Search for pentaquarks: the experimental program at CLAS

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

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

“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…

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

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!!

First observation: LEPS/SPring-8

T. Nakano et al., PRL91, 012002 (2003)

+

n→K+K-(n) (the neutron is bound inside 12C)

The Jefferson Lab electron acceleratorNewport News, USA

Emax ~ 6 GeV

Imax ~ 200 ADuty Factor ~ 100% E/E ~ 2.5 10-5

Beam Pol ~ 80%


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




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




CLAS is designedto measure exclusive reactions with multi-particle final states

The bremsstrahlung photon tagger

Tagging range: (20% - 95%)E0E= E0 – Ee (E0

known, E

e measured)

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)


CLAS-d: pK+K- event selectionMain background: p+-

pp- p+-

22;

K

cc

Kmp

p

c

Rtt

pK+K-

c

pm

d → p K+K─ (n) in CLAS

pK+

K-K-

K+

p


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


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.


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


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+)

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)


K. Hicks et al., hep-ph/0411265, found inconsistencies in Djerba’s approach


Open issue:needed measurements of Θ+ in different final states

(no K+K-)


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


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


V. Kubarovski et al., PRL92, 032001 (2004)

cut

to understand production mechanismlook at M(nK+K-)

+ production through N* resonance decays?



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?

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:

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

_

Decuplet partners of Θ+: search for 3/2

−−,

ddssu

uudds

uussd

Exotic, S = -2, Q = -2Non exotic, S = -2, Q = 0

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


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


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)


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


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

n

non-spectator neutron events

CLAS G10: data quality

d → K-pK+(n)

50% of the high-field data

(1520)


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



p

nd

K+

n

K+

p

Decay modes under study:

• Λ→pπ-

Θ+→K+n





p

n

K+

p

p

K0

• Λ→pπ- Θ+→K0p K0 → π+π- d


Decay modes under study:

• Λ→pπ-

Θ+→K+n




p

n

K+

p

n

K+

d


Decay mode:

• Λ→pπ-

Θ+→K+n

n

p

n

K+

p

p

K0

• Λ→pπ- Θ+→K0p K0 → π+π-

d


Decay mode:

K0


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



Xp pXp

XpKp )( pKKp )(

6% of statistics

p -


XKp

pXKp

nKp

nKp

nKp

K0n

nKp

6% of statistics

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

Search for pentaquarks: the experimental program at CLAS

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