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. 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. 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. 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. 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. Approximately similar number of experiments (BES, Belle, BaBar, HERA-B, CDF, PHENIX, SPHINX, HyperCP) report about non-observing it. The problem is still open! The problem is still open!

3. Analysis Model Photoproduction experiment is performed by detecting scattered lepton simultaneously with the other particles. 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. 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. 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: P γ =P e before -P e after. γ * four momentum is easy to calculate by four momentum of electron before and after interaction: P γ =P e before -P e after. X four momentum is calculated by four momenta of all particles before and after interaction: P X =P e before +P D -P e after -P P -P K. X four momentum is calculated by four momenta of all particles before and after interaction: P X =P e before +P D -P e after -P P -P K.

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

5. Cuts applied 1. The proton momentum is required to be in the range 4.0<P P <9.0 GeV. 2. The kaon momentum is required to be in the range 2.0<P K <15.0 GeV. 3. The identified lepton is required to be electron for the 1998 data and positron for the 1999, 2000 data. 4. DCA between proton and kaon tracks is required to be less than 0.4 cm. 5. Mid-point of that DCA distance from the z axis is required to be less than 0.8 cm. 6. That mid-point z coordinate is required to be in the range -18<z<18 cm. 7. DCA between reconstructed Λ and electron is required to be less than 1.5 cm. Mid-point of that DCA distance from the z axis is required to be less than 0.8 cm. Mid-point of that DCA distance from the z axis is required to be less than 0.8 cm. That mid-point z coordinate is required to be in the range -18<z<18 cm. That mid-point z coordinate is required to be in the range -18<z<18 cm. The reconstructed Λ decay length is required to be less than 7cm. The reconstructed Λ decay length is required to be less than 7cm. Events with reconstructed invariant mass of Φ(1020) in the range 1.01<M Φ <1.03 GeV, if we interpret proton as kaon, are discarded. Events with reconstructed invariant mass of Φ(1020) in the range 1.01<M Φ <1.03 GeV, 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<M Λ <1540MeV cut applied

20. Conclusions 1. Λ(1520) events can be observed. Applying several cuts we can make the peak for that particle more clear. 2. 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. 3. The missing mass distribution in the region where Θ + mass is supposed to be is very low. No events can be observed. 4. 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.