arXiv subjects

German Valencia

Publications and source records attributed to German Valencia.

At least 73 records · Page 4Linked to original sources

arXiv2014

Top-quark forward-backward asymmetry from a color-octet t-channel resonance

We consider new physics contributions to the top-quark forward-backward asymmetry from a neutral $V^0_8$ or charged $V^+_8$ color-octet vector exchanged in the $t$-channel. We study the phenomenological constraints on these particles arising from the Tevatron and LHC7 measurements and compare them with those on their color singlet counterparts $Z^\prime$ and $W^\prime$. We find that the color octets fare better than the singlets in that they generate a lower $A_C$, a lower high-invariant mass cross-section at LHC7 and a lower same sign top-pair cross-section. However, they also generate a lower $A_{FB}$ than their color-singlet counterparts.

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arXiv2013

CP violating anomalous couplings in $Wγ$ and $Zγ$ production at the LHC

The ATLAS and CMS collaborations have recently published new limits on CP conserving anomalous couplings from the $Wγ$ and $Zγ$ production processes. We study the corresponding limits that can be placed on the CP violating anomalous couplings $κ_tilde_γ$ and $h^1_{γ, Z}$ at the LHC. We find that the process $pp --> Wγ$ at 14 TeV can place the 95% CL limit $|κ_tilde_γ| <~ 0.05$ with 10 fb^{-1} which is comparable to the existing LHC bound on the CP conserving anomalous couplings $κ_γ$. Similarly, the process $pp --> Zγ$ can place the 95% CL limits $|h^1_γ| <~ 20$ and $|h^1_Z| <~ 40$ respectively. None of these limits is derived from a truly CP-odd observable so that it is not possible to separate the effects of the CP violating anomalous couplings from the rest.

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arXiv2013

Interplay between new physics in one-loop Higgs couplings and the top-quark Yukawa coupling

After the discovery of a 126 GeV state at the LHC it is imperative to establish whether this particle really is the Higgs boson of the standard model. The early measurements have not yet pinpointed any of the Higgs couplings to fermions, the Yukawa couplings of the standard model. In this paper we study the values of the top-quark-Higgs coupling, $g_{ht\bar{t}}$, that are still allowed by the one-loop couplings of the Higgs to two gluons or two photons. We first assume that both the gluon fusion production of the Higgs and its decay into two photons proceed through loops with standard model particles only, albeit with an arbitrary top-Higgs coupling. We find that the current Higgs data still allows for 20% deviations in $g_{ht\bar{t}}$ from its standard model value. We then investigate the effect of new particles contributing to the effective one-loop couplings. Specifically we consider a color octet electroweak doublet extension of the scalar sector and find that in this case $g_{ht\bar{t}}$ is allowed to deviate by 40% from its standard model value by the current data.

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arXiv2013

Unitarity and vacuum stability constraints on the couplings of color octet scalars

The recent discovery of a 126 GeV boson at the LHC will be followed by a detailed examination of its couplings in order to determine whether this particle is the Higgs boson of the standard model or one of many particles of an extended scalar sector. One such extension with a rich phenomenology consists of a color octet electroweak doublet scalar. The most general renormalizable scalar potential contains twelve new parameters and it is therefore desirable to constrain them. We present theoretical constraints on these parameters obtained by requiring perturbative unitarity for two-to-two scalar scattering amplitudes at high energy and vacuum stability.

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arXiv2013

Constraints on anomalous color dipole operators from Higgs boson production at the LHC

Physics beyond the standard model (SM) can be parameterized with an effective Lagrangian that respects the symmetries of the standard model and contains many operators of dimension six. We consider the subset of these operators that is responsible for flavor diagonal anomalous color magnetic (CMDM) and electric (CEDM) dipole couplings between quarks and gluons. Invariance of these operators under the SM implies that they contribute to Higgs boson production at the LHC and we study the corresponding constraints that can be placed on them. For the case of the top-quark we first review constraints from top-quark pair production and decay, and then compare them to what can be achieved by studying $t\bar{t}h$ production. We also constrain the corresponding couplings for $b$-quarks and light quarks by studying $pp \to b\bar{b}h$ and $pp \to hX$ respectively.

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arXiv2013

Probing new physics with tau-leptons at the LHC

We discuss new physics that can show up in the $τ^+τ^-$ production process at the LHC but not in the dimuon or the dielectron channels. We consider three different generic possibilities: a new resonance in the Drell-Yan process in the form of a non-universal $Z^\prime$; a new non-resonant contribution to $q\bar{q}\to τ^+τ^-$ in the form of leptoquarks; and contributions from gluon fusion due to effective lepton gluonic couplings. We emphasize the use of the charge asymmetry both to discover new physics and to distinguish between different possibilities

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arXiv2013

CP violation in top-quark pair production and decay

As the LHC becomes a top-quark factory and enters the era of precision measurements it presents us with a unique opportunity to search for new sources of CP violation. I discuss T-odd triple product correlations and the special role they play in the search for CP violation in top physics. I motivate the searches by discussing two examples of new physics that induce these CP violating observables at the LHC.

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arXiv2012

B decays with $τ$-leptons in non-universal left-right models

Recent measurements of semileptonic $B$-meson decays into $τ$-leptons are somewhat higher than expected in the standard model. Although the deviations are less than 3$σ$, they suggest the possibility of new physics affecting primarily the $τ$-lepton. In this paper we examine these results within the context of non-universal left-right models. We find that strong constraints from $b\to s γ$ on $W-W^\prime$ mixing lead to a prediction of approximately equal enhancements for the $B\to D τν$ and the $B\to D^\star τν$ modes. The model predicts approximately the same enhancement for the inclusive semileptonic rate $\bar{B} \to X_c τν$ as well as for the leptonic decay $B_c^-\to τ^- ν$. An enhancement in the leptonic decay $B^-\to τ^-ν$ is also possible but is not uniquely correlated with the other modes. For this explanation to be viable, the mass of the non-universal $W^\prime$ would be below 1 TeV, within LHC reach.

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arXiv2012

Probing lepton gluonic couplings at the LHC

In an effective Lagrangian description of physics beyond the standard model, gluonic couplings of leptons that respect the symmetries of the standard model occur in operators of dimension eight. Normally the effect of such operators is much suppressed in processes that occur at energies below the scale of new physics. For this reason usual studies of the effective Lagrangian are limited to the lowest dimension operators which is typically six. We point out that the large parton luminosity for gluon-gluon interactions at the LHC places these gluonic couplings to leptons within observable reach for a new physics scale near one TeV.

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arXiv2011

An extended scalar sector to address the tension between a fourth generation and Higgs searches at the LHC

It is expected that the LHC will soon discover the Higgs boson, or that failure to find it will severely constrain its production cross-section over a large mass range. Either one of these results spells trouble for a fourth generation that significantly enhances the Higgs production cross-section at LHC. In fact the LHC has already ruled out a SM Higgs mass in the range of 120 GeV to 600 GeV with a fourth generation at the 95% C.L. In this paper we explore options within extended scalar sectors to maintain the viability of a heavy fourth generation if an enhanced (relative to the Standard Model) Higgs production cross-section is not observed.

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arXiv2011

Color-octet scalars and potentially large CP violation at the LHC

We consider the phenomenology of CP violation in a color-octet extended scalar sector for t\bar{t} production and decay at the LHC. In particular we study the effect of the two neutral color-octet scalars S_I and S_R that occur in the model. There are two new sources of CP violation: a phase in the couplings of S_{I,R} to top-quarks; and two phases in the quartic couplings of the scalar potential. In resonant production of a single S_{I,R} followed by its decay into t\bar{t} pairs through the parton level process gg to S_{I,R} to t\bar{t}, we find large raw CP asymmetries which can reach 12%. These raw asymmetries are, of course, diluted by standard model (SM) t\bar{t} pairs making observation of CP violation contingent on whether the resonance itself can be extracted from the SM background.

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arXiv2011

Tau lepton charge asymmetry and new physics at the LHC

We consider the possibility of studying new physics that singles out the tau-lepton at the LHC. We concentrate on the tau-lepton charge asymmetry in tau+tau- pair production as a tool to probe this physics beyond the Standard Model. We consider two generic scenarios for the new physics. We first study a non-universal Z' boson as an example of a new resonance that can single out tau-leptons. We then consider vector lepto-quarks coupling of the first generation quarks with the third generation leptons as an example of non-resonant new physics. We find that in both cases the charge asymmetry can be sufficiently sensitive to the new physics to provide useful constraints at the LHC.

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arXiv2010

D0 Dimuon Asymmetry in $B_s - \bar B_s$ Mixing and Constraints on New Physics

We study the consequences of the large dimuon asymmetry observed at D0. Physics beyond the standard model (SM) in $B_s-\bar B_s$ mixing is required to explain the data. We first present a detailed analysis for model independent constraints on physics beyond the SM, and then study the implications for theoretical models which modify the SM results in different ways, such as $Z'$ with FCNC and R-parity violating SUSY contributions.

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arXiv2010

CP violation in top-quark physics

In this talk I review how to search for CP violation in top-quark pair production and decay using T-odd correlations. I discuss two examples which illustrate many of the relevant features: CP violation in a heavy neutral Higgs boson; and CP violating anomalous top-quark couplings. I present some numerical results for the LHC and some for the Tevatron.

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arXiv2009

Ansatz for small FCNC with a non-universal $Z^\prime$

It is well known that a non-universal $Z^\prime$ induces tree-level FCNC which are severely constrained by experiment, most notably meson mixing. We point out that there is a class of models, with a down-quark mass matrix of the Georgi-Jarlskog form, in which the FCNC in the down-type quark sector vanish or are strongly suppressed. The largest FCNC in these models would occur in the $tc$ transition with a strength comparable to $V_{ts}$.

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arXiv2007

$D-\bar{D}$ mixing constraints on FCNC with a non-universal $Z^\prime$

The BaBar and Belle collaborations have recently reported evidence for $D^0-\bar D^0$ mixing. This measurement provides the first significant constraint on FCNC in the up-quark sector for non-universal $Z^\prime$ models. Attributing the observed $D-\bar D$ mixing to new physics, we comment on the resulting rare $D$ and $t$ decays. We also show that a CP violating semileptonic asymmetry as large as $\sim 30%$ is allowed by the experimental results.

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arXiv2006

$B_s-\bar B_s$ Mixing constraints on FCNC and a non-universal $Z^\prime$

$B_s - \bar B_s$ mixing has been measured recently by D0 and CDF. The range predicted in the standard model is consistent with data. However, the standard model central values for $ΔM_{B_s}$ and also $ΔM_ {B_d}$ are away from the data which may be indications of new physics. Using the observed values of $ΔM_{B_s}$ and $ΔM_{B_d}$ we study general constraints on flavor changing $Z^\prime$ interactions. In models with non-universal $Z^\prime$ couplings we find that significant enhancements over the standard model are still possible in the rare decay modes $B\to X_s τ^+ τ^-$, $B\to X_s ν\bar ν$ and $B_s\to τ^+ τ^-$. Tree level $Z^\prime$ contributions to $K\to πν\bar ν$ are now constrained to be very small, but one loop effects can still enhance the standard model rate by a factor of two.

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arXiv2005

Implications of a new particle from the HyperCP data on Σ^+ \to p μ^+ μ^-

The HyperCP collaboration has recently reported the observation of three events for the decay Σ^+ \to p mu^+ mu^- with an invariant mass m_{μ^+μ^-} for a muon and an anti-muon pair of \sim 214 MeV. They suggest that a new particle state X may be needed to explain the observed m_{μ^+μ^_} distribution. Motivated by this result, we study the properties of such a hypothetical particle. We first use K^+ \to pi^+ μ^+μ^- data to conclude that X cannot be a scalar or a vector particle. We then collect existing constraints on a pseudo-scalar or axial vector X and find that these possibilities are still allowed as explanations for the HyperCP data. Finally we assume that the HyperCP data is indeed explained by a new pseudo-scalar or axial vector particle and use this to predict enhanced rates for K_L \to π^+π^- X \to π^+π^-μ^+μ^- and Ω^- to Ξ^- X \to Ξ^- μ^+μ^-.

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