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Links 1 through 10 of 129 by Chad Orzel tagged particles

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This is a repost and update of a six year old post in which I listed what I think are the most interesting and pressing open problems in theoretical physics, or at least the area that I work in, quantum gravity. I thought it might be worthwhile to revisit. This list doesn't even pretend to be objective - it omits entire areas of theoretical physics - it is mainly a reflection of my personal interests; a summary of puzzles I find promising to spend brain time on.

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I am less impressed with this than a lot of people who have linked it. Someday, I hope to have enough time to explain why.

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Is supersymmetry, as a symmetry that might explain some of the puzzling aspects of particle physics at the energy scales accessible to the Large Hadron Collider [LHC], ruled out yet? If the only thing you’re interested in is the answer to precisely that question, let me not waste your time: the answer is “not yet”. But a more interesting answer is that many simple variants of supersymmetry are either ruled out or near death.

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Both claims that I’m about to describe use novel techniques, and their analyses have not been repeated by anyone else. At this point you should understand that both are tentative, and (based on the history of radical claims) the odds are against them. Both might be wrong. That said, both analyses look to me as though they’ve been reasonably well done, and if a mistake has been made, it will require someone far more expert in dark matter studies than I am to point it out.

So let me describe them in turn, to the best of my ability.

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So it is important to balance the OPERA mini-fiasco with another hot-off-the-presses neutrino story that illustrates why, even though mistakes in individual scientific experiments are common, collective mistakes in science are rare. A discipline such as physics has intrinsic checks and balances that significantly reduce the probability of errors going unrecognized for long. In the story I’m about to relate, one can recognize how and why scientists start to come to consensus.  Though quite suspicious of any individual experiment, scientists generally take a different view of a group of experiments that buttress one another.

The context of this story, though much less revolutionary than a violation of Einstein’s speed limit, still represents a milestone in our understanding of neutrinos, which has been advancing very rapidly over the past fifteen years or so. W

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The search for ways to unify and understand physical phenomena goes back to Kaluza and Klein, who in the 1920s tried to combine electromagnetism with gravity by adding a fourth spatial dimension to the usual three (plus time). More recent theoretical work has suggested that a theory of everything may need 11 spacetime dimensions. Boada et al. are suggesting an experimental strategy for investigating how matter behaves in extra dimensions. Their idea is to encode a fourth spatial dimension in an internal degree of freedom offered by atoms trapped in an optical lattice, and do it in such a way as to exactly reproduce the physics described by a 4D Hamiltonian. The authors show two ways of observing such effects: one is to look for single-particle effects, such as rates of decay of excited states as a function of dimensionality; another is to search for many-body effects such as insulator-to-superfluid transitions that depend on the number of dimensions.

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Keep in mind that the total number of 7 TeV = 7000 GeV proton-proton collisions that took place in ATLAS while they were accumulating the data for the plot above was about 100,000,000,000,000.  [The total 2011 data set was 5 times larger, but the corresponding plot won't appear for a few months.]  Of all these collisions, just two had mini-collisions that passed above 3500 GeV — half the collision energy of the protons.  In principle the energy of the mini-collisions can go up all the way to 7000 GeV, but the probability goes down and down, and it is so rare to get a 6000 GeV mini-collision that chances are we wouldn’t get one even with 100 times this much data.

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The Physics World staff try to predict the big physical science stories of the next year.

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