Status of the cross Status of the cross section analysis in section analysis in

ee!! e e

Yelena ProkYelena Prok

PrimEx Collaboration PrimEx Collaboration MeetingMeeting

March 18, 2006March 18, 2006

OutlineOutlineEvent SelectionKinematic and Geometric CutsComparison with Simple SimulationBinning and AcceptanceObtaining the YieldWhat remains to be done

Dedicated Compton RunsDedicated Compton Runs40 (good) runs with the 5 % r.l. 12C target Target density=2.193 g/cm3

Target thickness=380 £ 2.54 cm Atomic mass= 12 Atomic number=6

2.1<Ebeam<3.1 and 4.9<Ebeam<5.5 GeV

4 (good) runs with the 5 % r.l. 9Be target Target density=1.848 g/cm3

Target thickness=70.6 £ 2.54 cm Atomic mass=9 Atomic number=4 4.9<Ebeam<5.5 GeV

Event ReconstructionEvent Reconstruction Skim files were created from raw datafiles at least 2 clusters, energy sum of 2 clusters >0.5 GeV,

central hole (3.8 cm) is cut out

Standard PrimEx reconstruction program used, with the following parameters

Clustering algorithm: combination of “island” + 5x5 algorithm

Using Double Arm Compton Calibration Constants Z=731.9 cm (survey) Coordinate reconstruction (method 3, alignment

correction, depth correction) Non-linearity correction in energy reconstruction

Event SelectionEvent Selection

To select candidates for a given event, look at the time difference distribution between the tagger and HYCAL (bit 9), then take all photons in a defined time window. Here, time window of § 6 ns is used.

Data SelectionData SelectionElasticity cut:|(eElasticity cut:|(e11+e+e22)/ebeam-1|<0.1)/ebeam-1|<0.1

4.9<Ebeam<5.5 GeV 2.1<Ebeam<3.1GeV

Data SelectionData SelectionCluster Separation:Cluster Separation:

4.9<Ebeam<5.5 GeV 2.1<Ebeam<3.1 GeVsep>18 cm sep > 25 cm

Data SelectionData SelectionGeometric Cuts (1)Geometric Cuts (1)

4.9<E<5.5 2.1<E<3.1

Fiducial region clearly contaminated by pair production is cut outMost likely, not all of the contamination is removed by this cut

Data SelectionData SelectionGeometric Cuts (2)Geometric Cuts (2)

Due to the potential problems with cluster reconstructionin proximity to the central hole, a region of 6x6 cm is cut out

Particle 1More

energetic of the two

Particle 2, less energetic

Data SelectionData Selection A simulation using GEANT 3 was performed with

the incident photons in the energy ranges of 2.1 <E< 3.1 and 4.9<E<5.5 GeV and target parameters of the experiment. No radiative losses were included. The purpose of this simulation is to determine detector acceptance, check the experimental kinematic distributions and to determine a set of appropriate cuts for data selection.

For compatibility, the experimental and simulated data are arranged into pairs of particles, so that the first particle of the pair is always most energetic

Comparison with SimulationComparison with SimulationCluster SeparationCluster Separation

cm cm

4.9<E<5.5 2.1<E<3.1

Comparison with SimulationComparison with SimulationGeometryGeometry

4.9<E<5.5 GeV 2.1<E<3.1 GeV

Comparison with SimulationComparison with Simulation11 vs vs 22 (deg) (deg)

4.9<E<5.5 GeV 2.1<E<3.1 GeV

Data Binning and AcceptanceData Binning and Acceptance• Data is binned consistently with the

photon flux binning in tagger TChannels• A simulation was done for the energy

range of every bin• Same cuts were applied to the simulated

and experimental data• Expected cross section and geometric

acceptance determined for every energy bin

Bin # tchannels E min (GeV) Emax(GeV) Acceptance(%)

1 1+2 5.45 5.5 6.29

2 3+4 5.39 5.45 6.45

3 5+6 5.32 5.39 6.61

4 7+8 5.27 5.32 6.71

5 9+10 5.22 5.27 6.83

6 11+12 5.17 5.22 6.95

7 13+14 5.12 5.17 7.08

8 15+16 5.07 5.12 7.18

9 17+18 5.01 5.07 7.3

10 19+20 4.95 5.01 7.4

11 21 4.89 4.95 7.5

30 59+60 3.0 3.13 6.81

32 63+64 2.8 2.9 5.84

34 67+68 2.61 2.7 4.89

35 69+70 2.52 2.63 4.40

36 71+72 2.44 2.53 3.81

38 75+76 2.30 2.37 2.80

39 77+78 2.22 2.31 2.33

Counting pairsCounting pairs• Summary of cuts: -Minimum cluster energy = 1 GeV -Cluster separation > 18 (25) cm -Central Area § 6 cm is cut out -Area mostly affected by pair- production contamination is cut out -Elasticity cut: |(e1+e2)/Ebeam-1| <0.1

• All events that passed the cuts above are used to fill the following histograms:

e1+e2-Ecompton vs bin #

Ebeam-Ecompton vs bin #

1 -2 vs bin #

e1+e2-ebeam vs bin #

Counting pairsCounting pairs

Each type of histogram is fitted with a gauss+gauss and gauss+polynomial functions

Ebeam-Ecomp

e1+e2-Ecomp

e1+e1-Ebeam

Acceptance Corrected Normalized YieldAcceptance Corrected Normalized Yield(per atom)(per atom)

Acceptance corrected normalized yield NCNY:

NCNY=C/L/A C=counts under the first gaussian

function L=*th*NA* th=target thickness =target density NA=Avogadro’s Number =atomic mass A=geometric acceptance obtained

from simulation

Detection efficiency and reconstruction efficiency are not known and set to 1

12C target

Acceptance Corrected Normalized YieldAcceptance Corrected Normalized Yield (per electron) (per electron)

12C target

Acceptance Corrected Normalized Acceptance Corrected Normalized Yield (per electronYield (per electron))

9Be, 5% r.l. target

Run 4875

Yield Stability vs Run Number Yield Stability vs Run Number (for (for 1212C target)C target)

Bin 35Problem with flux

Bin 1 Bin 2

Bin 3 Bin 4

Bin 5 Bin 6

Bin 7 Bin 8

Bin 9 Bin 10

Bin 11

Bin 30 Bin 32

Bin 34

Bin 36 Bin 38

Bin 39

VETO EffectVETO Effect

Requiring that one of the clusters is charged and the other one is neutral throws away ~10-20% of the data that passed the rest of the cuts

4.9<E<5.5

2.1<E<3.1

VETO effect (cont)VETO effect (cont)

No VETO cut1 neutal, 1 charged cluster

Background reduced: ~ 40 %Signal reduced:~ 20 %

Angular DistributionsAngular Distributions

DATA vs DATA vs GEANTGEANT 2.1<E<3.14.9<E<5.5

fast

slow

fast

slow

e- e-

Summary and OutlookSummary and OutlookWhat has been done First look at the total cross

section in e! e with 12C and 9Be targets

Results are on average 5-15% below the expectation (at least, partially, this is due to the assumption that the detection and reconstruction efficiencies are 100 %.)

Results show no obvious time dependence

What needs to be done Check of systematic

effects in data analysis Compton generator for

primsim Full simulation which will

allow more realistic comparison with data, and evaluation of detection and reconstruction efficiencies

Better understanding of the backgrounds (more simulations)

Trigger Efficiency not completely understood

Radiative Corrections