Friday, 29 May 2015

Friday wrap-up: B to D*τν, ULO...

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...

Very busy at Planck 2015 this week, so will keep this short and sweet -- just the headlines.

  • This week LHCb and Belle have weighed in on $R(D^*) = Br(B\to D^*\tau\nu)/Br(B\to D^*\mu\nu)$ at the Flavor Physics and CP violation conference in Nagoya. Intriguingly, both measurements have observed a higher $R$ than the SM predicts -- the same effect seen in the previous B-factory analyses. Taken alone they are not much, but together probably quite significant. Add this to the growing pile of flavour anomalies...

  • There is talk of a ULO (unidentified lying object) in the beampipe at the LHC, already causing some beam failures. In the mean time the beam has been directed around it. We will see if this has any effect on the schedule. At the moment we are expecting more 13 TeV collisions in June, and physics in earnest from July.

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!

Friday, 15 May 2015

Friday wrap-up: IBL, SABRE...

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 News had a wrap-up of the 900 GeV collisions performed last week. It was cool to see an event display with the insertable B-layer in action (the fourth and most inner layer of the pixel layers on the right insert):

  • The Stawell Underground Physics Laboratory project here in Australia has received $1.75 mil from the federal government to match the State's contribution from earlier this year. The funding will go a significant way to constructing the clean room to host SABRE, the first southern hemisphere dark matter direct-detection experiment, the advantages of which I've mentioned earlier.
  • If the dark matter distribution in M87 is spiked at the centre, then this arXiv preprint claims that thermal relic dark matter is ruled out for an unprecedented $m_{DM}\lesssim 100$ TeV! As well, an apparent excess at high energies can be explained by $\mathcal{O}(1-100)$ TeV dark matter. I wonder if this paper will become another galactic centre excess for hep-ph?
  • The result has been on the arXiv for a while, but the CMS+LHCb $B_s (B^0)\to \mu^+\mu^-$ analysis was published in Nature, which I thought was interesting enough to note. As far as I can tell from a quick Inspire search, this is the first paper from the LHC Collaborations published in Nature. Note the 6 months from receipt to publication...
  • A few very interesting articles this week:
    • Nautilus: the story behind the OPERA superluminal neutrinos.
    • Aeon: on the pervasiveness and apparent non-falsifiability of inflation.
    • Scientific American: on physicists as philosophers.
    • Quanta: ultra-high energy cosmic rays, the Oh-My-God particle, and an EeV+ hotspot in the sky.
  • In video/audio media:
    • New physics frontiers at the 13 TeV LHC from CERN. [3 minutes]
    • A first video spot at Quanta Magazine: In Theory with David Kaplan (of Particle Fever fame) on what happens if you fall into a black hole. [2 minutes]
    • And if you'd like to learn more about the man responsible for backing the foundation that supports Quanta Magazine in the first place (and for Chern-Simons forms, and for Renaissance Technologies), you should watch the very interesting interview with James Harris Simons at Numberphile. [19 minutes]
    • Stephen Hawking on intelligence. [15 minute talk]
    • Excellent video at SmarterEveryday on how the window shutters on the space station work. [8 minutes]
  • Finally, updates on space missions: 
    • New Horizons can now make out all of Pluto's known moons.

    • The bright spots on Ceres we've been following now appear to be deposits of ice at the bottom of a crater.

Friday, 8 May 2015

Friday wrap-up: Collisions, displaced Higgs decays...

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 has seen collisions at injection energy, 450 GeV per beam. There's a little more you can read at symmetry magazine; still plenty of calibration to be done. Here's one of the events in the ATLAS detector, a far cry from the messy environment we'll see at 13 TeV...

    As well, CMS has a visualisation of one of their events on YouTube.
  • Pheno 2015 happened this week in Pittsburgh. Definitely worth perusing the interesting talks on the indico page.
  • I uploaded an arXiv preprint on Monday, "Constraining portals with displaced Higgs decay searches at the LHC." One of the primary purposes of the LHC is to study the properties of the newly discovered Higgs boson in great detail. Even though we have now measured its mass to within ~0.2%, it is still possible that it is decaying to exotic particles 20% of the time! So it is clearly sensible to search for exotic Higgs decays. I concentrated on one possibility: the decay to a pair of long-lived particles which each subsequently decay around 1 metre from the beam pipe...

    Such long-lived particles are well-motivated; typically all you need is an approximate symmetry (which is by definition technically natural à la 't Hooft) in your model which if exact would result in a stable particle. This appears to be coming into vogue at the moment as we see natural SUSY being pushed into compressed and long-lived areas of parameter space; for example, if you violate R-parity just slightly then the would-be neutralino dark matter candidate can become long-lived. I became interested in this sort of phenomenology from a much simpler standard model (SM) extension: by a real singlet scalar field $S$. In that case you can write down a potential term $\zeta \phi^\dagger\phi S^2$ which mixes the Higgs boson and a new mass eigenstate $s$ after symmetry breaking. As $\zeta\to 0$ the $S$ field decouples from the SM and becomes stable, so for small $\zeta$ it is long-lived. It is possible that the $S$ also directly couples to some dark sector uncharged under the SM forces, so-called Higgs portal models. Anyway, the Higgs we know and love can decay to two $s$ particles which, in the simplest case, decay to SM particles somewhere in the middle of the detector. Else the $s$ could have some complex cascade decay into hidden sector states which subsequently decay in the middle of the detector (often called hidden valley models).

    You can do a similar thing with a massive dark photon, the so-called vector portal. In fact, the possibilities are many and varied, which presents two complementary challenges: how do collaborations present their results in the most model-independent way possible? and how do phenomenologists reinterpret the results in the context of their own models? The point of my paper was to explore these questions...

    So I had a go at reinterpreting two searches already performed by ATLAS, in a very simple way: by running Monte Carlo simulations, calculating decay probabilities, and folding in the provided particle reconstruction efficiencies as a function of the decay distance of the long-lived particles. As phenomenologists we are reliant upon these provided efficiencies, as there is no tool available to reliably calculate them ourselves. As we move on the journey which constitutes the bulk of the paper, we learn some valuable life lessons about what efficiency table information the collaborations could provide to make the life of phenomenologists wanting to reinterpret their searches (and there are a few of us out there!) much easier. Those life lessons are summarised in the conclusion.

    As well, at the end of the day I was able to make my own contribution to the portal model exclusion space, with the pretty pictures below...

  • There's an article at Nautilus on "The Admiral of the String Theory Wars" AKA Peter Woit. As might be intuited, the article describes the string theory wars around the time when Woit released his book, "Not Even Wrong." It touches on his arXiv trackback controversy and feud with Polchinski. (Also I learned that Motl once compared Woit to bin Laden...). Woit said a few words about the article on his blog. There is also some discussion there on the following...
  • You should be able to find Amanda Peet's hour long Perimeter Institute public lecture on string theory on YouTube within a day or so.

Friday, 1 May 2015

Friday wrap-up: DM self-interactions, on-Z excess, AMS...

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...

  • This arXiv preprint answers my hanging question from a couple of weeks ago as to how the recent Abell 3827 (four-)galaxy cluster measurement is $10^4$ times more sensitive to dark matter self-interactions than the larger scale Bullet Cluster type measurement that hit the news in late March. The answer according to the authors: it isn't! The system is composed of four galaxies, each with a dark matter subhalo, infalling into a larger dark matter halo. The subhaloes are observed to have lagged behind after a long infall period, with the possible interpretation that the dark matter is experiencing some DM-DM drag force that the stars are not. The claim in this new preprint is that the Massey et al paper made the assumption that the stars and the associated DM subhalo develop completely independently. But clearly they are gravitationally bound! And this matters. When taken into account, it is clear that a much stronger dark matter self-interaction is necessary to explain the offsets of the subhaloes from the stars. They find a strength $\sigma/m_{DM}\sim 3 \text{ cm}^2\text{g}^{-1}$, in tension with the limit from the larger clusters (also from Massey et al...).
  • On the ATLAS on-Z excess, I count already six articles dedicated to discussing/explaining it. In particular, this one points out that an explanation in terms of the simplified General Gauge Mediation model taken as a benchmark in the ATLAS paper is inconsistent with other measurements. As shown below, the white band preferred by the on-Z excess is disfavoured by a collection of other measurements.

    There are at least a-few-papers which claim that a decay chain more like $\tilde g \to q\bar{q}\tilde \chi_2^0 \to q\bar{q} Z \chi_1^0$ with a somewhat compressed spectra can go some way to explaining the excess while remaining consistent with other observations.
  • On the AMS antiproton-proton "excess" there have been a few more preprints showing up on the arXiv. I took a quick look at this one, which does the sensible thing: notes that propagation models can fit the data fairly well, and that there is no unambiguous excess (though there are always some that see things another way...), nevertheless we can use these models along with the observations to bound the dark matter annihilation contribution at high energies -- which is the interesting physics after all!

    Above are the limits they derive on the annihilation cross-section into $b\bar{b}$ obtained assuming two different propagation models. What's interesting is that they compete with the Fermi dwarf spheroidal bounds for $m_{DM}\lesssim 100$ GeV, which is the region of interest for the galactic centre excess.
  • The Stawell Underground Physics Laboratory project aiming at setting up the southern hemisphere's first dark matter direct detection experiment looks like it's really coming along (It even has a Wiki page now)! There was a stakeholder event in Stawell on Tuesday and some buzz from the CAASTRO group on twitter...

  • Ellis, Gaillard, and Nanopoulos have uploaded "An Updated Historical Profile of the Higgs Boson" to the arXiv.
  • Another nice-couple of articles at Quanta Magazine on quantum phenomena.

arXiv-watch: Feb-Apr 2015

The top five cited articles (according to INSPIRE) of the last three months overall and of the last six months in hep-ph.

Hot topics are the Planck (and BICEP2/Keck) results for the final month in these blog entry installments, and the ATLAS+CMS combined Higgs mass measurement first presented at Moriond.

Joint Analysis of BICEP2/Keck Array and Planck Data
BICEP2 and Planck Collaborations (P. A. R. Ade (Cardiff U.) et al.). Feb 2, 2015. 17 pp.
Published in Phys.Rev.Lett. 114 (2015) 10, 101301
DOI: 10.1103/PhysRevLett.114.101301
e-Print: arXiv:1502.00612 [astro-ph.CO] | PDF


In hep-ph we are continually chasing excesses: hot topics are the galactic centre excess, the LHCb B→K*μμ and LFU measurements, and the CMS Higgs LFV decay measurement.

WIMPs at the Galactic Center
Prateek Agrawal (Fermilab), Brian Batell (CERN), Patrick J. FoxRoni Harnik (Fermilab). Nov 10, 2014. 34 pp.
FERMILAB-PUB-14-411-T, CERN-PH-TH-2014-219
e-Print: arXiv:1411.2592 [hep-ph] | PDF

Lepton Flavor Violation in B Decays?
Sheldon L. Glashow (Boston U.), Diego Guadagnoli (Annecy, LAPP), Kenneth Lane (Boston U.). Nov 3, 2014. 4 pp.
Published in Phys.Rev.Lett. 114 (2015) 091801
LAPTH-227-14, CERN-PH-TH-2014-229
DOI: 10.1103/PhysRevLett.114.091801
e-Print: arXiv:1411.0565 [hep-ph] | PDF