• John
  • Felde
  • University of Maryland
  • USA

Latest Posts

  • USLHC
  • USLHC
  • USA

  • James
  • Doherty
  • Open University
  • United Kingdom

Latest Posts

  • Andrea
  • Signori
  • Nikhef
  • Netherlands

Latest Posts

  • CERN
  • Geneva
  • Switzerland

Latest Posts

  • Aidan
  • Randle-Conde
  • Université Libre de Bruxelles
  • Belgium

Latest Posts

  • TRIUMF
  • Vancouver, BC
  • Canada

Latest Posts

  • Laura
  • Gladstone
  • MIT
  • USA

Latest Posts

  • Steven
  • Goldfarb
  • University of Michigan

Latest Posts

  • Fermilab
  • Batavia, IL
  • USA

Latest Posts

  • Seth
  • Zenz
  • Imperial College London
  • UK

Latest Posts

  • Nhan
  • Tran
  • Fermilab
  • USA

Latest Posts

  • Alex
  • Millar
  • University of Melbourne
  • Australia

Latest Posts

  • Ken
  • Bloom
  • USLHC
  • USA

Latest Posts

Posts Tagged ‘BSM’

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.

excited_atom_NASA

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.

emission_atom_NASA

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.

Words

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.

cms_dijet_spectrum

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!

Share

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.

CMS-Htautau1

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.

Share

Hi All.

In case you have been away from the Wonderful World of Physics for the past few weeks there is now evidence for the Standard Model Brout­-Englert­-Higgs Boson, with a mass of approximately 125 GeV/c2, from the ATLAS, CMS, CDF, DZero, and the combined CDF+Zero experiments [Moriond 2012 Conference, FNAL press release]. This is really exciting, and measurements of Higgs-related processes will definitely have a profound impact on the viability of Beyond the Standard Model theories like supersymmetry and technicolor.

Enough about Higgs, though. Of the many, MANY reasons for constructing the Large Hadron Collider and the Detector Experiments, one of my personal favorites is

to search for evidence of quantum gravity in TeV-scale proton collisions.

We know pretty well that gravity exists. (If you have issue with this, buy two apples and while eating one let go of the other.) We also know things like electrons, muons, & photons exist. (Flip on a light switch or buy a Geiger counter.) What we are less sure about is how, on an elementary level, are electrons, muons, & photons affected by gravity?

Figure 1: An example of a black hole (center) demonstrating Hawking radiation, which is when a black hole radiates, or emits,  particles (e & γ) through interaction with virtual particles.

Over the past few decades, there has been a ton of research investigating this very question, resulting in very fruitful and fascinating discoveries. For example: black holes can radiate photons and other gauge bosons by interacting with particles that have spontaneously been produced through quantum mechanical fluctuations. This is the famous Hawking radiation (See Fig. 1) [3]. Two other examples that come to mind both attempt to explain why gravity appears to be so much weaker than either the strong nuclear force (QCD) or the electroweak force (EWK). Their argument is that all Standard Model particles are restricted to three spatial dimensions, whereas new physics, include quantum gravity, exists in more than three spatial dimensions. The difference between the two theories is that the Large Extra Dimensions (or ADD) model supposes that all additional spatial dimensions are very small (<10-20 cm) but that each dimension is not too difference from what we experience everyday (See Fig. 2) [4,5]. The Randall-Sundrum model, on the other hand, proposes that there exists only a single extra dimension but that this spatial dimension is “warped” and unlike anything we have ever experienced [6,7]. I have not even mentioned string theory, but I am sure you can imagine that the list goes on for a while.

 

Figure 2: In the ADD (Large Extra Dimension) model, an electron (e-) and positron (e+) may annihilate and produce a graviton (G) and photon (γ). A defining feature is that the Standard Model particles (e±,γ) are restricted to the move in 3 spatial dimensions, whereas the graviton may propagate in additional dimensions.

Microscopic Black Holes

Despite the number of models trying to describe gravity at the most elementary level, there is actually a phenomenon that is surprisingly common to most all of them: they all predict the existence of microscopic black holes, or at least something very close to it. Now here is where I can easily dig myself a hole, so I want to be clear. The black hole-like objects these models predict are vastly different from the star-devouring black holes we have grown to know and love. Those exist at the center of galaxies and other places like that. The most obvious difference is that astronomical black holes are, well, astronomically huge. The black holes that I am talking about, if they exist, are significantly smaller than a proton.  The term “microscopic” makes these things sound much bigger than they are. Secondly, the masses of micro-black holes are comparable to the energy of the LHC; consequently, they will evaporate (via Hawking radiation) and disintegrate (decay) within moments of being produced. In the off chance that a stable micro-black hole is generated, then after about 10-25 seconds the thing will decay and burst into a blaze of  glory quarks & gluons (See Figs. 1 (above) & 3 (below)). Research has also concluded that these things are harmless and CERN has gone out of its way to inform the public of this.

Figure 3: "-->--" is the path the microscopic black hole travels (exaggerated) while evaporating, before decaying. Click to enlarge.

Admittedly, the fun part of writing this post was trying figure out a way to describe just how a microscopic black hole event, if it existed, would look in an LHC collider detector. Hawking radiation is straight forward enough to draw (Fig. 1), but things are a bit more involved when you want to show that some of those photons and Z bosons decaying into, say, electrons and positrons. So I got a little carried away and drew things by hand. Figure 3 shows a “typical” a micro-black hole, if they exist, briefly zipping around the detector radiating photons (γ), Z’s, W±’s, and gluons (g), before bursting into a bunch more bosons all at once. These bosons will then do whatever particles normally do in a particle detector and make a mess (shower and hadronize). A very distinguishing feature that I want to highlight is the number of particles that are produced in a single micro-black hole event, this is called particle multiplicity. If they exist, then the average micro-black hole event will result in a very high multiplicity (number) of final-state particles.

This is really important because in a typical proton-proton collision, things are not as busy. To clarify: plenty of things happen in proton collisions; micro-black hole events are just a bit busier. When protons collide, only two or three primary particles are produced and these then decay in predictable ways. In addition, the incident protons fragment and hit the side walls (“end caps”) of the detectors.

Figure 4: Typical proton-proton collision at the Large Hadron Collider as seen from a Detector Experiment. Click to enlarge.

This is it though. This is how experimentalists test whether these gravity-motivated theories correctly describe nature. What differentiates microscopic black hole events from any other proton-proton event is the number of final-state particles seen by the detector. In other words: particle multiplicity! There are not too many Standard Model processes that will result in, say, 10~15 final-state particles. If suddenly a experiment group sees a bunch of 15-particle events, then more refined searches can be performed to determine the root cause of this potential signal of new physics.

Recent Results from ATLAS and CMS

The most recent results from the ATLAS and CMS Experiments on their searches for microscopic black holes are both from March 2012. In these papers, ATLAS reports using 1.3 fb-1 of data, which is the equivalent of 91 trillion proton-proton collisions; CMS reports using a whopping 4.7 fb-1, or the equivalent of 329 trillion collisions. Both groups have opted to look for events with a large number of final-state particles, specifically in the central/barrel region of the detector in order to sidestep the fact that fragmenting protons increase the multiplicity in the detectors’ side walls (end caps). ATLAS, in particular, requires that two of the final-state particles are muons with the same electric charge. This subtle requirement actually has a significant impact on the search by minimizing the number of Standard Model processes that may mimic the signal, but at the cost of reducing the number of expected micro-black hole events. In order to optimize their search, CMS sums the magnitudes of all final-state particles’ momenta. This is a bit clever because with so many additional particles this sum is expected to be significantly larger than for a typical Standard Model process.

Sadly, as you have probably guessed, neither group has seen anything like a micro-black hole. 🙁 At any rate, here is a really cool micro-black hole candidate observed by with the CMS detector. It is most likely NOT an actual mico-black hole event, just a couple Standard Model processes that passed all the analysis requirements. Pretty, isn’t it.

Figure 5: A candidate microscopic black hole event observed with the Compact Muon Solenoid Experiment. Click to enlarge.

 

 

Happy Colliding

– richard (@bravelittlemuon)

 

 

Partial Bibliography

  1. ATLAS Collaboration, Search for strong gravity signatures in same-sign dimuon final states using the ATLAS detector at the LHC, Phys. Lett. B 709 (2012) 322-340, arXiv:1111.0080v2
  2. CMS Collaboration,Search for microscopic black holes in pp collisions at sqrt(s) = 7 TeV, Submitted to the Journal of High Energy Physics,  arXiv:1202.6396v1
  3. S. Hawking, Particle Creation by Black Holes, Commun. Math. Phys. 43 (1975) 199–220, euclid.cmp/1103899181
  4. N. Arkani-Hamed, S. Dimopoulos, and G. Dvali, The hierarchy problem and new dimensions at a millimeter, Phys. Lett. B 429 (1998) 263–267, arXiv:hep-ph/9803315v1
  5. N. Arkani-Hamed, S. Dimopoulos, and G. Dvali, Phenomenology, astrophysics and cosmology of theories with submillimeter dimensions and TeV scale quantum gravity, Phys. Rev. D 59 (1999) 086004, arXiv:hep-ph/9807344v1
  6. L. Randall and R. Sundrum, Large Mass Hierarchy from a Small Extra Dimension, Phys. Rev. Lett. 83 (1999) 3370–3373, arXiv:hep-ph/9905221v1
  7. L. Randall and R. Sundrum, An Alternative to Compactification, Phys. Rev. Lett. 83(1999) 4690–4693, arXiv:hep-th/9906064v1
  8. S. Dimopoulos and R. Emparan, String balls at the LHC and beyond, Phys. Lett. B 526(2002) 393–398, arXiv:hep-ph/0108060v1
  9. R. Casadio, S. Fabi, B. Harms, & O. Micu, Theoretical survey of tidal-charged black holes at the LHC, arxiv.org/abs/0911.1884v1
Share