Download SEARCH FOR HEAVY LEPTONS FROM TIME

1
Victor F. Weisskopf
SEARCH FOR HEAVY LEPTONS
FROM TIME-LIKE PHOTONS AT CERN
From
Lepton Physics at CERN and Frascati - Edited by Nicola Cabibbo
World Scientific Series in 20th Century Physics - Vol. 8 - 1995 - pp. 45-47
WORLD SCIENTIFIC
SINGAPORE - NEW JERSEY - LONDON - HONG KONG
1995
2
The "heartbeat of the proton"
Victor F. WEISSKOPF
Once Nino came to my office to tell me about his ideas of studying
lepton pair production at PS. I was still not Director General, but Research
Director at CERN. In addition to (ee) and () pairs, he wanted to
search for (e) pairs as a signature of a new lepton carrying its own lepton
number. He told me that if such a lepton existed with one GeV mass, it
would have escaped detection in hadron accelerator experiments for two
reasons: i) it would decay with a lifetime of order 10 11 sec and ii) because
there is no    mechanism for such a heavy new lepton: for its
production a time-like photon would be needed. Time-like photons could be
produced in hadronic interactions: for example in (p̅p) annihilation. This
was before Lederman-Schwartz and Steinberger had discovered the two
neutrinos. To think of a ''sequential'' Heavy Lepton and to work out the
possible ways to get it in a hadron machine was for me extremely
interesting. Nino had just finished his first high precision work on the
muon (g2). It was some time after the Rochester Conference in 1960. I
gave Nino the following suggestion: if you want to search for something so
revolutionary as a Heavy Lepton carrying its own lepton number you should
work out a proposal for a series of experiments where the study of lepton
pairs (ee) and () could be justified in terms of physics accepted by
the community. In addition a high intensity antiproton beam was needed.
He came later to tell me that he had two very good friends, both excellent
engineers:
Mario Morpurgo and Guido Petrucci.
A very high intensity
antiproton beam could be built to study the electromagnetic form factor of
the proton in the time-like region. If the proton was "point-like" in the time-
3
like region, the rate of time-like photons yielding (ee) and () pairs
could be accessible to experimental observation, thus allowing to establish
some limits on the new Heavy Lepton mass, or to see it, via the (e )
channel.
The "official" theme was: to establish if the proton had a structure or
not in the time-like region. Thus a powerful system able to detect (ee)
and () pairs could be built. Nino established in 1963 the existence of a
time-like structure of the proton studying the (ee) channel and in 1964
studying the () channel. The set up was able to do what he wanted: a
simultaneous detection of electrons and  pairs, therefore (e) as well.
Unfortunately the proton was not a point-like particle in the time-like region
and therefore the source of time-like photons originated in (p̅p) annihilation
was very depressed. In fact, using the (ee) and the () channels, Nino
established that at 6.8 (GeV/c)2 time-like four momentum transfer, the crosssection was 500 times below the expected point-like value. This result had
attracted a lot of attention. Bogoliubov was very interested when in 1964
Nino went to Dubna to present the () results at the International
Conference on "High Energy Physics". Yang had a model that predicted a
point-like structure of the proton in the time-like region. I called this series
of experiments as measuring the "heartbeat of the proton".
Of course
there were no (e) events, neither in the (p̅p) nor in the (p) channel.
Nevertheless a series of experiments was performed on "standard" physics,
such as the discovery of many rare decay modes of mesons and the
measurement of the () mixing.
All these experiments could be done because Nino had invented what
is now known as the ''preshower'' method to reject with high efficiency
pions in favor of "electrons". Once it was clear that in hadronic interactions
there are very few time-like photons, he asked me if I would give the green light
4
in order to consider the use of the (e) technology in the newly being
developed Frascati (ee) collider. There the "time-like" photons were
very abundant and the (e) method would have been the best in order to
see if a Heavy Lepton carrying its own lepton number existed. Of course he
got the green light and in 1970 he got the first limit on the Heavy Lepton
mass together with a series of high precision QED measurements.
But one story I Will never forget in connection with the "heartbeat of
the proton". After he succeeded with his friends Mario (Morpurgo) and
Guido (Petrucci) to build the highest intensity antiproton beam at CERN,
Nino came to my office and said more or less the following: "Viki, by
changing the voltage of the electrostatic separator and a few other trivial
details, in one night, I will be able to establish if the antideuteron exists with
the correct expected deuteron mass". I told him that this was an experiment
where he would get the Nobel prize if he found nothing.
"But, there is a
but", I added. "If you do not succeed in one night and if you destroy the
beam, then I will not defend you. My green light is only valid if you really
can check the existence of the antideuteron in a single night”. Next
morning, when I arrived at CERN, Nino was there With his graph where the
antideuteron signal was exactly where it was expected to be.
I remember the year, 1965, not the day. It was (and is) the birthday of
Peter Standley who was at that time the PS Division Leader. I called him in
my office and the antideuteron discovery at CERN was my and Nino's gift to
our mutual friend Peter. I decided not to have a press-release and Nino
agreed. A few weeks later we read in the newspaper that the antideuteron
had been discovered by Lederman and Ting in the United States. They
had decided to have a press-release. Nevertheless Nino's paper in Nuovo
Cimento preceeds Leon's and Sam's publication in Physical Review.