I am back from a six week tour of Europe (Greece for Planck conference, UK for seminar talks, Italy for ICTP Summer School and talk in Rome) followed by a proper two week holiday (Hawai'i for lava and turtles)... hence the inactivity here. In my absence, the diboson excess has been hot, the first 13 TeV results have been already trickling out, and in other good news it is 92% probable we are even doing something "worthwhile" -- hey, that's almost 2σ!
Let me attempt an incomplete summary of the last month...
- It's been almost two months now since the ATLAS diboson excess hit the arXiv (see Resonaances for a description), and many theorists/phenomenologists have now had the time to digest and interpret the result. The paper has been cited 37 times, and I count 31 dedicated studies. Let's take a stroll through them in the hopes of learning (in some Bayesian sense -- of course you will have to unweight for selection bias) what is a likely explanation if the signal persists... [This is only a quick survey and probably not completely accurate; send me a message or leave a comment if you believe I've done any of these papers a grave injustice...]
Paper Authors Candidate Comment 1507.07406 Faraggi, Guzzi $Z', W'$ String inspired GUTs 1507.07102 Lane, Prichett $\rho, a_1$ Vector or axial triplet in composite Higgs 1507.06499 Fritzsch $Z^*, W^*$ Excited states of composite weak bosons 1507.06312 Kim et al. - EFT study 1507.06018 Bian et al. $\rho$ Vector triplet in composite Higgs 1507.05299 Anchordoqui et al. $Z'$ Leptophobic, string inspired 1507.05310 Chao $H$ 2HDM 1507.05028 Omura et al. $H$ 2HDM 1507.04431 Chen, Nomura $H, H^\pm$ 2HDM 1507.03553 Sanz Exotic glueballs Perhaps within composite Higgs framework 1507.03428 Fukano et al. Dilaton e.g. scale-invariant generic heavy vector triplet model 1507.03098 Cacciapaglia et al. Pseudoscalar Weak singlet with Wess-Zumino-Witten (effective) couplings 1507.02483 Chiang et al. Composite Spin-0 Hidden confining gauge theory coupled to SM via D5 operators 1507.01923 Dobrescu, Liu $W'$ $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ model 1507.01638 Allanach et al. $Z', W'$ (motivated by EFT) within $SU(2)_L$ or $SU(2)_R$ vector triplet 1507.01914 Carmona et al. Vector resonances Composite Higgs (non-custodial) 1507.01681 Abe et al. Vector resonances Partially composite [G221 model with one dynamical SU(2)] 1507.01584 Heeck, Patra $W_R$ $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ 1507.01185 Abe et al. $Z', W'$ G(221) 'three site moose model' e.g. KK excitations of weak bosons 1507.00900 Cacciapaglia, Frandsen - Unitarity study 1507.00268 Cao et al. $Z', W'$ In G221 and G331 models 1507.00013 Brehmer et al. $W_R$ $SU(2)_L\times SU(2)_R\times U(1)'$ 1506.08688 Thamm et al. Composite $Z', W'$ Within vector triplet 1506.07511 Gao et al. $W_R$ $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ 1506.06767 Alves et al. $Z'$ $U(1)_{d-u}$ 1506.06739 Aguilar-Saavedra $(VVX)$ Triboson final state mimicking a VV resonance 1506.06736 Dobrescu, Liu $W'$ $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ model 1506.06064 Cheung et al. $W'$ $SU(2)_L\times SU(2)_R\times U(1)'$ 1506.04392 Franzosi et al. Composite $Z', W'$ Within vector triplet 1506.03931 Hisano et al. $Z'$ Leptophobic 1506.03751 Fukano et al. Technirho Vector triplet within walking technicolour (composite) model
Looks like the most popular explanation is a $W'$ within an extra vector triplet, either arising from an extended gauge sector (minimally a G221 model) or as a low-lying composite state. Less popular, but still well represented, are explanations via a leptophobic $Z'$ or a heavy Higgs in a 2HDM with the second Higgs doublet coupling strongly to the first generation quarks. A notable absence is any (minimal) SUSY explanation.
Many (but certainly not all) of these models tend to predict observable $WZ$ and $WW$ resonances ($ZZ$ is difficult for a spin-1 due to Landau-Yang), usually in conjunction with $Wh$ (just by naive equivalence theorem). These are channels to keep an eye on during Run II.
- The first 13 TeV results are already being released! E.g. check out all-these ATLAS notes which have appeared in the last couple of weeks (just in time for EPS-HEP). For the record, CMS had the first as far as I know (charged hadron pseudorapidity distributions).
In particular, ATLAS released a plot (below) which beautifully agrees with the standard model as per usual: top quark pairs at 13 TeV just where they're supposed to be!
- The EPS-HEP conference ran this week from 22-29 July. The slides are available on Indico here. I was impressed by the live webcast of plenary sessions, the daily newsletters, and the well-used hashtag which almost made it possible to attend the whole conference online. Some highlights for me...
- LHCb presented preliminary results in their search for long-lived light scalars (see this talk [pdf] from Andrea Mauri) in $B\to K^* s \to K^*(\mu^+\mu^-)_{displaced}$ decays; they see no significant signal above background. Last year I gave a talk to the LHCb rare decays group motivating such a search, so it is very exciting to now see results! Below are the limits they set on the $B$ meson branching fraction for different lifetimes.
The simplest model which can give this phenomenology is the standard model plus a real singlet scalar (Higgs portal), as described in an earlier post here. The pertinent free parameters of that model are the light scalar mass and a mixing parameter, and this new result will constrain that parameter space. To get a feel for how much, I picked off the limit lines (sans the statistical fluctuations which can be scraped from the vector plot once the preprint is out) and translated them. [Here I am taking data from an unpublished plot presented at a conference... have I learned nothing from BICEP?] Anyway, the exclusion result is shown in orange in the following figure (the grey shaded regions indicate lifetimes of 0.1mm, 1mm, 1cm,... for more details on the plot see here):
Interestingly, LHCb competes with the BaBar exclusion curve (grey) even for very low masses. It was not obvious at all that LHCb would be able to do this, since for these low masses the long-lived light scalars are very boosted and many will escape their detector. Looking forward to reading the preprint when it is out!
- Two months ago we mentioned the new LHCb result on $R(D^*)=Br(B\to D^*\tau\nu)/Br(B\to D^*\mu\nu)$. The heavy flavour averaging group (HFAG) have now released their combination average; it's 3.9σ from the SM. (See talk from Marta Calvi [pdf]).
- LHCb published in Nature Physics their exclusive measurement of $|V_{ub}|$ in $\Lambda_b$ decays, an important result in resolving the $V_{ub}$ puzzle. You can read the LHCb release here. It has been on the arXiv since April, so it's not a "hot off the press" result, nevertheless it is now for some reason being picked up by various news sources as a blow for supersymmetry (see-these-four-examples). Good to know that if we see nothing in LHC Run II there is at least one way to sell the null result to the media... even though as a scientist such a result would be extremely interesting!
- LHCb have claimed the discovery of pentaquarks (paper here and EPS-HEP slides from Sheldon Stone here [pdf]), a $J/\psi p$ resonance in $\Lambda_b\to J/\psi p K$ decays.
This comes 12 years after SPring-8 first announced (the later ruled out) evidence for such states. One cool thing about the LHCb result is that you can even see it by eye in the Dalitz plot (below as line in $m^2_{J/\psi p}$); the LHCb team cannot account for it with any known $\Lambda^*$ resonance or interference. The best fit is in fact found by including two new $uudc\bar{c}$ pentaquark states.
There's a good Quantum Diaries post from Adam Davis about it here (see also nature news, symmetry, Jon Butterworth, and Tommaso Dorigo + comments).
- This week the XENON Collaboration released an arXiv paper, "Search for Event Rate Modulation in XENON100 Electronic Recoil Data". They see a 2.8σ annual modulation signal in low energy single scatterings with a phase consistent with DAMA/LIBRA (!) ... and then pour a serious amount of cold water on the measurement. In order of decreasing temperature, here are the buckets they use: (1) There is no globally significant modulation in the data. (2) The phase of the annual modulation signal deviates from that expected for a standard dark matter halo by 2.5σ. (3) The amplitude is much lower than that expected if DAMA/LIBRA was correct. (4) A 2.5σ annual modulation signal is seen in low energy multiple scatterings as well.
Some comments now... Bucket (1) is lukewarm; we should only be interested in annual modulation for a dark matter hypothesis and there is no look-elsewhere effect. For buckets (2) and (3) let's look first at their Figure 4.
Bucket (2) is room temperature. The phase of an annual modulation hypothesis is found to be inconsistent from the standard stationary halo expectation by 2.5σ. However, it is plain to see that it is consistent with the DAMA/LIBRA phase. If there is some bulk rotation/movement in the halo, perhaps this can be explained? Bucket (3) is certainly chilly, but there are two things to keep in mind. The amplitude is calculated for a particular model (WIMP-electron scattering with axial vector coupling), and the two experiments have very different targets (NaI crystal versus Xenon). Unfortunately we cannot compare apples with apples here and a conversion must take place, for which there is more information in a second XENON paper. For the last bucket let's look at their Figure 3.
Bucket (4) is potentially large and freezing; a dark matter explanation should not induce an annual modulation in low energy multiple scatterings, and it appears to at 2.5σ. However, I can find nowhere in the paper where they quote the phase of this modulation! If indeed the phase is consistent with the single scattering phase, then this would be evidence for a background origin. Note that XENON100 is in Gran Sasso, as is DAMA/LIBRA, thus such a measurement would have implications for the DAMA/LIBRA result. So, XENON, what is the phase of the annual modulation in low energy multiple scatterings?
- Those following this blog will know we have been documenting somewhat the status of the galactic central excess of gamma rays seen in the Fermi data. The excess (over standard astrophysical backgrounds) is undeniably there, but the question of course to be answered is its origin: dark matter, or some (not yet fully understood) baryonic astrophysics? The most popular explanation in the latter set is by some population of millisecond pulsars (i.e. point sources) [see Sabine Hossenfelder's post here]. Recently, some-studies have analysed the Fermi data to see if the excess prefers a diffuse (e.g. dark matter) or point source origin. Both of the studies find a preference for a point source origin...
This morning I stumbled upon a (days old) CERN Seminar from Tracy Slatyer, who may be in a unique position to comment on the issue, being an author of one of those new studies, as well as an author on one of the well cited papers arguing a dark matter interpretation. Below is the conclusion slide from the Slatyer talk, where it is interesting to see that the game has changed, with preference now for a point source origin over dark matter.
This is science in action; it sounds like some very interesting new astrophysics will be revealed by the time the book is closed on this excess, and this should be celebrated.
- The PASCOS conference happened at ICTP at the end of last month; a very many interesting plenary talks (~30 mins each) are available as videos and worth a peruse.
- There's a public kaggle competition to design machine learning algorithms to best discover $\tau\to 3\mu$ decays at LHCb.
- Frank Wilczek's new book on beauty in nature is out. See Peter Woit's blog for a good summary and further links.
- Over the coming weeks, Stephen Hawking will be answering (some) submitted questions on artificial intelligence in a reddit AMA.
- In audio/video media:
- BBC documentary on dark matter, Dancing in the Dark. [50 mins]
- Pentaquarks on Sixty Symbols. [8 mins]
- A 20 minute short on SUSY and the LHC from Motherboard.
- A Brief History of Everything featuring Neil deGrasse Tyson at MinutePhysics. [8 mins]
- Physics Girl on New Horizons. [5 mins]
- Two bike-physics videos at MinutePhysics.
- How many chess games are possible? on Numberphile. [12 mins]
- Frank Wilczek talks on his new book at Google [40 mins], at the LSE [>1hr], or on radio [30 mins].
- The Kepler mission has discovered the first ~Earth-sized planet within the habitable zone of a Sun-like star [see xkcd]. There has been significant hype; for a no-nonsense take see Bad Astronomy. You can read the actual paper [pdf] here; they state, "The likelihood that this planet has a rocky composition lies between 49% and 62%."
- And while I was away, New Horizons flew by Pluto! In the tradition of ending each post with stunning shots of space, this probably takes the cake: the money shot in natural colour, a surface shot, and the farewell. Truly magnificent. (For more information, Nat Geo has a good story).
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