Skip to comments.Racing to the 'God Particle'
Posted on 08/17/2002 4:50:36 AM PDT by JohnHuang2Edited on 06/29/2004 7:09:22 PM PDT by Jim Robinson. [history]
Physicists from all over the world are racing to prove the existence of a particle that's surmised to be at the heart of the matter. Literally.
Dubbed the "God particle" by Nobel Prize-winning physicist Leon Lederman, the Higgs boson is a controversial particle believed to bestow mass on all other particles.
(Excerpt) Read more at wired.com ...
Fermilab may have better luck with supersymmetry. A few years ago, CDF discovered an event with two electrons and two photons, and a large amount of missing energy and momentum. The event was consistent with the production and decay of a pair of scalar electrons (a scalar electron is the supersymmetric partner to the electron). Still, there was a very remote chance that a known physics process could produce that event topology. If that were the case, they'd never see another one.
Deep rumor is that they've recently seen another one.
To claim discovery of Higgs, Fermilab needs to reach the blue band. To see any evidence of Higgs at all, they need to reach the green band. If you pick a Higgs mass along the bottom axis, you can read from the side axis how much integrated luminosity--how much data--they'll need.
According to the Run II schedule, Fermilab should receive 15 fb^-1 of data by the time LHC turns on in 2008. If that's the case, they'll be able to detect hints of the Higgs up to a mass of 180 GeV (about 180 times the proton mass), and claim discovery of any Higgs up to almost 120 GeV. But they are only on schedule to get about half of that, in which case they'll only be able to see hints of Higgs up to about 120 GeV, and won't be able to discover it at all. (Higgs has already been ruled out up to about 113 GeV.)
[Geek alert: The unit of data is called an "inverse femtobarn" (or fb^-1). A "barn" is a unit of cross-sectional area, 10^-24 square centimeters. A "femtobarn" is 10^-39 square centimeters. Because we measure event interactions by their effective cross-sectional size, it is convenient to measure integrated luminosity as an inverse cross section; then integrated luminosity times cross-section equals a number of events. So for a process with a cross section of 10^-39 square centimeters, you'd expect to see an average of one event per inverse femtobarn of data.]
if there's any way you can translate the article (and your reply) into "something that I can understand", it would be appreciated.
I know a few things about sub-atomic physics (like QED, and what quarks are) but I have no idea what this article is talking about. (I have the feeling though, that it's all just too far over my head.)
Let's say that for a given collision energy and a given Higgs mass, there's a chance in a hundred billion that a Higgs will be produced in each collision. So if we crashed a proton into an antiproton 100 billion times, we'd have about a 2/3 chance of seeing one Higgs particle.
Typically in high energy physics, we require a 3-sigma effect to claim that we see evidence for something, and a 5-sigma effect to claim discovery. Since in a counting experiment, sigma--the standard deviation--goes as the square root of the number of counts, we'd need 9 Higgs events to claim evidence for the existence of the Higgs, and 25 events to claim discovery (which would require several trillion collisions).
But it's not so easy as all that. First of all, there are numerous inefficiencies. Particle detectors aren't 100% hermetic, and not all events can be reconstructed. But worse is the fact that there are background events: events from other physics processes that look very much like the signal events you're trying to find. Most of the background gets eliminated by placing cuts on the data (requiring that the decay particles be above a certain energy, for example). But this reduces the efficiency, because some of the real Higgs events will fail the cuts, so that means you need even more data. And then there will always be some background left, which means that the sigma is not so simple as the square root of the number of events.
This also requires that you be able to calculate the background very accurately. Fortunately, in the case of the Higgs, the background is probably the most thoroughly studied in the history of particle physics. At this point, all that remains is counting events and applying basic statistics.
So, there's this particle and we're trying to find its mass, but it's hard to do because we want to be really, really, really, sure before we say for sure we found it.
The article says,
"Without the Higgs, all fundamental particles would be massless, etc..."
So, apparently the Higgs does exist. And once we prove it for sure, we'll understand the nature of things better. (I think).
VadeRetro: Higgs boson, which creates mass,
A point of clarification. The Higgs particle is thought to generate the masses of the fundamental particles of matter: the quarks and the leptons. There are other mechanisms by which mass is generated, however. For example, the masses of the protons and neutrons are much larger than the sum of the masses of the constituent quarks, so most of the "baryonic" mass density of the universe comes from mechanisms other than the Higgs mechanism. (The main focus of the intensely computational field of "Lattice QCD" has traditionally been to calculate the proton mass straight from the equations of quantum chromodynamics.)
You have an electron flying along in a particle accelerator. Its mass, it being a lepton, comes from the Higgs boson. Why can't we see this directly?
Ding! Ding! Ding! We have a winner.
Just as we need to distinguish the photon from the electromagnetic field, we need to distinguish the Higgs particle from the Higgs field.
We know that the electromagnetic field can be quantized, which is to say, described as an infinite superposition of discrete photons, provided that these photons are virtual (i.e. they operate below the "resolution" of the Heisenberg uncertainty principle, and are therefore not visible as actual light). Nevertheless, the existence of virtual particles does have a measurable effect on physical phenomena, even if the virtual particles themselves do not possess reality in the same way that the photons in a sunbeam do.
If the Standard Model is correct, then there is a Higgs field that is responsible for the elementary particle masses. The particles interact with that field just as a charged particle might interact with a magnetic field. This field is quantizable, just like an electromagnetic field, and we call the associated quantum particle the Higgs boson. As it turns out, however, this particle isn't massless like a photon or a graviton, but is very heavy--at least 100 times heavier than a proton or neutron.
This tells you right away that the Higgs bosons that give an electron its mass must be virtual. There just isn't enough energy there to make a Higgs boson "real": the Higgs mass is at least 200,000 times heavier than the electron. But the Heisenberg uncertainty principle allows the electron to "borrow" Higgs bosons from the vacuum.
Not enough energy to realize the Higgs. Now, if this electron happens to smash into, oh, say a positron with enough center-of-mass energy, a real Higgs boson might be prized loose from the vacuum. This is exactly what we try to do at particle accelerators.
Ohhhh! It vectors a fiieeeeeeeeeeeld!
Thanks. One of the best features of FR is having you around for "Ask Mr. Physicist." Almost as good as Dave Barry's "Mr. Language Person" and no doubt providing a bit better grounded answers.
I was doing fine till I saw your graphic.