Thursday, 24 December 2015

750 and what next

A week has passed since the LHC jamboree, but the excitement about the 750 GeV diphoton excess has not abated. So far, the scenario from 2011 repeats itself. A significant but not definitive signal is spotted in the early data set by the ATLAS and CMS experiments. This announcement is wrapped in multiple layers of caution and skepticism by experimentalists, but is universally embraced by theorists. What is unprecedented is the scale of theorist's response, which took a form of a hep-ph tsunami.    I still need time to digest this feast, and pick up interesting bits among general citation fishing.  So today I won't write about the specific models in which the 750 GeV particle could fit: I promise a post on that after the New Year (anyway, the short story is that, oh my god, it could be just anybody). Instead, I want to write about one point that was elucidated by the early papers,  namely that the diphoton resonance signal is unlikely to be on its own, and there should be accompanying signals in other channels. In the best case scenario, confirmation of the diphoton signal may come by analyzing the existing data in other channels collected this year or in run-1.

First of all, there should be a dijet signal. Since the new particle is almost certainly produced via gluon collisions,  it must be able to decay to gluons as well by time-reversing the production process. This would show up at the LHC as a pair of energetic jets with the invariant mass of 750 GeV. Moreover, in simplest models the 750 GeV particle decays to gluons most of the times. The precise dijet rate is very model-dependent, and in some models it  is too small to ever be observed, but typical scenarios predict order 1-10 picobarn dijet cross-sections. This would mean that thousands of such events have been produced in the LHC run-1 and this year in run-2. The plot on the right shows one example of a parameter space (green) overlaid with contours of dijet cross section (red lines) and limits from dijet resonance searches in run-1 with 8 TeV proton collisions (red area). Dijet resonance searches are routine at the LHC, however experimenters usually focus on the high-energy end of the spectrum, far above 1 TeV invariant mass. In fact, the 750 GeV region is not covered at all by the recent LHC searches at 13 TeV proton collision energy.

The next important conclusion is that there should be matching signals in other diboson channels at the 750 GeV invariant mass. For the 125 GeV Higgs boson, the signal was originally discovered  in  both the γγ and  the ZZ final states, while in  the WW channel the signal is currently similarly strong. If the 750 GeV particle were anything like the Higgs, the resonance should actually first show in the ZZ and WW final states (due to the large coupling to longitudinal polarizations of vector bosons which is a characteristic feature of Higgs-like particles).  From the non-observation of anything interesting in run-1 one can conclude that there must be little Higgsiness in the 750 GeV particle, less than 10%.  Nevertheless, even if the particle has nothing to do with the Higgs (for example, if it's a pseudo-scalar), it should still decay to diboson final states once in a while. This is because a neutral scalar cannot couple directly to photons, and the coupling has to  arise at the quantum level through some other new electrically charged particles, see the diagram above. The latter couple not only to photons but also to Z bosons, and sometimes to W bosons too.  While the details of the branching fractions are highly dependent, diboson signals  with comparable rates as the diphoton one are  generically predicted.  In this respect, the decays of the 750 GeV particle to one photon and one Z boson emerge as a new interesting battleground.  For the 125 GeV Higgs boson, decays to Zγ have not been observed yet, but in the heavier mass range the sensitivity is apparently better.  ATLAS made a search for high-mass Zγ resonances in the run-1 data,  and their limits already put non-trivial constraint on some models explaining the 750 GeV excess. Amusingly, the ATLAS Zγ search has a 1 sigma excess at 730 GeV...   CMS has no search in this mass range at all, and both experiments are yet to analyze the run-2 data in this channel.  So, in principle,  it is well possible that we learn something interesting even before the new round of collisions starts at the LHC.

Another generic prediction is that there should be vector-like quarks or other new colored particles just behind the corner. As mentioned above, such particles are necessary to generate an effective coupling of the 750 GeV particle to photons and gluons. In order for those couplings to be large enough to explain the observed signal,  at least one of the new states should have mass below ~1.5 TeV. Limits on vector-like quarks depend on what they decay to,  but the typical sensitivity in run-1 is around 800 GeV. In run-2, CMS already presented a search for a charge 5/3 quark decaying to a top quark and a W boson, and they were able to improve the run-1 limits on the new quark's mass from 800 GeV up to 950 GeV. Limits on other type of new quarks should follow shortly.

On a bit more speculative side, ATLAS claims that the best fit to the data is obtained if the 750 GeV resonance is wider than the experimental resolution. While the statistical significance of this statement is not very high, it would have profound consequences if confirmed. Large width is possible only if the 750 GeV particle decays to other final states than photons and gluons. An exciting possibility is that the large width is due to decays to a new hidden sector with new light particles very weakly or not at all coupled to the Standard Model. If these particles do not leave any trace in the detector then the signal is the same monojet signature as that of dark matter: an energetic jet emitted before the collision without matching activity on the other side of the detector. In fact, dark matter searches in run-1 practically exclude the  possibility that the large width can be accounted for uniquely by invisible decays (see comments #2 and #13 below).  However, if the new particles in the hidden sector couple weakly to the known particles, they can decay back to our sector, possibly after some delay, leading to complicated exotic signals in the detector. This is the so-called hidden valley scenario that my fellow blogger has been promoting for some time. If the 750 GeV particle is confirmed to have a large width, the motivation for this kind of new physics will become very strong. Many of the possible signals that one can imagine in this context are yet to be searched for.    

Dijets, dibosons, monojets, vector-like quarks, hidden valley...  experimentalists will have hands full this winter.  A negative result in any of these searches would not strongly disfavor the diphoton signal, but would provide important clues for model building. A positive signal would break all hell loose, assuming it hasn't yet. So, we are waiting eagerly for further results from the LHC,  which should show up  around the time of the Moriond conference in March. Watch out for rumors on blogs and Twitter ;)

Tuesday, 15 December 2015

A new boson at 750 GeV?

ATLAS and CMS presented today a summary of the first LHC results obtained from proton collisions with 13 TeV center-of-mass energy. The most exciting news was of course the 3.6 sigma bump at 750 GeV in the ATLAS diphoton spectrum, roughly coinciding with a 2.6 sigma excess in CMS. When there's an experimental hint of new physics signal there is always this set of questions we must ask:

0. WTF ?
0. Do we understand the background?
1. What is the statistical significance of  the signal?
2. Is the signal consistent with other data sets?
3. Is there a theoretical framework to describe it?
4. Does it fit in a bigger scheme of new physics?

Let us go through these questions one by one.

The background.  There's several boring ways to make photon pairs at the LHC, but they are expected to produce a  spectrum smoothly decreasing with the invariant mass of the pair. This expectation was borne out in run-1, where the 125 GeV Higgs resonance could be clearly seen on top of a nicely smooth background, with no breaks or big wiggles. So it is unlikely that some Standard Model processes (other than a statistical fluctuation) may produce a bump such as the one seen by ATLAS.

The stats.   The local significance is 3.6 sigma in ATLAS and 2.6 sigma in CMS.  Naively combining the two, we get a more than 4 sigma excess. It is a very large effect, but we have already seen this large fluctuations at the LHC that vanished into thin air (remember 145 GeV Higgs?). Next year's LHC data will be  crucial to confirm or exclude the signal.  In the meantime, we have a perfect right to be excited.

The consistency. For this discussion, the most important piece of information is the diphoton data collected in run-1 at 8 TeV center-of-mass energy.  Both ATLAS and CMS have a small 1 sigma excess around 750 GeV in the run-1 data, but there is no clear bump there.  If a new 750 GeV  particle is produced in gluon-gluon collisions,  then the gain in the signal cross section at 13 TeV compared to 8 TeV is roughly a factor of 5.  On the other hand, there was 6 times more data collected at 8 TeV by ATLAS (3.2 fb-1 vs 20 fb-1). This means that the number of signal events produced in ATLAS at 13 TeV should be about 75% of those at 8 TeV, and the ratio is even worse for CMS (who used only 2.6 fb-1).  However, the background may grow less fast than the signal, so the power of the 13 TeV and 8 TeV data is comparable.  All in all, there is some tension between the run-1 and run-2 data sets,  however a mild downward fluctuation of the signal at 8 TeV and/or a mild upward fluctuation at 13 TeV is enough to explain it.  One can also try to explain the lack of signal in run-1 by the fact that the 750 GeV particle is a decay product of a heavier resonance (in which case the cross-section gain can be much larger). More careful study with next year's data  will be needed to test for this possibility.

The model.  This is the easiest part :)  A resonance produced in gluon-gluon collisions and decaying to 2 photons?  We've seen that already... that's how the Higgs boson was first spotted.  So all we need to do is to borrow from the Standard Model. The simplest toy model for the resonance would be a new singlet scalar with mass of 750 GeV coupled to new heavy vector-like quarks that carry color and electric charges. Then quantum effects will produce, in analogy to what happens for the Higgs boson, an effective coupling of the new scalar to gluons and photons:

By a judicious choice of the effective couplings (which depend on masses, charges, and couplings of the vector-like quarks) one can easily fit the diphoton excess observed by ATLAS and CMS. This is shown as the green region in the plot.
 If the vector-like quark is a T', that is to say, it has the same color and electric charge as the Standard Model top quark, then the effective couplings must lie along the blue line. The exclusion limits from the run-1 data (mesh) cut through the best fit region, but do not disfavor the model completely. Variation of this minimal toy model will appear in a 100 papers this week.

The big picture.  Here sky is the limit. The situation is completely different than 3 years ago, where there was one strongly preferred (and ultimately true) interpretation of the 125 GeV diphoton and 4-lepton signals as the Higgs boson of the Standard Model. On the other hand,  scalars coupled to new quarks appear in countless model of new physics. We may be seeing the radial Higgs partner predicted by little Higgs or twin Higgs models, or the dilaton arising due to spontaneous conformal symmetry breaking, or a composite state bound by new strong interactions.  It could be a part of the extended Higgs sector in many different context, e.g. the heavy scalar or pseudo-scalar in the two Higgs doublet models.  For more spaced out possibilities, it could be the KK graviton of the Randall-Sundrum model, or it could fit some popular supersymmetric models such as the  NMSSM. All these scenarios face some challenges.  One is to explain why the branching ratio into two photons is large enough to be observed, and why the 750 GeV scalar is not seen in other decays channels, e.g. in decay to W boson pairs which should be the dominant mode for a Higgs-like scalar.  However, these challenges are nothing that an average theorist could not resolve by tomorrow morning.  Most likely, this particle would just be a small part of the larger structure, possibly having something to do with electroweak symmetry breaking and the hierarchy problem of the Standard Model.  If the signal is a real thing, then it may be the beginning of a new golden era in particle physics....