Tag Archives: CERN

Higgs Boson and Dark Matter, Part 1

The discovery of the Higgs boson at the Large Hadron Collider (the LHC, at CERN, near Geneva) was announced on the 4th of July, 2012. This new particle was the important missing component of the Standard Model of particle physics; the Standard Model has had great success in describing the three non-gravitational forces: electromagnetism, the weak nuclear force, and the strong nuclear force.

The mass of the Higgs is about 126 GeV (giga electron-Volts) and by way of comparison the proton mass is a bit under 1 GeV.  The Higgs particle is highly unstable, with a decay lifetime of only about one tenth of one billionth of one trillionth of a second (10^-22 seconds). While the  Higgs field pervades all of space, the particle requires very high energy conditions to “pop out” of the field for a very short while. The only place where these conditions exist on Earth is at the LHC.

The Higgs boson is not detected directly at LHC, but inferred (with high confidence) through the detection of its decay products. The main decay channels are shown in the pie chart below, and include bottom quarks, W vector bosons (which mediate the weak force), gluons (which mediate the strong force holding quarks inside protons and neutrons), tau leptons (very heavy members of the electron family), Z vector bosons (also weak force mediators), and even some photon decay channels. While the two photon channel is rare, much less than 1% of the decays, it is the most important channel used for the Higgs detection because of the “clean” signal provided.


What is the relationship between the Higgs and dark matter? In an earlier blog, http://wp.me/p1mZmr-3K , I discussed why the Higgs particle itself cannot be the explanation for dark matter. Dark matter must be stable; it must persist over the nearly 14 billion year lifetime of the universe. In today’s universe it’s very difficult and expensive to create a Higgs particle and it vanishes immediately.

But in the very early universe, at a tiny fraction of a second after its creation (less than the present-day Higgs boson lifetime!), the “temperature” and energy levels were so high that the Higgs particle (or more than one type of Higgs particle) would have been abundant, and as today it would have decayed to many other, lighter, particles. Could it be the source of dark matter? It’s quite plausible, if dark matter is due to WIMPs – an undiscovered, stable, weakly interacting massive particle. That dark matter is due to some type of WIMP is currently a favored explanation among physicists and cosmologists. WIMPs are expected from extensions to the Standard Model, especially supersymmetry models.

One possible decay channel would be for the Higgs boson to decay to two dark matter WIMPs. In such a decay to two particles (a WIMP of some sort and its anti-particle), each would have to have a rest mass energy equivalent of less than half of the 126 GeV Higgs boson mass; that is, the dark matter particle mass would have to be 63 GeV or less.

There may be more than one type of Higgs boson, and another Higgs family particle could be the main source of decays to dark matter. In supersymmetric extensions to the Standard Model, there is more than one Higgs boson expected to exist. In fact the simplest supersymmetric model has 5 Higgs particles! 

Interestingly, there are 3 experiments which are claiming statistically significant detections of dark matter, these are DAMA/LIBRA, COGENT,  and CDMS-II. And they are all suggesting a dark matter particle mass in the neighborhood of just 10 GeV. Heavy, compared to a proton, but quite acceptable in mass to be decay products from the Higgs in the early universe. It’s not a problem that such a mass might be much less than 63 GeV as the energy in the decay could also be carried off by additional particles, or as kinetic energy (momentum) of the dark matter decay products.

At the LHC the search is underway for dark matter as a result of Higgs boson decays, but none has been found. The limits on the cross-section for production of dark matter from Higgs decay do not conflict with the possible direct detection experiments mentioned above.

The search for dark matter at the LHC will actively continue in 2015, after the collision energy for the accelerator is upgraded from 8 TeV to 14 TeV (trillion electron-Volts). The hope is that the chances of detecting dark matter will increase as well. It’s a very difficult search because dark matter would not interact with the detectors directly. Rather its presence must be inferred from extra energy and momentum not accounted for by known particles seen in the LHC’s detectors.



AMS Positron Excess: Due to Dark Matter or not?

The first results from the Alpha Magnetic Spectrometer (AMS), which is an experiment operating in orbit on the International Space Station (ISS), have been released.

ImageIt’s been two years since the delivery via Space Shuttle to the ISS (in May, 2011) of the AMS-02 instrument, which was especially designed to explore the properties of antimatter. And it’s been a long time coming to get to this point, since the experiment was first proposed in 1995 by the Nobel Prize-winning M.I.T. physicist, professor Samuel Ting. A prototype instrument, the AMS-01, flew in a short-duration Space Shuttle mission in 1998, and had much lower sensitivity.

Over 16 countries across the globe participate in the AMS mission, and the instrument underwent testing at the CERN particle physics research center near Geneva and also in the Netherlands before being launched from Cape Canaveral, Florida.

The lifetime of the mission is expected to extend for over 10 years. In this first data release, with over 30 billion cosmic rays detected, the AMS has detected among these over 400,000 positrons, the positively charged antiparticles to electrons. This is the most antimatter that has ever been directly measured in space.

It is hoped that the AMS can shed light on dark matter, since one of the possible signatures of dark matter is the production of positrons and electrons when dark matter particles annihilate. This assumes that some type of WIMP is the explanation for dark matter, WIMP meaning a “weakly interacting massive particle”. Weakly interacting signifies that dark matter particles (in this scenario highly favored by many physicists) would interact through the weak nuclear force, but not the electromagnetic force. Which basically explains why we can’t easily detect them except through their gravitational effects. Massive particle refers to a particle substantially more massive than a proton or neutron, which have rest masses of just under 1 GeV (1 giga volt) in energy terms. A WIMP dark matter particle mass could be 10 to 1000 times or more higher. The lightest member of the neutralino family is the most-favored hypothesized WIMP dark matter candidate.

The figure below shows the positron relative abundance versus energy, based on 18 months of AMS operational data. The energy of detected positrons ranges from 1/2 GeV to over 300 GeV. The abundance, shown on the y-axis, is the fraction of positrons relative to total electrons and positrons detected at a given energy. The spectrum shows a clear trend to relatively fewer positrons as the energy grows to 10 GeV and then a substantially increasing relative number of positrons at higher energies. This general shape for the spectrum was seen with previous experiments including Fermi, Caprice94 and Pamela, but is much clearer with the AMS due to the higher resolution and significantly greater number of positrons detected. It is particularly this increase in positrons seen above 10 GeV that is suggestive of sources other than the general cosmic ray background.

PositronspectrumFigure: Positron relative fraction (y-axis) versus energy (x-axis)

So what is the source of the energetic positrons detected by the AMS? Some or all of these could be produced when two dark matter WIMP particles meet one another. In the WIMP scenario the dark matter particle such as the neutralino is neutrally charged (no electromagnetic interaction, remember) and also its own antiparticle. And when a particle meets its antiparticle what happens is that they mutually annihilate. The energy of the pair of colliding dark matter particles is transformed into lighter particles, including electron-positron pairs and energetic photons including gamma rays.

Another likely source is pulsars, which are rotating neutron stars with magnetic fields. Since neutron stars are compact and rotate quickly, and their magnetic field strengths are high, electrons and positrons can be accelerated to very high energy. In particular, the Geminga pulsar is the closest energetic pulsar and has been suggested as a major source of these extra positrons.

More data is needed, especially at higher energies above 100 GeV. Over the next few years as AMS continues to operate and the number of positrons detected climbs to 1 million and above, this spectral shape will be better determined. And as the shape of the high-energy portion of the spectrum becomes clearer, it will help elucidate whether dark matter or pulsars or something else are the primary source of the positrons.

You can follow AMS-02 on Facebook here.







Why the Higgs Boson is not Dark Matter

The Higgs boson is considered a necessary part of the Standard Model of particle physics. In the Standard Model there are 3 main forces of nature: the electromagnetic force, the weak nuclear force, and the strong nuclear force. The Standard Model does not address gravity and we do not yet have a proven theory for the unification of gravity with the other 3 forces.

On July 4th CERN, the European particle physics lab near Geneva, announced that two experiments using the Large Hadron Collider accelerator, ATLAS and CMS, have both amassed strong statistical evidence (around 5 sigma) for a new particle. This new particle has a mass of about 126 GeV* and “smells” very much like it is the long sought after, and elusive, Higgs boson. The prediction of the Higgs dates from 1964. For comparison, the proton mass is about 0.94 GeV, so the Higgs is around 134 times more massive. Further work is necessary to determine all of its properties, but at this point it looks as if the new particle decays into other particles in the expected manner. It is these decay products that are actually detected.

This decades-long search has proceeded in fits and starts, principally at CERN in Europe and Fermilab in the U.S., with different accelerators and detectors. Over time the experiments were able to exclude possible masses for the Higgs, since the rate of creation of different decay products varies for different putative masses. By the end of 2011 it looked like there was a preliminary signal, not yet of sufficient statistical strength, but that the mass would have to be in the range of about 115 to 130 GeV.


The CMS detector at the Large Hadron Collider. Credit: Mark Thiessen/National Geographic Society/Corbis

One of my professors, Steven Weinberg, won the Nobel Prize in Physics years ago for his work on unifying the electromagnetic force and the weak force. While the Standard Model and the body of work in particle physics provides a theoretical underpinning for all of the particles which we observe, and their quantum properties, and describes a unification of the strong force (which holds together the quarks inside a proton or neutron) with these other two forces, it also requires an additional mechanism to explain why most particles have non-zero masses.

The Higgs mechanism is the favored explanation, and it predicts a particle as the mediator to provide masses to other particles. The Higgs mechanism is theorized as an all-pervasive Higgs field, which slows down particles as they move through it. As you swim through water you feel a drag that slows you down. A fish with a very hydrodynamic design will feel less drag. In the particle world, more massive ones slow down more than the lighter ones, since they interact more strongly with the Higgs field.

The particle corresponding to this mechanism is known as the Higgs boson. Particles can have quantum spin that is a multiple of ½ or an integer multiple. Bosons have integer multiple spins. Actually the spin of the Higgs boson is zero. All of the force mediator particles such as the photon (spin = 1), which mediates electromagnetism, are bosons.

The Large Hadron Collider is in some sense recreating the conditions of the very early universe by smashing particles together at 7000 GeV, or 7 TeV. The Higgs originally would have been created in Nature in the very early part of the Big Bang, around the first one-trillionth of a second. The appearance of the Higgs broke the unification, or symmetry, between the electromagnetic and weak forces that Steven Weinberg demonstrated are one at very high energies. And the Higgs gave mass to particles.

Without the Higgs mechanism, all particles would be massless, and thus travelling at the speed of light, and structure in the universe – stars, planets, galaxies, human beings, would not be possible. Even the existence of the proton itself requires that quarks have mass, although most of the proton mass comes from the energy of the gluons (strong force mediation particle) and ‘virtual’ quark-antiquark pairs found inside it.

The Higgs boson cannot be the explanation for dark matter for a very simple reason. Dark matter must be stable with a very long lifetime, persisting over the universe’s present age of 14 billion years. It mostly sits in space doing nothing except providing additional gravitational interaction with ordinary matter. The favored candidate for dark matter is the least massive supersymmetric particle; being the least massive, it would have nothing to decay into. Supersymmetry is a theoretical extension beyond the Standard Model. No supersymmetric particles are detected as of yet, but the theory has a lot of support and has the benefit of stabilizing the mass of the Higgs itself.

The Higgs boson, on the other hand, decays very rapidly. There are various decay channels, including into quarks, W/Z bosons, leptons or photons, producing these in pairs (two Zs, two top quarks etc.). Sometimes even four particles are produced from a single Higgs decay. It is these decay products that are actually detected in the Large Hadron Collider at CERN.

There are a few experiments that are claiming to have directly detected dark matter. The favored mass range from the COGENT and DAMA/LIBRA experiments is around 10 GeV for dark matter, much more than a proton, but less than 10% of the Higgs’ mass. Now that the Higgs appears to have been found, work will proceed on confirming and elucidating its properties. And the next great hunt for particle physics may be the direct detection of dark matter particles and the beginning of a determination if supersymmetry is real.

* GeV = Giga-electronVolt or 1 billion electron Volts. 1 TeV (Tera-electronVolt) = 1000 GeV






http://www.youtube.com/watch?v=ktEpSvzPROc – Don Lincoln of Fermilab on how we search for the Higgs at particle accelerators

http://www.youtube.com/watch?v=r4-wVzjnQRI&feature=related – BBC documentary