Search for elusive “di-Higgs production” reaches new milestone
19 November 2021 | By
Seeing one Higgs boson is already a challenge, but what would it take to spot two at the same time? According to the Standard Model, the Higgs boson can interact with itself, resulting in the simultaneous production of two Higgs bosons ("di-Higgs production"). This is expected to be a very rare process – so rare that, according to the Standard Model, physicists will need around 20 times more data than available today to be able to measure it. This will be possible with the High-Luminosity upgrade of the LHC!
However, physics beyond the Standard Model may enhance the di-Higgs production rate, by altering the strength of the Higgs self-interaction or through an intermediate heavy particle. ATLAS physicists are looking for signs of di-Higgs production in LHC Run-2 data (collected 2015–2018); if spotted early, it would be a clear indication of the existence of new physics phenomena.
As the Higgs boson can decay in many different ways, physicists are searching for di-Higgs events in various decay channels. The latest ATLAS searches look for events where one Higgs boson decays into two bottom quarks, and the other decays into either two bottom quarks (bbbb), two tau leptons (bbττ) or two photons (bbγγ).
Each search has its pros and cons. While bbbb occurs more frequently than bbττ or bbγγ, it can be challenging to separate it from other processes produced in the LHC collisions. Whereas although bbγγ occurs very infrequently, photons are easy to pick out amongst other processes. A happy medium is bbττ, which has a higher production rate than bbγγ and lower background rates than bbbb. However, tau leptons can decay in many ways, making them sometimes tricky to identify.
In a new result presented at the Higgs 2021 conference, ATLAS physicists combined all three of these decay channels, taking advantage of their unique strengths to reach the best limits yet on di-Higgs production. Figure 1 shows the limits on the magnitude of the Higgs self-interaction relative to the Standard Model prediction (described by κλ), which has been constrained to be between –1.0 and 6.6 (the Standard Model value of κλ would give unity).
The new ATLAS result combines three di-Higgs decay channels, taking advantage of their unique strengths to reach the best limits yet on di-Higgs production.
In addition, Figure 2 shows that the observed exclusion upper limits on the signal strength from bbγγ and bbττ lie at 4.3 and 4.6 times the Standard Model prediction individually, though when combined the observed upper limit goes down to 3.1. This is a factor of three improvement with respect to the previous ATLAS results using partial Run-2 dataset, where the 1.5 gain comes from the improved data analysis strategy of the channels.
ATLAS physicists also combined the latest results from bbγγ, bbττ and bbbb to set limits on the production rates of heavy hypothetical particles decaying into Higgs boson pairs (Figure 3). In low mass (mx) regions, the bbγγ channel is the most dominant in the combination; while at higher masses, bbbb is the more dominant channel. By combining all three channels in one analysis, physicists were able to achieve the best sensitivities across a wide range of new-particle masses.
ATLAS physicists are looking forward to adding additional di-Higgs decay channels to their search – using both the current dataset and the coming LHC Run-3 dataset.
Utafiti wa 'di-Higgs' unafikia hatua mpya - Kuona boson moja ya Higgs tayari ni changamoto, lakini inahitaji nini kuona mbili kwa wakati mmoja? Kulingana na Mfano wa Kiwango, bosoni ya Higgs inaweza kuingiliana yenyewe, ikisababisha uzalishaji wa wakati mmoja wa bosoni mbili za Higgs ("uzalishaji wa di-Higgs"). Hii inatarajiwa kuwa mchakato wa nadra sana - nadra sana kwamba, kulingana na Mfano wa Kiwango, wataalamu wa fizikia watahitaji karibu mara 20 zaidi ya data inayopatikana leo ili kuweza kupima. Hii itakuwa inawezekana na High-Luminosity kuboresha ya LHC! Hata hivyo, fizikia zaidi ya Mfano wa Kiwango inaweza kuongeza kiwango cha uzalishaji wa di-Higgs, kwa kubadilisha nguvu ya mwingiliano wa Higgs mwenyewe au kupitia chembe nzito ya kati. Wataalamu wa ATLAS wanatafuta ishara za uzalishaji wa di-Higgs katika data ya LHC Run-2 (iliyokusanywa 2015-2018); ikiwa itagunduliwa mapema, itakuwa ishara ya wazi ya kuwepo kwa matukio mapya ya fizikia. Kwa kuwa bosoni ya Higgs inaweza kuharibika kwa njia nyingi tofauti, wanasayansi wanatafuta matukio ya di-Higgs katika njia mbalimbali za kuharibika. Katika uchunguzi wa ATLAS, Higgs boson moja imeharibika kuwa quark mbili za chini (BBB), leptoni mbili za tau (BBT) au fotoni mbili (BBB). Kila utafutaji una faida na hasara zake. Wakati bBBB hutokea mara nyingi kuliko bbB au bbB, inaweza kuwa changamoto kuitenganisha na michakato mingine inayotokana na mgongano wa LHC. Wakati ingawa bbγγ hutokea mara chache sana, photons ni rahisi kuchagua kati ya michakato mingine. "Happy medium ni ""bbb"" ambayo ina kiwango cha juu cha uzalishaji kuliko ""bbc"" na kiwango cha chini cha nyuma kuliko ""bbb.""" Hata hivyo, leptoni za tau zinaweza kuharibika kwa njia nyingi, na nyakati nyingine ni vigumu kuzitambua. Katika matokeo mapya yaliyowasilishwa katika mkutano wa Higgs 2021, wanasayansi wa ATLAS waliunganisha njia hizi zote tatu za uharibifu, wakitumia nguvu zao za kipekee kufikia mipaka bora zaidi ya uzalishaji wa di-Higgins. Mchoro wa 1 unaonyesha mipaka ya ukubwa wa mwingiliano wa Higgs binafsi kuhusiana na utabiri wa Mfano wa Kiwango (kuelezewa na KLC), ambayo imepigwa marufuku kuwa kati ya <1.0 na 6.6 (thamani ya Mfano wa Kiwango ya KLC ingetoa umoja). Matokeo ya ATLAS mpya huchanganya njia tatu za kuharibika kwa di-Higgs, kwa kutumia nguvu zao za kipekee kufikia mipaka bora zaidi ya uzalishaji wa di-Higgs. Kwa kuongezea, Mchoro wa 2 unaonyesha kwamba mipaka ya juu ya kutengwa iliyoonekana kwenye nguvu ya ishara kutoka kwa bbg na bbg iko kwa mara 4.3 na 4.6 ya utabiri wa Mfano wa Kiwango, ingawa wakati unaunganishwa, kikomo cha juu kinaanguka hadi 3.1. Hii ni sababu ya tatu ya uboreshaji kwa heshima ya matokeo ya awali ya ATLAS kwa kutumia sehemu ya Run-2 dataset, ambapo faida ya 1.5 inatokana na mbinu ya kuboresha uchambuzi wa data ya njia. Katika utafiti wa ATLAS, wataalamu wa fizikia waliunganisha matokeo ya hivi karibuni kutoka kwa bbg, bbb na bbbb ili kuweka mipaka kwa viwango vya uzalishaji wa chembechembe nzito za nadharia zinazoharibika katika jozi za bosoni ya Higgs. Katika maeneo ya chini ya wingi (mx), kituo cha bbγγ ni cha kutawala zaidi katika mchanganyiko; wakati katika wingi wa juu, bbbb ni kituo cha kutawala zaidi. Kwa kuchanganya njia zote tatu katika uchambuzi mmoja, wanasayansi waliweza kufikia unyeti bora katika wigo mpana wa wingi wa chembe mpya. Wataalamu wa ATLAS wanatazamia kuongeza njia za ziada za kuharibika kwa di-Higgs kwa utafutaji wao - kwa kutumia seti ya data ya sasa na seti ya data ya LHC Run-3 inayokuja.