Particle Physics by MikeJenny

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									            WP2.3 - Robust Spin
            Polarisation Status
                                Ian Bailey, University of Liverpool
                                                 on behalf of

                              Helical Collaboration
       I.R. Bailey, P. Cooke, J.B. Dainton, L.J. Jenner, L.I. Malysheva (University of Liverpool / Cockcroft
                                                        Institute)
                                     D.P. Barber (DESY / Cockcroft Institute)
                                                 P. Schmid (DESY)
                       G.A. Moortgat-Pick (IPPP, University of Durham / Cockcroft Institute)
       A. Birch, J.A. Clarke, O.B. Malyshev, D.J. Scott (CCLRC ASTeC Daresbury Laboratory / Cockcroft
                                                        Institute)
            E. Baynham, T. Bradshaw, A. Brummit, S. Carr, Y. Ivanyushenkov, A. Lintern, J. Rochford
                                       (CCLRC Rutherford Appleton Laboratory)



LC-ABD: WP2.3 (robust spin polarisation) and WP5.1 (helical undulator).

   EUROTeV: WP3 (damping rings) and WP4 (polarised positron source).
                  WP2.3 Aims and Overview
• Developing reliable software tools that   Summary of major milestones
allow the machine to be optimised for       achieved to date:
high polarisation as well as luminosity.
                                            •Updated SLICKTRACK software
• Currently carrying out simulations of     package to include full non-
depolarisation effects in damping rings,    commuting spin rotations
beam delivery system, main linac and
during bunch-bunch interactions.            •Simulated spin dynamics in ILC
• Developing simulations of spin                •damping rings
transport through the positron source.
                                                •beam delivery system
•We require relative uncertainty on
polarisation to be less than 0.1% at IP         •main linac
for precision physics.
                                            •Evaluated theoretical
•Luminosity-weighted polarisation           uncertainties in beam-beam
cannot be directly measured with a
                                            interactions at the ILC
polarimeter.
                                            •Implemented first polarised
Collaborating with                          incoherent pair-production
     T. Hartin (Oxford)                     processes in CAIN.
      P. Bambade, C. Rimbault (LAL)
     J. Smith (Cornell)
     S. Riemann, A. Ushakov (DESY)
                    Depolarisation Processes


Both stochastic spin diffusion through
photon emission and classical spin
precession in inhomogeneous magnetic
fields can lead to depolarisation.


            ( g  2)
 spin             orbit              Photon emission
                2
1 mrad orbital deflection  30° spin
precession at 250GeV.


Largest depolarisation effects are
expected at the Interaction Points.
                                           Spin precession
                          Software Tools

                   Undulator          Collimator / Target         Capture
                                                                  Optics
Physics




                                                                               e+ source
         Electrodynamics             Standard Model            T-BMT
Process                                                        (spin spread)

Packages SPECTRA,                    GEANT4, FLUKA             ASTRA
              URGENT
                Damping ring            Main Linac /           Interaction
                                           BDS                   Region
Physics  T-BMT                       T-BMT                   Bunch-Bunch
Process (spin diffusion)
Packages SLICKTRACK,                 SLICKTRACK              CAIN2.35
              (Merlin)               (Merlin)                (Guinea-Pig)
Packages in parentheses will be evaluated at a later date.
              Beam-Beam Simulations
                                  Theoretical work ongoing into:
•Require ΔP/P < 0.1% at IP
•Compare with CAIN simulations        •validity of anomalous moment
of depolarisation at IP (below)       expression used to implement T-
                                      BMT equation in strong fields
                                      (now validated by Gudi!)

                                      •higher-order QED processes
                                      (eg second-order incoherent Breit-
                                      Wheeler - ICBW)

                                      •spin correlations in pair-
                                      production processes

•In addtion - the estimated           •validity of equivalent photon
theoretical uncertainty in            approximation (EPA) for
                                      incoherent pair production
luminosity is ~1% at ILC
                                      processes
energies                              (eg not valid for Bremstrahlung!)
              Beam-Beam Simulations(2)
Incoherent pair-production                 Polarised cross-sections for
dominates at ILC energies                  incoherent Breit-Wheeler pair
                                           production added to CAIN:




•Equivalent Photon Approximation




                                                                                       Tony Hartin
requires proper treatment of initial
                                       Beamstrahlung photons have little circular
and final state polarisation.
                                       polarisation  final state e+ /e- are largely
•Not currently included in CAIN        unpolarised.
            SLICKTRACK Simulations
Damping Rings                           Beam Delivery System

Simulations show negligible         •   Simulated 2-mrad beam line
depolarisation of vertically-       •   Spin precession ~332°
aligned spins - even when           •   Loss of polarisation ~0.06%
injection energy is close to
spin resonances.                    •   Cross-checked with BMAD
                                          •   (J. Smith, Cornell - ILC-NOTE-2007-012)
Behaviour of horizontal
component of spins more
complex (eg misaligned spin     11 mrad NLC-style
rotators)                           Big Bends

In general, horizontal
component does NOT fully                                 IR2
                                                        2 mrad
decohere
Effect of very large positron
source energy spread (+/-                                         IR1
25MeV) on horizontal                                             20 mrad
component being investigated.
            SLICKTRACK Simulations (2)
 Main Linac
•SLICKTRACK has been updated to
include acceleration and ‘linear mode’   •Expect spin precession ~26° for 11km
                                         Earth-following linac (250GeV)
•First results (w/o misalignments)
show negligible depolarisation           •Inconsistent with BMAD result?

•I.e. polarisation reduced by factor     •Further investigation needed.
~cos(10-4 radians)
                                                           d
             WP2.3 - Remaining Milestones
                  and Future Plans
The motivation for a complete cradle to grave spin tracking simulation has grown
as the high energy physics community has increasingly identified the importance
of polarised beams as a means to offset any reduction in ILC luminosity.
This work has been identified as high priority by the Global R&D board.
We have simulated most of the machine but not yet in a fully integrated fashion.
We need (LC-ABD2, etc):
    Further development of SLICKTRACK for full non-linear orbital motion study
    of BDS and study of main linac (collaborating with E. Forest and Kai Hock)
    Development of positron source simulation including effects such as beam
    jitter and the investigation of spin flip techniques.
    Further theoretical work on beam-beam interactions (beamstrahlung models,
    higher-order QCD, etc)
    A continued rolling study of the whole machine to allow optimum use of
    polarisation as a tool for the ILC.
    Benchmarking from BMAD, Merlin, etc.

								
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