instrumentation of the very forward region of the tesla detector – summary of the workshop on...

Instrumentation of the very forward region of the TESLA detector –

summary of the Workshop on Forward Calorimetry and

Luminosity Measurement, Zeuthen, 13-14 November 2002

L.Suszycki†‡

Faculty of Physics and Nuclear Techniques, University of Mining and Metallurgy,Cracow, Poland

ECFA DESY Linear Collider Workshop

Prague, 15th-18th November 2002

†For the LCAL group: K. Afanaciev, V. Drugakov, G. Klämke, E. Kousnetzova, W. Lohmann, A. Stahl, M. Ternick

‡For the LAT group: H. Abramowicz, A. Eskreys, S. Kananov, D. Kisielewska, A. Kowal, A. Levy, L. Suszycki, W. Wierba

November 17, 2002 L.Suszycki: Instrumentation... 2

Very forward region of TESLA

Two calorimeters:

• LCAL – Luminosity CALorimeter covering angles 5 – 27.5 mrad, actually it is“Beam Calorimeter”

• LAT – Low Angle Tagger covering angles 27.5 – 83.1 mrad, actually it is“Luminosity Calorimeter”


Tasks for the very forward region detectors

• Precision luminosity measurement

• Beam diagnostics

• Detection and measurement of electrons and photons at small angles

• Extension of the energy flow measurement down to small angles

• Shielding of the tracking detectors against backscattered beamstrahlung


Beamstrahlung• Energy deposition via e+e- pairs ~20 TeV/bunch cr.

One year of runs makes a dose ~10 Mgy/year in LCAL a need of radiation hard sensors

• Radial distribution up to ~4 - 5cm LAT should be save

• Azimuthal anizotropy caused by magnetic field


LCAL design

• Shape

• Two technology options: crystal PbWO4 – Moliere

radius ~2 cmLSO crystals considered

diamond-tungsten sandwich - Moliere radius ~1 cmSegment size ~half of RM

• Readout via optical fibres


LCAL segmentation

R-z projection x-y projection

30 layers = 30 radiation lengths 12 rings


LCAL calorimeter• Half barrel of

the diamond-tungsten sandwich calorimeter

8L.Suszycki: Instrumentation...November 17, 2002


Background in LCALExample of a 250 GeV electron event

1. Generated 2. Background added 3. Reconstructed


Detection of particles in LCAL

Simple algorithm to find electron or gamma:

• search for cells with signal > 3 sigma of background

• require longitudinal chain of such cells

Efficiency and energy resolution

depend on number of ADC bits 10 bits sufficient


Energy measurement in LCALEnergy resolution as functions of : R Energy


Angular resolution of LCALEnergy dependence R dependence


Fake events in LCAL• High energetic particles in background• Beamstrahlung fluctuations

Electron energy spectrum

generated (tail >20 GeV only is shown) ...and reconstructed


Beam diagnostics in LCALMeasurement of

x and z

ExerciseInput:

x = 600 nm, z = 250 mResult:x = 597 nm, z = 241 m

More about: see A.Stahl talk at this workshop


LAT geometry

• 14 `cylinders` in R24 `sectors` in 40 `rings` in z13440 cells assumed for MC studies

• Conical setup

• R: 4 to 12 cmz: 140 to 200 cm

• Silicon-tungsten sandwich 40 X0 deep


Bhabha scattering• Elastic e+e- e+e-

radiative e+e- e+e-

• Example of elastic Bhabha event (scale ratio 8:1!)

• Born approximation

d/d ~ -3

tot(27.5, 83.1mrad)5nb


Luminosity measurementR = L · obs , where obs = theor acceptance

For L = 3.4 · 1034 cm-2s-1 rate R 170 Hz `one minute` luminosity possible on-line

• Systematic errors:1. from detector acceptance

Since tot(min, max) ~ min-2 - max

-2 min-2

then L/L = 2min/ min = 2rmin / rmin

With rmin 5 cm, L/L = 10-4 needs rmin = 2.5 m

• Challenging for mechanics and thermal stability!Use of interferometry to monitor position and shape?

2. from theoryAt LEP energy theor. error achieved 5. 10-4 .At TESLA may be harder

18L.Suszycki: Instrumentation...November 17, 2002


Luminosity measurement (cont`d)Luminosity spectrum due to• ISR• Beamstrahlung• Beam energy spreadcan be measured through acolinearity of Bhabha events -

- resolution better than 0.1 mrad necessary

Background and corrections• Beamstrahlung• Synchrotron radiation• Beam-gas bremsstrahlung• Thermal photons from the rest gas• Beam size effectSo far no estimations done...


LAT shower example

Only photons (blue) and electrons (red) over 5 MeV are displayed


LAT calorimetryShowers well contained within ~ 30 radiation lengths:


LAT – showers...

Two examples of hits generated by 250 GeV electrons:Upper plots show all hits, lower plots show the shower cores

Strong scattering may affect energy measurement


LAT energy response and resolution


LAT angular uniformity Energy response Energy resolution

Much poorer performance in the first bin (first cylinder) the effect increasing with energy


LAT angular resolution

rec calculated using a simple energy weighting

• Accuracy not satisfactory

• Background not incorporated

• More sophisticated algorithm must be used for Bhabha measurement


LAT fiducial volume

Remark: LAT size is 27.5 to 83.1 mrad


LAT fiducial volume (cont`d).

Energy deposit as a function of polar angle All events cut Edep > 2.5 GeV


LAT performance improvedEnergy cut Edep > 2.5GeV applied

Energy response and resolution Angular resolution


Summary and outlookLCALLCAL• Advanced studies of the several technologies• Detection of hard electrons and gammas with high efficiency feasible• Fake events due to beamstrahlung fluctuations may be a problem• Beam diagnostics looks promising• R&D startedLAT• First MC studies done• Problems with energy resolution and angular resolution• Shape and segmentation is still an open question

flat LAT “ l=5m” option is very recommended:1. Makes mechanical design more realistic2. Improves resolution

• Single module for tests will be prepared soon

The LCAL+LAT proposal will be recommended by the PRC, so let`s look forward in this adventure!

instrumentation of the very forward region of the tesla detector – summary of the workshop on...

Documents

e e pairs

r energy

energy measurement

lcal group

lcal luminosity calorimeter

lcal designshape

energy flow measurement

luminosity calorimeterl