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Alessandro Strumia

Publications and source records attributed to Alessandro Strumia.

At least 73 records · Page 4Linked to original sources
arXiv2019

Colored Dark Matter

We explore the possibility that Dark Matter is the lightest hadron made of two stable color octet Dirac fermions ${\cal Q}$. The cosmological DM abundance is reproduced for $M_{\cal Q}\approx 12.5$ TeV, compatibly with direct searches (the Rayleigh cross section, suppressed by $1/M_{\cal Q}^6$, is close to present bounds), indirect searches (enhanced by ${\cal Q}{\cal Q}+\bar{\cal Q}\bar{\cal Q}\to {\cal Q}\bar{\cal Q}+{\cal Q}\bar{\cal Q}$ recombination), and with collider searches (where ${\cal Q}$ manifests as tracks, pair produced via QCD). Hybrid hadrons, made of $\cal Q$ and of SM quarks and gluons, have large QCD cross sections, and do not reach underground detectors. Their cosmological abundance is $10^5$ times smaller than DM, such that their unusual signals seem compatible with bounds. Those in the Earth and stars sank to their centers; the Earth crust and meteorites later accumulate a secondary abundance, although their present abundance depends on nuclear and geological properties that we cannot compute from first principles.

hep-ph↗
arXiv2018

Cosmological Abundance of Colored Relics

The relic cosmological abundance of stable or long-lived neutral colored particles gets reduced by about a few orders of magnitude by annihilations that occur after QCD confinement. We compute the abundance and the cosmological bounds on relic gluinos. The same post-confinement effect strongly enhances co-annihilations with a lighter Dark Matter particle, provided that their mass difference is below a few GeV. Charged colored particles (such as stops) can instead form baryons, which can be (quasi)stable in some models.

hep-ph↗
arXiv2018

Dark Matter in the Standard Model?

We critically reexamine two possible Dark Matter candidate within the Standard Model. First, we consider the $uuddss$ exa-quark. Its QCD binding energy could be large enough to make it (quasi) stable. We show that the cosmological Dark Matter abundance is reproduced thermally if its mass is 1.2 GeV. However, we also find that such mass is excluded by the stability of Oxygen nuclei. Second, we consider the possibility that the instability in the Higgs potential leads to the formation of primordial black holes while avoiding vacuum decay during inflation. We show that the non-minimal Higgs coupling to gravity must be as small as allowed by quantum corrections, $|ξ_H| < 0.01$. Even so, one must assume that the Universe survived in $e^{120}$ independent regions to fluctuations that lead to vacuum decay with probability 1/2 each.

hep-ph↗
arXiv2018

Super-cool Dark Matter

In dimension-less theories of dynamical generation of the weak scale, the Universe can undergo a period of low-scale inflation during which all particles are massless and super-cool. This leads to a new mechanism of generation of the cosmological Dark Matter (DM) relic density: super-cooling can easily suppress the amount of DM to the desired level. This is achieved for TeV-scale DM, if super-cooling ends when quark condensates form at the QCD phase transition. Along this scenario, the baryon asymmetry can be generated either at the phase transition or through leptogenesis. We show that the above mechanism takes place in old and new dimension-less models.

hep-ph↗
arXiv2018

New infra-red enhancements in 4-derivative gravity

4-derivative gravity provides a renormalizable theory of quantum gravity at the price of introducing a physical ghost, which could admit a sensible positive-energy quantization. To understand its physics, we compute ghost-mediated scatterings among matter particles at tree-level, finding a new power-like infra-red enhancement typical of 4-derivative theories, that we dub '$ghostrahlung$'. Super-Planckian scatterings get downgraded to Planckian by radiating hard gravitons and ghosts, which are weakly coupled and carry away the energy.

hep-th↗
arXiv2018

Cosmological Implications of Dark Matter Bound States

We present generic formulae for computing how Sommerfeld corrections together with bound-state formation affects the thermal abundance of Dark Matter with non-abelian gauge interactions. We consider DM as a fermion 3plet (wino) or 5plet under SU(2)$_L$. In the latter case bound states raise to 14 TeV the DM mass required to reproduce the cosmological DM abundance and give indirect detection signals such as (for this mass) a dominant $γ$-line around 85 GeV. Furthermore, we consider DM co-annihilating with a colored particle, such as a squark or a gluino, finding that bound state effects are especially relevant in the latter case.

hep-ph↗
arXiv2018

Asymptotically Safe Standard Model Extensions?

We consider theories with a large number $N_F$ of charged fermions and compute the renormalisation group equations for the gauge, Yukawa and quartic couplings resummed at leading order in $1/N_F$. We construct extensions of the Standard Model where SU(2) and/or SU(3) are asymptotically safe. When the same procedure is applied to the Abelian U(1) factor, we find that the Higgs quartic can not be made asymptotically safe and stay perturbative at the same time.

hep-ph↗
arXiv2018

Bounds on Dark Matter annihilations from 21 cm data

The observation of an absorption feature in the 21 cm spectrum at redshift $z\approx 17$ implies bounds on Dark Matter annihilations for a broad range of masses, given that significant heating of the intergalactic medium would have erased such feature. The resulting bounds on the DM annihilation cross sections are comparable to the strongest ones from all other observables.

astro-ph.CO↗
arXiv2018

Agravity up to infinite energy

The self-interactions of the conformal mode of the graviton are controlled, in dimensionless gravity theories (agravity), by a coupling $f_0$ that is not asymptotically free. We show that, nevertheless, agravity can be a complete theory valid up to infinite energy. When $f_0$ grows to large values, the conformal mode of the graviton decouples from the rest of the theory and does not hit any Landau pole provided that scalars are asymptotically conformally coupled and all other couplings approach fixed points. Then, agravity can flow to conformal gravity at infinite energy. We identify scenarios where the Higgs mass does not receive unnaturally large physical corrections. We also show a useful equivalence between agravity and conformal gravity plus two extra conformally coupled scalars, and give a simpler form for the renormalization group equations of dimensionless couplings as well as of massive parameters in the presence of the most general matter sector.

hep-th↗
arXiv2017

Dark Matter as a weakly coupled Dark Baryon

Dark Matter might be an accidentally stable baryon of a new confining gauge interaction. We extend previous studies exploring the possibility that the DM is made of dark quarks heavier than the dark confinement scale. The resulting phenomenology contains new unusual elements: a two-stage DM cosmology (freeze-out followed by dark condensation), a large DM annihilation cross section through recombination of dark quarks (allowing to fit the positron excess). Light dark glue-balls are relatively long lived and give extra cosmological effects; DM itself can remain radioactive.

hep-ph↗
arXiv2017

Naturalness of asymptotically safe Higgs

We extend the list of theories featuring a rigorous interacting ultraviolet fixed point by constructing the first theory featuring a Higgs-like scalar with gauge, Yukawa and quartic interactions. We show that the theory enters a perturbative asymptotically safe regime at energies above a physical scale $Λ$. We determine the salient properties of the theory and use it as a concrete example to test whether scalars masses unavoidably receive quantum correction of order $Λ$. Having at our dispose a calculable model allowing us to precisely relate the IR and UV of the theory we demonstrate that the scalars can be lighter than $Λ$. Although we do not have an answer to whether the Standard Model hypercharge coupling growth towards a Landau pole at around $Λ\sim 10^{40}$ GeV can be tamed by non-perturbative asymptotic safety, our results indicate that such a possibility is worth exploring. In fact, if successful, it might also offer an explanation for the unbearable lightness of the Higgs.

hep-ph↗
arXiv2017

(Higgs) vacuum decay during inflation

We develop the formalism for computing gravitational corrections to vacuum decay from de Sitter space as a sub-Planckian perturbative expansion. Non-minimal coupling to gravity can be encoded in an effective potential. The Coleman bounce continuously deforms into the Hawking-Moss bounce, until they coincide for a critical value of the Hubble constant. As an application, we reconsider the decay of the electroweak Higgs vacuum during inflation. Our vacuum decay computation reproduces and improves bounds on the maximal inflationary Hubble scale previously computed through statistical techniques.

hep-ph↗
arXiv2017

Agravity

We explore the possibility that the fundamental theory of nature does not contain any scale. This implies a renormalizable quantum gravity theory where the graviton kinetic term has 4 derivatives, and can be reinterpreted as gravity minus an anti-graviton. We compute the super-Planckian RGE of adimensional gravity coupled to a generic matter sector. The Planck scale and a flat space can arise dynamically at quantum level provided that a quartic scalar coupling and its $β$ function vanish at the Planck scale. This is how the Higgs boson behaves for $M_h\approx 125$ GeV at $M_t\approx 171$ GeV. Within agravity, inflation is a generic phenomenon: the slow-roll parameters are given by the $β$-functions of the theory, and are small if couplings are perturbative. The predictions $n_s\approx 0.967$ and $r\approx 0.13$ arise if the inflaton is identified with the Higgs of gravity. Furthermore, quadratically divergent corrections to the Higgs mass vanish: a small weak scale is natural and can be generated by agravity quantum corrections.

hep-ph↗
arXiv2017

Z' models for the LHCb and g-2 muon anomalies

We revisit a class of Z' explanations of the anomalies found by the LHCb collaboration in $B$ decays, and show that the scenario is tightly constrained by a combination of constraints: (i) LHC searches for di-muon resonances, (ii) pertubativity of the Z' couplings; (iii) the $B_s$ mass difference, and (iv) and electro-weak precision data. Solutions are found by suppressing the Z' coupling to electrons and to light quarks and/or by allowing for a Z' decay width into dark matter. We also present a simplified framework where a TeV-scale Z' gauge boson that couples to standard leptons as well as to new heavy vector-like leptons, can simultaneously accommodate the LHCb anomalies and the muon g-2 anomaly.

hep-ph↗
arXiv2016

Fundamental partial compositeness

We construct renormalizable Standard Model extensions, valid up to the Planck scale, that give a composite Higgs from a new fundamental strong force acting on fermions and scalars. Yukawa interactions of these particles with Standard Model fermions realize the partial compositeness scenario. Under certain assumptions on the dynamics of the scalars, successful models exist because gauge quantum numbers of Standard Model fermions admit a minimal enough 'square root'. Furthermore, right-handed SM fermions have an SU(2)$_R$-like structure, yielding a custodially-protected composite Higgs. Baryon and lepton numbers arise accidentally. Standard Model fermions acquire mass at tree level, while the Higgs potential and flavor violations are generated by quantum corrections. We further discuss accidental symmetries and other dynamical features stemming from the new strongly interacting scalars. If the same phenomenology can be obtained from models without our elementary scalars, they would reappear as composite states.

hep-ph↗
arXiv2016

On gravitational and thermal corrections to vacuum decay

We reconsider gravitational corrections to vacuum decay, confirming and simplifying earlier results and extending them by allowing for a non-minimal coupling of the Higgs to gravity. We find that leading-order gravitational corrections suppress the vacuum decay rate. Furthermore, we compute minor corrections to thermal vacuum decay in the SM by adding one-loop contributions to the Higgs kinetic term, two-loop contributions to the Higgs potential and allowing for time-dependent bounces.

hep-ph↗
arXiv2016

The Higgs of the Higgs and the diphoton channel

LHC results do not confirm conventional natural solutions to the Higgs mass hierarchy problem, motivating alternative interpretations where a hierarchically small weak scale is generated from a dimension-less quantum dynamics. We propose weakly and strongly-coupled models where the field that breaks classical scale invariance giving mass to itself and to the Higgs is identified with a possible new resonance within the LHC reach. As an example, we identify such resonance with the 750 GeV diphoton excess recently reported by ATLAS and CMS. Such models can be extrapolated up to the Planck scale, provide Dark Matter candidates and eliminate the SM vacuum instability.

hep-ph↗