ECT Trento Experimental overview of Bound-Free e+ e- Pair Production in Heavy Ion collisions. Per Grafstrom CERN

Transcription

1 ECT Trento Experimental overview of Bound-Free e+ e- Pair Production in Heavy Ion collisions Per Grafstrom CERN 1

2 Bound Free Pair Production : or Electron Capture from Pair production: BFPP ECPP In ion atom collisions a charge change can occur through either ionization or electron capture. At lower collision energies a bare ion hitting a target atom might pick up an electron from the target atom either with simultaneous emission of a photon: Radiative Electron capture (REC) without emission of photon : Non Radiative Electron Capture (NRC) At highly relativistic energies pair production starts to dominate i.e the electron which will be bound is created in the very strong electromagnetic field between the nuclei. 2

3 First ever observation of anti-hydrogen CERN

4 Sheldon Datz ORNL Pioneering work opened up field of molecular beam chemistry Full collaboration at dinner at my home with wives around one table Fermi award 2000 Together with Sidney Drell and Herbert York 10% of the ATLAS collaboration meeting at a football stadium 4

5 Outline First observation at the Bevalac Measurements at Brookhaven AGS Measurements at the CERN fixed target heavy ion program Measurements at RHIC Measurements at LHC 5

6 First observation of Bound-Free Pair Production U92+ on different targets: Experimental signature: U91+ and a positron 6

7 LBL Bevalac 0.96 GeV/nucleon U92+ bare uranium ions on fixed targets of Au, Ag, Cu and Mylar γ ~2 Charge separation positron detection Results: barn for Au target and a Z dependence of ZT ZT2 expected 7

8 The same group one year later - energy and projectile dependence ( 0.4 GeV to 1.3 GeV/nucleon ) Increase fast with energy nearly as ln2 γ Expected at relativistic limit ln γ Using La57+ beam gives also projectile dependence using the U92 data: Zp Zp5 expected 8

9 5 years later (1998) GeV/nucleon measurement at Brookhaven AGS Au79+ as projectile γ ~12 ZT2 behaviour confirmed 9

10 At the same time. measurement at CERN with 158 GeV /nucleon bare Pb 82+ beams Measurements done within the SPS Heavy Ion program γ ~

11 Basic idea: Letting the bare Pb82+ beam hitting a target and measure the yield of Pb81+ ions leaving the target for different target thicknesses. Use the full SPS beam line as a high resolution spectrometer With this method the total capture cross section is measured only works at high energies Radiative electron capture: σrec Zt/γ Non radiative electron capture: σ NRC Zt5/γ Capture via pair production: σ ECPP Zt2 ln γ Zp5 dependence for all three process 11

12 800 meters Momentum resolution 7 x 10-4 Mass and charge selection of Pb81+ Mass and charge selection of Pb 82+ with first main bend and collimator

13 Loss cross section three order of magnitudes bigger (kbarn)than ECPP Need to measure PB81+ yield as a function of target thickness for each target type. Beyond a certain thickness -equilibrium F(81) = Feq( 1-exp(-(σc + σi)t)) exp(-σn t) F(81)= fraction of Pb81+ ions σc = total capture cross section σi =total ionization cross section Target thickness σn =total cross section for beam loss by nuclear reactions t= target thickness Feq = σc /(σc + σi ) is the equilibrium Pb81+ ions fraction 13

14 Results: Correction for radiative and non radiative capture σrec Zt/γ σ NRC Zt5/γ σ ECPP Zt2 ln γ Errors of order 10 % Clear ZT2 dependence 14

15 BFPP is a limiting factor in the performance of Heavy Ions colliders RHIC and LHC 1. Luminosity life time is mainly determined by burn off from ultra peripheral collisions ( intra beam scattering also important at RHIC) With our measurement at the SPS at γ =168 we extrapolated the BFPP cross section to RHIC and LHC energies using ln γ scaling σrhiccap ~ 95 barn and σlhccap ~ 200 barn This is the luminosity limiting factor together with the burn off from electromagnetic dissociation! 2. Pb81+ beam has the potential to quench the supra conducting magnets 15

16 25 Watt (design luminosity) 16

17 First observation of BFPP in a Collider 63 Cu 29 + ions with 100 GeV/nucleon 17

18 Not a direct measurement but an observation of the hadronic showers that were produced when the ions struck the beam pipe. Rigidity change 18

19 Use p-i-n diodes on the magnet cryostat to measure Result: Location of the shower maximum at meter from IP (within meter according to optics calculation) Event rates estimated from p-i-n diodes within a factor 2 of the 4kHz expected. 19

20 BFPP at the LHC Work ongoing on tracking, shower simulation and beam loss response in order to extract rough estimate of the cross section. 20

21 Can we measure the BFPP cross section properly using ATLAS Roman Pots? All 6 σ envelopes 20 mm Blue: PB82+ beam Green: PB81+ beam Red: 1n loss Yellow: 2n loss 10 mm ATLAS Roman Pots First impression: NOT feasible ~ 400 meters 21

22 Conclusions No real conclusions I have summarized the experimental knowledge of BFPP cross sections as of today Looking forward to hear in the next talk how to proceed for better measurements at the LHC. Also looking forward to hear how BFPP can be used for «tagging» of ultra peripherial collisions 22

23 BACK UP 23

24 Additional Operational aspects: See chamonix Bumps shallow angle factor 5 Quench more robust 1-2 design lumi without quench New instrumented collimators 24