Studies of Charmed Particle Production and Decay in the

CHORUS Experiment

Emiliano Barbuto

University of Salerno and INFN (Italy)

For the CHORUS Collaboration

The CHORUS Experiment• Short Baseline: L = 0.6 Km, <E> = 27 GeV

• Appearance: beam, / 10-6,

• Accelerators: West Area Neutrino Facility (WANF) from SPS (CERN)

XN

Conceptual DesignDetect interactions produced by oscillation in the beam:

))((

)(0

0

n

nh

The signature of a interaction is the path of the particle in the emulsion: its trajectory is subjected to a kink. Detecting a kink in the emulsion, on a particle trajectory surviving some kinematical cuts, is the direct evidence of a interaction.

CHORUS Detector

Veto

Emulsion target + Fiber tracker

Calorimeter

Air-core magnet

Muon Spectrometer

Nuclear emulsion (770 Kg) is the active target for neutrino interactions, target + scintillating fibers track the interaction back to the emulsion target.

Air-core magnet spectrometer provides hadron sign and momentum.

Showers energy and missing Pt are detected by lead-fiber “spaghetti” calorimeter.

Muon Id, sign and momentum are provided by an iron-core muon spectrometer.

Electronic veto rejects particles not generated by interactions in the emulsion target.

2

2

22.0035.0

c

GeVp

p

p

GeVmrad

ElectronsEE

E

HadronsEE

E

hadrons 10@60

)(%14

)(%32

)70%(15%10 GeVpp

p

Scan-back Procedure

• Topological + kinematical information of selected events are given to the emulsion laboratories

• Good quality tracks (scan-back tracks) are selected and searched in the interface emulsion sheets (CS and SS)

• Tracks found in these sheets are projected back in the nuclear emulsion target (36 Target Emulsion Sheets)

• Tracks are searched in the emulsion target sheets up to the interaction vertex detecting any possible decay.

• By means of kinematical cuts applied to select decay signal and to reduce scanning load some events are predicted.

Associated charm production in CC neutrino interaction

Physics motivation:This phenomenon is invoked to explain prompt trimuons and like sign dimuons in high energy neutrino interactions. Predicted value of the cross section was smaller by more than one order of magnitude than the measurement performed. Because of the large background from non prompt dimuons, no definite conclusions could be reached.

An important contribution from CHORUS:Due to the good spatial resolution of nuclear emulsion (1m) we can perform a direct observation of the associated charm production eliminating background due to non prompt dimuons. A direct observation of associated charm production was never performed so far.

c

c

cXcN

c

cXcN

Trimuon

Like-sign Dimuon

XN

Background

Automatic selection of Neutral D0

The impact parameter (IP) technique can detect decay and D0 decay because, in both topologies, has a large impact parameter with some emulsion tracks belonging to the event.

- -

-

D0

Large IPLarge IP

By computing intersection of tracks reconstructed in emulsion we can automatically select events with large IP. 1000 CC interactions were selected and checked by eye, these gave 98 D0-like events confirmed by eye-check.

The principle of the search:

The hadronization fractions evaluated using the Herwig event generator are given in the following table. More than 70% of the associated charm events show up with at least one neutral charmed meson. Therefore it is efficient to start with already located D0-like events.

Eye CheckBlue lines = Scan-back track (muon), matching the electronic detector

Red lines = tracks matching the electronic detector

Black lines = tracks not matching the electronic detector (to follow down in the next emulsion sheets in order to detect any possible decay)

Emulsion Sheet (Side View)

Dashed lines = Neutral particles (not visible in emulsion)

Solid lines = Charged particles (visible in emulsion as sequences of grains)

Bold lines = Nuclear fragments (visible in emulsion as thick black tracks)

All particles are not visible in the plastic support

Video Image of Neutrino Interaction (Front View – Microscope View)

D0

-Emulsion Layer 350 m

Emulsion Layer 350 m

Plastic Support 100 m

200 m

200 m

Eye CheckNeutral charmed hadron identification in emulsion

Non-planarity of two prongs decay: in D0

decays, neutral particles (not visible) cause the non-planarity of the two charged prongs with respect to D0 direction (reject K0 and two-body decays that have no neutrals). We require the angle ||0 between the two prongs plane and the D0 direction.

*ctL

D0

-

Flight length: in agreement with D0 life-time

Daughter electronically reconstructed: to join the secondary interaction to the primary we are observing and no black tracks at secondary vertex

Plane of the two daughter particles

D0 direction

Charmed partner search

98 D0-like events selected

Following-down by eye all primary tracks not matching predicted tracks (>20mrad and ||<400mrad) along 5 plates down-stream of the vertex plate(4000m).

When D0 decay is validated the “Charmed partner search” starts.

Neutral Partner Charged Partner

Nuclear emulsion was scanned in a fiducial volume of 2mm 2mm 7plates using volume scanning technique (Net-scan) that will be presented later.

No partner found 1 candidate found

L = 7560 mKink = 310mrad

D0

L = 340mt = 2.810-13s(D0) = 4.1510-13sCoplanarity: = (48 5)mrad(not compatible with 0)

Momentum by MCS: p = 0.78 MeV/cIonization in emulsion: dE/dx Proton

Transverse Momentum: p > 0.33 MeV/c

Event 72620473

L = 1010 mKink = 420mrad

Possible Interpretation

Background

Process Note Event Yield (10-3)

Relevant cuts applied

N -c+XK0

s (from primary) decays 0.6 Decay in plate

acoplan.||>10mrad

N -c+XK0

s (from c+ decay) decays 1.6 Decay in plate

acoplan.||>10mrad

N -c+X (from primary) decays 1.3 Decay in plate

acoplan.||>10mrad

N -c+X (from c+ decay) decays 0.2 Decay in plate

acoplan.||>10mrad

N -D0X White kink 16 p>250MeV

N -D0X And K decays Negligeable L<4mm; p>250MeV

Overall 204

In emulsion, most hadronic interactions can be distinguished from decays by their visible nuclear recoil or Auger electrons. The remaining interactions, “white kinks”, quasi elastic scatters of hadrons on nuclei, have a long interaction length and a steeply falling p distribution.

The Background is mainly topological...

... and it is low: (204)10-3 events expected on the sample taken into account.

Cross-section measurement

Systematic error: the energy-dependence of the cross section in the MC simulation affects the result through the energy dependence of the detection efficiency.

44.11.1

1.108.3 109.5

syststatN

N

N

dEEEEN

ccCC

CCobs

CC

cc

CCCCCC

ccccccccobs

Statistical error: defined by the 68.27% confidence interval as derived in the unified approach to the analysis of small signals (observed events, computed background) by Feldman, Cousins.

Weighted over the flux.

ConclusionsCross section for these events is larger than expected (agreement with the indications of previous experiments).

It is obtained in a topological search with very low background, supporting the interpretation of these events as due to associated charm production.

D0 production in CC interactionsUsing the same volume scanning technique, adopted for the neutral partner search in the associated charm production, we can perform a general search for D0 production in located CC interactions around the primary interaction vertex.

• Detecting a CC event with D0 decay is that it must be reconstructed by CHORUS electronic detector (Electronic reconstruction efficiency);

• Event must be reached by the standard scan-back procedure adopted in the emulsion scanning laboratories (Location efficiency).

Procedure: large samples of neutrino interactions were generated by JETTA (derived from LEPTO and JETSET); Q.E. reactions generated by RESQUE (8.5% relative to D.I.S.). These events were processed with the reconstruction program of the experiment to simulate the CHORUS electronic detector response. The tracks in the emulsion and the scanning procedure for the event location were also simulated to evaluate the location efficiency.

The ratio of the detection efficiencies of events with a D0 in the final state to that of all CC events is:

.35.0.007.0871.0

0

syststatCC

D

Systematic error is evaluated by varying the conditions of the simulation.

• Emulsion Scanning of the Volume around the event (scanning of emulsion sheets to record tracks’ segments around the detected event position)

• Reconstruction of the event (Automatic event reconstruction using data recorded in the emulsion sheets)

• Selection of decays (to reduce eye-check load and select a pure sample of interesting decay topologies)

• Manual check (to confirm decays selected by automatic reconstruction)

• Post scanning analysis (multiple scattering analysis, electron pair detection for decays rejection)

Volume scanning techniquesOnce the event is reconstructed by the electronic apparatus and located by the scan-back procedure (generally the muon is followed back up to the primary vertex) no decay is still detected.

It is necessary to search in a volume around the primary vertex to find a possible decay. To perform this, some volume scanning techniques were used. The general procedure is the following:

Volume scanning techniquesThe Scanning volume

8 plates 800 m = 6400 m

1500 m

1500 m

Volume scanning techniques

Net-Scan technique (Emulsion Laboratory - University of Nagoya - Japan)

It is based on an hardware equipment (track selector) interfacing a camera grabbing images from emulsion. The “Track Selector” reconstructs tracks in emulsion along a selected direction within a depth of about 100m. It is the technique that allowed to perform the detection and the analysis of the D0 decays sample in CHORUS. The Track Selector also provided a huge statistics for the computation of the oscillation limit in CHORUS.

The key point is the analysis of tomographic images of nuclear emulsion for reconstructing tracks left by the charged particles.

Total-Scan technique (Emulsion Laboratory - University of Salerno - Italy)

It is based on an software system (SySal) analysing emulsion images coming from a frame grabber board. SySal is a multi-tracking system reconstructing tracks along the whole emulsion volume. SySal gave its contribution to statistics for the computation of the oscillation limit in CHORUS.

Event ReconstructionStep 1: Emulsion Sheets AlignmentA first plate to plate alignment allowing for relative translation of the emulsion sheets is performed by comparing the pattern of segments in a plate with the corresponding pattern in the next plate.

Total-Scan reconstruction with about 6000 segments.

Segments in different plates are associated within some tolerances in posiitons and slopes and tracks are reconstructed.

On the right a Total-Scan reconstruction with about 200 tracks (having at least 3 segments)

Step 2: Tracks Reconstruction

Step 3: Fine AlignmentA, more accurate, alignemnt of sheets is performed using tracks passing through the whole volume

Step 4: Vertex DetectionA 2 cut is applied to select well reconstructed tracks; tracks passing through the scanning volume are rejected; the remnant part of tracks is tried to be associated into vertices.

A Total Scan reconstructed event consisting of 2 vertices.D0 production (Flight length = 137 m).

+ N - + D0 + X D0 + + h- + + Y

Decay topologies in emulsion

V2 V4

C1

Dashed lines = Neutral particles (not visible in emulsion)

Solid lines = Charged particles (visible in emulsion as sequences of grains)

Bold lines = Nuclear fragments (visible in emulsion as thick black tracks)

C3

C5

Background events

Electron pair

Low momentum scattering

Hadron Interactions

Manual Scanning25693 CC interactions were analyzed = 22415 (p<30GeV/c) + 3278 (p>30GeV/c)851 events were selected by the previous criteria for eye-check to confirm the decay topology

Topology Accepted Events

V2 226

V4 57

C1 121

C3 124

C5 7

Total 535

The confirmed D0 sample contains 283 candidates

Rejected Events Why?

174 Low momentum

68 Hadron Interaction

42 -conversion

2 -ray

30 other

316

Reconstruction efficiencyDuring the event reconstruction procedure there are different steps to take into account for the coputation of reconstruction efficiency.

The geometrical acceptance of the reconstruction volume: geo

The efficiency of the reconstruction algorithm: net

The efficiency of the selection criteria: sel

These coefficients are different for each topology.

V2(%) V4(%)

geo 96.60.2 96.30.5

net 88.50.4 95.20.6

sel 68.60.6 76.41.1

Combined 58.60.7 70.11.3

It takes into account if the event is contained in the scanning volume.

If there is enough information to detect the event and if the sheet alignement algorithm and the vertexing algorithm are able to provide a decay alarm.

If the decay selection criteria applied in the analysis do not reject the event.

Detection efficiency

Detection efficiency as a function of E for 2-prongs (left) and 4-prongs (right) topology.

Systematic errors: samples generated with different structure functions and different fragmentation functions (4.6%), different sets of spurious emulsion data having different track densities and alignment accuracies (2%).

Background Evaluation: by processing with the same chain of programs CC interactions with no D0 in the final state the background is evaluated to be (3.6 1.0)10-4 per located CC event. In the present sample of 25693 events this corresponds to 9.22.6 background events, mainly K0

s and 0 decays.

Final result 2

0

10.11.0.13.099.1 syststatCC

D

27.29.1

2

101.202

4...

100.41.232

4

V

VVAW

V

V

D0 production rate as a function of neutrino energy. The result of this analysis are shown as solid lines and compared with those of the E531 experiment (dashed lines). The data points of E531 have been scaled with their measurement of the D0 rate compared to the total charm production rate. The curve shows a fit based on the slow rescaling model to NOMAD charm data multiplied by the (D0/charm) cross-section ratio measured in the present experiment. The experimental results well fit the model.

Conclusions

CHORUS hybrid detector and fast emulsion scanning systems, adopted in the experiment, resulted to be useful, not only to reach important results for what concerns neutrino oscillation, but, also for charm physics.

A study on associated charm production was performed giving a value for relative cross section and providing the first direct evidence of this phenomenon.

The experimental result confirms a cross section that is one order of magnitude larger than the theoretical prediction, in agreement with previous experimental values.

A study on D0 production, induced by neutrino interaction, was also possible, reaching an high statistics and an accurate value for relative cross section.

This result is in agreement with previous experimental values of the cross section and with the slow rescaling model.