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Dark Matter searches at colliders – part I April 23, 2008

Posted by dorigo in cosmology, personal, physics, science.
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Yesterday I gave a seminar on searches for dark matter at the Tevatron and LHC in Padova, to a wide audience. This was a one-afternoon-workshop intended to educate students and publicize the LHC experiments, but it gathered more audience than undergraduates: quite a few of the Department staff came to listen.

My talk was the last one in a tightly packed agenda, and it indeed started with some 40 minutes of delay, as I had predicted. However, despite the late time -5.40 in the afternoon is about time to catch a train on normal workdays, even for me- the audience stayed to listen.

I already posted my slides here, but since they are in italian, I feel the need to give a summary of my seminar in English here, now that I have some more time to do so. I will do this in at least two parts, because I am swamped with other obligations these days!

I started my seminar by comparing the Tevatron and the LHC (in the aerial view of Fermilab above, the Tevatron ring is compared to the size of LHC, overimposed as a red circle courtesy M.Schmitt): the former collides protons against antiprotons, the latter collides protons with other protons. The crucial differences are however not the projectiles, but two parameters: energy E_{tot}=2 TeV and luminosity L=10^{32} cm^{-1} s^{-2} at the Tevatron, and E_{tot}=14 TeV and L=10^{34} cm^{-1} s^{-2} at LHC. While E sets the limit of investigation in new physics phenomena – particles more massive than a few hundred GeV cannot be produced at the Tevatron – L is a parameter which dictates the rate of rare processes. The dumb product of the increases in E and L offered by LHC is a factor 1000, which can be thought as a rule of thumb for the increase in discovery reach of the ATLAS and CMS detectors with respect to their smaller, older brothers CDF and D0. Sure, discovery reach scales only with the square root of the collected data (proportional to L), but cross sections of rare phenomena scale with more than the square of the energy increase: for instance, top production at LHC is 100 times more frequent, at equal L.

I had to mention the huge legacy that the Tevatron offers to LHC: twenty years of investigations, discoveries, and measurements. The top quark mass is known with a 0.8% accuracy thanks to CDF and D0’s recent measurements. This grants CMS and ATLAS a standard candle with which to calibrate their calorimeter response to hadronic jets: it will be extremely important in the initial phase of running, when top quark pairs will be available for a check of the jet energy scale. But the Tevatron’s high precision studies of electroweak physics will do much more for the LHC: the tuning of parton distribution functions performed by CDF and D0 with detailed QCD studies will be crucial to tune the simulation and understand the cross section of rare phenomena.

I then spent five minutes discussing why the important quantity at a hadron collider is the momentum flow in a plane orthogonal to the direction of the beams. While in electron-positron colliders the center-of-mass of the collision is at rest (unless beams are asymmetric in energy on purpose, such as at BaBar or Belle), and particle momenta are equally important regardless of their outgoing direction, a hadron collision of high energy is in fact a collision between quarks and gluons. These constituents of hadrons (drawn as colored lines in the cartoon above, where protons are the black circles) carry a variable fraction of their container’s momentum, and as a result the collision center-of-mass may move in either direction along the beam. What characterizes a hard interaction is instead the momentum flowing orthogonally from this direction (the two red and blue lines exiting at large angle from the protons direction in the cartoon): transverse momentum is therefore a measure of the acceleration that the proton constituents participating in the collision underwent during the mindboggingly brief moment of their interaction.

As a quark or gluon escapes the collision point, it extends a gluon string. The QCD potential grows linearly with distance decelerating the outgoing parton, until it finds it energetically favorable to break in two, materializing a quark-antiquark pair at its midpoint. The process continues until a stream of colorless hadrons are created. These then decay with strong and weak interactions, producing a final stream of particles which collectively carries memory of the originating parton’s momentum. It is what we call a hadronic jet.

Jets are measured in the detector elements called calorimeters (see a description in two parts here and here) by destroying the particles they contain, both charged and neutral ones, in electromagnetic and nuclear interactions with heavy elements – typically tiles of lead or iron. What is measured in these devices is the total track length – the sum of paths of all secondary particles produced in the shower originated by the chain of interactions in the absorber. That quantity is proportional to the energy of the incident bodies. Ultimately, the originating quark or gluon energy and its direction are reconstructed with an accuracy sufficient to understand the characteristics of the process which caused its emission.

In general, a hadronic collision produces jets of particles. Sometimes, though, rarer and fancier objects -ones that are not present in the projectiles- are produced: leptons and photons of high energy. These do not feel the strong interactions, and are due to electroweak interactions, which involves the exchange of W and Z bosons, or heavy quarks which decay weakly. In general, electrons and muons are objects that the detectors are trained to detect with high efficiency. But for dark matter, the signal which is by far the most important of all is an indirect one: missing transverse energy.

Missing transverse energy -the energy carried away by a body which leaves the detector unseen- is reconstructed thanks to the law of conservation of momentum: the incoming projectiles carry no momentum in the direction orthogonal to the beam, and so the final products of a collision must balance their momenta in the transverse plane. When this does not happen, it may be due to an imperfect reconstruction of momenta -a likely cause only if missing Et is not large and not significantly different from zero-, or to the escape of a high-energy neutrino. A dark matter candidate would similarly cause the same imbalance.

The graph above shows an event with two electrons (giving pink energetic deposits) and large energy imbalance -indicated by the downward arrow. Most probably, this rare event collected by CDF is the decay of a pair of Z bosons: p \bar p \to ZZ \to e^+ e^- \nu \bar \nu, where the two neutrinos escape giving collectively a trace of their creation by the energy imbalance they leave behind.

Missing transverse energy is defined as the opposite of the vector sum of all detected energetic deposits in the calorimeters, in the transverse plane. It is measured with a resolution with depends on the total transverse energy detected: in fact, its resolution scales with the square root of total transverse energy. The reason is the way energy is extracted from the number of track segments caused by hadronic showers: integer numbers follow Poisson statistics, and their uncertainty scales with the square root of the number -and so does energy, and so does missing transverse energy.

Why is a dark matter candidate going to cause missing energy in the detector ? Because dark matter particles cannot be electrically charged -or they would have been found quite easily in the Universe-, they cannot feel strong or electromagnetic interactions -or they would create exotic atoms we do not see-, and they are massive -they need to, if they are to solve the matter-energy balance equation of the Universe, which foresees that dark matter makes up for 20% of the total budget as compared to baryonic matter’s 4%.

One of the most appealing candidates for dark matter is the Supersymmetric particle called Neutralino. Supersymmetry is a model extending the Standard Model of particle physics. It predicts the existence of a new partner for each known quark, lepton, or boson we know – only, with different values of spin. This multiplication of known bodies is the price to pay for a theory that solves one big issue in the standard model: the inconsistency of the mass of the Higgs boson, which must be light if the Standard Model is to be consistent with the many measurements colliders performed at the electroweak scale, but should be far heavier to avoid having to invoke a delicate and unnatural cancelation of huge contributions from virtual divergent diagrams that are present in the theory. WIth Supersymmetry, the Higgs mass is “stabilized at the electroweak scale“: supersymmetric particles cancel automatically the unwanted loop effects of SM particles. SUSY also predicts a unification of forces at a common, very high-energy scale, in a way that is pleasing to the eye but admittedly not called for by any intrinsic requirement.

(To be continued in Part II)

Comments

1. Nick - April 23, 2008

Very cool stuff. Do you know of any good books or resources dealing with high energy physics? I’m still kind of lost in the dark when it comes to what all the different particles are and what actually happens in a collision. It would also be interesting to see the mathematics behind all of this.

I’m glad you didn’t write too much more otherwise it might have been too overwhelming! Now I have enough time to take in this part, before part II comes out.

Thanks,
Nick

2. forrest noble - April 24, 2008

Tomasso,

Dark matter is an omni-present aether that you can see in most every collision as “quark jets”: The energy of motion of these particles we call zero point fluctuations. The smallest of these dark matter stings are maybe millions of time smaller than the hypothetical Higgs boson or even an electron. They are not a boson or fermion since they have no intrinsic spin and are neutral entities. An electron neutrino after millions of years of traveling through matter and the aether, slows down to maybe 1/4 the speed of light. At this slower momentum their spin greatly increases and accordingly so does their interaction with matter.

These are the pushing gravitons which are the most influential agents of pushing gravity. All of the smaller particles in this field, which influxes into all matter and is radiated away as EM radiation, create a low-pressure area in the field surrounding all matter which influxes field material into the surrounding vortex of spinning matter, in a never ending cycle. This explains the basic mechanisms of gravity where the density of dark matter controls field motion surrounding matter as well as controlling the ultimate speed of light.

Individual dark matter particles will seemingly be beyond the capabilities of particle physics to see for maybe millennia, if ever. But the longest strings of this dark matter can be seen as quark jets. Of course there are no quarks or gluons. They’re just a good mathematical construct for the time being until they figure out that looped strings are the constituents and building blocks of matter. No extra dimensions are needed, only XYZ & T.

your friend forrest forrest_forrest@netzero.net

3. dorigo - April 24, 2008

Hi Nick,

I am not sure if I understand the level of complexity you are looking at for a book on particle physics. You say you want the formulas… This restricts the choice to more “serious” books, because the more divulgative ones are very careful to _avoid_ formulas, in order to not lose readers…

A very good introductory book, which has the formulas but is also descriptive, is D.H.Perkins “Particle Astrophysics”, Oxford UP.

Dear Forrest,

I do not know how to deal with your comments. One impulse would be to ignore them, because they contain too many unsupported claims. On the other hand this is not in the spirit of my blog. You seem to talk as if you were God explaining things to humans, but you fail to realize that by putting together concepts you do not explain you can sound knowledgeable only to a small part of readers here: others tag you very quickly as a unrecoverable.

Cheers,
T.

4. nc - April 24, 2008

‘I do not know how to deal with your comments. One impulse would be to ignore them, because they contain too many unsupported claims. … You seem to talk as if you were God explaining things to humans, but you fail to realize that by putting together concepts you do not explain you can sound knowledgeable only to a small part of readers here: others tag you very quickly as a unrecoverable.’ – Tommaso

If you seriously compare the style Forrest has to certain popularist books asserting the facts about the universe written by string theory experts for lay people, it’s not that different. Assertion: the universe is 11 dimensional supergravity brane on 10 dimensional supersymmetric bulk. Assertion: gravity results from spin-2 gravitons. Actually, if you read Forrest’s statement, he mentions dark matter ‘stings’ by which he presumably means ‘strings’!

Here are is my reaction to Forrest’s statement:

‘Dark matter is an omni-present aether that you can see in most every collision as “quark jets”: The energy of motion of these particles we call zero point fluctuations.’

The word ‘aether’ is too vague to have any meaning in physics. Naming something you haven’t any significant concrete evidence for is the kind of thing that is done in religion, not science. The ‘zero point fluctuations’ again are too vague to be scientific. The zero point concept seems to me to be false, because the vacuum isn’t chaotic at energies lower than the IR cutoff. Schwinger calculated that to get a chaotic vacuum with pair production of virtual fermions which annihilate back into field quanta, you need an electric field to exceed 1.3 * 10^18 volts/metre. This is only the case very close to electrons. At bigger distances, there are no such fluctuations in the vacuum. If there were, the fermions would be polarized around charges until the charges were cancelled out completely. This doesn’t happen; field strengths only diminish geometrically (not by dielectric polarization of the vacuum) at large distances. Hence, we observe a minimum charge for the electron at large distances.

‘The smallest of these dark matter stings are maybe millions of time smaller than the hypothetical Higgs boson or even an electron. They are not a boson or fermion since they have no intrinsic spin and are neutral entities.’

This is classic stringy conjecture; totally uncheckable, non falsifiable, pseudo-scientific religion. Just throw in some random equations, sign the paper off as Witten or Bogdanov, and it could be hyped by Lubos Motl on arXiv.

‘An electron neutrino after millions of years of traveling through matter and the aether, slows down to maybe 1/4 the speed of light. At this slower momentum their spin greatly increases and accordingly so does their interaction with matter.’

You need to provide evidence in science or it looks stringy!

‘These are the pushing gravitons which are the most influential agents of pushing gravity. All of the smaller particles in this field, which influxes into all matter and is radiated away as EM radiation, create a low-pressure area in the field surrounding all matter which influxes field material into the surrounding vortex of spinning matter, in a never ending cycle. This explains the basic mechanisms of gravity where the density of dark matter controls field motion surrounding matter as well as controlling the ultimate speed of light.’

You’re not as far off reality here as most readers would assume. Field quanta (the gauge bosons which are exchanged between gravitational, colour, electromagnetic and isospin charges to create force fields) certainly do exist. In 1983 the weak field quanta were discovered at CERN. !n accelerating charge – e.g one that is spinning or is orbiting a proton – should be radiating. This was the key problem in Bohr’s atomic model, according to Rutherford’s letter to Bohr. However, if such radiation is the exchange radiation (gauge bosons), then you get rid of the entire objection to the Bohr atom, because you then have to replace the classical Coulomb field potential by a random exchange of field quanta between charges. This causes chaos on small scales, because the exchange of field quanta between the electric charges is not continuously and smooth by is random and induces chaotic motion in the small electron. At least, that’s what Feynman claimed from the path integral formulation of QFT when he wrote that the chaotic motion of an electron inside an atom is due to individual quantum interactions (Feynman, QED, Penguin, London, 1990, page 84-5).

‘Individual dark matter particles will seemingly be beyond the capabilities of particle physics to see for maybe millennia, if ever. But the longest strings of this dark matter can be seen as quark jets. Of course there are no quarks or gluons. They’re just a good mathematical construct for the time being until they figure out that looped strings are the constituents and building blocks of matter. No extra dimensions are needed, only XYZ & T.’

This again is just assertions with no evidence, i.e. not so different from religion, the writings of nostradamus, or string popularization.

5. Nick - April 24, 2008

Looks like it might be a good book! I’m almost done with my math here, only have diff-eq left, but I’m assuming i’ll learn even more math in my higher level physics class. Calc definitely shouldn’t be a problem though.

You’re right, a book on this type of physics would have pretty high level math though. Maybe I’m looking for more of an explanatory guide at this point.

Thanks!
Nick

6. IceBogan - April 24, 2008

Tomasso,
You said unification was “pleasing to the eye but admittedly not called for by any intrinsic requirement”. Without unification, the simple relationship between the charges of quarks and electrons, and thus the exact electrical neutrality of atoms, would be just a wild coincidence. That seems pretty compelling to me!

7. forrest noble - April 24, 2008

Hi Tomasso, Thanks for not ignoring my post. I will address your comments each time on a point by point bases and not burn up the blog with extended “proofs”. For this please e-mail me. Unrecoverable? Nice word, I’m sure there have been others over the last 50 years with similar sentiments (or worse).

You said “putting together concepts you do not explain.” It is seemingly easy however to understand the concept of dark matter. I will present these assertions in the form of a couple of Predictions from my book. Dark Matter and the aether defined: Particles or strings not visible to us because it emits no radiation that we can observe, but it is detectable gravitationally.
.
The “Aether” has already been found: A major portion of dark matter, black holes, and the entire ZPF comprises the whole of an omni-present Aether field. This aether field is now called both dark matter which includes the ZPF — along with other entities. The ZPF is only a part of the total dark matter. This ZPF has been associated, by experiment, to be the source of potential energy that can interact with matter, form virtual particles, and is intimately related to the determination of a particle’s mass therefore gravity. There are vast quantities of evidence presented in both astronomy and physics to support the existence of this field. It is an energy source which can manifest its influence in the smallest measurable way (less than 1 : mV). Its physical constituents have both potential and kinetic energy and are believed to be primarily neutral in charge, without intrinsic spin. This is not just Ipan theory, but the experimental conclusions of many of today’s physicists.

Dark matter consists of many entities. The greatest quantity related to gravitational influences would be the fields themselves and the field particles within them, which today is called dark matter and its energy of motion, Zero Point Fluctuations/ Field (ZPF). The next would be highly compressed dark matter which we call black holes. The largest constituent would accordingly be fermions: protons, electrons, and electron neutrinos. The third largest constituent would probably be neutral hydrogen atoms, then molecular hydrogen. Next would be nuclei such as helium and larger nuclei, and lastly stellar and planetary remnants of nearly countless sizes and varieties of atomic and molecular matter formed in huge thinly dispersed clouds. Additionally there are vast quantities of photons and the energy of their orbiting EM radiation which encircles both galaxies and galaxy clusters, which also has extensive gravitational influences.

As far as “old” electron neutrinos being a likely candidate as a WIMP another prediction in in order.

The neutrino model of a Pushing “Graviton”

The earth is about 8 1/3 light minutes away from the sun and is 4.22 light years away from the next closest star Proxima Centauri. According to the Theoretical Physics Section, the speed of all particles and matter moving against the motion of the surrounding field (dark matter) would accordingly slow down as time progresses. A current example would be Voyagers one and Two. The rate of this deceleration would be dependent upon the density and motion of the engulfing Aether field (dark matter) which is determined by the solar vortex, the age and size of the galaxy. According to the Pan Theory, as the speed of these electron neutrinos slows down, the rate of their spin would greatly increases which initiates a newly forming vortex surrounding the neutrino, of field material created by virtue of the vastly increasing particle spin. Accordingly the older a neutrino, the greater its gravitational mass would become before it would eventually be absorbed by the surrounding field after transferring its momentum to matter by contact. Its energy equivalent however would remain the same because of its decreased energy of linear motion.

The primary factor to their influence concerning gravity is because of their vastly expanded vortex.

They would collectively interact with matter continuously which would accordingly be evidenced by the major force of gravity which they create, as apposed to their faster solar cousins (solar electron neutrinos) that rarely interact with matter and have practically no field vortex because of their high speed and very slow spin. These slower, older neutrinos energy of linear motion and vortex would be absorbed transferring a vector force against matter that it interacts with in the direction of their linear motion.

Of course the discovery of these slower moving neutrinos would not necessarily mean that they were stellar neutrinos that have slowed down. But consider the first factor proposed which was the inflowing vortex field. What factor would be presented to explain why these pushing field-vectors would represent an even force surrounding earth? Another type of pushing “graviton” however would still confirm much of the premises and predictions of Pushing Particle-Gravity and related field theory.

Nick concerning your comments,

As you can see I have defined aether for my book and this blog above. You can see its meaning, at least according to my definition. If you don’t like the word aether just use the words dark matter or field material. The only other significant facet of this aether is the same that Michelson and Morley tested for. There problem was all measurements of the speed of light were made perpendicular to the flow of the aether which accordingly would be directly into the earth where it in turn would be radiated away as EM radiation. There accordingly would be a difference in the speed of light up vs. down. Down would be about 32 feet per second faster than the perpendicular speed. Up would be about 32 feet per second slower. The difference in the speed of light up vs. down would be roughly 64 feet per second maximum.

“You need to provide evidence in science or it looks stringy!” Of course that’s true. Theories are a dime a dozen. My book has more than 70 predictions and a book full of “evidence”. It is 250 pages long with related mathematics but nothing too complicated. If you
google the ZPF I think you will see there is over 30 years of evidence to support its existence. Few particle physicists today dispute this. The question is whether it is made up of particles, energy, or both. I would assert that it is dark matter particles that have kinetic energy.

I would assert that there is no faith or metaphysics involved with my comments or theory. Each assertion is backed by specific evidence, mathematics and each also fits with all the other pieces of the puzzle. The title of the book that I’m currently leaning toward is “The Pan Theory, a General Unified Model of: cosmology, the forces of physics, classical mechanics and quantum theory.

your friend forrest

8. dorigo - April 25, 2008

Hi all,

I am leaving to a weekend on the Alps, and will answer these interesting comments when I am back. Sorry for delaying the discussion here… Sometimes my family takes the precedence.

Cheers,
T.

9. Nick - April 25, 2008

Of course, have a good time!

Nick

10. dorigo - April 27, 2008

Hello IceBogan,

I mean that there is no fundamental reason why the unification of all three couplings should occur at the same energy. We could first see two of them unify, and then match the third higher up. Or am I missing something fundamental ?

Thank you
T.

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