Strangelet Search in Cosmic Rays at Mountain Altitude and elsewhere

1. Dr. Sandhya Dey(Mandal) Research Scientist IRHPA – Project P.I.-Prof. Sibaji Raha(Director- Bose Institute) Centre for Astroparticle Physics and Space Science Bose Institute Kolkata -INDIA Strangelet Search in Cosmic Rays at Mountain Altitude and elsewhere

2. What are Strangelets? • Small stable lumps of SQM is what is being referred to as strangelets. • Such strangelets are nuggets of a hypothetical state of matter called Strange Quark Matter (SQM) consisting of roughly equal numbers of up, down and strange quarks. A unique experimental signature of strangelets will be an unusually low charge to mass ratio (Z/A<<1/2) compared to ordinary nuclei.

3. research goal • To look for rare events (e.g. strangelets) in cosmic rays at mountain altitude. • Theoretical studies [Madsen (2005), Biswaset. al. (2012)] point to significant measurable strangelet flux in our part of the galaxy. • Search for strangelets remain an active field of research as their discovery (or exclusion) will of significant interest for physics including astrophysics

4. Reports of events with unusual Z/A ratio • There are several observations of exotic nuclear fragments with unusual charge to mass ratio in different cosmic ray experiments. • In 1990, Saito et al. analyzed the data of 1981 balloon borne experiment and claimed to have identified two events which were consistent with A ~ 370 and Z ~ 14 and was explained in the scheme of strange quark matter. • In 1993 Ichimura et al. reported an event with unusually long m.f.p. called the ‘exotic track’ event with Z ~ 20 and A ~ 460. The report was based on an analysis of a 1989 balloon borne experiment using solid state nuclear track detector. • In a paper in 2001 Fujii et. al. reported detection of a possible SQM candidate, an anomalous massive nuclei of charge Z~14 and M~370 amu in a hybdrid system combining active (Cherenkov and scintillation )and passive detectors (CR-39) in a 19 hr balloon flight.

5. Other space based ongoing experiment looking for SQM • The Alpha Magnetic Spectrometer (AMS) is a space based particle physics experiment led by Samuel Ting of MIT. The prototype AMS-01 flew in space in 1998 aboard the Space Shuttle Discovery. Analysis of data from that mission has given hints of some interesting events, such as one with Z=8, A=54 [Aguilar et.al.(2002)]. AMS-02, which is an improvement on AMS-01, has been deployed on ISS in 2011 and is currently collecting data. One of the primary goals of this mission is the search for strangelets.

6. Use of passive detectors in cosmic ray detection Passive detectors like Nuclear Track Detectors (NTDs) offer an alternative low cost detector option to look for rare events in cosmic rays. Such detectors have various advantages : Since they do not require power for their operation or data collection, storage or transmission facilities, it is relatively easy and less-expensive to set up passive detector modules. Existence of natural thresholds of registration. Thus providing a natural and easy way of background suppression. Permanent track recorder. They offer a good charge resolution.

7. Choice of PET for strangelet search • Research carried out at Bose Institute, Kolkata [D. Bhowmiket. al. (2011), S. Deyet.al.(2011)]revealed that a particular kind of polymer, identified as Polyethylene Terephthalate (PET)with chemical formula (C10H8O4)n can be very effective as a NTD. • It was found that PET has a much higher detection threshold compared to other commercially available NTDs and as such is significantly better at background suppression. Another advantage of PET is that it is substantially cheaper compared to detector material like CR-39. • Systematic studies were carried out to study the charge response of PET using particle accelerators at IUAC, New Delhi; GSI, Damstadt, Germany and REX- ISOLDE at CERN, Switzerland and a calibration curve obtained for PET. • Indian patent application [No. 1473/KOL/2011] titled “A novel solid state nuclear track detector for charged particles” filed on 18 Nov. 2011 • We propose to use both PET (higher detection threshold) and the standard CR-39 (with lower detection threshold) as they can complement each other.

8. Work done at Bose Institute Open air exposure to Nuclear Track detector at different mountain altitudes to study change in characteristics of detectors. Characterization and calibration of PET and CR-39 with ions from accelerators in India & abroad. Study of cosmic radiation at high mountain altitude with PET & CR-39.

9. Characterization and calibration of PET and CR-39. Calibration with ions from different accelerators, 16O, 32S, 56Fe from IUAC, New Delhi. Study with ions 238U from GSI, Darmstadt, Germany. Calibration with ions 84Kr and 131Xe from CERN ISOLDE

10. Leica DM 4000 M

11. 16O exposures at IUAC, New Delhi experimental arrangement

12. Traks of oxygen ion on pet

13. Experimental setup for Fe & S at IUAC, Delhi Small (5 cm x 5 cm) pieces of PET were mounted on aluminum holders and were placed on top of two movable arms inside the GPSC.

14. Experiment with sulpher beam Beam energy : 125 MeV (~ 3.9 MeV/A) Beam current : 1 pnA Detectors were placed making angles between 20-160 degrees with respect to the beam direction. The incident energies were between 120 - 65 MeV

15. Experiment with iron beam Beam energy : 150 MeV (~ 2.7 MeV/A Beam current: 1 pnA The two arms inside the GPSC with the detectors mounted on top of them were placed at angles between 20 – 145 degrees with respect to the beam direction. So the energies of the scattered ions impinging on the detectors were between 140 – 50 MeV.

16. Experiment with Xe and Kr beam at CERN Experiment utilized 20° beam line of the REX-ISOLDE facility at CERN 2.82 MeV/u 131Xe and 78Kr beams were produced in the ISOLDE GPS target and accelerated with the REX-ISOLDE linear accelerator. The 78Kr beam contained an admixture of 49Ti ions which provided an additional data point for calibration.

17. Kr and Ti tracks on PET

18. Calibration curves PET CR-39

19. Detectors exposed to cosmic rays. Darjeeling Ooty Hanle Kolkata

20. Detectors at mountain altitudes. To detect exotic events strangelets predicted to be present in cosmic radiation at mountain altitude, we have placed plastic detectors (PET) along with CR-39 for preliminary study . The locations have been chosen are Darjeeling (Latitude- 28° N Longitude-88° E and Altitude 2.13 km asl), Ooty (Latitude- 11.4° N Longitude-76.7° E and Altitude 2.23 km asl) Hanle.(Latitude-32° 42’N,Longitude- 79° 04’E, Altitude 4.5 km asl).

21. Exposure at Darjeeling • At Darjeeling first cosmic ray exposure was given on PET. Both CR-39 and PET have been used. • Exposure was given to several sets of detectors. Several different arrangements were made. Horizontal, inclined, bending. • Duration of exposure was around six months for each exposure. • Both CR-39 and PET plates are etched in appropriate etching condition(6.25N conc and temp 55°C for PET and 70°C) in aqueous solution of NaOH. Duration of etching are 1-12 hours.

22. Observation at Darjeeling • Charge particle flux~1.5x 10-5/(cm2.s.sr) recorded on PET over different sets of exposures of various durations.

23. Analysis of tracks on PET found at Darjeeling • VT/VG=2.94±0.42 corresponding to these tracks. • Tracks very short ranged (~10 μm) making particle identification difficult. • Origin of these charged particle tracks on PET not very clear. Characterization will require further studies of local radiation environment of Darjeeling.

24. Tracks on CR-39 at Darjeeling • Flux recorded on CR-39 ~ 9.299 x 10-7/(cm2.s.sr) for non stop tracks • CR-39

25. Short time exposure at Darjeeling(CR-39) • 15 days exposure at Darjeeling. • CR-39 coated with 35 um plastic prevents radon alphas. • Track diameter shows different category charge particles present at this altitude. • These tracks are of cosmic origin.

26. Tracks on PET after short time exposure at Darjeeling • After scanning of 25 cm2 of PET film reveals only one track. • Track dia is smaller(~2um) than those(10um) of long time exposure. • The fact is that bulk etch rate is less for short time exposure.

27. Detectors exposed at Ooty • Charged particles flux recorded with CR-39 detectors for nonstop tracks is 1.954x10-6/(cm2.s.sr)

28. Design of detectors for Hanle

29. Flux obtained at Hanle • Charge particles flux recorded with CR-39 detectors 2.116x10-7/(cm2.s.sr). Main contributions are from proton and alpha.

30. Minor axis distribution

31. Cosmic ray flux at mountain altitude • Non stop tracks can be considered as cosmic ray particles which have gone through the plate and produces conical tracks. • Flux of such tracks in CR-39 after 6 hour etching. • At Ooty - 1.954x10-7/(cm2.s.sr). • At Hanle- 2.116x10-6/(cm2.s.sr). • At Darjeeling- 9.299x10-7/(cm2.s.sr).

32. Cosmic ray flux at mountain altitude • Non stop tracks can be considered as cosmic ray particles which have gone through the plate and produces conical tracks. • Flux of such tracks in CR-39 after 6 hour etching. • At Ooty - 1.954x10-7/(cm2.s.sr). • At Hanle- 2.116x10-6/(cm2.s.sr). • At Darjeeling- 9.299x10-7/(cm2.s.sr).

33. Conclusion : • Flux obtained at Hanle(Lat-32° 42’N,Long- 79° 04’E, Alt. 4.5 asl). =2.23 x flux obtained at Darjeeling(Lat - 28° N Long -88° E and Alt 2.13 asl). =10.8 x flux obtained at Ooty(Lat - 11.4° N Long -76.7° E and Alt - 2.23 asl). • Flux of non stop particles is changing with altitude and also with latitude. So we can conclude these are due to cosmic rays. • For large area exposure the site at hanle and further work is in progress .

34. IMPORTANT OBSERVATION AT HANLEY: • One un-usually heavy ion track is observed on the bottom side of a CR-39 plate. • Micro photograph of track is shown in the next slide. • It has punched through CR-39 plate and entered to the PET film then stopped after few micron(~33 micron) travel. • CR-39 plate was placed on a bunch of PET film. Bunch contained three PET film each of 100 µ thick. • Range of the ion in the detector is greater than 700µ. Thickness of the CR-39 detector used was 700µ. • From the track parameters of this track on PET it is identified as particle of Z~26±5.

35. Heavy ion track observed in the bottom surface of CR-39 .

36. Distribution of Reduced Etch Rate

37. Identification of high energy event • Range in PET =33±2 µm • Charge response Vt/Vg =6.53+±0.67 • Then observed event at Hanle is identified with the help of calibration curve and dE/dx vs. range relation. • dE/dx is determined from our calibration curve and found to be 35.3±5 MeV/mg/cm2. • The event identified as ion of charge (Z)=26±2 • Energy of the ion (iron) when it enters to the PET is ~150 MeV. • Track length in CR-39 is determined (~24.5±1.75 µ). Then range from the tip in CR-39 is ~65µ). • Energy of ion on CR-39 is determined to be ~2GeV.

38. Identification of heavy ion from range

39. This is a very very rare event • Flux of iron at the altitude of Hanley is ~10-20 /cm2/s. Which is less than 1 particle per square km/century. • We have observed it after searching an area of ~100 cm2, which has exposed for ~one year. • Calculated flux over this area is 3.6x10-10

40. Future plan • Set up of large area passive detector arrays at Hanle (alt. 4500 m) in the Himalayan region to begin a systematic search for rare events in cosmic rays. • Search for rare events in cosmic rays through the deployment of Nuclear Track Detectors (NTDs) in space as part of a recoverable payload in astrosat project.

41. References Witten, E., Phys. Rev. D30 (1984) 272. Farhi, E. and Jaffe, R.L., Phys. Rev. D30 (1984) 2379. Madsen, J., Phys. Rev.Lett.70 (1993) 391. Madsen, J., Phys. Rev.D71(2005) 014026. SayanBiswas, J.N. De, Partha S. Joarder, Sibaji Raha, DebapriyoSyam. Physics Letters B 715 (2012) 30 Banerjee, S., Ghosh, S.K., Raha, S., Syam, D., Phys. Rev. Lett. 85 (2000) 1384. Saito,T., Phys. Rev. Lett. 65 (1990) 2094. Ichimura, M., et. al., Nuovo Cimento 106A (1993) 843. Aguilar, M., et. al.,Phys.Rep. 366 (2002) 331. Fuji,M., et.al.,Rad.Meas. 34 (2001) 255. Ziegler,J.F.,Biersack, J.P., 2003. The Stopping and Range of Ions in Matter (SRIM Computer Code), Version:2003.26 .

42. References • J. Madsen, Phys. Rev. D 71(2005) 014026 • SayanBiswas, J.N. De, Partha S. Joarder, SibajiRaha, DebapriyoSyam. Physics Letters B 715 (2012) 30 • Banerjee, S., Ghosh, S.K., Raha, S., Syam, D., Phys. Rev. Lett. 85 (2000) 1384. • T. Saito, Phys. Rev. Lett. 65 (1990) 2094 • M. Ichimura, et. al., Nuovo Cimento 106A (1993) 843 • M. Fuji, et. al., Rad. Meas. 34 (2001) 255 • M. Aguilar, et. al., Phys. Rep. 366 (2002) 331 • D. Bhowmik, S. Dey, A. Maulik, SibajiRaha, S. Saha, Swapan K. Saha, D. Syam. Nucl. Instr. and Meth. B 269 (2011) 197 • S. Dey, D. Gupta, A. Maulik, SibajiRaha, Swapan K. Saha, D. Syam, J. Pakarinen, D. Voulot, F. Wenander. Astroparticle Physics 34 (2011) 805

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