Fundamental Physics with Cosmic Rays. Kate Scholberg MIT NEPPSR 2003

Transcription

1 Fundamental Physics with Cosmic Rays Kate Scholberg MIT NEPPSR 2003

2 OUTLINE Introduction to cosmic rays Cosmic rays in particle physics history A few selections from the smorgasbord: Ultrahigh energy cosmic rays SUSY dark matter Supernova neutrinos Relic big bang neutrinos

3 Questions of Fundamental Physics What are the elementary particles and their interactions? What are the neutrino masses and mixings? Is nature supersymmetric? What is the Universe made of, and how did it all come about? What is the dark matter? Why is there a matter-antimatter asymmetry?

4 Cosmic Ray Primer "Cosmic Ray" "a particle from space" naturally occurring various sources (Sun, supernovae, AGN, GRB) wide energy range many species, charged and neutral p, n, A, e ±, γ, µ ±, ν,... (Are photons CR? Depends...)

5 More terminology: PRIMARY CR: directly from outer space (stable, charged component mostly protons) SECONDARY CR: created in collisions with atmosphere (includes muons, short-lived component) (c) 1999 K. Bernlohr

6 Charged cosmic ray fluxes for different species Dominated by protons up to ~ TeV (composition less well known at higher energies)

7 Charged cosmic rays: affected by Earth's dipole magnetic field Many interesting trapping and bouncing effects...

8 Low energy primary CR cannot enter geomagnetic field Cutoff rigidity ~1-10 GeV per nucleon, depending on latitude Rigidity p/(ze) solar wind effects at <~ 1 GeV

9 Another comment: charged cosmic rays don't point back to where they came from! Gyromagnetic radius: R= 3.3 x p/(zb) R in cm, E in GeV, B in µg For Galactic field of 3 µg, R=10 12 cm(<0.1 A.U.) for 1 GeV/c proton Charged CR follow tangled path, nearly isotropic Neutral CR (γ,ν) point back to source R~ diameter of Galaxy at ~ ev/c

10 Charged Primary Cosmic Ray Energy Spectrum geo magnetic cutoff higher energies preferentially escape Galaxy Sun Supernovae Other sources???

11 OUTLINE Introduction to cosmic rays Cosmic rays in particle physics history A few selections from the smorgasbord: Ultrahigh energy cosmic rays SUSY dark matter Supernova neutrinos Relic big bang neutrinos

12 Cosmic rays have figured prominently in the history of particle physics... Victor Hess, 1912: gold leaf electroscope to 5000 m altitude First identification of "cosmic radiation"

13 Years of high drama ensue...

14 Anderson's discovery of the positron, 1932 The antimatter predicted by Dirac! Wilson cloud chamber photo

15 Neddermeyer and Anderson, 1937: discovery of the muon in cloud chamber "Mesotron": mass intermediate between electron and proton

16 Late 1940's: Powell and others Mountaintop observatories and photographic emulsion: discovery of the pion π µ + ν µ e + ν + ν

17 Discovery of strangeness: "V particles" Rochester and Butler, 1946 First kaon The "particle zoo" followed...

18 Over the next ~40 years, accelerators dominated new discoveries in particle physics... Cosmic ray physicists mostly focused on understanding sources, composition, propagation,......but, now since the 1990's, cosmic rays have again come to the forefront as a tool for fundamental physics, complementing accelerators!

19 Cosmic rays answer where accelerators can't reach... Neutrino mass & oscillations: Atmospheric, solar, and supernova ν's Ultrahigh energy cosmic rays: exotic matter, Z-bursts, cosmic ν's Relic big bang neutrinos Strange matter, QCD Primordial black holes SUSY dark matter: annihilation signals, direct detection Matter-antimatter asymmetry: antimatter searches Gravitational waves Neutrino astrophysics

20 OUTLINE Introduction to cosmic rays Cosmic rays in particle physics history A few selections from the smorgasbord: Ultrahigh energy cosmic rays SUSY dark matter Supernova neutrinos Relic big bang neutrinos

21 Cosmic Ray Spectrum M G T Peta Exa Zetta extra galactic component 1 per sq km per century above ev

22 Greisen-Zatsepin-Kuzmin (GZK) cutoff Cosmic rays with energies greater than 5 x ev will be absorbed by the Cosmic Microwave Background p + γ N + π Mean free path 50 Mpc at ev Galaxy: 20 kpc Andromeda: 0.7 Mpc

23 But some CR observed above the GZK cutoff... What are they??

24 "Bottom up" mechanisms: particles accelerated to high energies How? SN can only accelerate up to ev Origin in AGN, GRB, magnetars? Need very large fields, confined spaces not clear how it works... Any observed anisotropy should lead to sources

25 "Top-down": source is something exotic, involving new fundamental physics? e.g. Superheavy ( GeV) particles decay to UHE Standard Model particles... BB relic long-lived dark matter? No cutoff because Galactic origin Expect excess toward Galactic center

26 Other ideas: Topological defects? Other exotic primaries? uhecrons (light SUSY hadrons), glueballinos,... Violation of Lorentz invariance? Strong neutrino interactions? "Z-bursts"? Again, we need to look at anisotropy, correlations with objects, spectrum, composition to distinguish the models

27 Detection Techniques observe gigantic air showers 1 per sq km per century above ev Requires huge area! Air Fluorescence: glow of excited N molecules Fly's Eye, Hi-Res, TA Air shower array: observe particles on ground AGASA

28 Recent results Hi-Res: fluorescence AGASA: air shower Exp'ts don't agree? Is GZK cutoff there or not?

29 The Pierre Auger Experiment Argentina air fluorescence and air shower array 3000 km 2, expect UHE events per year

30 And the farther future: site a detector in space air fluorescence from above to view huge area! EUSO for ISS OWL/Airwatch stereo view satellites

31 Summary of UHECR Nucleons are absorbed by the CMB above ~10 20 ev within 50 Mpc... Observed post-gzk events have a mysterious origin "Bottom-up": exotic astrophysics "Top-down": exotic physics Need to characterize anisotropy, spectrum Gigantic area detectors required... AGASA, Hi-Res Auger EUSO, OWL

32 OUTLINE Introduction to cosmic rays Cosmic rays in particle physics history A few selections from the smorgasbord: Ultrahigh energy cosmic rays SUSY dark matter Supernova neutrinos Relic big bang neutrinos

33 The DARK MATTER Mystery Many independent measurements Baryonic matter (ordinary stuff) only ~5%! Galactic rotation curves Gravitational lensing, microlensing Cosmic microwave background Large scale structure Nucleosynthesis High z redshift surveys "DARK ENERGY" Nonbaryonic dark matter ~25%!!

34 One appealing hypothesis to explain non-baryonic dark matter: Weakly Interacting Massive Particles (WIMPs) that froze out after the Big Bang e.g. NEUTRALINO χ lightest stable supersymmetric particle 50 GeV/c 2 < m χ < 3 TeV/c 2 accelerator bound (LEP) cosmological bound

35 Neutralinos could make up the Galactic halo χ χ χ χ χ χ χ χ χ χ χ χ Local halo density ~ 0.3 GeV cm -3 (but could be clumpy)

36 Signature of neutralino dark matter: Look for ANNIHILATION PRODUCTS χχ gauge bosons quarks leptons e + p d γ... Here, have background of SECONDARIES from CR collisions look for ANOMALIES in the energy distribution "bump in the spectrum"

37 Look for anomalous POSITRONS background from secondaries should be smooth χχ annihilation would give bump around ~ GeV

38 A hint from a balloon experiment, HEAT? Positron fraction vs energy hep-ph/ Bump at ~10 GeV seen with different instruments

39 Interpretation in terms of SUSY DM Baltz et al. astro-ph/ N Fits require "boost factor" to enhance signal (plausible for clumpy DM)

40 SUSY parameter space dots represent allowed models

41 Look for anomalous ANTIPROTONS background from secondaries A Also: antideuterons In this case, low energies may have less background But: geomagnetic cutoff, solar wind effects

42 Can also look for χχ annihilation via γ-ray products χχ gauge bosons quarks leptons hadronize γ's in showers Continuum emission at ~1/10 m χ Or, spectral line from direct χχ -> γ's

43 The Alpha Magnetic Spectrometer for ISS Sensitivity to charged cosmic rays up to 1 TeV, and γ's GeV

44 Summary of Dark Matter Search Non-baryonic dark matter (e.g. χ) indirect signature: χχ annihilation products Positrons Antiprotons Gamma rays Neutrinos in >~ 10 GeV range in ~< 1 GeV range in GeV range from Galactic center, halo from Earth center, sun, Galactic center (trapped WIMPs)

45 OUTLINE Introduction to cosmic rays Cosmic rays in particle physics history A few selections from the smorgasbord: Ultrahigh energy cosmic rays SUSY dark matter Supernova neutrinos Relic big bang neutrinos

46 Core Collapse Supernovae: Copious producers of ν's Expect ~3 ±1 /century in our Galaxy

47 The Supernova Neutrino Signal E b ~ GM 2 core R nstar ~ ergs < 1% in em radiation, k.e., 99% in ν's of all flavors ~1% ν e from 'breakout', 99% νν from cooling Energies: <Eν e > ~ 12 MeV <Eν e > ~ 15 MeV ( ) <Eν µ,τ > ~ 18 MeV Deeper ν-sphere => hotter ν's Timescale: prompt after core collapse t~10's of seconds (possible sharp cutoff if BH forms)

48 Neutrino Luminosity: Generic Features Burrows et al very short (ms) ν e spike at shock breakout 1 s sum of ν µ,τ and anti-ν's roughly equal luminosity per flavor cooling 50 s luminosity decrease over 10's of seconds

49 SN1987A Type II in LMC (~55 kpc) Water Cherenkov: IMB E th ~ 29 MeV, 6 kton 8 events Kam II E th ~ 8.5 MeV, 2.4 kton 11 events Liquid Scintillator: Baksan E th ~ 10 MeV, 130 ton 3-5 events Mont Blanc E th ~ 7 MeV, 90 ton 5 events?? Confirmed baseline model... but still many questions

50 What Can We Learn from a Galactic Supernova Neutrino Signal? NEUTRINO PHYSICS ν absolute mass from time of flight delay ν oscillations from spectra (flavor conversion in supernova core, in Earth) CORE COLLAPSE PHYSICS explosion mechanism proto nstar cooling, quark matter black hole formation ASTRONOMY FROM EARLY ALERT from flavor, energy, time structure of burst ~hours of warning before visible SN, + some pointing with ν's progenitor and environment info unknown early effects?

51 Neutrinos: What Do We Now Know? 2-flavor oscillation signals m e x sin 2 2θ LSND signal still there: wait for BooNE Atmospheric e signal confirmed by K2K beam suppression + spectrum Solar ν oscillation confirmed by SNO NC; only LMA now allowed; and now KamLAND confirms with reactor ν's!

52 "Standard" 3-flavor picture: Parameters: 2 m 2, 3 angles, δ CP, (2 δ M ) MNS mixing matrix U C 23 S 23 0 S 23 C 23 C 13 0 S 13 e i S 13 e i 0 C 13 C 12 S 12 0 S 12 C "Normal" hierarchy m 23 2 (atm.) m 12 2 (solar) { { µ e µ e Absolute mass scale? τ µ τ τ { or "Inverted" hierarchy { 2 m 12 { 2 m 23 e e µ µ τ τ µ τ Kinematic limits: m ν < 2.2 ev 0νββ limits: <m ν > < 0.35 ev Cosmology (WMAP): m ν < 0.23 ev

53 Remaining Questions (that supernova neutrinos might shed light on) What is the absolute mass scale? What is the mass hierarchy? "Normal" hierarchy m 23 2 (atm.) m 12 2 (solar) { { µ e µ e µ τ τ or What is U e3? Is it non-zero? τ "Inverted" hierarchy { 2 m 12 { 2 m 23 e e µ µ µ τ τ τ

54 Neutrino Absolute Mass: Expect time of flight delay t(e) = 0.515(m ν /E) 2 D Look for: energy-dependent time spread flavor-dependent delay SN1987A: m ν < 20 ev for ν e t=0 from black hole collapse? grav wave signal?

55 Example: ν e signal for black hole cutoff Beacom et al. hep-ph/ energy-dependent delay for ν e Current detectors: ~few ev level limits possible, at best...no longer relevant?

56 Perhaps more promising: Neutrino Oscillations, Mass Hierarchy Energies: <Eν e > ~ 12 MeV <Eν e > ~ 15 MeV ( ) <Eν µ,τ > ~ 18 MeV Flavor-energy hierarchy is robust Flavor transformations in stellar matter spectral distortion e.g. expect hot ν e or ν e Also: matter effects in Earth can modify signal compare NC, ν e, ν e rates and spectra

57 Some signatures (assuming LMA, U e3 2 large, 3-flavor picture) relatively ν e in neutronization peak completely transformed hard ν e during cooling Earth matter effects for ν e ν e in neutronization peak partly transformed hard ν e during cooling Earth matter effects for ν e } } Normal hierarchy Inverted hierarchy Sensitivity to U e3 2 as low as 10-4 to 10-5 Some SN model-dependence...

58 Supernova Neutrino Detectors Need ~ 1kton for ~100 interactions Must have bg rate << rate in burst Also want: Timing Energy resolution Pointing Flavor sensitivity (neutral current) Detector Types Scintillator C n H 2n Water Cherenkov H 2 O Heavy Water D 2 O Long string water Cherenkov H 2 O 'High Z' Pb, Fe

59 Example: Super-Kamiokande Mozumi, Japan 50 kton of water (32 kton inner + outer detector) Now resumed operation after 2001 accident Events expected for collapse at 8.5 kpc, > 5 MeV: νe + p νx + 16O νe + 16,18O νe + 16O 7000 ν + e- ν + e- 200 x x νx + 16O* 300 (5-10 from breakout) 5 16,18 F + e 50 Pointing: 16 N + e+ 60 ~4o at 8.5 kpc e+ + n

60 Summary of Types of SN Neutrino Detectors Primary sensitivity is to ν e, NC for heavy water, high Z Pointing for water Cherenkov, heavy water, argon All real-time except radiochemical All have energy resolution except long string, radiochemical

61 Distance sensitivity (kpc) Distance for 90% CL detection, 1/month threshold λ= Hz/kton Andromeda λ=0.001 Hz/kton Distance sensitivity depends on: Mass Background rate λ LMC Far side of Galaxy λ=0.01 Hz/kton Detector Mass (kton) E th ~ 5 MeV T = 10 s

62 Summary of Future SN Neutrino Detectors Galactic sensitivity Extra Galactic

63 Summary of Supernova Neutrinos A Galactic core collapse will yield a vast quantity of information... Neutrino absolute mass: few ev sensitivity from time of flight delay (not better than lab?) Oscillation info: mass hierarchy, θ 13 from spectral distortion, Earth matter effect Many detectors with Galactic sensitivity online now... next generation extra-galactic?

64 OUTLINE Introduction to cosmic rays Cosmic rays in particle physics history A few selections from the smorgasbord: Ultrahigh energy cosmic rays SUSY dark matter Supernova neutrinos Relic big bang neutrinos

65 Relic neutrinos which froze out after the Big Bang, t ~ 1 sec Expect T=1.95 K, sub-ev! Nonrelativistic? Number density 113/cm 3 per family Very very very hard to detect... A experimental Holy Grail...

66 One idea: Z-bursts Ultra-high energy neutrinos interact with relic BB ν background at Z-pole produce UHE CR E res = M Z2 /2m ν = 4.2 x ev (m ν /1 ev)

67 Summary Cosmic rays have a venerable history, and are in vogue again! The next progress in fundamental physics may come from a non-standard approach... Sumptuous dining ahead!