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Sterile Neutrinos S LIGHT EXTENDED NEUTRINO SECTORS: FOUNDATIONS AND PHENOMENOLOGY MINI-WORKSHOP ON NEUTRINO THEORY SEPTEMBER 22, 2O2O BIBHUSHAN SHAKYA REFERENCES Neutrino masses and sterile neutrino dark matter from the PeV scale S. B. Roland, B. Shakya, J. D. Wells, 1412.4791 PeV neutrinos and a 3.5 keV X-ray line from a PeV scale supersymmetric neutrino sector S. B. Roland, B. Shakya, J. D. Wells, 1506.08195 Sterile neutrino dark matter from freeze-in B. Shakya, 1512.02751 Cosmological imprints of frozen-in light sterile neutrinos S. B. Roland, B. Shakya, 1609.06739 Sterile Neutrino Dark Matter with Supersymmetry B. Shakya, J. D. Wells, 1611.01517 Exotic Sterile Neutrinos and Pseudo-Goldstone Phenomenology B. Shakya, J. D. Wells, 1801.02640 … 2 PARAMETER SPACE FOR STERILE NEUTRINOS from Roland, Shakya, Wells, 1412.4791 [hep-ph] 3 PARAMETER SPACE FOR STERILE NEUTRINOS Sterile Neutrino Dark Matter: X-ray + Lyman-alpha rule out DM from Dodelson-Widrow (DW) production from Roland, Shakya, Wells, 1412.4791 [hep-ph] need alternate mechanism, e.g. freeze-in, that does not need active- Abazajian, 1705.01837 sterile mixing 4 PARAMETER SPACE FOR STERILE NEUTRINOS Direct Searches rely on decay channels+lifetime dictated by active-sterile mixing from 1504.04855 5 NEED FOR “NEW” PHYSICS IN THE NEUTRINO SECTOR (WITH LIGHT STERILE NEUTRINOS) “unnatural” parameters in the (sterile) neutrino sector: • < GeV scale masses for sterile neutrinos • tiny Yukawa couplings ( y < 10-7 ) • production of sterile neutrino DM beyond DW Hints of an underlying structure? additional structure? new particles? new symmetries? novel phenomenology? 6 Exotic Signals in the Sterile Neutrino Sector Exotic SignalsPreliminary in notes the on Sterile the phenomenology Neutrino associated Sector with a light pseudo-Goldstone from a broken global U(1)0 (unrelated to lepton number) in the neutrino sector. Preliminary notes on the phenomenology associated with a light pseudo-Goldstone from a broken global U(1)0 (unrelated to lepton number) in the neutrino sector. MOTIVATION The U(1)B L is appealing but by no means the only Exotic Signals in the Sterile Neutrino Sector − possibility. We consider instead a global symmetry U(1)0 Preliminary notes on the phenomenology associated with a light pseudo-Goldstone from a broken MOTIVATIONLight sterile neutrinos below the electroweakThe U(1)B scaleL is are appealingthat the butN byi (but no means none of the the only SM fields) are charged un- global U(1)0 (unrelated to lepton number) in the neutrino sector. − well motivated by many argumentspossibility. (dark matter, We lepto- consider insteadder. This a global forbids symmetry the DiracU(1) as0 well as Majorana mass terms in the above equations. However, with an oppo- Light sterile neutrinosgenesis below the etc) electroweak and are being scale searched are that for the at aN varietyi (but none of of the SM fields) are charged un- MOTIVATION The U(1)B L is appealing but by no means the only experiments. The searches are performedder. This in− the forbids tradi- the Diracsitely as charged well asφ Majorana, we can write mass down the higher dimen- well motivated by many arguments (dark matter, lepto- possibility. We consider instead a global symmetry U(1) tional decay channels induced by theterms mixing in of the the above active equations.sional operator However, with0 an oppo- genesis etc)Light and sterile are neutrinos being searched below the forelectroweak at a variety scale are of that the Ni (but none of the SM fields) are charged un- and sterile sectors as dictated by the seesaw mechanism. experiments.well motivated The searches by many are arguments performed (dark matter, in the lepto- tradi- der.sitely This charged forbids theφ, we Dirac can as write well as down Majorana the higher mass dimen-1 However, the natural mass scale oftermssional Majorana in theoperator above neutrinos equations. However, with an oppo- LhNφ (3) tional decaygenesis channels etc) and induced are being by searched the mixing for at of a the variety active of L ⊃ ⇤ and sterileexperiments. sectors as The dictated searchesis at the by are UVthe performed seesaw cuto↵ scale in mechanism. the (GUT tradi- or Planck),sitely charged so ifφ they, we can are write down the higher dimen- sional operator 1 However,tional the decay natural channels masslight, induced scale this by of is the Majorana indicative mixing of neutrinos the of active some deeper structure in the which,LhN onceφ φ gets a vev, reproduces(3) the Dirac mass term. and sterile sectors as dictated by the seesaw mechanism. sterile neutrino sectors, such as some protecting symme- 1L ⊃Here⇤ ⇤ is some UV-cuto↵ scale. To keep things general, is at theHowever, UV cuto the↵ naturalscale (GUT mass scale or Planck), of Majorana so if neutrinos they are LhNφ (3) try that keeps their masses at a low scale. This structure ⇤ light, thisis at is the indicative UV cuto↵ scale of some (GUT deeper or Planck), structure so if they in are the which, once φ getsL a⊃ vev,we reproduces do not explicitly the Dirac write mass down term. a term that gives rise to sterile neutrinolight, this sectors, is indicative suchcan of also as some some give deeper protecting rise structure to other symme- in light the degreeswhich,Here of once⇤ freedomisφ somegets a in vev, UV-cuto the reproduces↵thescale. Majorana the Dirac To keep mass mass things term. term, general, and take the sterile neutrino try thatsterile keeps neutrino their masses sectors,neutrino at such a low as sector, some scale. protecting and This consequently structure symme- newHerewe decay⇤ dois not some channels explicitly UV-cuto for↵ writescale.mass down To keepM ato things term be that general, a free gives parameter rise to instead. Spontaneous can alsotry give that rise keeps to their other massessterile light at neutrinos, degrees a low scale. of whichfreedom This structure can in completely the we do change not explicitly their write phe- downbreaking a term that of the givesU rise(1)0 towith φ gives rise to a light pseudo- the Majorana mass term, and take the sterile neutrinoh i neutrinocan sector, also give and rise consequently to othernomenology. light new degrees We decay of would freedom channels like in theto for studythemass this Majorana possibility.M to mass be a term, free and parameterGoldstone take the sterile instead.⇢, which neutrino Spontaneous inherits the couplings of φ; its mass neutrino sector, and consequently new decay channels for mass M to be a free parameter instead. Spontaneous sterile neutrinos, which canMotivate completely from change hidden their sectors. phe- Even if GUT scale see- depends on details of the underlying model (explicit soft sterile neutrinos, which can completely change their phe- breakingbreaking of the ofU the(1)Uwith(1)0 withφ givesφ risegives to a rise light to pseudo- a light pseudo- saw, if neutrino-like fields in hidden sectors exist,0 inte- term,h i or from quantum gravity), but for generality we nomenology.nomenology. We would We would like like to study to study this this possibility. possibility. GoldstoneGoldstone⇢, which⇢, which inherits inheritsh i the couplings the couplings of φ; its mass of φ; its mass grating out the GUT scale neutrinos generates a low en- MotivateWHYMotivate fromARE hidden from STERILE hidden sectors. sectors. Even NEUTRINOS Even if if GUT GUT scale scale see- see- LIGHT?dependsdepends on details on details of the of underlying thealso underlying take model this (explicit model mass soft (explicitm⇢ to be soft a free parameter. saw, ifsaw, neutrino-like if neutrino-like fieldsergy fields in e hidden in↵ective hidden sectors seesaw! sectors exist, exist, Plausible inte- inte- toterm, haveterm, additionalor or from from quantum struc- quantum gravity), gravity), but for but generality for generality we we grating out the GUTture scale there. neutrinos generates a low en- also take this mass m to be a free parameter. grating out the GUT scale neutrinos generates a low en- also take this mass⇢ m⇢ to be a free parameter. ergy e↵ergyective e↵ective seesaw! seesaw! Plausible Plausible to to have have additional additional struc- struc- PHENOMENOLOGY Whyture there. are the active (Standard Model) neutrinos light? ture there. PHENOMENOLOGY FRAMEWORK PHENOMENOLOGYThe phenomenology depends on four free parame- 1. symmetry protectionFRAMEWORK The phenomenology dependsters: on fourφ , ⇤ free,m parame-⇢,M (other phenomenologically relevant FRAMEWORKThe operators traditionally associated with right- h i 2. mass partner is a singlet ters:Theφ , ⇤ phenomenology,m⇢,M (other phenomenologically dependsparameters on are four relevantmφ freeand parame- its mixing with the SM Higgs, The operators traditionallyhanded, sterile associated neutrinos with right- are theparameters Dirach i and are Majoranam and its mixing with the SM Higgs, ters: φ , ⇤,mφ ⇢,M (otherbut phenomenologically these are only tangentially relevant relevant to neutrino phe- handed, sterile neutrinos are the Dirac and Majorana but theseh arei only tangentially relevant to neutrino phe- The operators traditionallymasses:y associated with right- parameters are m andnomenology its mixing with and wethe ignore SM Higgs, these for now). masses: ν<h> N nomenology and we ignoreφ these for now). handed, sterile neutrinos are the Dirac and Majorana ¯butc these are only tangentiallyIf m⇢ relevant<M, we to have neutrino a new, phe- “exotic” decay channel for ¯ c yijLihNj + MiNIfi Nmi⇢.<M, we have(1) a new, “exotic” decay channel for masses: yijLihNj + MiNi Ni. (1) the sterile neutrino into the pseudo-Goldstone and an L ⊃ L ⊃ thenomenology sterile neutrino and into we ignore the pseudo-Goldstone these for now). and an When M y h , this setup giveactive riseIf neutrino,m to<M the, familiarN we have⇢⌫. This aactive new, has “exotic” a neutrino, decay width decayN (as- channel⇢⌫. This for has a decay width (as- When M y h , this setup give¯ c rise to the familiar ⇢ ! ! yhijiLihNj + MiNi Nih.
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