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Home , W, W and Z bosons

Discovery of the W± and Z0

Status of the ~1980

Planning the Search for W± and Z0

SppS, UA1 and UA2

The analyses and the observed events

First measurements of W± and Z0

1 A Bit of History...

1960’: Glashow, Salam, Weinberg: electroweak unification: • consistent with observed interactions (exchange of W± ):

Þ

• But: predicted interactions (exchange of , Zo) which had never been observed...

2 Neutral Currents

• Until 1973 all observed weak interactions were consistent with only a charged boson. • CERN, 1973: first neutral current interaction observed (see Martin & Shaw, p. 185): - nm + nucleus ® nm + p + p + p • suddenly very urgent to observe W±, Zo bosons directly to test electroweak theory.

3 Planning the Search

To find W ± and Z0, needed to understand: – how they could be produced • in order to find or design a collider that could create them. – how they would decay • in order to design detectors that could see them.

4 W and Production

• For W± and Z0, electroweak theory predicted: – their masses: W+ • mW± @ 83 ± 3 GeV +, • mZ0 @ 94 ± 3 GeV – they would couple to Z0 and . l-, q Þ Needed a or collider capable of creating with masses in the 100-GeV range. 5 SPS ® SppS

• No machine in existence could reach this energy – CERN’s ISR collider (p-p): Ös = 61 GeV – some new colliders being planned (.g. LEP) but would not be ready soon – SPS was a accelerator for fixed target

experiments (Eproton = 400 GeV; but for fixed target Ös = Ö2mE, = of target Þ too low!) Þ Rubbia, van der Meer: upgrade SPS into SppS (Super proton- Synchrotron)

6 The SppS

· Ös = 540 GeV • 3 bunches , 3 bunches , 1011 particles per bunch • Luminosity = 5 1027 cm-2sec-1 • first collisions in December 1981

7 W and Z Decay Modes

W + ® Z0 ® + + - • e ne • l l (l = e, m, ) + • m nm • nl nl (l = e, m, t) + • t nt • q q (q = u,d,,s,t,) • u d • c s • t b W - ® the conjugates of the

above 8 Choosing a Decay Mode

Recall that at p-p() colliders: – most events are due to soft collisions, which have low-p^ final-state particles.

– much high-p^ background is jets from and scattering (‘QCD background’). ß Most promising decay modes are the leptonic ones. 9 Signatures in the Detectors

+ + 0 + - W ® l nl Z ® l l

• a high-p^ • two or two or with – high p • large missing E^ ^ (due to the ) – opposite charge

10 Detector Requirements

The detector(s) must be capable of: – charged lepton • detection, • identification, • and/or energy measurement.

– Missing-E^ measurement.

11 The SppS Experiments

6 detectors: – UA1, UA2, UA3, UA4, UA5, UA6 * – UA1 and UA2 were the only ones able to see W±, Z0

* UA = Underground Area.

12 The UA1 Detector

• all-purpose detector • Excellent hermeticity (i.e. very few gaps) - good for missing E^ measurement • tracker and electromagnetic calorimeter immersed in magnetic field • Magnet return yoke = hadronic calorimeter • 8-layer muon detector

13 UA1, cont’d

Advantages Disadvantages

• magnetic field in • ecal not great: tracker – poor granularity • hermetic – no position detection in barrel • muon detection – difficult to calibrate

14 UA1’s W ® e n Search

• Using 18 nb-1 (~109 collisions) from late 1982...

• Need a high-p^ electron. Looked for events with:

– an ecal cluster with E^ > 15 GeV

– an isolated high-p^ track pointing to cluster – ecal energy measurement matches tracker energy measurement – no associated energy in hadronic calorimeter Þ 39 such events ! 15 UA1’s W ® e n Search, cont’d

• Looked closely at those 39 events: – 5 events had: • no jets

• missing E^ @ electron E^. – The other 34 events had: • one or two jets • no missing energy. • Similar analysis performed on end-cap region yielded one more event with an electron and no jets. • Parallel analysis concentrating on finding events with

missing E^ yielded the same 6 events.

16 UA1’s W ® e n Search: Background Evaluations

Could these events be something other than W±s?

– A high-p^ hadron or mostly-neutral jet misidentified as an electron? – p0,h0 or g ® e+e-, with one e missed? – Jet with electron (rest undetected) + jet with (rest undetected)? • Used knowledge of: – detector response – expected rates of such background events – deliberate searches for such background events in the data to conclude that these backgrounds were negligible.

17 UA1 announced their observation the following February:

Physics Letters 122B (1983) p103:

“Experimental Observation of Isolated Large Transverse Energy Electrons with Associated

Missing Energy at Ös = 540 GeV’’

18 The UA2 Detector

• Principally for W±, Z0 decays to

high-p^ electrons • well instrumented in central region • Inner tracker – no central magnetic field

Þvertexing only for high-p^ tracks • finely-segmented calorimeters – electron ID – energy measurement

19 UA2, cont’d

Advantages Disadvantages

• good ecal, esp. in • no magnetic field in barrel region: central region – good granularity • no endcap-region – tower structure points to origin calorimetry – everything could be • no muon detection calibrated in-beam

20 UA2’s W ® e n Search

UA2 performed a similar analysis on the data they collected during the same period (November- December 1982), and found 4 W ® e n events.

Physics Letters 122B (1983) p476:

“Observation of Single Isolated Electrons of High Transverse Momentum in Events with Missing Transverse Energy at the CERN pp Collider’’

21 Z search

Z0 ® leptons rarer than W± ® leptons Þ no Z0 discovery in 1982 data.

More collisions in 1983 produced enough additional data for Z0 to be observed...

22 Observation of Z0

• For Z0 ® e+e-, need:

– one high-p^ electron and one high-p^ , chosen ~ as for W± search.

– No missing E^. – UA1: 3 events, UA2: 4 events • For Z0 ® m+m- (UA1 only), need:

– two oppositely-charged isolated high-p^ tracks in central tracker with matching tracks in muon chambers

– no missing E^. – 1 event found 23 Determination of W Mass

• Two methods:

– lepton E^ spectrum

peaks at mw/2 Þ • compare measurement to Monte Carlo prediction • can be affected by transverse momentum of W – transverse mass method (see next slide...)

24 W Transverse Mass

In the plane transverse to the beam:

e p^

n p^

2 e n 2 e n 2 – MT = (E^ + E^ ) - (p^ + p^ )

– neglecting me,mn: 2 e n mT = 2E^ E^ (1-cosf) • compare measurement to Monte Carlo prediction • ~independent of transverse momentum of W± 25 Determination of Z Mass

Invariant mass of the lepton system

forms a peak at mz0 .

26 (see Martin & Shaw, Appendix A)

• Consider a system of N particles:

E = E1 + E2 + … + EN

p = p1 + p2 + … + pN • The invariant mass of the system (M) is defined by: M2c4 = E2 - |p|2c2 • M has the same value in any reference frame.

27 W, Z Mass Measurements

Using all data from 1982-3, and combining results from UA1 and UA2:

mW± = 82.1 ± 1.7 GeV

mZ0 = 93.0 ± 1.7 GeV Current values:

mW± = 80.43 ± 0.04 GeV

mZ0 = 91.188 ± 0.002 GeV

28