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WIMPS (Weakly interacting massive particles (WIMPs)) AND THEIR RELATIONS


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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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WIMPS (Weakly interacting massive particles (WIMPs)) AND THEIR RELATIONS

  1. 1. 27 Oct 08 Feng 1 WIMPS AND THEIR RELATIONS Jonathan Feng University of California, Irvine 27 October 2008 MIT Nuclear and Particle Physics Colloquium
  2. 2. 27 Oct 08 Feng 2 DARK MATTER • Unambiguous evidence for new physics • Intimately connected to central problems – electroweak symmetry breaking – structure formation • Remarkable precision WDMh2 = 0.1099 ± 0.0062 WMAP (2008)
  3. 3. 27 Oct 08 Feng 3 OPEN QUESTIONS • What particle forms dark matter? • What is its mass? • What is its spin? • What are its other quantum numbers and interactions? • Is dark matter composed of one particle species or many? • How was it produced? • When was it produced? • Why does WDM have the observed value? • What was its role in structure formation? • How is dark matter distributed now? • Is it absolutely stable?
  4. 4. 27 Oct 08 Feng 4 CANDIDATES • Observational constraints – Not baryonic (≠ weakly- interacting) – Not hot (≠ cold) – Not short-lived (≠ stable) • Masses and interaction strengths span many, many orders of magnitude HEPAP/AAACDMSAGSubpanel(2007) • Focus on WIMPs, superWIMPs, WIMPless dark matter
  5. 5. 27 Oct 08 Feng 5 THE “WIMP MIRACLE” (1) Assume a new (heavy) particle c is initially in thermal equilibrium: cc ↔` f f (2) Universe cools: cc ` f f (3) cs “freeze out”: cc f f (1) (2) (3)→ ←/ → ← / / Kolb, Turner
  6. 6. 27 Oct 08 Feng 6 • The amount of dark matter left over is inversely proportional to the annihilation cross section: WDM ~ <sAv>-1 • What is the constant of proportionality? • Impose a natural relation: sA = ka2/m2 , so WDM ~ m2 Remarkable “coincidence”: WDM ~ 0.1 for m ~ 0.1 – 1 TeV Cosmology alone tells us we should explore the weak scale HEPAP LHC/ILC Subpanel (2006) [band width from k = 0.5 – 2, S and P wave]
  7. 7. 27 Oct 08 Feng 7 STABILITY • This all assumes the new particle is stable. Why should it be? • LEP’s Cosmological Legacy Problems ↕ Discrete symmetry ↕ Stability • In many theories, dark matter is easier to explain than no dark matter New Particle States Standard Model Particles Stable
  8. 8. 27 Oct 08 Feng 8 Dark Matter! Propose Discrete Symmetry Find Problems Evaluate Precision Constraints Make a Model Predict DM Signals Recent proliferation of examples: • Supersymmetry: R-parity à Neutralinos Goldberg (1983); Ellis et al. (1984) • Universal Extra Dimensions: KK-parity à Kaluza-Klein DM Servant, Tait (2002); Cheng, Feng, Matchev (2002) • Branes: Brane-parity à Branon DM Cembranos, Dobado, Maroto (2003) • Little Higgs: T-parity à T-odd DM Cheng, Low (2003) WIMPS
  9. 9. 27 Oct 08 Feng 9 NEUTRALINOS • The neutralino is the classic WIMP – c Î ( g̃, Z̃, H̃u, H̃d ) – ~ 50 GeV – 1 TeV – weakly-interacting – naturally the lightest standard model superpartner in many models • Particle physics alone à neutralinos have all the right properties to be WIMP dark matter
  10. 10. 27 Oct 08 Feng 10 WDMh2 stringently constrains models Feng,Matchev,Wilczek(2003) Focus point region Co-annihilation region Bulk region Yellow: pre-WMAP Red: post-WMAP Too much dark matter Cosmology excludes many possibilities, favors certain regions RELIC DENSITY
  11. 11. 27 Oct 08 Feng 11 WIMP DETECTION Correct relic density à Efficient annihilation then c c q q Efficientannihilationnow (Indirectdetection) Efficient scattering now (Direct detection) Efficientproductionnow (Particlecolliders)
  12. 12. 27 Oct 08 Feng 12 DIRECT DETECTION • WIMP properties: v ~ 10-3 c Kinetic energy ~ 100 keV Local density ~ 1 / liter • Detected by nuclear recoil in underground detectors. Two approaches: – Background-free detection – Annual modulation • Spin-independent scattering – Theories: cq – Experiments: c-nucleus – Meet in the middle: cp
  13. 13. 27 Oct 08 Feng 13 THEORETICAL PREDICTIONS • Model-dependent, but in SUSY we can say something. There are two classes of annihilation processes: • Neutralino DM à gravity-mediation SUSY flavor and CP problems à heavy sleptons and squarks Relic density à mixed Bino-Higgsino neutralinos with s ~ 10-8 pb • Many SUSY models (mSUGRA, general focus point SUSY, gaugino- mediated, more minimal SUSY, 2-1 models, split SUSY) will be tested in the next few years
  14. 14. DIRECT DETECTION: DAMA • Annual modulation expected Drukier, Freese, Spergel (1986) • DAMA: 8s signal with – T ~ 1 year, max ~ June 2 27 Oct 08 Feng 14 DAMACollaboration(2008)
  15. 15. CHANNELING • DAMA’s results have been puzzling, in part because the allowed region is excluded by experiments • This may be ameliorated by astrophysics and channeling: in crystalline detectors, efficiency for nuclei recoil energy à electron energy depends on direction • Channeling reduces threshold, shifts allowed region to lower masses. Consistency restored? Gondolo, Gelmini (2005) Drobyshevski (2007), DAMA (2007) 27 Oct 08 Feng 15
  16. 16. 27 Oct 08 Feng 16 TAKING STOCK • WIMPs are astrophysically identical – Weakly-interacting – Cold – Stable • Is this true of all DM candidates? • No. But is this true of all DM candidates motivated by particle physics and the “WIMP miracle”? • No! SuperWIMPs: identical motivations, but qualitatively different implications
  17. 17. 27 Oct 08 Feng 17 SUPERWIMPS • G̃ not LSP • Assumption of most of literature SM LSP G̃ • G̃ LSP • Completely different cosmology and particle physics SM NLSP G̃ Supersymmetry: Graviton à Gravitino G̃ Mass ~ 100 GeV; Interactions: only gravitational (superweak) Feng, Rajaraman, Takayama (2003)
  18. 18. 27 Oct 08 Feng 18 • Suppose the gravitino G̃ is 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,, axinos, etc.) G̃ 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); …
  19. 19. 27 Oct 08 Feng 19 CHARGED PARTICLE TRAPPING • SuperWIMPs are produced by decays of metastable particles, which can be charged. • Charged metastable particles will be obvious at colliders, can be trapped and moved to a quiet environment to study their decays. • Can catch 1000 per year in a 1m thick water tank Feng, Smith (2004) Hamaguchi, Kuno, Nakawa, Nojiri (2004) De Roeck et al. (2005) Charged particle trap Reservoir
  20. 20. 27 Oct 08 Feng 20 IMPLICATIONS FROM CHARGED PARTICLE DECAYS • Measurement of t , ml ̃ and El à mG̃ and GN – Probes gravity in a particle physics experiment! – Measurement of GN on fundamental particle scale – Precise test of supergravity: gravitino is graviton partner – Determines WG̃: SuperWIMP contribution to dark matter – Determines F : supersymmetry breaking scale, contribution of SUSY breaking to dark energy, cosmological constant Hamaguchi et al. (2004); Takayama et al. (2004)
  21. 21. 27 Oct 08 Feng 21 SUPERWIMP COSMOLOGY Late decays can modify CMB black body spectrum (µ distortions) Fields,Sarkar,PDG(2002) Late decays can modify BBN (Resolve 6,7Li problems?) Fixsenetal.(1996)
  22. 22. 27 Oct 08 Feng 22 • 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 SMALL SCALE STRUCTURE 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
  23. 23. 27 Oct 08 Feng 23 WIMPLESS DARK MATTER • 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 Lee, Yang (1956); Gross, Harvey, Martinec, Rohm (1985) • What one seemingly loses – The WIMP miracle – Non-gravitational signals
  24. 24. 27 Oct 08 Feng 24 HIDDEN SECTORS • Can we recover the WIMP miracle, but with hidden DM? • Consider gauge-mediated SUSY breaking with one or more hidden sectors • Each hidden sector has its own gauge groups and couplings
  25. 25. 27 Oct 08 Feng 25 • Particle Physics Superpartner masses, interaction strengths depend on gauge couplings • Cosmology W depends only on the SUSY breaking sector: WX ~ WWIMP ~ WDM Any hidden particle with mass ~ mX will have the right thermal relic density (for any mX) THE WIMPLESS MIRACLE Feng, Kumar (2008)
  26. 26. 27 Oct 08 Feng 26 • The thermal relic density constrains only one combination of gX and mX • These models map out the remaining degree of freedom • This framework decouples the WIMP miracle from WIMPs, gives a new class of candidates with WIMP pedigree, but with a range of masses/couplings WIMPLESS DARK MATTER mX gX Feng,Tu,Yu(2008)
  27. 27. 27 Oct 08 Feng 27 • This requires that an mX particle be stable. Can one be? STABILITY MSSM mX sparticles, W, Z, t q, l 0 p, e, g, n, G̃ Flavor-free MSSM O(1) Yukawas mX sparticles, W, Z, q, l, t̃ (or t) 0 g, g, n, G̃ • If the hidden sector is a flavor-free MSSM, a natural NLSP candidate, the stau (or tau), would be stabilized by charge conservation.
  28. 28. WIMPLESS DETECTION • WIMPless DM may have only gravitational effects • But connectors with both MSSM and hidden charges may mediate interactions with the SM • Related ideas Arkani-Hamed, Finkbeiner, Slatyer, Weiner (2008) Pospelov, Ritz (2008) 27 Oct 08 Feng 28 X X f f Y l l Feng,Kumar,Learned,Strigari(2008)
  29. 29. 27 Oct 08 Feng 29 CONCLUSIONS • WIMPs and related ideas have never been more motivated – WIMP miracle – Cosmological legacy of LEP à stable new particle • The WIMP miracle motivates three classes of candidates – WIMP dark matter – superWIMP dark matter – WIMPless dark matter • If anything discussed here is realized in nature, life will be very interesting in the coming years