1. Search for Pentaquarks at
Avetik Airapetian
(University of Michigan)
for the HERMES Collaboration
HERMES Experiment
Topology of the search for resonances decaying into longlived daughters
Results
Conclusions
EPIPHANY05, 6-8 January 2005
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

2. Introduction The turning Point
Pentaquarks, minimum content is 5 quarks
Exotic, antiquark has a different flavor than other 4 quarks
Importance: “Should exotic states be discovered, the conventional quark model would have to be modified or even abandoned” E. Golowich PRD v4 N1 1971
“The end of the constituent quark model?” F. Close HADRON-03
2004 “existence ranges from very likely to certain, but further confirmation is desirable” – “***” definition by PDG
The Prediction by DPP based on χQSM
(Z. Phys. A359(1997))
Assuming N(1710) is known member of
mass and width of is derived
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

3. The HERMES Spectrometer
Beam: 27.6 GeV e+/e- from HERA accelerator
Track reconstruction: Dp/p < 2%, Dq < 0.6 mrad
Particle ID: TRD, Preshower, Calorimeter (hadron/lepton sep.)
dual radiator RICH (p, K, p separation)
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

4. Particle Identification
hadron/positron separation
combining signals from:
TRD, calorimeter, preshower, RICH
hadron separation
Dual radiator RICH for p, K, p
Aerogel n=1.03
Gas C4F10 n=1.0014
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

5. A. Airapetian Cracow Epiphany Conference 2005
Event Reconstruction
Define appropriate event topology
Hadron identification: RICH
p: GeV p: 4 – 9 GeV
Reject events from L(1116)  pp- within ±2s of ML
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

6. A. Airapetian Cracow Epiphany Conference 2005
Event Reconstruction
Define appropriate event topology
Hadron identification: RICH
p: GeV p: 2 – 15 GeV
Select events from L(1116)  pp- within ±2s of ML
Select events from Ξ(1321)  Λ within ±2σ of
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

7. DCA and Decay Length cm Red is Signal ,Black is Background cm
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

8. Results(Phys.Lett.B585:213,2004, hep-ex/0312044)
Peak at:
M= 1528 ± 2.6 MeV
= 8 ± 2 MeV
Significance:
Unbinned fit is used: result is more stable
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

9. The Signal and its Background
Peak at:
M= 1527 ± 2.3 MeV
= 9.2 ± 2 MeV
Significance:
mixed event background
PYTHIA6
excited S* hyperons
(not included in Pythia6)
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

10. A. Airapetian Cracow Epiphany Conference 2005
A Non-Zero Width for Q+?
Monte-Carlo with complete detector Simulation
generated: M=1540 MeV , =2 MeV
Reconstructed : M= MeV , =6.2 MeV
Observed width FWHM: 19 – 24 MeV
Detector resolution (from MC) FWHM: 10 – 14.6 MeV
re-fit spectra with Breit-Wigner convolved with a Gaussian (fixed by MC)
HERMES intrinsic width: Γ = 17 ± 9 ± 3 MeV
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

11. A. Airapetian Cracow Epiphany Conference 2005
Q+ Isospin
Well established L(1520)  pK- with acceptance:1.5%
No peak structure for
Q++  pK+ zero counts at 91% CL
Q+ not isotensor probably isoscalar
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

12. Final spectra for and arXiv:hep-ex/0412027
UL for (1860) cross-section is nb
UL for (1860) cross-section is nb
Cross-section for (1530) is nb
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

13. Production Cross Sections and UL
Integrated luminosity: 290 pb-1
Acceptance from MC: % for L(1520) % for Q % for (1530) % for (1860) % for (1860)
HERMES estimate: s(L(1520)) = 62 ± 11 nb s(Q)= nb ± 25% (stat)
(additional factor 2 from production kinematics)
σ( )= nb
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

14. PYTHIA pz and pt spectra
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

15. HERMES Contribution to exotics search
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

16. Comparison with other experiments
nK+
World Average: ±2.1 MeV
Large variation in mass not uncommon for new, decaying particles
but need to better estimate exp. uncertainties
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

17. Summary of Positive Results
⇒Certainly high statistic data are needed
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

18. Summary of Null Results
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

19. A. Airapetian Cracow Epiphany Conference 2005
How real is the
check for
“kinematic reflections”
detector acceptance and cuts (PYTHIA6 MC / Toy MC)
Q+ vs S*+
is Q+ a pentaquark or a previously unobserved S*+?
add a fourth hadron
is the peak still there?
can we guess the production process for the Q+?
can we suppress background?
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

20. A. Airapetian Cracow Epiphany Conference 2005
Q+ vs S*+
Is peak a new S*+ or a pentaquark state?
If peak is S*+ ⇒ also see a peak in M(Lp+)
b.r.(Lp+)/(pKs)  3/2
S*+
HERMES Preliminary
HERMES Preliminary
HERMES Preliminary
No peak in Lp+ spectrum near 1530 MeV
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

21. Q+ Mass spectrum with additional p
standard cuts applied
+ K* and L veto
signal/background: :1
signal/background: :3
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005

22. Conclusions and Outlook
Direct reconstruction of Q+ invariant mass
Mass:
Intrinsic Width:
Significance: ~ 4 s
Q+ is probably an isosinglet ,No in that mass vicinity
Requiring additional p improves signal/background, it eliminates KS contamination from various processes
Missing X-- and σ UL estimated in reaction
Cross-sections for , Λ(1520) and (1530) are estimated in quasireal photoproduction( <<0.1GeV2)
7-Januar-2004
A. Airapetian Cracow Epiphany Conference 2005