Tevtron accelerator at Fermilab near
the 1980s the first Fermilab Director, the late Robert R. Wilson, somehow found
enough funds in the Fermilab operating budget to develop and construct in the
synchrotron tunnel a parallel ring of superconducting magnets with about twice
magnetic fields of the original magnets. Protons
were accelerated first in the warm magnets and then in the superconducting
magnets. This increased the maximum
proton energy of the machine to 800 GeV, leading to effective fixed-target
proton+proton center-of-mass collision energies of 38.8 GeV.
1985, after the Isobel Collider construction at Brookhaven National Laboratory
was cancelled, the U. S. Department of Energy allowed Fermilab, following the
lead of CERN, to develop a sub-facility for producing, storing, cooling, and
injecting antiprotons into the magnet ring.
These negatively charged antimatter particles were accelerated in the
opposite direction in the magnet rings to produce proton-antiproton collisions
at selected locations and to raise the effective center-of-mass collision
energies to 1,600 GeV. In other
words, operating the synchrotron in collider mode increased the collision energy
by a factor of about 41. Later
superconducting magnet improvements raised the proton/antiproton beam energies
980 GeV and raised the collision energies to 1,960 GeV, the present collision
energy of the machine. This facility
at Fermilab is now called the Tevatron because the collisions are in the TeV
(1,000 GeV) energy region.
large CDF and D0 collider detectors operating at the Tevatron have maintained
very active physics programs over the past 25 years, producing many important
results including the discovery of the top quark, and publishing many hundreds
of papers (each with many hundreds of authors) in the major physics journals.
However, the holy grail of particle physics, the discovery of a new
result that would “break” quantum chromodynamics, the standard model of
particle physics, and lead to new physics beyond the standard model has never
been achieved. Essentially all
results coming out of the Tevatron up to the end of 2010 have been consistent
with the standard model.
now, the Tevatron, as all large particle accelerators must eventually do, has
reached the end of its active life. These
are difficult economic times for funding large and expensive physics projects,
and recently the U. S. Department of Energy has decreed that by the end of
September, 2011, the Tevatron will cease operation permanently.
is some irony in this decision. In
April, 2011, the CDF (Collider Detector at Fermilab) Collaboration submitted a
paper to the “fast” and prestigious physics journal Physical Review Letters
announcing a new 3.2 standard deviation result, the observation of a “bump”
of mass-energy around 144 GeV. [For
reference, the mass-energy (E = mc2) of a proton is 0.938 GeV and the
mass-energy of a W boson is 80.4 GeV]. This
new mass-bump was found in proton-antiproton collisions involving Z-meson
production. Its presence is
inconsistent with the standard model and seems to suggest new physics.
This announcement is reminiscent of the discovery in 1974 of the J/psi
particle, mass 3.1 GeV, that became the key observation in the development of
quantum chromodynamics, the present standard model of particle physics.
However, it is also reminiscent of “observations” made in past
decades, e.g., the penta-quark at several laboratories, the leptoquark at DESY,
and quark substructure at Fermilab, that have turned out to be incorrect in the
light of closer scrutiny.
of the characteristic decay processes expected from the long-sought Higgs boson,
the so-called “God-particle” that gives mass to other particles, is its
decay into a W boson. The W boson,
one of the fundamental mediating particles of the weak interaction, may in turn
decay (1) in a “leptonic” mode into an electron or muon and a neutrino,
which happens about 1/3 of the time or (2) in a “hadronic” mode into pairs
of energetic quarks, which happens about 2/3 of the time.
Decay mode (1) shows up in the detectors as an energetic electron or muon
with missing energy (because the neutrinos are not detected), while decay mode
(2) shows up as two energetic “jets” of strongly interacting particles.
A number of groups have studied “di-boson” processes involving twin
instances of decay mode (1).
sub-group of the CDF Collaboration had chosen the somewhat more difficult path
of focusing on di-boson events containing evidence of decay mode (1) combined
with decay mode (2). This process is
interesting because the Higgs boson is expected to have a decay mode as a W and
a pair of heavy quarks, but this di-boson channel has more problems with
background. Nevertheless, it offers
certain technical experimental advantages and could be a path to Higgs
their analysis, the CDF group attempted to pull from the many millions of 1,960
GeV proton-antiproton collision events recorded by the CDF detector that subset
that might be characteristic of di-boson processes. They focused on events
showing one leptonic decay characteristic of a W meson, accompanied by two
hadronic jets. Using the
tracking data of the particle constituents of the jets, they calculated the
rest-mass of some hypothetical initial particle that might produce the pair of
jets. As expected, they observed a
mass peak around 80-90 GeV that is characteristic of a W or Z boson decaying
into two quarks. But the surprise
came in looking at the data on the high side of that peak.
There they found an excess of events in the mass region between 120 and
160 GeV. Simulation of all of
the known process predicted by the QCD Standard Model to participate in this
decay process provided a “background” that could be subtracted from the data
to highlight the observed excess. This
background-subtracted plot showed a “bump” centered at 144±5
GeV, with a width that was just that expected for the di-baryon channel’s mass
resolution. The implication of the
latter observation is that the hypothetical particle producing the bump is “narrow”
and long-lived, with a decay width that does not significantly modify the mass
resolution of the di-baryon channel.
There is no known particle with a mass in this region that might be
expected to produce such a bump. In
particular, a 144 GeV Higgs boson decaying in this channel with the observed
strength, which is about 300 times the expected Higgs strength, would have long
ago been observed in di-leptonic W+W decays studied at CERN’s LEP facility and
at the Tevatron itself. Further, a
Higgs decaying to produce a W boson and two quarks should preferentially produce
heavy bottom quarks, and this preference is inconsistent with the channel in
which the mass bump is observed, which contains few bottom-quark jets.
new bump has a statistical significance of 3.2 standard deviations.
That means that, assuming that all systematic errors have been correctly
dealt with, there is about a 0.1% chance that the observation could be a product
of random chance. That sounds pretty
solid, but experience with other 3-standard-deviation effects indicates that a
bogus one shows up to titillate the physics community about once per decade.
The result cannot be taken seriously, at the level of recording it in
that Particle Data Book and using it as the basis for a new standard model,
until we have a 5 standard deviation effect that has been confirmed by its
observation from another experiment, either the D0 experiment at the Tevatron or
one of the collider detectors, ATLAS or CMS, at CERN’s LHC.
We note that the CDF Collaboration has analyzed only about half of the
available 1,960 GeV Tevatron collision data, so their statistical precision is
likely to improve.
unexpectedly, within a few days of the appearance on the ArXiv preprint server
of the CDF paper describing the bump, theoretical explanations of the
observation began to appear like April dandelions.
At this writing, about 10 weeks after the preprint appeared, there are 35
such theoretical papers involving a spectrum of mutually exclusive extensions
beyond the standard model. One of
the papers, by Estai Eichten,
how real is this result? How likely
is it to withstand the scrutiny of the physics community?
How likely is it to be confirmed by other ongoing particle physics
experiments, particularly the D0 experiment at Fermilab and the new collider
or CMS, at
this column for further results.
Followup note (11/22/2014): The "bump" described in the column was not reproduced by further analysis or experiments.
Mass Distribution of Jet Pairs Produced in Association with a W boson in p-p-bar
Collisions at √s
= 196 TeV”, The
CDF Collaboration, submitted to Physical Review Letters, preprint
ArXiv: 1104.0699v2 [hep-ex], 2 May 2011.
at the Tevatron”, Estai Eichten, Kenneth Lane, and Adam Martin, submitted to
Physical Review Letters; preprint ArXiv: 1104.0976v1 [hep-ph], 6 Apr
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