16.451 Lecture 2: Research frontiers 19/9/2003

2001 - FYPC

Most recent long range planning reports: FYPC (Canada), NSAC (USA)

Collision productsat RHIC

2002 - NSAC

PMT’s at SNO

• The boundaries between “particle” and “nuclear” physics are somewhat fuzzy. One frontier remains, as always, the attainment of higher and higher energy scales for laboratory-based experiments to search for new particles beyond what are includedin the “Standard Model” of nuclear and particle physics that we know today. (http://public.web.cern.ch/public/about/future/future.html)

• Great emphasis is being placed on precision measurements of the structure of the proton, neutron, and other relatively simple bound quark systems, with the aim ofbridging the gap between QCD-based models and the underlying microscopic (but stillincalculable) theory. (see e.g. http://www.jlab.org)

• “Traditional” nuclear physics spectroscopy studies are entering a renaissance with the development of new instrumentation of unprecedented resolution combined with new facilities dedicated to radiactive isotope production – the ‘nuclear map’ will be extended into “Terra Incognita” via experimental programs at Canada’s TRIUMF-ISAC facility (http://www.triumf.ca) and others in the USA and Europe. One goal is to shed light on current problems in nuclear astrophysics, ultimately solving the problem of nucleosynthesis of elements in the universe beginning with the Big Bang and continuing with cataclysmic supernova explosions into the present day.

• With recent evidence from Canada’s SNO facility (http://www.sno.phy.queensu.ca)and others that neutrinos have nonzero rest mass, there are hints that the verysuccessful “Standard Model” of fundamental particles and interactions will soon have to be revised -- stay tuned!

Research frontiers.... 2

Fundamental interactions in nuclei (2 protons, 1 fm apart)

1. Strong interaction (QCD) scale: 1

- responsible for nuclear binding- alpha decay, nuclear fission and fusion processes

2. Electromagnetic interaction scale: 0.01

- correction to binding energies, N>Z for heavy nuclei - gamma decay of excited states

3. Weak interaction scale: 0.0000001

- nuclear beta decay- mirror symmetry violation

4. Gravitational interaction scale: 10-36

- forget it!

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Simplest Nucleus: the proton! Static properties – a lot to explain... 4

Particle DataGroup – refereesa compendium ofcredible datain nuclear andparticle physics(revised annually)

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Properties of the proton:

Intrinsic spin: S = ½ (fermion) (listed as J in the table)

important consequence: Pauli exclusion principle – no twoidentical fermions can occupy the same quantum state.

Intrinsic parity: = + (listed as P in the table)

symmetry of the intrinsic wave function for + parity:

)()( rr

Mass: m = 1.67 x 10-27 kg, or rest energy mc2 = 938.3 MeV

lighter than the neutron – the only stable 3-quark system

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• precision mass measurement: m/m ~ 10-10 !!!!

Precision mass measurements: Penning Trap technique1

1Also precision magnetic moment measurements, especially for the electron – more later!

The Nobel Prize in Physics 1989

Hans DehmeltUniversity of Washington

Wolfgang PaulUniversitat Bonn

Norman F. RamsayHarvard University

for the development of the ion trap technique

for invention of the separated oscillatory fields method and its use in the hydrogenmaser and other atomic clocks

http://www.nobel.se/physics/laureates/1989/illpres/

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Basic idea:

• confinement in electric and magnetic fields leads to motion in characteristic orbits(orbits are quantized – hence the analogy to atomic systems)

• oscillation frequency is proportional to (e/m) ratio for the charged particle

• resonant electrical signal from oscillations can be detected by an external circuit

• linewidth must be very narrow to achieve high precision -- some tricks:

- very stable B field (superconducting magnet)- carefully constructed and tuned or “compensated” electrode structure- cooling of electronics to liquid He temperature for low noise

• comparison of signals for reference and to-be-measured particle for calibration

Ref: Brown & Gabrielse, Rev. Mod. Phys. 58, 1986 p. 233 7

Basic Penning Trap Configuration:

• uniform, axial B field (superconducting solenoid) plus quadrupole E field:

• particles orbit around B field at cyclotron frequency, c = eB/m; radius given by energy.

• vertical confinement due to E; axial oscillations about horizontal midplane of trap

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Motion analysis (simple version!)

• cylindrical coordinates: (,,z); • B = constant along z• radial () and axial (z) electric field

2

22

2

2/

d

zVVwithVE o

zzd

V

m

ez

Bd

V

m

e

zo

o

22

22

A superposition of three motions for a given particle energy near the center of the trap:

1. circular orbits around the magnetic field at the cyclotron frequency c’ = eB/m - m 2. vertical oscillations (along z) at the axial frequency z

3. slow circular orbits in the horizontal plane at the magnetron frequency m = z2/ 2 c

BvEqF

:forceLorentz

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Particle Orbits:

Bm

ec

2d

V

m

e oz

Bd

Vom 22

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Typical parameter values 11