Preliminary comparison of ATLAS Combined test-beam data with G4: pions in calorimetric system

Andrea Dotti, Per Johansson

Physics Validation of LHC Simulation

23rd February 2005

Outline:

ATLAS CTB Setup

Brief description of CTB Data and G4 data

Summary of results for different energies, eta values and analysis methods

Conclusions

Work to be done before mass production

ATLAS CTB Setup

Beam

ID

Calorimetry

Muons

Only first chamber is shown

...

CTB Data Set

Considered a “good” run list

 at = 0.35 and = 0.2 energies from 20 to 350 GeV

Fit method to reconstruct energy in TileCal, cubic fit for energy reconstruction in LArg (this is not “perfect” for LArg, it is only a backup solution, while waiting for the Optimal Filter method)

Used LArg information to separate e/, MuTag (scintillator)+ TileCal to identify muons, beam line instrumentation (quality cuts)

G4

G4 data simulated with the ATLAS/CTB Sw release for preproduction studies

Physics list QGSP_GN (for standalone 2002 studies: QGSP2.7, small difference between the two expected <1%)

Energy in LArg and Tile reconstructed with Optimal Filter method (not the same reconstruction methods as for the data)

No photostatistics applied for Tile (small effect expected)

Not fully optimized to the CTB setup (beam divergence, momentum smearing)

Noise is applied (at the level of samples)

Exactly the same cuts used on data have been applied

More details on Energy Reconstruction (TileCal as example)

G4 Hit in scintillator

Q=fem*pC/GeV*Ehit(MeV)

Signal (charge) in TileCal:

factors from both data and MC

Signal Shaping:

shape obtained by calibration system

Energy Reconstruction

Algorithm

This is what we have

in real data

Energy Reconstruction: analysis

First study: simple approach

The energy is reconstructed summing all the cells in a small eta phi region (±0.1) around impact point

Second study: noise cut and cryo correction

The energy is reconstructed summing all the cells with 0<<0.7 if E>2.2

2.2 obtained from electrons contamination: the value for which we obtain the best linearity (20 – 180 GeV). The pions reconstructed energy doesn't depend too much on the noise cut.

Correction for energy lost in cryo added

Ecryo = sqrt(Eback*EtileA)

The two analysis give very similar results.

Sum in eta phi region: 180 GeV =0.2

LArg + Tile

Gaussian fit gives:

MC: 139 GeV  = 23.8

DA: 132 GeV = 20.2

5000 Evts in MC (blue)

~5000 Evts in Data (black)

Sum in eta phi region only TileCal: 180 GeV =0.2

Tile Only

Pions showering only

in Tile

~15 GeV

Sum of cells above noise: total energy
worst case for LArg (20 GeV) =0.35

Cubic fit is not precise enough?

Good agreement

with Tile

LArg

Tile

LArg + Tile

Sum of cells above noise: total energy
worst case for Tile (350 GeV) =0.35

Cubic fit is better at high energy

Poor agreement with Tile: too much energy in G4

LArg

Tile

LArg + Tile

Sum of cells above noise: total energy
middle case (180 GeV) =0.35

Two factors can play a role in

the disagreement:

1 – The differences in the energy reconstruction. But the effect should be small for Tile. We shall try to use the same energy reconstruction algorithms

2- The physics list doesn't describe precisely the energy scale. TileCal participated in QGSP/LHEP validation (see: CERN-LCGAPP-2004-10) can we expect differences in G4 since then? different physics lists (QGSP2.7 vs QGSP_GN) and G4 versions 5.2 vs 6.2).

Can also be a mixture of the two effects

LArg

Tile

LArg + Tile

Response to pions h=0.35

±20%

The agreement is poor:

agreement between data and

G4 is ±10%

Another study (Gia Khoriauli) using Calo Calibration Hits shows a better agreement.

Summing nonEM+EM energy from all hits (scintillator+absorber+...) the agreement is ~5%.

Is this and indication that the problem is in energy reconstruction (from hit to reco energy)?

Note: Gia is using topo_clusters for data

Black: Data

Red: MC

Ebeam (GeV)

Ereco (GeV)

Ebeam (GeV)

Ereco(DATA)/Ereco(MC)

Comparison with 2002 Standalone: preliminary

We approximated e/ (Tile standalone) with Ebeam/ and fitted the peak of pions showering only in Tile obtaining for the point at 180 GeV:

e/ (CTB) = 1.23

e/ (CTB-G4) = 1.12

e/ (2002) = 1.23

e/ (2002-G4) = 1.2

We obtain the same value for data, but the difference in G4 is 7%. Indicates a problem in G4 simulation or in the energy reconstruction method

Conclusions

For previous study (standalone TB) ±5% agreement between G4/Data was reached (and was considered sufficient). Still lot of work has to be done. We need to improve both in analysis and the simulation

At this stage it is difficult to verify in details the shower development simulation (it was the main concern in standalone comparison)

We need to disentangle G4 and energy reconstruction method to verify each step in the simulation

The disagreement between G4/Data depends on the energy (LArg simulation is better at high energies, the opposite for TileCal)

Work to be done:

Important step: check all the constants and methods that reconstruct energy starting from a G4-Hit: sampling fraction, pC/GeV, noise contribution, ....

Select pions showering in TileCal in G4 data and compare the results with standalone simulation results (we want to obtain the same level of agreement ±5%)

Reconstruct events with same algorithm of data (for LArg use parabolic fit while waiting for OFC), try different combinations

Compare G4/Data for e/ and e/h ratios

Check Shower Profile

Comparison between CTB and G4: preliminary results for Calorime

2005-02-22T14:35:33