# Tag Archives: LHC

## WIMPZillas: The Biggest WIMPs

In the search for direct detection of dark matter, the experimental focus has been on WIMPS – weakly interacting massive particles. Large crystal detectors are placed deep underground to avoid contamination from cosmic rays and other stray particles.

WIMPs are often hypothesized to arise as supersymmetric partners of Standard Model particles. However, there are also WIMP candidates that arise due to non-supersymmetric extensions to the Standard Model.

The idea is that the least massive supersymmetric particle would be stable, and neutral. The (hypothetical) neutralino is the most often cited candidate.

The search technique is essentially to look for direct recoil of dark matter particles onto ordinary atomic nuclei.

The only problem is that we keep not seeing WIMPs. Not in the dark matter searches, not at the Large Hadron Collider whose main achievement has been the detection of the Higgs boson at mass 125 GeV. The mass of the Higgs is somewhat on the heavy side, and constrains the likelihood of supersymmetry being a correct Standard Model extension.

The figure below shows WIMP interaction with ordinary nuclear matter cross-section limits from a range of experiments spanning from 1 to 1000 GeV masses for WIMP candidates. Typical supersymmetric (SUSY) models are disfavored by these results at higher masses above 40 GeV or so as the observational limits are well down into the yellow shaded regions.

Perhaps the problem is that the WIMPs are much heavier than where the experiments have been searching. Most of the direction detection experiments are sensitive to candidate masses in the range from around 1 GeV to 1000 GeV (1 GeV or giga-electronVolt is about 6% greater than the rest mass energy of a proton). The 10 to 100 GeV range has been the most thoroughly searched region and multiple experiments place very strong constraints on interaction cross-sections with normal matter.

WIMPzillas is the moniker given to the most massive WIMPs, with masses from a billion GeV up to  potentially as large as the GUT (grand Unified Theory) scale of $10^{16} GeV$.

The more general term is Superheavy Dark Matter, and this is proposed as a possibility for unexplained ultra high energy cosmic rays (UHECR). The WIMPzillas may decay to highly energetic gamma rays, or other particles, and these would be detected as the UHECR.

UHECR have energies greater than a billion GeV ($10^9 GeV$) and the most massive event ever seen (the so-called Oh My God Particle) was detected at $3 \cdot 10^{11} GeV$. It had energy equivalent to a baseball with velocity of 94 kilometers per hour. Or 40 million times the energy of particles in the Large Hadron Collider.

It has taken decades of searching at multiple cosmic ray arrays to detect particles at or near that energy.

Most UHECR appear to be spatially correlated with external galaxy sources, in particular with nearby Active Galactic Nuclei that are powered by supermassive black holes accelerating material near, but outside of, their event horizons.

However, they are not expected to be able to produce cosmic rays with energies above around $10^{11} GeV$, thus the WIMPzilla possibility. Again WIMPzillas could span the range from $10^9 GeV$ up to $10^{16} GeV$.

In a paper published last year, Kolb and Long calculated the production of WIMPzillas from Higgs boson pairs in the early universe. These Higgs pairs would have very high kinetic energies, much beyond their rest mass.

This production would occur during the “Reheating” period after inflation, as the inflaton (scalar energy field) dumped its energy into particles and radiation of the plasma.

There is another production mechanism, a gravitational mechanism, as the universe transitions from the accelerated expansion phase during cosmological inflation into the matter dominated (and then radiation-dominated) phases.

Thermal production from the Higgs portal, according to their results, is the dominant source of WIMPzillas for masses above $10^{14} GeV$. It may also be the dominant source for masses less than about $10^{11} GeV$.

They based their assumptions on chaotic inflation with quadratic inflation potential, followed by a typical model for reheating, but do not expect that their conclusions would be changed strongly with different inflation models.

It will take decades to discriminate between Big Bang-produced WIMPzilla style cosmic rays and those from extragalactic sources, since many more $10^{11} GeV$ and above UHECRs should be detected to build statistics on these rare events.

But it is possible that WIMPzillas have already been seen.

The density is tiny. The current dark matter density in the Solar neighborhood is measured at 0.4 Gev per cc. Thus in a cubic meter there would be the equivalent of 400,000 proton masses.

But if the WIMPzillas are at energies $10^{11} Gev$ and above (100 billion GeV), a cubic kilometer would only contain 4000 particles at a given time. Not easy to catch.

References

http://cdms.berkeley.edu/publications.html – SuperCDMS experiment led by UC Berkeley

http://pdg.lbl.gov/2017/reviews/rpp2017-rev-dark-matter.pdf – Dark matter review chapter from Lawrence Berkeley Lab (Figure above is from this review article).

http://home.physics.ucla.edu/~arisaka/home3/Particle/Cosmic_Rays/ – Ultra high energy cosmic rays

https://arxiv.org/pdf/1708.04293.pdf – E. Kolb and A. Long, 2017 “Superheavy Dark Matter through Higgs Portal Operators”

## 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.

## Supersymmetry in Trouble?

There’s a major particle physics symposium going on this week in Kyoto, Japan – Hadron Collider Physics 2012. A paper from the LHCb collaboration, with 619 authors, was presented on the opening day, here is the title and abstract:

# First evidence for the decay Bs -> mu+ mu-

A search for the rare decays Bs->mu+mu- and B0->mu+mu- is performed using data collected in 2011 and 2012 with the LHCb experiment at the Large Hadron Collider. The data samples comprise 1.1 fb^-1 of proton-proton collisions at sqrt{s} = 8 TeV and 1.0 fb^-1 at sqrt{s}=7 TeV. We observe an excess of Bs -> mu+ mu- candidates with respect to the background expectation. The probability that the background could produce such an excess or larger is 5.3 x 10^-4 corresponding to a signal significance of 3.5 standard deviations. A maximum-likelihood fit gives a branching fraction of BR(Bs -> mu+ mu-) = (3.2^{+1.5}_{-1.2}) x 10^-9, where the statistical uncertainty is 95% of the total uncertainty. This result is in agreement with the Standard Model expectation. The observed number of B0 -> mu+ mu- candidates is consistent with the background expectation, giving an upper limit of BR(B0 -> mu+ mu-) < 9.4 x 10^-10 at 95% confidence level.

In other words, the LHCb consortium claim to have observed the quite rare decay channel from B-mesons to muons (each B-meson decaying to two muons), representing about 3 occurrences out of each 1 billion decays of the Bs type of the B-meson. Their detection has marginal statistical significance of 3.5 standard deviations (one would prefer 5 deviations), so needs further confirmation.

What’s a B-meson? It’s a particle that consists of a quark and an anti-quark. Quarks are the underlying constituents of protons and neutrons, but they are composed of 3 quarks each, whereas B-mesons have just two each. The particle is called B-meson because one of the quarks is a bottom quark (there are 6 types of quarks: up, down, top, bottom, charge, strange plus the corresponding anti-particles). A Bs-meson consists of a strange quark and an anti-bottom quark (the antiparticle of the bottom quark). Its mass is between 5 and 6 times that of a proton.

What’s a muon? It’s a heavy electron, basically, around 200 times heavier.

What’s important about this proposed result is that the decay ratio (branching fraction) that they have measured is fully consistent with the Standard Model of particle physics, without adding supersymmetry. Supersymmetry relates known particles with integer multiple spin to as-yet-undetected particles with half-integer spin (and known particles of half-integer spin to as-yet-undetected particles with integer spin). So each of the existing Standard Model particles has a “superpartner”.

Yet the very existence of what appears to be a Higgs Boson at around 125 GeV as announced at the LHC in July of this year is highly suggestive of the existence of supersymmetry of some type. Supersymmetry is one way to get the Higgs to have a “reasonable” mass such as what has been found. And there are many other outstanding issues with the Standard Model that supersymmetric theories could help to resolve.

Now this has implications for the interpretation of dark matter as well. One of the favored explanations for dark matter, if it is composed of some fundamental particle, is that it is one type of supersymmetric particle. Since dark matter persists throughout the history of the universe, nearly 14 billion years, it must be highly stable. Now the least massive particle in supersymmetry theories is stable, i.e. does not decay since there is no lighter supersymmetric particle into which it can decay. And this so called LSP for lightest supersymmetric particle is the favored candidate for dark matter.

So if there is no supersymmetry then there needs to be another explanation for dark matter.