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A "New Physics" Bump at Fermilab?

by John G. Cramer

Alternate View Column AV-159
Keywords: Fermilab, CDF, collaboration, resonance, structure, 144 GeV
Published in the October-2011 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 5/15/2011 and is copyrighted ©2011 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

The Tevtron accelerator at Fermilab near Chicago has had a distinguished history.  It began operation in 1972 as a large national synchrotron facility, a circular tunnel 3.9 miles in circumference containing a ring of “warm” conventional magnets used to accelerate protons to energies up to 400 GeV.  Because of the relativistic mass-increase effect, if a 400 GeV proton is directed at a proton target at rest in the laboratory, the effective proton+proton collision energy is only 27.4 GeV.  The rest of the available energy ends up in simply moving the entire center of mass of the colliding system forward and does not contribute to the dynamics of the collision itself.  This is the disadvantage of fixed-target experiments and a strong argument for moving to colliders.

In 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.

In 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.

The 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.

But 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.

There 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.

One 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).

A 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 discovery.

In 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.

The 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.

Not 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, Kenneth Lane , and Adam Martin, is of particular interest.  It involves the invocation of “technicolor”, a previously hypothesized fifth force of nature to be added to the strong, weak, electromagnetic, and gravitational interactions, that bears some resemblance to the color-charge structure of the strong interaction.  According to the model, technicolor is responsible for the breaking of symmetry between the weak and electromagnetic interactions and generates the masses of the Z and W bosons through a new dynamic interaction.  If technicolor is a valid extension of the standard model, the ongoing searches for the Higgs boson will come up dry, and instead, new particles characteristic of technicolor will be discovered, the CDF bump perhaps representing the first of these.  In particular, the technicolor model asserts that the Tevatron collisions are producing a technicolor-flavored rho meson with a mass of about 290 GeV that decays into a W boson and a technicolor-flavored pi meson with a mass around 160 GeV.  The authors are able to fit the CDF data with this model, and they suggest several other possible experimental observations that the model would predict.  Other theory papers focused on the CDF bump use string theory, or supersymmetry, or new “dark” forces, or “leptophobic” W’ bosons, or even the Higgs boson itself to explain the observations.

But 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 detectors, ATLAS or CMS, at CERN ’s LHC?  I would not venture a guess, but I look forward to seeing how the saga of the CDF bump unfolds.  This could be the “crack in the wall” that allows us to go beyond the standard model and develop a new theory of fundamental interactions with more insights into what is really going on in the universe.

Watch this column for further results.  

Followup note (11/22/2014): 
The "bump" described in the column was not reproduced by further analysis or experiments.


Fermilab Bump:

“Invariant 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.

 “Technicolor at the Tevatron”, Estai Eichten, Kenneth Lane, and Adam Martin, submitted to Physical Review Letters; preprint ArXiv: 1104.0976v1 [hep-ph], 6 Apr 2011.

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