Light WIMPs! Dan Hooper. Fermilab/University of Chicago. The Ohio State University. April 12, 2011

Transcription

1 Light WIMPs! Dan Hooper Fermilab/University of Chicago The Ohio State University April 12, 2011

2 Based on A Consistent Dark Matter Interpretation for CoGeNT and DAMA/LIBRA Dan Hooper, Juan Collar, Jeter Hall, and Dan McKinsey, PRD 82 (2010), arxiv: Dark Matter Annihilation in the Galactic Center as Seen by the Fermi Gamma Ray Space Telescope Dan Hooper and Lisa Goodenough Phys. Lett. B 697 (2011), arxiv: Particle Physics Implications for CoGeNT, DAMA, and Fermi Matthew Buckley, Dan Hooper, and Tim Tait, arxiv: Light Z Bosons at the Tevatron Matthew Buckley, Dan Hooper, Joachim Kopp and Ethan Neil, arxiv:

3 Why WIMPs? The thermal abundance of a WIMP T >> M X, WIMPs in thermal equilibrium T < M X, number density becomes Boltzmann suppressed

4 Why WIMPs? The thermal abundance of a WIMP T >> M X, WIMPs in thermal equilibrium T < M X, number density becomes Boltzmann suppressed Including the effects of expansion pulls the density away from its equilibrium value:

5 Why WIMPs? The thermal abundance of a WIMP Numerically, this yield a thermal relic abundance given by: For comparison,

6 Why WIMPs? The thermal abundance of a WIMP Numerically, this yield a thermal relic abundance given by: For comparison, A weak scale interaction yields approximately the observed dark matter abundance - Numerical coincidence? Or an indication that dark matter originates from electroweakscale physics?

7 In Defense of Light WIMPs This thermal abundance argument works (roughly) equally well for WIMPs with masses between ~1 GeV and several TeV Historically, we have focused on ~40 GeV to ~1 TeV WIMPs for a number of reasons (neutralino/chargino mass ratios in minimal supersymmetry models, for example) which do not apply more generally to other dark matter candidates Papers have been written, analyses have been carried out, and experiments have been designed (and funded) with this bias in mind I know of no compelling argument for why dark matter should not consist of ~1-10 GeV particles

8 Direct Detection A WIMP striking a nucleus will impact a recoil of energy: For m X >> M nucleus and v~300 km/s, this corresponds to E recoil ~1-100 kev

9 Direct Detection A WIMP striking a nucleus will impact a recoil of energy: For m X >> M nucleus and v~300 km/s, this corresponds to E recoil ~1-100 kev The rate of WIMP-nucleus scattering is given by: DM velocity distribution

10 Direct Detection It is often not appreciated how stringent direct detection constraints have become XENON

11 Direct Detection Over the past decade, direct detection experiments have improved in sensitivity at a rate of about an order of magnitude every few years Within the next ~5 years, most of the parameter space associated with SUSY and other weak-scale physics beyond the SM frameworks will fall within the reach of direct detection experiments Many of the most simple dark matter models are already ruled out

12 WIMP Annihilation and Elastic Scattering From the observed density of dark matter, we can estimate the couplings of the WIMP (in lieu of resonances, coannihilations, etc.) Annihilation rate to quarks can be used to estimate the elastic scattering cross section with nuclei WIMP q WIMP WIMP WIMP q q q

13 WIMP Annihilation and Elastic Scattering Case Example: Dirac Fermion or Scalar WIMP with vector-like interactions (such as a heavy 4th generation neutrino, or a sneutrino) ν Z q ν Z ν ν q q q

14 WIMP Annihilation and Elastic Scattering Case Example: Dirac Fermion or Scalar WIMP with vector-like interactions (such as a heavy 4th generation neutrino, or a sneutrino) ν ν Z q q This naïve estimate is strongly excluded (by a factor of ~10 3!) by the null results of CDMS, XENON, and other direct detection experiments! ν q Z ν q Ways for a WIMP to evade these constraints include: Resonances Coannihilations Interactions with suppressed elastic scattering rates Dominant annihilations to particles other than quarks Non-standard early universe Light WIMPs (<10 GeV)

15 Signals From Direct Detection DAMA/LIBRA Over the course of a year, the motion of the Earth around the Solar System is expected to induce a modulation in the dark matter scattering rate Drukier, Freese, Spergel, PRD (1986)

16 Signals From Direct Detection DAMA/LIBRA Over the course of a year, the motion of the Earth around the Solar System is expected to induce a modulation in the dark matter scattering rate The DAMA collaboration reports a modulation with a phase consistent with dark matter, and with high significance (8.9σ)

17 Signals From Direct Detection CoGeNT The CoGeNT collaboration recently announced their observation of an excess of low energy events Although it has less exposure than other direct detection experiments, CoGeNT is particularly well suited to look for low energy events (and low mass WIMPs) CoGeNT Collaboration, arxiv:

18 CoGeNT and DAMA Intriguingly, the CoGeNT and DAMA signals could both arise from the elastic scattering of the same ~6-8 GeV dark matter particle Hooper, J. Collar, J. Hall, D. McKinsey, C. Kelso, PRD

19 Consistency With CDMS The recent low threshold analysis by CDMS claims to be in tension with this interpretation The event spectrum reported by CDMS, however, is not very different from that observed by CoGeNT (J. Collar, ) My opinion: If all systematic uncertainties (associated with CoGeNT, CDMS, velocity distributions, etc.) were properly taken into account, a consistent picture could easily appear

20 Signals in Other Experiments? CRESST In recent talks by members of the CRESST collaboration, a 4.6σ excess over known backgrounds has been reported (paper expected soon) The excess events appear in the oxygen band, implying a low WIMP mass The best fit point was reported to be m=13 GeV, with σ=3x10-40 cm 2, although these values are likely to be surrounded by considerable error bars

21 Signals in Other Experiments? CRESST In recent talks by members of the CRESST collaboration, a 4.6σ excess over known backgrounds has been reported (paper expected soon) The excess events appear in the oxygen band, implying a low WIMP mass The best fit point was reported to be m=13 GeV, with σ=3x10-40 cm 2, although these values are likely to be surrounded by considerable error bars COUPP In the winter Aspen workshop, it was reported that COUPP has observed ~5-10 nuclear recoil candidate events in each of their low threshold runs (7 and 10 kev) More recently, they have been running with a 15 kev threshold, and have not observed any such rate (not neutrons) Event spectrum consistent with a ~5-10 GeV WIMP with ~10-40 cm 2 cross section

22 The Key Test: An Annual Modulation At CoGeNT Published CoGeNT excess consists of ~10 2 events, from winter season; insufficient to observe any annual variation in rate If CoGeNT and DAMA are observing elastically scattering dark mater, we predict a ~5-15% annual modulation at CoGeNT (10-30% higher rate in summer than in winter) 2-3σ detection of this effect should be possible with existing data (alternatively, the dark matter interpretation could be ruled out at 95 to 99% CL) Unlike DAMA, CoGeNT can measure both the signal rate and the modulation amplitude (a much stronger discriminant than the modulation amplitude alone) (1 year projection) Kelso, Hooper, arxiv: ; Hooper, Collar, Hall, McKinsey, PRD, arxiv:

23 The Indirect Detection of Dark Matter 1. WIMP Annihilation Typical final states include heavy fermions, gauge or Higgs bosons χ χ W + W-

24 The Indirect Detection of Dark Matter 1. WIMP Annihilation Typical final states include heavy fermions, gauge or Higgs bosons χ χ 2.Fragmentation/Decay Annihilation products decay and/or fragment into combinations of electrons, protons, deuterium, neutrinos and gamma-rays e + W + W- ν q q π 0 p γ γ

25 The Indirect Detection of Dark Matter 1. WIMP Annihilation Typical final states include heavy fermions, gauge or Higgs bosons χ χ 2.Fragmentation/Decay Annihilation products decay and/or fragment into combinations of electrons, protons, deuterium, neutrinos and gamma-rays 3.Synchrotron and Inverse Compton Relativistic electrons up-scatter starlight/cmb to MeV-GeV energies, and emit synchrotron photons via interactions with magnetic fields γ e + W + W- ν p q e + q γ π 0 γ

26 The Indirect Detection of Dark Matter Neutrinos from annihilations in the core of the Sun Gamma Rays from annihilations in the galactic halo, near the galactic center, in dwarf galaxies, etc. Positrons/Antiprotons from annihilations throughout the galactic halo Synchrotron and Inverse Compton from electron/positron interactions with the magnetic fields and radiation fields of the galaxy

27 An Essential Test: Searches For Gamma Rays From Dark Matter Annihilations With Fermi The Fermi Gamma Ray Space Telescope has been collecting data for more than two and a half years In August 2009, their first year data became publicly available Fermi s Large Area Telescope (LAT) possesses superior effective area (~ cm 2 ), angular resolution (sub-degree), and energy resolution (~10%) than its predecessor EGRET Unlike ground based gamma ray telescopes, Fermi observes the entire sky, and can study far lower energy emission (down to ~300 MeV)

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30 Where To Look For Dark Matter With Fermi?

31 Where To Look For Dark The Galactic Center -Brightest spot in the sky -Considerable astrophysical backgrounds Matter With Fermi?

32 Where To Look For Dark The Galactic Center -Brightest spot in the sky -Considerable astrophysical backgrounds Matter With Fermi? The Galactic Halo -High statistics -Requires detailed model of galactic backgrounds

33 Where To Look For Dark The Galactic Center -Brightest spot in the sky -Considerable astrophysical backgrounds Matter With Fermi? The Galactic Halo -High statistics -Requires detailed model of galactic backgrounds Individual Subhalos -Less signal -Low backgrounds

34 Where To Look For Dark The Galactic Center -Brightest spot in the sky -Considerable astrophysical backgrounds Matter With Fermi? The Galactic Halo -High statistics -Requires detailed model of galactic backgrounds Extragalactic Background -High statistics -potentially difficult to identify Individual Subhalos -Less signal -Low backgrounds

35 Dark Matter In The Galactic Center Region The region surrounding the Galactic Center is complex; backgrounds present are not necessarily well understood This does not, however, necessarily make searches for dark matter in this region intractable The signal from dark matter annihilation is large in most benchmark models (typically hundreds of events per year) To separate dark matter annihilation products from backgrounds, we must focus on the distinct observational features of these components

36 Dark Matter In The Galactic Center Region The characteristics of a signal from dark matter annihilations: 1) Signal highly concentrated around the Galactic Center (but not entirely point-like) 2) Distinctive bump-like spectral feature

37 Astrophysical Backgrounds In The Galactic Center Region Known backgrounds of gamma rays from Inner Galaxy include: 1) Pion decay gamma rays from cosmic ray proton interactions with gas (p+p p+p+π 0 ) 2) Inverse Compton scattering of cosmic ray electrons with radiation fields 3) Bremsstrahlung 4) Point sources (pulsars, supernova remnants, the supermassive black hole)

38 Astrophysical Backgrounds In The Galactic Center Region Much of the emission is concentrated along the disk, but a spherically symmetric component (associated with the Galactic Bulge) is also to be expected The Fermi First Source Catalog contains 69 point sources in the inner +/-15 of the Milky Way Build a background model with a morphology of disk+bulge+known point sources

39 Astrophysical Backgrounds In The Galactic Center Region Fit one energy bin at a time, and one angular range around the Galactic Center (no assumptions about spectral shape, or radial dependence) Fit to intensity of the disk (allow to vary along the disk), width of the disk (gaussian), intensity of the flat (spherically symmetric) component Include point sources, but do not float Provides a very good description of the overall features of the observed emission (between ~2-10 from the Galactic Center)

40 The Inner Two Degrees Around The Galactic Center If the Fermi data contains a signal from dark matter annihilations in the Galactic Center, we should expect to see departures from the background model within the inner ~1 degree The key will be to observe both the morphological and spectral transitions in the data

41 Dashed=disk Dotted=bulge Solid=disk+bulge Between 1 and 10 from the GC, our background model does very well

42 Dashed=disk Dotted=bulge Solid=disk+bulge Between 1 and 10 from the GC, our background model does very well Inside of ~0.5, backgrounds utterly fail to describe the data A new component is clearly present in this inner region, with a spectrum peaking at ~1-4 GeV

43 The Spectrum Of The Excess Emission We have been able to cleanly extract the spectrum of the central emission (not disk or bulge) Emission peaks around 1 to 4 GeV No statistically significant excess above ~6-7 GeV

44 The Dark Matter Interpretation The spectral shape of the excess can be well fit by a dark matter particle with a mass in the range of 7 to 10 GeV (similar to that required by CoGeNT and DAMA), annihilating primarily to τ + τ - (possibly among other leptons)

45 The Dark Matter Interpretation The spectral shape of the excess can be well fit by a dark matter particle with a mass in the range of 7 to 10 GeV (similar to that required by CoGeNT and DAMA), annihilating primarily to τ + τ - (possibly among other leptons) The angular distribution of the signal is well fit by a flux distribution that scales with r -α, with α=2.36 to 2.66; if interpreted as dark matter, this implied an inner profile ρ(r)~r -γ, with γ=1.18 to 1.33 (in good agreement with the best fit to the Via Lactea simulation, for example)

46 The Dark Matter Interpretation The spectral shape of the excess can be well fit by a dark matter particle with a mass in the range of 7 to 10 GeV (similar to that required by CoGeNT and DAMA), annihilating primarily to τ + τ - (possibly among other leptons) The angular distribution of the signal is well fit by a flux distribution that scales with r -α, with α=2.36 to 2.66; if interpreted as dark matter, this implied an inner profile ρ(r)~r -γ, with γ=1.18 to 1.33 (in good agreement with the best fit to the Via Lactea simulation, for example) The normalization of the signal requires the dark matter to have an annihilation cross section (to τ + τ - and hadronic channels) of σv = 4.6x10-27 to 5.3x10-26 cm 3 /s (in agreement with the value of 3x10-26 cm 3 /s predicted for a simple thermal relic)

47 Challenges: Other Interpretations? Very concentrated, but not point-like, emission (scales with roughly r -2.5 ) Hard and peaked spectral shape (dn/de ~ E -1 )

48 Other Interpretations? Unresolved Point Sources? Perhaps a population of ~50 or more unresolved points sources distributed throughout the inner tens of parsecs of the Milky Way could produce the observed signal - millisecond pulsars, for example

49 Other Interpretations? Unresolved Point Sources? Perhaps a population of ~50 or more unresolved points sources distributed throughout the inner tens of parsecs of the Milky Way could produce the observed signal - millisecond pulsars, for example Two problems: 1) Why so many in the inner 20 pc, and so few at 100 pc? -With typical pulsar kicks of km/s, millisecond pulsars should escape the inner region of the galaxy, and be distributed no more steeply than r -2 (assuming that none are created outside of the inner tens of parcecs)

50 Other Interpretations? Unresolved Point Sources? Perhaps a population of ~50 or more unresolved points sources distributed throughout the inner tens of parsecs of the Milky Way could produce the observed signal - millisecond pulsars, for example Two problems: 1) Why so many in the inner 20 pc, and so few at 100 pc? -With typical pulsar kicks of km/s, millisecond pulsars should escape the inner region of the galaxy, and be distributed no more steeply than r -2 (assuming that none are created outside of the inner tens of parsecs) 2) The spectral shape of gamma rays from pulsars has been measured (46 are in the FGST s catalog), and is not consistent with the spectrum observed from the Galactic Center (a different population or class of pulsars that those observed by Fermi?)

51 Other Interpretations? Confusion with the Galactic Center s Supermassive Black Hole? Above ~8 GeV, the observed spectrum agrees very well with an extrapolation of the power-law emission reported by HESS (at ~200 GeV to ~20 TeV) Much of the central emission observed by FGST could potentially originate from the SMBH, but this would require a dramatic departure from the power-law observed at higher energies The data also significantly prefers an extended source for the peaked emission (but this is difficult to state in a way that is independent of the details of the PSF) I eagerly await further study of this issue by the Fermi Collaboration

52 Other Signatures of Light WIMP Annihilation in the Inner Milky Way? If dark matter annihilations are generating the gamma ray emission observed from the Galactic Center, where else in the sky, and in what other channels, might signals of this appear?

53 Other Signatures of Light WIMP Annihilation in the Inner Milky Way? For years, it has been argued that the WMAP data contains an excess synchrotron emission from the inner ~20 around the Galactic Center, and that this cannot be explained by known astrophysical mechanisms Previous studies have shown that this emission could be accounted for electrons produced in dark matter annihilations WMAP Haze (22 GHz) Finkbeiner, astro-ph/ ; Hooper, Finkbeiner, Dobler, PRD (2007); Dobler, Finkbeiner, ApJ (2008)

54 Synchrotron Emission and The WMAP Haze Using the halo profile, mass, annihilation cross section and annihilation channels determined by the Fermi GC data, we proceed to calculate the corresponding synchrotron spectrum and distribution Set B-field model to obtain the spectrum and angular profile observed by WMAP (almost no additional freedom) The resulting synchrotron intensity is forced to be very close to that observed A dark matter interpretation of the Galactic Center gamma rays (almost) automatically generates the WMAP Haze Annihilations to e + e -, µ + µ -, τ + τ - B~10 µg in haze region D. Hooper and Tim Linden, arxiv:

55 Dwarf Spheroidal Galaxies The FGST collaboration has recently placed some relatively stringent limits on dark matter from observations of a number of satellite galaxies (dwarf spheroidals) of the Milky Way The most stringent limits come from those dwarfs which are 1) dense, 2) nearby, and 3) in low background regions of the sky

56 Dwarf Spheroidal Galaxies Light dark matter particles produce more annihilation power, and brighter indirect detection signals Current constraints from observations of dwarf spheroidal galaxies and isotropic diffuse emission are not very far from the signals predicted in light of our GC analysis Although limits have not been presented for masses as low as 7-8 GeV, or for annihilations to τ + τ -, predicted signal should look very much like that found in this region Fermi Collaboration arxiv: (First 11 months of data)

57 And Now Something (seemingly) Completely Different Last Wednesday, the CDF collaboration announced the observation of a 3.2σ excess of W+dijet events This implies the existence of a new particle(s) with a mass of GeV Two basic possible explanations exist: 1) A new particle with modest couplings to quarks (g~0.2) but much smaller couplings to leptons (g<0.04, to evade LEP constraints) 2) Two (or more) new particles lead to the events, the last of which is the ~ GeV state being observed (could appear within the context of technicolor, or R-parity SUSY models, for example)

58 And Now Something (seemingly) Completely Different Focusing on the first case (only one new particle), the simplest thing we can consider is a new gauge boson (Z or W )

59 And Now Something (seemingly) Completely Different Focusing on the first case (only one new particle), the easiest thing we can consider is a new gauge boson (Z or W ) But what does this have to do with dark matter?

60 Dark Forces at the Tevatron The new gauge boson must have couplings to quarks, but little or no coupling to leptons - a leptophobic Z To cancel the anomalies associated with this new gauge boson, we must to introduce a new particles, including heavy vector-like fermions (called exotics ) Within the leptophobic Z models in the existing literature, some of the new particles carry baryon number (but do not necessarily have QCD color); the lightest of these will be stable and will couple to the Z with approximately (within a factor of a few) the same strength as the Z couples to quarks

61 Dark Forces at the Tevatron If the dark matter (the lightest baryon number charged new state) is a scalar or a Dirac fermion, then its couplings to the Z lead to a sizable elastic scattering cross section with nuclei: X X After putting in the coupling and mass of the Z that is required to explain the CDF anomaly, this becomes: Z q q For a ~10 GeV-10 TeV WIMP, this is ruled out by direct detection For a ~5-10 GeV WIMP, this is approximately the same cross section that is implied by CoGeNT and DAMA

62 Dark Forces at the Tevatron If the dark matter (the lightest baryon number charged new state) is a scalar or a Dirac fermion, then its couplings to the Z lead to a sizable elastic scattering cross section with nuclei: X X After putting in the coupling and mass of the Z that is required to explain the CDF anomaly, this becomes: Z q q For a ~10 GeV-10 TeV WIMP, this is ruled out by direct detection For a ~5-10 GeV WIMP, this is approximately the same cross section that is implied by CoGeNT and DAMA CDF may be observing the force that mediates the interactions between the dark and visible sectors of our universe!

63 Looking Forward Although evidence for light WIMPs has appeared in a variety of experiments (CoGeNT, DAMA, Fermi, WMAP, CRESST), the case for this interpretation is not yet incontrovertible

64 Looking Forward Although evidence for light WIMPs has appeared in a variety of experiments (CoGeNT, DAMA, Fermi, WMAP, CRESST), the case for this interpretation is not yet incontrovertible In the near future (weeks? months?), the presence or absence of a statistically significant annual modulation from CoGeNT (in addition to a more detailed spectrum measurement) will have the most to add to this discussion - this will be the definitive test

65 Looking Forward Although evidence for light WIMPs has appeared in a variety of experiments (CoGeNT, DAMA, Fermi, WMAP, CRESST), the case for this interpretation is not yet incontrovertible In the near future (weeks? months?), the presence or absence of a statistically significant annual modulation from CoGeNT (in addition to a more detailed spectrum measurement) will have the most to add to this discussion - this will be the definitive test Other results to bear include: -Details of the CRESST excess (soon?) -Fermi collaboration study of the Galactic Center (summer?) -Analysis of full CDF+D0 data set (weeks?) -Planck s view of the WMAP haze (2012?) -Low threshold results from COUPP (summer?)

66 Summary Although many direct and indirect searches for dark matter are designed to detect ~50 GeV to ~1 TeV particles, WIMPs as light as ~1 GeV are well motivated and viable candidates for dark matter

67 Summary Although many direct and indirect searches for dark matter are designed to detect ~50 GeV to ~1 TeV particles, WIMPs as light as ~1 GeV are well motivated and viable candidates for dark matter From the first two years of publicly available FGST data, we have identified a component of gamma rays concentrated around the inner around the Galactic Center, with a spectrum sharply peaked at 2-4 GeV

68 Summary Although many direct and indirect searches for dark matter are designed to detect ~50 GeV to ~1 TeV particles, WIMPs as light as ~1 GeV are well motivated and viable candidates for dark matter From the first two years of publicly available FGST data, we have identified a component of gamma rays concentrated around the inner around the Galactic Center, with a spectrum sharply peaked at 2-4 GeV The spectrum and morphology of the observed emission can be easily accounted for with annihilating dark matter distributed with a cusped (and perhaps adiabatically contracted) profile (ρ α r -1.2 ), with a mass of 7-10 GeV, and an annihilation cross section of σv~4.6x10-27 cm 3 /s to 5.3x10-26 cm 3 /s, primarily to τ + τ - (possibly among other leptonic final states)

69 Summary Although many direct and indirect searches for dark matter are designed to detect ~50 GeV to ~1 TeV particles, WIMPs as light as ~1 GeV are well motivated and viable candidates for dark matter From the first two years of publicly available FGST data, we have identified a component of gamma rays concentrated around the inner around the Galactic Center, with a spectrum sharply peaked at 2-4 GeV The spectrum and morphology of the observed emission can be easily accounted for with annihilating dark matter distributed with a cusped (and perhaps adiabatically contracted) profile (ρ α r -1.2 ), with a mass of 7-10 GeV, and an annihilation cross section of σv~4.6x10-27 cm 3 /s to 5.3x10-26 cm 3 /s, primarily to τ + τ - (possibly among other leptonic final states) The required mass range is remarkably similar to that inferred from the signals reported by CoGeNT and DAMA/LIBRA

70 Summary If CoGeNT is observing the elastic scattering of dark matter, their existing data should contain an observable annual modulation, providing a definitive test of the dark matter interpretation

71 Summary If CoGeNT is observing the elastic scattering of dark matter, their existing data should contain an observable annual modulation, providing a definitive test of the dark matter interpretation The most simple interpretations of the recently announced CDF W+dijets anomaly is of a new leptophobic gauge boson; such a particle invariably requires other new particle content, including a possible dark matter candidate - the elastic scattering cross section this state is naturally predicted to be near the value reported by CoGeNT (~10-40 cm 2 )

72 Summary If CoGeNT is observing the elastic scattering of dark matter, their existing data should contain an observable annual modulation, providing a definitive test of the dark matter interpretation The most simple interpretations of the recently announced CDF W+dijets anomaly is of a new leptophobic gauge boson; such a particle invariably requires other new particle content, including a possible dark matter candidate - the elastic scattering cross section this state is naturally predicted to be near the value reported by CoGeNT (~10-40 cm 2 ) Although the possibility that astrophysical backgrounds are generating these various signals cannot be excluded at this time, new results from CoGeNT, CRESST, Fermi, Planck, COUPP, CDF/D0 and others will bring us much closer to resolving these questions in the near future

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74 What Are We Looking At Here? (comments on model building) Requirements Stable particle with a mass of ~7-8 GeV At non-relativistic velocities, annihilates primarily to τ + τ - (perhaps among other leptonic final states) Non-relativistic annihilation cross section (to τ + τ - ) of σv~3.3x10-27 cm 3 /s to 1.5x10-26 cm 3 /s (or 1-5 x cm 3 /s for annihilations to e + e -, µ + µ -, τ + τ - ) Elastic scattering cross section with nucleons of σ SI ~10-40 cm 2 (from CoGeNT+DAMA) Are these requirements difficult to accommodate?

75 What Has Been Discovered Here? (comments on model building) Using SUSY as a example In the MSSM, neutralinos can annihilate to fermions (including τ + τ - ) through sfermion, Z, or A exchange Z couplings are limited by LEP, and A leads to mostly bb final states σv χχ τ τ τ ~ 4x10-27 cm 3 /s x N 11 4 (85 GeV / m τ ~ )4 ~

76 What Has Been Discovered Here? (comments on model building) Using SUSY as a example In the MSSM, neutralinos can annihilate to fermions (including τ + τ - ) through sfermion, Z, or A exchange Z couplings are limited by LEP, and A leads to mostly bb final states σv χχ τ τ τ ~ 4x10-27 cm 3 /s x N 11 4 (85 GeV / m τ ~ )4 ~ Gamma Ray signal is easy to accommodate

77 What Has Been Discovered Here? (comments on model building) Using SUSY as a example The elastic scattering of neutralinos with nucleons can result from scalar higgs or squark exchange Amplitude for quark exchange is much too small, and in the MSSM, even higgs diagrams lead to values of σ SI that fall short by a factor of ~10 or more

78 What Has Been Discovered Here? (comments on model building) Using SUSY as a example The elastic scattering of neutralinos with nucleons can result from scalar higgs or squark exchange Amplitude for quark exchange is much too small, and in the MSSM, even higgs diagrams lead to values of σ SI that fall short by a factor of ~10 or more If we extend the MSSM by a chiral singlet, however, the lightest neutralino can scatter much more efficiently with nucleons Light singlet-like higgs Belikov, Gunion, Hooper, Tait, arxiv:

79 What Has Been Discovered Here? (comments on model building) Using SUSY as a example This model can also be used to predict the abundance of neutralino dark matter, resulting from thermal freeze-out in the early universe Stau exchange diagrams alone would lead to the overproduction of neutralino dark matter by a factor of ~10 (Ω χ h 2 ~1) The higgs exchange diagrams, however, are more efficient, and lead to Ω χ h 2 ~0.1 In this simple SUSY model, the cross section implied by CoGeNT and DAMA forces us to the prediction of Ω χ h 2 ~0.1 Belikov, Gunion, Hooper, Tait, arxiv:

80 What Has Been Discovered Here? (comments on model building) More generally speaking Relatively large couplings and/or light mediators are needed to provide the large cross section implied by CoGeNT and DAMA Preferential annihilation to τ + τ - requires either exchanged particles which share the quantum numbers of tau leptons (ie. staus) or that possess leptophillic couplings (to a Z for example) MSSM does not provide a dark matter candidate that can produce these signals, but (slightly) extended supersymmetric models can Simple models can accommodate these signals, but they are not the models most particle theorists have been studying Buckley, Hooper, Tait, arxiv: