Current Status of Pentaquark States OUTLINE Why are pentaquarks interesting? Status of observations Outlook for new experiments Summary X−− X+

Properties of quarks u p d u n d s K− u s u Protons are made of (uud) Quark Flavor Charge (Q) Baryon number Strangeness (S) u +2/3 +1/3 d −1/3 s −1 − 2/3 +1 u d +2/3 −1/3 p d u +2/3 −1/3 n s −1/3 u −2/3 K− s u +1/3 +2/3 Protons are made of (uud) Neutrons are made of (ddu) K+

Interactions understood in terms of quarks Very high energy “free” quarks not found, only particles that contain quarks

Asymptotic Freedom The discovery which is awarded this year's Nobel Prize is of decisive importance for our understanding of how the theory of one of Nature's fundamental forces works, the force that ties together the smallest pieces of matter – the quarks.

Strong coupling constant as vs energy Confinement Asymptotic Freedom

Quarks are confined inside colorless hadrons Quarks combine to “neutralize” color force q q q All hadrons could be categorized as qq mesons and qqq baryons... …at least until recently.

Families of quarks S=+1 S= 0 I3 = Q ─ ½ (B+S) −½ +½ S=−1

Baryons built from meson-baryon, or qqqqq Hadron multiplets K p Mesons qq Baryons qqq N S X W─ Baryons built from meson-baryon, or qqqqq Q+ X+ X−−

What are pentaquarks? Example: uudss, non-exotic 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 + 1/3 – 1/3 = 1 Strangeness = 0 + 0 + 0 − 1 + 1 = 0 Example: uudds, exotic Baryon number = 1/3 + 1/3 + 1/3 + 1/3 – 1/3 = 1 Strangeness = 0 + 0 + 0 + 0 + 1 = +1

The Anti-decuplet predicted by Diakonov et al. In the Chiral Soliton Model, nucleons and Deltas are rotational states of the same soliton field. Z.Phys. A359, 305 (1997) Rotational excitations include G~15 MeV The mass splittings are predicted to be equally spaced Anchor JP=½+ G~140 MeV

First observation of Q+ at SPring-8 Phys.Rev.Lett. 91 (2003) 012002 = 1.540.01 MeV < 25 MeV Gaussian significance 4.6s background

Experimental evidence for Q+ Many experiments Dedicated experiments starting to take data Selected examples only No attempt at completeness

Quark lines for production of Q+ g K− us us K+ Q+ n ddu ddu n Q+ is composed of (uudds) quarks

JLab accelerator CEBAF Continuous Electron Beam Energy 0.8 ─ 5.7 GeV 200 mA, polarization 75% 1499 MHz operation Simultaneous delivery 3 halls

CEBAF Large Acceptance Spectrometer Torus magnet 6 superconducting coils Electromagnetic calorimeters Lead/scintillator, 1296 photomultipliers Liquid D2 (H2)target + g start counter; e minitorus Drift chambers argon/CO2 gas, 35,000 cells Gas Cherenkov counters e/p separation, 256 PMTs Time-of-flight counters plastic scintillators, 684 photomultipliers

gd → p K+K─ (n) in CLAS K+ p K- Sectors 3 & 6 Sectors 1 & 4

Deuterium: nK+ invariant mass distribution Mass = 1.542 GeV < 21 MeV Significance 5.2±0.6 s Q+ NQ = 43 events Two different Background shapes Distribution of L*(1520) events Further analysis of the deuterium data find that the significance of the observed peak may not be as large as indicated.

Evidence for pentaquark states Spring8 DIANA JLab-d ELSA ITEP SVD/IHEP JLab-p HERMES ZEUS pp  S+Q+. COSY-TOF CERN/NA49 H1

But…

Search for pentaquarks in HERA-B HERA-B hep-ex/0403020 L*(1520) Q+ ? M(pK−) (GeV) M(pK0s) (GeV) Null result at HERA-B pA →K0p X Q+(1540) L*(1520) < 0.02 √s ~ 41.6 GeV

BaBar null results other channels searched Q+? M(pK0s) (GeV) BABAR hep-ex/0408064 other channels searched Q+? M(pK0s) (GeV) M(pK0s) (GeV)

A new cousin: observation of exotic X5−− M=1.862± 0.002 GeV X50 ssddu Q+ X5−− X50 X(1530) NA49 CERN SPS Phys. Rev. Lett. 92 (2004) 042003

But, can it be reproduced?? HERA-B hep-ex/0403020 X(1530) X−p+ + X+p− X−p− + X+p+ Mass (GeV/c2) HERA-B collaboration at DESY (Germany) Null result with much higher statistics! They also have a null result for the Q+

Second Generation Dedicated Experiments

LEPS-2/Spring8: deuterium target Dedicated experiment Aimed for 4x statistics of 2003 result Preliminary results announced at N*2004 MX(K−) (GeV)

Current activities at Jlab Pentaquark experiments in Hall B g10 (completed) gd → Q+ Eg~1 – 3.5 GeV g11 (completed) gp → Q+ Eg~1 – 3.5 GeV eg3 (December) gvd→ X5−−, X5− Eg> 3.9 GeV High energy data gp → Q+, X5 Eg~1.5 – 5.4 GeV Pentaquark experiment in Hall A E04-012 (completed), search for excited Q++ and Q0 states.

Reaction gd→pK+K−(n) G2 G10 Published final state. Fully exclusive reaction. G2 L(1520) L(1520) G10

g11 – 6% of data MX(p+p−) (GeV) M2X(p+p) (GeV) M(K+K−) (GeV) L f MX(p+p−) (GeV) M2X(p+p) (GeV) M(K+K−) (GeV) M(p−p) (GeV)

eg3 ─ Search for exotic X5−− In 20 days expect 46 events nb-1 <s> s ~ 0.4 –1.5 nb s µ GX K+ g n S− X5−− p− X− W. Liu and C.M. Ko, nucl-th/0312119

BEACH2004 Conference Summary (Joel Butler) The existence of pentaquaks is an experimental issue Treat each state as a separate entity. Don’t use the non-existence or confusion about higher mass states to cast doubt on Q+ and vice versa Proponents of signals should listen to all criticisms and address them Reflections Justification of cuts, smooth evolution of signal significance with cut values. Be able to demonstrate the stability of signals with respect to cuts Elimination of ghost tracks, Ks-L ambiguities Treatment of backgrounds in fits More data will become available and the truth will eventually emerge. Be patient!

Summary Pentaquarks have revived the interest in hadronic physics and spectroscopy. Jlab is playing and will continue to play a central role in the discovery of the pentaquark or in the rise and fall of the pentaquark.