The Higgs boson – what it is and how you would find it

The Higgs Boson is an elementary particle predicted by the Standard Model of particle physics. It is associated with the Higgs mechanism that was developed with the contributions of physicists Brout, Englert, Guralnik, Hagen, Kibble and Higgs in 1964. This mechanism proposed an elegant solution to one of the standing problems in particle physics – how particles acquire mass. The elementary particles of nature have masses varying by many orders of magnitude. For instance, the top quark, the heaviest known particle yet, has a mass that is roughly 340000 times that of the electron. Neutrinos on the other hand, have masses that are constrained by experiment to only a small fraction of the electron mass. The Higgs mechanism suggests that all particles acquire their mass by the strength of their couplings to a field that permeates the whole universe – the Higgs field. The top quark does not get its higher mass because it is bigger in size; in fact it is probably no larger than an electron. It simply couples more strongly to the Higgs field to get its enormous mass at around 173 GeV. Photons do not couple to this field at all and hence are massless. The much sought after Higgs boson is the quanta of this hypothetical field, very similar to the photon being the quanta of the electromagnetic field. One of the main goals of the LHC physics program is to find evidence of this quanta, the Higgs boson.

Higgs bosons, if they exist, can be created in proton – proton collisions at the LHC but would decay instantaneously. Any search for them has to be carried out by analyzing data for signatures of Higgs decay. This is not a trivial task however, because such decays are impossible to distinguish from some other well known Standard Model processes. Collision data from LHC would have to be analyzed, and deviations from known background processes would point towards some form of new physics and possibly the existence of the Higgs boson. One would then have to compare the rates in different Higgs decay channels to see if the observed rates match those from predictions for a Higgs decay.

In December 2011, ATLAS and CMS, the two main experiments at LHC, announced their 2011 Higgs search results which led to much hype in the physics world and the media – both experiments were indeed seeing some excess in several Higgs decay channels around 125 GeV. The mass of a proton is about 0.94 GeV, so the finding suggests a Higgs boson mass that is about 133 times heavier than the proton (notice the bump in the above plot from ATLAS at around 125 GeV).

However, it is still very early to come to a conclusive statement about a discovery. Standard Model backgrounds are subject to statistical fluctuations which show themselves at various levels in experiments. A discovery would only be claimed if the deviations from expected backgrounds are over 5 standard deviations. The results announced by CMS and ATLAS in December 2011 were 1.9 and 2.6 standard deviations above expected backgrounds respectively. Though this is interesting, it is far from sufficient to provide a conclusive answer about the existence of the Higgs boson. The search will have to continue in 2012 as more data from LHC becomes available, and physicists will finally be able to give a conclusive answer to either confirm or exclude the Standard Model Higgs boson, ending a puzzle that has been around for several decades.

Serdar Gozpinar is a 5th year graduate student in the high-energy physics group at Brandeis working on the ATLAS experiment at CERN.

A new twist on interfacial tension

In a mixture of two molecular components, the surface tension is defined as the energetic cost per unit area of moving molecules from the bulk and bringing them to the interface. The higher the magnitude of the surface tension, the greater the tendency of two components to demix. Surface tension allows trees to carry nutrients from the roots out to the branches, and water striders to walk on the surface of water.

The interface between hydrophobic and hydrophilic components has very high interfacial tension. A common way to adjust the magnitude of surface tension is to add amphiphilic molecules (like soaps), which contain both hydrophilic and hydrophobic components. These amphiphilic molecules prefer to be at the interface between the two components, and effectively lower the interfacial tension, allowing the components to mix more easily. This is how detergent causes oily stains to dissolve in water.

In a recently published article in Nature, an interdisciplinary team of researchers at Brandeis headed by Zvonimir Dogic, and consisting of experimental, theoretical, and computational physicists as well as biologists, has demonstrated a new way of controlling interfacial tension using a molecular property called chirality, or lack of mirror symmetry. The study was performed on a model system of two-dimensional colloidal membranes composed of the rod-like bacteriophage virus fd, which are about one micrometer in length and 7 nanometers in diameter. The electrostatically repulsive virus particles are condensed into membranes through the depletion mechanism by adding non-adsorbing polymer to a virus suspension. Because the fd rods are chiral, they tend to twist by a small angle with respect to neighboring rods. However, the geometry of the membrane prevents twisting in the structure’s interior; only along the perimeter can the rods twist. Thus, increasing the strength of chirality of the rods both lowers the energy of the rods along the membrane’s edge and increases the frustration of untwisted rods in the bulk, lowering the interfacial tension. This contrasts the standard method of controlling interfacial tension using amphiphilic molecules, since the rod-like particles are completely homogenous, and do not contain any hydrophilic components.

The strength of chiral interactions in fd is temperature sensitive; the rods are achiral at 60o C, and the strength of chirality increases with decreasing temperature. By increasing the strength of chiral interactions in-situ, the team of researchers was able to dynamically vary the membrane’s interfacial tension in order to drive a dramatic transition from a membrane to several twisted ribbon structures (Movie 1). The twisted ribbons have much more interfacial area than the membranes, but are much “twistier” structures, and are therefore favored when the strength of chirality is relatively high. Additionally, the team was able to drive the same membrane-to-ribbon transition using optical tweezers, as shown in Movie 2. Membranes and ribbons are only two of a myriad of structures that were observed in the fd system. This work presents a powerful new method to control the assembly of materials by tuning interfacial tension with chirality.

Shear-induced jamming

From breakfast cereals to sand on a beach, granular materials are all around us. Under different conditions, these materials can exhibit liquid-like behavior (flowing) as well as solid-like behavior. The transition between solid and liquid phases has been known as the jamming transition.

The basic concept of jamming is pretty intuitive. A simple example of what can induce jamming is the following: compacting loose sand inside a container increases its density. When the container is removed, the sand can form a self-supporting pile, hence becoming jammed. Jamming has been studied extensively in numerical simulations of systems composed of idealized grains without frictional forces.  These studies find a critical density at which jamming occurs. Since these idealized granular materials are non-cohesive (no attractive forces between them)  they can become solids only through externally imposed pressure, such as through compaction, and therefore a critical density makes sense.  Real granular materials, however, have friction, and how this affects jamming is not well understood.

An experimental image of typical Shear Jammed state in a 2-D frictional granular material. The shear strain is applied in the horizontal direction. Red colored grains form the backbone of the system, which provides rigidity with respect to external shear

Newly published in Nature, are results of a collaboration between Prof. Bulbul Chakraborty’s group at Brandeis and Prof. Behringer’s group at Duke University, which show a new class of jammed states in frictional granular materials. This new class of “Shear-Jammed” states exhibits a richer phenomenology than previously seen. An initially unjammed or loose granular material can become jammed not just by increasing its density, but by applying shear strain on it while holding the density fixed. Shear-Jammed states are inherently anisotropic in their stress and grain-to-grain contact network (see photo above). The transition from an unjammed to shear-jammed state is clearly marked by a percolation of the strong force chains in all directions (see video below). The phenomenon of shear-jamming does not currently have a fundamental theoretical description. Ongoing work in Prof. Chakraborty’s group attempts to construct a theoretical framework for this non-equilibrium phase transition using a generalization of equilibrium statistical ensembles.

This video shows the evolution of the strong force cluster and transition from unjammed to fragile and eventually to SJ. The video shows experimental states created under pure shear. Green colored grains form the strong force cluster defined in the paper. Initially, the system is unjammed. As the fraction of force bearing grains increases with increasing strain, the strong force cluster percolates in the compressive (vertical) direction and we call the state fragile.  Eventually the system becomes percolated in all directions with sufficient number of force bearing grains. We call these states Shear Jammed.

see also:

Higgs Boson Search Webcast

Craig Blocker writes:

The Higgs boson has been sought for many years by high energy physics experiments.  It is the only particle in the standard model of particle physics that has not yet been observed and plays a crucial role since it gives mass to the other fundamental particles.

Tomorrow at 8:00 am EST (Tues., Dec. 13), CERN is giving a seminar (webcast) about the status of the search for the Higgs at the LHC by the two general purpose detector collaborations, ATLAS and CMS.  Although the data is not quite strong enough yet to claim a discovery, the evidence is becoming strong.

The Physics Department will show a web cast of this seminar in Abelson 131. The Brandeis high energy group has been a member of the ATLAS experiment for many years and has been instrumental in building parts of the detector that are key to this measurement.  I will be there to answer any questions.

Although it is early in the morning for most, if you are interested, please come to what should be a very interesting talk.

Update:

Eisenbud Lectures: “The Mathematics of Dynamic Random Networks”

This year’s Eisenbud Lectures in Mathematics and Physics will be given by Dr. Jennifer Chayes, Distinguished Scientist and Managing Director of Microsoft Research New England. Dr. Chayes is well known for her work on the phase transitions in combinatorial and computer science problems; she is a world expert on the study of random, dynamically growing graphs, which can be used to model real-world social and technological networks.

Dr. Chayes received her PhD in mathematical physics from Princeton.  After postdoctoral fellowships at Harvard and Cornell, she was on the faculty at UC Los Angeles before co-founding the theory group at Microsoft Research in Redmond, Washington.  In 2008 she co-founded Microsoft Research New England. She is a fellow of the American Association for the Advancement of Science, the Fields Institute, and the Association for Computing Machinery; she is also a National Associate of the National Academies.

The Eisenbud Lectures are the result of a generous donation by Leonard and Ruth-Jean Eisenbud, intended for a yearly set of lectures by an eminent physicist or mathematician working close to the interface of the two subjects. Dr. Chayes’ distinguished career working on fundamental issues in mathematics, physics, and computer science makes her an ideal speaker for this series.

The lectures will take place at 4 PM on Tuesday Nov. 29 and at 4:30 PM on Thursday Dec. 1. both in Abelson 131.  A full description of the lectures can be found below. Driving directions, maps, links to the MBTA, and so forth can be found at: http://www.brandeis.edu/about/visiting/directions.html.  If you need parking, please contact Catherine Broderick at cbroderi@brandeis.edu.  A reception will be held after the first lecture on Tuesday November 29th from 5pm – 7pm in the Faculty Club Lounge at Brandeis.  All are welcome.

Everybody should come out to hear this year’s lectures!  They promise to be a lot of fun.

THE MATHEMATICS OF DYNAMIC RANDOM NETWORKS
During the past decade, dynamic random networks have become increasingly important in communication and information technology.  Vast, self-engineered networks, like the Internet, the World Wide Web, and online social networks, have facilitated the flow of information, and served as media for social and economic interaction.  I will discuss both the mathematical challenges and opportunities that exist in describing these networks:  How do we model these networks – taking into account both observed features and incentives?  What processes occur on these networks, again motivated by strategic interactions and incentives, and how can we influence or control these processes?  What algorithms can we construct on these networks to make them more valuable to the participants?  In this talk, I will review the general classes of mathematical problems which arise on these networks, and present a few results which take into account mathematical, computer science and economic considerations.  I will also present a general theory of limits of sequences of networks, and discuss what this theory may tell us about dynamically growing networks.

LECTURE 1:  Models and Behavior of the Internet,  the World Wide Web and Online Social Networks
Although the Internet, the World Wide Web and online social networks have many distinct features, all have a self-organized structure, rather than the engineered architecture of previous networks, such as phone or transportation systems.  As a consequence of this self-organization, these networks have a host of properties which differ from those encountered in engineered structures:  a broad “power-law” distribution of connections (so-called “scale-invariance”), short paths between two given points (so-called “small world phenomena” like “six degrees of separation”), strong clustering (leading to so-called “communities and subcultures”), robustness to random errors, but vulnerability to malicious attack, etc.    During this lecture, I will first review some of the distinguishing observed features of these networks, and then discuss some of the models which have been devised to explain these features.  I will also discuss processes and algorithms on these networks, focusing on a few particular examples.

LECTURE 2:  Convergent Sequences of Networks
In the second lecture of this series, I will abstract some of the lessons of the first lecture.  Inspired by dynamically growing networks, I will ask how we can characterize general sequences of graphs in which the number of nodes grows without bound.   In particular, I will define various natural notions of convergence for a sequence of graphs, and show that, in the case of dense graphs and even some sparse graphs, many of these notions are equivalent.  I will also give a construction for a function representing the limit of a sequence of graphs.  I’ll review examples of some simple growing network models, and illustrate the corresponding limit functions.  I will also discuss the relationship between these convergent sequences and some notions from mathematical statistical physics.

Dynamics of double-strand break repair


In a new paper in the journal Genetics, former Brandeis postdoc Eric Coïc and undergrads Taehyun Ryu and Sue Yen Tay from Professor of Biology Jim Haber’s lab, along with grad student Joshua Martin and Professor of Physics Jané Kondev, tackle the problem of understanding the dynamics of homologous recombination after double strand breaks in yeast. According to Haber,

The accurate repair of chromosome breaks is an essential process that prevents cells from undergoing gross chromosomal rearrangements that are the hallmark of most cancer cells.  We know a lot about how such breaks are repaired.  The ends of the break are resected and provide a platform for the assembly of many copies of the key recombination protein, Rad51.  Somehow the Rad51 filament is then able to facilitate a search of the entire DNA of the nucleus to locate identical or nearly identical (homologous) sequences so that the broken end can pair up with this template and initiate local copying of this segment to patch up the chromosome break.  How this search takes place remains poorly understood.

The switching of budding yeast mating type genes has been a valuable model system in which to study the molecular events of broken chromosome repair, in real time.  It is possible to induce synchronously a site-specific double-strand break (DSB) on one chromosome, within the mating-type (MAT) locus.  At opposite ends of the same chromosome are two competing donor sequences with which the broken ends of the MAT sequence can pair up and copy new mating-type sequences into the MAT locus.

Normally one of these donors is used 9 times more often than the other.  We asked if this preference was irrevocable or if the bias could be changed by making the “wrong” donor more attractive – in this case by adding more sequences to that donor so that it shared more and more homology with the broken ends at MAT.  We found that the competition could indeed be changed and that adding more homologous sequences to the poorly-used donor increased its use.


In collaboration with Jané Kondev’s lab we devised both a “toy” model and a more rigorous thermodynamic model to explain these results.  They suggest that the Rad51 filament carrying the broken end of the MAT locus collides on average 4 times before with the preferred donor region before it actually succeeds in carrying out the next steps in the process that lead to repair and MAT switching.

Dynamics of homology searching during gene conversion in Saccharomyces cerevisiae revealed by donor competition Eric Coïc , Joshua Martin, Taehyun Ryu, Sue Yen Tay, Jané Kondev and James E. Haber. Genetics. 2011 Sep 27 2011 Sep 27

Three Leopards and a Shower

Dan Perlman passed along these notes from Briana Abrahms ’08, a Brandeis physics major whose focus has shifted to conservation issues and is in currently working in Botswana. Briana’s blog has more information, you can read it at http://www.conservationconnections.blogspot.com/

My Crash Course in Large Carnivores (Aug. 1, 2011)

Dear family and friends,

Greetings from Botswana! As many of you know, I’ve taken a six-month research position with the Botswana Predator Conservation Trust (BPCT) located outside the Moremi Game Reserve in northwestern Botswana. BPCT is a non-profit organization that works closely with the Botswana government to study and protect Botswana’s five large carnivores: lions, hyenas, African wild dogs, leopards, and cheetahs. (Read more at www.bpctrust.org!) A quick note about African wild dogs because its name can cause some confusion: African wild dogs are a distinct species (Lycaon pictus) just like the Gray wolf or the Spotted hyena, and do not refer to feral dog populations, as the name suggests. Because of habitat loss, disease, and competition with other carnivores, African wild dogs are one of the most endangered predators in Africa, with less than 1% of its former population remaining.

Within each of the five species that BPCT studies, several ‘representatives’ – usually one of the dominant animals in a pack – are radio collared and collect GPS data on their movements. So most of what we do on a day-to-day basis is drive around, see what animals we can pick up with our radio antennae, and then track them. We are the only organization who are permitted to go off road to look for animals, so we do a lot of exciting off-roading into the bush! Once we find the animal(s), we download the GPS data from its collar and make observations about what they are doing (eating, hunting, resting, caring for offspring, etc.) and what other animals it’s with at that time. The purpose of this is to collect data on the basic ecology and behavior of these species, for example: How much space does this species use? How does it share the landscape with other species? What does it eat? How do animals rise to dominance in a pack? The list goes on and on.

I got incredibly lucky on my first day here to witness a successful wild dog hunt that led to a steenbok kill (steenboks are like little antelope). Apparently this is really rare to see – the director of BPCT who’s been working here for over 20 years says he can count on one hand the number of times he’s seen a wild dog kill. And I saw it on my first day! Here’s a crudely edited video that I took with my camera, with footage of the camp I’m staying at, the wild dog hunt, and some of the other things I’ve seen. Be sure to check it out and notice the radio collars on some of the animals. Warning!: the video includes lions copulating and wild dogs killing and eating the steenbok, which can be a bit gruesome. (If the link doesn’t work for you, just search for ‘Botswana Day 1 – Wild Dog Hunt’ on YouTube).

A Tale of Three Leopards and a Shower (Oct. 15, 2011)

Hi all!

Here goes again with another monthly email. Last night I had what was probably my most exciting night at dog camp. Now that the dry season is in full swing here, we’ve been seeing more non-human visitors to our camp in search of water, which is generally found either in a bird bath near our kitchen area or our shower. Yesterday evening I came back from the field and met my coworkers Krys and Neil on their way out to find Chalak, a collared male leopard whose signal they had picked up very close to camp. We’d been very eager to find him because earlier this week he’d been seen mating with not one but TWO uncollared females within minutes of each other, which is very unusual because leopards are solitary and same-sexes generally don’t tolerate each other, at least from what BPCT researchers have seen.

Sure enough, about a half hour later I got a radio message from Krys saying they’d found Chalak and his two lady friends walking towards camp. It was dark by then, and I was alone in camp getting dinner ready in the kitchen. A few minutes later, I hadn’t heard anything more from Krys, but I did hear the loud snarling noise that one only hears when leopards are mating close by. It was obvious that they were somewhere in camp, though I couldn’t see them. The good thing was that I could localize where Chalak and one of the females were from the sounds of their mating, but I had no idea where the other female was.

As I stood in the kitchen near our radio, that question was solved as I saw one of the females emerge out of the bushes and head over to our bird bath, twenty feet away from where I was standing. Our kitchen (which is open, no walls) was the only structure around and there was nowhere safer for me to go, so I radioed Krys to let her know the situation and then I stayed put and kept an eye on the female. She didn’t seem interested in my presence. Then, just a few minutes later, I saw Chalak follow her out of the bushes and lay down by the bird bath. That really got my heart going – Chalak is a huge leopard, almost twice the size of the females, and I was standing twenty feet away from him with nothing in between. Again, though, his promiscuous evening had made him very thirsty and he was only interested in getting some water. After a few more minutes, the other female came, so now camp was occupied by three leopards and myself by my lonesome! I had quite the adrenaline rush. Not long after Krys and Neil finally came and pulled the truck right up to the kitchen. I climbed over a fridge in order to not exit the kitchen near the leopards and hopped into the truck. That was a huge relief. We tried to scare Chalak and the females off with the car to discourage them from using our camp as a drinking hole, but Chalak was so habituated to cars that he wasn’t bothered by it approaching him. Eventually he and the females made their way to the shower, where we heard some (probably hot and steamy) leopard mating roars. Krys and Neil ended up driving me to my tent and then parking the truck next to their tent so nobody had to walk around by themselves. This morning the leopards were out of camp but I saw one of them from my tent in the grassland behind camp, so they are still around. And thus the saga continues!

On another note, I decided to make a little “Day in the Life” video (shot very unprofessionally with my tiny digital camera) to hopefully give you a better sense of what I actually do here, how I spend my time, etc. I made this on October 7th, which turned out to be a pretty good day to choose for this project. Enjoy!

Cheers,
Briana

Complex Fluids Workshop on Sep 23

On Friday, Sep 23 2011, Brandeis will play host to the 48th New England Complex Fluids Meeting, run by the New England Complex Fluids Workgroup, of which the Brandeis Complex Fluids group is a charter participant. These quarterly meetings foster collaboration among researchers from industry and academia in the New England area studying Soft Condensed Matter, offer the opportunity to exchange ideas, and help introduce students and post-docs to the local academic and industrial research community.

The workshop, to be held in the Shapiro Campus Center, will have four talks by invited speakers, each about 30 minutes long with ample time for questions. In addition, everyone who attends is encouraged to give a five minute update (soundbite) of their current work.

Schedule

9:30 AM – Krystyn Van Vliet (Materials Science and Engineering, MIT), Chemomechanics of responsive gels
10:15 AM – Jeremy England (Physics, MIT), Shape Shifting: the statistical physics of protein conformational change

Soundbites: 11:30 – 12:30 PM Five minute updates of current research

1:30 PM – Francis Starr (Physics, Wesleyan), DNA-linked Nanoparticle Assemblies
2:15 PM – Jennifer Ross (Physics, UMass Amherst), Controlling Microtubules Through Severing

More Soundbites: 3:30 PM – 4:30 PM

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