A brand new experimental consequence has shaken the world of particle physics. The W boson, it seems, is so much heavier than we thought. This poses a problem to our most profitable and examined idea concerning the material of the universe to this point: the Commonplace Mannequin. And whereas this one experimental consequence may not be sufficient by itself to overthrow the speculation, it already factors within the course of a idea that might, writes Martin Bauer.
Not too long ago the mass of the W boson has been measured by the Collider Detector at Fermilab with unprecedented precision and a shocking consequence. The latest consequence wildly disagrees with all earlier measurements of the W boson’s mass, however this result’s no fluke. To provide you an concept of the precision of this newest measurement and the way unlikley it’s that this result’s a mistake, contemplate this: when you weigh your self a number of occasions with totally different scales you’ll anticipate to see some discrepancy. However an equal discrapancy just like the one between the newest measurement and the earlier measurements of the W boson’s mass would statistically happen solely after you’ve got weighed your self 1 billion occasions.
Hints of a brand new elementary pressure
Measuring the properties of the W boson exactly is essential. From stellar fusion to carbon courting, the mass of the W boson impacts many calculations that underly our understanding of the Universe. For example, its mass is linked to the lifetimes of different particles, which in flip are essential for understanding how the Universe developed after the Huge Bang. However maybe a very powerful consequence of this newest measurement of the W boson’s mass is that it places it in rigidity with our most profitable idea about particle physics: the Commonplace Mannequin.
At this level you’re in all probability questioning: What’s a W boson, what’s it good for and what are the precise implications of this shocking consequence?
The W boson is a elementary particle mediating the weak pressure, a pressure that we by no means expertise straight as a result of it solely acts on subatomic distances. All different identified elementary forces give rise to sure programs: eg photo voltaic programs sure collectively by gravity, atoms sure collectively by the electromagnetic pressure and atomic nuclei sure by the sturdy pressure. The weak pressure doesn’t give rise to such sure programs, however is essential for a lot of pure phenomena that have an effect on our on a regular basis life. The fusion course of within the solar is initiated by hydrogen remodeling into heavy hydrogen by way of the weak pressure. With out the W boson the solar could be very dim. All atoms we all know are created from protons, neutrons and electrons but there are various totally different particles round. Why do they by no means make atoms or extra difficult constructions? As a result of they decay quickly as a result of weak pressure.
What makes the weak pressure so distinctive is the truth that the W boson can change the fees of different particles it interacts with. It might probably flip an electron (cost -1) right into a neutrino (cost 0), or a neutron (cost 0) right into a proton (cost +1). Strategies like carbon courting straight depend on this property. The sluggish decay of neutrons into protons in carbon isotopes lets us date archeological artefacts and could be unattainable with out the W boson.
Measuring the mass of the W boson straight is extraordinarily difficult.
A lot in order that my experimental colleagues dubbed it ‘the toughest measurement in high-energy physics’. There are a selection of causes that make it so troublesome:
First, W bosons are very heavy for elementary particles. A single W boson weighs 80 occasions as a lot as a proton. Accelerators are the one place on this planet the place the big power may be leveraged to supply them. However as soon as they’re produced, W bosons decay instantly and figuring out their mass requires measurements of the decay merchandise and reconstructing it from these measurements. To make issues worse, the colliders that produce W bosons inevitably additionally produce tons of and tons of of different particles. Determining whether or not a W boson has been produced and making a measurement of its mass is quite like discovering connecting puzzle items in an enormous field filled with them after which placing them collectively accurately – and a few are lacking and there’s a couple of approach they might match.
In distinction to many anomalies which have challenged the standing of the Commonplace Mannequin earlier than, it’s statistically virtually unattainable for the CDF consequence to be a fluke.
The CDF Collaboration has carried out this train with a dataset of 4 million W bosons collected from 2002 to 2011 on the Tevatron collider at Fermilab, about an hour exterior of Chicago. Weighing the W boson took the collaboration over a decade to finish. And the outcomes are spectacular. Not solely did CDF ship essentially the most exact measurement of the W mass each achieved, their consequence disagrees with all earlier measurements, placing it in direct rigidity with numerous experiments which have confirmed essentially the most profitable idea high-energy physicists has ever developed: the Commonplace Mannequin of Particle Physics. It can’t be overstated how exceptional that is: The CDF measurement is off by lower than one in a thousand, however the precision achieved is one in ten thousand!
This huge precision is the rationale why this measurement caught the eye of physicists worldwide. In distinction to many anomalies which have challenged the standing of the Commonplace Mannequin earlier than, it’s statistically virtually unattainable for the CDF consequence to be a fluke. But when it isn’t a fluke, what’s it then? Did the CDF experimentalists uncover a primary signal of latest physics of their information?
Whereas a single measurement not often establishes a brand new idea of physics, it will possibly determine constructions that might change the Commonplace Mannequin prediction.
Within the wake of the CDF announcement a twofold problem emerges: A complete evaluation of all of the instruments and strategies utilized to extract the W mass from the mountain of knowledge is underway. Experimentalists will probe time and again whether or not there’s any unaccounted uncertainty, any rationalization within the measurement course of for this virtually unattainable discrepancy.
Within the meantime, theoretical physicists are asking the query a couple of totally different, extra speculative origin. The construction of boson plenty is a prediction of the Commonplace Mannequin. This construction could be very inflexible. It’s not doable to easily change the mass of the W boson with out adjustments in different observables comparable to decays of heavy quarks and leptons, the mass measurement of the Z boson, and numerous different measurements which have confirmed the Commonplace Mannequin construction over and over . That is until there’s something else that adjustments this construction, one thing that has been missed by different experiments to this point, one thing new and sudden, manifesting itself within the collisions of the Tevatron collider.
Plenty of preliminary research present that the presence of one other Higgs boson, or novel heavy particles interacting with the W boson, as predicted in some theories of Darkish Matter, may each be chargeable for the discrepancy.
Theorists will rigorously scrutinize the Commonplace Mannequin calculations. It’s clear that if the CDF measurement stands, the Commonplace Mannequin alone won’t be sufficient to clarify all of our experimental information anymore. And whereas a single measurement not often establishes a brand new idea of physics, it will possibly determine constructions that might change the Commonplace Mannequin prediction.
Plenty of preliminary research present that the presence of one other Higgs boson or novel heavy particles interacting with the W boson, as they’re predicted in some theories of Darkish Matter, may each be chargeable for the discrepancy. If these new particles actually shift the W mass, they can be produced elsewhere. Guided by theoretical research, information from the Giant Hadron Collider (LHC) at Cern might be combed for indicators of those new particles and the outcomes will slender down doable explanations or produce additional hints concerning the nature of the underlying physics.The LHC collides with protons a file 7 occasions the power of the Tevatron with which CDF took their information. It has simply began its third run and is projected to gather extra information than the primary two runs mixed.
Even when these searches come up empty-handed, finally LHC information will enable for an equally or much more exact measurement of the W mass and ensure or contradict the CDF measurement.
The complexity of the ‘hardest measurement in high-energy physics’ makes this a really arduous puzzle to resolve. It will likely be with us for some time. However we physicists have lengthy given up the idea that we will all the time predict the place new physics might be discovered. Too typically have we been blindsided by sudden discoveries.
If we will say something with certainty proper now, it’s that continued funding for elementary analysis is indispensable. It took ten years to take the information and one other ten years for the evaluation and if we had minimize funding at any level the ‘hardest measurement in high-energy physics’ would by no means have been accomplished and what we’ll be taught from the spectacular measurement by CDF would have remained buried within the information.