# Presentation: 553225

## The Path to Discovery of the Pentaquark: an Exotic Baryon

- Announcements from LEPS (Japan), ITEP (Russia), CLAS (USA), and ELSA (Germany),  provide evidence for the existence of an exotic baryon, a pentaquark with strangeness S=+1, called the +.

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Media Interest

- The pentaquark discovery received wide media coverage:
- Newspapers (July, 2003):
- New York Times, USA Today, L.A. Times, Boston Globe, Cleveland Plain Dealer, Dallas Morning News, Washington Times, Richmond Times, MSNBC (web), and others...
- Le Figaro (Paris), Allgemeine Frankfurter (Germany), Times of India, HARRETZ (Israel), Italy, Netherlands, and many newspapers in Japan.
- Magazines:
- US News & World Report, The Economist, Discover Magazine, Science, Nature, Physics World (IOP), Cern Courier...
- The reason?  In part, because the idea is simple to explain.

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Media Graphic (from the AIP)

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Media Graphic (from the JPS)

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Historical Bias Against S=+1 Baryons

**(PDG ****1986****; Phys. Lett. B170, 289)**
***The evidence for strangeness +1 baryon resonances was reviewed in our 1976 edition,******1****** and more recently by Kelly******2****** and by Oades.******3****** Two new partial-wave anaIyses******4****** have appeared since our 1984 edition. Both claim that the P******13****** and perhaps other waves resonate.***
***However, ******the results permit no definite conclusion- the same story heard for ***_***15 years***_***.****** The standards of proof must simply be much more severe here than in a channel in which many resonances are already known to exist. ******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******.***

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Outline

- What is the pentaquark?
- How was it predicted by theory?
- Why is it a new kind of particle?
- Experimental evidence (since October 2002):
- LEPS (4.6   peak at mass = 1.54 GeV)
- ITEP (4.5 , peak at mass = 1.539 GeV)
- CLAS (5.5 , peak at mass = 1.542 GeV)
- SAPHIR (4.8 , peak at mass = 1.540 GeV)
- NEW: WA21... scattering (6.7 , 1533 +/- 5 MeV)
- Theorists response to the pentaquark ‘discovery’
- What next in experimental investigation?

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## A short review...

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Symmetries and Conservation Laws

- A conservation law implies a symmetry of nature:
- Conservation of momentum  gauge invariance
- Conservation of energy  time reversal invariance
- Other conservation principles for particles:
- Conservation of baryon number  flavor SU(n)f
- Conservation of strangeness   hypercharge
- Conservation of isospin  chiral symmetry
- Gell-Mann used these symmetries (and group theory) to develop the quark model.

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## The Particle Zoo (I)

| Name | Lifetime | Mass | Strange | Charge |
| --- | --- | --- | --- | --- |
|    p |   stable |   938.3 |      0 |    +1 |
|    n |   stable |   939.6 |      0 |     0 |
|  |  3. e-10 |   1115. |     -1 |     0 |
|  |   1. e-9 |   1189. |     -1 |    +1 |
|  |   1. e-19 |   1193. |     -1 |     0 |
|  |   1. e-10 |   1197. |     -1 |    -1 |
|  |   3. e-10 |   1315. |     -2 |     0 |
|  |   2. e-10 |   1321. |     -2 |    -1 |

**Baryons (J******** =1/2****+****)**

- Is there a pattern here?  What about the ?

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## The Particle Zoo (II)

| Name | Lifetime | Mass | Strange | Charge |
| --- | --- | --- | --- | --- |
|    K+ |   1. e-8 |   493.7 |     +1 |    +1 |
|    K0S |   1. e-10 |   497.7 |   +1/-1 |     0 |
|  |   3. e-8 |   139.6 |      0 |    +1 |
|  |   1. e-16 |   135.0 |      0 |     0 |
|  |   3. e-8 |   139.6 |      0 |    -1 |
|    K0L |   5. e-8 |   497.7 |    -1/+1 |     0 |
|    K- |   1. e-8 |   1321. |     -1 |    -1 |

**Mesons (J******** =0****-****)**

- Is this better?  How do the K0 mesons fit in?

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Strangeness vs. Charge

****** ****To center, define: ****Y = B+S**** and ****I****3**** = Q–Y/2****.**

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Hypercharge vs. Isospin

**Mesons (J=0)**
**     ****B=0**

**-1      0     +1**

**I****3**

****** ****Now the objects can be treated as QM rotations**

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## The standard baryon decuplet

- Each unit of
- strangeness
- costs about
- 150 MeV.

- The Gell-Mann/Okubo relation: equal mass spacings

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## The anti-decuplet from the chiral soliton

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Parameters of the chiral soliton model

- There are 3 equations and 3 unknowns:
- The soliton model parameters are ,  and   These are related to two “moments of inertia” of rotations in spin and isospin space, and the chiral symmetry breaking.
- Experimental (known) values:
- Mass splittings of groups, and the in-medium quark condensate  = 0.5(mu+md) <N|(uu+dd)|N> = 0.045 GeV.
- Specifically:
- Octet:                m()-m(N) = (/2)+2+(/4)
- Decuplet:           m(*)-m() = (/8)+-(5/16)
- Anti-decuplet:  m(N*)-m() = (/4)++(/8)
- These mass splittings are satisfied to ~1-2% !

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## The + is has quark structure (uudds):
Experiment conserves baryon number and strangeness
Prediction by D. Diakonov, V. Petrov, and M.Polyakov, 
Z. Phys. A 359, 305 (1997)
chiral soliton model: mass = 1530 MeV
+ width predicted ~15 MeV
JP=1/2+ (requires NK+ orbital L=1)
Mass fixed by the N* (JP=1/2+) at mass = 1710 MeV
This is the only well-known P11 above the Roper resonance
Similarly, there is a P11 state  at mass = 1880 MeV
Only given “2-star” status by the PDG

- Experiment conserves baryon number and strangeness
- Prediction by D. Diakonov, V. Petrov, and M.Polyakov,
- Z. Phys. A 359, 305 (1997)
- chiral soliton model: mass = 1530 MeV
******+** width predicted ~15 MeV
- JP=1/2+ (requires NK+ orbital L=1)
- Mass fixed by the N* (JP=1/2+) at mass = 1710 MeV
- This is the only well-known P11 above the Roper resonance
- Similarly, there is a P11 state ****** **at mass = 1880 MeV
- Only given “2-star” status by the PDG

- What is the ?

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## QCD does not prohibit q4q states, but early searches failed to produce evidence for  pentaquarks. This led people to believe that all baryonic matter comes in only one form: 3-quark states.
The +, if found, is the first hard evidence of a new class of particle: the pentaquark.
One of the central activities at Jefferson Lab is to understand N* resonances.  Do pentaquarks contribute to the resonance spectrum?

- The +, if found, is the first hard evidence of a _new class of particle_: the pentaquark.
- One of the central activities at Jefferson Lab is to understand N* resonances.  Do pentaquarks contribute to the resonance spectrum?

- Why is the + important?

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Super Photon ring-8 GeV       SPring-8

- Super Photon ring-8 GeV**       ****SPring-8**

**Third-generation synchrotron radiation facility**
**Circumference: 1436 m**
**8 GeV**
**100 mA**
**62 beamlines**

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Laser Electron Photon facility at SPring-8

****

**Laser Electron Photon facility at SPring-8**

**in operation since 2000**

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## LEPS detector

- 1m

**TOF**
**wall**

**MWDC 2**

**MWDC 3**

**MWDC 1**

**Dipole Magnet (0.7 T)**

**Liquid Hydrogen**
**Target (50mm thick**)

**Start counter**

**Silicon Vertex**
**Detector**

**Aerogel**
**Cerenkov**
**(n=1.03)**

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Charged particle identification

- (mass) = 30 MeV(typ.) for 1 GeV/c Kaon

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Detected nuclear reactions 

- *N* (1020) *N**K**+**K**- **N*****

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

##  analysis at LEPS at Spring-8.

- LEPS Collaboration (T. Nakano *et al.*), PRL **91**: 012002, 2003; hep-ex/0301020
- Look in  ****12C N K- ****+N K- K+ n
- elementary process:   ****n ******+**K-  nK+K-
- Detect K-, look at missing mass MM****K-
- Remove events with energetic protons
- Estimate background from LH2 target

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## (1520) from LEPS at Spring-8.

- Make Fermi motion correction:
-  ****p  ****(1520)K+  K+(p)K-same nucleon is struck in both cases; know proton.
- Dashed: events where recoil proton detected, shows clear ****(1520)** **peak
- Solid: proton veto showing no ****** **peak

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Observation + from LEPS at Spring-8.

- Apply same Fermi motion correction to MM****K-
- Solid: signal sample
- Dashed: background from protons in upstream H2 target, normalized to signal above 1590 MeV
- 19 +/- 2.8 events above background of 17
- Mass 1540 +/- 10 MeV
- Width < 25 MeV @ 90% CL

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

##  from DIANA@ITEP...

- DIANA Collaboration  hep-ex/0304040
- Xe bubble chamber, 850 MeV K+ beam from proton synchrotron at ITEP
-  K+ Xe  **+ **Xe’  (K0p) Xe’
- 1539 +/- 2 MeV, width < 9 MeV (detector resolution), statistical significance 4.4 .
- Criticism: not exclusive final state...

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## All measured events DIANA@ITEP...

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## ...with cuts to suppress p and K0 reinteractions in Xe nucleus

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## CEBAF Large Acceptance Spectrometer

- CEBAF Large Acceptance Spectrometer

**Electromagnetic calorimeters**
- Lead/scintillator, 1296 photomultipliers

**Gas Cherenkov counters**
- e/ separation, 256 PMTs

**Time-of-flight counters**
- plastic scintillators, 684 photomultipliers

**Drift chambers**
- argon/CO2 gas, 35,000 cells

**Torus magnet**
- 6 superconducting coils

**Liquid D****2 ****(H****2****)target +**
-  start counter; e minitorus

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Event detection in CLAS@JLab

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## The CLAS Photon Tagger

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Exclusive reaction on deuterium

- CLAS Collaboration (S. Stepanyan, K. Hicks, *et al.*), hep-ex/0307018
- Requires FSI – both nucleons involved
- No Fermi motion correction necessary
- FSI not rare: in ~50% of (1520) events both nucleons detected with p>0.2 GeV/c

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Time difference for K-p and K+p

- Time difference for K-p and K+p

**tight timing**

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Neutron found via missing mass

**“****loose” timing cuts**

**“****tight” timing cuts**

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## +: Background Rejection

- Remove events with IM(KK) (1020)
- Remove events with IM(pK) (1520)
- Limit K+ momentum due to MC studies **p****K******** < 1.0GeV/c**

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## M(nK) = MM(dpKX)
~42 events in the narrow peak at 1542+/-5 MeV with FWHM of 21 MeV/c
Estimated significance
    5.3+/-0.5 
Spectrum of the events associated with (1520)

- ~42 events in the narrow peak at 1542+/-5 MeV with FWHM of 21 MeV/c
- Estimated significance
-     5.3+/-0.5 
- Spectrum of the events associated with (1520)

****+ the g2 Exclusive Result

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Variations of cuts for the + analysis 

- M(nK) = MM(dpKX)
- a) no longer has cuts on the (1520) or on the K+ momentum, giving only a 4.8 fit.
- b) has tighter timing cuts, which require that each K+ comes within 0.75 ns of the proton, giving 6.0 .

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## + on hydrogen g6 data in CLAS

****+ on hydrogen g6 data in CLAS

- exclusive channel
****p  ****+K+K- (n)
- Production via *t*-channel K0 exchange
- Largest cross section at big cos equivalent with small *t*(M. Polyakov)

- +

- +

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## +: Channel Identification

- Missing mass selects neutrons:
- p  KKX
- Invariant mass of {K} selects K*0

- neutrons

- K*0

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## +: CLAS proton target

- Result of “g6a&b” analysis of channel  p+KK+(n)
- Invariant mass of {K+n} after selecting
-      cos *(+ K-) > 0.5
- Background shape taken from spectrum without angle (small-*t)*  cut
- Estimate 4.8  significance

- After angle cut

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

##  photoproduction with the SAPHIR detector (Bonn)

- The reaction p  ******+** Ks0,
-      where Ks0 +-
- 	and ******+**  nK+,
- Bremsstrahlung tagged photons have energy up to 2.6 GeV
- The neutron is identified in a kinematical fit

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## The SAPHIR result

- 1540 +/- 4 MeV, width < 25 MeV  (90% CL)

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Neutrino scattering 

- Courtesy of
- Dolgolenko
- (ITEP)

- Reanalysis of
- bubble chamber
- experiments from
- WA21, WA25, WA59,
- E180, E632

- M(Ksp) spectrum

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Additional questions

- The ******+**** **signal was observed on deuteron, nuclear targets, and proton experimentally.
- The existing information does not really answer questions required of a newly discovered subatomic particle:
- Parity and spin
- Isospin
- Width (Lifetime)
- Excited states
- Form factors

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Theoretical interpretations

- Chiral soliton model (Diakonov, Petrov, Polyakov) 1997:
- the original motivation;  is “rotational excitation” of soliton
- Flavor-spin quark interaction could lower the p-wave pentaquark state below the s-wave state (Stancu, Riska)
- Assumes an s-wave NK+ molecule would “fall apart”
- Diquark-triquark structure: 2 quasi-particle (Karliner, Lipkin)
- Diquark (spin-0) and [diquark (spin-1) + s-bar] lower hyperfine
- Double diquark structure: J = 1⁄2+ (Jaffe, Wilczek)
- Spin-0 diquarks act as pseudo-bosons and inhibit decay
- K+N phase shifts reanalyzed (Arndt, Strakovsky, et al.)
- Width of ~few MeV or less, or chi-square increases a lot
- Lattice Gauge calculation (Csikor, Fodor, Katz, Kovacs)
- S=+1 pentaquark J = (1/2)+  has mass 1.54 +/- 0.05 GeV
- Many others !!

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## New data set being analyzed 
Will double the statistics.
New experiment E03-113 (Hicks/Stepanyan) approved by PAC, to run in February 2004 
will provide 20x more statistics.
obtain angular distribution of the decay of + as well as the energy dependence of the cross section.
Continuing analysis effort with existing data
p      K0 shows surprising cross section suppression 

- Will double the statistics.
- New experiment E03-113 (Hicks/Stepanyan) approved by PAC, to run in February 2004
- will provide 20x more statistics.
- obtain angular distribution of the decay of ******+** as well as the energy dependence of the cross section.
- Continuing analysis effort with existing data
- p      K0 shows surprising cross section suppression

- What is next at CLAS?

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)

## Exciting development if holds up!

- Ken Hicks, Ohio University

- Physics School (Bad Honnef, 03/10/7)