Q Search at LEPS/SPring-8

1
Q Search at LEPS/SPring-8

• T. Nakano
• (RCNP, Osaka University)

• Introduction
• Evidences for Q
• Counter-evidences for Q
• Preliminary results from new LEPS data
• Summary

Exotic Hadron WS _at_ NWU, May 27, 2005
2
What are penta-quarks?

• Baryon.
• Minimum quark content is 5 quarks.
• Exotic penta-quarks are those where the
  antiquark has a different flavor than the other 4
  quarks
• Quantum numbers cannot be defined by 3 quarks
  alone.

Example uudds
Baryon number 1/3 1/3 1/3 1/3 1/3 1
Strangeness 0 0 0 0 1 1
e.g. uuddc, uussd
c.f. L(1405) uudsu or uds
3
Prediction of the Q Baryon
D. Diakonov, V. Petrov, and M. Polyakov, Z. Phys.
A 359 (1997) 305.

• Exotic S1
• Low mass 1530 MeV
• Narrow width 15 MeV
• Jp1/2

M 1890-180Y MeV
4
Baryon masses in constituent quark model
mu md 300 350 MeV, msmu(d)130180 MeV

• Mainly 3 quark baryons
• M 3Mq (strangeness)(symmetry)
• 5-quark baryons, naively
• M 5Mq (strangeness) (symmetry)
• 17001900 MeV for Q

5
Theory

• DPP predicted the Q with M1530MeV, Glt15MeV,
  and Jp1/2.
• Naïve QM (and many Lattice calc.) gives
  M17001900MeV with Jp1/2-.
• But the negative parity state must have very wide
  width (1 GeV) due to fall apart decay.

Ordinary baryons
Positive Parity?

• Positive parity requires P-state excitation.
• Expect state to get heavier.
• Need counter mechanism.
• diquark-diquark, diquark-triquark, or strong
  interaction with pion cloud?

For pentaquark
6
Evidence for Penta-Quark States
This is a lot of evidence
Nomad
7
First evidence from LEPS
g n?KK-n
M 1.54?0.01 MeV G lt 25 MeV

• Background level was estimated by using events
  from a LH2 target.
• Assumption
• Background is from non-resonant KK- production
  off the neutron/nucleus
• is nearly identical to non-resonant KK-
  production off the proton

Q
No L(1520) peak in events without a proton.
Phys.Rev.Lett. 91 (2003) 012002 hep-ex/0301020
8
Mass
Final state
K n
Ks p
(Ks p )
A few difference from zero, but 20 difference
from the KN threshold.
9
Width

• There is some inconsistency
• Most measurements are only upper limits.
• DIANA has G lt 9 MeV.
• The cross-section implies G0.9 MeV.
• HERMES G 13 - 9 stat. (- 3 sys.) MeV
• ZEUS G 8 - 4 stat. (- 5 sys.) MeV
• Arndt et al. and Cahn et al. analysis of KN phase
  shifts suggests that G lt 1 MeV !!
• The small width is the hardest feature for
  theorists to understand

10
Negative Results

• HERA-B (Germany)
• reaction pA at 920 GeV
• measured K-p and K0p invariant mass
• Clear peak for L(1520), no peak for Q
• production rate Q/L(1520)lt0.02
• BES (China)
• reaction ee- ? J/y ? QQ-
• limit on B.R. of 10-5

And many unpublished negative results (HyperCP,
CDF, E690, BaBar, LEP,,,).
11
Q Search at CDF pK0S Mass Spectrum
12
(No Transcript)
13
Slope for mesons
Slope for baryons
Slope for pentaquarks??
14
CLAS New high statistics exp.Search for Q in g
p?KKsn
Limit on Q productionTotal cross-section lt 14
nb
How could other low energy experiments with lower
statistics could observe the signal?
R. De Vita, APS April meeting, 2005
15
LEPS New LD2 and LH2 runs

• Data taken from Oct. 2002 to Jun. 2003.
• 21012 photons on a 15cm-long LD2 target.
• Less Fermi motion effect.
• LH2 data were taken in the same period with
  1.41012 photons on the target.

of photons LH2LD2 23 we expect of events
from protons LH2LD2 23 of events LH2LD2
13
16
Search for Q in g n?KK-n

• A proton is a spectator (undetected).
• Fermi motion is corrected to get the missing
  mass spectra.
• Tight f exclusion cut is essential.
• Background is estimated by mixed events.

g p?KK-p
g n?KK-n
L(1520)
Can be consistent with the CLAS high stat result?
preliminary
preliminary
? Next talk by S.I. Nam
MMgK (GeV)
MMgK- (GeV)
17
LEPS detector
Good acceptance in the forward angle.
g
1m
18
New way to search for Q

• 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
19
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
LH2
LD2
?
L(1520)
MMd(?,K-p) GeV/c2
K/K0
p/n
Formation momentum
Q
n/p
PK GeV/c
20
Final State Interaction
I.
III.
p
K-
?
?
K
K-
p
K
p
p
p
K
n
n
n
n
Large at low Pp
Small, especially at low PK
p
II.
p
L(1520)
?
?
K-
K
K-
p
K
p
K-
K
n
n
n
n
Contributions from II and III are suppressed.
21
Event selection
?(1520)
K mass is smeared by Fermi motion. (assumed
proton at rest)
LD2 data after selecting L(1520)
LD2
select
inelastic events
?p?K-pKp
MMp(?,K-p) GeV/c2
M(K-p) GeV/c2
Select ?(1520) in 1.501.54 GeV/c2 ?
calculate K- p missing mass
of g d ? K- p X reaction
22
K-p missing mass in 1.50ltM(K-p)lt1.54 GeV/c2
g d L(1520) X
1.53 GeV
K- p
Good understanding of the background spectrum
shape is crucial .
preliminary
You see the peak because you want it. I see two
dips. by Prof. I at KEK
MMd(?,K-p) GeV/c2
23
Major background process

• Quasi free L(1520) production must be the major
  background.
• The effect can be estimated from the LH2 data

?
L(1520)
p
n
n
K
24
f contribution
f
K momentum is estimated by missing momentum
technique. ? Smeared by Fermi motion.
LD2 data
M(K-K) GeV/c2
25
K-p invariant mass after f exclusion
Eg gt 2.2 GeV
all energy
L(1520)
LH2
LH2
non-resonant KKp events
non-resonant KKp events
M(K-p) GeV/c2
M(K-p) GeV/c2
Non-resonant KKp (phase space) contribution
reproduces the spectrum shape under L(1520) well.
26
K-p missing mass for L(1520) and non-resonant
events in the
non-resonant KKp
L(1520)
MMd(?,K-p) GeV/c2
MMd(?,K-p) GeV/c2
Energy dependences are set to be the same. No
angular dependence has been introduced in the
both reactions.
27
K-p missing mass for L(1520)
Quasi-free L(1520) and non-resonant KKp
f
preliminary
preliminary
MMd(?,K-p) GeV/c2
MMd(?,K-p) GeV/c2
28
K-p missing mass in the sideband regions
1.46lt M(K-p) lt 1.50 GeV/c2
1.54lt M(K-p) lt 1.58 GeV/c2
LD2
LD2
MMd(?,K-p) GeV/c2
MMd(?,K-p ) GeV/c2
f Energy and angular dependences were obtained
by fitting to the data. KKp Enegy dependence ?
fit to the data. Angulardependence ? flat.
29
Test by side-band subtraction
Narrow gate (1.52ltM(K-p)lt1.54 GeV/c2) L N
?(1520) is enhanced. Wide
gate (1.46ltM(K-p)lt1.58 GeV/c2) L 3N
Non-resonant KKp is enhanced. ? L
1.5 (LN) - 0.5 (L3N)
narrow gate wide gate
L
N
N
N
M(K-p) GeV/c2
30
K-p missing mass for non-resonant KKp events
(phase space distribution)
1.46lt M(K-p) lt 1.50 GeV/c2
sidebands averaged
1.50lt M(K-p) lt 1.54 GeV/c2
1.54lt M(K-p) lt 1.58 GeV/c2
MMd(?,K-p) GeV/c2
MMd(?,K-p) GeV/c2
31
Side-band subtraction in KK invariant mass
Since f is not related with ?(1520), f peak
disappears.
LD2
LD2
M(K,K-) GeV/c2
M(K,K-) GeV/c2 after side-band subtraction
32
K-p missing mass in L(1520) and sideband region.
LD2
LH2
preliminary
preliminary
MMd(?,K-p) GeV/c2
MMd(?,K-p) GeV/c2
1.50 lt M(K-p) lt 1.54
0.5(1.46 lt M(K-p) lt 1.50) 0.5(1.54 lt M(K-p) lt
1.58)
33
Side-band subtracted K-p missing mass
Excess is related with ?(1520).
Most events are below 1.52 GeV/c2
Bump structure
LD2
LH2
preliminary
preliminary
MMd(?,K-p) GeV/c2
MMd(?,K-p) GeV/c2
Events in MMd(?,K-p) lt 1.52 GeV/c2 LH2LD2
11.3
of photons LH2LD2 23 ? s(p) gtgt s(n) for
quasi free L(1520) production.
34
Energy dependence
2.1 GeV lt Eg lt 2.4 GeV(Emax)
1.75 GeV lt Eg lt 2.1 GeV
LD2
LD2
preliminary
preliminary
MMd(?,K-p) GeV/c2
MMd(?,K-p) GeV/c2
35
K-p missing mass in high and low energy region

• 1.53 GeV Peak
• No change in the peak position.? not likely due
  to kinematical reflections.
• Narrower width in the low energy region.?
  consistent with the detector resolution.

Below 2.1 GeV
Above 2.1 GeV

• 1.60 GeV Bump
• Only seen in the low energy region. ? threshold
  effects?
• Not seen in LH2 data
• Associated with L(1520).

preliminary
MMd(?,K-p) GeV/c2
36
f contribution removed by angle cut
Require cos qKp gt 0
LD2
LH2
Excess remains.
preliminary
preliminary
MMd(?,K-p) GeV/c2
MMd(?,K-p) GeV/c2
37
K-p missing mass in sideband regions
?
p
LD2
K-
No peak!
K/K0
Q
?
L(1520)
K/K0
Q
Q formation cross-section by simple kaon
re-scattering should be small.
MMd(?,K-p) GeV/c2
A theoretical estimation by A. Titov is small.
38
Width Comparison with a MC spectrum
MC gives 10 MeV resolution while Real data
resolution is about 8 MeV. The statistical
uncertainty of 2 MeV is mainly due to
fluctuations of the background at the tails of
the peak.
preliminary
MMd(?,K-p) GeV/c2
39
Summary

• Evidence for an S1 baryon around 1.54 GeV with
  a narrow width has been observed by several
  experimental groups.
• There are some inconsistencies in the measured
  masses and widths.
• No signal has been observed in high energy
  experiments with high statistics and good mass
  resolution.
• If the Q does exist, its production in high
  energy reactions must be highly suppressed.
• The Q is not seen in the new CLAS high
  statistics data from a proton.
• If the Q does exist, its production must have a
  large isospin asymmtery.

40
Summary of LEPS new result

• Peak is seen at 1.53 GeV/c2 in the missing mass
  of the (g,L(1520)) reaction from a deuteron.
• If is is real, the width seems to be very
  narrow.
• The peak cannot be seen in the K-p invariant
  mass region outside of the L(1520).
• If the peak is due to the Q, its production by
  re-scattering seems to be small in our kinematic
  region.
• Enhancement (bump structure) around 1.6 GeV is
  also observed in the (g,L(1520)) reaction but
  only in the low Eg region.
• Non-resonant KKp and f contributions can be
  removed by sideband subtraction.
• For quasi-free L(1520) production, s(p)gtgts(n).

41
K-p invariant mass after f exclusionLD2 data
Eg gt 2.2 GeV
all energy
LD2
LD2
M(K-p) GeV/c2
M(K-p) GeV/c2
42
K-p invariant mass after subtracting KKp
Eg gt 2.2 GeV
Eg gt 2.2 GeV
L(1520)
LH2
LD2
Y
L(1405)
M(K-p) GeV/c2
M(K-p) GeV/c2
Enhancement of L(1405) and Y productions from
neutron?