1. 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 1

2. isospin-forbidden reaction since T = 0 for the d, 4 He, 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  3 He +  °  = 13  b (Isospin allowed) d + d  4 He +  °  = 13  2 pb (forbidden, new result) Rough estimate of cross section ratio : Comparison of precise measurement and theory, accounting for all known CSB effects, tests our understanding of CS as a symmetry of the strong interaction 2

3. 00 Cooler CSB the search for d+d  0 Ed Stephenson Physics Colloquium 9/24/03 slides courtesy of Dr. E. Stephenson, Indiana University full set: http://www.iucf.indiana.edu/Experiments/COOLCSB 3

4. CHARGE SYMMETRY BREAKING atomic nucleus proton neutron Simple notion: charge symmetry The proton and neutron are the same except for electromagnetic properties. Isospin: the quantum number for CS Proton and neutron have T(I) = 1/2 But they are different: m N – m P = 1.3 MeV (The neutron decays in 887 s: n  p + e – + ν e ) uu u d dd quarks inside nucleons: CS says up and down are the same except for charge z = 2/3 z = –1/3 Nuclear charge symmetry breaking comes from: electromagnetic interactions among quarks m d > m u How much does each contribute? 4

5. Observation of the Isospin-forbidden d+d  4 He+π 0 Reaction near Threshold d + d  4 He + π 0 isospin: 0 0 0 1 CHARGE SYMMETRY says that the physics is unchanged when protons and neutrons are swapped, or when up and down quarks are swapped. The pion wavefunction is not symmetric under up-down exchange. Deuterons and helium reverse exactly. Thus, an observation of this process is also an observation of charge symmetry breaking. 5

6. For a reaction to occur in a fixed target experiment, m 1 has to hit m 2 with enough energy to make the particles in the final state. The minimum kinetic energy required is called the threshold energy: Threshold Energy: m1m1 m2m2 m 3 - m n T thr = - Q 2m 2 m 1 +  m f m 2 + Q = m 1 +  m f m2 -m2 -    dd++ Examples:   3 Hedp++ T thr = 225.4 MeV T thr = 198.7 MeV 6

7. Experimental approach: Search just above threshold (225.5 MeV) (No other π channel open for d+d.) Capture forward-going 4 He. Pb-glass arrays for π 0  γγ. Efficiency on two sides ~ 1/3. Insensitive to other products (γ beam = 0.51) 6  bend in Cooler straight section Target upstream, surrounded by Pb-glass Magnetic channel to catch 4 He (~100 MeV) Reconstruct kinematics from channel time of flight and position. Target density = 3.1 x 10 15 Stored current = 1.4 mA Luminosity = 2.7 x 10 31 /cm 2 /s Expected rate ~ 5 /day Pb-glass measures photon energy via Cerenkov light from high energy e- produced in a ‘shower’ initiated by high energy photon collisions 7

8.  d   d For a fixed target experiment just above threshold,    dd++ in the lab close to threshold:.   cone angle  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. 8

9. Target D 2 jet Pb-glass array 256 detectors from IUCF and ANL (Spinka) + scintillators for cosmic trigger 228.5 or 231.8 MeV deuteron beam Separation Magnet removes 4 He at 12.5  from beam at 6  20  Septum Magnet Focussing Quads MWPCs Scintillator  E-1 MWPC COOLER-CSB MAGNETIC CHANNEL and Pb-GLASS ARRAYS separate all 4 He for total cross section measurement determine 4 He 4-momentum (using TOF and position) detect one or both decay  ’s from  0 in Pb-glass array Scintillators  E-2 E Veto-1 Veto-2 Note: MWPC = multiwire proportional chamber – gives tracking information for trajectory angles scintillators measure flight time (ToF)  velocity  momentum 9

10. Pb-glass Hit Patterns beam goes into X  0   from p+d  3 He+  0 LEFTRIGHT cosmic ray muon color scale: red > pink > blue 10

11. p + d 3 He +   Mass (Mev/c 2 * 100) `Missing Mass’ measured with proton beam: Mass of   : 134.98 MeV/c 2 Resolution is ~ 100 KeV. conservation of energy: W  E p + E d – E( 3 He) = m  E p from beam energy deuteron at rest in target E( 3 He) 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 11

12. SEPARATION OF  0 AND  EVENTS MWPC 1 X-position (cm) Y-position (cm) Time of Flight (ΔE 1 - ΔE 2 ) (ns) needed TOF resolution  GAUSS = 100 ps MWPC spacing = 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 that the peak is detectable! 12

13. RESULTS (measured at two different beam energies) 231.8 MeV 50 events σ TOT = 15.1 ± 3.1 pb 228.5 MeV 66 events σ TOT = 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 correct π 0 mass with 60 keV error. First ever convincing observation of both the  ° and  reactions! 13 Bottom line: time to revise all the textbooks!!!

14. Back to the neutron: Lifetime: 885.7  0.8 sec (world average) 14 isospin

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

16. Outline of the method: decay rate: 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 ?... 16

17. 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 17

18. 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. 10 B is about 0.2 barns, lecture 4) key feature: cross sections scale as 1/velocity at low energy (barns) 10 B + n   + 7 Li + 2.79 MeV  =  o v o /v ;  o = 3838 b, v o = 2.2 km/s kinetic energy of ionized fragments can be converted into an electrical signal  detector 18

19. Put this together for detector counting rates as shown: incoming n beam decay volume, length L decay proton, to detector transmitted beam 10 B neutron detector Decays and transmitted detector counts accumulated for time T (DC beam, thermal energy spread) neutrons must be captured to be detected! 19

20. 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 N det /N decay as a function of trap length L  slope gives  protons spiral around field lines when let out of the trap proton detector thin 10 B foil to capture beam n’s  particle detectors for capture products variable length Penning trap (16 electrodes) 20

21. Amazing results: proton energy from timing: about 34 keV (after kick – raw energy is only 0.75 keV) Rate versus trap length L Result: 893.6  5.3 seconds (1990) PDG average: 885.7  0.8 (2003) 21