Part 1: Have Pentaquark States been seen?

Part 1: Have Pentaquark States been seen?
New Hadrons: Facts and Fancy Part 1: Have Pentaquark States been seen? Volker D Burkert Jefferson Lab It has been more than two years since evidence for a pentaquark baryon state was announced by the LEPS collaboration at SPring8 in Japan, followed by a flurry of confirmations from about ten experiments. You might ask why am I here to address the question if “Have Pentaquark States been seen?”? EINN 2005 – September, Milos, Greece

The initial evidence for the Q+
CLAS-D LEPS DIANA SAPHIR 4.4 5.2 4.8 4.6 CLAS-p HERMES Neutrino SVD 7.8 6.7 5.6 This looks like a lot of evidence! ~5 This is a collection of the published evidence for the Q+ until April 2005, eleven experimental results. The numbers within the graphs give the quoted significance of the signal typically 4-6s. The counts in the signals are small, a few tens, with the exception of the ZEUS data. However, combined this looks like a lot evidence. I will make some comments later regarding the significance. ZEUS pp  S+Q+. COSY-TOF K+N ->X ~5 4.6 3

Non-evidence for Pentaquarks
FOCUS BES FOCUS BABAR X-- HERA-B HyperCP SPHINX + more CDF CDF CDF DELPHI X-- Q0c

The Q+ width Measured width dominated by experimental resolution
U. Meissner Measured width dominated by experimental resolution Analysis of K+A data give limit of few MeV. K+D -> X U. Meissner K+D , K+Xe X Sibirtsev et al.: GQ < 1 MeV Very narrow for a hadronically decaying particle with mass ~100 MeV above threshold: Q+(1540) -> nK+, pK0 In most cases the observed width of the state is dominated by experimental resolution. Estimates of the natural width have to rely on estimates using the elastic K+n -> K+ n or K+-deuteron total cross section measurements. Several estimates show that the Q+ width cannot be more than about 1 MeV. The analysis of K+D total cross section data gives a width of 0.9 +/- 0.3 MeV. The data and fits are shown in here. From selected data sample What can we say about event rates? K+n -> Q+ => G~1MeV, K-p -> L* => G~16 MeV. s(Q+)/s(L*) ~ 1.4% for formation. GQ = 0.9 +/-0.3 MeV W. Gibbs, Phys.Rev.C70, (2004)

Are the results on Q+ consistent?
Low energy – effective degrees of freedom, meson-baryon coupling High energy – quark degrees of freedom, fragmentation Can the “positive” low statistics results be reproduced with high statistics, e.g. LEPS, CLAS_d, CLAS_p, SAPHIR, COSY, HERMES,..? Is fragmentation at high energy an efficient way of generating pentaquarks? Why do some experiment (ZEUS, HERMES, SVD) see a signal while others don’t? Are these results consistent? Positive results are at low energy or in specific regions of phase space at high energy experiments. Negative results are often at high energy where quark fragmentation processes are relevant. So we have to address the questions: 1) Are the positive low energy results reproducible? 2) Can we say something about what to expect on production rates? 3) Is fragmentation a good way of generating the Q+ ? And, is it in current or target (baryon) fragmentation? Some of the questions are addressed in abstracts and papers submitted to this conference.

Pentaquark talks at Workshop
C. Alexandrou - Pentaquarks on the Lattice Paul Stoler - Searches for Pentaquarks on proton targets at JLab R. Mizuk - Limit of the Q+ width from KN scattering at Belle S. Niccolai - Searches for Pentaquark baryons on deuterium with CLAS N. Muramatsu - Studies of Q+ with LEPS at SPring8 A. Geiser - Study of Q+ and other Pentaquark searches at HERA J. M. Izen - Search for Pentaquark baryons at BaBaR S. Kabana - Search for exotic baryons at RHIC U. Meissner - What do we know about the width of Q+ ? Discussion - Where are we and what to do next in the Pentaquark search?

C. Alexandrou Takahashi et al., Pentaquark04 and hep-lat/ : JKN and J’KN on spatial lattice size ~1.4, 1.7, 2.0 and 2.7 with a larger number of configurations

Summary of LQCD C. Alexandrou Group Method of analysis/criterion
Conclusion Alexandrou and Tsapalis Correlation matrix, Scaling of weights Can not exclude a resonance state. Mass difference seen in positive channel of right order but mass too large Chiu et al. Correlation matrix Evidence for resonance in the positive parity channel Csikor et al. Correlation matrix, scaling of energies First paper supported a pentaquark , second paper with different interpolating fields produces a negative result Holland and Juge Negative result Ishii et al. Hybrid boundary conditions Negative result in the negative parity channel Lasscosk et al. Binding energy Mathur et al. Scaling of weights Sasaki Double plateau Evidence for a resonance state in the negative parity channel. Takahashi et al. Correlation matrix, scaling of weights J. Negele, Lattice 2005 Maybe evidence for a resonance state?

Upper limit on the Q+ cross section (95%c.l.)
Paul Stoler: Upper limit on the Q+ cross section (95%c.l.) s g p  Q+ K0 < GeV/c2 CLAS: gp -> K-K0n Fit with a sum of smooth function and a Gaussian with fix width and centroid. M(nK+)(GeV) Counts/4 MeV preliminary g K0 Results put stringent limits on possible production mechanism, e.g. implies very small coupling to K* K* n p Q+ K+

Comparison with SAPHIR results
P Stoler: Comparison with SAPHIR results SAPHIR Observed Yields SAPHIR N(Q+)/N(L*) ~ 10% CLAS N(Q+)/N(L*) < 0.2% (95%CL) Cross Sections sg p  Q+ K0 ~ 300 nb reanalysis 50 nb (unpublished) sg p  Q+ K0 < 2 nb Counts Counts cosqCM(K0) > 0.5 cosqCM(K0) > 0.5 M(nK0) (GeV) M(nK+) (GeV) Coun ts L(1520) Coun ts cosqCM(K0) > 0.5 preliminary cosqCM(K0) > 0.5 Q+(1540) ? M(nK0) (GeV) M(nK+) (GeV)

p K γD ® n Upper limit on Q+ production with CLAS. g n -> Q+K-
Silvia Niccolai Upper limit on Q+ production with CLAS. no signal p K γD - + n G10 Set upper limit on cross section < 450pb Upper limit on elementary cross section: The new data show no signal g n -> Q+K- sQ+ < 4-5 nb (95% CL) model dependent. The second experiment repeats a previous low statistics measurement that observed a Q+ signal, with higher statistics. The Q+ excitation is expected to occur on the neutron through gn -> K- Q+ where the proton is a spectator. For the proton to be detected in the CLAS detector a rescattering involving the proton is required. No signal is observed and an upper limit determined. The previous results were re-analysed using the rescaled new data as background and fitted with a Gaussian to the previous data giving a significance of about 3s. G2 Background from G10 In previous result the background is underestimated. New estimate of the original data gives a significance of ~3s, possibly due to fluctuations.

Upper limit on GQ+ from CLAS Results
Theoretical cross section are for GQ+=1 MeV Publication Reaction Jp Experimental width 1/2- 1/2+ 3/2- 3/2+ GQ+ M. Guidal et al., 0.01nb 0.22nb <5.7 MeV hep-ph/ 0.2nb 1nb 55nb 10nb < 4 MeV S. Nam et al., (2.7)nb 8nb (1)nb <(0.5) MeV hep-ph/ 2.7nb 200nb 25nb < 1.7 MeV Y. Oh et al., NP A745 ~0.4nb ~1.6(100)nb < 0.8 MeV hep-ph/ ~1.7nb ~8.7(75)nb < 0.5 MeV C.M. Ko and W. Liu 15(30)nb < 0.08 MeV nucl-th/ 15(30)nb < 0.25 MeV W. Roberts 2nb 5.2(~10)nb 15.4nb 1.8nb < 0.24 MeV nuc-th/ 3.5nb 11.2(~20)nb 48nb 4.nb < 0.4 MeV ( ) – with K* exchange

Old and New Signal from LEPS/SPring8
N. Muramatsu gD -> K-X Mix K＋, K－,  from different events in LH2 data. (1520) contribution was removed from the sample. (1520) ＋ preliminary preliminary MMC(γ,K＋) GeV/c2 MMC(γ,K－) GeV/c2

K-p missing mass spectrum
N. Muramatsu * fitted in MM<1.52 GeV/c2 ~5σ excess at 1.53 GeV/c2 corresponding + 1.6 GeV bump was also seen. + preliminary 1.6 GeV bump Counts/5 MeV preliminary Counts/5 MeV * from sidebands MMd(γ,K－p) GeV/c2 MMd(γ,K－p) GeV/c2 Signal emerges with selection of L*(1520) in M(K-p)

Pentaquark Studies at HERA
A. Geiser ep -> eK0spX High energy production mechanism? M(GeV) ZEUS => Signal seen at medium Q2 and forward rapidity in both pKs and pKs

ZEUS – Cross section on Q+
A. Geiser The signal is seen at different minimum Q2 values. They go ahead and extract a cross section for theta+ production as shown on the right figure. . M(GeV) Q+/L* ~ 5% (independent of Q2)

Pentaquark in fragmentation?
Quark fragmentation Pentaquark less suppressed ? Baryon fragmentation s e d u Needs fewer quark pairs from the vacuum Q+ u u d q q Q+ e The same argument applies to scattering experiments such as ZEUS and H1. In deep inelastic electron scattering at small Bjorken x a sea quark maybe knocked out of the proton. In order to dress it self to hadronize into a pentaquark, 4 q-qbar pairs are needed as in to e+e- collisions, and may cause a suppression of pentaquarks. However, the proton that lost a sea-antiquark only needs one additional quark and a re-arrangement of the proton remnants to turn into a pentaquark. Baryon fragmentation may therefore be a much more favorable way of producing pentaquarks. That this may indeed be so has been concluded in a ZEUS paper submitted to the conference. Pentaquark strongly suppressed ?

BELLE – Low energy K+N scattering
R. Mizuk e+e- -> K+/- X, K+/-A -> pK0, pK- Detector Tomography momentum spectra of K+ and K- 1 / 50MeV The BELLE experiment used scattering of secondary charged kaons off their vertex detectors to study the production and formation of the Theta+. While production can occur for kaons at higher momentum, formation of the Q+ can only occur over a narrow momentum bin corresponding to the natural width convoluted with the Fermi momentum in the nucleus. The formation process can be used to estimate the width of the state. From K+D scattering the width is determined at 0.9 +/- 0.3. momentum, GeV/c K+ n Q+ Momentum range possibly contributing to Q+ formation. => Determine resonance width

Belle – Limit on Q+ Width
R. Mizuk K+A -> pK0s GQ+ from K+A -> pK0sX & K+D -> inclusive analysis Belle limit 90%CL 397 fb-1 DIANA No no signal Using nearly 400 fb-1 of collected data the exclusive process K+n -> Ksp is studied, and no signal is found in the mass spectrum. From this an upper limit for the width can be determined. The collaboration claims an upper limit of 0.64MeV (90% CL) for the Q+ at the mass 1539MeV. This limit would be somewhat below the range of the K+D analysis. However, given the uncertainty in mass of the Theta+ signal a ~1 MeV limit would be a better choice if the mass range from 1530 to MeV were chosen. This limit is marginally consistent with the K+D and K+A analysis. ΓΘ+= Δm NΘ+ Nch σch 107mb Bi Bf Belle: G < 0.64 MeV (90% M = GeV G < 1 MeV (90% CL) @ M = 1.525–1.545 GeV Cahn,Trilling,PRD69,11501 (2004). Not inconsistent with previous results.

Upper limit on cross section ratio
σ(KN Θ(1540)+ X) σ(KN Λ(1520) X) NΘ+ NΛ* εpK+ εpKs B(Λ(1520) pK-) B(Θ(1540)+  pK0) B(K0 KS π+π-) = <2.5% at the 90% C.L. εpKs/εpK+ from MC assuming that Θ+ and Λ(1520) kinematics is similar. Comparison with other experiments Experiment Reaction Energy σ(Θ+)/σ(Λ*) CDF ppΘ+X 1960 GeV <3% HERA-B pAΘ+X 42 GeV <2% SPHINX 12 GeV Belle KAΘ+X 2 GeV <2.5% LEPS γAΘ+X 60% HERMES eDΘ+X 7 GeV 200% Slide1 Consistent with <1 MeV width. ?

Hadron production in e+e- collisions
J.M. Izen Slope for p.s. mesons Slope: Pseudoscalar mesons: ~ 10-2/GeV/c2 (need to generate one qq pair) Slope for baryons Baryons: ~ /GeV/c2 (generate two pairs) Slope for Pentaquark ? The blue and red points show the production rates for baryons versus their masses for different CMS energies. The rate drops at about 10^-4 per GeV in mass. At the mass of the Q+ the upper limit from BABAR is 5 times 10^-5 per event a factor of a few below the sigma*at the same mass and a factor of 20 below the Lambda*. Is the particle mass is the only relevant parameter for the production rate? Clearly not. If we include mesons, the slope is about 10-2/GeV for pseudo scalar mesons. These hadrons are produced in quark fragmentation processes the number of quark-antiquark pairs needed to produce a hadron may be relevant. For mesons only a single pair is needed. For protons two pairs are needed. And for pentaquarks 4 pairs would be needed. If I naïvely extrapolate from mesons to baryons to pentaquarks a slope of 10^8 /GeV could be relevant, as shown in this line. There is no way to normalize this line. If I arbitrarily let it intersect where the meson and baryon lines intersect, at the proton mass, the rate for pentaquarks may just be too low to be seen in the BABAR experiment. Pentaquarks: ~ 10-8 /GeV/c2 (?) (generate four pairs) Q+ + Q- G= 1Mev X- - +X++ G= 1MeV BR(X-- X-p-)=0.5

Tomography with pKs0 Vertices
BaBar Preliminary 233 fb–1 e+e– data Tomography with pKs0 Vertices Y [cm] Y [cm] Veto beamspot R  2 cm e– e+ SVT X [cm] -0.2 < y < 0.7 cm -3.1< x < -2.2 cm Ta Be X [cm] SVT support tube DCH inner wall Beam pipe z profile

e-Be Electroproduction
Preliminary 233 fb–1 e+e– data e-Be Electroproduction (quasi-real photoproduction) HERMES (e+dKS0p + X) ZEUS (e-pe-KS0p + X) Zeus: ~5 M=15223 MeV *(1480) ZEUS Hermes: ~5 M=15283 MeV HERMES HERMES/BaBaR comparison disturbing. ZEUS : High Q2, no signal for Q2<1 BaBaR: Quasi-real photons HERMES: acceptance loss in low mass region? (PID requires p(p) >4.1 GeV/c; p(KS0)>3 GeV/c)

STAR d-Au results Q D A Q++ candidate ?
S. Kabana Possible mechanism for production of multi-q/g states: Coalescence of out of QGP? pK+ and pK- from 18.6 M d+Au at 200 GeV D Q ? STAR Preliminary M (GeV/c2) A Q++ candidate ? Also a Q+ candidate ??

SVD-2 – New Analysis Two independent data set:
Private info from SVD group Two independent data set: KS decays inside or outside the Vertex Detector Ep = 70 GeV M = 1523 MeV s = MeV Nevnt = 165 M = 1522 MeV s = MeV Nevnt = 205 The SVD group re-analyzed their published results and increased sensitivity. This Increased the number of events nearly by an order of magnitude. New analysis improves Q+ signal by factor 8. The results maybe in conflict with new WA89 results (to be published shortly).

Cascade Pentaquark X5--(1862) ?
pp -> pXX HERA-B CDF State not produced in quark fragmentation or is severely suppressed. gA FOCUS Returning to the other pentaquark candidates, a lot of new evidence has come out against the exotic X– and its 5-quark partners claimed by CERN experiment NA49. There is overwhelming evidence against the signal which the HERA-B group claims to be incompatible with their results. The HERA-B data are shown in the left graph with no signal visible in the Xp channels.. Another example is CDF that also see no signal while in both cases the X(1530) is clearly visible, allowing to extract upper limits for the ratio. FOCUS gA < /B range

Charmed Pentaquark Q0c(3100) ?
FOCUS FOCUS events H1 expected Upper limit factor 4 lower than H1 results. Claim is that results are incompatible with H1. The situation with the anti-charm Q0 is not much better. Evidence was only found in the H1 experiment at HERA. The claim is contradicted by the FOCUS experiment showing a small non resonant distribution. The large peak would have been expected from the H1 results in the FOCUS data. New data from ZEUS submitted to this conference give upper limit for the ratio Q0c/D*p that is 4 times lower than the H1 signal. Signal also in photoproduction Claim kinematical uniqueness. Possible production mechanism - PGF. FOCUS experiment claims incompatibility with H1.

Pentaquark Status @ EINN 2005
Group Signal Backgr. Significance publ. Comments SPring s 3.2s SPring s SAPHIR s 5.2s DIANA s 3.4s CLAS(d) s 4.4s CLAS(p) s 4.7s   s HERMES  3.6s ZEUS  s COSY  s SVD  s SVD s Improved analysis NA  s H  s SPring s L*(K+n) STAR 2, , s Q++ candidate s/ b+s G11 CLAS-p G10 CLAS-d ? BELLE Q+ ? BABAR This slide summarizes the remaining evidence for any pentaquark. The new CLAS results eliminated two signals one on hydrogen and one on deuterium. The two non-Theta+ signals at H1 and NA49 are highly questionable and contradict later results from ZEUS and HERA-B with higher sensitivity. Finally, the very new CLAS data on deuterium, presented here for the first time, should be confronted with the LEPS deuterium results from SPring8 which have lower statistics. On the side favoring pentaquark signal is some positive news: The SVD-2 improved analysis gives many more events with high significance. SprinG8 claims a signal in L*(K+n) with 5s, and there is a signal of a doubly charged Q++ from STAR at RHIC. X5 HERA-B, CDF, COMPASS ZEUS, FOCUS, BABAR Q0c

Concluding comments Q+(1540)
Two high statistics experiments on protons and deuterium (CLAS) contradict results that observed ~5s signals with same kinematics. Within hadronic models the new CLAS results put limits on the widths. For most models GQ+ < 0.5 MeV. New limit from Belle not (yet?) in contradiction with 1 MeV limit. Increase in statistics could change that. Need to examine if gD and gA experiments with lower statistics are consistent with CLAS cross section limit on neutrons . Sensitivity to Q+ at high energy appears in baryon fragmentation not in quark fragmentation. This could explain some of the null results at high energies. Babar is challenging the HERMES results on Q+. New data/analysis from LEPS, SVD-2, and STAR, support Q observations.

Outlook The Q pentaquark is not in good health, but it is still alive.
- new analyses underway from COSY, SVD-2, SPring8 - measurements planned at SPring8, JLab - H1, ZEUS, HERMES high luminosity run until 2007 - higher statistics data from STAR, PHENIX - more statistics and analyses from B-factories Should the Q pentaquark not survive the next few years of intense scrutiny we will have to examine why so many experiments observed the signal in the first place. So, is it time to write the obituary for pentaquark states?

Part 1: Have Pentaquark States been seen?

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