The Light Dark Matter eXperiment

NorCal HEP-EXchange 2018

December 1, 2018

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Omar Moreno on behalf of the LDMX Collaboration

The Search for Dark Matter

There is strong evidence for the existence of Dark Matter

arXiv:0404175

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But it’s particle nature continues to elude us!

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Broad and impressive program has been built to understand ~GeV - TeV WIMP Dark Matter, but searches for them in the most favorable areas have yielded nothing → will be ruled out or found in the coming years by next gen experiments (e.g. SuperCDMS, LZ or LHC)

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Non-thermal

Non-thermal

10^-22 eV

~100M☉

m_pl ~ 10¹⁹ GeV

MeV

GeV

m_z

< 100 MeV

N_eff*/BBN

> 100 TeV

Too much

Light Dark Matter

WIMP’s

Galactic Rotation Curves

Structure of Cosmic Microwave Background

Gravitational Lensing

O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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The Search for Dark Matter

There is strong evidence for the existence of Dark Matter

arXiv:0404175

​

​

​

​

​

​

But it’s particle nature continues to elude us!

​

Broad and impressive program has been built to understand ~GeV - TeV WIMP Dark Matter, but searches for them in the most favorable areas have yielded nothing → will be ruled out or found in the coming years by next gen experiments (e.g. SuperCDMS, LZ or LHC)

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What about light (< 1 GeV) thermal DM?

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Non-thermal

Non-thermal

10^-22 eV

~100M☉

m_pl ~ 10¹⁹ GeV

MeV

GeV

m_z

< 100 MeV

N_eff*/BBN

> 100 TeV

Too much

Light Dark Matter

WIMP’s

Galactic Rotation Curves

Structure of Cosmic Microwave Background

Gravitational Lensing

O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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Searching For Light Dark Matter

Light Dark Matter in the broad vicinity of the weak scale is a natural and simple generalization of WIMPs. Light thermal dark matter requires a new force to achieve the correct thermal relic abundance (WIMP’s limited by Lee-Weinberg Bound to 2 GeV). Phys. Rev. Lett. 39, 165

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Given the complex structure of the Standard Model, a "Dark Sector" where dark matter interacts via a light mediator is an obvious scenario to test. It has been the focus of a broad array of searches and experiments for many years now.

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Let’s focus on the simplest scenario where DM is charged under a new U(1)’ gauge field mediated by a U(1)’ gauge boson (dark/heavy photon, A′ )

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Electron accelerators can play a major role in testing models of light dark matter!

Accelerator Based

Indirect Detection

Direct Detection

kinetic mixing between SM photon and the dark photon → induces weak coupling to electric charge

Equivalent

εe

A^μ → A^μ + εA’^μ

If m_A′ > 2m_χ

O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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The Light Dark Matter eXperiment

The Light Dark Matter eXperiment is a e^- fixed target missing momentum search for light dark matter

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A zero background experiment can test all thermal targets over most of the MeV-GeV range (with 10¹⁶ e^- on target)!

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Electromagnetic

Calorimeter

Tagging Tracker

Recoil Tracker

~10% W Target

Full B ⊗

Fringe B ⊗

χ

σ∝є² → N_signal ≃N_e- ✕ y|_{1 MeV}

χ

p_i

p_f

p_miss = p_i - p_f

O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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Dark Bremsstrahlung Kinematics

Since dark photons couple to electric charge, they will be produced through a process analogous to bremsstrahlung off heavy targets

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but with different rates and kinematics

• Production is sharply peaked at x ≈ 1 → A’ takes most of the beam energy
• Recoil is produced very soft and at wide angles → Large missing momentum

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Recoil kinematics allow efficient signal definition providing a factor of 30 background rejection!

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Target

O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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Dark Bremsstrahlung Kinematics

Since dark photons couple to electric charge, they will be produced through a process analogous to bremsstrahlung off heavy targets

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but with different rates and kinematics

• Production is sharply peaked at x ≈ 1 → A’ takes most of the beam energy
• Recoil is produced very soft and at wide angles → Large missing momentum

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p_t is also an important experimental handle as it depends on the mass of the A’

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O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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Dark Bremsstrahlung Kinematics

Since dark photons couple to electric charge, they will be produced through a process analogous to bremsstrahlung off heavy targets

​

​

​

​

​

​

but with different rates and kinematics

• Production is sharply peaked at x ≈ 1 → A’ takes most of the beam energy
• Recoil is produced very soft and at wide angles → Large missing momentum

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p_t is also an important experimental handle as it depends on the mass of the A’

Goal: achieve zero background without using p_T as a signal discriminator

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O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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Missing Momentum Backgrounds

O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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LDMX Design Considerations

Beam that allows individual tagging and reconstruction of 10¹⁶ incident e^-

• A low-current, multi-GeV, e^- beam with high repetition rate (10¹⁶/year ≈ 1 e^- /3 ns).
• The possibilities are Sector 30 Transfer Line @ SLAC (4/8 GeV) and CERN and CEBAF @ JLab ( < 11 GeV)
• large beamspot (~10 cm²) to spread out otherwise extreme rates and radiation doses

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Tracking and calorimetry capable of high rates and radiation tolerance

• High resolution, low mass tagging/recoil trackers
• High energy resolution EM calorimeters

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Requirements for 10¹⁶ experiment close to limits of available technologies ➡ Two-stage approach to LDMX: 4×10¹⁴ “Phase I” (1 e^-/25 ns @ 4 GeV) followed by 10¹⁶ “Phase II” (O(1 e^-/ns) @ >8 GeV)

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Electromagnetic

Calorimeter

Tagging Tracker

Recoil Tracker

~10% W Target

Full B ⊗

Fringe B ⊗

O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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LDMX Phase I Detector Concept

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18D36 Dipole

1.5 T

Hadronic Calorimeter

Electromagnetic Calorimeter

Tagger Tracker

Recoil Tracker

.1X0 W

Target

e^-

Scintillator pads surround the target

O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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LDMX Phase I Detector Concept

Silicon strip trackers will be similar to the HPS Silicon Vertex Tracker

• Fast (2 ns time resolution)
• Meets radiation tolerance requirements

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Tagging tracker → 7 measurement stations composed of two sensors at small angle stereo

• Used to select against off-energy e^-

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Recoil tracker → 4 stations composed of sensor pairs at small angle stereo + “axial only” layers

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18D36 Dipole

1.5 T

Hadronic Calorimeter

Electromagnetic Calorimeter

Tagger Tracker

e^-

Scintillator pads surround the target

Recoil Tracker

.1X0 W

Target

O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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LDMX Phase I Detector Concept

Silicon strip trackers will be similar to the HPS Silicon Vertex Tracker

• Fast (2 ns time resolution)
• Meets radiation tolerance requirements

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Tagging tracker → 7 measurement stations composed of two sensors at small angle stereo

• Used to select against off-energy e^-

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Recoil tracker → 4 stations composed of sensor pairs at small angle stereo + ‘axial only’ layers

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Single 18D36 dipole magnet → Two field regions

• Tagging tracker in central 1.5T field
• Recoil Tracker in fringe field

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18D36 Dipole

1.5 T

Hadronic Calorimeter

Electromagnetic Calorimeter

Tagger Tracker

e^-

Scintillator pads surround the target

Recoil Tracker

.1X0 W

Target

O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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LDMX Phase I Detector Concept

Si-W calorimeter developed for CMS upgrade

• Fast, dense, granular for high occupancies → Allows for exploitation of both longitudinal and transverse shower shapes
• Deep (40 X₀) for extraordinary EM containment

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For LDMX

• Easily withstands the effective fluence of 10¹³ n/cm² caused by 10¹⁴ e^-’s on target
• Can provide fast trigger for trackers (~3 μs)
• Is capable of MIP tracking which will help with background rejection.

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18D36 Dipole

1.5 T

Hadronic Calorimeter

Tagger Tracker

Target

e^-

Scintillator pads surround the target

Electromagnetic Calorimeter

O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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LDMX Phase I Detector Concept

Makes use of CMS upgrade hardware

• Steel absorber/plastic scintillator
• SiPM readout via WLS fibers

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Surround ECal as much as possible

• Many PN events have a high multiplicity of soft neutral hadrons
• Also catches wide-angle brems (≳ 25 deg.)

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18D36 Dipole

1.5 T

Hadronic Calorimeter

Tagger Tracker

Target

e^-

Scintillator pads surround the target

O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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Trigger

Trigger makes use of Ecal and trigger scintillator pad downstream of the target to reject beam backgrounds

• Apply a cut on the sum of the first 20 Ecal layers
• Scintillator pad used to count the number of incident electrons → Allows setting of trigger threshold

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3 x 10^-4 background rejection!

Efficiency

Trigger Threshold

1.2 GeV

Signal efficiency

Background efficiency

O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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Design Studies Status

Developed Geant4 based simulation framework (ldmx-sw) to study the performance and background veto power of a preliminary LDMX design

• Initial design studies used photo/electro-nuclear and muon conversion samples equivalent to 10¹⁴ e^- on target
• Found that Geant4 was over-producing events with an enormous amount of momentum transfer to the recoiling nucleus → mitigated by patching Geant4 Bertini cascade model and validating against available data
• Using machine learning techniques that makes use of information from each subsystems, we were able to reject all backgrounds at the 10¹⁴ e^- on target level

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Detailed summary of these design studies can be found in https://arxiv.org/abs/1808.05219

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Signal Event

m_A’ = 100 MeV, p_e-= 1207 MeV

ECal Photonuclear

ECal Muon conversion

Hard Brem

O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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LDMX Reach

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Sensitivity extends to lower masses

LDMX Phase I @ 4 GeV 0.1-0.3 X0 target

LDMX Phase II @ 8 GeV

O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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Summary and Outlook

Accelerator-based DM searches have unique sensitivity in the MeV-GeV range.

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LDMX can robustly reach all thermal targets over most of the MeV-GeV range and probe other physics models.

​Broad physics potential: LDMX can probe sub-GeV dark sectors that couple weakly to electrons, and the physics of photo- and electro-nuclear collisions.

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• Sub-GeV dark matter production
• Sub-GeV invisibly decaying mediators
• Displaced vertex signatures that arise from visibly decaying mediators
• Displaced electron-positron showers that arise from 'DM co-annihilation' models
• Sub-GeV axion-like particles
• Milli-charge particles
• Dark Vectors decaying to neutrinos
• Photonuclear and electronuclear measurements of interest for neutrino experiments --> drive to understand nuclear final state interactions

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O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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Backup

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LDMX Collaboration

Robert Johnson

Ruth Pottgen, Torsten Akesson

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Takashi Maruyama, Omar Moreno, Tim Nelson, Philip Schuster, Natalia Toro

Owen Colegrove, Joe Incandela, Alex Patterson

Josh Hiltbrand, Jeremy Mans

Gordan Krnjaic, Nhan Tran, Andrew Whitbeck

Bertrand Echenard, David Hitlin

O. Moreno (SLAC National Accelerator Laboratory) 2018 NorCal HEP-EXchange December 1, 2018

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