Physics Enters the Dark Age

Transcription

1 Physics Enters the Dark Age Brooks Thomas University of Hawaii Colloquium at Macalester College, April 29th, 211

2 A Tour of the Dark Side of the Universe: Past, Present, and Future 1). A brief history of dark-matter physics What (little) we know about dark matter How we know what we do know 2). What dark matter could tell us about our universe What the properties of the dark matter could tell us about other aspects of particle physics A few theoretical frameworks for dark matter and how to test them 3). Why everyone's so excited all of a sudden Why the next few years are likely to be a golden age for darkmatter physics, and what we hope to learn A few results already out there which may potentially be signals of dark matter

3 Dark Matter: a Central Theme Astronomy Cosmology X-ray telescopes gamma-ray telescopes Structure formation CMB Measurements Supernovae Nucleosynthesis Neutrino Physics Neutrino observatories Dark-Matter ElectroweakSymmetry Breaking Physics Direct Detection Solid-state Physics Hidden Sectors Missing-Energy Supersymmetry Signatures Particle Theory Collider Physics Current efforts to resolve the dark matter mystery are uniting researchers from a large number of subdisciplines of physical science from particle theory to cosmology to solid-state physics and beyond.

4 Part I First Inklings of the Dark Side (1933-9)

5 A long time ago, in a galaxy cluster far, far away... galaxies were moving as if there was a lot more mass in the cluster than observations suggested. Fritz Zwicky The This discrepancy was first noticed by Fritz Zwicky in Jan Oort had noticed a similar anomaly in the motions of local stars the year before. Possible Explanations? Maybe the laws of physics are different in the. Maybe the coma cluster isn't in equilibrium. Maybe there's a lot of matter in the cluster that doesn't emit light. Jan Oort Zwicky coined the term dark matter to refer to this extra matter.

6 2 21 Rotation Curves Rotation Curves of Galaxies R v Vera Rubin Rotation curves: radial velocity vs. distance from galactic center. In 5, Vera Rubin showed that even within any single galaxy, luminous matter made up only a small fraction of that galaxy's mass. Objects far from the galactic center rotate far faster than one would expect if only luminous matter were present. 22

7 So what was this Dark Matter? Astrophysical candidates abounded: Jupiters and Failed Stars? Neutrinos? Black Holes? Neutron Stars? Brown Dwarfs? And there were some more exotic, hypothetical possibilities with funny-sounding names: Axions WIMPS Gravitinos Q-balls or maybe there was really no dark matter, and gravity really does work differently at large distances. Modified Newtonian Dynamics (MOND) [Milgrom, 3]

8 Part II The Era of Precision Cosmology (9-29) Over the last two decades, experiment and observation have tightly constrained the properties of the dark matter. Overwhelming evidence now suggests a very exciting result: The dark matter really is something exotic and mysterious something not made of normal baryonic matter. I'll now try to explain why we believe this, and what we still have yet to learn about this elusive stuff of which our universe is made.

9 2 Rotation Curves 21 WMAP (21) COBE (9) The Cosmic Microwave Background Light emitted when electrons and nuclei came together to form neutral atoms. Offers a snapshot of what the universe looked like 38, years after the Big Bang. Very isotropic, but small deviations can tell us a lot about what the universe was like at early times! 22

10 Rotation Curves COBE A flat universe made of... 23% Matter 73% Dark Energy WMAP And what does it tell us? Most of the matter in our universe is dark! Neutrinos make up only 1% of the matter in the universe, and (at least the usual kind) can't be the dark matter. Breakdown of the Matter Density

11 Rotation Curves COBE WMAP Structure Formation How did structures like galaxies, clusters, voids, etc. form in such an initially homogeneous, isotropic place? Small fluctuations large inhomogeneities Baryonic matter experiences pressure and can't clump together well until after last scattering. Cold dark matter can clump together very early and seed the formation of structure quite efficiently. Pressure Gravity Current models suggest that baryonic matter alone is insufficient for generating the sorts and sizes of structure we observe.

12 Rotation Curves 2 COBE WMAP Big Bang Nucleosynthesis: The Cosmic Crucible As the universe cools, nucleons interact and form light elements. Reproducing observed abundances requires just the right initial conditions... but amazingly enough, theoretical prediction and observation agree! This agreement aids in constraining... The number of neutrino species The density of baryonic matter The rate of any heavy particles decaying while BBN is going on. And a lot more...

13 Rotation Curves COBE The Bullet Cluster (26) WMAP Bullet Cluster Mapping the mass in pair of colliding galaxy clusters Mass bends the path light takes Gravitational Lensing Luminous matter only (X-Rays) The Bullet Point: The center of mass of the entire system is displaced from the center of mass of the luminous matter. All mass (Gravitational Lensing) The most direct evidence for the existence of dark matter to date. Hard to explain with modified gravity alone!

14 Rotation Curves COBE The Bullet Cluster (26) WMAP Bullet Cluster Mapping the mass in pair of colliding galaxy clusters Mass bends the path light takes Gravitational Lensing Luminous matter only (X-Rays) The Bullet Point: The center of mass of the entire system is displaced from the center of mass of the luminous matter. All mass (Gravitational Lensing) The most direct evidence for the existence of dark matter to date. Hard to explain with modified gravity alone!

15 The present state of our knowledge about dark matter:

16 Known Knowns It's out there (~23% of the energy density [WMAP, galaxy clusters, BBN, supernovae] in the universe). It's cold (acts like massive matter). [Structure formation] It's stable (or at least very long-lived). [BBN, X-ray/gamma-ray telescopes, etc. ] Most of it isn't normal (atomic) matter. [BBN, Structure formation, WMAP] Its interactions with normal matter are extremely weak. [Colliders, structure formation, heavy-isotope searches ]

17 Known Unknowns Does it interact only through gravity, or does it feel other forces (e.g. the weak nuclear force)? Is it made of only one kind of particle, or many? What is the mass of that particle (or particles)? Why is there so much of it around? How was it made? (Thermally? Non-thermally?) What can its properties tell us about the underlying theory which describes our universe?

18 Unknown Unknowns (There may be a few... nature has been know to surprise us!) However, one thing is clear: most of the dark matter, whatever it may be, is not made of normal baryonic matter. One of the only robust indications (along with neutrino masses) of physics that cannot be explained by the Standard Model plus gravity alone.

19 So what could the dark matter be? Many theories, motivated by other considerations, turn out to provide natural candidates for dark matter: WIMPs (weakly-interacting massive particles) Axions (light fields kind of like pions, but with highly suppressed couplings) SuperWIMPS (much lighter, very weakly coupled: often result from WIMP decays) Primordial Black Holes Other stuff... The Dark Matter Menagerie

20 The WIMP: The Industry Standard Weak-scale mass: Relic Abundance ThermalFreeze-out mwimp ~ 1 1 GeV/c2 SM Increasing annihilation rate SM Advantages: Temperature [GeV] Theoretically well-motivated. Naturally have the right relic abundance: WIMP miracle. Interact with normal matter via the weak force direct detection possible (but not easy) Examples: Neutralinos (Supersymmetry) KK Photon (Extra dimensions) Inert Higgs (Multi-Higgs models)

21 One dark-matter candidate... or many? In collaboration with K. Dienes [NSF, Arizona U.] The dominant paradigm has been to consider scenarios in which the dark-matter abundance is made up by one stable particle (or maybe two or three), but maybe things aren't quite so simple. It could be that a large number of particles contribute nontrivially to that abundance, some of which are only quasi-stable. It is even possible that ΩDM could be partitioned evenly among a vast number of particles, each of which only contributes a minute fraction of the total. As long as the individual abundances are balanced against decay rates in just the right way, this is a viable dark-matter scenario!

22 Dynamical Dark Matter : The Basic Picture:

23 Not at all: such a situation arises naturally in theories with large extra dimensions.... Horribly contrived, you say? Quantized! These theories inherently contain large numbers of fields: each corresponds to a different set of (quantized) momenta in the extra dimensions. Other mass contributions too mixed tower of massive particles Unstable fields Just the right properties for dynamical dark matter!

24 Constraints on Dynamical Dark Matter (There are many, but they can easily be satisfied) GC stars SN7A Diffuse photon spectra Eötvös experiments Helioscopes (CAST) Collider limits DM overabundant Thermal production

25 Part III Physics in a New Dark Age (29- ) Over the next decade, a new generation of experiments will probe the nature and properties of the dark matter more deeply than ever before.

26 Rotation Curves COBE New Collider Data 2 WMAP Bullet Cluster Tevatron (Batavia, IL) Fermilab Tevatron: (running until October) Proton-antiproton (pp) collider Center-of-mass energy: 1.96 TeV LHC (CERN, Switzerland) CERN LHC: Proton-proton (pp) collider Center-of-mass energy: 7 TeV (later 1 TeV)

27 Rotation Curves Seeing by not seeing COBE Tevatron 21 2 WMAP Bullet Cluster LHC 22 Missing Transverse Momentum Collider signals come in the form of missing momentum. ( M N o t iss det ing ec E n te d e rg y ) Proton DM g u g g d Proton Detector u g g u g g d u g DM Can't conclusively discover the DM, but can provide important evidence.

28 Rotation Curves COBE Tevatron 21 2 WMAP Bullet Cluster 22 Planck LHC More precise measurements of the CMB Planck (first-year results)...and other astrophysical data as well: Visible/IR galaxy surveys with the James Webb Space Telescope (214) Gamma-ray astronomy with FERMI, HESS, VERITAS, etc. Cosmic-ray data from AMS (launched two hours ago!), etc. Info on dark energy from supernovae (JDEM, SNAP?)

29 Rotation Curves COBE Tevatron 2 WMAP Bullet Cluster Direct Detection LHC Planck The Basic Principle (DM Scatters) N (Nucleus recoils) DM (Detected) As the Earth moves through the galactic halo, DM particles pass through us. Interactions with normal matter are very weak, but with a sensitive detector and enough time, we might detect a few scattering events. Detector Types Noble liquids (e.g. liquid Xenon) Ge, Si diode arrays NaI crystals Gas target (directional detection)

30 Rotation Curves Experiments Already operating: XENON1 CDMS CoGeNT DAMA and many more... Soon to begin: SuperCDMS (211) XENON1T (213) EURECA (214?) COBE Tevatron 2 WMAP Bullet Cluster Interaction cross-section [cm-2] SuperCDMS LHC Planck XENON1T EURECA NEW this month! WIMP MASS [GeV] XENON1 ZEPLIN III CDMS CRESST DAMA CoGeNT EDELWEISS

31 Rotation Curves COBE Tevatron Indirect Detection 2 WMAP Bullet Cluster 21 SuperCDMS LHC Planck XENON1T EURECA The Basic Principle Dark matter in the galactic halo annihilates (or decays) to ordinary particles Some of the particles emitted in our direction will reach Earth and can be detected Auspicious channels: Antimatter (e+, 2H-, p-, etc.) Gamma rays Neutrinos 22 SM particles

32 Rotation Curves COBE Tevatron 2 WMAP Bullet Cluster 21 SuperCDMS LHC Planck XENON1T EURECA Indirect Detection with neutrinos (a special case) Interactions too weak/rare to observe directly. Method: detect muons produced by neutrinos interacting with rock. 22

33 Rotation Curves COBE Tevatron A lot of recent activity and a lot more to come! 2 WMAP Bullet Cluster SuperCDMS LHC Planck XENON1T EURECA Electrons, Positrons FERMI (28) Photons H.E.S.S. (23) ATIC IV (27) PAMELA (26) Neutrinos General antimatter GAPS (211) AMS (211) Super-Kamiokande (Upgrade: 28) IceCUBE (21)

34 Rotation Curves COBE Tevatron 2 WMAP Bullet Cluster Some tantalizing hints: SuperCDMS LHC Planck XENON1T EURECA Observed In 28, the PAMELA satellite reported seeing an excess of positrons in cosmic rays. Expe c te d In the same year, ATIC reported a similar excess in cosmic-ray electrons.

35 Rotation Curves COBE Tevatron 2 WMAP Bullet Cluster 21 SuperCDMS LHC Planck XENON1T EURECA Is it dark matter? Perhaps... but there are reasons to be skeptical: FERMI data show a much smaller deviation from theoretical predictions. Similar anomalies not seen in antiproton channel. 22 FERMI (21) Flux is a 1-1 times too big for a thermal relic. Standard astrophysics (nearby pulsars) can account for the reported excess.

36 Rotation Curves COBE Tevatron The DAMA puzzle 2 WMAP Bullet Cluster SuperCDMS LHC Planck XENON1T EURECA Direct detection looking for dark matter annual modulation in the WIMP wind. Taking data since 6. Clear signal of modulation, with a period of one year and a peak around June 2nd. 8σ significance!

37 Rotation Curves COBE Tevatron 2 WMAP Bullet Cluster SuperCDMS LHC Planck XENON1T EURECA Is it dark matter? Perhaps... but there are reasons to be skeptical: Seemingly to be in conflict with XENON1 data (consistency requires very contrived models). Potential backgrounds (like that from cosmic-ray muons) modulate annually too. Doesn't seem to agree with other potential signals from other experiments (e.g. CoGeNT). New data and new data-analysis techniques should help resolve this puzzle soon.

38 The Upshot: Dark matter physics is increasingly becoming a nexus around which contributions from numerous areas of physics, both theoretical and experimental, are coalescing. Over the next decade, we can look forward to... New results (and perhaps a discovery) at direct-detection experiments New data from x-ray and gamma-ray telescopes Potential signals at neutrino observatories New measurements of the particle content in cosmic-rays A trove of data imminent from the Tevatron and LHC New, more accurate measurements of the CMB New prospects for axion detection via cavity experiments More data on the properties of dwarf galaxies and subhalos Thus, after nearly a century, we may finally be on the doorstep of unraveling the dark-matter mystery.

39 So, am I excited to be living in the dark ages? You bet!