Tag Archives: detector

Shown here is "event zero," the first detection of a trapped electron in the MIT physicists' instrument. The color indicates the electron's detected power as a function of frequency and time. The sudden “jumps” in frequency indicate an electron collision with the residual hydrogen gas in the cell.

Courtesy of the researchers

Source: MIT News

New tabletop detector “sees” single electrons

Magnet-based setup may help detect the elusive mass of neutrinos.

Jennifer Chu


MIT physicists have developed a new tabletop particle detector that is able to identify single electrons in a radioactive gas.
As the gas decays and gives off electrons, the detector uses a magnet to trap them in a magnetic bottle. A radio antenna then picks up very weak signals emitted by the electrons, which can be used to map the electrons’ precise activity over several milliseconds.

Shown here is "event zero," the first detection of a trapped electron in the MIT physicists' instrument. The color indicates the electron's detected power as a function of frequency and time. The sudden “jumps” in frequency indicate an electron collision with the residual hydrogen gas in the cell. Courtesy of the researchers Source: MIT News
Shown here is “event zero,” the first detection of a trapped electron in the MIT physicists’ instrument. The color indicates the electron’s detected power as a function of frequency and time. The sudden “jumps” in frequency indicate an electron collision with the residual hydrogen gas in the cell.
Courtesy of the researchers
Source: MIT News

The team worked with researchers at Pacific Northwest National Laboratory, the University of Washington, the University of California at Santa Barbara (UCSB), and elsewhere to record the activity of more than 100,000 individual electrons in krypton gas.
The majority of electrons observed behaved in a characteristic pattern: As the radioactive krypton gas decays, it emits electrons that vibrate at a baseline frequency before petering out; this frequency spikes again whenever an electron hits an atom of radioactive gas. As an electron ping-pongs against multiple atoms in the detector, its energy appears to jump in a step-like pattern.
“We can literally image the frequency of the electron, and we see this electron suddenly pop into our radio antenna,” says Joe Formaggio, an associate professor of physics at MIT. “Over time, the frequency changes, and actually chirps up. So these electrons are chirping in radio waves.”
Formaggio says the group’s results, published in Physical Review Letters, are a big step toward a more elusive goal: measuring the mass of a neutrino.

A ghostly particle
Neutrinos are among the more mysterious elementary particles in the universe: Billions of them pass through every cell of our bodies each second, and yet these ghostly particles are incredibly difficult to detect, as they don’t appear to interact with ordinary matter. Scientists have set theoretical limits on neutrino mass, but researchers have yet to precisely detect it.
“We have [the mass] cornered, but haven’t measured it yet,” Formaggio says. “The name of the game is to measure the energy of an electron — that’s your signature that tells you about the neutrino.”
As Formaggio explains it, when a radioactive atom such as tritium decays, it turns into an isotope of helium and, in the process, also releases an electron and a neutrino. The energy of all particles released adds up to the original energy of the parent neutron. Measuring the energy of the electron, therefore, can illuminate the energy — and consequently, the mass — of the neutrino.
Scientists agree that tritium, a radioactive isotope of hydrogen, is key to obtaining a precise measurement: As a gas, tritium decays at such a rate that scientists can relatively easily observe its electron byproducts.
Researchers in Karlsruhe, Germany, hope to measure electrons in tritium using a massive spectrometer as part of an experiment named KATRIN (Karlsruhe Tritium Neutrino Experiment). Electrons, produced from the decay of tritium, pass through the spectrometer, which filters them according to their different energy levels. The experiment, which is just getting under way, may obtain measurements of single electrons, but at a cost.
“In KATRIN, the electrons are detected in a silicon detector, which means the electrons smash into the crystal, and a lot of random things happen, essentially destroying the electrons,” says Daniel Furse, a graduate student in physics, and a co-author on the paper. “We still want to measure the energy of electrons, but we do it in a nondestructive way.”
The group’s setup has an additional advantage: size. The detector essentially fits on a tabletop, and the space in which electrons are detected is smaller than a postage stamp. In contrast, KATRIN’s spectrometer, when delivered to Karlsruhe, barely fit through the city’s streets.
Tuning in
Furse and Formaggio’s detector — an experiment called “Project 8” — is based on a decades-old phenomenon known as cyclotron radiation, in which charged particles such as electrons emit radio waves in a magnetic field. It turns out electrons emit this radiation at a frequency similar to that of military radio communications.
“It’s the same frequency that the military uses — 26 gigahertz,” Formaggio says. “And it turns out the baseline frequency changes very slightly if the electron has energy. So we said, ‘Why not look at the radiation [electrons] emit directly?’”
Formaggio and former postdoc Benjamin Monreal, now an assistant professor of physics at UCSB, reasoned that if they could tune into this baseline frequency, they could catch electrons as they shot out of a decaying radioactive gas, and measure their energy in a magnetic field.
“If you could measure the frequency of this radio signal, you could measure the energy potentially much more accurately than you can with any other method,” Furse says. “The problem is, you’re looking at this really weak signal over a very short amount of time, and it’s tough to see, which is why no one has ever done it before.”
It took five years of fits and starts before the group was finally able to build an accurate detector. Once the researchers turned the detector on, they were able to record individual electrons within the first 100 milliseconds of the experiment — although the analysis took a bit longer.
“Our software was so slow at processing things that we could tell funny things were happening because, all of a sudden, our file size became larger, as these things started appearing,” Formaggio recalls.
He says the precision of the measurements obtained so far in krypton gas has encouraged the team to move on to tritium — a goal Formaggio says may be attainable in the next year or two — and pave a path toward measuring the mass of the neutrino.
Steven Elliott, a technical staff member at Los Alamos National Laboratory, says the group’s new detector “represents a very significant result.” In order to use the detector to measure the mass of a neutrino, Elliott adds, the group will have to make multiple improvements, including developing a bigger cell to contain a larger amount of tritium.
“This was the first step, albeit a very important step, along the way to building a next-generation experiment,” says Elliott, who did not contribute to the research. “As a result, the neutrino community is very impressed with the concept and execution of this experiment.”
This research was funded in part by the Department of Energy and the National Science Foundation.
Figure 1 (left) Exclusion limits for production of Higgsino production as a function of Higgsino mass and branching fraction. (right) Most sensitive search channel as a function of Higgsino mass and branching fraction. Credit: CERN

Recent results in the search for supersymmetry : CERN CMS

By Frank Wuerthwein, Keith Ulmer and Guillelmo Gomez Ceballos.


Among the leading candidates to describe physics beyond the standard model of particle physics is Supersymmetry, a new symmetry that posits the existence of a partner particle for each known particle in the standard model. Supersymmetry, or “SUSY” as it has come to be known, may help explain the nature of dark matter and the large difference in strength between the fundamental forces of nature. Each year, new experimental results and theoretical developments are reported in the “SUSY” conference series, with the 2014 edition (SUSY2014) happening this week in Manchester, England[1].

Figure 1 (left) Exclusion limits for production of Higgsino production as a function of Higgsino mass and branching fraction. (right) Most sensitive search channel as a function of Higgsino mass and branching fraction. Credit: CERN
Figure 1 (left) Exclusion limits for production of Higgsino production as a function of Higgsino mass and branching fraction. (right) Most sensitive search channel as a function of Higgsino mass and branching fraction. Credit: CERN

Experimental evidence for SUSY has been sought for many years at multiple colliders, including a vast array of search results from the CMS experiment at the Large Hadron Collider at CERN. With data from Run 1 of the LHC collected through the end of 2012, the full set of results thus far has not revealed any striking signs of physics beyond the standard model [2]. New searches presented at SUSY2014 have begun to probe increasingly complicated potential decay chains and to combine multiple searches to access more challenging new physics scenarios. Below we highlight some of the most recent results first presented this summer at SUSY14 and ICHEP 2014 [3].

Figure 2: Exclusion limits versus gluino and neutralino masses for a variety of gluino decay branching fractions from the “razor” search. Credit: CERN
Figure 2: Exclusion limits versus gluino and neutralino masses for a variety of gluino decay branching fractions from the “razor” search. Credit: CERN

Search for new physics in the final states hh, Zh, and ZZ plus MET

After its discovery only two years ago, the Higgs boson is already a powerful tool in the search for new physics. Earlier this year, CMS submitted for publication [4] a set of searches for associate production of W, Higgs, and missing transverse energy (“MET”, indicative of particles escaping the detector). At ICHEP this summer, CMS presented the first combined searches for hh, Zh, and ZZ plus MET. No excess above standard model backgrounds is observed. Figure 1 shows the interpretation of the results in terms of limits on higgsino pair production as a function of the higgsino mass and decay branching fraction. Within the framework of Gauge Mediated Supersymmetry Breaking (GMSB), the neutral higgsino decays to a gravitino and either a higgs or Z boson. The left plot in Figure 1 shows that CMS excludes higgsino production up to ~ 300GeV when the higgsino decays at equal rate to either of these two decays. The right plot in Figure 1 indicates that four different final states dominate the sensitivity in different parts of the 2D parameter space, clearly demonstrating that searches for new physics with one or two higgs bosons in the final state benefit greatly from combining many different decay channels.

Figure 3: Dilepton invariant mass distribution for “same flavor” events, compared to the background prediction from “opposite flavor” events. Credit:CERN
Figure 3: Dilepton invariant mass distribution for “same flavor” events, compared to the background prediction from “opposite flavor” events. Credit:CERN

Search for gluino pair production via the decays to top pairs, bottom pairs, or top and bottom plus MET

Up to now, CMS searches for gluino pair production inspired by “natural SUSY” (i.e. SUSY in which the masses of the SUSY partners are not much higher than those of the Higgs boson) have focused on final states with either four top or four b-quarks plus MET. In contrast, theoretically any combination of MET plus 4 quarks, top or bottom, is well justified. At ICHEP, CMS presented the first complete exploration of sensitivity across the full set of possible final states and branching fractions. Figure 2 shows the corresponding exclusion curves in the gluino vs neutralino mass plane. This search employs the so-called “razor” variables, and its sensitivity is dominated by all-hadronic final states. The more top quarks there are in the final state for a given gluino mass, the less momentum is left for all the decay products, and the harder it is thus to distinguish signal from background. Accordingly, the sensitivity decreases as the number of top quarks per event increases.

Figure 4: MSSMvsSM limit in the MSSM mmod+h scenario. At each mA - tanβpoint a Hypothesis test is performed testing the MSSM (A+H+h+BKG) hypothesis against the SM (hSM+BKG) hypothesis. Credit: CERN
Figure 4: MSSMvsSM limit in the MSSM mmod+h scenario. At each mA – tanβpoint a Hypothesis test is performed testing the MSSM (A+H+h+BKG) hypothesis against the SM (hSM+BKG) hypothesis. Credit: CERN

Searching for SUSY with an “Edge”

The dilepton invariant mass distribution for leptons from the decays χ20 to l+l- χ10, or similar decays via a slepton as an intermediate state, display the striking feature of a kinematic “edge” [5, 6]. As these decays conserve lepton flavor, this edge is present only in same-flavor events, i.e. ee and μμ, and is completely absent in the “opposite flavor” lepton sample, i.e. eμ events. In contrast, backgrounds for which each of the two leptons come from a different W decay, e.g. top pairs, WW, etc., will have identical dilepton distributions for same and opposite flavor. Thus, the eμ sample in data provides a perfect model of the background dilepton mass distribution – modulo effects from the trigger and lepton reconstruction. The kinematic edge is a sufficiently striking signature to reveal new physics even at relatively modest hadronic activity, HT and MET, i.e. in the presence of sizeable top and Z backgrounds.

CMS presented a search for such an “edge” in dilepton events with jets and MET at SUSY2014 using the full 8TeV data sample [7]. Figure 3 overlays the dilepton mass distribution in ee plus μμ (data points), with the corresponding one from eμ (pink histogram). The blue shaded region depicts the systematic error envelope for the background prediction. A small excess is visible below the Z peak. A signal region of 20GeV < mll < 70GeV was chosen before data taking. Within this region, 860 events are observed with an expected standard model background yield of 730 ± 40. The small excess is consistent with a 2.6 sigma fluctuation of the standard model background. For more details see [8].

Search for additional neutral MSSM Higgs bosons in the H→ττ decay channel

Another highlight among the CMS results presented at the SUSY2014 conference is the search for additional neutral Higgs bosons decaying to τ leptons, which is the most promising channel to search for such Higgs bosons in the context of the minimal SUSY extension of the standard model, the MSSM. Following the release of a preliminary result based on the full data set of the 2011/2012 data taking period [8], additional results based on a new interpretation of the data have been presented at this conference for the first time [9]. While the data selection has not changed, extensive work has set the ground for a new interpretation of the data in the context of modern benchmark models. In particular, the new models take into account the presence of the recently discovered Higgs boson with a mass of 125 GeV, as proposed in [10]. Also for the first time the model-dependent exclusion contours as a function of the mass of the CP-odd Higgs boson, A, and the ratio of the vacuum expectation values of the two SUSY Higgs doublets, tanβ, have been derived, taking the presence of the newly discovered Higgs boson properly into account in the test statistic. As recently demonstrated by CMS [11], all observations of the new boson are so far compatible with the SM expectation within ~10% accuracy, which justifies the standard model hypothesis to be the better choice for the test statistic. The hypothesis test now becomes a search based on a model with three Higgs bosons compared against the standard model with only one Higgs boson. Traditional limits, based on the test statistic excluding the Higgs boson from the standard model hypothesis have also been made public on the CMS web-pages [12]. Also made available to the public is an extended database of results based on a model-independent single-resonance search model, which will be extremely valuable to theorists engaged in model building. Figure 1 shows the exclusion contour in a modified mh,max scenario, also referred to as mh,mod+ exploiting the new statistical treatment for the statistical inference.

By Frank Wuerthwein, Keith Ulmer and Guillelmo Gomez Ceballos.


[1] http://www.susy2014.manchester.ac.uk

[2] https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsSUS

[3] http://ichep2014.es

[4] https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsSUS13006

[5] http://cds.cern.ch/record/1194507/files/SUS-09-002-pas.pdf

[6] https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsSUS11011

[7] https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsSUS12019

[8] CMS Collaboration, “Search for Neutral MSSM Higgs Bosons Decaying to Tau Pairs in pp Collisions”, (2013), CMS-PAS-HIG-13-021.

[9] CMS Collaboration, “Search for Neutral MSSM Higgs Bosons Decaying to Tau Pairs in pp Collisions”, to be submitted to JHEP.

[10] M. S. Carena et al, “MSSM Higgs boson searches at the Tevatron and at the LHC: Impact of different benchmark scenarios” Eur. Phy. J C 73, 2552 (2013) (arXiv:hep-ph/0511023).

[11] CMS Collaboration, “Precise determination of the mass of the Higgs boson and studies of the compatibility of its couplings with the standard model”, (2014), CMS-PAS-HIG-14-009.

[12] https://indico.hep.manchester.ac.uk/contributionDisplay.py?contribId=288….

Source: CERN CMS