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Search for pentaquarks: the experimental program at CLAS
S. Niccolai, IPN Orsay
for the CLAS collaboration
Epiphany ConferenceKrakov, 1-6-2005
• Introduction• Published CLAS results on Θ+ • Pentaquark program at CLAS• Outlook
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
What are pentaquarks?• New form of quark matter: baryons whose minimum quark content is 5 (qqqqq)
• “Non-exotic” pentaquarks The antiquark has the same flavor as one of the other quarksDifficult to distinguish from 3-quark baryons
s+1/
3 d−1/
3u
+2/3
d−1/3
u+2/3
Baryons with S = +1 cannot be made by qqq !!!!!
• “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
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“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”
1986, Particle Data Group:
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)• + listed in PDG 2004 (***)!• Research on pentaquarks is one of the central programs in many labs
• 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…
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The pentaquark anti-decupletExperimental searches were motivated by predictions of chiral soliton model
Diakonov, Petrov, Polyakov, Z.Physics A359 (1997)
S = +1
S = 0
S = -1
S = -2
I=-3/2 I=-1/2 I=1/2 I=3/2I=1
: exotic states
I=-1 I=0
uudds
ddssu uussd
+ (1539)
N(1650-1690)
(1760-1810)
(1862)
Baryon octets, decuplets and antidecuplets are seen as rotational excitations of
the same soliton field
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The pentaquark anti-decupletExperimental searches were motivated by predictions of chiral soliton model
Diakonov, Petrov, Polyakov, Z.Physics A359 (1997)
S = +1
S = 0
S = -1
S = -2
: exotic states
I=-3/2 I=-1/2 I=1/2 I=3/2I=1I=-1 I=0
+ (1539)
N(1650-1690)
(1760-1810)
(1862)
uudds
ddssu uussd
narrow width (few MeV)
decay modesnK+,pK0
J=1/2+
can be detected experimentally!!
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First observation: LEPS/SPring-8
T. Nakano et al., PRL91, 012002 (2003)
+
n→K+K-(n) (the neutron is bound inside 12C)
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The Jefferson Lab electron acceleratorNewport News, USA
Emax ~ 6 GeV
Imax ~ 200 ADuty Factor ~ 100% E/E ~ 2.5 10-5
Beam Pol ~ 80%
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
The CLAS detector at JLAB
• 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
(e/ separation, 216 PMTs)
Bremsstrahlung photon tagger
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
The CLAS detector at JLAB
Bremsstrahlung photon tagger
Performances:• large acceptance for charged particles
8°<<142°, 0.2< pp<4 GeV/c, 0.1<p<4 GeV/c• good momentum and angular resolution
p/p ≤ 1.5%, ≤ 1 mrad
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
The CLAS detector at JLAB
Bremsstrahlung photon tagger
CLAS is designedto measure exclusive reactions with multi-particle final states
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The bremsstrahlung photon tagger
Tagging range: (20% - 95%)E0E= E0 – Ee (E0
known, E
e measured)
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CLAS-d: first + exclusive measurement
d → K-pK+(n)
Exclusivity:• No Fermi motion corrections• Final state identified with less background
• Experimental data from 1999 run• Tagged photons with Eγmax = 3 GeV • Target: 10 cm long liquid deuterium
Possible reaction mechanism
non-spectator proton:pK- rescattering required
to detect p in CLAS(pmin(p) = 300 MeV/c)
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
CLAS-d: pK+K- event selectionMain background: p+-
pp- p+-
22;
K
cc
Kmp
p
c
Rtt
pK+K-
c
pm
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d → p K+K─ (n) in CLAS
pK+
K-K-
K+
p
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
CLAS-d: neutron ID
• 15% of non pK+K- events within ±3 of the peak
• Almost no background under the neutron peak with tight timing cut, tK
d → K-pK+(n)
The neutron is detectedby missing mass
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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→p(n), →K+K-
d→(1520)K+(n),
(1520)→pK-
• Both reactions proceed predominantly on the proton (neutron is a spectator)
• Kinematics of both reactions are not a good match for + production.
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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→+K-p, +→nK+)
Before pK+ cut
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
CLAS-d: result
d → K-pK+(n)
S. Stepanyan et al., PRL91, 252001 (2003)
Distribution of (1520) events
Gaussian background
Simulated background
M = (1.542 ± 0.005) GeV/c2
= 21 MeV/c2
• Only ~40 events in the + peak
• Statistical significance: 4.6 - , depending on the background shape.
• No significant + signal was found in the spectator analysis (like SPring-8) due to not optimal torus field settings (limited forward acceptance for K-).
• No ++ peak observed in M(pK+)
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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)
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Is “Θ+” a kinematical reflection?
K. Hicks et al., hep-ph/0411265, found inconsistencies in Djerba’s approach
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Is “Θ+” a kinematical reflection?
Open issue:needed measurements of Θ+ in different final states
(no K+K-)
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
First observation on the proton: CLAS-pp→K-+K+(n)
E = 3.0 – 5.47 GeV
After PID: neutron identified by missing mass
No clear Θ+ signal!!
Cut on : M(K+K-)<1.06 GeV/c2
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First observation on the proton: CLAS-pp→K-+K+(n)
V. Kubarovski et al., PRL92, 032001 (2004)
M = (1.555 ± 0.010) GeV/c2
Statistical significance: (7.8 ± 1.0) σMeV/c
t-channel process a) selected andbackground processes eliminatedwith the cuts (c.m.): cosθ*
+ > 0.8 and cosθ*K+ < 0.6
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First observation on the proton: CLAS-pp→K-+K+(n)
V. Kubarovski et al., PRL92, 032001 (2004)
cut
to understand production mechanismlook at M(nK+K-)
+ production through N* resonance decays?
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
First observation on the proton: CLAS-pp→K-+K+(n)
Cut on M(nK+)in + region
outside+ region
• -p cross section data in PDG have a gap in the mass range 2.3–2.43 GeV
• intermediate baryon state?
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S. Niccolai, IPN OrsayKrakow, 1-6-2005
Summary on Θ+: positive results
Spring-8
DIANA
CLAS-d
CLAS-p
SAPHIR
ITEP
HERMES
COSY
ZEUS
IHEP
• ~ 10 MeV gap between measured masses• statistical significance between 4 and 7.8 σ• often heavy cuts• backgrounds not well understood
• 10 observations• different reaction mechanisms
BUT:
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Summary on Θ+: negative results
• high statistics• clearly see some of the known resonances
NEEDED DEDICATED, HIGH-STATISTICS, EXCLUSIVE MEASUREMENTS!!!!!
HERA B: pA→pK0XCDF: pp→pK0XPhenix: Au+Au→nK-XBaBar: e+e-→pK0XBelle: e+e-→B0B0→ppK0
BES:e+e-→J/Ψ→ΘΘ
__
_ __
BUT:
• in e+e- : there are no quarks in initial state no theoretical predictions on production
• in the other cases: fragmentation processes, + could be suppressed (Titov et al., Phys. Rev. C 70, 042202 (2004)) high multiplicity in the final state, combinatorial backgrounds
_
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Decuplet partners of Θ+: search for 3/2
−−,
ddssu
uudds
uussd
Exotic, S = -2, Q = -2Non exotic, S = -2, Q = 0
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Decuplet partners of Θ+: search for 3/2
−−,
p+A 920 GeV/cHERA-Bhep-ex/0403020
Combined spectra
C. Alt, et al., Phys.Rev.Lett.92, 042003 (2004)
Ecm = 17.2 GeV
M=1.862± 0.002 GeVStatistical significance = 5.6σ
MeV/c
NA49 pp
No peaks observed!
Ξ0 also observed No Ξ0
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
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
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
Pentaquark Searches at CLAS
Search for *++ on the proton - G11 run p → various final states (data taking completed)
Search for pentaquarks - EG3 run vd→--, X (data taking just finished)
Exotic hadron spectroscopy - G12 run p at 6 GeV, high luminosity (to run in 2005/6)
Experimental program approved and underway at CLAS
Search for + on deuterium - G10 run d→K-K+p(n) and other final states (data taking completed)
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
CLAS G10: search for Θ+ on deuterium
Reactions channels to study:
d→pK-K+n Θ+→nK+
d→ΛK+(0)n(p); Θ+→nK+,pK0
d→pK- (p) Θ+→pK0
”n”→K-K+n Θ+→nK+
(with Fermi momentum corrections, to compare with SPring-8)
• data taking finished end of May• 10 billion of events collected • data processing almost done
• results in the spring
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
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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 7
• 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 (, )
• Divide data set into two independent sets. Analyze one set, then apply the
same analysis (cuts, etc) to the second set
• Inspect + candidates in CLAS event display
• Full simulation of possible background final states
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n
non-spectator neutron events
CLAS G10: data quality
d → K-pK+(n)
50% of the high-field data
(1520)
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
CLAS G10: γd→ΛΘ+
p
nd
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
Reaction already studiedon 3He CLAS data, but statistics were too low
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
CLAS G10: γd→ΛΘ+
p
nd
K+
n
K+
p
Decay modes under study:
• Λ→pπ-
Θ+→K+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
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
p
n
K+
p
p
K0
• Λ→pπ- Θ+→K0p K0 → π+π- d
CLAS G10: γd→ΛΘ+
Decay modes under study:
• Λ→pπ-
Θ+→K+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
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p
n
K+
p
n
K+
d
CLAS G10: γd→ΛΘ+
Decay mode:
• Λ→pπ-
Θ+→K+n
n
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p
n
K+
p
p
K0
• Λ→pπ- Θ+→K0p K0 → π+π-
d
CLAS G10: γd→ΛΘ+
Decay mode:
K0
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
CLAS G11: search for Θ+ on proton
Reaction channels to study:
p→K0K+(n) Θ+→nK+
p→K0K0p Θ+→pK0
p→pK- Θ++→pK+
p→K-Kn) Θ+→nK+
• 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 +
• search for ++
• data are being processed
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Krakow, 1-6-2005 S. Niccolai, IPN Orsay
CLAS G11: data quality
Xp pXp
XpKp )( pKKp )(
6% of statistics
p -
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CLAS G11: data quality
XKp
pXKp
nKp
nKp
nKp
K0n
nKp
6% of statistics
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Summary and outlook possible existence of pentaquarks gave new boost to hadronic
physics and QCD spectroscopy 10 Θ+ signals published so far, but:
several reports of non-observations need of high-statistics dedicated experiments to:
• low statistics• discrepancies in measured masses
• establish existence of Θ+
• study its properties• find possible other pentaquarks (5’s ?)
STAY TUNED…
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