1. Search For Pentaquark Q+ At HERMES
Yajun Mao
(School of Physics, Peking University)
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

2. Yajun Mao, CCAST Workshop, 9-14 Aug, 2004
Outline
Particle Identification
Search For pKS Resonance
Systematic Studies
Summary and Conclusions
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

3. The HERMES Spectrometer
Beam: 27.6 GeV e+/e- from HERA accelerator
Reconstruction: Dp/p < 2%, Dq < 0.6 mrad
Particle ID: TRD, Preshower, Calorimeter (hadron/lepton sep.)
RICH (p, K, p separation)
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

4. Particle Identification
hadron/positron separation
combining signals from: TRD, calorimeter, preshower, RICH
hadron separation
Dual radiator RICH for p, K, p
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

5. Yajun Mao, CCAST Workshop, 9-14 Aug, 2004
Event Reconstruction
Define appropriate event topology
Hadron identification: RICH
p: GeV p: 4 – 9 GeV
Lambda is well identified  protons are cleanly identified
Reject events from L(1116)  pp- within ±2s of ML
invariant mass calibration ± 2MeV
direct reconstruction: detection of each decay particle, invariant mass reconstruction
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

6. Yajun Mao, CCAST Workshop, 9-14 Aug, 2004
Detector Mass Calibration
HERMES Mass[MeV] ± ± ± ±0.3
PDG Mass[MeV] ± ±0.13
s width (data)[MeV] ± ± ± ±0.3
s width (MC)[MeV]
Decay Pcm[MeV/c] (Lp-)
HERMES can precisely measure the above known particles
The MC well reproduce the data
The systematic error estimated is about 1.9MeV
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

7. Yajun Mao, CCAST Workshop, 9-14 Aug, 2004
Reconstruction
after all cuts
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

8. Yajun Mao, CCAST Workshop, 9-14 Aug, 2004
Results
add p to invariant mass
Peak at:
M= 1528 ± 2.6 MeV
= 8 ± 2 MeV
significance: 3.7
Unbinned fit is used: result independent of bin size and starting point
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

9. The Signal and Its Background
Peak at:
1527 ± 2.3 MeV
= 9.2 ± 2 MeV
Significance: 4.3
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

10. Yajun Mao, CCAST Workshop, 9-14 Aug, 2004
The Signal Width
Q+ Monte Carlo with complete detector simulation
- generated:
M=1540 MeV , s=2 MeV
- reconstructed:
M= MeV , s=6.2 MeV
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

11. Yajun Mao, CCAST Workshop, 9-14 Aug, 2004
A Non-Zero Width For Q+
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 width: Γ = 17 ± 9 ± 3 MeV
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

12. Yajun Mao, CCAST Workshop, 9-14 Aug, 2004
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
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

13. Comparison of Experiment Results
nK+
World Average: ±2.4 MeV
Large variation in mass not uncommon for new, decaying particles
but need to better estimate exp. uncertainties
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

14. Summary of Positive Results
hep-ex/ , PRL
hep-ex/ , ?
hep-ex/
hep-ex/
hep-ex/
hep-ex/
hep-ex/
hep-ex/ , PLB
Proc. Mong. Acad Sci.
hep-ex/ , PRL
hep-ex/ , PRL
hep-ex/ , PLB
hep-ex/ , ?
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

15. Summary of Negative Results
0 null results published, only 3 on arXiv so far ⇒ need null results to be published
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

16. Yajun Mao, CCAST Workshop, 9-14 Aug, 2004
Open Questions
How real are positive results?
check if peaks are generated by
kinematical reflections
ghost tracks
acceptance or cuts
How real are negative results? need to published
scrutinized as hard as positive results
Mass?
Width?
Spin and Parity
etc.
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

17. Yajun Mao, CCAST Workshop, 9-14 Aug, 2004
How Real Is The Peak?
check for
kinematical reflections
detector acceptance and cuts (PYTHIA6 MC / Toy MC)
Q+ vs S*+
is Q+ a pentaquark or a previously unobserved S*+
add a forth hadron
is the peak still there
can we guess the production process for the Q+
can we supress background
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

18. Kinematical Reflection
particle miss-identification
ghost tracks
PID “leaks”
expect peak in M(p-p)
if p+ is p and KS is L(1116)
Spectrum of events associated with L(1116)
remove L(1116) contribution
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

19. Yajun Mao, CCAST Workshop, 9-14 Aug, 2004
Q+ vs S*+
Is peak a new S*+ or a pentaquark state
If peak is S*+ ⇒ also see a peak in M(Lp+)
if member of baryon octect: b.r.(Lp+)/(pKs)  3/2
if member of decuplet: ~ 3/2 (M. Polyakov)
S*+
No peak in Lp+ spectrum near 1530 MeV
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

20. Yajun Mao, CCAST Workshop, 9-14 Aug, 2004
The Mass Spectrum
relatively large BG, although good PID for proton and KS
what if we require one additional hadron?
could additional hadron help remove KS from other process?
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

21. Requiring 4th Hadron As p
Black requires 4th hadron (all species)
only p
Red specifies 4th hadron as p
K & p
Green specifies 4th hadron as not p
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

22. Why Does Additional p Help?
remove
remove add new cut: |M(KSp)-892|<75 MeV
high statistics K*
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

23. The Mass of (K* removed)
|M(pp-) – 1.116|<0.006GeV
there is a L(1116) peak from
but it only contributes to background
add L veto as a new cut
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

24. Mass Spectrum After Removing K*
standard cuts applied
+ K* veto
+ L veto
 background is greatly reduced
signal/noise:
from 1:3 to 2:1
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

25. Production Process At HERMES
can 4th hadron come from exclusive processes?
associated K- or KS from exclusive processes goes backward
even decay pions from KS are inaccessible
PID threshold requires p(Q+) > 7 GeV
tagged pions events can’t come from these exclusive processes
 production process has to be at least partially inclusive
inclusive processes increase with higher energy
exclusive processes decrease with higher energy
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

26. HERMES Conclusions and Outlook
Direct reconstruction of Q+ invariant mass
Mass:
Intrinsic Width:
Significance: ~ 4 s
Q+ is probably an isosinglet
Requiring additional p improves signal/background, it eliminates KS contamination from various processes
Production process is at least partially inclusive
Anticipate doubling statistics by end of this summer
Will soon report on X-- search and Q+ from TOF (low p)
14-August-2004
Yajun Mao, CCAST Workshop, 9-14 Aug, 2004