Searches for light new physics. Theoretical overview Maxim Pospelov U of Minnesota and FTPI Physics Beyond Colliders at CERN: Beyond the Standard Model Working Group Report J. Beacham (Ohio State U., Columbus (main)), C. Burrage (U. Nottingham), D. Curtin (Toronto U.), A. De Roeck (CERN), J. Evans (Cincinnati U.), J.L. Feng (UC, Irvine), C. Gatto (INFN, Naples & NIU, DeKalb), S. Gninenko (Moscow, INR), A. Hartin (U. Coll. London), I. Irastorza (U. Zaragoza, LFNAE) et al. Show all 33 authors Jan 20, 2019 - 150 pages •CERN-PBC-REPORT-2018-007 •e-Print: arXiv:1901.09966 [hep-ex] 1 Particle “content” of the Universe is largely unknown Atoms DM In Energy chart they are g 4%. In number density DR chart ~ 5 ×10-10 relative to g n We have no idea about DM number densities. (WIMPs ~ 10-8 cm-3; axions ~ 109 cm-3. Dark Radiation, Dark Forces – We don’t know). Number density chart for axionic universe: axions Lack of precise knowledge about nature of dark matter leaves a lot of room for existence of dark radiation, and dark forces – dark sector in general. 2 New IR degrees of freedom = light (e.g. sub-eV) beyond-Standard-Model states Typical BSM model-independent approach is to include all possible BSM operators once very heavy new physics is integrated out 2 + LSM+BSM= - mH (H SMHSM) + all dim 4 terms (ASM, ySM, HSM) + 2 (Wilson coeff. /L ) × Dim 6 etc (ASM, ySM, HSM) + … But is this framework really all-inclusive – it is motivated by new heavy states often with sizeable couplings? The alternative possibility for New Physics – weakly coupled light new physics - is equally viable 3 New IR degrees of freedom = light (e.g. sub-eV) beyond-Standard-Model states Typical BSM model-independent approach is to include all possible BSM operators once very heavy new physics is integrated out 2 + LSM+BSM= - mH (H SMHSM) + all dim 4 terms (ASM, ySM, HSM) + 2 (W.coeff. /L ) × Dim 6 etc (ASM, ySM, HSM) + … all lowest dimension portals (ASM, ySM, H, ADS, yDS, HDS) × portal couplings + dark sector interactions (ADS, yDS, HDS) SM = Standard Model DS – Dark Sector 4 1.1 Kinetic mixing 1.1 Kinetic mixing ConsiderConsider a QED-like a QED-like theory theory with with one one (or (or several) several) extra extra vector vector particle(s), particle(s), coupled coupled to the to the electromagneticelectromagnetic current. current. A mass A mass term, term, or or in in general general a a mass mass matrix matrix for for the the vector vector states, states, is is protectedprotected against against additive additive renormalization renormalization due due to to the the conservation conservation of of the the electromagnetic electromagnetic 2 2 current.current. If the If mass the mass matrix matrix for for such such vector vector states states has a a zero zero determinant, determinant, det( det(MV )=0,thenMV )=0,then the theorythe theory contains contains one one massless massless vector, vector, to to be be identified identified with with a a photon, photon, and and several several massive massive vectorvector states. states. This is thee model of ‘paraphotons’, introduced by Okun in early 1980s [6], that can be This is the model of ‘paraphotons’, introduced by Okun in early 1980s [6], that can be reformulated in equivalent language using the kinetic mixing portal. Following Holdom [7], reformulated in equivalent language using the kinetic mixing portal. Following Holdom [7], one writes a QED-like theory withγ two U(1) groups, supplemented by the cross term in the one writeskinetic a Lagrangian, QED-like theory and a mass with term two forU(1) one groups, of the vector supplemented fields. by the cross term in the kinetic Lagrangian,A and simple a mass term model for one⇥ of theof vector dark fields. sector ⇥ 1 2 2 = ⌅,A + ⇤,A Fµ⇥F + m (A ) . (1.1) γ µ⇥ A µ L L L −⇥2 21 2 2 = + F F + m (A ) . (1.1) ⌅,A ⇤,A µ⇥ µ⇥ A µ ⌅,A and ⇤,A are theL standardL QED-typeL − 2 Lagrangians,2 L L ⇤ 1 ⌅,A and ⇤,A are the standard QED-type2 ¯ Lagrangians, L L ⌅,A = Fµ⇥ + ⌅[γµ(i⌥µ eAµ) m⌅]⌅ Figure 1: The interaction throughL the exchange−4 by a mixed−γ A− propagator between the 1 2 ⇥ = 1 F 2+ ⌅¯[γ (i⌥ eA− ) m ]⌅ SM particles and particles ⌅ charged⌅,A= under(F µ new)⇥ +¯⇤U[(1)γ µ(⇥i⌥group.µ gA In)µ them limit]⇤⌅, of mA 0the(1.2) L⇤,A −4 µ⇥ µ µ − µ − ⇤ apparent electromagentioc chargeL of ⌅−is4 e⇥. − − ⇧ 1 2 with Fµ⇥ and F standing for the fields strength tensors. States ⌅ represent the QED µ⇥ ⇤,A = e(Fµ ⇥) +¯⇤[γµ(i⌥µ gAµ ) m⇤]⇤, (1.2) electron fields, andL states −⇤ 4are similar particles, charged− under− ”dark” U(1). In the limit In the simplest example, a new fermionic fieldγ charged under both U(1)’s will gener- with Fofµ⇥⇥ and0, theFµ ⇥ twostanding sectors become for the completely fields strength decoupled. tensors. In eq. States (1.1), the⌅ massrepresent term for theA QED ⇧ ⇥ 2 ate anexplicitly additional breaks contribution the second toU(1), the but mixing is protected angle that from scalesadditive as renormalization,∆⇥ g⇥e/(12⇤ and) hence electron2 fields,2 and states ⇤ are similar particles, charged under ”dark”⇤ U(1). In⇥ the limit γ EM log(⇥isUV technically/M ) . In principle, natural. Using the two the sectors equations can of be motion, ”several⌥ F loop= removed”,eJ , the so interaction that one term of ⇥ 0, the two sectors become completely decoupled.µ Inµ⇥ eq. (1.1),⇥ the mass term for A can⇧ entertaincan be rewritten a wide rangeas of mixing⇤ angles. explicitly breaks the second U(1), but⇥ is protected from additiveEM renormalization, and hence F F = A (e⇥)J , (1.3) § “Effective”Figure 1: The interaction charge through of the theµ⇥ exchange“darkµ⇥ bysector”µ a mixed γ particleµA⇥ propagator c betweenisEM Q = the e × e 2.is If technically both groups natural. are unbroken, Usingm theV equations− 20, then of⌅ represent motion,⇥ −⌥ theµFµ ”millicharged⇥ = eJ⇥ , the particles” interaction term SM particles and particles ⌅ charged under new U(1)⇥ group. In the limit of mA 0the showing thatapparent the new electromagentioc vector particle⇧ charge of couples⌅ is e⇥. 2 to the2 electromagnetic current⇧ with strength, canwith be electric rewritten charge(if as momentumqχ = e⇥.For scalemV =0,atq > mV q). At<m q <V ,m theV one particles can say⌅ can that be thought reduced by a small factor ⇥. The⇥ ⌥ generalization| | of (1.1)EM to the SM2 is straightforward,2 by of as neutral particlesparticle with c has a a non-vanishing non-FvanishingF = electric AEM charge charge(e⇥)J radius,radius,, rχ 6⇥m−. The (1.3) In the simplest example, aµ new⇥ fermionicµ⇥ fieldµ charged underµ both U(1)’s will⌃ gener- V subsituting the QED U(1) with− the2 hypercharge ⇥U(1) of the SM. 2 diagram, describing basicate an additionalinteraction contribution between to the mixing the angletwo that sectors scales as is∆⇥ showng⇥e/(12 in⇤ ) Fig. 1. § Dark photonlog(⇥2 /M )can2. In principle, “communicate” the two sectors can interaction be ”several loop removed”, between⇤ so that SM one⇥ and showing that the new vectorUV particle couples to the electromagnetic current with5 strength, There is a multitudecan entertain of a notations wide range of mixing and names angles. referring to one and the same model. We 3.reduced If there are by ano small statesdark factormatter. charged⇥ .Very under The lightU generalization(1) c⇥ (orcan they be possible. are of very (1.1) heavy), to the(Y and SM) m isV straightforward,is taken to by shall call the A state as ”dark photon”. It can also be called as V , a vector state coupled be zero, then the two2. sectors If both groups decouple are unbroken, evenmV at0, non-zero then ⌅ represent⇥. This the ”millicharged leads to particles” the suppression subsituting the QED U(1) with the hypercharge⇧ 2U(1)2 of the SM. to the hyperchargewith current. electric charge Weqχ = choosee⇥.FormV to=0,at callq the<mV mixing, the particles angle⌅ can be⇥,andthroughoutthis thought ⌥ | | 2 2 of all interactions forof a as dark neutral photon particles with inside a non-vanishing a medium, electric if chargemV radius,becomesr 6⇥m smaller− . The than the chapter assume ⇥ 1. In contrast, one does not have to assumeχ ⌃ a smallnessV of g coupling, characteristicThere is a plasmamultitudediagram, frequency, of describing notations and basic all interaction and processes names between thewith referring two sectors emission is to shown one or in Fig. aborption and 1. the same of dark model. We which can be comparable⌅ to the gauge couplings of the SM, g g . 3. If there2 are no states charged under U(1) (or they are very heavy), and mSM(isY taken) to shallphotons call decouple the A state as asm ”darkV [8]. photon”. It can⇥ also be called as⇤ VV , a vector state coupled ⇤be zero, then the two sectors decouple even at non-zero ⇥. This leads to the suppression to the hyperchargeAthough the current. modelof all interactions of this We for type a choose dark is photon exceedingly to inside call a medium, the simple, if mixingmV onebecomes can angle smaller already than⇥,andthroughoutthis the learn a number of 4. New vectorinstructive boson, features. interactingcharacteristic plasma with frequency, the SM and via all processes the electromagnetic with emission or aborption current, of dark conserves chapter assume ⇥ 1.photons In contrast, decouple as m one2 [8]. does not have to assume a smallness of g coupling, all discrete symmetries⌅ (parity, flavour,⇤ V CP etc).