115 research outputs found

### Naturalness and Supersymmetry

Supersymmetry solves the gauge hierarchy problem of the Standard Model if the
masses of supersymmetric partners of the SM particles are close to the weak
scale. In this thesis, we argue that the supersymmetric Standard Model, while
avoiding the fine tuning in electroweak symmetry breaking, requires
unnaturalness/fine tuning in some (other) sector of the theory. For example,
Baryon and Lepton number violating operators are allowed which lead to proton
decay and flavor changing neutral currents. We study some of the constraints
from the latter in this thesis. We have to impose an R-parity for the theory to
be both natural and viable. In the absence of flavor symmetries, the
supersymmetry breaking masses for the squarks and sleptons lead to too large
flavor changing neutral currents. We show that two of the solutions to this
problem, gauge mediation of supersymmetry breaking and making the scalars of
the first two generations heavier than a few TeV, reintroduce fine tuning in
electroweak symmetry breaking. We also construct a model of low energy gauge
mediation with a non-minimal messenger sector which improves the fine tuning
and also generates required Higgs mass terms. We show that this model can be
derived from a Grand Unified Theory despite the non-minimal spectrum.Comment: LaTeX, 154 pages, figures. Ph.D. thesis (UC Berkeley, May 1998

### Using Energy Peaks to Measure New Particle Masses

We discussed in arXiv:1209.0772 that the laboratory frame distribution of the
energy of a massless particle from a two-body decay at a hadron collider has a
peak whose location is identical to the value of this daughter's (fixed) energy
in the rest frame of the corresponding mother particle. For that result to hold
we assumed that the mother is unpolarized and has a generic boost distribution
in the laboratory frame. In this work we discuss how this observation can be
applied for determination of masses of new particles, without requiring a full
reconstruction of their decay chains or information about the rest of the
event. We focus on a two-step cascade decay of a massive particle that has one
invisible particle in the final state: C -> Bb -> Aab, where C, B and A are new
particles of which A is invisible and a, b are visible particles. Combining the
measurements of the peaks of energy distributions of a and b with that of the
edge in their invariant mass distribution, we demonstrate that it is in
principle possible to determine separately all three masses of the new
particles, in particular, without using any measurement of missing transverse
momentum. Furthermore, we show how the use of the peaks in an inclusive energy
distribution is generically less affected by combinatorial issues as compared
to other mass measurement strategies. For some simplified, yet interesting,
scenarios we find that these combinatorial issues are absent altogether. As an
example of this general strategy, we study SUSY models where gluino decays to
an invisible lightest neutralino via an on-shell bottom squark. Taking into
account the dominant backgrounds, we show how the mass of the bottom squark,
the gluino and (for some class of spectra) that of the neutralino can be
determined using this technique.Comment: 42 pages, 11 figure

### Top Compositeness and Precision Unification

The evolution of Standard Model gauge couplings is studied in a
non-supersymmetric scenario in which the hierarchy problem is resolved by Higgs
compositeness above the weak scale. It is argued that massiveness of the top
quark combined with precision tests of the bottom quark imply that the
right-handed top must also be composite. If, further, the Standard Model gauge
symmetry is embedded into a simple subgroup of the unbroken composite-sector
flavor symmetry, then precision coupling unification is shown to occur
at~10^{15} GeV, to a degree comparable to supersymmetric unification.Comment: v2: few extra comments added; slightly shorter version published in
PR

### Probing the Randall-Sundrum geometric origin of flavor with lepton flavor violation

The ``anarchic'' Randall-Sundrum model of flavor is a low energy solution to
both the electroweak hierarchy and flavor problems. Such models have a warped,
compact extra dimension with the standard model fermions and gauge bosons
living in the bulk, and the Higgs living on or near the TeV brane. In this
paper we consider bounds on these models set by lepton flavor violation
constraints. We find that loop-induced decays of the form l->l'+gamma are
ultraviolet sensitive and uncalculable when the Higgs field is localized on a
four-dimensional brane; this drawback does not occur when the Higgs field
propagates in the full five-dimensional space-time. We find constraints at the
few TeV level throughout the natural range of parameters, arising from
muon-electron conversion in the presence of nuclei, rare muon decays, and rare
tau decays. A "tension" exists between loop-induced dipole decays such as
mu->e+gamma and tree-level processes such as muon-electron conversion; they
have opposite dependences on the five-dimensional Yukawa couplings, making it
difficult to decouple flavor-violating effects. We emphasize the importance of
the future experiments MEG and PRIME. These experiments will definitively test
the Randall-Sundrum geometric origin of hierarchies in the lepton sector at the
TeV-scale.Comment: 27 pgs, 15 figs; v2: numerical bug in tau decays fixe

### Composite Higgs-Mediated FCNC

We discuss how, in the presence of higher-dimensional operators, the Standard
Model (SM) fermion masses can be misaligned in flavor space with the Yukawa
couplings to the Higgs boson, even with only one Higgs doublet. Such
misalignment results in flavor-violating couplings to the Higgs and hence
flavor-changing neutral current (FCNC) processes from tree-level Higgs
exchange. We perform a model-independent analysis of such an effect.
Specializing to the framework of a composite Higgs with partially composite SM
gauge and fermion fields, we show that the constraints on the compositeness
scale implied by epsilon_K can be generically as strong as those from the
exchange of heavy spin-1 resonances if the Higgs is light and strongly coupled
to the new states. In the special and well motivated case of a composite
pseudo-Goldstone Higgs, we find that the shift symmetry acting on the Higgs
forces an alignment of the fermion mass terms with their Yukawa couplings at
leading order in the fermions' degree of compositeness, thus implying much
milder bounds. As a consequence of the flavor-violating Higgs couplings, we
estimate BR(t -> c h) ~ 10^{-4} and BR(h -> tc) ~ 5 x 10^{-3} both for a
pseudo-Goldstone (if t_R is fully composite) and for a generic composite Higgs.
By virtue of the AdS/CFT correspondence, our results directly apply to
5-dimensional Randall-Sundrum compactifications.Comment: 14 pages; v2: important change in the model-independent analysis of
section 2, a comment added in the conclusions on how our results are modified
by relaxing the assumptions of linear couplings and flavor anarchy. A few
typos corrected, two references added. All conclusions unchanged. Version
published in PR

### Mass Measurement Using Energy Spectra in Three-body Decays

In previous works we have demonstrated how the energy distribution of
massless decay products in two body decays can be used to measure the mass of
decaying particles. In this work we show how such results can be generalized to
the case of multi-body decays. The key ideas that allow us to deal with
multi-body final states are an extension of our previous results to the case of
massive decay products and the factorization of the multi-body phase space. The
mass measurement strategy that we propose is distinct from alternative methods
because it does not require an accurate reconstruction of the entire event, as
it does not involve, for instance, the missing transverse momentum, but rather
requires measuring only the visible decay products of the decay of interest. To
demonstrate the general strategy, we study a supersymmetric model wherein
pair-produced gluinos each decay to a stable neutralino and a bottom
quark-antiquark pair via an off-shell bottom squark. The combinatorial
background stemming from the indistinguishable visible final states on both
decay sides can be treated by an "event mixing" technique, the performance of
which is discussed in detail. Taking into account dominant backgrounds, we are
able to show that the mass of the gluino and, in favorable cases, that of the
neutralino can be determined by this mass measurement strategy.Comment: 42 pages, 12 figures, Journal-submitted versio

### Top quark mass determination from the energy peaks of b-jets and B-hadrons at NLO QCD

We analyze the energy spectra of $single$ b-jets and B-hadrons resulting from
the production and decay of top quarks within the SM at the LHC at the NLO QCD.
For both hadrons and jets, we calculate the correlation of the peak of the
spectrum with the top quark mass, considering the "energy-peak" as an
observable to determine the top quark mass. Such a method is motivated by our
previous work where we argued that this approach can have reduced sensitivity
to the details of the production mechanism of the top quark, whether it is
higher-order QCD effects or new physics contributions. As part of the NLO
improvement over the original proposal, we assess the residual sensitivity of
the extracted top quark mass to perturbative effects both in top quark
production and decay. For a 1% jet energy scale uncertainty (and assuming
negligible statistical error), the top quark mass can then be extracted using
the energy-peak of b-jets with an error +- (1.2 (exp) + 0.6(th)) GeV. We note
that recently the CMS collaboration reported a top quark mass measurement based
on the original proposal (with b-jets) so that our result contributes to a
precise evaluation of the associated theory uncertainty. In view of the
dominant jet energy scale uncertainty in the measurement using b-jets, we also
investigate the extraction of the top quark mass from the energy-peak of the
corresponding B-hadrons which, in principle, can be measured without this
uncertainty. The calculation of the B-hadron energy spectrum is carried out
using fragmentation functions at NLO. The dependence on the fragmentation scale
turns out to be the largest theoretical uncertainty in this extraction of top
quark mass. Future improvement of the treatment of bottom quark hadronization
can reduce this uncertainty, rendering methods based on the B-hadron
energy-peak competitive for the top quark mass measurement.Comment: 5 figures, 12 page

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