The search for Pentaquarks at
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Transcript of The search for Pentaquarks at
1Exotic States 2005E.C. Aschenauer
The The search for search for Pentaquarks atPentaquarks at
on behalf of the HERMES Collaboration
E.C. Aschenauer
DESY
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HERA: e+/e- (27GeV) - proton collider
HERA @ DESY
Polarized Deep Inelastic Scattering
HERMES @ HERA
self-polarised electrons: e
<Pb>~ 53±2.5 %
27.5 GeV (e/e)
<Pb>~ 53±2.5 %
1H→ <|Pt|> ~ 85 ±3.8 %
2H→ <|Pt|> ~ 84 ±3.5 %
1H <|Pt|> ~ 74 ±4.2 %
pure nuclear-polarized atomic gas, flipped at 90s time
intervals
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The HERMES Spectrometer
Kinematic Range: 0.02 < x < 0.8 at Q2 > 1 GeV2 and W > 2 GeV
Reconstruction: p/p < 2%, < 1 mrad
Particle ID: TRD, Preshower, Calorimeter
1997: Cherenkov, 1998: RICH + Muon-ID
Internal Gas Target: unpol: H2,D2,He,N,Ne,Kr, Xe He , H , D , H
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hadron/positron separationcombining signals from: TRD, calorimeter, preshower, RICHTRD, calorimeter, preshower, RICH
Aerogel; n=1.03
C4F10; n=1.0014
hadron separationDual radiator RICH for , K, p
Particle IdentificationParticle Identification
K
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direct reconstruction: detection of each decay particle, invariant mass reconstruction
+ + 0Se +D +X K p +X
Particle Reconstruction
Hadron identification: RICH
: 1 - 15 GeV p: 4 – 9 GeV
Suppress contamination from
(1116) p
Define appropriate event topology
Invariant Mass calibration ± 2MeV
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Resultsafter all cuts0K
Sadd p to invariant mass0K
S
Peak at:
1528 ± 2.6 MeV
= 8 ± 2 MeV
Significance: 3.7
Un-binned fit (root):Results are more stable
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The signal and its background
Peak at:
1527 ± 2.3 MeV
= 9.2 ± 2 MeV
Significance: 4.3
Mixed event backgroundexcited * hyperons (not included in Pythia6)
PYTHIA
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Is there a ++
Clear Signal for
(1520) pK-
with acceptance: 1.5%
No peak structure for
pK+
Zero counts at 91% CL
+ not isotensor probably isosinglet
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generated: M=1540 MeV , =2 MeV
The signal width
+ Monte Carlo with complete detector simulation
reconstructed: M=1539.5 MeV , =6.2 MeV Detector resolution: FWHM: 10-14.6 MeV
measured: FWHM: 19-24MeV
intrinsic width: = 17 +/- 9 +/- 3 MeV
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How real is the +
check for “kinematical reflections” detector acceptance and cuts (pythia6 MC / Toy MC)
vs
is the a pentaquark or a previously unobserved
?
add a fourth hadron is the peak still there? can we suppress background?
can we guess the production process for the ? What about -( )? It would purely be produced in fragmentation.
~ 4 / 1 no clear - signal strong influence from target remnant to
+
uudds
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vs *+
Is peak a new or a pentaquark state ?
If peak is ⇒ also see a peak in M() B.R.: ()/(ps) 3/2
HERMES preliminary HERMES preliminary
- *+
*-
No peak in spectrum near 1530 MeV
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More on the + spectra
signal / background 2:1 standard cuts applied + K* and veto
signal / background: 1:3
add additional
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Comparison with World Data
Kn
0sK p
Mean:1532.1±2.1 MeV
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What about the --
+ --
- - - - - - -
e +D +X + p
Define appropriate event topology
Hadron identification: RICH: 0.25 - 15 GeV p: 2 – 15 GeV Select events from (1116) p-
within ±3 of M
Select events from (1321) -
within ±3 of M
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Final spectra for –-and 0
0
UL for -- (1860) cross-section: 1.0 - 2.1 nbUL for 0 (1860) cross-section: 1.2 - 2.5 nbCross-section for 0(1530): 8.8 – 24 nb
Cross section and UL very sensitive to pt and pz distributions
Mixed event background
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Production cross sections and UL
Integrated luminosity: 290 pb-1
all measurements done in quasi-real photo-production (Q2<<1GeV2)
Acceptance from MC:
- (1520): 1.5%
- +: 0.05%
- 0(1530): 0.036-0.1%
- 0(1860): - 0.065%
- --(1860): - 0.031% HERMES estimate:
((1520)) = 62 ± 11 nb
(+)= 100-220 nb ± 25% (stat) (additional factor 2 from production kinematics)
(0(1530)) = 8.8 – 24 nb
Pythia-6 pt and pz spectra
or pt and pz spectra frommeasurements of HERMES
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HERMES contribution to exotic searches
Phys. Lett. B 585 (2004) 213
0
hep-ex/0412027
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Conclusion and Outlook
Direct reconstruction of + invariant mass
Mass: FWHM-Resolution: Significance: ~ 4
1528±2.6(stat)±2.1(syst) MeV19±5(stat)±2(syst)
~ 17 9 3MeV
+ is probably an iso-singlet Additional improves signal / background
0eD X K p+Xs
no signal observed for--and0 for
all cross sections and c.s. U.L determined in quasi-real photo-production (Q2<<1 GeV2)
--/0 - -/+eD X
Continue data taking for more statistics