Friday, 13 March 2015

Friday wrap-up: ATLAS on-Z excess, CMS kinematic edge, new dwarfs, dark matter annihilation...

Wherein I list some (mostly) recent happenings, ramble a bit, and provide links, in an order roughly determined by importance and relevance to particle physics. Views are my own. Content very definitely skewed by my own leanings and by papers getting coverage, and it may not even be correct. It is a blog after all...

  • ATLAS released a preprint yesterday (submitted to EPJC), Search for supersymmetry in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum..., that is interesting for two reasons.

    1. Remember the 2.6σ kinematic edge excess that CMS observed in their similar analysis originally released as a PAS in August last year? If not then see Tommaso Dorigo or Collider Blog for a summary... or don't, since ATLAS don't see any hint of it!

    2. They call CMS's excess, and then they raise, with a 3.0σ excess in a different signal region (SR). So let's talk about that...

    The search is for an on-Z opposite-sign same-flavour (OSSF) lepton pair + jets + MET. They are motivated by a gravitino LSP SUSY scenario with pair-produced gluinos which decay via $\tilde g\to qq\tilde\chi_1^0, \tilde\chi_1^0\to Z\tilde G$ (though it seems to me like something as simple as a vector-like quark could also work). Anyway, after typical preselection and requiring two OSSF leptons (if more than two are present they take the leading leptons), they define the on-Z signal region as: $$81< m_{l^+l^-}/\text{GeV}<101, \\ n_{jets}\ge2, \\ E_T^{miss}>225\text{ GeV,} \\ H_T>600\text{ GeV,} \\ \Delta\phi(jet_{12},E_T^{miss}>0.4,$$where $H_T$ is the scalar sum of the jet and lepton $p_T$ in the event, and the $\Delta\phi$ cut is designed to reject background from mismeasured jets faking large $E_T^{miss}$. And backgrounds are tough... $Z/\gamma^*+jets$ with mismeasured jets producing difficult-to-model instrumental $E_T^{miss}$ is potentially worrisome, but it is made negligible by the $\Delta\phi$ cut. Flavour-symmetric backgrounds (with a truth-level flavour ratio $ee:\mu\mu:e\mu$ of 1:1:2) from $t\bar{t}$, $WW$, single top, and $Z\to\tau\tau$ are dominant; they are estimated with a data-driven method using opposite-flavour data as a control region. Fake leptons are estimated from data.  Diboson, $t\bar{t}V$, $t\bar{t}VV$, and $t+Z$ are estimated from MC, making sure not to double count the flavour-symmetric component.

    The expected and observed number of events as a function of invariant mass in the dielectron and dimuon channels are shown below:

    For the sum of both channels the expected background is 10.6±3.2 with 29 events observed, which ends up corresponding to a 3.0σ excess.

    Now, CMS did a similar search in the on-Z SR in their paper and didn't see anything. So are the results consistent? It's possible. The CMS SR wasn't quite as tight as the one employed by ATLAS. After similar preselection, for an on-Z signal region defined as $$81< m_{l^+l^-}/\text{GeV}<101, \\ n_{jets}\ge2, \\ E_T^{miss}>200\text{ GeV,}$$CMS have an expected background of $\approx$ 87.3±12.1 with 72 events observed. So who knows, maybe if CMS demanded $H_T>600$ GeV they would see something too, or maybe not... 
  • The biggest news of the week comes from Tuesday's astro-ph listings. This is not my area, so I can't comment intelligently, but anyone can read an abstract and look at Figures, so I will just sum up here for completeness and convenience (click the figures to make them larger)...

    1. Fermi-LAT released their Pass 8 constraints on dark matter annihilation (already largely known from preliminary results). They rule out dark matter masses $\lesssim 100$ GeV for a thermal relic annihilating to $b\bar{b}$ or $\tau\tau$. Those results are cutting into the best fit regions for the galactic centre excess.

    2. The Dark Energy Survey (DES) Collaboration has located eight new dwarf satellite galaxy candidates (of the Milky Way and/or Magellanic Clouds), and an independent Cambridge group has located nine using the publicly released DES deep photometry data. You can read the press release here.

    3. The new satellite candidates are prime spots to look for dark matter annihilation... so Fermi-LAT went and did it already! Assuming that the new candidates are dwarf spheroidals, they set a limit on the annihilation cross-section that rivals their Pass 8 results with known dwarfs above.

    4. But the story isn't over yet, because an independent group (which includes the Cambridge group that found nine candidates) has reported a gamma-ray excess, consistent with DM annihilation, in one of the new dwarf candidates. [Edit: The candidate is Reticulum II or DES J0335.6−5403, the green line in the above Fermi-LAT plot, which appears by eye to be the only line of all the candidates to have a weakened limit in the 10−few×100 GeV DM mass region, the region that would produce the excess.]

    And the dark matter annihilation saga continues...
  • Protons bunches half-circled the LHC beam pipe last weekend for the first time since the long shutdown began! Injector tests sent bunches from the SPS into the LHC ring and through ALICE and LHCb on their way to beam dumps. Both ALICE and LHCb recorded splash events when the beam was made to collide with a target.

    You can play with the LHCb event here. First fully circulating beam is expected at the end of the month.
  • PRL has published the Planck/BICEP2/Keck joint analysis, along with a Viewpoint article which tells some of the story -- we are reminded of the following: "... alternative models may be detectable with the next generation of experiments, some of which claim a sensitivity to r as small as 0.01. The competition is fierce, with at least six funded ground-based experiments underway (including the third version of BICEP), several balloon-borne experiments, and a number of proposed space missions."
  • There's a nice feature at ScienceNews about the AMS experiment, the positron excess, and Samuel Ting; on the (unreleased) preliminary antiproton data he remarks: "intriguing".
  • Published in Nature yesterday, the Cassini orbiter has detected tiny rock grains emitted from the plumes of the Saturnian moon Enceladus, hinting at a subsurface ocean. You can read the articles at NASA, ESA, or Scientific American. Meanwhile a team using Hubble have used observations of aurora to indirectly suggest that there is a subsurface ocean on Ganymede, Jupiter's largest moon. Nice to see that there are complementary ways to measure these things.
  • Today Rosetta is trying to listen for a signal from the Philae lander on Comet 67P/Churyumov-Gerasimenko. ESA released a cartoon video about it a few days ago [3 minutes]. The Lander Project Manager says, "It will probably still be too cold for the lander to wake up, but it is worth trying."

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