SEARCHES FOR ANTIMATTER DIETRICH MÜLLER University of Chicago SLAC SUMMER INSTITUTE 2011
OUTLINE Early History Baryon Asymmetry of the Universe? Current Limits on Antimatter Nuclei from Distant Galaxies Antiprotons t in Cosmic Rays Positrons in Cosmic Rays Outlook and conclusionsc o s 2/26/11 dmuller SSI 2011 2
EARLY HISTORY 1932 Anderson and Neddermeyer (Caltech): Discovery of Positron (1936 Nobel for Anderson) 1955 Segre et al. (Berkeley) Discovery of Antiproton (1959 Nobel for Segre and Chamberlain) 2/26/11 dmuller SSI 2011 3
POSITRON TRACK IN CLOUD CHAMBER AND MAGNETIC FIELD QUANTITIES TO IDENTIFY THE PARTICLE: Momentum Lorentz Factor Anderson & Neddermeyer 1933 Direction Charge sign Charge number Z 2/26/11 dmuller SSI 2011 4
COMMENT First observation of apparently positive electrons in cloud chamber pictures by Skobeltsyn in 1920 s, but ignored then. Dirac theory proposed in 1928/29: Thus, Anderson knew what to look for! It was not known that primary cosmic rays are mostly protons, not positrons, until 1940 s (M. Schein) 2/26/11 dmuller SSI 2011 5
COMMENT ON ANTI-PROTONS Discovery of pbar was made at Bevalac accelerator, energy barely sufficient for pbar production. A single antiproton event from cosmic-ray interactions in the atmosphere was seen by E. Amaldi, also in 1955, by observation of annihilation star in nuclear emulsion. 2/26/11 dmuller SSI 2011 6
ANTIMATTER IN THE UNIVERSE? General Assumption: Baryon Symmetry y at Big Bang: t=0: B=0 Baryogenesis now: Universe dominated by matter Universe remains baryon symmetric: Anti-matter domains exist 2/26/11 dmuller SSI 2011 7
COMMENT ON BARYOGENESIS It is not easy! Sakharov Conditions (1967): (1) ViolationofBnumber of B-number conservation (2) Violation C- and CP conservation (3) Interactions outside of thermal equilibrium 2/26/11 dmuller SSI 2011 8
BARYON-SYMMETRIC UNIVERSE? Anti-matter in anti-stars and anti-galaxies? 2/26/11 dmuller SSI 2011 9
Anti-matter domains in the universe, cannot be close to our galaxy! Otherwise, annihilation gamma rays would be seen: p+p π +, π -, π 0 e +, e -, γ, neutrinos (~100 MeV energies) COMMENT BY TARLE AND SWORDY (Scientific American 1998): If a human and an anti-human shook hands,the resulting explosion would be equivalent to 1,000 one-megaton nuclear blasts, each capable of destroying a small city 2/26/11 dmuller SSI 2011 10
BARYON-SYMMETRIC UNIVERSE? Anti-matter in anti-stars and anti-galaxies? If these exist, they must be at least 100 Mpc away: Otherwise, annihilation gamma rays would be observable. These would not look different from regular stars when seen in visible light. Can Cosmic rays from such distances reach us? 2/26/11 dmuller SSI 2011 11
COSMIC RAYS FROM DISTANT GALAXIES C.R. particle needs 300 Myears for straight-line traversal of 100 Mpc. Even a minute intergalactic magnetic field (~10-12 Gauss) could stretch this time to approach the age of the Universe. Et Extra-galactic ti cosmic rays may have difficulty entering the Galaxy. Any search must look for heavy anti-nuclei; anti-protons can be locally produced! 2/26/11 dmuller SSI 2011 12
Illustration of Particle Transport between Galaxies 2/26/11 dmuller SSI 2011 13
UPPER LIMITS FOR RELATIVE ANTI-HELIUM ABUNDANCE IN COSMIC RAYS GV 2/26/11 dmuller SSI 2011 14
AMS-1 (Space Shuttle,1998) 2/26/11 dmuller SSI 2011 15
ANTIHELIUM UPPER LIMIT vs. RIGITY FROM AMS-1 Alcaraz et al 1999 2/26/11 dmuller SSI 2011 16
ANTIHELIUM UPPER LIMIT vs. RIGITY FROM AMS-1 PREDICTED LIMITS: 3x10-8 FOR BESS-POLAR ~ 10-9 FOR AMS-2 Alcaraz et al 1999 2/26/11 dmuller SSI 2011 17
COMMENT: Anti-helium if it exists could either come from primordial nucleo-synthesis, or from material processed in stars Heavier anti nuclei (anti-carbon, anti-oxygen, etc.) must have been produced by nucleo-synthesis in anti-stars 2/26/11 dmuller SSI 2011 18
ANTI-MATTER FRACTION IN HEAVY C. R. NUCLEI (Z= 3 to 8) AMS 1 data (1998) from thesis Cristinziani (unpublished?) 2/26/11 dmuller SSI 2011 19
COSMIC-RAY PROTONS IN THE ISM Accelerated in primary sources in the galaxy (presumably SN-remnants) Energy spectrum ~ E -2.7 27 Interactions: p+h p+p+p+p p+h π +, π -, π 0, + e +,e, -, γ, + Relative intensities of secondary positrons and of secondary antiprotons of order 10-4 2/26/11 dmuller SSI 2011 20
INDIVIDUAL ENERGY SPECTRA OF THE MAJOR PRIMARY NUCLEI (Particle Physics Data Handbook, 2008 edition) 2/26/11 dmuller SSI 2011 21
ANTIPROTONS IN THE COSMIC RAYS Can be produced in the interstellar p+p p collisions, expected level pbar/p 10-4 Background on balloons : atmospheric pions First identified in 1990 s (IMAX, BESS) Kinematic cut-off in intensity below 1 GeV (softened by solar modulation) Search for new Physics at low energies 2/26/11 dmuller SSI 2011 22
BESS INSTRUMENT (Balloon-borne Experiment with a Superconducting magnet rigidity Spectrometer) G > 3,000 cm 2 sr 2/26/11 dmuller SSI 2011 23
BESS 2/26/11 dmuller SSI 2011 24
BESS-Polar 2/26/11 dmuller SSI 2011 25
Balloon Flight of BESS-polar 25 days in 2007/8 2/26/11 dmuller SSI 2011 26
AT FLOAT 40 km altitude (3 mbar) 2/26/11 dmuller SSI 2011 27
DETECTION OF ANTIPROTONS WITH BESS 2/26/11 dmuller SSI 2011 28
Antiproton Fraction measurements consistent with purely secondary production secondary from p-p collisions neutralino primordial black hole 2/26/11 dmuller SSI 2011 29
MEASUREMENT OF Pbar IN SPACE WITH PAMELA (2011) ENERGY SPECTRUM pbar/p RATIO 2/26/11 dmuller SSI 2011 30
ANTI-DEUTERONS PERHAPS FROM EVAPORATING PRIMORDIAL BLACK HOLES NOT FOUND IN COSMIC RAYS Data from BESS: Fuke et al. PRL 95,081101 (2005) 2/26/11 dmuller SSI 2011 31
SUMMARY OF BESS RESULTS for Pbar and anti-d Mitchell & Yamamoto (2009) 2/26/11 dmuller SSI 2011 32
POSITRONS IN COSMIC RAYS Electrons and Positrons discovered in 1960 s in c.r. (e + +e - )/p 10-2 ; e + /(e + +e - ) 10-2 at a few GeV Direct acceleration in cosmic-ray sources possible for negative electrons but unlikely for positrons Interstellar secondary production: p+p collisions, leading to pions which decay into positrons and electrons: expected ratio e + /p 10-4 2/26/11 dmuller SSI 2011 33
COMMENT ON DETECTION OF ELECTRONS (e + + e - ) Rejection against protons (and pions) required at least at the 10-3 level Commonly used device: Electro-magnetic calorimeter which will also register hadron-induced showers! Background problems in many early observations Problem is worse by factor 100 for positron detection 2/26/11 dmuller SSI 2011 34
EARLY MEASUREMENTS OF POSITRON FRACTION e + /(e + +e - ) (Magnet spectrometers with spark chambers on balloons) 2/26/11 dmuller SSI 2011 35
POSITRONS FROM EXOTIC SOURCES? Pair-production in pulsar magnetospheres? WIMPs as dark matter candidates: Annihilation i of neutralinos leading to electron-positron pairs (and characteristic gamma rays)? Hadronic production in giant molecular clouds? 2/26/11 dmuller SSI 2011 36
TREE INSTRUMENT (Chicago 1970 s) measures e + + e - Transition Radiation Detector (TRD): Proton rejection 100 Shower Counter (longitudinal profile) Proton rejection 100 2/26/11 dmuller SSI 2011 37
THE EARTH S MAGNETIC FIELD AS SPECTROMETER Müller & Tang ApJ 312, 183 (1987) 2/26/11 dmuller SSI 2011 38
FERMI-LAT IN THE EARTH S MAGNETIC FIELD 2/26/11 dmuller SSI 2011 39
POSITRON- FRACTION Müller & Tang, 1987 2/26/11 dmuller SSI 2011 40
HEAT e +/- Electron/Positron Spectrometer TRD High Energy Antimatter Telescope (1990 s) Chicago Michigan Indiana Irvine Penn State (D. Müller, P.I.) Magnet Shower Counter (longitudinal) Total rejection: 10 5 or better 2/26/11 dmuller SSI 2011 41
The Positron Fraction, e+/(e + +e - ) (Measurements in the 1990 s) 2/26/11 dmuller SSI 2011 42
68 GeV e+ PAMELA Launched 2006 Positron identification from magnet spectrometer plus imaging calorimeter 2/26/11 dmuller SSI 2011 43
QUESTIONS INTENSIFY WITH RECENT POSITRON OBSERVATIONS Positron Fraction Electron/Positron pairs from WIMP-decay??? 2/26/11 dmuller SSI 2011 44
Low E positron ratio from PAMELA corrected for solar modulation 2/26/11 dmuller SSI 2011 45
e + /e - pair production in nearby pulsar (GEMINGA) 2/26/11 dmuller SSI 2011 46
e+/e- from 200 GeV neutralino 2/26/11 dmuller SSI 2011 47
M. Schubnell (Michigan) (2009) Contributions from background protons at level 2x10-4 Note: cosmic-ray protons have harder energy spectrum than electrons 2/26/11 dmuller SSI 2011 48
Proton contamination Geminga pulsar 200 GeV neutralino Analysis by M. Schubnell (2009) 2/26/11 dmuller SSI 2011 49
OUTLOOK AMS: Alpha Magnetic Spectrometer (S. Ting and many collaborators) Flight on Space Station Launch 20xx 2/26/11 dmuller SSI 2011 50
AMS UPGRADE 2010 0.87 Tesla 0.125 Tesla 2/26/11 dmuller SSI 2011 51
CONCLUSIONS There is no indication for anti-nuclei heavier than antiprotons. The upper limit for α/α currently stands around 10-7 ; AMS promises to push this down to 10-9. Antiprotons in the cosmic rays seem to be exclusively generated by nuclear interactions in the interstellar medium. Intensity and energy spectrum from 0.1 to 100 GeV are well fitted by standard cosmic-ray models There is a clear anomaly in the reported intensity of positrons between 10 and 100 GeV. The explanation may require new physics or new astrophysics. Confirmation of this discovery may become the most important result from AMS. 2/26/11 dmuller SSI 2011 52
CONCLUSIONS, cont d.: Even if the anti-particles in the cosmic rays are all just ordinary secondary particles, their intensities and energy spectra will provide long-sought detail about the propagation of cosmic rays through the Galaxy. AMS will provide this information with much greater accuracy than balloon instruments could. 2/26/11 dmuller SSI 2011 53
GALACTIC COSMIC-RAY SOURCES? c.r. * Source GALAXY Observer Characteristics of sources of primary cosmic rays must be deduced from measurements near Earth. Production characteristics for secondary particles from proton interactions throughout the Galaxy are essentially known. 2/26/11 dmuller SSI 2011 54