1. New results from the CHORUS Neutrino Oscillation Experiment Pasquale Migliozzi CERN XXIX International Conference on High Energy Physics UBC, Vancouver, B. C., Canada July 23-29 1998

2. Outline  The CHORUS experiment  Data  Present results  Outlook  Conclusions

3.  Search for using the WBB at CERN at the Dark Matter scale (  m 2 ~1 eV 2 )  Philosophy: direct detection of  decay (kink) The CHORUS experiment EMULSION AS “ACTIVE” TARGET

4. The neutrino beam The mean energy of the  beam at the CHORUS emulsion surface is The anti-neutrino contamination of the beam is The prompt   contamination

5. target and target tracker target and target tracker  Purpose of electronic detector predict tracks back into emulsion kinematical cuts suppress background, e.g. D decays (particle id, kinematics)  Target: ~800 kg nuclear emulsion (<1  m space resolution) Observe the  decay topology (kink) in emulsion

6. Data (data taking finished in 1997)

7. Microscope event view 150  m 120  m Emulsion to the beam Good tracks appear as dots (grains) Tracking implies connection of dots in different layers

8. Automatic scanning: Track Selector (developed in Nagoya)

9. 1  channel  Identify the muon track and require 0>P  >-30 GeV/c  no other leptons at the primary vertex  require a kink along the muon track with P T >250 MeV  Flight length cut (l<5plates)  Antineutrino charm production (D - ) with the primary lepton not identified 1.6 x 10 -6 x N CC Signal definition (BR=~18%)Background

10. 0  channel  Identify a non-muon track and require - 1>P h >-20 GeV/c  no other leptons at the primary vertex  require a kink along the hadron track with P T >250 MeV  Flight length cut (l<3plates)  Antineutrino charm production (D - ) with the primary lepton not identified 2.3 x 10 -6 x N CC  Neutral current events with a  - kink-like interaction (white kink) 2 x 10 -5 x N CC Signal definition (BR=~68%)Background

11. Analysis steps  Event reconstruction by electronic detectors  track predictions in emulsion  Automatic scanning in interface emulsion sheets  Automatic scanning in bulk emulsion: vertex location and rejection of not-  candidates by search for short decays (decay inside the vertex plate) search for long decays (decay outside the vertex plate)  Semi-automatic eye scan and remeasuring of retained events  Event topology and kinematics (emulsion+electronic detector)    candidate or identified background

12. Vertex location Vertex location procedure Location efficiency scan-back track

13. Vertex validation Impact parameter measurement Small IP = normal  event Big IP = interesting event (  ?) Semi-automatic eye check scan-back track

14. Offline selection  Offline selection small impact parameter between parent and daughter kink point is in the vertex plate  Semi-automatic eye check the events selected automatically are checked by eye for the final judgement  - kink detection ( parent search )   or h Principle Principle: Parent track (  ) can be detected by wider view and general angle scanning at the vertex plate general scanning area Impact parameter scan-back track

15. Experimental check of the  detection efficiency  Computed using Montecarlo simulation  Checked using events with 2 muons in the final state (~2% of the total) locate the muon tracks in the emulsion scan the emulsion searching for a kink charm dimuon candidates (muonic D + decays)  Yield in agreement with Montecarlo assuming  charm /  CC = 5% (measured)

16. A special event CERN-EP/98-87 Parallel session 5 July 25th talk given by O. Melzer

17. Results New data: 31974N  +  362N  events have been analysed No   candidate has been found! where if i=4 the  is not identified @ 90% C.L.

18. CHORUS efficiencies

19. The exclusion plot (@ 90% C.L.)

20. Outlook  Phase I (the present till the end of the year) we will just complete the scanning of the whole statistics  we gain a factor 3 in the 1  sample  we gain a factor 6.7 in the 0  sample  P  ~2 x 10 -4  Phase II (towards the final result) improvements on the reconstruction algorithms are going on because of the increasing scanning power the kink finding efficiency will improve by about a factor 2  if no  candidate will be found, the proposal sensitivity will be reached P  ~1 x 10 -4

21. Conclusions  Neutrino data taking finished in 1997  Emulsion data taking in progress  CHORUS has shown that automatic scanning procedures are reliable and fast  No  candidate has been found so far  The proposal sensitivity can be reached