particle physics – Page 4 – Of Particular Significance

A Few Remarks on the W Boson Mass Measurement

Based on some questions I received about yesterday’s post, I thought I’d add some additional comments this morning.

A natural and persistent question has been: “How likely do you think it is that this W boson mass result is wrong?” Obviously I can’t put a number on it, but I’d say the chance that it’s wrong is substantial. Why? This measurement, which took several many years of work, is probably among the most difficult ever performed in particle physics. Only first-rate physicists with complete dedication to the task could attempt it, carry it out, convince their many colleagues on the CDF experiment that they’d done it right, and get it through external peer review into Science magazine. But even first-rate physicists can get a measurement like this one wrong. The tiniest of subtle mistakes will undo it.

And that mistake, if there is one, might not even be their own, in a sense. Any measurement like this has to rely on other measurements, on simulation software, and on calculations involving other processes, and even though they’ve all been checked, perhaps they need to be rechecked.

Another question about the new measurement is that it seems inconsistent not only with the Standard Model but also with previous, less precise measurements by other experiments, which were closer to the Standard Model’s result. (It is even inconsistent with CDF’s own previous measurement.) That’s true, and you can see some evidence in the plot in yesterday’s post. But

it could be that one or more of the previous measurements has an error;
there is a known risk of unconscious experimental bias that tends to push results toward the Standard Model (i.e. if the result doesn’t match your expectation, you check everything again and tweak it and then stop when it better matches your expectation. Performing double-blinded experiments, as this one was, helps mitigate this risk, but it doesn’t entirely eliminate it.);
CDF has revised their old measurement slightly upward to account for things they learned while performing this new one, so their internal inconsistency is less than it appears, and
even if the truth lies between this new measurement and the old ones, that would still leave a big discrepancy with the Standard Model, and the implication for science would be much the same.

I’ve heard some cynicism: “Is this just an old experiment trying to make a name for itself and get headlines?” Don’t be absurd. No one seeking publicity would go through the hell of working on one project for several years, running down every loose end multiple times and checking it twice and cross-checking it three times, spending every living hour asking oneself “what did I forget to check?”, all while knowing that in the end one’s reputation will be at stake when the final result hits the international press. There would be far easier ways to grab headlines if that were the goal.

Someone wisely asked about the Z boson mass; can one study it as well? This is a great question, because it goes to the heart of how the Standard Model is checked for consistency. The answer is “no.” Really, when we say that “the W mass is too large,” what we mean (roughly) is that “the ratio of the W mass to the Z mass is too large.” One way to view it (not exactly right) is that certain extremely precise measurements have to be taken as inputs to the Standard Model, and once that is done, the Standard Model can be used to make predictions of other precise measurements. Because of the precision with which the Z boson mass can be measured (to 2 MeV, two parts in 100,000), it is effectively taken as an input to the Standard Model, and so we can’t then compare it against a prediction. (The Z boson mass measurement is much easier, because a Z boson can decay (for example) to an electron and a positron, which can both be observed directly. Meanwhile a W boson can only decay (for example) to an electron and a neutrino, but a neutrino can only be inferred indirectly, making determination of its energy and momentum much less precise.)

In fact, one of the ways that the experimenters at CDF who carried out this measurement checked their methods is that they remeasured the Z boson mass too, and it came out to agree with other, even more precise measurements. They’d never have convinced themselves, or any of us, that they could get the W boson mass right if the Z boson mass measurement was off. So we can even interpret the CDF result as a measurement of the ratio of the W boson mass to the Z boson mass.

One last thing for today: once you have measured the Z boson mass and a few other things precisely, it is the consistency of the top quark mass, the Higgs boson mass and the W boson mass that provide one of the key tests of the Standard Model. Because of this, my headline from yesterday (“The W Boson isn’t Behaving”) is somewhat misleading. The cause of the discrepancy may not involve the W boson at all. The issue might turn out to be a new effect on the Z boson, for instance, or perhaps even the top quark. Working that out is the purview of theoretical physicists, who have to understand the complex interplay between the various precise measurements of masses and interactions of the Standard Model’s particles, and the many direct (and so far futile) searches for unknown types of particles that could potentially shift those masses and interactions. This isn’t easy, and there are lots of possibilities to consider, so there’s a lot of work yet to be done.

POSTED BY Matt Strassler

ON April 8, 2022

The W Boson Isn’t Behaving

The mass of the W boson, one of the fundamental particles within the Standard Model of particle physics, is apparently not what the Higgs boson, top quark, and the rest of the Standard Model say it should be. Such is the claim from the CDF experiment, from the long-ago-closed but not forgotten Tevatron. Analysis of their old data, carried out with extreme care, and including both more data and improved techniques, calibrations, and modeling, has led to the conclusion that the W boson mass is off by 1/10 of one percent (by about 80 MeV/c² out of about 80,400 MeV/c²). That may not sound like much, but it’s seven times larger than what is believed to be the accuracy of the theoretical calculation.

New CDF Result: 80,443.5 ± 9.4 MeV/c²
SM Calculation: 80,357± 4 [inputs]± 4[theory] MeV/c²

*The new measurement of the W mass and its uncertainty (bottom point) versus previous ones, and the current Standard Model prediction (grey band.)*

What could cause this discrepancy of 7 standard deviations (7 “sigma”), far above the criteria for a discovery? Unfortunately we must always consider the possibility of an error. But let’s set that aside for today. (And we should expect the experiments at the Large Hadron Collider to weigh in over time with their own better measurements, not quite as good as this one but still good enough to test its plausibility.)

A shift in the W boson mass could occur through a wide variety of possible effects. If you add new fields (and their particles) to the Standard Model, the interactions between the Standard Model particles and the new fields will induce small indirect effects, including tiny shifts in the various masses. That, in turn, will cause the relation between the W boson mass, top quark mass, and Higgs boson mass to come into conflict with what the Standard Model predicts. So there are lots of possibilities. Many of these possible new particles would have been seen already at the Large Hadron Collider, or affected other experiments, and so are ruled out. But this is clearly not true in all cases, especially if one is conservative in interpreting the new result. Theorists will be busy even now trying to figure out which possibilities are still allowed.

It will be quite some time before the experimental and theoretical dust settles. The implications are not yet obvious and they depend on the degree to which we trust the details. Even if this discrepancy is real, it still might be quite a bit smaller than CDF’s result implies, due to statistical flukes or small errors. [After all, if someone tells you they find a 7 sigma deviation from expectation, that would be statistically compatible with the truth being only a 4 or 5 sigma deviation.] I expect many papers over the coming days and weeks trying to make sense of not only this deviation but one or more of the other ones that are hanging about (such as this one.)

Clearly this will require follow-up posts.

Note added: To give you a sense of just how difficult this measurement is, please see this discussion by someone who knows much more about the nitty-gritty than a theorist like me ever could.

POSTED BY Matt Strassler

ON April 7, 2022

A Prediction from String Theory

(An advanced particle physics topic today…)

There have been various intellectual wars over string theory since before I was a graduate student. (Many people in my generation got caught in the crossfire.) But I’ve always taken the point of view that string theory is first and foremost a tool for understanding the universe, and it should be applied just like any other tool: as best as one can, to the widest variety of situations in which it is applicable.

And it is a powerful tool, one that most certainly makes experimental predictions… even ones for the Large Hadron Collider (LHC).

These predictions have nothing to do with whether string theory will someday turn out to be the “theory of everything.” (That’s a grandiose term that means something far less grand, namely a “complete set of equations that captures the behavior of spacetime and all its types of particles and fields,” or something like that; it’s certainly not a theory of biology or economics, or even of semiconductors or proteins.) Such a theory would, presumably, resolve the conceptual divide between quantum physics and general relativity, Einstein’s theory of gravity, and explain a number of other features of the world. But to focus only on this possible application of string theory is to take an unjustifiably narrow view of its value and role.

The issue for today involves the behavior of particles in an unfamiliar context, one which might someday show up (or may already have shown up and been missed) at the LHC or elsewhere. It’s a context that, until 1998 or so, no one had ever thought to ask about, and even if someone had, they’d have been stymied because traditional methods are useless. But then string theory drew our attention to this regime, and showed us that it has unusual features. There are entirely unexpected phenomena that occur there, ones that we can look for in experiments.

(more…)

POSTED BY Matt Strassler

ON March 20, 2022

Has a New Force of Nature Been Discovered?

There have been dramatic articles in the news media suggesting that a Nobel Prize has essentially already been awarded for the amazing discovery of a “fifth force.” I thought I’d better throw some cold water on that fire; it’s fine for it to smoulder, but we shouldn’t let it overheat.

There could certainly be as-yet unknown forces waiting to be discovered — dozens of them, perhaps. So far, there are four well-studied forces: gravity, electricity/magnetism, the strong nuclear force, and the weak nuclear force. Moreover, scientists are already fully confident there is a fifth force, predicted but not yet measured, that is generated by the Higgs field. So the current story would really be about a sixth force.

Roughly speaking, any new force comes with at least one new particle. That’s because

every force arises from a type of field (for instance, the electric force comes from the electromagnetic field, and the predicted Higgs force comes from the Higgs field)
and ripples in that type of field are a type of particle (for instance, a minimal ripple in the electromagnetic field is a photon — a particle of light — and a minimal ripple in the Higgs field is the particle known as the Higgs boson.)

The current excitement, such as it is, arises because someone claims to have evidence for a new particle, whose properties would imply a previously unknown force exists in nature. The force itself has not been looked for, much less discovered.

The new particle, if it really exists, would have a rest mass about 34 times larger than that of an electron — about 1/50th of a proton’s rest mass. In technical terms that means its E=mc² energy is about 17 million electron volts (MeV), and that’s why physicists are referring to it as the X17. But the question is whether the two experiments that find evidence for it are correct. (more…)

POSTED BY Matt Strassler

ON November 25, 2019

The New York Times Remembers A Great Physicist

The untimely and sudden deaths of Steve Gubser and Ann Nelson, two of the United States’ greatest talents in the theoretical physics of particles, fields and strings, has cast a pall over my summer and that of many of my colleagues.

I have not been finding it easy to write a proper memorial post for Ann, who was by turns my teacher, mentor, co-author, and faculty colleague. I would hope to convey to those who never met her what an extraordinary scientist and person she was, but my spotty memory banks aren’t helping. Eventually I’ll get it done, I’m sure.

(Meanwhile I am afraid I cannot write something similar for Steve, as I really didn’t know him all that well. I hope someone who knew him better will write about his astonishing capabilities and his unique personality, and I’d be more than happy to link to it from here.)

In this context, I’m gratified to see that the New York Times has given Ann a substantive obituary, https://www.nytimes.com/2019/08/26/science/ann-nelson-dies.html, and appearing in the August 28th print edition, I’m told. It contains a striking (but, to those of us who knew her, not surprising) quotation from Howard Georgi. Georgi is a professor at Harvard who is justifiably famous as the co-inventor, with Nobel-winner Sheldon Glashow, of Grand Unified Theories (in which the electromagnetic, weak nuclear, and strong nuclear force all emerge from a single force.) He describes Ann, his former student, as being able to best him at his own game.

“I have had many fabulous students who are better than I am at many things. Ann was the only student I ever had who was better than I am at what I do best, and I learned more from her than she learned from me.”

He’s being a little modest, perhaps. But not much. There’s no question that Ann was an all-star.

And for that reason, I do have to complain about one thing in the Times obituary. It says “Dr. Nelson stood out in the world of physics not only because she was a woman, but also because of her brilliance.”

Really, NYTimes, really?!?

Any scientist who knew Ann would have said this instead: that Professor Nelson stood out in the world of physics for exceptional brilliance — lightning-fast, sharp, creative and careful, in the same league as humanity’s finest thinkers — and for remarkable character — kind, thoughtful, even-keeled, rigorous, funny, quirky, dogged, supportive, generous. Like most of us, Professor Nelson had a gender, too, which was female. There are dozens of female theoretical physicists in the United States; they are a too-small minority, but they aren’t rare. By contrast, a physicist and person like Ann Nelson, of any gender? They are extremely few in number across the entire planet, and they certainly do stand out.

But with that off my chest, I have no other complaints. (Well, admittedly the physics in the obit is rather garbled, but we can get that straight another time.) Mainly I am grateful that the Times gave Ann fitting public recognition, something that she did not actively seek in life. Her death is an enormous loss for theoretical physics, for many theoretical physicists, and of course for many other people. I join all my colleagues in extending my condolences to her husband, our friend and colleague David B. Kaplan, and to the rest of her family.

POSTED BY Matt Strassler

ON August 28, 2019

A Catastrophic Weekend for Theoretical High Energy Physics

It is beyond belief that not only am I again writing a post about the premature death of a colleague whom I have known for decades, but that I am doing it about two of them.

Over the past weekend, two of the world’s most influential and brilliant theoretical high-energy physicists — Steve Gubser of Princeton University and Ann Nelson of the University of Washington — fell to their deaths in separate mountain accidents, one in the Alps and one in the Cascades.

Theoretical high energy physics is a small community, and within the United States itself the community is tiny. Ann and Steve were both justifiably famous and highly respected as exceptionally bright lights in their areas of research. Even for those who had not met them personally, this is a stunning and irreplaceable loss of talent and of knowledge.

But most of us did know them personally. For me, and for others with a personal connection to them, the news is devastating and tragic. I encountered Steve when he was a student and I was a postdoc in the Princeton area, and later helped bring him into a social group where he met his future wife (a great scientist in her own right, and a friend of mine going back decades). As for Ann, she was one of my teachers at Stanford in graduate school, then my senior colleague on four long scientific papers, and then my colleague (along with her husband David B. Kaplan) for five years at the University of Washington, where she had the office next to mine. I cannot express what a privilege it always was to work with her, learn from her, and laugh with her.

I don’t have the heart or energy right now to write more about this, but I will try to do so at a later time. Right now I join their spouses and families, and my colleagues, in mourning.

POSTED BY Matt Strassler

ON August 6, 2019

A Broad Search for Fast Hidden Particles

A few days ago I wrote a quick summary of a project that we just completed (and you may find it helpful to read that post first). In this project, we looked for new particles at the Large Hadron Collider (LHC) in a novel way, in two senses. Today I’m going to explain what we did, why we did it, and what was unconventional about our search strategy.

The first half of this post will be appropriate for any reader who has been following particle physics as a spectator sport, or in some similar vein. In the second half, I’ll add some comments for my expert colleagues that may be useful in understanding and appreciating some of our results. [If you just want to read the comments for experts, jump here.]

Why did we do this?

Motivation first. Why, as theorists, would we attempt to take on the role of our experimental colleagues — to try on our own to analyze the extremely complex and challenging data from the LHC? We’re by no means experts in data analysis, and we were very slow at it. And on top of that, we only had access to 1% of the data that CMS has collected. Isn’t it obvious that there is no chance whatsoever of finding something new with just 1% of the data, since the experimenters have had years to look through much larger data sets? (more…)

POSTED BY Matt Strassler

ON February 19, 2019

Breaking a Little New Ground at the Large Hadron Collider

Today, a small but intrepid band of theoretical particle physicists (professor Jesse Thaler of MIT, postdocs Yotam Soreq and Wei Xue of CERN, Harvard Ph.D. student Cari Cesarotti, and myself) put out a paper that is unconventional in two senses. First, we looked for new particles at the Large Hadron Collider in a way that hasn’t been done before, at least in public. And second, we looked for new particles at the Large Hadron Collider in a way that hasn’t been done before, at least in public.

And no, there’s no error in the previous paragraph.

1) We used a small amount of actual data from the CMS experiment, even though we’re not ourselves members of the CMS experiment, to do a search for a new particle. Both ATLAS and CMS, the two large multipurpose experimental detectors at the Large Hadron Collider [LHC], have made a small fraction of their proton-proton collision data public, through a website called the CERN Open Data Portal. Some experts, including my co-authors Thaler, Xue and their colleagues, have used this data (and the simulations that accompany it) to do a variety of important studies involving known particles and their properties. [Here’s a blog post by Thaler concerning Open Data and its importance from his perspective.] But our new study is the first to look for signs of a new particle in this public data. While our chances of finding anything were low, we had a larger goal: to see whether Open Data could be used for such searches. We hope our paper provides some evidence that Open Data offers a reasonable path for preserving priceless LHC data, allowing it to be used as an archive by physicists of the post-LHC era.

2) Since only had a tiny fraction of CMS’s data was available to us, about 1% by some count, how could we have done anything useful compared to what the LHC experts have already done? Well, that’s why we examined the data in a slightly unconventional way (one of several methods that I’ve advocated for many years, but has not been used in any public study). Consequently it allowed us to explore some ground that no one had yet swept clean, and even have a tiny chance of an actual discovery! But the larger scientific goal, absent a discovery, was to prove the value of this unconventional strategy, in hopes that the experts at CMS and ATLAS will use it (and others like it) in future. Their chance of discovering something new, using their full data set, is vastly greater than ours ever was.

Now don’t all go rushing off to download and analyze terabytes of CMS Open Data; you’d better know what you’re getting into first. It’s worthwhile, but it’s not easy going. LHC data is extremely complicated, and until this project I’ve always been skeptical that it could be released in a form that anyone outside the experimental collaborations could use. Downloading the data and turning it into a manageable form is itself a major task. Then, while studying it, there are an enormous number of mistakes that you can make (and we made quite a few of them) and you’d better know how to make lots of cross-checks to find your mistakes (which, fortunately, we did know; we hope we found all of them!) The CMS personnel in charge of the Open Data project were enormously helpful to us, and we’re very grateful to them; but since the project is new, there were inevitable wrinkles which had to be worked around. And you’d better have some friends among the experimentalists who can give you advice when you get stuck, or point out aspects of your results that don’t look quite right. [Our thanks to them!]

All in all, this project took us two years! Well, honestly, it should have taken half that time — but it couldn’t have taken much less than that, with all we had to learn. So trying to use Open Data from an LHC experiment is not something you do in your idle free time.

Nevertheless, I feel it was worth it. At a personal level, I learned a great deal more about how experimental analyses are carried out at CMS, and by extension, at the LHC more generally. And more importantly, we were able to show what we’d hoped to show: that there are still tremendous opportunities for discovery at the LHC, through the use of (even slightly) unconventional model-independent analyses. It’s a big world to explore, and we took only a small step in the easiest direction, but perhaps our efforts will encourage others to take bigger and more challenging ones.

For those readers with greater interest in our work, I’ll put out more details in two blog posts over the next few days: one about what we looked for and how, and one about our views regarding the value of open data from the LHC, not only for our project but for the field of particle physics as a whole.

POSTED BY Matt Strassler

ON February 13, 2019

Pop went the Weasel, but Vroom goes the LHC

At the end of April, as reported hysterically in the press, the Large Hadron Collider was shut down and set back an entire week by a “fouine”, an animal famous for chewing through wires in cars, and apparently in colliders too. What a rotten little weasel! especially for its skill in managing to get the English-language press to blame the wrong species — a fouine is actually a beech marten, not a weasel, and I’m told it goes Bzzzt, not Pop. But who’s counting?

Particle physicists are counting. Last week the particle accelerator operated so well that it generated almost half as many collisions as were produced in 2015 (from July til the end of November), bringing the 2016 total to about three-fourths of 2015.

The key question is how many of the next few weeks will be like this past one. We’d be happy with three out of five, even two. If the amount of 2016 data can significantly exceed that of 2015 by July 15th, as now seems likely, a definitive answer to the question on everyone’s mind (namely, what is the bump on that plot?!? a new particle? or just a statistical fluke?) might be available at the time of the early August ICHEP conference.

So it’s looking more likely that we’re going to have an interesting August… though it’s not at all clear yet whether we’ll get great news (in which case we get no summer vacation), bad news (in which case we’ll all need a vacation), or ambiguous news (in which case we wait a few additional months for yet more news.)

POSTED BY Matt Strassler

ON June 9, 2016

Tag: particle physics

The Simplest Way to Shift the W Boson Mass?

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A Few Remarks on the W Boson Mass Measurement

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The W Boson Isn’t Behaving

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A Prediction from String Theory

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Has a New Force of Nature Been Discovered?

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The New York Times Remembers A Great Physicist

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A Catastrophic Weekend for Theoretical High Energy Physics

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A Broad Search for Fast Hidden Particles

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Breaking a Little New Ground at the Large Hadron Collider

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Pop went the Weasel, but Vroom goes the LHC

POSTED BY Matt Strassler

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