study appearing this week<\/a> in the journal Physical Review Letters, the team reports results from the Large Hadron Collider (LHC) \u2014 a massive underground, ring-shaped accelerator in Geneva, Switzerland. Rather than focus on the accelerator\u2019s particle collisions, the MIT team searched for instances when particles barely glanced by each other.<\/p>\nWhen particles travel at close to the speed of light, they are surrounded by an electromagnetic halo that flattens when particles pass close but don\u2019t collide. The pancaked energy fields produce extremely high-energy photons. Occasionally, a photon from one particle can ping off another particle, like an intense, quantum-sized pinprick of light.<\/p>\n
The MIT team was able to pick out such near-miss pinpricks, or what scientists call \u201cphotonuclear interactions,\u201d from the LHC\u2019s particle-collision data. They found that when some photons pinged off a particle, they kicked out a type of subatomic particle, known as a\u00a0D0\u00a0meson, that the scientists could measure for the first time.<\/p>\n
D0\u00a0mesons are subatomic particles that\u00a0contain\u00a0a charm quark, a rare type of quark not normally found in ordinary nuclear matter. Quarks are the fundamental building blocks of all matter, and are bound by gluons, which are massless particles that are the invisible glue, or \u201cstrong force\u201d that holds matter together. The rare charm quarks can only be created in high-energy interactions. As such, they provide an especially clean, unambiguous probe of quarks and gluons inside a nucleus.<\/p>\n
Through their measurements of D0 mesons , the researchers could estimate how tightly gluons are packed, and, essentially, how strong the strong force is within a particle\u2019s nucleus.<\/p>\n
\u201c<\/strong>Our result gives an indication that when nuclear matter is squeezed together, then gluons start behaving in a funny way,\u201d says lead author Gian Michele Innocenti, an assistant professor of physics at MIT. \u201cWe need to know how these gluons behave in these extreme conditions because gluons keep the universe together. And at this point, photonuclear interactions are the best way we have to study gluon behavior.\u201d<\/p>\nThe study\u2019s co-authors include members of the CMS Collaboration \u2014 a global consortium of physicists who operate and maintain the Compact Muon Solenoid (CMS) experiment, which is one of the largest detectors within the LHC that was used to collect the study\u2019s data.<\/p>\n
Bringing a \u201cbackground\u201d into focus<\/strong><\/p>\nWith each run, the Large Hadron Collider fires off needle-thin beams of particles in opposite directions around a 27-kilometer-long underground ring. When the beams cross paths, particles can collide. If the collisions happen to take place in a region of the ring where the CMS detector is set up, the detector can record the collisions, and scientists can then analyze the aftermath to reconstruct the fragments that make up the original particles.<\/p>\n
Since the LHC began operations in 2008, the focus has been overwhelmingly on the detection and analysis of \u201chead-on\u201d collisions. Physicists have known that by accelerating particle beams, they would also produce photonuclear interactions \u2014 near-miss events where a particle might collide not with another particle, but with its cloud of photons. But such light-nucleus interactions were thought to be simply noise.<\/p>\n
\u201cThese photonuclear events were considered a background that people wanted to cancel,\u201d Innocenti says. \u201cBut now people want to use it as a signal because a collision between a photon and a nucleus can essentially be like a super-high-accuracy microscope for nuclear matter.\u201d<\/p>\n
When a photon pings off a particle, the abundance, direction, and energy of the produced\u00a0D0 meson relates directly to the energy and density of the gluons in the nucleus. If scientists can detect and measure this photon interaction, it would be like using an extremely small and powerful flashlight to illuminate the nuclear structures. But until now, it was assumed that photonuclear interactions would be impossible to pick out amid the various physics processes that can occur in such collisions.<\/p>\n
\u201cPeople didn\u2019t think it was possible to remove the huge mess of all these other collisions, to zoom in on single photons hitting single nuclei producing a\u00a0D0 meson,\u201d Innocenti says. \u201cWe had to devise a system to recognize those very rare photonuclear interactions while data was being taken of particle collisions.\u201d<\/p>\n
Illuminating charm<\/strong><\/p>\nFor their new study, Innocenti and his colleagues first simulated what a photonuclear interaction would look like amid a shower of other particle collisions. In particular, they simulated a scenario in which a photon pings off a nucleus and produces a D0 meson.\u00a0Although these events are rare,\u00a0D0 mesons\u00a0are among the most abundant particles that\u00a0contain\u00a0a charm quark.\u00a0The team reasoned that if they could detect signs of a charm quark in D0\u00a0mesons that are produced in a photonuclear interaction, it could give valuable information about the gluons that hold the nucleus together.<\/p>\n
With their simulations, the researchers then developed an algorithm to detect photonuclear interactions. They implemented the algorithm at the CMS detector to search for signals in real-time during the LHC\u2019s particle-colliding runs.<\/p>\n
\u201cWe had to collect tens of billions of collisions in order to extract a few hundred of these rare instances where a photon hits a nucleus and produces one of these exotic D0 meson particles,\u201d Innocenti explains.<\/p>\n
From this enormous dataset, the team\u00a0identified\u00a0a clean sample of these rare events by exploiting CMS\u2019s advanced detector capabilities to select near-miss events and reconstruct the properties of the\u00a0D0 mesons.<\/p>\n
Through this process, the team detected instances of D0 meson production and then worked back to calculate properties of the particles\u2019 charm quarks and the gluons that would have held them together in the original nucleus.\u00a0<\/p>\n
\u201cWe are constraining what happens to gluons when they are squeezed in ions that are very large that are traveling very fast,\u201d Innocenti says. \u201cSo far,\u00a0our data confirms what people expect in terms of high-density nuclear matter. In reality, this is the first time we\u2019ve shown this kind of measurement is feasible. \u201d<\/p>\n
The team is working to improve the measurement\u2019s accuracy in order to provide a clearer picture of how quarks and gluons are arranged inside a nucleus.<\/p>\n
\u201cGluons are a very strong force that keeps the universe together,\u201d Innocenti says. \u201cThe description of the strong force is at the basis of everything we see in nature. Now we have a way to either fully confirm, or show deviations from, that description.\u201d<\/p>\n
This work was supported, in part, by the U.S. Department of Energy, including support from a DOE Early Career Research Program award, and it builds on the contributions of a large MIT team of graduate students, undergraduate researchers, scientists, and postdocs.<\/p>\n","protected":false},"excerpt":{"rendered":"Particle accelerators reveal the heart of nuclear matter by smashing together atoms at close to the speed of…\n","protected":false},"author":2,"featured_media":407200,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":"","_share_on_mastodon":"0"},"categories":[271],"tags":[183974,18,183973,19,17,38451,11873,183977,38045,183978,5474,183976,452,183975,133],"class_list":["post-407199","post","type-post","status-publish","format-standard","has-post-thumbnail","category-physics","tag-cms-collaboration","tag-eire","tag-gian-innocenti","tag-ie","tag-ireland","tag-laboratory-for-nuclear-science","tag-large-hadron-collider","tag-lhc-particle-accelerator","tag-mit-physics","tag-particle-near-misses","tag-particle-physics","tag-photonuclear-collisions","tag-physics","tag-quarks-and-gluons","tag-science"],"share_on_mastodon":{"url":"https:\/\/pubeurope.com\/@ie\/116302590512573148","error":""},"_links":{"self":[{"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/posts\/407199","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/comments?post=407199"}],"version-history":[{"count":0,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/posts\/407199\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/media\/407200"}],"wp:attachment":[{"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/media?parent=407199"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/categories?post=407199"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/tags?post=407199"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}