Search for pentaquarks: the
experimental program at CLAS
S. Niccolai, IPN Orsay
for the CLAS collaboration
•Epiphany
Introduction Conference
• Published CLAS results on Θ+
Krakov, 1-6-2005
• Pentaquark program at CLAS
• Outlook
What are pentaquarks?
• New form of quark matter: baryons whose
minimum quark content is 5 (qqqqq)
d
−1/3
u
s
+1/3
+2/3
u
d
−1/3
+2/3
• “Non-exotic” pentaquarks
 The antiquark has the same flavor as one of the other quarks
 Difficult to distinguish from 3-quark baryons
• “Exotic” pentaquarks (qqqqQ)
 The antiquark has a different flavor from the other 4 quarks
 Quantum numbers different from any 3-quark baryon
Example: uudds (exotic):
Baryon number = 1/3 + 1/3 + 1/3 + 1/3 - 1/3 = 1
Strangeness = 0 + 0 + 0 + 0 + 1 = +1
Baryons with S = +1 cannot be made by qqq !!!!!
Krakow, 1-6-2005
S. Niccolai, IPN Orsay
• Existence of pentaquark is not forbidden by QCD
• General idea of a five-quark states has been around since late 60’s
(Jaffe, Lipkin, Strotteman…)
However…
1986, Particle Data Group:
“The general prejudice against baryons not made of three
quarks and the lack of any experimental activity in this
area make it likely that it will be another 15 years before
the issue is decided”
Almost 15 years later...
• 10 experiments reported evidences for pentaquark states since 2003
• Over 200 theory papers published
• Started series of dedicated international workshops: JLab (2003),
SPring-8 (2004), Genova (2005)
• Q+ listed in PDG 2004 (***)!
• Research on pentaquarks is one of the central programs in many labs
The pentaquark anti-decuplet
Experimental searches were motivated by predictions of chiral soliton model
Diakonov, Petrov, Polyakov, Z.Physics A359 (1997)
uudds
Q+ (1539)
Baryon octets, decuplets
and antidecuplets are seen
as rotational excitations of
the same soliton field
S = +1
N(1650-1690)
: exotic states
S(1760-1810)
X+(1862)
ddssu
I=-3/2
uussd
I=-1
I=-1/2
I=0
I=1/2
I=1
I=3/2
S=0
S = -1
S = -2
The pentaquark anti-decuplet
Experimental searches were motivated by predictions of chiral soliton model
Diakonov, Petrov, Polyakov, Z.Physics A359 (1997)
uudds
Q+ (1539)
decay modes
nK+,pK0
narrow width
(few MeV)
S = +1
can be detected
experimentally!!
Jp=1/2+
N(1650-1690)
: exotic states
S(1760-1810)
X+(1862)
ddssu
I=-3/2
uussd
I=-1
I=-1/2
I=0
I=1/2
I=1
I=3/2
S=0
S = -1
S = -2
First observation: LEPS/SPring-8
n→K+K-(n)
T. Nakano et al., PRL91, 012002 (2003)
(the neutron is bound inside 12C)
Q+
The Jefferson Lab electron accelerator
Newport News, USA
Emax ~ 6 GeV
Imax ~ 200 mA
Duty Factor ~ 100%
sE/E ~ 2.5 10-5
Beam Pol ~ 80%
The CLAS detector at JLAB
Bremsstrahlung photon tagger
Krakow, 1-6-2005
• Toroidal magnetic field (6 supercond. coils)
• Drift chambers (argon/CO2 gas, 35,000 cells)
• Time-of-flight scintillators (684 PMTs)
• Electromagnetic calorimeters
(lead/scintillator, 1296 PMTs)
• Cherenkov Counters
S. Niccolai,
(e/p separation, 216
PMTs) IPN Orsay
The CLAS detector at JLAB
Performances:
Bremsstrahlung photon tagger
Krakow, 1-6-2005
• large acceptance for charged particles
8°<q<142°, 0.2< pp<4 GeV/c, 0.1<pp<4 GeV/c
• good momentum and angular resolution
Dp/p ≤ 1.5%, Dq, Df ≤ 1 mrad
S. Niccolai, IPN Orsay
The CLAS detector at JLAB
Bremsstrahlung photon tagger
Krakow, 1-6-2005
CLAS is designed
to measure exclusive reactions
with multi-particle final states
S. Niccolai, IPN Orsay
The bremsstrahlung photon tagger
E = E0 – Ee
(E0 known, Ee measured)Tagging range: (20% - 95%)E0
CLAS-d: first Q+ exclusive measurement
d → K-pK+(n)
• Experimental data from 1999 run
• Tagged photons with Eγmax = 3 GeV
• Target: 10 cm long liquid deuterium
Exclusivity:
• No Fermi motion corrections
• Final state identified with less background
Possible reaction mechanism
non-spectator proton:
pK- rescattering required
to detect p in CLAS
(pmin(p) = 300 MeV/c)
CLAS-d: pK+K- event selection
Main background: pp+p-
pp+p-
ppp-
pK+K-
m
Krakow, 1-6-2005
p
 c
DtK  t 
R
; c 
c  c
p
p 2 + mK2
S. Niccolai, IPN Orsay
d → p K+K─ (n) in CLAS
p
K-
Kp
K+
K+
CLAS-d: neutron ID
d → K-pK+(n)
The neutron is detected
by missing mass
• 15% of non pK+K- events
within ±3s of the peak
• Almost no background
under the neutron peak
with tight timing cut, DtK
Krakow, 1-6-2005
S. Niccolai, IPN Orsay
CLAS-d : cut on known resonances
• Several other known processes
can contribute to the pK+K-(n)
final state:
d→fp(n), f→K+Kd→L(1520)K+(n),
L(1520)→pK• Both reactions proceed
predominantly on the proton
(neutron is a spectator)
• Kinematics of both reactions are
not a good match for Q+
production.
Krakow, 1-6-2005
S. Niccolai, IPN Orsay
CLAS-d: background reduction
• Cut on the missing neutron momentum, pn>0.08 GeV/c
• Cut on the K+ momentum, pK+<1 GeV/c, based on the 3-body
phase space Monte Carlo (d→Q+K-p, Q+→nK+)
Before pK+ cut
Krakow, 1-6-2005
S. Niccolai, IPN Orsay
CLAS-d: result
S. Stepanyan et al., PRL91, 252001 (2003)
M = (1.542 ± 0.005) GeV/c2
= 21 MeV/c2
• Only ~40 events in the Q+ peak
• Statistical significance: 4.6s - 5.8s,
depending on the background shape.
• No significant Q+ signal was found in
the spectator analysis (like SPring-8)
due to not optimal torus field settings
(limited forward acceptance for K-).
d → K-pK+(n)
Gaussian
background
Simulated background
• No Q++ peak observed in M(pK+)
Distribution of L(1520) events
Krakow, 1-6-2005
S. Niccolai, IPN Orsay
Is “Θ+” a kinematical reflection?
High mass mesons (a2(1320), f2(1270)) decaying to
K+K- can produce structures in M(NK) at low mass
(A. Dzierba et al., hep-ph/0311125)
Is “Θ+” a kinematical reflection?
K. Hicks et al., hep-ph/0411265, found
inconsistencies in Djerba’s approach
Is “Θ+” a kinematical reflection?
Open issue:
needed measurements of Θ+ in different final states
(no K+K-)
First observation on the proton: CLAS-p
After PID: neutron identified
by missing mass
p→K-p+K+(n)
E = 3.0 – 5.47 GeV
Cut on f: M(K+K-)<1.06 GeV/c2
No clear Θ+ signal!!
Krakow, 1-6-2005
S. Niccolai, IPN Orsay
First observation on the proton: CLAS-p
p→K-p+K+(n)
V. Kubarovski et al., PRL92, 032001 (2004)
M = (1.555 ± 0.010) GeV/c2
Statistical significance: (7.8 ± 1.0) σ
  26 MeV/c2
t-channel process a) selected and
background processes eliminated
with the cuts (c.m.):
cosθ*p+ > 0.8 and cosθ*K+ < 0.6
First observation on the proton: CLAS-p
p→K-p+K+(n)
V. Kubarovski et al., PRL92, 032001 (2004)
 Q+ production through
N* resonance decays?
cut
to understand production mechanism
look at M(nK+K-)
First observation on the proton: CLAS-p
p→K-p+K+(n)
outside
Q+ region
Cut on M(nK+)
in Q+ region
• intermediate baryon state?
• p-p cross section data in PDG
have a gap in the mass range
2.3–2.43 GeV
Krakow, 1-6-2005
S. Niccolai, IPN Orsay
Summary on Θ+: positive results
• 10 observations
• different reaction
mechanisms
Krakow, 1-6-2005
IHEP
ZEUS
COSY
HERMES
ITEP
SAPHIR
CLAS-p
CLAS-d
DIANA
Spring-8
BUT:
• ~ 10 MeV gap between
measured masses
• statistical significance
between 4 and 7.8 σ
• often heavy cuts
• backgrounds not well
understood
S. Niccolai, IPN Orsay
Summary on Θ+: negative results
HERA _
B: pA→pK0X
CDF: pp→pK0X _
Phenix: Au+Au→nK-X
_ 0X _
BaBar: e+e-→pK
0
Belle: e+e-→B0B0→ppK
_
BES:e+e-→J/Ψ→ΘΘ
• high statistics
• clearly see some of the known resonances
BUT:
• in e+e- :
 there are no quarks in initial state
_
 no theoretical predictions on QQ production
• in the other cases:
 fragmentation processes, Q+ could be suppressed (Titov et al.,
Phys. Rev. C 70, 042202 (2004))
 high multiplicity in the final state, combinatorial backgrounds
NEEDED DEDICATED, HIGH-STATISTICS,
EXCLUSIVE MEASUREMENTS!!!!!
Decuplet partners of Θ+:
search for X3/2−−, X3/20
uudds
ddssu
Exotic, S = -2, Q = -2
uussd
Non exotic, S = -2, Q = 0
Decuplet partners of Θ+:
search for X3/2−−, X3/20
NA49
pp Ecm = 17.2 GeV
C. Alt, et al., Phys.Rev.Lett.92, 042003 (2004)
HERA-B
p+A 920 GeV/c
hep-ex/0403020
Combined spectra
Xp + Xp+
M=1.862± 0.002 GeV
Statistical significance = 5.6σ
  18 MeV/c2
No peaks observed!
Ξ0 also observed
No Ξ0
Pentaquark today: open issues
• confirmation of existence of Θ+ with high statistics
• precise determination of the mass of Θ+
• properties of Θ+: spin, isospin, parity
• production mechanisms
• possible excited states of Θ+
• coupling N* to Θ+
• confirmation existence of other exotic members of decuplet
An experimental program is currently
underway at CLAS to address these issues
Krakow, 1-6-2005
S. Niccolai, IPN Orsay
Pentaquark Searches at CLAS
Experimental program approved and underway at CLAS
 Search for Q+ on deuterium - G10 run
d→K-K+p(n) and other final states
(data taking completed)
 Search for Q+, Q*+, Q*++ on the proton - G11 run
p → various final states
(data taking completed)
 Search for X pentaquarks - EG3 run
vd→X5--, X5 X
 Exotic hadron spectroscopy - G12 run
p at 6 GeV, high luminosity
Krakow, 1-6-2005
(data taking just finished)
(to run in 2005/6)
S. Niccolai, IPN Orsay
CLAS G10: search for Θ+ on deuterium
 tagged photons in the energy range 0.8 GeV to 3.59 GeV
 target: 24 cm long liquid deuterium
 data were taken at 2 settings of CLAS toroidal magnet (2250 A and 3375 A),
lower field to increase acceptance for K- (Spring-8)
 at each setting integrated luminosity (2.5pb-1) is about 10 times higher than in
published deuterium data
Reactions channels to study:
• data taking finished end of May
• 10 billion of events collected
• data processing almost done
• results in the spring
d→pK-K+n Θ+→nK+
d→ΛK+(0)n(p); Θ+→nK+,pK0
d→pK- K0 (p) Θ+→pK0
”n”→K-K+n
Θ+→nK+
(with Fermi momentum corrections,
to compare with SPring-8)
Krakow, 1-6-2005
S. Niccolai, IPN Orsay
CLAS G10: how to avoid a fake signal
• Consider only signals with statistical uncertainty <10%, and statistical
significance greater than 7s
• Study of the NK system:
– for pK0 and nK+ final states
– in both the missing mass and invariant mass distributions
• Production of NK system together with a hyperon (L, S)
• Divide data set into two independent sets. Analyze one set, then apply the
same analysis (cuts, etc) to the second set
• Inspect Q+ candidates in CLAS event display
• Full simulation of possible background final states
Krakow, 1-6-2005
S. Niccolai, IPN Orsay
CLAS G10: data quality
n
d → K-pK+(n)
non-spectator
neutron events
f
L(1520)
50% of the
high-field data
CLAS G10: γd→ΛΘ+
L
p
d
K+
Q+
Reaction already studied
on 3He CLAS data, but
statistics were too low
n
• No possibility of kinematical reflections
(only one K, from Θ+ decay, in the final state)
• S=+1 both for nK+ and pK0, thanks to Λ
• No background channels to remove
Krakow, 1-6-2005
S. Niccolai, IPN Orsay
CLAS G10: γd→ΛΘ+
p
p
L
p
d
K+
Decay modes under study:
• Λ→pπ-
Θ+→K+n
n
Q+
n
K+
• No possibility of kinematical reflections
(only one K, from Θ+ decay, in the final state)
• S=+1 both for nK+ and pK0, thanks to Λ
• No background channels to remove
Krakow, 1-6-2005
S. Niccolai, IPN Orsay
CLAS G10: γd→ΛΘ+
p
p
L
p
d
K+
Decay modes under study:
p
Q+
n
K0
• Λ→pπ-
Θ+→K+n
• Λ→pπ-
Θ+→K0p
K0 → π+π-
p+
p
• No possibility of kinematical reflections
(only one K, from Θ+ decay, in the final state)
• S=+1 both for nK+ and pK0, thanks to Λ
• No background channels to remove
Krakow, 1-6-2005
S. Niccolai, IPN Orsay
CLAS G10: γd→ΛΘ+
p
p
L
p
d
K+
Decay mode:
• Λ→pπ-
Θ+→K+n
n
Q+
n
K+
n
L
CLAS G10: γd→ΛΘ+
p
p
L
p
d
K+
Decay mode:
• Λ→pπ-
Θ+→K0p
p
Q+
n
K0
p+
p
L
K0
K0 → π+π-
CLAS G11: search for Θ+ on proton
• data taking finished end of July
• 10 times more statistics than old p
• Eγ = 0.8 – 3.59 GeV
• search for ground and first excited
states of Q+
• search for Q++
• data are being processed
Reaction channels to study:
p→K0K+(n)
Θ+→nK+
p→K0K0p
Θ+→pK0
p→K-K+p+(n) Θ+→nK+
p→pK- K+
Krakow, 1-6-2005
Θ++→pK+
S. Niccolai, IPN Orsay
CLAS G11: data quality
6% of statistics
p  p + pX
p  p +p  X
p
p-
p  K + ( pp  ) X
p  ( K + K  ) p
L
Krakow, 1-6-2005
f
S. Niccolai, IPN Orsay
CLAS G11: data quality
6% of statistics
p  p + K +p n
p  p +p  K + n
p  p +p  K + X
n
p  p  K +p + n
p  K +p +p n
S+
S
K0
L(1520)
p  K + pX
L
S0
Summary and outlook
 possible existence of pentaquarks gave new boost to hadronic
physics and QCD spectroscopy
 10 Θ+ signals published so far, but:
• low statistics
• discrepancies in measured masses
 several reports of non-observations
 need of high-statistics dedicated experiments to:
• establish existence of Θ+
• study its properties
• find possible other pentaquarks (X5’s ?)
 the new CLAS experimental program should solve these issues, with:
• two experiments completed (G10, G11), results coming soon
• one experiment finishing in these days (EG3)
• one experiment to run later on in 2005