The SLAC Hybrid Program

Bubble Chamber Physics

From

The mid 1970’s – early 1980’s

DORNAN FEST

Imperial College, September 2009

Andy White, University of Texas at Arlington

Overview

Connections

The SLAC Rapid Cycling Bubble Chamber

BC-60 – Line reversed Regge Interactions

BC-65 – Search for direct lepton production

BC-68 – Exotic meson production in pbar-N

BC-72 – Lifetimes and other properties of charmed particles.

Connections

I first met/worked with Peter when I was at Westfield College – studies of N -> N in resonance region.

I started work at I.C. in Fall 1973…soon asked to go to SLAC to work on new experiment with rapid cycling bubble chamber group – led by Joe Ballam.

Visit to SLAC was initially for “six months” – turned into ~2.5 years!

Great experience at SLAC…the right place to be in 1974 with J/ discovery in November – changed the whole of HEP! (Letter to Peter at I.C.)

I returned to I.C. and worked on SHF experiments analysis, then on to ALEPH…

Connections (2)

Then…LEP proposal era: ELECTRA -> ALEPH

1985 – I had an opportunity for a Research Professor position at the University of Florida – joined D0 at Fermilab, built the Intercryostat Detector.

1991 – I was hired to start a new HEP group at the University of Texas at Arlington – the closest group to the SSC!

1993 After the SSC we joined ATLAS – built the Intermediate Tile Calorimeter.

My group now has 5 faculty, 6 postdocs, 7 graduate students, plus technicians, software experts etc., and the ATLAS Tier 2 center for the southern U.S.

Connections (3)

I had 12 great years at I.C. working with Peter and will always be grateful for the opportunities he gave me, the skills I learned, and the confidence to go forward into a career in HEP.

The SLAC Hybrid Facility

A Rapid Cycling Bubble Chamber

with external charged particle tracking and triggering chambers

and particle i.d.

with the ability to selectively flash the camera lights based on the result of a trigger

algorithm.

The SLAC Hybrid Facility

- Beam (e -> etc.) from End Station C (later B for the back-scattered laser beam)

- Beam pulse ~1.5s, 5cm x 15cm parallel in bubble chamber

- 40-inch Rapid Cycling (≤ 15 Hz) Bubble Chamber

- Bubble chamber magnet ≤ 26 Kg

- Upstream scintillator, Cerenkov, MWPC to define incoming particles

- Downstream MWPC, in fringe field of the magnet, to define trigger particle trajectory and measure momentum.

- Large pressurized Cerenkov tank plus large hodoscope for trigger particle i.d.

- Muon veto (for K-p)

The 40-inch Rapid Cycling Bubble Chamber

Impressive machine at 15 Hz!

The SLAC Hybrid Facility

Triggering the camera lights~2-3 millisconds available from passage of beam until the bubbles start to wander and destroy evidence of charged particle trajectories.

Decision based on incoming/outgoing particle inforation from wire chambers/Cerenkov’s using an algorithm.

A Data General NOVA-840 was available for real-time running of the algorithm. The cycle time was ~1s so there was time for ~2000 cycles.

Peter took up the challenge of fitting the calculation into the available time, writing the code in assembly language!

(I wrote an equivalent FORTRAN version…so our colleagues could understand the algorithm)

The SLAC Hybrid Facility

SLAC Hybrid Facility

~2ms to decide whether to flash lights – otherwise the bubbles move/destroy tracks.

Experiment BC-60

Study of hypercharge exchange reactions using +, K- beams at 7 GeV/c and 11.6 GeV/c.

Trigger particles- require ptrig > 3 GeV/c

Experiment BC-60

Data taking 1976 – 1978

~ 3 million pictures taken

1 picture/ 250 beam pulses (~/10 reduction vs. regular bubble chamber experiment)

Example

Experiment BC-60

The processes +p -> K++ and K-p -> -+ should be dominated at high energy and small |t| by a pair of exchange reggeons (corresponding to the K*(890) and K**(1420)).

For exchange degenerate trajectories with equal residues, equal cross sections and zero polarization are expected (“strong exchange degeneracy”).

For degenerate trajectories, but unequal residues, expect equal cross sections, but non-zero polarizations equal and opposite in sign. (“weak exchange degeneracy”)

[K.W. Lai and J, Louie, Nuc. Phys. B19 (1970) 205]

The SHF for BC-60

Beam i.d. and trajectory

Bubble chamber

Outgoing trigger particle i.d.

Outgoing trigger particle

trajectory

Experiment BC-60

Experiment BC-60Differential cross sections

For +p -> K++ and K-p -> -+ take results of 4-C fits to primary and decay vertices.

Only events with + -> n+ because of problems detecting p0

Simple exponential behavior in low t region, no turnover in forward direction.

K- cross sections have larger energy dependence than +.

Effective trajectories: using d/dt = f(t) s 2eff(t)-2 extract eff(t) from the two energies for each reaction.

Consistent with trajectory through K*(890) and K**(1420) poles.

Consistent with weak exchange degeneracy; some violation at 7 GeV/c, good at 11.6 GeV/c.

Experiment BC-65

Search for direct e production in interactions at 18 GeV/c.

- e/ ratio had been measured at ~10-4 (increasing with decreasing pT)

- Mechanisms proposed to explain ratio: e.m. (l lbar pairs), singles from semi-leptonic decay,…

- e.m. decays of vector mesons (0, 0, 0,…) not sufficient to account for all prompt leptons.

- Other possibilities: Drell-Yan, e.m. decays of “hadron clusters”, internal conversion of massive virtual photons from quark bremstrahlung, decay of charmed particles, heavy leptons, intermediate bosons,…

Experiment BC-65

3 tantalum plates (1 X0 each) were installed in the bubble chamber.

-> advantages of seeing all charged tracks at the interaction vertex, distinguish single e from e+e- pairs.

Beam ~2-3 particles/pulse.

Trigger: beam in miss scintillator algorithm -> cameras

Electron identification:

(a) low energy: spirals, brems, -rays, ionization.

(b) higher energy: e.m. interactions in the plates.

-> separation of e from at better than 1 in 104 level.

Calibration data taken with e and .

Model used for p and n -> 0 interactions in Ta plates.

Calibration data – electron shower in plates

Pion interaction in plates

-p charge exchange interaction in plate

Experiment BC-65

Momentum distribution of misidentified = “electron candidates”

Model of interactions in Ta plates

Experiment BC-65

Results:

22 candidate events found for single e, with all opposite sign tracks identified as

hadrons

Distributions of pT, Evis for showers (in plates) are similar to hadron calibration data, but dissimilar to electron data.

Experiment BC-65

Evidence favors hadronic origin of candidates.

Limit on single e relative to inelastic pion production:

e/ < 2.4 x 10-5

and…limit as function of pT

Relation of results to charm production (decay in semi-leptonic mode):

Charmed meson pair production < 13 b at 90% C.L.

Experiment BC-68

Experiment BC-68

History: A neutral state with M ~2.02 GeV/c2, coupled to NNbar had been observed in an earlier experiment but not subsequently confirmed.

at 6, 9 GeV/c

using the SHF with Cerenkov’s, upstream for pbar/- separation, and downstream for /(K or p) separation, and hydrogen and deuterium bubble chamber fills.

[Original results reported by P. Benkheiri et al., P.L. B 69 (1977) 483

-> two narrow p-pbar states at 2.02 and 2.20 GeV/c2 in -p at 9 and 12 GeV/c.]

Our experiment

Experiment BC-68

Search for Baryonium State(s)

q

q

qbar

qbar

Angular momentum

Experiment BC-68

Suppress background where pbar is diffractively produced or by meson exchange

Experiment BC-068

Combined significance 7.6 s.d. !

2.02 GeV/c2

Our paper

Experiment BC-72

Experiment BC-072- SHF was moved from End station C line to End station B line

- Searching for short decay lengths in the bubble chamber.

- Added special high resolution camera (resolution ~ 45m) to see production/decay vertices separated.

- 2.4 million triggered pictures -> 0.58 million hadronic interactions

- Bubble chamber operated in mode to retard bubble growth and increase bubble density. Run at 10-12 Hz.

Experiment BC-72

Experiment BC-72

Experiment BC-72

Detail of lead-glass stack

Experiment BC-072

- Trigger – two components (OR-ed):

1) Minimum energy deposition in the lead glass system

2) Set of hits in downstream PWC’s consistent with track coming from BC fiducial volume.

- Data taken Summer 1980 – Spring 1982.

- Double scan of film from high resolution camera

- Charm candidates: visible decay, or significant impact parameter

- High magnification prints made of charm candidates -> measured

- 43 “topologically unambiguous” decays found

Experiment BC-72

Experiment BC-72

Experiment BC-072

Experiment BC-72

- 62 Charm events found

- Using the unambiguous 22 neutral and 21 charged decays:

(D0) = (6.8+2.3-1.8) x 10-13s, (D) = (7.4+2.3

-2.0) x 10-13s

(D0)/ (D) = 1.1+0.6-0.3

Inclusive charm (20 GeV) = 56+24-23 nb.

- Evidence of a non D Dbar component to charm production

(some +c and some D*++)

Experiment BC-72

The SLAC Hybrid Facility

Final personal note:

I am now regularly visiting SLAC again, working on the development of Digital Hadron Calorimetry for the SiD Detector Concept for the International Linear Collider.

I am working with some of the same people we worked with ~30 years ago on the SHF: Ken Moffeit, Jim Brau, Dieter Freytag and in the same building.

- it brings back a lot of good memories of working with Peter and I.C. on the SHF!