Towards Direct Detection Signals · Towards Direct Detection Signals CDMS, DAMA, Iand Constraining...

Towards Direct Detection SignalsTowards Direct Detection SignalsCDMS, DAMA, andCDMS, DAMA, and

Constraining the WIMP mass and cross sectionConstraining the WIMP mass and cross section

Richard Schnee

Syracuse University

TASI

June 24, 2009

TASI 2009 DM Experiment Richard Schnee

Q inner

Q outer

A

B

D

C

Rbias

I bias

SQUID array Phonon D

Rfeedback

Vqbias

1 µ tungsten

380µ x 60µ aluminum fins

•250 g Ge or 100 g Si crystal

•1 cm thick x 7.5 cm diameter

•Photolithographic patterning

! Thousands of W transition-edgesensors

! Fractional area coverage

•Collect athermal phonons

Measure ionization in low-field

(~volts/cm) with segmented

contacts to allow rejection of

events near outer edge

Z-sensitive

Ionization and

Phonon-mediated

detector

Qouter

Qinner

@50 mK

CDMS II: Ionization & Athermal Phonons


CDMS ZIP Detector Phonon Sensors

Al

quasiparticle

trapAl Collector W

Transition-Edge

Sensor

Si or Ge

quasiparticle

diffusion

phonons

• Measurement of athermal

phonon signals allows position

imaging, additional

discrimination based on timing

superconducting

normal

W Transition-Edge Sensor:

sensitive thermometer

T (mK)Tc ~ 80mK

RT

ES

(!

)

4

3

2

1

~ 10mK

Richard Schnee TASI 2009 DM ExperimentPage 4

CDMS II Background RejectionCDMS II Background Rejection

• Effect is evenlarger forotherwisetroublesomesurfaceelectron-recoils

•Set all data-selection cuts“blind” to yieldabout 0.6expectedleakage eventsin WIMP-searchdata

photons from 133Ba source

(30x WIMP-search

background!)

Neutrons from 252Cf

source

133Ba surface events

•Nuclear recoils have lower ionization yield and slowerphonon signals than electron-recoil backgrounds


CDMS WIMP-search Data

Photons

Surface events

Nuclear recoil region

J. Filippini, UCB

PRL 102, 011301,

arXiv:0802.3530

•2006-2007data

•3.75 kg Ge

•400 kg daysexposure(before cuts)

•No WIMPcandidates

•3x data inhand, resultsby end ofsummer


• Research and development towards 300x mass scale-up (GeODM)

! Critical issue is manufacturability (R&D in progress)

! 20x Larger detectors exploiting (inexpensive) dislocation-free Ge crystals

! “Smarter” readout to simplify infrastructure, reduce heatload on cryogenics

! means in hand to stay

background-free at 15x

exposure from current

results

• Improved analysis

• 2.5x Thicker detectors

• Reduced exposure to radon

daughters

• New channel

layout

CDMS Future: SuperCDMS & GeODM

• CDMS background-free, best proven discrimination of backgrounds

• ~4x mass scale up underway

6”


~100 evt/ton/year

CDMSXENON10

CRESST

ZEPLIN II

EDELWEISS

ZEPLIN III

DAMA

Current WIMP Limits

90% CL upper limits on spin-independent interaction assuming

standard halo

• Excludes regions of

supersymmetry

parameter space

under some

frameworks

! Kim et al. 2004 in

light blue (MSSM)

• Starting to probe

Constrained MSSMTrotta et al. 2008 in

blue (68% CL) & green

(95% CL)


Neutrino Limiting Background

• Ton-scale dark matter experiments will detect 100s - 1000s of 8B

neutrinos from sun coherently scattering with nucleus below 5-10 keV

! Directional detectors can reject/subtract

• A 100-ton detector would have background from atmospheric neutrinos

! Limits ultimate detector size L. Strigari, 2009


Current WIMP Detection? DAMA/LIBRACurrent WIMP Detection? DAMA/LIBRA

• If WIMPs exist, expect annualmodulation (Drukier, Freese, &

Spergel 1986)

• DAMA/LIBRA do not distinguishbetween WIMP signal andbackground directly, but inferWIMPs from annual modulation

0

25

50

75

100

125

-0.5 -0.1 0.3 0.7 1.1 1.5

Background

WIMP Signal

95

97

99

101

103

105

-0.5 -0.1 0.3 0.7 1.1 1.5

±2%

JuneJune

Dec Dec

JuneJune

Dec DecDec Dec

in lowest-energy single-scatter interactions, assumingbackgrounds don’t modulate:

December

June

log


NaI

NaI

NaI

NaI

PM

T

PM

T

DAMA ApparatusDAMA Apparatus

• 250-kg NaI scintillator array• 25 crystals, each 9.7 kg, 10 cm x 10 cm x 25.4 cm

• Each crystal viewed by 2 PMTs through suprasil-B

lightguides, 5.5-7.5 photoelectrons/keVee

• 0.82 ton-years of exposure!

• Located at Gran Sasso

Underground Lab (4000 mwe)

+ Photon and Neutron shielding

• Detectors separated by lots of Cu

• No surrounding scintillator veto

• Extreme efforts taken to avoid

contamination• Etching followed by HP N2

atmosphere

• Installation in N2 atmosphere usingbreathing apparatus

Ultraclean copperAncient leadpolyethylene


DAMA Annual Rate Variation

modulation amplitude is only significant at low energy

arXiv:0804.2738

arXiv:0804.2741

Sum of residuals after subtracting time-averaged rates

in each energy bin in each detector

Consistent with a variety of models (fewer when DC rate spectrum also

considered)Richard Schnee TASI 2009 DM ExperimentPage 12

DC SpectrumDC Spectrum

• Average spectrum measures background level

• Impressive raw background rate: ~0.1/kg/keVr/day

• QF in NaI is 11, so 1 keVr ~ 0.09 keVee, keVee is what is

usually plotted

efficiency corrected

S. Golwala


DC SpectrumDC Spectrum

• Compare background spectra between DAMA/NaI and

DAMA/LIBRA

• Continuum background ~2X lower than DAMA/NaI" any explanation must not depend on absolute level of continuum

background

• Slight but statistically significant increase in bgnd ~2-2.5 keV

bgnd has gone up in 2-2.5 keV region!?

efficiency correctedupper: DAMA/NaIlower: DAMA/LIBRA

S. GolwalaRichard Schnee TASI 2009 DM ExperimentPage 14

MultiplesMultiples

• No modulation

seen in multiples

• Folded with

modulation

phase

• open circles:singles

• filled triangles:multiples

• Prime evidence that

signal is not a

simple modulation of

background

S. Golwala


Other Modulation Experiments

!"#$%#&&'()*$+,'-

KIMS (Yangyang Lab, Korea)Published SI Limit from 4 x 8.7 kg CsI(Tl) crystals (3409 kg-d)

12 detectors (104.4 kg) now installed

pulse shape discriminationliquid scint veto, 1 dru bkgd

ANAIS (Canfranc, Spain) Tests on one 10.7 kg of NaI (Tl)

viewed by 2 PMTs with acrylic light guides Bkgd: 232Th in Cu welds, 40K, ! internal contamination

Eventual array of 10 selected crystalsstored underground since 1988.

pulse shape discrimination, ancient lead shield


•Could some background be causing the annual modulation?Such a background would have to fulfill the annual modulationcharacteristics of a standard WIMP:

• Rate = cos(t)

• 1 year period

• Known phase

• Low energies only

• Single hits only

• Amplitude < 7% at maximum

• Consistent signal between NaI/LIBRA and differentdetectors

Possibilities Other Than DM?Possibilities Other Than DM?

Many differences between summer/winter

Most likely to be affected by systematic effect

Not very powerful test


Consistency of Consistency of DAMA/NaI DAMA/NaI & DAMA/LIBRA& DAMA/LIBRA

• Funny math in comparing 2-6 keV between expts:

• “difference is 0.008±0.004, only 2#”; actually, it is 0.0093±0.0037,

2.5#, 1.2% chance

• Differential rates: 5-6 keV is 0.0169±0.0056 in DAMA/NaI, 0.0010±0.0031 in DAMA/LIBRA, also 2.5# different


Potassium 3.2 keV PeakPotassium 3.2 keV Peak

• 40K chainelectron capture to

excited state of 40Ar

10.66%

excited state of 40Ar

decays by 1460.85 keV

gamma emission

daughter Ar

atom is missing

K-shell electron now,

so 3.2 keV emerges

as Auger and x-rays when

an atomic electron is capture

and the shells rearrange

S. Golwala


The Potassium 3.2 keV PeakThe Potassium 3.2 keV Peak

• Event topology

• 3.2 keV appears in same detector as parent/daughter nucleus

• 1461 keV can deposit all or some of its energy in parent detector, all or some in

other detectors, all or some in copper

• Many kinds of events will be seen. Two interesting examples:

• 1461 keV deposits full energy in another detector. Identified as double-scatter

event with 1461 keV in one detector, 3.2 keV in another. Used to measure

natural K contamination = 13 ppb.

• 1461 keV deposits all energy in Cu. Event appears as 3.2 keV single-scatter.

Inferred contribution to single-scatter DC rate is < 0.1 /( kg keV day).

• Unless rate is grossly wrong,

implies need ~50% percent

modulation in detection or in K in

detector (due to change in

humidity?) - surprisingly high.

• Would like to see check of 1461-

3 keV doubles for modulation


Muon-Induced Muon-Induced 3 keV Peak?3 keV Peak?

• Modulation of muon rate

• DAMA claim that fast neutrons cannot be produced at largeenough flux for this modulation to appear as a ratemodulation in DAMA

• What if muon spallation in NaI detectors creates metastable

isotope that decays with emission of 3 keV? (Spencer Klein)

• Lifetime must be > 500 !s trigger holdoff time

• Beam test of NaI could see such a line

• MACRO measured

annual modulation

of muon rate,

phase is correct to

explain DAMA

signal


Possible Modulation of Noise Cut EffectivenessPossible Modulation of Noise Cut Effectiveness

• Cut on sharpness of pulse used to remove “PMT noise” events.

• How do they know what the tail of the PMT noise distribution looks like,

and that this tail does not modulate?

• Dark current in PMTs exponentially temperature dependent; not clear

that temperature stability measurements are good enough.

• Total number of noise events

doesn’t modulate

• Efficiency of cut for particle

interactions does not

modulate

• Poorly motivated but cannot

be ruled out


CDMSXENON10

WARP

ZEPLIN II

EDELWEISS

Not aNot a Standard WIMPStandard WIMPSpin-independent interactions

• “Standard”(theoreticallyfavored, >40GeV, spin-independentinteraction on I )excluded


CDMSXENON10

Not aNot a Standard WIMPStandard WIMPSpin-independent interactions

• “Standard”(theoreticallyfavored, >40GeV, spin-independentinteraction on I )excluded

• Low-massWIMP with SIinteraction onNa also barelyexcluded

CDMS Si

CoG

EN

TRichard Schnee TASI 2009 DM ExperimentPage 24

CRESST I

NAIAD

Super-K

PICASSO

COUPP

XENON10

Not aNot a Standard WIMPStandard WIMPSpin-dependent on proton

• To be WIMP, need atleast 2 of

• Low-mass WIMPscattering on Nanot I

• Spin-dependentinteraction (mostlyon proton)

• Non-standardWIMP halo

• Or

• Ion Channeling

No experimentalevidence

• Non-standardinteraction (~10suggestions onarXiv)

What could we learn fromWhat could we learn from directdirect

detection of detection of WIMPsWIMPs??

Let’s start with the WIMP mass.


Direct Detection Mass Determination BasicsDirect Detection Mass Determination Basics

•Measure number of WIMP recoils and their recoil energies• Assume for what follows that detector cannot determine

directionality (so no diurnal modulation), not enoughstatistics/stability to search for annual modulation

•Energy spectrum results from assumed WIMP velocitydistribution + elastic scattering kinematics

• Standard assumption is Maxwellian (isothermal halo)

• See Lewin & Smith, Astropart. Phys. 6, 87 (1996) for review ofdetails.

• Recoil energy spectrum is roughly exponential e-aE with coefficient

So a $ M-2 for M << MT

a = constant for M >> MT

!

a"

1+M

T

M

#

$ %

&

' (

2

v0

2

Mass of target nucleus

WIMP Mass

Characteristic galactic velocity


Dependence of Spectrum on MassDependence of Spectrum on Mass

Maximum energy due

to assumed galactic

escape velocity

Ge

!

a"

1+M

T

M

#

$ %

&

' (

2

v0

2

• Larger WIMP mass M

yields harder energy

spectrum (but small

effect for large

masses)


Dependence of Spectrum on Mass, Dependence of Spectrum on Mass, vv00

Ge 270 km/s

170 km/s

• Larger WIMP mass M

yields harder energy

spectrum (but small

effect for large

masses)

• Uncertainty on v0

translates into

uncertainty on M

• For small M,

!

a"

1+M

T

M

#

$ %

&

' (

2

v0

2

!

"M

M="v

0

v0


Dependence of Spectrum on Halo ModelDependence of Spectrum on Halo Model

•Dark mattervelocity distributionis not well known

•Different halomodels result indifferent energyspectra

•Double-edgesword

• At first, willincreaseuncertainty on M

• Ultimately, mayallow inferenceof which halomodel is right

Isothermal, triaxial, Evans

halo models with various

parameters

Ge M = 60 GeV/c2

Craig Copi (Case Western)Richard Schnee TASI 2009 DM ExperimentPage 30

Dependence of Spectrum on Interaction(s)Dependence of Spectrum on Interaction(s)

• Different nuclear formfactors

• spin-independent

• spin-dependent on p

• spin-dependent on n

• With a single targetisotope, distinguishonly by difference inenergy spectrum

• Unless assume SDinteractions areinsignificant

• Detections withseveral detectorisotopes would makeproblem much easier.

• Spectral shapes are very similar for low WIMP masses

• Differences for high WIMP masses may confuse mass determination forsmall statistics Jeff Filippini (UC Berkeley)


Best-case ScenarioBest-case Scenario

• Sweep all uncertainties and confusion under the rug• Assume SI interaction only, with well-known velocity distribution

• If dominated by experimental statistical uncertainties, how

well do we measure WIMP mass & WIMP-nucleon cross

section for given # events detected?


Mass Determination with Mass Determination with GeGe, , XeXe, , SiSi

David Jackson, Rick Gaitskell (Brown U.), RWS

•Same exposure for

each

• 10 keV threshold

for Ge, Si


for Xe

• Ge gives slightly

better precision

than Xe, followed

by Si

•50 GeV WIMP typically

measured to range 25-

160 GeV with 10 events

in Ge

•Get only lower limits

for more massive WIMP

No upper

limit





each


for Ge, Si


for Xe


better precision

than Xe, followed

by Si


measured to range 43-

70 GeV with 100 events

in Ge





each


for Ge, Si


for Xe


better precision

than Xe, followed

by Si


measured to ±10% with

1000 events in Ge


Mass and Cross Section DeterminationMass and Cross Section Determination

•Uncertainties on mass can translate into uncertainties onWIMP-nucleon cross section #

• All under the assumption we know the WIMP local mass density

60 GeV 120 GeV 250 GeV

10 events expected (30% Poisson uncertainty on #)

1#

90%

99%

10-6

10-7

10-8Cro

ss S

ectio

n (

pb)

David Jackson, Rick Gaitskell (Brown U.), RWSRichard Schnee TASI 2009 DM ExperimentPage 36


60 GeV 120 GeV 250 GeV


1#

90%

99%

10-6

10-7

10-8

Cro

ss S

ectio

n (

pb)


•Uncertainties on mass can translate into uncertainties onWIMP-nucleon cross section #

• All under the assumption we know the WIMP local mass density




60 GeV

120 GeV 250 GeV

1#

90%

99%

David Jackson, Rick Gaitskell (Brown U.), RWSRichard Schnee TASI 2009 DM ExperimentPage 38

Direct Detection and LHCDirect Detection and LHC

• For many models, LHCwill constrain WIMP massto 10%

• However, it is difficult tomeasure WIMPproperties well

• Given WIMP mass fromLHC, direct detection candetermine WIMP-nucleoncross section much betterthan LHC alone• Statistical uncertainties

simply Poisson based onnumber of WIMPs detected

SuperCDMS

15000 kg-d

(region of %

coanihilation)

• Presumes same particle for both, WIMP comprises entire halo

• If relax second assumption, direct detection provides lower limiton WIMP-nucleon cross section

Baltz, Battaglia, Peskin &

Wizansky hep-ph/0602187


ConclusionsConclusions

• Determination of WIMP mass is much better for low WIMP

masses

• Systematic uncertainties on WIMP mass due to halo are of

order ± 20% - ±50% (should be explored in more detail)

• Better measurements of WIMP mass from colliders may be

combined with information from direct detection to better

constrain WIMP-nucleon cross section (and hence particle-

physics parameters)

250 GeV/c269 GeV55 GeV1000

100 GeV/c2101 GeV45 GeV100

50 GeV/c2none30 GeV10

Upper

99% Limit

Lower

99% Limit

Events

Detected

No upper limit

for WIMPs

with M >

For 60 GeV WIMP

Towards Direct Detection Signals · Towards Direct Detection Signals CDMS, DAMA, Iand Constraining...

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