{"id":3980,"date":"2016-08-18T18:33:54","date_gmt":"2016-08-18T18:33:54","guid":{"rendered":"http:\/\/www.particlebites.com\/?p=3980"},"modified":"2016-08-18T18:33:54","modified_gmt":"2016-08-18T18:33:54","slug":"what-is-model-building","status":"publish","type":"post","link":"https:\/\/www.particlebites.com\/?p=3980","title":{"rendered":"What is “Model Building”?"},"content":{"rendered":"

One thing that makes physics, and especially particle\u00a0physics, is unique in the sciences is\u00a0the split\u00a0between theory and experiment. The role of experimentalists is clear: they build and conduct experiments, take data and analyze\u00a0it\u00a0using\u00a0mathematical, statistical, and numerical techniques to separate signal from background. In short, they seem to do all of the real<\/em> science!<\/p>\n

So what is it that theorists do, besides sipping espresso and scribbling on chalk boards? In\u00a0this post we describe one type\u00a0of theoretical\u00a0work called model building<\/strong>.\u00a0This usually falls under the umbrella of phenomenology, which in physics refers to\u00a0making connections between mathematically defined theories (or models) of nature\u00a0and actual experimental observations of nature.<\/p>\n

One common scenario is that one experiment observes something unusual: an\u00a0anomaly<\/strong>.\u00a0Two things immediately happen:<\/p>\n

    \n
  1. Other experiments find ways to\u00a0cross-check\u00a0to see\u00a0if they can confirm the anomaly.<\/li>\n
  2. Theorists\u00a0start\u00a0figure out the broader implications if the anomaly is real.<\/li>\n<\/ol>\n

    #1 is the\u00a0key step in the scientific method, but in this post we’ll illuminate what #2 actually entails. The scenario\u00a0looks a little like this:<\/p>\n

    \"An<\/a>
    An unusual experimental result (anomaly) is observed. One thing we would like to know is whether it\u00a0is\u00a0consistent with other experimental observations,\u00a0but\u00a0these other observations may not\u00a0be simply\u00a0related to the anomaly.<\/figcaption><\/figure>\n

    Theorists, who have spent\u00a0plenty of time mulling over the open questions in physics, are\u00a0ready to apply their favorite models of new physics to see if they fit. These are the models that they know lead to elegant mathematical results, like grand unification or a solution\u00a0to the Hierarchy problem<\/a>. Sometimes theorists are more utilitarian, and start with “do it all” Swiss army knife theories called effective theories<\/em> (or simplified models<\/em>) and see if they can explain the anomaly in the context of existing constraints.<\/p>\n

    Here’s what usually happens:<\/p>\n

    \"Usually<\/a>
    Usually the nicest models of new physics don’t fit! In the explicit example, the minimal supersymmetric Standard Model doesn’t include\u00a0a good candidate\u00a0to explain\u00a0the 750 GeV diphoton bump.<\/figcaption><\/figure>\n

    Indeed, usually one\u00a0needs to\u00a0get creative and\u00a0modify the nice-and-elegant theory to make sure it can explain the anomaly while avoiding other experimental constraints. This makes the theory a little less elegant, but sometimes nature isn’t elegant.<\/p>\n

    <\/a>
    Candidate theory\u00a0extended with a module (in this case, an additional particle). This additional model is “bolted on” to the theory to make it fit\u00a0the experimental observations.<\/figcaption><\/figure>\n

    Now we’re feeling pretty good about ourselves.\u00a0It can take quite a bit of work to\u00a0hack the\u00a0well-motivated original theory in a way that both explains the anomaly and avoids\u00a0all other known experimental observations.\u00a0A good theory can do\u00a0a couple of other things:<\/p>\n

      \n
    1. It\u00a0points the way to\u00a0future experiments that can\u00a0test it.<\/li>\n
    2. It can use the additional structure to explain other anomalies.<\/li>\n<\/ol>\n

      The picture for #2 is as follows:<\/p>\n

      \"A<\/a>
      A good hack to a theory can explain multiple anomalies. Sometimes that makes the hack\u00a0a little more cumbersome. Physicists often develop their own sense of ‘taste’ for when a module\u00a0is elegant enough.<\/figcaption><\/figure>\n

      Even at this stage, there can be a lot of really neat physics to be learned. Model-builders can develop a reputation for particularly clever, minimal, or inspired\u00a0modules.\u00a0If a\u00a0module is really successful, then people will start to think about\u00a0it as\u00a0part of a pre-packaged deal:<\/p>\n

      \"A<\/a>
      A really successful hack may eventually\u00a0be thought of as it’s own variant of the original theory.<\/figcaption><\/figure>\n

      Model-smithing\u00a0is a\u00a0craft\u00a0that blends\u00a0together a lot of the fun of understanding how\u00a0physics works—which bits of common wisdom can be bent or broken to\u00a0accommodate an unexpected experimental result? Is it possible to find a simpler theory\u00a0that can explain more observations?\u00a0Are the observations pointing to an\u00a0even deeper guiding principle?<\/p>\n

      Of course—we should also say that sometimes, while theorists are having fun developing their favorite models,\u00a0other experimentalists have gone on to refute the original anomaly.<\/p>\n

      \"Sometimes<\/a>
      Sometimes anomalies go away and the models built to explain them don’t hold together.<\/figcaption><\/figure>\n

      But here’s the mark of a really, really good model: even if the anomaly goes away and the particular\u00a0model\u00a0falls out of favor, a good model will have taught other\u00a0physicists something really neat about what can be done within the a given theoretical framework. Physicists get\u00a0a feel for the kinds of modules that are out in the market (like an app store) and they develop a library of\u00a0tricks to attack\u00a0future anomalies. And if one is really fortunate,\u00a0these insights can point the way to even bigger connections between physical principles.<\/p>\n

      I cannot help but end this post without one of my favorite physics jokes, courtesy of T. Tait:<\/p>\n

      \n

      A theorist and an experimentalist are having coffee. The theorist is really excited, she tells the experimentalist, “I’ve got it—it’s a model\u00a0that’s\u00a0elegant, explains everything, and it’s completely predictive.”<\/p>\n

      The experimentalist listens to\u00a0her colleague’s idea and realizes\u00a0how to test those predictions. She\u00a0writes several grant applications, hires a team of postdocs and graduate students, trains them, \u00a0and builds the new experiment. After years of design, labor, and testing, the machine is ready to take data. They run for several months, and the experimentalist pores over the results.<\/p>\n

      The experimentalist\u00a0knocks on the theorist’s\u00a0door the next day and says, “I’m sorry—the experiment doesn’t find what you were predicting. The theory is dead.”<\/p>\n

      The theorist frowns a bit: “What a shame. Did you know I spent three whole weeks of my life<\/em> writing that paper?”<\/p>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

      So what is it that theorists do, besides sipping espresso and scribbling on chalk boards? In this post we describe one type of theoretical work called model building. <\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[47],"tags":[],"class_list":["post-3980","post","type-post","status-publish","format-standard","hentry","category-beyond-standard-model"],"_links":{"self":[{"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/posts\/3980","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=3980"}],"version-history":[{"count":8,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/posts\/3980\/revisions"}],"predecessor-version":[{"id":4624,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/posts\/3980\/revisions\/4624"}],"wp:attachment":[{"href":"https:\/\/www.particlebites.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3980"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3980"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3980"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}