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Doojin Kim
KEK Theory Meeting on Particle Physics Phenomenology
December 6th, 2018
PRL119 (2017) 161801, PLB780 (2018) 543, PRD98 (2018) 075027, JHEP1808 (2018) 155, more in progress,
in collaboration with H. Alhazmi, W. Bonivento, A. Chatterjee, A. De Roeck, K. Dienes, G. Giudice,
K. Kong, P. Machado, Z. Moghaddam, J.-C. Park, S. Shin, B. Thomas, L. Whitehead, J. Yu
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop
Contents
I. Physics Motivation
II. Signatures and Search
III. Phenomenology: Experimental Sensitivities
IV. Conclusions
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-1-
Current Status of Conventional Dark Matter Searches
No observation of dark matter signatures via non-gravitational interactions (many
searches/interpretations designed/performed under WIMP/minimal dark-sector
scenarios) merely excluding more parameter space in dark matter models
[Cushman, Calbiati and McKinsey (2013); Baudis (2014)] [CMS mono-photon search (2014)]
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-1-
Current Status of Conventional Dark Matter Searches
No observation of dark matter signatures via non-gravitational interactions (many
searches/interpretations designed/performed under WIMP/minimal dark-sector
scenarios) merely excluding more parameter space in dark matter models
[Cushman, Calbiati and McKinsey (2013); Baudis (2014)] [CMS mono-photon search (2014)]
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-2-
Conventional Approach
Traditional approaches for DM searches:
Weak-scale mass
Weakly-coupled
Minimal dark sector
Elastic scattering
Non-relativistic
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop
Modified approaches for DM searches:
Other mass scale: e.g., PeV, sub-GeV,
MeV, keV, meV, …
Weaker coupling to the SM: e.g.,
vector portal (dark photon), scalar
portal, axion portal, …
“Flavorful” dark sector: e.g., more
dark matter species, unstable heavier
dark sector states, …
Inelastic scattering (i.e., up-scatter to
an “excited” state)
Relativistic
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Conventional vs. Nonconventional Approach
Traditional approaches for DM searches:
Weak-scale mass
Weakly-coupled
Minimal dark sector
Elastic scattering
Non-relativistic
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop
Modified approaches for DM searches:
Other mass scale: e.g., PeV, sub-GeV,
MeV, keV, meV, …
Weaker coupling to the SM: e.g.,
vector portal (dark photon), scalar
portal, axion portal, …
“Flavorful” dark sector: e.g., more
dark matter species, unstable heavier
dark sector states, …
Inelastic scattering (i.e., up-scatter to
an “excited” state)
Relativistic
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Conventional vs. Nonconventional Approach
Traditional approaches for DM searches:
Weak-scale mass
Weakly-coupled
Minimal dark sector
Elastic scattering
Non-relativistic
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-3-
Two-component Boosted DM Scenario
𝜒0
𝜒0
𝜒1
𝜒1
SM
SM
A possible relativistic source: BDM scenario (cosmic frontier), stability of the two DM species ensured by separate symmetries, e.g., 𝑍2 ⊗ 𝑍2
′ , 𝑈 1 ⊗ 𝑈 1 ′, etc.
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-3-
Two-component Boosted DM Scenario
𝜒0
𝜒0
𝜒1
𝜒1
SM
SM
A possible relativistic source: BDM scenario (cosmic frontier), stability of the two DM species ensured by separate symmetries, e.g., 𝑍2 ⊗ 𝑍2
′ , 𝑈 1 ⊗ 𝑈 1 ′, etc.
𝑌0 𝑌1
Freeze-out first
Dominant relic
“Assisted” freeze-out mechanism[Belanger, Park (2011)]
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-3-
Two-component Boosted DM Scenario
𝜒0
𝜒0
𝜒1
𝜒1
SM
SM
A possible relativistic source: BDM scenario (cosmic frontier), stability of the two DM species ensured by separate symmetries, e.g., 𝑍2 ⊗ 𝑍2
′ , 𝑈 1 ⊗ 𝑈 1 ′, etc.
𝑌0 𝑌1
Freeze-out first
Dominant relic
“Assisted” freeze-out mechanism[Belanger, Park (2011)]
Freeze-out later
𝑌1Negligible, non-relativistic relic
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-4-
“Relativistic” Dark Matter Search
Heavier relic 𝜒0: hard to detect it due to tiny/negligible coupling to SM
Lighter relic 𝜒1: hard to detect it due to small amount
𝜒0
𝜒0
𝜒1
𝜒1
SM
SM
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-4-
“Relativistic” Dark Matter Search
𝜒0
𝜒0
𝜒1
𝜒1
𝜒1
(Galactic Center at CURRENT universe) (Laboratory)
becomes boosted, hence relativistic!(𝛾1 = 𝑚0/𝑚1)
[Agashe, Cui, Necib, Thaler (2014)]
Heavier relic 𝜒0: hard to detect it due to tiny/negligible coupling to SM
Lighter relic 𝜒1: hard to detect it due to small amount
𝜒0
𝜒0
𝜒1
𝜒1
SM
SM
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-5-
Boosted DM Detection
Flux of boosted 𝜒1 near the earth
Setting 𝜎𝑣 𝜒0𝜒0→𝜒1𝜒1 to be ~10−26 cm3s−1 and assuming Navarro-Frenk-White DM halo profile, a
standard profile, one finds
ℱ𝜒1 ∝ (interaction strength) × (𝜒0 number)2
~0.8 × 10−7cm−2s−1𝜎𝑣 𝜒0𝜒0→𝜒1𝜒110−26cm3s−1
20 GeV
𝑚0
2
from DM number density
ℱ𝜒1~10−7cm−2s−1 for 𝒪(20 GeV) mass of 𝜒0
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-5-
Boosted DM Detection
Flux of boosted 𝜒1 near the earth
Setting 𝜎𝑣 𝜒0𝜒0→𝜒1𝜒1 to be ~10−26 cm3s−1 and assuming Navarro-Frenk-White DM halo profile, a
standard profile, one finds
ℱ𝜒1 ∝ (interaction strength) × (𝜒0 number)2
~0.8 × 10−7cm−2s−1𝜎𝑣 𝜒0𝜒0→𝜒1𝜒110−26cm3s−1
20 GeV
𝑚0
2
from DM number density
ℱ𝜒1~10−7cm−2s−1
Too small for small-volume detectors (e.g., conventional
WIMP detectors) large-volume (neutrino) detectors
motivated
Super-/Hyper-Kamiokande (SK/HK): active
volume of 22.4 kt/560 kt
SK
SKY
for 𝒪(20 GeV) mass of 𝜒0
HK
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-6-
SK Official Results for Elastic Boosted Dark Matter Search
[SK Collaboration, arXiv:1711.05278]m0 (GeV)
m1=200 MeV, mX=20 MeV, g’=0.5 m0 (GeV)
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop
Contents
I. Physics Motivation
II. Signatures and Search
III. Phenomenology: Experimental Sensitivities
IV. Conclusions
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop
Benchmark Model: Building Blocks
Vector portal (e.g., dark photon scenario)
Fermionic DM
𝜒2: a heavier (unstable) dark-sector state
Flavor-conserving interaction elastic scattering
Flavor-changing interaction inelastic scattering
ℒint ∋ −𝜖
2𝐹𝜇𝜈𝑋
𝜇𝜈 + 𝑔11 𝜒1𝛾𝜇𝜒1𝑋𝜇 + 𝑔12 𝜒2𝛾
𝜇𝜒1𝑋𝜇 + h. c.+(others)
𝑋
𝜒1𝜒1𝑔11
𝑋
𝑒−
𝑒+
𝜖
𝑋
𝜒2𝜒1𝑔12
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop
Benchmark Model: Building Blocks
Vector portal (e.g., dark photon scenario)
Fermionic DM
𝜒2: a heavier (unstable) dark-sector state
Flavor-conserving interaction elastic scattering
Flavor-changing interaction inelastic scattering
ℒint ∋ −𝜖
2𝐹𝜇𝜈𝑋
𝜇𝜈 + 𝑔11 𝜒1𝛾𝜇𝜒1𝑋𝜇 + 𝑔12 𝜒2𝛾
𝜇𝜒1𝑋𝜇 + h. c.+(others)
𝑋
𝜒1𝜒1𝑔11
𝑋
𝑒−
𝑒+
𝜖
𝑋
𝜒2𝜒1𝑔12
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Today’s focus
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop
Inelastic Boosted Dark Matter (iBDM) Signatures
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop
Inelastic Boosted Dark Matter (iBDM) Signatures
𝑒−/𝑝
𝜒2
𝑋∗ 𝜖
𝑔12
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop
Inelastic Boosted Dark Matter (iBDM) Signatures
𝑒−/𝑝
𝑒−
𝑒+
𝜒2
𝑋∗𝑋(∗)
𝜖𝜖
𝑔12 𝑔12
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop
Inelastic Boosted Dark Matter (iBDM) Signatures
𝑒−/𝑝
𝑒−
𝑒+
𝜒2
𝑋∗𝑋(∗)
𝜖𝜖
𝑔12 𝑔12
𝑝- or 𝑒-scattering (primary) Decay (secondary)
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(cf. DM vs. iDM in Paddy’s talkBDM vs. iBDM)
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop
Inelastic Boosted Dark Matter (iBDM) Signatures
𝑒−/𝑝
𝑒−
𝑒+
𝜒2
𝑋∗𝑋(∗)
𝜖𝜖
𝑔12 𝑔12
𝑝- or 𝑒-scattering (primary) Decay (secondary)
Everything happens inside a detector fiducial volume.
The secondary interaction point may be displaced due to either long-lived
𝜒2 – when it decays via an off-shell 𝑋 (i.e., 𝑚2 < 𝑚1 + 𝑚𝑋) – or
on-shell 𝑋 – when kinetic mixing parameter is sufficiently small.
If 𝛿𝑚 = 𝑚2 − 𝑚1 is large enough, other final states (e.g., 𝜇+𝜇−, 𝜋+𝜋−, etc) are available.
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(cf. DM vs. iDM in Paddy’s talkBDM vs. iBDM)
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop
𝒑-Scattering vs. DIS
𝜒2
𝑋∗
𝜖
𝑝
If a momentum transfer is too large, a proton may break apart.
What is large? A Super-K simulation study [Fechner et al, PRD
(2009)] showed about 50 % events accompany (at least) a pion
or a secondary particle for 𝑝𝑝 ≈ 2 GeV.
We categorize any event with 𝑝𝑝 < 2 GeV as the 𝑝-scattering
(i.e., simplified step-function-like transition). 𝑝
-on
ly e
ven
ts f
ract
ion
𝑝𝑝
1
0.5
2 GeV
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop
𝒑-Scattering vs. DIS: “Semi”-Numerical Study
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We study 𝜎𝜒1𝑝cut/𝜎DIS where 1.07 GeV < 𝑝𝑝 < 2 GeV (Cherenkov detector) is applied to 𝜎𝜒1𝑝 while no cuts
are imposed to 𝜎DIS.
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop
𝒑-Scattering vs. DIS: “Semi”-Numerical Study
-10-
We study 𝜎𝜒1𝑝cut/𝜎DIS where 1.07 GeV < 𝑝𝑝 < 2 GeV (Cherenkov detector) is applied to 𝜎𝜒1𝑝 while no cuts
are imposed to 𝜎DIS.
𝑚𝑋
[GeV
]
𝑚1 [GeV]
𝜎𝜒1𝑝cut/𝜎DIS, 𝐸1 = 50 GeV
/𝜎DIS, 𝐸1 = 50 GeV
DIS more important? Maybe, not (too much penalty on the p-scattering, not a fair game!
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop
𝒑-Scattering vs. DIS: “Semi”-Numerical Study
-10-
We study 𝜎𝜒1𝑝cut/𝜎DIS where 1.07 GeV < 𝑝𝑝 < 2 GeV (Cherenkov detector) is applied to 𝜎𝜒1𝑝 while no cuts
are imposed to 𝜎DIS.
𝑚𝑋
[GeV
]
𝑚𝑋
[GeV
]
𝑚1 [GeV] 𝑚1 [GeV]
𝜎𝜒1𝑝cut/𝜎DIS, 𝐸1 = 50 GeV 𝑁DIS/year/kt, 𝐸1 = 50 GeV,𝑚2 = 1.5𝑚1
/𝜎DIS, 𝐸1 = 50 GeV
𝑔12 = 1, 𝜖 = 10−4
DIS more important? Maybe, not (too much penalty on the p-scattering, not a fair game!)
The expected number of DIS events is indeed small (even before applying cuts).
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-11-
Background Consideration
Atm.-𝜈 may induce multi-track events (which could be backgrounds)
The dominant source
𝝂𝒆-induced C.C. events
Other subdominant sources
N.C. events: smaller cross section
𝜈𝜏-induced: too small flux, hence negligible
𝜈𝜇-induced C.C.: leaving an energetic (primary) muon (which can be tagged easily)
𝜈𝑒±
𝑝/𝑛
𝑒±
𝑊𝜋±
𝜋±𝑒±
𝜈
𝑒±
𝜈𝜈𝜈
e.g. 𝜋± → 𝜇±𝜈 → 𝑒±𝜈𝜈𝜈, 𝜋± → 𝑒±𝜈
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-12-
Expected Number of ν-induced Background Events
𝜈𝑒-flux [SK Collaboration, 1502.03916] 𝜈𝑒-cross section [Formaggio, Zeller, 1305.7513]
RES RES
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-12-
Expected Number of ν-induced Background Events
𝜈𝑒-flux [SK Collaboration, 1502.03916] 𝜈𝑒-cross section [Formaggio, Zeller, 1305.7513]
Most DIS events result in messy final states, not mimicking signal events, while a majority of
resonance events may create a few mesons in the final state [Formaggio, Zeller, 1305.7513].
12 events/kt/yr are potentially relevant, i.e., 4600 events/yr for 380 kt (a full fiducial
mass of HK)
RES RES
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-13-
SK Study on Multi-track Events
[SK Collaboration (2012)]
Energy of hardest e-like track
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-13-
SK Study on Multi-track Events
[SK Collaboration (2012)]
Energy of hardest e-like track
Assuming that all 4600 events belong to the category of 1.33-2.5 GeV, ~1000 events/yr and ~100 events/yr involve
two and three tracks.
Decent level of particle identification at HK Cherenkov detectors can suppress such events further (e.g.,
identifying decaying particles such as pions).
Decent vertex resolution (28 cm for 𝐸 = 0.5 GeV) allows for “displaced” signal events: typical displaced vertex
> 50 cm in much of well-motivated parameter space.
Zer0 background search is achievable! (dedicated study in progress)
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop
Contents
I. Physics Motivation
II. Signatures and Search
III. Phenomenology: Experimental Sensitivities
IV. Conclusions
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-14-
Reminder for the Signal of Interest
Remember that dark photon is a “player” in the benchmark model, allowing us to study
phenomenology of dark photon!
𝑒−
𝑒−
𝑒+
𝜒2
𝑋𝑋
Scattering (primary) Decay (secondary)
𝜖𝜖
𝑔12 𝑔12
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-15-
Event Selection
Selection criteria based on detector performance of Super-/Hyper-K
𝑝th,𝑒 > 0.1 GeV
𝑝th,𝑝 > 1.07 GeV and 𝑝𝑝 < 2 GeV
Δ𝜃𝑝/𝑒−𝑖 = 3° for 𝑝𝑝/𝑒 < 1.33 GeV, Δ𝜃𝑝/𝑒−𝑖 = 1.2
° for 𝑝𝑝/𝑒 > 1.33 GeV (𝑖 = the other
particles)
Both primary and secondary interaction points should appear inside the detector.
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-16-
Exploring Dark Photon Parameter Space
Case study 1: mass spectra of 𝑚𝑋 > 2𝑚1, 𝜒1𝑇 → 𝜒2𝑇, 𝜒2 → 𝑋∗ → 𝜒1𝑒
−𝑒+ (𝑇 = 𝑒− or 𝑝)
1-year data collection, a fiducial mass of 380kt, 𝑔12 = 1, and a zero background assumption are taken.
HK can probe the uncovered parameter region by up to an order of magnitude in the 𝜖 axis.
No sensitivity is available in the p-scattering channel with 𝐸1 = 0.5 GeV due to threshold (𝑝th,𝑝 = 1.07 GeV vs.
𝑝th,𝑒 = 0.1 GeV).
p-scattering begins to have signal sensitivity with larger 𝐸1, but explores not much due to threshold and angular
resolution (𝜃𝑝/𝑒 = 3° for 𝑝 > 0.1 GeV). A clever search strategy is needed?!
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-17-
Exploring Dark Photon Parameter Space: Modified Selection
Case study 1: mass spectra of 𝑚𝑋 > 2𝑚1, 𝜒1𝑇 → 𝜒2𝑇, 𝜒2 → 𝑋∗ → 𝜒1𝑒
−𝑒+ (𝑇 = 𝑒− or 𝑝)
We require “displaced” (more than 45 cm) events only, reducing 𝑝th,𝑒 to 10 MeV (for which the vertex resolution is
45 cm).
The case in the left-hand side does not benefit much, as a majority of events are not substantially displaced.
The modified selection scheme allows the proton channel (in the right-hand side) to access some unexplored
parameter space.
The combination between the original and the modified selection schemes would improve the signal sensitivity.
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-18-
Exploring Dark Photon Parameter Space
Case study 2: mass spectra of 𝑚𝑋 < 2𝑚1, 𝜒1𝑇 → 𝜒2𝑇, 𝜒2 → 𝑋(∗) → 𝜒1𝑒
−𝑒+ (𝑇 = 𝑒− or 𝑝)
1-year data collection, a fiducial mass of 380kt, 𝑔12 = 1, and a zero background assumption are taken.
HK can probe the unexplored parameter space in the 𝜖 axis.
A transition happens at 𝛿𝑚 = 𝑚𝑋 where 𝜒2 decays to an 𝑒−𝑒+ pair through on-shell 𝑋 ↔ off-shell 𝑋.
p-scattering begins to have signal sensitivity with larger 𝐸1, but explores not much due to threshold (𝑝th,𝑝 = 1.07
GeV vs. 𝑝th,𝑒 = 0.1 GeV) and angular resolution (𝜃𝑝/𝑒 = 3° for 𝐸 > 0.1 GeV). A clever search strategy is
needed?!
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-19-
Exploring Dark Photon Parameter Space: Modified Selection
Case study 2: mass spectra of 𝑚𝑋 < 2𝑚1, 𝜒1𝑇 → 𝜒2𝑇, 𝜒2 → 𝑋(∗) → 𝜒1𝑒
−𝑒+ (𝑇 = 𝑒− or 𝑝)
We require “displaced” (more than 45 cm) events only, reducing 𝑝th,𝑒 to 10 MeV (for which the vertex resolution is
45 cm).
The case in the left-hand side does benefit a little, as some fraction of events are substantially displaced but some
final state particles deposit a small amount of energy.
The modified selection scheme does not help the proton channel access some unexplored parameter space.
The combination between the original and the modified selection schemes would improve the signal sensitivity.
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-20-
Model-independent Sensitivity
Any model-independent sensitivity just like 𝜎 vs. 𝑚DM in conventional WIMP searches?
Experimental sensitivity with e.g., 90% C.L.
𝜎𝜖: scattering cross section between 𝜒1 and (target) electron
ℱ: flux of incoming boosted 𝜒1 (possibly BDM production mechanism-dependent)
𝐴: acceptance
𝑡exp: exposure time
𝑁𝑒: total # of target electrons
𝑁90: 90% C.L. for a given background assumption
Controllable
Determined by distance between the primary (ER) and the secondary vertices, cuts, energy threshold,
etc. Depending on analyses, some factors can be absorbed into 𝜎𝜖.
𝑁sig = 𝜎𝜖ℱ𝐴𝑡exp𝑁𝑒 > 𝑁90
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-21-
Example Model-independent Sensitivity
𝑁90 for a zero-BG assumption
Evaluated under the assumption
of cumulatively isotropic 𝜒1 flux
ℓlab different event-by-event, so taking
ℓlabmax for more conservative limit
𝜎𝜖ℱ >2.3
𝐴( ℓlab)𝑡exp𝑁𝑒
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-22-
Interpretations
★
★
♦
♦
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Doojin Kim, University of Arizona 3rd KIAS-NCTS-KEK Workshop-23-
Conclusions and Outlook
The boosted (light) dark matter search is promising and provides a new direction to
study dark matter phenomenology.
Theoretical/experimental studies have been actively conducted and in progress.
These ideas can be tested in Hyper-Kamiokande experiment.
To get around detector acceptance issues, hence maximize the signal sensitivity in HK, it is
highly desired to design clever search strategies.
𝑣𝐷𝑀Scattering
Non-relativistic(𝑣𝐷𝑀 ≪ 𝑐)
Relativistic(𝑣𝐷𝑀~𝑐)
elastic Direct detection Boosted DM (eBDM)
inelastic inelastic DM (iDM) inelastic BDM (𝒊BDM)
thank you !