Search for Θ + Pentaquark by looking at missing mass spectrum in reaction γ * D Λ(1520)X in...

20
Search for Search for Θ Θ + Pentaquark by Pentaquark by looking at missing looking at missing mass spectrum in mass spectrum in reaction reaction γ γ * * D D Λ Λ (1520)X in HERMES (1520)X in HERMES data data Grigor Aslanyan Grigor Aslanyan Yerevan State University Yerevan State University September 7, 2005 September 7, 2005
  • date post

    20-Dec-2015
  • Category

    Documents

  • view

    214
  • download

    0

Transcript of Search for Θ + Pentaquark by looking at missing mass spectrum in reaction γ * D Λ(1520)X in...

Search for Search for ΘΘ++ Pentaquark by looking Pentaquark by looking

at missing mass at missing mass spectrum in reaction spectrum in reaction

γγ**D D ΛΛ(1520)X in (1520)X in HERMES dataHERMES data

Grigor AslanyanGrigor AslanyanYerevan State UniversityYerevan State University

September 7, 2005September 7, 2005

About About ΘΘ++

• Pentaquark states are not prohibited by any Pentaquark states are not prohibited by any known law of QCD.known law of QCD.

• From early stages of quark models, such type of From early stages of quark models, such type of states were proposed to exist.states were proposed to exist.

• In 1997 Diakonov and co. predicted existence of In 1997 Diakonov and co. predicted existence of set of exotic particles, for example set of exotic particles, for example ΘΘ++=uudds =uudds with mass ~1530 MeV.with mass ~1530 MeV.

• Since 2003 several experiments (LEPS, DIANA, Since 2003 several experiments (LEPS, DIANA, CLAS, SAPHIR, ITEP, HERMES, ZEUS, SVD, COSY) CLAS, SAPHIR, ITEP, HERMES, ZEUS, SVD, COSY) report about observing report about observing ΘΘ++ states. states.

• Approximately similar number of experiments Approximately similar number of experiments (BES, Belle, BaBar, HERA-B, CDF, PHENIX, (BES, Belle, BaBar, HERA-B, CDF, PHENIX, SPHINX, HyperCP) report about non-observing SPHINX, HyperCP) report about non-observing it.it.

• The problem is still open!The problem is still open!

Analysis ModelAnalysis Model• Photoproduction experiment is performed by Photoproduction experiment is performed by

detecting scattered lepton simultaneously with detecting scattered lepton simultaneously with the other particles.the other particles.

• We look at the reaction We look at the reaction γγ**D D ΛΛ(1520)X, where X (1520)X, where X is supposed to be the is supposed to be the ΘΘ++ pentaquark. pentaquark.

• ΛΛ(1520) is reconstructed from detected proton (1520) is reconstructed from detected proton and kaon.and kaon.

• γγ** four momentum is easy to calculate by four four momentum is easy to calculate by four momentum of electron before and after momentum of electron before and after interaction: Pinteraction: Pγγ=P=Pee

beforebefore-P-Peeafterafter..

• X four momentum is calculated by four X four momentum is calculated by four momenta of all particles before and after momenta of all particles before and after interaction: Pinteraction: PXX=P=Pee

beforebefore+P+PDD-P-Peeafterafter-P-PPP-P-PKK..

Data SelectionData Selection• Polarized and unpolarized deuterium data from Polarized and unpolarized deuterium data from

1998-2000 was used (to have RICH detector 1998-2000 was used (to have RICH detector available).available).

• The runlist wasThe runlist was chosen using the latest LOGRUN chosen using the latest LOGRUN fifiles which are marked by the HERMES DQles which are marked by the HERMES DQ group as group as analyzableanalyzable..

• Events were required to contain at least three Events were required to contain at least three tracks, one lepton identitracks, one lepton identifified using PID detectors ed using PID detectors responresponsse: PID3 + PID5 > 0e: PID3 + PID5 > 0 and two opposite and two opposite charged hadrons using the cut charged hadrons using the cut -1 -100 < PID3 +PID5 00 < PID3 +PID5 < 0.< 0.

• UUsing link between track and RICH we require that sing link between track and RICH we require that one hadron isone hadron is identiidentifified by RICH ased by RICH as p protonroton (antiproton) and another one as (antiproton) and another one as KK±±..

Cuts appliedCuts applied1.1. The proton momentum is required to be in the range 4The proton momentum is required to be in the range 4..0<P0<PPP<9<9..00 GeVGeV..

2.2. The kaon momentum is required to be in the range 2The kaon momentum is required to be in the range 2..0<P0<PKK<15<15..00 GeV.GeV.

3.3. The identiThe identifified lepton is required to be electron fored lepton is required to be electron for the the 1998 data and 1998 data and positron forpositron for the the 1999, 2000 data. 1999, 2000 data.

4.4. DCA between proton and kaon tracks is required to be less than 0DCA between proton and kaon tracks is required to be less than 0..44 cm.cm.

5.5. Mid-point of that DCA distance fromMid-point of that DCA distance from the the z axis is required to be less than z axis is required to be less than 00..88 cm.cm.

6.6. That mid-point z coordinate is required to be in the range That mid-point z coordinate is required to be in the range -1-18<z<188<z<18 cm.cm.

7.7. DCA between reconstructed Λ and electron is required to be less thanDCA between reconstructed Λ and electron is required to be less than 11..55 cm.cm.

• Mid-point of that DCA distance fromMid-point of that DCA distance from the the z axis is required to be less than z axis is required to be less than 00..88 cm.cm.

• That mid-point z coordinate is required to be in the rangeThat mid-point z coordinate is required to be in the range -1 -18<z<188<z<18 cm.cm.

• The reconstructed Λ The reconstructed Λ decay lengthdecay length is required to be less than 7cm. is required to be less than 7cm.

• Events with reconstructed invariant mass of Φ(1020) inEvents with reconstructed invariant mass of Φ(1020) in the range the range 11..0011<M<MΦΦ<1<1..0303 GeVGeV, if we interpret proton as kaon, are discarded, if we interpret proton as kaon, are discarded..

ΛΛ and and ΛΛ invariant masses invariant massesNo cuts applied (148±42 events for No cuts applied (148±42 events for ΛΛ))

ΛΛ and and ΛΛ invariant masses invariant massesP and K clearing cuts applied (107±33 events for P and K clearing cuts applied (107±33 events for ΛΛ))

ΛΛ and and ΛΛ invariant masses invariant massesElectron/positron cuts applied (81±31 events for Electron/positron cuts applied (81±31 events for ΛΛ))

ΛΛ and and ΛΛ invariant masses invariant massesP and K tracks DCA<0.4cm cut applied (72±27 events P and K tracks DCA<0.4cm cut applied (72±27 events

for for ΛΛ))

ΛΛ and and ΛΛ invariant masses invariant massesDCA mid-point r<0.8cm cut applied (59±25 events DCA mid-point r<0.8cm cut applied (59±25 events

for for ΛΛ))

ΛΛ and and ΛΛ invariant masses invariant massesDCA mid-point |z|<18cm cut applied (62±24 events for DCA mid-point |z|<18cm cut applied (62±24 events for

ΛΛ))

ΛΛ and and ΛΛ invariant masses invariant massesΛΛ and e tracks DCA<1.5cm cut applied (62±24 events and e tracks DCA<1.5cm cut applied (62±24 events

for for ΛΛ))

ΛΛ and and ΛΛ invariant masses invariant massesDCA mid-point r<0.8cm cut applied (66±24 events DCA mid-point r<0.8cm cut applied (66±24 events

for for ΛΛ))

ΛΛ and and ΛΛ invariant masses invariant massesDCA mid-point |z|<18cm cut applied (65±23 events DCA mid-point |z|<18cm cut applied (65±23 events

for for ΛΛ))

ΛΛ and and ΛΛ invariant masses invariant massesΛΛ decay length<7cm cut applied (55±23 events for decay length<7cm cut applied (55±23 events for

ΛΛ))

ΛΛ and and ΛΛ invariant masses invariant massesΦΦ(1020) events discarded (55±22 events for (1020) events discarded (55±22 events for ΛΛ))

Missing mass d(Missing mass d(γγ**,PKX),PKX)No cuts appliedNo cuts applied

Missing mass d(Missing mass d(γγ**,PKX),PKX)The same cuts applied as for The same cuts applied as for ΛΛ(1520)(1520)

Missing mass d(Missing mass d(γγ**,PKX),PKX)ΛΛ(1520) invariant mass in region 1500<M(1520) invariant mass in region 1500<MΛΛ<1540MeV <1540MeV

cut appliedcut applied

ConclusionsConclusions1.1. ΛΛ(1520) events can be observed. Applying (1520) events can be observed. Applying

several cuts we can make the peak for that several cuts we can make the peak for that particle more clear.particle more clear.

2.2. A little peak can be seen for A little peak can be seen for ΛΛ but we can't be but we can't be sure that it's not a statistical fluctuation. sure that it's not a statistical fluctuation. Applying cuts makes the statistics for that Applying cuts makes the statistics for that particle very low and no events can be particle very low and no events can be observed.observed.

3.3. The missing mass distribution in the region The missing mass distribution in the region where where ΘΘ++ mass is supposed to be is very low. mass is supposed to be is very low. No events can be observed.No events can be observed.

4.4. We need much more data collected and much We need much more data collected and much higher statistics to observe higher statistics to observe ΛΛ events more events more clearly and to be able to apply the method clearly and to be able to apply the method described here for searching described here for searching ΘΘ++ pentaquarks. pentaquarks.