Evidence for the Pentaquark An Exotic Baryon Announcements

Evidence for the Pentaquark: An Exotic Baryon Announcements from several experiments around the world provide evidence for the existence of an exotic baryon, a pentaquark with strangeness S=+1. Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

What are pentaquarks? • Minimum content: 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 Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Historical Bias against Pentaquarks NOTE ON THE S = + 1 BARYON SYSTEM (PDG 1986; Phys. Lett. B 170, 289) The evidence for strangeness +1 baryon resonances was reviewed in our 1976 edition, 1 and more recently by Kelly 2 and by Oades. 3 Two new partial-wave ana. Iyses 4 have appeared since our 1984 edition. Both claim that the P 13 and perhaps other waves resonate. However, the results permit no definite conclusion- the same story heard for 15 years. The standards of proof must simply be much more severe here than in a channel in which many resonances are already known to exist. The general prejudice against baryons not made of three quarks and the lack of any experimental activity in this area make it likely that it will be another 15 years before the issue is decided. Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Summary of Experiments • Evidence (since October 2002): – LEPS (4. 6 s, peak at mass = 1. 54 Ge. V) – ITEP (4. 5 s, peak at mass = 1. 539 Ge. V) – CLAS (5. 5 s, peak at mass = 1. 542 Ge. V) – SAPHIR (4. 8 s, peak at mass = 1. 540 Ge. V) – NEW: n scattering (6. 7 s, m = 1533 +/- 5 Me. V) – NEW: p. K 0 from HERMES(~5 s, m = 1527 Me. V) – NEW: X-- from CERN (~8 s, m = 1865 Me. V) Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Baryon Groups ms=150 Me. V W− Seminar (SLAC, Nov. 4, 2003) ? Ken Hicks, Ohio University

Hadron multiplets Mesons qq Baryons qqq K p K N S X W─ Baryons built from meson-baryon, or qqqqq + Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

The Anti-decuplet predicted by Diakonov et al. Symmetries give an equal spacing between “tiers” Seminar (SLAC, Nov. 4, 2003) Width < 15 Me. V ! Ken Hicks, Ohio University

A di-quark model for pentaquarks JW hep-ph/0307341 JM hep-ph/0308286 SZ hep-ph/0310270 (ud) L=1 s (ud) Decay Width: L=1, one unit of orbital angular momentum needed to get J=1/2+ as in c. SM Uncorrelated quarks: JP = 1/2− G» 200 Me. V (2 6 ) 2 » 8 Me. V Mass Prediction for X−− is 1. 75 instead of 2. 07 Ge. V Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Why is the + important? • QCD does not prohibit q 4 q states, but early searches have failed to produce evidence for pentaquarks. What is the mechanism of nonperturbative QCD that allows the + to exist with such a small width? (Naïve quark models cannot explain the narrow width; the soliton model can. ) • The + is the first evidence of a new class of particle: the pentaquark. • One of the central activities at Jefferson Lab is to understand N* resonances. Do pentaquarks contribute to the resonance spectrum? Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Super Photon ring-8 Ge. V • • • SPring-8 Third-generation synchrotron radiation facility Circumference: 1436 m 8 Ge. V 100 m. A 62 beamlines Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

LEPS detector g Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University 1 m

Charged particle identification K/p separation (positive charge) Events p- p+ p K+ d K- Mass/Charge (Ge. V) Momentum (Ge. V) Reconstructed mass p+ K+ Mass(Ge. V) s(mass) = 30 Me. V/c 2 for 1 Ge. V/c Kaon Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Summary of data taking 30 Hz for 800 k. Hz@tagger Charge veto Target (LH 2) AC(n=1. 03) Start counter (SC) Optimized for g p f p K+ K- p Small distance between LH 2 and SC High index of AC Seminar (SLAC, Nov. 4, 2003) • Total number of trigger 1. 83*108 trigger Dec, 2000 to Jun, 2001 • Number of events with reconstructed charged tracks 4. 37*107 events • About a half of events were produced in SC g p K 0 Q+ p + p - K + n Ken Hicks, Ohio University

Detected nuclear reactions K─ g n ( p) Q K ( p) + - g + + K n Q + n p - L (1520) K p n p K+ g p (n) L* (1520)K + (n) * K+ g p n K─ L* p n g. N f(1020) N K+K- N Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Identification of Q+ g n → K - Q+ → K - K + n • K- missing mass gives Q+ mass • K+K- missing mass gives n Problems: • no neutron target • CH start counter (n is part of C!) • “background” · f K+K(produced from n & p) Non-resonant KK L(1520) Q+? Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Fermi motion correction L K+p- missing mass Correction works better at small kaon angles. K+ missing mass (corrected) L K+ missing mass Seminar (SLAC, Nov. 4, 2003) S- K+ missing mass (corrected) MMg. K+ (corrected) = MMg. K+- MMg. K+p-+ Mn Ken Hicks, Ohio University LH 2 target Startcounter (CH) Test-case: g n → K+ S-→K+ p- n g p → K+ L→K+ p- p

Proton recoil cut K+ tof dc dc ssd dc § gn K+K-n no recoil proton (proton is a spectator) § gp K+K-p slow recoil proton is present § proton is too slow to be seen in full detector, but might be seen in SSD vertex detector. g target (SC) Kp dipole Remove all events for which proton is detected in SSD, or for which predicted proton track does not hit SSD. Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Effectiveness of proton recoil cut g p(n) K+K-p(n) • select KK events from startcounter • construct K+ missing mass (corrected) plot • apply p-recoil cut MMcg. K+ = MMg. K+- MMg. K+K-+ Mn gp L(1520)K+ K- p • reverse p-recoil cut (proton is present) No L(1520) peak in events with a spectator proton. Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

+ signal: K- missing mass g n → K - Q+ → K - K + n • select KK events from SC • apply KK invariant mass cut (f) • apply KK missing mass cut (0. 9<MMkk<0. 98) • apply proton recoil cut • construct K- missing mass plot Background shape can be determined from events from LH 2 target using the same cuts except for: • Proton-recoil cut is removed • L(1520) events are removed (only from p) Seminar (SLAC, Nov. 4, 2003) Q+ Background? Ken Hicks, Ohio University

+: published mass plot Background level is estimated by a fit in a mass region above 1. 59 Ge. V. M = 1. 54 0. 01 Me. V G < 25 Me. V Gaussian significance 4. 6 s Assumption: • Background is from non-resonant K+K - production off the neutron/nucleus • … is nearly identical to non-resonant K+K- production off the proton background Phys. Rev. Lett. 91 (2003) 012002 hep-ex/0301020 Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

DIANA at ITEP: 850 Me. V K+ beam K+ Xe + N (K 0 p) N Cuts to suppress p and K 0 reinteraction in Xe nucleus hep-ex/0304040 All Events Seminar (SLAC, Nov. 4, 2003) M=1539± 2 Me. V G < 9 Me. V Ken Hicks, Ohio University

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 Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

The CLAS Photon Tagger Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

CLAS: particle ID by time-of-flight Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Photoproduction on deuterium I • In the analysis we assume gn a Q+K- a n. K+K- with Fermi correction a la Spring-8 applied • No statistical significant result obtained! • Production of + off a single nucleon proceeds via t -channel kaon exchange like L(1520) • The t-channel meson, K- in the case of +, is emitted mostly in forward direction. • The limited forward acceptance of CLAS together with the in-bending of negative charged particles due to the magnetic field, are unfavorable circumstance for direct + photoproduction detection. Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

CLAS: Exclusive photoproduction II CLAS Collaboration (S. Stepanyan, K. Hicks, et al. ), hep-ex/0307018 • Requires FSI – both nucleons involved – No Fermi motion correction necessary – FSI puts K- at larger lab angles: better CLAS acceptance – FSI not rare: in ~50% of L(1520) events both nucleons detected with p>0. 2 Ge. V/c Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Kaon times relative to proton pp+p- ppp- Dt (p-K─) (ns) Seminar (SLAC, Nov. 4, 2003) p. K+K─ Dt (p-K+) (ns) Ken Hicks, Ohio University tight timing

Neutron found via missing mass “loose” timing cuts Seminar (SLAC, Nov. 4, 2003) “tight” timing cuts Ken Hicks, Ohio University

CLAS: Q+ on Deuterium (result) M(n. K+) = MM(gd p. K-X) • ~42 events in the narrow peak at 1542+/-5 Me. V with FWHM of 21 Me. V/c • Estimated significance 5. 3+/-0. 5 s • Spectrum of the events associated with L(1520) Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Variations of cuts for the Q+ analysis M(n. K+) = MM(gd p. K-X) • a) no longer has cuts on the L(1520) or on the K+ momentum, giving only a 4. 8 s fit. • b) has tighter timing cuts, which require that each K+ comes within 0. 75 ns of the proton, giving 6. 0 s. Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

+ CLAS - Exclusive Processes III Guo, Kubarovsky, et al. • exclusive channel p+K+K- (n) gp Q+ g p+ M(n. K+) p- 7. 8 s proton N* KK+ + n cut Seminar (SLAC, Nov. 4, 2003) Combined analysis of all CLAS data on protons minimal cuts - forward p+, backward K+ indications of production from heavy N*(2420) Ken Hicks, Ohio University

+ CLAS - Exclusive Processes III g N* ? p+ p- proton N* K K+ + n Why not seen in p. N elastic ? Need 2. 6 Ge. V/c pion beam. PDG shows gap from 2. 3 – 2. 7 Ge. V/c with no elastic data! Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Neutrino scattering Courtesy of Dolgolenko (ITEP) Reanalysis of bubble chamber experiments from WA 21, WA 25, WA 59, E 180, E 632 M(Ksp) spectrum Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

HERMES Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

A new cousin: observation of exotic X−− M=1. 862± 0. 002 Ge. V + X 0 X(1530) X−− CERN SPS hep-ex/0310014 Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Theoretical questions • The Q+ signal was observed on deuteron, nuclear targets, and proton experimentally. • The existing information does not completely answer questions required of a newly discovered subatomic particle: – – – Parity and spin Isospin Width (Lifetime) Excited states Form factors Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Theoretical interpretations • Chiral soliton model (Diakonov, Petrov, Polyakov) 1997. – the original motivation for experimental analysis • Is it Q+ an isotensor? (S. Capstick, P. Page, W. Roberts) – I=2 would suppress production and decay) • Flavor-spin quark interaction could lower the p-wave pentaquark state below the s-wave state (Stancu, Riska) – An s-wave NK+ molecule would likely “fall apart” • Diquark-triquark structure: 2 quasi-particle (Karliner, Lipkin) – Diquark (spin-0) and [diquark (spin-1) + s-bar] lower hyperfine • Double diquark structure: Jp = ½+ (Jaffe, Wilczek) – Spin-0 diquarks act as pseudo-bosons and inhibit decay • Lattice Gauge calculation (Lee, Bennhold, et al. ) – S=+1 pentaquark might have negative parity (? ) Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Summary and Conclusions • Chiral soliton model: the original motivation • Now 6 independent experiments give evidence for +. – CLAS result on deuterium confirmed the LEPS result • A second pentaquark: the X-- from CERN. – Still needs to be confirmed by another experiment • There is still a lot of experimental work needed: – Spin, parity, (isospin), width, Eg dependence, etc. • New experiment is scheduled at CLAS in Feb. 2004 – Expect x 20 increase in statistics • Evidence from other experiments is still needed – The + should be seen at RHIC, KEK, etc. Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

Mass (Ge. V) Summary of the S=+1 particle data LEPS : g. C→(n. K+) K−X DIANA : K+Xe→(p. K 0) X CLAS : gd→(n. K+) K−p SAPHIR : gp→(n. K+) K 0 ITEP : n d, Ne→(p. K 0) K 0 HERMES : e+d→(p. K 0) X Parenthesis show + decay products Upper Limit on Width (Me. V) Seminar (SLAC, Nov. 4, 2003) Searches for isospin p. K+ partner have found nothing Ken Hicks, Ohio University

Exciting development if holds up! Seminar (SLAC, Nov. 4, 2003) Ken Hicks, Ohio University

The unclaimed Q+(1540) ? CERN: K+p p. K 0 X A. Berthon, et al. , Nucl. Phys. B 63, 54 (1973) Seminar (SLAC, Nov. 4, 2003) Bubble Chamber Data 1. 54 Ge. V Ken Hicks, Ohio University