DarkLight: Searching for Dark Forces (and more) · 2016-12-28 · EINN 2015, Paphos, Cyprus November 5, 2015. Outline 1 DarkLight: An Overview 2 Relevance of M˝ller Scattering to

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



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

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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



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

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9







9







9







9

Outline



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

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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

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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



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




data




Experimental Goals




21




data




Experimental Goals




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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


25


25


25


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

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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

Documents

DarkLight: Searching for Dark Forces (and more) · 2016-12-28 · EINN 2015, Paphos, Cyprus November 5, 2015. Outline 1 DarkLight: An Overview 2 Relevance of M˝ller Scattering to