# Presentation: 321608

## How to Measure a Triangle:  Probing the Matter/Antimatter Asymmetry in the Universe

**James D. Olsen**
*Princeton University*

***2002 Sambamurti Lecture***
***Brookhaven National Laboratory***

## Overview

- A little background
- A brief history and the Standard Model
- The Big Bang, and a Big Question
- Matter and antimatter
- Symmetries
- Parity and CP violation
- The triangle and the elephant
- CP violation in the Standard Model
- Why B’s are better than K’s
- The BaBar experiment
- Discovery of CP violation in B decays
- A final thought from my favorite politician...

- J. Olsen

- July 18, 2002

## A Brief History of the 20th Century...

- 1900-1950:
- Relativity and quantum mechanics emerge as the pillars of 20th century physics
- Antimatter predicted (1928) and positron discovered (1933)
- Quantum Electrodynamics sets the template for particle theories
- 1950-1983: If you build it, they will come
- 100’s of new particles and “resonances” are discovered in new accelerators at Berkely, Brookhaven, and elsewhere
- Quark model (1964) brings order from chaos
- Electroweak theory postulated by Glashow, Weinberg, and Salam
- Dramatically confirmed with the discovery of the W and Z bosons (1983)
- 1983-present:
- The Standard Model is established as the most likely theory of particle interactions.  But there are still some loose ends...

- J. Olsen

- July 18, 2002

## The Standard Model of Particle Physics

- The modern theory of particle physics is called the “Standard Model”
- The underlying principle is that Nature can be described in the context of forces acting on particles
- Quantum Field Theory is the official language
- The Fundamental Forces:
- Electromagnetism (light, atomic and molecular binding)
- Weak (beta, and other, decays)
- Strong (binds quarks inside protons, neutrons, etc...)
- Unification of all forces (including gravity)?  Not in this talk!
- The Fundamental Particles:
- Quarks (nuclear building blocks)
- Leptons (the “light” particles: electrons, neutrinos, etc...)

- J. Olsen

- July 18, 2002

## The Particle Rubik’s Cube

- J. Olsen

- July 18, 2002

## Big Bang: From Particles to People...

- Pre-heat oven to 10degrees, add quarks/leptons/forces
- Reduce temperature and stuff “hadrons” with quarks using a strong glue
- Mesons – quark/antiquark pairs
- Kmeson = down + antistrange
- B meson = down + antibottom
- Baryons – three quarks or three antiquarks
- Proton = up + up + down
- Neutron = up + down + down
- Continue reducing temperature, electrons will bind to the protons → atoms (watch out for clumping galaxies)
- Slowly cool atoms to form molecules, proteins, cells, and, after 15 billion years in the kitchen, people!

- J. Olsen

- July 18, 2002

## Big Question:  Where is the Antimatter?

- Added equal amounts matter and antimatter...

- Only matter comes out

- In fact, all matter should have annihilated; lucky for us it didn’t!

**R. Kolb**

- J. Olsen

- July 18, 2002

## Mom: “So what is antimatter anyway?”

- The marriage of relativity and quantum mechanics embodied in the Dirac equation predicts that for every particle there is an antiparticle with opposite charge and magnetic moment
- All other attributes are identical: mass, lifetime, etc...
- Neutral (fundamental) particles are their own antiparticles
- The discovery of the positron in 1933 confirmed the prediction, but does *every* particle have an antiparticle?
- Antiproton and antineutron were discovered in 1955-6
- The laws of physics at that time treated particle and antiparticle equally, so how could an imbalance arise in the early universe?

- J. Olsen

- July 18, 2002

## Symmetry and Conservation Laws

- Symmetry is a deep and fundamental concept in physics
- Nöether’s theorem states that for every symmetry in Nature there is a conserved quantity
- Conservation laws separate the theory wheat from the chafe by requiring the fundamental interactions to obey the corresponding symmetries
- Some well-known examples:
- Lorentz invariance → conservation of energy-momentum
- Rotational invariance → conservation of angular momentum (spin)
- These dynamical symmetries refer to the fundamental structure of space-time itself.  What about discrete symmetries related to sub-atomic particles?

- J. Olsen

- July 18, 2002

## Discrete Symmetries:  C, P, and T

- Three very important discrete symmetries:
- Charge conjugation (C): particle            antiparticle
- Parity (P): x → -x, y → -y, z → -z
- The mirror image of any physical process should be possible
- Time reversal (T):  t → -t
- Before 1956, all interactions were assumed to obey all three symmetries independently
- In practical terms, it means that we cannot tell particle from antiparticle, left-handed from right-handed, or the direction in time; they are relative, not absolute concepts
- To distinguish these characteristics you need to break the symmetry!  So what happened in 1956?

- J. Olsen

- July 18, 2002

## T. D. Lee (Columbia)

**T. D. Lee (Columbia)**

**C.N. Yang (IAS)**

- J. Olsen

- July 18, 2002

## Is Parity Conserved in Weak Interactions?

**T. D. Lee (Columbia)**

**C.N. Yang (IAS)**

- J. Olsen

- July 18, 2002

## NO!

**All electrons spin **
** ****left-handed about their**
**direction of motion**

**C.N. Yang (IAS)**

**T. D. Lee (Columbia)**

**The mirror image**
**does not exist!**

- Slide from A. J. Smith and R. N. Cahn

- Is Parity Conserved in Weak Interactions?

- Is Parity Conserved in Weak Interactions?

- J. Olsen

- July 18, 2002

## C and P Bad, CP Good?

- Wu and others found that left-handed positrons do not exist either, so C and P are maximally violated!
- However, the combined operation (CP) of swapping particle/antiparticle and left-handed/right-handed restores the symmetry:

- J. Olsen

- July 18, 2002

## C and P Bad, CP Good?

- Wu and others found that left-handed positrons do not exist either, so C and P are maximally violated!
- However, the combined operation (CP) of swapping particle/antiparticle and left-handed/right-handed restores the symmetry:

- J. Olsen

- July 18, 2002

## CP Violation

- In 1964, Princeton researchers (Cronin and Fitch) working at the Brookhaven AGS observed the CP-violating decay KL→
- Completely unexpected!
- Unlike parity violation, CP violation did not fit into existing models
- Fundamentally altered our understanding of the weak force

- J. Olsen

- July 18, 2002

## A Nobel Prize, and a Cartoon!

**Jim Cronin          Val Fitch**

**(Hank Martin, New Yorker )**

**Then**

**Now**

- from A. J. Smith

- J. Olsen

- July 18, 2002

## So What?

- CP violation is one of the necessary ingredients to produce a matter/antimatter asymmetry in the early universe!

- R N Cahn

- J. Olsen

- July 18, 2002

## A Cosmological Fight to the Death...

- -- The Smithsonian

- J. Olsen

- July 18, 2002

## Intermission

- Calculations showed that the level of CP violation observed in the Cronin-Fitch experiment failed, by billions, to explain the matter/antimatter asymmetry in the universe.  Hmmm.....
- Meanwhile, a mechanism to describe CP violation in the Standard Model was developed by Kobayashi and Maskawa (1973), with inspiration from Cabibbo (1963)
- Despite tireless efforts by experimentalist, for 37 years the question of whether the CKM mechanism was the source of CP violation in the K-meson system remained unanswered...

- J. Olsen

- July 18, 2002

## Intermission

- Calculations showed that the level of CP violation observed in the Cronin-Fitch experiment failed, by billions, to explain the matter/antimatter asymmetry in the universe.  Hmmm.....
- Meanwhile, a mechanism to describe CP violation in the Standard Model was developed by Kobayashi and Maskawa (1973), with inspiration from Cabibbo (1963)
- Despite tireless efforts by experimentalist, for 37 years the question of whether the CKM mechanism was the source of CP violation in the K-meson system remained unanswered...

- J. Olsen

- July 18, 2002

## Beauty is Better than Strangeness

- B mesons have several advantages over K mesons when it comes to studying CP violation:
- CP-violating observables are much larger (0.5 vs 0.002)
- Many more decays modes → can cross-check measurements in several decay modes to look for (in)consistencies
- Less theoretical uncertainty → tighter constraints on theory
- The B’s allow for direct confrontation of the Standard Model with experiment, and the possibility to distinguish between competing models of CP violation
- In 1999, two dedicated CP violation experiments using B mesons began taking data: BaBar at Stanford, and Belle at Tsukuba, Japan

- J. Olsen

- July 18, 2002

## Finally, the Triangles!

- Weak interactions can change one quark flavor to another

*c*

*W***

*b*

- J. Olsen

- July 18, 2002

## Finally, the Triangles!

- Weak interactions can change one quark flavor to another
- The strength of the interaction is proportional to one of the elements of the “CKM matrix”

- J. Olsen

- July 18, 2002

## Finally, the Triangles!

- Weak interactions can change one quark flavor to another
- The strength of the interaction is proportional to one of the elements of the “CKM matrix”
- Unitarity (V*V = 1): “something has to happen!”
- Leads to “triangles” in the complex plane

*CP violation is proportional to the area!*

- J. Olsen

- July 18, 2002

## Finally, the Triangles!

- Weak interactions can change one quark flavor to another
- The strength of the interaction is proportional to one of the elements of the “CKM matrix”
- Unitarity (V*V = 1): “something has to happen!”
- Leads to “triangles” in the complex plane

*CP violation is proportional to the area!*

- J. Olsen

- July 18, 2002

## Finally, the Triangles!

- Weak interactions can change one quark flavor to another
- The strength of the interaction is proportional to one of the elements of the “CKM matrix”
- Unitarity (V*V = 1): “something has to happen!”
- Leads to “triangles” in the complex plane

- B mesons:

*CP violation is proportional to the area!*

- “Unitarity Triangle”

- J. Olsen

- July 18, 2002

## It’s easy, and not so easy...

- The angles of the Unitarity Triangle are observable as CP-violating asymmetries in the time spectra of Band anti-B  decays to well-defined states of CP symmetry
- CP violation demonstrated if any angle is different from 0 or 180!
- Task of the B Factories is to measure the angles and sides of the Unitarity Triangle with unprecedented precision
- But, the relevant decays are rare (1 in 10,000), and the B meson lives for only 1.5 *trillionths* of a second
- Need lots of B’s → B Factories!!!
- Even at B Factories, the B flies on average only .25 millimeters before decay → precision detectors needed to “see” the B decay

- J. Olsen

- July 18, 2002

## Stanford Linear Accelerator Center

- BABAR

**2 miles**

- J. Olsen

- July 18, 2002

- J. Olsen

- July 18, 2002

## BaBar Detector:  Peel the Onion

- DIRC PID)
- 144 quartz bars
- 11000 PMs

- 1.5T solenoid

- EMC
- 6580 CsI(Tl) crystals

- Drift Chamber
- 40 stereo layers

- Instrumented Flux Return
- iron / RPCs  (muon / neutral hadrons)

- Silicon Vertex Tracker
- 5 layers, double sided strips

- e+ (3.1GeV)

- e (9 GeV)

- J. Olsen

- July 18, 2002

## BaBar Detector:  Peel the Onion

- DIRC PID)
- 144 quartz bars
- 11000 PMs

- 1.5T solenoid

- EMC
- 6580 CsI(Tl) crystals

- Drift Chamber
- 40 stereo layers

- Instrumented Flux Return
- iron / RPCs  (muon / neutral hadrons)

- Silicon Vertex Tracker
- 5 layers, double sided strips

- e+ (3.1GeV)

- e (9 GeV)

**Me!**
- 6’3”, xxx lbs

- J. Olsen

- July 18, 2002

## How many physicists does it take to herd an elephant?

- J. Olsen

- July 18, 2002

## How many physicists does it take to herd an elephant?

- ~600!

**How many physicists does it take to herd an elephant?**

- J. Olsen

- July 18, 2002

## It’s Really a B Factory!

*90 million B/anti-B pairs produced!*

- All previous B data recorded since the Big Bang

- J. Olsen

- July 18, 2002

## Experimental Setup

- Electron and positron collide producing an Upsilon meson boosted in the lab frame

- Y(4S)

- J. Olsen

- July 18, 2002

## Experimental Setup

- Electron and positron collide producing an Upsilon meson boosted in the lab frame

- Y(4S)

- Upsilon decays to B/anti-B pair in coherent angular momentum state

- J. Olsen

- July 18, 2002

## Experimental Setup

- Electron and positron collide producing an Upsilon meson boosted in the lab frame

- Y(4S)

- Upsilon decays to B/anti-B pair in coherent angular momentum state

- Start the clock when one B (call it Btag) decays, “tag” it’s flavor

*t = 0*

***e******+***

- J. Olsen

- July 18, 2002

## Experimental Setup

- Electron and positron collide producing an Upsilon meson boosted in the lab frame

- Y(4S)

- Upsilon decays to B/anti-B pair in coherent angular momentum state

- Start the clock when one B (call it Btag) decays, “tag” it’s flavor

*t = 0*

***e******+***

*CP eigenstate*

- After a time t, the second B (call it BCP) decays into a CP eigenstate that is fully reconstructed

- J. Olsen

- July 18, 2002

## A Real Event: 

- Beam’s eye view

- Zoom in...

- J. Olsen

- July 18, 2002

## What Do We Expect?

*With a perfect detector*

*Smeared by finite detector resolution*

- J. Olsen

- July 18, 2002

## What Do We Expect?

*With a perfect detector*

*Smeared by finite detector resolution*

- Matter/Antimatter Asymmetry

- 

- 

- 

- J. Olsen

- July 18, 2002

## Observation of CP Violation in B Decays

- Matter/antimatter asymmetry visible to the naked eye!
- First observation July 2001
- Latest measurement (yesterday!):

- sin2 = 0.741 ± 0.067 (stat) ± 0.033 (syst)

- (hep-ex/0207042)

- So  = 24 degrees ≠ 0 or 180!

- J. Olsen

- July 18, 2002

## What is the Predicted Value?

- A triumph for the Standard Model!

- J. Olsen

- July 18, 2002

## So What Does it All Mean?

- The observation of CP violation in B decays, and the extraordinary agreement with the Standard Model prediction, leave little doubt that the CKM paradigm is the common source of CP violation in B and K mesons
- But this still leaves us billions of times short of describing the cosmological CP violation that led to our matter-dominated Universe!
- Is it “New Physics”, or something less exotic?
- We are now in a new phase of the experiments, looking at different, and rarer decay modes
- The B Factories continue taking data at ever-higher rates in order to squeeze the Triangle until it cracks!

- J. Olsen

- July 18, 2002

## A Final Thought...

*   **-- Winston Churchill*

- J. Olsen

- July 18, 2002