Direct WIMP searches with the LUX-ZEPLIN experiment IBLES OLCINA YTF - DURHAM 12/01/2017
Outline 1. The dark matter puzzle 2. Direct detection of WIMPs 3. The LZ experiment o Sensitivity to WIMP interactions o WIMP parameter reconstruction 4. Summary Ibles Olcina WIMP SEARCHES WITH LZ 1 1
The dark matter puzzle Gravitational mass is missing at all scales (and all times...) Astrophysical e evidence Cosmological e evidence v Rotation c curves of spiral galaxies v We can estimate the value of some cosmological v Virial t theorem applied to parameters from the gravitational bound systems: temperature anisotropies in galaxies in some clusters move the CMB r radiation far too fast to be held by the amount of luminous matter (e.g. the Coma Cluster) v Large S Scale S Simulations (LLS) of the evolution of the (L v X-ray e emission and gr gravitational universe favour the Λ -CDM lensing techniques: mismatch le between the position of intergalactic gas and the regions with the highest gravitational fields (e.g. Bullet Cluster) Ibles Olcina WIMP SEARCHES WITH LZ 2 2
Checklist for a good dark matter candidate q No E EM o or s strong i interaction : otherwise we would have “seen” it or “found” it in atoms q No Non-ba baryonic : no more room for baryons (BBN, LLS) q St Stable : its lifetime should be comparable to the age of the Universe q Cold r relic (non-relativistic at freeze out): hot matter is ruled out from N-body simulations Favoured candidates v WIM IMPs : mass in the GeV-TeV range. If weakly interacting, they would be thermally produced in the early Universe with the correct relic density ( WIMP miracle ) Axions : very light particles ( 𝑛 # < 0.01 eV). They could be v Ax detected through their coupling to photons v Sterile n neutrinos : RH neutrinos that only interact gravitationally. Their mass is constraint to be less than 10 keV Ibles Olcina WIMP SEARCHES WITH LZ 3 3
Direct detection of WIMPs Strategy The Milky Way is embedded in a halo of dark matter → A continuous WIMP flux should be crossing the Earth as the Solar system moves around the halo → Hence, look for nuclear recoils from elastic scatterings of WIMPs from atomic nuclei using terrestrial detectors → Expected signal rate is of the order of < 1 event/kg/year Background sources must be understood in exquisite detail to be successful in the search: background : elastic • Nuclear R Recoil ( (NR) b neutron scatters, 𝜉 -N coherent scattering, daughter nuclei from radioactive decay ~ tens to hundreds counts/kg/day background : gamma • Electron R Recoil ( (ER) b rays, beta and conversion electrons, 𝜉 -e scattering Ibles Olcina WIMP SEARCHES WITH LZ 4 4
Differential WIMP recoil rate B 𝑒𝑆 𝜍 0 𝜏 𝑔 ⊕ (𝑤) 2 9 𝐺 9 (𝐹 - ) 𝑒 A 𝑤 = = 𝑒𝐹 - 𝑤 2𝑛 4567 𝜈 2 C DEF (G H ) Ibles Olcina WIMP SEARCHES WITH LZ 5 5
Differential WIMP recoil rate Nuclear to nucleon interaction: B 𝑒𝑆 𝜍 0 𝜏 𝑔 ⊕ (𝑤) 𝐵 9 𝜏 I J5 Astrophysics SI (scalar) interaction 2 9 𝐺 9 (𝐹 - ) 𝑒 A 𝑤 = = 𝜏 2 ∝ O 𝐾 + 1 Particle physics 𝑒𝐹 - 𝑤 2𝑛 4567 𝜈 2 JV 𝜏 I,U SD (axial-vector) interaction C DEF (G H ) 𝐾 J5 = 10 KLM cm 2 : For a SI interaction and 𝑛 4567 = 100 GeV, 𝜏 I The “spherical cow” galactic model v Stationary and isothermal DM halo with 𝜍 0 = 0.3 GeV/cm 3 v Maxwell-Boltzmann WIMP velocity distribution Ibles Olcina WIMP SEARCHES WITH LZ 6 6
Differential WIMP recoil rate Nuclear to nucleon interaction: B 𝑒𝑆 𝜍 0 𝜏 𝑔 ⊕ (𝑤) 𝐵 9 𝜏 I J5 Astrophysics SI (scalar) interaction 2 9 𝐺 9 (𝐹 - ) 𝑒 A 𝑤 = = 𝜏 2 ∝ O 𝐾 + 1 Particle physics 𝑒𝐹 - 𝑤 2𝑛 4567 𝜈 2 JV 𝜏 I,U SD (axial-vector) interaction C DEF (G H ) 𝐾 J5 = 10 KLM cm 2 : For a SI interaction and 𝑛 4567 = 100 GeV, 𝜏 I The “spherical cow” galactic model 𝐹 XYZ[ v Stationary and isothermal DM halo with 𝜍 0 = 0.3 GeV/cm 3 v Maxwell-Boltzmann WIMP 𝐹 \#] velocity distribution Ea Easy! Build the detector, measure a WIMP energy spectrum and infer mass and cross section from it Ibles Olcina WIMP SEARCHES WITH LZ 7 7
No so easy... Ideally, any direct detection experiment would like to: 1. Discriminate between electron and nuclear recoils 2. Reconstruct the energy of each interaction accurately Ibles Olcina WIMP SEARCHES WITH LZ 8 8
The LZ experiment Two-phase Xenon detector Any particle interacting with the detector will produce UV scintillation photons (S1 signal) and ionization electrons (S2 signal) • Position reconstruction: S2 signal in the top PMT array determines (x,y) and z-position is calculated from time difference between S1 and S2 Energy reconstruction: 𝐹 UZ = 𝑋 𝑜 ` + 𝑜 a /𝑀 • 𝑋 : work function (average energy/quantum) 𝑜 ` , 𝑜 a : calculated from pulse areas of S1 and S2 signals 𝑀 : Linhard factor, to account for heat energy loss • Particle discrimination: ER and NR events are distributed along separate bands in the S2-S1 plane Ibles Olcina WIMP SEARCHES WITH LZ 9 9
The LZ detector The inner vessel is about 1.5m in diameter and • 2.6m in height and contains 7 tonnes of active liquid Xe It incorporates two monitored veto systems to • reject gammas and neutrons: § Xe skin surrounding the TPC § Liquid scintillator outer tank with enhanced neutron capture rate Located at the Sandford Underground Research • Facility (SURF) in Lead, South Dakota (US) Installation is expected to start in mid-2018 and • commissioning by beginning of 2019 Ibles Olcina WIMP SEARCHES WITH LZ 10 10
LZ sensitivity to WIMP SI interactions Exposure Running time: 1000 live days Target mass: 5.6 tonnes Best sensitivity Baseline: 2 .3e-48 cm 2 Goal: 1 .1e-48 cm 2 Ibles Olcina WIMP SEARCHES WITH LZ 11 11
LZ sensitivity to WIMP SI interactions We can proudly say that we are the best at not finding dark matter... Ibles Olcina WIMP SEARCHES WITH LZ 12 12
WIMP parameter reconstruction In the case of discovery, we would like to estimate the value of the most relevant WIMP parameters. For that, we need to construct a precise likelihood function: U ℒ 𝜾, 𝝃, 𝜈 = 𝜈 U 𝒚: data 𝑜! 𝑓 Ki j 𝑔(𝒚 𝒋 |𝜾, 𝝃) 𝜾: parameters of interest nop 𝝃: nuisance parameters 𝝃 = 𝝃 𝒕 ∪ 𝝃 𝒄 Poisson probability of measuring 𝜈 = 𝜈 [ + 𝜈 s n events if mean is 𝜈 i q (𝜾,𝝃 q ) i r 𝝃 r 𝑔 𝒚 𝜾, 𝝃 = 𝑔 [ 𝒚 𝜾, 𝝃 [ + 𝑔 s (𝒚|𝝃 s ) Model PDF : i i J5 } 𝒚 = {𝑇1, 𝑇2} , 𝜾 = {𝑛 4567 , 𝜏 I • 𝑔 [ 𝒚 𝜾, 𝝃 [ : signal PDF • 𝑔 s (𝒚|𝝃 s ) : background PDF, which is broken into the different • background components Ibles Olcina WIMP SEARCHES WITH LZ 13 13
Summary • There has been an impressive increase in sensitivity in direct dark matter experiments over the past two decades. How far we can push this limit? • The LZ experiment will probe WIMP interactions practically as far as it is allowed by new neutrino backgrounds and many theoretical models will be tested • In the case of discovery, most likely a combination of results from different experiments will be necessary to completely characterise the new particle Ibles Olcina WIMP SEARCHES WITH LZ 14 14
BACKUP Ibles Olcina WIMP SEARCHES WITH LZ 15 15
Dark matter searches Production Missing energy at accelerators • LHC, ... • Annihilation Into fermion pairs, photons, • neutrinos, ... FERMI, AMS, ... • Scattering Nuclear recoils at terrestrial • underground labs XENON, LUX, PICO, CDMS, ... • Ibles Olcina WIMP SEARCHES WITH LZ 16 16
Total background rate for the LZ exposure Ibles Olcina WIMP SEARCHES WITH LZ 17 17
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