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Showing posts with label LHC restart. Show all posts
Showing posts with label LHC restart. Show all posts

Friday, 26 June 2015

Friday wrap-up: first physics, CMS magnet, diboson excess...

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...

Long time no blog -- turns out to be hard to keep up with everything in the midst of travel. So here are some "links without thinks" from the last month...
  • As of June 3 the LHC has physics data being taken at 13 TeV! The live blog from the day can be found here [1 of 4], and there are five highlights videos on the CERN YouTube channel. The media hype for this milestone was significantly larger than for the first 13 TeV collisions 21 May. I wonder if this was a calculated decision from CERN... it seems that's what would have been preferred after the 7 TeV restart in 2010 (see Particle Fever e.g. 1:00:20).

    ATLAS made public a dijet animation and the below pp collision events with 17 vertices(!) from June 3...


  • There was a rumour a couple of weeks back that the CMS magnet was in trouble. The earliest article I am aware of is this one (dated 8 June), which with the help of Google Translate suggests oil contamination of the liquid helium involved in cooling the 4T superconducting magnet. You can read the immediate response of CMS physicists here. Now CMS has released a statement (14 June), which is partly quoted below.

    CMS has been taking collision data since the 13TeV startup of the LHC on 3 June. During this period, the CMS magnet has been kept off due to an issue with the cooling system... The issue with the magnet cooling system was identified in the final preparatory phase leading to collisions in the LHC. While preparing for beam in CMS, a problem was found in the system that feeds liquid helium to the CMS superconducting magnet. The problem was diagnosed to be due to oil, which is used in the initial compression stages, reaching the so-called 'cold-box’ of the cryogenic system. The cold-box is a complex system with several sets of filters protecting three turbines along the path of the helium towards the magnet. In order to clean the oil contamination essentially all components of the cold-box have been extracted and replaced... CMS is confident that, following the LHC technical stop and the beam conditioning run that will start at the end of this week, after the low-intensity and commissioning period, the full magnetic field will be available for the 13 TeV LHC run.
  • Certainly a month couldn't go by without another excess to keep people busy! Now it is in an ATLAS search for diboson resonances.


    The excess is of ~3σ local significance in WZ, WW and ZZ channels, and 2.5σ global. Jester has a write-up and some thoughts, and notes that there is a small excess at around the same mass scale in similar CMS searches. Along with the CMS WH resonance and right-handed W excesses at around 2 TeV, who knows, maybe there's life out there in the desert yet... and not too far away...

    (image credit John Pritchett)

    PS: I hear the ATLAS $h\to \mu\tau$ analysis is hopefully out within weeks (context)...
  • The OPERA experiment has observed (i.e. at 5σ significance) tau neutrino appearance in a muon neutrino beam.
  • The Perimeter Institute Convergence conference June 20-24 is archiving its very interesting talks here.
  • The CMB@50 Conference held at Princeton from 10-12 June has been "storified" by Renée Hložek, links at her blog.
  • In space news and pretty pictures:
    • The Philae lander on Comet 67P/Churyumov-Gerasimenko is awake again, and ready to do science...

      Landing
    • ... Pluto and Charon in colour from New Horizons...


      (Only a couple of weeks now until closest approach: “Color observations are going to get much, much better, eventually resolving the surfaces of Charon and Pluto at scales of just kilometers.”)
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Friday, 22 May 2015

Friday wrap-up: 13 TeV, natural leptogenesis...

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...

It has been a busy week for me, finishing off a preprint (see below) and making slides in preparation for a trip, leaving tonight. I'll be at the PLANCK 2015 conference in Greece next week, travelling around the UK for two weeks from July 1 (giving talks on displaced Higgs decays and natural leptogenesis), attending the ICTP Summer School from June 15, and finishing up with a talk in Rome. The blog might fall rather quiet as a result (or perhaps the opposite)...

  • We uploaded a preprint to the arXiv today, "Natural leptogenesis and neutrino masses with two Higgs doublets."

    The Type I see-saw model is the most straight-forward way to extend the standard model to incorporate neutrino masses. In the model, the neutrino masses are suppressed by the inverse of the possibly very large mass scale of the right-handed neutrinos, $m_\nu \propto v^2/M_N$. As well, Type I see-saw can explain the baryon asymmetry of the universe (BAU) via the out-of-equilibrium decays of the lightest right-handed neutrino $N_1$; this is dubbed hierarchical thermal leptogenesis. It only works if the well-known Davidson-Ibarra bound is satisfed,$$M_{N_1}\gtrsim 5\times 10^8\text{ GeV}.$$
    In 1997, Vissani pointed out that the newly measured neutrino mass scale implies a naturalness problem when the right-handed neutrino masses are $\gtrsim 10^7$ GeV. In such a case the quantum corrections to the Higgs mass (~100 GeV) are greater than 1 TeV. This can be thought of as a fine-tuning in a mass parameter at a very large energy in order to reproduce the observed Higgs mass at a low energy. It is possible that nature is just fine-tuned in this way, but it is at least aesthetically unappealing from the theoretical point of view, and the quest for models which are not fine-tuned in this way (i.e. natural) has motivated much of modern high-energy physics (c.f. SUSY). Over the new year we worked on checking whether there were any holes in Vissani's rough bound in the full three-flavour Type I see-saw model. Our conclusion: nope. The minimal Type I see-saw model suffers a naturalness problem when$$M_{N_1}\gtrsim 4\times 10^7\text{ GeV},$$obviously encompassing the region where hierarchical thermal leptogenesis is possible.

    In this new preprint we make the observation that in an imaginary world where the Higgs VEV is less than 30 GeV (instead of 246 GeV), there is no longer a conflict between the Davidson-Ibarra and Vissani bounds! Of course we do not have the freedom to change the Higgs VEV, but we can think about models containing a second Higgs doublet with such a property, and imagine that this second Higgs doublet is the one involved in the see-saw and thermal leptogenesis. Such models are an extension of two-Higgs-doublet models (2HDMs) with right-handed neutrinos: $\nu$2HDMs.

    It turns out that naturalness concerns force such models into a corner of parameter space that naturally accommodates a SM-like Higgs, predicts extra scalar states that can't be too much heavier than the TeV-scale, and a lightest right-handed neutrino with mass $10^3\text{ GeV}\lesssim M_{N_1} \lesssim 10^8\text{GeV}$ in order to reproduce the BAU.

    For 2HDM aficionados, it turns out that all the 2HDM Types have viable parameter space. Type II/Flipped are most constrained, with $100\gtrsim \tan\beta \gtrsim 5$ necessary if the model is to remain perturbative up to the right-handed neutrino mass scale. Other Types can work up to $\tan\beta\approx 700$ (such a large $\tan\beta$ is kept natural by an "almost-symmetry"). The inert doublet model, giving masses to neutrinos radiatively (Ma's scotogenic model), also has a region of viable parameter space.

    Below are the "money plots" from the paper. As you can see, the models are well-constrained from all sides by separate considerations, and in fact B physics is indirectly excluding models with $M_{N_1}\sim 10^7\text{ GeV}$ for Type II/Flipped $\nu$2HDMs.


  • There was a story on the Stawell Underground Physics Laboratory project getting underway here in Australia (see last week) in the Unimelb magazine.
  • It has been great to see the #girlswithtoys hashtag trending over the past week on twitter. Here is a story from Kate Clancy, who started the hashtag in response to a sexist comment made by a scientist in a radio interview last week.
  • Some hubbub on the 5154 author joint ATLAS/CMS Higgs mass paper published in PRL this week.
  • I didn't stumble across a space image this week, but here is the Oracle of Dodona; those attending PLANCK will have the pleasure of visiting!

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Saturday, 18 April 2015

Friday wrap-up: 6.5 TeV, AMS, young pulsars, dark matter...

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...

  • On this day last week we had the first 6.5 TeV beam fly around the LHC, and a day later we had both beams at once. You can read a little more at symmetry magazine.


  • AMS have presented their results on the cosmic ray proton/helium/lithium flux and the antiproton-proton ratio at a CERN mini-conference. Recordings of the talks are here and slides here. The press release is here. The plot they are pushing is the following one:

    Figure 1. Antiproton to proton ratio measured by AMS. As seen, the measured ratio cannot be explained by existing models of secondary production.

    The claim in the press release is, "This behavior cannot be explained by secondary production of antiprotons from ordinary cosmic ray collisions," with the suggestion that a new primary source(s) may be needed. But Sam Ting made sure during his talk to emphasise that it cannot be explained by existing secondary production models, and when presenting this figure he noted that there are many secondary production models, but "this is the one we choose" -- I wonder why... (an aside: he also in the same talk referred to his $J/\psi$ particle first as $J$ and then as $\psi$ on the same slide!). The point is that these models are very uncertain and the claim in the press release is unfounded. Indeed, there was a preprint on hep-ph yesterday which had the following to say: "Our first and main result is that there is no unambiguous antiproton excess that can be identified in the first place, and thus, at this stage, no real need for primary sources of antiprotons. Within errors, secondary astrophysical production alone can account for the data." Their Figure 2:


    So, don't believe the hype.
  • There was an arXiv preprint on Wednesday suggesting that young pulsars can explain the galactic centre excess. Their money plot is the following comparison of the expected dark matter spectrum with that from a prototypical young pulsar (Geminga).


  • Spectroscopic measurements of Reticulum II (here, here, and here) confirm it is an ultra-faint dwarf galaxy. Recall from the Hooper paper on the observed gamma ray excess, "In order for this excess to be compatible with the lack of significant gamma-ray detections from other dwarf galaxies... Reticulum II must contain a high density of dark matter, corresponding to $J \gtrsim 10^{19.6}\text{ GeV}^2/\text{cm}^{-5}$." One the new preprints has the J-factor measured at $10^{18.8\pm0.6}\text{ GeV}^2/\text{cm}^{-5}$ within 0.2 degs, and $10^{18.9\pm0.6}\text{ GeV}^2/\text{cm}^{-5}$ within 0.5 degs, and another has it at $10^{19.5^{+1.0(+1.6)}_{−0.6(−1.3)}}\text{ GeV}^2/\text{cm}^{-5}$ within 0.5 degs. So measurements seem to be disfavouring a dark matter interpretation. Also, it is of note that there is a radio source (likely a blazar) located 0.1 degs from the Ret II location which could be responsible for excess gamma rays...
  • An arXiv preprint (press release here) has observed that, in a system of four colliding elliptical galaxies, "each of the central galaxies retains a dark matter halo, but that (at least) one of these is spatially offset from its stars." The abstract adds, "With such a small physical separation, it is difficult to definitively rule out astrophysical effects operating exclusively in dense cluster core environments – but if interpreted solely as evidence for self-interacting dark matter, this offset implies a cross-section $\sigma_{DM}/m \sim (1.7\pm0.7)\times10^{−4} \text{ cm}^2/\text{g}\times(t_{infall}/10^9\text{yrs})^{−2}$, where $t_{infall}$ is the infall duration."

    We should keep in mind that these limits are based on the assumption that the interaction is velocity-independent, which is not true of a low-mass mediator. I don't have anything illuminating to add, but I do find it interesting that the previous study of colliding galaxy clusters set an upper limit of $\sigma_{DM}/m < 0.47 \text{ cm}^2/\text{g}$, and this measurement is almost four orders of magnitude smaller! Are measurements of these kinds of systems that much more sensitive? Why doesn't this system set an even stronger upper limit?
  • The "Evidence for dark matter in the inner Milky Way" saga appears to have reached an end. Both the comment and the reply to comment were updated this week, with the former reproducing a plot from a 1988 paper which is "in essence, identical to that of Iocco et al," and the latter writing, "In our letter we made a claim based on a specific technical point: that current data are constraining enough to make the claim robust against statistical and systematic errors. We believe we have made this point clear with our letter and two replies, and we shall not continue the discussion on the arXiv."
  • The Dark Energy Survey has produced a dark matter map (arXiv here, nature article here) of part of our sky using graviational lensing. Below is a heat map showing the mass density along with locations of galaxy clusters superimposed as grey dots. The map supports the standard picture that dark matter drives large-scale structure formation.

    photo
  • The EPS HEPP prizes were given out this week; the main prize went to James D. Bjorken “for his prediction of scaling behaviour in the structure of the proton that led to a new understanding of the strong interaction”, and to Guido Altarelli, Yuri L. Dokshitzer, Lev Lipatov, and Giorgio Parisi “for developing a probabilistic field theory framework for the dynamics of quarks and gluons, enabling a quantitative understanding of high-energy collisions involving hadrons”.
  • Strassler has been tackling the issue of dark matter searches at the LHC this week. He has a blog post and a new article for the layman.
  • Paul Jackson has written a blog post for the ATLAS blog on CoEPP and our conference in February.
  • Our understanding of the particle zoo as a function of time at Scientific American.
  • Frank Wilczek has a new book coming out in July, "A Beautiful Question: Finding Nature’s Deep Design."
  • Lastly, space images...
    • The first colour image of Pluto (and Charon) from New Horizons:

    • And here is our first view of Ceres from Dawn's new address:

      Ceres' North Pole
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Friday, 10 April 2015

Friday wrap-up: LHC first beam, mono-Z'...

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...

  • Protons circulated the LHC for the first time since long shutdown last Sunday! If you want to relive the day you can read backwards through the live blog. Beams circled both ways (first Beam 2 [anticlockwise] then Beam 1) at the injection energy of 450 GeV, after the collimators placed along the way were each opened up in turn. For ATLAS and CMS the collimators placed ~100m away created some spectacular beam splash events. Below are the first recorded splash events from CMS and from ATLAS.




    Images from the day can be found here, and you can check the current LHC status here (or a more detailed status here). Since then there have been a bunch of technical tests and splashes. There's also an article from CERN about the plan up until collisions. In the immediate future we wait for both beams to ramp-up to 6.5 TeV. The magnets are trained and ready as of Saturday, and first probe beam reached 6.5 TeV this morning...


  • An interesting observation which Strassler brought up, and I have noticed as well: there is much more talk about dark matter as a goal for LHC Run II, in lieu SUSY. The CERN press release wrote, "The Brout-Englert-Higgs mechanism, dark matter, antimatter and quark-gluon plasma are all on the menu for LHC season 2," and Nature wrote, "In 2012, more than two years after the machine first started up, LHC experiments found the long-awaited Higgs boson... The second run does not have such an obvious target. Instead, physicists will scour the data for signs of phenomena that do not fit with the standard model of particle physics in hopes of solving mysteries such as the origins of dark matter."
  • On that (dark matter at the LHC) front, the arXiv awoke on Wednesday to a new mono-X, i.e. these-three-preprints discussing the mono-Z'. The salient processes are the three depicted in the below diagram (where the leptons could be replaced by jets, or a thin jet if the Z' is light and boosted), plus dark Higgs-strahlung.


    It's a sensible thing to go and look for, and I'm surprised it's taken so long to be looked at in a general context... though obviously it was high time! Now inviting speculation on the next mono-X... mono-W'? mono-stoponium? neutrinoless mono-beta decay?
  • Lastly, as is the tradition, a space image... here is a gravitationally lensed galaxy nearly 12 billion light-years away, imaged by ALMA.

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Friday, 3 April 2015

Friday wrap-up: LHC restart, dark photon, a drama in five acts...

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...

  • On the LHC restart, the short circuit was fixed on Monday by melting the offending metal fragment with an injection of 400 amps of current for a few milliseconds. Now the news is that "first beams could be circulating in the machine sometime between Saturday and Monday... Particle collisions at an energy of 13 TeV could start as early as June." 
  • The NA48/2 Collaboration has submitted their search for the dark photon in $\pi^0\to \gamma A'$ decays, ruling out the remaining parameter space for the dark photon as an explanation for the muon $(g-2)$ anomaly.


  • On the 'Evidence for dark matter in the inner Milky Way' front, we now have a 'Reply to Comment on "Evidence for dark matter in the inner Milky Way."' Will we see a 'Comment on "Reply to Comment on "Evidence for dark matter in the inner Milky Way"'? Stay tuned... 
  • In a blog post from Tim Head, it turns out that machine learning can do pretty well at telling the difference between interesting and uninteresting papers on the arXiv, using only the title and abstract as input.
  • Arkani-Hamed and Maldacena had a preprint out on Monday: "Cosmological Collider Physics." The paper is about how we might recognise the presence of new particles with inflaton interactions by their impact on primordial cosmological fluctuations. It is quite long (49+12 pages) and far outside my area, so I will leave it at that... I wonder if Tim Head's algorithm would classify the paper as interesting?
  • NPR ran a story on the CRAYFIS project aimed at detecting high-energy cosmic rays with a network of smartphones, bringing >7000 people to the service.
  • An arXiv preprint from Melbourne CoEPP and collaborators has pointed out that ATLAS and CMS have been led by theorists to perform searches for an unphysical dark matter EFT. To quote from the paper:

    As an example of a problem encountered with an $SU(2)_L$ violating EFT, consider the following operator:$$\frac{1}{\Lambda^2}(\overline{\chi}\gamma^\mu \chi)(\overline{u}\gamma_\mu u+\xi\overline{d}\gamma_\mu d)$$This Lagrangian violates $SU(2)_L$, unless $\xi=1$. The case of unequal $u$ and $d$ couplings was considered in Ref. [13], where a very strong constructive(destructive) “interference effect” was found for $\xi=-1(+1)$, the degree of which depends on the energy scale. The analysis of Ref. [13] was subsequently repeated by the LHC experimental collaborations ATLAS [14, 15] and CMS [16, 17]. We shall demonstrate that the large cross section enhancement for $\xi=+1$ is in fact due the production of longitudinally polarized W’s as a result of breaking gauge invariance.
  • APS has spotlighted some lattice QCD results from investigators including CoEPP Adelaide node members which indicate that the Λ(1405) resonance has a molecular quark pair+triplet structure (arXiv version).
  • Scientific American has a story on dark matter and the dinosaurs, inspired by the Rampino paper from February. They are refreshingly skeptical, and they mention the work of Randall-Reece and Abbas-Abbas, which was my main problem with prior articles -- overall a nice piece of science journalism!
  • In video/audio media
  • I have only seen one April Fools' arXiv article making the rounds this week: A Farewell to Falsifiability. From the abstract, "... some far-thinking physicists have proposed instead that we should give up on the notion of Falsifiability itself. We endorse this suggestion but think it does not go nearly far enough. We believe that we should also dispense with other outdated ideas, such as Fidelity, Frugality, Factuality and other "F" words. And we quote a lot of famous people to support this view." There is some discussion on Peter Woit's blog as to whether this article from Ashoke Sen (Milner Prize winner) is also to be taken as an April Fools' submission, or just some pop-sci...
  • This week I became aware of a 1979 document on the CERN server from Ellis et al. called "Can one tell QCD from a hole in the ground? : a drama in five acts." It is indeed a drama in five acts, about QCD, with illustrations and figures!


  • Lastly, some loosely science-related images from the week...

    Here is a photo from César Cantú of lightning in the ash cloud of Mexico's Colima volcano (hat-tip Bad Astronomy).

    IMG_0360 2400x1600.jpg

    Typhoon Maysak from Samantha Cristoforetti on the International Space Station.

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Friday, 27 March 2015

Friday wrap-up: LHC delay, B to Kμμ, Higgs at N3LO, dark matter self-interaction...

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...

  • The LHC did not see a circulating beam this week due to a short circuit in one of the magnets, likely due to debris in a diode box. Delay could be days or months. Read the CERN press release here, and there's some more information at Nature News. [Edit: more from CERN here.]
  • The ATLAS/CMS combined Higgs mass measurement is now up on the arXiv (~6000 authors!).


  • There has been a bit more talk on the LHCb $B\to K^*\mu\mu$ excess which was updated from 1/fb to 3/fb of data last week. The Conf Note is now up. There's a short blog post on interpretation from David Straub, and the Straub/Altmannshofer Proceedings paper has quite a nice summary. See also the Resonaances post and the previous-two posts also there.

    Shown below is the old result (blue) and new result (black) against theory (orange) for the angular observable of interest.


    The point is that the 4--8 GeV^2 bins each deviate from the theory prediction, and a naive combination suggests a 3.7σ tension. This happens to be the same tension seen in the analysis with 1/fb, which suggests that the new data have still fluctuated up, but not quite as much. The effect has not gone away. But I am with Tommaso... I would like to know what is the probability of seeing such a deviation or larger in any of the angular observables they looked at, assuming the null. Probably this is difficult to do; likely there are significant correlations between bins of different observables(?)... still, it would be nice to know a number for the global significance as well.

    The question now is whether we are seeing new physics or underestimated theory uncertainty. The data are not suggesting any problem with form factors, but an unexpectedly large charm-loop contribution near $J/\psi$ is a possible explanation. If it is new physics, then the discrepancy can be explained by a single operator$$O_9=(\bar{s}\gamma_\mu P_L b)(\bar{l}\gamma^\mu l)$$quantified by a parameter $C_9^{NP}$. This could also go some way to explaining the 2.6σ deviation from lepton-flavour universality (quantified by $R_K$) also measured by LHCb. According to the Straub/Altmannshofer Proceedings there are sensible ways to proceed if we want to discover the culprit. Here are a few of them:

    1. Keep looking at q^2 dependence of $C_9$; the new physics effect should be q^2 independent.

    2. Measure $B\to K^*\mu\mu$ and $B\to K^*ee$ branching ratios and angular observables. If the same new physics is responsible for this and $R_K$ then you might see "spectacular" deviations.

    3. Search for lepton flavour violations in $B\to K^*\mu e$.

    Certainly these are all measurements to look forward to...
  • If you want more insight into what's going on at Moriond, their twitter stream is excellent.
  • There's another excess that you might see making the rounds soon. They're everywhere! This week it is >2σ in a search for WH resonances in $l\nu b\bar{b}$.


    Of interest are the three events at ~1800 GeV. It is only seen in the electron channel, so if it is new physics, it is not probably not a WH resonance. It is almost not worth mentioning, but CMS also saw an excess in their right-handed W search in $lljj$. There they saw a >2σ discrepancy in the electron channel at around the same mass scale.


    I have not had time to read about these in detail, but if they can be linked, they will be linked. Watch the arXiv...
  • Higgs gluon-fusion production cross-section has been computed at N3LO (and Moriond talk here [pdf]), "the first ever complete computation of a cross-section at N3LO at a hadron collider."
  • Hooper and Linden have weighed in on the Reticulum II gamma-ray excess (seen in one of the newly discovered DES dwarf satellites). From the conclusion: "In order for this excess to be compatible with the lack of significant gamma-ray detections from other dwarf galaxies (most importantly, Segue 1 and Ursa Major II), Reticulum II must contain a high density of dark matter... A measurement of Reticulum II’s J-factor that is much smaller than this value would place serious doubt as to any dark matter interpretation of its excess."
  • There was lot of hype relating to a lazy LHC article in the media this week. On the plus side, Backreaction has an excellent rebuke (on the article and the paper that spawned it), and there you can also learn a bit about rainbow gravity.
  • A paper published in Science used the Chandra and Hubble Space Telescopes to observe 72 galaxy cluster collisions and subsequently constrain dark matter self-interaction. This number now supersedes the constraint from the bullet cluster. You can read the press release or watch the astrophysicists involved (along with others) discussing it on YouTube [1 hour].

    Images of galaxy clusters
  • A paper appeared on the arXiv today, signed by a number of physicists from many different institutions, commenting that the Nature paper on inner galaxy dark matter appearing a month or so back was not even wrong: "Considerable confusion may stem from the use of the term ‘inner’. The Sun’s orbit encompasses roughly 90% of the stellar mass. By this standard, we live in the outskirts of the Galaxy. That some DM is needed interior to the Solar circle is neither surprising nor new."
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Friday, 20 March 2015

Friday wrap-up: Higgs mass combination, ttH, LHCb rumour...

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...

  • Long Shutdown (LS1) is over, and we should have the first fully circulating beam next week!

    You can track LHC and CMS status here and even view live events. CMS are currently doing their cosmic run with the magnet only turned up a couple of days ago; I managed to catch a nice one bending in the B field:


  • The ATLAS/CMS preliminary Higgs mass combination was presented for the first time at Moriond (talk here [pdf]). The result is $$m_H=125.09\pm0.24\, [\pm0.21\text{ (stat.)}\pm0.11\text{ (syst.)] GeV}$$

  • ATLAS has submitted their results on $t\bar{t}H$ production with $H\to b\bar{b}$ and released a Conf Note on $H\to WW,\tau\tau,ZZ$. Both see a small excess over the SM (of some interest only because CMS has $\mu_{ttH}$ at $>2\sigma$ above SM). I assume that those results enter into the following plot also presented in the Moriond talk(?). At least it is something to keep half an eye on in Run 2, while it likely goes away...


  • Also released for Moriond are new results on Higgs decays to dark photons: both $H\to\gamma_d\gamma_d\to 4l$ and $H\to Z\gamma_d\to 4l$ (talk here [pdf]). Nothing seen, but a nice result, especially looking into Run 2 when they are likely to explore significant parameter space for the Higgsed dark photon model.
  • There's a rumour (via Jester) that the LHCb $B\to K^*\mu\mu$ analysis for the full 3/fb of data is due out soon, and it confirms the anomaly already observed in the previous analysis. If it is today then that would line up with the heavy flavour day at Moriond. See these-two Resonaances posts for an easily digestible recap, or dig deeper with any of these...

    Edit: Now confirmed; significance $\approx 3.7\sigma$... see LHCb news and the talk slides [pdf]. Below is new result (black) and old result (blue). Conf Note out soon.


  • Super-Kamiokande has set the best limits on spin-dependent dark matter annihilating indirectly to neutrinos in the Sun.


  • Quanta Magazine has an interview with Steven Weinberg which touches on his view of the history of science and where it's at now, including some discussion on the multiverse.
  • In video/audio media:
    • Dancing in the Dark documentary on the LHC and dark matter is on BBC iPlayer for the next month or so [60 minutes], and a radio programme as well [43 minutes].
    • A video about the LHC from Fermilab featuring Don Lincoln. [6 minutes]
    • It was Super Pi Day last week... here is ViHart's anti Pi Day rant. [4 minutes]
    • Visualisation of today's upcoming eclipse from NASA Goddard. [2 minutes]
    • Hunting new craters on the moon, from NASA Goddard. [5 minutes]
  • NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft has observed two unexpected phenomena in the Martian atmosphere: an unexplained high-altitude dust cloud and aurora that reaches deep into the Martian atmosphere. No mention in the NASA release or the Nature News, but I wonder if perhaps they are seeing again the plumes that hit the news a few weeks back?
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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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