New Constraints On The Higgs Boson Width

[ohwilleke]

TL;DR Summary

A new ATLAS measurement limits the Higgs boson width (i.e. the inverse of its mean lifetime) of 4.5 MeV is close to the Standard Model prediction of 4.1 MeV and has only modest uncertainty.

The New Measurement

The ATLAS experiment at the Large Hadron Collider (LHC) has experimentally limited the "width" of the Standard Model Higgs boson with a rest mass of about 125 GeV to 4.5 + 3.3 - 2.5 MeV, with a maximum value of 10.5 MeV at a 95% confidence level.

In the Standard Model, the theoretically calculated width of its sole 125 GeV mass Higgs boson is 4.1 MeV (which implies a mean lifetime of 1.56 * 10^-22 seconds) (actually it's 4.07 MeV to three digit precision).

The result is consistent with the Standard Model expectation at the 0.16 sigma level. It also tightly observationally constrains deviations from the Standard Model width of the Higgs boson.

Existing physics instruments aren't powerful enough to directly measure the Higgs boson's width, although they can indirectly bound it with experimental observations, as the linked latest experimental measurement did.

The Paper and Its Citation

This Letter reports on a search for off-shell production of the Higgs boson using 139 fb−1 of pp collision data at s√= 13 TeV collected by the ATLAS detector at the Large Hadron Collider. The signature is a pair of Z bosons, with contributions from both the production and subsequent decay of a virtual Higgs boson and the interference of that process with other processes. The two observable final states are ZZ→4ℓ and ZZ→2ℓ2ν with ℓ=e or μ. In the ZZ→4ℓ final state, a dense Neural Network is used to enhance analysis sensitivity. The background-only hypothesis is rejected with an observed (expected) significance of 3.3 (2.2) standard deviations, representing experimental evidence for off-shell Higgs boson production. Assuming that no new particles enter the production of the virtual Higgs boson, its total width can be deduced from the measurement of its off-shell production cross-section. The measured total width of the Higgs boson is 4.5+3.3−2.5 MeV, and the observed (expected) upper limit on the total width is found to be 10.5 (10.9) MeV at 95% confidence level.

ATLAS Collaboration, "Evidence of off-shell Higgs boson production from ZZ leptonic decay channels and constraints on its total width with the ATLAS detector" CERN-EP-2023-3 https://arxiv.org/abs/2304.01532 (April 4, 2023)(submitted to Phys. Lett. B.).

Background: What Is A Particle's Width?

The width of a particle in this sense is the mean lifetime of a particle expressed in terms of electron volts rather than seconds. Width is inversely related to mean lifetime. The larger the width, the shorter the mean lifetime. The smaller the width, the longer the mean lifetime. One divided by width equal mean lifetime subject to a unit conversion constant from electron volts to seconds.

For comparison purposes, the width of the top quark is about 1,320 MeV, the width of the W boson is 2,085 ± 42 MeV, and the width of the Z boson is 2,495.2 ± 2.3 MeV. These imply mean lifetimes on the order of 10^-25 seconds.

A particle that can't decay and is stable, like an electron or a proton, has a width of zero.

How Is Width Be Calculated In The Standard Model?

The width of a particle, in this sense, can be calculated theoretically in the Standard Model for any particle in the Standard Model fundamental or composite, from its Standard Model properties.

This is done by identifying every possible way that the Standard Model fundamental or composite particle is allowed to decay in the Standard Model, calculating the likelihood that this will happen in a given time period given the Standard Model experimentally determined parameters, converting all of these probabilities into width units, and then adding up all of the widths for particular decay paths to get a total width of the particle.

If a particle has an experimentally measured width greater than the Standard Model prediction, then that means that you missed a possible decay channel of the particle (possibly via a non-Standard Model particle, and possibly because you just screwed up and missed a possibility).

What Does The New Measurement Mean For New Physics?

The 95% confidence interval boundary implies that overlooked beyond the Standard Model decays of the Higgs boson not considered in determining the Standard Model predicted value can't have combined widths of more than 6.4 MeV without being in tension with this observation. So, this measurement significantly limits the extent to which there can be beyond the Standard Model particles that get their rest mass via the Higgs mechanism. Articulating the precise limitation that this places on such particles is something I won't theorize about myself without a paper to back it up.

Since all fundamental particles in the Standard Model (with the possible exception of neutrinos) get their rest mass via the Higgs mechanism, the width of the Higgs boson, like the anomalous magnetic moment of the muon (muon g-2), the decays of the W and Z bosons, and the relative masses of the W boson and the Z boson, is a significant precision global constraint on possible undiscovered fundamental particles (e.g. particles that could give rise to dark matter particles, or fifth forces).

Collectively, these measurements place tight limits on the masses and properties of any beyond the Standard Model particles.

 

[malawi_glenn]

So, this measurement significantly limits the extent to which there can be beyond the Standard Model particles that get their rest mass via the Higgs mechanism.

*BSM particles that get their rest mass via the Higgs mechanism and that are lighter than mH/2. You know this but omitted it on purpose?

 

[malawi_glenn]

The width of a particle, in this sense, can be calculated theoretically in the Standard Model for any particle in the Standard Model fundamental or composite, from its Standard Model properties.

This is done by identifying every possible way that the Standard Model fundamental or composite particle is allowed to decay in the Standard Model, calculating the likelihood that this will happen in a given time period given the Standard Model experimentally determined parameters, converting all of these probabilities into width units, and then adding up all of the widths for particular decay paths to get a total width of the particle

Kudos to you for writing these introductory explanations. But each paragraph is comprised of one senten each. And each paragraph contains "Standard Model" three times. It is therefore very hard to read.

 

[malawi_glenn]

So, this measurement significantly limits the extent to which there can be beyond the Standard Model particles that get their rest mass via the Higgs memechanism

No. What if particle X has a mass of 3×mH?

 

[mfb]

*BSM particles that get their rest mass via the Higgs mechanism and that are lighter than mH/2. You know this but omitted it on purpose?

You could in principle see these in a width measurement, but searches for Higgs to invisible decays (tagged by other particles produced together with the Higgs) are more sensitive.
What ATLAS is looking for here is the coupling of the Higgs to potential unknown particles, they don't have to be light because the Higgs doesn't decay to anything here (it's an off-shell measurement).

 

[malawi_glenn]

You could in principle see these in a width measurement, but searches for Higgs to invisible decays (tagged by other particles produced together with the Higgs) are more sensitive.
What ATLAS is looking for here is the coupling of the Higgs to potential unknown particles, they don't have to be light because the Higgs doesn't decay to anything here (it's an off-shell measurement).

Yes, in principle, but at this stage and probably never at LHC - no.

If the BSM particle X has HXX or HHX couplings, you would have another propagator suppression apart from the Breit-Wigner suppression from the H-propagator for the processes they considered in this paper. Of course higgs does not need to decay in the processes considered here. However, if mX > mH/2, the inclusion of particle X in your theory will only contribute slighly to $\Gamma_H$. Thus only if mX < mH/2 you can have a "significant" contribution to $\Gamma_H$ and thus influence the observables considered in this paper.

We are nowhere near of putting "tight" limits on BSM particles heavier than mH/2 which couples to H by measuring $\Gamma_H$.

The original post makes too far reached conclusions.

 

[ohwilleke]

*BSM particles that get their rest mass via the Higgs mechanism and that are lighter than mH/2. You know this but omitted it on purpose?

My intent in saying "significantly limits the extent to which there can be beyond the Standard Model particles" was simply to say that it imposes real global limitations without specifying exactly what those limitations were.

I think you are reading more into a vague statement than it really says.

 

[malawi_glenn]

My intent in saying "significantly limits the extent to which there can be beyond the Standard Model particles" was simply to say that it imposes real global limitations without specifying exactly what those limitations were. It is not to say that the limitations are unbounded.

Well you are a lawyer, you know how to put the truth in a way that favors your favorite outcome by playing word games.

TL;DR Summary: A new ATLAS measurement limits the Higgs boson width (i.e. the inverse of its mean lifetime) of 4.5 MeV is close to the Standard Model prediction of 4.1 MeV and has only modest uncertainty.

I'd say the uncertainty is still pretty huge.

 

[ohwilleke]

I'd say the uncertainty is still pretty huge.

The uncertainty in some of the early measures of Higgs boson width had uncertainties that were much larger.

For example, in "Measurement of the properties of a Higgs boson in the four-lepton final state", CMS Collaboration (Submitted on 18 Dec 2013), the 95% confidence interval upper bound was 3,400 MeV.

A conference paper from 2017 had the 95% confidence interval upper bound at 1,700 MeV.

 

[malawi_glenn]

The uncertainty in some of the early measures of Higgs boson width had uncertainties ten to a hundred times larger.

Yes, but then you write "has less uncertainties than earlier measurments" instead.

 

[ohwilleke]

Yes, but then you write "has less uncertainties than earlier measurments" instead.

I've edited to be more substantiated and clear and the spell things correctly.

 

[Vanadium 50]

I'm a bit puzzled why this is considered so interesting on PF.

1. Its not the first measurement.
2. It may not be the best measurement. If it is, it's not hugely better.
3. It's in the category of "we should check"., But we already know the width can't be mroe than maybe 25 or 30% discrepant from other measurements (below) and if this measurement were to be discrepant the reaction would not be "the width is wrong!" but rather "we need to revisit the assumptions used in calculating the width."

If the H width were 10x larger because some other particle or particles had a partial width 9x larger than the SM what would happen"
1. ggF production would also need to go up by a factor of 9-ish to cover the observed rates into gamma gamma, WW* and ZZ*. This is possible, but then the fraction of VBF production falls by this same factor, and now it becomes discrepant.
2. We know the HWW and HZZ couplings are the same in production and decay, so you can't adjust them to fix #1 without moving the discrepanvy elsewhere.
3. VBF production is as predicted, so the WWH and ZZH couplings are close the the SM values.

So there may be some wiggle room at the 20-30% level by carefully peanut-buttering the discrepancies around. but the only way you can get a large change is to invoke new physics to change the width and different new physics to hide the fact you've done that. And possibly turtles all the way down.

It doesn't m,ean it shouldn't be checked, but it's in the spirit of comparing two numbers that should be equal to see if they are.