A few weeks ago, I reported on the completion of a large project, with which I’ve been personally involved, to investigate how particle physicists at the Large Hadron Collider [LHC] could be searching, * not only in the future but even right now*, for possible “Exotic Decays” of the newly-discovered Higgs particle .

By the term “exotic decays” (also called “non-Standard-Model [non-SM] Decays”), we mean “decays of this particle that are not expected to occur unless there’s something missing from the Standard Model *(the set of equations we use to describe **the known elementary particles and forces and the simplest possible type of Higgs field and its particle)*.” I’ve written extensively on this website about this possibility *(see here, here, here, here, here, here, here and here)*, though mostly in general terms. In our recent paper on Exotic Decays, we have gone into nitty-gritty detail… the sort of detail only an expert could love. This week I’m splitting the difference, providing a detailed and semi-technical overview of the results of our work. This includes organized lists of some of the decays we’re most likely to run across, and suggestions regarding the ones most promising to look for (which aren’t always the most common ones.)

Before I begin, let me again mention the twelve young physicists who were co-authors on this work, all of whom are pre-tenure and several of whom are still not professors yet. *[ When New Scientist reported on our work, they unfortunately didn't even mention, much less list, my co-authors.]* They are (in alphabetical order): David Curtin, Rouven Essig, Stefania Gori, Prerit Jaiswal, Andrey Katz, Tao Liu, Zhen Liu, David McKeen, Jessie Shelton, Ze’ev Surujon, Brock Tweedie, and Yi-Ming Zhong.

**The Project**

With an immense variety of possible exotic decays of the observed Higgs particle, a thorough search poses a severe challenge. Experimenters at ATLAS and CMS, the two general-purpose experiments at the LHC, have their work cut out for them. But the payoff would also be huge; if any such decay turned up, it would mean a revolutionary discovery of new forces and (almost always) new particles, ones that lie outside the Standard Model, which has stood as the predictive framework for particle physics for nearly 40 years.

Writing a paper that completely covers all the possibilities would have been impossible. We had to choose a more limited goal, and so the decision was to restrict ourselves to

- decays which do not produce any unknown long-lived particles (except ones that are always completely invisible to the detector);
- decays in which the number of observable electrons, muons, taus, photons, quarks, or gluons (or their anti-particles) is at most 4.

It’s important to realize, then, that ** we left out a lot**. But at least, with these restrictions, we had a finite list of cases (about 20 sub-classes) to consider, and it took us under 200 pages to do so.

Perhaps our most important observations are that:

CMS and ATLAS could add to our knowledge by looking more carefully at the data they already have, rather than waiting a year or two for more.**Discoveries are still possible with the existing data set.**- Some completed ATLAS and CMS searches for other phenomena, which weren’t intended for this purpose, nevertheless, according to our estimates,
on non-SM Higgs decays. Of course, dedicated searches would certainly do even better.*already put interesting and even sometimes stringent limits* - There are many opportunities coming when the LHC turns on again in 2015. One of the big challenges is
, which faces even greater strain as the LHC collision energy and collision rate increase,**to assure that the trigger system**. This important issue must be studied**can avoid discarding certain types of non-SM Higgs decays***this year*!

Today, I’ve created a page summarizing what we know about *possible Higgs decays to two new spin-zero particles, which in turn decay to quark pairs or lepton pairs according to our general expectation that heavier particles are preferred in spin-zero-particle decays*. A number of theories (including models with more Higgs particles, certain non-minimal supersymmetric models, some Little Higgs models, and various dark matter models) predict this possibility.

Over the coming week or so, I’ll be adding more pages with other classes of decays, and pointing you to them as they’re ready.

Matt,

You mentioned the following: “decays in which the number of observable electrons, muons, taus, photons, quarks, or gluons (or their anti-particles) is at most 4.”

Could you explain the main reasons for the limit of “at most 4″.?

Is it because the amount of possible decay paths to evaluate for larger sets (larger than 4) of particle is too large to tackle?

Is it because there is another technical limitation, like say, the information already captured at the LHC precludes the analysis of decays with larger sets?

Any other reason I can’t imagine at this time?

Kind regards, GEN

As you suspected: the number of options simply spirals out of control as one goes above 4.

Although this wasn’t a reason we didn’t explore them, more complex decays are also often more technically difficult to discover. The Higgs mass-energy of 125 GeV is fixed, and unless it’s produced with a very high velocity, which is rare, its total energy is rarely above 200 GeV. If it decays to 4 particles, those particles typically have 30-50 GeV, and it is common for one of them to have energy below 20. If it decays to 8 particles, divide those numbers by two. Well, particles with 10 GeV of energy are commonly going to be lost — just too hard to detect amid all of the debris from the proton-proton collisions. Moreover, with 8 particles, the probability that one of them goes into a region of the detector which is not instrumented (e.g., a region with cables) is becoming high. Finally, the background processes (i.e. other, Standard Model processes that mimic the signal) are harder and harder to model as the number of particles increases. So as the numbers of particles go up, the strategies required become more and more difficult, optimization becomes more and more non-obvious, efficiency for detecting the events becomes lower and lower, and it’s harder and harder for theorists to study the processes and help determine the best strategies.

How these larger sets of decay particles appear?

Due to collisions that are not head-on collisions, like say, collisions happening at an angle that is different from PI (180 degrees)?

No: it can just be a cascade of decays, such as h –> X X, X –> Y Y, Y –> quark+antiquark.

mmm, these collisions (with different numbers of decay particles) follow a distribution, and they just appear, I guess, as this is a QFT, after all!

It has nothing to do with the collisions. The collisions are the same as always; the Higgs is produced in the same ways as usual; only the decay of the Higgs is different.

Does “more Higgs particles” imply more or different Higgs fields than the field associated with the Higgs particle already detected? If so can all of these fields be unified?

Yes, it implies more Higgs fields, whose ripples are more types of Higgs particles. They’re quite easy to come by.

“Unified” depends on your point of view. Generally, at this stage, we’re not really worrying about that issue. To unify a group of things, you have to have the complete list of them, which means you have to first discover all of them (or most of them, so you can guess what might be missing). But to show that there’s something interesting going on, you only need to discover one of them. Let’s start with that!

Matt: In the triggering process, did they keep all the events where an unknown particle was produced with an energy around 125 GeV. If I remember right, Higgs was mainly identified in two photon and four leptons channels, everything else was background(?) Just curious how many such events are saved in the data set?

“did they keep all the events where an unknown particle was produced with an energy around 125 GeV?” You can’t do that, for a host of reasons. (a) You can’t identify events as having unknown particles; if you could, that would make discoveries pretty easy. You can only identify them by collecting all events that possibly could contain such particles, and then looking for deviations from the predictions of the Standard Model. (b) Unless the events are already easy to trigger on for other reasons, then they almost certainly include particles that are challenging to measure well, if at all (jets from quarks and gluons, taus, neutrinos). In this case you may not be able to reconstruct the mass of any particle that MIGHT have been produced to better than 10-20%, especially in the quick moment that the trigger system has availale. (c) Stray jets are always produced, and it’s not generally clear whether you should include them in your best guess as to the mass of any new particle that MIGHT have been produced; so this leads to ambiguities. (d) The number of events in which two jets are produced with an observed invariant mass of ~110-140 GeV/c^2, which (because of measurement uncertainties) could in principle have come from a particle of mass 125 GeV/c^2, exceeds Higgs production by hundreds of thousands. You can’t store all those events.

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What interest me more than what residues the Higgs gives is whether you could measure with this big machine how the Higgs mechanism does its work. What it is supposed to do is couple the gravitation potential of fermions to the gravitation potential of the massive bosons. Or with other words how this mechanism couples space curvature caused by fermions to space curvature that is caused by massive bosons.

Are you sure that the Higgs is an elementary particle? Or with other words that it is not a composite?

Did you find any means to explain the origin of space curvature. If not why does this community then state that the Higgs provides mass to (certain kinds of) elementary particles?

Without the Higgs mechanism the Higgs is just another particle. What are its potentials? What is the difference between these potentials and the Higgs field. How does the Higgs particle generate its Higgs field? How does any particle generate its fields?

Ok — a few things to fix here. First, the gravitational potential has nothing to do with this; http://profmattstrassler.com/2012/10/15/why-the-higgs-and-gravity-are-unrelated/ . So the last two sentences of your first paragraph are mistaken. If you have a little univeristy math background you may want to read http://profmattstrassler.com/articles-and-posts/particle-physics-basics/how-the-higgs-field-works-with-math/ , which is based on http://profmattstrassler.com/articles-and-posts/particle-physics-basics/fields-and-their-particles-with-math/ .

We are not sure the Higgs particle is elementary, although so far it is behaving like an elementary particle. Measurements continue. http://profmattstrassler.com/articles-and-posts/the-higgs-particle/taking-stock-of-the-higgs-jan-2013/2-perhaps-composite/

There’s no connection with space or space-time curvature; see above article. There is absolutely no connection between the Higgs field (which gives things their rest mass) and gravity (which in Einstein’s theory, unlike Newton’s, responds to their energy and momentum.)

Finally, you have things backwards; all particles are ripples in their own fields. The Higgs particle is a ripple in the Higgs field just as the photon is a ripple in the electromagnetic field and a muon is a ripple in a muon field, etc. The fields that particles emit — such as the electric field emitted by muon — is a result of a fundamental interaction between electric fields and muon fields. I have written extensively about these things on this website; try googling “Of Particular Significance particles ripples” and things like that and you should find the answers to your questions.

The Higgs is not like any other particle, at least for now.

It is the first scalar boson that is detected in nature.

Even though the Higgs boson is not a force mediator (like the photon or the Z boson), it is an elementary particle, like the vector bosons that mediate forces.

Gaston,

Exactly, which is why many prefer to call it the Higgs particle rather than Higgs boson. Spin zero perhaps contributes an extra layer of suspicion.

However, would it not be incredible to one day discover that the Higgs actually does mediate a force, one that (besides gravity) is evidently unaccounted for in the SM? Could that even be possible??

That’s really not right. “Boson” means any integer spin, not just spin zero. And we know the Higgs particle is a boson, because it decays to two photons, a combination of particles whose spin must be an integer.

The reason I generally don’t use “boson” is that it sounds more mysterious than it is. There are lots of other bosons in nature; photons, W’s, Z’s, gluons, gravitons, composite objects like pions, hydrogen atoms, helium nuclei, etc.

The Higgs IS predicted to mediate a force in the Standard Model: http://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-forces-of-nature/the-strength-of-the-known-forces/

Thanks Matt. I thimk my very poorly worded first paragraph failed to convey my meaning, which is the following: While all bosons are, by definition, particles with integer spin, suspicion is cast on the Higgs because it is the only known elementary particle with spin zero. I need to be much more careful with my verbiage ;-)

All bosons are particles.

Particles are either fermions or bosons, either elementary or composite.

Fermions (both elementary and composite) have a non integer value of spin, and are affected by Pauli’s exclusion principle when under bounded systems.

Bosons (both elementary and composite) have an integer value of spin, and are not affected by Pauli’s exclusion principle under no circunstance.

The name used for any given particle is just a naming convention, and the convenient conventions could change over time.

We could just call it “Peter’s thang”, but it does not sound “scientificky” enough, isn’t it?

A nice example is the laser beam, which wouldn’t be phycially possible if photons (a type of spin 1 boson) obeyed the exclusion principle.

First “apparently-elementary” scalar boson.

And it

isa force mediator, see below. Note this force is not related to how the Higgs field gives masses to particles, however.I read out there somewhere that only vector bosons could be force mediators, or something of that sort, but clearly it is the wrong concept.

I apologize for conveying wrong info.

Kind regards, GEN

Gravitons are spin-two (tensor bosons) and they do the trick also.

To wrap things up:

“All bosons in principle are capable of mediating forces”: right or wrong?

“All known bosons do mediate some force”: right or wrong?

A) I think that’s true, but maybe I’m forgetting something at the moment. Worth emphasizing that it’s the boson FIELD, not its particle, which is responsible. http://profmattstrassler.com/articles-and-posts/particle-physics-basics/mass-energy-matter-etc/the-energy-that-holds-things-together/

B) True for all known *elementary* bosons, and true for pions, which are hadrons (http://profmattstrassler.com/articles-and-posts/largehadroncolliderfaq/whats-a-hadron-as-in-large-hadron-collider/ ). Not true, say, for hydrogen atoms, at least not in the sense we mean it here.

As far as I know (I’m not an expert), forces are mediated by vector bosons (bosons with spin larger than zero).

The Higgs, both predicted by the theory and validated by experiments at the LHC, is a scalar boson.

As far as we know, this Higgs boson could not be the mediator of a force.

If I remember right, Matt has remarked earlier that exchange of Higgs also can result in a force. It may be difficult to detect in presence of other forces, but just because it has spin zero, it is not disqualified as a force carrier, I think!! Actually in QFT exchange of any particle can be thought of as exerting a force on the particles exchanging that. I would like Matt to correct this statement or support it!!

Correct: http://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-forces-of-nature/the-strength-of-the-known-forces/ .

Only bosons can really exert strong long-distance forces. Fermions can only do this by combining in some way.

Matt,

When you say ” Fermions can only do this (exert strong long-distance forces) by combining in some way”: could give some more details of how this works?

Look up Yukawa forces; a boson (a pion, in particular) is formed as a bound state made from fermions, and helps bind protons and neutrons into nuclei. This is what I called the “residual strong nuclear force” in my articles on nuclei.

Thanks for the reply. I would like to understand why fermion exchange has a shorter range than boson exchange, If it is a long argument, do you have a reference handy?

Generally it’s because you need at least two fermions exchanged to produce a force. But that argument has a loophole.

So, to me, this leads to at least two questions:

a) Is the LHC capable to detect this Higgs “force”, some time in the future, either with its current design or with some possible modification?

b) How important would this validation of the theory be in terms of its prospect for “New Physics”?

The LHC can potentially do it if measurements of W particle pair production are accurate enough by 2025 or so. IF not, a proposed International Linear Collider would be able to; precision measurements of top quark/anti-quark pair production are needed. Alternatively, dark matter might show up in dark matter detectors through Higgs force interactions, but that’s a long shot.

I don’t think this validation is going to be that important; if the theory’s invalid in this regard, we’ll probably discover that in some other way first.

Matt said

There’s no connection with space or space-time curvature; see above article. There is absolutely no connection between the Higgs field (which gives things their rest mass) and gravity (which in Einstein’s theory, unlike Newton’s, responds to their energy and momentum.)

I am not sure I understand the statment above. If the Higgs field gives some particles(it is my understanding that not all particles get their mass from the Higgs field) their mass and if in GR mass of an object tells space how to curve, which tells objects how to move in space(gravity), then how is there not connection between the HIggs field and gravity.

Or is there no connection between the concept of mass on the quantum level and mass on the scale of the universe?

Don,

Matt treated this exact issue very nicely here:

http://profmattstrassler.com/2012/10/15/why-the-higgs-and-gravity-are-unrelated/

The Higgs field gives particles their mass (as particle physicists mean it: that is, what some people call their “rest” mass, or “invariant” mass. I call it “mass” on this website.)

In GR, energy and momentum,

not [rest] mass, tell space how to curve. Some people confuse the issue by defining energy and mass to be equal, up to a factor of c^2; but that definition of mass is called “relativistic” mass, and is NOT, generally, what the Higgs provides. (I don’t use “relativistic mass” on this website; I call it “energy”.)A photon always has energy; therefore every photon has zero rest mass and non-zero relativistic mass. Do you want to call it a massless particle or not?

This is continually confusing to non-experts, because there are two entirely different interpretations of the equation E=mc^2, and if you confuse one for the other, you will remain confused forever about this issue. See http://profmattstrassler.com/articles-and-posts/particle-physics-basics/mass-energy-matter-etc/more-on-mass/the-two-definitions-of-mass-and-why-i-use-only-one/ Actually in general relativity this can even become a little more confusing, but I’ll skip that point.

If a particle is at rest, or moving very slowly, then its energy equals its rest mass times c^2. In that case ONLY, the curvature of spacetime is proportional to the [rest] mass, which is what the Higgs provides. That’s the case in which Newton’s theory and Einstein’s theory of gravity agree.

But photons have no rest mass, yet a photons will most definitely curve space-time. That’s because they have energy, even though they have no rest mass.

All of this is to say that when you said “If the Higgs field gives some particles(it is my understanding that not all particles get their mass from the Higgs field) their mass and if in GR mass of an object tells space how to curve”, you can either say that your second clause is simply wrong (because it’s energy and momentum which does the job) or you can say that you used one definition of mass [rest mass] in the first clause and a different definition of mass [relativistic mass] in the second clause. I prefer to say it the first way because I don’t want two definitions of mass running around.

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Thank you, Matt, for taking the time to explain this. It is more clear, however still somewhat confusing to me. I will go back and read your references on this subject more carefully. Something still is not adding up for me, but as you say it can be confusing for non experts and I am clearly no expert. Thanks, again.

It would definitely have been easier if the term “mass” hadn’t gotten multiple definitions!

The key point is that there’s a famous formula relating energy, momentum and mass for particles:

E^2 – (pc)^2 = (mc^2)^2

where “m” means “rest mass” and c is the speed of light. This is the formula that particle physicists stick with; you notice E = mc^2 only if a particle is at rest, i.e. p=0. Then it is easy to say what is happening. The Higgs field produced the electron’s m; the particle’s motion, however, determines p, and the combination determines E. In Newton’s theory, m would have produced gravity. But in Einstein’s theory,

E and p produce the gravity. Now it is true thatif p = 0, E and m are proportional, so Newton’s theory looks just like Einstein’s for a stationary (or very slow) object, like the Earth relative to the Sun. But in Newton’s theory, a massless particle (m=0, E=pc) produces no gravity because m is zero, while in Einstein’s theory a massless particle DOES produce gravity because E isn’t zero. The fact that light rays will deflect each other gravitationally is the statement that E produces the gravity, not the [rest] mass m.Now if you simply DEFINE relativistic mass to be E/c^2, as many people do (or something similar), then indeed it will appear from this point of view that relativistic mass (also known as “energy”, up to a constant) produces gravity. But that’s not the same as rest mass. A photon has a relativistic mass but no rest mass. Particle physicists hate this kind of confusing talk, and are very clear about it within their own community: photons have energy and no mass, and the energy is what generates the gravity.

I’ve never heard a modern professional theoretical particle physicist or string theorist use relativistic mass as a concept.If I’m not mistaken, for a few years after Einstein published his paper on GR, there was some arguing among physicists about “rest mass” and “relativistic mass”, until the issue was settled and from that moment on, nobody would use these two terms anymore, as you have stated that is the norm among experts nowadays.

Einstein took both points of view during his life, which confused matters further. His view late in life is concordant with the one particle physicists use today, whose value only became clear as particle physics developed further.

One sees articles for the public regularly using the old view, and first-year physics textbooks are incredibly confusing on the matter.

It is my understanding that we should always consider the pythagorean relationship as the “complete picture” regarding mass, energy and momentum: E^2 – (pc)^2 = (mc^2)^2

With this in mind as the only complete picture for any particle and for any physical body, we should always be on the safe side.

But for this to be useful, we must know for each particle and body what numerical values apply for each term: in the case of a massless particle like the photon, in this case, massless means that it does not have rest mass, so, the term (mc^2)^2 will be equal to zero since m is the rest mass, which is equal to zero, and this is what precisely it is meant when we say “the photon is massless”.

Matt

Thank you. This explanation and rereading your earlier posts on the subject greatly relieved my confusion. I deeply appreciate the time you took to explain this yet again. I also just completed professor Lederman’s book, Beyond the God Particle(he seem intent on maintaining that name). That book in combination with your explanations have been most enlightening.

Also, thanks to, Doc, for providing an additional reference. I am beginning my day with a deeper understanding of this subject, which brings me great joy.

Sent from my iPhone

>

Howdy! This article couldn’t be written any better!

Going through this post reminds me of my previous roommate!

He always kept talking about this. I most certainly will forward this

post to him. Pretty sure he will have a great read.

Thank you for sharing!

Thank you Matt for a very interesting and clear presented Blog. I understand that the Higgs mechanism is to explain, via the Standard Model, how elementary (not composite) particles obtain mass and especially so called massless particles. Is it not essential that the fundamental principles are first fully understood otherwise there is a risk to science, where the starting point is evidentially based on an incorrect assumption or understanding, of questioning the validity in any further developments.

If you take Einstein’s formulation E = hf , the energy of photons related to their wave frequency, then you have a reference or starting point of (h) Planck’s constant; the minimum quanta of energy, which suggests, in this formulation, that photons emanate with a minimum rest energy.

Long wave radio frequencies of <= 1 Hz plainly points to Einstein's formulation being incorrect at these frequencies. (At 1 Hz photons moving towards the speed of light are instead at rest or, at the lower frequencies, are below the minimum quanta of energy.) Not going into too much detail to keep this comment short, on investigation, this inaccuracy in Einstein's formulation extends but diminishes towards 10^10 Hz. Using Einstein's relativistic equation – denoted by gamma (γ) – rectification of this inaccuracy at lower frequencies is by E = hγ. Thereby, if a photon was at rest (where γ = 1) would retain a rest energy of h (and a rest mass of 4.14 x 10^-21 MeV/c^2).

Further, from investigation, the values of γ and f converge towards 10^10 Hz allowing the eventual infinity, produced by γ, to be renormalized by: γ = f (validating E = hf). The renormalization of γ would then establish particle velocities (v) to be directly related to their frequencies: v = c – (1/f); used to ascertain γ in reconciling the inaccuracy in Einstein's original formulation at lower frequencies.

But then again I am not a scientist so what do I know! http://scienceau.com

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