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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