Friday 30 September 2011

Live from Fermilab: Chronicle of a Death Foretold

2:38 pm: It's over. The heart stopped 2:38 pm, the last store number was 9158. Good night.
2:37 pm: Helen Edwards all too eagerly pressed the big red button to dump the beam. Soon she will press the big green button to ramp down.
2:35 pm: Stop Helen, I'm afraid
2:35 pm: The heart is still beating but the brain is dead: Tevatron no longer records the data.
2:34 pm: ...though I must say that the CDF show was much more entertaining.
2:32 pm: D0 run terminated. They're ramping down.
2:29 pm: Somehow the whole ceremony reminds me of this scene.
2:28 pm: Time for D0, the better of the 2 Tevatron experiments ;-) Bill Lee from the D0 control room.
2:25 pm: The CDF run has been terminated, 2 million events collected. CDF no longer takes data.
2:22 pm: There is now a story of chickenpox children sacrificed at the altar of science. You don't want to know how it ends.
2:16 pm: Ben Kilminster live from the CDF control room says that back in 1985 there was only one monitor there. There was also no blogs, Twitter or Facebook. Clearly there is some progress...
2:15 pm: Soon the detectors will start shutting down. They don't to watch it...
2:10 pm: Tour of the control room. Looks like space movies from the 70s with lots of color lights blinking.
2:o4 pm: It started. Booooo. Pier Oddone, the director of Fermilab, speaking.
2:o1 pm: Nothing's happening yet. The stream shows photos of serious faces staring at monitors or parts of the accelerator.
1:57 pm: I wonder what will happen to the buffaloes... Will they all be slaughtered and served at the funeral party in the Wilson Hall autrium?
1:50 pm: Except for the top quark, is the Tevatron going to be remember for anything? In the coming years their measurement of the top quark and the W boson mass will remain the most precise one - the LHC will have to struggle hard to beat it. Moreover, a number of measurements - especially various production asymmetries - cannot be repeated at the LHC.
1:45 pm: The Tevatron will die today but the ghost will linger on a bit longer. Physics analyses based on the full dataset are expected only in about 5 months, for the winter 2012 Moriond conference. After that the trickle will be slowing down, but papers and analyses should will be coming up for several more years.
1:40 pm: Streaming of the execution will begin in about 5 minutes.
1:30 pm: Memorial photo of the D0 collaboration in the pit. Not much time left...
1:10 pm: Dismantling of the Tevatron will begin in about a week, shutdown, as soon as the superconducting magnets are warmed up to the room temperature. The CDF detector will also be shut down today, while D0 will be operating for 3 more months to get a sample of cosmic events for calibration purposes. I'm not aware of any plans of reusing parts of these detectors for other experiments.
12:50 pm: With the shutdown of the Tevatron, Fermilab is losing its dearest child and the place at the forefront of high-energy physics, but for a while it will remain an important laboratory running smaller scale experiments. The dark matter detector COUPP, or the neutrino experiment MINOS will be producing important results that may even make it to blogs ;-) Construction of Mu2e, an interesting experiment to study lepton flavor violation, will begin in 2013. In the long run, however, the future of Fermilab looks bleak. Most likely it will share the fate of other once great US labs, like BNL or SLAC: sliding slowly into insignificance.
12:10 pm: One more statistically significant departure from the Standard Model was reported by the Tevatron: the dijet mass bump in W+2j events at CDF. Unfortunately, the effect was not confirmed by D0. It's not clear if this will be sorted out anytime soon...

12:10 pm:
The shutdown of the Tevatron should be viewed as a part of the bigger program of shutting down fundamental research in the US. It makes sense: since manufacturing could be outsourced to China, no reason why research could not.
12:05 pm: Here you can see the current status of the accelerator. The luminosity is low but the old chap should make it all the way to the end.
11:45 am: Wonder how the execution will be carried out? In the state of Illinois they do it as follows:
...Helen Edwards, who was the lead scientist for the construction of the Tevatron in the 1980s, will terminate the final store in the Tevatron by pressing a button that will activate a set of magnets that will steer the beam into the metal target. Edwards will then push a second button to power off the magnets that have been guiding beams through the Tevatron ring for 28 years...
I think there should be 3 people, each pressing a button, only one of which is actually connected to the kicker...11:40 am: It's a beautiful autumn day here in Fermilab today, unusually beautiful. Nature refuses to mourn.
11:05 am: The Tevatron has 3-4 more hours to live.
11:00 am: Except for the top asymmetry, another Tevatron's measurement returned a result grossly inconsistent with the Standard Model, namely, the dimuon charge asymmetry at D0. Although the interpretation of this result in terms of anomalous CP violation in the B-meson sector has been put to doubt by recent LHCb measurements of related processes, formally the D0 result still stands 10:45 am: The gravestone is ready even before the actual death:
10:15 am: So, Tevatron Run I got the top quark. Run II, which started in 2001, had 2 major goals: find the Higgs and find new physics. From this perspective one must admit that, \begin{evenif} insert here how great job was done \end{evenif}, Run II was a disappointment.
9:50 am: Except for the top quark, what were the most important findings of the Tevatron? See the list at Tommaso's blog.9:30am: Tevatron's observation of the anomalous top-antitop forward-backward asymmetry is currently the strongest hint that there may be new physics. The fact it is the strongest is not really encouraging ;-)
9:15am: A bit of nostalgia: a page in Particle Data Group from 1996
9:00am: The LHC is leading the game in most of the Higgs search channels, but for the moment the Tevatron has a far better sensitivity to a light Higgs boson decaying to a pair of b-quarks. Interestingly, they see no excess in this channel (the excess in the combination comes mostly from the H to WW channel), even though they should if the Higgs is there...
8:40am: The eulogies have begun. For the next 2 hours I'll listen to the summary of the most important results obtained by the D0 collaboration.
8:30am: They're still accumulating antiprotons; a sort of life support in case the Tevatron trips before the scheduled time.
8:10am: The last store of protons and antiprotons is circulating in the ring since last evening. Current luminosity: 100 ub/sec, more than 3 times below the peak luminosity. Clearly, the Tevatron is already flatlining.
8:00am: The Tevatron will go down in history as the place where back in 1995 they discovered the top quark - probably the heaviest elementary particle.
7:50am: Tevatron's first beam was in 1983 so he's dying at 28. One year more than Janis Joplin, Jimmy Hendrix, Jim Morrison, Kurt Cobain and Amy Winehouse. What's similar is that death is coming is when the career is already on the decline.
I'm wide awake, it's morning in Fermilab. Putting on my best suit and setting off to the funeral. In less than 7 hours the Tevatron will be no more...

Friday 23 September 2011

The Phantom of OPERA

Those working in science are accustomed to receiving emails starting with "dear sir/madam, please look at the attached file where I'm proving einstein theory wrong". This time it's a tad more serious because the message comes from a genuine scientific collaboration... As everyone knows by now, the OPERA collaboration announced that muon neutrinos produced at CERN arrive to a detector 700 kilometers away in Gran Sasso about 60 nanoseconds earlier than expected if they traveled at the speed of light (incidentally, trains traveling the same route arrive always late). The paper is available on arXiv, and the video from the CERN seminar is here.

OPERA is an experiment who has had some bad luck in the past. Its original goal was to study neutrino oscillations by detecting the appearance of tau neutrinos in a beam of muon neutrinos. However due to construction delays their results arrive too late to have any impact on measuring the neutrino masses and mixing; other experiments have in the meantime achieved a much better sensitivity to to these parameters. Moreover, the "atmospheric" neutrino mass difference, which enters the probability of a muon neutrino oscillating into a tau one, turned out to be at the lower end of the window allowed when OPERA was being planned. As a consequence, a fairly small number of oscillation events is predicted to occur on the way to Italy, leading to the expectation of about 1-2 tau events to be recorded during experiment's lifetime (they were lucky to already get 1). However they will not walk off the stage quietly. What was meant to be a little side analysis returned the result that neutrinos travel faster than light, confounding the physics community and wreaking havoc in the mainstream media.

I'm not very original in thinking that the result is almost certainly wrong. The main experimental reason, already discussed on blogs, is the observation of neutrinos from the supernova SN1987A. Back in 1987, three different experiments detected a burst of neutrinos, all arriving within 15 seconds and 2-3 hours before the visible light (which agrees with models of supernova explosion). On the other hand, if neutrinos traveled as fast as OPERA claims, they should have arrived years earlier. Note that the argument that OPERA is dealing with muon neutrinos while supernovae produce electron ones is not valid: electron neutrinos have enough time to oscillate to other flavors on the way from the Large Magellanic Clouds. One way to reconcile OPERA with SN1987A would be to invoke a strong energy dependence of the neutrino speed (it should be steeper than Energy^2), since the detected supernova neutrinos are in the 5-40 MeV range, while the energy of the CERN-to-Gran-Sasso beam is 20 GeV on average. However OPERA does not observe any significant energy dependence of the neutrino speed, so that is an unlikely explanation either.

From the point of view of theory the chances that the OPERA result being true are no better as there is no sensible model of tachyonic neutrinos. At the same time, we've been observing neutrinos in numerous experiments and in various different settings, for example in beta decay, from terrestrial nuclear reactors, from the Sun, in colliders as missing energy, etc. Each time they seem to behave like ordinary fermions obeying all rules of the local Lorentz invariant quantum field theory.

We should weigh this evidence against the analysis of OPERA which does not appear rock solid. Recall that OPERA was conceived to observe tau neutrino appearance, not to measure the neutrino speed, and indeed there are certain aspects of the experimental set-up that call for caution. The most worrying is the fact that OPERA has no way to know the precise production time of a neutrino it detects, as it could be produced anytime during a 10 microsecond long proton pulse that creates the neutrinos at CERN. To go around this problem they need a statistical approach. Namely, they measure the time delay of the neutrino arrival in Gran Sasso with respect to the start of the proton pulse at CERN. Then they fit the time distribution to the templates based on the measured shape of the proton pulse, assuming various hypotheses about the neutrino travel time. In this manner they find that the best fit is for the travel time is 60 nanoseconds smaller than what one would expect if the neutrinos traveled at the speed of light. However, one could easily imagine that the systematic errors of this procedure have been underestimated, for example, the shape of the rise and the fall-off of the proton pulse have been inaccurately measured. OPERA does a very good job arguing that the distance from CERN to Gran Sasso can be determined to 20 cm precision, or that synchronizing the clocks in these two labs is possible to 1 nanosecond precision, but the systematic uncertainties on the shape of the proton pulse are not carefully addressed (and, during the seminar at CERN, the questions concerning this issue were the ones that confounded the speaker the most).

So what's next? Fortunately OPERA appears to be open for discussion and scrutiny, thus the issue of systematic uncertainties should be resolved in the near future. Simultaneously, the MINOS collaboration should be able to repeat the measurement with similar if no better precision, and I'm sure they're already sharpening their screwdrivers. In the longer timescale, OPERA could try to optimize the experimental setting for the velocity measurement. For example, they might install a near detector on the CERN site (where there should be no effect if the current observation is due to neutrinos traveling faster than light, or there should be a similar effect if there is an unaccounted for systematic error in the production time). Or they could use shorter proton pulses, so that the neutrino production time can be determined without statistical gymnastics (it appears feasible - the LHC currently works with 5 ns bunches). I bet, my private level of confidence being 6 sigma, that the future checks will demonstrate that neutrinos are not superluminal... in the end the character from the original book turned out to be 100% human. But, of course, the ultimate verdict belongs not to our preconceptions but to experiment.

Wednesday 14 September 2011

Summer's almost gone

The season of the year known as summer conferences is over now. What will follow is probably two quiet months when particle physicists make provisions for winter. The LHC has recently restarted at a higher-than-ever luminosity in a bid to double the 2011 data set. Interesting new results are therefore not expected before November when the current run will end. All in all, this is a perfect moment for a post of the sort d'où venons nous blabla. Here's a summary of the most important events and cultural phenomenons of the past summer.
  • Higgs Chase
    It was like one of these action movies where a fugitive surrounded by hundreds of police with guns and helicopters manages to escape disguised as a hostage. The odds of discovering Higgs this summer were significant, but the bugger chose its parameters so as to maximize the difficulty of being found. Nevermind, next time. A plot from this talk of Bill Murray, which is just an extrapolation of the current search sensitivity to larger data sets, shows that we're very close now to ultimate answers. With 5 inverse femtobarns of data CMS alone should be able to get a 3-4 sigma exclusion even in the worst possible case of a 115 GeV Higgs. ATLAS looks worse on that plot because their pT threshold for detecting leptons is set higher than that of CMS. This fact does not matter too much for a moderately heavy Higgs, but for a light one it punishes them (it's then easier to miss the H → WW → 2l 2ν decay which would produce rather soft leptons). If this can be improved we'll get an even better reach after combination of ATLAS and CMS data, close to the CMSx2 line. So by the end of the year we should know much more than today: 5 sigma discovery probably no, 3 sigma hint or exclusion probably yes.
  • Apocryphal Combinations
    That was definitely the hit of the summer, on par with James Blunt. Although experts warn about their quality, although CERN authorities threaten with corporal punishment to anyone caught watching one, bootleg combinations of ATLAS and CMS Higgs results are thriving on blogs and even in LHC experimenters' talks. Coming next are apocryphal data analyses and, who knows, maybe apocryphal colliders.
  • Conference Revival
    During the past decade particle physics conferences have been a sad and boring display. With the LHC running at full speed things have changed a lot. At least in theory, a spectacular discovery may now occur anytime which creates big expectations and excitement around the major conferences. Of course, in the 21st century there is no logical reason to present results at conferences. Experimental results could be presented for example once they're ready, and the presentations more efficiently via internet. Nevertheless, one should not neglect the important convivial aspects of conferences which play a similar role to maypole festivities in pagan societies. Not to mention that the conference deadlines provides an efficient whip for PhD students to finish the analysis in time.
  • SUSY Scorned
    They say you don't kick a man when he's down. Another approach, more popular where I come from, is that it is precisely the best moment as he has limited options to retaliate. I'm somewhat torn between these two approaches. On one hand, watching someone once noble being tarred-and-feathered is always delightful. On the other hand, I sympathize with the view that the backlash against SUSY that is currently unfolding in the mainstream media has no logical grounds. Before the LHC one had to believe in a hundred of new particles just behind the corner who conspire not to break any of the accidental/approximate symmetries of the Standard Model such as the baryon, flavor, or CP symmetry, and in addition their contributions to the electroweak scale accidentally cancel at the 1% level. Now one has to believe in a hundred of new particles just behind the corner who conspire not to break any of the accidental symmetries of the Standard Model, whose contributions to the electroweak scale accidentally cancel at the 1% level, and who do not produce spectacular signals in the early LHC data. In this sense, the summer 2011 LHC results only infinitesimally changed the situation of supersymmetry.
  • Nihil Novi
    Unfortunately, it's not only SUSY that is missing; new physics in whatever form obstinately refuses to show up. Not even a rumor these days... Especially disappointing is that the LHC, unlike the Tevatron, does not see any non-standard effects in top physics. Before, I was estimating the chances of LHC discovering new physics at about 50%. Now it is closer to 33%. The moment we discover the Higgs looking roughly like the Standard Model one, these chances will drop face down...
  • Sunset of Tevatron
    This summer we have watched the Tevatron falling helter-skelter into obsolete. In most of the analyses the sensitivity of the LHC is now far superior. There are some notable exceptions though, such as the top and W mass precision measurement, light Higgs decays to b-quarks, and the forward-backward asymmetry of the top quark production. For these and other reasons, even though Tevatron's life-support will be switched off at the end of this month, the ghost will haunt us a little longer.

To finish with important events of this summer, Resonaances now has a Twitter account. Well, these days every celebrity has one ;-) Disappointingly, you won't learn from it what I had for breakfast, or about my views on the political tensions in southern Uzbekistan. It is going to be a low-traffic twitter limited to announcing new posts on Resonaances, pointing to interesting papers, blog posts and articles elsewhere, and spreading lesser rumors and gossips.

Wednesday 7 September 2011


Until yesterday CRESST was a sort of a legend: everybody heard of them but nobody ever saw them. That is to say, the CRESST excess has been informally discussed for a long time. Moreover, the events from one of the modules displaying an excess of events in the oxygen band have been shown at conferences since more than a year. However we were in the dark about the significance of the excess, backgrounds and systematic effects. Now CRESST has finally come out with a paper that spells out the excess and provides interesting details.

CRESST is in a way the fanciest of all dark matter experiments. Located in the Gran Sasso underground laboratory, it uses the target of CaWO4 crystals cooled down to 10 miliKelvins. When a particle scatters inside the target the deposited energy is converted into phonons and scintillation light, both of which can be detected. The light-to-phonon ratio helps discriminating the dark matter signal from backgrounds, for example electrons and photons produce mostly light. Furthermore, that ratio depends on the atom of the crystal molecule on which the scattering occurred: it is largest for oxygen, intermediate for calcium, and smallest for tungsten. This leads to characteristic bands in the light yield vs. recoil energy plane that you can see in the plot above showing events from one of the eight CRESST modules used in this analysis. These bands provide another handle on the signal, as heavy dark matter would show up mostly via scattering on tungsten, while the light one would pop up in the oxygen band.

At the same time CRESST is paying a price for their innovative technology, as they have to deal with incalculable and sometimes unexpected backgrounds. Apart from the usual neutron background and the leakage of e/γ events into the signal region they had to face α particles and Pb atoms emitted from the clamps holding the crystals, not to mention the exhaust fumes from the nearby DAMA detector. Some of these backgrounds will be reduced in future runs, but for the moment CRESST needs to estimate their contribution in the signal region using sideband analysis. Having done so, CRESST finds that a fraction (slightly less than a half) of the 67 events in the signal region cannot be understood in terms of the known backgrounds. Therefore they study the likelihood of the background plus dark matter signal hypothesis assuming vanilla elastic scattering of dark matter on the target. Here is their result for the preferred mass and cross section of the dark matter particle:
The likelihood function has 2 minima corresponding to 4.7 and 4.2 sigma rejection of the background-only hypothesis. We can safely forget about the deeper one: for these parameters Xenon100, CDMS and Edelweiss would see an elephant in their data. The shallower minimum, where the preferred dark matter mass is 9-15 GeV, also seems excluded by orders of magnitude. This one however lies in the tantalizing proximity to the CoGeNT and DAMA preferred region; actually the mass region (though not the cross section) perfectly agrees with the DAMA low-mass region. Some argue that CDMS and Xenon collaboration grossly overestimate their sensitivity near the threshold. This may be imagined in the case of 5-7 GeV dark matter, in which case combining experimental and astrophysical uncertainties with some good will and the presumption of innocence one can try to argue that the CoGeNT signal is marginally consistent with the Xenon and CDMS exclusion limits. On the other hand, 10 GeV dark matter would produce observable signals further away from the threshold of these 2 experiments, and it's unlikely it could escape their attention. Therefore, given CRESST is facing pesky backgrounds very similar to the suspected signal (both in spectral shape and the order of magnitude), the hypothesis of unknown and/or underestimated backgrounds faking the signal is currently the most probable one.

Summarizing, the new CRESST results are welcome and illuminating but they do not change significantly the landscape of dark matter searches. Clearly, experiment is closing in on IDM; what is not clear is whether that stands for Inelastic or Italian Dark Matter ;-)

See also Lubos, Matt, and again Matt.