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Les Houches

2021 Session

Wikis of Previous sessions

Les Houches Themes



Responsible: Grégory Moreau [TH](, Susan Gascon-Shotkin [EXP](

Participants (please add your name and your email): Jérémie Quevillon (, Antoine Lesauvage (, Aoife Bharucha (, Bill Murray (, Nishita Desai ( Jose Miguel No (, Jon Butterworth (, Natascia Vignaroli (, Kohsaku Tobioka (, Giacomo Polesello (, Sophie Williamson (, Shilpi Jain(, Shankha Banerjee (, Biplob Bhattacherjee(, José Zurita (, Thomas Flacke (

⇒ First MEETING Friday June 21 at 17:00pm in the QCD Room.

⇒ Second MEETING Saturday June 22 at 14:30pm in the Library.

⇒ Dedicated MEETING on Exotic Higgs decays $h \to Z \phi$ Monday June 24 at 16:00 in the Auditorium

⇒ Third MEETING (on all possible light scalar subjects) on Tuesday June 25 at 14:30 in the QCD Room

IMPORTANT: Please see the 'EXECUTIVE SUMMARY' of potential subjects for the proceedings AT THE END OF THIS WIKI and recommendations for preparation of contributions to Friday's summary talk (please send to Gregory, Susan and Jose by Thursday morning)

Down to which masses the LHC can test (pseudo-)scalars? And how?

Below 80-90GeV, for a generic (pseudo-)scalar $\phi$ (light Higgs, radion, dilaton, axion, ALP's…), the decay $\phi$ → Z*Z is kinematically closed and $\phi$ → $\gamma\gamma$ becomes challenging to trigger on, so that the gluon-gluon Fusion mechanism to produce $\phi$ is not promising anymore (see for instance 1607.08653).

  • Quick contur study using the BNT ALP UFO file. All couplings set to zero except the photons which is scanned over. 2D scan . Over a wide range of $\phi$ masses ($M_{ALP}$ on the plot) the sensitivity extends down to $C_{\gamma\gamma}/\Lambda \approx 3$ TeV$^{-1}$. Here the scan is performed over the following ranges: $m_a:$ 1 to 50 GeV, $g_{a\gamma\gamma}$: 10 to $10^{-6}$, $g_{aZ\gamma}$: 10 to $10^{-6}$
  • Checking at an example point in the above scan, $M_{ALP} = 1.8$ GeV, $C_{\gamma\gamma}/\Lambda = 5.2$ TeV$^{-1}$, all other couplings set to zero. The $\phi$ decays 100% to $\gamma\gamma$ as expected (width $10^{-7}$ GeV). The main production mechanisms are $\gamma, W$ or $Z$ + $\phi$. $gg \rightarrow \phi$ is zero. The different subprocess cross sections are visible in the example Herwig output file: lhc-s101-runpoint_0095.out
  • Some more scans as suggested by Aoife and Jose:

For instance, Higgs composite models with additional light scalars:1710.11142, 1902.08690.

Motivation: the produced Vector boson V helps to trigger on the signal (for example: Radion+Z production within the RS model studied in 1702.03984).

Context here: EFT with on-shell scalar ($\phi$) and scalar couplings to vector bosons only so look at the decay $\phi \to \gamma \gamma$ [here $\phi$ is not a stable DM candidate leading to missing $E_T$]. Looking at scalar couplings to fermions, other production and decay modes ($\phi \to \tau \tau$ or $\phi \to b \bar b$ [reconstructing the invariant scalar mass]) could be studied as well.

Gauge invariant (pseudo-)scalar couplings to ZZ, WW, $\gamma\gamma$, $\gamma$Z, gluon-gluon: $$\frac{\phi}{\Lambda_1}\vert D_\mu H\vert^2 \ \ (for \ scalars \ only) \ \ and \ \ \frac{\phi}{\Lambda_2} Tr[V_{\mu\nu} \tilde V^{\mu\nu}] \ .$$

Two other effective parameters: $\phi$ mass ($m_\phi$) and Branching for $\phi \to \gamma \gamma$ ($B_\gamma$).

Exhaustive list of hard PROCESSES (no focus on W production for EXP reasons):

  • $q \bar q$/$e^+e^-$ → Z*/$\gamma$* → Z $\phi$ followed by Z→$\ell^+\ell^- \ $ [first looked at Les Houches Session of 2017]. The LH2017 Contur study (which also considers inclusive $\phi$ production) is updated here.

$e^+e^-$ → Z* → Z$\phi$ or $\gamma$ $\phi$ at FCC [names: GP?]

$qq$ → Z* → Z$\phi$ interfaced to DELPHES [names: SG, AL]

  • $q \bar q$ → $Z^{(*)}$ → $\gamma$ $\phi \ $ [existing analysis from ATLAS?]
  • $q \bar q$ → $\gamma$* → $\gamma$ $\phi \ $ and $q \bar q$ → gluon* → gluon $\phi \ $ 1409.4792
  • VBF production of $\phi q q$ (possible tag on the 2 jets if above 40GeV?)
  • ggF+ISR: g g → $\phi$ with a gluon radiated off one of the 2 initial state gluons [credit: Thomas Flacke]. A scalar recoiling against a hard jet could allow to trigger on it. For example, motivated in extended Higgs sectors (e.g. light Higgses in 2HDM) when the 125GeV is SM like (2HDM “alignment limit”) and also for ALPs, see 1710.01743.
  • Associated $tt\phi$ production ?
  • In Z decays:

Z → $\gamma\phi$, $\phi\to\gamma\gamma$. Triphoton final state [names: GP?]

Exotic Higgs decays in $h \to Z \phi$

  • t'→ t$\phi$, $\phi$ → $\gamma\gamma$ [singlet pseudo-scalar in context of composite Higgs, see for example 1803.00021].
  • $qq \to Z^* \rightarrow h \phi$.
  • fat photons [names: SG, AL, KT]

Promising DECAYS: $\phi\to\gamma\gamma$ (and $\mu\mu$, $bb$, $\tau\tau$, Missing E_T ?) [names: TF, JZ]

General comment: If the particle is lighter than hadronic threshold ($m_a <3m_\pi<450\rm MeV$ for CP-odd scalar), the particle is easily long-lived in the reasonable parameter space. Many literature ignores loop-factor $16\pi^2$, which sometimes leads to less physical parameter space. A scale $\Lambda/C_{\gamma\gamma}=1\rm TeV$ in 1708.00443 parameterization corresponds to a scale $f_a =6\rm GeV$ in the decay constant (SSB scale).So ALP being invisible is quite possible with sub-GeV mass and $f_a >100\rm GeV$. Estimates with ALP as missing energy in 1409.4792 (mono-jet), 1701.05379 (mono-W, mono-Z, mono-Higgs) and 1901.03061.

For boosted configurations: for example gamma gamma + j(l)etc, 'fat' photon ids…

ALP UFO file from Bauer, Neubert, Thamm with fix for the ALP mass / Higgs/ Z mass bug from Peter Richardson.

ALP UFO file from 1701.05379 (Brivio, Gavela, Merlo, Mimasu, No, del Rey, Sanz) (with fix for the ALP width).

ALP linear UFO file from Brivio, Gavela, Merlo, Mimasu, No, del Rey, Sanz (also with fix for the ALP width - GP).

Readme file Conversion of couplings between two UFOs above - GP.

fat_photon_phenomenology from

$h^* \to Z \phi$ [UFO files from effective ggh coupling for kinematics and rescale rate with ggF cross section?]

A model apart from ALP for $h\to Z\phi$ is flipped 2HDM by0904.0705. However, the model with light A seems to be very constrained by Higgs mass and Higgs coupling measurements. Another straightforward model is 2HDM plus singlet but in this case the Higgs portal (SM+singlet) is simpler (refs).

'Executive Summary' of possible project list for the proceedings and input needed for the Friday summary talk:

  • Processes + models:
    • h–>Zphi—>llgamma gamma (or mu+mu-, or tau+tau- with recast from h→aa→2mu2tau, CMS-PAS-HIG-15-011) [BB, SJ, SB]
    • Z*–>Zphi–>llgamma or gammaphi–>3gamma
      • (Above 2 with 'Bauer et al.' model)
    • h*–>Zphi–>llgamma gamma (derive UFO from h*–>ZZ* with add'l functionality?) [SJ]
    • qqbar/e+e—>Z*/gamma*–>Zphi–>llgamma gamma (started LH 2017)
    • qqbar–>Z*/gamma*–>phi + 2j (VBF version of above)
      • (Above 2 with EFT model)
    • qqbar–>gluon*–>gluon + phi–>gamma gamma jet? (associated prdn)
    • ggf prod. with same final state (but probably different kinematics): gg–>phi–>gamma gamma + ISR gluon
    • tprime–tphi, phi–>gamma gamma (singlet pseudo-scalar, composite Higgs)
  • Theoretical: recasting as 2HDM and other benchmark models [JQ]
  • Experimental:
    • 'fat' photons [SJ]
    • statistical tool to produce proto-limits after proto-selections [SJ]: Such a tool is now ready. This uses likelihood ratio as a test statistic i.e. L(s+b)/L(b). Uses Cls method to quote the upper limits which is commonly and widely used by CMS and ATLAS. This has been cross-checked against the CMS limit tool for cut and counting experiment. But the present code can be extended to do a shape based limit.

Contributions on the above projects to the Friday closing talk (by the Higgs convenors) should include (~1-2 slides):

  • Your names (add initials to above)
  • Current status at end of LH (some description, plots/numbers + other info)
  • What you hope to accomplish for the proceedings (~mid-November)

Please send to Gregory, Susan and Jose by Thursday morning (so we can give a synthesis to the speaker by ~Thursday noon and also circulate among us)

Thank you all for all the work, and let's not stop here! Here are the slides shown in the summary session on June 28, 2019.

2019/groups/higgs/lightscalars.txt · Last modified: 2019/06/28 11:15 by suzanne.gascon