29/03/04 A. Geiser, Recent Spectroscopy Results from ZEUS 1

Recent Spectroscopy Results from the (H1 and) ZEUS Experiments

Selected main topic: evidence/search for pentaquarks

IntroductionEvidence for a resonance consistent with p and its antiparticle (ZEUS) ( Evidence/search for a charmed pentaquark (H1/ZEUS) -> see talk by K. Lipka tomorrow)

Achim Geiser, DESY Hamburg for the ZEUS collaboration

XXXIXth Rencontres de Moriond QCD and Hadronic Interactions at High Energy La Thuile, Italy, 29. March 04

hep-ex/04040xx (soon)

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QCD: only colourless states can exist as free particles (confinement)

conventional wisdom: bound multiquark states (hadrons): qq (mesons) qqq (baryons) but: other colourless combinations

not forbidden: qqqq (tetraquarks) qqqqq (pentaquarks)

... can they be bound tightly enough

to have narrow width?

What are pentaquarks?

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some QCD models: yes!

e.g. Diakonov et al. 1997: uudds = m~1530 MeV, < 15 MeV

recently observed by several fixed target experiments:

n K+, LEPS, CLAS, SAPHIR (establishes exotic quantum numbers) m~1540+-5 MeV, width ~ resolution

p K0s, DIANA, ITEP, HERMES,

SVD, COSY-TOF (more precise measurements) m~1527+-3 MeV, width ~ resolution

evidence for uussd, ddssu by NA49 (not seen by HERA-B)

Evidence for pentaquarks

fixed target=> pentaquark uses valence quarks

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s = 300 (318) GeV ep CM energy before (after) 1998

W = m(p) p CM energy

Q2 = -q2 photon virtuality, squared momentum transfer

Q2 > 1 GeV2: DIS Q2 < 1 GeV2 : photoproduction

uudds = „established“, I=0 no , (?) what about uudds = - ? (this talk) (can only be produced in fragmentation)

what about uuddc/uuddc = c ? -> talk K. Lipka tomorrow

(need high CMS energy)

look in high energy collider experiments, e.g. deep inelastic ep scattering (DIS) at

HERA:

Pentaquarks in high energy collisions?

pq2

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inclusive DIS event sample: Q2 > 1 GeV2

data 1996-2000, 121 pb-1

K0 selection: pT(K0) > 0.3 GeV

eliminate and photon conversion candidates

866800 K0‘s

Search for + p K0s / - p K0

s in ZEUS

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several resonances observed

produced in gluon-rich environment

f0(1710) = glueball candidate??

Intermezzo: look for K0s K0

s

resonances

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select protons by defining ionization band in dE/dx

dE/dx > 1.15 mips, p(p)<1.5 GeV => ~60% proton purity

Search for + p K0s / - p K0

s in ZEUS

p pK+ K-

+

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well established narrow d-wave uds state, isospin 0, charge 0 isospin 1 partner is expected to have higher mass

=> do NOT expect + or X++ in same place

select p,K using dE/dx resonance seen in p K- + c.c., not seen in p K+ + c.c.

mass and natural width consistent with PDG: M = 1519.5 +- 1.0 MeV = 15.6 +- 1.0 MeV

no evidence for hypothetical ++(1530) (I=1 pentaquark)

Check: look for (1520) p K+ + c.c.

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select p/p and K0s

in inclusive DIS (Q2>1 GeV2)

plot m(pK0s) + m(pK0

s)

structure emerges for increasing Q2

or low W around m~1525 MeV

Search for + p K0s / - p K0

s in ZEUS

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fit bg + single Gaussian

peak at1522+-2 MeV

width consistentwith resolution

statisticalsignificance~ 3.9

still structure left ofpeak?also try different background shape

Search for + p K0s / - p K0

s in ZEUS

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fit bg + two Gaussians(2nd Gaussian parametrizes potential resonance or empirical shoulder in background shape)

m = 1521.5 +- 1.5 (stat) +2.8

-1.7 (sys) MeV

Gaussian width 6.1 +- 1.5 MeV still compatible withexperimental resolution of ~2 MeV(Breit-Wigner fit: = 8+-4 MeV)

significance ~ 4.6

signal seen in both charges (inset)if interpreted as + + -

antipentaquark!

Search for + p K0s / - p K0

s in ZEUS

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summary of fit results:

p(fluctuation)m = 1500-1560 MeV = 1.5-12 MeV: 6 x 10-5

other systematic checks (not part of the default selection):

remove proton candidates consistent with misidentified from K* -> K0 momentum imbalance: require E(p) > E(K0)=> peak significance increases further (including Q2>1 GeV2), but complicated background shape -> do not use

check that reflections from Ds+- -> K0 K+- or other charm resonances

can not produce such a peak

assign proton mass to pion enriched sample using dE/dx; mix K0 and p from different events -> no peaks

Search for + p K0s / - p K0

s in ZEUS

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pentaquarks and other exotic multiquark states are allowed in principle in QCD, but are they narrow enough to be seen?

recent evidence from fixed target experiments, suggested by some QCD models: yes! (+ = uudds, m ~ 1530 MeV)

ZEUS finds evidence for a narrow resonance in p K0s and p K0

s

in DIS events at HERA, consistent with + + - m = 1521.5 +- 1.5(stat) +2.8

-1.7(sys) MeV width still consistent with exp. resolution of 2 MeV ( = 8+-4 MeV)

if pentaquark interpretation confirmed: first observation of the - state (antipentaquark)

new era in spectroscopy: this is just the beginning!

Summary and Conclusions