then you know that there were some particles produced that were invisible to your detector.

Generalized Questions

In context of this thread started on the woodpecker and the listening for the sounds of species, I couldn’t help think of sound in this case, as events discovered, as the “Oh My God Particle.”

Would the “landscape” be relevant here as we might interpret it in Sun-Earth relation. In light of Lagrange points and how particles would make it through, from early cosmological events?

Thanks

It’s not that the particles were there and are not now. It’s just that the particles that JoAnne was referring to do not interact with the matter of the detector, as she said. Thus, they don’t leave signatures (no tracks, no energy deposits, etc.). We infer their existence from energy and momentume conservation: the colliding beams come on one and the same axis. This means that the initial momentum of the system before the collision is all in one direction. Therefore, the final momentum of the system after the collision should also be in the same direction. If, when you add up the momenta of all the visible particles after the collision, you end up with a significant net momentum on the plane transverse to the beam axis (and after you have calibrated your detector and you are sure it is not a systematic effect), then you know that there were some particles produced that were invisible to your detector.

I realized I didn’t respond to your earlier point about the LC seeing the Higgs if the latter decays in non-standard ways. Yes of course, you will infer it from the recoil. I didn’t mean to dismiss this option altogether (I said that the LC will be not SO much a discovery machine as…), I just emphasized the main motivation for building the LC. As you also said at the end of your post, we are building the physics case for the LC mostly as the place to sort out whatever we find at the LHC (i.e., unravel the underlying theory).

I now realize that my first posting was rather dense. Sorry. See, that was my very first posting at any web blog ever. I now know I should explain things in more detail.

Do all agree this is the case that if particles were there, and are not now, that “something” has to answer this?? JoAnne, Lee Smolin, Peter Woit. Anyone for that matter?

When I said that “measuring the properties would be important in order to distinguish among the different models”, I meant exactly what you so nicely described in the last two paragraphs of your reply. I just had a narrower definition of “discovery” in mind, meaning discovery of the particle itself, whereas you imply discovery of the underlying theory. But we agree on the basis – w/o the LC we will probably not be able to tell what we will be seeing at the LHC, if we see something.

Here at Snowmass, in about 45 minutes, we are having a panel discussion on `What will it take to start construction on the International Linear Collider.’ Naturally, I will post about the results. The question is a political one, i.e., what will it take for the politicians to fund the ILC. Scientifically, we know that we want the ILC no matter what the LHC finds.

To respond to Adam, I am afraid that you are correct in the sense that the ILC will not get funded if the LHC comes up empty-handed. This is very unfortunate as in that case we would need the ILC even more than ever!

It is possible that the LHC will not be able to see the Higgs if it decays in a non-standard way. There are many such models with regions of parameter space where this can happen. However, the ILC is a model independent machine and detects the Higgs no matter how it decays, even if it decays into particles which are invisible to the detector. The scenario I just described is our worst nightmare for the LHC.

However, I sincerely doubt if accelerator science will come to an end. Current acceleration techniques have hit a dead end, but there is alot of interesting research on advanced acceleration techniques (see Caolionn O’Connell over at Quantum Diaries). I am confident that the accelerator folks will come up with a feasible method of obtaining high gradients (i.e., high accelerating energy per meter) at a reasonable cost. We are talking about obtaining tens to hundreds of GeV per meter, thus making very high energy colliders feasible in the future. I should say that more efficient acceleration techniques has followed a natural progression with historical precedent. Recall that Fermi envisioned an accelerator with the circumference equal to that of the moon in order to test his theory.

Now, to respond to Anna – my goodness! The ILC IS a discovery machine – it does MUCH more than measure properties. I just gave one example above where the ILC can discover particles that the LHC cannot. In addition, there are 2 steps for discovery: 1) to discover a new particle, 2) to discover what that new particle is and what theory it originates from. The LHC is very good at doing the first step, but it simply cannot carry out the second. We need the ILC for that.

One example that has come up many times here at Snowmass is that of Supersymmetry. The main signature for supersymmetry at the LHC is missing energy – particles which do not interact with the detector, but we know were there due to energy and momentum conservation. However, that’s the main signature for a group of other theories (such as extra dimensions or Little Higgs) as well. And how do we know which theory gives rise to the missing energy if it is observed at the LHC? The answer is that it depends on what the physics actually is and what the values of the model parameters actually are. In some cases we can be lucky and have already divised clever methods of tests that work at the LHC, but most of the time there are no such tests that work. In other words, we will know that the LHC has discovered missing energy, but we won’t know whether it is Supersymmetry, extra dimensions, Little Higgs, or something else we haven’t thought of yet. This is the main crux of the physics case for the ILC.

First, to answer Adam’s question: yes, there is the next accelerator we’d like to build, the electron-positron Linear Collider, which will be under threat if we don’t find anything new at the LHC. The latest indications from the funding agencies are that they would like to wait for the first LHC results before they decide on the Linear Collider.

However, the Linear Collider in itself is not going to be so much a discovery machine as a study-the-properties one. If we find a new particle or particles at the LHC, we can run the Linear Collider at the appropriate energy to produce them copiously and do precision measurements of all their properties (i.e., mass, couplings, spin, parity, etc.) This would be important not only for its own sake, but also to distinguish among the different models beyond the Standard Model.

If on the other hand the LHC does not discover any new particles (and given that its discovery potential will be all the way up to 1 TeV), we will probably need to go back to the drawing board and rethink our models. Of course, such a result, albeit negative, would be exciting in its own right.

-cvj