Unsolved Mysteries of the Universe: Looking for Clues in Surprising Places

Transcription

1 The 64 th Compton Lecture Series Unsolved Mysteries of the Universe: Looking for Clues in Surprising Places Lecture 5: Using the Fine Structure Constant to Push on the Standard Model Brian Odom THE ENRICO FERMI INSTITUTE

2 The Fine Structure Constant α = Hydrogen Energy Levels 2 e c If there were no fine structure 2P 1S With fine structure 2P 1S Predicted Emission Spectrum 4.5 x 10-5 ev 10.2 ev (121.6 nm) 10.2 ev (121.6 nm)

3 Coupling Constants The fine structure constant quantifies the strength of the coupling of light to matter, or the strength of electromagnetism. We will talk only about the low-energy limit

4 What is its Value? It is one of the greatest damn mysteries of physics: a magic number that comes to us with no understanding by man." -Richard Feynman It has been a mystery ever since it was discovered more than fifty years ago, and all good theoretical physicists put this number up on their wall and worry about it. -Richard Feynman Despite many attempts, α remains one of the free parameters of the standard model. We cannot predict it; we must measure it.

5 What is its Value? α or α but not quite α (96) Gabrielse, Hanneke, Kinoshita, Nio, and Odom, Phys Rev Lett 97, (2006)

6 A Good Thing that it is Small α This allows us to use perturbation theory for calculations 1 st order: 2 nd order: rd order: +

7 Old Quasar Light Is it Really Constant? Murphy, et. al. Mon. Not. R. Astron Soc. 345 (2003) ( figure from Chris Churchill )

8 Old Quasar Light Is it Really Constant? Srianand, et al. Phys. Rev. Lett.. 92, 12 (2004) figure from Chris Churchill systematics errors can be tricky

9 Is it Really Constant? experiment z Δα α α α (yr -1 ) Lab atomic transitions (Yb/Cs) 0 < 6 x < 200 x Oklo nuclear phenomenon 0.14 < 0.1 x 10-7 < 0.5 x Meteorite Re/Os abundance 0.45 < 8 x 10-7 < 20 x QSO absorp., Srianand et. al < 6 x 10-7 < 6 x QSO absorp., Murphy et. al (12) x (14) x ( but I heard yesterday that the lab results have improved substantially) An interesting note: these experiments have fantastic sensitvity to variations of α, but they are not the best ways to measure its actual value So far, there is no evidence that α varies with time. But, it does vary in some theories. We keep looking.

10 Electron Magnetic Moment 0 th order: 1 st order: + If the electron is a point particle (has no size), the Standard Model allows precise prediction of the magnetic moment, provided we know α

11 The g - Factor q μ = g 2m S Classical, non-relativistic Dirac equation as singleparticle wave equation Quantum Electrodynamics (QED) g =1 g = 2 g =

12 Why Measure the g - Factor? Determination of α, using QED calculations Precision test of QED Probe for electron sub-structure Precision test of Lorentz, CPT symmetry Complement to the muon g factor measurement (which is a great search for new particles) Prospects for improved proton to electron mass ratio

13 Does the Electron Have Size? So far we have no reason to believe it has size but we keep looking.

14 Testing Quantum Electrodynamics Δα / α (ppb) electron g, Harvard 2006 h / m Cs, optical transitions, mass ratios electron g, UW 1987 h / m Rb, mass ratios quantum Hall effect h / m n ac Josephson effect & γ p,h muonium h.f. structure α -1

15 Different Levers for New Physics experiment z Δα α α α (yr -1 ) Lab atomic transitions (Yb/Cs) 0 < 6 x < 200 x Oklo nuclear phenomenon 0.14 < 0.1 x 10-7 < 0.5 x Meteorite Re/Os abundance 0.45 < 8 x 10-7 < 20 x QSO absorp., Srianand et. al < 6 x 10-7 < 6 x QSO absorp., Murphy et. al (12) x (14) x ( but I heard yesterday that the lab results have improved substantially) Some people use high precision over short times. Some people use enormous times and less precision.

16 Different Levers for New Physics Higher energy means shorter wavelength and the ability to look for even smaller structure or we can do precision measurements, like measurement of the g - factor

17 g from the Standard Model g α α α α = 1 + C1 C2 C3 C4... non QED π π π π

18 An Electron in a Penning Trap B Field E Field magnetron motion axial motion cyclotron motion motion frequency hυ/k b damping axial 200 MHz 9.6 mk 1 Hz cyclotron GHz 7.2 K 0.02 Hz spin GHz 7.2 K Hz magnetron 130 khz 6.4 μk Hz

19 Do a Frequency Measurement! Definition: q μ = g 2m S g in free space: g 2 = ωs ω c g-2 in free space: g 2 a 1 g = + = 1+ ω 2 2 ω (3 orders of magnitude for free) c g-2 in a Penning trap: 2 1 a z 2( c + 2 ) 2 ( ω + 1 δ) + ω ( ω + 1 δ) g =1+ ω ω ω δ 2 2 c 2 z c 2 [ Brown and Gabrielse. Rev. Mod. Phys. 58, 1 (1986) ]

20 Cylindrical Penning Trap Construction

21 Dilution Refrigerator and Magnet 10' 1" x10

22 The Lab

23 A tabletop experiment if you have a high ceiling

24 Detection of a Single Electron The axial oscillator is coupled to a tuned-circuit amplifier Signal Out Axial motion is driven to increase signal amplitude (a. u.) frequency ν z (Hz)

25 Single Quantum Jumps In free space, cyclotron lifetime = 0.08 s In our cylindrical traps, we have achieved a 16 s lifetime number of n=1 to n=0 decays τ = 16 s axial frequency shift (Hz) time (s) decay time (s) [ Peil and Gabrielse. Phys. Rev. Lett. 83, 1287 (1999) ]

26 Benefits of Cooling Down Relativistic Corrections Thermal Jumps 4.2 K cyclotron quantum number K 2.0 K 1.6 K.08 K time (minutes) [ Peil and Gabrielse. Phys. Rev. Lett. 83, 1287 (1999) ] Eliminates relativistic error from ω c uncertainty Reducing thermal jumps permits single-quantum cyclotron spectroscopy

27 T-Dependent Magnetism BAD An unpleasant surprise: temperature (mk) B field shift (ppb) time (hours) Shift of -10 ppb / mk at 75 mk!!! We observed a huge shift of B-field vs. trap temperature Heat load changes are unavoidable as: Amplifier cycles on/off Anomaly drive is applied 10 ppb / mk is far too much!

28 It s the Trap! magnetic field shift (ppb) temperature (Kelvin) temperature -1 (Kelvin -1 ) Nuclear paramagnetism makes standard Penning trap materials (copper, MACOR) incompatible with a stable B-field below 1 K

29 New Silver Trap

30 Prototype Silver Tripod

31 Silver Trap Improvement magnetic field shift (ppb) expanded 200 x copper trap silver trap copper trap silver trap temperature (Kelvin) temperature -1 (Kelvin -1 ) New silver trap decreases T-dependence of the field by ~ 400 With the silver trap, sub-ppb field stability is easily achieved

32 More Benefits of Cooling Down T z (K) υ z (MHz) B 2 (T/m 2 ) H U. Wash. Harvard Δ H Δ Δ = UW Δ UW 0.09 [ Van Dyck et al., QED, Kinoshita, ed. 1990] Harvard cyclotron line quantum jump fraction ppb quantum jump fraction ppb frequency khz frequency khz

33 Scatter of Measurements UW 1987 Harvard UW uwave power (a.u.) uwave power (a.u.)

34 Cavity Shift Systematic Parametric response of large e - cloud maps cavity mode structure [ Tan and Gabrielse. App. Phys. Lett. 55, 2144 (1989) ] Modes coupling to centered single e - cloud are easily identified TM 1n1 TE 1n1

35 Lifetime Shifts Perform single e - experiments between TE 127 and TM 143 Cyclotron lifetime data shows qualitatively correct behavior Q = 6500 E Q = 1400 M Fixing mode ω s and fitting for Qs gives reasonable results

36 Apparent g Factor Shift

37 And the Number is Harvard g-factor measurement: Fully quantum measurement eliminates relativistic shift ( 1 ppt per quantum level ) Low temperature allows quantum spectroscopy and narrows lines Cylindrical trap allows first quantitative treatment of cavity shift Results : g / 2 = (76) (0.76 ppt) -1 α = (90) (32) (96) (0.70 ppb)

38 New Value for α