Symmetry factor

There is a curious article that I read on CNN’s website. It is called “Chimps on treadmills offer evolution insight” (given here via Reuters, as CNN promptly removed it from its website (to conserve diskspace?)). The basic premise of this investigation (here is the original article published in the Proceedings of the National Academy of Sciences) was to study energy consumption of humans vs. primates in a treadmill exercise. The conclusion was that “…people walking on a treadmill used just a quarter of the energy relative to their size compared to chimpanzees knuckle-walking on four legs.” Which in turns implies that this reduced energy consumption steered pre-humans to evolve into bipedal humans.

As far as physics is concerned, the conclusion of this study is “nearly flawless.” An analogy can be found in quantum mechanics: a particle trapped in a “false vacuum” state will eventually leak out into a lower-energy “true vacuum” state. So, in a sense, it is nice to have minimization of energy as a guiding principle of evolution. But surely not a dominant one. Otherwise people would still have been jumping on trees — why walk (and use more energy) if you can just sit on a tree! Well, maybe that is how prehistoric sloths evolved into modern-day sloths… but yet again, maybe all technological evolution is built on a principle of Total Laziness (minimization of energy) — people got tired of walking — so they invented horseback riding; they got tired of rough riding on those smelly horses — so they invented cars; … well, you got the idea…

There is a lesson that can be applied to everyday life: if you exercise regularily and want to use your treadmill time more effciently, walk on all four of your limbs. And don’t tell me that evolution is not applicable to today’s world.

Just wanted to share some phrases by physicists that caught my attention during a recent conference I went to (CHARM-2007):

“The errors are consistent within the error” (comparing recent experimental measurement with the older one).

“I cannot measure [...], I don’t have my own accelerator. But you do!” (theorist replying to a question from an experimentalist).

“Don’t listen to what I say, listen to what I think!” (from an experimentalist who misspoke about a result, but caught himself while doing it)

“I’m not sure how to get to my conclusions now” (from a speaker having technical difficulties with a remote that advances slides)

Also, this phrase was repeated to me — it is from another meeting. That meeting wasn’t even a conference, it was a meeting on super-B factory. One of the proposals call for essentially moving SLAC’s experiment (BaBar) to Europe, reassembling it there in the new, undevelloped site and finally upgrading it to high luminocity. So a question was posed: why bother digging a new tunnel in Europe, de-assembling an experiment in the US and reassembling it back when you can just upgrade it at SLAC, one well-known physicist said: “We don’t do this anymore” [in the US]. Sadly he was referring to (almost) an executive decision to stop flavor physics programs in the US…

My colleagues tell me that CDF has become the third collaboration to observe D0-anti-D0 mixing, i.e. flavor oscillations in the charm sector. Their results were reported at the Fermilab seminar today (see website here) as part of CDF results to be presented at the Lepton-Photon conference in Korea next week. They are consistent with earlier observations — BaBar and Belle saw those oscillations a couple of months ago. And I cannot pass mentioning it again: their results are also consistent with our theoretical predictions reported here.

Day 4 was the last day of the conference. And really was even shorter than the previous day. But they finally put all the talk on the web — please see them here.

The first section started with the talk by Mikhail (Borisovich) Voloshin about charmed meson spectroscopy. I really enjoyed his talk. He talked about mass splittings among ground and exciting states in D-system both from quark mass difference and from electromagnetic effects. Kinda bread-and-butter physics. At the end he also talked difficulties with interpretation of Ds(2317) states as parity doublers of usual pseudoscalar D-states. In particular, he explained that the old calculation of E. Eichten et al from 2003 actually is not self-consistent and requires extra assumptions (now, of course, we explained the existence of this state via a mixture of 2-quark and 4-quark states). Then he proposed something that I didn’t quite understood: that these states are really 0- states with “charm quark dressed up by 0- glue field.” He referred to an old paper by NSVZ (Novikov, Shifman, Vainshtein and Zakharov) from the 80’s, where the idea was explained. I don’t know how this mechanism works, maybe someone can teach me… Then A. Polosa talked about modification of the 4-quark picture to explain X(3872) charmonium state. They propose 2 states around 3872 MeV instead of one — one containing c, anti-c and u and anti-u and another one containing c, anti-c and d and anti-d. So these states decay into different decay channels. The way I see it, their mechanism cannot be right, as those states must mix among themselves, so both states would decay into both channels. The session ened with two experimental talks about spectroscopy of charmed particles.

The second section contained several talks about future experimental facilities: BESIII, LHC-b, PANDA, and Super-B. Take a look at the slides yourself. The conference ended with a question-and-answer section about the future of flavor physics.

I must say that contrary to recent claims, flavor physics is not dead, it just moved to Europe and Asia with the rest of high energy experimental particle physics.

The third day of the conference was really a half-day. The reason is that the organizers scheduled a conference hike in the beautifull Treman Park with subsequent barbeque. The day stared with experimentral talks on semileptonic and leptonic charm decays — here CLEO-c has a fantastic advantage of an experiment in pristine enviroment. This, of course, is partially off-set by huge statistics of other experiments. At the end of the first section Aida El-Khadra talked about lattice approaches to formfactors and decay constants. And yes, she did show that plot of HPQCD collaboration that they show for three years already. Some of their results are quite amazing and dwarf the experimental results (and also somewhat inconsistent with them). F. Follana elaborated on HPQCD collaboration’s results. Then there were several talks on semileptonic decays, including Richard Hill’s talk on z-expansion. The point is that one has no idea what q^2-dependence semileptonic form-factors have, so meaningful comaprision of theory and experiment is a bit unclear — that is, if you want to claim model-independent theory. He puts some meaning into how to do that.

 At the end, Peter Cooper reported on the number of doubly-charmed baryon production at SELEX. Now, it is known (for once, from the calculations  of my former postdoc A. Onishchenko) that the production cross-section that they measure is _orders_ of magnitude higher than what is predicted. Jon Rosner made an intersting suggestion that those states could even be highly-excited single charm states. The rest of the session was about charmed baryons and their decays. In particular, H.-Y. Cheng gave an extensive discussion of charmed baryon spectroscopy.  

The conference hike was probably ok, I didn’t go (I did it when I lived in Ithaca) — it’s hard to go up and down with a kid’s stroller. It also started to rain midway through the hike. In taht respect, giving out a “conference umbrella” instead of usual conference bag was entirely appropriate. 

Here is the day two of the CHARM-2007 conference. The day started with Brendan Casey talking about rare D-decays. This is quite interesting, since these decays are sensitive to indirect effects of new physics. Among other things, he talked about D_s decays to fully leptonic final states. These decays are rare not because they are generated by quantum effects (so-called penguins), but simply because of the structure of weak interactions inducing the so-called helicity suppression of the decay — the decay  probability is proportional to square of the lepton mass in the final state. This means that the largest decay probability would be for tau+tau-neutrino final state. The interesting this about that transition is that it is also sensitive to effects of the charged Higgs, required in supersymmetric extensions of the Standard Model. He reported on the ratio of Ds -> tau nu/Ds -> mu nu decay probabilities corrected for the kinematical effects and took a ratio of that ratio (he-he) to the theoretical calculation. I cannot verify this at the moment (I cannot look again at his transparencies), but he says it is 0.78 +- 0.12, which is away from expected unity. This might mean that there is a charged Higgs contribution there (I’m speculating). There were also talks by H. Muramatsu and Fred Harris (for R. Ping, who didn’t get his visa) on charmonium decays. In particular, it was reported on the status of the so-called “rho-pi” (or 12%) puzzle. Then  Zaza Metreveli gave a great talk about spectroscopy and ways of using it for things like measuring strong coupling in charmonium decays.

The second installment was more theoretical. It started with Estia Eichten’s talk on charmonium states above threshold — he talked about the weird X(3872) state and various ways of explaining it. He didn’t like our cool paper on the subject. Bob McErlath talked about new physics in heavy quarkonium decays. I was surprised to learn that there are essentially no model-independent constraints on the neutralino mass, one of the primary candidates for the Dark Matter particles in SUSY (it has to be heavier than 2 eV to insure that dark matter is dominated by its cold component). In fact, he claims that it can be so light that heavy quarkonium can decay into it. Who knew… Nora Brambilla talked about theory of extraction of mass and strong coupling in heavy quarkonium decay in non-relativistic QCD. It’s been kown for years that the definition of quark mass in QCD is not trivial, as quarks never appear as free states due to confinement. So one needs a convenient definition — and there are several. Jo Dudek gave a nice talk on radiative charmonium decays in lattice QCD.

After lunch there were many talks on hadronic decays of charmed mesons. Jon Rosner talked about extraction of strong phases with U-spin triangles (U-spin is a subgroup of flavor SU(3)). Alex Bondar gave a telephone talk about his elegant method of extraction of phase gamma (or phi_3 in Japan) in B decays to charmed meson with D-meson Dalitz plot analysis. We also had talks on various ways of doing Dalitz plot analysis. The problem there is more of the “memory” than physics — Dalitz plot analyses were popular in the sixties, so not so many people are fully versed in those analyses nowdays. In particular, the simplest analysis that is done by many people nowdays (using the so-called isobar method) violates unitarity.

Stay tuned for day 3 and final day of the conference. 

I’ve been slow in posting this conference report — mainly because I’ve been writing my talk… Ayways, it’s a great time to be at a charm conference — several collaborations recently announced evidence for observations of charm mixing, a bunch of new (and weird) hadrons have been recently discovered, which happened to contain charm quark, etc. So it is a good time for such a conference.

This particular workshop, CHARM-2007, has only plenary talks, so I’ll only report about things that I find interesting (hm…). The first section of the Day 1 contained three talks from Brian Lang, H.Hu and Bruce Yabsley and dealt with charmonum production at ~ 4 GeV and at higher energies at KEK. In particular, Bruce was talking about a problem that was hanging over theorists for quite some time now — production of J/psi  plus two charmed quarks at Belle energies. The problem there is that NRQCD, an approximation to QCD for non-relativistic quarks, underestimates the observation, even taking into account more exotic mechanisms such as gamma-gamma processes. Bruce claims that with the refined experimental analysis Belle sees two newish charmonium resonaces recoiling against the J/psi: X(3940) and X(4160) — not very well established resonances.

In the next section, Eric Swanson talked about some theoretical (quark model-motivated) approaches to charmonium production, including the X-states, some of which are believed to be the molecular states of two D-mesons and their excitations. J Radenmacker gave a nice report on charmonium production at the Tevatron, inlcuding polarization of J/psi and its excitation, psi’. There is a funny problem there — NRQCD prdicts that at high p_T (transverse momentum), all heavy quarkonia, i.e. both charmonia and bottomonia would have to be produced transversely polarized. This is easy to explain — at high p_T the dominant mechanism for heavy quarkonia production is gluon fragmentation, so produced quarkonium should retain polarizatin of that gluon. Since gluon is massless, it only has transverse polarization, so the resulting quarkonium should be transversely polarized. Now, what’s interesting is that this prediction fails miserably at the Tevatron. In fact, the produced psi’s show the opposite trend: more longitudinal polarization! And he showed new data that confirms that… so it is a problem — maybe charm quark is not heavy enough for NRQCD to work at leading or next-to-leading order in velocity expansion… also p_T maybe not high enough… It sort of works for bottomonium, but the statistics is not that great there. The last talk of that section was by A. Knospe, who gave a crush course in heavy ion collisions and charm production at RHIC.

The rest of the first day was about charm mixing and CP-violation. Personally, I think that’s the hottest topic of this conference, but I might be biased. :-)  Abe Seiden talked about mixing at BaBar, Werner Sun talked about TQCA analysis of mixing at CLEO-c (they don’t see mixing — not enough statistics, but the “tendency” is towards negative values of th elifetime difference, which is just the opposite of what other collaboratons see) and Brian Petersen talked about Heavy Flavor Averaging Group (HFAG) averages of charm mixing parameters. Them there was a talk of my long-time collaborator Gene Golowich about theory of charm mixing, reporting, among other things, on posiible New Physics in charm mixing. I talked about CP-violation in charmed particle decays amd G. Mancinelli talked about experimental prospects for measuring CP-violation in charm.    

All in all, it is great workshop. One problem (that I thought was resolved some time ago) was that several physicists from China and Russia did not get their visas on time, so could not actually come to Cornell. Their talks were given by their collaborators. This is very frustarting! The thing about it is that those people are well-known experts in their fields and have absolutely no desire to stay in the US illigally. So I don’t see why there were not issued visas on time. What will happen is that at some point well-known physicists would stop holding high-profile conferences inthe US, which would be a shame…

I am leaving for CHARM-2007 conference in beautiful Ithaca tomorrow. It is always nice to come back to Cornell where I did my postdoc some while ago. I will try to blog from there to cover the conference activities, if anyone is interested. I also wanted to add a couple of links here

1. This comes via Peter Woit’s blog: apparently the only problem with the intermediate-scale and time project (i.e. something that would be done at Fermilab before ILC is built — in other words, what BTeV was supposed to be) is its name. So very appropriately it is now called “Project X“. They are even asking commnity input for name proposals. Personally, I like SNuFL (Superconducting Neutrino and Flavor Linac), but can also propose “High intEnsity Proton aCcelerAtor PUT at FNAL.” What do you think?

2. There is a new peer-reviewed journal in High Energy Physics called “Advances in High Energy Physics” (here is its Editorial Board). It will publish both reviews and research articles in HEP. They employ a different concept from journals available so far: access to the content is free anywhere in the world (which is great for people in developing countries), but there are old-good per-page publication charges (not so great for people in developing countries). It will be interesting to see how well this publication model works.

3 . Terrible news from Minneapolis (I-35W bridge collapse). This bridge is located right next to the University of Minnesota. I might be mistaken, but it is what people take to get to/from the airport (I’ve been doing it every two years during the Continuous Advances in QCD conference at the Univ. of Minnesota)…