ect trento experimental overview in heavy ion collisions...
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Experimental overview of Bound-Free e+ e- Pair Productionin Heavy Ion collisions
Per GrafstromCERN
ECT Trento
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Bound –Free Pair Production : BFPPor
Electron Capture from Pair production: 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.
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First ever observation of anti-hydrogen CERN 1995
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Sheldon DatzORNL
Pioneering work opened up field of molecular beam chemistry
10% of the ATLAScollaboration meeting at a football stadium
Full collaboration at dinner at my home with wives around one table
Fermi award 2000Together withSidney Drell and Herbert York
OutlineOutline
First observation at the Bevalac
Measurements at Brookhaven AGS
Measurements at the CERN fixed target heavy ion program
Measurements at RHIC
Measurements at LHC5
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First observation of Bound-Free Pair Production
U92+ on different targets: Experimental signature:U91+ and a positron
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LBL Bevalac 0.96 GeV/nucleon U92+ bare uranium ionson fixed targets of Au, Ag, Cu and Mylar
Results: 2.19 +- 0.25 barn for Au target and a Z dependence of ZT
2.8+-0.25
ZT2 expected
γ ~ 2
Charge separation
positron detection
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The same group one year later - energy and projectile dependence ( 0.4 GeV to 1.3 GeV/nucleon )
Using La57+ beam gives also projectile dependence using the U92 data: Zp
6.54+-0.65
Zp5 expected
Increase fast with energynearly as ln2 γExpected at relativistic limit ln γ
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5 years later (1998) 10. 8 GeV/nucleon measurement at Brookhaven AGS
ZT2 behaviour
confirmed
γ ~12Au79+ as projectile
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At the same time….measurement at CERN with 158 GeV /nucleon bare Pb82+ beams
γ ~ 168
Measurements done within the SPS Heavy Ion program
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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/γNon radiative electron capture: σ NRC ∼ Zt
5/γCapture via pair production: σ ECPP ∼ Zt
2 ln γ
Zp5 dependence for all three process
Mass and charge selection of Pb 82+
with first main bend and collimator
Mass and charge selection of Pb81+
800 meters
Momentum resolution 7 x 10-4
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Target thickness
Loss cross section ∼ three order of magnitudes bigger (kbarn)than ECPPNeed to measure PB81+ yield as a function of target thickness for each target type.Beyond a certain thickness -equilibrium
F(81)= fraction of Pb81+ ionsσc = total capture cross sectionσi =total ionization cross sectionσn =total cross section for beam loss by nuclear reactionst= target thicknessFeq = σc /(σc + σi ) is the equilibrium Pb81+ ions fraction
F(81) = Feq( 1-exp(-(σc + σi)t)) exp(-σN t)
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Results:
Errors of order 10 %
Correction for radiative and non radiative capture
σREC ∼ Zt/γσ NRC ∼ Zt
5/γσ ECPP ∼ Zt
2 ln γ
Clear ZT2 dependence
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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” fromultra peripheral collisions ( intra beam scattering also important at RHIC)
2. Pb81+ beam has the potential to quench the supra conducting magnets
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!
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25 Watt (design luminosity)
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First observation of BFPP in a Collider
63Cu 29 + ions with 100 GeV/nucleon
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Not a direct measurement but an observation of the hadronic showers thatwere produced when the ions struck the beam pipe .
Rigidity change
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Use p-i-n diodes on the magnet cryostat to measure
Result: Location of the shower maximum at 140.5 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.
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BFPP at the LHC
Work ongoing on tracking ,shower simulationand beam loss response in order to extract rough estimate of the cross section.
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10 mm
ATLAS Roman Pots
Blue: PB82+ beamGreen: PB81+ beamRed: 1n lossYellow: 2n loss
20 mmAll 6 σ envelopes
~ 400 meters
Can we measure the BFPP cross section properly using ATLAS Roman Pots?
First impression:NOT feasible
ConclusionsConclusions 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
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BACK UP
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Additional……
Operational aspects:See chamonixBumps shallow angle factor 5Quench more robust 1-2 design lumi without quench 20152018 New instrumented collimators