Experimental Review of Pentaquarks: Positive and Null Results

1
Experimental Review of PentaquarksPositive and
Null Results

• Forum on Pentaquarks (DESY)
• February 1, 2005
• Ken Hicks (Ohio University)

2
Outline

• Preliminary Comments and Opinions
• Evidence for the Q
• The Null Experiments
• Some common myths
• New Data (SPring-8)
• Conclusions

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Preliminary Remark
Congratulations to the ESA on a MAJOR success
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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.
• Lets make scientifically sound statements.

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More Opinions

• If the Q exists, data suggests it likely favors
  certain production mechanisms.
• This is an exotic baryon.
• It may have an exotic production mechanism.
• To make solid scientific statements
• Calculate the expected rate of production.
• Understand the rate of the background.
• Compare with acceptance-corrected data.

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If it exists, what is it?

• The first Q search was motivated by the chiral
  soliton model of DPP.
• Is it is possible that there is another
  interpretation of the Q? We should not be
  biased toward a particular theory.
• Lattice QCD suggests that the Q has negative
  parity (opposite to DPP).
• But these are not gold-plated calculations

Diakonov, Petrov and Polyakov, Z. Phys. A359, 305
(1997).
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Positive results
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Comparison of Q Experiments
Where Reaction Mass Width ss
LEPS gC ? KK- X 1540 - 10 lt 25 4.6
DIANA KXe ?K0p X 1539 - 2 lt 9 4.4
CLAS gd ? KK-p(n) 1542 - 5 lt 21 5.2
SAPHIR gp ? KK0(n) 1540 - 6 lt 25 4.8
ITEP nA ? K0p X 1533 - 5 lt 20 6.7
CLAS gp ? pK-K(n) 1555 - 10 lt 26 7.8
HERMES ed ? K0p X 1526 - 3 13 - 9 5
ZEUS ep ? eK0p X 1522 - 3 8 - 4 5
COSY pp ? K0pS 1530 - 5 lt 18 4-6
Gaussian statistical significance estimated
background fluctuation
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Evidence for Pentaquark States
This is a lot of evidence
Nomad
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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.

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CLAS deuterium result

• Mass 1.542 GeV
• lt 21 MeV
• Significance 5.20.6 s

NQ 43 events
Q
Significance ?
?
Two different background shapes
Events in the L(1520) peak.
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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? --gt Its only 8
  months!!

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Results from ZEUS
NOTES 1. Q peak is evident only for Q2 gt 20
GeV2. --gt ZEUS suggests that this condition
gives the Q enough transverse momentum to get
into their detector acceptance. 2. There is an
assumption of background shape. --gt A different
background changes the stat. signifig.
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HERMES Q spectra
add additional p

• signal / background 13

• signal / background
• 21
• standard cuts applied
• K and L veto

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Results from H1
(From Karin Daum)
Apply mass difference technique M(Dp)m(K??
p)-m(K??)MPDG(D)
Background well described by D MC and wrong
charge D from data
narrow resonance at M3099? 3(stat.) ? 5 (syst.)
MeV

• Signal is visible in different data taking
  periods
• But no signal seen in ZEUS data (question
  different D accep.?)

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Null Results
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Published Null Experiments
Group Reaction Limit Sensitivity?
BES ee- J/Y --gt QQ lt1.1x10-5 No?
Belle ee- Y(2S) --gt pK0 lt0.6x10-5 ??
BaBar ee- U(4S) --gtpKs0 lt1.1x10-4 ??
ALEPH ee- -gtZ -gt pKs0 lt0.6x10-5 ??
HERA-B pA --gt pKs0X lt0.02xL No?
CDF pp --gt pKs0X lt0.03xL No?
HyperCP pCu --gt pKs0X lt0.3 K0p No?
PHENIX AuAu --gtnK- not given ??
Belle KSi --gtpKs0X lt0.02xL Yes?
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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?

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Titov inclusive production (fragmentation
region)
fast
slow
Ratio pentaquark to baryon production


Regge exchange dominates
(2 diquarks as quasi-partons)
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Slope for mesons
Slope for baryons
Slope for pentaquarks??
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Hadron production in ee-
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)
we dont know the production mechanism!!
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Some common myths
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Myth 1

• Kinematic reflection of the a2 and f2 tensor
  mesons explain the CLAS data

Near theshold (Eglt3 GeV) pion exchange dominates
Regge exchange. --gt For T(a20 and f2), the
g-p-T vertex violates C-parity! --gt calculations
using diagrams that do not violate C-parity (Y.
Oh et al., hep-ph/0412363) give sT far too small
to explain CLAS data as a2/f2 reflections.
Some people use a Regge trajectory (p, p1, p2,
etc.)
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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. --gt The same track must be
used twice! --gt All pentaquark (pK0) data
analysis has been checked, and no such tracking
error is found.
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New Data
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New data LEPS deuterium
Minimal cuts vertex, MMgKKMN, no f, Eg lt 2.35
GeV
L(1520)
Q
Preliminary
Preliminary
MMgK (GeV)
MMgK- (GeV)
in collaboration with T. Nakano
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LEPS Fermi motion corrections
L(1520) resonance
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Fermi motion corrections Q
MMgK- (GeV)
MMgK- (GeV)

• No large differences among the three Fermi
  motion corrections.

MMgK- (GeV)
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LEPS K-p detection mode(New and Preliminary
results)

• Inclusive production
• T is identified by K-p missing mass from
• deuteron. ? No Fermi correction is needed.

?
T
?
T
L(1520)
p
K-
p
K-
n
n
p
p
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Event selections in K-p mode
K mass
?(1520)
Non-resonant KKp
?p?K-pKp
p- mis-ID as K-
MMp(?,K-p) GeV/c2
M(K-p) GeV/c2
?(1520) is tightly selected in 1.501.54 GeV/c2
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K-p missing mass for events in the L(1520) peak
Small enhancement at 1.53 GeV. But the statistics
is not large enough.
Hydrogen target data
MMd(?,K-p) GeV/c2
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A possible reaction mechanism

• Q can be produced by re-scattering of K.
• K momentum spectrum is soft for forward going
  L(1520).

PK obtained by missing momentum
?
L(1520)
K/K0
p/n
Formation momentum
Q
n/p
PK GeV/c
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K-p missing mass for events with missing momentum
gt 0.35 GeV/c
sideband regions
VERY PRELIMINARY!
MMd(?,K-p) GeV/c2
MMd(?,K-p) GeV/c2
select
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Summary

• There is reason for caution about the existence
  of the Q.
• 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 Q is interesting, but
  not conclusive.
• CLAS data internal review in 1 month.

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Outlook

• There are several new experiments that will help
  settle the existence question
• SPring-8 LEPS (deuterium higher statistics)
• JLAB CLAS (g10, g11, eg3)
• COSY TOF
• DESY?
• We still need to understand the null experiments
  
• background? production mechanism?

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Model-independent Parity
At threshold S-wave dominant
T 1
K, or K
If S 0, then Li even, P even gt P(Q)
If S 1, then Li odd, P odd gt
P(Q) -
Thomas, Hosaka, KH, Prog. Theor. Phys. 111, 291
(2004). See full calculation C. Hanhart et al.,
hep-ph/0410293.
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Width Indirect Limits

• Nussinov (hep-ph/0307357) GQlt 6 MeV
• Arndt et al. (nucl-th/0308012) GQlt 1 MeV
• Haidenbauer (hep-ph/0309243) GQlt 5 MeV
• Cahn, Trilling (hep-ph/0311245) GQ 0.9 MeV
• Sibertsev et al. (hep-ph/0405099) GQlt 1 MeV
• Gibbs (nucl-th/0405024) GQ 0.9 MeV

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Width Possible Q Signal?
Input mass
Conclude width G must be 1 MeV
Gibbs, nucl-th/0405024
Widths range 0.6-1.2 MeV 0.9 MeV solid
background (non-reson.)
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Comments Width and Parity

• If the KN database is correct, it is likely that
  the Q width is G1 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 Q.
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A di-quark model for pentaquarks
JW hep-ph/0307341
JM hep-ph/0308286
L1, one unit of orbital angular momentum needed
to get J1/2 as in cSM Uncorrelated quarks JP
1/2-
Additional width suppression may come from w.f.
overlap.