semester projects - university of toledo

Semester Projects

Draft One in a two days: Thursday, April 7th (an extension from the former deadline). This is an Absolute Deadline, late drafts will be marked down by 3% per day off the final project grade!!!

First presenter, Ryan Yockey, April 7th. (2% bonus!) Powerpoint/PDF/Keynote, come to class 10 min early and we can set up.

Thursday, April 7, 2011

Exam #2

A bit long: graded on 4 questions, each worth 25 points. Additional questions counted up to 5 points extra credit.

Great Job: Average: 87% SD: 7.5%.


“Hard Line” across the event horizon

From the perspective of the outside ship, the ship descending into the black hole would move ever more slowly, approaching the event horizon at infinitesimal speed.

The “pulses” (like the 1‘s and 0‘s of a binary stream) of information along the hard line would face similar gravitational time dilation, until one pulse would never end.


Swapping Matter & Energy

Stars turn matter into energy constantly: 4 H’s have larger mass than 1 He.

LHC, the world’s largest super-collider, sends a beam of 3×1014 protons circling around for collision. They travel 99.9999991% the speed of light!

Their energy is 7 TeV (trillion electron volts), about 7500× larger than their rest mass!

This kinetic energy, upon collision, is converted to showers of new particles.


Matter to Energy

The only known means of converting 100% of the mass of some object to energy (radiation), is to combine matter and antimatter.

Antimatter is rare; normally this happens of very small scales


Matter to EnergyFusion in the sun is very diffuse: about 300 W/m3

The human body is around .07m3, and we generate around 100W: 1500 W/m3!

so we generate more energy per volume than the sun’s core! A “Tokomak”: magnetic

confinement for fusion


Trailing QuestionsHow can we “Know” the conditions of the early universe, before the CMB?

Before the CMB formed, no real information is coming to us, so we infer it based on properties of the universe (H/He abundance, structure) and known physical laws.


More specifically

Big Bang Nucleosynthesis: theory of how much H, He, etc. was formed.

Lasted only 17 minutes, starting about 3 minutes after the Big Bang.

We know the rate of fusion to Deuterium (heavy hydrogen) limited the process.

BB theory generically predicts proton neutron abundances (about 7 to 1!) based simply on the mass of the neutron and proton. Bottleneck after Beryllium: no stable nucleus with 4 or 8 “nucleons” (stars solve this by combining 3 helium nuclei at the same time!).

Hydrogen = 1p + 1e

Deuterium = 1p + 1e + 1n

Helium = 2p + 2e + 2n


A bit more on Dark Mater

There are many possibilities for Dark Matter, including normal (“Baryonic”) and non-baryonic.

Baryonic dark matter: ordinary particles,neutrons, electrons, atoms, that just happen to be hiding.

Non-Baryonic matter: an unknown class of particles of any mass.


Dark Gas between galaxies

The “Lyman Alpha Forest” seen in absorption of distant quasar spectra.


Brown Dwarfs

Failed low mass stars below 0.08M⨀.


“Red” Dwarf StarsNormal but very faint stars

HST searched for red dwarf stars in the halo of the Galaxy

Surprisingly few red dwarf stars were found, < 6% of mass of galaxy halo

Expected 38 red dwarfs: Seen 0!


“Ghost” Galaxies

AKA “Low Surface Brightness” Galaxies.

Many galaxies are below our threshold for detection: mostly dark matter.


“Macho’s”

Massive Compact Halo Objects

Many have been discovered through gravitational micro-lensing

Not enough to account for Dark Matter

And few in the halo


Black Holes

Primordial Black Holes

Too small? They would evaporate by Hawking radiation.

Can’t be too large, or they would impact the orbits of stars in the outer Galaxy.


Black Hole Machos

Isolated black holes in the Galactic bulge

Distorts lensing light curve.


Gravitation Lensing

“Weak” Lensing: minor distortions like looking through an old window


“Strong” Lensing: minor distortions like looking through an old window

Gravitation Lensing


Dust

Dust is made of elements heavier than Helium, which were previously produced by stars (<2% of total)

Dust absorbs and reradiates background light

Dust clouds of the dark Pipe nebula


NeutrinosThere are about 100 million neutrinos per m3

More (or less) types of neutrinos would lead to more (or less) primordial Helium than we see

Neutrinos with mass affect the formation of structure in the Universe

Much less small scale structure would be present

Observed structure sets limits on how much mass neutrinos may have, and on their contribution to dark matter.


AxionsExtremely light particles, with typical mass of 10-6 eV/c2

Interactions are 1012 weaker than ordinary weak interaction

Density would be 108 per cubic centimeter

Velocities are low

Axions may be detected when they convert to low energy photons after passing through a strong magnetic field


WIMPsWeakly Interacting Massive Particles

Predicted by Supersymmetry (SUSY) theories of particle physics

Supersymmetry tries to unify the four forces of physics by adding extra dimensions

WIMPs would have been easily detected in accelerators if M < 15 GeV/c2

The lightest WIMPs would be stable, and could still exist in the Universe, contributing most if not all of the Dark Matter


Finding WIMPsCryogenic Dark Matter Search

6.4 million events studied - 13 possible candidates for WIMPs

All are consistent with expected neutron flux

Cryostat holds T= 0.01 K

CDMS Lab 35 feet under Stanford


Dark Matter Search Results fromthe CDMS II ExperimentThe CDMS II Collaboration*†

Astrophysical observations indicate that dark matter constitutes most of the mass in our universe,but its nature remains unknown. Over the past decade, the Cryogenic Dark Matter Search(CDMS II) experiment has provided world-leading sensitivity for the direct detection of weaklyinteracting massive particle (WIMP) dark matter. The final exposure of our low-temperaturegermanium particle detectors at the Soudan Underground Laboratory yielded two candidate events,with an expected background of 0.9 T 0.2 events. This is not statistically significant evidence for aWIMP signal. The combined CDMS II data place the strongest constraints on the WIMP-nucleonspin-independent scattering cross section for a wide range of WIMP masses and exclude newparameter space in inelastic dark matter models.

Awide variety of observational evidence(1) indicates that ~85% of the matter inour universe is in some nonluminous

form that has thus far eluded laboratory identifi-cation. The inferred properties of this dark mattersuggest that it is composed of elementary parti-cles beyond those described in the Standard Mod-el of particle physics. weakly interacting massiveparticles (WIMPs) (2) are a class of candidates toconstitute this dark matter that are particularlywell-motivated by independent considerations ofcosmology and particle physics (3–5). If WIMPsconstitute the dark matter in our galaxy, theyshould occasionally scatter elastically off atomicnuclei in a terrestrial target (6, 7). Laboratorysearches for such scattering events (8–10) establishtheir rate to be less than 0.1 per day per kilogramof target mass, and researchers have begun to testthe most interesting classes of WIMP models.

The Cryogenic DarkMatter Search (CDMS II)experiment seeks to detect recoiling atomicnuclei (nuclear recoils) from WIMP-scatteringevents using particle detectors operated at cryo-genic temperatures (<50 mK) (8, 11). Each de-tector is a semiconductor disk ~10 mm thick and76 mm in diameter, which is photolithographi-cally patternedwith sensors to detect the phononsand ionization generated by incident particles.These detectors have extraordinary power to dis-tinguish nuclear recoils (produced by interactionsof WIMPs or neutrons) from the far more com-mon electron recoils produced by incident pho-tons and electrons. Nuclear recoils generate lessionization than electron recoils of the same de-posited energy, allowing event-by-event rejectionof electron-recoil events with a misidentificationrate of <1 in 104. Electron recoils within a fewmm of the detector surface can suffer from re-duced ionization collection, but these may beidentified by the relatively fast arrival of their

phonon signals. Combining the ratio of ioniza-tion to phonon recoil energy (ionization yield)with the timing of the phonon signals gives anoverall misidentification rate of <1 in 10−6 forelectron recoils.

CDMS II operated an array of 30 such de-tectors (19 Ge and 11 Si) in a low-radioactivityinstallation in the Soudan Underground Labora-tory, Minnesota, USA (11). The depth of theexperimental facility (713 m below the surface)greatly reduces the rate of background eventsfrom particle showers induced by cosmic rays.Nearly all remaining events from this sourcewere identified using a layer of plastic scintillatorsurrounding the detector volume. Inner layers oflead and polyethylene further shielded the de-tectors against environmental radioactivity. Datataken during four periods of stable operation

between July 2007 and September 2008 wereanalyzed for this work. Because of their greatersensitivity to spin-independent WIMP scattering,only Ge detectors were used to search for WIMPscatters. After excluding periods of poor detec-tor performance, a total exposure to WIMPs of612 kg-days was considered for this work.

After detector calibration, we defined a seriesof criteria to identify candidate WIMP-scatteringevents. WIMP candidates were required to de-posit 10 to 100 keVof energy in a single detector,have the ionization and phonon characteristics ofa nuclear recoil, and have no identifiable energydeposition in the rest of the array or in thescintillator shield. These criteria are describedin more detail in the supporting online material(SOM) text. To avoid unconscious bias, we per-formed a “blind analysis” in which the exactselection criteria were defined without priorknowledge of the content of the signal region orits vicinity. The fraction of nuclear recoil eventsaccepted by these criteria was measured using acalibration sample of nuclear recoil events in-duced by a 252Cf source. Despite the great dis-crimination power of this experiment, a smallexpected rate of misidentified background eventsremains. In the exposure considered here, we ex-pected to misclassify 0.8 T 0.1 (statistical) T0.2(systematic) surface electron recoils as WIMPcandidates. We also expect neutrons produced bycosmic rays and radioactivity to generate an aver-age of ~0.1 nuclear recoils, which would be in-distinguishable from WIMP scatters.

After finalizing all selection criteria, we “un-blinded” to examine the contents of the WIMPacceptance region (SOM text). We observed twocandidate events at recoil energies of 12.3 keVand 15.5 keV (Figs. 1 and 2). These events

REPORTS

*To whom correspondence should be addressed: Jodi Cooley.E-mail: [email protected]†All authors and their affiliations appear at the end of thispaper.

Fig. 1. Ionization yield versusrecoil energy for events consistentwith all signal criteria, excludingyield and timing. The top (bottom)plot shows events for detector T1Z5(T3Z4) (see SOM text for detectornomenclature). The solid red linesindicate the ionization yield accept-ance region. The vertical dashed linerepresents the recoil energy thresh-old, and the slopingmagenta dashedline is the ionization threshold.Events with phonon timing charac-teristics consistent with our selec-tion criteria are shown with roundmarkers. The candidate events arethe round markers between the redlines.

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Constraints from the Universe itself

“Hot Dark Matter”: rapidly moving particles, washes out small scale structure.

“Cold Dark Matter”: Slowly moving dark matter particles readily clump and form small structures early.

CDM HDM


“Concordance”

ΛCDMThursday, April 7, 2011

semester projects - university of toledo

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