XCC: An X-ray FEL-based γγ\gamma\gamma Compton Collider Higgs Factory

This report describes the conceptual design of a $\gamma\gamma$ Higgs factory in which 62.8 GeV electron beams collide with 1 keV X-ray free electron laser (XFEL) beams to produce colliding beams of 62.5 GeV photons. The Higgs boson production rate is 80,000 Higgs bosons per 10$^7$ second year, roughly the same as the ILC Higgs rate at $\sqrt{s}$=250 GeV. The electron accelerator is based on cold copper distributed coupling (C$^3$) accelerator technology. Unlike the center-of-mass energy spectra of previous optical wavelength $\gamma\gamma$ collider designs, the sharply peaked $\gamma\gamma$ center-of-mass energy spectrum of XCC produces model independent Higgs coupling measurements with precision on par with $e^+e^-$ colliders. For the triple Higgs coupling measurement, the XCC center-of-mass energy can be upgraded to 380 GeV, where the cross section for $\gamma\gamma\rightarrow HH$ is twice that of $e^+e^- \rightarrow ZHH$ at $\sqrt{s}$=500 GeV. Design challenges are discussed, along with the R\&D to address them, including demonstrators.Comment: 30 pages, 21 figures, submitted to JINST. arXiv admin note: substantial text overlap with arXiv:2203.0848

Nuclear Activity in the Low Metallicity Dwarf Galaxy SDSS J0944-0038: A Glimpse into the Primordial Universe

Local low metallicity dwarf galaxies are relics of the early universe and hold clues into the origins of supermassive black holes (SMBHs). In recent work, coronal lines have been used to unveil a population of candidate accreting black holes in dwarf galaxies with gas phase metallicities and stellar masses well below the host galaxies of any previously known AGNs. Using MUSE/VLT observations, we report the detection of [Fe X] $\lambda$6374 coronal line emission and a broad H$\alpha$ line in the nucleus of SDSS J094401.87$-$003832.1, a nearby ($z=0.0049$) metal poor dwarf galaxy at least fifty times less massive than the LMC. The [Fe X] $\lambda$6374 emission is compact and centered on the brightest nuclear source, with a spatial extent of $\approx$100 pc. The [Fe X] luminosity is $\approx 10^{37}$ erg s$^{-1}$, within the range seen in previously identified AGNs in the dwarf galaxy population. This line has never been observed in gas ionized by hot stars. While it can be produced in supernova ejecta, the [Fe X] flux from SDSS J094401.87$-$003832.1 has persisted over the ~19 year time period between the SDSS and MUSE observations, ruling out supernovae as the origin for the emission. The FWHM of the broad component of the H$\alpha$ line is $446 \pm 17$ km s$^{-1}$ and its luminosity is $\approx 1.5\times10^{38}$ erg s$^{-1}$, lower than the broad line luminosities of previously identified low mass broad line AGNs. These observations, together with previously reported multi-wavelength observations, can most plausibly be explained by the presence of an accreting intermediate mass black hole in a primordial galaxy analog. However, we cannot rule out the possibility that current stellar population models of metal poor stars significantly under-predict the stellar ionizing photon flux, and that metal poor stars can produce an extreme ionizing spectrum similar to that produced by AGNs.Comment: 12 pages, 5 figures, 1 table, submitted to ApJL. Comments welcom

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements