Alternate View Column AV-87
Keywords: DESY leptoquark high energy physics standard model HERA ZEUS H3 lepton quark
Published in the November-1997 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 06/14/97 and is copyrighted ©1997 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.
These days, experimental particle physicists spend much of their time banging their heads against a brick wall called "The Standard Model of Quantum Chromo- Dynamics". It's all very frustrating. The Standard Model is in good agreement with essentially all of the data collected by particle physics experiments during the past decades, yet it is not a theory that provides any deep understanding of the inner workings of the universe. It depends on about two dozen arbitrary "constants": particle masses, force strengths, and interconnection strengths. We have no idea where these constants come from or how they are related to each other. We know there must be a better, more fundamental theory behind this facade. Therefore, particle physicists, along with their other activities, have been trying to make the Standard Model "break", to find places where its predictions fail, to find a crack in the brick wall which might provide some inkling of what lies behind it.
So far this has been a lonely and unrewarding quest. New experiments occasionally come along which point to a breakdown of the Standard Model, but up to now they have invariably been proved wrong by more careful analysis or subsequent experiments with better data. A case in point is the energetic jet data from the CDF experiment at FermiLab which suggested possible substructure of the quark. (See my AV column "Inside the Quark" in the September-1996 issue of Analog.) The CDF group found an unexpected excess of "jets" (clumps of energetic particles moving in the same direction) with energies above 200 GeV in their data. They found that they could not explain this excess of high energy jets using the Standard Model, as interpreted by standard theoretical procedures, and they pointed out that the data might represent new physics, possibly an indication that the quark is a composite object made of even more fundamental particles.
This quark substructure "discovery" lasted about half a year. In that time, particle theorists found that minor changes in their assumptions about the velocity profiles of quarks and gluons within the proton could explain the data without significant changes to the Standard Model itself. My column discussing the CDF data (because the Analog publication pipeline takes about five months) appeared in print at just about the time when the quark substructure effect was being explained away.
Does this mean that the quark has no substructure?
No. In only means that even if a substructure is
there, finding it is probably beyond the capabilities
of presently available particle accelerators. To
observe such effects directly, the collisions would
have to be much more violent than those we can
However, new data from two groups at the DESY (pronounced "daisy") accelerator laboratory in Hamburg may suggest indirect evidence for quark substructure. Let me begin by describing the DESY collider, which is called HERA (Hadron-Electron Ring Accelerator). It is remarkable that the entire HERA synchronotron ring accelerator has been constructed in a tunnel bored through the rock and subsoil directly beneath the city of Hamburg. This placement of a major particle accelerator in a tunnel under a major metropolitan area points to an essential difference in attitudes toward scientific research in Germany and the USA. Imagine the public outcry if, in 1987 the US Department of Energy had announced that it intended to construct the Superconducting Super Collider in a tunnel beneath New York City, Chicago, or San Francisco! Imagine the hearings, the environmental impact statements, the protests of outraged citizens, the feeding frenzies of the lawyers, ... But somehow, the Germans have done the equivalent of this with essentially no public opposition. With careful planning, the support of state and local governments, and deeply ingrained public respect for science, the HERA collider was constructed deep beneath Hamburg.
HERA is also unique in colliding dissimilar particles: 27.5 GeV electrons with 820 GeV protons. The SPS collider at CERN and the Tevatron collider at FermiLab produce head-on collisions of protons with antiprotons. The LEP collider at CERN and the Tristan collider in Japan collide electrons and positrons. The new LHC collider now being constructed at CERN will collide protons with protons. Only in HERA collisions does a lepton (an electron) collide with a quark (one of the three inside a proton).
The HERA experiments H3 and ZEUS, after searching their three-year inventory of data, are now reporting observation of events that seem to be incompatible with the Standard Model. Both experiments observe rare events in which the incoming electron strikes some "object" deep within the proton that bounces it back in the reverse direction with almost its initial energy and momentum, producing a jet in the opposite direction. The probability that random fluctuations in the "standard" behavior of the quarks and gluons in the proton could produce the observed effects is only a few parts per thousand. Thus, unless new calculations alter the odds, there seems to be a very good chance that the Standard Model has been broken by the HERA data.
In order to produce the effect observed in the HERA data, the object struck within the proton would have to have two characteristics: (1) it would have to be very massive, and (2) there would have to be a very large force between the electron and the struck object, a force on the scale of the strong interaction. The problem with this combination of characteristics is that while massive components can be found within a proton, the Standard Model requires that leptons do not have strong interaction either with other leptons or with quarks and gluons. As far as we know leptons only participate in the weak interaction, which has only about one one-millionth the strength of the strong interaction. This suggests that the object struck by the electron (or created in an electron-quark collision) is an entirely new particle, christened the "lepto-quark", which is very massive and which has strong interactions with leptons. If such a particle existed, it would "break" the standard model, requiring extensive revision of the basic theory and perhaps bringing onto the fundamental particle scene a whole new generation of particles.
This possibility has already been anticipated by some particle theorists. Even before the new HERA data was reported, many theoretical papers were published that speculated on the properties of lepto-quarks and described ways in which experimental data might be analyzed to find them. More than 14 sub-species of lepto-quarks with differing properties have been proposed in various theoretical papers which use an array of theoretical models with names like grand unification, supersymmetry, technicolor, and preon substructure.
In a previous column ("Inside the Quark", Analog, September-1996) I discussed preon models which describes quarks and leptons as constructed of even more fundamental particles called "preons". Such models can include lepto-quarks by combining preons in alternate ways. In most of these variants of the Standard model the lepto-quark is a massive boson (integer spin) which has non-zero quantum numbers for being both a lepton and a baryons (i.e., it has the quantum characteristics of both an electron and a proton). Like a quark the lepto-quark should have a net color charge and a fractional electric charge.
The particle physics community is now awaiting
more data from HERA and more careful analysis of the
available data. The burning question is, has the
Standard Model been broken, or will the HERA data be
explained away like the "quark substructure" data from
FermiLab? Are we on the threshold of a new
breakthrough discovery about the fundamental
substructure of the universe, or must we continue to
grind out more confirmations of the Standard Model,
punctuated by "discoveries" that are ultimately
explained away? I'll try to keep you up to date on
these questions in future columns.
There are other questions raised by discoveries like this one that are often directed at particle physicists: "So what?" Suppose the Higgs boson is found, or a lepto-quark is discovered, or some new and previously unanticipated force of nature is found to exist, or a better and more deeply penetrating successor to the Standard Model emerges from current research in particle physics. What difference does that make to our everyday lives? What new commercial products will be built using these discoveries? How will it ease the burden on the taxpayers who ultimately pay for the research? Or even, how much will the populace be entertained, as they are to some extent by NASA's space shots and planet closeups?
As a physicist I have found three ways of answering questions like these. First, if we look back at the scientific discoveries and related technical applications of the previous century, it is clear that key concepts and discoveries cannot be anticipated. We must first allow science to progress and then use the ideas, concepts, and insights which emerge. We can track, for example, the personal computer, the smoke detector, the television, and the MRI imaging technique back to their origins in the discoveries of current control using solid-state physics, of transuranic elements produced using nuclear reactions, of electron beams manipulated and focused using electromagnetic fields, and of nuclear magnetic resonances in atoms. But it would have been impossible for the early 20th century scientists working in these fields to anticipate the impacts of their discoveries. We know from experience that, overall, basic scientific research always pays off, but we cannot predict where or when the payoffs will come.
Second, gaining deeper understanding of the way the universe works alters our world view of our society, our planet, our universe, and our place in it. We come to realize how delicately tuned are the physical constants of the universe which make life and civilization possible, how rare and unlikely intelligent life and civilization must be. I refer interested readers to my column "The 'Real World' and the Standard Model", which is about this aspect of particle physics. It was published in the May-1996 issue of Analog.
The third kind of answer, the one I like best for its honest and straightforward arrogance, was given by Robert R. Wilson, then Director of FermiLab, when he was testifying before a congressional committee in the 1970's. A rather hawkish US Senator asked him whether current research particle physics would contribute to the defense of our nation. Wilson, resisting the trap of spinning some extrapolative fairy tale about ultimate impact or resorting to NASA-style "spin-off" hyperbole, answered, "Senator, particle physics research is not likely to aid in the defense of our nation, but it will make our nation worth defending."
He meant that when our civilization loses its compulsion
to push back the frontiers of knowledge, to want to find
out what is behind the next intellectual barrier, to
discover the answer to the next question, to peer ever
more deeply into the heart of the universe and learn the
secrets of its innermost mechanisms, then perhaps we are
done as a civilization. We should, when we reach that
point, retire to a vegetable existence of manmade
diversions, to MTV and soap operas and virtual reality
and political posturings and video games, and allow the
inevitable plagues and asteroids and tidal waves of
evolution and change to wash us away, to be replaced on
the planet by a more promising species.
Comparison of ZEUS Data with Standard Model Predictions, The ZEUS Collaboration, Zeitscrift fuer Physik (accepted for publication, 1997).
See also the DESY/HERA web site at: http://www.desy.de/pr-info/desy-recent-hera-results-feb97_e.html
and the ZEUS high Q2 web site at: http://www-zeus.desy.de/~ukatz/ZEUS_PUBLIC/hqex/hqex_highx.html
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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This page was created by John G. Cramer on 07/10/97.