Higgs boson production in partonic and electromagnetic interactions with heavy ions

TL;DR

This paper evaluates Higgs boson production via partonic and electromagnetic interactions in heavy-ion collisions at current and future colliders, quantifying cross sections and experimental feasibility.

Contribution

It provides the first comprehensive calculation of Higgs production cross sections in heavy-ion collisions, including electromagnetic interactions, and assesses detection prospects at colliders.

Findings

Electromagnetic-to-partonic Higgs cross section ratio is 10^-5 to 10^-4.

At FCC energies, this ratio increases by a factor of 10-100 in heavy-ion collisions.

A 5σ Higgs detection is feasible within one month at the FCC.

Abstract

Higgs boson production in partonic and electromagnetic (photon-photon) interactions of light- and heavy-ions (A-A) at the LHC and future colliders is summarized. Parton-induced cross sections --including gluon-gluon, vector-boson fusion, and associated W, Z, t processes-- are computed at NNLO with MCFM 8.0 using nuclear parton distribution functions. Photon-photon cross sections are computed with MadGraph 5.0 for ultraperipheral A-A interactions with both ions surviving the collision. In the center-of-mass energy range $\sqrt{s} \approx 5$--100 TeV, the ratio of electromagnetic-to-partonic Higgs cross sections is $\rm R_{e.m./parton} \approx 10^{-5}$--10$^{-4}$. At FCC energies, this ratio is 10--100 times larger in A-A than in p-p thanks to the strong nuclear coherent $\gamma$ fluxes. The feasibility of Higgs boson measurements at LHC, HE-LHC, and FCC, in the most-favourable $H\to\gamma\gamma,b\bar{b}$ decay channels in partonic and e.m. interactions respectively, is determined taking into account standard acceptance and efficiency losses on the final states, plus selection criteria to minimize the respective continuum backgrounds. Whereas $3\sigma$ evidence for partonic and e.m. Higgs production requires about $\times$35 and $\times$200 ($\times$7 and $\times$30) larger integrated luminosities than those expected for a nominal 1-month run at the LHC (HE-LHC), a $5\sigma$ observation of both production modes is warranted in just one month at the FCC.

Full text

1 Introduction

The Higgs boson remains unobserved in heavy-ion collisions at the LHC due to the lower nucleon-nucleon c.m. energies ($\sqrt{\rm s_{{}_{\mathrm{NN}}}}$), as well as integrated luminosities ($\mathcal{L}_{\rm\tiny{int}}$), in A-A compared to p-p collisions. Three planned or proposed CERN future hadron colliders: the high-luminosity LHC (HL-LHC) [1], high-energy LHC (HE-LHC) [1, 2], and Future Circular Collider (FCC) [3], will noticeably increase the available c.m. energies and $\mathcal{L}_{\rm\tiny{int}}$ values, thereby making the observation of the scalar boson potentially feasible in nuclear collisions. The motivations for such a measurement are twofold. First, when produced in standard parton-induced processes, the H boson can be used to probe the properties of the quark-gluon plasma produced in A-A collisions [4] (although this possibility has been challenged in Ref. [5]). Secondly, the Higgs boson can also be exclusively produced via A-A$\xrightarrow{\gamma\gamma}$(A)H(A) in ultraperipheral heavy-ion collisions (UPCs) where the ions survive their electromagnetic (e.m.) interaction. This latter unique production mode exploits the huge e.m. fields generated by the collective action of $Z$ individual proton charges, leading to photon-photon cross sections enhanced by up to factors $Z^{4}\approx 5\cdot 10^{7}$, for Pb-Pb, compared to proton or e± beams [6]. The UPC production provides an independent measurement of the H-$\gamma$ coupling not based on Higgs decays but on $s$-channel production, and of the total Higgs width by combining it with the LHC diphoton decay branching ratio, via $\rm\Gamma_{\rm H}^{\rm tot}=\Gamma(H\to\gamma\gamma)/\mathcal{B}(H\to\gamma\gamma)$ [7].

The parton-initiated cross sections –including gluon-gluon, vector-boson fusion, and associated W, Z, t processes– have been studied for A-A collisions at the LHC and FCC in Ref. [8], including their expected yields after analysis cuts in the cleanest $\rm H\to\gamma\gamma$ decay. The Higgs UPC-production was first considered 30 years ago [9], and detailed studies of the dominant ${\rm H}\to b\overline{b}$ decay channel, including realistic acceptance and efficiencies for signal and $\gamma\gamma\to b\overline{b}$ continuum background presented in Refs. [10] for LHC, HE-LHC, and FCC. This work here extends previous studies by including also light-ion collisions, and compares the results of both production modes.

2 Higgs boson in partonic A-A collisions

The Higgs cross sections in p-Pb and Pb-Pb collisions over $\sqrt{\rm s_{{}_{\mathrm{NN}}}}\approx 3$–100 TeV have been computed at NNLO accuracy in Ref. [8] with mcfm (v.8.0) [11], using the CT10 proton PDFs [12] and EPS09 nuclear PDFs (nPDFs, including its 30 eigenvalues sets) [13], and fixing the default renormalization and factorization scales at $\rm\mu_{R}=\mu_{F}=m_{\rm H}/2$. The PDF uncertainties, obtained adding in quadrature the CT10 and EPS09 uncertainties, are $\sim$10%. The computed nucleon-nucleon cross sections are scaled by the mass number A or A2 to obtain the corresponding p-A or A-A cross sections respectively, including those for future runs with lighter-ions (Xe-Xe, Kr-Kr, Ar-Ar, and O-O), not considered so far. Compared to the corresponding p-p results at a given c.m. energy, antishadowing nPDF modifications slightly enhance (deplete) the nucleon-nucleon yields by $\sim$3% at the LHC (FCC). The ${{\cal{O}}}\left(5-10\%\right)$ theoretical $\mu_{R,F}$ scale uncertainties cancel out in the ratios of (p-Pb,Pb-Pb)/(p-p) cross sections at the same colliding energy. The collision energy dependence of the parton-induced Higgs cross sections for all colliding systems is shown Fig. 1 (left, solid curves). The cross sections increase by about a factor of $\times$4 and $\times$20 between LHC ($\sigma_{\rm PbPb\to H+X}\approx$ 500 nb), and HE-LHC ($\sim$2 $\mu$b) and FCC ($\sim$11.5 $\mu$b) respectively.

A first Higgs measurement in nuclear collisions will use the clean diphoton and 4-lepton final-state channels as done in p-p [14], with very low branching ratios but small and/or controllable backgrounds. After branching ratios ($\mathcal{B}=0.23\%$ for $\gamma\gamma$, 0.012% for $4\,\ell$), and taking into account typical ATLAS/CMS analysis cuts111$p_{T}^{\gamma}>30,40$ GeV, $|\eta^{\gamma}|<2.5,\,5$ (LHC, FCC); $p_{T}^{\ell}>20,15,10,10$ GeV, $|\eta^{\ell}|<2.5,\,5$ (LHC, FCC); $R_{\rm isol}^{\gamma,\ell}=0.3$., which result in $\sim$50% acceptance and efficiency signal losses, one expects about 10, 50, and 1 500 Higgs bosons per month in Pb-Pb at the LHC (5.5 TeV), HE-LHC (10.6 TeV), and FCC (39 TeV) with $\mathcal{L}_{\rm\tiny{int}}=$ 10, 10, and 110 nb*-1* respectively, on top of the corresponding $\gamma\gamma$ and $4\ell$ non-resonant backgrounds. In the $\gamma\gamma$ case, the backgrounds include the irreducible QCD diphoton continuum plus 30% of events coming from misidentified $\gamma$-jet and jet-jet processes. For the nominal HL-LHC and HE-LHC luminosities, the significances of the diphoton signal (S) over the background (B), computed via S/$\sqrt{\rm B}$ at the Higgs peak, are 0.5 and 1.2$\sigma$, and thus one would need a factor of $\times$35 and $\times$7 larger integrated luminosities to reach $3\sigma$ evidence at both machines. A straightforward way to gain a factor of $\sim$10 in $\mathcal{L}_{\rm\tiny{int}}$ would be to run one full-year, instead of one heavy-ion month, but even in this case evidence at the HL-LHC would require an extra $\times$3 increase in the instantaneous luminosity. The situation is much more favourable at HE-LHC and FCC. Figure 2 shows the expected diphoton invariant mass distributions for Pb-Pb at HE-LHC ($\times$7 the nominal 1-month luminosity, left) and FCC (nominal luminosity, right). The significances of the H$(\gamma\gamma)$ peaks are 3$\sigma$ and 5.5$\sigma$ respectively.

3 Higgs boson in electromagnetic A-A collisions

The madgraph 5 (v.2.6.5) Monte Carlo event generator [15] is employed to compute the UPC cross sections for Higgs boson and diquark continuum ($b\overline{b}$, as well as misidentified $c\overline{c}$ and light-quark $q\overline{q}$, pairs) [10], by convolving the Weizsäcker-Williams equivalent $\gamma$ fluxes of the ions (as a function of impact parameter $b$) [16] and/or protons [17] with the elementary $\gamma\gamma\to\rm H$ cross section (with H-$\gamma$ coupling parametrized in the Higgs effective field theory). We exclude nuclear overlap by imposing $b_{1}>R_{A_{1}}$ and $b_{2}>R_{A_{2}}$ for each $\gamma$ flux (where $R_{A}$ is the radius of nucleus A), and applying a correcting factor [18] on the final cross section that depends on the ratio of Higgs mass over $\sqrt{\rm s_{{}_{\gamma\gamma}}}$. The computed exclusive $\gamma\gamma\to$ Higgs cross sections are shown in Fig. 1 (left, dashed curves) as a function of $\sqrt{\rm s_{{}_{\mathrm{NN}}}}$ for all colliding systems. The larger the charge of the ions, the larger the UPC cross sections. In the Pb-Pb case, the Higgs cross sections increase by about a factor of $\times$10 and $\times$100 between LHC ($\sigma_{\rm\gamma\gamma\to\,H}\approx$ 15 pb), and HE-LHC ($\sim$150 pb) and FCC ($\sim$1.8 nb) respectively. Note that, below $\sqrt{\rm s_{{}_{\mathrm{NN}}}}\approx 5$ TeV, the e.m. production cross section in Pb-Pb is larger than the partonic one in p-p collisions. The ratio of e.m.-to-partonic Higgs cross sections is shown in Fig. 1 (right). For A-A collisions, one finds $\rm R_{e.m./parton}\approx 10^{-5}$–10*-4* due to the strong nuclear coherent $\gamma$ fluxes. This ratio in practice amounts to $\rm R_{e.m./parton}\approx 0.01$–0.03 when one further accounts for the $\mathcal{B}=0.27\%,56\%$ of the most-favourable $\rm H\to\gamma\gamma,b\overline{b}$ decay channels for a measurement in partonic and e.m. interactions respectively. As a function of c.m.  energy, $\rm R_{e.m./parton}$ decreases for p-p (as the partonic cross sections increase faster than the e.m. ones) but increases for A-A (as the energy of the coherent $\gamma$ fluxes rise) up to a point where it levels off and starts to drop. At FCC energies, the ratio is 10–100 larger in A-A than in p-p.

The feasibility of measuring e.m. exclusive Higgs production is studied in its dominant H $\to b\overline{b}$ decay. Taking into account the expected beam luminosities, the most competitive system to try a measurement is Ar-Ar (Kr-Kr) at the HL-LHC (HE-LHC). At the FCC, the combination of $\times$100 larger cross sections and $\times 10$ larger $\mathcal{L}_{\rm\tiny{int}}$ yield $\sim$200 Higgs produced per month ($\sim$110 in the $b\overline{b}$ channel). The following acceptance and reconstruction performances are assumed: jet reconstruction over $|\eta|<2$ ($5$ for FCC), 7% b-jet energy resolution (resulting in $\sim$6 GeV dijet mass resolution at the Higgs peak), 70% b-jet tagging efficiency, and 5% (1.5%) b-jet mistagging probability for a $c$ ($q$) quark. These selection criteria lead to a $\sim$50% signal loss, and a total reduction of the misidentified $c\overline{c}$ and $q\overline{q}$ non-resonant backgrounds by factors of $\sim$400 and $\sim$4500 respectively. The remaining irreducible $b\overline{b}$ continuum is further suppressed through kinematical cuts on the jet $p_{T}^{jet}\approx$ 55–67 GeV, invariant dijet mass $m_{b\overline{b}}\approx$ 116–134 GeV, and dijet angular distribution $|\cos\theta_{j_{1}j_{2}}|<0.5$. The final signal significance is derived from the number of counts around the Gaussian Higgs peak over the dijet continuum. Evidence for UPC Higgs at the HL-LHC requires $\times$200 times larger-than-nominal $\mathcal{L}_{\rm\tiny{int}}$ values in Ar-Ar(6.3 TeV), so as to reach S/$\sqrt{\rm B}\approx 10/\sqrt{12}\approx 3$ (Fig. 3, left). The same analysis for Kr-Kr(12.5 TeV) at the HE-LHC indicates that $\times$30 larger $\mathcal{L}_{\rm\tiny{int}}$ values are needed than currently designed. On the other hand, Pb-Pb collisions at $\sqrt{\rm s_{{}_{\mathrm{NN}}}}$ = 39 GeV with the FCC nominal $\mathcal{L}_{\rm\tiny{int}}=110\;\mbox{nb}^{-1}$ per month, yield $\sim$20 signal counts over about the same number for the sum of backgrounds. Reaching a $5\sigma$ statistical significance would just require to combine the measurements of the first run from two different experiments or accumulating two 1-month runs in a single one (Fig. 3, right).

4 Conclusions

The expected Higgs production cross sections and yields from parton-induced and exclusive photon-photon processes in heavy- and light-ion collisions have been studied in the diphoton and $b\overline{b}$ final states respectively at the HL-LHC, HE-LHC, and FCC. Evidence for the scalar boson, over the expected continuum backgrounds, seems out of reach at the HL-LHC with the currently expected integrated luminosities, but can be possible in the partonic mode at HE-LHC. Full observation in both production modes requires a future hadron collider such as the FCC.