Beam Conditions

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Beam Conditions
Andreas Morsch ALICE CERN/EP-AIP
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Outline

• AA and p-A Collisions
• Maximum Luminosity
• Collision Systems
• pp
• Luminosity reduction
• Vacuum and Beam Halo

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Interaction with Machine

• Baseline Documents finished 2001
• D. Brandt "Review of the LHC Ion Program", LHC
  Project Report 450
• A. Morsch "ALICE Luminosity and Beam
  Requirements", ALICE-INT-2001-10
• Link-Person D. Brandt (SL-AP)
• Committees
• LEMIC (permanent)
• LCC (invited for special topics)
• Identify critical issues and actions in
  collaboration with LHC/SL

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ALICE Global Running Scenario

• First 5-6 years
• 2-3y Pb-Pb (highest energy density)
• 2y Ar-Ar (vary energy density)
• 1y d(a)-Pb (nucl. str. func., comparison)
• pp normal runs at 14 TeV centre of mass energy
• Later options depending on the physics outcome of
  the first years
• dedicated pp or pp-like (aa) collisions at 5.5
  TeV
• possibly another A-A system out of the LHC
  proposed menu of possible candidates (O-O, Kr-Kr,
  Sn-Sn)
• maybe another d-A system (d-Ar ?)
• low energy Pb-Pb run

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Critical Issues Pb-Pb (1)

• LHC Intensity Limit
• Pb losses due to electromagnetic processes (500
  barn !)
• Quench limit close to nominal luminosity (1027
  cm-2s-1)
• Concern that actual limit could be ltlt 1027
  cm-2s-1 (official value 5 1026 cm-2s-1)
• SPS bunch intensity limit
• Tune shift and longitudinal intra-beam scattering
• New RF scheme proposed (split bunches)
• Limit close to nominal luminosity

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Critical Issues Pb-Pb (1)

• Luminosity Life-Time limited by
• Intra Beam Scattering
• Electromagnetic Cross-Section
• 1 Exp. 6.7 h _at_10 h ltLgt 0.44 L0
• 2 Exp. 3.7 h ltLgt 0.34 L0
• 3 Exp. 2.7 h ltLgt 0.28 L0

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1 Experiment
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2 Experiments
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3 Experiments
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Luminosity Lifetime

• Lifetime ? Intensity
• Increase intensity
• Increase b to obtain nominal luminosity
• Or even better decrease b during run and keep
  luminosity constant.
• However, intensity limited by SPS !
• Concern that average luminosity will be ltlt 0.5 L0

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Beta-Tuning Scheme
Luminosity lifetime problem can be solved using a
2 times higher beam intensity and start run with
a b of 2 m (see next slide ) !
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Beta-Tuning Scheme
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Intermediate Mass Ions

• At least one intermediate mass collision systems
  during the first 5 years (Ar-Ar)
• Reason Change energy density
• Other options proposed by machine (O-O, Kr-Kr and
  Sn-Sn) also interesting and possibility to use
  them should be left open
• Two operation luminosities
• low luminosity matching PbPb rate
• high luminosity maximize dimuon rate
• Ar-Ar
• low 2.8 1027 cm-2s-1 high 1029 cm-2s-1
• OO
• low 5.5 1027 cm-2s-1 high 2 1029 cm-2s-1

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Critical Issues Intermediate Mass Ions (2)

• SPS intensity limit
• long. intra beam scattering and tune shift
• No official numbers yet, but in-officially
• O Ö
• Ar 60 of requested luminosity (high
  luminosity option)

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Critical Issues pA (1)

• For rare signals
• dA or aA preferred
• Deuteron source
• Same source for protons and deuterons
• Challenge time to switch between p and d
• a seems to be better option
• Under investigation

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Critical Issues pA (2)

• Required luminosity reached through asymmetric
  beams
• high intensity p(d), low-intensity ion beam
• check possible tune-shift problems (seems to be
  ok)

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Critical Issues pp

• Luminosity Reduction from 1034cm-2s-1 to
• 3 1030 cm-2s-1 (rare signals, 20 overlapping
  events)
• 1 1029 cm-2s-1 (minimum bias)
• Two possibilities
• De-focussing L µ1/b
• Beam separation L µ exp(-d2/4 s2)
• Nominal ALICE operation
• b 10 m / 5.0 s separation gives reduction by
  1/20 x 1/500 1/10000 ( to reach 1030
  cm-2s-1 )
• LHC early operation 20 of nominal luminosity
• still ca. factor 1000 reduction needed for 3 1033
  cm-2s-1 operation

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Luminosity Reduction

• ALICE expects best conditions for pp running with
  de-focussed beams and no displacement
• Be prepared for some weeks of very low intensity
  LHC running (lt 1033 cm-2 s-1)
• Study maximum b as a function of beam intensity
  and beam emittance.
• Inner triplet can yield 0.5 m lt b lt 250 m (no
  crossing)
• Problem beam crossing and beam separation.
• Request sent to SL/AP

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Critical Issues ppBeam Halo

• ALICE runs at reduced luminosity but full beam
  intensity
• Halo/collision by 4 orders of magnitudes worse
  than for dedicated high-luminosity experiments
• Recent dynamic gas pressure estimates are up to
  factor 100 higher than previous calculations.
  (LHC Project Note 274)

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Dynamic Gas Pressure Estimates
after some weeks of running
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Dynamic Gas Pressure Estimates
first weeks of running
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Halo Issues for ALICE

• Muon trigger
• First estimates give up to 200 Hz/cm2 from hadron
  halo and 15 Hz/cm2 from muons !!!!
• Impact on RPC aging at such rates not yet
  evaluated.
• TPC/ITS
• Pile up
• To be studied

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Beam Halo Calculation

• LHC Project Note 273

LHC Project Note 273
Origin of muon halo
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Main Concerns

• PbPb (integrated) Luminosity
• Vacuum gas pressure / beam halo in pp