The ortho-positronium lifetime puzzle & new Physics P.Crivelli ETH, Zürich, Switzerland Under the supervision of Prof.Andre Rubbia
Introduction The Positronium, the bound state of electron and positron, is a pure quantum electrodynamical system. ideal to test of QED calculations for bound states. 2 possible ground states The triplet spin state Ortho-Positronium (o-ps) The singlet spin state Para-Positronium 142ns 125ps Due to odd-parity under C-transformation, o-ps decays in three photons.
Decay rate The decay rate of o-ps in vacuum is given by: In matter there is the probability that the bounded positron annihilates in 2γ with an electron which is not is partner electron (so called pick-off annihilation or collisional quenching). In matter the decay rate of o-ps can increase!
Positronium formation The positron from a radioactive source interacting with an electron can form positronium. Three different techniques have been used: 1) Gas 2) A vacuum surface interface 3) Low density SiO 2 powder Once o-ps is formed it interacts with the surrounding environment which may increase the decay rate in case of pick-off annihilation or decrease it in case of electric fields. It is necessary to remove these effects to determine the decay rate in vacuum.
Principle of lifetime measurements Gas: measurements in different gas densities and then the lifetime is extrapolated to 0 (vacuum). Large systematic error due to unexpected low thermalisation rate of o-ps in gases and non linear dependence of pickoff rate with densities. Cavity: o-ps formed on the cavity surface, it migrates in vacuum, the pick-off is much less compared to gas and powder (just 4 collision/τ ops with the walls ). Powder: The background from the 2γ collisional quenching is measured with a germanium detector and then it is subtracted to obtain the lifetime in vacuum, (extrapol. 10 4 collision/τ ops 0 collisions still open question).
History of the o-ps lifetime measurements Recent Adkins et al. calculations up to α 2 (Phys.Lett.2000). Gas measurements Small error dominated by statistic 5σ discrepancy in vacuum experiment Bigger error dominated by the systematic 95 Powder (Corrected 2γ/3γ time dependence) Agreement for measurements in powder
Motivation of our experiment 1 The experimental situation is confusing and should be clarified. The long standing 5σ discrepancy between measured and predicted decay rate of ortho-positronium in vacuum led to an extensive search of exotic decays. In fact the discrepancy might be explained by an exotic decay of o-ps if the BR 10-3. Exotic decay of o-ps in γx,γγx, invisible (but not in vacuum), 2γ, 4γ has been searched and definitely they are not the source of this discrepancy. 1 ETHZ, INR Moscow, CNRS-IN2P3 France
Test for old idea of mirror world Left-right world asymmetry is still a great puzzle, why the weak interaction has left handed chirality? Already in 1956 Lee & Yang in their famous nobel-prize-won paper on parity violation, pointed out that the existence of a shadow/mirror world will restore the symmetry. Glashow: o-ps puzzle could be explained by o-ps o-ps M oscillation which results in o-ps invisible decay in vacuum! ( Matter destroy coherence)
Other strong motivations to search for o-ps invisible (not in vacuum) Extra-dimensions model of Randall-Sundrum type predicts BR(o-Ps invisible) 10-9. (see Mod.Phys.Lett.A,Vol.17, No.26, (2002) pp.1713-1724) Do fractional charged particles exist? Particles with eq 10-3 naturally appears in GUT theories, o-ps could decay apparently invisibly since such particles would mostly penetrate any type of calorimeter without interaction (recent SLAC search could be improved). New light vector X-boson which e.g. may contribute to (g-2) µ (recent (g-2) µ of BNL).
Our Plans We started our activity in January and we foresee mainly 2 research lines Search for new physics (Source Exp.) 1)Search for three body decay of o-ps, which could solve the discrepancy.(results published Phys.Lett. B542: 29-34, 2002) 2) Search for invisible decay (not in vacuum) of o-ps with sensitivity of BR < 10-8 -10-9. 3) CP,CPT test in leptonic syst. o-ps lifetime puzzle (Beam Exp.) 1) Development of a slow positron beam for a precise measurement of the o-ps lifetime in vacuum with systematic differences from Michigan experiment and 2)Search for invisible decay of o-ps in vacuum.
Search for the exotic decay of ortho- Positronium (o-ps) + e e (o Ps) γ + X 1 + X 2 Two weak interacting massive particles The signature of such an event will be a single photon detected in a hermetic calorimeter accompanied by no other energy deposition. In this exotic decay the photon has a continuum of energy, thus this search is more difficult than the previous ones which were based on the peak detection arising from the 2 body decay. The second goal was to optimize our calorimeter for the next steps of the experiment, performing a MC simulation based on the acquired data to use
Photograph of the calorimeter
Front and top view of the calorimeter Region of positronium formation
Description of the source + 3 EC(9.4%) This gamma is emitted 3 ps after the positron. + 2 22 Na Tagged through scintillating fiber. 90.6% e + (0.543MeV) γ(1.27mev) 0.06% e + (1.830MeV) + 0 22 Ne
Positrons tagging The positrons are tagged when the signals from the 2PMT s are in coincidence, then the gate opens. 22 Na source 3.6kBq
Target o SiO 2 grains ( 50-100 A ) After the fiber the positrons enter the Aerogel Positronium can form inside the grain It can migrates in the inter granular space where it decay almost freely e + e (sin glet) γγ τ 125ps (in vacuum) or e + e (triplet) γγγ τ 142ns (in vacuum) Collisional quenching In Aerogel pores filled with nitrogen: τ 132ns
Time spectra between tagged positron and photon detected in the calorimeter (with aerogel) Peak from 2γ annihilation Constant background Exponential decay from 3γ
Calorimeter 5,2 cm thickness 20 cm length 24 x BGO kindly lend us From Prof. Fetcher (PSI) The resolution of the crystals determined with a fit is about 16% at 1.27Mev (FWHM) Energy spectra
Example of signal signature γ from the o-ps decay 1.27MeV γ X 1 X 2
Events selection One of the photons with energy between 40 and 700 kev from the decay is asked to be in the trigger BGO. For the same event the 1.27MeV (not more then one) should be present in one of the other crystals. events/20 kev 500 400 300 200 100 Selection of the 1.27 MeV 0 800 1000 1200 1400 1600 1.27 γ energy, kev
o-ps decays selection In order to decrease the background from the 2 photons, we select the 3 photons from the o-ps applying a cut on the time. The lower limit has been set at 160ns, which suppress the 2γ strongly enough, but it doesn t decrease too much statistic. The upper limit is set at 800ns, because then the background from accidentals becomes dominant. events/10 nsec 10 6 10 5 10 4 10 3 200 400 600 800 t e+γ, nsec
Results After selection, the sum is calculated. E VETO = E1.27 E TriggerBGO all The sum EVETO + ETriggerBGO = 1MeV (Mass of Positronium) 1400 1200 1000 800 600 400 200 0 Signal region Pile up 0 200 400 600
Signal region: and E VETO 20keV 40keV E γ 400keV E VETO, kev Sensivity of the calorimeter 20keV 35 30 25 20 15 10 5 0 One event has been observed 0 200 400 600 E TRIG, kev The single photon maximal energy depends on the mass of the two exotic particles. For the 511 kev photons from the 2 photons decay the unhermeticity of the calorimeter is in the order of 10-3.
Background estimation The expected background is extrapolated assuming a linear fit (in log scale) of the projected energy in the VETO. events/5 kev 10 3 10 2 c) 10 1 10-1 0 100 200 300 400 500 For E VETO 20keV, 1.6 event (± 0.8) is expected, which is consistent with the measurements. E VETO, kev
Calculation of the upper limit BR(o Ps γ + X 1 + X 2 ) ε ε 3γ 1γ N up o Ps γ+ X N 1 o Ps 3γ N up o Ps γ+ X 1 + X 3.8 has been calculated with Poisson 2 = statistic for 1 event observed and 0-background expected (conservatively). Using a Monte-Carlo simulation (assuming phase space) we estimate the different detection efficiencies for a photon from the three photon and from the single photon decay: 3.0<ε γ3 / ε γ1 <3.7 for: 0 M x1 +M x2 900keV The number of o-ps decays in the target is measured from the decay curve, the measured lifetime is 6.6% less than in vacuum, it follows that 5 No Ps 3γ 3.2x10 + X 2
Conclusions Branching ratio 10 2 10-3 -4 EXCLUDED + BR(e e (o Ps) γ + X1 + X2 ) 1 0-5 0 200 400 600 800 1000 m1+m2(kev) 4.4x10-5 at 90% CL It follows that this decay mode can not explain the discrepancy (the limit is 20 times smaller)!
Search for invisible decay of o-ps Status of the experiment: Based on a Monte-Carlo simulation in Geant 3 (tuned with the first phase of the experiment) we predicted that we could reach the sensitivity BR < 10-8 -10-9, if we could increase the size of the detector. From our study the geometry should have 98 crystals! 24 crystals (present) 56 crystals 98 crystals Background from 2γ escaping detection 2x10-5 3.3x10-8 10-10
Measurement of the o-ps lifetime and invisible decay of o-ps in vacuum: The first continous beam of positron is expected for the end of the year. SOURCE MODERATOR 4Mbq 22 Na source produced by PSI irradiating a pure 27 Al piece. e- GUN PRE-BUNCHER * ION PUMP e+ BEAM CORRECTOR B Detector e+ FILTER 1 M COILS