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### HYPERNUCLEI

Author: Luka Debenjak

Adviser: doc. dr. Simon Širca

University of Ljubljana

Faculty of mathematics and physics

Contents

- Introduction
- The baryon-baryon interaction
- Production of the hypernuclei
- The hypernuclear g-ray spectroscopy
- Weak decays of hypernuclei
- The (e,e’K) setup at MAMI
- Conclusion

Hypernuclei

Introduction

First hypernucleus found in photoemulsions by Danysz and Pniewski (1952)

p: cosmic ray

A: nucleus

f: track of hypernucleus

B: decay of hypernucleus

Identified reaction:

Hypernuclei

Introduction

- Hypernucleus: nucleus with A and Z with one or more hyperons
- Hyperons (Y): baryons with strangeness:
- p, n, Y: distinguishable particles
placed in independent potential

well with Pauli exclusion principle

Shell model

- Produced hypernucleus can be in an excited state
- Hypernucleus in its ground state: the hyperon occupies the lowest shell (1s)

Hypernuclei

Double L-hypernuclei

S=-2

Studied with

Single L-hypernuclei

S=-1

Studied with

Ordinary nuclei with protons and neutrons

S=0

Hypernuclei

The baryon-baryon interaction

- Why study hypernuclei:
- hyperon inside nucleus as a unique probe of nuclear interior
- enables us to study direct baryon-baryon or meson-baryon interactions

- Difficult to study the YY and NY: no hyperon beams and targets
Hypernuclei as a micro-laboratory for YY and NY interactions

Hypernucleus: nuclear core with hyperon in hyperon–nucleus effective potential

V(r): Gaussian shape, depend on the model

Hypernuclei

Radial integrals of two-body matrix elements

Parameters depend on the model of and

Once the potential (parameters) is set:

Hypernuclei

Production of the hypernuclei

- Strangeness exchange reaction, a)
- Associated production reaction, b)
- Photo-production
- Electro-production, c)

Low recoil momentum: hyperon bound inside the nucleus

Hypernuclei

The (e,e’K) reaction

Experiments done under small angles

- Excellent energy resolution
- Well described by first order
perturbation calculation

(one photon exchange)

Two spectrometers are needed for e’ and K at extremely forward angles

Excellent trajectory reconstructionand particle identification

Excellent resolution

of the energy spectrum

Jefferson Lab, MAMI,…

Hypernuclei

An example of excitation energy spectrum

Best fit: solid line

Theory: dashed line

Ls,Lp: after replacing s- or p-shell proton

Hard to distinguish the finer structure!

g-ray spectroscopy

Hypernuclei

The hypernuclear g-ray spectroscopy

Energy released by neutrons or protons or g-rays when Y lower shells

Limited up to the L p-orbit!

The region of high excitation energy in hypernuclei can not be explored with g-ray spectroscopy

Excellent resolution with Ge detectors (Hyperball)

Hypernuclei

An example of the g-ray spectroscopy of

Small spacing in twin peaks: spin dependent YN

Contribution to the energy spacings:

Hypernuclei

The (e,e’K ) setup at MAMI

- At MAMI spectrometer Kaos is used
- Bmax = 1.95 T pmax= 1.6 GeV/c
- pmax/pmin = 2
- Trajectories measured by:
MWPCs

- Time of flight and trigger information:
segmented scintillator array

- High momentum particle identification (p /K separation):
aerogel threshold Čerenkov detector

- Electron detection:
Scintillation fibres with MAPMT read-out

double spectrometer

Hypernuclei

Conclusion Effective potential as a function of five parameters:

- The main properties of the hypernuclei physics was shown, mostly L-hypernuclei
- Hyperon as a deep nuclear probe for NN, YN, YY interactions
- Studied by
- production mechanisms (small recoil momentum)
- g-ray spectroscopy (limited up to the Lp-orbit)

Hypernuclei

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