Subir Sarkar

Transcription

1 Trinity 2016 Oxford ² The universe observed ² Relativistic world models ² Reconstructing the thermal history ² Big bang nucleosynthesis ² Dark matter: astrophysical observations ² Dark matter: relic particles ² Dark matter: direct detection ² Dark matter: indirect detection ² Cosmic rays in the Galaxy ² Antimatter in cosmic rays ² Ultrahigh energy cosmic rays ² High energy cosmic neutrinos ² The early universe: constraints on new physics ² The early universe: baryo/leptogenesis ² The early universe: inflation & the primordial density perturbation ² Cosmic microwave background & large-scale structure Subir Sarkar

2 Particularly interesting (to high energy physicists) because of the PAMELA anomaly PAMELA has measured the positron fraction: (Gast & Schael, ICRC 09) Anomaly excess above astrophysical background Source of anomaly: Dark matter? Pulsars? Supernova remnants?

3 confirmed by AMS-02

4 resulting in a bold claim The new results from AMS on the positron fraction published in Physical Review Letters show that items (1)-(4) highlighted in Figure 3 have been unambiguously resolved, yielding observations of a new phenomenon. They are consistent with a dark-matter particle (neutralino) of mass on the order of 1 TeV. To determine if the observed new phenomenon is from dark matter or from astrophysical sources such as pulsars, AMS is now making measurements to determine the rate of decrease of the positron fraction beyond the turning point (item 5), as well as to determine the antiproton fraction...

5 However e lose energy readily during propagation, so only nearby sources dominate at such high energies the usual background calculation is then irrelevant Delhaye et al, A&A 501:821,2009 ' yr 1TeV E 1 TeV 100 GeV 10 GeV 1 GeV 100 MeV Kobayashi et al, ApJ 601:340,2004 Might there be primary sources of e + (with a hard spectrum) in our Galactic neighbourhood?

6 This is not the first time an anomalous excess over background has been seen The inclusive jet differential cross section has been measured for jet transverse energies, E T, from 15 to 440 GeV, in the pseudorapidity region 0.1 η 0.7. The results are based on 19.5 pb -1 of data collected by the CDF Collaboration at the Fermilab Tevatron collider. The data are compared with QCD predictions for various sets of parton distribution functions. The cross section for jets with E T > 200 GeV is significantly higher than current predictions based on O(α s3 ) perturbative QCD calculations. Various possible explanations for the high-e T excess are discussed. Abe et al, PRL 77:438,1996 it turned out to be a mis-estimation of the QCD background not new physics!

7 What particle physicists have learnt through experience (UA1 monojets, NuTeV anomaly, CDF high E T excess, ) Yesterday s discovery is today s calibration and tomorrow s background! Richard Feynman Val Telegdi is also now a major issue for astroparticle physics viz. just how well do we know the astrophysical background for signals of apparently new particle physics?

8 A nearby cosmic ray accelerator?. Rise in e + fraction could be due to secondaries being produced during acceleration which are then accelerated along with the primaries (Blasi, PRL 103:051104,2009)... generic feature of a stochastic acceleration process, if τ 1 è2 < τ acc (Cowsik 1979, Eichler 1979) This component naturally has a harder spectrum and fits PAMELA/AMS-02 data (adjusting just one parameter) RXJ , HESS (age~3000 yr, d~0.8 kpc) Ahlers, Mertsch & Sarkar,PRD80:123017,2009

9 Diffusive (1 st -order Fermi) shock acceleration Flux: Conservation equation: density change acceleration convection injection Steady state: log f i.e. = 4 for strong shock (u 1 /u 2 = 4) log p

10 Diffusive (1 st -order Fermi) shock acceleration Acceleration determined by compression ratio: Solve transport equation: u f x = D 2 f x du dx p f p Solution for: f x f inj (p), lim x f where, f 0 (p) = p 0 dp p p p f inj (p ) + Cp As long as f inj (p) is softer than p at high energies: f(x, p) p

11 DSA with secondary production Secondaries have same spectrum as primaries (Feynman): Only particles with are accelerated Bohm diffusion: Fraction of accelerated secondaries is Steady state spectrum: log n p 2 > p 1 è rising positron fraction! log p

12 Diffusion near shock front Ø Ø Diffusion coefficient not known a priori in neighbourhood of shock Bohm diffusion sets a limit: Ø Ø Ø Actual rate may be parametrised by: D = D Bohm /K B, K B = B 2 / B 2 Can try and determine diffusion rate from simulations (difficult!) So we fix K B by fitting to the Fermi e excess can then predict e + /(e + + e - ) for PAMELA/AMS-02, and other secondary-to-primary ratios (e.g B/C)

13 The downstream spectrum, integrated over SNR lifetime where is downstream volume Mertsch, Sarkar, Phys.Rev.D90,:061301(R),2014

14 It is not just the few (optically) observed SNRs which contribute to observed cosmic rays there must be many other hidden SNRs (if there are ~3 SN/century and cosmic rays diffuse in Galaxy for ~10 7 yr)!" ( 10 GeV 100 GeV Known!" ( Simulated!" ' 1 TeV H!" '?::2! =03*.>-!" &,*30 45!" % A07- EF #! B4>.C+ ='&D$ &D( ;!%( ;<!G& ;<!""' 84/9: ,*30 45!" %!" $!" $!"# $%&%'() %! *+"#"!&,)#!" #!"!!!" )*+,-./0 12/!"# $%&%'() %! *+"#"!&,)#!" #!"!!!" )*+,-./0 12/ Ahlers, Mertsch & Sarkar,PRD80:123017,2009

15 Statistical distribution of SNRs in our neighbourhood %&'() "# "$ # $!#!"$!"#!"#!"$!# $ # "$ "# %&'() Case & Bhattacharya, ApJ 504:761,1998 $! #" #! "!! " #! #" $! $" Draw source positions from this distribution Inject e - & e + normalized to observables (HESS ) Propagate to Earth accounting for synchrotron and inverse-compton scattering energy losses Confront total e - +e + flux at Earth with Fermi data The best fit to data is closest to real distribution Ahlers, Mertsch, Sarkar,PRD80:123017,2009

16 Parameters of the Monte Carlo Di usion Model D cm 2 s 1 from GCR nuclear 0.6 secondary-to-primary ratios L 3 kpc b GeV 1 s 1 CMB, IBL and B energy densities Source Distribution t max yr from E min 3.3 GeV SNR 10 4 yr from observations N from number of observed SNRs Source Model R 0 e GeV 1 fit to e flux at 10 GeV 2.4 average -ray spectral index E max 20 TeV typical -ray maximum energy E cut 20 TeV DSA theory R GeV 1 -rays K B 15 free parameter (for fixed ) Ahlers, Mertsch & Sarkar, PRD80:123017,2009

17 Normalising the source spectra Cassiopeia A, HESS Normalisation of primary : fit absolute flux at low energies Normalisation of secondary : Source Other name(s) J E max d Q [(cm 2 s TeV) 1 ] [TeV] [kpc] [(s TeV) 1 ] HESS J RX J (Vela Junior) 2.1 ± ± 2 > HESS J RCW 86, SN 185 (?) 2.54 ± ± HESS J CTB 37B, G ± ± HESS J RX J , G ± ± ± HESS J CTB 37A 2.30 ± ± HESS J G ± ± HESS J a W 28, GRO J ± ± HESS J b W 30, G ± HESS J W 41, G ± MAGIC J IC ± Cassiopeia A 2.4 ± ± J Monoceros 2.53 ± ± 0.17 N/A Mean Mean, excluding sources with > Mean, excluding sources with > Ahlers, Mertsch & Sarkar, PRD80:123017,2009

18 Fitting the e + + e - flux The propagated primary e - spectrum is much too steep to match the Fermi LAT data... but the accelerated secondary e + + e - component has a harder spectrum so does fit the bump! Ahlers, Mertsch & Sarkar, PRD80:123017,2009

19 The postdicted positron fraction 1 Standard Solar modulation Charge-sign dependent Solar modulation Positron fraction 10 1 PAMELA Ahlers, Mertsch, Sarkar, PRD80:123017, Energy GeV

20 Nearby pulsars as source of. Highly magnetized, fast spinning neutron stars -rays and electron/positron pairs produced along the magnetic axis Spectrum speculated to be harder than background from propagation:

21 Combination of Galactic contribution and two nearby pulsars, Geminga (157 pc) and B (290 pc), can fit PAMELA excess (and perhaps also Fermi bump) Hooper, Blasi & Serpico, JCAP 01:025,2009 However ~40% of rotational energy must be released as energetic e + plausible? Fermi should detect expected anisotropy towards B in ~5 years?

22 What about the antiproton-to-proton ratio? Blasi & Serpico, PRL 103:081103,2009 Bohm-like ISM ISM+B term Total Dark matter ( ) p- /p Pulsars Acceleration of secondaries 1e-05 B term A term Kinetic Energy, T [GeV] Secondary acceleration model predicts rise beyond 100 GeV will be tested soon by AMS-02 (if we see this then pulsars are ruled out)

23 Can solve problem analytically but more complicated than for since energy losses must now be included Transport equation: with boundary condition: Solution: Mertsch & Sarkar, PRL 103:081104,2009

24 Nuclear secondary-to-primary Ratios Dark matter Pulsars Acceleration of secondaries (TBD) S/P If we see this rise convincingly, then both dark matter and pulsar origin models would be ruled out! Ti Fe ratio Since nuclei are accelerated in the same sources, the ratio of secondaries to primaries (e.g. B/C or Ti/Fe) must also rise with energy beyond ~100 GeV spallation during propagation only spallation during acceleration as well our fit ATIC-2 Zatsepin et al., arxiv: energy per nucleon GeV Mertsch & Sarkar, PRL 103:081104,2009

25 We fit the AMS-02 p, He fluxes to fix the spectral indices and normalisation, and the e - flux (in accordance with radio data) E 3 J e,e 3 J e + [GeV 2 m 2 s 1 sr 1 ] e (AMS-02) e + (AMS-02) R max = 10 3 GV R max = GV R max = 10 4 GV kinetic Energy E [GeV] E 2.7 Jp, E 2.7 JHe [(GeV/n) 1.7 m 2 s 1 sr 1 ] 10 4 p (AMS-02) He (AMS-02) 10 3 R max = 10 3 GV R max = GV R max = 10 4 GV kinetic Energy E [GeV/n] Mertsch, Sarkar, Phys.Rev.D90: ,2014

26 positron fraction B/C 10 1 R max = 10 3 GV R max = GV R max = 10 4 GV kinetic Energy E [GeV] R max = 10 3 GV R max = GV R max = 10 4 GV AMS-02 CREAM TRACER (AMS-02) kinetic Energy E [GeV/n] pbar/p We can then predict secondary to primary ratios the only free parameter is the maximum energy of the cosmic accelerator (taken to be 1, 3, 10 TeV for illustration) Measurements of B/C and by AMS-02 at higher energies then calibrate/test our model (PAMELA) R max = 10 3 GV R max = 10 4 GV kinetic Energy E [GeV] R max = GV Mertsch, Sarkar, Phys.Rev.D90: (R),2014

27 and here are our predictions confronted with the latest AMS-02 data Looks like the cutoff is closer to ~500 GV? 10 3 (PAMELA) pbar/p B/C R max = 10 4 GV kinetic Energy E [GeV] R max = 10 3 GV R max = GV R max = 10 4 GV R max = 10 3 GV R max = GV AMS-02 CREAM TRACER kinetic Energy E [GeV/n] Mertsch, Sarkar, Phys.Rev.D90, (R),2014

28 Summary AMS-02 confirms the rising positron fraction in cosmic rays (first observed by PAMELA) and this has been interpreted as due to the annihilation of ~TeV mass dark matter particles But the claim is seriously undermined by uncertainties in the modelling of the astrophysical foreground/background These uncertainties must be quantified through a better understanding of the conventional physics before claims for new physics are made (just as establishing new phenomena in the lab depends on precise knowledge of SM processes) this is increasingly important as we come to rely on astroparticle arguments to motivate BSM physics (in the absence of signals at the LHC)