Search for Strangelets at high altitude

1. Search for Strangelets at high altitude S. K. Saha Bose Institute, Kolkata 4th Workshop on AstroParticle Physics Darjeeling, 10-12 December, 2009

2. What are Strangelets ? • There are several observations of exotic nuclear fragments with unusual charge to mass ratio in different cosmic ray experiments. • These massive particles observed as exotic events in cosmic rays might be strangelets as suggested by several theoretical work [Bjorken & McLerran(1979), Witten(1984), Farhi & Jaffe(1984), Madsen(1994)]. • Strangelets are small, stable lumps of Strange Quark Matter (SQM). • Normal baryonic matter is composed of only up and down quarks. • SQM contains up, down and also strange quarks in almost equal measure. • According to some theorists ( Witten, PRD 1984) SQM represents the true ground state of Quantum Chromodynamics. • Very low baryon number strangelets are likely to be unstable but strange matter in bulk is stable.

3. The Strange Quark Matter Hypothesis Comparison of the energy per baryon of 56Fe and nuclear matter with the energy per baryon of 2-flavor (u, d quarks) and 3-flavor (u, d, s quarks) strange quark matter. Theoretically the energy per baryon of strange quark matter may be below 930 MeV, which would render such matter more stable than nuclear matter. The theoretical possibility that strange quark matter may constitute the true ground state of the strong interaction rather than 56Fe was proposed by Bodmer (1971), Witten (1984), and Terazawa (1990).

4. Strangelet search Experiments to find SQM • An experiment (SLIM collaboration) is going on at Mt. Chacaltya, Bolivia for searching SQM in cosmic rays using 440 sq. m of CR-39 at an altitude of 5260 m. Preliminary results from the analysis of ~383 sq. m with an average exposure time of 4.07 years were reported in December 2007. • The Alpha Magnetic Spectrometer (AMS) is a space based particle physics experiment led by MIT. The AMS-01 flew in space in 1998 aboard the Space Shuttle Discovery. A reanalysis of data from this mission has given hints of some interesting events, such as one with Z=8, A=54. A larger AMS-02 is scheduled to fly to the International Space Station in 2010 (originally scheduled in 2003) and will remain active for at least three years before it is returned to earth. • The Lunar Soil Strangelet Search (LSSS) is a search for strangelets using the accelerator at Yale. The experiment studies a sample of 15 grams of Lunar Soil from Apollo 11. If strangelets have been trapped in the lunar surface layer during the 500 million years of exposure, the experiment with sensitivity of 10-17 will easily detect it. In a recent paper published in PRL in August 2009 they have reported that no strangelets were found over a mass range of A=42 to 70 for nuclear charges 5,6,8,9 and 11.

5. Where can SQM be found and how can they reach earth ? • SQM , if stable is almost unavoidable in the core of dense stellar objects like pulsars and compact X-ray sources. • Strangelets may be produced when two strange stars in a binary system collide. • A strange star – black hole collision may also release lumps of quark matter. • Certain calculations suggest a galactic production rate equivalent to 10 -10 M⊙ yr -1 • Strangelets in many ways behave like ordinary cosmic ray nuclei, most likely acceleration mechanism being Fermi acceleration in supernova shocks. • Strangelets, produced in the Early Universe or in violent astrophysical processes, could be present in the cosmic radiation (Rujula and Glashow, Nature 312 1984) • Calculations describing production of stranglets in binary strange star system and its galactic propagation were recently published (Madsen PRD 71, 2005). This studies point to significant measurable strangelet flux in our part of the galaxy.

6. Inside the earth’s atmosphere • Strangelets have small positive charge, consequently they have unusual charge to mass ratio (Z/A <<1) • According to one model [S. Banerjee, S.K. Ghosh, S. Raha and D. Syam, Phys. Rev. Lett. 85 (2000) 1384] an initially small strangelet propagating through the earth’s atmosphere would pick up mass by preferentially absorbing neutrons over protons from the nuclei of atmospheric atoms, protons being Coulomb repelled. As a consequence Z/A ratio gets even more skewed. • At the same time the strangelet looses energy through ionisation of the surrounding media which effectively puts a lower limit to the altitude at which it can be detected. • The authors predicted that under suitable conditions, small strangelets of initial mass A~64 amu and charge Z~2, impinging on the top of the terrestrial atmosphere with ~0.6, will evolve to mass A~340 and Z~10-20 on its way down to mountain altitude ~3.6 km above sea level and be left with enough K.E. (~ 8.5 MeV ) with a ~0.01 for them to be detected with SSNTD. • In a recent work Wu et al. has shown that strangelets with large mass and energy have the chance to penetrate the atmosphere to reach the sea level [Wu et al., J. Phys. G 34 (2007) 597]

7. Site at Hanle (altitude 4.5 km) • Hanle in Ladakh offers many crucial advantages when it comes to setting up of large passive detector arrays to look for strangelets in cosmic rays. • Computer simulations of strangelet propagation through the earth’s atmosphere has shown that the chances of finding such rare particles with energies sufficient enough for them to be detectible increases as one goes up in altitude and Hanle is basically the highest accessible place in India where one can set up a large area (400 sq. m) array. • The easy accessibility to that area as well as the presence of significant scientific and communications facilities there makes any further scientific activity considerably easier and cheaper. • The passive detectors exposed to the atmosphere undergo degradation. Hanle is a cold desert with very little rain of 120mm per year (same as Sahara Desert) and very low humidity of 3-45%. It is also virtually free of pollutants. Our pilot studies has established the fact that the degradation that plastic detectors suffer while in Hanle is significantly less compared to the detectors placed at Darjeeling or Ooty, places which are at lower altitudes (2200m) and which experience heavy rainfall and high humidity.

8. Pilot study at Hanle Two A4 size PET detectors were placed as shown in the adjacent figure. Two small pieces of standard SSNTD CR-39 were attached to the PET detectors. The objective was to find out the level of degradation with long term exposure and whether it detects any particle track and its nature. These were recovered after 320 days. The figures below show some initial results for PET and CR-39. In PET (left) we can see a track due to a heavy-ion. We can see tracks in CR-39 (right) of different opening diameters from various directions perhaps due to protons and alpha particles. Currently we are analyzing the data.

9. Site at Sandakphu (altitude ~3.6 km) • We have also explored a site at Sandakphu for setting up an array of detectors of 400 sq. m. • The abundance of such objects may be extremely low (5-10/100 m2/year) compared to that of primary cosmic rays (1000/cm2/sec). • With this large area detector either at Sandakphu or Hanle we hope to see about 20-40 signatures of SQM in a year’s exposure.

10. Detector development for Strangelet search at Mountain Altitude Large Area Detector is required • As explained, to investigate this type of exotic events, a very large area (~ 400 m2) detector will be required and that has to be placed at 3 to 4 km altitude. With that arrangement one can expect to detect about 20 to 40 strangelets in one year exposure time, if the predictions are correct. Choice of Detector • The suitable choice of detector for such large area cosmic ray exposure at mountain altitude is no doubt the polymer or plastic detectors, known as Solid State Nuclear Track Detector (SSNTD). What are SSNTDs? • SSNTDs are basically dielectric solids e.g. polymers (plastics), inorganic glasses, mineral crystals and some poor semiconductors. Among polymers or plastic detectors CR-39 and Lexan are two widely used detectors for studying CR nuclear abundance. They are sensitive to Z/ of the incident ion.

11. The standard SSNTDs are not suitable for strangelet detection • Though there are several efficient polymer detectors, e.g. CR39, Lexan, etc., available for nuclear track detection, none of these is suitable for the present purpose for the following reason: • The detection threshold Z/ of widely used CR39 (Z/6) and Lexan (Z/57) are both much lower in comparison to the predicted Z/ range of 1000 – 2000 for strangelets in the mountain altitude. As a consequence, a huge low-Z noise will be recorded which is most undesirable in search of objects (SQM) whose abundance is very low compared to that of primary cosmic rays. • So, a detector which is sensitive only to higher values of Z/ is preferable, so that the primary cosmic rays (mainly protons) do not get detected.

12. Choice of OHP as Solid State Nuclear Track Detector • In an earlier work it has been reported that 14.5 MeV/n and 5.9 MeV/n 132Xe ion can form etch pits (tracks) in commercially available overhead projector (OHP) films by applying SSNTD technique. [S Ghosh, J Raju, P P Choubey, K K Dwivedi Nucl.Tracks Radiat. Meas. 19 77 (1991)]. No work was reported on that material since then. • In our work we have chosen a particular brand (CENTURY de’Smat, India) of overhead projector (OHP) films with good surface quality. Elemental analysis revealed that the plastic was Polyethylene Terephthalate(PET) with chemical formula C5H4O2. It was reconfirmed with IR spectroscopy. The density of the film, measured over 17 samples, was found to be 1.380.08 g/cm2. • To characterize the detector we have to measure its charge response characteristics as a function of specific energy loss. The detectors must be calibrated by impinging known ions with known energy on them using particle accelerators and natural radioactive sources.

13. Calibration with 16O beam at IUAC, New Delhi • In order to check the response parameter of PET it was exposed to 95 MeV 16O beam from the pelletron at the IUAC, New Delhi. A gold foil was used to diffuse the beam over a circular area of diameter of about 3cm. • The beam energy just outside the flange was calculated to be 53.6 MeV considering the loss of energy at the diffuser and other absorbers. • A mechanical arrangement was made so that one could expose a number of targets in a short period of time. • We exposed the targets at varying distance from the flange to determine the response of the detectors at various energies as the incident energy degraded in the air.

14. Etch pits for 16O in PET detector

15. Calibration with 16O beam at NSC The charge response parameter VT/VB for PET detector at four different incident energies against the corresponding Z/b values. If the response curve is extrapolated to lower values of Z/b, the detection threshold will be obtained at around Z/b ~140. • This is a much higher threshold as compared to that of the widely used CR-39 and Lexan for which the z/b thresholds are only 6 and 57 respectively. • Clearly this is a very suitable detector for strangelet search.

16. Calibration with 56Fe and 32S beam at IUAC, New Delhi • PET detectors were also exposed to 2.7 MeV/n 56Fe and 3.9 MeV/n 32S beam from the pelletron at the IUAC, New Delhi. A gold foil was used as a scatterer inside the General Purpose Scattering Chamber (GPSC). • We mounted the detectors at 50 cm from the gold foil target both at forward and backward angles. We exposed the targets at different angles from the target to determine the response of the detectors at various energies. Half of the detectors were placed at a 30 degree orientation to see tracks at an angle.

17. 238U-ion Exposure at GSI, Darmstadt, Germany Beam energy : 11.3 MeV/n 238U-ion • Energy degrader: Aluminium (Al) foils of different thicknesses ( 84.0 m, 73.5 m and 63.0 m ) • Detector sample: 3 circular pieces of PET 100 m thick of area 3.5X3.5 cm2 were exposed. • Detectors exposed to 238U were etched maintaining the same etching conditions as in previous experiments. • Scanning and measurements were done as in previous experiments.

18. Response to heavy fission fragments and a-particles • We exposed PET films as well as CR39 (Intercast Europe Co., Italy), inside a vacuum chamber (at ~10-4 mb) to the normally incident -particles and fission fragments from a 252Cf source of strength ~ 1 Ci. CR39 films were exposed as a standard polymer detector to compare its response with that obtained from the PET films. • The exposed PET films and CR39 samples were etched using standard SSNTD techniques with 6.25N NaOH for 3 hours at 55 C and 70 C for PET and CR39 respectively. • Measurements on the etched plates were made under the x100 dry objective of a Leica DMR microscope, connected to a computer for scanning, with automated stage movements. We measured the diameters as well as the cone length of the etch pits on CR39 and PET samples.

19. Response to heavy fission fragments and a-particles The radiation damage created by heavy fission fragments form good etch-cones in PET films. The circular surface openings in CR39 etched for 6 hours (top) and one such etch cone in 4 hours etched PET (bottom) are shown on the right. The diameter distribution of the tracks in CR39 shows two distinct peaks which correspond to the -particles and the fission fragments from the decay of 252Cf and that in PET shows only one peak. This indicated that while CR39 detected both, PET films detected only the heavy ions from 252Cf which have z/b ~ 900-1600 and not a with z/b ~35.

20. Calibration with 56Fe, 32S and 16O ions from IUAC, New Delhi and 238U from GSI, Darmstadt. Charge response parameter Vt/Vg of Polyethylene Terepthalate as a function of specific energy loss dE/dX. From this curve we get dE/dX of an unknown charged particle and along with the Range, we can estimate its charge, mass and energy.

21. Exposure at open air cosmic rays • During the standardization of this polymer, we also observed its behaviour in open air cosmic ray exposures at mountain altitude here at Darjeeling (altitude ~ 2.2 km). We exposed it in open air for six months. • The purpose was to find out whether the PET detector could detect any Cosmic Ray nuclear track at all and secondly to see whether the quality of the recorded nuclear tracks in this material was sufficiently good for measurements of track parameters. • We found that it recorded good quality tracks with much less track density, as expected, in comparison to those recorded by CR39 exposed simultaneously.

22. RESULTS • We analysed over 8000 image frames each with an area of 4.510-5 cm2. • In a single image frame, for only 2% of the total scanned area there was generally only one track, as shown here. • The image frames for the rest 98% of the scanned area had no tracks at all. • A total of 188 isolated circular etch cone openings were found within the scanned area of the PET detector.

23. The measured flux density corresponding to these 188 tracks is 1.1310-5cm2 – sr –sec. • In another set up the PET detectors were exposed for longer duration (525 days 20 hrs 21 min). The recorded flux 0.910-5/cm2 – sr - sec is in good agreement with the flux of the tracks obtained from our first open air exposure. • The flux measured on the CR39(Intercast) detector exposed simultaneously with the stack of PET detectors under identical atmospheric condition has been found to be 1.510-3/cm2 – sr - sec, which is ~100 times higher than the flux observed in PET detectors.

24. Results with CR - 39 • In case of CR39, almost all single image frame of the total scanned area, tracks were found. • It clearly supports our observation that the PET detectors have higher limit for nuclear track detection threshold as compared to that of CR39.

25. Charge response parameter Fig. shows charge response parameter VT/VG of the cosmic ray tracks observed in PET detector. We find that the charge response parameter is typically 1.250.25. This agrees well within the error with the measured value of VT/VG1.5 that we obtained with oxygen beam from the pelletron with Z/b160at IUAC, New Delhi.

26. Unusual Event • During the analysis of one such PET detector we have found a highly unusual event. • This figure shows the tracks in a particular image frame observed under a Leica Digital Microscope. These are six almost identical tracks in a single image frame. • We analysed over 8000 such image frames each with an area of 4.510-9 m2. • Out of all image frames only 2% frames contained only one track and the rest 98% showed no track at all.

27. The measured flux of this particular event is ~ 1.110-6m2-sr-sec. So the possibility that they are Cosmic Ray heavy ions is ruled out, which has a flux of 10-52m2-sr-sec for Fe group and many order of magnitudes lower for ultra-heavy group at Darjeeling altitude. • So, it is possible that the particles came simultaneously and they may be emanated from a single vertex. It is seen that the angles of incidence of the six cones are within ~ 80 to 160. • So, there is a chance that they may correspond to particles radiating from a single vertex not very far from the detector. A least-square-fit calculation yields a vertex at a height of ~350mm above the PET detector.

28. From dE/dX and Range we estimated z~24, A~52 for these particles. The datum also corresponds to energy of about 1MeV/nucleon. It gives b~0.05 for each of the six tracks in this particulr event. • If these six particles indeed emanated from a single vertex, as the case seems to be, then the parent particle would have Z > 144 and M > 312. • It could be that the observed tracks are due to multifragmentation of an encounter involving a 56Fe26 ion in the cosmic ray and a 238U92 floating in the air. The energy of 1MeV/nucleon would also support that.

29. Muon detectors and air shower array • We plan to have an air shower array of 1 sq. m plastic scintillators detectors at Darjeeling. Initially we plan to install seven detectors in an hexagonal pattern with one at the centre, which we would like to expand to fifty in future. We have acquired the necessary electronics for setting up the initial array of seven detectors. We will use this initial array as a training experiment and man power developemnt. • We have plan to install a muon detector array similar to that in Ooty. This will provide us information on the composition of the primary particle. It also allows discrimination between the detected g-rays and protons. • We plan to study solar phenomena such as CME and Forbush Decrease with this facility. It was shown in a previous talk that the facility at GRAPES array in Ooty and the facility at Akeno, Japan has an overlapping region of the direction for the study of interplanetary medium through CME’s, flares etc. Darjeeling facility may help to improve accuracy of the data with added statistics. • We have a weather monitoring facility at Darjeeling and the study of long term cosmic ray variation and its effect on weather can also be studied here.

30. Summary • We have reviewed the status for the search of strangelets. • We have found that the OHP can be used to detect cosmic ray events and it has a much higher detection threshold compared to the CR-39 or Lexan used for strangelet search in other experiments. • We have explored sites at Hanle in Kashmir and at Sandakphu. A pilot study has established the suitability for such experiment at Hanle. • During preliminary analysis of the data we have found a very unusual event that we have interpreted as a multifragmentaion event involving cosmic rays. • We also plan to have an array initially of seven plastic scintillators and a muon detection array in Darjeeling. • During the next one year we hope to start taking data at high altitude.