mardi 22 décembre 2015

Sailing with three nonzero vev scalars in the wind of Adimensional gravity, Planck Mass excursion : 250

Is it enough to circumnavigating the grand loop of physics?
Today is the Winter solstice. Younger I would spend the shortest day and part of its longest night in a sofa reading the Treasure Island. Nowadays I am old enough to indulge myself : dreaming of a different kind of quest where gold coins have been traded for quanta of heavy scalar fields, taking part in a mutiny against the TeV scale physics doxa, looking for broader theoretical horizons and a clearer phenomenological outlook... Here is my last reading for the ending year 2015 :
The current treatment is devoted to simultaneously address a number of these problems within a unified and consistent classically scale invariant framework. Specifically, an analysis of the slow-roll inflationary paradigm, dark matter, and the neutrino mass generation mechanism is presented, in which the various scales are free of the mutual quadratic destabilizations [1a, 1b]. 
The model concerns the minimal addition of a complex gauge singlet to the SM content, in a scale- and CP symmetric manner. It has been previously demonstrated [2] that, within such an extended scalar sector, the ColemanWeinberg mechanism [3] may be successfully realized, while accommodating the recently discovered 125 GeV Higgslike state [4a,4b], and, therefore, remedy the failure of the mechanism within the ordinary SM scalar sector. In particular, the dynamical generation of a nonzero VEV for the CP-even component of the complex singlet scalar via this mechanism can be transmitted to the electroweak sector via the Higgs portal operators, inducing the nonzero VEV for the SM Higgs boson, and thereby giving rise to a successful spontaneous breaking of the electroweak symmetry. The classical scale symmetry, subsequently, guarantees the absence of any quadratic destabilization between the singlet and the electroweak scales. The CP-odd singlet component cannot decay due to the imposed CP-invariance, and constitutes a dark matter candidate. In addition, incorporating the see-saw mechanism [5] within this framework, by including three flavors of the right-handed Majorana neutrinos, yields nonzero masses for the SM neutrinos. The dark matter and collider phenomenology of the theory around the TeV scale were studied in [6, 7], whereas the possibility for realizing a strongly first-order electroweak phase transition, important for the baryogenesis paradigm, was exhibited in [8]. Hence, this technically-natural (minimal and yet comprehensive) framework presents an extremely promising and economical theoretical route to pursue, from a model-building perspective. 
In this analysis, we further investigate incorporating the slow-roll inflationary paradigm within the described framework, while additionally accounting for the gravitational effects by utilizing a renormalizable and scale symmetric theory of gravity, known as the Agravity [9]. Within this fully scale invariant framework, the dynamically-generated nonzero VEV of the CP-even singlet scalar induces the Planck scale via the scalar non-minimal couplings, in addition to the aforementioned generation of the electroweak scale via the Higgs portal operators. These induced scales are, then, shown to be free of quadratic divergences, and thus stable, as a consequence of the scale symmetry. The availability of a renormalizable gravitational UV-completion candidate, consistent with the symmetries of theory, allows for the proper trans-Planckian field excursions, while the stability of the vacuum and the perturbativity of the couplings are satisfied to large trans-Planckian energies. Identifying the pseudo-Nambu-Goldstone boson of the (approximate) scale symmetry with the inflaton field, we explore the viability of the inflationary paradigm according to the latest observational values by the Planck collaboration [10],[ Studies of the classically scale-invariant inflation, in various contexts, are available within the literature. See, e.g. [11, 12]]  and the reheating of the Universe due to the decay of the inflaton. Moreover, we demonstrate that the aforementioned pseudoscalar dark matter candidate constitutes a WIMPzilla [13], satisfying the observed relic abundance [10]. Hence, several important and pressing issues, faced by the contemporary physics, are captured within a single consistent framework.
... within the framework under consideration, it is sufficient to demand that the gauge, fermionic, and scalar couplings remain perturbative up to at least an energy scale of 250 {times the reduced Planck massP, where the internal consistency of the theory is guaranteed...  
In our developed model, the sole relevant scalar degree of freedom along the flat direction with a non-vanishing (loop-induced) potential is the σ boson. In this section we analyze the consequences of identifying this degree of freedom, in its canonical form, with the inflaton... and confront our model with the available cosmological data. For this purpose, we consider the slow-roll inflation paradigm...
The nS − r plane, incorporating several observational data sets by the Planck collaboration [10] at 68% C.L. (darker region) and at 95% C.L. (lighter region). Model’s predictions for both the small and large field inflation scenarios are also depicted (dashed lines) for the e-folding numbers, N = 60, 80, and the full range of the mixing angle, sin ω2 {between scalar singlet (σ) and gravity (κ) sectors}. Near the sin ω2 → 0 limit, the model reduces to the ordinary chaotic inflation scenario, indicated by the diagonal line labeled as “ϕ2 -model” within the plot

Marginalized probability distributions displayed within the sin ω2 − M/M¯P plane (left) and the N − M/P  plane (right), with 68% C.L. (darker inner region) and 95% C.L. (lighter outer region), obtained by the parameter estimation using the Planck TT+lowP (red) and Planck TT, TE, EE+lowP (blue) data sets {N stands for the e-folding number, M for a mass scale related to the σ singlet mass and for the reduced Planck mass}.

The best-fit locations in the parameter space are determined as ... (log10(M/P), log10 sinω2, N) = (−1.42, −1.01, 65.05) ... for the Planck TT, TE, EE+lowP data set. One deduces from this statistical analysis that the most likely value of the mixing angle compatible with the observation is limited to sinω2∼0.1, whereas the corresponding most-favorable value of the mass combination lies around M∼1017 GeV, with a most-likely e-folding number in the vicinity of N∼65.

Dominant final state products of the χ WIMPzilla pair-annihilation. Various scalar final states (including non-identical pairs) are obtained via the contact interactions, as well as the s, t, u-channel processes (diagrams in the top row from left to right), whereas the fermionic and vector boson final states only proceed through the s-channel (bottom row). The relevant mediator(s) are indicated within each diagram. Note that the tree-level σσh, σσκ, σσσ, and σκh couplings are absent due to the classical scale symmetry
... the pseudoscalar dark matter, χ, constitutes a WIMPzilla candidate [13], with a mass much larger than the reheating temperature, which becomes non-relativistic at the time of the reheating. As the inflaton mass is also larger than the reheating temperature, hence, only a sufficiently light scalar graviton, κ, can thermalize and contribute to {the effective number of thermalized relativistic degrees of freedom at Treh equal to} 107.75 for Mκ  Treheating... 
Furthermore, one observes that the WIMPzilla mass, compatible with the dark matter relic abundance, is confined below ∼1013 GeV. This (rather general) result is a direct consequence of the constraints imposed by the inflation, combined with the WIMPzilla nature of the dark matter. In particular, the small amplitude of the primordial scalar perturbations, As∼10-9, implies an accordingly small amplitude for the inflaton potential,... M < 0.1P, within vast regions of the parameter space... This, in turn, introduces a relatively light inflaton..., with a mass several orders of magnitude below the reduced Planck mass, which results in moderate reheating temperatures ... Within the WIMPzilla paradigm, the dark matter mass satisfies the condition Mχ < 2000 Treh, and hence may not be too heavy for moderate reheating temperatures. In addition, small values of M, as favored by the inflationary constraints, require the masses of the bosonic and fermionic degrees of freedom to reside in the relative vicinity of one another...
We have identified the viable regions of the parameter space, which simultaneously accommodate {the perturbativity and vacuum stability, inflation, and the dark matter constraints}... In particular, we have reached the important (and rather general) conclusion that within a classically scale invariant framework in which the pseudo-Nambu-Goldstone boson of the (approximate) scale symmetry is identified with the inflaton, the masses of the inflaton and the WIMPzilla, as well as the resulting reheating temperature are (much) smaller than the reduced Planck scale, while satisfying the inflationary and the relic abundance observational values.

(Submitted on 18 Dec 2015)

mardi 15 décembre 2015

They collide hadrons. Don't they?

They kill softly physical models too
No diboson excess around 2 TeV in the data collected at 13TeV for CMS as far as I understand it ...
More to come later (after work ;-)


The 2TeVbump@LHC chronicle probably comes to a dead end
The preliminary results presented by the ATLAS collaboration point to the same conclusion as CMS (slides below) as far as the former reported deviations from the standard model predictions near 2 TeV as concerned... They were probably bad correlated statistical flukes.

LHC2 : Open to us the gates beyond the realms of the Standard Model!

Waiting for a sort of higher energy particle physics model Bingo
Neither the usual wanderer in this blog nor other science enthusiasts ignore any more (particularly after the good teasing by Jon Butterworth in his most recent post from the Guardian) that the CERN scientists have scheduled a seminar for 3 p.m (2 p.m UTC/GMT) today to deliver what could be exciting new results from their first fishing in the uncharted 13 teraelectronvolts.

To wait and see this potentially memorable event (commented in real time by Tommaso Dorigo) I suggest to bring the "bingo grid" below and have a look on the review article it is extracted from  in order to understand it and imagine the rules of the game one could indulge to play watching the slides of the seminar. 

The aim of this paper is to summarize in a single document both the experimental and the theoretical situation of the ’diboson’ excess in preparation for the data coming from the LHC 13 TeV run. Should the excess be confirmed the reader could easily use this document as a first point to check which one of the possible approaches are more likely to be able to explain not only the existing excesses but any other that may come in different channels. By having a comprehensive analysis of the different ideas we hope to present a clear explanation of the situation... 

For the purpose of a condensed phenomenological overview of models addressing the diboson excess, in {the t}able {below} 
we summarize the main production modes and decay channels of the particles responsible for the excesses in the different setups. We classify the models according to the nature of the relevant resonance(s), here spin and charges, and not according to the UV theory (which might be weakly or strongly coupled). Analyses not fitting in this simple format are listed under ’Unconventional’

(Submitted on 14 Dec 2015)

samedi 12 décembre 2015

A purported excess at 700 GeV for what kind of events at LHC2?

//last edition December 13 2015
A bump on a heavy diquark-invariant mass distribution for Higgsmas 2015 
The blogosphere has started to propagate a rumor about what physicists from CERN might talk about next week (see the former post). It's all about "a 750 GeV diphoton excess at >3σ, seen by both ATLAS and CMS" and "seems very hard to accommodate with the absence of such a bump in Run 1" to quote Jackson Clarke at Syymmetries. To celebrate Higgsmas 2015, I propose below another subject with slightly different premises (same energy range anomaly but in a very different channel) but which might have connection too with a potential future, who knows. Last but not least the authors of the following work propose an explanation for the absence of their hypothetical excess in LHC1 data so we can already learn some physics from the rumor eventually.  
At present the Standard Model (SM) successfully describes all experimental data in particle physics. Moreover, it is theoretically consistent and applicable up to the Planck scale, MPl ≈ 1.2×1019 GeV. On the other hand, there are many natural questions, which cannot be answered within the SM framework. For example, chiral anomalies are canceled only when quarks and leptons are considered simultaneously. At the same time these two sectors are completely independent within the SM. 
The hope that the different gauge coupling constants of the SU(3)C×SU(2)W×U(1)Y  SM group meet at a single unification point has failed [1]. Therefore, if we, nevertheless, expect such unification, new physics should be introduced at some scale above the electroweak unification. In this paper we will consider the one-loop approximation to the gauge coupling evolution modifying only the Higgs sector of the SM. The matter sector of the SM consists of electroweak doublets: fermionic and bosonic ones. The SM contains only one bosonic doublet of the Higgs fields. We will assume that the number of the bosonic doublets N above some scale could be greater than one, while the number of the fermionic doublets is not modified. At present there are practically no limitations on the number N of the Higgs doublets from the precision low-energy measurements [2]. 
... For one SM Higgs doublet, N=1, there is no unique scale μ, where {there is a unification of the Standard Model gauge couplings}. However, if at some scale μ  new states start to make additional contribution to the gauge coupling evolution, this unification point can be found... 
the physically acceptable result μ>MZ is possible only for N≥8. Therefore, the lightest states, which can provide unification, correspond to N=8 and the scale μ=692+144-120 GeV... In the following only this possibility will be discussed.
... the extension of the SM with seven additional Higgs doublets looks awkward. Here we will propose a different interpretation of the given result. In [4] it was shown that the introduction of the weak-doublet spin-1 bosons Vµ=(Zµ ,W⋆-µ) with the internal quantum numbers identical to the SM Higgs doublet is motivated by the hierarchy problem. It means that each spin-0 Higgs doublet is associated with the corresponding spin-1 doublet and vice versa... 
Their interactions with SU(2)W×U(1)Y  gauge fields are similar to the interactions of the SM Higgs doublet due to identical internal quantum numbers. The massive vector boson has three physical degrees of freedom and contributes to the gauge coupling evolution in the one-loop approximation three times more strongly than the scalar boson. Therefore, introduction of one pair of scalar and vector doublets is equivalent to the four Higgs doublets content. So, the solution with N = 8 can be interpreted as an extension of the SM Higgs sector with one additional Higgs doublet and two corresponding vector doublets. That is exactly the set of fields which was proposed in [5]. It was shown [5] that the second pair of scalar and vector doublets with opposite hypercharges is necessary to cancel the chiral anomaly.
...we concentrate here on a less-known issue connected with the production and decay of vector doublets. Interactions of vector doublets resemble scalar Higgs doublet couplings. Therefore, experimental signatures should be within the scope of those for the Higgs searches, although with obvious differences due to different spins. 
For example, the leading channel for the Higgs production at the LHC, gluon-gluon fusion through the t-quark loop, is not operative or suppressed for production of vector bosons due to the Landau–Yang theorem [12]. Vector fields cannot have nonzero vacuum expectation value unless Lorentz symmetry violation exists. Therefore, analogs of the Higgs-strahlung and weak vector boson fusion production processes are also absent for the vector doublet boson production. For the same reason the new vector boson cannot decay into two photons or two Z bosons, which are used as very clean channels for precise reconstruction of the Higgs mass. The only highly suppressed processes of heavy quark–antiquark fusion can produce resonantly the new vector bosons (Fig{ure below}).

resonant mechanism for the production of the new vector bosons
...the extension of the SM Higgs sector leads to a unification scale around 5×1013 GeV. This value has many specific features. For example, if the Majorana mass of a sterile right-handed neutrino is of the order of the unification scale, then the light neutrino states should have the mass of the expected order mν∼ v2/2μ ≈ 0.6 eV due to the see-saw mechanism [20]. Here v is the vacuum expectation value of the Higgs field. This not so high unification scale is closer to the allowed heavy Majorana neutrino masses for successful baryogenesis through leptogenesis [21]. This scale does not destroy naturality from the Planck scale [22] δmh∼ μ3/(4π)3M2Pl ≈0.5 GeV. On the other hand, the new lightest states at the scale μ≈700 GeV maintain naturality, solving the hierarchy problem [4]. The introduction of the spin-1 doublets with the vector degrees of freedom replaces the introduction of many scalar states with degenerate masses... 

The tb and bt¯-invariant mass distributions at √ s = 13 TeV.
...the gauge coupling unification can be achieved in the one-loop approximation by extension of the Standard Model scalar sector only with extra Higgs doublets. As a result, the unification scale is lower than other scales known in the literature and does not depend on the extra Higgs doublets, whereas a new physics scale, at which new scalar degrees of freedom become active, can be well below 1 TeV. However, this scale of new physics is reached at total number of 8 Higgs doublets, which looks very awkward. 
Therefore, we assume that spin-1 vector bosons can play the role of some of scalar degrees of freedom. In this case we get a compact fields content: two Higgs and two spin-1 doublets. However, such light states were not found in the first LHC run. The reason, as we see it, is in accepting the hypothesis of family universality of vector doublet interactions with quarks and leptons. If the vector doublet interactions resemble the Higgs fermion couplings, the new spin-1 bosons cannot be produced in light quark–antiquark annihilation from the proton beams and cannot decay into light lepton pairs as well. 
This means that the production and the decay of the new heavy bosons should be associated only with heavy band t-quarks. Moreover, the increasing gluon luminosity due to higher centre-of-mass energies in the second LHC run will lead to an order of magnitude higher cross sections for the considered processes than in the first LHC run. In conclusion, we would like to stress out that the new channel gb → tW⋆- → tb can be very useful for early new physics search.

(Submitted on 25 Sep 2015)

lundi 7 décembre 2015

The most wanted particle after 4 fb-1 of data at LHC Run 2 : a Z' ?

A handful of new particles for Higgsmas? May be...
What have we learned from the LHC this year, our first year of data-taking at 13 TeV, the highest collision energies we have ever achieved, and the highest we might hope to have for years to come?

We will get our first answers to this question at a CERN seminar scheduled for Tuesday, December 15, where ATLAS and CMS will be presenting physics results from this year’s run. The current situation is reminiscent of December 2011, when the experiments had recorded their first significant datasets from LHC Run 1, and we saw what turned out to be the first hints of the evidence for the Higgs boson that was discovered in 2012.
Quantum Diaries blog 12/05/2015

Afficher l'image d'origine
Event display of a candidate electron-positron pair with an invariant mass of 2.9 TeV reported by the CMS collaboration for an integrated luminosity of 65 pb-1 at LHC13 that could be interpreted as a Z' resonance in in a model with SU(3)C×SU(2)L×SU(2)R×U(1)B−L gauge structure, a Higgs sector with only a bidoublet and an SU(2)R doublet, and a flavor symmetry that controls the masses of the right-handed neutrinos.

The Z' production cross section at the 13 TeV LHC, computed at leading order with MadGraph and multiplied by K(Z') = 1.16 is shown in Figure 5 {below}. For MZ' = 2.9 TeV (corresponding to gR = 0.48 for MW' = 1.9 TeV) the production cross section is ∼ 19 fb. Assuming similar acceptance-times-efficiency at Run 2 as at Run 1, we predict approximately 5 dilepton Z' events after 5 fb-1. Intriguingly, after 65 pb-1 CMS has already observed a dielectron event with invariant mass of 2.9 TeV; the probablity for this event to be due to the SM background is ∼ 10-3 [43]. The scattering angle of the electron, in the Collins-Soper frame, for this event is negative, as expected for the Z' in this model. Note also that the Z' resonance is narrow in this model. For a Z' mass of 2.9 TeV (corresponding to gR = 0.48) the total width is ΓZ' ≈ 38 GeV; for MZ' = 4.5 TeV (gR = 0.4) we find ΓZ' ≈ 130 GeV. 
 Figure 5: Production cross section, including a K-factor of 1.16, for a Z' boson of mass 2.9, 3.5, 4, and 4.5 TeV at the LHC with √s=13 TeV. The thicker region of each curve denotes the range of gR for which 2 ≥ MW' ≥ 1.8 TeV, with the marked point at MW'=1.9 TeV. 

Bogdan A. Dobrescu, Patrick J. Fox (Submitted on 6 Nov 2015)

A bunch of Geminid meteors? For sure!

Afficher l'image d'origine

The two nights before the December 15 meeting at CERN are expected to be peak nights for observing the 2015 Geminid meteor shower. Then why not wishing upon a shooting star for future experimental evidence of new particles beyond the Higgs completed standard model? Of course it requires less efforts than an educated guess from a few anomalies but it is worth a bet on one event in a large hadron collider... 

//last edition December 9 2015.