Experimental Review of Pentaquarks: Positive and Null Results SLAC Seminar February 17, 2005 Ken...
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Transcript of Experimental Review of Pentaquarks: Positive and Null Results SLAC Seminar February 17, 2005 Ken...
Experimental Review of Pentaquarks:
Positive and Null Results
SLAC SeminarFebruary 17, 2005
Ken Hicks (Ohio University)
February 17, 2005 K. Hicks, Ohio U.
Outline
• The prediction• The excitement• The pendulum swings back• Some common “myths”• The quest for higher statistics• Conclusions
February 17, 2005 K. Hicks, Ohio U.
The Prediction
February 17, 2005 K. Hicks, Ohio U.
The Anti-decuplet predicted by Diakonov et al.
Symmetries give an equal spacingbetween “tiers”
Narrow width!
February 17, 2005 K. Hicks, Ohio U.
Postdiction: diquark model
ud ud sJW hep-ph/0307341
L=1
(ud)
(ud)
s
L=1, one unit of orbital angular momentum needed to get J=1/2+ as in SM
Uncorrelated quarks: JP = 1/2−
62
1suuudsududDecay Width: MeV
MeV8
62
2002
Additional width suppression may come from w.f. overlap.
February 17, 2005 K. Hicks, Ohio U.
PDG Review Table
February 17, 2005 K. Hicks, Ohio U.
Width: Indirect Limits
• Nussinov (hep-ph/0307357): < 6 MeV
• Arndt et al. (nucl-th/0308012): < 1 MeV
• Haidenbauer (hep-ph/0309243): < 5 MeV
• Cahn, Trilling (hep-ph/0311245): ~ 0.9 MeV
• Sibertsev et al. (hep-ph/0405099): < 1 MeV
• Gibbs (nucl-th/0405024): ~ 0.9 MeV
February 17, 2005 K. Hicks, Ohio U.
Width: Possible Signal?Input mass
background(non-reson.)
Gibbs, nucl-th/0405024
Widths range:0.6-1.2 MeV0.9 MeV = solid
Conclude:width must be ~1 MeV
February 17, 2005 K. Hicks, Ohio U.
Comments: Width and Parity
• If the KN database is correct, it is likely that the + width is ~1 MeV.
• If the width is 1 MeV, the parity is almost surely positive.– negative parity width goes up by ~50.
• If the lattice results are correct, the width is almost surely negative.
This problem of width/parity is the most worrisome aspect to the existence of the +.
February 17, 2005 K. Hicks, Ohio U.
Positive results
February 17, 2005 K. Hicks, Ohio U.
Opinions on Pentaquarks:
• There are valid criticisms for both positive and null experimental results.– A “scorecard” approach will not work.
We need better, higher-statistics, data.
• Science versus emotion– There have been strong statements on
both sides of the existence question.– Let’s make scientifically sound
statements.
February 17, 2005 K. Hicks, Ohio U.
Comparison of + Experiments
Where Reaction Mass Width ’s*LEPS C K+K- X 1540 +-
10< 25 4.6
DIANA K+Xe K0p X 1539 +- 2
< 9 4.4
CLAS d K+K-p(n) 1542 +- 5
< 21 5.2
SAPHIR p K+K0(n) 1540 +- 6
< 25 4.8
ITEP A K0p X 1533 +- 5
< 20 6.7
CLAS p +K-K+
(n)1555 +- 10
< 26 7.8
HERMES e+d K0p X 1526 +- 3
13 +- 9
~5
ZEUS e+p e’K0p X
1522 +- 3
8 +- 4 ~5
COSY pp K0p+ 1530 +- 5
< 18 4-6
*Gaussian statistical significance: estimated background fluctuation
February 17, 2005 K. Hicks, Ohio U.
Evidence for Pentaquark StatesSpring8 SAPHIR
JLab-p
HERMES
ITEP
pp ++.
COSY-TOFDIANA
SVD/IHEP
JLab-d
ZEUSCERN/NA49
H1
Nomad
This is a lot of evidence
February 17, 2005 K. Hicks, Ohio U.
Critical Comments
• For many experiments, the background shape is not clearly known.
• Some experiments have harsh angle cuts that could affect the mass spectra.
• In all cases, the signal is weak compared with standard resonances.– Cuts are necessary to lower background.
February 17, 2005 K. Hicks, Ohio U.
CLAS: deuterium result+
Mass = 1.542 GeV< 21 MeVSignificance 5.2±0.6
N= 43 events
?
Significance = ?
Two different background shapes
Events in the (1520) peak.
February 17, 2005 K. Hicks, Ohio U.
Official CLAS statement
• “Further analysis of the deuterium data find that the significance of the observed peak may not be as large as indicated.”– We really need a calculation of the background before
the statistical significance of the peak can be known.
• Eventually the new experiment, with much higher statistics, will settle the question.– The g10 experiment (x10 statistics) is now complete,
and final results are expected at end of Feb. 2005.– “Why is it taking so long?” --> It’s only 8 months!!
February 17, 2005 K. Hicks, Ohio U.
M(nK+) [GeV]
CLAS: + from the proton
no cuts
p→+K- K+ (n)
K*0
Prominent K*0
February 17, 2005 K. Hicks, Ohio U.
CLAS: + from the protonp→+K-K+(n)
CLAS Collaboration PRL 92, 032001-1 (2004).
E = 3 – 5.5 GeV
M(nK+) [GeV]
M=1555±10 MeV< 26 MeV
Cos*(+) > 0.8
Cos*(+) > 0.8
Cos*(K+) < 0.6
n
K+
K−
+
p
N*
−
February 17, 2005 K. Hicks, Ohio U.
Results from ZEUS
NOTES:1. + peak is evident only for Q2 > 20 GeV2.--> ZEUS suggests that this condition gives the + enough transverse momentum to get into their detector acceptance.
2. There is an assumption of background shape.--> A different backgroundchanges the stat. signifig.
February 17, 2005 K. Hicks, Ohio U.
HERMES + spectra
signal / background 2:1 standard cuts applied + K* and veto
signal / background: 1:3
add additional
February 17, 2005 K. Hicks, Ohio U.
Null Results
February 17, 2005 K. Hicks, Ohio U.
Production Mechanism?
• If the + exists, data suggests it likely favors certain production mechanisms.– This is an exotic baryon.– It may have an exotic production
mechanism.
February 17, 2005 K. Hicks, Ohio U.
Published Null Experiments
Group Reaction Limit Sensitivity?
BES e+e- J/ --> * <1.1x10-5 No?
Belle e+e- (2S) --> pK0 <0.6x10-5 ??
BaBar e+e-
(4S) -->pKs0 <1.1x10-4 ??
ALEPH e+e- ->Z -> pKs
0
<0.6x10-5 ??
HERA-B pA --> pKs0X <0.02x* No?
CDF pp* --> pKs0X <0.03x* No?
HyperCP pCu --> pKs0X <0.3%
K0pNo?
PHENIX AuAu -->n*K- not given ??
Belle K+Si -->pKs0X <0.02x* Yes?
February 17, 2005 K. Hicks, Ohio U.
+: Null ResultsHERA-B data on Carbon target:invariant massof pK0 shows no peak.
Could kinematics be an issue? If is not produced by fragmentation, HERA-B may not see it.
920 GeV beam!
February 17, 2005 K. Hicks, Ohio U.
Critical Comments
• Inclusive versus Exclusive measurement– inclusive has better resolution, but more
background (especially at higher energy)• Backgrounds: combinatorial and from
other resonances. Can we estimate?• Production mechanism: projectile or
target fragmentation?– Is it calculable in some model?
February 17, 2005 K. Hicks, Ohio U.
Titov: inclusive production (fragmentation region)
...)...)(( KNNN
A
q
p
px
q
h
p
py
23 104.210
R
...)...)(( KNNN
7.0z
fast slow
;)1( )2(4zR ;
maxh
h
p
pz
Ratio: pentaquark to baryon production
Regge exchange dominates(2 = diquarks as quasi-partons)
February 17, 2005 K. Hicks, Ohio U.
February 17, 2005 K. Hicks, Ohio U.
Slope for mesons
Slope for baryons
Slope for pentaquarks??
February 17, 2005 K. Hicks, Ohio U.
Hadron production in e+e-
Slope: Pseudoscalar mesons: ~ 10-2/GeV/c2 (need to generate one qq pair)
Baryons: ~ 10-4 /GeV/c2 (need two more pairs)
Pentaquarks: ~ 10-6 /GeV/c2 (?) (need 4 more pairs)
Slope for Pentaquark??
Slope forbaryons
Slope for p.s.mesons
we don’t know the production mechanism!!
February 17, 2005 K. Hicks, Ohio U.
Some common “myths”
February 17, 2005 K. Hicks, Ohio U.
Myth #1• “Kinematic reflection of the a2 and f2
tensor mesons explain the CLAS data”
Some people use a Regge trajectory (, 1, 2, etc.)
Near theshold (E<3 GeV)pion exchange dominates Regge exchange.--> For T=(a2
0 and f2), the --T vertex violates C-parity!--> calculations using diagramsthat do not violate C-parity (Y. Oh et al., hep-ph/0412363)give T far too small to explainCLAS data as a “reflection”.
February 17, 2005 K. Hicks, Ohio U.
Myth #2• “Ghost tracks could be responsible for the
peaks seen in the pK0 mass spectra”
This only can happen if there is an error in the tacking software.--> The same track mustbe used twice!--> All pentaquark (pK0)data analysis has been checked, and no such tracking error is found.
February 17, 2005 K. Hicks, Ohio U.
New Data
February 17, 2005 K. Hicks, Ohio U.
Detected nuclear reactions
pK
nKnp
nKpKpn
)1520(
)()1520()(
)()(
*
*
n
K─
K+
n+
p p
p
K+
K─
p
n n
N (1020) NK+K-
N
S=-1
S=+1
background
February 17, 2005 K. Hicks, Ohio U.
New data: LEPS deuterium*
MM (GeV) MM (GeV)
PreliminaryPreliminary
Minimal cuts: vertex, MMKK=MN, no , E < 2.35 GeV
*in collaboration with T. Nakano
February 17, 2005 K. Hicks, Ohio U.
LEPS: Fermi motion corrections
MM (GeV)
MM (GeV) MM (GeV)
•No large difference among the three Fermi motion correction methods
(1520) resonance
February 17, 2005 K. Hicks, Ohio U.
Fermi motion corrections: +
MM (GeV)
MM (GeV) MM (GeV)
• No large differences among the three Fermi motion corrections.
February 17, 2005 K. Hicks, Ohio U.
LEPS: K - p detection mode(New and Preliminary results)
• Inclusive production:• Θ + is identified by K -p missing mass from deuteron. ⇒ No Fermi correction is needed.
γ
p
n
Θ +
K -
p
γ
p
n
Θ +
K -
p
(1520)
February 17, 2005 K. Hicks, Ohio U.
Event selections in K - p mode
MMp(γ,K - p) GeV/c2
γp→K - pKππ - mis-ID as K -
K + mass
M(K - p) GeV/c2
Λ(1520)
Λ(1520) is tightly selected in 1.50–1.54 GeV/c2
Non-resonantKKp
February 17, 2005 K. Hicks, Ohio U.
K - p missing mass for events in the (1520) peak
MMd(γ,K - p) GeV/c2
Small enhancement at 1.53 GeV.
But the statistics is not large enough.
Hydrogentarget data
February 17, 2005 K. Hicks, Ohio U.
A possible reaction mechanism• + can be produced by re-scattering of K+.• K momentum spectrum is soft for forward going (1520).
γ
p/n
n/p
(1520)
K+/K0
PK GeV/c
PK obtained by missing momentum
Formation momentum
February 17, 2005 K. Hicks, Ohio U.
G10 run (E-03-113): March 13 - May 16
• Tagged photons in the energy range from 0.8 GeV to 3.59 GeV;
• Target – 24 cm long liquid deuterium at Z=-25cm;
• Trigger – two charged particles in CLAS.
• Data are taken at 2 settings of CLAS toroidal magnet.
• At each setting integrated luminosity (25pb-1) is about
10 times higher than in published deuterium data.
February 17, 2005 K. Hicks, Ohio U.
First analysis meeting (11/29)
• Discuss corrections: Eloss, E, momentum;
• Kinematic fitting;• PID, cuts• Physics channels to be analyzed:
nKnKKpd ; KpK
0)0(0 ,);( SpKnKpnKd ppKp ; 00;; SS KpKKpd Kp
;XpKd pK
February 17, 2005 K. Hicks, Ohio U.
CLAS: d’K+K-p(n) high-statistics
Eloss and E corrections. Kinematic fit
• Kinematic fit works. After corrections, it has small effect on masses and resolutions.
February 17, 2005 K. Hicks, Ohio U.
Silvia Niccolai - d’K+(n) and d’K0p (both data set)
February 17, 2005 K. Hicks, Ohio U.
Summary• There is reason for caution about the
existence of the +. – Need better experiments (pos. and null).
• Experiments need to have better control over the background shape.– Can backgrounds be calculated?
• The new LEPS data for the + is interesting, but not conclusive.– CLAS: high-statistics data under review.