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DarkLight: Searching for Dark
Forces (and more)
Charles Epstein
EINN 2015, Paphos, Cyprus
November 5, 2015
Outline
1 DarkLight: An Overview2 Relevance of Møller Scattering to
DarkLight3 Improved Møller Calculation4 Measurement of Møller Scattering
Outline
1 DarkLight: An Overview2 Relevance of Møller Scattering to
DarkLight3 Improved Møller Calculation4 Measurement of Møller Scattering
A Search for Dark Forces
• Given strong astrophysicalevidence for dark matter,it is natural to presumethat Dark Matter has itsown Dark Forces
• A “dark photon” mighthave mass <1 GeV and asmall coupling to theStandard Model
• DarkLight searching10-100 MeV range
arXiv:1504.00607, arXiv:1406.2980
2
Dark Interactions in ep Collisions at 100 MeV
DarkLight will search for a dark photon in electron-proton scattering
A0 production
Dark forces in ep ! ep e+e scattering
A0
p
e
e+
e
A0
p
e
e+
e
Select only events with an extra electron positron pair
Invariant mass gives the mass of the dark force boson –straightforward way to search for the A0
Reconstruct tracks of all four final state particles→ Kinematic redundancy
3
Dark Interactions in ep Collisions at 100 MeV
DarkLight will search for a dark photon in electron-proton scattering
e-
p
e-
e+
me+e-
e-
e-
e+
p
Beam
Reconstruct tracks of all four final state particles→ Kinematic redundancy
3
Requirement: Huge LuminositySmall resonance on an already rare process!
Irreducible background
p
e
e+
e
p
e
e+
e
p
e
e+
e
All of these QED processes are indistinguishablefrom A0 production and decay!
L = 6× 1035 cm−2 s−1(Phase 1)
→ 2× 1036 cm−2 s−1(Phase 2)
Goal: 1 ab−1 in one month4
The JLab Low Energy Recirculator Facility
• Next-generation EnergyRecovering Linac (ERL)
• 100 MeV, 10mA→ 1 MW of power
• Provides intensity necessaryto observe a dark photon
The JLab FEL
The Je↵erson Lab freeelectron laster (FEL) is theonly currently-operating FELusing a continuous wavesuperconducting energyrecovering linac
The FEL linac provides aunique high-intensityelectron source
5
DarkLight Experimental Layout
!
! 13!
will also be using explosion bonded vacuum flanges to transition from the aluminum scattering chamber to the stainless steel beam line. We plan on reusing the gas feed system from the OLYMPUS experiment with a few modifications to accommodate the higher gas flows. This will include changing some of the gauges and purchase and installation of a higher capacity mass flow controller. For the Moller dump we will use a large stainless steel tank filled with a graphite slug that has a small hole drilled in it for the beam. This will also require it’s own support structure and alignment system. The vacuum system and its components can be seen schematically in Fig. 5.
Fig. 5. Schematic layout of the phase-I DarkLight experiment with a human figure for scale. Solenoidal spectrometer magnet for the DarkLight experiment An existing solenoidal magnet, in the possession of the Stony Brook U, group, will be available for the summer 2015 experiment and will be used in the full experiment at a later date. This magnet and its power supply were constructed by Toikin from Japan (Tokyo Electro-Communications University) and was last used in experiment E906 at BNL in early 2000’s. Since then the magnet and its power supply have been stored on the AGS floor. The Stony Brook group, looking for a spectrometer magnet during the construction and tests of future PHENIX-upgrade and EIC detector components, located them and moved them to Stony Brook. The magnet is 1280 mm (long) x 1180 mm (wide) x 1180 mm (high). The radius of the inner cylindrical volume is 356 mm. This is a water-cooled normal conducting magnet. It has a maximum field of 0.5 Tesla. The field ahs been mapped and a TOSCA model has been constructed. The map and model are in good agreement.
• 100 MeV e− on gas 1019/cm2 H2 target in 0.5T solenoid
• Target chamber surrounded by lepton tracker
• Silicon recoil proton detector inside target chamber
6
DarkLight Experimental Layout
e-
e-
e+
p
• 100 MeV e− on gas 1019/cm2 H2 target in 0.5T solenoid
• Target chamber surrounded by lepton tracker
• Silicon recoil proton detector inside target chamber
6
The DarkLight Experiment
DarkLight will be comprised ofPhase 1 (preliminary) and Phase 2 (complete) experiments
Phase 1 Primary Objectives (2016-17)
1 Accelerator studies: dense internal gas target in ERL
2 Measure SM cross-sections: radiative Møller
3 Pilot A′ search
Goals for Phase 2
• Full A′ search
• Streaming readout
• Invisible A′ search
7
The DarkLight Experiment
DarkLight will be comprised ofPhase 1 (preliminary) and Phase 2 (complete) experiments
Phase 1 Primary Objectives (2016-17)
1 Accelerator studies: dense internal gas target in ERL
2 Measure SM cross-sections: radiative Møller
3 Pilot A′ search
Goals for Phase 2
• Full A′ search
• Streaming readout
• Invisible A′ search
7
Outline
1 DarkLight: An Overview2 Relevance of Møller Scattering to
DarkLight3 Improved Møller Calculation4 Measurement of Møller Scattering
DarkLight
Solenoid
Tracker
BeamMøller Cone
H2 Target
e-
e-
e+
p
• Sweep Møller e− with solenoid: still significant backscattering
• Møller photons produce high rate of secondaries
8
Various experiments rely on e±e− scattering
• OLYMPUS: 2γ exchange in e+p/e−p cross-sections @ 2 GeV• Separate e+/e− beams: normalize to Møller/Bhabha
• PRad: proton form factor at low Q2: charge radius• Normalize ep to Møller at forward angles
• DarkLight: high-rate noise background @ 100 MeV
• Future low-energy machines: MESA, Cornell ERLs
→ Experiments are depending on knowledge of low-energy Møllerscattering, but few precise calculations or data exist in this regime
9
Various experiments rely on e±e− scattering
• OLYMPUS: 2γ exchange in e+p/e−p cross-sections @ 2 GeV• Separate e+/e− beams: normalize to Møller/Bhabha
• PRad: proton form factor at low Q2: charge radius• Normalize ep to Møller at forward angles
• DarkLight: high-rate noise background @ 100 MeV
• Future low-energy machines: MESA, Cornell ERLs
→ Experiments are depending on knowledge of low-energy Møllerscattering, but few precise calculations or data exist in this regime
9
Various experiments rely on e±e− scattering
• OLYMPUS: 2γ exchange in e+p/e−p cross-sections @ 2 GeV• Separate e+/e− beams: normalize to Møller/Bhabha
• PRad: proton form factor at low Q2: charge radius• Normalize ep to Møller at forward angles
• DarkLight: high-rate noise background @ 100 MeV
• Future low-energy machines: MESA, Cornell ERLs
→ Experiments are depending on knowledge of low-energy Møllerscattering, but few precise calculations or data exist in this regime
9
Various experiments rely on e±e− scattering
• OLYMPUS: 2γ exchange in e+p/e−p cross-sections @ 2 GeV• Separate e+/e− beams: normalize to Møller/Bhabha
• PRad: proton form factor at low Q2: charge radius• Normalize ep to Møller at forward angles
• DarkLight: high-rate noise background @ 100 MeV
• Future low-energy machines: MESA, Cornell ERLs
→ Experiments are depending on knowledge of low-energy Møllerscattering, but few precise calculations or data exist in this regime
9
Various experiments rely on e±e− scattering
• OLYMPUS: 2γ exchange in e+p/e−p cross-sections @ 2 GeV• Separate e+/e− beams: normalize to Møller/Bhabha
• PRad: proton form factor at low Q2: charge radius• Normalize ep to Møller at forward angles
• DarkLight: high-rate noise background @ 100 MeV
• Future low-energy machines: MESA, Cornell ERLs
→ Experiments are depending on knowledge of low-energy Møllerscattering, but few precise calculations or data exist in this regime
9
Outline
1 DarkLight: An Overview2 Relevance of Møller Scattering to
DarkLight3 Improved Møller Calculation4 Measurement of Møller Scattering
Radiative corrections, minimizing approximations
+
Desired andProduced:
• Monte Carlo Event Generator
• Full NLO QED radiative corrections• Møller, Bhabha, Pair Annihilation
• Does not ignore electron mass
10
Approach to the radiative corrections
Separate “elastic” from radiative events by an Eγ cutoff: ∆E
Treat types of events separately
• Eγ > ∆E : Hard-photon bremsstrahlung• Full e e → e e γ cross-section
• Eγ < ∆E : “Close enough” to elastic kinematics• Apply soft-photon radiative corrections
11
Hard-photon Bremsstrahlung: Eγ > ∆E
Calculated exactly using FeynArts/FormCalc
e−1 + e−2 → e−3 + e−4 + γ
12
Soft Radiative Corrections: Eγ < ∆E
• Integrate soft photon over all Ω and up to Eγ = ∆E
• Soft-photon + loop-level IR divergences cancel
dσ
dΩ= (1 + δ)
dσ
dΩ
∣∣∣∣Born
δ = δ(∆E , s, t, u)
Expect that as ∆E gets smaller, δ gets smaller
• Many formulations use ultra-relativistic approximations: me → 0
• Only recently worked out beyond URA• Møller: I. Akishevich, et al (2015)• Møller/Bhabha: N. Kaiser (2010)
13
Soft Radiative Corrections: Eγ < ∆E
• Integrate soft photon over all Ω and up to Eγ = ∆E
• Soft-photon + loop-level IR divergences cancel
dσ
dΩ= (1 + δ)
dσ
dΩ
∣∣∣∣Born
δ = δ(∆E , s, t, u)
Expect that as ∆E gets smaller, δ gets smaller
• Many formulations use ultra-relativistic approximations: me → 0
• Only recently worked out beyond URA• Møller: I. Akishevich, et al (2015)• Møller/Bhabha: N. Kaiser (2010)
13
Ultra-relativistic approximations are unsuitable
• Higher-order me terms often neglected• For high beam energies, typically a good approximation
• Effects visible at low energy:at angles where m2
e/t or m2e/u no longer 1
δ grows as ∆E gets smaller
• Unphysical → implies higher rate at smaller energy windows
14
Improper behavior of δ
0
2
4
Delta E @MeVD
020
4060
80 Theta CM @ºD
-0.05
0.00
0.05
δ(∆E , θ)
(Tsai, 1960)
15
What δ should look like
0
2
4
Delta E @MeVD
020
4060
80 Theta CM @ºD
-0.05
0.00
δ(∆E , θ)
(Kaiser, 2010)
16
Bremsstrahlung Comparison
Soft-photon corrections and bremsstrahlungshould agree near Eγ ∼ ∆E
Rearrange:
Soft-Photond3σ
dΩ3 dEγ
=dσ
dΩ3
∣∣∣∣Born
× d
d∆E
δ(Ω3,∆E )
Bremsstrahlung d3σ
dΩ3 dEγ
=
∫4π
d5σ
dΩ3 dEγ dΩγ
dΩγ
17
Electron Cross-Section at fixed angles (2 GeV)
106
107
108
109
1010
1011
5 10 15 20
Møl
ler
Cro
ssSe
ctio
n[p
b/
MeV
/ra
dian
]
CM Photon Energy [MeV]∆E
20CM
40CM
90CM
18
Ratio of hard/soft cross-sections
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
2 4 6 8 10 12 14
Rat
io:
Har
dB
rem
sstr
ahlu
ng/
Soft
Pho
ton
CM Photon Energy [MeV]∆E
20CM
40CM
90CM
19
Electron Cross-Section at high photon energies
100
102
104
106
108
1010
1012
22 22.1 22.2 22.3 22.4 22.5 22.6
MøllerCross
Section[pb/MeV
/radian]
CM Photon Energy [MeV]
Eγ0
20CM
40CM
90CM
20
Outline
1 DarkLight: An Overview2 Relevance of Møller Scattering to
DarkLight3 Improved Møller Calculation4 Measurement of Møller Scattering
Møller Scattering at 100 MeV
Why measure unpolarized low-energy Møller scattering?
Quantities withfew precision
data
• Distribution of E at fixed θ: radiative tail• Verify bremsstrahlung calculation
• Electron-electron cross-section vs θ
• Goal: understand calculations beyond URA
Experimental Goals
• Scattering angles 25-45: cannot neglect me
• Measure electrons with momentum 1-5 MeV/c
• Momentum resolution δp/p ∼ 1%
21
Møller Scattering at 100 MeV
Why measure unpolarized low-energy Møller scattering?
Quantities withfew precision
data
• Distribution of E at fixed θ: radiative tail• Verify bremsstrahlung calculation
• Electron-electron cross-section vs θ
• Goal: understand calculations beyond URA
Experimental Goals
• Scattering angles 25-45: cannot neglect me
• Measure electrons with momentum 1-5 MeV/c
• Momentum resolution δp/p ∼ 1%
21
Møller Scattering at 100 MeV
Why measure unpolarized low-energy Møller scattering?
Quantities withfew precision
data
• Distribution of E at fixed θ: radiative tail• Verify bremsstrahlung calculation
• Electron-electron cross-section vs θ
• Goal: understand calculations beyond URA
Experimental Goals
• Scattering angles 25-45: cannot neglect me
• Measure electrons with momentum 1-5 MeV/c
• Momentum resolution δp/p ∼ 1%
21
Electron energy spectrum
e−e− → e−e−γ single-e− cross-section: 25± 0.5
101
102
103
104
105
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5Cross-Section[H
z/0.02465MeV
/c/1/1 ]
Electron Momentum [MeV/c]
22
Electron energy spectrum
e−e− → e−e−γ single-e− cross-section: 45± 0.5
101
102
103
104
105
0 0.2 0.4 0.6 0.8 1 1.2 1.4Cross-Section[H
z/0.00712MeV
/c/1/1 ]
Electron Momentum [MeV/c]
22
Møller Experiment Layout
Two Spectrometers
Dipole
Detector
Top View Beamline View
r = 3
0 cm
fixed Dipole
• Pointlike target: 5µm diamond-like carbon foil
• 90 bending dipole magnet, scintillating fiber detector
• Map out electron energy spectrum at various θ
• Relative luminosity monitor: elastic e-C scattering23
Møller Experiment Layout
= 25°
= 45°
Bellows
Target
Entirely vacuum until detector: eliminate multiple scattering
24
Engineering Drawing 25 (MIT-Bates)
25
Engineering Drawing 30 (MIT-Bates)
25
Engineering Drawing 35 (MIT-Bates)
25
Engineering Drawing 40 (MIT-Bates)
25
Engineering Drawing 45 (MIT-Bates)
25
Main Spectrometer Magnet
Main spectrometer is a C-magnet→ allows high-energy elastics to escape
26
Detector: Array of scintillating fibers
0.5mm
• Four layers of fibers crossed at 90
• Detector Size: 15 × 5cm
• Light collection: SiPM array
27
SensL Matrix System SiPM Arrays
Array + Front End Board Readout Board
• 12×12 array: 50.2 mmsquare (4.2 mm pitch)
• Up to 16 sensors
• Common clock, buffer,coincidence, USB out
28
Fiber Array Holder - Open
29
Fiber Array Holder - Closed
30
Fiber frame with SiPM connectors
31
SiPM to fiber connector
32
SiPM Array with Connector: 3D Printing
33
Summary
DarkLight Phase 1 aims to run in 2016
• Proof of A′-search principle; accelerator studies
• Opportunity to measure additional physics
Will measure radiative Møller scattering in Phase 1
• Have written a new generator based on a complete calculation
• Aim to take data to verify the calculation
• Building an additional detector to do so
34
Summary
DarkLight Phase 1 aims to run in 2016
• Proof of A′-search principle; accelerator studies
• Opportunity to measure additional physics
Will measure radiative Møller scattering in Phase 1
• Have written a new generator based on a complete calculation
• Aim to take data to verify the calculation
• Building an additional detector to do so
34
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