Physics news Update 775

Article Tools

Enlarge text

Shrink text
Print this page

Print

Subscribe
E-mail alert
RSS feed

Save and Share
Save to Digg

Digg this

Del.icio.us
Save to Furl

Furl

Number 775, April 26, 2006 by Phil Schewe and Ben Stein

Testing Special Relativity and Newtonian Gravity

Lorentz invariance says that the laws of physics are the same for an observer at rest on the Earth or one who is rotated through some angle or traveling at a constant speed relative to the observer at rest. Looking for a crack in the universe in the form of a very faint field pervading the Cosmos, one that exerts a force on electron spin, would mean the end of Lorentz invariance.
An important ingredient in Einstein's theory of special relativity, Lorentz invariance has been borne out in numerous experiments. A new experiment conducted at the University of Washington, in Seattle, has sought such an anomalous field and not found it even at an energy scale of 10^-21 electronvolt. This is the most stringent search yet -- by a factor of 100 -- for Lorentz-invariance-violating effects involving electrons.
The Washington work, described at this week’s American Physical Society's (APS) April Meeting in Dallas by Claire Cramer, is part of an ongoing battery of tests carried out with a flexible and sophisticated torsion-balance apparatus. In this case, a pendulum is made of blocks whose magnetism arises from both the orbital motion of an electron around its nucleus and from the intrinsic spin of the electron itself. Carefully choosing and arranging the blocks, one can create an assembly that has zero magnetization and yet still have an overall nonzero electron spin. Cramer refers to this condition as a "spin dipole," analogous to the case of an electric dipole, an object with zero net charge but which, because of a displaced arrangement of positive and negative charge, possesses a net electric field.
The existence of a preferred-direction, Lorentz-violating spin-related force would have shown up as a subtle mode in the rotation of the pendulum. The conclusion: any such quasi-magnetic field would have to be weaker than about a femtogauss, or 10^-15 gauss.
At the APS meeting, Eric Adelberger, leader of the Washington group, summarized some of the other efforts underway in his lab such as the search for evidence of extra dimensions in the form of departures from Newtonian gravity -- for instance, the inverse-square dependence -- at a size scale of tens of microns. In fact, he said that something strange was happening at a measurement scale of about 70 microns; the most likely explanation of this, he conceded, was an experimental artifact.

Quark-Gluon Plasma: Has It Been Observed?

Barbara Jacak of Stony Brook University in Long Island, N.Y., is a member of the PHENIX team, a large detector collaboration studying the high-energy smashup of gold nuclei at Brookhaven National Lab's Relativistic Heavy Ion Collider (RHIC). Delivering a plenary talk at this week's American Physical Society April Meeting in Dallas, Jacak argued that new experimental data provide evidence that in collisions the gold nucleus, including its complement of neutrons and protons, and all their quark constituents, are being melted into a true plasma of quarks and gluons. This plasma possesses the highest energy density of any substance made in a lab -- up to 15 gigaelectronvolt per cubic femtometer.
At last year's APS April meeting all the RHIC teams unanimously agreed that a peculiar liquid of quarks had been created in the collisions. Peculiar and unexpected: instead of a gas of weakly interacting quarks, the collision fireball ensuing from a head-on interaction of the two nuclei resulted in a liquid of strongly-interacting quarks (see PNU 728). But this wasn't quite the same thing as claiming that this fluid was a true plasma.
To form a plasma, the quarks must reside outside their customary groupings of two or three; two quarks (a quark-antiquark couplet) together are called a meson while three-quark groupings are called baryons. Mesons and baryons in turn are collectively referred to as hadrons. One of the observed properties of hadrons is that they are "color-neutral" (just as ordinary atoms are charge neutral), "color" being the fanciful name for the strong-nuclear-force equivalent of electrical charge. For example, a proton would normally consist of a red, blue, and green quark which (in a color sense) adds up to zero.
And just as an electrical plasma is one in which the particles are charged so a nuclear plasma would be one in which the particles possess color. At last year's April meeting the observation that the matter is liquid was presented. According to Jacak, further studies over the past year now provide, at least for her and a growing number of RHIC scientists, the necessary proof for a plasma state.
One notable fact supporting the plasma contention is the fact, apparent in recent data analysis, that charm-quark jets are being suppressed. In the fireball, charm quarks are being produced, albeit at much lower rates than the light quarks (up, down, strange).
Because of their heft the charm quarks (or, to be precise, the jets of hadrons they engender) ought to be able to punch their way out of the plasma to be observed in outside detectors, but they’re not. What seems to be happening is this: the plasma of mostly light quarks are taking up or engulfing the heavy quarks through frequent and intense interactions.
As Jacak says, it's as if a strongly rushing river were picking up stones off the riverbed and pulling them along with the stream. A river of hadrons (quarks bundled up into color-neutral clumps) wouldn't be able to do this as readily as a river of mostly unattached quarks.

Back to Physics News Update