A Highly Selective Dilepton Trigger System

Based on Ring RecognitionAlberica Toia

II Physikalisches InstitutJustus-Liebig-Universität Gießen, Germany

for the HADES Collaboration

Overview• HADES dilepton second level trigger:

– The Matching Unit• Performances of the Ring Recognition algorithm (J.Lehnert)

– Identification of pair(investigation of close pairs)

• Opening angle distribution• Properties of rings

– Ratios online/offline

Overview• HADES dilepton second level trigger:

– The Matching Unit• Performances of the Ring Recognition algorithm (J.Lehnert)

– Identification of pair(investigation of close pairs)

• Opening angle distribution• Properties of rings

– Ratios online/offline

Lepton SignaturesThe Matching Unit

• Combines electron signatures • Determines p from

p (MeV/c)

Momentum

Lepton selection

The HADES magnet

Dilepton Selection• Combines leptons into un-like

sign pairs with a given opening angle and invariant mass range

Invariant Mass

m (MeV/c2)

Trigger condition % evts

>= 1 online ring 10.5%

>= 1 online lepton 2.9%

>= 2 online rings 4.6%

>= 2 online leptons 1.7%

>= 1 online dilepton 0.3%

Opening angle (°)

Data: Nov01C-C 1.5AGeV full fieldOpening Angle

-> RICH is the most selective element of the second level trigger

No Shower IPU included-> higher lepton reduction

-> different acceptance for e+ and e-

Analysis StrategyWhich leptons are lost with a given trigger condition?

-> need for a calibration reaction pp -> pp-> need for a calibration reaction pp -> pp

Offline analysis

RICHMDC

TOF

SHOWER Offline LEPTONS

Found by online ???

• Correlation of electron signatures from different detectors

• More complex algorithms than online are possible

real leptons ???

Comparison of candidates found by online hardware algorithms with results from full offline analysis

Pairs of Rings

x

xx

x xx

x

xx

x

• Opening angle of online pairs• Correlation with Mini Drift Chambers (MDC)

• fakes suppression• position resolution

xx x

IPU opening angle

All rings found by online

Fakes accompanying real rings

Correlated with MDC • local maximum needs to be extended to the 8 neighbouring pads ???• real rings main source of fake rings (~ 1 ring diameter)-> hint for smeared out rings (conversion pairs ???)• unphysical distribution at large opening angle (phase space)

Matching Rings

RICH

MDC

META

high B field

low B field

META - MDC

p ~

1/

ME

TA

- M

DC

e-e+

positrons

electrons

MDC opening angle

magnet coils Analysis steps:

• Before B field: - RICH-MDC < 2°- RICH-MDC < 5°• After B field: - MDC-META no cut

- MDC-META < 5°- 5 ns < tof < 8

ns-> best candidate selected

Further coincidence with TOF / Shower („META“ - MultiplicityElectronTriggerArray)

• „Singles“ still contaminated by unresolved close pairs • Different behaviour of „singles“ and „doubles“• No clean discriminating criteria

1 ring matches 2 MDC („double“)

2 rings match 2 MDC („singles“)

1 ring matches 1 MDC cluster: unresolved close pair

Properties of Rings

# pads per ring

doubles

singles

MDC opening angle

alldoublessingles

Comparison Online - Offline

Online Ringcorrelated with MDC

correlated with METACorrelated Online Ring

Online Pairs

Online Ringcorrelated with MDC

correlated with METACorrelated Online Ring

Offline Ringcorrelated with MDC

correlated with METACorrelated Offline Ring

Offline Pairs

Offline Ringcorrelated with MDC

correlated with METACorrelated Offline Ring

PAIRS RATIO (PAIR~ SINGLE LEPTON 2) :• (Online AND Offline)/Online • (Offline AND Online)/Offlineas a function of opening angle in MDC

Singles found by offline

Correlated with online 52%

Doubles found by offline Correlated with online72%

Ratio of Pairs: online/offline

Loss of pairs at ~ 4° < < 10° (destructive interference of ring and veto region)

Rings found by offline

Correlated with online 66%

opening angle

opening angle

opening angle

Pair Ratio (PAIR~ SINGLE LEPTON 2) as a function of opening angle in MDC

Ratio of Pairs: offline/online

Rings found by online

Correlated with offline 66%

Doubles found by online

Correlated withoffline 78%

Singles found by online

Correlated with offline 53%online / offline ~ offline / online

-> need for an absolute reference system, i.e.: pp -> ppas a calibration reaction

opening angle

opening angle

opening angle

Pair Ratio (PAIR~ SINGLE LEPTON 2) as a function of opening angle in MDC

Online AND offline / offline Offline AND online / online

Rings found by offline

Correlated with online 66%

Doubles found by offline

Correlated with online 72%

Singles found by offline

Correlated with online 52%Singles found by online

Correlated with offline 53%

Doubles found by online

Correlated with offline 78%

Rings found by online

Correlated with offline 66%

opening angle

opening angle

opening angle opening angle

opening angle

opening angle

Simulation of Lepton Pairs

2 leptons per event shot from target• 100 < p < 1000 MeV

• 25° < < 40°• 65° < < 80°

• noise 1%

opening angle (deg)

Single lepton efficiency

ASYMPTOTIC ~ 98%

ASYMPTOTIC ~ 90%

--- stronger drop at 1° < < 3° due to higher photon multiplicity (veto)--- lower saturation efficiency due to lower photon multiplicity

Summary and OutlookAlberica Toia

Ingo Fröhlich, Adrian Gabriel, Daniel Kirschner, Wolfgang Kühn, Jörg Lehnert, Erik Lins, Markus Petri, Jim Ritman, Daniel Schäfer, Michael Traxler

II. Physikalisches InstitutJustus-Liebig-Universität Gießen, Germany

• Low magnetic field: • possibility of resolving close pairs (conversion)• better understanding of the ring properties

• Characterization of relevant figures of the online Ring Recognition Estimation and methods for:

• Single lepton efficiency• Pair ratios online/offline and viceversa• Reduction factor

• Characterization for different thresholds• Full analysis of simulated data• Lower limit for ring recognition efficiency• Need for a calibration reaction

• Low magnetic field: • possibility of resolving close pairs (conversion)• better understanding of the ring properties

• Characterization of relevant figures of the online Ring Recognition Estimation and methods for:

• Single lepton efficiency• Pair ratios online/offline and viceversa• Reduction factor

• Characterization for different thresholds• Full analysis of simulated data• Lower limit for ring recognition efficiency• Need for a calibration reaction