created and emitted!

Easy to do: just heat any collection of atoms; the electrons are shaken, and emit photons.

What is a photon?

The electric field from the photons is perpendicular to their direction of travel.

This macroscopic effect is reflecting a property of the individual photons.

What is a photon?

In a sense, each photon carries two numbers specifying the oscillation perpendicular to its motion.

There is no third number needed for oscillations parallel to motion. Deeply related to photon having no mass.

What is a photon?

Q: How can we detect individual photons? A: It depends on the photon’s energy…

You have detectors for photons with energy of 1½ to 3 eV: your eyes!

Absorption of single photon changes shape of huge rhodopsin molecule.

Complex amplification mechanism results in an electrical signal going to the brain.

What is a photon?

Artificial “electronic eyes” for photon detection are highly evolved as well. CMS uses “lead tungstate” crystals: 45% lead, 40% tungsten, and 15% oxygen.

What is a photon?

80,000 crystals!

Provides fast detection of photons with energies of 1 GeV to 1000 GeV.

Energy converted to visible light, then electrical signal.

What is a photon?

Beta decay, n → p + electron + neutrino, has an intermediate “virtual” step,

W bosons are the quanta of the weak force! Weak force changes n to p (and vice versa) by emitting W bosons.

What is mass?

Long, heroic story: neutrons and protons are made from (whimsically named) quarks, called up (u) and down (d).

Neutron: Proton:

So yet deeper view of beta decay is: d → u + electron + neutrino. Weak force changes one type of quark into another when W emitted!

What is mass?

So some truly elementary particles of matter are up and down quark, neutrino, electron.

What is mass?

What is mass?

Then a “copy” of the electron, 200 times more massive, was discovered: the muon.

Muons from reactions initiated by cosmic rays are going through us now.

Muons play an important role in the Higgs search.

Over time, two complete “copies” , more massive but otherwise identical to the first set, were discovered.

Complete mystery.

What is mass?

Point-like elementary particles of matter Q

UA

RK

S

LEP

TON

S

Most massive point-like particle is top quark, 173 GeV.

Now we can look into the strength of forces. Very long story short: The weak force appears weak because mass of W is large! If the W mass were zero, weak force would be about same strength as EM force.

What is mass?

W bosons are massive. mc2 = 80 GeV (!)

You need to shake matter with a lot of energy in order to emit a “real” W boson! (First done at CERN in 1982-83.) So the question, “Why is the weak force weak?” leads us to “Why do W bosons have a large mass?”

Why do W bosons have any mass at all?

What is mass?

What do we mean by “mass”???

For a composite object such as a proton, the mass is m = E/c2 , where E includes all the energies of the constituents. Most of the mass of the proton comes from kinetic energies of its constituents, not their masses.

What is mass?

Such “origin of mass” for a composite particle, from pure energy of its constituents, is no longer a mystery.

The “origin of mass” of “point-like” particles such as quarks, electrons, muons, and W bosons, is the problem the Higgs mechanism solves, as we shall see.

There may be other “origins of mass” as well, for neutrinos, for dark matter! The Higgs boson is firstly about the masses of the quanta of the weak force.

What is mass?

In the 1950’s, promising “Yang-Mills” theories for all the forces appeared to require that m=0, i.e., that all the force quanta, and all the quarks and leptons, had no mass and moved at the speed of light!

In 1960, Sheldon Glashow used such an approach to write down combined equations of the electromagnetic and weak forces.

What is the electroweak theory?

Start with two forces (Glashow, 1960):

W force: 3 massless quanta W+, W0, W−. B force: 1 massless quantum B0. The two forces’ equations are combined.

The W0 and B0 quanta get mixed! The physical force quanta are mixtures:

photon γ : sin2θW W0 and cos2θW B0 Z0 : cos2θW W0 and sin2θW B0. [I’ve written coefficients as probabilities] New constant of nature: sin2θW = 0.23


There are then four force quanta for the combined “electroweak force”: photon γ, Z0, W+, W−. These are the “modern” quanta, except THEY WERE ALL MASSLESS. And the forces’ strengths were about equal.

Glashow was essentially stuck. He and others failed to make a viable theory with the W± massive and the associated “weak force” weak.


There were deep reasons for the problem. It goes back to our demonstration: Massless particles, such as the photon, cannot oscillate along their direction of motion. Massive particles can. This caused big problems in the math – not enough variables.


Breakthrough ideas to allow non-zero masses, known as the “Higgs mechanism”, came from a lot of directions in the 1960’s.

I use “Higgs” as a composite person (partly Peter Higgs).

It all started from nothing: the vacuum! In classical physics, a vacuum exists in a volume where you remove all matter.

What is the Higgs field?

In Quantum Field Theory, this vacuum is filled with the lowest-energy configuration allowed by Quantum Mechanics, and is teeming with quanta from force and matter fields.

Key postulate of the “Higgs mechanism” is: There exists a new force field, the Higgs field, and the average value of the Higgs field in the vacuum became non-zero as the early universe cooled. Jargon: “spontaneous broken symmetry”


The non-zero average value in vacuum of the Higgs field has a “description”, and an inferred value (246 GeV), but still does not (yet) have a deeper explanation. In 1967-68, Steven Weinberg and Abdus Salam applied this idea to Glashow’s theory. They wrote down a Higgs field with four parts, related in a particular way for a Yang-Mills theory.


Then they introduced the non-zero average value of the Higgs field.

The four parts became a massive Higgs boson and three new massless bosons according to a theorem by Goldstone.

This added three variables to the theory, just what was needed to make it possible to describe the oscillations of the W+, W−, and Z0 in their direction of motion!


This is the “Higgs mechanism”! There was not a fourth Goldstone boson to do the same for the photon, so it stayed massless.

The colloquial way theorists say it: “The W+, W−, and Z0 eat the Goldstone bosons and acquire a mass.”

Cartoon by Flip Tanedo


Once this non-zero average value of the Higgs field exists, it can also give masses to the quarks, electrons, muons – to all point-like particles. This is often stated (much too loosely for my taste) as “The Higgs boson is the origin of mass”



e ν e

u d

c s

µ ν µ

t b

τ ν τ

Masses in GeV

0.002

0.005

0.0005

1.3

0.1

173

4.2

0.1 1.8

<10-9 <10-9 <10-9

Using the same Higgs field is an economical way to allow quarks and leptons to have masses, and is at least partly required. Deeply related to other properties. But! Values of masses are still unpredictable, now just expressed in terms of unpredictable couplings of the particles to the Higgs fields. We need to test experimentally…


Added bonus: Since the W boson was invented, there had been a festering problem that the probability of two W’s hitting each other became greater than 1 (or incalculable) at high energy.

This was also solved by the Higgs field!


1967-1979: Very slow reaction, followed by some confusion. Eventually all the dust settled, key prediction of Z0 was confirmed.


Nobel prizes for theory of electroweak force 1979: Glashow, Salam, and Weinberg "for their contributions to the theory of the unified weak and electromagnetic interaction… including, inter alia, the prediction of the weak neutral current". 1999: 't Hooft and Veltman "for elucidating the quantum structure of electroweak interactions in physics“ 2008: half to Nambu "for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics"

There was just one little loose end…


In spite of decades of attempts, there was no direct experimental evidence for the Higgs field or the Higgs Boson!

What do the experiments see?

How to make Higgs bosons?

You need to shake the Higgs field!

We need some equipment…not just film in a drawer.


In Switzerland and France outside Geneva:

The Large Hadron Collider.

Large is an understatement! Hadrons referred to here are protons. Collide hadrons is what it does..

Tiny bunches of counter-circulating protons colliding head-on 20 million times a second, using E=mc2 to create previously unobserved particles.

Energy of each proton is 8 TeV (Aim for 13-14 TeV and 40 million/s in 2014)

Air pressure is lower than on the moon inside the two 16-mile-long vacuum pipes, where 100 tons of liquid helium maintain a temperature colder than outer space.

In tiny volumes inside colliding protons, the LHC creates temperatures a billion times hotter than the center of the sun.

In a multi-step process, Higgs bosons are radiated from briefly-created W bosons and top quarks. A Higgs boson decays while still inside the debris of the colliding protons.

LHC experiments should be able to detect at least five different classes of decay products. I’ll discuss two:

1) The decay to two Z0 bosons, which in turn decay to electrons or muons.

2) The decay to two photons, γγ.


Assembly of CMS What do the experiments see?

INSERTION OF THE SILICON TRACKER


2000 square feet of silicon wafers: from cartoon to reality


INSERTION OF THE SILICON TRACKER


March 31, 2010 What do the experiments see?

We see the decay products of the particles we make. How do we know the mass of a decayed particle?

We measure the energies of the decay products and add them. That’s the energy of the decaying particle – plug that in for E.

Vector sum of products’ momenta is p.

Solve for m, the “invariant mass”.


Computer reconstruction of collision data resulting in two muons.

We take many such events and compute invariant mass of each muon pair.


66

u,d,s c b quark-antiquark resonances

Early LHC data: we plotted frequency of values of computed invariant m of two muons (µ+µ−)

Quickly reprised 50 years of HEP, then new territory.


0

100 mµµc2 (GeV)

Higgs boson can decay to two Z0 bosons. Each in turn quickly decays to µµ or to ee, so we observe µµµµ, µµee, or eeee.


We plot frequency of invariant m for each four-lepton event. CMS, ATLAS both see excess near 125 GeV!


Search for Higgs Boson decay to two photons We use the 80,000 crystals to measure energy and direction. (E=pc)


Look for collisions with two photons, and calculate invariant mass mγγ .

Template blocks


CMS, ATLAS both see excess near 125 GeV! CMS and ATLAS each conclude “observation” of a new particle! CERN announces discovery.



Peer-reviewed papers: Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC. “…compatible with the production and decay of the Standard Model Higgs boson.” Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC “..consistent, within uncertainties, with expectations for the standard model Higgs boson.”


We still don’t really know!

Is there 1 Higgs boson? Or 5? Is it point-like? Or composite? Are all probabilities as predicted, or not? Are the angular distributions of the decay products as predicted, or not?

Any red answer leads us to “new physics”!

What next?

Meanwhile our broad search for “New Physics” continues – Supersymmetry, Dark matter, Substructure of “point-like” objects, New forces … a long list. (For my broad public lecture, google Robert Cousins Big Bang Machine 2010)

What next?

We are grateful for the past and continuing support from over one hundred funding agencies world-wide that contribute to the physics program of the LHC. The key U.S. federal agencies are: All of the universities involved also support this research directly or indirectly.

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