Wednesday, 27 May 2009

News and Gossips from the LHC

Since two weeks Planck stands first of all for a satellite CMB observatory, but it's also the name for an annual series of conferences on physics beyond the standard model. This year's edition is taking place in the furnace of Padova. Since Tommaso is around, he will surely describe everything in great detail, including color of the tie of each speaker, while I should later write a summary of the interesting ideas discussed here in case there is any. But for now I'd like to share a handful of interesting facts about the progress of the LHC that I learned from a supercool talk delivered here by Jörg Wenninger. I guess most of what's below is not new and should be familiar to those closely following the LHC saga.

One interesting fact I was not aware of: a quench (a phase transition from superconductivity to normal conductivity) of an LHC magnet can be induced by just a few milijoules of energy. That energy may be provided by a bunch of strayed protons from the beam . To avoid quenching, LHC cannot lose more than a millionth part of its beam. For comparison, the Tevatron loses about one thousandth of its beam during acceleration. In that respect, Jörg was very convincing that the LHC would ever work ;-) But then, miracles do happen, sometimes.

Another interesting part of the talk was the explanation why the LHC will run at 10 TeV in the center of mass, instead of the nominal 14 TeV. The story goes as follows. Before installing, the LHC magnets have to be "trained", that is to say, to undergo a series of quenches to let their coils settle down at stable positions. After being installed in the tunnel they are supposed to come back to their test performance with no or few quenches. It turns out that the magnets provided by one of the three manufacturing companies need an extraordinary number of quenches to settle down. Although the company in question was not pointed at, everybody knows that the name is Ansaldo. In the case of that company, the number of quenches required for stable operation at 7 TeV per beam is currently unknown, it is probably somewhere between a hundred and a thousand. At the moment it is not clear if the LHC will ever reach 14 TeV; 12-13 TeV might be a more realistic goal.

The talk gave also a detailed account of the incident of September 19 known as the 9/11 of particle physics. Although the evidence has evaporated, one can quite reliably outline the sequence of events. An abnormally large resistance in one of the magnets acted as a heat source that quenched the superconducting cable at one interconnection. In case of a quench the current should start flowing or a few minutes through the copper bus-bar that encloses the cable until the energy stored in the magnet is removed. However, due to bad soldering of an interconnection the current could not flow normally and an electric arc was created. This melted copper, punctured the helium enclosure which led to spilling of 6 tons of helium into the tunnel. The logo of the company that made the faulty magnet is always erased in the pictures, although everybody knows that the name is Ansaldo.

So what's next? The repairs of the damaged sector are almost finished. The current plan is to head for collisions this year (with a caveat "depends how one defines collisions"). Beam commissioning is scheduled for September/October and the first collisions could happen in November. The schedule is very tight and, moreover, the quality control of has revealed problems like bad soldering or reduced electrical contact in a number of places, including sectors that are already cold. The rumor is that some of the LHC magnets in reality turned out to be electric kettles.

The slides here.

Wednesday, 20 May 2009

Bon Voyage, Planck

The rumors of new imminent delays at the LHC imply that this year we need to look for action elsewhere. Fortunately, experimental astroparticle physics is truly enjoyable these days. FERMI and PAMELA are probing high energy cosmic rays, and there is still a plenty of hope that telltale signals of dark matter will be uncovered. That celebrity couple may soon be outshined by another competitor at shorter wavelengths - the Planck cosmic microwave background observatory. Contrary to my fears, the Ariane rocket which carried Planck into space was not shot down by an Imperial cruiser. Planck is now on its way to the L2 point and in one and half year or so we will bath in a plenty of new precise cosmological data.

Planck is the third in a row, after COBE and WMAP, to chase after small anisotropies of the CMB. At first sight the mission comes close to Lord Kelvin's nightmare: Planck will measure what its predecessors have measured, but more precisely, with better resolution, and in more colors. From the propaganda plot on the right one can see that one practical virtue of Planck is the access to higher multipoles of the CMB temperature anisotropy. Probing more acoustic peaks and the damping tail will allow us to precisely determine the cosmological parameters and put the currently ruling Lambda-CDM cosmological model to a thorough test.

Doesn't sound too exciting? Of course, there is always a good chance that something unexpected will emerge from the data. However, I'm going to argue that even confirming the boring cosmological standard model may provide us with extremely interesting pieces of information. In particular, there is one important question to which Planck, with a little bit of luck, may provide an answer: what is the scale of inflation?

The past missions have collected some shreds and pieces of information about inflation. First of all, we know that the highly primitive model of inflation - a single scalar field slowly rolling down its potential - perfectly describes all available data. That is to say, the power spectrum of the primordial density fluctuations that ultimately produced the CMB temperature fluctuations can be explained by quantum fluctuations of that scalar field. Furthermore, we know something about the potential that provides for vacuum energy driving the accelerated expansion during inflation. In particular, the overall scale of the potential can be inferred from the amplitude of the temperature fluctuations observed by COBE and WMAP. This yields
$(V/\epsilon)^{1/4} \sim 3 \cdot 10^{16}$ GeV
where $\epsilon = (V'/V)^2/2M_{Pl}^2$ is one of the so-called slow-roll parameters. The slow-roll parameters must be small during inflation, of order 0.01 or less, which sets the upper bound on the scale of inflation. But, in principle, there's no lower limit on $\epsilon$, and at this point we cannot make a definitive statement about the magnitude of V.

Planck has a good chance to ultimately pinpoint the scale of inflation. The hopes are based on Planck's fantastic ability to measure the CMB polarization. Thomson scattering at the last scattering surface results in linear polarization of the CMB photons. The polarization can be decomposed into the E-mode (gradient) and the B-mode (curl), each of which is then decomposed into multipoles, much as the temperature fluctuations. The lower multipoles of the E-mode have already been detected by WMAP; the B-mode is more tricky and it is waiting for Planck.

The importance of the B-mode follows from the fact that, at the linear level, it is not produced by scalar density perturbations, but only by tensor perturbations, that is by the primordial gravity waves. The amount of tensor perturbations is directly related to the scale of the inflationary potential. The larger V, the higher is the ratio of tensor to scalar primoridial perturbations. As an example, Planck's sensitivity to the primordial B-mode for the tensor-to-scalar ratio = .1 is plotted on the right. If the tensor-to-scalar ratio is high enough for Planck to detect the primordial B-mode, then we will have the first evidence of the existence of a very high-energy scale in particle physics. (who said neutrinos? it's not certain if they're really Majorana, and besides who cares about neutrinos anyway).

But of course the tensor-to-scalar ratio can be too small for Planck to measure. In the worst case scenario Planck may share the tragic fate of LEP: a successful experiment without much success. Let's cross our fingers.

More info in Planck Bluebook.

Friday, 1 May 2009

All eyes on Denver

Tomorrow (Saturday) morning, the FERMI/GLAST collaboration is going to announce their first results at the APS meeting in Denver. FERMI/GLAST is a satellite cosmic gamma-ray observatory, but it also has capabilities to measure the electron+positron spectrum. The latter is eagerly awaited by the particle physics community. Last year, the measurements of the cosmic ray positron fraction by PAMELA and of the combined electron+positron flux by ATIC have sparked some 150 theory papers and one paparazzi affair. Recall that PAMELA sees an excess of positrons over the background (whatever the background means) in the 10-100 GeV range, while ATIC claims there is a clear bump in the spectrum at around 700 GeV. One tantalizing interpretation of these data is that the excess positrons originate from annihilation or decay of TeV scale dark matter particles.

If you can't wait till tomorrow have a look at the plot extracted from a theory paper of two weeks ago. The solid black line on that plot by sheer accident reproduces pretty well the FERMI/GLAST data to come. The sexy ATIC bump is gone and is replaced with milder features: a shallow deep around 100 GeV followed by a mild rise toward 800 GeV, and then a steep decline consistent with the earlier HESS measurements. These new results neither exclude (there's still an excess) nor significantly support the dark matter cause (there's no smoking gun features). Next week arXiv will be flooded with papers refitting the earlier theoretical models to the new data.

Update: FERMI/GLAST has revealed the new measurement of the e+e- cosmic ray spectrum but the plot is not available yet - it will be published coming Monday. According to those who saw Denver's talk, the spectrum is indeed similar to the one plotted above, although the low energy (20-80 GeV) data points lie slightly below the background curve and the dip is even less pronounced. Also, FERMI's data stop at 1 TeV so the high-energy decline cannot be clearly seen. So at this point everything is clear: it's either dark matter or a pulsar or an alien civilization or maybe the galactic propagation model needs refining ;-). More insight should come from FERMI's measurement of the diffuse photon spectrum which is expected by late summer.

Update 2: and here is the original plot from Fermi's today paper on arXiv:
Also HESS got its foot in the door and just published new results for the electron+positron flux above 340 GeV, consistent with those of FERMI and inconsistent with ATIC.

Higgs Was At LEP

Everybody knows that the LEP experiment set a stringent limit on the Higgs boson mass - it has be larger than 114.4 GeV. The common expectation is that Higgs is just around the corner, and will be hunted down and roasted alive at the LHC or at the Tevatron. But this is not the only conceivable scenario that future may unfold: the world can be Higgsless, or the Higgs may be too wide or too invisible or too whatever to be detected at a hadron collider. There is yet another possibility that is definitely a bit crazy but is nevertheless not completely excluded. Namely, it is possible that the Higgs is lighter than 115 GeV and therefore kinematically available at LEP but ... we missed it.

How could Higgs have been missed? The point is that the 114.4 GeV limit strictly applies to a particle that walks, talks and couples just like the Standard Model Higgs boson. If we meddle with the Higgs couplings then, with a bit of skill, we can make Higgs effectively invisible to LEP. One obvious way to achieve that is to suppress the production rate. At LEP, Higgs would be dominantly produced by the process petnamed Higgsstrahlung where the e+e- collision first produces a Z boson which then radiates a Higgs boson. The LEP limits can be relaxed by suppressing the Higgs-Z-Z vertex by a factor of 3-4, see the black line on the plot below. However, this is not a theoretically plausible direction, as the electroweak precision observables suggest the existence of a light Higgs particle whose coupling to W and Z bosons is not suppressed. Besides, on the more philosophical side, a particle with a reduced coupling to Z should not be called Higgs (but rather, a scalar particle that slightly mixes with the Higgs). So let's leave the Higgs-Z-Z vertex alone. In that case we can still try to hide the Higgs from LEP by meddling with the Higgs decays.

The LEP collaboration was not that stupid and they also searched for the Higgs decaying in a non-standard way. You could think that Higgs could be hidden by making it invisible, that is to say, it could decay to some light, almost non-interacting particles that leave the detector undetected. This does not work: the signature involving a Z boson plus missing energy (carried out by the invisible stuff) is not easy to miss in a lepton collider, and in consequence the limit on the invisible is 114 GeV, almost as strong as that on the standard Higgs. Thus, paradoxically, to make Higgs invisible one must make it decay into something visible. LEP has concluded the following:
  • Higgs decaying to a pair of jets of any flavor (rather than dominantly into b-jets as the standard Higgs) has to be heavier than 113 GeV
  • Fermiophobic Higgs decaying dominantly to off-shell WW and ZZ has to be heavier than 110 GeV
  • Higgs decaying dominantly into two photons has to be heavier than 117 GeV
All in all, Higgs lighter than 110 GeV decaying into a two-body final state is excluded. But the situation is far less clear if the final state contains more particles. For example, the Higgs can undergo a cascade decay: it first decays into a pair of light scalars or pseudoscalars which subsequently decay into a pair of quarks or leptons each. In that case we deal with a four-body final state, for example with four b-quarks or four tau-leptons (typically, the pseudoscalars decays into the heaviest quark or lepton that is kinematically available). This is of course impossible in the Standard Model, while in the MSSM it occurs only in an obscure corner of the parameter space. But in several popular extensions of the Standard Model, for example in the NMSSM (MSSM adorned by a singlet superfield) or in little Higgs theories such cascade decays appear often and willingly.

The possibility of avoiding the LEP bounds via the cascade decays was first pointed out by Radovan Dermisek and Jack Gunion in the context of NMMSM. In that model, there are new pseudoscalar states in the Higgs sector which can naturally be light and to which the true Higgs (the one that couples to Z with the largest strength) can decay. These pseudoscalars then decay into a pair of b quarks each, or into tau quarks if the pseudoscalar is lighter than twice the b-quark mass. The former possibility was excluded by a subsequent LEP analysis - the limit on the Higgs decaying into four b-jets is now 110 GeV - but the four-tau or the four-light-jet final states allow for a much lighter Higgs particle. See the exclusion limits for the case of four-tau cascade decay - the allowed region on this plot is almost non-existing but there is no limit above the Higgs mass of 85 GeV. The reason why that analysis stopped at 85 GeV is not physical but psychological: in the MSSM there is no parameter space that would allow to consider Higgs heavier than 85 GeV. This is a clinical case of the damage that happens when experimenters take theorists and their theories too seriously (following this logic, if the MSSM did not allow for a light Higgs one could completely skip the LEP experiment).

Hiding the Higgs is a nice prank in itself, but there are also some theoretical and phenomenological motivations for playing this game. Firstly, the electroweak precision observables are best fitted by a fairly light Higgs mass with the central value of order 80 GeV,
and the light Higgs of 90-100 GeV would alleviate the tension. Secondly, LEP saw a 2.3 sigma excess of Higgs-like bbar events around the mass of 100 GeV. That cannot be interpreted as the standard Higgs (the number of events would have been five times much higher), but can be perfectly explained by the Higgs decaying most of the times into four light quarks or leptons and one fifth of the times into the b quarks. Recall that in the final year of LEP a smaller excess created much larger theoretical activity.

Of course, a light elusive Higgs is a nightmare for the LHC. Fortunately, theories that motivate such a scenario typically predict a lot of new phenomena at the TeV scale to provide enough fun for the LHC experiment. Just that some people will have to wait a bit longer for their Nobel prize.

Here is the review of the non-standard Higgs decays.