Experimental Status of Pentaquark States Phys Rev Lett

Experimental Status of Pentaquark States Phys. Rev. Lett. 91 (2003) 012002 Introduction Experimental evidence Production mechanisms What do we know about the Q+? Exotic cascades states Special Thanks CLAS Collaborators SPring-8 uudds Mass = 1. 54 Ge. V Elton S. Smith / APS Meeting / Denver May 1 -4, 2004 1

Quarks are confined inside colorless hadrons Quarks combine to “neutralize” color force q q q Mystery remains: Of the many possibilities for combining quarks with color into colorless hadrons, only two configurations were found, till now… Elton S. Smith APS, Denver May 1 -4, 2004 2

What are pentaquarks? § Minimum quark content is 4 quarks and 1 antiquark § “Exotic” pentaquarks are those where the antiquark has a different flavor than the other 4 quarks § Quantum numbers cannot be defined by 3 quarks alone. Example: uudss, non-exotic Baryon number = 1/3 + 1/3 – 1/3 = 1 Strangeness = 0 + 0 − 1 + 1 = 0 Example: uudds, exotic Baryon number = 1/3 + 1/3 – 1/3 = 1 Strangeness = 0 + 0 + 1 = +1 Elton S. Smith APS, Denver May 1 -4, 2004 3

Pentaquarks – two approaches Chiral soliton model: (Diakonov, Petrov, Polyakov) Pentaquark comes out naturally from these models as they represent rotational excitations of the soliton [rigid core (q 3) surrounded by meson fields (qq)] Quark cluster models, e. g. di-quark description (Jaffe, Wilczek) (ud) s L=1 (ud) Soliton: (simplified) L=1, one unit of orbital angular momentum needed to get J=1/2+ as in c. SM Meson fields Lattice QCD => JP = 1/2 Elton S. Smith APS, Denver May 1 -4, 2004 4

The Anti-decuplet of SU(3)f Ten observations Null Results? X 5−− X 5 0 One experiment D. Diakonov, V. Petrov, hep-ph/0310212 Elton S. Smith APS, Denver May 1 -4, 2004 (revised version) 5

Experimental Evidence § Many experiments § No dedicated experiments to date ─ but, … dedicated experiments are starting to take data For new data from SPring-8 see Hicks Session J 2 Sun 10: 45 § Walk through the analysis from CLAS § Selected examples from other experiments Elton S. Smith APS, Denver May 1 -4, 2004 6

Quark lines for the reaction g us us K+ Q+ n K− ddu n Q+ is composed of (uudds) quarks Elton S. Smith APS, Denver May 1 -4, 2004 7

Production mechanisms g n p K+ n (p) K+(K 0) Q+ pspec Control Reactions K+ g g n p K− K− S− n p− pspec Elton S. Smith p n APS, Denver K+ K− L*(1520) p K− nspec May 1 -4, 2004 8

JLab accelerator CEBAF Continuous Electron Beam • Energy 0. 8 ─ 5. 7 Ge. V • 200 m. A, polarization 75% • 1499 MHz operation • Simultaneous delivery 3 halls Elton S. Smith APS, Denver May 1 -4, 2004 9

CEBAF Large Acceptance Spectrometer Torus magnet 6 superconducting coils Electromagnetic calorimeters Lead/scintillator, 1296 photomultipliers Liquid D 2 (H 2)target + g start counter; e minitorus Drift chambers argon/CO 2 gas, 35, 000 cells Gas Cherenkov counters e/p separation, 256 PMTs Time-of-flight counters plastic scintillators, 684 photomultipliers Elton S. Smith APS, Denver May 1 -4, 2004 10

gd → p K+K─ (n) in CLAS K+ K- p Elton S. Smith APS, Denver May 1 -4, 2004 11

Particle identification by time-of-flight Elton S. Smith APS, Denver May 1 -4, 2004 12

Reaction gd→p. K+K-(n) n Clear peak at neutron mass. n 15% non-p. KK events within ± 3 s of the peak. n Almost no background under the neutron peak after event selection with tight timing cut. Reconstructed Neutrons Elton S. Smith APS, Denver May 1 -4, 2004 13

Deuterium: n. K+ invariant mass distribution Q+ NQ = 43 events Mass = 1. 542 Ge. V G< 21 Me. V Significance 5. 2± 0. 6 s Two different Background shapes Distribution of L*(1520) events Elton S. Smith APS, Denver May 1 -4, 2004 14

Searching for Q+ on a proton target gp→p+K- K+ (n) Prominent K*0 no cuts K*0 M(n. K+) [Ge. V] Elton S. Smith APS, Denver May 1 -4, 2004 15

Searching for the Q+ on a proton target gp→p+K-K+(n) Eg = 3 – 5. 5 Ge. V g Cosq*(p+) > 0. 8 p− Q+ p 7. 8 s p+ n Q+ N* K− K+ M=1555± 10 Me. V G< 26 Me. V Cosq*(p+) > 0. 8 Cosq*(K+) < 0. 6 cut CLAS Collaboration PRL 92, 032001 -1 (2004). M(n. K+) [Ge. V] Elton S. Smith APS, Denver May 1 -4, 2004 16

Q+ ─ N* production mechanism? g cuts outside cuts Q+ p− p N* ? N* p+ n Q+ K− K+ § What do p-p scattering data say? M(n. K+K−) [Ge. V] Elton S. Smith § p-p cross section data in PDG have a gap in the mass range 2. 3– 2. 43 Ge. V. APS, Denver May 1 -4, 2004 17

Diffractive mechanism? g K- K*0 p+ p Q+ Require forward K 0* Cos q*(K−p+) > 0. 5 Elton S. Smith APS, Denver May 1 -4, 2004 18

Q+ NOT produced in association with K*0 events Non K*0 events Elton S. Smith APS, Denver May 1 -4, 2004 19

HERMES e+d→(p. Ks 0) X Airapetian et al. Hep-ph/0312044 27. 6 Ge. V positrons M=1528± 3. 3 Me. V G ~ 17± 9 Me. V Mass (K 0 p) [Ge. V] Elton S. Smith APS, Denver May 1 -4, 2004 20

New results from COSY-TOF hep-ex/0403011 pp → S+ K 0 p M=1530± 5 Me. V G< 18 Me. V s ~ 0. 4± 0. 1 mb 2. 95 Ge. V M(K 0 p) Ge. V/c 2 The TOF spectrometer at the COSY facility in Juelich, Germany found evidence for the Q+ in the reaction: p + p → S+ + Q+. Elton S. Smith APS, Denver May 1 -4, 2004 21

From ZEUS at DESY… ZEUS hep-ex/0403051 ep →e. Ks 0 p X M=1521. 5± 1. 5 Me. V G~ 8± 4 Me. V √s ~ 310 Ge. V Q 2 > 20 Ge. V 2 Mass (K 0 p) [Ge. V] Elton S. Smith APS, Denver May 1 -4, 2004 22

Mass (Ge. V) What do we know about this S=+1 state? LEPS : g. C→(n. K+) K−X DIANA : K+Xe→(p. K 0) X CLAS-d : gd→(n. K+) K−p CLAS-p : gp→(n. K+) p+K− SAPHIR : gp→(n. K+) K 0 ITEP : n d, Ne→(p. K 0) K 0 HERMES : e+d→(p. K 0) X COSY-TOF: pp→(p. K 0) S+ Upper limit or estimate of G (Ge. V) Elton S. Smith ZEUS : ep→e (p. K 0) X SVD-2 : p. A→(p. K 0) X ( ) Strangeness undetermined Decay products in parenthesis APS, Denver May 1 -4, 2004 23

Search for pentaquarks in HERA-B hep-ex/0403020 L*(1520) Q+ ? M(p. K 0 s) (Ge. V) M(p. K−) (Ge. V) p. A →K 0 p X Null result at HERA-B Q+(1540) √s ~ 41. 6 Ge. V L*(1520) Elton S. Smith APS, Denver < 0. 02 May 1 -4, 2004 24

There is much more to learn § Spin, parity ─ Chiral soliton model predicts Jp=½+ (p-wave) ─ Quark model naïve expectation is Jp=½− (s-wave) ─ Lattice calculations predict Jp=½− § Isospin ─ Likely I=0, since searches for p. K+ partners unsuccessful § Width (lifetime) ─ Measurements mostly limited by experimental resolution. ─ Theoretical problem remains why the state is so narrow. ─ Analysis of existing K+d scattering data indicate that G < 1 -2 Me. V. § Complete determination of the pentaquark multiplet Elton S. Smith APS, Denver May 1 -4, 2004 25

A new cousin: observation of exotic X 5−− M=1. 862± 0. 002 Ge. V ssddu X 50 Q+ X(1530) X 5−− X 50 NA 49 CERN SPS Phys. Rev. Lett. 92 (2004) 042003 Elton S. Smith APS, Denver May 1 -4, 2004 26

But, can it be reproduced? ? HERA-B hep-ex/0403020 X(1530) X− p + + X+ p − X− p − + X+ p + Mass (Ge. V/c 2) § HERA-B collaboration at DESY (Germany) § Null result with much higher statistics! § They also have a null result for the Q+ Elton S. Smith APS, Denver May 1 -4, 2004 27

Predictions depend on dynamics Decay modes are sensitive to dynamical picture Number of states and mass spectra Ss X X 5 Ns X 5 BR(X 5 → K− S−) BR(X 5 → p− X−) S N LS Q BR(X 5 → p 0 X−) BR(X 5 → p− X 0) Q N Di-quark Soliton Note: Models adjusted to experiment Elton S. Smith APS, Denver May 1 -4, 2004 28

Search for exotic cascades X 5−− and X 5− Experiment 04 -10, scheduled to run this fall X −− → p− X− L → p− p X − → p− L Electron beam gn→K+K+X 5−− Two decay vertices, Negatives bend outwards Elton S. Smith APS, Denver May 1 -4, 2004 X− ct = 4. 9 cm L ct = 7. 9 cm <gb> ~ 1. 5 29

Current activities at Jlab § Pentaquark experiments in Hall B ─ g 10 (currently taking data) gd → Q+ Eg~1 – 3. 5 Ge. V ─ g 11 (starts in mid-May) gp → Q+ Eg~1 – 3. 5 Ge. V ─ eg 3 (November) gvd→ X 5−−, X 5− Eg> 3. 9 Ge. V ─ High energy data gp → Q+, X 5 Eg~1. 5 – 5. 4 Ge. V § Pentaquark experiment in Hall A ─ E 04 -012 (May), search for excited Q++ and Q 0 states. Elton S. Smith APS, Denver May 1 -4, 2004 30

Summary § A key question in non-perturbative QCD is the structure of hadrons. § We have reviewed the evidence for the existence of a new class of colorless hadrons with quantum numbers which cannot be generated from solely three quarks: ─ There is substantial corroborating evidence for an exotic baryon with S = +1, which would have a minimal quark content of (uudds). ─ The observation of a doubly negative S=− 2 baryon (ddssu) is consistent with a second corner of the anti-decuplet the family of pentaquarks, but needs additional confirmation. § Dedicated experiments are being mounted which should easily establish (or refute) the observations to date. Elton S. Smith APS, Denver May 1 -4, 2004 31