{"id":4731,"date":"2016-10-11T03:11:52","date_gmt":"2016-10-11T03:11:52","guid":{"rendered":"https:\/\/particlebites.com\/?p=4731"},"modified":"2016-10-11T03:11:52","modified_gmt":"2016-10-11T03:11:52","slug":"does-the-higgs-talk-to-itself","status":"publish","type":"post","link":"https:\/\/www.particlebites.com\/?p=4731","title":{"rendered":"Does the Higgs talk to itself?"},"content":{"rendered":"
Article:\u00a0<\/strong>Indirect probes of the trilinear Higgs Coupling
\nAuthors:<\/strong>\u00a0Martin Gorbahn, Ulrich Haisch
\nReference:<\/strong>\u00a01607.03773<\/a><\/div>\n

Hello particle munchers,<\/p>\n

I\u2019m back to further discuss our good friend the Higgs boson.<\/p>\n

After\u00a0its discovery in 2012, there are many properties of the Higgs boson which have since been established. In addition to its spin-0 nature and measuring its mass with high precision<\/a> (125 GeV), the existence of its couplings to a number of Standard Model particles has also been established<\/a>.<\/p>\n

What has yet to be established is\u00a0whether the Higgs can \u201ctalk to itself\u201d. More precisely, does the Higgs have self interactions involving multiple Higgs bosons (see diagrams in Figure 1) and how strong are these self interactions? Measuring these interactions gives us direct\u00a0information on the Higgs scalar potential, which is responsible for not only the electroweak symmetry breaking\u00a0mechanism<\/a>\u00a0leading to\u00a0the generation of mass for\u00a0the Standard Model particles, but also has implications for the stability of our universe (see Footnote 1<\/strong>)\u00a0.<\/p>\n

The Higgs potential can be written in a very simple form (just a polynomial in the Higgs field H) as shown in Figure 1. The $latex H^2$ term represents\u00a0the Higgs mass while the $latex H^3$ and $latex H^4$ terms represent the Higgs self interactions we are interested in.\u00a0In the Standard Model\u00a0$latex \\lambda_3$\u00a0and $latex \\lambda_4$ have a precise relation reflecting the underlying symmetries of the SM. By measuring interactions involving 3 and 4 Higgs bosons we can determine these parameters\u00a0and therefore test directly this prediction of the SM. A deviation from from the Standard Model prediction would signal the presence of new physics\u00a0which, needless to say, would send theorist into a drunk frenzy not seen since\u2026oh wait, never mind<\/a> :\/.<\/p>\n

\"Figure<\/a>
Figure 1: V(H) represents the Higgs potential. We have also indicated which terms in the potential correspond to interactions between 3 and 4 Higgs bosons.<\/figcaption><\/figure>\n

Typically one attempts to measure $latex \\lambda_3$\u00a0and $latex \\lambda_4$ using $latex h \\to hh$\u00a0and $latex h\\to hhh$ decays respectively (see\u00a0Footnote 2<\/strong>). The\u00a0$latex h\\to hhh$ rate is far\u00a0too small at the LHC to be useful for extracting $latex \\lambda_4$. Thus, most theoretical and experimental studies (see here<\/a> and references therein)\u00a0have focused on measuring\u00a0$latex \\lambda_3$ via\u00a0$latex h\\to hh$ decays.<\/p>\n

Since\u00a0the $latex h\\to hh$ rate is also small, it requires\u00a0producing many Higgs bosons so that after a long time and massive amounts of data, enough of them will decay off-shell into pairs of\u00a0Higgs bosons to be seen at colliders.\u00a0At LHC energies the production rates are not quite large enough to be able see this process if it conforms to the SM prediction. However, if new physics somehow makes the rate much larger than found in the SM, then perhaps it can be seen at the LHC<\/a>.<\/p>\n

The paper highlighted above is\u00a0interesting because it\u00a0proposes a new, but indirect, way to measure $latex \\lambda_3$ at the LHC\u00a0by\u00a0looking at gluon fusion Higgs\u00a0production\u00a0and $latex h\\to \\gamma\\gamma$ decays (where $latex \\gamma$ is a photon), which i’ll focus on in todays post. If you recall my earlier post<\/a> about how we see the Higgs boson in its decays to pairs of photons, you might remember that the interaction between the Higgs and photons is generated through loops of virtual<\/a> charged particles. At leading one loop order, there can only be charged particles running in the loop and thus no Higgs bosons. This means that at leading order we are not sensitive to $latex \\lambda_3$. However, if we go to next to leading order at\u00a0two loops one can have Higgs bosons contribute as shown in Figure 2.\u00a0In this case we see the Higgs boson can enter in the loops and in particular the $latex \\lambda_3$ coupling at the vertex with 3 Higgs bosons indicated by the box.<\/p>\n

\"Figure<\/a>
Figure 2: Two loop contributions to Higgs decays to pairs of photons. The box indicates the Higgs self interaction we are interested in (figure from arxiv: 1607.03773).<\/figcaption><\/figure>\n

Since these two loop processes are next to leading order, they are in general very small. However, the high precision with which $latex h\\to \\gamma\\gamma$ will eventually be measured at the LHC<\/a> allows for these tiny effects to be potentially probed. What is interesting is that the authors find that these indirect methods are\u00a0competitive and complementary to $latex h\\to hh$ decays at the LHC. They find\u00a0it will be possible to\u00a0eliminate regions of parameter space in beyond the Standard Model theories which can not be ruled out with the more conventional and\u00a0direct\u00a0$latex h\\to hh$ decays.<\/p>\n

Though the precision is still not great, due to the importance of establishing the precise form of the Higgs potential, it is crucial to have as many ways as possible of constraining\u00a0its parameters. Since the LHC will just begin to probe the parameters of the Higgs potential before the end of running, a future collider which could produce far larger numbers of Higgs bosons will be crucial in this endeavor of determining if indeed the\u00a0Higgs talks to itself.<\/p>\n

Footnotes<\/b><\/p>\n

    \n
  1. I’ll discuss this interesting topic\u00a0of the vacuum stability of the universe more in a future post, but see here<\/a> for a great discussion or here<\/a> for a famous paper about it. Or just ask Stephen Hawking<\/a>.<\/li>\n
  2. You might ask how one Higgs boson can decay to multiple\u00a0Higgs bosons when clearly one Higgs boson has less mass than two or more Higgs bosons. This of course is due to the subtleties of quantum mechanics<\/a> which leads to the fact that particles can decay `off mass shell<\/a>\u2019 (more precisely with a 4-momentum squared which is not equal to its physical mass). This means that particles which naively are too light to decay to a particular more massive final state can still do so at the cost of a massive kinematic suppression.<\/li>\n<\/ol>\n

    Further reading:\u00a0<\/b><\/p>\n

      \n
    1. Very similar study<\/a>\u00a0proposing\u00a0additional indirect probes of $latex \\lambda_3$<\/li>\n
    2. A basic introduction<\/a> to the physics of the Standard Model Higgs boson<\/li>\n<\/ol>\n","protected":false},"excerpt":{"rendered":"

      Article:\u00a0Indirect probes of the trilinear Higgs Coupling Authors:\u00a0Martin Gorbahn, Ulrich Haisch Reference:\u00a01607.03773 Hello particle munchers, I\u2019m back to further discuss our good friend the Higgs boson. After\u00a0its discovery in 2012, there are many properties of the Higgs boson which have since been established. In addition to its spin-0 nature and measuring its mass with high … <\/p>\n

      Continue reading “Does the Higgs talk to itself?”<\/span><\/a><\/p>\n","protected":false},"author":7,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[],"tags":[],"class_list":["post-4731","post","type-post","status-publish","format-standard","hentry"],"_links":{"self":[{"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/posts\/4731","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\/7"}],"replies":[{"embeddable":true,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=4731"}],"version-history":[{"count":0,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/posts\/4731\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.particlebites.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4731"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4731"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4731"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}