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Light quark Yukawa couplings from Higgs kinematics

Yotam Soreq, Hua Xing Zhu, Jure Zupan

TL;DR

The paper addresses the question of whether light-quark Yukawa couplings can deviate from their SM expectations and proposes to use normalized Higgs kinematic distributions, $1/\sigma_h \cdot d\sigma_h/dp_T$ and $1/\sigma_h \cdot d\sigma_h/dy_h$, to isolate potential $q\bar q \to h$ contributions. It introduces $\bar{\kappa}_q = y_q^{\rm exp}/y_b^{\rm SM}$ and demonstrates that enhancing $y_u$ or $y_d$ shifts the Higgs production towards forward rapidity and softens the $p_T$ spectrum, with normalized shapes reducing theoretical uncertainties. Using LO simulations with reweighting to higher-order predictions, the authors extract 8 TeV ATLAS bounds $[\bar{\kappa}_u]_{8\text{ TeV},p_T}<0.46$ and $[\bar{\kappa}_d]_{8\text{ TeV},p_T}<0.54$, and project 13 TeV sensitivities to $\bar{\kappa}_u<0.36$ and $\bar{\kappa}_d<0.41$, suggesting potential flavor non-universality in the down sector with sufficient luminosity. The work highlights that while strange Yukawa remains challenging via these observables, charm could be probed with dedicated channels, and stresses the need for higher-order QCD calculations and mass-dependent effects to sharpen future constraints. Overall, the approach provides a robust, indirect probe of light-quark Yukawas complementary to direct channels and total width measurements, with meaningful implications for HL-LHC physics.

Abstract

We show that the normalized Higgs production $p_T$ and $y_h$ distributions are sensitive probes of Higgs couplings to light quarks. For up and/or down quark Yukawa couplings comparable to the SM $b$ quark Yukawa the $\bar u u$ or $\bar d d$ fusion production of the Higgs could lead to appreciable softer $p_T$ distribution than in the SM. The rapidity distribution, on the other hand, becomes more forward. We find that, owing partially to a downward fluctuation, one can derive competitive bounds on the two couplings using ATLAS measurements of normalized $p_T$ distribution at 8\,TeV. With 300 fb${}^{-1}$ at 13\,TeV LHC one could establish flavor non-universality of the Yukawa couplings in the down sector.

Light quark Yukawa couplings from Higgs kinematics

TL;DR

The paper addresses the question of whether light-quark Yukawa couplings can deviate from their SM expectations and proposes to use normalized Higgs kinematic distributions, and , to isolate potential contributions. It introduces and demonstrates that enhancing or shifts the Higgs production towards forward rapidity and softens the spectrum, with normalized shapes reducing theoretical uncertainties. Using LO simulations with reweighting to higher-order predictions, the authors extract 8 TeV ATLAS bounds and , and project 13 TeV sensitivities to and , suggesting potential flavor non-universality in the down sector with sufficient luminosity. The work highlights that while strange Yukawa remains challenging via these observables, charm could be probed with dedicated channels, and stresses the need for higher-order QCD calculations and mass-dependent effects to sharpen future constraints. Overall, the approach provides a robust, indirect probe of light-quark Yukawas complementary to direct channels and total width measurements, with meaningful implications for HL-LHC physics.

Abstract

We show that the normalized Higgs production and distributions are sensitive probes of Higgs couplings to light quarks. For up and/or down quark Yukawa couplings comparable to the SM quark Yukawa the or fusion production of the Higgs could lead to appreciable softer distribution than in the SM. The rapidity distribution, on the other hand, becomes more forward. We find that, owing partially to a downward fluctuation, one can derive competitive bounds on the two couplings using ATLAS measurements of normalized distribution at 8\,TeV. With 300 fb at 13\,TeV LHC one could establish flavor non-universality of the Yukawa couplings in the down sector.

Paper Structure

This paper contains 4 sections, 4 equations, 7 figures.

Table of Contents

  1. Introduction
  2. Light Yukawa couplings from Higgs distributions
  3. Current constraints and future prospects
  4. Conclusions

Figures (7)

  • Figure 1: The $1/\sigma_h\cdot d\sigma_h/dy_h$ (left) and $1/\sigma_h\cdot d\sigma_h/dp_T$ (right) normalized distributions at $\sqrt s=13$ TeV collision energy for several values of up quark Yukawa couplings, $\bar{\kappa}_u=0$ (SM, blue), $\bar{\kappa}_u=1$ (orange), $\bar{\kappa}_u=4$ (green).
  • Figure 2: The upper panels show the rapidity distribution $d\sigma_h/dy_h$ (left), where the Higgs decay to $\gamma\gamma$, and the normalized rapidity distribution $1/\sigma_h\cdot d\sigma_h/dy_h$ (right) calculated at LO, NLO and NNLO (red, black, blue lines respectively) using HNNLOCatani:2007vq, see text for details. The lower panels show NLL (black) and NNLL (blue) predictions for $d\sigma_h/dp_T$ (left) and $1/\sigma_h\cdot d\sigma_h/dp_T$ (right), obtained using HqT2.0Bozzi:2005wkdeFlorian:2011xf. Blue bands denote scale dependence when varying $m_h/4<\mu<m_h$.
  • Figure 3: The predictions for the $1/\sigma_h\cdot d\sigma_h/dy_h$ (left) and $1/\sigma_h\cdot d\sigma_h/dp_T$ (right) normalized distributions at $\sqrt s=13$ TeV collision energy . The tree-level MadGraph 5+Pythia is shown in solid black, while the QCD NNLO rapidity distribution computed using HNNLO code Catani:2007vq, shown in the left panel (the NNLO+NNLL resumed $p_T$ distributions computed using HqT2.0Bozzi:2005wkdeFlorian:2011xf, shown in the right panel), are denoted by blue lines. Theoretical uncertainties, denoted by the blue bands, are estimated by varying the resummation scale between $m_h/4$ and $m_h$.
  • Figure 4: The $1/\sigma_h\cdot d\sigma_h/dy_h$ (left) and $1/\sigma_h\cdot d\sigma_h/dp_T$ (right) when switching on up (orange), down (green) and strange (red) Yukawa coupling.
  • Figure 5: Left: The ATLAS 8TeV measurement of the normalized Higgs $p_T$ distribution (black) Aad:2015lha, and the theoretical predictions for the SM (blue), $\bar{\kappa}_u=2$ (red), $\bar{\kappa}_d=2$ (orange). Right: The resulting $1\sigma$ ($2\sigma$) allowed regions for the up and down Yukawa are denoted by dark gray (light gray) shadings, while the dashed line denotes the $2\sigma$ expected sensitivity.
  • ...and 2 more figures