EXPERIMENTSWITH INTERNAL T MAINZ ENERGY-RECOVERING...

EXPERIMENTS WITH INTERNAL TARGETS AT THE

MAINZ ENERGY-RECOVERING SUPERCONDUCTING ACCELERATOR

Harald MerkelJohannes Gutenberg-Universitat Mainz

International Workshop on (e,e′p) ProcessesBled (Slowenia), July 4th, 2017

The MESA Accelerator

MAGIX

High resolution magnetic spectrometersInternal targets: Gas Jet Target or Polarized Target

Physics program

Electric and Magnetic Radius of the ProtonAstropyhsical S-FactorSearch for exotic particlesFew body physics

Summary

Harald Merkel, Bled 2017 2/33

Energy Recovery Linac - Idea

Radio Frequency

RF-Cavity

Recirculation: l = n · λ

Injection External Beam

Wave length λ

Recirculating Linear Accelerator→ increase beam energy


Energy Recovery Linac - Idea

Radio Frequency

RF-Cavity

Recirculation: l = n · λ

Last Recirculation: l = (m + 1/2) · λ

Target

Injection Beam Dump

Wave length λ

Recirculating Linear Accelerator→ increase beam energy

Last return path: l = (m+ 12)λ→ Phase shift

Energy feed back to cavities→ increase beam current


MAMI - Floorplan

Using Halls of former A4 Experiment

Extension with new Hall

MAMI stays in operation!


MESA - Mainz Energy Recovery Superconducing Acclerator

Super-conducting, recirculating LINAC

Energy of up to 155 MeV

Operation for external targets, 1 mA, polarized beam

Operation in Energy Recovery Mode

Energy of up to 105 MeVHigh beam current (up to 10 mA)Large fraction of the beam can be used for an Internal Target

...funded, under construction!


Magix - Design Considerations

Using the full power of the MESA beam quality

Beam intensity

⇒ High count rate capability (MHz)

Nuclear Physics: Separate energy levels at 100 MeV at 100keV distance

⇒ momentum resolution δpp < 10−4

Precision measurement of electron scattering cross sectionsproportional to Mott cross section

dσ

dΩ≈ 1

Q4 ≈1

sin4θ/2

⇒ angular resolution δθ < 0.05

Low energetic particles (e, p, below π threshold)

Negligible energy loss in window-less gas targetVacuum until first detector layerExcellent position detection in first layer, modest angular detection later

⇒ Focussing spectrometers


Magix - Optics

0 500 1000 1500 2000 2500

z [mm] scale 1:180

x [

mm

]

0

500

1000

1500

2000

2500

0

0.175

0.35

0.525

0.7B [T]

-60

-50

-40

-30

-20

-10 0

10

20

30

40

50

60

0 5

00

1000

1500

200

0 2

500

3000

3500

y [mm]

pa

th le

ng

th o

ff c

en

tra

l b

ea

m [

mm

]

Vertical: Point-to-Point

⇒ Momentum Resolution

Horizontal: Parallel-to-Point

⇒ Angular Resolution


Magix


Magix - Possible Targets

Simple “Tube”-Target

Polarized Gas

Beam

Target cell

Polarized Hydrogen target

Flow is limited by polarizator (Laser driven/Atomic beam)

Luminosities of up to L = 2×1031 s−1cm−2

Polarization |~Pe| ≈ 80%

Established technique, e.g. BLAST, OLYMPUS, ...


Supersonic Jet Target

Hypersonic gas jet by Laval nozzle

Cool down to Cluster mode

Gas jet caught by vacuum system

Expensive part: Pumping system 1 mbar at target→ 10−11 mbar at RF-Cavity

Beam quality allows for narrow flow delimiters

Luminosities of up to L = 1036 s−1cm−2 at 10 mA


Supersonic Jet Target

Stabilized Temperature Tnozzle

Gas flow: 40 l/min

Density > 1019 Particles/cm2

First cluster jet seen!

S. Grieser, A. Khoukaz, ... (U Munster)


Physics Program at Magix

Physics Program to employ the strenghtes of MESA and Magix

High beam intensity⇔ low target density

Excellent beam quality⇒ Precision physics

High degree of beam/target polarization

Tuneable to very low energies

Selected Examples:

Electric and Magnetic Radius of the Proton

Astrophysical S-Factors

Search for exotic particles:

Search for Dark PhotonsInvisible Decay of Dark PhotonsDark matter beam

Tests of ab-initio Calculations in Few-Body Physics


Proton Form Factor


Form Factor of the Nucleon

Elastic Cross Section (Rosenbluth-Formula):

dσ

dΩe=

(dσ

dΩe

)Mott

1(1+ τ)

[G2

E(Q2)+

τ

εG2

M(Q2)]

with τ = Q2

4m2p

ε =(1+2(1+ τ) tan2 θe

2

)−1

Structure of the Nucleon:

GE(Q2): Electric Form Factor → related to charge distributionGM(Q2): Magnetic Form Factor → related to distribution of magnetic moments

Normalization: GpE(Q

2 = 0) = 1 GnE(Q

2 = 0) = 0

GpM(Q

2 = 0) = 2.79 GnM(Q

2 = 0) = −1.91

Proton Charge/Magnetic Radius:

r2E = −6

ddQ2 GE(Q2)

∣∣∣∣Q2=0

r2M = − 6

µp

ddQ2 GM(Q2)

∣∣∣∣Q2=0


Radius of the Proton

> 5σ Discrepancy between atomic physics and electron scattering

Situation still unclear

Serious problem far beyond nuclear physics: e.g. Rydberg Constant

Triggered extensive experimental program in atomic and nuclear physics


Radius of the Proton

Experiment: Elastic scattering — Rosenbluth-Separation

Example kinematics:

Q2 = 0.0001 GeV2

E0 = 38.0 MeVE ′ = 37.9 MeVθ′ = 15

L = 1036 1s cm2

⇒ Count Rate N = 300 MHz/msr

“infinite” count rate

Very clean experiment

No target walls

Four-Momentum-Transfer 0.00001 GeV2 < Q2 < 0.03 GeV2

⇒ Classical determination of electric Radius


Magnetic Radius of the Proton

Magnetic Radius from limit Q2→ 0

Suppressed by τ = Q2

4m2p

in cross section

dσ

dΩe=

(dσ

dΩe

)Mott

1ε(1+ τ)

[ε G2

E(Q2)+ τ G2

M(Q2)]

Beam-Recoil polarization is limited by proton recoil momentum |~pp|> 300 MeVc

Beam-Target polarization:

A(θ∗,φ∗) = AI sinθ∗ cosφ

∗+AS cosθ∗

AI = −2√

τ(1+ τ) tanθ

2GE GM

G2E +

(τ+2τ(1+ τ) tan2 θ

2

)G2

M

AS = −2 τ

√1+ τ+(1+ τ)2 tan2 θ

2tan

θ

2G2

M

G2E +

(τ+2τ(1+ τ) tan2 θ

2

)G2

M

φ∗ = 0θ∗ = 0, π

2

⇒ A⊥ =

Al

As∼ GE

GM


Magnetic Radius of the Proton - Asymmtry

]2/c2 [GeV2Photon Virtuality q

0.03 0.025 0.02 0.015 0.01 0.005

[

%]

Asym

metr

y A

70

60

50

40

30

20

10

0

Asymmetry (Calc) o

= 42.0e

θSetting o

= 61.8e

θSetting o

= 79.3e

θSetting o

= 96.9e

θSetting o

= 116.2e

θSetting o

= 141.8e

θSetting

= 100 MeV0

Beam Energy E

= 80%beam

Beam polarization P

= 75%T

Target polarization P

Time per setting t = 10d

2cm1 s31

10×Luminosity L= 2

(Conservative) assumptions for target ≈ Blast target

Statistical error only (systematic error should be small!)


Magnetic Radius of the Proton - Errors

— Belushkin (Disp. Analysis 2007)MESA projected errorJones (JLab 2000)Gayou (JLab 2001)Dietrich (MAMI 2001)Pospischil (MAMI 2001)Punjabi (JLab 2005)Jones (JLab 2006)MacLachlan (JLab 2006)Crawford (Bates 2007)Zhan (JLab 2011)

– Bernauer (MAMI 2010)

Q2 / (GeV2/c2)

µpG

p E/G

p M

1010.10.01

1.1

1

0.9

0.8

0.7

0.6

Maybe a little bit optimistic...


Astrophysical S-Factors


Nuclear Astrophysics α(12C,16 O)γ

Cross sections of stellar burning

Crucial for Nucleo-Synthesis

Supernova Explosions

Abundance of elements

Temperature of the sun⇒ Gamow Window

← He-Burning σ≈ 10−17 barn

Gamow Window

0+

0

0+

6049

3−

6129

2+

6917

1−

7116

2−

8871

12C+α

7162

16O


Astrophysical S-Factor for α(12C,16 O)γ

S-Factor: divide out “trivial” cross section σ(E) = 1E e−2π

z1 z2 α

v ·S(E)

Very low count rates

Reduce backgrund⇒ best results from underground labors, e.g. LUNA at Gran Sasso

Still large errors

Extrapolation to Gamow-Window


Astrophysical S-Factor for α(12C,16 O)γ

α

EE'

γ*

Silicon Strip Detector

O16 C

12

Spectrometer

Time reversal (enhancement by factor 10 due to spin weight):

γ+16 O→12 C+α

Covering the Threshold: Electroproduction in limit Q2→ 0

e+16 O→ e′+12 C+α ⇔ γ∗+16 O→12 C+α

Electron has large momentum, but virtual photon energy goes to zero!

High resolution for e-detection⇒ high resolution of γ detection

Detection of slow recoil α⇒ gas target, recoil detector ⇒ e+α coincidence


Search for Exotic Particles


Search for exotic particles: Motivation

Overwhelming evidence for Physics beyond the Standard Model

One of the most pressing Problems: Dark Matter, Dark Energy

Particle Physicists approach:

New unkown particlesResidual interaction with SM very likely (produced in the same Big Bang)...... but very weak

Direct search

Expensive experiments: XENON, CRESST, ...Very senistiveNo Signal yet

If particles, is there a whole Dark Sector of particles?

⇒ look e.g. for gauge bosons of dark sector


Search for exotic particles: Dark Photons

How to model new physics:

Postulate new symmetry group

Symmetry Breaking to restore Standard Model at low energies

Consequences:

Extra U(1) gauge bosons ubiquitous in well motivated extensions

large gauge symmetries must be broken

U(1)s are the lowest-rank local symmetries

U(1) gauge bosons may be hidden (no interaction with SM)

Example: U(1) gauge factors in string compactifications:

E8×E8→ E6×E8→ SU(3)c×SU(2)L×U(1)Y︸︷︷︸standard model

×U(1)hidden

from breaking of second E8

No reason for U (1) boson to be heavy! ⇒ Dark Photon


Search for exotic particles: Dark Photons

γ'

Z

e− e−

Z Z

e− e−

Z

(a) (b) γ'

Dark photon: Force carrier of the Dark Sector

Radiative production

e+Z → e+Z + γ′

→ e++ e− (detected in Magix)

] 2 [GeV/c'γm-210 -110

ε

-410

-310

-210

e(g-2)

KLOE 2013

KL

OE

201

4

WASA

HADESPHENIX

σ 2±µ(g-2)

favored

E774

E141

AP

EX

A1NA48/2

B A B AR2009

B A B AR2014

BESIII

MESA


Dark Photons at MESA: Invisible Decay

0o

90o

180o

270o

360o

10−2

10−1

1

10

102

103

104

105

106

107

108

dσ

/dE

’dΩ

’dΩ

γ [nb/G

eV

sr2

]

θγγ

Bethe−Heitler Cross Section

γ’−Produktion mγ’=50 MeV

e+ p → e′+ p+ γ′

→ χ+ χ

γ ′ detection via missing mass m2γ ′ = (e+ p− e′− p′)2

No restriction by decay

Background: virtual Compton scattering: e+ p→ e′+ p+ γ + radiative tail

Recoil angular resolution needed→ Silicon strip detectors


Beam-Dump Experiments: Motivation

1 10 100 1000 10410!5010!4910!4810!4710!4610!4510!4410!4310!4210!4110!4010!39

10!1410!1310!1210!1110!1010!910!810!710!610!510!410!3

WIMP Mass !GeV"c2#

WIMP!nucleoncrosssection!cm2 #

WIMP!nucleoncrosssection!pb#

7BeNeutrinos

NEUTRINO C OHER ENT SCATTERING

NEUTRINO COHERENT SCATTERING(Green&ovals)&Asymmetric&DM&&(Violet&oval)&Magne7c&DM&(Blue&oval)&Extra&dimensions&&(Red&circle)&SUSY&MSSM&&&&&&MSSM:&Pure&Higgsino&&&&&&&MSSM:&A&funnel&&&&&&MSSM:&BinoEstop&coannihila7on&&&&&&MSSM:&BinoEsquark&coannihila7on&&

8BNeutrinos

Atmospheric and DSNB Neutrinos

CDMS II Ge (2009)

Xenon100 (2012)

CRESST

CoGeNT(2012)

CDMS Si(2013)

EDELWEISS (2011)

DAMA SIMPLE (2012)

ZEPLIN-III (2012)COUPP (2012)

SuperCDMS Soudan Low ThresholdSuperCDMS Soudan CDMS-lite

XENON 10 S2 (2013)CDMS-II Ge Low Threshold (2011)

SuperCDMS Soudan

Xenon1T

LZ

LUX

DarkSide G2

DarkSide 50

DEAP3600

PICO250-CF3I

PICO250-C3F8

SNOLAB

SuperCDMS

Direct detection experiments:

No clear signal yet

Limit of sensitivity (solar ν background) will be reached soon

Lower masses (i.e. low recoil energy) not accessible

P. Cushman, et al., arXiv:1310.8327


Beam-Dump Experiments: Idea2

Beam

e

Dump

10 m 10 mDirt

Detector

1 m

1 m

1 m

OptionalShieldingDetector

FIG. 1: Schematic experimental setup. A high-intensitymulti-GeV electron beam impinging on a beam dump pro-duces a secondary beam of dark sector states. In the basicsetup, a small detector is placed downstream so that muonsand energetic neutrons are entirely ranged out. In the con-crete example we consider, a scintillator detector is used tostudy quasi-elastic -nucleon scattering at momentum trans-fers > 140 MeV, well above radiological backgrounds, slowneutrons, and noise. To improve sensitivity, additional shield-ing or vetoes can be used to actively reduce cosmogenic andother environmental backgrounds.

.

A0a)

Z

e

e

p, n

b)

A0

Z

FIG. 2: a) pair production in electron-nucleus collisionsvia the Cabibbo-Parisi radiative process (with A0 on- or o↵-shell) and b) scattering o↵ a detector nucleus and liberatinga constituent nucleon. For the momentum transfers of inter-est, the incoming resolves the nuclear substructure, so thetypical reaction is quasi-elastic and nucleons will be ejected.

Production in beam dump, e.g. via pair production (actually: we don’t care...)2

MeV GeV dark matter are those whose interactionswith ordinary matter are mediated by new GeV-scale“dark” force carriers (for example, a gauge boson thatkinetically mixes with the photon). Such models readilyaccount for the stability of dark matter and its observedrelic density, are compatible with observations, and haveimportant implications beyond the dark matter itself. Inthese scenarios, high energy accelerator probes of sub-GeV dark matter are as ine↵ective as direct detectionsearches, because the missing energy in dark matter pairproduction is peaked well below the Z ! backgroundand is invisible over QCD backgrounds[? ? ].

Instead, the tightest constraints on light dark matterarise from B-factory searches in (partly) invisible decaymodes [? ], rare kaon decays [? ], precision (g 2) mea-surements of the electron and muon [? ], neutrino ex-periments [? ], supernova cooling, and high-backgroundanalyses of electron recoils in direct detection [? ]. Theseconstraints and those from future B-factories and neu-trino experiments leave a broad and well-motivated classof sub-GeV dark matter models largely unexplored. Forexample, with a dark matter mass > 70 MeV, existingneutrino factories and optimisitic projections for futureBelle II sensitivity leave a swath of parameter space rel-evant for reconciling the (g 2)µ anomaly wide open(see Figure 3). More broadly, the interaction strengthbest motivated in the context of models with kineticallymixed force carriers (mixing 105 . . 103) lies justbeyond current sensitivity across a wide range of darkmatter and force carrier masses in the MeVGeV range.These considerations, along with the goal of greatly ex-tending sensitivity to any components of MeVGeV darkmatter beyond direct detection constraints motivates amuch more aggressive program of searches in the comingdecade.

The experimental setup we consider can dramaticallyextend sensitivity to long-lived weakly coupled states (seeFig. 3), including GeV-scale dark matter, any componentof dark matter below a few GeV, and milli-charged parti-cles. This includes a swath of light force carrier parame-ters motivated by the (g2)µ anomaly, extending beyondthe reach of proposed neutrino-factory searches and BelleII projections (see Figure 3). The setup requires a small1 m3-scale detector volume tens of meters downstreamof the beam dump for a high-intensity multi-GeV elec-tron beam (for example, behind the Je↵erson Lab Hall Aor C dumps or a linear collider beam dump), and couldrun parasitically at existing facilities. All of the above-mentioned light particles (referred to hereafter as “”)can be pair-produced radiatively in electron-nucleus col-lisions in the dump (see Fig. 2a). A fraction of theserelativistic particles then scatter o↵ nucleons, nuclei, orelectrons in the detector volume (see Fig. 2b).

Within a year, Je↵erson Laboratory’s CEBAF (JLab)[53] will produce 100µA beams at 12 GeV. Even a simplemeter-scale instrument capable of detecting quasi-elasticnucleon scattering, but without cosmic background re-jection, positioned roughly 20 meters downstream of the

A0a)

Z

e

e

p, n

b)

A0

Z

FIG. 2: a) pair production in electron-nucleus collisionsvia the Cabibbo-Parisi radiative process (with A0 on- or o↵-shell) and b) scattering o↵ a detector nucleus and liberatinga constituent nucleon. For the momentum transfers of inter-est, the incoming resolves the nuclear substructure, so thetypical reaction is quasi-elastic and nucleons will be ejected.

Hall A dump has interesting physics sensitivity (upper,dotted red curves in Fig. 3). Dramatic further gainscan be obtained by shielding from or vetoing cosmogenicneutrons (lower two red curves), or more simply by us-ing a pulsed beam. The lower red curve corresponds to40-event sensitivity per 1022 electrons on target, whichmay be realistically achievable in under a beam-year atJLab. The middle and upper red curves correspondto background-systematics-limited configurations, with1000 and 2 · 104 signal-event sensitivity, respectively, per1022 electrons on target. Though not considered in de-tail in this paper, detectors sensitive to -electron elas-tic scattering, coherent -nuclear scattering, and pionproduction in inelastic -nucleon scattering could haveadditional sensitivity. With a pulsed beam, comparableparameter space could be equally well probed with 1 to3 orders of magnitude less intensity. A high-intensitypulsed beam such as the proposed ILC beam could reacheven greater sensitivity (orange curve). The parameterspaces of these plots are explained in the forthcomingsubsection.

The beam dump approach outlined here is quite com-plementary to B-factory + invisible searches [50], withbetter sensitivity in the MeVGeV range and less sen-sitivity for 1 10 GeV (see also [54]). Compared tosimilar search strategies using proton beam dumps, thesetup we consider has several virtues. Most significantly,beam-related neutrino backgrounds, which are the lim-iting factor for proton beam setups, are negligible forelectron beams. MeV-to-GeV are also produced withvery forward-peaked kinematics (enhanced at high beamenergy), permitting large angular acceptance even for asmall detector. Furthermore, the expected cosmogenic

We now have a “Dark Matter Beam”!

Dark Matter particles have enough recoil energy!

Detection with simple detector, e.g. scintillator cube

... or with sophisticated DM Detector ...


New Hall

Dedicated new hall for experiment

Space for high resolution experiment

Additional space for beam dump experiment: 1 mA beam on target for 50%/a


FLUKA Simulation

Neutrons can be shielded

Below pion threshold: negligible ν background

Clean conditions, detailed layout of hall needed for further design


Magix Sensitivity

Reasonable sensitivity for low mass region

Multidimensional plot: Assumptions for dark photon mass, mχ

Calculations: G. Krnjaic, Perimeter Institute


Tests of ab-initio Calulations in Few-Body Physics

Ab initio calculations e.g. with Effective Field Theory

Consistent chiral expansion of elementary NN-interaction

Consistent expansion of Few-Body-Systems

Very promising, but...

⇒ Talk by Jacek


Waiting for the Accelerator...


Initial State Radiaion

Final State Interaction contains known form factor (elastic Li ne)

Initial State Interaction: Cross section changes linear with form factor

Disentangle via comparison Simulation↔ Data


First Results

102

103

104

105

106

107

Cou

nts

/(0.

1m

C·5

MeV

)

-3%

-2%

-1%

0%

1%

2%

3%

100 150 200 250 300 350 400 450 500

Dat

a/Sim

ula

tion

−1

Electron energy E ′ [MeV]

SimulationData at 495MeVData at 330MeVData at 195MeV

H(e, e′)nπ+ andH(e, e′)pπ0 ContributionsBackground

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

Systematic uncertainty

Radiative corrections of O(α2)necessary

Better than 1% achievable

First measurement of GEp(Q

2)in range0.001GeV2 < Q2 < 0.004GeV2

Systematic errors:

Target wallsBackscattering on target framePion productionBackscattering entrance flangeRadiative corrections

⇒ Cluster Jet Target

M. Mihovilovic, A. B. Weber, et al., PLB 771 (2017) 194–198


Summary

MESA:

High beam current in Energy Recovery mode

Excellent beam quality

MAGIX:

High Resolution Spectrometers

High density or high polarization internal target

Multi-purpose setup for precision physics

Physics Program

Precision form factors: Magnetic Radius of the Proton

Nuclear Astrophysics: S-Factor measurements

Few-Body physics

Search for exotic particles

Contributions from other groups are very welcome!

EXPERIMENTSWITH INTERNAL T MAINZ ENERGY-RECOVERING...

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