Thursday, September 9, 2021

Discovery Of A New Beyond The Standard Model Spin-0 Particle With A Mass Of 151.5 GeV Claimed


At least two new pre-prints are claiming that there is evidence at 4.8 sigma of global statistical significance (more than five sigma locally) of a new scalar particle S with a mass of 151.5 GeV in Large Hadron Collider (LHC) data, which is being framed as a beyond the Standard Model Higgs boson.

This is something hypothesized in many beyond the Standard Model theories with two of more "Higgs doublets". In the simplest version of two Higgs doublet models (2HDM), there is a second scalar Higgs boson, a pseudo-scalar Higgs boson, a positively charged Higgs boson, and a negatively charged Higgs boson. More than two Higgs double theories add additional Higgs bosons in groups of four. The couplings of these extra Higgs bosons to other particles varies from theory to theory.

The fact that this claimed discovery hasn't yet spurred a flood of new papers to explain it, that it hasn't triggered a rush of new blog posts and twitter reports by most of the leaders in the Physics blogosphere, that it hasn't penetrated science media outlets targeted at educated laymen, and that it isn't published under the banner of any of the major experimental collaborations at the LHC, suggest that the physics community is viewing the claims skeptically. 

The physics community has been repeatedly burned by claimed new particle discoveries that haven't panned out. 

The community has been burned in the past because, contrary to what theory would expect, empirically, due to poorly modeled systemic errors, the chance of large anomalies do not fall off exponentially as you'd expect in a normal bell curve, and anomalous five sigma observations happen up to 100,000 times more often than expected. See David C. Bailey. "Not Normal: the uncertainties of scientific measurements." Royal Society Open 4(1) Science 160600 (2017).

Still, I'd be remiss if I didn't mention it, given the extraordinary claims made.

The Papers

The first of the papers is this one:
In recent years, hints for "multi-lepton anomalies" have been accumulated by the analysis of Large Hadron Collider (LHC) data, pointing towards the existence of beyond the Standard Model (SM) Higgs bosons: a new scalar particle S with a mass mS in the range between 130 GeV and 160 GeV, produced from the decay of a heavier new scalar particle, H. 
Motivated by this observation, we perform a search for the signatures of S within this mass region, which has been studied by CMS and ATLAS as a by-product of the SM Higgs searches in the side-bands of the kinematic regions. Combining the γγ and Zγ channels, with associated leptons, di-jets, bottom quarks and missing energy, we obtain a local (global) significance of 5.1σ (4.8σ) for a mass of m(S)=151.5 GeV and provide the preferred ranges for the corresponding (fiducial) cross sections. 
This is a strong indication for a scalar resonance S decaying into photons, and, to a lesser extent to Zγ, in association with missing energy, jets or leptons. Hints for the decays into, or production in association with, bottom quarks are statistically less significant. In order to test this hypothesis, we propose a search for H→γγbb⎯⎯⎯,τ+τ−bb⎯⎯⎯ in asymmetric configurations that has not yet been performed by ATLAS and CMS.
Andrea Crivellin, et al., "Accumulating Evidence for the Associate Production of a Neutral Scalar with Mass around 151 GeV" arXiv:2109.02650 (September 6, 2021).

The second paper is a conference presentation by one of the co-authors of the first paper:
In this presentation an account of the multi-lepton (electrons and muons) anomalies at the LHC is given. These include the excess production of opposite sign leptons with and without b-quarks, including a corner of the phase-space with a full hadronic jet veto; same sign leptons with and without b-quarks; three leptons with and without b-quarks, including also the presence of a Z. 
Excesses emerge in corners of the phase space where a range of SM processes dominate, indicating that the potential mismodeling of a particular SM process is unlikely to explain them. A procedure is implemented that avoids parameter tuning or scanning the phase-space in order to nullify potential look-else-where effects or selection biases. 
The internal consistency of these anomalies and their interpretation in the framework of a simplified model are presented. Motivated by the multi-lepton anomalies, a search for narrow resonances with S→γγ,Zγ in association with light jets, b-jets or missing transverse energy is performed. The maximum local (global) significance is achieved for m(S)=151.5 GeV with 5.1σ (4.8σ).
Bruce Mellado, "The anatomy of the multilepton anomalies at the LHC and a candidate for a singlet scalar" arXiv:2109.03800 (September 8, 2021). 


Could It Be An Overlooked W Boson Pair Background?

Lubos Motl has also done a blog post about the claimed discovery which is a Higgs-like scalar, much like several previous LHC anomalies at 28 GeV, 400 GeV and 1000 GeV, and a pre-LHC anomaly at about 96 GeV. He comments that:
This new scalar resonance isn't produced directly but instead, it seems to be a product of a decay of a heavier particle, arguably yet another and heavy Higgs, perhaps one at 400 GeV or 1000 GeV mentioned previously. . . .

The dominant decay of this new scalar particle S seems to be S→γγ while S→Zγ also contributes. The decay S→bb¯… seems to be there but it is weak, not very important for the statistical significance, and it may be absent, too. S→ZZ∗ seems to be surprisingly absent. The authors also mention that a 96 GeV scalar-like hint from LEP and CMS could be justified by a better story if that particle resulted from the decay of this 151.5 GeV pal.

A new scalar particle, if real, may be interpreted as a part of the extended supersymmetric Higgs sector, it may play other roles, too. It may also be a mirage. In particular, I am a bit worried that this mass is close to twice W-boson mass or something like that. Maybe in the calculation of the cross section, some events are incorrectly assumed to be identified as a W+W− final state and subtracted from new events, and because they see more of these events, they are seen as an excess. I don't claim to fully understand this vague alternative and boring explanation, however.

The body text of the first paper seems to indicate an uncertainty of about ± 3 GeV). The authors state:

The spectra of the channels in which the excesses are most significant are shown in Fig. 1 and the combination of all channels in Fig. 2, excluding the bb channel due to the large resolution. Note that the background curves here do not exactly correspond to the SM hypothesis but rather to Eq. (1) in the combined fit to Eq. (1) and Eq. (2). For improved visualization, each plot combines the spectra from ATLAS and CMS (if available) for the same final states by using a signal over background (NS/NB) re-weighting, such that NS is the number of signal at the peak (±3 GeV) of the crystal-ball function and NB are the corresponding background events within this range. Combining all spectra (being a function of a common mass m(S)) the local p-value that a background-only hypothesis is true is shown in Fig. 3 as a function of m(S). The maximal local significance for a resonance is 5.1σ at m(S) = 151.5 GeV, where it varies by 1σ when shifting the mass by 1 GeV.

The authors also hypothesize a mass of a heavy Higgs boson that decays into this particle:

[A]ssuming that S is pair produced by the decays of H with a mass around 270 GeV (as suggested by the multi-lepton anomalies) via pp → H → SS∗ . We remark that also the process including H → Sh(∗) is possible in principle, but we will ignore it for now. 

And, the paper also notes that:

These signatures can be explained by the decay of a neutral scalar H into a lighter one S and the SM Higgs, i.e. H → Sh, SS, as realized e.g. within the 2HDM+S model (also called N2HDM).

The combined mass of an S scalar (as the authors hypothesize it) and an (ordinary) Higgs boson would be about 276.8 GeV. The combined mass of two S scalars would be about 303 GeV and this decay mode seems to be the one emphasized in the paper.

Two W bosons would have a mass of about 160.7 GeV (prior to adjustments due to energy scale that would reduce that combined mass at high enough energies bringing them closer to the 151.5 GeV target identified by the papers whose abstracts, at least, don't reveal the magnitude of the uncertainty in that value. The W boson pair would also have about the right decay width (the greater the decay width, the more rapidly a particle decays), to fit this resonance.

Non-W Boson Background Confounds Seem Less Likely

These masses are far in excess of any possible hadrons (i.e. composite particles of quarks and gluons), even exotic tetraquark, pentaquark or hexaquark hadrons that could hypothetically exist. 

These masses also exceed the masses of any single Standard Model boson (the most massive of which is the Higgs boson at 125.25 GeV).

The proposed S particle is less massive than a top quark (with a mass of about 173 GeV), but a top quark can't decay to a diphoton state and nearly always decays to a bottom quark and a W+ boson, so confusion with that is unlikely. 

The combined masses also aren't very close to the combines masses of a top quark and top anti-quark pair (about 346 GeV), or a two Higgs bosons and a bottom quark and bottom anti-quark pair (258.9 GeV).

Implications Of Its Apparent Decay Width

Lubos Motl also notes that the resonance has a 2 GeV decay width.

The Standard Model Higgs boson has theoretically calculated much narrower width of 4 MeV (which implies a mean lifetime of 10^-22 seconds). It isn't obvious why the S particle proposed, or a heavy neutral Higgs boson from which it decays, would have a so much greater width than the Standard Model Higgs boson, and one can greatly narrow the parameter space of possible couplings of these hypothetical scalar particles from this width. If this resonance is a real beyond the Standard Model particle, it can't, for example, have the same couplings as the Standard Model Higgs boson.

The top quark has a measured width of about 1.99 GeV which is consistent with its predicted width in the Standard Model of about 1.4 GeV (which is equivalent to a mean lifetime of about 5  * 10^-25 seconds). The W boson and Z boson, respectively, have a width of about 2.1 GeV and 2.5 GeV (which is equivalent to a mean lifetime of about 3 * 10^-25 seconds). Both the W and Z boson widths are measured to superb accuracy, which rules out particles that couple to these bosons with masses less than the W or Z boson. This is because such a coupling would change the decay width, which can be calculated in the Standard Model, from the experimentally measured value.

A particle that decays via the weak force but has a width greater than the width of the W boson could be problematic (one hypothesis for the reason that there are only three generations of fundamental fermions is that a more massive fermion would have a width greater than the W boson, which isn't possible). The hypothetical S boson proposed doesn't exceed that threshold, however, although it comes close.

Also, as the paper notes, the H and S bosons proposed in the paper couldn't have very significant couplings with the Standard Model Higgs boson h, because if this was the case the measured properties of the Standard Model Higgs boson would differ from what is observed to a far greater degree.

Prior Experimental Exclusions Of BSM Higgs Bosons 

There is also the question of how to reconcile this result with previous work that has excluded a new heavy neutral Higgs boson with a mass of under 1,496 GeV in the context of supersymmetric theories that have two Higgs doublets. A comprehensive model independent exclusion, however, is difficult because a heavy neutral Higgs boson would have different predicted decays with different theoretically proposed couplings to other particles.

Charged Higgs bosons heavier than a top quark are excluded for masses from 173 GeV up to 1,103 GeV, and for charged Higgs bosons with masses below 80 GeV are also excluded. But, the Particle Data Group does not report an exclusion for charged Higgs bosons with masses from about 80 GeV to 173 GeV. But, to produce a Higgs boson and S boson, one would need a pair of charged Higgs bosons in this allowed mass range, and not just one heavy neutral Higgs boson as assumed in the paper.


This anomaly deserves monitoring, but despite the claimed high significance, isn't something to get too excited about yet. 


Andi said...

Note that the real width of the particle can be much smaller as the one from the fit, since the latter is dominated by the detector resolution. Also charged Higgs bosons are not excluded up to 1,103 GeV, this is only the case for a specific 2HDM, the type II one. H anyway stands for an additional neutral Higgs boson. A potentially underestimated WW background might be related to the multi-lepton anomalies, but not the 151 GeV excess, mainly appearing in photons. Anyway, the WW background in the SM is well understood.

andrew said...

"the real width of the particle can be much smaller as the one from the fit, since the latter is dominated by the detector resolution"

Fair point.

"Also charged Higgs bosons are not excluded up to 1,103 GeV, this is only the case for a specific 2HDM, the type II one. H anyway stands for an additional neutral Higgs boson."

Nonetheless, this is all fairly well tread territory. The models that are picked as benchmarks to do exclusions for at the LHC are picked because they are representative of the well motivated possibilities that have been proposed. I acknowledge in my post that you can adjust the hypothetical particle couplings to evade almost any exclusion limitations. Likewise, While this paper proposes both an S and an H, one would think that similar searches would show some hint of both, and an 2HDM + S is really digging deep into the archives of explanatory theories and isn't very well motivated.

"A potentially underestimated WW background might be related to the multi-lepton anomalies, but not the 151 GeV excess, mainly appearing in photons. Anyway, the WW background in the SM is well understood."

Mr. Lubos is a total crackpot when it comes to COVID and politics, but he actually is a pretty darned good bump hunter in the HEP field. I'm inclined to give him the benefit of the doubt here on how the WW background could be mishandled by basically a poor choice of cutoffs used to select the data you are going to analyze.

If your paper is claiming a 5.1 sigma detection of a new particle, I expect your paper to do a more thorough vetting of non-BSM ways that this result could have arisen than this preprint does. The lack of enthusiasm in the HEP community also suggests that I'm not alone in my skepticism despite such a facially strong claim.

neo said...

so do we need a 100tev collider at cern?

neo said...

btw can you blog this

andrew said...

@neo I've read the post and glanced at the underlying paper. I'm very intrigued. I haven't groked it enough to write about it yet.

andrew said...

@neo "so do we need a 100tev collider at cern?"

Not yet. The cost-benefit isn't there right now. The money would be better spent on neutrino physics, astrophysics, Lattice QCD computational power, and lower energy hadronic physics and precision measurements.

neo said...

Not yet. The cost-benefit isn't there right now.

what about upgrade to the magnets he-lhc

neo said...

@neo I've read the post and glanced at the underlying paper. I'm very intrigued. I haven't groked it enough to write about it yet.

i'd be interested in read this