Sunday 31 January 2016

750 ways to leave your lover

A new paper last week straightens out the story of the diphoton background in ATLAS. Some confusion was created because theorists misinterpreted the procedures described in the ATLAS conference note, which could lead to a different estimate of the significance of the 750 GeV excess. However, once the correct phenomenological and statistical approach is adopted, the significance quoted by ATLAS can be reproduced, up to small differences due to incomplete information available in public documents. Anyway, now that this is all behind, we can safely continue being excited at least until summer.  Today I want to discuss different interpretations of the diphoton bump observed by ATLAS. I will take a purely phenomenological point of view, leaving for the next time  the question of a bigger picture that the resonance may fit into.

Phenomenologically, the most straightforward interpretation is the so-called everyone's model: a 750 GeV singlet scalar particle produced in gluon fusion and decaying to photons via loops of new vector-like quarks. This simple construction perfectly explains all publicly available data, and can be easily embedded in more sophisticated models. Nevertheless, many more possibilities were pointed out in the 750 papers so far, and here I review a few that I find most interesting.

Spin Zero or More?  
For a particle decaying to two photons, there is not that many possibilities: the resonance has to be a boson and, according to young Landau's theorem, it cannot have spin 1. This leaves at the table spin 0, 2, or higher. Spin-2 is an interesting hypothesis, as this kind of excitations is predicted in popular models like the Randall-Sundrum one. Higher-than-two spins are disfavored theoretically. When more data is collected, the spin of the 750 GeV resonance can be tested by looking at the angular distribution of the photons. The rumor is that the data so far somewhat favor spin-2 over spin-0, although the statistics is certainly insufficient for any serious conclusions.  Concerning the parity, it is practically impossible to determine it by studying the diphoton final state, and both the scalar and the pseudoscalar option are equally viable at present. Discrimination may be possible in the future, but  only if multi-body decay modes of the resonance are discovered. If the true final state is more complicated than two photons (see below), then the 750 GeV resonance may have  any spin, including spin-1 and spin-1/2.

Narrow or Wide? 
The total width is an inverse of particle's lifetime (in our funny units). From the experimental point of view, the width larger than detector's  energy resolution  will show up as a smearing of the resonance due to the uncertainty principle. Currently, the ATLAS run-2 data prefer the width 10 times larger than the experimental resolution  (which is about 5 GeV in this energy ballpark), although the preference is not very strong in the statistical sense. On the other hand, from the theoretical point of view, it is much easier to construct models where the 750 GeV resonance is a narrow particle. Therefore, confirmation of the large width would have profound consequences, as it would significantly narrow down the scope of viable models.  The most exciting interpretation would then be that the resonance is a portal to a dark sector containing new light particles very weakly coupled to ordinary matter.    

How many resonances?  
One resonance is enough, but a family of resonances tightly packed around 750 GeV may also explain the data. As a bonus, this could explain the seemingly large width without opening new dangerous decay channels. It is quite natural for particles to come in multiplets with similar masses: our pion is an example where the small mass splitting π± and π0 arises due to electromagnetic quantum corrections. For Higgs-like multiplets the small splitting may naturally arise after electroweak symmetry breaking, and  the familiar 2-Higgs doublet model offers a simple realization. If the mass splitting of the multiplet is larger than the experimental resolution, this possibility can tested by precisely measuring the profile of the resonance and searching for a departure from the Breit-Wigner shape. On the other side of the spectrum is the idea is that there is no resonance at all at 750 GeV, but rather at another mass, and the bump at 750 GeV appears due to some kinematical accidents.
   
Who made it? 
The most plausible production process is definitely the gluon-gluon fusion. Production in collisions of light quark and antiquarks is also theoretically sound, however it leads to a more acute tension between run-2 and run-1 data. Indeed, even for the gluon fusion, the production cross section of a 750 GeV resonance in 13 TeV proton collisions is only 5 times larger than at 8 TeV. Given the larger amount of data collected in run-1, we would expect a similar excess there, contrary to observations. For a resonance produced from u-ubar or d-dbar the analogous ratio is only 2.5 (see the table), leading to much more  tension. The ratio climbs back to 5 if the initial state contains the heavier quarks: strange, charm, or bottom (which can also be found sometimes inside a proton), however I haven't seen yet a neat model that makes use of that. Another possibility is to produce the resonance via photon-photon collisions. This way one could cook up a truly minimal and very predictive model where the resonance couples only to photons of all the Standard Model particles. However, in this case, the ratio between 13 and 8 TeV cross section is very unfavorable, merely a factor of 2, and the run-1 vs run-2 tension comes back with more force. More options open up when associated production (e.g. with t-tbar, or in vector boson fusion) is considered. The problem with these ideas is that, according to what was revealed during the talk last December, there isn't any additional energetic particles in the diphoton events. Similar problems are facing models where the 750 GeV resonance appears as a decay product of a heavier resonance, although in this case some clever engineering or fine-tuning may help to hide the additional particles from experimentalist's eyes.

Two-body or more?
While a simple two-body decay of the resonance into two photons is a perfectly plausible explanation of all existing data, a number of interesting alternatives have been suggested. For example, the decay could be 3-body, with another soft visible or invisible  particle accompanying two photons. If the masses of all particles involved are chosen appropriately, the invariant mass spectrum of the diphoton remains sharply peaked. At the same time, a broadening of the diphoton energy due to the 3-body kinematics may explain why the resonance appears wide in ATLAS. Another possibility is a cascade decay into 4 photons. If the  intermediate particles are very light, then the pairs of photons from their decay are very collimated and may look like a single photon in the detector.
   
 ♬ The problem is all inside your head   and the possibilities are endless. The situation is completely different than during the process of discovering the  Higgs boson, where one strongly favored hypothesis was tested against more exotic ideas. Of course, the first and foremost question is whether the excess is really new physics, or just a nasty statistical fluctuation. But if that is confirmed, the next crucial task for experimentalists will be to establish the nature of the resonance and get model builders on the right track.  The answer is easy if you take it logically ♬ 

All ideas discussed above appeared in recent articles by various authors addressing the 750 GeV excess. If I were to include all references the post would be just one giant hyperlink, so you need to browse the literature yourself to find the original references.

34 comments:

mfb said...

"The problem with these ideas is that, according to rumors, there isn't any additional energetic particles in the diphoton events."
Not just rumors, this was discussed in the presentations: the events in the excess region and in the sidebands look similar. The number of events is low, of course, but I guess a top pair or W/Z in every excess event would have left some trace (higher jet multiplicity, additional leptons, missing transverse energy, ...)

Anonymous said...

Would it be fair to say that if it's spin-2 it's not the graviton because it's way too massive, though it would have some gravity-like effect, albeit at much shorter distance than gravity?

Anonymous said...

"Such at least are some of the thoughts summoned by _Ficciones_; the variants are numberless..."

- Anthony Kerrigan, from his Introduction to Jorge Luis Borges' _Ficciones_ (1962)

Jester said...

Anon 4:12, apart from the mass there may be important differences between the hypothetical spin-2 particle and the usual massless graviton that mediate gravity. In particular, the couplings of the spin-2 guy to matter do not have to be universal, unlike for the massless graviton in the Einstein gravity. Nevertheless, we usually just call it a "massive graviton".

Anonymous said...

Jester, but what would the ramifications be of finding such a thing? I know there are various multiple-Higgs models, are there any multiple graviton (or multiple spin-2) models? Previously from what I'd read about theoretical investigations into the possibility of gravitons with mass I'd got the impression that they would still need to have a very small mass to work properly, as fields with quanta of large mass would be short distance. This would suggest you'd still need another graviton particle.

Jester said...

Models with extra dimensions of spacetime provide a consistent realization of a tower of massive gravitons on top of the massless one.

Jester said...

mfb: that's true this was mentioned though not quantified AFAIK. I'll edit that as soon as i have better Internet access

Vladimir Kalitvianski said...

How does emission of soft photons change the (Landau) statement about an integer spin of the decaying something?

Sebastian Jaeger said...

Dear Jester,

I don't think Landau-Yang forbids J>2 odd.
J>2 does imply orbital angular momentum though.

Jester said...

Thanks, indeed i'm not sure about J>2 odd so i need to check this. In the meantime i will modify the statement.

Jester said...

Vladimir: Landau-Yang refers only to 2-body decays. Unfortunately, this loophole is of no use for the diphoton final state (no emission possible at leading order).

Ervin Goldfain said...


The Landau-Yang (LY) theorem forbids the decay of a massive spin 1 particle into a pair of massless on-shell photons. The "everyone's model" that Jester is alluding to must also comply with the LY theorem as applied to vector-like quarks, unless some other mechanism comes into play.

Vladimir Kalitvianski said...

Jester, Leading order is not correct just because it lacks soft photons. We should speak of the exact theory result rather than of the "leading order".

Anonymous said...

Hi, in "Narrow or Wide?" part, you conclude that resonance could be a portal to a dark sector containing new light particles very weakly coupled to ordinary matter. How did you get this from large width assumption? Doesn't large width imply strong interaction? Thx

mfb said...

A large width means a short lifetime and a small branching fraction to photons. So where are all the other decays and the corresponding effects in precision experiments? If those decays are to invisible particles, that could work.

Jester said...

There's no strict implication, just plausible arguments. A large decay width must come from decays into something, however decays to SM particles are constrained by experiment. So the decays should be into some exotic particles. These exotics must be strongly coupled to the 750 GeV resonance (as you notice), however they must be weakly coupled to the SM particles otherwise they would have been seen already.

Rastus Odinga Odinga said...

"In particular, the couplings of the spin-2 guy to matter do not have to be universal, unlike for the massless graviton in the Einstein gravity. Nevertheless, we usually just call it a "massive graviton"."

Is there any chance of stopping this intensely noxious terminology from becoming standard? If it doesn't couple universally then it isn't a graviton. Why not call it a massive photon, that would be just as stupid and no more misleading in the long run --- after all, wrong spin, wrong couplings, who cares? Calling this thing a massive graviton will just encourage the people who have generated a vast number of foolish papers on that topic.

Alex Small said...

Even worse, what if the popular press hears about a possible "massive graviton"?

Jester said...

Admin note: please keep comments on topic. If you have a comment about authors of any paper, try to contact them directly.

RBS said...

It must be a stupid question but if true graviton couples universally how do we get photon, gluons and massless particles in general?

Ervin Goldfain said...

RBS,

The graviton, photon, weak boson and gluon are all integer spin particles that arise as a result of certain symmetries exhibited by physical theories. It is customary to refer to them as “force carriers”. For example, the graviton emerges by demanding invariance of the theory to arbitrary changes of spacetime coordinates. As hypothetical quantum of the gravitational field, the graviton is massless and couples to everything that has energy (mass).

Likewise, photons, weak bosons and gluons arise as “gauge bosons” by demanding that the theory stays unchanged under local gauge transformations. These transformations are at the core of the principle of local gauge invariance. Local gauge invariance constrains all “force carriers” to stay massless. In the Standard Model of elementary particles, the Higgs mechanism enables the weak bosons to acquire mass while leaving the photon massless.

RBS said...

You are right Ervin, thank you! It's mass vs energy terminology that confused me. A photon has zero rest mass but still couples to gravity because it carries energy.

Rastus Odinga Odinga said...

"Even worse, what if the popular press hears about a possible "massive graviton"?"

That would actually be good, because hype in the pop sci press nowadays only serves to discredit the idea in question.

RBS said...

Still trying to grasp this notion of "universal coupling": so let's say we managed to accelerate a mass particle like electron to near light speed. It's gotten huge mass, sends tons of massless gravitons, bends space around like crazy and eventually maybe, breaks it apart (somewhere around Planck's mass). That's fine. But to get it to this situation we'll have to send it extremely (and more and more) energetic photons, right? But the photon itself would have to couple to gravitons then, so even before getting to the electron it'll do all the havoc above. Is that right? Would an extremely energetic photon bend space time around it (like reverse gravity lens)? Can we hope to detect these space time distortions when energetic photons pass by?

mfb said...

The mass of the electron is unchanged. Where "mass" is used in the way all physicists used it in the last decades. There is a concept of relativistic mass - just forget it.
The energy increases with speed, the mass does not.
"bends space around like crazy and eventually maybe, breaks it apart (somewhere around Planck's mass)" - no. And that misconception is one of the reasons relativistic mass is not used any more.
"Would an extremely energetic photon bend space time around it (like reverse gravity lens)?" - every photon does so, but the effect is some tens orders of magnitude too weak to measure it.

Haelfix said...

The Bayesian posteriror estimate on what sort of physics we are looking at goes like this: As you increase the physical width of the resonance, the plausible models (assuming this isn't a side by side mis-trigger) goes from generic to plausible to weird and ends up somewhere in never never land.

Ervin Goldfain said...

RBS,

I am not the owner of this blog but your comments are clearly off topic. I suggest you raise these questions elsewhere, at Quora or Physics Stack Exchange.

The short answer is yes, all photons carry energy and distort spacetime, but this distortion is truly negligible even for ultra high energy cosmic rays where the kinetic energies are typically greater than 10^18 eV.

For more on spacetime warps and their detection, see:


https://en.wikipedia.org/wiki/Warp-field_experiments

Anonymous said...

RBS,

Merely accelerating an electron to near light speed will do nothing interesting. Look at an electron at rest in the lab - there exists a (Lorentz-boosted) reference frame in which this electron has total energy near (or even greater than) the Planck energy. But it doesn't collapse into a black hole, spacetime doesn't "break apart", nothing really happens.

If you want to see gravitational effects, you need huge energy density, but in the center-of-mass reference frame, not in an arbitrary one.

Anonymous said...

What about mini/micro black hole generation and subsequent evaporation? Did anyone exclude this possibility?

RBS said...

OK thanks to all who helped clarify the conundrum)

mfb said...

@Anonymous: They should not have a narrow mass - after some threshold (let's say 750 GeV) they should be produced up to several TeV. Anything that starts at 750 GeV would leave a massive signal at TeV energies.
Example spectra from CMS (8 TeV): http://arxiv.org/abs/1303.5338
From ATLAS for e/mu (13 TeV): https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2015-072/fig_07a.png

Anonymous said...

so quiet about the impending GW discovery? this time is for real :)

mfb said...

I'll be excited in 4 days. Either by a discovery, or by the amount of unsubstantiated rumor.
If the numbers in circulation are right (3 solar masses emitted as gravitational waves), the emitted power of gravitational waves towards the end should be the most powerful thing we saw in the universe so far, some large multiple of 10^46 W.

Jester said...

Yes, im of course aware of this, and I'm as excited as everybody. But I know very little about the topic, so I'm not sure if i can add to the discussion.