High-x and High-Q² Analyses
High-x and High-Q² Analyses
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In the years 1994 to 1996, ZEUS has collected more than 20 inverse picobarns of positron-proton scattering data. A first analysis of these data covered the hitherto unexplored region of very high x and Q². An excess of events is observed in this region: We found two ep->eX events for Q²>35000;GeV², where we only expect 0.145±0.013 events, corresponding to a Poisson probability of 0.0096. Four events are observed for x>0.55 and y>0.25, where 0.91±0.08 events are expected (Poisson probability 0.014).
Meanwhile, also high-Q² Charged Current results are available. Also, both for Neutral and Charged Current Scattering the first 13.3 inverse picobarns of data from the 1997 run period have been analysed.
Finally, ZEUS and H1 have presented their results on the measurements
of the high-Q² cross section using their full 1994-1997 data
set. Curious? Find the relevant links further down the page!
Just to make you familiar with the type of events this is all about: Here is one of these events (in fact it is the highest-Q² neutral-current deep-inelastic lepton-nucleon reaction ever observed by mankind):
A short introduction into the kinematics of deep-inelastic scattering and some explanation on what is shown on this event picture can be found in the events page which is introduced a little further down this page.
This section contains an overview of all ZEUS high-x and high-Q² results:
Of course the ZEUS results are documented in a paper which is published in Zeitschrift für Physik C74(1997)207. This paper has first been made available to the public as DESY-Preprint 97-025 (gzipped postscript file, 296 kbyte).
The results have first been presented in a DESY seminar at Wednesday, 19 February 1997. The ZEUS talk was held by Bruce Straub. The copies of his transparencies are available as a gzipped postscript file (189 kbyte).
Since all the ZEUS evidence for a deviation from the Standard Model prediction comes from five events, we feel that it is fair to let you have a look at the details of each of these events.
One of the most frequently asked questions is: "Is there also a similar effect in Charged Current ep scattering?" Meanwhile, ZEUS has released preliminary results on this reaction channel. Here are the transparencies of the talk at the Stony Brook conference, where these results were first presented to the public.
Of course it is of superior interest whether the effect can be comfirmed in the new data taken in the 1997 run period. A first bunch of these data have been analysed by both the H1 and the ZEUS collaborations meanwhile. Here are the transparencies of the talk at the Lepton-Photon '97 conference, where Bruce Straub presented this update of the high-Q² analyses.
Well, we've more than doubled our total collected luminosity in 1997,
and we are able to increase the statististical precision of the
high-Q² analyses. In addition, a lot has been learned about
technical issues and systematic effects. The latest results, covering
both NC and CC, are based on the full 1994-1997 data set and habe been
presented by Arnulf Quadt in a DESY seminar on 13.03.98 (yes, it was a
Friday). You might like to have a look at the
transparencies...
You might wonder whether the ZEUS observation is significant and ask whether the other HERA ep experiment, H1, has seen comparable evidence for an excess of events at high x and Q².
They have! If you havn't seen the H1 results yet, you should visit the H1 high-Q² events page.
Although it is still too early to speculate about possible origins of these observations, you might be interested in a synopsis of physical scenarios which could yield these extra events at high x and Q². A detailed review of physics beyond the Standard Model in ep collisions can be found in the "Beyond the Standard Model" section of the "Future Physics at HERA" proceedings. A short introduction to the relevant kinematic variables can be found on the ZEUS high-x, high-Q² events page.
There are various theoretical scenarios predicting different types of such states (which also include fundamental states coupling to positron and quark).
One type of hypothesized particles which might explain the observed effect are leptoquarks, which are suggested e.g. by Grand Unfication Theories (GUT's). Leptoquarks are expected to be produced as narrow resonances in the invariant positron-quark mass (respectively in x), which would be widened by the experimental resolution and also by QED and QCD radiation. Several leptoquark types can be constructed from the weak-isospin multiplets of quarks and leptons, several of them coupling to positrons and quarks and some in addition also to neutrinos and quarks. The latter implies that there also might be an effect in charged current ep scattering.
One of the most topical theoretical scenarios beyond the Standard Model is Supersymmetry (SUSY), which assigns bosonic (fermionic) "superpartners" to all existing fermions (bosons) and in addition extends the Standard Model Higgs sector. Supersymmetric models involve a new discrete quantum number called R-parity. This quantum number is 1 for "conventional" particles and -1 for their superpartners. If R-parity is not conserved (which means that lepton or baryon number conservation is violated), squarks (the superpartners of quarks) couple to positron and quark and can hence be resonantly produced exactly like leptoquarks.
The Standard Model describes positron-quark scattering via the exchange of virtual bosons (photons or the heavy gauge bosons, Z and W). In the case of additional positron-quark scattering mechanisms with an associated high mass scale (where "high" means beyond the HERA center-of-mass energy), the cross-section for ep scattering at high Q² is expected to be higher than predicted by the Standard Model. Such so-called contact-term interactions might arise from very heavy bosons (possibly mediating new types of forces), from heavy leptoquark-type objects being exchanged between positron and quark, and also if positrons and quarks are composite and share the same sub-components.
Non-standard evolution of the parton densities with Q²
All the excitement is based on our confidence that the Standard Model prediction can be evaluated with an accuracy of better than 10%. A large fraction of this uncertainty is due to the extrapolation of parton distributions in the proton to the high values of Q² and x relevant to the observed excess of events. The input to this extrapolation are high-precision structure functon measurements by fixed-target experiments at high x and low Q² on the one hand and the application of next-to-leading order perturbative QCD calculations (pQCD) on the other hand.
The fixed-target structure function data have small errors and are consistently reported by different experiments, so there is little doubt as to their accuracy. The pQCD calculations have proven to describe very successfully the Q² dependence of proton structure functions and parton distributions in the huge kinematic range covered by experimental data. In fact, no obvious way has been found to reconcile the observed access of events with the fixed-target data and pQCD.
However, one might certainly speculate whether the validity range of pQCD is limited at some high Q² scale (i.e. whether there is a fundamental theory of which QCD is the low-energy limit, or whether possibly some up-to-now hidden non-perturbative aspect of QCD has not been taken into account properly.
Of course the experimental observation of a possible deviation from the Standard Model has raised considerable interest in the theory community, and meanwhile there are various publications dealing with possible origins of the observed excess of events at high x and Q².
A list of these papers, with links to the postscript sources in the hep archive, can be found in the High-x and high-Q² references page. This list will be updated periodically. Please communicate errors or missing items to Uli Katz
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