Search for pentaquarks: the experimental program at CLAS

1
Search for pentaquarks the experimental program
at CLAS

• S. Niccolai, IPN Orsay
• for the CLAS collaboration

Epiphany Conference Krakov, 1-6-2005

• Introduction
• Published CLAS results on T
• Pentaquark program at CLAS
• Outlook

2
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 quarks
• Difficult to distinguish from 3-quark baryons

Baryons with S 1 cannot be made by qqq !!!!!
3

• Existence of pentaquark is not forbidden by
  QCD
• General idea of a five-quark states has been
  around since late 60s (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

4
The pentaquark anti-decuplet
Experimental searches were motivated by
predictions of chiral soliton model Diakonov,
Petrov, Polyakov, Z.Physics A359 (1997)
S 1
Baryon octets, decuplets and antidecuplets are
seen as rotational excitations of the same
soliton field
S 0
S -1
S -2
I-1/2
I-1
I-3/2
I3/2
I1
I0
I1/2
5
The pentaquark anti-decuplet
Experimental searches were motivated by
predictions of chiral soliton model Diakonov,
Petrov, Polyakov, Z.Physics A359 (1997)
Q (1539)
can be detected experimentally!!
N(1650-1690)
S(1760-1810)
X(1862)
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The pentaquark anti-decuplet
Experimental searches were motivated by
predictions of chiral soliton model Diakonov,
Petrov, Polyakov, Z.Physics A359 (1997)
Q (1539)
N(1650-1690)
S(1760-1810)
X(1862)
5-quark states predicted also by lattice QCD and
other models Skyrme model, MIT bag model, QCD
sum rules,
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First observation LEPS/SPring-8
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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
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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/p separation, 216 PMTs)

Bremsstrahlung photon tagger
10
The CLAS detector at JLAB

• Performances
• large acceptance for charged particles
• 8ltqlt142, 0.2lt pplt4 GeV/c, 0.1ltpplt4 GeV/c
• good momentum and angular resolution
• Dp/p 1.5, Dq, Df 1 mrad

Bremsstrahlung photon tagger
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The CLAS detector at JLAB
CLAS is designed to measure exclusive reactions
with multi-particle final states
Bremsstrahlung photon tagger
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The bremsstrahlung photon tagger
Tagging range (20 - 95)E0
Eg E0 Ee (E0 known, Ee measured)
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CLAS-d first Q exclusive measurement

• Experimental data from 1999 run
• Tagged photons with E?max 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)
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CLAS-d pKK- event selection
Main background ppp-
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gd ? p KK- (n) in CLAS
K-
p
K
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CLAS-d neutron ID
The neutron is detected by missing mass

• 15 of non pKK- events within 3s of the peak
• Almost no background under the neutron peak with
  tight timing cut, DtK

17
CLAS-d cut on known resonances

• Several other known processes can contribute to
  the pKK-(n) final state
• gd?fp(n), f?KK-
• gd?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.

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CLAS-d background reduction

• Cut on the missing neutron momentum, pngt0.08
  GeV/c
• Cut on the K momentum, pKlt1 GeV/c, based on the
  3-body phase space Monte Carlo (gd?QK-p, Q?nK)

Before pK cut
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CLAS-d result

• 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-).
• No Q peak observed in M(pK)

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Is T a kinematical reflection?
High mass mesons (a2(1320), f2(1270)) decaying to
KK- can produce structures in M(NK) at low mass
(A. Dzierba et al., hep-ph/0311125)
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Is T a kinematical reflection?
K. Hicks et al., hep-ph/0411265, found
inconsistencies in Djerbas approach
22
Is T a kinematical reflection?
Open issue needed measurements of T in
different final states (no KK-)
23
First observation on the proton CLAS-p
gp?K-pK(n)
After PID neutron identified by missing mass
Eg 3.0 5.47 GeV
Cut on f M(KK-)lt1.06 GeV/c2
No clear T signal!!
24
First observation on the proton CLAS-p
gp?K-pK(n)
V. Kubarovski et al., PRL92, 032001 (2004)
t-channel process a) selected and background
processes eliminated with the cuts (c.m.)
cos?p gt 0.8 and cos?K lt 0.6
M (1.555 0.010) GeV/c2 Statistical
significance (7.8 1.0) s G 26 MeV/c2
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First observation on the proton CLAS-p
gp?K-pK(n)
V. Kubarovski et al., PRL92, 032001 (2004)

• Q production through
• N resonance decays?

to understand production mechanism look at
M(nKK-)
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First observation on the proton CLAS-p
gp?K-pK(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
• 2.32.43 GeV

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Summary on T positive results

• 10 observations
• different reaction mechanisms

BUT

• 10 MeV gap between
• measured masses
• statistical significance
• between 4 and 7.8 s
• often heavy cuts
• backgrounds not well understood

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Summary on T negative results

• high statistics
• clearly see some of the known resonances

BUT
NEEDED DEDICATED, HIGH-STATISTICS, EXCLUSIVE
MEASUREMENTS!!!!!
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Decuplet partners of T search for X3/2--, X3/20
Exotic, S -2, Q -2
Non exotic, S -2, Q 0
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Decuplet partners of T search for X3/2--, X3/20
pp
NA49
Ecm 17.2 GeV
C. Alt, et al., Phys.Rev.Lett.92, 042003 (2004)
hep-ex/0403020
Combined spectra
M1.862 0.002 GeV Statistical significance
5.6s G 18 MeV/c2
No peaks observed!
?0 also observed
No ?0
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Pentaquark today open issues

• confirmation of existence of T with high
  statistics
• precise determination of the mass of T
• properties of T spin, isospin, parity
• production mechanisms
• possible excited states of T
• coupling N to T
• confirmation existence of other exotic members
  of decuplet

An experimental program is currently underway at
CLAS to address these issues
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Pentaquark Searches at CLAS
Experimental program approved and underway at CLAS

• Search for Q on deuterium - G10 run
• gd?K-Kp(n) and other final states
  (data taking completed)

• Search for Q, Q, Q on the proton - G11 run
  
• gp ? various final states
  (data taking completed)
• Search for X pentaquarks - EG3 run
• gvd?X5--, X5- X
  (data taking just finished)
• Exotic hadron spectroscopy - G12 run
• gp at 6 GeV, high luminosity
  (to run in 2005/6)

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CLAS G10 search for T 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

• data taking finished end of May
• 10 billion of events collected
• data processing almost done
• results in the spring

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CLAS G10 how to avoid a fake signal

• Consider only signals with statistical
  uncertainty lt10, 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

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CLAS G10 data quality
50 of the high-field data
L(1520)
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CLAS G10 ?d??T
Reaction already studied on 3He CLAS data, but
statistics were too low

• No possibility of kinematical reflections
• (only one K, from T decay, in the final
  state)
• S1 both for nK and pK0, thanks to ?
• No background channels to remove

37
CLAS G10 ?d??T
p
p-
L
p
K
n
Q
n
K

• No possibility of kinematical reflections
• (only one K, from T decay, in the final
  state)
• S1 both for nK and pK0, thanks to ?
• No background channels to remove

38
CLAS G10 ?d??T
p
p-
L
p
K
p
Q
n
p
K0
p-

• No possibility of kinematical reflections
• (only one K, from T decay, in the final
  state)
• S1 both for nK and pK0, thanks to ?
• No background channels to remove

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CLAS G10 ?d??T
p
p-
L
p
K
n
Q
n
K
L
n
40
CLAS G10 ?d??T
p
p-
L
p
K
p
Q
n
p
K0
p-
L
K0
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CLAS G11 search for T on proton

• data taking finished end of July
• 10 times more statistics than old gp
• E? 0.8 3.59 GeV
• search for ground and first excited states of Q
• search for Q
• data are being processed

42
CLAS G11 data quality
6 of statistics
p-
p
L
f
43
CLAS G11 data quality
6 of statistics
S-
n
K0
L(1520)
S0
L
S
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Summary and outlook

• possible existence of pentaquarks gave new boost
  to hadronic physics and QCD spectroscopy
• 10 T signals published so far, but
• several reports of non-observations
• need of high-statistics dedicated experiments
  to

• low statistics
• discrepancies in measured masses

STAY TUNED

• establish existence of T
• study its properties
• find possible other pentaquarks (X5s ?)

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CLAS-d variation of cuts

• No cuts on the K momentum..
• Tighter timing cuts.
• DT (P-K) lt 0.75 ns.