Strangeness production by p annihilation at rest on light nuclei (mostly 4 2 He)

1. Strangeness production by p annihilation at rest on light nuclei (mostly 42He) TullioBressani DipartimentodiFisicaSperimentale, Universita’ di Torino IstitutoNazionalediFisicaNucleare, Sezionedi Torino Workshop on Strangeness on Nuclei ECT*, 4-8 October, 2010

2. Outline of the talk • Introduction. Recent (2007-2010) analyses of old data collected with OBELIX (1994-1996) have shown unexpected and interesting results on annihilation of p at rest on light nuclei (4He mostly, 3He, d) • A brief summary of the OBELIX spectrometer’s performances • Analysis method to select multinucleon annihilation • Exp results on K+, K- production on nuclei; strong enhancements (up to x32) in some specific annihilation channels • Peaks in the (Λ,p), (Λ,d) I.M. spectra • 2K+ production on 4He • Possible confirmations with p in flight (FAIR, PANDA) and d at rest (J-PARC)

7. 2. A brief summary of the OBELIX spectrometer’s performances • Built in 1989-1991. Operational at CERN LEAR in 1991-1996 (premature closure of the LEAR facility). • Optimized for meson spectroscopy and N-N interactions studies • Ω/4π70% • p/p for charged particles (π±, k±, p, d): from ~2% to 4% depending on the momentum • p threshold for detection of charged particles: 80 MeV/c for π, 150 MeV/c for k, 300 MeV/c for p, 500 MeV/c for d • PID with E/x (J.D.C.) and T.O.F. • Resolution on I.M. (2 charged particles): ~20 MeV • Detection of  and π0 through an e.m. calorimeter (not used in the present analyses) • Triggerable on k± (not used in the present analyses) • Gaseous/liquid/solid/ targets (very easy interchange) • Excellent beam (~3×106 p at 300 MeV/c, ~2×105 p stopped) • Main limitations (especially for searches on AKNC): DAQ speed: ≤ 30 ev/sec (not due to FEE, very performing  FADC’s at 100 MHz, but to DAQ hardware and software)

8. No detection for secondary vertices (meson spectroscopy at the annihilation vertex)

9. OBELIX Usual (with secondary vertex) k0s ,Λ I.M. p p I.M. Primary ann. vertex Secondary decay vertex ONLY primary ann. vertex (5mm) Global detection efficiency for k0s and Λ: 35%, 40% • Unusual procedure adopted in the present analysis for long-lived k0s and Λ • ~ 200 days of run in 1991-1996 on meson spectroscopy and N-N interaction • ~ 5×108 events on annihilation in H2 • ~ 1,5 Days of run on 4He, 3He (unofficial) ~ 3×106 events in total

10. Ptot, Etot  measured MU= (E2U – p2u)½ unseen ptot= [(E0 - Etot)2 – M2U]½ Spectator, partecipant protons (><300 MeV/c) Unseen neutrons Missing π (charged by ΔΩ, π0) Missing K (charged by ΔΩ, K0)

12. Enhancement of K+, K- production in some specific annihilation channels • Partial results were already presented in: • P. Montagna et al., Nucl. Phys. A 700 (2002) 159; • A. Panzarasa et al., Nucl. Phys. A 747 (2005) 448; • P. Salvini et al., Nucl. Phys. A 760 (2005) 349. • Very systematic study as a function of • A (11H, 21H, 32He, 42He) • Number of involved nucleons (14) • Multiplicity of charged prongs (3-6) • Missing mesons (undetected π± or π0) • Annihilation type (pn- or pp-like, qualitative) • Charged particles spectra (unpublished)

13. A lot of information! More interesting observations For total charged Kaon production (K+, K-, K+K-) the yield in 4He and 3He is close to that in 11H (a bit larger due to the unmeasured Λproduction, absent in 11H)

16. For K+ from 42He a yield x2 over that from 11H in all channels, with noticeable exceptions: K+π+π- enhancement factor ~ 15 K+π+π- π- enhancement factor ~ 9 K+π+ π+ π- π- enhancement factor ~ 32 pK+π+ π- π- enhancement factor ~ 18 for events involving more nucleons! There is also an increase of K- in some specific channels, although less than for K+ The ratio K+/K- varies largely upper and below one in specific channels

17. Recent analysis also on π±total energy spectra for different multiplicities

19. Conclusion: no evidence that thermodinamical equilibrium is reached in that annihilation process

20. 2kH 2kH nk0X Λp nk0sX pπ- p π+π- nX p4He p4He n π+π-k0X pπ- p π+π-k0s,LX Λp n π+π-k0s,LX 5. Peaks in (Λ,p), (Λ,d) I.M. spectra Search for peaks on Λp, Λd I.M. triggered by results obtained with experiments using stopped k- [FINUDA-M. Agnello et al., PRL94 (2005), 212303; Phys. Lett. B 654 (2007) 80] Results published on NPA 789 (2007), 222 Reaction analysed: • Λ’s difficult to be detected since: • resolution on pπ- I.M. ~ 20 MeV • low rate (forbidden in N-N annihilation, ~1% of the total ann. rate in nuclei)

21. Hard to recognize Λ: huge background due to many N and Δ with large Γ overlapping in this region. Control of this background (or experimental phase space) by (pπ+) I.M. distributions Trial to overcome this difficulty by looking at the (2pπ-) I.M. conditioned by the (pπ-) I.M. to be that of the Λ. Search by varying both the central value (Λ1115 MeV) and width (from ± 10 MeV to ± 70 MeV) and cut in the (Etot, ptot) plot.

22. Consistent results obtained by this search Statistical significance of the presumed peak: 3σ Mass: 2212.1±4.9 MeV (mass of the free particles 2373 MeV) Width: 24.4±8.0 MeV Production rate: 1.5×10-4 (lower limit)

23. k0X Λd k0sX pπ- d π+π- X 3kH 3kH p4He p4He π+π-k0X pπ- d π+π-k0s,LX Λd π+πk0s,LX Same procedures adopted for the study of the 5-prong reactions: P.ID. for charged particles (π±, k±, p, d):

24. I.M. of the pdπ- system Less background, less statistics Statistical significance of the presumed peak : 2.6 σ Mass: 3190 ± 15 MeV (mass of the free particles 3311 MeV) Width: ≤ 60 MeV Production rate: 0.39×10-4 (lower limit)

25. Recent new analysis of (Λ,p) I.M spectra (EPJ A 40 (2009), 11) Careful inspection of the (ptot, Etot) plot Selection of the K0S region (C Left)

27. Careful study (MC+data) on the angular distributions with suitable cuts (in the hypothesis of a peculiar nature of the peaks: decay of an AKNC) Final results including a systematic error: Statistical significance: 4.7 σ Mass: 2223.2 ±3.2stat ±1.2sys MeV Width < 33.9 ±6.2 MeV Production rate: 1.2x10-4 (lower limit)

28. 2 k+2π-π+(3n) 2k+Σ-Σ+(nπ-) 2 k+(nπ-)(nπ+)nπ- 2k+k-Λ2n 2 k+k-(pπ-)2n 2 k+ k-pπ-(2n) p4He p4He p4He p4He 2k+Σ-Λn 2 k+(nπ-)(pπ-)n 2 k+ p2π-(2n) 2 k+2π-(2nps) 2k+Σ-Σ-ps 2 k+nπ-nπ-ps 6. Search for 2k+ emitted following p absorption at rest by 4He Results published in NPA 797 (2007), 109 The process is forbidden by energy conservation in a p-p annihilation (lack of ~114 MeV) May occur only on  2 nucleons, with production of Hyperons. Reaction investigated with positive results Yield: 1.7±0.4×10-5 Yield: 2.71±0.47×10-4 Yield: 1.20±0.3×10-4 Yield: 2.8±1.4×10-5

29. Selection of k+ (dE/dx, β) 2k+2π- events 5 prongs

30. Exclusion of channel with π0 production: analysis of ptot vs Etot plots 2k+2π- events • Confidence in the results given by • comparison with 2k- (surely background) • Comparison with similar data on H2 (no 2k+2k- ) • Excess 2k+-2k- (33±14 restrictive, 34±8)

31. 7. Possible confirmation with p in flight (PANDA/FAIR) and d at rest (J-PARC) PANDA: beautiful detector FAIR: p up to 15 GeV Much more energy (17 GeV) at disposal for the formation of the blob of QGP or whatever if it is formed. Very large amount of channels with strangeness, not limited to only K (Λ, Σ, Ξ-, Ω-multi K production) Comparison of the production on some nuclei, parasiting other dedicated measurements with nuclear targets with the production on p target Care to: C.M. energies for the comparison (Fermi motion, …..) Acceptance

32. If the results of this explorating search will be encouraging for the interpretation of QGP formation, dedicated runs initially quite short (scans?) should be scheduled with a proper choice of: -targets -energies If AKNC existence and formation will be confirmed by experiments running now or scheduled soon, they should be formed in all runs at PANDA with nuclear targets at all energies. There should be a large window of phenomena (A-dependence, decay channels) still open. Other approach: comparison on p and d annihilations at rest on p, d, 3He, 4He targets at J-PARC. An evaluation of the d flux of 1 GeV/c, with the 50 GeV proton beam and a ~30 msr acceptance was ~200 sec-1