[Jlab-seminars] Upcoming Physics Seminars: January 13, 2017 (Jan C. Bernauer), January 20, 2017 (Randolf Pohl)

[Stephanie Tysor]

*Physics Seminar*

*Jan C. Bernauer*

*Massachusetts Institute of Technology*

**

/Two-photon Exchange in Proton Elastic Scattering/


Abstract:


/Recent interest in the proton electromagnetic form factors is partly 
motivated by the discrepancy found in the determination of the 
electric-to-magnetic form factor ratio using different techniques. 
Results from scattering experiments using the Rosenbluth technique 
indicate that the form factor ratio is constant as a function of $Q^2$ 
while experiments employing polarization show a clear, roughly linear, 
decline of the ratio. A possible explanation is the typically 
unaccounted for contribution of hard two-photon exchange to the 
scattering process. Theoretical calculations show large variations, many 
indicating an effect of the right sign and magnitude. Direct 
verification was sought by experiments at VEPP-3, Jefferson Lab and by 
the OLYMPUS collaboration at DESY. In the talk, I will discuss the
OLYMPUS experiment and results and summarize our current understanding 
of TPE and the proton form factors in the context of the three experiments./

//

*Friday, January 13, 2017*

*11:00 am*

*CEBAF Auditorium*

*_________________________________________________________________________________*


*Physics Seminar*

*Randolf Pohl*

*Johannes Gutenberg-University, Mainz Germany*

*Max-Planck Institute of Quantum Optics, Garching Germany*

**

/Shrinking the Proton –Laser Spectroscopy for Nuclear Physics and 
Fundamental Constants/
Abstract:

/For more than a decade, the rms charge radius of the proton was known to be
0.88fm, with about 1% uncertainty [1]. Two methods, elastic electron 
scattering
and precision laser spectroscopy of atomic hydrogen, yielded consistent 
values.

In 2010, our result from laser spectroscopy of the exotic "muonic 
hydrogen" atom
yielded a 4% smaller value, 0.84 fm, with an uncertainty of less than 0.1%
[2,3]. In muonic hydrogen, a negative muon orbits a proton with a 200 times
smaller Bohr orbit than in regular hydrogen, which increases the 
sensitivity of
muonic hydrogen to the proton charge radius by 200^3 ~ 10 million!
Since 2010, the discrepancy increased to more than 7 sigmas [4], making 
it one
of the biggest discrepancies in the Standard Model.

I will discuss the so-called "proton radius puzzle" [5], report on more
measurements in muonic atoms [6], and the result of a new measurement in 
regular
atomic hydrogen./

[1] P.J. Mohr et al. (CODATA 2006), Rev. Mod. Phys. 80, 633 (2008 
<callto:80,%20633%20%282008>)
[2] Pohl et al. (CREMA coll.), Nature 466, 213 (2010 
<callto:466,%20213%20%282010>)
[3] Antognini et al.,(CREMA coll.), Science 339, 417 (2013 
<callto:339,%20417%20%282013>)
[4] Olive et al. (PDG 2014), Chin. Phys. C40, 090001 (2014 
<callto:090001%20%282014>)
[5] Pohl et al., Annu. Rev. Nucl. Part. Sci 63, 175 (2013 
<callto:63,%20175%20%282013>)
[6] Pohl et al. (CREMA coll.), Science 353, 669 (2016 
<callto:353,%20669%20%282016>)//

*Friday, January 20, 2017*

*11:00 am*

*CEBAF Auditorium//*

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