Wednesday, September 27, 2017

Proposed experiments seeking hidden sectors, millicharged particles

Off-topic I, LIGO: Virgo, the Italian sister (guess why she isn't a brother and what's the name of her bed partner), has finally joined her two LIGO brothers and they observed the GW170814 black hole merger together. Her signal is clearly weaker but it's there. Three detectors allowed precise localization of the event in both directions and will allow to test new consistency conditions that follow from GR. See LIGO's Twitter for more.

Off-topic II, quantum computing: a new PLB article promotes a faster hardware for quantum computers, with some photon pulses running around a room many times. See Science Alert for a summary. Because the qubit is embedded in an infinite-dimensional Hilbert space, the scheme may be easily made fault-tolerant.

Bob Henderson wrote about two proposed experiments to search for new (particle) physics outside the LHC's detectors:
How the Hidden Higgs Could Reveal Our Universe’s Dark Sector (Quanta Magazine)
There may be new Higgs-like bosons, superpartners predicted by supersymmetry, but completely new things – no physics beyond the Standard Model has been found as of today.

In particular, Henderson mentions a 2014 paper proposing the milliQan experiment and a 2016 paper proposing MATHUSLA (appeared in PLB).

MilliQan is clearly a distortion of the name Millikan who has performed famous oil droplet experiments to find the value of the elementary charge. Helpfully enough, Millikan's name starts with "milli" which is 1/1,000. ;-) Consequently, milliQ may be interpreted as a "tiny charge, about 1/1,000 of the charge of the electron" or another elementary particle.

The milliQan experiment should be sensitive to \(1\GeV\) particles, plus minus some 1-2 orders of magnitude, whose charges are between 0.001 and 0.1 of the electron charge. It's fun that one may propose such a thing and build it. But do I believe that such fractional charges exist? I would bet No. I really think that they don't exist at all, for some theoretical principles. In particular, grand unificiation and similar schemes would ban such a fragmentation of the elementary charge. On top of that, I think that the Millikan experiment by itself is a rather good empirical answer to the question and the answer it gave was that the electron charge is the elementary one.

But even if one assumes that this intuition is completely wrong and such particles are allowed, would they be found by this experiment? Would they have the masses that are close to \(1\GeV\), plus minus two orders of magnitude? Clearly, this condition reduces the probability of a find further. While the LHC hasn't found anything yet, it's still totally reasonable to imagine that new excesses suddenly start to grow in 2017 and a 5-sigma discovery will be made in a few years.

For this new experiment, it looks much less likely to me. But of course, if it's cheaper than say 1% of the LHC, it seems sensible to me to pay for such an experiment.

MATHUSLA, the other experiment that Henderson begins with, is named after a monster that is rumored to have lived 1,000 years ago. Well, even rumors about monsters living at the present are usually false – what about monsters a millennium ago? ;-) At any rate, MATHUSLA is spuposed to be a big, barn-like experiment in which some very long-lived particles – which are invisible inside the LHC and escape the LHC – are encouraged to decay to ordinary particles by the hay. I didn't quite understand whether the interior of the barn is important and how.

And the ordinary particles that result from such decays of the long-lived new hypothetical particles – whose lifetime times the speed of light is between millimeters and kilometers – are detected on the roof of the MATHUSLA barn. This kind of experiment is meant to be sensitive particularly to models with very extended, huge hidden sectors with many particles and/or their copies.

Those models are academically plausible and some of the arguments that they may provide us with new ways to solve the hierarchy problem are plausible after a few bottles of wine. Sorry, I can't accept them without the wine because the addition of an unnaturally large number of sectors is a sort of fine-tuning by itself. But I simply cannot get rid of the feeling that such experiments addressing such models are pure random guesswork. I don't really see the great new possibilities that the authors have discovered. In other words, I don't know what it would mean to "independently rediscover those things" and I don't understand how I could be proud about such a rediscovery. You may clearly generalize existing models in many ways – change the number of colors or factors of the gauge group, colors, or generations, and many similar things, from two to three, to five, to ten factorial, to infinity. I think that the last two possibilities aren't more natural or attractive than those smaller numbers at the beginning. At least without extra arguments, they don't seem to be.

Moreover, even if the experiment found something, I don't think it's clear at all that one should say that it provides us with evidence in favor of the particular models with extended hidden sectors that are being used to justify the experiment. There could also be "more minimal" models that incorporate such a new particle – which could be added to the Standard Model separately, in the well-known "who ordered that" way.

But of course, I can be wrong about all these guesses. I can misunderstand something important. I can be like Sheldon who was asked by Howard what's the name of the astronaut who will go to outer space with Howard's toilet. "Mohammed Lee," Sheldon answered because in the case of ignorance, the combination of the most frequent first name and most frequent last name gave him a mathematical edge. It turned out that the name was "Howard Wolowitz". Sheldon wouldn't have guessed it even if he had a million of attempts. ;-)

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