-
H Jung,
A de Roeck,
Z. J. Ajaltouni,
S. Albino, G. Altarelli,
F Ambroglini,
J Anderson,
G. Antchev,
M Arneodo,
P Aspell, [......],
Ch. Weiss,
Ch. White,
J Whitmore,
R Wolf,
J Wu,
A. Yagues-Molina,
S. A. Yost,
G. Zanderighi,
N. Zotov,
M zur Nedden
[show abstract]
[hide abstract]
ABSTRACT: 2nd workshop on the implications of HERA for LHC physics. Working groups: Parton Density Functions Multi-jet final states and energy flows Heavy quarks (charm and beauty) Diffraction Cosmic Rays Monte Carlos and Tools
04/2009;
-
[show abstract]
[hide abstract]
ABSTRACT: In view of the fact that the data on neutrino mixing are still compatible with a situation where Bimaximal mixing is valid in first approximation and it is then corrected by terms of order of the Cabibbo angle, arising from the diagonalization of the charged lepton masses, we construct a model based on the discrete group S4 where those properties are naturally realized. The model is supersymmetric in 4-dimensions and the complete flavour group is S4 x Z4 x U(1)_FN, which also allows to reproduce the hierarchy of the charged lepton spectrum. The only fine tuning needed in the model is to reproduce the small observed value of r, the ratio between the neutrino mass squared differences. Once the relevant parameters are set to accommodate r then the spectrum of light neutrinos shows a moderate normal hierarchy and is compatible, within large ambiguities, with the constraints from leptogenesis as an explanation of the baryon asymmetry in the Universe. Comment: 30 pages, 5 figures; added references
03/2009;
-
M Dittmar,
S. Forte,
A. Glazov,
S. Moch, G. Altarelli,
J Anderson,
R. D. Ball,
G. Beuf,
M Boonekamp,
H Burkhardt, [......],
C Royon,
G. P. Salam,
D. Salek,
A. M. Stasto,
R. S. Thorne,
M. Ubiali,
J. A. M. Vermaseren,
A Vogt,
G Watt,
C. D. White
[show abstract]
[hide abstract]
ABSTRACT: We provide an assessment of the state of the art in various issues related to experimental measurements, phenomenological methods and theoretical results relevant for the determination of parton distribution functions (PDFs) and their uncertainties, with the specific aim of providing benchmarks of different existing approaches and results in view of their application to physics at the LHC. We discuss higher order corrections, we review and compare different approaches to small x resummation, and we assess the possible relevance of parton saturation in the determination of PDFS at HERA and its possible study in LHC processes. We provide various benchmarks of PDF fits, with the specific aim of studying issues of error propagation, non-gaussian uncertainties, choice of functional forms of PDFs, and combination of data from different experiments and different processes. We study the impact of combined HERA (ZEUS-H1) structure function data, their impact on PDF uncertainties, and their implications for the computation of standard candle processes, and we review the recent F_L determination at HERA. Finally, we compare and assess methods for luminosity measurements at the LHC and the impact of PDFs on them.
02/2009;
-
M Dittmar,
S. Forte,
A. Glazov,
S. Moch,
S. Alekhin, G. Altarelli,
J Andersen,
R. D. Ball,
J. Blumlein,
H. Bottcher, [......],
A. Sabio Vera,
G. P. Salam,
F. Siegert,
A. Stasto,
H Stenzel,
C Targett-Adams,
R. S. Thorne,
A Tricoli,
J. A. M. Vermaseren,
A Vogt
[show abstract]
[hide abstract]
ABSTRACT: We provide an assessment of the impact of parton distributions on the determination of LHC processes, and of the accuracy with which parton distributions (PDFs) can be extracted from data, in particular from current and forthcoming HERA experiments. We give an overview of reference LHC processes and their associated PDF uncertainties, and study in detail W and Z production at the LHC. We discuss the precision which may be obtained from the analysis of existing HERA data, tests of consistency of HERA data from different experiments, and the combination of these data. We determine further improvements on PDFs which may be obtained from future HERA data (including measurements of $F_L$), and from combining present and future HERA data with present and future hadron collider data. We review the current status of knowledge of higher (NNLO) QCD corrections to perturbative evolution and deep-inelastic scattering, and provide reference results for their impact on parton evolution, and we briefly examine non-perturbative models for parton distributions. We discuss the state-of-the art in global parton fits, we assess the impact on them of various kinds of data and of theoretical corrections, by providing benchmarks of Alekhin and MRST parton distributions and a CTEQ analysis of parton fit stability, and we briefly presents proposals for alternative approaches to parton fitting. We summarize the status of large and small x resummation, by providing estimates of the impact of large x resummation on parton fits, and a comparison of different approaches to small x resummation, for which we also discuss numerical techniques. Comment: Report of Working Group I for the HERA-LHC Workshop
11/2005;
-
M. Dittmar,
S. Forte,
A. Glazov,
S. Moch,
S. Alekhin, G. Altarelli,
Jeppe R. Andersen,
R.D. Ball,
J. Blumlein,
H. Bottcher, [......],
M. Ciafaloni,
D. Colferai,
A. Cooper-Sarkar,
G. Corcella,
L. Del Debbio,
G. Dissertori,
J. Feltesse,
A. Guffanti,
C. Gwenlan,
J. Huston
[show abstract]
[hide abstract]
ABSTRACT: We provide an assessment of the impact of parton distributions on the determination of LHC processes, and of the accuracy with which parton distributions (PDFs) can be extracted from data, in particular from current and forthcoming HERA experiments. We give an overview of reference LHC processes and their associated PDF uncertainties, and study in detail W and Z production at the LHC.We discuss the precision which may be obtained from the analysis of existing HERA data, tests of consistency of HERA data from different experiments, and the combination of these data. We determine further improvements on PDFs which may be obtained from future HERA data (including measurements of F{sub L}), and from combining present and future HERA data with present and future hadron collider data. We review the current status of knowledge of higher (NNLO) QCD corrections to perturbative evolution and deep-inelastic scattering, and provide reference results for their impact on parton evolution, and we briefly examine non-perturbative models for parton distributions. We discuss the state-of-the art in global parton fits, we assess the impact on them of various kinds of data and of theoretical corrections, by providing benchmarks of Alekhin and MRST parton distributions and a CTEQ analysis of parton fit stability, and we briefly presents proposals for alternative approaches to parton fitting. We summarize the status of large and small x resummation, by providing estimates of the impact of large x resummation on parton fits, and a comparison of different approaches to small x resummation, for which we also discuss numerical techniques.
10/2005
-
G. Altarelli
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 07/2003;
-
[show abstract]
[hide abstract]
ABSTRACT: We explore interpretations of the anomaly observed by H1 and ZEUS at HERA in deep-inelastic e + p scattering at very large Q 2 , in terms of possible physics beyond the Standard Model. Since the present data could be compatible with either a continuum or a resonant solution, we discuss both the possibilities of new effective interactions and the production of a narrow state of mass M ¸ 200 GeV with leptoquark couplings. We compare these models with the measured Q 2 distributions: for the contact terms, constraints from LEP 2 and the Tevatron allow only a few choices of helicity and flavour structure that could roughly fit the HERA data. The data are instead quite consistent with the Q 2 distribution expected from a leptoquark state. We study the production cross sections of such a particle at the Tevatron and at HERA, the latter in the cases where it is produced from either a valence or a sea quark. The absence of a signal at the Tevatron disfavours the likelihood that any such...
06/1998;
-
[show abstract]
[hide abstract]
ABSTRACT: We review the analysis of polarized structure function data using perturbative QCD at next-to-leading order. We use the most recent experimental data to obtain updated results for polarized parton distributions, first moments and the strong coupling. We also discuss several theoretical issues involved in this analysis and in the interpretation of its results. Finally, we compare our results with other similar analyses in the recent literature. Comment: 29 pages, Latex, uses epsfig, 11 eps figures included. Talks given by G. Altarelli and G. Ridolfi at the Cracow Epiphany Conference on Spin Effects in Particle Physics, January 9-11, 1998, Cracow, Poland. Fig. 5 added and a few typos corrected
03/1998;
-
[show abstract]
[hide abstract]
ABSTRACT: 1. Introduction 2. Status of the Data 3. Precision Electroweak Data and the
Standard Model 4. A More General Analysis of Electroweak Data
4.1 Basic Definitions and Results
4.2 Experimental Determination of the Epsilon Variables
4.3 Comparing the Data with the Minimal Supersymmetric Standard Model 5.
Theoretical Limits on the Higgs Mass 6. Conclusion
12/1997;
-
G. Altarelli
[show abstract]
[hide abstract]
ABSTRACT: I present a concise review of the possible evidence for new physics at HERA and of the recent work towards a theoretical interpretation of the signal. It is not clear yet if the excess observed at large Q^2 is a resonance or a continuum (this tells much about the quality of the signal). I discuss both possibilities. For the continuum case one considers either modifications of the quark structure functions or contact terms. In the case of a resonance, a leptoquark, the most attractive possibility that is being studied is in terms of s-quarks with R-parity violation. In writing this script I updated the available information to include the new data and the literature presented up to August 1, 1997. Comment: 10 pages, LaTeX, uses fleqn,twoside,espcrc2, Talk given at SUSY'97
08/1997;
-
[show abstract]
[hide abstract]
ABSTRACT: An excess of events at large Q2 with a positron in the final state has been observed at HERA which, if confirmed, would be a signal of new physics. It is not clear at present if a signal of comparable rate is also seen in the charged current channel (with an antineutrino in the final state). In this note we analyse the implications of the presence of such a signal in models of new physics based on contact terms, leptoquarks and squarks with R-violating decays. We find that in all cases the most likely possibility is that the charged current signal is absent. As a consequence if this signal is present the resulting indications are very selective. In particular for squarks only charged current events with multi-quark final states are possible with quite definite predictions on the spectrum of supersymmetric particles.
06/1997;
-
[show abstract]
[hide abstract]
ABSTRACT: We explore interpretations of the anomaly observed by H1 and ZEUS at HERA in deep-inelastic e^+ p scattering at very large Q^2. We discuss the possibilities of new effective interactions and the production of a narrow state of mass 200 GeV with leptoquark couplings. We compare these models with the measured Q^2 distributions: for the contact terms, constraints from LEP2 and the Tevatron allow only a few choices of helicity and flavour structure that could roughly fit the HERA data. The data are instead quite consistent with the Q^2 distribution expected from a leptoquark state. We study the production cross sections of such a particle at the Tevatron and at HERA. The absence of a signal at the Tevatron disfavours the likelihood that any such leptoquark decays only into e^+ q. We then focus on the possibility that the leptoquark is a squark with R-violating couplings. In view of the present experimental limits on such couplings, the most likely production channels are e^+d -> scharm_L or perhaps e^+d->stop, with e^+s->stop a more marginal possibility. Possible tests of our preferred model include the absence both of analogous events in e^- p collisions and of charged current events, and the presence of detectable cascade decays whose kinematical signatures we discuss. We also discuss the possible implications for K->pi nu nubar, neutrinoless double-beta decay, the Tevatron and for e^+ e^- ->q qbar and neutralinos at LEP2. Comment: 28 pages, Latex, epsfig, 8 figures. Note added on contact terms
03/1997;
-
[show abstract]
[hide abstract]
ABSTRACT: We show that the reported anomalies in $R_b$ and $R_c$ can be interpreted as
the effect of a heavy vector boson \zprime\ universally coupled to $u$- and
$d$-type quarks separately and nearly decoupled from leptons. This extra vector
boson could then also naturally explain the apparent excess of the jet rate at
large transverse momentum observed at CDF.
01/1996;
-
[show abstract]
[hide abstract]
ABSTRACT: Using the recent relatively precise experimental results on the pion structure function obtained from Drell-Yan processes, we quantitatively test an old model where the structure function of any hadron is determined by that of its constituent quarks. In this model the pion structure function can be predicted from the known nucleon structure function. We find that the data support the model, at least as a good first approximation.
Physics Letters B. 10/1995;
-
Nuclear Physics B - Proceedings Supplements 02/1995; 39:106-117. · 0.88 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The divergent large-order behaviour of the perturbative series relevant for the determination of $\as$ from $\tau$ decay is controlled by the leading ultraviolet (UV) renormalon. Even in the absence of the first infrared (IR) renormalon, an ambiguity of order $\Lambda^2/m_\tau^2$ is introduced. We make a quantitative study of the practical implications of this ambiguity. We discuss the magnitude of UV renormalon corrections obtained in the large-$N_f$ limit, which, although unrealistic, is nevertheless interesting to some extent. We then study a number of improved approximants for the perturbative series, based on a change of variable in the Borel representation, such as to displace the leading UV renormalon singularity at a larger distance from the origin than the first IR renormalon. The spread of the resulting values of $\as(m^2_\tau)$ obtained by different approximants, at different renormalization scales, is exhibited as a measure of the underlying ambiguities. Finally, on the basis of mathematical models, we discuss the prospects of an actual improvement, given the signs and magnitudes of the computed coefficients, the size of $\as(m^2_\tau)$ and what is known of the asymptotic properties of the series. Our conclusion is that a realistic estimate of the theoretical error cannot go below $\delta\as(m^2_\tau) \sim \pm 0.060$, or $\delta\as(m^2_{\sss Z}) \sim \pm 0.006$. Comment: 32 pages, epsfig.sty.
01/1995;
-
[show abstract]
[hide abstract]
ABSTRACT: We analyse the available data on the polarized asymmetries $A_1$ for proton, neutron and deuteron targets. We use a homogeneous and updated set of unpolarized structure functions to derive $g_1$ from $A_1$, and we accurately correct for the scaling violations in order to obtain $g_1(x,Q^2)$ with the same $Q^2$ for all $x$ values. The contribution to the $Q^2$ evolution of a possible large gluon polarized density is also considered. The implications for the Ellis-Jaffe and for the Bjorken sum rules are discussed. Comment: 11 pages (LaTex) + 4 figures (topdrawer, included at the end) CERN-TH 7023/93
11/1993;
-
[show abstract]
[hide abstract]
ABSTRACT: Production and detection of new heavy vector bosons is studied at present (or near future)p
[`(p)]\bar p
collider experiments. Application to CERN and Tevatron experiments is considered in detail, but some results for the UNKp
[`(p)]\bar p
collider, at
Ös\sqrt s
= 6TeV, are also presented. Rather than considering a number of detailed models, we prefer to discuss some general, although necessarily schematic, classes of new vector bosons ranging from strongly interacting Higgs sector models to extended gauge models. We study in particular signals and backgrounds in the
l[`(l)] jj\ell \overline \ell jj
channels, where are leptons andj are hadron jets.
Zeitschrift für Physik C 01/1989; 45(1):109-121.
-
[show abstract]
[hide abstract]
ABSTRACT: A thorough analysis is presented of the hadronic transverse energy produced in association withW
andZ
0 weak bosons atp
[`(p)]\bar p
colliders. The complete rapidity and transverse energy distributions are derived, including the perturbative annihilation and Compton terms and the resummation of soft gluons. The first non leading corrections to the corresponding Sudakov exponent are included in full and their effects are discussed. To make contact with the actual data a number of non perturbative inputs is also necessary. We extract this information from minimum bias measurements. A detailed comparison of the resulting theoretical predictions with the available experimental data is finally presented.
Zeitschrift für Physik C 08/1986; 32(3):369-375.
-
[show abstract]
[hide abstract]
ABSTRACT: We develop in detail the phenomenological implications of our previous analysis of Drell-Yan processes and of vector boson production. Production cross-sections andy,q
T distributions ofW andZ
o in the energy range up to
ÖS\sqrt S
=(10–20) TeV are discussed. The problem ofW andZ
o production at super colliders is complicated by the very small values of
Ö{t} = Q \mathord/
\vphantom Q ÖS ÖS \sqrt \tau = {Q \mathord{\left/ {\vphantom {Q {\sqrt S }}} \right. \kern-\nulldelimiterspace} {\sqrt S }}
involved, whereQ=M
W.Z. The novel theoretical features of Drell-Yan processes at very small
Ö{t}\sqrt \tau
are therefore analysed. The dependence of cross-sections and distributions on the massQ of the vector boson is also discussed at different energies
ÖS\sqrt S
and the pattern of deviations from naive dimensional scaling is investigated. Particular attention is devoted to the tail of events at largeq
T where aW/Z
o is produced in association with hadronic jets. The probability ofW/Z
o production atq
T>q
T
o
is studied as a function ofq
T
o
and
ÖS\sqrt S
. The behaviour of the average value ofq
T, which is also determined by the largeq
T tail, is evaluated at various values of ÖS\sqrt S
. Detailed predictions forW andZ
o production at
ÖS\sqrt S
=630 GeV, the energy of the present run at the CERN collider are presented. Finally, ordinary Drell-Yan lepton pair production at the collider is also considered.
Zeitschrift für Physik C 11/1985; 27(4):617-632.
