arXiv subjects

Fatemeh Elahi

Publications and source records attributed to Fatemeh Elahi.

24 records · Page 2Linked to original sources

arXiv2017

Thermal dark matter via the flavon portal

Dark matter (DM) is added to the Froggatt-Nielsen (FN) mechanism, and conditions for its successful freezeout identified. Requesting the FN scale $Λ_{\text{FN}}$ to be the cutoff of the theory renders freezeout scenarios surprisingly few. Fermionic DM is typically charged under $U(1)_{\text{FN}}$, with the dominant annihilation channel a CP-even flavon + CP-odd flavon. A minimal case is when the DM-flavon coupling strength is $\mathcal{O}(1)$, with several implications: (1) the DM mass is $\mathcal{O}$(100 GeV - 1 TeV), thanks to the WIMP coincidence, (2) requiring perturbativity of couplings puts a lower $and$ upper limit on the flavor scale, 2 TeV $\lesssim Λ_{\text{FN}} \lesssim 14~$TeV, on account of its relation to DM mass and couplings, (3) DM is a "secluded WIMP" effectively hidden from collider and direct detection searches. Limits on the masses of dark matter and mediators from kaon mixing measurements constitute the best constraints, surpassing Xenon1T, Fermi-LAT, and the LHC. Future direct detection searches, and collider searches for missing energy plus a single jet/bottom/top, are promising avenues for discovery.

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arXiv2017

Using the (Modified) Matrix Element Method to constrain $L_μ- L_τ$ Interactions

In this paper, we explore the discriminatory power of the matrix element method (MEM) in constraining the $L_μ-L_τ$ model at the LHC. The $Z'$ gauge boson associated with the spontaneously broken $U(1)_{L_μ-L_τ}$ symmetry only interacts with the second and third generation of leptons at tree level, and is thus difficult to produce at the LHC. We argue that the best channels for discovering this $Z'$ are in $Z \to 4μ$ and $2μ+\displaystyle{\not}E_T$. Both these channels have a large number of kinematic observables, which strongly motivates the usage of a multivariate technique. The MEM is a multivariate analysis that uses the squared matrix element to quantify the likelihood of the testing hypotheses. We find that with $300 \, \text{fb}^{-1}$ of integrated luminosity, we are sensitive to the couplings of $ g_{Z'} \gtrsim 0.002 ~ g_1$ and $M_{Z'} < 20 \text{ GeV}$, and $g_{Z'} \gtrsim 0.005 g_1$ and $20\, \text{GeV} <M_{Z'} < 40 ~ \text{GeV}$, which is about an order of magnitude improvement over the cut-and-count method for the same amount of data.

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arXiv2016

Constraints on $L_μ- L_τ$ interactions at the LHC and beyond

In this paper we examine the constraints dedicated LHC multi lepton searches can place on $Z'$ bosons coming from gauged muon number minus tau number, $L_μ-L_τ$. As the $L_μ-L_τ$ gauge boson does not couple to proton constituents or electrons at tree level, the current bounds are fairly loose, especially for $M_{Z'} \gtrsim 1\, \rm GeV$. For $2m_μ < M_{Z'} < M_Z/2$ we develop search strategies using the $pp \rightarrow Z \to 4\, μ$ channel. The cleanliness of the final state, combined with the fact that $pp \to Z \to 4 e$, $Z \to 2e\,2μ$ can be used as background control samples, allow us to spot $L_μ-L_τ$ $Z'$ with couplings $\mathcal O(10^{-3})$ times the Standard Model couplings. For lighter $Z'$, we propose the mode $pp \rightarrow 2μ+ \displaystyle{\not} E_T$. The presence of missing energy means there is a wider set of backgrounds to consider in this final state, such as Drell-Yan production of leptonically decaying $τ$ pairs, however we find these can be controlled with careful cuts. Combining the $4\, μ$ and $2\,μ+ \displaystyle{\not} E_T$ modes, we find that with $\sim3 \, \text{ab}^{-1}$ of integrated luminosity we are sensitive to couplings $g_{Z'} \gtrsim 0.005\, g_1$ and $0.5\, \rm GeV \le M_{Z'} \le 40\, \rm GeV$ and $g_{Z'} \gtrsim 0.001\, g_1$ for $M_{Z'} < 2\, m_μ$. This region includes the parameter space where $L_μ-L_τ$ models can ameliorate the muon $g-2$ anomaly. We repeat these analyses at a future $e^+e^-$ Z-factory, where we find improved sensitivity. Specifically, given $2.6\, \rm ab^{-1}$ of luminosity, we can exclude $g_{Z'} \gtrsim 0.001\, g_1$ for $2\, m_μ \le M_{Z'} \le M_Z/2$.

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arXiv2015

UltraViolet Freeze-in

If dark matter is thermally decoupled from the visible sector, the observed relic density can potentially be obtained via freeze-in production of dark matter. Typically in such models it is assumed that the dark matter is connected to the thermal bath through feeble renormalisable interactions. Here, rather, we consider the case in which the hidden and visible sectors are coupled only via non-renormalisable operators. This is arguably a more generic realisation of the dark matter freeze-in scenario, as it does not require the introduction of diminutive renormalisable couplings. We examine general aspects of freeze-in via non-renormalisable operators in a number of toy models and present several motivated implementations in the context of Beyond the Standard Model physics. Specifically, we study models related to the Peccei-Quinn mechanism and Z' portals.

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arXiv2015

Higgs portals to pulsar collapse

Pulsars apparently missing from the galactic center could have been destroyed by asymmetric fermionic dark matter ($m_X = 1-100$ GeV) coupled to a light scalar ($m_ϕ= 5-20$ MeV), which mixes with the Higgs boson. We point out that this pulsar-collapsing dark sector can resolve the core-cusp problem and will either be excluded or discovered by upcoming direct detection experiments. Another implication is a maximum pulsar age curve that increases with distance from the galactic center, with a normalization that depends on the couplings and masses of dark sector particles. In addition, we use old pulsars outside the galactic center to place bounds on asymmetric Higgs portal models.

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arXiv2014

Catching sparks from well-forged neutralinos

In this paper we present a new search technique for electroweakinos, the superpartners of electroweak gauge and Higgs bosons, based on final states with missing transverse energy, a photon, and a dilepton pair, $\ell^+\,\ell^- + γ+ \displaystyle{\not} E_T$. Unlike traditional electroweakino searches, which perform best when $m_{\widetildeχ^0_{2,3}} - m_{\widetildeχ^0_1}, m_{\widetildeχ^{\pm}} - m_{\widetildeχ^0_1} > m_Z$, our search favors nearly degenerate spectra; degenerate electroweakinos typically have a larger branching ratio to photons, and the cut $m_{\ell\ell} \ll m_Z$ effectively removes on-shell Z boson backgrounds while retaining the signal. This feature makes our technique optimal for `well-tempered' scenarios, where the dark matter relic abundance is achieved with inter-electroweakino splittings of $\sim 20 - 70\,\text{GeV}$. Additionally, our strategy applies to a wider range of scenarios where the lightest neutralinos are almost degenerate, but only make up a subdominant component of the dark matter -- a spectrum we dub `well-forged'. Focusing on bino-Higgsino admixtures, we present optimal cuts and expected efficiencies for several benchmark scenarios. We find bino-Higgsino mixtures with $m_{\widetildeχ^0_{2,3}} \lesssim 190\,\text{GeV}$ and $m_{\widetildeχ^0_{2,3}} - m_{\widetildeχ^0_1} \cong 30\,\text{GeV}$ can be uncovered after roughly $600\,\text{fb}^{-1}$ of luminosity at the 14 TeV LHC. Scenarios with lighter states require less data for discovery, while scenarios with heavier states or larger mass splittings are harder to discriminate from the background and require more data. Unlike many searches for supersymmetry, electroweakino searches are one area where the high luminosity of the next LHC run, rather than the increased energy, is crucial for discovery.

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