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Posts Tagged ‘BSM Physics’

This article appeared in Fermilab Today on April 21, 2015.

Fermilab's Mu2e groundbreaking ceremony took place on Saturday, April 18. From left: Alan Stone (DOE Office of High Energy Physics), Nigel Lockyer (Fermilab director), Jim Siegrist (DOE Office of High Energy Physics director), Ron Ray (Mu2e project manager), Paul Philp (Mu2e federal project director at the Fermi Site Office), Jim Miller (Mu2e co-spokesperson), Doug Glenzinski (Mu2e co-spokesperson), Martha Michels (Fermilab ESH&Q head), Mike Shrader (Middough architecture firm), Julie Whitmore (Mu2e deputy project manager), Jason Whittaker (Whittaker Construction), Tom Lackowski (FESS). Photo: Reidar Hahn

Fermilab’s Mu2e groundbreaking ceremony took place on Saturday, April 18. From left: Alan Stone (DOE Office of High Energy Physics), Nigel Lockyer (Fermilab director), Jim Siegrist (DOE Office of High Energy Physics director), Ron Ray (Mu2e project manager), Paul Philp (Mu2e federal project director at the Fermi Site Office), Jim Miller (Mu2e co-spokesperson), Doug Glenzinski (Mu2e co-spokesperson), Martha Michels (Fermilab ESH&Q head), Mike Shrader (Middough architecture firm), Julie Whitmore (Mu2e deputy project manager), Jason Whittaker (Whittaker Construction), Tom Lackowski (FESS). Photo: Reidar Hahn

This weekend, members of the Mu2e collaboration dug their shovels into the ground of Fermilab’s Muon Campus for the experiment that will search for the direct conversion of a muon into an electron in the hunt for new physics.

For decades, the Standard Model has stood as the best explanation of the subatomic world, describing the properties of the basic building blocks of matter and the forces that govern them. However, challenges remain, including that of unifying gravity with the other fundamental forces or explaining the matter-antimatter asymmetry that allows our universe to exist. Physicists have since developed new models, and detecting the direct conversion of a muon to an electron would provide evidence for many of these alternative theories.

“There’s a real possibility that we’ll see a signal because so many theories beyond the Standard Model naturally allow muon-to-electron conversion,” said Jim Miller, a co-spokesperson for Mu2e. “It’ll also be exciting if we don’t see anything, since it will greatly constrain the parameters of these models.”

Muons and electrons are two different flavors in the charged-lepton family. Muons are 200 times more massive than electrons and decay quickly into lighter particles, while electrons are stable and live forever. Most of the time, a muon decays into an electron and two neutrinos, but physicists have reason to believe that once in a blue moon, muons will convert directly into an electron without releasing any neutrinos. This is physics beyond the Standard Model.

Under the Standard Model, the muon-to-electron direct conversion happens too rarely to ever observe. In more sophisticated models, however, this occurs just frequently enough for an extremely sensitive machine to detect.

The Mu2e detector, when complete, will be the instrument to do this. The 92-foot-long apparatus will have three sections, each with its own superconducting magnet. Its unique S-shape was designed to capture as many slow muons as possible with an aluminum target. The direct conversion of a muon to an electron in an aluminum nucleus would release exactly 105 million electronvolts of energy, which means that if it occurs, the signal in the detector will be unmistakable. Scientists expect Mu2e to be 10,000 times more sensitive than previous attempts to see this process.

Construction will now begin on a new experimental hall for Mu2e. This hall will eventually house the detector and the infrastructure needed to conduct the experiment, such as the cryogenic systems to cool the superconducting magnets and the power systems to keep the machine running.

“What’s nice about the groundbreaking is that it becomes a real thing. It’s a long haul, but we’ll get there eventually, and this is a start,” said Julie Whitmore, deputy project manager for Mu2e.

The detector hall will be complete in late 2016. The experiment, funded mainly by the Department of Energy Office of Science, is expected to begin in 2020 and run for three years until peak sensitivity is reached.

“This is a project that will be moving along for many years. It won’t just be one shot,” said Stefano Miscetti, the leader of the Italian INFN group, Mu2e’s largest international collaborator. “If we observe something, we will want to measure it better. If we don’t, we will want to increase the sensitivity.”

Physicists around the world are working to extend the frontiers of the Standard Model. One hundred seventy-eight people from 31 institutions are coming together for Mu2e to make a significant impact on this venture.

“We’re sensitive to the same new physics that scientists are searching for at the Large Hadron Collider, we just look for it in a complementary way,” said Ron Ray, Mu2e project manager. “Even if the LHC doesn’t see new physics, we could see new physics here.”

Diana Kwon

See a two-minute video on the ceremony

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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.

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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
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