1Exotic States 2005E.C. Aschenauer

The The search for search for Pentaquarks atPentaquarks at

on behalf of the HERMES Collaboration

E.C. Aschenauer

DESY

2Exotic States 2005E.C. Aschenauer

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

3Exotic States 2005E.C. Aschenauer

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

4Exotic States 2005E.C. Aschenauer

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

5Exotic States 2005E.C. Aschenauer

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

6Exotic States 2005E.C. Aschenauer

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

7Exotic States 2005E.C. Aschenauer

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

8Exotic States 2005E.C. Aschenauer

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

9Exotic States 2005E.C. Aschenauer

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

10Exotic States 2005E.C. Aschenauer

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

11Exotic States 2005E.C. Aschenauer

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

12Exotic States 2005E.C. Aschenauer

More on the + spectra

signal / background 2:1 standard cuts applied + K* and veto

signal / background: 1:3

add additional

13Exotic States 2005E.C. Aschenauer

Comparison with World Data

Kn

0sK p

Mean:1532.1±2.1 MeV

14Exotic States 2005E.C. Aschenauer

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

15Exotic States 2005E.C. Aschenauer

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

16Exotic States 2005E.C. Aschenauer

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

17Exotic States 2005E.C. Aschenauer

HERMES contribution to exotic searches

Phys. Lett. B 585 (2004) 213

0

hep-ex/0412027

18Exotic States 2005E.C. Aschenauer

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