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Weakly interacting massive particles (WIMPs) are hypothetical particles that are thought to constitute dark matter. There exists no clear definition of a WIMP, but broadly, a WIMP is a new elementary particle which interacts via gravity and any other force (or forces), potentially not part of the standard model itself, which is as weak as or weaker than the weak nuclear force, but also non-vanishing in its strength. A WIMP must also have been produced thermally in the early Universe, similarly to the particles of the standard model according to Big Bang cosmology, and usually will constitute cold dark matter. Obtaining the correct abundance of dark matter today via thermal production requires a self-annihilation cross section of {\displaystyle \langle \sigma v\rangle \simeq 3\times 10^{-26}\mathrm {cm} ^{3}\;\mathrm {s} ^{-1}} \langle \sigma v\rangle \simeq 3\times 10^{-26}\mathrm {cm} ^{3}\;\mathrm {s} ^{-1}, which is roughly what is expected for a new particle in the 100 GeV mass range that interacts via the electroweak force. Because supersymmetric extensions of the standard model of particle physics readily predict a new particle with these properties, this apparent coincidence is known as the "WIMP miracle", and a stable supersymmetric partner has long been a prime WIMP candidate.[1] However, recent null results from direct-detection experiments along with the failure to produce evidence of supersymmetry in the Large Hadron Collider (LHC) experiment[2][3] has cast doubt on the simplest WIMP hypothesis.[4] Experimental efforts to detect WIMPs include the search for products of WIMP annihilation, including gamma rays, neutrinos and cosmic rays in nearby galaxies and galaxy clusters; direct detection experiments designed to measure the collision of WIMPs with nuclei in the laboratory, as well as attempts to directly produce WIMPs in colliders, such as the LHC.

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  1. 1. WIMP AND RELATED MIRACLES Jonathan Feng, UC Irvine Manoj Kaplinghat, Jason Kumar, Huitzu Tu, Haibo Yu Inaugural Workshop, Center for Particle Cosmology University of Pennsylvania 11 December 2009
  2. 2. 11 Dec 09 INTRODUCTION • The WIMP miracle is the fact that weak scale particles make good dark matter • It is at the heart of many expectations for connections between particle physics and cosmology and is the driving motivation behind most dark matter searches • Executive Summary – The WIMP miracle is more restrictive than you might think – The WIMP miracle is less restrictive than you might think Feng 2
  3. 3. 11 Dec 09 Feng 3 THE WIMP MIRACLE • Fermi’s constant GF introduced in 1930s to describe beta decay n à p e- n • GF ≈ 1.1 105 GeV-2 à a new mass scale in nature mweak ~ 100 GeV • We still don’t understand the origin of this mass scale, but every attempt so far introduces new particles at the weak scale _
  4. 4. • Assume a new (heavy) particle X is initially in thermal equilibrium • Its relic density is • mX ~ 100 GeV, gX ~ 0.6 à WX ~ 0.1 11 Dec 09 Feng 4 • Remarkable coincidence: particle physics independently predicts particles with the right density to be dark matter Kolb, Turner X X q q _ THE WIMP MIRACLE
  5. 5. 11 Dec 09 Feng 5 WIMP DETECTION Correct relic density à Lower bound on DM-SM interaction c c q q Efficientannihilationnow (Indirectdetection) Efficient scattering now (Direct detection) Efficientproductionnow (Particlecolliders)
  6. 6. 11 Dec 09 DIRECT DETECTION • CDMS will announce new results next Friday • From correspondence with CDMS collaborators, I can tell you that – CDMS has not discovered DM – or these people would make incredible poker players Feng 6
  7. 7. 11 Dec 09 Feng 7 CURRENT STATUS • Direct detection searches for nuclear recoil in underground detectors • Spin-independent scattering is typically the most promising • Theory and experiment compared in the (mX, sp) plane – Expts: CDMS, XENON, … – Theory: Shaded region is the predictions for SUSY neutralino DM – what does this mean?
  8. 8. 11 Dec 09 Feng 8 NEW PHYSICS FLAVOR PROBLEM • New weak scale particles generically create many problems • One of many possible examples: K-K mixing • Three possible solutions – Alignment: q small – Degeneracy: squark Dm << m: typically not compatible with DM, because the gravitino mass is ~ Dm, so this would imply that neutralinos decay to gravitinos – Decoupling: m > few TeV d s d s d̃, s̃, b̃ d̃, s̃, b̃ g̃g̃ q _
  9. 9. 11 Dec 09 Feng 9 THE SIGNIFICANCE OF 10-44 CM2 • Decoupling is the strategy taken in many theories – focus point SUSY, inverted hierarchy models, more minimal SUSY, 2-1 models, split SUSY,… • This eliminates many annihilation diagrams, collapses predictions • Universal prediction: sp ~ 10-44 cm2 Stay tuned!
  10. 10. INDIRECT DETECTION 11 Dec 09 Feng 10 PAMELA (2008) ATIC (2008) Solid lines are the predicted spectra from GALPROP (Moskalenko, Strong) Fermi (2009) e+ + e-
  11. 11. Fermi (2009) ARE THESE DARK MATTER? 11 Dec 09 Feng 11 • Pulsars can explain PAMELA Zhang, Cheng (2001); Hooper, Blasi, Serpico (2008) Yuksel, Kistler, Stanev (2008) Profumo (2008) ; Fermi (2009) • For dark matter, there is both good and bad news • Good: the WIMP miracle motivates excesses at ~100 GeV – TeV • Bad: the WIMP miracle also tells us that the annihilation cross section should be a factor of 100-1000 too small to explain these excesses. Need enhancement from – astrophysics (very unlikely) – particle physics
  12. 12. SOMMERFELD ENHANCEMENT • If dark matter X is coupled to a hidden force carrier f, it can then annihilate through XX à f f • At freezeout: v ~ 0.3, only 1st diagram is significant, s = sth Now: v ~ 10-3, all diagrams significant, s = Ssth, S ~ min {pa/v, amX/mf}, boosted at low velocities Sommerfeld (1931) Hisano, Matsumoto, Nojiri (2002) • If S ~ 100-1000, seemingly can explain excesses, get around WIMP miracle predictions Cirelli, Kadastik, Raidal, Strumia (2008) Arkani-Hamed, Finkbeiner, Slatyer, Weiner (2008)11 Dec 09 Feng 12 f f ff… f f X X
  13. 13. CONSTRAINTS ON SOMMERFELD ENHANCEMENTS • Unfortunately, this scenario is internally inconsistent, at least in its original form • Large S requires large a and small mf • This also maximizes the annihilation cross section; requiring that X be all the dark matter à upper bounds on S • These scenarios also induce dark matter self-interactions XX à XX, are excluded for light f by halo ellipticity Spergel, Steinhardt (1999); Miralda-Escude (2000) Ackerman, Buckley, Carroll, Kamionkowski (2009) Feng, Tu, Yu (2009), Buckley, Fox (2009) 11 Dec 09 Feng 13 Feng, Kaplinghat, Yu (2009)
  14. 14. WAYS OUT? • X is only part of the dark matter: No, flux ~ n2 <sv> S ~ a-1 • Resonant Sommerfeld enhancement: No Dent et al. (2009), Zavala et al. (2009) • Alternative production mechanisms, cosmologies at freezeout: Yes – but then why consider Sommerfeld enhancement? • Boosts part Sommerfeld (~100), part astrophysical (~10)? Maybe 11 Dec 09 Feng 14 Data DarkMatter Pulsars
  15. 15. 11 Dec 09 DOES THE WIMP MIRACLE REQUIRE WIMPS? • The WIMP miracle seemingly implies vanilla dark matter – Weakly-interacting – Cold – Collisionless • Are all WIMP miracle-motivated candidates astrophysically equivalent? • No! There are dark matter candidates that preserve the virtues of WIMPs, including the WIMP miracle, but have qualitatively different implications Feng 15
  16. 16. 11 Dec 09 Feng 16 THE SUPERWIMP MIRACLE • G̃ not LSP • Assumption of most of literature SM LSP G̃ • G̃ LSP • Completely different cosmology and particle physics SM NLSP G̃ An example: in supersymmetry, Graviton à Gravitino G̃ Mass ~ 100 GeV; Interactions: only gravitational (superweak) Feng, Rajaraman, Takayama (2003)
  17. 17. 11 Dec 09 Feng 17 • Suppose gravitinos G̃ are the LSP • WIMPs freeze out as usual • But then all WIMPs decay to gravitinos after MPl 2/MW 3 ~ seconds to months SUPERWIMP RELICS Gravitinos naturally inherit the right density, but interact only gravitationally – they are superWIMPs (also KK gravitons, quintessinos, axinos, etc.) G̃ (+ g, e, …) WIMP ≈ Feng, Rajaraman, Takayama (2003); Bi, Li, Zhang (2003); Ellis, Olive, Santoso, Spanos (2003); Wang, Yang (2004); Feng, Su, Takayama (2004); Buchmuller, Hamaguchi, Ratz, Yanagida (2004); Roszkowski, Ruiz de Austri, Choi (2004); Brandeburg, Covi, Hamaguchi, Roszkowski, Steffen (2005); …
  18. 18. 11 Dec 09 • SuperWIMPs are produced in late decays with large velocity (0.1c – c) • Suppresses small scale structure, as determined by lFS, Q • Warm DM with cold DM pedigree WARM SUPERWIMPS Dalcanton, Hogan (2000) Lin, Huang, Zhang, Brandenberger (2001) Sigurdson, Kamionkowski (2003) Profumo, Sigurdson, Ullio, Kamionkowski (2004) Kaplinghat (2005) Cembranos, Feng, Rajaraman, Takayama (2005) Strigari, Kaplinghat, Bullock (2006) Bringmann, Borzumati, Ullio (2006) Kaplinghat(2005) Sterile n Dodelson, Widrow (1993) SuperWIMP Feng 18 Cembranosetal.(2005)
  19. 19. CHARGED PARTICLE TRAPPING • SuperWIMP DM à metastable particles, may be charged, far more spectacular than misssing ET (1st year LHC discovery) • Can collect these particles and study their decays • Several ideas ‒ Catch sleptons in a 1m thick water tank (up to 1000/year) Feng, Smith (2004) ‒ Catch sleptons in LHC detectors Hamaguchi, Kuno, Nakawa, Nojiri (2004) ‒ Dig sleptons out of detector hall walls De Roeck et al. (2005) Charged particle trap Reservoir 7 Sep 09
  20. 20. 11 Dec 09 Feng 20 THE WIMPLESS MIRACLE • Start over: What do we really know about dark matter? – All solid evidence is gravitational – Also solid evidence against strong and EM interactions • A reasonable 1st guess: dark matter has no SM gauge interactions, i.e., it is hidden Kobsarev, Okun, Pomeranchuk (1966); many others • What one seemingly loses • Connections to central problems of particle physics • The WIMP miracle • Signals Can hidden dark matter be rehabilitated? Feng, Kumar (2008); Feng, Tu, Yu (2008)
  21. 21. 11 Dec 09 Feng 21 CONNECTIONS TO CENTRAL PROBLEMS IN PARTICLE PHYSICS • We want hidden sectors • Consider SUSY – Connected to the gauge hierarchy problem – Hidden sectors are already required to break SUSY • Hidden sectors each have their own – particle content – mass scales mX – Interactions, gauge couplings gX
  22. 22. 11 Dec 09 • What can we say about hidden sectors in SUSY? • Generically, nothing. But in SUSY models that solve the new physics flavor problem (gauge-mediated models, anomaly-mediated models) the superpartner masses are determined by gauge couplings mX ~ gX 2 • This leaves the relic density invariant! Feng 22
  23. 23. • The thermal relic density constrains only one combination of gX and mX • These models map out the remaining degree of freedom; candidates have a range of masses and couplings, but always the right relic density • This decouples the WIMP miracle from WIMPs (is this what the flavor problem is really trying to tell us?) THE WIMPLESS MIRACLE 11 Dec 09 Feng 23 WIMPs WIMPless DM
  24. 24. How is hidden dark matter stabilized? If the hidden sector is standard model-like, the most natural possibility is that the DM particle has hidden charge, and so is stabilized by charge conservation (cf. the electron) SIGNALS MSSM mw sparticles, W, Z, t ~GeV q, l 0 p, e, g, n, G̃ Hidden, flavor-free MSSM mX sparticles, W, Z, q, l, t̃ (or t) 0 g, g, n, G̃ 11 Dec 09 Feng 24
  25. 25. • Such WIMPless DM self-interacts through Rutherford scattering – Highly velocity-dependent – constrained by existence of non- spherical halos, bullet cluster • Related to “dark photons” where there is hidden U(1) only Ackerman, Buckley, Carroll, Kamionkowski (2008) • With dark SM, weak interactions can give the right W, lots of freedom 11 Dec 09 Feng 25 DM-DM SIGNALS Feng,Kaplinghat,Tu,Yu(2009)
  26. 26. DM-SM SIGNALS • Alternatively, hidden DM may interact with normal matter through non- gauge interactions 11 Dec 09 Feng 26 q Y q X X
  27. 27. EXAMPLE • Assume WIMPless DM X is a scalar, Y is a fermion, interact with b quarks • May explain DAMA without contradicting CDMS, XENON – mX ~ 5 GeV (WIMPless miracle) – Naturally gives large sSI • Such Y’s look like exotic 4th generation quarks, will be seen at Tevatron, LHC 19 Jun 09 Feng 27 Feng,Kumar,Learned,Strigari(2008) X bL YL lblb bR YR X
  28. 28. 11 Dec 09 Feng 28 CONCLUSIONS • The WIMP miracle connects central questions in particle physics and astrophysics • WIMP searches are progressing rapidly on every front – Direct detection – Indirect detection – LHC • Proliferation of DM candidates that satisfy the WIMP miracle, but have completely different implications for particle physics, astrophysics – SuperWIMP dark matter – WIMPless dark matter • In the next few years, all of these DM models will be stringently tested