# Presentation: 043885

## Pentaquarks:Discovering new particles

## Outline

- Historical introduction
- Review of the Quark Model
- Prediction of the +
- Experimental evidence
- Data from CLAS
- What do we know about the +?
- Other 5-quark states

- S=+1
- B=+1
- Mass = 1.54 GeV

## Families within families of matter

- Molecule

- Nucleus

- Quark

- DNA

- Atom

- Proton

- 10-7 m

- 10-9 m

- 10-10 m

- 10-14 m

- 10-15 m

- <10-18 m

## Families of atoms

- Mendeleev (1869)

## Properties of quarks

| Quark Flavor | Charge (Q) | Baryon number | Strangeness (S) |
| --- | --- | --- | --- |
| u | +2/3 | +1/3 | 0 |
| d | −1/3 | +1/3 | 0 |
| s | −1/3 | +1/3 | −1 |
| u | − 2/3 | −1/3 | 0 |
| d | +1/3 | −1/3 | 0 |
| s | +1/3 | −1/3 | +1 |

- Protons are made of (*uud*)
- Neutrons are made of (*ddu*)

- p

- K+

- K−

- n

## Families of quarks

- S= 0

- S=−1

- S=+1

- I3 = Q ─ 1⁄2 (B+S)

- +1⁄2

- −1⁄2

- 0

## Families of baryons

- ?

## Production and decay of ─ → ─

**V.E. Barnes et. al., Phys. Rev. Lett. 8, 204 (1964)**

## Interactions understood in terms of quarks

- Very high energy

- “free” quarks not found, only
-  particles that
- contain quarks

## Electromagnetic and color forces

- QCD explains particle interactions

## Quarks are confined inside colorless hadrons

- Mystery remains:
- Of the many possibilities for
- combining quarks with color into
- colorless hadrons, only two
- configurations were found, till now...

**Particle Data Group ****1986**** reviewing evidence for***** exotic baryons states***
***“...******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. ***
**PDG dropped the discussion on pentaquark searches after ****1988****.**

## What are pentaquarks?

- Minimum quark content is 4 quarks and 1 antiquark
- “Exotic” pentaquarks are those where the antiquark has a different flavor than the other 4 quarks
- Quantum numbers cannot be defined by 3 quarks alone.

- Example: uudds, exotic

- Baryon number = 1/3 + 1/3 + 1/3 + 1/3 – 1/3 = 1

- Strangeness = 0 + 0 + 0 + 0 + 1 = +1

- Example: uudss, non-exotic

- Baryon number = 1/3 + 1/3 + 1/3 + 1/3 – 1/3 = 1

- Strangeness = 0 + 0 + 0 − 1 + 1 = 0

## Hadron multiplets

- Baryons qqq

## The Anti-decuplet predicted by Diakonov et al.

- In the Chiral Soliton Model, nucleons and Deltas are
- rotational states of the same soliton field.

- The mass splittings are
- predicted to be equally
- spaced

- ~15 MeV

- ~140 MeV

**Z.Phys. A359, 305 (1997)**

**Z****+**

- Anchor

## Quark lines for the reaction

**Quark lines for the reaction**

- n

- K+

- K−

****

********

- n

- ddu

- ddu

- + is composed of (uudds) quarks

## Production mechanisms

## Experimental Evidence

- Several experimental observations
- No dedicated experiments to date
- Walk through the analysis from CLAS

## Exclusive measurement using d reactions

- CLAS Collaboration (S. Stepanyan, K. Hicks, *et al.*), hep-ex/0307018
- Requires FSI – both nucleons involved
- No Fermi motion correction necessary
- FSI puts K- at larger lab angles: better CLAS acceptance
- FSI not rare: in ~50% of (1520) events both nucleons detected with
-     p > 0.15 GeV/c

## JLab accelerator CEBAF

## CEBAF Large Acceptance Spectrometer

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

**Torus magnet**
- 6 superconducting coils

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

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

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

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

## d →  p K+K─ (n) in CLAS

## Particle identification by time-of-flight

## “Kaon” vertex times relative to proton

- t (p-K─)
- (ns)

- t (p-K+) (ns)

- pK+K─

## Reaction d→pK+K-(n)

- Clear peak at neutron mass.
- 15% non-pKK events within ±3 of the peak.
- Almost no background under the neutron peak after event selection with tight timing cut.

- Reconstructed Neutrons

## Identification of known resonances

- Remove events with IM(KK) → (1020) by IM > 1.07 GeV
- Remove events with IM(pK) → (1520)
- Limit K+ momentum due to d →p K phase space pK < 1.0GeV/c

## Deuterium: nK+ invariant mass distribution

- Distribution of (1520) events

******+**

- Mass = 1.542 GeV
- < 21 MeV
- Significance 5.2±0.6 

## Searching for the + on a proton target

## First observation of + at SPring-8

******1.54********0.01 MeV**
** ****< 25 MeV**
**Gaussian significance 4.6******

**background**

**Phys.Rev.Lett. 91 (2003) 012002**

## DIANA at ITEP 850 MeV K+ beam

- All Events

- Cuts to suppress p and K0
- reinteraction in Xe nucleus

- K+ Xe  **+ **N(K0p) N

**hep-ex/0304040**

**M=1539****±****2 MeV**
**** **< 9 MeV**

## SAPHIR detector at ELSA

- The reaction p  ******+** Ks0,  where Ks0 +- and ******+**  nK+

- M=1540±4 MeV
-  < 25 MeV

## Reanalysis of bubble chamber neutrino data

- Enlargement of signal region

- M = 1533±5 MeV,  < 20 MeV

**ITEP group: hep-ex/0309042**

-  Ne,D  ****Ks0p) X

## HERMES

## What do we know about this S=+1 state?

**LEPS      : ********C****→****(nK****+****)**** K****−****X**

- Number of Events in Peak

- Mass (GeV)

**DIANA   : ****K****+****Xe→****(****pK****0****)**** X**

**CLAS     : ********d****→****(nK****+****)**** K****−****p**

**SAPHIR  : ********p****→****(nK****+****)**** K****0**

**ITEP      : ************d,Ne****→****(pK****0****)**** K****0**

**HERMES  : ****e****+****d****→****(pK****0****)**** X**

- Parenthesis show +
- decay products

- Searches for isospin pK+
- partner have found nothing

## There is much more to learn

- Spin, parity
- Chiral soliton model predicts Jpc=1⁄2+ (p-wave)
- Quark model naïve expectation is Jpc=1⁄2− (s-wave)
- Isospin
- Likely I=0, as predicted by the chiral soliton model.
- Width (lifetime)
- Measurements limited by experimental resolution.
- Naïve estimates predict ~200 MeV.
- Theoretical problem remains why the state is so narrow.
- Analysis of existing K+d scattering data indicate that         < 1-2 MeV. (e.g. nucl-th/0308012)
- Complete determination of the pentaquark multiplet

## A di-quark model for pentaquarks

**JW hep-ph/0307341**

**JM hep-ph/0308286**

- L=1, one unit of orbital angular momentum needed to get J=1/2+ as in SM
- Lattice QCD: JP = 1/2−

**SZ hep-ph/0310270**

- Mass Prediction for ******−−**** **is 1.75 instead of 2.07 GeV

## A new cousin: observation of exotic −−

- (1530)

**CERN SPS hep-ex/0310014**

## Current activities 

- Workshop@ JLab Penta-Quark 2003, Nov 6-8
- www.jlab.org/intralab/calendar/archive03/pentaquark/
- Approved experiment for 30 PAC days with deuterium target scheduled for early next year (spokespersons Ken Hicks, S. Stepanyan)
- New proposals under discussion, for example to search for −−...
- Theory papers appearing daily on the preprint server.
- Of course there is worldwide interest to continue to probe the nature of pentaquarks experimentally.

## Summary

- A key question in non-perturbative QCD is the structure of hadrons
- We have presented evidence for an exotic baryon with
-     S = +1, which would have a minimal quark content of five quarks (uudds).
- This baryon represents a new class of colorless hadrons.
- The additional observation of a doubly negative S=−2 baryon (ddssu) establishes a second corner of the anti-decuplet  the family of pentaquarks.
- The observation of new members of the family of 5-quark states gives credibility to the existence of pentaquarks.
- Hadronic physics is being driven by experimental discoveries.
- Electromagnetic probes are playing a major role in the investigation of these new states.

- Index of popular articles on the Pentaquark **http://www.jlab.org/news/articles/**