University of Trieste INFN section of Trieste. ALP signatures in low background photon measurements

Transcription

1 University of Trieste INFN section of Trieste ALP signatures in low background photon measurements Valentina Lozza March 5 th 2010

2 Summary Axion Like Particles: a brief introduction Experimental searches for Axion Like Particles Detectors CAST (Cern Axion Solar Telescope) experiment at CERN BaRBE setup Preliminary results at CAST G-APD cooled at liquid nitrogen temperature Conclusions Pag.1

3 Axion origin "...I called this particle the axion, after the laundry detergent, because that was a nice cutely name that sounded like a particle and because this particle solved a problem involving axial currents..." Frank Wilczek Existence predicted in 1977 by Peccei and Quinn as a solution for the strong CP problem related to QCD; Mechanism to acquire mass = interaction with gluons: Coupling with two photons = most important interaction from an experimental point of view; Cold dark matter candidate Pag.2

4 Coupling g aγγ Interaction of ALPs with electromagnetic wave: The probability that an initial axion state becomes a final photon state is Transferred momentum which in perfect vacuum condition Γ = 0 becomes: 2 ql 2 2 sin g aγγ ( BL) 2 P a γ = ql 2 Damping factor Coherence factor Chance for experiments using a high magnetic field to see ALPs Pag.3

5 ALP production: the Primakoff effect in magnetic field Production Reconversion Scalar/pseudoscalar particle reconverted photon Primary photon beam Scalar/pseudoscalar particle Virtual photon of the magnetic field Virtual photon of the magnetic field Pag.4

6 Experiments to search for Axion Like Particles The present experimental scenario for ALP searches can be divided in two classes, depending on the ALP source: Laboratory experiments where ALPs could be produced via the interaction of a laser beam with the virtual photons of an external magnetic field. - Shining light through a wall: ALPs, BMV, OSQAR, LIPS, GammeV - Polarization experiments: PVLAS, Q&A, BMV Astrophysical experiments where ALPs are produced by astrophysical or cosmological sources. - Helioscope experiments: CAST, Tokio Helioscope - Crystal searches: DAMA - Relic dark matter searches: ADMX -Haloscope - Limits based on helium-burning stars evolution and luminosity - Bounds based on the axion flux produced in supernovae core which should increase supernovae luminosity (conversion to γ-rays) Pag.5

7 Laboratory experiments earth based laboratory - photon beam - production magnet - P γ a B 2 Production Reconversion - wall = absorber completely blocking primary laser beam - reconversion magnet regenerated photons have the same energy as the produced particle, hence as the primary photons Laser Magnet Particle beam Photons are Blocked by mirror Particle beam Magnet Photons Production Granite absorber Regeneration Regeneration experiments Pag.6

8 1.17 ev or 2.34 ev linearly polarized photon beam Photon regeneration at PVLAS Magnet # 1 (B M = 5 T & L = 1 m) Particle beam Rotating table (f = Hz) granite optical bench Magnet # 2 (B = 2.2 T & L = 50 cm) Optical fiber Multimode Core = 600 μm L = 2 m Detector Only one detector is needed since on off measurements are obtained by rotating the magnet # 1 P of production (pseudoscalar) B 2 sen 2 (θ/2) max per θ = π, min per θ = 0 P of production (scalar) B 2 cos 2 (θ/2) max per θ = 0 min per θ = π Pag.14

9 Astrophysical experiments CAST astronomical observatory Production Weakly interacting particles are produced via interactions of solar photons with the solar magnetic field Reconversion - low energy tails of the spectrum - Magnet: B = 10 T & L = 10 m - mobile mount to perform solar tracking. - Eγ spectrum peaked at a few kev never investigated ALPs Magnet γ or X-ray Pag.7

10 CAST (Cern Axion Solar Telescope) experiment at CERN ALPs γ or X-ray Pag.8

11 CAST: set-up Present permanent position BaRBE set-up Detectors Pag.9 SunRise side Micromegas + Visible detector CCD + X ray telescope SunSet side Micromegas Micromegas

12 CAST: expected photon energy Peak energy = 3 kev Mean energy = 4.2 kev 57 Fe M1 nuclear transition Solar corona problem G.G. Raffelt: Stars as laboratories for fundamental physics. The Astrophysics of Neutrinos, Axions, and Other Weakly Interacting Particles, The University of Chigago Press, London (1996) chapter Pag.10

13 BaRBE setup at CAST Sunset Sunrise 9 mm ε = 50% Galilean Telescope 45 mm B = 10 T L = 10 m Particle beam from the Sun Magnet on mobile mount Optical Switch Detector 1 Multimode optical fiber Core = 200 μm L = 40 m 50% time of light 50% time of background Detector 2 Acquistion system Pag.11

14 Long term installation Semitransparent mirror to reflect visible photons and not to absorb X-rays Polypropylene (5 μm) + Aluminium (10nm) Detector placed far from the magnet using an optical fiber Top view SRMM line Mirror flange Galilean telescope Pag.12

15 Request -high QE in visible range - low background ( < 1 Hz) ( fiber coupled ) Detectors 1 PMT (THORN EMI 9893/350B) G-APD (id-quantique id100) - active area diameter 9 mm - maximum 350 nm ~ 25% -20 C - shielded from ambient light - active area diameter 20 μm - maximum 530 nm ~ 35% C - shielded from ambient light Pag.13

16 Detectors 2 Working voltage = V Pag.14

17 Detectors 3 Pag.15

18 Results at CAST: background measurements PMT DCR = 0.39 ± 0.04 Hz o Difference between light and dark counts (1 σ errorbars). o Different colors represent different type of measurements. o Light and dark counts corrected for afterpulsing effect (11%). November 2007 runs Pag.16

19 PMT Results at CAST: solar tracking No signal over background Pag.17 (1 σ error bars).

20 Coupling constant of axions with two photons Results obtained during the two campaigns at CAST give g aγγ < GeV -1. Need to improve this limit: Increase the integration time but g aγγ ( T 1 ) acq 1/8 Reduce the sensor background Possible choices TES: lowest background but small area, cooled to mk DEPFET: high energy (few kev) LN 2 cooled G-APD: commercially available easy to use Pag.18

21 G-APD cooled at liquid nitrogen: Cryostat To the pumping system Pirani pressure gauge Multimode optical fiber Electrical feedthrough Pag.19 Thermocouple sensor

22 G-APD cooled at liquid nitrogen: Cold finger Focussing system Optical fiber lens Thermistor Multimode optical fiber Thermo Electric Cooler Pag.20

23 Breakdown voltage vs Temperature Typical curve obtained at 0 C for the dark counts. The DCR is around 11 khz Breakdown voltage Peak voltage Breakdown and peak voltages as a function of the sensor temperature. Following this trend the sensor temperature, when the cold finger is filled with liquid nitrogen, is 130 K. Pag.21

24 Dark count rate vs Temperature 10 5 Dark count rates measured at different cooling temperatures. A decrease of a factor 10 5 is seen between the room temperature measurement and the one done at cryogenic temperature. Pag.22

25 Light count rate vs Temperature Dark count and light count rates as function of the sensor temperature. Upper plot: Light-dark count rate at a fixed blue LED emission rate of 1.4 khz Middle plot: Light and dark count rates Lower plot: Measured sensor temperatures Data are corrected for the afterpulsing effect which increases as temperature decreases The quantum efficiency of the sensor does not change appreciably lowering the temperature Pag.23

26 Conclusions o The measurement carried out with the G-APD cooled at cryogenic temperatures showed a decrease in the sensor background of a factor 10 5 without affecting the quantum efficiency. o The afterpulsing effect becomes important at low temperatures, however it was corrected via software increasing the sensor dead time. o The results obtained at CAST are well above the limit set on g aγγ by X-rays detectors. However the mechanism which will produce solar axions is not known. Low-energy tails can be enhanced by unknown processes (solar corona). Pag.24