Experimental Review of Pentaquarks: Positive and Null Results

Experimental Review of Pentaquarks:

Positive and Null Results

Forum on Pentaquarks (DESY)February 1, 2005

Ken Hicks (Ohio University)

February 1, 2005 K. Hicks, Ohio U.

Outline

• Preliminary Comments and Opinions

• Evidence for the +

• The Null Experiments• Some common “myths”• New Data (SPring-8)• Conclusions


Preliminary RemarkCongratulations to the ESA on a MAJOR success:


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.


More Opinions

• If the + 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.


If it exists, what is it?• The first + search was motivated by

the chiral soliton model of DPP.– Is it is possible that there is another

interpretation of the +? We should not be biased toward a particular theory.

• Lattice QCD suggests that the + has negative parity (opposite to DPP).– But these are not “gold-plated”

calculationsDiakonov, Petrov and Polyakov, Z. Phys. A359, 305 (1997).


Positive results


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


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


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.


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.


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!!


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.


HERMES + spectra

signal / background 2:1 standard cuts applied + K* and veto

signal / background: 1:3

add additional


Background well described by D* MC and “wrong charge D” from data

Apply mass difference technique

M(D*p)=m(K p)-m(K)+MPDG(D*)

no enhancement in D* Monte Carlo

no enhancement in wrong charge D

• Signal is visible in different data taking periods

• But no signal seen in ZEUS data (question: different D* accep.?)

narrow resonance at M=3099 3(stat.) 5 (syst.) MeV

Results from H1(From Karin Daum)


Null Results


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?


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?


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)


Slope for mesons

Slope for baryons

Slope for pentaquarks??


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!!


Some common “myths”


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 a2/f2 “reflections”.


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.


New Data


New data: LEPS deuterium*

MM (GeV) MM (GeV)

PreliminaryPreliminary

Minimal cuts: vertex, MMKK=MN, no , E < 2.35 GeV

*in collaboration with T. Nakano


LEPS: Fermi motion corrections

MM (GeV)

MM (GeV) MM (GeV)

•No large difference among the three Fermi motion correction methods

(1520) resonance


Fermi motion corrections: +

MM (GeV)

MM (GeV) MM (GeV)

• No large differences among the three Fermi motion corrections.


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)


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


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


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


K － p missing mass for events with missing momentum > 0.35 GeV/c

MMd(γ,K － p) GeV/c2MMd(γ,K － p) GeV/c2

sideband regions

select

VERY PRELIMINARY!


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 data: internal review in ~1 month.


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?


Model-independent Parity

p

p

At thresholdS-wave dominant

If S = 0, then Li = even, P = even ==> P() = +

If S = 1, then Li = odd, P = odd ==> P() = -

T = 1 K, or K*

Thomas, Hosaka, KH, Prog. Theor. Phys. 111, 291 (2004).See full calculation: C. Hanhart et al., hep-ph/0410293.


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


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


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 +.


A di-quark model for pentaquarks

ud ud sJW hep-ph/0307341

JM hep-ph/0308286

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.

Experimental Review of Pentaquarks: Positive and Null Results

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