Re: Higgs boson signal found?

On Apr 27, 1:01 am, franklinhu <frankli...@xxxxxxxxx> wrote:
On Apr 26, 1:13 pm, PD <thedraperfam...@xxxxxxxxx> wrote:

On Apr 26, 2:36 pm, franklinhu <frankli...@xxxxxxxxx> wrote:

On Apr 26, 6:14 am, PD <thedraperfam...@xxxxxxxxx> wrote:

On Apr 25, 11:55 pm, franklinhu <frankli...@xxxxxxxxx> wrote:

On Apr 25, 8:51 am, PD <thedraperfam...@xxxxxxxxx> wrote:

The physics blogs are abuzz with the (leaked) circulation of an
internal report -- which means that it hasn't been checked enough yet
-- of a resonance at 115 GeV into the decay channel of two photons.
This is the kind of resonance and decay channel one would expect from
the Higgs.

Can someone explain why a Higgs should have a resonance and decay
channel like this? What makes it different from all the rest of the
stuff you might observer in an experiment? What about the theory makes
any predictions about this? What are they really looking for?


All short lived particles have production and decay channels. These
are generally predictable by theory, though the cross-section (rate
per luminosity) may be dependent on the mass of the particle. This is
how particles are searched for.

It seems to me that we don't know a whole lot about what a Higgs
particle is.

Actually, we know quite a bit about the Higgs. Actually, what we know
is like what we know about a family. A family might consist of Carl,
Bob, June, and Trina. Each of these has different traits, but if it's
Carl that's in the room, then you know pretty much how he behaves. If
you don't see Carl's behavior, then you can rule out seeing Carl, but
then you have to ask whether you see behavior you'd expect of June.
And so on.

If we know so little about it, how can we possibly
predict what the production/decay channels will look like? You say the
decay channel is 2 photons - that seems pretty pedestrian or simple
for a decay channel. Aren't there thousands of things that could decay
that way?

Yes, there are, and those are called "background". So what you can't
say is that *all* events of this signature are the Higgs. But what you
do is *count* such events, especially counting them as functions of
other parameters such as reconstructed energy. What you then know is
that background will account for *so many* in a certain bin of that
parameter. If you see significantly more in a particular bin, then you
may have evidence for something more than background. This is the
signal you're looking for. What we look for is diphoton events that
are suspiciously clumped around a particular mass range.

So, I ask again, just what is it about the Higgs particle that allows
us to predict a 2 photon decay channel? Some references would be nice..

If you go to the Wiki article on "Higgs boson" and proceed to the
bottom of the article, you will see a long list of references, all of
which are instructive. You can actually start with the Wiki article
itself if you like. However, you're going to have to recognize that
most or all of these articles are aimed at an audience that is
familiar with the basics of elementary particle physics and how the
business of particle-hunting is done. That is, you'll have to read
something that gives you a core understanding of *particle-physics*
meanings to terms like "cross-section", "branching ratios",
"luminosity", "event reconstruction" and the like. For this, you
should read an introductory textbook on experimental particle physics.
Ferbel and Das have a nice one.- Hide quoted text -

- Show quoted text -

After a bit of searching, I ran across this very informative thesis on
the search for the Higgs at LHC

This specifically addresses why they are looking at the 2 photon
channel. Although, it seems that the Higgs can break apart into just
about anything.

Well, not everything, but there are a lot of channels, yes.

The only reason why they are looking at the 2 photon
channel is because it can be precisely recognized in the detectors and

Please keep in mind that this is not the only channel that is being
looked at.

These 2 photon events are caused by just about anything and
one would expect that just about every possible energy can be
measured. When plotted on a graph, it should show every possible
energy being detected at about the same probability - or a flat graph.

That's not quite right. Read what I wrote. It is a steeply falling
spectrum, not flat.

What they are looking for is a bump in that graph that shows slightly
more events around a particular energy level.

Around a particular reconstructed mass, yes.

Well, it seems that there is nothing "Higgs" specific about this
search at all. This is just a raw search for any particle at all that
could result in a 2 photon decay. It could be anything, anything at
all even if they do find some "bump" in the data.

That's true. But then again, if you find footprints in the jungle that
are the size and shape you would expect from the feet of a tiger, you
don't know for sure that those footprints aren't made by something
else that just happens to have feet like a tiger and weighs about the
same as a tiger and has a stride like a tiger. On the other hand, most
folks would just say they'd found the footprints of a tiger.

If you were looking for an unknown particle with 2 photons and you
were looking at the entire energy range, you would see a mile-high
skyscraper "bump" at 1MeV (probably the most commonly seen event) -
why don't we declare that we've found a particle of mass 1MeV by this
same logic?

We DO, and this is how the pi-zero was found, as well as other
particles that decay into this channel.

If we found a signal this big at any other energy, we
would most definitely declare the discovery of a new particle - but
not at this very, very particular energy. Why???

The Higgs WOULD be the discovery of a new particle. It's hypothesized
but has never been seen. When it's seen, it will be the discovery of a
new particle.

I think you're asking, why wouldn't we say that this bump at 115 GeV
is something new OTHER THAN the Higgs? The answer is, some very well
might, and it will take other studies in other decay channels to nail
it down as the Higgs. But I suspect there is just a difference between
how you brain works and how those of physicists may work. I believe
you would RATHER the bump be a signal of something completely new and
unexpected than the signal of something already anticipated. You
aren't completely alone in that sentiment, but think about it: Science
WORKS by making predictions and then testing whether the predictions
are borne out by data. If a theory makes a prediction and then you go
out and look for data that would either confirm or deny that
prediction, and you see evidence of confirmation, does it make sense
to say, "Well, the evidence seems to confirm the theory, but I'd
rather believe that that theory is wrong ANYWAY and believe that the
evidence is pointing to something else entirely"?