# LOG#058. LHC: last 2012 data/bounds.

**Posted:**2012/12/12

**Filed under:**Experimental H.E.P., Physmatics, Science |

**Tags:**127 GeV Higgs boson, BSM, LHC, LHC results, SM, Standard Model Leave a comment

Today, 12/12/12, the following paper arised in the arxiv http://arxiv.org/abs/1212.2339

This interesting paper reviews the last bounds about Beyond Stantard Model particles (both, fermions and bosons) for a large class of models until the end of this year, 2012. **Particle hunters, some theoretical physicists are!** The fundamental conclusions of this paper are encoded in a really beautiful table:

There, we have:

1. Extra gauge bosons . They are excluded below 1-2 TeV, depending on the channel/decay mode.

2. Heavy neutrinos . They are excluded with softer lower bounds.

3. Fourth generation quarks and vector-like quarks are also excluded with bounds.

4. Exotic quarks with charge are also excluded below 0.6 TeV.

We continue *desperately* searching for deviations to the Standard Model (SM). SUSY, 4th family, heavy likely right-handed neutrinos, tecnifermions, tecniquarks, new gauge bosons, Kalula-Klein resonances (KK particles), and much more are not appearing yet, but we are seeking deeply insight the core of the matter and the deepest structure of the quantum vacuum. We know we have to find “something” beyond the Higgs boson/particle, but what and where is not clear for any of us.

Probably, we wish, some further study of the total data in the next months will clarify the whole landscape, but these data are “bad news” and “good news” for many reasons. They are bad, since they point out to no new physics beyond the Higgs up to 1 TeV (more or less). They are good since we are collecting many data and hopefully, we will complement the collider data with cosmological searches next year, and then, some path relative to the Standard Model extension and the upcoming quantum theory of gravity should be enlightened, or at least, some critical models and theories will be ruled out! Of course, I am being globally pesimist but some experimental hint beyond the Higgs (beyond collider physics) is necessary in order to approach the true theory of this Universe.

And if it is not low energy SUSY (it could if one superparticle is found, but we have not found any superparticle yet), what stabilizes the Higgs potential and provides a Higgs mass, i.e, what does that “job”/role? What is forbidding the Higgs mass to receive Planck mass quantum corrections? For me, as a theoretical physicist,** this question is mandatory!** If SUSY fails to be the answer, we really need some good* theoretical* explanation for the “light” mass the Higgs boson seems to have!

Stay tuned!

# LOG#057. Naturalness problems.

**Posted:**2012/12/02

**Filed under:**Physmatics, Quantum Gravity, The Standard Model: Basics |

**Tags:**CKM matrix, cosmic coincidence, cosmological constant, cosmological constant problem, critical energy density, curvature, dark energy, dark energy density, Dirac large number hypothesis, electroweak scale, energy, energy density, flatness problem, flavour problem, gauge hierarchy problem, Higgs boson, Higgs mechanism, Hubble constant, inflation, inflationary cosmologies, little hierarchy problem, mass, matter density, naturalness, naturalness problem, neutrino mass hierarchy, neutrino masses, neutrino oscillations, NO, NOSEX, parameter, parameter space, Planck era, Planck scale, PMNS matrix, QCD, QFT, quark-lepton complementarity, SM, Standard Cosmological Model, Standard Model, strong CP problem, theta term, types of naturalness, vacuum, vacuum energy, W boson, Z boson 8 Comments

In this short blog post, I am going to list some of the greatest “naturalness” problems in Physics. It has nothing to do with some delicious natural dishes I like, but there is a natural beauty and sweetness related to naturalness problems in Physics. In fact, they include some hierarchy problems and additional problems related to stunning free values of parameters in our theories.

**Naturalness problems** arise when the “naturally expected” property of some free parameters or fundamental “constants” to appear as quantities of order one is violated, and thus, those paramenters or constants appear to be very large or very small quantities. That is, naturalness problems are problems of untuning “scales” of length, energy, field strength, … A value of 0.99 or 1.1, or even 0.7 and 2.3 are “more natural” than, e.g., Equivalently, imagine that the values of every fundamental and measurable physical quantity lies in the real interval . Then, 1 (or very close to this value) are “natural” values of the parameters while the two extrema or are “unnatural”. As we do know, in Physics, zero values are usually explained by some “fundamental symmetry” while extremely large parameters or even can be shown to be “unphysical” or “unnatural”. In fact, renormalization in QFT was invented to avoid quantities that are “infinite” at first sight and regularization provides some prescriptions to assign “natural numbers” to quantities that are formally ill-defined or infinite. However, naturalness goes beyond those last comments, and it arise in very different scenarios and physical theories. It is quite remarkable that naturalness can be explained as numbers/contants/parameters around 3 of the most important “numbers” in Mathematics:

**REMEMBER: Naturalness** of X is, thus, being 1 or close to it, while values approaching 0 or are unnatural. Therefore, if some day you heard a physicist talking/speaking/lecturing about “naturalness” remember the triple and then assign “some magnitude/constant/parameter” some quantity close to one of those numbers. If they approach 1, the parameter itself is natural and unnatural if it approaches any of the other two numbers, zero or infinity!

I have never seen a systematic classification of naturalness problems into types. I am going to do it here today. We could classify naturalness problems into:

1st.** Hierarchy problems**. They are naturalness problems related to the energy mass or energy spectrum/energy scale of interactions and fundamental particles.

2nd. **Nullity/Smallness problems**. These are naturalness problems related to free parameters which are, surprisingly, close to zero/null value, even when we have no knowledge of a deep reason to understand why it happens.

3rd.** Large number problems (or hypotheses).** This class of problems can be equivalently thought as nullity reciprocal problems but they arise naturally theirselves in cosmological contexts or when we consider a large amount of particles, e.g., in “statistical physics”, or when we face two theories in very different “parameter spaces”. Dirac pioneered these class of hypothesis when realized of some large number coincidences relating quantities appearing in particle physics and cosmology. This Dirac large number hypothesis is also an old example of this kind of naturalness problems.

4th. **Coincidence problems**. This 4th type of problems is related to why some different parameters of the same magnitude are similar in order of magnitude.

The following list of concrete naturalness problems is not going to be complete, but it can serve as a guide of what theoretical physicists are trying to understand better:

1. **The little hierarchy problem**. From the phenomenon called neutrino oscillations (NO) and neutrino oscillation experiments (NOSEX), we can know the difference between the squared masses of neutrinos. Furthermore, cosmological measurements allow us to put tight bounds to the total mass (energy) of light neutrinos in the Universe. The most conservative estimations give or even as an upper bound is quite likely to be true. By the other hand, NOSEX seems to say that there are two mass differences, and . However, we don’t know what kind of spectrum neutrinos have yet ( normal, inverted or quasidegenerated). Taking a neutrino mass about 1 meV as a reference, the little hierarchy problem is the question of why neutrino masses are so light when compared with the remaining leptons, quarks and gauge bosons ( excepting, of course, the gluon and photon, massless due to the gauge invariance).

Why is

We don’t know! Let me quote a wonderful sentence of a very famous short story by Asimov to describe this result and problem:

*“THERE* IS AS YET INSUFFICIENT *DATA* FOR A MEANINGFUL *ANSWER*.”

2. **The gauge hierarchy problem.** The electroweak (EW) scale can be generally represented by the Z or W boson mass scale. Interestingly, from this summer results, Higgs boson mass seems to be of the same order of magnitue, more or less, than gauge bosons. Then, the electroweak scale is about . Likely, it is also of the Higgs mass order. By the other hand, the Planck scale where we expect (naively or not, it is another question!) quantum effects of gravity to naturally arise is provided by the Planck mass scale:

or more generally, dropping the factor

Why is the EW mass (energy) scale so small compared to Planck mass, i.e., why are the masses so different? The problem is hard, since we do know that EW masses, e.g., for scalar particles like Higgs particles ( not protected by any SM gauge symmetry), should receive quantum contributions of order

*“THERE* IS AS YET INSUFFICIENT *DATA* FOR A MEANINGFUL *ANSWER*.”

3. **The cosmological constant (hierarchy) problem.** The cosmological constant , from the so-called Einstein’s field equations of classical relativistic gravity

is estimated to be about from the cosmological fitting procedures. The Standard Cosmological Model, with the CMB and other parallel measurements like large scale structures or supernovae data, agree with such a cosmological constant value. However, in the framework of Quantum Field Theories, it should receive quantum corrections coming from vacuum energies of the fields. Those contributions are unnaturally big, about or in the framework of supersymmetric field theories, after SUSY symmetry breaking. Then, the problem is:

Why is ? Even with TeV or PeV fundamental SUSY (or higher) we have a serious mismatch here! The mismatch is about 60 orders of magnitude even in the best known theory! And it is about 122-123 orders of magnitude if we compare directly the cosmological constant vacuum energy we observe with the cosmological constant we calculate (naively or not) with out current best theories using QFT or supersymmetric QFT! Then, this problem is a hierarchy problem and a large number problem as well. Again, and sadly, we don’t know why there is such a big gap between mass scales of the same thing! This problem is the biggest problem in theoretical physics and it is one of the worst predictions/failures in the story of Physics. However,

*“THERE* IS AS YET INSUFFICIENT *DATA* FOR A MEANINGFUL *ANSWER*.”

4.** The strong CP problem/puzzle. **From neutron electric dipople measurements, theoretical physicists can calculate the so-called -angle of QCD (Quantum Chromodynamics). The theta angle gives an extra contribution to the QCD lagrangian:

The theta angle is not provided by the SM framework and it is a free parameter. Experimentally,

while, from the theoretical aside, it could be any number in the interval . Why is close to the zero/null value? That is the strong CP problem! Once again, we don’t know. Perhaps a new symmetry?

*“THERE* IS AS YET INSUFFICIENT *DATA* FOR A MEANINGFUL *ANSWER*.”

5. **The flatness problem/puzzle.** In the Stantard Cosmological Model, also known as the model, the curvature of the Universe is related to the critical density and the Hubble “constant”:

There, is the total energy density contained in the whole Universe and is the so called critical density. The flatness problem arise when we deduce from cosmological data that:

At the Planck scale era, we can even calculate that

This result means that the Universe is “flat”. However, why did the Universe own such a small curvature? Why is the current curvature “small” yet? We don’t know. However, cosmologists working on this problem say that “inflation” and “inflationary” cosmological models can (at least in principle) solve this problem. There are even more radical ( and stranger) theories such as varying speed of light theories trying to explain this, but they are less popular than inflationary cosmologies/theories. Indeed, inflationary theories are popular because they include scalar fields, similar in Nature to the scalar particles that arise in the Higgs mechanism and other beyond the Standard Model theories (BSM). We don’t know if inflation theory is right yet, so

*“THERE* IS AS YET INSUFFICIENT *DATA* FOR A MEANINGFUL *ANSWER*.”

6. **The flavour problem/puzzle.** The ratios of successive SM fermion mass eigenvalues ( the electron, muon, and tau), as well as the angles appearing in one gadget called the CKM (Cabibbo-Kobayashi-Maskawa) matrix, are roughly of the same order of magnitude. The issue is harder to know ( but it is likely to be as well) for constituent quark masses. However, why do they follow this particular pattern/spectrum and structure? Even more, there is a mysterious lepton-quark complementarity. The analague matrix in the leptonic sector of such a CKM matrix is called the PMNS matrix (Pontecorvo-Maki-Nakagawa-Sakata matrix) and it describes the neutrino oscillation phenomenology. It shows that the angles of PMNS matrix are roughly complementary to those in the CKM matrix ( remember that two angles are said to be complementary when they add up to 90 sexagesimal degrees). What is the origin of this lepton(neutrino)-quark(constituent) complementarity? In fact, the two questions are related since, being rough, the mixing angles are related to the ratios of masses (quarks and neutrinos). Therefore, this problem, if solved, could shed light to the issue of the particle spectrum or at least it could help to understand the relationship between quark masses and neutrino masses. Of course, we don’t know how to solve this puzzle at current time. And once again:

*“THERE* IS AS YET INSUFFICIENT *DATA* FOR A MEANINGFUL *ANSWER*.”

7. **Cosmic matter-dark energy coincidence.** At current time, the densities of matter and vacuum energy are roughly of the same order of magnitude, i.e, . Why now? We do not know!

*“THERE* IS AS YET INSUFFICIENT *DATA* FOR A MEANINGFUL *ANSWER*.”

And my weblog is only just beginning! See you soon in my next post! 🙂

# LOG#047. The Askaryan effect.

**Posted:**2012/10/17

**Filed under:**Physmatics, Relativity |

**Tags:**Askaryan effect, Askaryan radiation, cherenkov effect, cherenkov radiation, coherent radiation, cosmic rays, electromagnetism, electroweak theory, G. Askaryan, matter-radiation interactions, microwaves, neutrino detection, neutrino detectors, neutrino experiments, neutrino telescopes, neutrinos, New Physics, optical band of the spectrum, origin of cosmic rays, Physmatics, radio waves, Relativity, special relativity, Standard Model, superluminality, UHE neutrinos Leave a comment

I discussed and reviewed the important Cherenkov effect and radiation in the previous post, here:

https://thespectrumofriemannium.wordpress.com/2012/10/16/log046-the-cherenkov-effect/

Today we are going to study a relatively new effect ( new experimentally speaking, because it was first detected when I was an undergraduate student, in 2000) but it is not so new from the theoretical aside (theoretically, it was predicted in 1962). This effect is closely related to the Cherenkov effect. It is named Askaryan effect or Askaryan radiation, see below after a brief recapitulation of the Cherenkov effect last post we are going to do in the next lines.

We do know that charged particles moving faster than light through the *vacuum* emit Cherenkov radiation. How can a particle move faster than light? The *weak* speed of a charged particle can exceed the speed of light. That is all. About some speculations about the so-called tachyonic gamma ray emissions, let me say that the existence of superluminal energy transfer has not been established so far, and one may ask why. There are two options:

1) The simplest solution is that superluminal quanta just do not exist, the vacuum speed of light being the definitive upper bound.

2) The second solution is that the interaction of superluminal radiation with matter is very small, the quotient of tachyonic and electric fine-structure constants being . Therefore superluminal quanta and their substratum are hard to detect.

A related and very interesting question could be asked now related to the Cherenkov radiation we have studied here. What about neutral particles? Is there some analogue of Cherenkov radiation valid for chargeless or neutral particles? Because neutrinos are electrically neutral, conventional Cherenkov radiation of superluminal neutrinos does not arise or it is otherwise weakened. However neutrinos *do carry electroweak charge* and may emit certain *Cherenkov-like radiation* via weak interactions when traveling at superluminal speeds. The Askaryan effect/radiation is this Cherenkov-like effect for neutrinos, and we are going to enlighten your knowledge of this effect with this entry.

We are being bombarded by cosmic rays, and even more, we are being bombarded by neutrinos. Indeed, we expect that ultra-high energy (UHE) neutrinos or extreme ultra-high energy (EHE) neutrinos will hit us as too. When neutrinos interact wiht matter, they create some shower, specifically in dense media. Thus, we expect that the electrons and positrons which travel faster than the speed of light in these media or even in the air and they should emit (coherent) Cherenkov-like radiation.

## Who was Gurgen Askaryan?

Let me quote what wikipedia say about him: Gurgen Askaryan (December 14, 1928-1997) was a prominent Soviet (armenian) physicist, famous for his discovery of the self-focusing of light, pioneering studies of light-matter interactions, and the discovery and investigation of the interaction of high-energy particles with condensed matter. He published more than 200 papers about different topics in high-energy physics.

Other interesting ideas by Askaryan: the bubble chamber (he discovered the idea independently to Glaser, but he did not published it so he did not win the Nobel Prize), laser self-focussing (one of the main contributions of Askaryan to non-linear optics was the self-focusing of light), and the acoustic UHECR detection proposal. Askaryan was the first to note that the outer few metres of the Moon’s surface, known as the regolith, would be a sufficiently transparent medium for detecting microwaves from the charge excess in particle showers. The radio transparency of the regolith has since been confirmed by the Apollo missions.

If you want to learn more about Askaryan ideas and his biography, you can read them here: http://en.wikipedia.org/wiki/Gurgen_Askaryan

## What is the Askaryan effect?

The next figure is from the Askaryan radiation detected by the ANITA experiment:

The **Askaryan effect** is the phenomenon whereby a particle traveling faster than the phase velocity of light in a dense dielectric medium (such as salt, ice or the lunar regolith) produces a shower of secondary charged particles which contain a charge anisotropy and thus emits a cone of coherent radiation in the radio or microwave part of the electromagnetic spectrum. It is similar, or more precisely it is based on the Cherenkov effect.^{}

High energy processes such as Compton, Bhabha and Moller scattering along with positron annihilation rapidly lead to about a 20%-30% negative charge asymmetry in the electron-photon part of a cascade. For instance, they can be initiated by UHE (higher than, e.g.,100 PeV) neutrinos.

1962, Askaryan first hypothesized this effect and suggested that it should lead to strong coherent radio and microwave Cherenkov emission for showers propagating within the dielectric. Since the dimensions of the clump of charged particles are small compared to the wavelength of the radio waves, the shower radiates coherent radio Cherenkov radiation whose power is proportional to the square of the net charge in the shower. The net charge in the shower is proportional to the primary energy so the radiated power scales quadratically with the shower energy, .

Indeed, these radio and coherent radiations are originated by the Cherenkov effect radiation. We do know that:

from the charged particle in a dense (refractive) medium experimenting Cherenkov radiation (CR). Every charge emittes a field . Then, the power is proportional to . In a dense medium:

We have two different experimental and interesting cases:

A) **The optical case,** with . Then, we expect random phases and .

B) **The microwave case**, with . In this situation, we expect coherent radiation/waves with .

We can exploit this effect in large natural volumes transparent to radio (dry): pure ice, salt formations, lunar regolith,…The peak of this coherent radiation for sand is produced at a frequency around , while the peak for ice is obtained around .

The first experimental confirmation of the Askaryan effect detection were the next two experiments:

1) 2000 Saltzberg et.al., SLAC. They used as target silica sand. The paper is this one http://arxiv.org/abs/hep-ex/0011001

2) 2002 Gorham et.al., SLAC. They used a synthetic salt target. The paper appeared in this place http://arxiv.org/abs/hep-ex/0108027

Indeed, in 1965, Askaryan himself proposes ice and salt as possible target media. The reasons are easy to understand:

1st. They provide high densities and then it means a higher probability for neutrino interaction.

2nd. They have a high refractive index. Therefore, the Cerenkov emission becomes important.

3rd. Salt and ice are radio transparent, and of course, they can be supplied in large volumes available throughout the world.

The advantages of radio detection of UHE neutrinos provided by the Askaryan effect are very interesting:

1) Low attenuation: clear signals from large detection volumes.

2) We can observe distant and inclined events.

3) It has a high duty cycle: good statistics in less time.

4) I has a relative low cost: large areas covered.

5) It is available for neutrinos and/or any other chargeless/neutral particle!

Problems with this Askaryan effect detection are, though: radio interference, correlation with shower parameters (still unclear), and that it is limited only to particles with very large energies, about .

**In summary:**

*Askaryan effect = coherent Cerenkov radiation from a charge excess induced by (likely) neutral/chargeless particles like (specially highly energetic) neutrinos passing through a dense medium.*

## Why the Askaryan effect matters?

It matters since it allows for the detection of UHE neutrinos, and it is “universal” for chargeless/neutral particles like neutrinos, just in the same way that the Cherenkov effect is universal for charged particles. And tracking UHE neutrinos is important because they point out towards its source, and it is suspected they can help us to solve the riddle of the origin and composition of cosmic rays, the acceleration mechanism of cosmic radiation, the nuclear interactions of astrophysical objects, and tracking the highest energy emissions of the Universe we can observe at current time.

Is it real? Has it been detected? Yes, after 38 years, it has been detected. This effect was firstly demonstrated in sand (2000), rock salt (2004) and ice (2006), all done in a laboratory at SLAC and later it has been checked in several independent experiments around the world. Indeed, I remember to have heard about this effect during my darker years as undergraduate student. Fortunately or not, I forgot about it till now. In spite of the beauty of it!

Moreover, it has extra applications to neutrino detection using the Moon as target: GLUE (detectors are Goldstone RTs), NuMoon (Westerbork array; LOFAR), or RESUN (EVLA), or the LUNASKA project. Using ice as target, there has been other experiments checking the reality of this effect: FORTE (satellite observing Greenland ice sheet), RICE (co-deployed on AMANDA strings, viewing Antarctic ice), and the celebrated ANITA (balloon-borne over Antarctica, viewing Antarctic ice) experiment.

Furthermore, even some experiments have used the Moon (an it is likely some others will be built in the near future) as a neutrino detector using the Askaryan radiation (the analogue for neutral particles of the Cherenkov effect, don’t forget the spot!).

## Askaryan effect and the mysterious cosmic rays.

Askaryan radiation is important because is one of the portals of the UHE neutrino observation coming from cosmic rays. The mysteries of cosmic rays continue today. We have detected indeed extremely energetic cosmic rays beyond the scale. Their origin is yet unsolved. We hope that tracking neutrinos we will discover the sources of those rays and their nature/composition. We don’t understand or know any mechanism being able to accelerate particles up to those incredible particles. At current time, IceCube has not detected UHE neutrinos, and it is a serious issue for curren theories and models. It is a challenge if we don’t observe enough UHE neutrinos as the Standard Model would predict. Would it mean that cosmic rays are exclusively composed by heavy nuclei or protons? Are we making a bad modelling of the spectrum of the sources and the nuclear models of stars as it happened before the neutrino oscillations at SuperKamiokande and Kamikande were detected -e.g.:SN1987A? Is there some kind of new Physics living at those scales and avoiding the GZK limit we would naively expect from our current theories?