Physicists detect process even rarer than the long-sought Higgs particle

Jul 15, 2014 by Karen Mcnulty Walsh
Candidate event for WW → WW scattering in the ATLAS detector at the Large Hadron Collider.

(Phys.org) —Scientists running the ATLAS experiment at the Large Hadron Collider (LHC), the world's largest and most powerful "atom smasher," report the first evidence of a process that can be used to test the mechanism by which the recently discovered Higgs particle imparts mass to other fundamental particles. More rare than the production of the Higgs itself, this process—a scattering of two same-charged particles called W bosons off one another—also provides a new stringent test of the Standard Model of particle physics. The findings, which so far are in agreement with predictions of the Standard Model, are reported in a paper just accepted by Physical Review Letters.

"Only about one in 100 trillion proton-proton collisions would produce one of these events," said Marc-André Pleier, a physicist at the U.S. Department of Energy's Brookhaven National Laboratory who played a leadership role in the analysis of this result for the ATLAS collaboration. Complicating matters further, finding one such rare event is not enough. "You need to observe many to see if the production rate is above or on par with predictions," Pleier said. "We looked through billions of proton-proton collisions produced at the LHC for a signature of these events—decay products that allow us to infer like Sherlock Holmes what happened in the event."

The analysis efforts started two years ago and were carried out in particular by groups from Brookhaven, Lawrence Berkeley National Laboratory, University of Michigan, and Technische Universität Dresden, Germany. Preliminary results were presented by Pleier at the "Rencontres de Moriond – QCD and High Energy Interactions" conference in March 2014. Now finalized based on a total of 34 observed events, the measured interaction rate is in good agreement with that predicted by the Standard Model, a theory describing all known and their interactions.

"The Standard Model has so far survived all tests, but we know that it is incomplete because there are observations of dark matter, dark energy, and the antimatter/matter asymmetry in the universe that can't be explained by the Standard Model," Pleier said. So physicists are always looking for new ways to test the theory, to find where and how it might break down.

Brookhaven Lab/ATLAS physicist Marc-André Pleier adjusting detector components.

"This process of W boson interactions is one we could never test," Pleier said, "because we didn't have enough energy or large enough data sets needed to see this very rare process—until we built the LHC."

Now with the LHC data in hand, the measured rate agrees with the prevailing theory's predictions and establishes a signal at a significance level of 3.6 sigma—strong evidence, according to Pleier. "The probability for this measurement to be a mere background fluctuation is very small—about one in 6000," he said. But the physicists would like to be more certain by collecting more data to reduce uncertainties and increase the level of significance.

There's another reason for continuing the quest: "By measuring this process we can check whether the Higgs particle we discovered does its job the way we expect it to," Pleier said. "A wealth of models in addition to the Higgs mechanism exists to try to explain how fundamental particles get their mass. Measurements of such scattering processes can thus be both a fundamental test of the Standard Model and a window to new physics."

To test the Higgs mechanism, the scientists compare distributions of decay products of the W scattering process—how often they observe particular products at a particular energy and geometrical configuration.

"It's like a fingerprint," Pleier said."We have a predicted fingerprint and we have the fingerprint we measure. If the fingerprints match, we know that the Higgs does its job of mass generation the way it should. But if it deviates, we know that some other physics mechanism is helping out because the Higgs alone is not doing what we expect."

Again, so far, the data indicate that the Higgs is working as expected.

"For the first time, we can rule out certain models or predictions that we could not before," Pleier said. "To complete the job, we need more data, at higher energy, so we can see the fingerprint more clearly."

The LHC will resume data taking at increased collision energies—13 tera-electronvolts (TeV) instead of 8 TeV—in spring of 2015. The datasets collected will be up to 150 times the size of the currently available data and will allow for a detailed behind-the-scenes look at the Higgs at work.

Explore further: Evidence found for the Higgs boson direct decay into fermions

More information: Scientific paper: "Evidence for Electroweak Production of W±W±jj in pp Collisions at √s = 8 TeV with the ATLAS Detector": arxiv.org/abs/1405.6241

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indio007
1.9 / 5 (24) Jul 15, 2014
"The Standard Model has so far survived all tests, but we know that it is incomplete because there are observations of dark matter, dark energy, and the antimatter/matter asymmetry in the universe that can't be explained by the Standard Model,

So it has survived all tests except observation?
George_Rajna
Jul 15, 2014
This comment has been removed by a moderator.
Torbjorn_Larsson_OM
4.5 / 5 (19) Jul 15, 2014
@indio007: Of course not, which is why Pleier said "incomplete". The SM doesn't predict any of that, so it isn't affected by their observation. It is the same as when you can't use the existence of effective gravity to say that quantum theory has failed, it isn't supposed to predict classical types of theory.

Read the article closer: "If the fingerprints match, we know that the Higgs does its job of mass generation the way it should. But if it deviates, we know that some other physics mechanism is helping out because the Higgs alone is not doing what we expect."

Again, so far, the data indicate that the Higgs is working as expected."
indio007
2.3 / 5 (20) Jul 15, 2014
@Torbjorn_Larsson_OM

Oh I love when people use "fingerprints" as an analogy. Are you aware there is one scientific study of fingerprint uniqueness? It was done by DOJ and promptly classified.
There are people with matching fingerprints but the meme continues to replicate like all well constructed mind viruses.

Anyhow, dark matter has never been observed, dark energy has never been observed.

There is a theoretical model. The model doesn't work without adding force effects and claiming what's the cause is unseen.

Occam's razor says the theoretical model is wrong.
vinlud
4.7 / 5 (19) Jul 15, 2014
@indio007

Dark matter not observed? https://en.wikipe...evidence
That doesn't mean we know what it is though, but its existence is not predicted by the SM, hence why we know it is incomplete.
Occam's razor says you're not really trying to understand this stuff
arom
1.2 / 5 (18) Jul 15, 2014
There's another reason for continuing the quest: "By measuring this process we can check whether the Higgs particle we discovered does its job the way we expect it to," Pleier said. "A wealth of models in addition to the Higgs mechanism exists to try to explain how fundamental particles get their mass. Measurements of such scattering processes can thus be both a fundamental test of the Standard Model and a window to new physics."
To test the Higgs mechanism, the scientists compare distributions of decay products of the W scattering process—how often they observe particular products at a particular energy and geometrical configuration.

Unfortunately the problem is no conventional explanation how the Higgs mechanism works and how fundamental particles get their mass; maybe this idea could help us to visualize ….
http://www.vacuum...=9〈=en
theon
1.3 / 5 (12) Jul 16, 2014
Horrible post. Very late we are informed what it is about: Higgs to two W's. Before that we have to swallow a lot of mambo jambo, including the nearly ruled out and otherwise impossible option of dark matter production (by WIMPs).
indio007
1.2 / 5 (17) Jul 16, 2014
@indio007

Dark matter not observed? https://en.wikipe...evidence
That doesn't mean we know what it is though, but its existence is not predicted by the SM, hence why we know it is incomplete.
Occam's razor says you're not really trying to understand this stuff


Dark matter has ZERO direct observations. If dark matter was observed they would change the name.
Please don't cite wikipedia crap to me.

DM is a band aid (more like emergency surgery) for general relativity's failure.

People are trying to prove DM bullshit with gravitational lensing bullshit.
It's elementary to prove gravitational lensing is nonsense using the Law of Gauss.
Mike_Massen
5 / 5 (14) Jul 16, 2014
indio007 relies on a simplistic narrow view with this blurt
Occam's razor says the theoretical model is wrong.
It seems you havent noticed that Occam's razor has NO metric, No discipline, NO definitive theorem, NO quantitative aspect, can & has been applied arbitrarily.

Just because a theory is complex does not automatically mean its wrong - & just because many people cannot conceive of managing complexity well does not mean it should not be pursued. Feeble intellectual processes are the basis of the need for an Occam's razor type pattern for over-simplification & rejection, it is lazy & inappropriate thinking indeed !

The so called Standard Model has observational/evidentiary support, clearly the process is asymptotic & obviously the fundamental nature of this process is probabilistic & at many levels.

To attempt determinism as you have indio007 lurched into is at odds with all that is known already of the value of the scientific process, arbitrary dismissal is not smart.
theon
1 / 5 (12) Jul 16, 2014
Forgot to say: already the title of this post is pretty disappointing, "see how learned we scientists are". Yes, if you study one decay channel of the Higgs, this will even be rarer than its production rate. Could any layman have expected something else?
George_Rajna
Jul 16, 2014
This comment has been removed by a moderator.
PhotonX
5 / 5 (9) Jul 17, 2014
Oh I love when people use "fingerprints" as an analogy. Are you aware there is one scientific study of fingerprint uniqueness? It was done by DOJ and promptly classified. There are people with matching fingerprints but the meme continues to replicate like all well constructed mind viruses.
Oh, please. As Robert Sheckley wrote "Don't pick at the metaphor, it leaves a nasty scab." Likewise, it's absurd to attack the word 'fingerprint' as though you've scored an intellectual point. The word 'profile' could replace 'fingerprint' in that usage--would you then be crowing that not every profile is unique? The point was that there was a distinct signature that was predicted, and the observation matched that signature, and you go off about classified fingerprint reports that may or may not actually exist as though that's pertinent to the article.
.
At another site there's a bonehead who insists telepathy is real, except that the government swoops down and classifies any scientific study that supports it. How is that relevant? It isn't, and that's the point.
indio007
1 / 5 (7) Jul 17, 2014
@PhotonX, I guess you didn't see the irony of using an unobserved assertion (absolute uniqueness of fingerprints) used as logical support. It reaks of desperation.
There are soooo many unseen unproven and straight up fraudulent items supporting modern physics, everything is suspect.

Your last paragraph is apples to oranges. WTH does what some guy on some website making unsupported claims that any telepathy studies are classified by the government have to do with fingerprint's "common knowledge" that has been held by the vast majority of the population for nearly 100 years?

Your example is a red herring.

Johnpaily
1 / 5 (3) Jul 18, 2014
We need to understand nature and universe not through multibillion dollar accelerators and powerful telescopes but IMAGINATION. Einstein said imagination is important. To understand nature we need to imagine the "principle and design" working behind quantum particle that gives it the property of spin and displacement in space and its capacity to form into atoms and higher structures to the whole universe. We need to imagine how space and time manifest out of these quantum particles. http://www.scribd...nologies
Dr_toad
Jul 18, 2014
This comment has been removed by a moderator.
rockwolf1000
5 / 5 (5) Jul 19, 2014
We need to understand nature and universe not through multibillion dollar accelerators and powerful telescopes but IMAGINATION. Einstein said imagination is important. To understand nature we need to imagine the "principle and design" working behind quantum particle that gives it the property of spin and displacement in space and its capacity to form into atoms and higher structures to the whole universe. We need to imagine how space and time manifest out of these quantum particles. http://www.scribd...nologies


What do you think is behind those "multi billion dollar accelerators and powerful telescopes"?

Imagination.

Vision.

Followed by physics and planning. And more planning.

All requiring imagination and vision.
Nowadays it costs a lot of $$$$ to validate one's imagination. The low hanging fruit has been harvested already.
ShotmanMaslo
3.7 / 5 (3) Jul 19, 2014
There is a theoretical model. The model doesn't work without adding force effects and claiming what's the cause is unseen.

Occam's razor says the theoretical model is wrong.


No, the opposite si true.

Dark matter is the simple explanation favored by Occams razor, because it just means adding yet another particle to the mix, the existence of which is nothing unexpected or extraordinary and in fact predicted by supersymmetry. No new assumptions are neccessary, its just more of the same.

On the other hand, contrived modified gravity theories tend to be disfavored by Occams razor.
DoieaS
1 / 5 (2) Jul 19, 2014
Dark matter is the simple explanation favored by Occams razor, because it just means adding yet another particle to the mix
From your perspective even the epicycle model was favored by Occams razor, because it just required to add yet another epicycle to the mix, when a new planet has been observed. Whereas the problem of this approach with Occams razor is right in the necessity to ADD new component into existing model. Your reply just illustrates, that the proponents of mainstream physics don't actually understand the principles of scientific method and during time they managed to turn it on its head.
Mike_Massen
not rated yet Jul 20, 2014
Don't u guys going on about Occam's razor understand:-

It seems you havent noticed that Occam's razor has NO metric, NO discipline, NO definitive theorem, NO quantitative aspect, can & has been applied arbitrarily.
MaleMatters
1 / 5 (1) Jul 21, 2014
The Higgs particle imparts mass to other particles. At cern.com, it says the Higgs particle has a mass of 126 GeV.

Where does the Higgs particle get its mass?