The Low Energy Physics Frontier of the Standard Model at the MAMI accelerator

Concettina Sfienti
Johannes Gutenberg-Universität - Institut für Kernphysik, Mainz
The low energy
physics frontier
@Mami:
Results and
Perspectives
Sunday, April 13, 2014

The low energy
physics frontier
@Mami:
Results and
Perspectives

Why? How?
Who? Where?
The low energy
physics frontier
@Mami:
Results and
Perspectives

EU-DE-MZ-JGU-KPH

The MAMI Legacy

Upgrade to ...
MAMI-C
Harmonic Double Sided Microtron (2007)
up to E = 1.6 GeV
HIGH
Intensity
up to 100 µA
Resolution
σE < 0.100 MeV
Polarization
up to 80% @ 40µA
Reliability
85% (7000 h/y)
The MAMI Legacy

The Wheelers
!"!"#$%&'()*"+&,-%(.*/"
01(%%"#$%&'()*+,-.+/"23%&'()4%'%(+"
01213'"456'78#9'

The Wheelers
!"!"#$%&'()*"+&,-%(.*/"
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!"#$%&'($)*+,+-$./'0&12-3!
! $!"##$%&41&566,1'*(78)*+,+-6$
! !"#8.&,9:#$;1<6,'($4'((=$>!).$

The Wheelers
!"!"#$%&'()*"+&,-%(.*/"
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01213'"456'78#9'
!"#$%&'($)*+,+-$./'0&12-3!
! $!"##$%&41&566,1'*(78)*+,+-6$
! !"#8.&,9:#$;1<6,'($4'((=$>!).$
!"#$%&'()*$+(,-&.,/$(/$(/$01$0-&2.3$23&40'(/5$
!"#$%&'!("#')*+,,&-."&*/012'3$
! $%678$9&-,'(:0)0'$$

The Realm

The Middle Earth of the Nuclear Realm
LRP Nuclear Science Advisory Committee(2008)
Resolution
Complexity
Simplicity
Hot and dense quark-gluon matter
Hadron structure
Nuclear structure
Nuclear reactions
Nuclear astrophysics
Applications of nuclear science
Hadron-Nuclear interface

What is it all about?

Scales and Phases of Nuclear Matter
Courtesy R.F.Carsten (WNSL)
LOOKING INWARD
Nucleon
Nuclear
Structure
Nuclear
Astrophysics
Neutron
Stars
Hypernuclei
Hot and
Dense
Matter
OUTWARD LOOKING

First Constraint
„If the Lord Almighty had consulted me before embarking upon creation,
I would have recommended something simpler.“
King Alphonse X. of Castille and Léon (1221-1284), on having the Ptolemaic system of epicycles explained to him
QCD in the
non-perturbative
regime
F. Wilczek “QCD made simple” (http://www.frankwilczek.com/)

First Constraint
Modern nuclear
physics is about...
!Linking QCD
to many body
systems
UNEDF SciDAC Collaboration
Universal Nuclear Energy Density Functional

The Beauty of the Electromagnetic Probe
Clean probe of
hadron structure
" Electron-vertex well-
known from QED
" One-photon exchange
dominates
" Higher-order exchange
diagrams are suppressed
" Vary the wavelength of the probe to view deeper
inside the hadron

The Proton

The Proton
...ask Prof. Wiki

The Proton
Mass(MeV)
One Millennium Quest
Generation of Mass
Not the elementary mass of the fermions
! Higgs Sector
But the actual mass of the “Macroscopic”
Hadron and its Composites

Nuclear Physics @ A=1 ≡ Nucleon Properties
Introduction – Nucleon properties
Proton / Neutron
mass
mp = 938.272046(21) MeV/c2
Discovered by E. Rutherford (1919)
mn = 939.565379(21) MeV/c2
Discovered by J. Chadwick (1939)
size
moments of electric charge
and magnetization distribution
derived from
form factor measurements
shape
departure from
spherical symmetry
determined in N −→ ∆
measurements
stiffness
electric and magnetic
polarizabilities extracted from
Virtual Compton scattering
(VCS)

“ Diamonds are for ever ...

Form Factors are e!ernal ”
http://hyperphysics.phy-astr.gsu.edu/
Rutherford Scattering
+ relativistic electrons
+ nuclear recoil
+ magnetic moments
= Mott Scattering

Form Factors are e!ernal ”
http://hyperphysics.phy-astr.gsu.edu/
Rutherford Scattering
+ relativistic electrons
+ nuclear recoil
+ magnetic moments
= Mott Scattering
Hofstadter, R., et al., Phys. Rev. 92, 978 (1953).
d"
d#
=
d"
d#
$
%
&
'
(
)
Mott
* G(q)
2

Form Factors from Elastic ep scattering
Classical picture
form factor: G(q2) = 1
e
∞
0
ρ(r)
sin qr
qr
4πr2
dr
dipole
e.g. proton
formfactor|G(q
2
)|!
gaussian
e.g.
6
Li
momentum transfer |q| !
oscillating
e.g.
40
Ca
Fourier
⇔
Transform
exponential
chargedensity!(r)
gaussian
radius r
sphere with
diffuse edge
charge distribution: ρ(r) = e
(2π)3
∞
0
G(q2
)
sin qr
qr
4πq2
dq

Cross section for one photon exchange
(Rosenbluth-cross section + Separation at constant Q2)
... and the slope of GE
charge distribution
distribution of
magnetic moments
# Separated GE(Q2) and GM(Q2)
$ but contribution from two photon exchange (TPE)
Form Factors from Elastic ep scattering

● Statistical precision σ 0.1%
● δθ 0.5 mrad vertical and horizontal
● Control of luminosity and systematic errors
The latest MAMI measurement
The experiment designed for ...
high precision by redundancy
% All quantities measured by more than one method

Rosenbluth with a twist
“Super-Rosenbluth Separation”:
fit of form factor models DIRECTLY to cross sections
● All Q2 and ε data are used in one ﬁt
● No projection to constant Q2
% no limit of kinematics
● One “estimator”
% stat. theory “robust estimator”

Strong Interactions
Hadron Structure
Hadron Spectroscopy
High-Energy Physics
Precision Physics
Atomic Physics
Astrophysics
The Low Energy Frontier
!#
$%'(#)*+%,-#
%+./01
...the
Atomic Physics
frontier

The Low Energy FrontierThe radius puzzle – Lamb shift in µH
Nature 466, 213-216 (8 July 2010)

“WE are famous!” (ED)
! !
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! !
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)!9%).)1
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*16#%.'01.;!01!('1;!=!9%)6#++#+!
....not just interesting:
Tests our theoretical understanding of proton
Radius of proton is dominant uncertainty in many QED processes

R. Pohl et al., Nature 466, (2010) 213
The Radius Puzzle
Discrepancy is between
muonic and electronic
measurements

The Radius Puzzle
measurements
Proton Radius Puzzle 57
muonic
hydrogen
1962
1963
1974
1980
1994
1996
1997
1999
2000
2001
2003
2007
2008
2010
2011
0.780
0.800
0.820
0.840
0.860
0.880
0.900
0.920
Orsay, 1962
Stanford, 1963
Saskatoon, 1974
Mainz, 1980
Sick, 2003
Hydrogen
Dispersion fit
CODATA 2006
MAMI, 2010
JLab, 2011
Sick, 2011
CODATA 2010
year !
Protonradius(fm)
Figure 1: Proton radius determinations over time. Electronic measurements seem
to settle around rp=0.88 fm, whereas the muonic hydrogen value [1,2] is at 0.84 fm.
Values are (from left to right): Orsay [10], Stanford [11], Saskatoon [12, 13],
Mainz [14] (all in blue) are early electron scattering measurements. Recent new
scattering measurements are from MAMI [4] and Jlab [15]. The green and cyan
points denote various reanalyses of the world electron scattering data [16–21]. The
red symbols originate from laser spectroscopy of atomic hydrogen and advances
Pohl, Gilman, Miller, Pachucki review, arXiv:1301.0905,
Novel beyond SM physics?
Novel hadron physics?
already excluded: missing atomic physics,
structures in FF, anomalous 3rd Zemach radius

The Radius Puzzle
measurements
New data are needed and they are coming ...
Range of new experiment
Belushkin (Dispersion Analysis 2007)
Price (CEA 1971)
Murphy (Saskatoon 1974)
Borkowski (MAMI 1975)
Simon (MAMI 1980)
Bernauer (MAMI 2010)
Four-Momentum Transfer Q2
/ (GeV2
/c2
)ElectricFormFactorGp
E/GDipole
0.010.0010.0001
1.02
1.01
1
0.99
0.98
0.97
0.96
Where should we determine the root-mean-square-radius?
r2
E = −6
d
dQ2
GE(Q2
)

Q2=0
Extrapolation to Q2
= 0, no absolute cross section!
...where should we determine the root-mean-
square-radius?
Extrapolation ! No absolute cross section

Possible experiments include:
Redoing atomic hydrogen
Light muonic atoms for radius
comparison in heavier systems
Electron scattering at lower Q2
(JLAB, MAMI)
Muon scattering (MUSE@PSI)
Improvements on Proton Radius

The Proton Radius Puzzle

Second constraint
© Jens Rydén
© Corgi Lane
© ryandury
It’s always
just water

Second constraint
© CERN © NASA
It’s always
just nuclear matter
© SciencePhotoLibrary

Second constraint
It’s always
just nuclear matter

Second constraint
Modern nuclear
physics is about...
!Unravelling
the phases of
nuclear matter
LRP Nuclear Science Advisory Committee(2008)

Second constraint
Each PHASE can exist in a
variety of states.
Each STATE is
characterized by defined
properties

Second constraint
Each PHASE can exist in a
variety of states.
Each STATE is
characterized by defined
properties
The EOS summarizes the
physically possible
combination of states.
PV =
nRT

A heavy nucleus (like 208Pb) is 18 orders
of magnitude smaller and 55 orders of
magnitude lighter than a neutron star
Yet bounded by the same EOS
The Equation of State of Nuclear Matter
© NASA
© SciencePhotoLibrary

Strong Interactions
Hadron Structure
Hadron Spectroscopy
High-Energy Physics
Precision Physics
Atomic Physics
Astrophysics
...the
Astrophysics
frontier
!#$%
'()*+
!,-.(,

Where do the neutrons go?
Pressure forces neutrons
out against surface tension
EOS
Neutron Skin Measurements

PV: the view in the mirror
#) !*+,#-./0 1111/./././.1111
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#$%' (#)$$!%*'+,
#-)'+! -!*#*$. /01!)2 #-)'+! /03)%*$.
2!)(4%! )(.22!$%.,
!!#$%*# #-)%+! 5 6
7!)8 #-)%+! !696: 5
Non-PV e-scattering
Electron scattering γ exchange provides Rp through nucleus FFs

1111/./././.1111
!*#*$. /01!)2 #-)'+! /03)%*$.
22!$%.,
!!#$%*# #-)%+! 5 6
7!)8 #-)%+! !696: 5
#) !*+,#-./0 1111/./././.1111
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2!)(4%! )(.22!$%.,
!!#$%*# #-)%+! 5 6
7!)8 #-)%+! !696: 5
Non-PV e-scattering
PV e-scattering
Electron also exchange Z, which is parity violating
Primarily couples to neutron

#) !*+,#-./0 1111/./././.1111
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Non-PV e-scattering
PV e-scattering
Electron also exchange Z, which is parity violating
Primarily couples to neutron
Detectable in PV asymmetry of electrons with different helicity
23456234562345623456 78(358978(358978(358978(3589:#:#:#:#5%'5%'5%'5%' 7';7';7';7'; 9#9#9#9#5%'5%'5%'5%' 4484448444844484
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Violating Electron Scattering
− also exchange Z, which is parity
olating
rimarily couples to neutron:
n
∝ 1 − 4 sin2
θW ≈ 0.076, Qneutron
weak ∝ −1
e−
Z
Detectable in parity violating asymmetry of electrons with
diﬀerent helicity
In Born approximation, Q2 M2
Z , from γ − Z interference:
APV =
σ+ − σ−
σ+ + σ−
=
GF Q2
4πα
√
2

1 − 4 sin2
θW −
Fn(Q2)
Fp(Q2)

For ﬁxed target exp., typical APV ∼ 10−7 − 10−4

Hard, harder, PV Experiments
3 Decades of Progess
sub-part per billion statistical
reach and systematic control
sub-1% normalization control
State-of-the-art:

Hard, harder, PV Experiments
sub-part per billion statistical
reach and systematic control
sub-1% normalization control
State-of-the-art:
3 Decades of Progess
Roca-Maza et al
± 3%
± 0.06 fm
PV Measurements@A1
(commissioning in Spring 2014 - 12C)

Diverse experiments but
consistent results
Too many model dependent
observables
Neutron skin Radii: Where are we?
Precise determination of NSkin in
208Pb set a basic constraint on
the nuclear symmetry energy
does not. Then, we have to conclude that a 3% accuracy in
APV sets modest constraints on L, implying that some of
the expectations that this measurement will constrain L
precisely may have to be revised to some extent. To narrow
down L, though demanding more experimental effort, a
1% measurement of APV should be sought ultimately in
PREX. Our approach can support it to yield a new accuracy
near Árnp 0:02 fm and L 10 MeV, well below any
previous constraint. Moreover, PREX is unique in that the
and Grants No. FIS2008-01661 from MEC and FEDER,
No. 2009SGR-1289 from Generalitat de Catalunya, and
No. N N202 231137 from Polish MNiSW.
[1] Special issue on The Fifth International Conference on
Exotic Nuclei and Atomic Masses ENAM’08, edited by
M. Pfu¨tzner [Eur. Phys. J. A 42, 299 (2009)].
[2] G. W. Hoffmann et al., Phys. Rev. C 21, 1488 (1980).
[3] J. Zenihiro et al., Phys. Rev. C 82, 044611 (2010).
[4] A. Krasznahorkay et al., Nucl. Phys. A731, 224 (2004).
[5] B. Kłos et al., Phys. Rev. C 76, 014311 (2007).
[6] E. Friedman, Hyperfine Interact. 193, 33 (2009).
[7] T. W. Donnelly, J. Dubach, and Ingo Sick, Nucl. Phys.
A503, 589 (1989).
[8] D. Vretenar et al., Phys. Rev. C 61, 064307 (2000).
[9] C. J. Horowitz, S. J. Pollock, P. A. Souder, and R.
Michaels, Phys. Rev. C 63, 025501 (2001).
[10] K. Kumar, P. A. Souder, R. Michaels, and G. M. Urciuoli,
http://hallaweb.jlab.org/parity/prex (see section ‘‘Status
and Plans’’ for latest updates).
[11] M. Centelles, X. Roca-Maza, X. Vinãs, and M. Warda,
Phys. Rev. C 82, 054314 (2010).
[12] I. Angeli, At. Data Nucl. Data Tables 87, 185 (2004).
[13] B. A. Brown, Phys. Rev. Lett. 85, 5296 (2000); S. Typel
and B. A. Brown, Phys. Rev. C 64, 027302 (2001).
[14] R. J. Furnstahl, Nucl. Phys. A706, 85 (2002).
[15] A. W. Steiner, M. Prakash, J. M. Lattimer, and P. J. Ellis,
Phys. Rep. 411, 325 (2005).
[16] B. G. Todd-Rutel and J. Piekarewicz, Phys. Rev. Lett. 95,
v090MSk7
HFB-8SkP
HFB-17
SkM*
Ska
Sk-Rs
Sk-T4
DD-ME2DD-ME1FSUGold
DD-PC1PK1.s24
NL3.s25
G2
NL-SV2PK1NL3NL3*
NL2
NL1
0 50 100 150
L (MeV)
0.1
0.15
0.2
0.25
0.3
∆rnp
(fm)
Linear Fit, r = 0.979
Nonrelativistic models
Relativistic models
D1S
D1N
SGII
Sk-T6SkX
SLy5
SLy4
MSkAMSL0SIVSkSM*SkMP
SkI2
SV
G1
TM1
NL-SHNL-RA1
PC-F1
BCP
RHF-PKO3Sk-Gs
RHF-PKA1PC-PK1
SkI5
FIG. 3 (color online). Neutron skin of 208
Pb against slope
of the symmetry energy. The linear fit is Árnp ¼ 0:101 þ
0:001 47L. A sample test constraint from a 3% accuracy in
APV is drawn.
PRL 106, 252501 (2011) P H Y S I C A L R E V I E W L E T T E R S
week ending
24 JUNE 2011
X. Roca-Maza, at al. Phys. Rev. Lett. 106, 252501 (2011)
Method
1 2 3 4 5 6
S(fm)
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Pb Neutron Skin Radius
208
PREX
Pb(p,p)208
Antiprotonic Atoms
EDP
PDR
Pionic Atoms

! Suggest what PV experiment can’t do:
good quality skin thickness measurements of a wide range of nuclei:
Do we need this systematics?
Neutron skin Radii: Where are we?
dy of neutron skins leadK ¼ À500þ125
À100 MeV. A value K ¼ À550 Æ 100 Mseems to be favored by the giant monopole resona(GMR) measured in Sn isotopes as is described in [Even if the present analyses may not be called deﬁnit
0
0.1
0.2
I = (N−Ζ) /Α
-0.1
0
0.1
0.2
0.3
S(fm)
40
20Ca 58
28Ni
54
26Fe
60
28Ni
56
26Fe 59
27Co
57
26Fe
106
48Cd
112
50Sn
90
40Zr
64
28Ni
116
50Sn
122
52Te
124
52Te
48
20Ca
96
40Zr
120
50Sn
116
48Cd
126
52Te
128
52Te
124
50Sn
130
52Te
209
83Bi
208
82Pb
232
90Th
238
92U
experiment
linear average
of experiment
prediction Eq. (2)
FSUGold
SLy4
FIG. 2 (color online). Comparison of the ﬁt described in th
text of Eq. (2) with the experimental neutron skins from anti
protonic measurements and their linear average S ¼
ð0:9 Æ 0:15ÞI þ ðÀ0:03 Æ 0:02Þ fm [20]. Results of the modern
Skyrme SLy4 and relativistic FSUGold forces are also shown.
M. Centelles, et al Phys. Rev. Lett. 102, 122502 (2009)
A. Trzcinska et al., Phys. Rev. Lett. 87, 082501 (2001);
J. Jastrzebski et al., Int. J. Mod. Phys. E 13, 343 (2004).
NSkin from antiprotonic-atoms:
correlation between
thickness and isospin
Neutron radius is not directly
measured
! strong dependence on
theoretical models.

If YES then ... shine light on the nucleus!
!#$%$'!#$%$'!#$%$'!#$%$' !!!!(((( )#')%*+!',)#')%*+!',)#')%*+!',)#')%*+!', -./0-./0-./0-./0
12.3#,$4.5 1678.0(9: ;;;;;;;;
!!!!! #$%'(% )*+, (-./ #$0.0*/*+1 $2
#$+$23 .2%42(+#$23
5.++(#46$#746.'+$# ! #8783847.++(#4#.%*3
5$3+43*7/(49:;4.#$=*7.+*$2
!!!!!
?4-

208Pb neutron skin from Coherent π0
D. Watts, et al,EPJ Web of Conferences 37, 01027 (2012)
Method
1 2 3 4 5 6 7
S(fm)
0.1
0.2
0.3
0.4
0.5
Pb Neutron Skin Radius
208
PREX
Pb(p,p)208
Antiprotonic Atoms
EDP
PDR
Pionic Atoms
Coh.Ph.!
Skin ~ 0.12 ±0.02(stat) fm
Systematic error currently
being finalised.
expect ~0.05 fm
p,d pickup
PREX@JLAB
Proton Scatt
Antiproton@CERN
Dipole
MAMI-C
q = 0 to 2
q =1st minima
q=2nd minima
q =3rd minima
Eγγγγ=155±5
Eγγγγ=165±5
Eγγγγ=175±5
Eγγγγ=185±5
Eγγγγ=195±5
(fm-1) |q| = pγγγγ-pππππ0 (fm-1)
PhD: Tarbert(Edinburgh)
Skin ~ 0.12 ±0.02(stat) fm
Systematic error currently
being finalised.
expect ~0.05 fm
p,d pickup
PREX@JLAB
Proton Scatt
Antiproton@CERN
Dipole
MAMI-C
q = 0 to 2
q =1st minima
q=2nd minima
q =3rd minima
Eγγγγ=155±5
Eγγγγ=165±5
Eγγγγ=175±5
Eγγγγ=185±5
Eγγγγ=195±5
|q| = pγγγγ-pππππ0 (fm-1) |q| = pγγγγ-pππππ0 (fm-1)
Neutronskinthickness(fm)
PhD: Tarbert(Edinburgh)
New experimental campaign@A2
(October 2012)
116,120,124Sn, 58Ni, 208Pb
(I = 0.03÷0.21)
208Pb Target

Concettina Sfienti
...perspective are excellent!
Near future (2013-2015) ....
Radius Puzzle: ISR and d-FF
Neutron form factor: new neutron detector
Nucleon polarizabilities with real photons
High resolution pion spectroscopy of light hypernuclei
PRECISION
COMPLEXITY
The Low Energy Frontier @ Mainz: Results and ...

Strong Interactions
Hadron Structure
Hadron Spectroscopy
High-Energy Physics
Precision Physics
Atomic Physics
Astrophysics
...the
discovery
(HE, Precision, Astroparticle)
frontier
!#$%%
'()(*%

Conventional strategies for DM searches
ventional strategies for dark matter search in particle physics
Direct Production:
Tevatron, LHC
Direct Search:
CDMS, DAMA/LIBRA,
XENON, CRESST, LUX,
COUPP, KIMS, ...
Indirect Search:
PAMELA, Fermi, HESS,
ATIC, WMAP, ...
A bottom up approach: Looking for interacting particles

Conventional strategies for DM searches
Assumptions:
There is dark matter (SUSY or something else)
Dark matter interacts with Standard Model
matter (besides gravity)
Dark matter interacts via a “dark force”
Question:
What is the character of this “dark force”?
Scalar, pseudo-scalar, vector bosons?
Massive or mass-less? Mass range?
Size of the coupling constant?
ventional strategies for dark matter search in particle physics
Direct Production:
Tevatron, LHC
Direct Search:
CDMS, DAMA/LIBRA,
XENON, CRESST, LUX,
COUPP, KIMS, ...
Indirect Search:
PAMELA, Fermi, HESS,
ATIC, WMAP, ...
A bottom up approach: Looking for interacting particles

A top down motivation
● Extra U(1) gauge bosons ubiquitous in well motivated
extension of SM
● U(1) gauge bosons may be hidden (no interaction with SM)
● No reason for U(1) boson to be heavy
● Dark matter couples to U(1) bosons γ and γ’
● Mixing parameter ε of γ/γ’ mixing
● Boson mass mγ’ 0 decay suppressed, macroscopic lifetime
Look for χ at high energies OR for γ’ at low energies!
B. Holdom Phys. Lett. B 166 (1986) 196
Kinetic mixing
Dark matter couples to U(1) bosons γ and γ
:
e
e
γ ′γ∗
χ
χ
_
L ⊃ −
1
4
FSM
µν Fµν
SM −
1
4
Fhidden
µν Fµν
hidden +
ε
2
FSM
µν Fµν
hidden + m2
γAhidden
µ Aµ
h
Renormalization of charge:

Dark Photon
Light weakly coupled
U(1) gauge boson
N. Arkani-Hamed, et al., Phys. Rev. D 79 (2009) 015014
Proton Radius Puzzle
D. Tucker-Smith and I. Yavin Phys. Rev. D83 (2011) 101702
(g-2)µ anomaly
M. Pospelov, Phys. Rev. D80 (2009) 095002
...it explains ...
terrestrial anomalies
(DAMA, CDMS, XENON)
satellite anomalies
(PAMELA, FERMI)
Probing Dark Forces @ GeV Scale

Probing Dark Forces @ GeV Scale
Prediction are testable:
Large cross section in leptons
' World wide effort (CERN, DESY,
JLAB, MAMI, all e+e- colliders, ...)Proton Radius Puzzle
D. Tucker-Smith and I. Yavin Phys. Rev. D83 (2011) 101702
(g-2)µ anomaly
M. Pospelov, Phys. Rev. D80 (2009) 095002
...it explains ...
terrestrial anomalies
(DAMA, CDMS, XENON)
satellite anomalies
(PAMELA, FERMI)
Dark Photon
Light weakly coupled
U(1) gauge boson
N. Arkani-Hamed, et al., Phys. Rev. D 79 (2009) 015014

Search for the Dark Photon @ MAMI
Bump Hunt: Quasi-photoproduction off 181Ta target
H. Merkel et al., Phys. Rev. Lett. 106 (2011) 251802
Beam current:! 100µA
Luminosity: L = 1.7·1035 (s·cm2)-1
Minimal angles for spectrometers
Geometry as symmetric as possible
(background reduction)
Coincidence time resolution! ≈ 1 ns FWHM
Almost no accidental background! ≈ 5%
Above background: only coincident e+e− pairs!
δme+e− 0.5MeV/c2

MixingParameter
2
Dark Photon Mass m ’ (MeV/c2
)
10
-7
10
-6
10-5
10
-4
10 100
(g-2)!(g-2)e vs.
BaBar
A1-2011
e
e #!
!
|(g-2)!| 2
E774
KLOE
APEX
Bump Hunt: Quasi-photoproduction off 181Ta target
% Fight background ....
... with high intensity ...
... and resolution.
Search for the Dark Photon @ MAMI
H. Merkel et al., Phys. Rev. Lett. 106 (2011) 251802
Feasibility:
Background (within 1%)
First exclusion limits 10−3
New ε/mγ’ scan
4 days beam time!!
APEX: S. Abrahamyan et al., Phys. Rev. Lett. 107 (2011) 191804
KLOE: F. Archilli et al., Phys. Lett. B. 706 (2012) 251
PRELIMINARY
Future: Low mass region 50 MeV/c2 and small dark photon coupling ε2

... MESA beyond MAMI ...
Harald Merkel, European Few Body 22, Krak´ow 09/2013
MESA: Mainz Energy recovering Superconducting Accelerator
Superconducting LINAC
Three recirculation arcs for external beam
high external current for “next generation” parity violation experiments
Energy recovery mode (half wave-length recirculation) for internal target experiments
Current Energy Luminosity
External Beam Mode: 150 µA 200 MeV 1039
cm−2
s−1
Energy Recovery Mode: 10 mA 150 MeV 1036
cm−2
s−1
Mainz Energy recovering Superconducting Accelerator

Concettina Sfienti
...perspective are excellent!
Near future (2013-2015) ....
Radius Puzzle: ISR and d-FF
Neutron form factor: new neutron detector
Nucleon polarizabilities with real photons
High resolution pion spectroscopy of light hypernuclei
PRECISION
COMPLEXITY DISCOVERY
POTENTIAL
The Low Energy Frontier @ Mainz: Results and ...

The Low Energy Physics Frontier of the Standard Model at the MAMI accelerator

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