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HERMES TRANSVERSE TARGETEXPERIMENTAL ISSUES
Contalbrigo Marco INFN Ferrara
CLAS Transverse Target Meeting
4th March, 2010 Frascati
The HERMES experiment
M. Contalbrigo 2Frascati: CLAS Transverse Target Meeting
Resolution: Dp/p ~ 1-2% Dq <~0.6 mrad
Electron-hadron separation efficiency ~ 98-99%
kinematic range ~ 7 GeV:
1 < Q2 < 10 GeV2
0.02 < x < 0.4
sself-polarised electrons:
e
27 GeV
Hyperfine energy levels as a function of the holding field
The HERMES target
M. Contalbrigo 3Frascati: CLAS Transverse Target Meeting
The 75 mm Al coated cell
N
27.5 GeV lepton beamHigh polarization ~ 80+/-3%
No dilution factor
ms polarization switching
No radiation damage
L ~ O(1031) cm-2 s-1
ONLINE ISSUES
Synchrotron radiation cone
Beam dynamic
M. Contalbrigo 5Frascati: CLAS Transverse Target Meeting
Beam trajectory
2mm shift atcell center
5 kW emitted powerat 50 mA beam
The holding magnetic field is required to inhibit depolarization mechanism by effectively decoupling the electrons and nucleons magnetic moments while providing the target spin direction.
Due to the RF fields induced by the bunched HERA beam, depolarization resonances could happen between different hyperfine states at certain B values.
The magnetic flux density has to be stabilized within 0.18 mT
HERMES Transverse Target Field
M. Contalbrigo 6Frascati: CLAS Transverse Target Meeting
M. Contalbrigo 7Frascati: CLAS Transverse Target Meeting
B Field drifts with Temperature
Automatic compensating system added: pair of correcting coils to the main coils
The magnetic flux density decreased with time due to the increasing temperature of the main yoke, pole and pole tips, affecting the magnetic permeability of the material (magnet is off during beam injection)
Additional correction coils mounted into the cell to increase spatial uniformity of the field
Before After
OFFLINE ISSUES
M. Contalbrigo 9Frascati: CLAS Transverse Target Meeting
Tracking
Two Transverse Magnet Correction algorithms:
TMC1: look up table
TMC2: inverted transfer matrix
In order to reconstruct the correct kinematics, it is necessary to measure lepton momentum and angle at the scattering vertex.
A correction must be applied to account for how much the trajectoryhas been deflected by the transverse target magnet between theinteraction point and the first drift-chamber plane.
Both corrections need accurate field maps
M. Contalbrigo 10Frascati: CLAS Transverse Target Meeting
Field maps
Field in un-measured regions extrapolated from: high order polynomials fitting measured points MAFIA simulations tuned in the measured region
M. Contalbrigo 11Frascati: CLAS Transverse Target Meeting
TMC-1
Gives a trajectory that reaches the (0,0,z) line
Look-up table: correction based on a reference track from a database.
Reference track selected to match position (closest distance cryterium)momentum and charge measured at the entrance of the HERMES spectrometer.
The database maps initial and final coordinates of a simulated sample of tracks.
The correction is defined from the reference track as the difference between the initial values extrapolated backward by assuming the trajectory is a straight line and the true values.
M. Contalbrigo 12Frascati: CLAS Transverse Target Meeting
TMC-2
Transfer function T:
Transfer function as commonly used in ion-optical system design.
Initial coordinates:
Reference particle has
Final coordinates:
Momentum deviation (in percent)
Real particle has
Parameters evaluated by numerically integrating the equations of motion by a Runge-Kutta algorithm over a testing set of particles at different starting position Z and momentum P.
M. Contalbrigo 13Frascati: CLAS Transverse Target Meeting
TMC-2
Approximated initial coordinate from a linearized transfer function L (all non-linear T terms are dropped) :
Iterative procedure starts from final coordinates (as defined by HERMES spectrometer):
New estimate of initial coordinate:
Corresponding final coordinate and its deviation from the true one:
Approximated error on initial coordinate:
Vertex is found as the point of minimum approach to the beam axis.
Does not require particle and beam trajectory coincide at a given point
M. Contalbrigo 14
Beam shift due to transverse field
Depends on beam charge!
Frascati: CLAS Transverse Target Meeting
Main shift due to the beam deviation in the upstream correction-coils
M. Contalbrigo 15
Correction of the beam shift due to the transverse target holding field
Beam shift due to transverse field
2004: electron beam 2005: positron beam
Frascati: CLAS Transverse Target Meeting
M. Contalbrigo 16
Slightly different efficiency depending on track multiplicity
Transverse magnet correctionsTMC1 vs TMC2
No effect in the observed azimuthal moments
Frascati: CLAS Transverse Target Meeting
M. Contalbrigo 17Frascati: CLAS Transverse Target Meeting
Resolution
Resolution similar to longitudinal target spin set-up
qx
zV
18
Azimuthal moments extraction
M. Contalbrigo
┴ ┴
┴
┴
┴
g1L
h1
Distribution Functions (DF)
€
FUTsin(φ−φS ) ∝ C −
ˆ h ⋅ pT
Mf1T
⊥D1
⎡
⎣ ⎢
⎤
⎦ ⎥
Azimuthal moments require careful study of instrumental effects
Frascati: CLAS Transverse Target Meeting
M. Contalbrigo 19Frascati: CLAS Transverse Target Meeting
The EVT RICH particle ID
To avoid inefficiency related to track spatial position (azimuthal angles)
Likelihood based on full event topology
Dual radiator Ring Imaging Cerenkov
No ring for p 2 rings for e, p
M. Contalbrigo 20Frascati: CLAS Transverse Target Meeting
The unbinned maximum likelihoodThe event distribution and probability density distribution for target polarization P
In a binned analysis residual acceptance dependence for integrated quantities
The unfolding of radiative effects procedure
M. Contalbrigo 21Frascati: CLAS Transverse Target Meeting
MCBORNMCEXPBgnSn '
Accounts for acceptance, radiative and smearing effects: depends only on instrumental and radiative effects
Probability that an event generated with kinematics w is measured with kinematics w’
MCEXPMCBORN
BgnSn 1'
Includes the events smeared within kinematical cuts
Remove systematics but introducestatistical correlations
The correction is averaged over the bin
One-dimensional analysis
Multi-dimensional analysis
dL
dL
n
nFLATMCMC
RAD
MCFLATMC
FLATMCMC
RAD
MCFLATMC
FLATMC
FLATMC
acc
acc
,2,2
,1,1
,2
,1
),(),()(
),(),()(
0
0
The multidimensional approach
M. Contalbrigo 22Frascati: CLAS Transverse Target Meeting
x
Best output for phenomenological models of TMDs
Q2
M. Contalbrigo 23Frascati: CLAS Transverse Target Meeting
MC tools
Sophisticated generator of the unpolarized cross-section
tuned to the HERMES multiplicities polarization dependence is introduced a-posteriori
randomly sort the spin state with probability defined by a given asymmetry model
Pythia:
Generator implementing models for TMDs and azimuthal moments
tuned to reproduce i.e. the observed dPhT/dz distribution
no radiative effects up to now
GMC_trans:
M. Contalbrigo 24Frascati: CLAS Transverse Target Meeting
Systematic error
Different tracking algorithms
alternative correction methods for bending inside the transverse magnet
standard tracking and improved version with refined Kalmann filter implementation, accounting for all B fields, misalignments and providing goodness of fit estimator.
Tracking:
Monte Carlo study comparing
reconstructed azimuthal moments with physical model in input to the simulation (evaluated at the average kinematics or integrated in 4p);
Acceptance/Resolution:
Monte Carlo study comparing
different beam position and slopes within ranges estimates by special alignment runs (dipole off);
detector aligned and misaligned geometry, the latter from survey measurements of the sub-detector positions;
indicator: top versus bottom detector halve response comparison.
Misalignment:
set of SIDIS events based on a Taylor expansion on :
The full kinematic dependence of the Collins and Sivers moments on
),,,( 2 hPzQxx
x
is evaluated from the real data through a fit of the full
)]sin();()sin();([1);,( SiSiversSiCollinstt cxAcxAPcPxf
hhCollins PzxcxcPcQczcxcccxA 222
254
23210 ...),(e.g.:
acceptance effects vanish model assumptions minimized
Full-differential physical model
M. Contalbrigo 25Frascati: CLAS Transverse Target Meeting
€
sin(φ ± φS )UT
acc,4 π(x) =
∫ σ UUacc,4 π (x ) ACollins,Sivers(x ;c i)
σ UUacc,4 π (x )∫
The extracted azimuthal moments and are folded with
the spin-independent cross section (known!) in 4 and within the
HERMES acceptance :
)( 4UU
);( iCollins cxA
)( .accUU
);( iSivers cxA
Testing the method:
Extraction:
Standard extraction method
New extraction method
Blue: within acceptance
Black: in 4
The method works
nicely at MC level!
Small effect on DATA systematic error
M. Contalbrigo 26Frascati: CLAS Transverse Target Meeting
Testing the method with GMC_TRANSArbitrary input model
M. Contalbrigo 27Frascati: CLAS Transverse Target Meeting
ConclusionTransverse data required special care during
Account for beam and scattered particles bending in target holding field
Account for full event topology in particle ID
Special algorithms to
minimize/correct instrumental effects (ML fits, unfolding, multi-D)
evaluate systematic effects (full differential model of the signal)
Preserve beam orbit
Minimize depolarizing effects
Data-taking:
Offline analysis: