Longitudinal Polarimeter at HERA

Report
Discovery of a Pentaquark
Andy Miller
August 2003
Review of recent experiments
1
Search for Exotic Baryon States
• Standard Quark Model
– classifies hadrons as
• mesons ( qq )
• baryons (qqq )
– also allows “non-standard” or exotic
hadron states
• multiquark mesons ( qqqq )
• multiquark baryons ( qqqqq )
-> appear as baryon resonances
• hybrid states ( qqg or qqqg )
• dibaryons ( qqqqqq )
• glueballs
-> no convincing previous evidence for exotic baryon states.
e.g. a state with S=+1 would be manifestly exotic (s with no s)
2
Baryon States
•All baryons observed so far
– classified as singlets, octets and decuplets of SU(3) flavor group
-> constructed of 3 quarks only
– have strangeness from S=-3 to S=0
baryon decuplet with JP=(3/2)+
baryon octet with JP=½+
Y
0
IZ
• Exotic Baryons with S=+1
– cannot be formed from only 3 quarks
– belong to higher SU(3) multiplet
-> minimal pentaquark state qqqqq with q flavor different from
other four flavors is natural candidate
3
Prediction in Chiral Soliton Model
D. Diakonov et al., Z. Phys. A 359, 305 (1997)
• all baryons are rotational excitations of a rigid object
• reproduces mass splittings witin 1% of
– baryon octet (JP=½+) and decuplet (JP=3/2+)
• predicts new anti-decuplet (among many Ncd artifacts)
• “only one” free parameter
the 3 corners
are exotic
pK0 or nK+
Identifying P11(1710) as
member of anti-decuplet:
prediction for Q+:
M=1.53 GeV, G  15 MeV
I=0
S=+1
JP=½+
with
Q+  K0 p or K+n
X-p- or S-K-
X0p+ or S+K0
Also older predictions:
Chemtob (1984); Praszalowicz (1987)
5
LEPS Experiment at SPring8 in Japan
Exclusive gn (12C)  K-K+n
T. Nakano et al., PRL 91, 012002-1 (2003)
• 8 GeV electrons  Compton backscattering: Egmax = 2.4 GeV (s ~ 15 MeV)
• Tagged photon beam for Eg>1.5 GeV, flux ~106/s
• Target:
2.5cm
LH2
0.5cm
(SC)
C:H1:1
• Detector: Silicon strip vertex detector
3 drift chambers
dipole magnet
B~0.7 T
TOF scintillator
Aerogel Cerenkov Counter
• Acceptance:
hor  0.4 rad
vert  0.2 rad
momentum resolution: 6 MeV for 1 GeV particles
• Fermi smearing correction of MM(gK-) based on correln with MM(gK- K+)
6
Experimental Evidence from Japan
• LEPS collaboration at SPring-8:
T. Nakano et al., PRL 91, 012002-1 (2003)
• Exclusive gn (12C)  K+K-n versus gp  K+K-p
a) solid histogram: CH spectrum of K-n b) solid histogram: CH spectrum of
missing mass (no res. expected)
K+n missing mass (Q+ expected?)
dashed histogram: CH spectrum of K-p
dashed histogram: LH2 spectrum of
missing mass (L(1520) expected)
K+p missing mass (no res. expected)
Conclusion by authors:
M = 1540 10 MeV, G  25 MeV, s (= Ns / Nb ) = 4.61
7
Experimental Evidence from Russia
DIANA collaboration at ITEP: Xenon bubble chamber
V.V. Barmin et al., (hep-ex/0304040
Inclusive K+Xe  K0s p X at PK+ ~480 MeV (Quasi-free K+ n  K0 p )
Pid via ionization density, momenta from ranges (no magnet)
a) solid histogram: K0 p invar. mass
spectrum without cuts
dashed histogram: background due to
non-resonant charge-exchange reaction
b) solid histogram: K0 p spectrum
with cuts imposed: K0 and p going
mainly forward and back-to-back
Conclusions by authors:
M = 15392 MeV, G  8 MeV, s = 4.4
8
Experimental Evidence from USA
S. Stepanyan et al., (hep-ex/0307018)
CLAS collaboration at Jefferson Lab (VA)
Exclusive gd  pK+K-(n)
10
Experimental Evidence from USA - II
exclusive gd  pK+K-(n) detectable via pK- FSI:
S. Stepanyan et al., (hep-ex/0307018)
Tagged photon beam from 2.5-3.1 GeV eMissing mass MX distribution of
gd  pK+K-X
• reactions with FSI will
– produce K- at larger angles
– make possible detection of
• proton
• neutron through missing mass
• in exclusive kinematics no correction
for Fermi smearing required
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Experimental Evidence from USA - III
S. Stepanyan et al., (hep-ex/0307018)
Production of f and L(1520) in the
reaction gd  pK+K-n
These resonances contribute to
background and are removed in analysis.
solid histogram: final M(nK+) spectrum
solid line: arbitrary fit of peak + bkgd
dashed histogram: events associated with
L(1520) production
Conclusion by authors:
M = 15425 MeV, FWhM = 21 MeV, s = 5.30.5
12
Experimental Evidence from Germany
J. Barth et al., (hep-ex/0307083)
SAPHIR detector at ELSA: 1-2.7 GeV tagged photon beam
Exclusive gp  K0 Q+  K0 K+ n
gp  K- Q++  K- K+ p
solid histogram: final M(nK+) spectrum
solid line: arbitrary fit of peak + bkgd
If Q isospin=1or 2,
expect ~70 times more
Conclusions by authors:
M = 154042 MeV, G 25 MeV, s = 4.8
Q+ Isospin = 0
13
Experimental Evidence from Neutrinos
A.E. Asratyan et al., (hep-ex/0309042)
Reanalysis of existing bubble chamber data from CERN, FNAL
Inclusive K0 p production by high energy n and n, mostly on neon
Curve: Gaussian plus linear
Width is consistent with
resolution
Conclusions by authors:
M = 15335 MeV, G 20 MeV, s = 6.7
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Experimental Evidence from HERMES
Inclusive quasi-real photo production with 27.5 GeV e+ on deuterium
g*d  K0s p X  p+p- p X
• Fit is Gaussian plus 4’th order
polynomial (using narrow bins)
• Width is consistent with
instrumental resolution
• Mass calibration checked to 1 MeV
with K0s, (1020), L(1116), L(1520)
M = 1526 2 2 MeV, G < 13 MeV,
s (= Ns / Nb )= 5.60.5
15
Experimental Evidence from K+N Scattering
R.A. Arndt et al., (nucl-th/0308012)
Effect on 2 of 1992 VPI K+N elastic decomposition
of adding narrow resonance
No isovector waves showed any negative 2 for above widths
Conclusion by authors:
Q+ widths beyond the few-MeV level are excluded
The existence of a Q+ in the P01 state is possible, if G  1 MeV
16
Summary of recent Evidence for Q+
Experiments
Results
Mass
(MeV)
SPring8
DIANA
CLAS
SAPHIR
ITEP (n)
HERMES
KN elastic
1540 10 5
One theory
1530 MeV
(QSM)
1539 2 “few”
1542 2 5
1540 4 2
1533 5
1526 2 2
I=0
S=+1
Width
(Mev)
s (= Ns / Nb )
G  25
G 8
FWhM = 21
G  25
G < 20
G  13
G  few MeV !
4.61
4.4
5.30.5
4.8
6.7
5.60.5
G15 MeV
JP=½+
Next:
• Determine width, other quantum numbers (parity!).
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The HERMES Experiment at DESY
Deeply Inelastic Scattering of Polarised Electrons
Probes the Orientation of Quark Spins Inside the Proton
TRIUMF leads a strong Canadian participation in this international collaboration
Scientific Highlights
 First data on spin alignment of both
Novel Features
• Nuclear-polarised atomic gas target
in high energy electron storage ring
• Dual-radiator Ring-Imaging Cerenkov
detector to identify pions, kaons…
quarks and antiquarks of 3 separate
flavours (up, down and strange)
 Discovery of single-spin azimuthal
asymmetries that provide a window on
transverse quark polarization
 First observation of spin asymmetries
for “Deeply Virtual Compton Scattering”,
which can reveal correlations between the
momentum and position of quarks inside
the proton (“femto-holography”)
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