Farewell to the Noughties - Theory

The LHC just started colliding protons at 7 TeV, marking the symbolic beginning of a new decade in particle physics. A good moment to complete the summary of the past one. Some time ago I made a list of most important particle-related experimental results. Time for theory. What significant developments took place in particle theory during the noughties?

Well...none, in the first approximation. The past decade has been marked by inertia and intellectual masturbation. The last truly novel ideas, like AdS/CFT, Randall-Sundrum, or ADD, were all born back in the 90s. No surprise that the list of 50 top cited articles last year contains only 3 particle theory papers written in the 00s, none of which in a prominent position.

Nevertheless, our understanding of particle theory has progressed, somewhat. Here is my subjective, biased, and utterly unfair summary of the most interesting developments.

• Extra dimensions are strong dynamics
  Warped extra dimensions a-la Randall-Sundrum have dominated new physics model building. Perhaps the most interesting aspect of that industry is a qualitative analogy between five-dimensional warped models and purely four-dimensional strongly coupled models. This of course is not completely unexpected given the AdS/CFT conjecture. Nonetheless it is interesting that the correspondence extends to more down-to-Earth and phenomenologically relevant examples, even if in a vulgarized form. And so, 5D Higgsless theories are large N technicolor models in disguise, 5D gravity in a black hole background captures some aspects of heavy ion physics, etc. Even low-energy QCD can, to a certain extent, be modeled this way, and some quantitative predictions for the parameters of the effective chiral lagrangian can be derived.
• Higgs can be stabilized without supersymmetry
  The bulk of particle theory is driven by the fact that the Higgs boson mass in the standard model receives large, quadratically divergent corrections at the quantum level. The common expectation, or maybe just wishful thinking, is that new symmetries and new particles appear at the TeV scale to fix that problem. The best known example - supersymmetry - is based on a boson-fermion interplay, for example, quantum corrections from the top quark are canceled by its scalar partners called stops. This is however not the only possibility, and the cancellations can occur between the same-statistics particles, for example top quark contributions can be canceled by another heavy colored fermion. This option has been known since 70s, but only during the last decade it was systematically understood and classified in the framework of little Higgs and gauge-Higgs unification theories. The end results is rather depressing though: all models we have constructed so far are just as good, or rather just as bad, as supersymmetry.
• QCD is boring but it's here to stay
  Theorists working on QCD have always been looked down upon by smartasses building new fancy models of the universe. Yet in the past and in the coming decade hadron colliders are the sad reality, and an input from QCD theory is necessary to isolate new physics from mundane background processes. Definitely, tons of good work in that direction has been done. At the most basic level, we have seen heroic computations of higher-order corrections to SM processes like W+jets, Z+jets or ttbar+jets, and so on, without which life at the LHC would be much harder. At a more sophisticated level, new jet algorithms better suited for hadron colliders have been developed, and new ways to search for new physics using jet substructure have been proposed. One should also mention the progress in theoretical handling of QCD, for example the soft-collinear effective theory.
• There is more to dark matter than meets the eye
  Models of dark matter are more numerous than stars in the sky, so why bother about another thousand spawned during the last decade? However, some recent proposals are important because they changed the way we search for dark matter. On one hand, models based on KK parity and T-parity prompted us to explore new collider signatures. Even more important was the impact on direct detection experiments. Not so long ago experimenters, brainwashed by MSSM preachers, searched only for spin-independent (coupled to nucleus' mass) or spin-dependent (coupled to nucleus' spin) elastic WIMP scattering. Experimental set-ups as well as data analyses were tailored for these 2 possibilities to the point that less standard dark matter signals would simply be discarded as background. This embarassing situation has been greatly improving in recent years. Thanks in part to inelastic dark matter models, or the recent offensive of light GeV scale elastic dark matter models, experimental analyses are becoming more flexible and developing alternative experimental techniques is being encouraged.
• There are more ways to compute scattering amplitudes
  Anybody who ever computed scattering amplitudes in gauge theories can't help the feeling that there is something wrong with the standard way of doing it. In the approach via Feynman diagrams, hundreds of complicated expressions at the end of the day magically combine into something far more simple. It is becoming more and more clear that gauge theories may hide surprising mathematical structures that control scattering amplitudes. During the last decade some of these structures have been uncovered thanks to e.g. BCFW recursion relations, CSW rules, or fancy twistor space techniques. More recently, a new approach based on Grassmannians suggests that the hidden simplicity extends to higher loop levels, at least in the maximally supersymmetric case. But this last one might be more appropriate for my Farewell to the Teenies...

So much for the last decade, now dying to see the new one. Clearly, it can't get much worse :-)

April Fools'10: Supersymmetry Discovered at the LHC?!

The floor has hardly been swept after celebrations of the first LHC 7 TeV collisions, and it seems that very soon champagne corks will pop again. That could well be rivers of champagne. I've just been informed that a couple of events captured during the first day of high-energy running contain unmistakable signatures of a particle predicted by supersymmetric theories!!!

The first of these events, reproduced on the right, has been observed in the ATLAS detector. An analysis of the event uncovered that the track marked in red corresponds a charged particle with the mass of approximately 800 GeV!!! This is a new, previously unknown particle, that can only be produced in very high energy collisions such as the ones now available at the LHC.

Let me quote the anonymous ATLAS member who sent me this event display

This is absolutely amazing, it's the greatest day in the history of particle physics.
We thought that reaching discoveries would be a long and painful process, but Nature has been extremely kind to us.

Indeed.

The other striking event comes from the CMS detector. Ironically, it has already been shown at the public presentation on March 30. At that time, however, the particle marked in red has been mistaken for an ordinary muon due to a glitch in the detector electronics that lead to an incorrect measurement of the particle momentum. But a reanalysis of the event lead to a conclusion that this is an 800 GeV particle, definitely not a muon!!! Most likely this particle is of the same kind as the one seen by ATLAS, which means we have an independent confirmation of the discovery.

The most plausible theoretical interpretation is that ATLAS and CMS have observed a chargino: a supersymmetric partner of the standard model W and Higgs bosons. A very interesting conclusion that can be drawn from these two events is that the chargino is quasi stable: rather than decaying immediately, it lives long enough to traverse the entire detector. Such long-lived charginos are predicted by a version supersymmetric theories known as gauge mediation. In that scenario, the chargino decays to a W boson and a gravitino (the supersymmetric partner of the graviton) who is the lightest supersymmetric particle. The decay proceeds very slowly because the gravitino, much as the graviton, has a tiny coupling to ordinary matter.

A few points are still unclear from the theoretical point of view. For example, we would expect supersymmetric particles to be produced in pairs, whereas both experiments observed a single chargino. It is possible that the other chargino might have been lost in a crack of the detector, or charginos are produced together with a different supersymmetric particle species that decays immediately. This conundrum should be resolved as soon as a larger event sample is acquired.

The ATLAS and CMS collaborations are currently embroiled in a fierce battle over the priority of the discovery. This is quite a delicate matter, since the CMS was the first to register a chargino, while ATLAS was the first to correctly interpret it. I believe CERN is refraining from an official announcement of the discovery until the dispute is settled. But we should expect the confirmation anytime soon... stay tuned!

Update:
This post is an April Fools' joke. This was obvious to everyone, of course, because there is no supersymmetry in Nature :-) The displayed events feature ordinary muons; these and more are available at ATLAS and CMS public pages.

Another experiment sees dark matter?

The echoes of the CDMS story have not completely faded away and already there is another one out there. This time it's not a rumor inflated by irresponsible bloggers :-) but a legitimate arXiv publication. Last Friday the CoGeNT collaboration put out a paper where they make - carefully phrased and full of caveats but still - a claim that they might be seeing a signal of dark matter in their data. The story has already been picked up by theorists (the usual speed-of-light constraints do not apply in our field :-) who argue that the CoGeNT signal is marginally consistent with all other dark matter experiments.

CoGeNT is one in the long list of direct detection experiments attempting to observe scattering of dark matter particles off ordinary atoms. The experiment bears some similarity to the more famous CDMS. (Actually, both experiments are located just meters away from each other in the Soudan mine.) It uses germanium as the target and it measures the ionization signal to find out if an interesting scattering event occurred inside the detector. Background discrimination in CDMS is much more sophisticated : any single event surviving their cuts may, with a large probability, be attributed to dark matter. CoGeNT's signal, on the other hand, is contaminated by hundreds of events from mundane backgrounds. For vanilla-flavor WIMP particles, that is ones with masses of 100 GeV or larger, CoGeNT's sensitivity is therefore much worse. However, one important advantage of CoGeNT is that its energy threshold above which a scattering can be observed is much smaller: 0.4 keV compared to 10keV in CDMS. For this reason CoGeNT is more sensitive to light, 1-10 GeV dark matter particles. A light particle, even giving away all its kinetic energy, could not give a germanium atom a kick large enough to be registered by CDMS (recall that that dark matter particles are expected to have small velocities today, $v \sim 10^{-3}$ of the speed of light).

The plot shows the CoGeNT results corresponding to 8 weeks of data-taking after applying their discrimination cuts. There is a lot of background there, including clear lines from radioactive pollution of the detector. But there is also an intriguing feature - an exponential rise at low energies near the threshold. Although this could well be due to a background, for example some other radioactive elements that they have not identified, the exciting thing is that scattering of light dark matter particles would produce a similar shape.

On the next plot, what looks like a geologic section of San Joaquin Valley is in reality a fit of dark matter parameters to CoGeNT's supposed signal. The region favored by CoGeNT is marked by red boundaries. It corresponds to the mass of 7-11 GeV and the cross section on matter of order $10^{-4}$ pb. The violet band is the region consistent with the dark matter interpretation of the 2 CDMS events and it cuts right through the CoGeNT region. The two green regions somewhat away are favored by the DAMA signal with and without including the channeling effect.

Of course, the hypothesis that the background has been underestimated is far more likely at this point. The good news is that, if this is dark matter, it will be confirmed very soon, probably this year. One more reason to bite your fingernails while waiting for the first XENON100 data; the CDMS collaboration may also dig deeper into their data below the 10 keV threshold. More excitement ahead.

How much is one inverse femtobarn?

Blog readers know this since ages, but today the news was made official.

Last week, the Chamonix workshop once again proved its worth as a place where all the stakeholders in the LHC can come together, take difficult decisions and reach a consensus on important issues for the future of particle physics. The most important decision we reached last week is to run the LHC for 18 to 24 months at a collision energy of 7 TeV (3.5 TeV per beam). After that, we’ll go into a long shutdown in which we’ll do all the necessary work to allow us to reach the LHC’s design collision energy of 14 TeV for the next run. This means that when beams go back into the LHC later this month, we’ll be entering the longest phase of accelerator operation in CERN’s history, scheduled to take us into summer or autumn 2011.

This announcement does not mention the luminosity goal, but both blogs and some Chamonix slides point to 1fb${}^{-1}$. How much is that? The Tevatron by the end of 2011 will have acquired 10-12 inverse femtobarns of luminosity. Using advanced calculus one concludes that 1 inverse femtobarn is less than 10 inverse femtobarns, but at the same time 7 TeV is more than 2 TeV. To unravel this, here is a handful of back-of-a-madgraph estimates of how many interesting events can the colliders get by the end of 2011.

Higgs Boson (120 GeV Higgs produced in gluon fusion)

Tevatron: 10 000 LHC: 11 000

Both experiments will have a similar sensitivity to the Higgs. Although 10k looks like whole lotta events, Higgs signatures are notoriously difficult to search. For example, one promising discovery channel at the LHC is when the Higgs decays into two photons, which happens roughly twice per thousand events for a 120 GeV Higgs. For this and other reasons, neither Tevatron nor the LHC has good prospects of discovering the Higgs, unless in lucky circumstances (e.g. production cross section larger than in the standard model, or Higgs mass sitting close to the sweet spot of 160 GeV).

Top Quark Pairs

Tevatron: 80 000 LHC: 130 000

Similarly as for the Higgs, the Tevatron and the LHC will acquire comparable top samples. There should be some, though not dramatic, improvement in top precision measurements. Who knows, maybe there will emerge some 3-sigmish discrepancies with the standard model. The general lesson is that the LHC will be competitive in measuring the standard model processes, but it cannot beat the Tevatron black and blue. What about beyond the standard model?

500 GeV Quark

Tevatron: 15 LHC: 300

This illustrates the obvious truth: LHC fares much better with particles who sit close to the kinematical limit of the Tevatron. In that case one finds that $7 \gg 2$: the energy advantage trumps the luminosity handicap. However, in that particular case the discovery is not guaranteed because of the large standard model background, for example from the top quark pair production. So let's try something easier.

1 TeV Z' (U(1)' gauge boson coupled to B-L with g'=0.1 and decaying to electrons or muons)

Tevatron: 5 LHC: 25

In this case the standard model background is almost non-existent, so 25 events might be enough to claim a discovery. But there is only a tiny sliver of parameter space which the Tevatron cannot reach but the first LHC run can. Make the Z' mass 1.3 TeV and the number of dilepton events at the LHC drops to 5. The final lesson to take home: the LHC can be lucky if Tevatron is extremely unlucky. Let's then hope for the worst, to some.

Farewell to the Noughties - Experiment

Year beginnings are always lazy in both theory and experiment (except for most important decisions being taken, but others write about it). So it's a perfect moment for writing all sorts of summaries. Like, for example, summarizing the entire decade. Here I would like to give the list of the most important experiments in the last decade from the point of view of a particle physicist. The other day I named the noughties the most depressing decade ever, and experiment is one of the main reasons. But not for the lack of trying. The pain is that all these beautiful experiments kept confirming the old truths rather than showing new horizons.

Auger

Cosmic rays tickle our imagination because of the huge energies involved: the most energetic beasts reach the stunning energies of order $10^{8}$ TeV in the Earth rest frame. (this translates to hundreds of TeV in the center of mass frame of the collision, still much more than what we can achieve in present colliders). Some earlier experiments suggested that some of these cosmic rays are TOO energetic. Above the energy threshold known as the GZK cutoff a cosmic particle should quickly lose its energy due to interactions with the cosmic microwave background. Observation of cosmic ray events above the GZK cutoff would defy the foundations of physics, maybe pointing to modifications of such fundamental symmetries as the Lorentz symmetry. Auger killed these reveries. The cosmic ray spectrum displays the superboring GZK cutoff at $5x10^{7}$ TeV, more or less where it should be. What a disappointment.

SNO

Neutrinos are the only subfield of particle physics that enjoyed experimental progress in the last decade. Of course, the really groundbreaking discovery of atmospheric neutrino oscillations falls into the previous decade. The SNO experiment only swiped the floor in the early noughties, by obtaining a solid proof that the solar neutrinos also oscillate. They demonstrated that the total number of neutrinos arriving from the Sun is more or less what we expect from our solar models, but that some of the neutrinos change the identity from electron to muon ones.

PAMELA

The true legacy of this experiment is still unclear at the moment of writing. One certain thing is that it turned out very influential, pushing particle theorists in a new direction. PAMELA has made precise measurements of cosmic ray protons, electrons, and their antiparticles, at energies extending to hundreds of GeV. The experiment reached the celebrity status after announcing that the positron spectrum displays a completely different shape than that predicted by standard models of our galaxy. Dark matter particles floating in our galaxy and annihilating into light SM particles provide one tantalizing explanation of that discrepancy. But huge astrophysical uncertainties involved in theoretical predictions make any strong conclusions impossible. It might be that in the future PAMELA will be promoted to the first harbinger of new physics. But more likely, downgraded to yet another false lead.

BaBar

Generally, flavor physics is best suited for botanists. Yet new physics hunters cannot afford the comfort of ignoring it. Because of approximate symmetries of the SM that suppress certain transitions between the generations of quarks, flavor physics is very sensitive to contributions from new hypothetical heavy particles. BaBar and its twin sister Belle produced kilograms of upsilon mesons (the ones made of a b-quark and a b-antiquark), which allowed them to precisely measure their properties. The results showed no major deviations from the predictions of the standard model, apart from a few glitches here and there that, maliciously, occur in observables under poor theoretical control. These results provide a strong hint that, apart from the Higgs boson, there is no new particles in the near energy reach. Scaring.

CDMS

Everybody hates them now: theorists for playing such a cruel game on them, while other experimentalists for getting too much attention. Yet they have been the leader in the field of dark matter direct detection for most of the decade. Depressingly, their leadership consisted in setting more and more stringent limits on the dark matter-nucleon cross section. One solid fact that has been established is that the dark matter particle is not a WIMP in its simplest form. That is to say, it cannot be a weak scale particle interacting via Z boson exchange with the weak coupling strength. But many other options are still wide open, so the hunt continues.

Tevatron Run-2

Great expectations, beautiful performance, gazillion events, hundreds of clever physicists devising clever tricks to extract tiny signals from the data. And nothing that would raise an eyebrow. Precise measurement of the W mass, or discovering omega bee baryons, is not the kind of story our grandchildren will want to listen. But the most depressing must be the that thousands of Higgs bosons have probably been produced at the Tevatron, maybe hundreds have been written on tape, but we just could not see it in all this hadronic mess. After LEP and B-factories, the Tevatron gave us yet another hint that the physics of electroweak symmetry breaking might be less rich than we hoped for.

WMAP

For more than 7 years WMAP has been making precise measurements of the anisotropies in the Cosmic Microwave Background. The only surprise was that our shaky theoretical models describe the data so well. The experiment turned cosmology into precision physics, bringing it dangerously close to Lord Kelvin's nightmare. But there is a glimmer of hope. WMAP's greatest achievement is a precise determination of the amount of various forms of matter in the universe. In particular, it solidly established that dark matter does exist. Which is the most tangible proof we have that the current standard model of particle physics is not the whole story. Maybe this decade we'll find out what's beyond.

Eyes on Xenon

Not the Higgs but dark matter is the true Holy Grail of high energy physics, given that only the purest can hope to discover it. For most of the past decade the leader of the quest to detect the dark matter particle has been the CDMS collaboration. Unfortunately, this amounted to setting better and better limits on the interaction strength of dark matter with nucleons, apart from this shadow of a hint of a possibility of two events announced last year. Although CDMS stays in the game and will continue taking data as super-CDMS, it is bound to lose the yellow shirt soon. For the moment, the primary contender is Xenon100 - a scaled up version of the Xenon10 detector that was in operation in 2006-2007 in Gran Sasso, Italy.

Xenon experiments use a completely different detection technology than solid state detectors such as CDMS. The detector is filled with xenon in the dual liquid/gas phase. When a xenon atom gets hit, it reports this fact to experimenters in two different ways. Photons produced when the atom returns from the excited state is promptly registered by the phototubes located around the detector volume. Besides, the electrons ionized from the atom drift slowly in the applied electric field, and they are registered after some delay. It turns out that the ratio of the scintillation (S1) and the ionization (S2) signals is different for nuclear recoils (that are due to WIMPs, once the experiment is shielded from neutrons) and electron recoils (that are due to ubiquitous backgrounds like photons).

Thus, by measuring the S1/S2 ratio xenon experiments are able reject most of the background. Furthermore, from the two signals and their relative delay it is possible to reconstruct where in the detector volume the hit occurred. Obviously, background events are more likely to occur near the walls of the tank. Therefore making a larger experiment not only increases the probability of registering a dark matter recoil event , but also decreases the background in the central volume - the property referred to as self-shielding. Add to this the good radioactive purity and relative availability of xenon (you just have to smash a lot of lightbulbs), and you understand why big xenon detectors are taking over the field of direct detection.

The prototype Xenon10 detector was not only a proof-of-principle but also a great success story. For some time, Xenon10 was providing the best constraint on the spin-independent WIMP-nucleon cross-section. In fact, it still sets the best limit for the WIMP masses in the 10-50 GeV range, while for larger masses it was later outraced by CDMS. After so much success, the group decided that things are going too smoothly, and set up a huge pillow fight to ease the tension. As a result, Xenon bifurcated into two rival experiments called Xenon and LUX; the latter was banished from sunny Italy into bottomless pits of South Dakota.

The two groups continued, each on its own, scaling up the same technology, each facing an orthogonal set of problems. Apparently, Xenon was the first to pull together. Last year calibrations were made and the physics run is due any time now. According to the official Xenon propaganda, just 40 live days is enough to push the limit on WIMP-nucleon cross section down to 6x$10^{-9}$ picobarns for a 100 GeV WIMP, almost a factor of 10 better than the current CDMS limit of 4x$10^{-8}$. If either of the two events reported by CDMS is really due to dark matter, by this summer we might have a discovery of the century. If not, the quest will continue, with more and more experiments joining in the race. One-ton monster versions of xenon experiments whose sensitivity should reach $10^{-11}$ picobarns are expected in the second half of this decade.

So tons of excitement ahead. As soon as first rumors appear, you know where to look ;-)

MMX

Welcome back after winter holidays! In the meantime the year 2009 has gone to past along with the whole damn decade. Nobody here is going to shed a tear for the noughties - definitely the most depressing decade in the history of particle physics. It closes the balance with *zero* major experimental discoveries, while particle theory has also produce little to write down in history books. The optimistic conclusion is that from this point things can only get better :-)

So what good do I expect in 2010? This year is going to be very special, in that we have two particle accelerators at the high energy frontier. Such a situation occurs for the first time in my life, I mean life as a physicist. Hopefully not for the last time...

All eyes are of course are turned toward the LHC. After the Baby Hadron Collider (BHC) phase last year, following the Aborted Hadron Collider (AHC) in 2008, this year the machine enters the difficult Coming-of-age Hadron Collider (CHC) phase. Even though discoveries are highly unlikely at this stage, we will be following with mouths wide open each step toward becoming the full-fledged LHC: first 7 TeV collisions, first inverse picobarns acquired, first W and Z bosons, and finally first top quarks on the European soil. Meanwhile, the Tevatron does not rust yet. The most fascinating is of course its quest for the Higgs: what mass range will they exclude, will they see a bump somewhere. And, one never knows, one of its many new physics searches may finally bring exciting results.

However, as we already got used to in this century, discoveries are much more likely to literally fall from the sky. End of last of year, the CDMS collaboration decided to go down in flames and announced a detection of statistically insignificant but thought-provoking two scattering events that could be triggered by dark matter particles. This year a much more sensitive dark matter detector called Xenon100 begins taking data. If any of the two CDMS events was really due to dark matter, Xenon100 should grab a discovery by this summer. That is definitely the most awaited result of the year.

Up in the sky, the Fermi gamma-ray telescope is still alive and taking data. This year should bring an answer if the haze - a population of energetic electrons and positrons in the center of the galaxy that is difficult to account for by astrophysical sources - really exists. Moreover, Fermi is continuing its search for subhalos - small satellite galaxies made entirely of dark matter that may glow in gamma rays due to dark matter annihilation. Deeper in space, the Planck satellite is sitting at the Lagrange point L2 and making precise measurements of the Cosmic Microwave Background since September last year. If all goes well we should have the first results this year, and we eagerly expect Planck's measurement of the CMB polarization that should greatly surpass in precision the polarization data of its predecessor WMAP. As usual, astrophysics will probably not bring a clear cut fundamental discovery, but may give us something to think about.

So, lots of things to get excited about, lots of rumors to spread. Even if the year 2010 will not turn very fruitful, at least it should not be boring.