UA2 at the CERN proton – antiproton collider

Luigi Di Lella

Physics Dept., University of Pisa

Daniel Foidevaux Fest

November 22, 2019

first measurement of the weak mixing angle qw

first quantitative prediction of the W± and Z mass values:

1973 Discovery of neutral – current neutrino interactions at CERN:

the first experimental evidence in favour of the unified

electro-weak theory.

mW = 60 – 80 GeV

mZ = 75 – 95 GeV

too large to be produced by any existing accelerators

The ideal machine to produce and study the W and Z bosons

in the most convenient experimental conditions: a high-energy e+e- collider

Zee -+ -+-+ WWee

still far in the future in the 1970’s (first operation of LEP in 1989)

1976: THE SHORTCUT TO W AND Z PRODUCTION(presented at the Neutrino 76 conference in Aachen)

Collider luminosity needed for

W and Z production requires to

increase the p phase-space density

between p production and injection

into the high-energy ring by ≥108

(‘’cooling’’)

Meeting of the CERN Research Board, November 25, 1976

ICE experiment:

“Initial Cooling

Experiment”

Recommendations:

To carry out an experiment on the cooling of protons;

To undertake a feasibility study of the construction of a system

of cooling and transfer rings to enable the injection of an intense

antiproton beam into the SPS.

Meeting of the CERN Research Board, May 25, 1978

Results from the ICE experiment (reported by S. van der Meer):

successful cooling achieved using the stochastic method

Recommend that CERN should go ahead with p p beams in the SPS;

Concern that the inclusion of a second intersection region may not be possible

due to budgetary limitations.

Meanwhile, two proposals for p p collider experiments

had been proposed to the SPS Experiments Committee

UA2 detector 1981 – 1985

Central region : track detector (“vertex detector”);

“preshower” detector;

electromagnetic and hadronic calorimeters:

each calorimeter cell: Dq = 10° , Df = 15°;no magnetic field.

20° – 40° regions : toroidal magnetic field (2 x 12 coils);track and preshower detectors;electromagnetic calorimetry (24X0 + 6X0):each calorimeter cell: Dq = 3.5° , Df = 15°;

No muon detectors

1981 – 82 configuration:

4 azimuthal sectors of the central calorimeter replaced by

an open magnetic spectrometer at 90° to the beam line

to study charged particle and p0 production at large angles

End of the 1982 run:

Removal of the magnetic spectrometer;

Full azimuthal coverage of the central calorimeter.

B

Meeting of the Research Board, June 29, 1978

Approval of Proposal P92, becoming the UA1 experiment

(UA1: Underground Area 1)

Proposal P93 is considered as a good experiment, which should

be considered for approval if budgetary conditions permit.

Meeting of the Research Board, December 14, 1978

Resources for a second intersection region are finally found

by the CERN Management.

Approval of Proposal P93, becoming the UA2 experiment

(following a “shoot-out” with another experimental proposal

which had been submitted two months earlier).

At the end of 1978, the UA2 Collaboration consisted of 6 Institutes:

Bern, CERN, Copenhagen (NBI), Orsay, Pavia, Saclay

~60 phycisists

Main Task of the LAL-Orsay group:

construction of the central tracker

(“vertex detector”):

Four Multiwire Proportional Chambers (MWPC)

with cathode strip readout;

A hodoscope of 24 scintillator strips;

Two drift chambers with 24 azimuthal cells,

6 wires / cell, two-dimensional information

by charge division;

A Tungsten converter 1.5 X0 thick, followed

by a fifth MWPC.

The UA2 vertex detector during installation (1981)

The UA2 detector

before data – taking in 1983

Sketch of the CERN accelerators in the early 1980s

CERN ANTIPROTON – PROTON COLLIDER (SppS) OPERATION

1981 - 90

1986: Upgrade of the antiproton source by the addition of

an Antiproton Collector ring (ACOL) around the Antiproton

Accumulator (AA) Luminosity increase by a factor ~5

1991: end of SppS operation (no longer competitive with Fermilab

Tevatron pp collider at 1.8 TeV total energy)

W discovery

Z discovery

Upgraded detector

First UA2 data-taking run (Autumn 1981):Evidence for hadronic jet production at high transverse momentum(the highlight at the International Conference on High-Energy Physics, Paris,1982)

Distribution of the

total transverse energy

(sum over the central

calorimeter cells)

SET (GeV)

Reconstruct cluster transverse energies:

sum over adjacent cells with E > 0.4 GeV

.....)3()2()1( TTT EEE

)(n

TE : transverse energy of nth cluster

T

n

Tn

E

Eh

)(

Define

Events with SET > 100 GeV

consist mainly of two high ET clusters

SET (GeV)

fra

ctio

n

h1

h1+h2

Transverse energy distribution

in the (q,f) plane

for four typical events

with SET > 100 GeV

Two-jet event

projection in a plane

perpendicular to the beam line

Events with SET > 100 GeV:

azimuthal separation DF12

between the two leading clusters

00° 90° 180°

DF12

Search for W e n

Methods for electron identification at high pT in absence of magnetic field

( with important contributions from Daniel ):

Calorimeter cluster consistent with single particle electromagnetic shower;

Isolated track matching the calorimeter cluster. Track ionization consistent

with 1 MIP (to reject photon conversions);

Track should match in space a charged cluster in the preshower detector

corresponding to ionization > 4 MIP.

Hadronic leakage

Energy in e.m. calorimeter

Pulse heights measured in preshower

1.5 X0 tungsten cylinder

Isolated track matching calorimeter cluster

A dangerous background:

isolated track close in space

to a high pT p0

(‘’overlap’’ background)

The search for W e n events in the 1982 data sample was based

on two parallel approaches:

Event selection by computer programs using electron identification

algorithms;

Visual event scanning on a loosely selected event sample using a

three-dimensional event display (Megatek) to validate and refine

the selection algorithms.

Daniel was one of the leading physicists in this effort, spending days and

nights at the Megatek display to understand the response of the central

tracking and preshower detector.

By the end of 1982 we had a convincing signal, which was presented

first at 3rd Topical Workshop on Proton-Antiproton Collider Physics in

Rome, January 12-14, 1983 (without claiming a discovery), and then

at a CERN seminar on January 21, 1983 (one day after the announcement

by UA1).

UA2 result from the analysis of the 1982 data

Six events containing an electron with pT 15 GeV

missing pT

electron pT

electron pT(GeV)

A paper was submitted to Phys. Lett. B on February 15, 1983

Photos from 1983

Winter meeting,

Moriond 1983

1983 collider run:

Detection of Z e+ e− decay

One Z e+ + e- candidate event was already present in the1982 data

It was presented at the 3rd “Topical Workshop on proton-antiproton

collider physics”, Rome, 12 – 14 janvier 1983:

“In the process of searching for electron pairs having a mass in excess

of 40 GeV, 27 events were retained for detailed inspection, none of which

could be considered as a legitimate e+e- candidate.

The event having M12 = 86 GeV has a good electron candidate in the central

calorimeter but its partner in the forward detector does not behave as a

single electron.”

27 events selected by calorimetric criteria

The only event with one identified electron

Details of the ‘Rome’ event

Electron track in the central region

Response of thepreshower detectorto the other track

in the forward region(most likely a genuine electron hitting one of the toroidal coils

near its surface)

Track identified as an isolated electron

pointing to both energy clusters

Two energy clusters with pT 25 GeV

in electromagnetic calorimeters;

energy leakage in hadronic calorimeters

consistent with electrons

A track identified as an isolated electron

pointing to at least one of the two clusters

24 events

8 events

mZ = 91.9 ± 1.3 ± 1.4 GeV(stat) (syst)

Search for Z e+ e− decay in the 1982 – 83 data

(with Daniel’s crucial contributions to electron identification)

The 8 final events include the ‘Rome’ event

A paper was submitted to Physics Letters B in August 1983

One of the 8 events is a Z e+e– g decaywith a hard photon (24 GeV) well separated

from the nearer electron.

Estimated probability from radiative corrections:

~ 1/200 Z e+e – (g) decays.

Nevertheless, several theoretical papers

were published interpreting this event in terms

of new physics beyond the Standard Model.

At the end of UA2 (1990), the final Z e+e–

decay sample consisted of ~250 events

with no other e+e– g event with non-collinear,

hard photons.

BEWARE OF STATISTICAL FLUCTUATIONS !

In 1984 three more groups joined the UA2 collaboration:

Heidelberg, Perugia, Pisa

UA2 collaboration meeting in Heidelberg

UA2 W e n and Z e+ e– event samples at the end of 1985

(Integrated luminosity: 142 nb–1 at 546 GeV; 768 nb–1 at 630 GeV)

mW = 80.2 ± 0.6 ± 0.5 ± 1.3 GeV

mZ = 91.5 ±1.2 ± 1.7 GeV

UA2 also measured:

W e n charge asymmetry in the forward directions

expected from V – A ;

W, Z production cross-sections and pT distributions.

Two-jet invariant mass distribution

First evidence for W and Z decay to q q pairs 2 hadronic jets(UA2 Collaboration, Phys.Lett.B 186 (1987) 452

Daniel’s contributions to the first phase of the UA2 experiment:

W and Z studies;

Search for exotic processesExamples:

Providing help and guuidance to Ph.D. students

(co-director of Lydia Fayard’s Ph. D. thesis)

Invited talk at the 5th Int. Conference on Physics in Collision, July 1985:

« Review of W and Z physics at the CERN p p Collider »

• Search for excited electrons: W e* n e g n ;

• Search for SuperSymmetric particles: 𝑍 → ෪𝑒+ ෦𝑒− → 𝑒+ 𝑒− 𝛾 𝛾 ;

𝑍 → ෪𝑊+ ෪𝑊− → 𝑒+ 𝑒− ǁ𝜈 ǁ𝜈 .

… and Daniel became the father of a boy:

In 1987 Daniel was awarded the Jean Thibaud prize

from the Lyon Science Academy.

1987 – 90: Running with an upgraded detector (UA2’)

and higher collider luminosity

A hint for a top quark with mass 30 – 50 GeV had been reported by UA1 in 1984

……….

However, more data from the 1984 – 85 runs had weakened this conclusion.

Search for physics beyond the Standard Model

Example: SUSY particles, expected to produce hadronic events with large pTmiss .

Main physics goals:

Search for the top quark:

At a total collision energy of 630 GeV, the top quark production rate is measurable

only if the decays W+ t b and W

– t b are energetically possible: Mtop < 70 GeV.

The top quark can then be detected by observing the decays t b e+ n , t b e– n

( lower electron pT and pTmiss than for W e n events + 2 jets from b and b decays).

Measurement of the missing transverse momentum

UA1 – UA2 comparison of pTmiss distributions in events containing pT > 15 GeV/c electrons

(shown by Daniel at his 1985 review talk «Review of W and Z physics at the CERN p p collider»

Before the analysis of the first p p collider data (1981 – 82), the importance of measuring

the missing transverse momentum (pTmiss) had not been fully acknowledged.

The lack of full calorimeter coverage in the UA2 detector could introduce unknown

systematic errors in the pTmiss measurement.

The effect of the incomplete UA2 calorimeter coverage is evident

UA2’ detector 1987 - 90

electromagnetic and hadronic calorimetry over ~4p

new tracking detectors

no magnetic field

no muon detector

UA2’ tracking detector

The UA2’ collaboration: Bern, Cambridge, CERN, Dortmund, Heidelberg, Melbourne, Milano,

Orsay (LAL), Pavia, Perugia, Pisa, Saclay (CEN)

( ~120 physicists in total)

Si : two cylindrical surfaces of Si pads

to measure dE/dx;

JVD: cylindrical drift chamber;

TRD: two modules of Transition Radiation Detectors;

SFD: Scintillation Fiber Detectors (6 stereo-triplets

+ 2 stereo-triplets behind a 1.5 X0 Pb cylinder)

ECPT: Proportional tubes (including a preshower)

Main Task of the LAL-Orsay group:

design, construction and setting up of the two TRD modules

UA2’ detector before closing the two End-Caps

Two photos taken on the occasion of the first transfer of the UA2’ detector

to the beam area

pTmiss distribution in the UA2’ detector

Events containing an electron

with pT > 15 GeV/c

Events containg an electron

with pT < 11 GeV/c (mostly

background)

UA2’ found no evidence for W t b decay

Lower limit to the top mass mtop > 71 GeV (90% C.L.)

UA2’ found no evidence for new physics beyond the Standard Model

(e.g., SUSY particles, leptoquarks, …)

UA2’ improved on several measurements of Standard Model processes

An example: single photon production at high pT

e n transverse mass distribution

2065 W e n events with the electronin the central calorimeter

Best fit: GeV 22.084.80 Wm

0019.00036.08813.0 Z

W

m

m

Two samples of Z e+ e- events :

Both electrons in central calorimeter ( 95 events)

Only one electron in central calorimeter (156 events)

Fit to the two distributions:

GeV 28.074.91 Zm

The most interesting UA2’ measurement: the mass ratio (mass ratio has no uncertainty from calorimeter calibration)

mW

mZ

GeV 17.033.035.80 Wm

1991: Use first precise measurement of mZ from LEP experiments:

mZ = 91.175 ± 0.021 GeV

to obtain a precise determination of mW :

bounds on the mass of the top quark:

GeV 160 50

60

+

-topm

(five years before the top quarkdiscovery at Fermilab)

Because of eletroweak radiative corrections, mW depends on mtop (Veltman 1977)2

Daniel gave invaluable contributions to the UA2’ experiment:

He took part in the design, construction and setting-up of

the two TRD modules of the central tracker;

At the data analysis stage, he had a leading role in the

study of events involving electron identification

(search for W t b, W and Z properties, …)

Daniel was the Ph.D thesis supervisor of Damir Buskulič and also

provided guidance to Guillaume Unal for his Ph.D. thesis on the

W to Z mass ratio.

In 1990 Daniel was invited to give a summary talk on W and Z

mass mesurements at hadron colliders at the Neutrino ‘90 conference

(this talk included W and Z results from the Fermilab Tevatron p p collider)

Also in 1990, Daniel joined CERN as a research physicist.

Additional personal comments

Daniel is a physicist with competences in both detector technologies

and data analysis.

He has initiative and critical sense.

He obviously enjoys doing physics. His love for physics is contagious.

He is an excellent teacher. He provided guidance to many newcomers

in UA2 and UA2’.

He has a nice personality and goes along well with his collaborators.

It was for me a real pleasure to work with him.

He speaks fluently at least six languages: English, French, German,

Greek, Italian, Russian.

Contribution list

………………….

Daniel during his presentation

at the event of November 22, 2002

Dear Daniel,

Many thanks for all your contributions to physics and

best wishes for many more enjoyable and productive years.