recent results for n interactions - ph.unimelb.edu.aumsevior/qnp2002/qnp2002.pdf · recent results...

Recent results for N interactionsMartin Sevior

[email protected]

University of Melbourne

Quark Nuclear Physics, 2002 – p.1/40

Outline of Presentation.

� Motivations for studying pion induced reactions at Lowenergies.

� The CHAOS detector at TRIUMF.

� Nuclear ( �� ) reactions.

� Pionic Hydrogen measurements.

� Precision � N scattering measurements.

� Precision � �� analyzing power measurements.


Motivations for N Measurements.

� Determination of the �NN coupling constant f�� .

� Measurement of �N scattering lengths as a test ofChPT.

� Measurement of the pion-nucleon

-term as a test of

ChPT and to determine the strange quark content ofthe proton.

� Investigation of isospin breaking.

� Benchmark for other experiments.


The NN coupling constantA fundamental ingredient for Nuclear Physics. Many calculations

have assumed the value of f

�� = 0.079 and which was used as

input for the KH80 Partial Wave Analysis.

Since the early 1990’s many investigators have suggested a

lower value (0.078 - 0.073)

It can obtained via PWA of N measurements and from the

Goldberger-Miyazawa-Oehme sum-rule calculations based on �

scattering lengths.

Precise data for N observables are needed.


The -N term

� �� ! "

It measures the size of the explicit chiral symmetry breaking.

The pion-nucleon sigma-term is the t = 0 value of the scalar form

factor and can be calculated from the mass spectrum of the

Bayrons.

The best ChPT calculation of # gives:# � $% & ' (*) +

This can be related to the pion-Nucleon interaction at the (un-

physical) Cheng-Dashen point of t = 2M

�� . (t = the four momentum

transfer). Quark Nuclear Physics, 2002 – p.5/40

The -N term

, � - �� . / �10 � 243 5 � � ( �� " � # � � ( �� "

. /

is the isoscalar amplitude for 6N scattering. Values for

,

range from 64 MeV (using KH80) to 90 MeV from analyses of

recent data.

This is a long standing discrepency which has been interpreted

as arising from a large static strange quark content in the proton.

However the large valuesof

,recently reported would require

an unphysically large strange quark content.,

requires precision N observables and a sophisticated PWA.

Precise data at low four-momentum transfer are particularly

useful.


The CHAOS collaborationP.A. Amaudruz, P.P.J. Delheij, L. Felawka, G. Hofman, B. Jamieson, D.

Ottewell, M. Pavan, K. Raywood

7

, G.R. Smith8

, K.Babcock, E.L.Mathie, R. Tacik, D.M. Yeomans, H. Xu

9

, J. T. Brack, J. Patterson, R.J.Peterson, R.A. Ristinen

:

, F. Bonutti, P. Camerini, E. Fragiacomo, N.Grion, R. Rui

;

, J. Clark, G. Moloney, M.E. Sevior

<, E.F. Gibson

=

, J.Breitschopf, H. Clement, H. Denz, R. Meier, G.J. Wagner, F. von

Wrochem

>, O. Patarakin

?

TRIUMF, Vancouver BC, Canada

@

Jefferson Lab, Newport News VA, USA

A

University of Regina, Regina, Saskatchewan, Canada

B

University of Colorada, Boulder, Colorado, USA

C

University of INFN Triests, Italy

D

University of Melbourne, Australia

E

California State University, Sacramento CA, USA

F

University of Tubingen, Tubingen, Germany

G

Kurchatov-Institute, Moskow, Russia

H


The CHAOS detector


Features of CHAOS�

Plane Geometry,

I � & � 2 JLK M.

�

Magnetic Field up 1.5 Tesla

�

4 concentric low mass detectors.

�

Particle Identification p, , N, e.

�

Fast 1st level trigger (40 Mhz)

�

Fast flexible 2nd level trigger - decisions in � 10 Nsec.

�

Good data rate, O 800 events/sec, 380 Kbytes/sec to tape.

�

Well developed, flexible software and analysis framework.


CHAOS


Measurements of Nuclear( P )

� Motivated by the work of Chanfrey et al. who firstsuggested that the � � interaction in the isospin 0channel may be substantially modified by the nuclearmedium.

� Data acquired at T � = 280 MeV for ( � / � Q ) and ( � / � /

)for Deuterium, carbon, Calcium and Lead.

� Data were not corrected for CHAOS out of planeacceptance.


CHAOS Acceptance for ( P )


Deuterium ( P )Substantial interference of amplitudes in the isospin 0channel in the low invariant mass region.


Deuterium ( P )The shape of deuterium results are well reproduced by

calculations from Vicnete-Vacis and Oset, Phys. Rev. C60,

064621 (1999)


Nuclear ( P )


Normalized resultsWe can remove acceptance artifacts by forming the ratio:

R S� � � T � ( S� �# SUV � WYX �Z T � ( [� � # [U


Crystal Ball Measurements


Crystal Ball Detector


Crystal Ball Results


Crystal Ball vs CHAOS

The Crystal Ball data confirm the CHAOS results.


Theoretical Calculations of ratios

See Vicente Vacas this conference for latest calculations.

Strong evidence for medium modifications of the both the pion

production process and the I=0 interaction.

Similar effect seen in A( \3 ) reactions.


N scattering experiments

�

Measurements of 6

p differential cross section in the

Coulomb Nuclear Interference region.

�

Measurements of 6^]� analyzing powers at low energy.

�

Improved measurements pionic Hydrogen energy levels and

widths.

�

Precise charge exchange total and differential cross

sections.

�

New charge exchange Analyzing Power measurements.


CHAOS CNI MotivationMeasurement of

_

requires an extrapolation of the isoscalar` a

amplitude to the unphysical Cheng-Dashen point at b =0, t = 2M

8c .The closest experimentally accessible points to this are the dNscattering lengths and forward scattered dN differential cross sectionsat low energy.The former can be measured in pionic Hydrogen, the latter are thesubject of the CHAOS Coulomb Nuclear Interference (CNI)experiment.


CHAOS CNI ExperimentThe difficulty with this experiment is background from e 0gf N /

which looks very similar to forward scattered interfaction.

The magnetic field of CHAOS sweeps most of the decay muons

away from one side of the spectrometer.


CHAOS CNI ExperimentWe still have a substantial flux of muons even after target

traceback and track consistency checks.

We take account of these with special purpose range stack to

distinguish from N’s


Status of CNIData taking is complete after 8 months of runningin 1999/2000 and having acquired 3.2 TBAngular distributions for h i

p and h jp have beenobtained at 15,20, 26, 33, 40, 45, 57 and 67 MeV.Analysis is in progress and will form the Ph.D.thesis of Holger Denz from Tübingen.


Pionic HydrogenThe energy levels and widths of pionic hydgrogen and deuterium fromprovide a unique measurement of the dN scattering lengths at 0energy.Desler-type formula relate the energy shift

kml nand width

ko n

to theelastic and charge exchange scattering lengths.

l 7qpr 7qp s j t u vc wx y c wxz|{ k} i ~�� n

o 7qpr 7qp s � �z�{ k } i }� n � u vc wx y c �� k } i ~�� n � 8

Similar formula’s are obtained for pionic deuterium.

Experiment was performed at PSI. They used the intense low energy

pion beam, a cyclotron trap and bent crystal spectrometer with CCD

readout to measure the X-rays. Quark Nuclear Physics, 2002 – p.27/40

Pionic Hydrogen ResultsPrevious results from PSI are published in H.-Ch. Schröder et al. Eur.Phys. J. C21, 433 (2001). For Pionic Hydrogen they obtain:

l 7qp s k j �� }� �� t � n�� o 7p s k� � � �� n� �

which implies u vc wx y c wx s k� �� d� �� n�� 7c

u vc wx y c �x s k j� � }� � d� �� n�� 7c

For Pionic Deuterium they obtain:

l 7p s k� � t� � � � } � n� �� o 7qp s k } �� } n� �

These define a complex scattering length

u c w � s k j� �� } } n i � k� �� t� � �� } } n�� 7c

Very important to take account of atomic effects that broaden and shift

the X-ray lines.


Next Generation Pionic H

Big focus on atomic effects.

Measurements at a variety of temperatures and pressures

including crygenic targets.


First Measurements


Crystal Ball Charge ExchangeThe Crystal ball experiment has made precision d �� d ¢¡ (chargeexchange) differential cross section measurements.Preliminary results.


Analyzing PowersAnalyzing Powers provide a probe that can be sensitive to weak partialwaves which be determined via interference.The analysing powers for d � £� at low energy and backward angles andfor d a £� at low energy and forward angles are particularly interesting.In these regimes the dominant partial waves are supressed allowingsensitivity to small partial waves.Because of this though the cross sections are very small (down to¤b/sr for d �p at 57 MeV)Require Novel experimental approaches to make the measurements.CHAOS employs a very thin polarized target and detects recoil protonsin the very thin detectors.


Analyzing Powers - CHAOSA large fraction of the CHAOS experimental program has focussed ondetermining these.

¥¦§¨© ª «¬ ® ª¯° ¯ ±²³ ¦° §´ µ¶ ª© · ¸ µ© ¹º¼» ½ ½ º ¯° ¾ ½¿À Á ª ÂÃ Ä ¯° ¾ ½¿À Á ª ÂÃ ÅÇÆ ® ª · µ ±¦ ¾±¦° ª ¦ · È ® ª É Á¿Ê · µ ±¨ È¦ µ° ¯° ¾ È ® ª ¾¯ ·® ª ¾ ±¦° ª ¦ · È ® ª ËÌ ¹Í · µ ±¨ È¦ µ° ÆÎÏ


CHAOS Final resultsI present our final results for these measurements.


p analyzing power measurements at PSI

R. Bilger, H. Clement, M. Cröni, H. Denz, R. Meier, J.Patzold

and G.J. Wagner

Ð

, B. van den Brandt, P. Mantle, J.A. Konter, S.

Mango

�

G. Hofman, M. Pavan

ÑG.R. Smith

Ò

K. Föhl

Ó

J.

Comfort

ÔE. Friedman

Õ

University of Tubingen, Tubingen, Germany

Ð

Paul Scherner Institute, CH-5252 Villigen PSI, Switzerland

TRIUMF, Vancouver BC, Canada

Ñ

Jefferson Lab, Newport News VA, USA

Ò

Department of Physics and Astronomy, Univ. of Edinburgh, UK

Ó

Arizona Stat University, Tempe, AZ 85287, USA

Ô

Racah Institute of Physics, The Hebrew University, Jerusalem, 91904, Isreal

Õ


Experimental ArrangementThere is substantial sensitivity to S-wave N amplitudes for

forward scattered / ]� analyzing powers.

The experiemental challenge is to isolate /p elastic scattering

events from the copius reaction on other nuclei in a polarized

target.

Use a novel polarized scintilation target to do this.


Experimental Arrangement -2

Incident pion

Scattered into LEPS

RecoilProton

PolarizedScintillatorTarget

P signal as function of pulse height cuts on the active

scintillator target.


Experimental Arrangement -3

The other arm is the LEPS spectrometer.


Preliminary ResultsPreliminary Results.


ConclusionsAfter 50 years of investigation there are still interesting

theoretical and experimental questions addressed in Ninteractions.

We may have observed partial restoration of chiral symmetry in

Nuclear ( 3 ) experiment.

Fundamental quantities like the NN coupling constant and theN #-term must be established unambiguosly

We await final results for the forward scattering /

p analysing

powers.

New results for N scattering in the Coulomb Nuclear

Interference region and for pionic hydrogen will provide tight

constraints for these quantities.


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