Exploring the dark Universe

Qiang Yuan (袁强)Purple Mountain Observatory, CAS

2018-10-11 @ NCTU

How to weigh an astronomical object?

Newton's law

rmv

rGMm 2

2

rGMv

Circular velocity of solar system planets

rGMv

Rotation of a spiral galaxy

Completely different from the square-root law!

What happens? Non-luminous (dark) matter

Another way: gravitational lensing

Bullet cluster: evidence of dark matter

Clowe et al. (2006)

PLANCK

Composition of the Universe

• The universe is made up of 68% dark energy, 27% dark matter and 5% ordinary matter

• We know little about the Universe!

Can we see dark matter if our telescopes are big enough？

Astronomical observations: NO!

CMB (PLANCK)

Light element abundance from BBN: b~0.05 (Fields and Sarkar 2004)

Angular spectrum of CMB anisotropies:b~0.05, dm~0.25

The nature of dark matter is completely different from baryonic matter!

Gravity's law (Newton, Einstein) is wrong?

Space-time Matter distribution

Modifying the gravity's law？

• It works sometimes, but not always (often failed)!• Einstein's law has been tested with very high precision in

many experiments (including gravitational wave detection)

Test of Einstein's gravity theory

Recent observations of S2's velocity when orbiting the SMBH Sgr A* precisely confirm general relativity.

GRAVITY collaboration (2018)

Dark matter is crucial to human beings

Primordial fluctuation

Dark matter can speed up galaxy formation. Without dark matter, there will be most probably no Milky Way, no solar system, no life either!

What is dark matter?

Dark matter is：stable, cold, non-baryonic, weakly interacting

The most likely candidate is some kind of weakly interacting massive particles (WIMP) beyond the standard model —— new physics!

Detection of (WIMP) dark matter particles

Underground direct detection

Ø Nuclear recoil from WIMP-nuclei collision

Ø Placed in deep underground laboratory to shield cosmic ray backgrounds

Jinping dark matter experimentsPandaX (2017)

CDEX (2018)

Yue, Q. (2016)

Current status

No signal has been successfully found. Stringent limits are placed.

arXiv:1709.00688

Collider detection

Missing energy events.

No signal of dark matter production has been identified yet in many colliders.

SM particle SM particle

Dark matter

Dark matter

Indirect detection

Detect the annihilation/decay products of dark matter in cosmic rays. Usually needs to go to space due to the block of atmosphere.

AMS02 (2013 PRL)

Excess of high energy positrons in cosmic rays

Dark matter?Pulsars? … epp

Gamma-ray excess from Galactic centerGordon and Macias (2013) Calore et al. (2015)

• Generalized NFW2 distribution• Spectrum peaks at 1-3 GeV• Consistent with dark matter

annihilation with 40 GeV mass and 10-26 cm3/s cross section

• Millisecond pulsars?

Goodenough & Hooper (2009)Vitale & Morselli (2009)Hooper & Goodenough (2011) ...

Cui, QY et al. (2017)Cuoco et al. (2017)

• The standard background model under-predicts cosmic ray antipotons in 1-10 GeV band, which could be explained by ~50 GeV dark matter annihilation

• Uncertainties of hadronic/nuclear interactions and solar modulation

Possible GeV antiproton excess?

Summary of dark matter searches

• Collider：Null！

• Direct：Null！1. positron exess

2. gamma-ray excess

3. antiproton excess

Inconclusive！

• Indirect：

Summary of dark matter searches

• Collider：Null！

• Direct：Null！1. positron exess

2. gamma-ray excess

3. antiproton excess

Inconclusive！

• Astronomers can not see dark matter, but they discover dark matter

• Physicsts can in principle “see” dark matter, but they find nothing

• Indirect：

Dark Matter Particle Explorer

Plastic Scintillator Detector (PSD; charge)

Silicon-Tungsten Tracker (STK; track)

BGO Calorimeter(BGO; energy)

Neutron Detector(NUD; particle ID)

A high energy resolution (~1%), high particle discrimination detector of cosmic rays and gamma-rays

Instrument Design

(Astropart.Phys. 95 (2017) 6–24)

Jiuquan Satellite Launch Center

Launched on 17th Dec. 2015

“Monkey King”

5 full scans of the sky 5M events/day4.6 billion in total

Observation overview

DAMPE 2.5 year counts map

BGO stability: 0.5%

STK stability: 0.7%PSD stability: 0.5%

NUD stability: 0.9%

Detector stability

Typical DAMPE events

4.7 TeV electron

Typical DAMPE events

Electron Gamma Proton

P vs. N-side

Energy measurement of electrons

1.005±0.016

Electron-proton discrimination

MC protonsMC electronsMC sumFlight data

z = Flast× (SumRms)4 /(8 × 106)

Candidate protonsCandidate electrons

0.5-1.0 TeV

SumRms [mm]

F las

t

(Nature 552 (2017) 63-66)

• We use the lateral (SumRMS) and longitudinal (energy ratio in last layer) developments of the showers to discriminate electrons from protons

• For 90% electron efficiency, proton background is ~2% @ TeV, ~5% @ 2 TeV, ~10% @ 5 TeV

(Nature 552 (2017) 63-66 + CALET result)

Ø Three different PID methods give very consistent results on event-by-event level

Ø Direct detection of a spectral break at ~1 TeV with 6.6 confidence level

530 days of data2.8 billion events1.5 million e+e- (>25 GeV)

Precise measurement of electron spectrum

The DAMPE collaboration• CHINA

• Purple Mountain Observatory, CAS, Nanjing • Institute of High Energy Physics, CAS, Beijing• National Space Science Center, CAS, Beijing• University of Science and Technology of China, Hefei• Institute of Modern Physics, CAS, Lanzhou

• ITALY• INFN Perugia and University of Perugia• INFN Bari and University of Bari• INFN Lecce and University of Salento

• SWITZERLAND• University of Geneva

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