Contalbrigo Marco INFN Ferrara CLAS Transverse Target Meeting 4th March, 2010 Frascati.

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


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


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



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


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


Field maps

Field in un-measured regions extrapolated from: high order polynomials fitting measured points MAFIA simulations tuned in the measured region


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.


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.


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


M. Contalbrigo 16

Slightly different efficiency depending on track multiplicity

Transverse magnet correctionsTMC1 vs TMC2

No effect in the observed azimuthal moments



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



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


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


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


x

Best output for phenomenological models of TMDs

Q2


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:


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


€

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


Testing the method with GMC_TRANSArbitrary input model


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:

Contalbrigo Marco INFN Ferrara CLAS Transverse Target Meeting 4th March, 2010 Frascati.

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