Lennert Thormaehlen
Institute for Theoretical Physics Heidelberg
20th Lomonosov Conference
24th August 2021

HIDDEN PHOTON DARK MATTER
IN THE LIGHT OFXENON1T AND STELLAR COOLING
Based on arxiv:2006.11243 in collaboration withGonzalo Alonso-Álvarez, Fatih Ertas, Joerg Jaeckel and Felix Kahlhoefer

• XENON1T observed an excess inelectron recoil events[arXiv:2006.09721]• 3.5 σ significance• Energy Resolution ∼20 % in signal
regionLennert Thormählen 24.08.21

THE XENON1T EXCESS• Upgrade XENONnT will confirm orrule out the observed excess2• After XENON1T announced their results, many possible explanations weresuggested:• Additional background fromTritium contamination• Solar axions• Neutrino magnetic moment• Inelastic DM• Boosted DM• …
• and hidden photon DM

Lennert Thormählen 24.08.21

POSSIBLE EXPLANATIONS

3

1. Can hidden photon DM explain the XENON1T excess?
2. Would such a hidden photon be a viable DM candidate?
3. How does it compare with stellar cooling limits and hints?

Lennert Thormählen 24.08.21

KEY QUESTIONS IN THIS TALK4

• A hidden photon (HP) is a gauge boson of a new U(1) symmetry with gaugefield.
• The HP can kinetically mix with the SM photon:• After a suitable field redefinition:• HP couples weakly to SM electromagnetic current

Lennert Thormählen 24.08.21

HIDDEN PHOTONS

5

HIDDEN PHOTONS IN XENON1T• Monoenergetic peak is smeared out by the detector resolution

XENON1T data[arXiv:2006.09721]

Lennert Thormählen 24.08.21• Absorption rate of non-relativistic HPs in xenon

Hidden photon fit

6

HIDDEN PHOTONS IN XENON1T• Global significance:• Lower significance thansolar axion in large partsdue to look-elsewhere• Results confirmed by[arXiv:2006.13159, 2006.13929, 2006.14521]

Lennert Thormählen 24.08.21• Best fit:7

• Light DM cannot be produced thermally• Non-thermal production mechanisms:a) Misalignment (large initial field values) [arXiv:1201.5902]b) Fluctuations during inflation [arXiv:1504.02102]

Higher inflation scales possible by including non-minimal coupling to gravity[arXiv:1905.09836]

Strong small scale fluctuations expected!
c) Decay product of e.g. axion field, dark Higgs, inflaton, cosmic strings etc.
[arXiv:1810.07188]

Lennert Thormählen 24.08.21

HIDDEN PHOTONS AS DM8

• Stability is ensured• Dominant decay channels:a) X ⟶ 𝛾𝛾𝛾[arXiv:0811.0326]

b) X ⟶ 𝜈𝜈Lennert Thormählen 24.08.21

HIDDEN PHOTONS AS DM[arXiv:1912.12152]⟹ A hidden photon of 2.8 keV is a viable DM candidate!

9STELLAR COOLING• Stellar cooling supplies strong constraints but anomalous cooling is observed

Lennert Thormählen 24.08.21• Stars can produce new light bosons in large abundances

10Figure taken from Giannotti, Irastorza, Redondo, Ringwald 2015 [arXiv:1512.08108]

HIDDEN PHOTONS AND HB STARS• For significant cooling, resonance condition needs to be fulfilled in somespherical shell inside HB stars.
• Maximal plasma frequency is reached at the core

Lennert Thormählen 24.08.21• Transverse modes resonantly convert to HPs if

11

HIDDEN PHOTONS AND HB STARS
Sharp edge from resonancecondition

Note:• Axion hint translated tohidden photons• The hint from the Rparameter could becustomised for HPs.
Bounds and Hints from: [arxiv:1512.08108]& [arxiv:1412.8379]

Lennert Thormählen 24.08.21• HB anomaly and XENON1T excess have a significant overlap

12

HIDDEN PHOTONS AND HB STARS
Significance of combined
analysis:
3.3 σIncluding unconstrained
Tritium abundance:
2.2 σ

Lennert Thormählen 24.08.21• Combined analysis gives shifts the best fit point to 2.5 keV.

13
Bounds and Hints from: [arxiv:1512.08108] & [arxiv:1412.8379]

• We can treat the HB exclusions and anomalies as prior knowledge in the HP
parameter space• Can assign a prior distribution to the Tritium abundance• Bayes factor:

Lennert Thormählen 24.08.21

BAYESIAN ANALYSIS• Result:• This is commonly interpreted as substantial evidence!

14

CONCLUSION• Such a particle could constitute all of the dark matter.

• Additional motivation comes from the HB cooling anomaly• There is substantial evidence for the HP explanation of the XENON1T excess
compared to the assumption of a tritium background

Lennert Thormählen 24.08.21• A hidden photon of (2 -3) keV could explain the XENON1T excess.

• To-Do’s:• Build larger, more sensitive xenon detector!
• Evaluate HB hint in detail for hidden photons• Investigate the time dependence of the signal15