size of the proton...the puzzle motivation hydrogen spectroscopy ep elastic scattering muonic...
TRANSCRIPT
Size of the proton
Sai Neha Santpur290E
22nd March 2017
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Outline
● The puzzle
● Motivation
● Hydrogen spectroscopy
● ep elastic scattering
● Muonic Hydrogen
● Possible explanations
● Future
● Summary
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The puzzle
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The puzzleEarly electron scattering experiments
Recent electron scattering experiments
>5
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The puzzle (Simplified version)
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Motivation
● Hydrogen – simplest atom and perfect for comparison between theory and experiment
● Looking back..
Balmer series Bohr model and quantum mechanics
Spin-orbit coupling Validated Dirac equation
Lamb shift Quantum Electrodynamics
Proton radius puzzle ???
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Hydrogen spectroscopy
● Lamb shift measurement
Lamb shift
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● Theoretically, simplified result for Lamb shift for S states can be written as
where R is Rydberg constant
and rp is the proton radius
– What does this tell us?
1. Lower n more sensitive to rp
2. Higher n Precise measurement of R
2. You need at least two transitions to comment on rp from Lamb shift measurements
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● Experimentally,
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ep elastic Scattering
where Q2 = Momentum transfer
GE and GM are electronic and magnetic form factors
= Q²/4m²
= lab scattering angle and
Here, proton radius is obtained by finding slope of GE when Q2=0
One photon echange
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Phys. Rev. C 93, 065207
Mainz A1 collaborationr
p = 0.895(20) fm
Modern scattering experiment values are more or less the same.
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Muonic hydrogen
● m ~ 105 MeV and me ~ 0.5 MeV
● Back of the envelope calculation:
– Probability of lepton being inside proton radius =
where mr = reduced lepton mass
– Probability of muon being inside proton radius is 8 million times more than electron!
– Muon must be more sensitive to proton radius.
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● Experimentally challenging!
Dominates
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2010 experiment
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● Using simplified expression for the transition after considering the Lamb shift and hyperfine splitting, the energy expression is
● Result: rp = 0.84184(67) fm
Previously agreed result: 0.8768(69) fm
>5 deviation!! Experimental artifact?
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● Using simplified expression for the transition after considering the Lamb shift and hyperfine splitting, the energy expression is
● Result: rp = 0.84184(67) fm
Previously agreed result: 0.8768(69) fm
>5 deviation!! Experimental artifact?
● Measured same transition in H and found
rp = 0.84087(39) fm
>7 deviation!!
No!2013 experiment
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Possible explanations
● Depends on which theorist you ask!
– Quantum gravity
– Extra dimensions
– New boson?
...
● Experimental error in measuring Rydberg Constant?
– Error of the order 1012 can explain this dicrepancy.
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Future
● PRad (New Proton Radius experiment)
● MUSE (MUon Scattering Experiment)
● Spectroscopy of electronic atoms and ions to determine Rydberg constant to very high precision.
● Muonic deuterium
● Muonic helium vs electronic helium
...
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PRad
● Experiment at Jlab
● ep elastic scattering method
● Data taken: May-June 2016
● Result: Coming soon!
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MUSE
● p elastic scattering
● This is messy– Muon beams have lower flux
– Need higher acceptance spectrometer
– Might decay into electrons
– Higher background due to ,e-
...
● Status: Not operational yet!
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Summary
● Proton radius measurement has significant discrepencies.
● Different experiments are using different techniques to make an independent measurement of the radius.
● Many more results to come in the future..
– which could either resolve the discrepancy or lead to some new physics
● Stay tuned!
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References● Muonic Hydrogen: https://www.jlab.org/indico/event/160/session/0/contribution/97/material/slides/0.pdf
● Nature 466: 213-216, 2010
● Science 339 (6118), 417-420
● http://www.physics.rutgers.edu/~rgilman/elasticmup/
● Annu. Rev. Nucl. Part. Sci. Vol 63 (2013)
● Particlebites.com
● https://www.jlab.org/prad/collaboration.html