1. The Pentaquark Q+ Search
At Hermes
Weilin Yu
(Peking University)
for Hermes Collaboration
2019/2/24
Weilin Yu QCD和轻强子暑期班
Weilin Yu QCD和轻强子暑期班

2. Outline Particle Identification Search For pKS Resonance
Systematic Studies
Summary and Conclusions
2019/2/24
Weilin Yu QCD和轻强子暑期班

3. The HERMES Experiment
2019/2/24
Weilin Yu QCD和轻强子暑期班

4. 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)
2019/2/24
Weilin Yu QCD和轻强子暑期班

5. Particle Identification
hadron/positron separation
combining signals from: TRD, calorimeter, preshower, RICH
hadron separation
Dual radiator RICH for p, K, p
2019/2/24
Weilin Yu QCD和轻强子暑期班

6. What is a Pentaquark Q+ Composed of 4 quarks and one antiquarks .
Has strangeness S=+1 .
Allowed by the Standard Quark Model.
Decay Mode:
Q+  Ks p or K+n
2019/2/24
Weilin Yu QCD和轻强子暑期班

7. 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 ±1.9MeV
direct reconstruction: detection of each decay particle, invariant mass reconstruction
2019/2/24
Weilin Yu QCD和轻强子暑期班

8. Identification
after all cuts
2019/2/24
Weilin Yu QCD和轻强子暑期班

9. Results 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
2019/2/24
Weilin Yu QCD和轻强子暑期班

10. The Signal and its Background
Peak at:
1527 ± 2.3 MeV
= 9.2 ± 2 MeV
Significance: 4.3
Known S*s
(excluded in Pythia6)
mixed event background
PYTHIA6
2019/2/24
Weilin Yu QCD和轻强子暑期班

11. A Non-Zero Width for Q+? HERMES intrinsic width: Γ = 17 ± 9 ± 3 MeV
Observed width FWHM: 19 – 24 MeV
Detector resolution (from MC) FWHM: 10 – 14.6 MeV
re-fit spectra with Breit-Wigner convoluted with a Gaussian (fixed by MC)
HERMES intrinsic width: Γ = 17 ± 9 ± 3 MeV
2019/2/24
Weilin Yu QCD和轻强子暑期班

12. 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
2019/2/24
Weilin Yu QCD和轻强子暑期班

13. 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/ , ?
2019/2/24
Weilin Yu QCD和轻强子暑期班

14. Summary of Null Results
0 null results published, only 3 on arXiv so far ( ) ⇒ need null results to be published
2019/2/24
Weilin Yu QCD和轻强子暑期班

15. 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
2019/2/24
Weilin Yu QCD和轻强子暑期班

16. 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
2019/2/24
Weilin Yu QCD和轻强子暑期班

17. 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?
2019/2/24
Weilin Yu QCD和轻强子暑期班

18. Mass Spectrum after 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
2019/2/24
Weilin Yu QCD和轻强子暑期班

19. Why does additional p help?
remove
remove add new cut: |M(KSp)-892|<75 MeV
high statistics K*
2019/2/24
Weilin Yu QCD和轻强子暑期班

20. Mass Spectrum After Removing K*
standard cuts applied
+ K* veto
+ L veto
 background is greatly reduced
signal/noise:
from 1:3 to 2:1
2019/2/24
Weilin Yu QCD和轻强子暑期班

21. 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
Anticipate doubling statistics this year
Will soon report on X-- search and Q+ from TOF (low p)
2019/2/24
Weilin Yu QCD和轻强子暑期班

22. 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
2019/2/24
Weilin Yu QCD和轻强子暑期班