Direct Detection of Dark Matter. Lauren Hsu Fermilab Center for Particle Astrophysics TRISEP Summer School, June 10, 2014

Transcription

1 Direct Detection of Dark Matter Lauren Hsu Fermilab Center for Particle Astrophysics TRISEP Summer School, June 10, 2014

2 Direct Detection of Dark Matter Lecture 1 How to detect dark matter Lecture 2 Review of current results Lecture 3 Future direct detection efforts

3 How to detect dark matter Outline I. Review of the dark matter problem II. WIMP scattering rates III. Experimental requirements for direct detection 3

4 The dark matter problem That isn t dark matter, sir you just forgot to take off the lens cap. 4

5 Zwicky s puzzle 1933: Fritz Zwicky analyzed velocity dispersion in Coma Cluster Coma Galaxy Cluster (SDSS) Individual galaxies move too fast for a bound system Posited existence of unseen matter in the cluster and named it dark matter 5

6 The modern view We have a consistent picture from multiple sources of astrophysical data We know a lot about dark matter stable, gravitationally interacting galactic rotation gravitational lensing nonbaryonic nonrelativistic BB nucleosynthesis CMB Spectrum Including that no standard model particle seems match! large scale structure formation 6

7 WIMP Detection 101 7

8 The Weakly Interacting Massive A sampling of available dark matter candidates Particle The WIMP Miracle Particles with mass and couplings at the weak scale yield cross sections that correspond to ~correct relic density of CDM early universe annihilation Freeze-out See lectures by Ann Nelson for alternatives to WIMPs 8

9 How to detect WIMPs See lectures by Adam for more on this technique χ χ and also for this FERMI-GLAST q q ~ q LHC Relic annihilation in the cosmos INDIRECT DETECTION χ χ man-made COLLIDER production Relic WIMPnucleon elastic scattering DIRECT DETECTION 9

10 Consider the relic WIMP distribution Observed energy spectrum & rate depend on WIMP distribution in dark matter halo Dark matter is distributed in a large extended, spherical halo around the Milky Way Schematic of DM Halo (1/2 cutaway) For comparison of direct detection experiments, assume an isothermal Maxwell-Boltzmann velocity distribution, with width = 220 km/s and v = esc 544 km/s Ve ~ 245 km/s - WIMP velocity relative to Earth Local density of WIMPs = 0.3 GeV/cm 3 If WIMPs are 100 GeV/c 2 particles, then ~10 million pass through your hand each second! 10

11 Scattering rate dissected Recoil energy of nucleus Reduced mass of WIMP- nucleon system 11

12 Scattering rate dissected σ 0 = σ SI + σ SD To first order, we will only consider the spin- independent and spin- dependent terms. There are other terms as well, but they tend to be suppressed see A. Ritz lecture last week. 12

13 Scattering rate dissected σ SI 2 = 4m r! f p N p π " + f n N # n $ 2 For spin- independent sca?ering, and small momentum transfer, sca?ering terms add coherently 13

14 Scattering rate dissected σ SI 2 = 4m r! f p N p π " + f n N # n $ 2 For spin- independent sca?ering, and small momentum transfer, sca?ering terms add coherently σ SI 2 = 4m r π f 2 A 2 In vanilla WIMP- nucleon coupling; assume same coupling for proton and neutron. Simplified expression depends only on atomic mass. Heavy nuclei have the advantage! 14

15 Scattering rate dissected σ SD = 32G F π 2 µ 2 ( J + 1) J! a p S p + a n S "# n 2 $ %& Spin- dependent sca?ering scales with the spin of the nucleus. Spins of individual nucleons can cancel NO COHERENT EFFECT! Many theories that predict SD sca?ering only are not as accessible to direct detecson experiments 15

16 Scattering rate dissected Choose your favorite nucleus to search for spin- dependent interacsons Tovey et al., PLB (2000) 16

17 Scattering rate dissected Roughly speaking, the form factor parameterizes coherence vs E r Coherence is lost (i.e. F(E R ) < 1) when the WIMP de Broglie wavelength is smaller than the nucleus. Since r nucleus ~ A 1/3 fm, choosing a large nucleus only helps you up to a a certain point! 17

18 Scattering rate dissected Integral over local WIMP velocity distribuson (Maxwell- boltzmann w/ assumed parameters on earlier slide) V min is the minimum WIMP velocity needed to produce recoil E r in your detector 18

19 Scattering rate for a heavy (100 GeV) WIMP Expected recoil spectrum is roughly exponential with << 1 event/kg/day expected, A 2 enhancement helps a lot with heavy WIMPs 100 GeV/c 2 WIMP-induced recoil spectrum Example experimental threshold Si (A 28) Ge (A 72) Xe (A 131) V esc = 544 km/s σ = cm 2 Implications for the experimentalist: In this example, one needs a detector that is O(100) kg running O(1) year to see one event (!) Rates are higher the lower your threshold, aim for ~5-10 kev At these rates and energies, you will be fighting a huge background from natural radioactivity, O(10 6 ) or more DEFINITION: differential rate unit (dru) = 1 event/kg/day/kev 19

20 Scattering rate for a light (10 GeV) WIMP Recoil spectrum drops off much more steeply with energy because kinematics matter much more for light WIMPs! 10 GeV/c 2 WIMP-induced recoil spectrum Example experimental threshold Si (A 28) Ge (A 72) Xe (A 131) V esc = 544 km/s σ = cm 2 A WIMP must have a minimum velocity to produce a recoil of a specific energy v min = E R m N 2m r 2 v DM è Experiments with lighter targets and lower thresholds have the advantage when looking for WIMPs with mass < 10 GeV/c 2 20

21 Designing an ideal WIMP detector 21

22 Typical backgrounds Most backgrounds are from trace radioactivity (U, Th, K contamination) or induced by cosmic rays (cosmogenic background) ELECTRON RECOILS (ER) Gamma: Most prevalent background Beta: on the surface or in the bulk Photon and electrons scatter from the atomic electrons WIMPs and neutrons scatter from the atomic nucleus NUCLEAR RECOILS (NR) Neutron: NOT distinguishable WIMP Alphas: almost always a surface event Recoiling parent nucleus: yet another surface event 22

23 Managing backgrounds (in 5-steps) 1) Choose highly radiopure materials for your detector and experimental setup. Build it in a state-of-the art clean lab (class ~1000 or better is often used). 23

24 1a) Screening and material assay Materials used for dark matter (and some neutrino) experiments must be thoroughly screened for radioactivity before use. Beta screener prototype MWPC (Beta Cage) HP Ge detector at SNOLAB XIA large-area Alpha detector ICP Mass Spectrometer at LNGS In many cases one is looking for isotope contamination at the level of parts per billion (ppb). The demands on radiopurity are so high that one needs a detector that is almost as well shielded and low in background as the dark matter detector itself! 24

25 1b) If you can t find it build it If the materials you come across aren t clean enough then build, extract or purify it yourself Distillation tower (at Fermilab) for extracting Ar depleted in 39Ar from natural gas wells Kr and Rn purification schematic for Xenon 1T Copper electroforming setup at PNNL 25

26 Managing backgrounds (in 5-steps) 1) Choose highly radiopure materials for your detector and experimental setup. Build it in a state-of-the art clean lab (class ~1000 or better is often used). 2) Cosmic muons produce fast neutrons via spallation. These are difficult to shield against and are a source of irreducible background. Go deep underground where the fast neutron flux is reduced. 26

27 2) Where to locate your experiment Homestake JinPing (CJPL) m.w.e. = meters water equivalent Most experiments use the earth as shielding from muons. The lower the muon rate, the lower the fast neutron rate. 27

28 Managing backgrounds (in 5-steps) 1) Choose highly radiopure materials for your detector and experimental setup. Build it in a state-of-the art clean lab (class ~1000 or better is often used). 2) Cosmic muons produce fast neutrons via spallation. These are difficult to shield against and are a source of irreducible background. Go deep underground where the fast neutron flux is reduced. 3) Unless you bury your detector 2 km deep in pristine glacial ice, you will have significant background from radioactivity. Surround your radiopure experiment with several tons of radiopure shielding 28

29 3a) Passive Shielding Trace U/Th/K and other isotopes in cavern walls and surroundings produce a constant flux of gammas and neutrons (via spontaneous fission or α,n) Lead shields against gammas; ~22 cm drops the gamma rate by ~106 Ancient lead or copper shields against 210Pb, and its daughters, found in standard lead Polyethylene or water moderates radiogenic and cosmogenic neutrons so that they produce recoils below the experimental threshold; 0.5 m of poly reduces the neutron scattering rate by ~104 SuperCDMS passive shielding 29

30 3b) Active Shielding Muon Veto: water cherenkov or scintillator; rejects muons passing through or near experiment (and the fast neutrons that come with them) Darkside Dar Neutron Veto Neutron Veto: liquid scintillator doped with isotope w/ high neutron capture cross-section; tags radiogenic neutrons that originate on contaminated material close to or within the experiment. SuperCDMS neutron veto schematic n LUX Muon Veto 30

31 Managing backgrounds (in 5-steps) 1) Choose highly radiopure materials for your detector and experimental setup. Build it in a state-of-the art clean lab (class ~1000 or better is often used). 2) Cosmic muons produce fast neutrons via spallation. These are difficult to shield against and are a source of irreducible background. Go deep underground where the fast neutron flux is reduced. 3) Unless you bury your detector 2 km deep in pristine glacial ice, you will have significant background from radioactivity. Surround your radiopure experiment with several tons of radiopure shielding 4) You will likely still have O(10 6 ) more ER than expected WIMP scatters in your detector, so make sure your experiment has some ability to distinguish ER from NR - at the level of one part in 10 6 or 10 7 is nice. 31

32 4) WIMP detection techniques Background discrimination Bubble chambers, superheated droplets TeO 2 bolometers HEAT HP Ge Point-contact Ge IONIZATION Dual phase liquid noble SCINTILLATION NaI cystals Single phase liquid noble 32

33 Textbook example w/ CDMS Some discrimination parameter (ionization yield in this case) analysis threshold 133 Ba 252 Cf 133 Ba (γ s) 132 Cf (n s) REJECTED ELECTRON RECOILS SIGNAL REGION (NUCLEAR RECOILS) Eionization ionization yield Ephonon = E ionization E phonon 1:10 4 rejection of gammas based on ionization yield alone Experiments that sit on the legs of the triangle exploit the fact that ER s and NR s deposit different fractions of the recoil energy in the form of HEAT, IONIZATION and SCINTILLATION. 33

34 Surface Events 1:10 4 sounds great, BUT wait! What are these events? Some discrimination parameter (ionization yield in this case) analysis threshold 133 Ba 252 Cf 133 Ba (γ s) 132 Cf (n s) REJECTED ELECTRON RECOILS SIGNAL REGION (NUCLEAR RECOILS) SURFACE EVENTS (betas, alphas, recoiling parent nuclei and x-rays) are a near-universal problem in direct detection. FIDUCIALIZATION of the target volume is necessary to reject these events. So ideally, your detector needs to be able to determine the position of an event as well as its energy. 34

35 Managing backgrounds (in 5-steps) 1) Choose highly radiopure materials for your detector and experimental setup. Build it in a state-of-the art clean lab (class ~1000 or better is often used). 2) Cosmic muons produce fast neutrons via spallation. These are difficult to shield against and are a source of irreducible background. Go deep underground where the fast neutron flux is reduced. 3) Unless you bury your detector 2 km deep in pristine glacial ice, you will have significant background from radioactivity. Surround your radiopure experiment with several tons of radiopure shielding 4) You will likely still have O(10 6 ) more ER than expected WIMP scatters in your detector, so make sure your experiment has some ability to distinguish ER from NR - at the level of one part in 10 6 or 10 7 is nice. 5) A team of talented students and postdocs who fine-tune rejection of background and maximize signal acceptance will extract the most out of the data. 35

36 Annual modulation and directional detection (yet more ways to distinguish WIMPs from backgrounds) 36

37 Annual modulation effect Earth s motion about the Sun produces small changes in velocity relative to the dark halo è Modulates expected rate of dark matter interactions detected on Earth If you see a signal, check for an annual modulation OR If you have irreducible backgrounds, use the modulation to pick out a signal A dark-matter-induced modulation will have extrema in June and December (whether it s max or min depends on target and threshold) 37

38 Annual modulation effect For example, you might see a signal like this DAMA/NaI and successor DAMA/LIBRA operate large arrays of NaI detectors. Their combined data yield a 9σ modulation consistent with dark matter. It has never been verified by another experiment, yet no one has a really good alternative explanation. 38

39 Directional Detection Similarly, due to the Sun s motion, the WIMP wind comes roughly from the direction of Cygnus. Low pressure TPC s preserve de/dx profile such that head to tail measurement can be made Detecting the direction of the nuclear recoil would help distinguish WIMPs (which should be coming from the direction of Cygnus) from backgrounds (which should be more isotropic). 39

40 End of Lecture 1 40

41 Homework 1 1) In spin-independent scattering, recall that coherence is lost when the momentum transferred to the nucleus (q) becomes large, thus corresponding to distance scales that are small with respect to the size of the nucleus. In other words when the debroglie wavelength is small with respect to the nucleus, one begins to probe the nuclear structure: # # λ = h/q < A 1/3 fm# # Use this relation to estimate the approximate recoil energy at which coherence is lost for a xenon nucleus. Use A = 131, which implies m xe 131 GeV/c 2 and hc = 197 MeV fm.# 2a) Using the expression for the minimum velocity needed to produce a recoil of energy E R, v min = E R m N 2m r 2 estimate the maximum recoil energy that can be produced by a 10 GeV/c 2 WIMP scattering in a xenon detector. In the expression above, m r is the reduced mass of the WIMP-nucleus system and m N is the mass of the nucleus. Use 544 km/s as the maximum WIMP velocity in the halo (note we are neglecting the motion of the sun and earth to simplify the problem).# Next do the same for silicon (A = 28) and compare the results and discuss the implications for the experimental thresholds.# # 41 #

42 Homework 1 cont d 2b) Give a qualitative description for how the differential 10 GeV/c 2 WIMP scattering rate on xenon might change once the Sun and Earth velocity are included. Discuss the implication for the experimental threshold. 2c) (Bonus) Derive the expression for v min assuming simple 2-body elastic scattering of the WIMP-nucleus system, with the nucleus initially at rest and the WIMP imparting the maximum recoil energy during the scatter (e.g. recoiling in the direction opposite of its incoming velocity).# 42