[Paper Review] Distinguishing `Higgs' Spin Hypotheses using gamma gamma and WW* Decays

John Ellis, Ricky Fok+3 more|arXiv (Cornell University)|Oct 18, 2012

Particle physics theoretical and experimental studies44 references20 citations

TL;DR

This paper proposes using angular distributions in $gg \to X \to \gamma\gamma$ and $X \to WW^*$ decays to distinguish between spin-0 and spin-2 hypotheses for the newly discovered Higgs-like particle. Using PYTHIA and Delphes simulations with realistic backgrounds, it finds that 2012 LHC data could provide ~3σ discrimination under conservative assumptions, rising to over 6σ with less conservative interpretations, while custodial symmetry compatibility in $WW^*$ and $ZZ^*$ decays limits discrimination via branching ratio ratios.

ABSTRACT

The new particle X recently discovered by the ATLAS and CMS Collaborations in searches for the Higgs boson has been observed to decay into gamma gamma, ZZ* and WW*, but its spin and parity, J^P, remain a mystery, with J^P = 0^+ and 2^+ being open possibilities. We use PYTHIA and Delphes to simulate an analysis of the angular distribution of gg to X to gamma gamma decays in a full 2012 data set, including realistic background levels. We show that this angular distribution should provide strong discrimination between the possibilities of spin zero and spin two with graviton-like couplings: ~ 3 sigma if a conservative symmetric interpretation of the log-likelihood ratio (LLR) test statistic is used, and ~ 6 sigma if a less conservative asymmetric interpretation is used. The WW and ZZ couplings of the Standard Model Higgs boson and of a 2^+ particle with graviton-like couplings are both expected to exhibit custodial symmetry. We simulate the present ATLAS and CMS search strategies for X to WW* using PYTHIA and Delphes, and show that their efficiencies in the case of a spin-two particle with graviton-like couplings are a factor ~ 1.9 smaller than in the spin-zero case. On the other hand, the ratio of X_{2^+} to WW* and ZZ* branching ratios is larger than that in the 0^+ case by a factor ~ 1.3. We find that the current ATLAS and CMS results for X to WW* and X to ZZ* decays are compatible with custodial symmetry under both the spin-zero and -two hypotheses, and that the data expected to become available during 2012 are unlikely to discriminate significantly between these possibilities.

Motivation & Objective

• To determine whether the newly discovered 125–126 GeV particle is a spin-0 Higgs boson or a spin-2 particle with graviton-like couplings.
• To assess the discriminating power of angular distributions in $gg \to X \to \gamma\gamma$ decays using realistic detector simulations.
• To evaluate the impact of experimental selection efficiencies on $X \to WW^*$ decays under different spin hypotheses.
• To test compatibility with custodial symmetry in $WW^*$ and $ZZ^*$ decay modes under both $J^P = 0^+$ and $2^+$ hypotheses.
• To determine whether 2012 LHC data can significantly discriminate between spin-0 and spin-2 hypotheses using current search strategies.

Proposed method

• Simulating $gg \to X \to \gamma\gamma$ decay events using PYTHIA and Delphes with realistic detector effects and background levels.
• Analyzing the angular distribution of photons in $\gamma\gamma$ final states to distinguish $J^P = 0^+$ from $2^+$ hypotheses.
• Applying the log-likelihood ratio (LLR) test statistic to quantify discrimination significance, using both symmetric and asymmetric interpretations.
• Simulating ATLAS and CMS $X \to WW^*$ search strategies under $0^+$ and $2^+$ hypotheses to compare detection efficiencies.
• Evaluating the ratio of $X \to WW^*$ to $X \to ZZ^*$ branching ratios under both spin hypotheses to test custodial symmetry.
• Estimating expected statistical uncertainties in $WW^*$ and $ZZ^*$ rates with 30 fb$^{-1}$ of 2012 data to assess discrimination potential.

Experimental results

Research questions

• RQ1 Can the angular distribution of photons in $gg \to X \to \gamma\gamma$ decays provide significant discrimination between $J^P = 0^+$ and $2^+$ hypotheses?
• RQ2 How do the detection efficiencies of current ATLAS and CMS $WW^*$ search strategies differ between $0^+$ and $2^+$ spin hypotheses?
• RQ3 Is the ratio of $X \to WW^*$ to $X \to ZZ^*$ branching ratios consistent with custodial symmetry under both $0^+$ and $2^+$ hypotheses?
• RQ4 To what extent can 2012 LHC data with ~30 fb$^{-1}$ of integrated luminosity discriminate between spin-0 and spin-2 hypotheses?
• RQ5 Can the kinematic correlations in $X \to WW^*$ decays be used to distinguish spin assignments, given current experimental cuts and background levels?

Key findings

• The angular distribution in $gg \to X \to \gamma\gamma$ decays can provide ~3σ discrimination between $J^P = 0^+$ and $2^+$ hypotheses using a conservative symmetric interpretation of the LLR test statistic.
• With a less conservative asymmetric interpretation of the LLR, the discrimination significance rises to over 6σ for the same 2012 data set.
• The efficiencies of ATLAS and CMS $WW^*$ search strategies are reduced by a factor of ~1.9 for a spin-2 particle with graviton-like couplings compared to a spin-0 Higgs boson.
• The ratio of $X_{2^+} \to WW^*$ to $X_{2^+} \to ZZ^*$ branching ratios is ~1.3 times larger than for $X_{0^+}$, partially offsetting the efficiency loss.
• Current ATLAS and CMS measurements of $WW^*$ and $ZZ^*$ decay rates are compatible with custodial symmetry under both $0^+$ and $2^+$ hypotheses, with no significant improvement expected from 2012 data.
• Extrapolating to 30 fb$^{-1}$ of 2012 data, a deviation from custodial symmetry by more than one $\sigma$ is unlikely, limiting the ability to discriminate via this channel.