16.451 Lecture 14: Two Key Experiments 21/10/2003

News flash from Indiana University:

Tour-de-force experiment: http://www.cerncourier.com/main/article/43/5/4

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???He o4 dd

• isospin-forbidden reaction since T = 0 for the d, 4He, and T=1 for °: “textbook case”

(technically speaking, this reaction breaks “charge symmetry” which is the symmetry under reversal of all up and down quarks in a wave function, or equivalently a quark “isospin flip”. The pion wave function is CS – odd; the others are CS even)

• Charge symmetry is broken by the electromagnetic interaction: up-down quark mass difference, and their electric charge differences

• reaction could proceed with very low cross section compared to isospin-allowed cases, but there was never any convincing evidence published until a couple of weeks ago

• compare similar cross-sections at reaction threshold:

p + d 3He + ° = 13 b (Isospin allowed)

d + d 4He + ° = 13 2 pb (forbidden, new result)

• Rough estimate of cross section ratio :

?102~~ 432

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12

2

coemV

sV

forbidden

allowedif rdV

Comparison of precise measurement and theory, accounting for all known CSBeffects, tests our understanding of CS as a symmetry of the strong interaction

2

0

Cooler CSB

the search for d+d0

Ed StephensonPhysics Colloquium9/24/03

slides courtesy of Dr. E. Stephenson, Indiana University

full set: http://www.iucf.indiana.edu/Experiments/COOLCSB

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CHARGE SYMMETRY BREAKING

atomicnucleus proton

neutron

Simple notion: charge symmetryThe proton and neutron are the sameexcept for electromagnetic properties.

Isospin: the quantum number for CSProton and neutron have T(I) = 1/2

But they are different: mN – mP = 1.3 MeV(The neutron decays in 887 s: n p + e– + νe)

u u

u

d

d d

quarksinsidenucleons:CS saysup anddown arethe sameexceptfor charge

z = 2/3

z = –1/3

Nuclear charge symmetry breaking comes from:

electromagnetic interactions among quarks

md > mu

How much does each contribute?

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Observation of the Isospin-forbiddend+d 4He+π0 Reaction near Threshold

d + d 4He + π0

isospin: 0 0 0 1

CHARGE SYMMETRYsays that the physics isunchanged when protonsand neutrons are swapped,or when up and downquarks are swapped.

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2uu dd

The pionwavefunctionis notsymmetric under up-down exchange.Deuterons and helium reverseexactly. Thus, an observation of thisprocess is also an observation ofcharge symmetry breaking.

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For a reaction to occur in a fixed target experiment, m1 has to hit m2 with enough energy to make the particles in the final state. Theminimum kinetic energy required is called the threshold energy:

Threshold Energy:

m1m2

m3 - mn

Tthr = - Q 2m2

m1 + mfm2 + Q = m1 + mfm2 -

dd + +

Examples:

3Hedp + +

Tthr = 225.4 MeV

Tthr = 198.7 MeV

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Experimental approach:

Search just above threshold (225.5 MeV) (No other π channel open for d+d.) Capture forward-going 4He. Pb-glass arrays for π0 γγ. Efficiency on two sides ~ 1/3. Insensitive to other products (γbeam = 0.51)

6 bend in Cooler straight section Target upstream, surrounded by Pb-glass Magnetic channel to catch 4He (~100 MeV) Reconstruct kinematics from channel time of flight and position.

Target density = 3.1 x 1015

Stored current = 1.4 mALuminosity = 2.7 x 1031 /cm2/s

Expected rate ~ 5 /day

Pb-glass measures photonenergy via Cerenkov lightfrom high energy e- producedin a ‘shower’ initiated by highenergy photon collisions

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d

d

For a fixed target experiment just above threshold,

dd + + in the lab close to threshold:

.

coneangle

particles emerge within a narrow cone about the 0-degree line. (Spectrometer with small forward acceptance will catch every .)

low-energy quickly decays into two photons which emerge nearly back to back in the lab.

Therefore, the apparatus must identify a forward in coincidence with two photons that have a large opening angle between them.

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Target

D2 jet

Pb-glass array

256 detectorsfrom IUCF andANL (Spinka) +scintillators forcosmic trigger

228.5 or 231.8 MeVdeuteron beam

Separation Magnet

removes 4He at 12.5from beam at 6

20 Septum Magnet

FocussingQuads

MWPCs

Scintillator

E-1 MWPC

COOLER-CSB MAGNETIC CHANNELand Pb-GLASS ARRAYS

•separate all 4He for total cross section measurement•determine 4He 4-momentum (using TOF and position)•detect one or both decay ’s from 0 in Pb-glass array

Scintillators

E-2EVeto-1Veto-2

Note: MWPC = multiwire proportional chamber – gives tracking information for trajectory angles

scintillators measure flight time (ToF) velocity momentum

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Pb-glass Hit Patterns

beam goesinto X

0 from p+d 3He+0LEFT RIGHT

cosmic ray muon

color scale: red > pink > blue

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p + d 3He +

Mass (Mev/c2 * 100)

`Missing Mass’ measured with proton beam:

Mass of :

134.98 MeV/c2

Resolution is~ 100 KeV.

conservation of energy:

W Ep + Ed – E(3He) = m

• Ep from beam energy

• deuteron at rest in target

• E(3He) from energy and momentum measured with the magnetic channel

• calculate W from data, should find a peak at the pion mass for reaction at threshold.

• then check in Pb glass array to see if pion was observed

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SEPARATION OF 0 AND EVENTS

MWPC1 X-position (cm)

Y-p

osi

tion

(c

m)

Time of Flight (ΔE1 - ΔE2) (ns)

needed TOFresolution

GAUSS = 100 ps

MWPCspacing= 2 mm

IDEA: Calculate missing mass from the four-momentum measured in the magnetic channel, using time-of-flight for z-axis momentum and MWPC X and Y for transverse momentum. Should see a peak for ° reaction and a broad background from

[Monte Carlo simulation for illustration. Experimental errors included.] 0 peak

TOT = 10 pb

background(16 pb)

missing mass (MeV)

.Background: d + d + 2

Need very good resolution so thatthe peak is detectable!

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RESULTS (measured at two different beam energies)

231.8 MeV50 events

σTOT = 15.1 ± 3.1 pb

228.5 MeV66 events

σTOT = 12.7 ± 2.2 pb

missing mass (MeV)

Events in these spectra must satisfy: correct pulse height in channel scintillators usable wire chamber signals good Pb-glass pulse height and timing

Peaks give the correctπ0 mass with 60 keVerror.

First ever convincingobservation of both the° and reactions!

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Bottom line: time to revise all the textbooks!!!

Back to the neutron:

Lifetime: 885.7 0.8 sec (world average)

eepn

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isospin

Case study: “state of the art” neutron lifetime measurement 15

Outline of the method:

N

dt

dNdecay rate:

eepn

measure rate by counting decay protons in a given time interval (dN/dt) and normalizing to the neutron beam flux (N)

incoming n beam

decay volume, length L

decay proton, to detector

transmitted beam

neutron detector

Ideally done with “cold neutrons”, e.g. from a reactor, moderated in liquid hydrogen...

Issues: 1. precise decay volume ? 2. proton detection ? 3. beam normalization ? ...

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Neutron beam distribution – definitely not monoenergetic:

• ~ MeV neutrons from a reactor are “moderated” by scattering in a large tank of water (“thermal”) or liquid hydrogen (“cold”)

• after many scatterings, they come to thermal equilibrium with the moderator and are extracted down a beamline to the experiment

• velocity distribution is “Maxwellian”: energies in the meV range (kT = 26 meV @ 293K)

Krane, Fig 12.4

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Neutron detection at low energy: (Krane, ch. 12)

• several light nuclei have enormous neutron capture cross sections at low energy: (recall, cross sectional area of a nucleus, e.g. 10B is about 0.2 barns, lecture 4)

• key feature: cross sections scale as 1/velocity at low energy

(barn

s) 10B + n + 7Li + 2.79 MeV

= ovo/v ; o = 3838 b, vo = 2.2 km/s

kinetic energy of ionizedfragments can be convertedinto an electrical signal detector

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Put this together for detector counting rates as shown:

incoming n beam

decay volume, length L

decay proton, to detector

transmitted beam

10B neutron detector

beamdetdet ~

)v(

vN

NN

oo

dt

dNN beam

decay decay

beam

/ N

N

dtdN

N

LN

Nconst

T

LNconstN

decay

detdetbeam )()(

TL v

Decays and transmitted detector counts accumulated for time T

beamN

(DC beam, thermalenergy spread)

neutrons must be captured tobe detected!

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Experimental details (all in vacuum, at ILL reactor, France):

• use Penning trap to confine decay protons

• let them out of the trap after accumulation interval T

• measure the ratio Ndet/Ndecay as a function of trap length L slope gives

protons spiralaround field lineswhen let out of the trap

protondetectordecayN

thin 10B foilto capture beam n’s

particledetectorsfor captureproducts

detN

variable lengthPenning trap(16 electrodes)

L

LN

N

decay

det~

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Amazing results:

proton energyfrom timing:about 34 keV(after kick –raw energy isonly 0.75 keV)

Rate versus trap length L

Result: 893.6 5.3 seconds (1990)

PDG average: 885.7 0.8 (2003)

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