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

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