Grigor Aslanyan Yerevan State University September 7, 2005

1. Search for Θ+ Pentaquark by looking at missing mass spectrum in reaction γ*D Λ(1520)X in HERMES data Grigor Aslanyan Yerevan State University September 7, 2005

2. About Θ+ • Pentaquark states are not prohibited by any known law of QCD. • From early stages of quark models, such type of states were proposed to exist. • In 1997 Diakonov and co. predicted existence of set of exotic particles, for example Θ+=uudds with mass ~1530 MeV. • Since 2003 several experiments (LEPS, DIANA, CLAS, SAPHIR, ITEP, HERMES, ZEUS, SVD, COSY) report about observing Θ+ states. • Approximately similar number of experiments (BES, Belle, BaBar, HERA-B, CDF, PHENIX, SPHINX, HyperCP) report about non-observing it. • The problem is still open!

3. Analysis Model • Photoproduction experiment is performed by detecting scattered lepton simultaneously with the other particles. • We look at the reaction γ*D Λ(1520)X, where X is supposed to be the Θ+ pentaquark. • Λ(1520) is reconstructed from detected proton and kaon. • γ* four momentum is easy to calculate by four momentum of electron before and after interaction: Pγ=Pebefore-Peafter. • X four momentum is calculated by four momenta of all particles before and after interaction: PX=Pebefore+PD-Peafter-PP-PK.

4. Data Selection • Polarized and unpolarized deuterium data from 1998-2000 was used (to have RICH detector available). • The runlist waschosen using the latest LOGRUN files which are marked by the HERMES DQgroup as analyzable. • Events were required to contain at least three tracks, one lepton identified using PID detectors response: PID3 + PID5 > 0and two opposite charged hadrons using the cut -100 < PID3 +PID5 < 0. • Using link between track and RICH we require that one hadron isidentified by RICH as proton(antiproton) and another one as K±.

5. Cuts applied • The proton momentum is required to be in the range 4.0<PP<9.0GeV. • The kaon momentum is required to be in the range 2.0<PK<15.0GeV. • The identified lepton is required to be electron for the 1998 data andpositron for the 1999, 2000 data. • DCA between proton and kaon tracks is required to be less than 0.4cm. • Mid-point of that DCA distance from the z axis is required to be less than0.8cm. • That mid-point z coordinate is required to be in the range -18<z<18cm. • DCA between reconstructed Λ and electron is required to be less than1.5cm. • Mid-point of that DCA distance from the z axis is required to be less than0.8cm. • That mid-point z coordinate is required to be in the range -18<z<18cm. • The reconstructed Λ decay length is required to be less than 7cm. • Events with reconstructed invariant mass of Φ(1020) inthe range 1.01<MΦ<1.03GeV, if we interpret proton as kaon, are discarded.

6. Λ and Λ invariant masses No cuts applied (148±42 events for Λ)

7. Λ and Λ invariant masses P and K clearing cuts applied (107±33 events for Λ)

8. Λ and Λ invariant masses Electron/positron cuts applied (81±31 events for Λ)

9. Λ and Λ invariant masses P and K tracks DCA<0.4cm cut applied (72±27 events for Λ)

10. Λ and Λ invariant masses DCA mid-point r<0.8cm cut applied (59±25 events for Λ)

11. Λ and Λ invariant masses DCA mid-point |z|<18cm cut applied (62±24 events for Λ)

12. Λ and Λ invariant masses Λ and e tracks DCA<1.5cm cut applied (62±24 events for Λ)

13. Λ and Λ invariant masses DCA mid-point r<0.8cm cut applied (66±24 events for Λ)

14. Λ and Λ invariant masses DCA mid-point |z|<18cm cut applied (65±23 events for Λ)

15. Λ and Λ invariant masses Λ decay length<7cm cut applied (55±23 events for Λ)

16. Λ and Λ invariant masses Φ(1020) events discarded (55±22 events for Λ)

17. Missing mass d(γ*,PKX) No cuts applied

18. Missing mass d(γ*,PKX) The same cuts applied as for Λ(1520)

19. Missing mass d(γ*,PKX) Λ(1520) invariant mass in region 1500<MΛ<1540MeV cut applied

20. Conclusions • Λ(1520) events can be observed. Applying several cuts we can make the peak for that particle more clear. • A little peak can be seen for Λ but we can't be sure that it's not a statistical fluctuation. Applying cuts makes the statistics for that particle very low and no events can be observed. • The missing mass distribution in the region where Θ+ mass is supposed to be is very low. No events can be observed. • We need much more data collected and much higher statistics to observe Λ events more clearly and to be able to apply the method described here for searching Θ+ pentaquarks.