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Richard Ruiz | Univ. of Pittsburgh | U.S.A.

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Excited Quarks and Early Discoveries at LHC Run II

Wednesday, February 4th, 2015

The LHC turns back on this year for Run II. What might we see day 1?

The highest-p_T jet event collected by the end of September 2012 (Event 37979867, Run 208781): the two central high-p_T jets have an invariant mass of 4.47 TeV, and the highest-p_T jet has a p_T of 2.34 TeV, and the subleading jet has a p_T of 2.10 TeV. The missing E_T and Sum E_T for this event are respectively 115 GeV and 4.97 TeV. Only tracks with p_T> 0.7 GeV are displayed. The event was collected on August 17th, 2012. Image and caption credit: ATLAS

The highest-p_T jet event collected by the end of September 2012 (Event 37979867, Run 208781): the two central high-p_T jets have an invariant mass of 4.47 TeV; the highest-p_T jet has a p_T of 2.34 TeV, and the subleading jet has a p_T of 2.10 TeV. The missing E_T and Sum E_T for this event are respectively 115 GeV and 4.97 TeV. Only tracks with p_T> 0.7 GeV are displayed. The event was collected on August 17th, 2012. Image and caption credit: ATLAS

In seven weeks CERN’s Large Hadron Collider (LHC), the largest and most energetic particle accelerator in history, is scheduled to turn back on. The LHC has been shutdown since December 2012 in order for experimentalists to repair and upgrade the different detector experiments as well as the collider itself. When recommissioning starts, the proton beams will be over 60% more energetic than before and probe a regime of physics we have yet to explore directly. With this in mind, today’s post is about a type of new physics that, if it exists, we can potentially see in the first days of LHC Run II: excited quarks.

Excited Quarks and Composite Quarks

Excited quarks are interesting little beasts and are analogous to excited atoms in atomic physics. When light (a photon) is shined onto an atom, electrons orbiting the nucleus will become energized and are pushed into higher, metastable orbits. This is called an excited atom.


After some estimable and often measurable period of time, an electron will radiate light (photon) and drop down to its original orbit. When this happens, we say that an excited atom has relaxed to its ground state.


In analogy, if quarks were bound states of something smaller, i.e., if they were composite particles, then we can pump energy into a quark, excite it, and then watch the excited quark relax back into its ground state.

Feynman diagram representing heavy excited quark (q*) production from quark (q)-gluon (g) scattering in proton collisions.

Feynman diagram representing heavy excited quark (q*) production from quark (q)-gluon (g) scattering in proton collisions.

Observing an excited quark would tell us that the quark model may not be the whole story after all. Presently, the quark model is the best description of protons and neutrons, and it certainly works very, very well, but this does not have to be the case. Nature may have something special in store for us. However, this is not why I think excited quarks are so odd and interesting. What is not obvious is that excited quarks, if they exist, could show up immediately after turning the LHC back on.

Early Dijet Discoveries at LHC Run II

Excited quarks participate in the strong nuclear force (QCD) just like ordinary quarks, which means they can absorb and radiate gluons with equal strength. This is key because protons at the LHC are just brimming with highly energetic quarks and gluons. Of particles in a proton carrying a small-to-medium fraction of the proton’s total energy, gluons are the most commonly found particle in a proton (red g curve below). Of those particles carrying a large fraction of the proton’s energy, the up and down quarks are the most common particles (blue u and green d curves below). Excited quarks, if they exist, are readily produced because their ingredients are the most commonly found particles in the proton.


Distributions of partons in a proton. The x-axis represents the fraction of the proton’s energy a parton has (x=1 means that the parton has 100% of the proton’s energy). The y-axis represents the likelihood of observing a parton. The left (right) plot corresponds to low (high) energy collisions. Credit: MSTW

When an excited quark decays, it will split back into quark and gluon pair. These two particles will be very energetic (each will have energy equal to half the mass of the excited quark due to energy conservation), will be back-to-back (by linear momentum conservation), and will each form jets (hadronization in QCD). Such collisions are called “dijet” events (pronounced: die-jet) and look like this

Words. Credit: CMS

Display for the event with the highest dijet mass (5.15 TeV) observed in CMS data. Image and caption credit: CMS

Although gluons and quarks in the Standard Model can mimic the signal, one can add up the energies of the two jets (which would equal the excited quark’s mass due to energy conservation) and expect to see a bump in the data centered about the mass of the excited quark. Unfortunately, the data (below) do not show such a bump, indicating that excited quarks with masses below a couple TeV do not exist.


Inclusive dijet mass spectrum from wide jets (points) compared to a fit (solid curve) and to predictions including detector simulation of multijet events and signal resonances. The predicted multijet shape (QCD MC) has been scaled to the data. The vertical error bars are statistical only and the horizontal error bars are the bin widths. For comparison, the signal distributions for a W resonance of mass 1.9 CMS.TeV and an excited quark of mass 3.6 CMS.TeV are shown. The bin-by-bin fit residuals scaled to the statistical uncertainty of the data are shown at the bottom and compared with the expected signal contributions. Image and caption credit: CMS

However, this does not mean that excited quarks do not or cannot exist at higher masses. If they do, and if their masses are within the energy reach of the LHC, then excited quarks are very much something we might see in just a few months from now.

Happy Colliding,

Richard Ruiz (@BraveLittleMuon)

Appreciation to Ms. Frost and her awesome physics classes at Whitney M. Young High School in Chicago, Illinois for motivating this post. Good luck on your AP exams!


Lepton Number Violation, Doubly Charged Higgs Bosons, and Vector Boson Fusion at the LHC

Wednesday, January 21st, 2015

Doubly charged Higgs bosons and lepton number violation are wickedly cool.

Hi Folks,

The Standard Model (SM) of particle physics is presently the best description of matter and its interactions at small distances and high energies. It is constructed based on observed conservation laws of nature. However, not all conservation laws found in the SM are intentional, for example lepton number conservation. New physics models, such as those that introduce singly and doubly charged Higgs bosons, are flexible enough to reproduce previously observed data but can either conserve or violate these accidental conservation laws. Therefore, some of the best ways of testing if these types of laws are much more fundamental may be with the help of new physics.

Observed Conservation Laws of Nature and the Standard Model

Conservation laws, like the conservation of energy or the conservation of linear momentum, have the most remarkable impact on life and the universe. Conservation of energy, for example, tells us that cars need fuel to operate and perpetual motion machines can never exist. A football sailing across a pitch does not suddenly jerk to the left at 90º because conversation of linear momentum, unless acted upon by a player (a force). This is Newton’s First Law of Motion. In particle physics, conservation laws are not taken lightly; they dictate how particles are allowed to behave and forbid some processes from occurring. To see this in action, lets consider a top quark (t) decaying into a W boson and a bottom quark (b).



A top quark cannot radiate a W+ boson and remain a top quark because of conservation of electric charge. Top quarks have an electric charge of +2/3 e, whereas W+ bosons have an electric charge of +1e, and we know quite well that

(+2/3)e ≠ (+1)e + (+2/3)e.

For reference a proton has an electric charge of +1e and an electron has an electric charge of -1e. However, a top quark can radiate a W+ boson and become a bottom quark, which has electric charge of -1/3e. Since

(+2/3)e = (+1)e + (-1/3)e,

we see that electric charge is conserved.

Conservation of energy, angular momentum, electric charged, etc., are so well-established that the SM is constructed to automatically obey these laws. If we pick any mathematical term in the SM that describes how two or more particles interact (for example how the top quark, bottom quark, and W boson interact with each other) and then add up the electric charge of all the participating particles, we will find that the total electric charge is zero:

The top quark-bottom quark-W boson vertices in the Standard Model, and the net charge carried by each interaction.

The top quark-bottom quark-W boson interaction terms in the Standard Model. Bars above quarks indicate that the quark is an antiparticle and has opposite charges.


Accidental Conservation Laws

However, not all conservation laws that appear in the SM are intentional. Conservation of lepton number is an example of this. A lepton is any SM fermion that does not interact with the strong nuclear force. There are six leptons in total: the electron, muon, tau, electron-neutrino, muon-neutrino, and tau-neutrino. We assign lepton number

L=1 to all leptons (electron, muon, tau, and all three neutrinos),

L=-1 to all antileptons (positron, antimuon, antitau, and all three antineutrinos),

L=0 to all other particles.

With these quantum number assignments, we see that lepton number is a conserved in the SM. To clarify this important point: we get lepton number conservation for free due to our very rigid requirements when constructing the SM, namely the correct conservation laws (e.g., electric and color charge) and particle content. Since lepton number conservation was not intentional, we say that lepton number is accidentally conserved. Just as we counted the electric charge for the top-bottom-W interaction, we can count the net lepton number for the electron-neutrino-W interaction in the SM and see that lepton number really is zero:


The W boson-neutrino-electron interaction terms in the Standard Model. Bars above leptons indicate that the lepton is an antiparticle and has opposite charges.

However, lepton number conservation is not required to explain data. At no point in constructing the SM did we require that it be conserved. Because of this, many physicists question whether lepton number is actually conserved. It may be, but we do not know. This is indeed one topic that is actively researched. An interesting example of a scenario in which lepton number conservation could be tested is the class of theories with singly and doubly charged Higgs boson. That is right, there are theories containing additional Higgs bosons that an electric charged equal or double the electric charge of the proton.


Models with scalar SU(2) triplets contain additional neutral Higgs bosons as well as singly and doubly charged Higgs bosons.

Doubly charged Higgs bosons have an electric charge that is twice as large as a proton (2e), which leads to rather peculiar properties. As discussed above, every interaction between two or more particles must respect the SM conservation laws, such as conservation of electric charge. Because of this, a doubly charged Higgs (+2e) cannot decay into a top quark (+2/3 e) and an antibottom quark (+1/3 e),

(+2)e ≠ (+2/3)e + (+1/3)e.

However, a doubly charged Higgs (+2e) can decay into two W bosons (+1e) or two antileptons (+1e) with the same electric charge,

(+2)e = (+1)e + (+1)e.

but that is it. A doubly charged Higgs boson cannot decay into any other pair of SM particles because it would violate electric charge conservation. For these two types of interactions, we can also check whether or not lepton number is conserved:

For the decay into same-sign W boson pairs, the total lepton number is 0L + 0L + 0L = 0L. In this case, lepton number is conserved!

For the decay into same-sign leptons pairs, the total lepton number is 0L + (-1)L + (-1)L = -2L. In this case, lepton number is violated!


Doubly charged Higgs boson interactions for same-sign W boson pairs and same-sign electron pairs. Bars indicate antiparticles. C’s indicate charge flipping.

Therefore if we observe a doubly charged Higgs decaying into a pair of same-sign leptons, then we have evidence that lepton number is violated. If we only observe doubly charged Higgs decaying into same-sign W bosons, then one may speculate that lepton number is conserved in the SM.

Doubly Charged Higgs Factories

Doubly charged Higgs bosons do not interact with quarks (otherwise it would violate electric charge conservation), so we have to rely on vector boson fusion (VBF) to produce them. VBF is when two bosons from on-coming quarks are radiated and then scatter off each other, as seen in the diagram below.

Figure 2: Diagram depicting the process known as WW Scattering, where two quarks from two protons each radiate a W boson that then elastically interact with one another.

Diagram depicting the process known as WW Scattering, where two quarks from two protons each radiate a W boson that then elastically interact with one another.

If two down quarks, one from each oncoming proton, radiate a W- boson (-1e) and become up quarks, the two W- bosons can fuse into a negatively, doubly charged Higgs (-2e). If lepton number is violated, the Higgs boson can decay into a pair of same-sign electrons (2x -1e). Counting lepton number at the beginning of the process (L = 0 – 0 = 0) and at the end (L = 0 – 2 = -2!), we see that it changes by two units!

Same-sign W- pairs fusing into a doubly charged Higgs boson that decays into same-sign electrons.

Same-sign W- pairs fusing into a doubly charged Higgs boson that decays into same-sign electrons.

If lepton number is not violated, we will never see this decay and only see decays to two very, very energetic W- boson (-1e). Searching for vector boson fusion as well as lepton number violation are important components of the overarching Large Hadron Collider (LHC) research program at CERN. Unfortunately, there is no evidence for the existence of doubly charged scalars. On the other hand, we do have evidence for vector boson scattering (VBS) of the same-sign W bosons! Additional plots can be found on ATLAS’ website.  Reaching this tremendous milestone is a triumph for the LHC experiments. Vector boson fusion is a very, very, very, very, very rare process in the Standard Model and difficult to separate from other SM processes. Finding evidence for it is a first step in using the VBF process as a probe of new physics.

Words. Credit: Junjie Zhu (Michigan)

Same-sign W boson scattering candidate event at the LHC ATLAS experiment. Slide credit: Junjie Zhu (Michigan)

We have observed that some quantities, like momentum and electric charge, are conserved in nature. Conservation laws are few and far between, but are powerful. The modern framework of particle physics has these laws built into them, but has also been found to accidentally conserve other quantities, like lepton number. However, as lepton number is not required to reproduce data, it may be the case that these accidental laws are not, in fact, conserved. Theories that introduce charged Higgs bosons can reproduce data but also predict new interactions, such as doubly charged Higgs bosons decaying to same-sign W boson pairs and, if lepton number is violated, to same-sign charged lepton pairs. These new, exotic particles can be produced through vector boson fusion of two same-sign W boson pairs. VBF is a rare process in the SM and can greatly increase if new particles exist. At last, there is evidence for vector boson scattering of same-sign W bosons, and may be the next step to discovering new particles and new laws of nature!

Happy Colliding

- Richard (@BraveLittleMuon)


The Standard Model of Particle Physics

Monday, December 1st, 2014

The Standard Model: What is it, how does it look, and how does it taste?


The Standard Model of Particle Physics in the cupcake representation. Credit: E. Bland

Hi All,

Today’s post is an introduction to the Standard Model of Particle Physics.  First off, saying “Standard Model of Particle Physics” is long-winded, so it is often shortened to “Standard Model” or abbreviated by “SM”. In short, the SM is presently the best description of how matter behaves, interacts, and works at very small distances and very high energies. High and small, of course, compared to our everyday experiences.

Elementary: In the SM is a collection of particles believed to be the elementary building blocks/constituents of all known matter and energy. By “elementary”, I mean that there are no smaller pieces inside these objects. To put this into perspective, humans (~2 meters) are made of cells (~10 μm); cells are made of molecules (~1 nm), or chains of atoms. Atoms (~100 pm = 1 angstrom) are made of electrons (?) that orbit a central nucleus (1~10 fm). A nucleus  is comprised of protons (1 fm) and neutrons (1 fm). Both of these are made of quarks (?) and gluons (?). However, as far as experiments have shown, SM particles are not made of smaller objects. Therefore, we call call them “elementary”, “fundamental”, and “point-like.” If there comes a day where we discover that quarks have sub-structure, then quarks will lose their “elementary” status.


Scale of the Universe. Credit: U Oregon.

Spin: Elementary particles are separated into three categories: matter, force carriers, and Higgs bosons (or fermions, gauge bosons, and the Higgs bosons). Fermions and gauge bosons have small but nonzero, intrinsic angular momentum, called spin. Angular momentum is a measure of how energetically and how quickly an object is rotating. Think of a bike wheel that never stops spinning and has only two speed: fast and half-fast. A standard unit of angular momentum at small distances is an ћ (pronounced: “h-bar”). This is like a mile or kilometer being a standard unit of distance, or a day being that for time. All gauge bosons carry the same amount of spin, 1ћ; all elementary fermions carry half as much spin, ћ/2. Particles that carry no spin are called scalars. More broadly, a boson is any particle with an integer amount of spin, i.e., 1 ћ, 2 ћ, 3 ћ, …, and a fermion is any particle with half-integer spin, i.e.,  ћ/2, 3 ћ/2, 5 ћ/2, … Spin is an example of a spacetime quantum number. Even sets of fermions make a composite boson; odd sets of fermions make composite fermions, like the proton.

Charge: There are 12 elementary fermions: the up (u), down (d), charm (c), strange (s), top (t), and bottom (b) quarks; the electron (e), muon (μ), and tau (τ) charged leptons; and the electron-neutrino (νe), muon-neutrino (νμ), and tau-neutrino (ντ) leptons. These labels/names represent another quantum number called flavor. In addition to spin, these particles also carry several different charges that cause them to be repelled or attracted when in proximity to each other, in other words to experience a force. There is the electric (or electromagnetic) charge, weak hyper charge, weak isospin charge, and strong (or color) charge. Quarks carry all charges; charged leptons carry all charges except for color; and neutrinos carry only weak charges. In fact, electric charge is a combination of hyper and isospin charges. Charges are examples of internal quantum numbers. In addition, each particle has a partner particle called an antiparticle. Particles and antiparticles have the same spacetime quantum numbers but opposite internal quantum numbers. For example: an electron is a spin-1/2 fermion with -1 electric charge; a positron (an antielectron) is a spin-1/2 fermion with +1 electric charge.

Forces: Gauge bosons are the mediators of the electromagnetic, weak, and color forces, and each force is associated with a conservation law. Fermions interact, exchange momentum, and scatter off each other by exchanging gauge bosons. For example, an electron and positron can interact by exchanging a photon. Throughout this whole process, the electric charges of the electron and positron were individually conserved.



The photon (γ) is the gauge boson for electromagnetism, and the rules of electromagnetism at small distances and high energies are called quantum electrodynamics, or QED.The gluon (g) is the gauge boson for the strong force, and its rules are called quantum chromodynamics (QCD). The strong force is responsible for holding the proton together: protons and neutrons are made up quarks that are bound together by gluons. Weak forces are responsible for certain types of radioactive decay and flavor-changing interactions. For example: an electron can radiate a W boson and become an electron-neutrino, and a top quark dominantly decays into a bottom quark and a W boson. The gauge bosons of weak isospin are the W1, W2, and W3 bosons; for weak hypercharge, this is the B boson. However, at low energies, weak charges are no longer conserved. What is conserved is the sum of isospin and hypercharge. The Ws, B, and three Higgs bosons (more on this in a bit) then combine, becoming the W+, W-, and Z bosons, collectively call the weak bosons. These are very massive particles, about 80 and 90 times more massive than the proton.

Higgs Bosons: In the SM, there are four Higgs bosons: H (sometimes call the Higgs boson), φ1, φ2, and φ3. All four Higgs are scalars (zero spin) and carry both weak isospin and weak hypercharge; two carry nonzero electric charge. A summary of all particles and how they can interact are described in this image:


Any two particles connected by a line can interact. Some bosons can interact with bosons of their own type.

Mass and Electroweak Symmetry Breaking: In the early universe, all elementary fermions and gauge bosons were massless. At some point, everything underwent a phase transition that broke the hypercharge and isospin conservation laws. During this phase transition, quarks and charged leptons acquired mass. The massless hypercharge and isospin gauge bosons along with φ1, φ2, and φ3 mixed and became the massive W+, W-, and Z bosons. Because of this, the W+, W-, and Z bosons can mediate weak interactions but, under the right conditions, behave like the scalars φ1, φ2, and φ3. This phenomenon is called electroweak symmetry breaking (EWSB). After EWSB, there is one remaining physical Higgs boson, H, which was only just discovered in 2012.

To summarize:

  1. The Standard Model of Particle physics is presently our best description of how matter behaves and interacts at very small distances and very high energies.
  2. Elementary particles are not made of any smaller particle and are divided into two categories: fermions (half-integer spin) and bosons (integer spin)
  3. Fermions make up matter (like protons and atoms), and come in 12 different varieties, or flavors.
  4. Gauge bosons mediate forces: the photon mediates electromagnetism, the gluon mediates the strong force, and the W+, W-, and Z bosons mediate the weak forces.
  5. The Higgs bosons are scalars (zero spin) and facilitate electroweak symmetry breaking. After EWSB, only one Higgs boson, H, remains.
  6. Only the photon and gluon are massless; everything else has a mass.
  7. Not everything in the SM agrees with data, but we have yet to find a better theory.


Happy Colliding

- Richard (@BraveLittleMuon)



Wednesday, August 6th, 2014

The particle with two names: The J/ψ Vector Meson. Again, under 500 words.


Trident decay of J/Psi Credit: SLAC/NOVA

Hi All,

The J/ψ (or J/psi) is a very special particle. Its discovery was announced in 1974 independently by two groups: one lead by Samuel Ting at Brookhaven National Laboratory (BNL) in New York and the second lead by Burton Richter at Standford Linear Accelerator Center (SLAC) in California. J/ψ is special because it established the quark model as a credible description of nature. Having been invented by Gell-Man and Zweig as a bookkeeping tool, it was not until Glashow, Iliopoulos and Maiani (GIM) that the concept of quarks as real particles was taken seriously. GIM predicted that if quarks were real, then they should come in pairs, like the  up and down quarks. Candidates for the up, down, and strange were identified, but there was no partner for the strange quark. J/ψ was the key.


Samuel Ting and his BNL team. Credit: BNL

Like the proton or an atom, the J/ψ is a composite particle. This means that J/ψ is made of smaller, more elementary particles. Specifically, it is a bound state of  one charm quark and one anticharm quark. Since it is made of quarks, it is a “hadron“. But since it is made of exactly one quark and one antiquark, it is specifically a “meson.” Experimentally, we have learned that the  J/ψ has an intrinsic angular momentum (spin) of 1ħ (same as the photon), and call it a “vector meson.” We infer that the charm and anticharm, which are both spin ½ħ, are aligned in the same direction (½ħ + ½ħ = 1ħ). The J/ψ must also be electrically neutral because charm and anticharm quarks have equal but opposite electric charges.


Burton Richter following the announcement of co-winning the 1976 Nobel Prize. Credit: SLAC

At 3.1 GeV/c², the J/ψ is a about three times heavier than the proton and about three-quarters the mass of the bottom quark. However, because so few hadrons are lighter than it, the J/ψ possesses a remarkable feature: it decays 10% of the time to charged leptons, like an electron-positron pair. By conservation of energy, it is forbidden to decay to heavier hadrons. Because there are so few  J/ψ decay modes, it is appears as a very narrow peak in experiments. In fact, the particle’s mass and width are so well-known that experiments like ATLAS and CMS use them as calibration markers.

Credit: CMS

Drell-Yan spectrum data at 7 TeV LHC Credit: CMS

The J/ψ meson is one of the coolest things in the particle zoo. It is a hadronic bound state that decays into charged leptons. It shares the same quantum numbers as the photon and Z boson, so it appears as a Drell-Yan processes. It established the quark model, and is critical to new discoveries because of its use as a calibration tool. In my opinion, not too shabby.

Happy colliding.

Richard (@BraveLittleMuon)


What are Sterile Neutrinos?

Sunday, July 27th, 2014

Sterile Neutrinos in Under 500 Words

Hi Folks,

In the Standard Model, we have three groups of particles: (i) force carriers, like photons and gluons; (ii) matter particles, like electrons, neutrinos and quarks; and (iii) the Higgs. Each force carrier is associated with a force. For example: photons are associated with electromagnetism, the W and Z bosons are associated with the weak nuclear force, and gluons are associated with the strong nuclear force. In principle, all particles (matter, force carries, the Higgs) can carry a charge associated with some force. If this is ever the case, then the charged particle can absorb or radiate a force carrier.

SM Credit: Wiki

Credit: Wikipedia

As a concrete example, consider electrons and top quarks. Electrons carry an electric charge of “-1″ and a top quark carries an electric charge of “+2/3″. Both the electron and top quark can absorb/radiate photons, but since the top quark’s electric charge is smaller than the electron’s electric charge, it will not absorb/emit a photon as often as an electron. In a similar vein, the electron carries no “color charge”, the charge associated with the strong nuclear force, whereas the top quark does carry color and interacts via the strong nuclear force. Thus, electrons have no idea gluons even exist but top quarks can readily emit/absorb them.

Neutrinos  possess a weak nuclear charge and hypercharge, but no electric or color charge. This means that neutrinos can absorb/emit W and Z bosons and nothing else.  Neutrinos are invisible to photons (particle of light) as well as gluons (particles of the color force).  This is why it is so difficult to observe neutrinos: the only way to detect a neutrino is through the weak nuclear interactions. These are much feebler than electromagnetism or the strong nuclear force.

Sterile neutrinos are like regular neutrinos: they are massive (spin-1/2) matter particles that do not possess electric or color charge. The difference, however, is that sterile neutrinos do not carry weak nuclear or hypercharge either. In fact, they do not carry any charge, for any force. This is why they are called “sterile”; they are free from the influences of  Standard Model forces.

Credit: somerandompearsonsblog.blogspot.com

Credit: somerandompearsonsblog.blogspot.com

The properties of sterile neutrinos are simply astonishing. For example: Since they have no charge of any kind, they can in principle be their own antiparticles (the infamous “sterile Majorana neutrino“). As they are not associated with either the strong nuclear scale or electroweak symmetry breaking scale, sterile neutrinos can, in principle, have an arbitrarily large/small mass. In fact, very heavy sterile neutrinos might even be dark matter, though this is probably not the case. However, since sterile neutrinos do have mass, and at low energies they act just like regular Standard Model neutrinos, then they can participate in neutrino flavor oscillations. It is through this subtle effect that we hope to find sterile neutrinos if they do exist.

Credit: Kamioka Observatory/ICRR/University of Tokyo

Credit: Kamioka Observatory/ICRR/University of Tokyo

Until next time!

Happy Colliding,

Richard (@bravelittlemuon)



Photon Colliders: Making Matter from Light Since 1994

Tuesday, June 17th, 2014

Protons are the brave casualties in the search for new physics, but sometimes everybody lives.

Hi Folks,

The CERN Large Hadron Collider (LHC), history’s largest and most energetic proton collider, is currently being tuned up for another round of new discoveries. A Higgs boson, incredibly rare B meson decays, and evidence for vector boson fusion have already been identified, so there is great anticipation on what we may find during Run II.

Our discoveries, however, come at the cost of protons. During 2012, the ATLAS, CMS, and LHCb experiments collected a combined 48 fb-1 (48 “inverse femtobarns“), and another 12 fb-1 in 2011. To translate, an “inverse femtobarn” is a measure of proton collisions and  is the equivalent of 70 trillion proton-proton collisions. Hence, 60 fb-1 is equivalent to about 4,200 trillion proton-proton collisions, or 8,400 trillion protons. We hope to generate almost twice as much data per year when everything starts back up winter/spring 2015. You know, suddenly, @LHCproton‘s many fears of its day job make sense:

With so many protons spent in the name of science, one can reasonably ask

Is it possible to find new physics at the LHC without destroying a proton?

The answer is


Sometimes, just sometimes, if we are very lucky, two protons can pass each other, interact and make new particles, but remain intact and unbroken. It sounds mind-boggling, but it has been one of the best tests of quantum electrodynamics (QED), the theory of light and matter at small distances and large energies.

From Maxwell to Photon Beams at the LHC

The idea is simple, the consequences are huge, and goes like this: Protons, like electrons, muons and W bosons, are electrically charged, so they can absorb and emit light. Protons, like electrons, do not just radiate light at random. Light is emitted following very specific rules and travel/propagate in very specific directions, dictated by Maxwell’s equations of electrodynamics. However, the rules of quantum mechanics state that at large enough energies and small enough distances, in other words an environment like we have at the LHC, particles of light (called photons) will interact with each other, with a predicted probability. Yes, you read that correctly, quantum mechanics states that light interacts with itself at small enough distances. The more protons we accelerate in the LHC, the more photons are radiated from protons that remain intact, and the more likely two photons will interact with each other, producing matter we can observe with detectors! An example of such a process that has already by observed is the pair production of muons from photons:

Muon pair production from photon scattering via elastic photon emission from protons. Credit: CMS, JHEP 1201 (2012) 052

To understand this more, lets take a look at Maxwell’s Equations, named after Scottish physicist J. C. Maxwell but really represent seminal contributions of several people. Do not worry about the calculus, we will not be working out any equations here, only discussing their physical interpretation. Without further ado, here are the four laws of electrodynamics:


The very first law, Gauss’ law, tells us that since the proton has an electric charge, it also it the source of an electric field (E). The bottom equation, Ampere’s Law, tells us that if we have a moving electric charge (a proton circling the LHC ring for example), then both the moving electric field (E) and the electric current (J) will generate a magnetic field (B). In the LHC, however, we do not just have a moving beam of protons, but an accelerating beam of protons. This means that the magnetic field is changing with time as the proton circles around the collider. The third equation, Faraday’s law, tells us that when a magnetic field (B) changes with time, an electric field (E) is generated. But since we already have an electric field, the two fields add together into something that also changes with time, and we end up back at Ampere’s law (the bottom equation). This is when something special happens. Whenever a charged particle is accelerated, the electric and magnetic fields that are generated feed into each other and create a sort of perpetual feedback. We call it electromagnetic radiation, or light. Accelerating charged particles emit light.


Schematic representation of the strength of the electric (blue) and magnetic (red) fields as light propagates through space. Credit: Wikipedia

Maxwell’s equations  in fact tell us a bit more. They also tell us the direction in which light is emitted. The crosses and dots tell us whether things are perpendicular (at right angles) or parallel to each other. Specifically, they tell us that the generated magnetic field is always at right angles to both the electric field and direction the proton is travelling, and that light travels perpendicular to both the electric and magnetic field.  Since protons are travelling in a circle at the LHC, their tangential velocity, which always points forward, and their radial acceleration, which always points toward the center of the LHC ring, are always at right angles to each other. This crucial bit fixes the direction of the emitted light. As the protons travel in a circle, the generated electric field points in the direction of acceleration (the center); the generated magnetic is perpendicular to both of these, so it points upward if the proton is travelling in a counter-clockwise direction, or downward if the proton is travelling in a clockwise direction. The light must then always travel parallel to the proton! Along side the LHC proton beam is a hyper focused light beam! Technically speaking, this is called synchrotron radiation.


(a) Relative orientation of an electrically charged particle travelling in a circle and its electromagnetic field likes. (b) Synchrotron radiation emitted tangent to a circular path traversed by an electrically charged particle. Credit: Wikipedia

The last but still important step is to remember that all of this is happening at distances the size of a proton and smaller. In other words, at distances where quantum mechanics is important. At these small distances, it is appropriate to talk about individual pieces (quanta) of light, called photons. That beam of synchrotron radiation travelling parallel to the proton beam can appropriately be identified as a beam of photons.  In summary, along side the LHC proton beam is the LHC photon beam! This photon beam is radiated from the protons in the proton beam, but the protons remain intact and do not rupture as long as the momentum transfer to the photon beam is not too large. A very important note I want to make is that the photon beams do not travel in a circle; they travel in straight lines and are constantly leaving the proton beam. Synchrotron radiation continuously drains the LHC beams of energy, which is why the LHC beam must be continuously fed with more energy.

Making Matter from Light Since 1994

Synchrotron radiation has been around for quite sometime. Despite recent claims, the first evidence for direct production of matter from photon beams came in 1994  from the DELPHI experiment at the Large Electron Positron (LEP) Collider, the LHC’s predecessor at CERN. There are earlier reports of photon-photon scattering at colliders but I have been unable to track down the appropriate citations. Since 1994, evidence for photon-photon scattering has been observed by the Fermilab’s CDF experiment at the Tevatron, as reported by the CERN Courier, and there is even evidence for the pair production of muons and W bosons at the CMS Experiment. Excitingly, there has also been so research to a potential Higgs factory using a dedicated photon collider. This image shows a few photon-photon scattering processes that result in final-state bottom quark and anti-bottom quark pairs.

Various photon-photon scattering processes that result in final-state bottom quark and anti-bottom quark pairs. Credit: Phys.Rev. D79 (2009) 033002


We can expect to see much more from the LHC on this matter because photon beams offer a good handle on understanding the stability of proton beam themselves and are a potential avenue for new physics.

Until next time, happy colliding.

- Richard (@BraveLittleMuon)


And So Summer (Conferences) Begin

Friday, May 2nd, 2014

The summer conference season starts Monday and QD will be there at every turn.

Hi Folks,

On Monday, the University of Pittsburgh in the US has honor of kicking off the 2014 summer conference season with the Phenomenology 2014 Symposium, and on its tail will be the Americas Workshop on Linear Collider at Fermilab also in the US, and beyond that are more results from around the world. Notably, the International Conference on High Energy Physics (ICHEP 2014) will be held in Valencia, Spain, and Supersymmetry 2014: The International Conference on Supersymmetry and Unification of Fundamental Interactions will be at the University of Manchester in England. Both ICHEP and SUSY are biannual conferences, and the last time ICHEP was held the Higgs discovery was announced. Whatever great result is announced this summer, QD will be there…. and possibly even giving the talk. :) So check out the list below and start planning your summer getaway. (Note: For a great list of conferences/schools/and workshops that is regularly maintained, see Heather Logan ‘s (Carleton) conference page).

Happy Colliding

- richard (@BraveLittleMuon)

Phenomenology 2014 Symposium (#Pheno14)

Dates: 5-7 May. Host: University of Pittsburgh. Homepage


Americas Workshop on Linear Colliders 2014 (#AWLC14)

Dates: 12-16 May. Host: Fermilab.  Homepage

International Conference From the Planck Scale to the Electroweak Scale (#Planck14)

Dates: 26-30 May. Host: Institut des Cordeliers. Homepage

Muon Accelerator Program 2014 Spring Workshop (#Muon14)

Dates: 27-31 May. Host: Fermilab. Homepage

Large Hadron Collider Physics (LHCP) Conference (#LHCP14)

Dates:  2-7 June. Host: Brookhaven National Laboratory and Columbia University. Location: Columbia University. Homepage.

International Conference on Neutrino Physics and Astrophysics (#Neutrino14)

Dates:  2-7 June. Host: Boston University, Harvard University, Massachusetts Institute of Technology, and Tufts University. Location: Boston University. Homepage

LoopFest XIII (#Loop13)

Dates: 18-20 June. Host: New York City College of Technology. Homepage


7th Future Circular Collider / Tri- Large Electron Positron Physics Workshop (#TLEP)

Dates: 19-21 June. Host: CERN. Homepage

International Conference on High Energy Physics 2014 (#ICHEP14)

Dates: 2-9 July. Location: Valencia, Spain. Homepage

Coordinated Theoretical-Experimental Project on QCD Summer School (#CTEQ)

Dates: 8 – 18 July. Host: Peking University (PKU), Beijing. Homepage

Pre-SUSY 14 Summer School (#SUSY14)

Dates: 15-19 July. Host: University of Manchester. Homepage

Supersymmetry 2014 (#SUSY14)

Dates: 21-26 July. Host: University of Manchester. Homepage

SLAC Summer Institute (#SSI14)

Dates: 4-15 August. Host: SLAC. Homepage


Top Quarks… So Many Top Quarks

Wednesday, April 30th, 2014

Thousands of paper on top quarks exist. Why?

There are literally thousands of papers, collaboration notes, and conference notes with the words “Top” and “Quark” in the title. As of this post, there are 3,477 since 1979 listed on inSpires. There are many, many more that omit the word “quark”. And sure, this is meager compared to the 5,114 papers with the words “Higgs Boson” written since ’74, but that is over 50,000 pages of top quarks (estimating 15 pages/paper). To be fair, there are also many, many more that omit the word “boson”. But for further comparison, there are only 395 papers with a title including the words “Bottom Quark“, 211 with “Bottomonium“, and 125 with “Bottom Hadron“. So why are there so many papers written about the top quark? The answer is that the top quark is weird special.


A single top quark candidate event at the Collider Detector experiment at Fermilab. Credit: CDF Collaboration

The top quark is very heavy, about 185 times heavier than the proton and ranks as the heaviest known elementary particle in all the particle kingdom. The second heaviest quark, the bottom quark, is only 4 or 5 times heavier than the proton. If you or I were a proton, then a medium-to-large school bus (without any people) would be a top quark. In fact, the top quark is so heavy it can decay into a real (on-shell) W boson, which is roughly half its mass. The only other particle that can do this is the Higgs. Though it is rare, exceedingly rare, the top quark can decay into real Z  and Higgs bosons as well. Not even the Higgs can top that last feat.

Top quark decaying into real, on-shell W boson and bottom quark. Credit: DZero Collaboration

However, the top quark is still a quark. It has an electric charge that is 2/3 as large as the proton. It has an intrinsic angular momentum (spin) equal to the proton’s or electron’s spin. The top quark is also colored, meaning that is interacts with gluons and is influenced by the strong nuclear force (QCD). When colored objects (quarks and gluons) are produced at collider and fixed target experiments, they undergo a process called hadronization. Hadronization is when two colored objects are far away from one another and the strong nuclear attraction between the two becomes so strong that a pair of colored objects will spontaneously be produced in the space between them. These new colored particles will then form bound states with the old colored states. However, the process hadronization means that we only observe the bound states of colored objects and not the colored objects themselves. Physicists have to infer their properties from the physics of bound states…. or do we?


Colored objects before (L), during (Center L and Center R), and after (R) hadronization.

The onset of hadronization is typically occurs about 10-24 seconds after the creation of a colored object. Yes, that is 0.000000000000000000000001 seconds. That is incredibly fast and well beyond anything that can be done at an experiment. The mean lifetime of the top quark on the other hand is about 10-25 seconds. In other words, the top quark is much more likely to decay in to a W boson, its principle decay mode, than hadronize. By looking at the decays of the W boson, for example to an electron and an electron-neutrino, their angular distributions, and other kinematic properties, we can measure directly the top quark’s quantum numbers. The top quark is special because it is the only quark whose spin and charge quantum numbers we can measure directly.


Top quark decaying into real, on-shell W boson and bottom quark. The W boson can subsequently decay into a charged lepton and a neutrino or into a quark and anti-quark. Credit: DZero Collaboration

The top quark tells us much about the Standard Model of particle physics, but it also may be a window to new physics. Presently, no one has any idea why the top quark is so much heavier than the bottom quark, or why both are orders of magnitude heavier than the electron and muon. This is called the “Mass Hierarchy Problem” of the Standard Model and stems from the fact that the quark and lepton masses in the theory are not predicted but are taken as input parameters. This does not mean that the Standard Model is “wrong”. On the contrary, the model works very, very well; it is simply incomplete. Of course there are new models and hypotheses that offer explanations, but none have been verified by data.

However, thanks to the 2012 discovery of the Higgs boson, there is a new avenue that may shed light upon the mass hierarchy problem. We now know that quarks and leptons interact with the Higgs boson proportionally to their masses. Since the top quark is ~40 times more massive than the bottom quark, it will interact with Higgs boson 40 times more strongly. There is suspicion that since the Higgs boson is sensitive to the different quark and lepton masses, it may somehow play a role in how masses are assigned.

Happy Colliding

- richard (@BraveLittleMuon)


Getting to the Bottom of the Higgs

Thursday, January 30th, 2014

Updated Friday, January 31, 2014: Candidate event of Higgs boson decaying to bottom quarks has been added at the bottom.

CMS has announced direct evidence of the Higgs coupling to bottom quarks. This is special.

Last week, the Compact Muon Solenoid Experiment, one of the two general purpose experiments at the CERN Large Hadron Collider (LHC), submitted two papers to the arXiv. The first claims the first evidence for the Higgs boson decaying directly to tau lepton pairs and the second summarizes the evidence for the Higgs boson decaying directly to bottom quarks and tau leptons. (As an aside: The summary paper is targeted for Nature Physics, so it is shorter and more broadly accessible than other ATLAS and CMS publications.) These results are special, and why they are important is the topic of today’s post. For more information about the evidence was obtained, CERN posted a nice QD post last month.

Event display of a candidate Higgs boson decaying into a tau lepton and anti-tau lepton in the CMS detector.

Fig 1. Event display of a candidate Higgs boson decaying into a tau lepton and anti-tau lepton in the ATLAS detector.

There is a litany of results from ATLAS and CMS regarding the measured properties of the Higgs boson. However, these previous observations rely on the Higgs decaying to photons, Z bosons, or W bosons, as well as the Higgs being produced from annihilating gluons or being radiated off a W or Z. Though the top quark does contribute to the Higgs-photon and Higgs-gluon interactions, none of these previous measurements directly probe how fermions (i.e., quarks and leptons) interact with the Higgs boson. Until now, suggestions that the Higgs boson couples to fermions (i) proportionally to their masses and (ii) that the couplings possess no other scaling factor were untested hypotheses. In fact, this second hypothesis remains untested.


Fig. 2: Event display of a candidate Higgs boson decaying into a tau lepton and anti-tau lepton in the CMS detector.

As it stands, CMS claims “strong evidence for the direct coupling of the 125 GeV Higgs boson” to bottom quarks and tau leptons. ATLAS has comparable evidence but only for tau leptons. The CMS experiment’s statistical significance of the signal versus the “no Higgs-to-fermion couplings” hypothesis is 3.8 standard deviations, so no rigorous discovery yet (5 standard deviations is required). For ATLAS, it is 4.1 standard deviations. The collaborations still need to collect more data to satisfactorily validate such an incredible claim. However, this should not detract from that fact that we are witnessing phenomena never before seen in nature. This is new physics as far as I am concerned, and both ATLAS and CMS should be congratulated on discovering it.

Event display of a candidate Higgs boson decaying into a tau lepton and anti-tau lepton in the CMS detector.

Fig. 3: Event display of a candidate Higgs boson decaying into a bottom quark and anti-bottom quark in the ATLAS detector. HT to Jon Butterworth for the link.

The Next Step

Once enough data has been collected to firmly and undoubtedly demonstrate that quarks and leptons directly interact with the Higgs, the real tests of the Standard Model of particle physics start up. In the Standard Model, the strength at which a fermion interacts with the Higgs is proportional to the fermion mass and inversely proportional to the ground state energy of the Higgs field. There is no other factor involved. This is definitively not the case for a plethora of new physics models, including scenarios with multiple Higgs bosons, like supersymmetry, as well as scenarios with new, heavy fermions (heavy bottom quark and tau lepton partners). This is definitely a case of using newly discovered physics to find more new physics.

Happy Colliding.

- Richard (@bravelittlemuon)

PS I was unable to find an event display of a Higgs boson candidate decaying into a pair of bottom quarks. If anyone knows where I can find one, I would be very grateful.

PSS Much gratitude toward Jon Butterworth for providing a link to Higgs-bbar candidate events.


Questions I Wish I Asked When Applying for Graduate School

Monday, October 21st, 2013

Grad School in the sciences is a life-changing endeavour, so do not be afraid to ask questions.

Hi Folks,

Quantum Diaries is not just a place to learn the latest news in particle physics; it is also a resource. It is a forum for sharing ideas and experiences.

In science, it is almost always necessary to have a PhD, but what is a PhD? It is a certification that the holder has demonstrated unambiguously her or his ability to thoroughly carry out an independent investigation addressing a well-defined question. Unsurprisingly, the journey to earning a PhD is never light work, but nor should it be. Scientists undertake painstaking work to learn about nature, its underpinnings, and all the wonderful phenomena that occur in everyday life. This journey, however, is also filled with unexpected consequences, disappointment, and sometimes even heartbreak.

It is also that time of year again when people start compiling their CVs, resumes, research statements, and personal statements, that time of year when people begin applying for graduate programs. For this post, I have asked a number of good friends and colleagues, from current graduate students to current post docs, what questions they wished they had asked when apply for graduate school, selecting a school, and selecting a research group.

However, if you are interested in applying to for PhD programs, you should always first yourself,  “Why do I want a research degree like a PhD?”

If you have an experience, question, or thought that you would like to share, comment below! A longer list only provides more information for applicants.

As Always, Happy Colliding

- Richard (@bravelittlemuon)

PS I would like to thank Adam, Amy, John, Josh, Lauren, Mike, Riti, and Sam for their contributions.

Applying to Graduate School:

“When scouting for grad schools, I investigated the top 40 schools in my program of interest.  For chemistry, research primarily occurs in one or two research labs, so for each school, I investigated the faculty list and group research pages.  I eliminated any school where there werre fewer than two faculty members whose fields I could see myself pursuing.  This narrowed down my list to about a dozen schools.  I then filtered based on location: I enjoy being near a big city, so I removed any school in a non-ideal location.  This let me with half a dozen schools, to which I applied.” - Adam Weingarten, Chemistry, Northwestern

“If there is faculty member you are interested in working for, ask both the professor and especially the students separately about the average length of time it takes students to graduate, and how long financial support might be available.” – Lauren Jarocha, Chemistry, UNC

“My university has a pretty small physics program that, presently, only specializes in a few areas. A great deal of the research from my lab happens in conjunction with other local institutes (such as NIST and NIH) or with members of the chemistry or biology departments. If you are interested in a smaller department, ask professors about Institutes and interdisciplinary studies that they might have some connection to, be it within academia or industry.” – Marguerite Brown, Physics, Georgetown

“If you can afford the application fees and the time, apply as broadly as you can.  It’s good to have options when it comes time to make final decisions about where to go. That said, don’t aim too high (you want to make sure you have realistic schools on your list, whatever “realistic” means given your grades and experience), and don’t aim too low (don’t waste time and money applying to a school that you wouldn’t go to even if it was the only school that accepted you, whether because of academics, location, or anything else).  Be as honest as possible with yourself on that front and get input from trusted older students and professors.  On the flip side, if you don’t get rejected from at least one or two schools, you didn’t aim high enough.  You want a blend of reach schools and realistic schools.” – Amy Lowitz, Physics, Wisconsin

Choosing a School

“One of the most common mistakes I see prospective graduate students make is choosing their institution based on wanting to work with a specific professor without getting a clear enough idea of the funding situation in that lab.  Don’t just ask the professor about funding.  Also ask their graduate students when the professor isn’t present.  Even then, you may have to read between the lines; funding can be a delicate subject, especially when it is lacking.” – Amy Lowitz, Physics, Wisconsin

“If you have a particular subfield/group you *know* you are interested in, check how many profs/postdocs/grads are in these groups, check if there are likely to be open slots, and if there are only 1 or 2 open slots make sure you know how to secure one. If they tell you there are currently no open slots, take this to mean that this group is probably closed for everything but the most exceptional circumstances, and do not take into account that group when making your decision.” – Samuel Ducatman, Physics, Wisconsin

“When choosing a school, I based my decision on how happy the grad students seemed, how energetic/curious the faculty appeared, and if the location would allow me to have extracurricular pursuits (such as writing, improv, playing games with people, going to the movies…basically a location where I could live in for 4-6 years).” – Adam Weingarten, Chemistry, Northwestern

“At the visitor weekend, pay attention to how happy the [current] grads seem. Remember they are likely to be primarily 1st years, who generally are the most happy, but still check. Pay attention to the other students visiting, some of them will be in your incoming class. Make sure there is a good social vibe.” – Samuel Ducatman, Physics, Wisconsin

“When I was visiting a prospective grad student, there was a professor at a university I was visiting whose research I was really interested in, but the university would only allow tuition support for 5 years. When I asked his students about graduation rates and times, however, the answer I got was, ‘Anyone who graduates in 5 years hasn’t actually learned anything, it takes at least 7 or 8 years before people should really graduate anyway. Seven years is average for our group.’ In some fields, there is a stigma associated with longer graduation times and a financial burden that you may have to plan for in advance.” – Lauren Jarocha, Chemistry, UNC

Choosing a Group

“When considering a sub-field, look for what interests you of course, but bear in mind that many people change their focus, many don’t know exactly what they want to do immediately upon entering grad school, and your picture of the different areas of research may change over time. Ask around among your contemporaries and older students, especially when it comes to particular advisers.” – Joshua Sayre, PhD, Physics, Pittsburgh
“If you know that you’re interested in an academic career that is more teaching oriented or research oriented, ask about teaching or grant writing opportunities, respectively. I know plenty of fellow students who didn’t start asking about teaching opportunities their 4th or 5th year of their program, and often by then it was too late. If you know that finding funding will be a big part of your future, joining a group where the students take an active part in writing grants and grant renewals is invaluable experience.” –  Lauren Jarocha, Chemistry, UNC
“For choosing groups, I attended group and subgroup meetings, met with faculty to discuss research and ideas, and read several recent publications from each group of interest.  What I did not do (and wish I had) was talk with the graduate students, see how they and the group operated.  For example, I am very motivated and curious to try new ideas, so in my current research group my PI plays a minimal role in my life.  The most important aspect is how well one’s working style fits with the group mentality, followed by research interest.  There’s a ton of cool, exciting research going on, but finding a group with fun, happy, motivated people will make or break the PhD experience.” – Adam Weingarten, Chemistry, Northwestern
“I went into [Condensed Matter Theory] and not [X] because (1) In the summer of my first year I had no research, and I came close to having no income because of this. I realized I needed someone who could promise me research/funding and real advising. The [X] group was pretty filled up (and there were some politics), so it was impossible to get more than this. (2) I thought the professors in CMT treated me with more respect then the [X] profs I talked to.” - John Doe, Physics
“I believe that choosing which grad schools to apply to should primarily be about the research, so this question is more for after you’ve (hopefully) been accepted to a couple schools.  If you are going into theoretical physics, and if you don’t have some sort of fellowship from them or an outside agency, ask them how much their theory students [teach].  Do they have to TA every semester for their funding?  Do they at least get summers off?  Or do they only have to TA for the first one or two years?  This shouldn’t be the primary factor in deciding where to go – research always is – but it’s not something that should be ignored completely.  Teaching is usually somewhat rewarding in my experience, but it adds absolutely no benefit to your career if you are focused on a professorship at a research university.  Every hour you spend steaching is an hour someone else is researching and you aren’t.  And 10-20 hours a week of teaching adds up.” – Michael Saelim, Physics, Cornell